handbook of zinc oxide volume 1

Handbook ofZinc Oxide and

Related Materials

Electronic Materials and Devices Series

Volume One

Materials

Edited by

Zhe Chuan Feng

Electronic Materials and Devices Series Feng

Handbook of Zinc O

xide and Related Materials

Volume One

Series Editors: Yongbing Xu and Jean-Pierre Leburton

Handbook of Zinc Oxide and Related Materials

Materials

Volume One

ISBN: 978-1-4398-5570-6

9 781439 855706

90000

K12599

Through their application in energy-efficient and environmentally friendly devices, zinc oxide (ZnO) and related classes of wide gap semiconductors, including GaN and SiC, are revolutionizing numerous areas, from lighting, energy conversion, photovoltaics, and communications to biotechnology, imaging, and medicine. With an emphasis on engineering and materials science, Handbook of Zinc Oxide and Related Materials provides a comprehensive, up-to-date review of various technological aspects of ZnO.

Volume One presents fundamental knowledge on ZnO-based materials and technologies. It covers the basic physics and chemistry of ZnO and related compound semiconductors and alloys. The first part of this volume discusses preparation methods, modeling, and doping strategies. It then describes epitaxial methods used to create thin films and functional materials. The book concludes with a review of alloys and related materials, exploring their preparation, bulk properties, and applications.

Covering key properties and important technologies of ZnO-based devices and nano-engineering, the handbook highlights the potential of this wide gap semiconductor. It also illustrates the remaining challenging issues in nanomaterial preparation and device fabrication for R&D in the twenty-first century.

Features• Presents the essentials on ZnO-based materials and technologies • Describes the key properties of ZnO and its alloys • Emphasizes the growth and characterization of novel nanostructures• Highlights the remaining issues in nanomaterial preparation for future

R&D

Materials Science

K12599_COVER_final.indd 1 8/9/12 10:20 AM


Related MaterialsVolume One

Materials

Handbook of Zinc Oxide and Related Materials: Volume One, Materials

Handbook of Zinc Oxide and Related Materials: Volume Two, Devices and Nano-Engineering


Series Editors

Jean-Pierre Leburton and Yongbing Xu

The MOCVD Challenge: A Survey of GaInAsP-InP and GaInAsP-GaAs for Photonic and Electronic Device Spplications, Second Edition, Manijeh Razeghi

Handbook of Zinc Oxide and Related Materials: Volume One, Materials, Edited by Zhe Chuan Feng

Handbook of Zinc Oxide and Related Materials: Volume Two, Devices and Nano-Engineering, Edited by Zhe Chuan Feng


Related MaterialsVolume One

Materials

Edited byZhe Chuan Feng

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vii

Contents

Preface...............................................................................................................................................ixEditor................................................................................................................................................xiContributors.................................................................................................................................. xiii

Part I ZnO Basic

1. Brief Historical Review of Research and Basic/Interdisciplinary Characterization of ZnO........................................................................................................3Zhe Chuan Feng

2. Pressurized Melt Growth of ZnO Single Crystals......................................................... 37Jeff Nause

3. New Design and Development of MOCVD, Process and Modeling for ZnO-Based Materials..................................................................................................... 47G.S. Tompa and S. Sun

4. p-Type ZnO: Current Status and Perspective.................................................................. 81Zhizhen Ye, Haiping He, Jianguo Lu, and Liping Zhu

Part II ZnO Epitaxy

5. ZnO Nanostructures and Thin Films Grown in Aqueous Solution: Growth, Defects, and Doping........................................................................................... 107S.J. Chua, C.B. Tay, and J. Tang

6. Second Harmonic Generation and Related Studies on ZnO Films.......................... 141Maria Cristina Larciprete and Mario Bertolotti

7. Optical Properties and Carrier Dynamics of ZnO and ZnO/ZnMgO Multiple Quantum Well Structures................................................................................. 167Bong-Joon Kwon and Yong-Hoon Cho

8. p-Type ZnO-N Films: Preparation and Characterization by Synchrotron Radiation................................................................................................. 205C.W. Zou and W. Gao

9. Optical Properties of MgZnO/ZnO Heterostructures Grown on Sapphire Substrates by Plasma-Assisted Molecular Beam Epitaxy.........................223Y.M. Lu, P.J. Cao, W.J. Liu, D.L. Zhu, X.C. Ma, D.Z. Shen, and X.W. Fan

viii Contents

Part III ZnO Alloys

10. The (Mg,Zn)O Alloy........................................................................................................... 257Holger von Wenckstern, Rüdiger Schmidt-Grund, Carsten Bundesmann, Alexander Müller, Christof P. Dietrich, Marko Stölzel, Martin Lange, and Marius Grundmann

11. Growth and Characterizations of Zn(Mg,Cd)O Alloys and Heterostructures Using Remote-Plasma-Enhanced MOCVD................................................................... 321Kenji Yamamoto and Jiro Temmyo

12. Structural and Optical Properties of Zn1−xCuxO Thin Films..................................... 351Ram S. Katiyar and Kousik Samanta

13. Structural and Magnetic Properties of ZnO Alloy Films with Cu, Cr, and Fe by RF Magnetron Sputtering Technique............................................. 373X.M. Wu, L.J. Zhuge, Z.F. Wu, and C.G. Jin

ix

Preface

Zinc.oxide.(ZnO).is.an.“old”.semiconductor.that.has.attracted.the.attention.of.researchers.for.a. long.time.because.of. its.applications. in.science.and.industry.such.as.piezoelectric.transducers,.optical.waveguides,.acousto-optic.media,.conductive.gas.sensors,.transparent.conductive.electrodes,.and.so.on.

ZnO,.which.crystallizes.in.the.wurtzite.structure,.is.a.direct.band-gap.semiconductor.with.a.room.temperature.band.gap.of.3.37.eV,.an.exciton.binding.energy.of.60.meV,.and.other.useful.properties..ZnO.can.be.grown.at.relatively.low.temperatures.below.500°C..The.band.gap.of.ZnO.can.be.tuned.by.forming.alloys.of.ZnMgO,.ZnCdO,.etc..Magnetic.semiconductors.can.be.obtained.from.ZnMnO,.ZnCrO,.and.so.on,.which.have.wonderful.applications.in.spintronics.and.other.fields.

Therefore,. ZnO. and. related. materials. as. well. as. quantum/nanostructures. have. now.received.increasing.attention.and.have.been.recognized.as.promising.candidates.for.effi-cient. UV/blue. light–emitting. diodes. (LEDs),. sensors,. photodetectors,. and. laser. diodes.(LDs)..A.strong.research.trend.has.formed..A.large.number.of.publications.and.books.have.now.appeared,.and.conferences.have.been.held..More.new.researchers,.contributors,.and.especially.new.graduate.students.have.devoted.themselves.to.these.fields.

In.recent.years,.research.and.development.on.wide.gap.semiconductors,.GaN-SiC-ZnO.and. related. materials,. and. quantum/nanostructures. have. been. very. active.. GaN-based.LEDs.are.forming.new.industries.worldwide..It.is.expected.that.LEDs.may.replace.tradi-tional.lightbulbs.and.tubes.to.achieve.a.new.lighting.echo..SiC.is.recognized.as.the.power.electronic.material.for.the.twenty-first.century..ZnO.is.rapidly.emerging.as.a.third.class.of.promising.wide.gap.semiconductors..ZnO.and.related.materials—together.with. two.other.classes.of.wide.gap.semiconductors,.GaN.and.SiC—are.currently.revolutionizing.an.increasing.number.of.applications.and.bring.apparent.benefits.to.vast.areas.of.develop-ment,.such.as.lighting,.communications,.biotechnology,.imaging,.energy.conversion,.pho-tovoltaic,.and.medicine,.with.energy-efficient/saving.and.environment-friendly.devices.

I.have.recently.published.four.review.books.on.SiC.and.III-Nitrides..The.current. two.volumes.on.ZnO.and.related.materials,.devices.and.nano-engineering,.provide.up-to-date,.comprehensive.reviews.of.various.technological.fields.on.ZnO.

The.research.and.application.on.these.materials.and.devices.are.developing.very.fast..Data,. even. if. published. recently,. need. to. be. updated. constantly.. This. two-volume. set...covers.the.state.of.the.art.in.the.field..These.books.are.oriented.more.toward.engineering.and.materials.science.rather.than.pure.science.

Handbook of Zinc Oxide and Related Materials: Volume One, Materials and Handbook of Zinc Oxide and Related Materials: Volume Two, Devices and Nano-Engineering.are. intended.for.a.wide. range. of. readers. and. covers. each. of. the. basic. and. critical. aspects. of. ZnO. science.and. technology.. Each. chapter,. written. by. experts. in. the. field,. reviews. the. important.topics. and. achievements. in. recent. years,. especially. after. 2005,. discusses. the. progress.made. by. different. groups,. and. suggests. further. works. needed.. This. volume. provides.useful.information.about.the.device.and.nanoscale.process;.the.fabrication.of.LEDs,.LDs,.photodetectors,.and.nanodevices;.and.the.characterization,.application,.and.development.of.ZnO-based.semiconductor.devices.and.nano-engineering.

Handbook of Zinc Oxide and Related Materials: Volume One, Materials consists. of. 13. well-written. chapters,. and. is. divided. into. 3. parts:. Part. I—ZnO. Basic,. Part. II—ZnO. Epitaxy,.

x Preface

and.Part. III—ZnO.Alloys.. It.presents. the.key.properties.of.ZnO-based.devices.and.nano-engineering,.describes.important.technologies,.and.demonstrates.the.remaining.challenging.issues.in.nanomaterial.preparation.and.device.fabrication.for.R&D.in.the.twenty-first.century..It.can.serve.well.material.growers.and.evaluators,.device.design.and.processing.engineers,.as.well.as.potential.users.of.ZnO-based.technologies,. including.newcomers,.postgraduate.students,.engineers,.and.scientists.in.the.ZnO.and.related.fields.

Zhe Chuan FengNational Taiwan University

and

Feng Research Laboratories

xi

Editor

Professor Zhe Chuan Feng.received.his.BS.(1962–1968).and.MS.(1978–1981).from.Peking.University,. Department. of. Physics,. Beijing,. People’s. Republic. of. China.. He. engaged. in.semiconductor.growth.process,.device. fabrication.and. testing,. semiconductor. laser.and.waveguide.optics,.and.teaching.activities.in.China.until.1982..He.then.moved.to.the.United.States.and.received.his.PhD.in.condensed.matter.physics.from.the.University.of.Pittsburgh.in. 1987.. He. also. worked. at. Emory. University. (1988–1992),. the. National. University. of.Singapore. (1992–1994),. EMCORE. Corporation. (1995–1997),. the. Institute. of. Materials.Research. and. Engineering. (1998–2001),. Axcel. Photonics. (2001–2002),. and. Georgia. Tech.(1995,.2002–2003),.with.much.success..Since.August.2003,.Dr..Feng.has.been.a.professor.at. the. Graduate. Institute. of. Photonics. and. Optoelectronics. and. in. the. Department. of.Electrical.Engineering,.National.Taiwan.University..His.current.research.interests.include.materials. research. and. MOCVD. growth. of. LED,. III-Nitrides,. and. SiC,. ZnO,. and. other.semiconductors/oxides.

Dr.. Feng. has. edited/coedited. nine. specialized. review. books. on. compound. semi-conductors. and. microstructures,. porous. Si,. SiC,. and. III-Nitrides,. ZnO. devices,. and.nano-.engineering. (including. the. current. two-volume. ZnO. books). and. has. published.approximately.500.scientific.papers.with.more.than.190.selected.by.the.Science.Citation.Index.(SCI).and.cited.nearly.2300.times..He.has.been.a.symposium.organizer.and.invited.speaker.at.various. international.conferences.and.universities,.has.served.as.a.reviewer.for. several. international. journals,. and. has. been. a. guest. editor. of. Thin Solid Films. and.Surface and Coatings Technology..He.has.also.been.a.visiting/guest.professor.at.South.China.Normal.University,.Huazhong.University.of.Science.and.Technology,.Nankai.University,.and.Tianjin.Normal.University..He.is.currently.a.member.of.the.International.Organizing.Committee.of.Asian.Conferences.on.Chemical.Vapor.Deposition.and.serves.on.the.board.of.directors.for.the.Taiwan.Association.for.Coating.and.Thin.Film.Technology.(TACT)..website:.http://www.ee.ntu.edu.tw/profile?id=57.

xiii

Contributors

Mario BertolottiDepartment.of.Basic.and.Applied.Sciences.

for.EngineeringSapienza.University.of.RomeRome,.Italy

Carsten BundesmannLeibniz.Institute.of.Surface.ModificationLeipzig,.Germany

P.J. CaoShenzhen.Key.Laboratory.of.Special.

Functional.MaterialsCollege.of.Materials.Science.and.

EngineeringShenzhen.UniversityShenzhen,.People’s.Republic.of.China

Yong-Hoon ChoDepartment.of.PhysicsandGraduate.School.of.Nanoscience.and.

TechnologyKorea.Advanced.Institute.of.Science.and.

TechnologyDaejeon,.Republic.of.Korea

S.J. ChuaDepartment.of.Electrical.and.Computer.

EngineeringNational.University.of.SingaporeandInstitute.of.Materials.Research.and.

EngineeringSingapore,.Singapore

Christof P. DietrichInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

X.W. FanLaboratory.of.Excited.State.ProcessesChangchun.Institute.of.Optics,.Fine.

Mechanics.and.PhysicsChinese.Academy.of.SciencesChangchun,.People’s.Republic.of.China

Zhe Chuan FengDepartment.of.Electrical.EngineeringInstitute.of.Photonics.and.OptoelectronicsNational.Taiwan.UniversityTaipei,.Taiwan

W. GaoDepartment.of.Chemical.and.Material.

EngineeringThe.University.of.AucklandAuckland,.New.Zealand

Marius GrundmannInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

Haiping HeState.Key.Laboratory.of.Silicon.MaterialsDepartment.of.Materials.Science.and.

EngineeringZhejiang.UniversityHangzhou,.People’s.Republic.of.China

C.G. JinDepartment.of.PhysicsandJiangsu.Key.Laboratory.of.Thin.FilmsSuzhou.UniversitySuzhou,.People’s.Republic.of.China

Ram S. KatiyarDepartment.of.PhysicsUniversity.of.Puerto.RicoSan.Juan,.Puerto.Rico

xiv Contributors

Bong-Joon KwonDepartment.of.PhysicsandGraduate.School.of.Nanoscience.and.

TechnologyKorea.Advanced.Institute.of.Science.and.

TechnologyDaejeon,.Republic.of.Korea

Martin LangeInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

Maria Cristina LarcipreteDepartment.of.Basic.and.Applied.Sciences.

for.EngineeringSapienza.University.of.RomeRome,.Italy

W.J. LiuShenzhen.Key.Laboratory.of.Special.



Jianguo LuState.Key.Laboratory.of.Silicon.MaterialsDepartment.of.Materials.Science.and.


Y.M. LuShenzhen.Key.Laboratory.of.Special.



X.C. MaShenzhen.Key.Laboratory.of.Special.



Alexander MüllerInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

Jeff NauseCermet,.Inc.Atlanta,.Georgia

Kousik SamantaCondensed.Matter.Physics.DivisionIndira.Gandhi.Centre.for.Atomic.ResearchKalpakkam,.India

Rüdiger Schmidt-GrundInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

D.Z. ShenLaboratory.of.Excited.State.ProcessesChangchun.Institute.of.Optics,.Fine.

Mechanics.and.PhysicsChinese.Academy.of.SciencesChangchun,.People’s.Republic.of.China

Marko StölzelInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

S. SunStructured.Materials.Industries,.Inc.Piscataway,.New.Jersey

J. TangDepartment.of.Electrical.and.Computer.

EngineeringNational.University.of.SingaporeSingapore,.Singapore

C.B. TayDepartment.of.Electrical.and.Computer.

EngineeringNational.University.of.SingaporeSingapore,.Singapore

xvContributors

Jiro TemmyoResearch.Institute.of.ElectronicsShizuoka.UniversityHamamatsu,.Japan

G.S. TompaStructured.Materials.Industries,.Inc.Piscataway,.New.Jersey

Holger von WencksternInstitute.for.Experimental.Physics.IIUniversität.LeipzigLeipzig,.Germany

X.M. WuDepartment.of.PhysicsandJiangsu.Key.Laboratory.of.Thin.FilmsSuzhou.UniversitySuzhou,.People’s.Republic.of.China

Z.F. WuDepartment.of.PhysicsandJiangsu.Key.Laboratory.of.Thin.FilmsSuzhou.UniversitySuzhou,.People’s.Republic.of.China

Kenji YamamotoResearch.Institute.of.ElectronicsShizuoka.UniversityHamamatsu,.Japan

Zhizhen YeState.Key.Laboratory.of.Silicon.MaterialsDepartment.of.Materials.Science.and.


D.L. ZhuShenzhen.Key.Laboratory.of.Special.



Liping ZhuState.Key.Laboratory.of.Silicon.MaterialsDepartment.of.Materials.Science.and.


L.J. ZhugeAnalysis.and.Testing.CenterSuzhou.UniversitySuzhou,.People’s.Republic.of.China

C.W. ZouDepartment.of.Chemical.and.Material.

EngineeringThe.University.of.AucklandAuckland,.New.Zealand

and

National.Synchrotron.Radiation.Laboratory

University.of.Science.and.Technology.of.China

Hefei,.People’s.Republic.of.China

Part I

ZnO Basic

3

1Brief Historical Review of Research and Basic/Interdisciplinary Characterization of ZnO

Zhe Chuan Feng

CONTENTS

1.1. Introduction.............................................................................................................................41.2. Brief.Historical.Review.of.Research.and.Development.on.ZnO.....................................51.3. Basic.Properties.of.ZnO.and.Related.Alloys.......................................................................61.4. Optical.Characterization.of.Bulk.ZnO.Materials...............................................................9

1.4.1. Bulk.ZnO.Materials.Grown.by.Modified.Melt.Growth.Technique.....................91.4.2. Raman.Scattering.from.Bulk.ZnO...........................................................................91.4.3. Photoluminescence................................................................................................... 111.4.4. Optical.Transmission.for.Bulk.ZnO....................................................................... 121.4.5. Summary.................................................................................................................... 14

1.5. ZnO.Film.on.Sapphire:.Rutherford.Backscattering.and.Optical.Characterization...... 141.5.1. ZnO.Thin.Layers.Grown.on.Sapphire.by.MOCVD............................................. 141.5.2. Photoluminescence.of.Epitaxial.ZnO.................................................................... 141.5.3. Optical.Transmission.for.ZnO/Sapphire............................................................... 151.5.4. Variable.Angle.Scanning.Ellipsometry................................................................. 161.5.5. Rutherford.Backscattering....................................................................................... 181.5.6. Atomic.Force.Microscopy........................................................................................ 191.5.7. Scanning.Electron.Microscopy............................................................................... 201.5.8. Summary.................................................................................................................... 20

1.6. Cr-Doped.ZnO.Films.on.Si.by.Sputtering.........................................................................221.6.1. Magnetron.Sputtering.of.Cr-Doped.ZnO.Thin.Layers.on.Si.Substrate............221.6.2. Combined.UV.Micro-PL.and.Raman.Spectra......................................................221.6.3. Multi-Phonon.Resonance.Raman.Scattering.from.Cr-Doped.ZnO..................221.6.4. Visible.Raman.Spectra.of.ZnO:Cr/Si.....................................................................231.6.5. X-Ray.Absorption.Near-Edge.Spectroscopy.on.O.K-Edge.................................251.6.6. Summary.................................................................................................................... 26

1.7. X-Ray.Photoelectron.Spectroscopy.on.Bulk.and.Epitaxial.ZnO.Materials.................. 271.7.1. XPS.on.ZnO—General............................................................................................. 271.7.2. XPS.of.ZnO.Bulk....................................................................................................... 271.7.3. XPS.of.Epitaxial.ZnO.by.MOCVD.......................................................................... 271.7.4. Summary.................................................................................................................... 29

1.8. More.Interdisciplinary.Studies.on.ZnO,.Alloys,.and.Nanostructures......................... 291.9. Conclusion............................................................................................................................. 32Acknowledgments......................................................................................................................... 32References........................................................................................................................................33

4 Volume One, Materials

1.1 Introduction

Research. and. development. on. advanced. semiconductors,. especially. wide. energy. gap.GaN-,.SiC-,.and.ZnO-based,.various.oxides,.related.materials,.and.quantum/nano.struc-tures,. have. been. very. extensive. in. recent. years.. Energy-efficient. and. environmentally.friendly. solid-state. light. sources,. in. particular. GaN-based. light-emitting. diodes. (LEDs).and.solar.cells,.are.currently.revolutionizing.an.increasing.number.of.applications,.and.bring.apparent.benefits.to.vast.areas.of.development,.such.as.lighting,.communications,.biotechnology,.imaging,.energy.conversion,.photovoltaic,.and.medicine.[1,2]..It.is.expected.that.LEDs.may.replace.traditional.light.bulbs.and.tubes.to.achieve.a.new.lighting.echo..Solar.cells.may.gradually.increase.their.share.in.energy.production..SiC.is.recognized.as.a.powerful.electronic.materials.for.the.twenty-first.century.[3,4]..Zinc.oxide.(ZnO).is.rapidly.rising.as.the.third.class.of.promising.wide-gap.semiconductor..New.oxides.and.compound.semiconductors.are.developing.amazingly,.which.may.also.be.incorporated.into.the.afore-mentioned.energy-saving.devices.

Indeed,.ZnO.is.an.“old”.semiconductor.that.has.been.drawing.attention.of.the.research-ers. for.a. long. time.because. of. its. applications. in. scientific.and. industrial. areas. such.as.piezoelectric. transducers,.optical.waveguides,.acousto-optic.media,. conductive.gas.sen-sors,.transparent.conductive.electrodes,.varistors,.and.so.on.[5].

ZnO,.crystallizing. in.the.wurtzite.structure,. is.a.direct.band-gap.semiconductor.with.a.room.temperature.band.gap.of.3.37.eV,.an.exciton.binding.energy.of.60.meV,.and.other.useful.properties..ZnO.can.be.grown.at.relatively.low.growth.temperatures.below.500°C.

The.band.gap.of.ZnO.can.be.tuned.via.divalent.substitution.on.the.cation.site.to.pro-duce.heterostructures..For.example,.Cd.substitution.leads.to.a.reduction.in.the.band.gap.to.~3.0.eV..In.the.case.of.ZnO.with.MgO.it.is.possible.to.tune.the.Eg.from.3.37.eV.(ZnO.band.gap). to.7.8.eV.(MgO.band.gap)..Substituting.Mg.on.the.Zn.site. in.epitaxial.films.can.increase.the.band.gap.to.approximately.4.0.eV.while.still.maintaining.the.wurtzite.structure.

Therefore,.ZnO.and. related.materials. as.well. as.quantum/nano.structures.have.now.received.increasing.attention.and.recognition.as.promising.candidates.for.efficient.UV/blue. light-emitting.diodes,. sensors,.photodetectors,.and. laser.diodes..A.strong.research.trend. has. been. formed.. A. large. number. of. publications,. conferences,. and. books. have.appeared..More.new.researchers,.contributors,.and.especially.new.graduate.students.have.devoted. themselves. to. these. fields.. The. two. volumes. of. the. current. book. on. ZnO. and.related.materials,.devices,.and.nano-engineering.provide.comprehensive.reviews.in.vari-ous.technological.fields.on.ZnO.

This. chapter,. as. the.first.one.of. these. two.volumes,.presents.a.brief.historical. review.of. research. and. development. on. ZnO,. some. basic. properties. and. various. characteriza-tion.technologies.applied.to. the.studies.on.bulk.and.epitaxial.ZnO,.ZnO.ternary.alloys.of.AlZnO.and.MgZnO,.nano-structural.ZnO,.and.ZnO.substrates.for.the.growth.of.GaN.and. InGaN. materials.. For. the. characterization. side,. the. author. would. like. to. introduce.basic.characterization.techniques,.advanced.optical,.structure.and.surface,.nuclear.science,.and.synchrotron.radiation,.as.applicable.to.different.material.systems.mentioned.in.this.chapter..This.information.should.serve.researchers.and.engineers,.especially.newcomers.and.students,.conveniently.to.get.familiar.with.the.research.developments.on.ZnO.and.to.easily.learn.the.techniques.that.might.not.be.familiar.to.them.previously..New.up-to-date.references.are.presented,.including.those.in.the.recent.2.years,.that.is,.2010–2011,.to.help.readers.catch.up.with.the.most.recent.work..All.the.experimental.data.and.graphs.were.

5Historical Review and Basic Characterization

produced.in.the.interest.of.a.group.of.authors.and.students..Most.of.them.are.presented.for.the.first.time.

The. experimental. samples. given. in. this. chapter. were. prepared. from. various. growth.technologies,.including.bulk.growth.technique,.metalorganic.chemical.vapor.deposition.(MOCVD),.pulsed. laser.deposition. (PLD),.etc.,. the.details.of.which.can.be. found. in. the.references.cited.

1.2 Brief Historical Review of Research and Development on ZnO

Zinc.oxide.(ZnO).powder.has.been.widely.used.as.a.white.paint.pigment.and.industrial.processing.chemical. for.nearly.150.years.. In. the.early.1900s,.white,.polycrystalline.ZnO.powder.was.extensively.applied.in.medical.technology.and.in.the.cosmetics.and.pharma-ceutical.industries.[6]..In.the.1930s,.some.research.activities.on.ZnO,.including.the.photolu-minescence.and.electroluminescence.properties.of.ZnO,.appeared.and.this.early.work.was.reviewed.and.documented.in.Refs.[7–9],.which.were.cited.by.Klingshirn.[10].

After. the. invention. of. the. semiconductor. transistor. before. 1950,. the. semiconductor.age. began. and. systematic. studies. on. ZnO. as. a. compound. semiconductor. were. also.started..Following.the.rediscovery.of.ZnO.and. its.potential.applications. in. the.1950s,.science.and.industry.alike.began.to.realize.that.ZnO.had.many.interesting.novel.prop-erties.that.were.worth.further.investigation..Research.on.ZnO.entered.into.a.“modern.rediscovery”.after.the.mid-1950s.[6]..In.1957,.the.New.Jersey.Zinc.Company.published.a.book,.Zinc Oxide Rediscovered.[8],.to.promote.the.ZnO.materials’.“frontier”.properties.of.semiconducting,.luminescent,.catalytic,.ferrite,.photoconductive,.and.photochemical.applications.[11].

In.1960,.the.good.piezoelectric.properties.of.ZnO.were.discovered,.which.led.to.the.first.electronic.application.of.ZnO.as.a.thin.layer.for.surface.acoustic.wave.devices.[12]..Efforts.have. been. made. toward. fabrication. of. large-size. ZnO. substrates. of. excellent. structural.perfection..A.variety.of.ZnO.substrate.growth.techniques.are.being.explored,.the.underly-ing.basis.for.which.was.developed.in.the.1960s.and.1970s..Owing.to.the.renewed.need.for.large.high-quality.single.crystals,.these.methods.are.being.revamped.for.producing.large.area.wafers.reproducibly.and.economically.[13].

Throughout.the.1960s,.a.series.of.studies.on.the.fundamental.properties.of.ZnO.were.conducted,.for.example,.on.the.phonon.properties.by.Raman.scattering.[14–17].and.infra-red.(IR).spectroscopy.[18,19],.and.also.on.its.energy.band.gap.and.structures,.excitons,.electron. and. hole. effective. masses,. and. electrical. transport. properties. of. undoped. or.intrinsic. ZnO. [6].. Numerical. values. for. ε-infinite. were. obtained. from. precise. below-band-gap.index-of-reflection.measurements.using.the.minimum.deviation.method.[20]..With. ZnO,. Au. Schottky. barriers. were. formed. in. 1965. [21,22]. and. LEDs. were. demon-strated.in.1967.[22,23].

Until. the. 1970,. doping. and. implantation. of. impurities. into. ZnO. were. studied. [24].. It.was.found.that.the.n-type.conductivity.can.be.obtained.rather.easily.from.ZnO,.and.that.as-grown,.unintentionally.doped.ZnO.is.always.n-type.because.of.high.concentration.of.background.donors.which.are.mainly.H,.O.vacancy,.VO,.Zn.interstitial.Zni,.and.group.III.elements,.especially.Al.[25]..However,.the.p-type.conductivity.in.ZnO.is.a.formidable.chal-lenge.and.this.doping.asymmetry.arises.because.wide-gap.semiconductors.either.have.a.low.valence-band.maximum.or.a.high.conduction-band.minimum.[26,27]..This.research.


wave.on.ZnO.peaked.around.the. late.1970s.and.the.early.1980s,.and.thereafter. interest.faded.away,.according.to.Klingshirn.[10,28].

From. Ellmer. and. Klein. [12],. the. trace. of. the. annual. publication. number. on. ZnO. in.1960–2005.showed. that. the.ZnO.publication.numbers.per.year.were.quite. few. in.early-middle.1960s,.slightly.increased.in.the.late.1960s,.reached.a.small.peak.in.1970,.continued.to.increase.to.reach.another.peak.value.of.about.500.in.the.1985,.decreased.down.till.1990,.and.then.increased.again.to.reach.about.1000.in.1996–2000..After.entering.the.twenty-first.century,.this.value.increased.rapidly.above.3000.in.2005..The.research.on.ZnO.experiences.a.very.vivid.renaissance.[29]..In.recent.years,.from.2005.till.now,.every.year,.the.number.of.publications.on.ZnO.and.related.materials,.including.nano-structures.and.device.applica-tions,.is.increasing..Important.achievements.and.results.can.be.seen.in.a.number.of.books.[5,6,11,12,22,28].and.review.articles.[10,12,13,24–26,29–32].in.the.recent.period.of.2006–2011.

During. the. past. two. decades,. research. and. development. (R&D). on. GaN,. III-nitrides,.and. devices. have. achieved. great. breakthroughs.. Energy-efficient. and. environmentally.friendly.solid-state. light.sources,. in.particular.GaN-based. light-emitting.diodes. (LEDs),.are. currently. revolutionizing. an. increasing. number. of. applications. and. industries. and.bringing. apparent. benefits. to. vast. areas. of. development,. such. as. lighting,. communica-tions,.biotechnology,.imaging,.and.medicine.[1,2]..It.is.expected.that.LEDs.may.replace.the.traditional.light.bulbs.and.tubes.to.achieve.a.new.lighting.echo..The.second.class.of.wide-gap.semiconductors,.SiC,.is.continually.attracting.a.great.deal.of.R&D.[3,4,33,34]..ZnO.and.related.materials.have.formed.a.third.class.of.wide-gap.semiconductors,.complementary.to.III-nitrides.and.SiC..These.three.classes.of.wide-gap.semiconductors.are.indeed.develop-ing.in.correlation,.because.of.their.many.common.or.similar.properties,.such.as.wurtzite.crystalline.structures.and.similar.range.values.of.lattice.constants.and.energy.band.gap.

1.3 Basic Properties of ZnO and Related Alloys

ZnO.is.a.direct.band.gap.semiconductor.with.a.room.temperature.(RT).band.gap,.Eg,.of.3.37.eV.and.an.exciton.binding.energy.of.60.meV..ZnO.normally.forms.in.the.hexagonal.(wurtzite).crystal.structure.with.lattice.constants.of.a.=.3.250.Å.and.c.=.5.207.Å..Figure.1.1.presents.the.relationship.diagram.of.the.energy.band.gap.versus.lattice.constant.for.main.compound.semiconductors,.including.ZnO..Two.major.groups.are.seen..One.is.with.the.lattice.constants.between.5.4.and.6.1.Å.for.mostly.traditional.III–V.and.II–VI.compounds..Another.group.involves.III-nitrides.(GaN,.AlN,.and.InN),.SiC.and.ZnO,.belonging.to.the.group.of.wide-gap.semiconductors..ZnO.is.situated,.in.Figure.1.1,.very.close.to.GaN.which.has. an. RT. Eg. of. 3.42. eV. and. an. exciton. binding. energy. of. 25. meV.. ZnO. has. a. smaller.c-plane.lattice.mismatch.of.1.8%.to.GaN,.and.a.perfect.lattice.match.in.the.a-axis.direction.to.InxGa1−xN.with.x.=.18%..These.make.the.ZnO.a.suitable.substrate.material.for.the.growth.of.GaN.and.InGaN.[35].

Some.important.properties.of.ZnO.are.listed.in.Table.1.1..More.can.be.found.in.the.books.on.ZnO.and.related.materials. [5–8,11,13,22,28].and.review.articles. [9,10,12,24–27,29–31],.which.are.cited.in.this.chapter..ZnO.is.normally.formed.in.hexagonal.(wurtzite).crystal.structure..The.Zn.atoms.are.tetrahedrally.coordinated.to.four.O.atoms,.where.the.Zn.d.electrons. hybridize. with. the. O. p. electrons,. as. shown. in. Figure. 1.2.. Electron. doping. in.nominally. undoped. ZnO. has. been. attributed. to. Zn. interstitials,. oxygen. vacancies,. or.hydrogen.[36,37]..The.intrinsic.defect.levels.that.lead.to.n-type.doping.lie.approximately.


3.00

1

3

2

Ener

gy g

ap (e

V)

4GaN

AIN

ZnO

Visible light

InNGaAs InP

CdSe

AIAsGaP

AIP

ZnS MgSe

MgS

ZnSeSiC

5

6

7

3.5 4.0 4.5Lattice constant (Å)

5.0 5.5 6.0 6.5

FIGURE 1.1Energy.band.gap.versus.lattice.constant.in.compound.semiconductors.

TABLE 1.1

Some.Important.Properties.of.ZnO

Property Value

Lattice.parameters.at.300.Ka0 0.32495.nmc0 0.52069.nma0/c0 1.602.(ideal.hexagonal.

structure.shows.1.633)Density 5.606.g.cm−3

Stable.phase.at.300.K WurtziteMelting.point 1975°CThermal.conductivity 0.6,.1–1.2Linear.expansion.coefficient.(/°C) A0:.6.531026

c0:.3.031026Static.dielectric.constant 8.656Refractive.index 2.008,.2.029Energy.gap 3.3.eV,.directIntrinsic.carrier.concentration <106.cm−3

Exciton.binding.energy 60.meVElectron.effective.mass 0.24Electron.Hall.mobility.at.300.K 200.cm2.V−1.s−1.(low.

n-type.conductivity)Hole.effective.mass 0.59Hole.Hall.mobility.at.300.K 5–50.cm2.V−1.s−1.(low.

p-type.conductivity)


0.01–0.05.eV.below.the.conduction.band..The.electron.Hall.mobility.in.ZnO.single.crys-tals.is.on.the.order.of.200.cm2.V−1.s−1.at.room.temperature..While.the.electron.mobility.is.slightly.lower.than.that.for.GaN,.ZnO.has.a.higher.theoretical.saturation.velocity..The.opti-cal.properties.of.ZnO,.studied.using.photoluminescence,.photoconductivity,.and.absorp-tion,.reflect.the.intrinsic.direct.band.gap,.a.strongly.bound.exciton.state,.and.gap.states.due.to. point. defects. [38,39]..A. strong. room. temperature. near-band-edge. UV. photolumines-cence.peak.at.~3.2.eV.is.attributed.to.an.exciton.state,.as.the.exciton.binding.energy.is.on.the.order.of.60.meV,.which.makes.it.promising.for.RT.lasing..In.addition,.visible.emission.is.also.observed.due.to.defect.states..A.blue-green.emission,.centered.at.around.530.nm.in.wavelength,.has.been.explained.within.the.context.of.transitions.involving.self-activated.centers.formed.by.a.doubly.ionized.zinc.vacancy.and.an.ionized.interstitial.Zn+,.oxygen.vacancies. [40],. donor–acceptor. pair. recombination. involving. an. impurity. acceptor. [41],.and/or.interstitial.O.[42,43]..A.broad.orange-red.photoluminescence.emission.at.~1.9.eV.can.be.observed.in.some.materials.and.has.been.assigned.to.defect.states.

The. valence. band. of. ZnO. is. split. by. crystal. field. and. spin. orbit. interaction. into.three. states. named. A,. B,. and. C. [10,24].. The. symmetry. of. the. upper. valence. subband.(A-subband).in.ZnO.is.Γ7,.while.B-subband.is.Γ9,.and.C-subband.is.Γ7.[10,24]..Detailed.research.and.discussions.can.be.seen.in.these.review.articles.and.reference.therein..In.addition,.ZnO.has.a.similar.thermal.expansion.coefficient.with.GaN.which.allows.for.almost.zero.thermal.strain.[44]..ZnO.substrates.are.conductive,.and.they.can.be.utilized.in.vertical.structures.allowing.for.multiple.electrodes.to.be.formed.on.both.surfaces.to.spread.current.further.[45]..ZnO.can.be.wet-etched.chemically.and.easily.removed.to.allow.for.a.thin.GaN.structure.[46]..ZnO.can.also.act.as.a.new.nonlinear.optical.(NLO).material. for. potential. application. in. integrated. optics,. in. addition. to. expensive. NLO.single.crystals.such.as.LiNbO3,.KTiOPO4,.and.LiTaO3,.and.other.ZnSe-.and.GaAs-based.semiconductor.heterostructures.[47].

FIGURE 1.2Wurtzite.structure.of.ZnO.with.big.symbols.for.Zn.atoms.and.small.symbols.for.O.atoms.


Furthermore,. ZnO. can. be. grown. at. relatively. low. growth. temperatures. by. thin. film.deposition. techniques. at. temperatures. <500°C,. which. enables. the. growth. of. ZnO. on.silicon.[48].and.glass.substrates..Its.refractive.index.shows.a.strong.variation.depending.on.the.dopant.used,.which.is.very.attractive.for.using.different.dopants.in.ZnO.for.optical.waveguide.applications..This.indicates.that.both.the.cladding.layer.with.lower.refractive.index.and.the.core.layer.with.higher.refractive.index.can.be.developed.with.ZnO.by.dop-ing. it. with. suitable. elements.. ZnO. doped. with. transition. metal. elements. has. attracted.interest.for.the.application.in.the.field.of.spintronics.[49]..Theoretical.predictions.and.some.experimental.results.have.indicated.that.it.is.a.promising.room.temperature.ferromagnetic.semiconductor..ZnO.is.also.well.poised.for.space.applications,.since.it.is.fairly.resistant.to.radiation.damage.compared.with.other.semiconductors.[50].

Therefore,.these.excellent.material.properties.from.ZnO.make.it.highly.suitable.for.appli-cations.in.UV.light.emitters,.varistors,.transparent.high.power.electronics,.surface.acoustic.wave.devices,.piezoelectric.transducers,.chemical.and.gas.sensing,.and.so.on.

1.4 Optical Characterization of Bulk ZnO Materials

1.4.1 Bulk ZnO Materials Grown by Modified Melt Growth Technique

A.series.of.bulk.ZnO.crystals.were.grown.at.Cermet.Inc..through.a.patented.technology.of.using.a.pressurized.melt.growth.approach.[51,52,Chapter.2.in.this.book]..The.method.involves.melting.and.crystallizing.materials.that.have.volatile.components.or.have.ther-modynamic.instabilities.at.or.near.the.material’s.melting.point.at.atmospheric.pressure..The.technique.consists.of.a.high-pressure.induction.melting.apparatus,.in.which.the.melt.is.contained.in.a.water-cooled.crucible..The.heat.source.used.during.the.melting.operation.is.radio.frequency.energy..The.highly.refractory.melt.produced.is.contained.in.a.cold.wall.crucible,.such.that.part.of.the.solid.thermal.barrier.between.the.molten.material.and.the.cooling.fluid.is.a.cooled.material.with.the.same.composition.as.the.melt..Large.ingot,.high.quality,.high.purity,.ZnO.crystals.have.been.crystallized,.oriented,.and.shaped.into.round.or. square. boules. and. eventually. processed. into. epitaxial-ready. substrates.. Centimeter-sized. single. crystals. were. obtained. from. these. boules. by. solidification.. Also,. the. melt.growth.enables.uniform.incorporation.of.different.dopants.

Three.optical.characterization.techniques,.including.Raman.scattering.(RS),.photolumi-nescence.(PL),.and.optical.transmission.(OT),.were.employed.at.room.temperature.(RT).to.assess.various.ZnO.bulk.wafer.samples,.including.undoped,.doped.with.Ga,.Er,.Co,.Ho,.Fe,.Mn,.and.co-doped.with.Mg.and.Li.

1.4.2 Raman Scattering from Bulk ZnO

Raman.scattering.was.performed.in.the.backscattering.configuration.under.the.excita-tion.of.514.5.nm.from.an.Ar+.laser.by.a.JY.T64000.Raman.Microscope.with.a.high.spec-tral.resolution.of.0.5.cm−1..Figure.1.3.shows.RT.Raman.spectra.under.excitation.of.514.5.nm.from.seven.ZnO.bulk.wafers.grown.by.Cermet.Inc..XRD.characterization.measure-ments.on.these.samples.have.exhibited.mainly.(0002).and.(0004).ZnO.wurtzite.crystal-line.patterns.[51,52]..Our.Raman.data.confirm.these.structural.features.of.our.bulk.ZnO.wafers..Wurtzite. (W).ZnO.belongs. to. the.space.group. C6

4v .with. two.formula.units. in.


the.primitive.cell,.having.Raman-active.phonon.modes,.E2.(low),.E2.(high),.A1. longitu-dinal.optical.(LO),.A1.transverse.optical.(TO),.E1(LO),.and.E1(TO).[53,54]..In.the.case.of.highly.oriented.ZnO.with.c-axis.along.the.surface.normal.direction,.only.E2.and.A1.(LO).modes.are.allowed. in. the.backscattering.geometry.and.expected. to.be.observed,.and.other.modes.are.forbidden.[53,54]..In.Figure.1.3,.all.bulk.ZnO.wafers.have.the.allowed.E2.dominant,.indicating.their.wurtzite.structure.with.the.wafer.surface.being.(0001).ori-ented,.that.is,.c-axis.

We.have.performed.Lorentzian.curve.fits.on.the.E2.mode.and.adjacent.spectral.region.for.all.Raman.spectra.from.all.ZnO.bulk.wafer.samples..A.typical.example.is.shown.in.Figure.1.4a..Assignments,.mode.peak.position,.and.half.width.for.all.samples.are.listed.in. Table. 1.2.. As. seen,. the. major. E2. mode. is. located. at. about. 438. cm−1,. indicating. un-strained.for.our.measured.ZnO.bulk.samples..The.shoulder.in.the.lower.frequency.side.

200

Mg, Li-doped (g)

Ga-doped (f)

Fe-doped (e)

Ho-doped (d)

Co-doped (c)

Er-doped (b)

Undoped (a)Bulk

438

203 332 cm–1

514 nm, RT

A1(TO)

E2 ZnO

300 400 500Raman shift (cm–1)

Ram

an in

tens

ity (a

. u.)

600 700 800

FIGURE 1.3RT.Raman.spectra.under.excitation.of.514.5.nm.from.seven.ZnO.bulk.wafers.

3600

5,000

10,000

15,000

20,000

25,000

380

ZnOBulkUndoped514.5 nm, RT

Lorentzian fits:

L1 - 437.8279

L2 - 433.8641

L3 - 412.6192

W1 - 5.6479

W2 - 9.1909

W3 - 39.2774

E2

E1(TO)

400 420Raman shift (cm–1)(a)

(cm–1)

Ram

an in

tens

ity

440 460 480 500 200

2000

3000 203 cm–1

4000

300

Bulk, undoped

400

514.5 nm, RT

Raman shift (cm–1)(b)

Ram

an in

tens

ity

500

537

E1(TO) A1(LO)

E2

723660

615576

ZnO332

600 700 800

FIGURE 1.4(a).Curve-fitted.Raman.spectrum,.in.the.region.of.E1(TO)-E2.modes,.of.an.undoped.ZnO.wafer;.(b).magnified.Raman.spectrum.of.an.undoped.ZnO.wafer.


of.the.E2.mode.could.be.de-convoluted.with.a.mode.at.about.434.cm−1..It.might.be.related.to.the.defect-.or.damage-broadening.and.the.detailed.origin.is.unknown.yet..A.weak.but.broad.mode.with.the.peak.position.between.408.and.412.cm−1.could.be.assigned.as.the. E1(TO). [53].. The. weak. appearance. of. this. forbidden. mode. reveals. some. degree. of.imperfection.of.our.experimental.ZnO.bulk.materials,.leading.to.the.partially.release.of.the.Raman.selection.rule.

Furthermore,.we.could.enlarge.the.Raman.spectra.to.see.other.weak.features.with.an.example.as.shown.in.Figure.1.4b..Two.modes.located.at.332.and.203.cm−1.were.observed.previously.from.ZnO.[13]..The.mode.at.332.cm−1.was.assigned.to.a.multiple-phonon.pro-cess.[8]..In.the.spectral.range.between.460.and.760.cm−1,.there.exists.a.broad.background.with.the.A1(LO).and.a.few.other.modes.superposed.on..The.origins.of.these.extra.modes.are.under.investigation.

1.4.3 Photoluminescence

RT.PL.was.measured.using.a.UV–Vis.Raman-PL.Microscope.under.the.excitation.of.325.nm.from.a.HeCd.laser..Figure.1.5.shows.RT.PL.spectra.of.seven.ZnO.bulk.wafers..The.PL.spectrum.from.the.undoped.ZnO.exhibits.a.dominant.band.near.3.27.eV,.which.is.due.to.

TABLE 1.2

Bulk.ZnO.Data.of.Raman.E2.and.A1(TO).Modes

ZnO SamplePeak L1, E2 (cm−1)

Width W1 (cm−1)

Peak L2 (cm−1)

Width W2 (cm−1)

Peak L3, E1(TO) (cm−1)

Width W3 (cm−1)

Undoped 437.83 5.65 433.86 9.19 412.62 39.28Er-doped 437.92 5.82 433.47 9.05 408.53 50.88Ho-doped 437.99 5.60 434.03 9.00 408.46 46.28Co-doped 437.89 5.60 434.13 9.03 412.48 36.94Fe-doped 437.92 5.65 433.79 9.29 412.15 36.72Ga-doped 436.92 10.52 424.21 6.22 411.40 16.99Mg,.Li-doped 437.78 6.26 433.95 9.75 411.92 29.05

2.0 2.5

(e) Fe-doped

(d) Co-doped

(c) Ho-doped

(b) Er-doped

(a) Un-doped bulk

ZnO 325 nm, RT

3.0Energy (eV)

PL in

tens

ity (a

. u.)

3.5

FIGURE 1.5RT.photoluminescence.spectra.of.seven.ZnO.bulk.wafers.


the.band.edge.emission,.or.band-to-band.recombination.including.contributions.of.exci-tons,.from.ZnO.crystal..The.doped.ZnO.samples.with.different.impurities.have.their.major.RT.PL.spectral.peaks.shifted.from.the.value.for.the.undoped.ZnO,.indicating.their.alloy.properties..Because.the.peak.positions.does.not.shift.away.too.much.from.the.peak.of.the.undoped.ZnO,.their.alloy.compositions.should.be.very.small,.for.example,.less.than.1%..These.PL.spectra.show.asymmetric.characteristic.

We.have.performed.the.spectral.curve.fittings.with.Gaussians.to.study.their.variations.from.sample.to.sample..Figure.1.6.shows.two.typical.examples.of.these.Gaussian.fits.on.two.RT.PL.spectra.from.bulk.ZnO.samples,.undoped.and.co-doped,.respectively..Table.1.3.shows.these.fitted.data..It.is.seen.that.the.major.PL.band.from.most.of.ZnO.bulk.wafer.sam-ples.is.located.at.3.26–3.28.eV,.indicating.the.near-band-edge.emission.from.wurtzite.ZnO.crystal..A.separated.PL.band.is.located.below.the.wurtzite.ZnO.characteristic.band,.79–168.meV.away,.depending.on.the.dopant.elements.and.doping.levels..Further.PL.measurements.in.the.low.temperature.would.provide.more.information.on.these.dopants.in.wurtzite.ZnO.

1.4.4 Optical Transmission for Bulk ZnO

OT.measurements.were.performed.using.a.Perkin.Elmer.UV–NIR.spectrometer.with.the.step.resolution.of.1.nm..Figure.1.7.shows.RT.UV-visible.optical. transmission.spectra.of.

3.00

200

400

600

800

1000

3.1

ZnOBulkUndoped

3.2 3.3

Gauss fits:P1 - 3.269 eV

P2 - 3.173 eVW1 - 77 meV

W2 - 78.8 meV

Energy (eV)(a)

PL in

tens

ity

3.4 3.5 3.00

50

100

150

200

250

300

350

400

3.1

ZnO

BulkCo-doped325 nm, RT

3.2Energy (eV)(b)

PL in

tens

ity

3.3

Gauss fits:

P1 - 3.274 eV

P2 - 3.152 eVW2 - 90.1 meV

W1 - 102.2 meV

3.4 3.5

FIGURE 1.6Gaussian.fits.of.two.typical.RT.PL.spectra.from.ZnO.bulk.samples.of.(a).undoped.and.(b).co-doped.

TABLE 1.3

Bulk.ZnO.Data.of.Transmission.and.Photoluminescence.Near-Band-Edge.Region

ZnO SampleOT Optical

Gap Eog (eV)Main PL

Peak E0 (eV)Width W1

(meV)2nd PL Fitted Peak E1 (eV)

Width W2 (meV)

E0 − E1 (meV)

Undoped 3.1548 3.2693 77 3.1734 78.8 95.9Er-doped 3.1468 3.2654 53.4 3.2329 168.3 32.5Ho-doped 3.1388 3.2587 47.5 3.2115 110.4 47.2Co-doped 2.8568 3.2744 102.2 3.1517 90.1 122.7Fe-doped 2.6721 3.2733 106.9 3.1475 102.9 125.8Ga-doped 3.1230Mg,.Li-doped 3.1629 3.2791 54.8 3.2515 142.7 27.6Mn-doped 1.9743


eight.ZnO.bulk.wafers,.undoped,.doped.with.Ga,.Er,.Co,.Ho,.Fe,.Mn,.and.co-doped.with.Mg.and.Li,.respectively,.grown.by.Cermet.Inc..In.order.to.precisely.distinguish.and.deter-mine.their.optical.gap,.Eog,.we.performed.differential.calculation.for.all.OT.spectra.in.the.wavelength.region.near.their.optical.gaps..Figure.1.8.exhibits.these.differential.spectra.of.UV–Vis.transmissions.for.seven.Cermet’s.ZnO.bulk.wafers..The.obtained.Eog.values.are.also.listed.into.the.Table.1.3.

It. is. interesting. to. find. that. the. RT. transmission. or. differential. transmission. spectra.exhibit.much.larger.differences.from.their.corresponding.RT.PL.spectra..For.example,.the.RT. PL. spectra. of. Co-. and. Fe-doped. ZnO. look. quite. similar,. with. major. emission. band.both.near.3.27.eV.close.to.the.band.value.from.undoped.ZnO..However,.their.transmission.or.differential.transmission.spectra.have.showed.very.large.difference.among.these.three.samples..These.mean.that.the.slight.doping.of.Co.and.Fe.impurities.into.bulk.ZnO.could.introduce.very.effective.impurity.bands,.leading.to.a.large.variation.in.their.optical.gap,.although.the.alloy.energy.band.might.alter.little.only..This.feature.could.be.very.useful.in.

3000

20

40

60

80

100

400 500Wavelength (nm)

Tran

smiss

ion

(%)

600 700

0390 400

20

40

60

80

100

800

Ho-doped

Er-doped

Mn-doped

Bulk ZnO

Mg, Li-dopedGa-doped

Co-dopedFe-dopedUn-dopedHo-dopedEr-doped

Un-doped

900

FIGURE 1.7RT.UV–visible.optical.transmission.spectra.of.seven.ZnO.bulk.wafers.

350 400 450Wavelength (nm)

Diff

eren

tial o

f tra

nsm

issio

n (a

. u.)

500

Un-doped

Er-doped

Ho-doped

Ga-doped

Co-dopedFe-doped

Bulk ZnO

550

FIGURE 1.8Differential.spectra.of.UV–Vis.transmissions.of.seven.Cermet.ZnO.bulk.wafers.


the.fabrication.of.optical.waveguide.devices.and.at.the.same.time.keeping.the.electronic.properties.less.varied..Further.investigation.in.this.interesting.topics.is.worthy.to.penetrate.

1.4.5 Summary

In.summary,.a.series.of.ZnO.crystal.wafers,.undoped.and.doped.with.various.of.dop-ants,. including.Ga,.Er,.Co,.Ho,.Fe,.Mn,.and.co-dopant.of.Mg-Li,.have.been.grown.by.a.modified.melt.growth.method.and.characterized.by.optical. techniques.of.Raman.scat-tering,. photoluminescence,. and. optical. UV–visible. transmission.. Raman. spectroscopic.measurements. have. identified. the. wurtzite. structure. with. (0001). orientation. along. the.normal.surface.direction.for.wafers.after.cutting.and.polishing..The.observation.of.weak.Raman.signals.from.selection.rule.broken.modes.indicated.a.small.degree.of.imperfec-tion.in.the.crystalline.structure.of.experimental.ZnO.samples..RT.PL.have.showed.most.of.the.ZnO.bulk.wafer.samples.with.the.major.band.edge.emissions.from.wurtzite.ZnO.crystal,.which.are.affected.only.slightly.by.dopings,.and.a.lower.energy.emission.band.depending.on.the.dopants..UV–visible.optical.transmission.measurements.revealed.the.optical.absorption.gap.varied.greatly,.depending.on.the.dopants.in.ZnO.crystal,.although.they.can.show.band.edge.emissions.closely..A.large.shift.in.the.optical.absorption.edge.has.been.observed.from.these.Mn/Co/Fe-doped.bulk.ZnO.crystals.in.comparison.with.undoped.ZnO,.with.information.of.the.impurity. levels.of. these.dopants.obtained..This.interesting.phenomenon.is.significant.and.can.be.useful.in.the.fabrication.of.optical.wave-guide.devices.based.upon.ZnO.materials.. In. this. study,. the.computer.fitting.processes.have. provided. an. easy. and. essential. way. to. deepen. our. understanding. on. the. optical.properties.of.the.ZnO.bulk.materials.

1.5 ZnO Film on Sapphire: Rutherford Backscattering and Optical Characterization

1.5.1 ZnO Thin Layers Grown on Sapphire by MOCVD

A.series.of.undoped.zinc.oxide.(ZnO).thin.films.were.grown.on.c-plane.sapphire.substrates.to.study.the.dynamics.of.ZnO.growth.by.MOCVD.in.a.highly.modified,.vertical. injec-tion,. commercial.MOCVD. reactor.. Temperature. was. kept. at. 500°C.. Diethylzinc. (DEZn).and.oxygen.(O2).were.used.as.precursors.for.Zn.and.O,.respectively,.and.nitrogen.(N2).was.employed.as.a.carrier.gas..Reactor.geometry.was.used.to.calculate.a.theoretical.growth.window.for.which.gas.flow.is.stable.and.in.the.laminar.flow.regime..Four.different.thick-ness.samples.were.prepared..O2.flow.rate.was.constant.at.8348.μmol.min−1.

1.5.2 Photoluminescence of Epitaxial ZnO

Figure.1.9.shows.a.room-temperature.photoluminescence.(PL).spectrum.with.325.nm.as.an.excitation.source..It.can.be.seen.that.all.the.ZnO.films.have.a.strong.UV.emission..This.peak.is.the.transition.of.free.excitons.with.LO.phonon.replicas..The.peaks.shown.for.SK94.are.3.45.eV.and.for.SK169,.SK268,.SK290.are.around.3.29.eV..It.has.also.been.observed.others.with.an.amount.of.about.0.16.eV..This.might.imply.that.sample.SK94.has.unintentionally.Al.doped.in.ZnO.films..Therefore,.the.higher.energy.PL.emissions.may.be.due.to.Al.diffu-sion.from.sapphire.and.forming.impurity.levels.inside.the.ZnO.layers.


A.very.weak.green.emission.can.be.seen.in.the.spectrum.of.samples.SK290.and.SK94,.while.the.broad.feature.at.around.2.35.eV.(green.emission).has.been.attributed.by.various.authors.to.the.recombination.of.free.electrons.with.holes.via.interstitial.zinc.[55],.or.via.defects.at.grain.boundaries.in.poly-crystalline.ZnO.investigated.by.cathodoluminescence..The.origin.of. this.peak. is. still. in.dispute,.but. it.was.usually.attributed. to. the.emission.related.to.native.point.defects.such.as.O.vacancy.and.OZn.[56,57]..For.samples.SK169.and.SK268,.the.green.peak.was.absolutely.absent,.suggesting.that.the.films.have.higher.quality.in.these.two.samples.

1.5.3 Optical Transmission for ZnO/Sapphire

The. optical. transmittance. was. recorded. with. a. UV–visible. spectrophotometer. (Perkin–Elmer.Lambda.900),.which.data.points.can.be.collected.from.190.to.800.nm.and.measured.at. room. temperature. with. calibration. in. atmosphere.. Figure. 1.10. shows. the. optical.

2.0 2.5

(4) (1)

(2)

(3)ZnO/sapphire

(1) SK169(2) SK290(3) SK94(4) SK268

Energy (eV)

PL in

tens

ity (a

. u.)

3.0 3.5

FIGURE 1.9RT.PL.spectra.of.ZnO/sapphire.samples.SK169,.SK290,.SK94,.and.SK268.

2.50.0

0.2

0.4

0.6

0.8

1.0

3.0Photon energy (eV)

Tran

smiss

ion

(%)

Tran

smiss

ion

(%)

3.5

(1)

(1) SK169(2) SK290(3) SK94

(4) SK268

(1)

(3)(2)

(4)3.30.00

0.05

0.10

0.15

0.20

0.25

0.30

3.4 3.5 3.6 3.7Photon energy (eV)

(3)(4)

(2)

ZnO/sapphire

FIGURE 1.10Optical.transmittance.of.four.ZnO/sapphire.samples.


transmission.(OT).spectra.in.the.wavelength.range.of.2.3–3.7.eV.for.four.ZnO.thin.films.on.sapphire.substrate..All.films.exhibit.an.average.optical.transmittance.of.85%–95%.in.the.visible.range.and.a.sharp.fundamental.absorption.edge..The.optical.energy.gap.for.samples.SK169,.SK290,.SK268.which.are.determined.from.the.absorption.edge.is.around.3.35.eV,.and.for.SK94.it.is.about.3.59.eV..To.obtain.the.optical.energy.gap.more.clearly,.it.used. the.first-order.derivatives.of. transmittance. to.calculate. the.absorption.bands.edge.of. ZnO. films.. Figure. 1.11. illustrates. the. first-order. derivatives. (dots). of. ZnO. films. and.Gaussian.curve.fittings.(solid.line).

1.5.4 Variable Angle Scanning Ellipsometry

Variable.angle.scanning.ellipsometry.(VASE).measurements.were.carried.out.by.a.variable.angle.spectroscopic.ellipsometer..The.measured.VASE.data.(Ψ.and.Δ).in.the.wavelength.region.of.200–1100.nm.under.an.incident.angle.of.65°.and.70°.are.shown.in.Figure.1.12..The.solid.and.dashed.lines.are.simulation.fittings.by.the.Cauchy.model.for.65°.and.70°.incidents,.respectively..From.the.experimental.data.of.Ψ.and.Δ,. the.refractive.index.and.extinction.coefficient.can.be.extracted.through.a.Cauchy.equation.fitting.of.Ψ.and.Δ..The.fitting.process.was.done.by.minimizing.the.mean-square.error.(MSE).automatically.by.the.WASE32.professional.fitting.program..Film.thickness.was.also.obtained.as.a.by-product..The.Cauchy.equation.for.the.refractive.index,.n,.and.the.extinction.coefficient,.k.as.a.func-tion.of.wavelength.in.our.model.is.expressed.by

.n A

B C( )λ

λ λ= + +2 4 . (1.1)

. k( )λ = 0 . (1.2)

A, B,.and.C.are.the.fitting.parameters,.and.λ.is.the.wavelength.of.light..However,.this.fact.can.also.be.verified.from.our.VASE.data.if.a.nonzero.k.is.assumed..The.stimulated.models.are.shown.in.Figure.1.13..Refractive.constants.“n”.and.extinction.coefficients.“k”.from.300.

3.1

(2)

ZnO/sapphire

(4) (1) (3)

(1) SK169(2) SK290(3) SK94(4) SK268

3.2 3.3Photon energy (eV)

Diff

eren

tial o

f T

3.4 3.5 3.6 3.7

FIGURE 1.11Gaussian.fitting.of.band.edges.in.ZnO.thin.films.


to.1100.nm,.as.well.as.real.and.imaginary.dielectric.constants.“ε1”.and.“ε2,”.are.obtained.as.shown.in.Figure.1.14..The.simulated.n,.as.a.function.of.wavelength,.is.plotted.in.Figure.1.12a,. one. can. see. the. rugged. experimental. refractive. index. curve. at. less. than. 380. nm.wavelength.range..This.corresponds.to.a.relatively.large.deviation.which.comes.from.the.strong.absorption.of.ZnO. interband. transition..The.band.gap.values.obtained.by. room.temperature.PL,.optical.transmission.and.first-order.derivatives,.and.variable.angle.scan-ning.ellipsometry.are.listed.at.Table.1.4.

Generated and experimental30

25

20

15

10

5

0300

(a)

(b)

475 650Wavelength (nm)

Wavelength (nm)

∆ in

deg

rees

Ψ in

deg

rees

Generated and experimental

825 1000

3000

20

40

60

80

100

475 650 825 1000

Model fitExp E 65Exp E 75

Model fitExp E 65Exp E 75

FIGURE 1.12SE.curves.of.Ψ.in.(a),.Δ in.(b),.and.model.fitting.results.in.the.range.of.300–1000.nm.for.ZnO/sapphire.SK290.

SK290 SK268

Roughness 25 nm Roughness 31 nm

ZnO 80 nm

Substrate Substrate

ZnO 212 nm

FIGURE 1.13The.SE.simulated.model.of.samples.SK290.and.SK268.


1.5.5 Rutherford Backscattering

Rutherford. backscattering. (RBS). was. used. to. measure. the. microstructure. of. ZnO. films.and.can.provide.a.direct.analysis.of.sample.composition.and.thickness..It.was.performed.at.2.×.1.7.MV.tandem.accelerator..A.collimated.2.023.MeV.He+.beam.was.used.for.RBS.mea-surement..The.sample.was.mounted.on.a.high.precision.(0.01°).three-axis.goniometer.in.a.vacuum.chamber,.so.that.the.orientation.of.the.sample.relative.to.the.He+.beam.could.be.precisely.controlled..The.backscattered.particles.were.accepted.by.an.Au-Si.barrier.detec-tor..The.detection.angle.was.165°.and.the.energy.resolution.of.the.detector.was.about.15.keV.

Figure.1.15.shows.the.measured.RBS.spectra.of.four.ZnO/sapphire.samples.with.different.thickness,.respectively..The.Zn.distribution.results.in.a.broadband.signal.at.high-energies.separated.from.the.rest.of.the.elements..Al.and.O.signals.overlap.in.the.low-energy.range.of.the.spectrum..Two.target.models.were.compared.within.the.analysis:.the.first.model.

GenOsc optical constants2.50

2.402.30

2.20

2.10

2.00

1.90

6.5

6.0

5.5

5.0

4.5

4.0

3.5300 475 650 825 1000

0.0

0.5

1.0

1.5

2.0

2.5

300(a)

(b)

475 650 825

ε1ε2

nk

10000.00

0.10

0.20

Extin

ctio

n co

effici

ent ΄

k΄

0.30

0.40

0.50

GenOsc optical constants

Wavelength (nm)

Wavelength (nm)

Inde

x of

refra

ctio

n “n

”Pe

al (d

iele

ctric

cons

tant

), ε 1

Imag

e (di

elec

tric

cons

tant

), ε 2

FIGURE 1.14(a).The.index.of.refraction.“n”.and.extinction.coefficient.“k”..(b).The.real.dielectric.constant.ε1.and.imaginary.dielectric.constant.ε2.of.SK.290.

TABLE 1.4

Comparative.Band.Gap.Values.Obtained.from.PL,.OT,.and.SE

Sample (Thickness)

From PL (eV)

From OT (eV)

From OT Derivatives (eV)

From SE (eV)

SK169.(13.nm) 3.290 3.372 3.312SK290.(90.nm) 3.293 3.346 3.270 3.333SK94.(165.nm) 3.452 3.590 3.481SK268.(242.nm) 3.291 3.337 3.290 3.360


consisted.of.a.smooth.ZnO.layer.on.a.sapphire.substrate.(model.1);.the.second.model.con-sisted.of.surface.roughness..The.fitting.curve.and.the.chi-square. (χ2). test.show.a.better.agreement.for.model.2,.due.to.a.better.fitting.of.the.low-energy.tail.of.the.Zn.band.and.the.onset.of.the.Al.signal..Figure.1.16a.and.b.shows.good.correlation.between.the.experimental.data. and. the. model.. The. concentrations. and. thickness. obtained. from. the. fitting. in. the.second.model.are.included.in.Table.1.5.

1.5.6 Atomic Force Microscopy

The.surface.morphology.of.ZnO.thin.films.were.examined.by.atomic. force.microscopy.(AFM)..Figure.1.17a.and.b.show.AFM.images.of.the.samples.SK268.and.SK290..The.scan.area.is.10.μm.×.10.μm,.the.RMS.roughness.of.samples.290.and.SK268.was.2.336.nm.and.3.054. nm,. respectively.. As. is. seen,. the. surface. of. sample. SK268. becomes. rougher. than.sample.SK290..The.roughness.layers.of.sample.SK290.and.SK268.were.18.nm.and.30.nm,.respectively,.which.are.corresponding.with.RBS.and.spectroscopic.ellipsometry.(SE).simu-lated.results.obtained.earlier.

150 200 250 300 350 400Channels

Al

O

ZnO/sapphire(1) SK169 (13 nm)(2) SK290 (90 nm)(3) SK94 (165 nm)(4) SK268 (242 nm)

Zn

Coun

ts (4)(3)

(2)(1)

FIGURE 1.15RBS.experimented.data.of.four.different.thickness.ZnO.thin.films.on.sapphire.substrate.

100

2000

1500

1000

Coun

ts

500

0150 200

Zn0.52O0.48(242 nm)/sapphire

SK268

Zn

AlO

RBS dataWithout roughness (χ2 = 1975) With roughness (χ2 = 1150)

250Channels(a)

300 350 400 220

0

100

SK268

RBS data

Coun

ts

200

300

240 260 280Channels(b)

300 320 340

Without roughness ( χ2 = 1975) With roughness (χ2 = 1150)

FIGURE 1.16(a).RBS.spectrum.of.a.ZnO/sapphire.sample..Solid.lines.represent.the.fitting.results.for.model.1.and.model.2,.respectively..The.calculated.errors.χ2.for.each.model.are.shown.in.the.legend..(b).Zoom.out.of.the.region.between.Zn.and.Al.signals.


1.5.7 Scanning Electron Microscopy

The.microstructure.of. the.ZnO.films.and. the. interface.of.ZnO/Al2O3.were.analyzed.by.scanning.electron.microscopy.(SEM).at.magnifications.of.up.to.×.105..Figure.1.18.shows.the.cross-sectional.SEM.images.of.samples.SK290.and.SK268..It.obtained.the.thickness.of.ZnO.epilayers.for.samples.SK290.and.SK268.with.74.90.and.202.0.nm,.respectively..The.rough-ness.layers.of.surface.on.samples.SK290.and.SK268.were.19.03.and.31.nm.which.are.similar.to.AFM.image..The.layer.thicknesses.and.roughness.were.determined.from.images.that.were.well.corresponded.by.the.simulated.results.from.RBS.and.SE.

1.5.8 Summary

The.optical.and.structural.properties.of.ZnO.films.on.sapphire.substrate.grown.by.met-alorganic. chemical. vapor. deposition. with. different. thickness. have. been. investigated..Photoluminescence.measurement.showed.clear.band.edge.structures.at.around.3.30.eV.which.are.smaller.than.optical.transmission.(OT).band.gap.about.0.03.eV..Refractive.and.

TABLE 1.5

Concentrations.and.Thickness.Obtained.from.RBS-Fits

Sample Zn (%) O (%)Roughness

(nm)Thickness

(nm)

SK.169 40 60 13 XSK.290 48 52 20 80SK.94 42 58 30 135SK.268 52 48 32 202

Digital instiScan sizeScan rateNumber of sarImage dataData scale

X 1.00 µm/divZ 20.00 µm/div

X 1.00 µm/divZ 40.00 µm/div

20040513--39102030.036

41

23

4

µm

µm

(a) (b)SK290 SK268

3

2

1

20040513--39102030.025

Digital insScan sizeScan rateNumber of sImage dataData scale

FIGURE 1.17AFM.image.of.sample.of.(a).SK290.and.(b).SK268.


extinction.coefficient.was.extracted.by.fitting.VASE.of.Ψ.and.Δ..From.the.simulated.VASE,.the. surface. roughness. of. sample. was. also. obtained.. RBS. shows. the. atomic. Zn:O. ratios.with.a.few.percentage.deviation.from.1:1,.and.thicknesses.in.the.range.of.10–230.nm,.layer.roughness.within.10–30.nm,.which.are.corresponding.to.results.from.atomic.force.micros-copy. (AFM),. and. scanning. electron. microscopy. (SEM).. VASE. and. RBS. characterization.

×50,0005.0 kV 100 nm WD 6.1 mmCCMS SEIY:0.105 µm

×50,0005.0 kV 100 nm W D5.9 mmCCMS

(a)

(b)

SEIY:0.212 µm

FIGURE 1.18Cross-sectional.SEM.image.of.samples.(a).SK290.and.(b).SK268.


demonstrated. the. existence. of. surface. roughness. which. was. proved. by. atomic. force.microscopy.image..The.interfacial.structure.of.ZnO.on.sapphire.has.been.studied.by.cross-sectional.SEM.and.shows.strong.correlation.with.the.earlier-simulated.model.from.RBS.and.VASE.

1.6 Cr-Doped ZnO Films on Si by Sputtering

1.6.1 Magnetron Sputtering of Cr-Doped ZnO Thin Layers on Si Substrate

A.series.of.transition.metal.element.Cr-doped.ZnO.thin.films.were.prepared.by.radio.fre-quency.(RF).magnetron.sputtering.deposition.technique.on.Si.substrates.under.different.deposition.temperature.and.other.conditions..Deposition.technology.and.details.can.be.found.from.references.[58,59,Chapter.13.in.this.book].

1.6.2 Combined UV Micro-PL and Raman Spectra

Figure.1.19.shows.UV.micro.(μ)-PL.spectra.for.a.set.of.four.sputter-deposited.Cr-doped.ZnO.films.with.different.sputtering.temperatures,.indicated.in.the.figure..Only.Sample.C4,.with.a.high.deposition.temperature.of.650°C,.exhibits.clear.PL.broadband.emission,.spreading.over.3.0–3.6.eV.energy.region;.whereas.samples.C1,.C2,.and.C3.show.mainly.multiple.sharp.lines. in. the.right.side.of. the.figure,.between.3.5.and.3.8.eV..These.are. indeed.the.multi-phonon. resonance. Raman. scattering. (RRS). features. [60].. Therefore,. Figure. 1.19. presents.a.combined.UV-excited.micro-PL.and.micro-Raman.spectra.on.these.ZnO:Cr/Si.samples.

1.6.3 Multi-Phonon Resonance Raman Scattering from Cr-Doped ZnO

We. could. replot. Figure. 1.19. with. the. x-axis. in. unit. of. Raman. shift,. cm−1,. as. shown. in.Figure.1.20..Combined.and.μ-Raman.ZnO:Cr..Sample.C4.exhibits.clear.PL.band.emission.

ZnO: Cr/SiC1 RTC2 300°CC3 500°CC4 650°C

6000

4000

2000

03.0 3.2

3.2

C4

C3C1

C23.4

3.4

3.22 eV

Energy (eV)

PL in

tens

ity

3.6 3.8

Excitation: 325 nm

FIGURE 1.19Miro-PL. spectra,. excited. by. He-Cd. (325. nm). laser,. of. four. Cr-doped. ZnO. films. under. different. sputtering.temperatures.


while.sample.C1,.C2,.and.C3.only.show.multi-phonon.resonance.Raman.scattering.(RRS)..High-order.LOs.appear.due. to. the. resonance.of. the.Raman.scattered.photons.with. the.ZnO.3.22.eV.bandgap..In.non-resonance.excitation.conditions,.Raman.scattering.of.A1(LO).and.E2.(high).phonons.occur.via.the.deformation.potential.mechanism.with.A1(LO).much.weaker.than.the.E2.(high).mode..For.the.resonance.case,.A1(LO).phonons.are.scattered.via.the.Fröhlich.mechanism.with.a.strong.enhancement.of.multiple.A1(LO).phonon.scattering..When.the.energy.of. the.Raman-scattered.photon.Eout.matches. the.ZnO.bandgap.Eg,.an.outgoing.resonance.(OR).occurs.with.an.incoming.photon.of.energy.Ein.scattered.m.times.by.LO.phonons:

. E E m h Eout in LO g/2= − ( )π ω ~ . (1.3)

where(h/2π).ωLO.is.the.energy.of.an.A1(LO).phononh/2π.is.the.Planck.constant

We.notice. from.Figure.1.19. that. the.LO.resonance.enhancement.occurs.over.an.energy.range.of.about.0.5.eV,.which.is.much.wider.than.the.half-width.of.the.fundamental.transi-tion.E0.band..Figure.1.21.shows. the.multiple-resonance.Raman.spectrum.of.sample.C1,.with.up.to.seven.LO.modes.

1.6.4 Visible Raman Spectra of ZnO:Cr/Si

Raman.spectra,.under.the.visible.laser.excitation.of.532.nm,.of.four.Cr-doped.ZnO.films.with.various.sputtering.temperatures.are.shown.in.Figure.1.22(a).through.(d)..The.peaks.at.300,.520,.and.618.cm−1.are.due. to. scattering. from.the.silicon.substrate..The.peak.at.about.437.and.573.cm−1.are,.respectively,.for.the.E2.(high).and.A1(LO).of.ZnO.signals..It.is.worth.mentioning.that.many.reports.show.that.the.E2.mode.of.ZnO.indicates.the.point.group. of. wurtzite. structure.. Other. peaks. can. be. treated. as. additional. modes,. which.were.related.to.defect-induced.modes..The.destruction.of.ZnO.crystal.structure.may.be.

6000

4000

2000

1LO Excitation: 325 nm

2 LOIn

tens

ity

3 LOC4

C2 300°CC1 RT

ZnO: Cr/Si

C3 500°CC4 550°C

C1

C2 C31000 2000 3000 4000

Raman shift (cm–1)5000 6000 7000

0

FIGURE 1.20Combined.UV.μ-Raman.and.μ-PL.spectra.of.four.Cr-doped.ZnO.thin.films.sputter-deposited.at.25°C,.300°C,.500°C,.and.650°C,.respectively.


caused.by.the.formation.of.Cr.clusters.and.generation.of.defects.such.as.vacancies.and.interstitials.[61].

We. have. measured. the. Raman. spectra. of. the. films. to. study. how. Cr. doping. influ-ences.the.Raman.scattering.of.ZnO.film.and.whether.the.anomalous.modes.(AMs).exist.in. Raman. spectra. of. Cr-doped. ZnO. film.. ZnO. has. a. wurtzite. structure. with. C6v. point.

18,00016,00014,00012,00010,0008,0006,0004,0002,000

0100

(a)200 300 400

Raman shift (cm–1)

Raman shift (cm–1) Raman shift (cm–1)

500 600 700 800

ZnO: Cr/SiC1

A1(LO)Si

Si

SiAMsAMs

×50

E2

Excitation: 532 nm

Inte

nsity

100

15,000Excitation: 532 nm

Si

Si

SiAMsAMsE2

×50

ZnO: Cr/SiC2

10,000

Inte

nsity

5,000

0200 300 400 500 600

Raman shift (cm–1)(b)700 800

15,000

10,000 Excitation: 532 nm

5,000Inte

nsity

AMs

×50

AMs

Si

C3

A1(LO)

ZnO: Cr/Si Si

E2Si

0100

(c)200 300 400 500 600 700 800

(d)

15,000

10,000 Excitation: 532 nm

5,000Inte

nsity AMs

A1(LO)

×50AMs

Si

C4ZnO/Si Si

E2 Si

0100 200 300 400 500 600 700 800

FIGURE 1.22Raman.spectra.(under.532.nm.excitation).and.peaks.assignment.of.four.Cr-doped.ZnO.thin.films.grown.on.Si,.C1–C4,.which.were.sputtering.under.(a).RT,.(b).300°C,.(c).500°C,.and.(d).650°C,.respectively.

8000570 cm–1

1 LO

2 LO

3 LO4 LO

5 LO

C1ZnO: Cr/Si

6 LO

Excitation: 325 nm

7 LO6000

4000

3200 4000 48002000

1000 2000 3000 4000 5000 6000 7000

Inte

nsity


FIGURE 1.21The.multiple.resonance.Raman.spectrum.of.sample.C1,.showing.up.to.seven.LO.modes.


group.symmetry..Group.theory.predicts. that. there.are.two.A1,. two.E1,. two.E2,.and.two.B1.modes.in.the.Raman.spectra.of.ZnO..Raman.spectra.of.the.Cr-doped.ZnO.films.with.various.sputtering.temperatures.are.shown.in.Figure.1.22a.through.d..The.peaks.at.300,.520,.and.618.cm−1.are.due.to.scattering.from.the.silicon.substrate..The.peaks.at.about.437.and.573.cm−1.are,.respectively,.for.E2.(high).and.A1(LO).of.ZnO.signals..It.is.worth.mention-ing.that.many.reports.show.that.the.E2.mode.of.ZnO.indicates.the.point.group.of.wurzite.structure..Other.peaks.can.be.treated.as.additional.modes,.which.were.related.to.defect-induced.modes..The.destruction.of.ZnO.crystal.structure.may.be.caused.by.the.formation.of.Cr.clusters.and.generation.of.defects.such.as.vacancies.and.interstitials.[61].

The.additional.mode. (AMs).at.275.cm−1.has.been.also.observed. in.Raman.scattering.of.N-doped.ZnO.[62]..However,.the.origin.of.this.additional.mode.is.still.ambiguous..Kaschner.et.al..[62].explained.the.occurrence.of.this.additional.mode.as.a.local.vibrational.mode.due.to.vibrating.nitrogen-related.complexes,.while.Bundesman.et.al..[63].argued.the.host.lattice.defects.as.its.origin.because.this.mode.occurs.also.for.Ga-,.Fe-,.Sb-,.and.Al-doped.ZnO.films.

1.6.5 X-Ray Absorption Near-Edge Spectroscopy on O K-Edge

Within.the.past.two.decades.synchrotron.radiation.(SR).x-ray.absorption.fine.spectros-copy.(XAFS).has.become.a.widely.used.technique.to.determine.the.local.atomic.struc-ture.of.materials,.which.is.of.great.importance.in.materials.science,.biology,.chemistry,.electronics,.geophysics,.metallurgy.science,.and.so.on..It.refers.to.the.oscillatory.struc-ture. in. the. x-ray. absorption. coefficient. just. above. an. x-ray. absorption. edge. and. this.turns. out. to. be. a. unique. signature. of. a. given. material.. The. region. closer. to. an. edge.is. often. dominated. by. strong. scattering. processes. as. well. as. local. atomic. resonances.in. the.x-ray.absorption.and. is. referred. to.as. the.x-ray.absorption.near-edge. structure.(XANES).typically.lying.within.the.first.40.eV.of.the.edge.position..The.XANES.on.the.oxygen.(O).K-edge.of.ZnO.has.been.measured.by.soft.x-ray.beam.line.(20A.at.National.Synchrotron.Radiation.Research.Center,.Hsingchu,.Taiwan).for.our.transition.metal.ele-ment.Cr-doped.ZnO.thin.films.

The. total. electron. yield. (TEY). and. total. fluorescence. yield. (TFY). were. combined. to.study.on.the.O.K-edge.of.ZnO:Cr.samples..Figure.1.23.plots.normalized.O.K-edge.XANES.

1.2

1.0

0.8O K-edge

TEY

0.6

0.4

0.2

0.0

–0.2525 530 535 540

C1 RTC4

544.5

C4 C3

C2C1

A5

544.8 545.1C1

A1

A2

A3A4

C3

C2

C2 300°CC3 500°CC4 600°C

Photon energy (eV)

Nor

mal

ized

inte

nsity

(arb

. uni

ts)

545 550 555

ZnO: Cr/Si

FIGURE 1.23Normalized.O.K-edge.XANES.spectra.of.Cr-doped.ZnO.films.grown.on.Si.base.with.various.deposited.tem-peratures.in.TEY.mode.


spectra.of.Cr-doped.ZnO.films.measured.by.TEY..Features.A1–A5.of.ZnO.are.attributable.to.electron.transition.from.O.1s.to.2p.(along.the.bilayer).and.O.2p.(along.the.c.axis).states.[64,65]..Features.A1.and.A2.slightly.increase.when.the.deposition.temperatures.keep.going.up,.implying.that.the.number.of.O.2p.unoccupied.decreases.with.the.reduction.of.sputter-ing.temperature;.feature.A4.and.feature.A5.have.no.significant.changes.from.the.XANES.spectra..Only.feature.A3.of.samples.C3.and.C4.drop.dramatically.compared.with.samples.C1and.C2..However,.the.TEY.measurement.could.provide.us.electron.structures.on.thin.film.surfaces..Figure.1.24.shows.the.TFY.XANES.spectra.of.four.ZnO.samples..Similarly,.features.of.B1–B4.are.attributed.to.the.transitions.from.O.1s.to.2p.(along.the.bilayer).and.O.2p▫.(along.the.c.axis).states..In.this.set,.the.feature.B1.exhibits.stronger.peak.in.samples.C1.and.C2.which.needs.to.be.investigated.further..On.the.other.hand,.features.B3.to.B4.can.be.identified.very.well.in.sample.C4,.also.can.be.grouped.into.two.sets.for.these.four.samples..The.first.set.for.ZnO.thin.films.C1.and.C2,.which.with.the.same.characteristics.of.XANES.spectra.and.the.other.are.samples.C3.and.C4..From.a.study.[66].it.is.shown.that.different.doping.levels.in.ZnO.will.affect.XANES.intensity.

1.6.6 Summary

In.summary,.the.magnetic.element.Cr-doped.ZnO.thin.films.on.silicon,.deposited.by.radio.frequency.(RF).magnetron.sputtering.on.silicon,.under.different.sputtering.temperatures,.was.studied,.on.the.optical.properties.and.electronic.structure.by.UV-325.nm.micro-Raman-PL,.visible.532.nm.Raman.spectroscopy,.and.synchrotron.radiation.x-ray.absorption.

The.Raman.experimental. results.clearly.show.that.high-order.LOs.appear.due. to. the.resonance.of.the.Raman.scattered.photons.with.the.ZnO.3.22.eV.bandgap..Several.Raman.modes.were.found.in.the.Raman.spectra.with.various.Cr-doping.contents..The.peak.at.about.275.cm−1. is.an.additional.mode. (AM).due. to.N-dopant. in.ZnO,. that. is,. as.defect-induced.mode..This.was.observed.while.increasing.sputtering.temperature.to.600°C,.due.to.the.resonance.with.the.ZnO.bandgap.

O.K-edge.x-ray.absorption.near-edge.structure.(XANES).has.revealed.the.variations.of.the.electronic.structure.of.sputter-deposited.Cr-doped.ZnO.films.grown.on.Si.substrate..Apparently,.the.doping.of.Cr.under.high.temperatures.can.improve.the.emission.efficiency.near.excitation.edge.

525

0

1

2

3

530 535 540Photon energy (eV)

545 550 555

C1 RTC2 300°CC3 500°CC4 600°C

B1

B2

B3B4

C1

ZnO:Cr/Si

C4C3

C2

O K-edgeTFY

Nor

mal

ized

inte

nsity

(a

rb. u

nits

)

FIGURE 1.24Normalized. O. K-edge. XANES. spectra. of. Cr-doped. ZnO. films. grown. on. Si. base. with. various. deposited.temperatures.in.TFY.mode.


1.7 X-Ray Photoelectron Spectroscopy on Bulk and Epitaxial ZnO Materials

1.7.1 XPS on ZnO—General

In.order. to.develop.the.ZnO.substrate.and.epitaxy. technology,.we.have. to.character-ize.and.investigate. the.surface.properties.of. the.ZnO.bulk..X-ray.photoelectron.spec-troscopy.(XPS). is.a.powerful. technology.for. this.purpose.and.for.ZnO.materials.and.structures.[67–69]..Recently,.XPS.has.been.successfully.employed.for.“x-ray.photoelec-tron.spectroscopy.measurement.of.n-ZnO/p-NiO.heterostructure.valence-band.offset”.[67],.“Energy.band.alignment.of.SiO2./ZnO. interface.determined.by.x-ray.photoelec-tron.spectroscopy”.[68],.and.“Pulsed.laser.deposited.ZnO:In.as.transparent.conducting.oxide”.[69].

Here,.we.present. the.use.of.XPS. for. the.characterization.of. the.surface.status.of.bulk.and.MOCVD-grown.epitaxial.ZnO.materials..Due.to.different.growth.conditions,.the.rela-tive.atomic.percentages.and.ion.boding.phases.were.very.diverse..The.energy.scale.of.the.XPS.spectra.was.calibrated.assuming.at.285.0.eV.the.B.E..of.the.C.1s.peak.value.due.to.sample.surface.contamination..Background.subtraction.of.Zn.2p3/2.of.ZnO.epilayers.was.carried.out.using.Shirley’s.iterative.method..The.baseline.of.background.is.a.combination.of.Shirley.and.linear.function.

We.focused.on.the.analyses.of.Zn.and.O.elements.to.discuss.the.relative.atomic.ratios.and. ion.boding.phases..We.used.XPSPEAK4.1. to.fit. the.measured.curves..The.Zn.2p3/2.peak.consisted.of.two.phases,.Zn-Zn.boding.at.1021.01.eV.and.Zn-O.at.1021.9.eV..Similar.analysis.of.O.1s.is.performed;.O.1s.peak.consisted.of.two.phases,.Zn-O.boding.at.530.2.eV.and.O-H.at.531.7.eV.

1.7.2 XPS of ZnO Bulk

Figure.1.25.shows.the.XPS.fine.scans.of.(a).Zn.2p3/2.and.(b).O.1s.of.a.bulk.ZnO.undoped.wafer.. In. this.analysis,. the.main.oxides.and.bulk.peaks.are.well. separated.. In.Zn.2p3/2.analysis,. the.Zn-Zn.and.Zn-O.ratio.was.1.55:1;.we.presumed.that.Zn-Zn.was. the.major.boding.phase.in.the.surface.region..In.O.1s.analysis,.the.Zn-O.and.O-H.ratio.was.2.96:1;.we.presumed.that.oxygen.had.better.combination.with.zinc.in.the.surface.region.in.the.growth.process..In.further.work.concerning.the.growth.of.GaN.on.ZnO,.we.can.make.the.most.of.the.surface.information.of.the.ZnO.bulk..The.atomic.percentages.and.FWHM.of.ZnO.bulk.are.calculated.and.summarized.in.Table.1.6.

1.7.3 XPS of Epitaxial ZnO by MOCVD

Three.MOCVD-grown.ZnO.thin.films.on.c-sapphire.substrate.are.involved.in.this.section..Two.were.measured.with.SE.in.the.previous.section..Here,.they.were.remeasured.with.the.obtained.film.thicknesses.slightly.different.from.the.previous.values,.which.might.be.due.to.the.non-uniformity.of.the.film.or.different.fittings.

For.sample.SK151,.with.150.nm.ZnO.epilayer,.the.Zn.2p3/2.peak.consisted.of.two.phases,.Zn-Zn.boding.at.1021.02.eV.and.Zn-O.at.1021.92.eV..However,.O.1s.peak.consisted.of.three.phases,.Zn-O.boding.at.530.04.eV,.O-H.at.530.86.eV,.and.Al2O3.at.532.10.eV,.respectively.

For.SK290,.with.75nm.epilayer,.the.Zn.2p3/2.peak.consisted.of.two.phases,.Zn-Zn.boding.at.1021.16.eV.and.Zn-O.at.1021.80.eV..O.1s.peak.consisted.of.three.phases,.Zn-O.boding.at.530.20.eV,.O-H.at.531.57.eV.and.Al2O3.phase.at.532.63.eV,.respectively.


In.the.case.of.SK268,.with.250nm.epilayer,.the.Zn.2p3/2.peak.consisted.of.two.phases,.Zn-Zn.boding.at.1021.05.eV.and.Zn-O.at.1021.90.eV..O.1s.peak.consisted.of.three.phases,.Zn-O. boding. at. 530.29. eV,. O-H. at. 531.11. eV. and. Al2O3. phase. at. 532.26. eV,. respectively,.Figures.1.26.through.1.28.

The.atomic.percentages.and.FWHMs.of. these.three.ZnO/sapphire.are.calculated.and.summarized.in.Tables.1.7.through.1.9,.respectively.

TABLE 1.6

Zn.and.O.Atomic.Percentage.and.Boding.Analysis.of.ZnO.Bulk

Zn 2p3/2 Zn ZnO O 1s ZnO O-H

Peak.energy.(eV) 1021.02 1021.92 Peak.energy.(eV) 530.27 531.94FWHM.(eV) 1.54 1.5 FWHM.(eV) 1.48 1.57Area.(counts)/ASF(3.6)

13743.49 8870.85 Area.(counts)/ASF(0.73)

17535.49 6029.392

Atomic.(%) 29.76 19.21 Atomic.(%) 37.97 13.06

ExpSumShirley backgroundO-H 531.7 eVZnO530.2 eV

ZnO 530.2 eV

O-H 531.7 eV

ZnO bulk36.0 k

34.0 k

32.0 k

30.0 k

28.0 k

XPS

inte

nsity

(cou

nts/

s)

26.0 k

24.0 k

22.0 k

540 538 536 534 532Binding energy (eV)(b)

530 528 526

1026

30 k

40 k

50 k

60 kXP

S in

tens

ity (c

ount

s/s)

70 k

80 k

90 k ZnO bulk

ZnO 1021.9 eVZn 1021.1 eV

ZnO 1021.9 eVZn 1021.1 eV

1024 1022 1020Binding energy (eV)(a)

1018

ExpFitting resultsShirley background

1016

FIGURE 1.25The.XPS.scans.of.(a).Zn.2p3/2,.(b).O.1s.for.a.ZnO.undoped.bulk.


1.7.4 Summary

In.summary,.x-ray.photoelectron.spectroscopy.(XPS).measurements.were.performed.for.typical.bulk.and.epitaxial.ZnO.samples..Fine.scans.and.simulation.on.Zn.2p3/2.and.O.1s.were.analyzed.in.detail,.to.specify.the.surface.status.of.bulk.and.film.ZnO..It.is.seen.that.the.Zn.2p3/2.signal.from.pure.Zn.with.respective.to.those.from.ZnO.environment.is.stronger.in.epitaxial.ZnO.than.that.in.bulk.ZnO..At.the.same.time,.the.O.1s.signal.from.ZnO.environment.with.respect.to.O-H.component.is.stronger.in.epitaxial.ZnO.than.that.in.bulk.ZnO.

1.8 More Interdisciplinary Studies on ZnO, Alloys, and Nanostructures

More. interdisciplinary. characterizations. were. applied. to. other. ZnO-based. materials,.alloys.of.AlZnO.and.MgZnO,.as.well.as.nanostructures.using.ZnO.as.substrate.materials.by.the.author.and.collaborators..We.describe.them.briefly.here.

XPS

inte

nsity

(cou

nts)

Binding energy (eV)(a)1030

30 k

40 k

50 k

60 k

70 kSK151 ZnO (150 nm)

80 k

1028 1026 1024 1022 1020 1018 1016

ZnO 1021.92 eV

ZnO/c-sapphire Zn 1021.02 eV

ExpFitting resultsShirley backgroundZn 1021.02 eVZnO 1021.92 eV

28.0 k

26.0 k

24.0 k

22.0 k

20.0 k


530 528 526

XPS

inte

nsity

(cou

nts) ZnO/c-sapphire

SK151 ZnO (150 nm)

ZnO 530.27 eV

O-H531.94 eV

ExpSumShirley backgroundZnO530.27 eVO-H531.94 eV

FIGURE 1.26The.XPS.fine.scans.of.(a).Zn.2p3/2,.(b).O.1s.for.SK151.ZnO.film.on.c-sapphire.


For.bulk.and.thin.film.ZnO,.we.have.studied.“Optical.Properties.of.Bulk.and.Epitaxial.ZnO”.[52],.“Characterization.of.Bulk.crystals.of.Transition.Metal.Doped.ZnO.for.Spintronic.Applications”.[51],.“Reflective.Second.Harmonic.Generation.from.ZnO.Thin.Films:.A.Study.on.The.Zn-O.Bonding”.[47],.“Metal.Organic.Chemical.Vapor.Deposition.And.Investigation.of.ZnO.Thin.Films.Grown.on.Sapphire”.[70],.“Temperature-Dependent.Excitonic.Luminescence.in.ZnO.Thin.Film.Grown.by.Metal.Organic.Chemical.Vapor.Deposition”.[71],.“Rapid.Thermal.Annealing.Effects.on.the.Structural.and.Optical.Properties.of.ZnO.Films.Deposited.on.Si.Substrates”.[72],.and.“Rutherford.Backscattering.And.Optical.Studies.for.ZnO.Thin.Films.On.Sapphire.Substrates.Grown.by.Metalorganic.Chemical.Vapor.Deposition”.[73].

We.have.also.conducted.research.on.ZnO.alloys.of.AlZnO.in.“Room.Temperature.Deposition.of.Al-Doped.ZnO.Films.on.Quartz.Substrates.by.Radio-Frequency.Magnetron.Sputtering.and.Effects.of.Thermal.Annealing”.[74],.and.MgZnO.in.“Characterization.of.MgxZn1−xO.Thin.Films.Grown.on.Sapphire.Substrates.by.Metalorganic.Chemical.Vapor.Deposition”.[75],.by.using.various.characterization.tools,.including.high.resolution.x-ray.diffraction.(HR-XRD).

More. characterization. and. investigation. on. nanometer. scale. thin. GaN. (~30. nm). and.InGaN. (10–100. nm). layers. grown. on. ZnO. substrates. by. MOCVD. have. been. done. on.subjects.of.“Metalorganic.Chemical.Vapor.Deposition.of.InGaN.Layers.on.ZnO.Substrates”.[35],.“Effect.of.an.Al2O3.Transition.Layer.on.InGaN.on.ZnO.Substrates.by.Organometallic.Vapor.Phase.Deposition”.[76],.“Suppression.of.Phase.Separation.in.InGaN.Layers.Grown.on.Lattice.Matched.ZnO.Substrates”.[77],.and.“Metalorganic.Chemical.Vapor.Deposition.of.GaN.layers.on.ZnO.Substrates.Using.α-Al2O3.as.a.Transition.Layer”.[78].

45.0 k

40.0 k

35.0 k

30.0 k

25.0 k

20.0 k

1028 1026 1024 1022 1020 1018 1016

Binding energy (eV)(a)

XPS

inte

nsity

(cou

nts)

ZnO 1021.80 eV

ZnO/c-sapphire SK290 ZnO (75 nm)

Zn 1021.16eV

Exp.Fitting resultsShirley backgroundZnO 1021.80 eVZn 1021.16 eV

18 k

17 k

16 k

15 k

14 k

13 k

536 534 532 530 528 526

Binding energy (eV)(b)

XPs i

nten

sity (

coun

ts)

ZnO/c-sapphire ZnO 530.02 eV

O-H 531.57 eV

Al2O3532.63 eV

SK290 ZnO (75 nm)

Exp.SumShirley backgroundZnO 530.02 eVO-H 531.57 eVAl2O3 532.63 eV



TABLE 1.8

Zn.and.O.Atomic.Percentage.and.Boding.Analysis.of.SK290.ZnO.(75.nm).on.c-Sapphire

Zn 2p3/2 Zn ZnO O 1s ZnO O-H Al2O3

Peak.energy.(eV) 1021.16 1021.80 Peak.energy.(eV) 530.02 531.57 532.63FWHM.(eV) 1.78 2.12 FWHM.(eV) 1.74 1.44 1.30Area.(counts)/ASF(3.6)

4916.75 9779.38 Area.(counts)/ASF(0.73)

9931.86 7341.00 2921.07

Atomic.(%) 14.09 28.03 Atomic.(%) 28.47 21.04 8.37

80 k

70 k

60 k

50 k

40 k

30 k

XPs i

nten

sity (

coun

ts)

1030 1028 1026 1024 1022 1020 1018 1016Binding energy (eV)(a)

Exp.

Shirkley backgroundSum

ZnO 1021.90 eVZn 1021.05 eV

ZnO 1021.90 eV

Zn 1021.05 eV

ZnO/c-sapphire

SK268 ZnO (250 nm)

32.0 k

30.0 k

28.0 k

26.0 k

24.0 k

22.0 k

20.0 k

XPs i

nten

sity (

coun

ts)


530 528 526

ZnO 530.29 eV

O-H 531.11 eV

ZnO/c-sapphireSK268 ZnO (250 nm)

Al2O3532.26 eV

Exp.Fitting resultsShirley backgroundZnO 530.29 eV

Al2O3 532.26 eVO-H 531.11 eV


TABLE 1.7




7215.35 15260.58 Area.(counts)/ASF(0.73)

10913.82 8373.31 4358.46

Atomic.(%) 15.64 33.09 Atomic.(%) 23.66 18.15 9.45


1.9 Conclusion

In.conclusion,.this.chapter.has.given.a.brief.historical.review.of.research.and.develop-ment.on.ZnO.and.related.materials.over.100.years,.especially.in.recent.years,.an.intro-duction.of.their.basic.properties,.and.basic/interdisciplinary.characterization.on.ZnO..The.multiple.techniques.employed.in.the.ZnO.materials.analyses.include.Raman.scat-tering,.photoluminescence,.optical.transmission,.variable.angle.scanning.ellipsometry,.atomic. force. microscopy,. scanning. electron. microscopy,. Rutherford. backscattering,.synchrotron. radiation. x-ray. absorption. near-edge. spectroscopy,. x-ray. photoelectron.spectroscopy,.and.so.on..These.led.to.a.comprehensive.and.deep.understanding.of.the.structural.and.optical.properties.of.bulk.and.film.ZnO.materials.studied..More.inter-disciplinary.characterizations.on.other.ZnO.films,.alloys.of.AlZnO.and.MgZnO,.and.nano-structural.layers.of.GaN.and.InGaN.deposited.on.the.bulk.ZnO.substrate.are.also.described.briefly.

Acknowledgments

The.author.would.like.to.pay.his.sincere.thanks.all.the.sample.providers,.collaborators,.and.students:.Profs.. Ian.T..Ferguson,.Weijie.Lu,.Dong-Sing.Wuu,.Sude.Yao,.Zhengyun.Wu,.X..Mei.Wu,.C..C..Yang,.Li-Chyong.Chen,.Kuei-Hsien.Chen,.Jer-Run.Yang,.K..Y..Lo,.Jin-Ming.Chen,.Jyh-Fu.Lee;.Drs..Jeff.Nauss,.Garry.Tampa,.Shangzhu.Sun,.Will.E..Fenwick,.Chia-Cheng.Wu,.Weifeng.Yang,.Hong-Ling.Tsai;.and.more.of.the.author’s.old.and.cur-rent. students:. Jiun-Bi. Wang,. Tsung-Lung. Huang,. Siou-Cheng. Lien,. Zhen-Sheng. Lee,.Yu-Cheng.Shin,.Yen-Ting.Chen,.Yi-Li.Tu,.Yi-Zhe.Huang,.Yee.Ling.Chung,.You.Ren.Lan,.Tse.Yang.Lin,.and.Yu.Hsiang.Lai;.and.collaborative.students.of.Lin.Li,.in.the.works.for.this. chapter.. The. work. at. National. Taiwan. University. was. supported. by. funds. from.National. Science. Council. of. Republic. of. China,. NSC. 93-2218-E-002-011,. 93-2215-E-002-035,.94-2215-E-002-019,.95-2221-E-002-118,.96-2221-E002-166,.NSC.97-2221-E-002-026-,.NSC.98-2221-E-002-015-MY3.and.from.National.Taiwan.University,.Excellent.Research.Project.(10R80908),.and.so.on.

TABLE 1.9




14989.68 7058.78 Area.(counts)/ASF(0.73)

3013.02 1656.56 1798.43

Atomic.(%) 52.56 24.75 Atomic.(%) 10.57 5.81 6.31


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37

2Pressurized Melt Growth of ZnO Single Crystals

Jeff Nause

2.1 Introduction

Semiconductors.as.a.class.of.material.have.found.diverse.applications.that.seem.to.be.grow-ing.at.an.exponential.rate..The.evolution.of.new.applications.necessitates.that.the.semicon-ductor.perform.better.under.more.intense.environments,.so.materials.scientists.develop.parallel.processes.that.will.suit.the.needs.of.the.application..They.either.alter.or.improve.the.method.of.creating.the.material,.or.they.form.a.new.material.to.meet.the.needs.of.the.application..Wide.band.gap.semiconductors.have.emerged.in.recent.years.as.a.promising.category.of.material.that.offer.some.advantages.over.traditional.semiconductors.and.have.facilitated.devices.and.detectors. that.operate. faster.and.more.efficiently.at.higher.pow-ers.under.more.severe.conditions..Single.crystal.zinc.oxide.(ZnO).is.one.such.wide.band.gap.semiconductor.with.great.potential.for.a.variety.of.commercial.applications.including.substrates,.UV.photodetectors,.acoustic.wave.devices,.light.emitting.diodes,.laser.diodes,.high.frequency.electronic.devices,.and.ultrafast.nuclear.particle.detectors..Table.2.1.com-pares.some.of.the.properties.of.ZnO.with.other.wide.band.gap.semiconductors.

Both.the.ready.availability.of.high-quality.ZnO.in.bulk.form.and.its.interesting.properties.establish.ZnO.as.a.leading.wide.band.gap.semiconductor.candidate.for.traditional.com-mercial.device.applications..For.more.severe.device.operating.temperatures,.ZnO.has.an.advantage. of. a. very. high. exciton. binding. energy. (60. meV). enabling. stability. at. higher.device.operating.temperatures..Further,.ZnO.is.an.excellent.candidate.for.extreme.operat-ing.conditions.required.for.some.space.and.military.applications.as.the.material.is.highly.resistant.to.radiation.damage.compared.even.to.GaN.[1].

CONTENTS

2.1. Introduction........................................................................................................................... 372.2. Background............................................................................................................................382.3. Pressurized.Melt.Growth.of.ZnO.Crystals....................................................................... 392.4. Crystal.Growth.Results........................................................................................................40

2.4.1. Bulk.Electrical.Properties........................................................................................432.5. Devices.Incorporating.Melt-Grown.ZnO..........................................................................43

2.5.1. Homojunction.LEDs.................................................................................................432.5.1.1. I–V.Characteristics.of.ZnO.p-n.Junction.................................................442.5.1.2. Electroluminescence.of.ZnO-Based.LEDs..............................................45

2.6. Conclusions............................................................................................................................45Acknowledgments.........................................................................................................................45References........................................................................................................................................46


2.2 Background

Single.crystals.can.be.grown.in.a.variety.of.ways.such.as.melt.growth,.vapor.transport,.and.solution.growth,.which.all. involve.a.physical.or.chemical.phase.change.to.produce.the.single.crystal..The.most.common.single.crystal.materials.used.for.electronic.device.growth.are.silicon,.gallium.arsenide,.quartz,.and.silicon.carbide..Economics.dictate.that.melt.growth.be.employed.where.possible..However,. thermodynamic.conditions.require.the.use.of.methods.other.than.melt.growth.for.growing.single.crystals.of.some.of.these.materials..Currently.95%.of.all.single.crystal.silicon.is.grown.by.the.Czochralski.method,.which. produces. the. silicon. needed. for. highly. integrated,. low. power. devices. [2].. High-purity.semi-insulating.GaAs.used.for.microwave.devices.is.grown.from.an.As-rich.melt.in. a. pressurized. inert. gas. atmosphere. with. a. liquid. encapsulant. in. a. variation. of. the.Czochralski.method..Optoelectronic.GaAs.devices.are.made.from.Si-doped.n-type.GaAs.crystals. grown. in. near-stoichiometric. horizontal. and. vertical. Bridgman. techniques. [3]..Melt. growth. processes. are. unsuitable. for. quartz. crystal. growth. due. to. a. catastrophic.phase.transformation.upon.cooling.from.the.melt..Instead,.synthetic.quartz.is.grown.in.large.industrial.autoclaves.by.hydrothermal.solution.convection.[4]..Stoichiometric.melt-ing.of.SiC.requires.pressures.exceeding.105.bar.at.temperatures.higher.than.3000°C.which.prevents.the.use.of.traditional.melt.growth.processes..Commercially,.SiC.is.grown.by.a.physical.vapor.transport.process.(seeded.sublimation.growth).[5].

As. the. need. for. high-quality,. large-area. ZnO. increases,. vigorous. research. is. being.conducted. to. grow. bulk. ZnO. crystals. using. variations. of. the. three. primary. meth-ods. described. earlier:. hydrothermal. solution. growth,. seeded. sublimation. growth,. and.pressurized.melt.growth..These. three. techniques.utilize.different.growth.mechanisms,.resulting.in.bulk.ZnO.crystals.grown.at.different.rates.and.subsequent.dissimilar.crystal-line.quality..Growth.by.hydrothermal.solution.takes.place.in.a.platinum-lined.autoclave.at.a.temperature.of.approximately.350°C..A.high.purity.ZnO.nutrient.mixed.with.a.solution.

TABLE 2.1

Properties.of.Several.Wide.Band.Gap.Semiconductors

Material ZnO GaN 4H-SiC 6H-SiC

Band.gap.(eV) 3.37 3.39 3.26 3.03Lattice Wurtzite Wurtzite Wurtzite WurtziteLattice.parameter.(Å) a.=.3.250

c.=.5.205a.=.3.189c.=.5.185

a.=.3.073c.=.10.053

a.=.3.081c.=.15.117

Melting.point.(K) 2250 2770 2070 2070Thermal.conductivity.(W/(cm·K))

0.6 1.3 3.0–3.8 3.0–3.8

CTE.(10−6/K) a.=.6.5c.=.3.0

a.=.5.6c.=.7.7

3.5–5.0 3.5–5.0

Electron.mobility.(cm2/(V·s))

196 1000 800 370

Saturation.velocity.(107cm/s)

3.0 2.5 2.0 2.0

Breakdown.voltage.(106V/cm)

5.0 5.0 2.2 2.4


of.Li2CO3,.KOH,.and.NaOH.precipitates.crystalline.ZnO.onto.seeds.at.a.rate.of.0.2.mm/day.[6]..Very.good.crystal.quality.is.achieved.using.this.method,.but.it.is.very.slow.and.incorporation.of.group.I.elements.hinders.some.device.architectures..Seeded.sublimation.growth.involves.heating.a.polycrystalline.ZnO.source.to.cause.sublimation.under.vacuum.and.eventual.deposition.on.a.seed.due.to.thermal.gradients..Good.crystal.quality.can.be.achieved.accompanied.by.a.propensity.for.twinning,.and.growth.rates.can.be.1.mm/day..The.melt.growth.process.employs.the.use.of.a.modified.Bridgman.configuration.produc-ing.very-good-quality.crystals.with.low.defects.in.less.time.(1–5.mm/h).than.the.preced-ing.processes.

2.3 Pressurized Melt Growth of ZnO Crystals

Researchers. at. Cermet,. Inc.. (Atlanta,. GA). are. focused. on. the. growth. of. ZnO. using. a.pressurized.melt.growth.approach..This.approach.utilizes.a.patented.method.of.melting.and.crystallizing.materials. that.have.a.high.melting.point. (particularly.above.~1450°C),.a. volatile. component. in. the. structure,. or. thermodynamic. instabilities. at. or. near. the.material’s.melting.point. (decomposes. into.atomic.components).at.atmospheric.pressure..The. technology. is. a. high-pressure. induction. melting. apparatus,. wherein. the. melt. is.contained.in.a.water-cooled.crucible.(rough.schematic.in.Figure.2.1).

(2)

(4) ZnO (8)

(6)

(1)

O2

H2O(3)

(7)

(5)

FIGURE 2.1Schematic.illustration.of.the.ZnO.crystal.growth.apparatus.


The.heat.source.used.during.the.melting.operation.is.radio.frequency.energy..Induced.fields. in. the. charge. material. produce. eddy. currents,. which. produce. joule. heating. in.the. material. until. a. molten. phase. is. achieved.. The. highly. refractory. melt. produced. is.contained.in.a.cold.wall.crucible,.such.that.part.of.the.solid.thermal.barrier.between.the.molten.material.and.the.cooling.fluid.is.cooled.material.with.the.same.composition.as.the.melt..The.cooled.material.prevents.the.molten.material. from.coming.into.direct.contact.with.the.cooling.surface,.which.eliminates.containment.problems.and.crucible.reactivity.regardless.of.the.melting.temperature.of.the.material..This.entire.melting.and.containment.process.is.carried.out.in.a.controlled.gas.atmosphere.ranging.from.1.atm.to.over.100.atm,.which.prevents.the.evolution.of.volatile.components,.as.well.as.the.decomposition.of.some.compounds.into.atomic.species..The.system.has.been.proven.at.temperatures.in.excess.of.3600°C.and.at.melt.environments.in.excess.of.100.atm.

In. standard. melt. growth. atmospheres,. ZnO. decomposes. upon. heating. into. a. highly.defective.ZnO1−x.structure..This.problem.has.been.overcome.by.providing.an.overpressure.of.oxygen.as.the.growth.atmosphere..A.thermodynamic.equilibrium.between.the.liquid.ZnO.and.the.oxygen.reservoir.is.established,.thereby.preventing.reduction.of.the.lattice..The.stoichiometric.ZnO.melt.is.contained.in.a.thin.layer.of.cooled,.polycrystalline.ZnO,.which.eliminates.crucible-introduced.impurities..The.process,.which.is.ultimately.scalable.to.large.dimensions,.has.been.used.to.melt.8.in..diameter,.kilogram-dimensioned.ingots.of.ZnO..From.these.large.ingots,.high-quality,.high-purity,.ZnO.crystals.have.been.crys-tallized. in. sizes. up. to. 2. in.. diameter,. oriented. and. shaped. into. round. or. square. boules.(Figure.2.2),.and.eventually.processed.into.epitaxial-ready.substrates.(Figure.2.3).

2.4 Crystal Growth Results

A. misconception. regarding. the. melt. growth. of. ZnO. is. that. high. levels. of. defects. are.introduced. into. the. crystal,. particularly. voids. and. inclusions.. High-quality. ZnO. crys-tals. have. been. grown. using. the. melt. growth. technique,. as. evidenced. by. the. following.

FIGURE 2.2Two.inch.round.ZnO.boule.


etch.pit.density.micrograph,.with.a.count.of.<104.defects/cm2.(Figure.2.4)..Furthermore,.these.defects.are.seen.to.typically.align.themselves.into.sub-grain.boundaries,.revealing.extremely.low.defect.densities.in.the.bulk.of.the.crystal.

Furthermore,.ZnO.commonly.does.have.a.slight.stoichiometric.flaw.in.that.interstitial.zinc.atoms.reside.in.the.lattice.coupled.with.oxygen.vacancies.[7].Additional.optical.stud-ies.of.melt.grown.ZnO.have.been.performed.by.Reschikov.[8].to.understand.the.defect.states. in.as.grown.ZnO.as.compared. to.ZnO.treated.under.various. thermal.conditions..Along.with.intense.and.sharp.excitonic.lines,.several.broad.bands.were.observed.presum-ably.related.to.deep.acceptors..Two.luminescence.bands.peaking.at.1.95.and.2.15.eV.at.10.K.were.studied.in.detail.for.different.temperature.treatments..The.author.concluded.that.the.1.95.eV.band.was.attributed.to.transitions.from.shallow.donors.to.yet.unidentified.deep.acceptors.

In.a.separate.study.[9],.Reschikov.endeavored.to.quantify.the.quantum.efficiency.of.melt.grown.ZnO.at.low.temperature..Figure.2.5.shows.the.low.temperature.photoluminescence.data.for.a.typical.melt.grown.ZnO.sample..The.spectrum.exhibited.very.strong.emission.from. 3. to. 3.4. eV,. which. was. attributed. to. free. and. bound. excitons,. their. excited. states,.and.phonon.replicas..The. strongest.peak. (3.3605.eV).had.a.FWHM.of.1.5.meV..A.weak.

FIGURE 2.3Two.inch.ZnO.wafer.

X= 262.1 Y=195.0 I 5/U SF2*0200

FIGURE 2.4Etch.pit.density.of.104/cm2.in.melt-grown.ZnO.


broad. band. extended. from. 1.7. to. 2.8. eV.. Using. the. process. described. by. Reschikov. [9],.the. PL. efficiency. of. undoped. ZnO. crystals. was. determined. to. be. approximately. 85%..The.defect-related.PL.constituted.approximately.0.1%.of.the.total.radiative.recombination,.with.virtually.all.of.the.PL.coming.from.excitonic.emission.

Subsequent.to.earlier-given.results.of.the.experiment.by.Reschikov.[9],.additional.sam-ples.of.hydrothermally.grown.and.vapor-grown.ZnO.were.measured.and.compared.to.the.results.of.the.melt.grown.material..It.was.found.that.one.source.of.hydrothermally.grown.ZnO.yielded.an.efficiency.of.approximately.50%..However,.the.defect-related.emission.con-stituted.the.majority.of.the.PL,.particularly.the.“orange”.band.attributed.to.Li.incorporation..Other.hydrothermally.grown.ZnO.demonstrated.an.efficiency.of.only.10%,.with.the.major-ity.of.this.PL.again.coming.from.defect-related.luminescence..Vapor-grown.ZnO.also.dem-onstrated.a.PL.efficiency.of.only.approximately.10%..By.way.of.comparison,.high-.quality.freestanding.GaN.templates.yielded.a.high.QE.(exciton.emission).of.approximately.20%.

Melt-grown. ZnO. has. the. capability. of. producing. high. structural. quality. single. crys-tal.material..By.taking.care.to.reduce.thermal.stresses.during.growth,.narrow.line.width.single. crystal. x-ray. rocking. curves. can. be. obtained.. Figure. 2.6. shows. an. x-ray. rocking.

Photon energy (eV)

10121012

1011

1010

109

1011

1010

109

108

107

106

1052

3.2 3.25 3.3 3.35 3.4

15 K40 K

2.5 3 3.5

PL in

tens

ity (r

el. u

nits

)

FIGURE 2.5Low-temperature.photoluminescence.spectrum.for.melt-grown.ZnO.

–10,000

0

10,000

20,000

30,000

40,000

50,000

31 32 33 34 35 36 37 38

Series 1

FIGURE 2.6X-ray.θ–2θ.rocking.curve.for.melt-grown.ZnO.


curve.characteristic.of.high-quality,.melt-grown.ZnO..In.this.example,.a.FWHM.of.approx-imately.20.as.have.been.accomplished,.indicating.very.high.crystalline.quality.material.

2.4.1 Bulk Electrical Properties

Undoped.ZnO.wafers.were.analyzed.using.the.room.temperature.Hall.technique.and.were.found.to.be.n-type..The.melt.growth.technique.offers. the.opportunity. to.dope.the.bulk.ZnO.with.group.III.metals.to.drastically.increase.the.electron.concentration..This.enhanced.electrical.conductivity.enables.vertical.current.devices.with.high.current.densities,.such.as.power.LEDs.and.LDs..Table.2.2.shows.the.basic.electrical.properties.of.undoped.ZnO.as.well.as.ZnO.doped.in situ.with.four.different.group.III.elements.

2.5 Devices Incorporating Melt-Grown ZnO

2.5.1 Homojunction LEDs

Using.the.bulk.ZnO.as.the.n-type.layer.in.a.ZnO.p-n.junction,.several.iterations.of.p-n.junc-tions.have.been.made..The.focus.over.time.has.been.to.improve.the.carrier.concentration.and.the.stability.of.the.p-type.layer.grown.on.the.substrate.

Early.in.the.development.stage.of.ZnO.epitaxy,.the.author.grew.p-type.ZnO.with.a.car-rier.concentration.shown.in.black.in.Table.2.3..Typically,.this.material.did.not.exhibit.stable.p-type.conduction.over.a.long.period.of.time..Recently,.however,.high.carrier.concentration.ZnO.was.grown.that.demonstrated.a.higher.carrier.concentration.and.was.stable.over.time.(blue.date.in.Table.2.3).

TABLE 2.2

Electrical.Properties.of.Undoped.and.Doped.Melt-Grown.ZnO

DopantResistivity

(Ωcm)Mobility

(cm2/(V s))Carriers (cm−3)

Undoped 3.50.×.10−1 215 8.50.×.1016

In 1.70.×.10−2 106 3.50.×.1018

Ga 6.60.×.10−3 74 1.30.×.1019

B 2.00.×.10−1 245 1.20.×.1017

Al 2.20.×.10−2 113 2.50.×.1018.

TABLE 2.3

Improvement.of.p-Type.ZnO.Properties.from.Prior.Capability.to.Current.Capability

Bulk Carrier Conc. (cm−3)

Mobility (cm2/(V s))

Resistivity (Ω·cm)

9.0.×.1016.(prior.art) 1.5 2176.4.×.1018.(current) 1.0 3.0


Homojunction. LEDs. have. been. fabricated. using. p-type. ZnO. epitaxial. applied. to. the.n-type.bulk.ZnO..Ohmic.contacts.were.applied.to.both.the.n-type.and.p-type.ZnO.active.layers..The.n-type.contact.used.was.Ti/Au.and.the.ohmic.contact.for.the.p-type.material.was.Ni/Au..The.finished.mesa.devices.are.shown.in.Figure.2.7.

2.5.1.1 I–V Characteristics of ZnO p-n Junction

The.I–V.characteristics.of.the.fabricated.homo-junction.are.presented.in.Figure.2.8..The.turn-on.voltage.was.~3.V,.although.the.turn-on.appears.very.soft..A.reverse.leakage.was.minimal.up.to.the.−4.V.test.range.

FIGURE 2.7Finished.mesa.structure.ZnO.LEDs.

Voltage (V)

5.00E-03

4.00E-03

3.00E-03

2.00E-03

1.00E-03

0.00E-03

–1.00E-03–6 –4 –2 0 2 4 6

Curr

ent (

A)

FIGURE 2.8I–V.curve.for.fabricated.ZnO.homojunctions.


2.5.1.2 Electroluminescence of ZnO-Based LEDs

The.aforementioned.sample.generated.a.very.small.amount.of.EL.center.at.an.emission.wavelength.of.384.nm.at.room.temperature.(Figure.2.9),.with.a.long.wavelength.shoulder.visible.to.the.naked.eye..The.long.wavelength.component.diminished.to.background.lev-els.at.a.wavelength.greater.than.approximately.420.nm.

2.6 Conclusions

Very-high-quality.ZnO.bulk.crystals.have.been.grown.using.the.pressurized.melt.growth.process..This.is.evidenced.by.the.extremely.high.optical,.structural,.and.electrical.quality.of.the.material..This.process.has.the.significant.advantage.of.growth.rate.and.scalability,.coupled.with.excellent.crystal.quality..Further,.large-area.ingots.have.been.grown,.from.which. wafers. have. been. produced.. Very. smooth. surfaces. have. been. achieved. on. these.resulting.wafers..This.process.has.been.shown.to.yield.wafers.that.are.ideally.suited.for.deposition.of.zinc.oxide.and.nitride.films.for.use.in.a.number.of.devices,.as.evidenced.by.active.emitters.produced.incorporating.these.substrates.

Acknowledgments

The.authors.would.like.to.acknowledge.the.assistance.of.Dr..Hadis.Morkoc.and.Dr..Michael.Reshchikov.for.AFM.and.PL.measurements..The.authors.would.also.like.to.acknowledge.the.financial.support.of.the.Missile.Defense.Agency,.Office.of.Naval.Research.and.Wright.Patterson.Air.Force.Base.

320

EL in

tens

ity (a

rb. u

nits

)

340

Cermet n-ZnO/p-ZnO040209 diodeUnder forward biasat 4V, 140 mA

360 380 400Wavelength (nm)

420 440 460 480

FIGURE 2.9Room.temperature.electroluminescence.from.a.ZnO.p–n.junction.


References

. 1.. Look,.D.C..et.al..Mat. Sci. Eng..B80.(2001),.383.

. 2.. Zulehner,.W..et.al..Mat. Sci. Eng..B73.(2000),.7.

. 3.. Rudolph,.P..et.al..J. Cryst. Growth..198/199.(1999),.325.

. 4.. Iwasaki,.F..et.al..J. Cryst. Growth..237–239.(2002),.820.

. 5.. Foti,.G..et.al..App. Surf. Sci..184.(2001),.20.

. 6.. Ohshima,.E..et.al..J. Cryst. Growth..260.(2004),.166.

. 7.. Reynolds,.R.W..et.al..Sol. St. Comm..101.(1997),.643.

. 8.. Reschikov.et.al..Mater. Res. Soc. Symp. Proc..957.(2007).

. 9.. Reschikov.et.al..Mater. Res. Soc. Symp..Proc..892.(2006).

47

3New Design and Development of MOCVD, Process and Modeling for ZnO-Based Materials

G.S. Tompa and S. Sun

CONTENTS

3.1. Introduction...........................................................................................................................483.2. Deposition.Approaches.and.MOCVD.Tool.Designs.......................................................48

3.2.1. Technique.Comparison............................................................................................483.2.2. General.MOCVD.Tool.Comparison....................................................................... 493.2.3. Chemistries.Employed............................................................................................. 513.2.4. Special.p-Type............................................................................................................ 523.2.5. General.Process.Parameters.................................................................................... 52

3.3. General.System.Design.Considerations............................................................................533.4. System.Solutions...................................................................................................................53

3.4.1. General.System.Schematic.......................................................................................533.4.2. Horizontal.Systems................................................................................................... 57

3.4.2.1. Modeling.....................................................................................................583.4.2.2. Tool.Design.and.Systems.......................................................................... 59

3.4.3. Vertical.Systems........................................................................................................603.4.3.1. Modeling..................................................................................................... 613.4.3.2. Tool.Design.and.Systems.......................................................................... 62

3.4.4. In-Line.and.Roll-to-Roll.Systems............................................................................643.4.4.1. Modeling.....................................................................................................643.4.4.2. Tool.Design.and.Systems..........................................................................66

3.4.5. Atomic.Layer.Deposition.(an MOCVD Subset Process)........................................663.4.6. MOCVD-Enhanced.Processes................................................................................ 70

3.5. Packing.Fractions.and.COO................................................................................................ 713.6. Example.Application.Results.............................................................................................. 73

3.6.1. Transport.Contact.Layers.for.GaN.LEDs.............................................................. 733.6.2. ZnO.as.the.Contact.Layer.for.Photovoltaics.......................................................... 743.6.3. Nanowires.as.Photonic.Nanosensors.................................................................... 743.6.4. Transistors..................................................................................................................773.6.5. ZnO.as.Phosphors.and.Electroluminescent.Materials........................................773.6.6. Display.Contacts....................................................................................................... 78

3.7. Conclusion............................................................................................................................. 79References........................................................................................................................................80


3.1 Introduction

Zinc.oxide.(ZnO).and.its.alloys.are.semiconductors.that.are.used.for.applications.such.as.transparent.contacts.for.photovoltaics.and.LEDs,.sensors,.phosphors,.and.potentially.lasers..Recently,. highly. conducting. ZnO. has. been. attractive. because. of. advantages. of. cost. and.toxicity.compared.with.indium.tin.oxide.(ITO).for.the.light-emitting.contact.layer.on.GaN-based.LED.structures.and.as.a.contact.layer.for.photovoltaics..While.far.behind.in.develop-ment.in.relation.to.GaN,.ZnO.also.has.potential.for.transistors.and.LEDs..ZnO.has.shown.significant.potential.as.a.transparent.transistor.for.displays.and.made.tremendous.progress.over.the.last.few.years.for.low-cost.high-speed.transistors..ZnO.visible.and.UV.LEDs.have.also.been.demonstrated.but.remain.elusive.with.respect.to.viable.devices..To.realize.these.applications.high-quality.materials.are.reproducibly.needed.with.good.uniformity.of.com-position,.thickness,.structure,.and.doping.levels..Metalorganic.chemical.vapor.deposition.(MOCVD).is.an.excellent.technology.for.not.only.developing.such.materials.but.also.meet-ing.these.requirements—it.offers.significant.flexibility.and.large.scalability.

The.different.applications.require.deposition. tools. tuned.to. their.needs—they.can.be.horizontal.tools,.vertical.tools,.or.tape.tools.that.provide.in-line.or.reel-to-reel.capabilities..Manufacturers.need.assurance.that.the.developed.products.have.viable.production.path-ways.before.their.introduction.to.the.market..Significant.effort.has.gone.into.developing.automated.reactors.for.wafer.and.for.tape.deposition.tools..A.particular.focus.at.our.facil-ity.has.been.on.vertical.reactor.high-speed.susceptor.rotation.design.MOCVD.systems.for.ZnO.devices.and.LED.contacts,.with.a.more.recent.effort.on.in-line.and.roll-to-roll.(R2R).application.tools..Processes.have.been.developed.for.depositing.films.of.single.and.multi-component.oxides.on.wafers..For.very.thin.and.conformal.applications.our.group.has.also.developed.the.alternating.layer.deposition.(ALD).process,.which.is.a.subset.of.MOCVD.processes. that. provide. extremely. precise. thickness. and. conformality. control—often. at.temperatures. lower. than. conventional. MOCVD.. Plasma-enhanced. processes. have. also.been.evaluated.for.both.MOCVD.and.ALD..At.our.facility,.we.have.demonstrated.reactor-scaled.performance. from.2. to.16. in..diameter.deposition.planes.using.advanced.design.and.modeling.software.to.define.tools.that.routinely.produce.a.wide.range.of.oxides.and.other.materials..Modeling.in.particular.has.enabled.rapid.scaling.to.greater.diameters.and.deposition.of.uniform.high-quality.insulating.and.conductive.n-type.ZnO.films.using.a.range.of.n-type.dopants..p-type.materials,.while.elusive,.have.also.been.demonstrated,.but.p-type.properties.remain.difficult.to.interpret..The.materials.have.been.qualified.by.a.range.of.techniques,.including.AFM,.x-ray.diffraction.(XRD),.photoluminescence.(PL),.Hall,.and.CV.measurements.among.others..The.efforts.have.produced.pin-hole.free.films.demon-strating.amorphous,.polycrystalline,.and.epitaxy.as.a.function.of.process.parameters.

The.following.sections.review.issues.with.tool.development.and.present.examples.of.the.tools.developed.

3.2 Deposition Approaches and MOCVD Tool Designs

3.2.1 Technique Comparison

The.three.main.thin.film.deposition.approaches.are.contrasted.in.Figure.3.1..They.are.(1).physical.vapor.deposition.(PVD).and.include.molecular.beam.epitaxy.(MBE),.pulsed.

49New Design and Development of MOCVD, Process and Modeling

laser.deposition.(PLD),.or.sublimation;. (2).spin/mist.deposition;.and.(3).metalorganic.chemical.vapor.deposition.(MOCVD).and.its.subset,.alternating.layer.deposition.(ALD)..PVD. techniques. are. generally. line. of. sight. and. therefore. are. difficult. for. producing.conformal.coatings..They.are.also.generally.relatively.slow.deposition.techniques,.are.limited.in.range.of.structural.control,.and.are.generally.very.difficult.to.rapidly.vary.composition.except.for.MBE.and.sometimes.PLD..Spin/mist.deposition.techniques.gen-erally. involve. introducing. a. liquid. to. the. surface. to. be. coated. that. is. thereat. decom-posed.thermally.to.produce.a.coating—these.films.are.often.limited.in.thickness.so.as.to.stress.cracks.on.thermal.processing.and.are.also.difficult.with.respect.to.conformal-ity..MOCVD,.and.its.variations,.introduce.precursors.to.a.heated.surface.which.gener-ally.drives.the.reaction.that.grows.the.desired.film.composition.and.structure..Because.precursors.are.introduced.as.gases.a.generally.wide.range.of.chemistries.and.process-ing. conditions. can. yield. equivalent. films.. The. composition. uniformity. (composition,.thickness,.and.structure). is.widely. tunable.by.temperature.and.concentrations.of. the.process.vapors.

For.transparent.conductive.coatings.the.historical.choice.has.been.indium.tin.oxide.(ITO).deposited.by.sputtering.because.of.widespread.and.longstanding.tool.and.process.technology.development..A.few.things.are.changing.this,.including.the.limited.supply.of. indium,.requirements.for.finer.degrees.of.film.property.control,.and.developments.in.MOCVD.techniques.for.ZnO.and.its.alloys..Further,.sputtering.appears.to.be.unable.to. provide. many. of. the. properties. needed. for. more. exotic. ZnO. applications. such. as.transistors.and.nanowires..Table.3.1.compares.some.of.the.properties.of.sputtering.and.MOCVD.

3.2.2 General MOCVD Tool Comparison

Figure. 3.2. compares. general. MOCVD. film. growth. tool. approaches.. The. three. general.reactor.styles.are.a.horizontal.tube.(or.the.wrapping.of.a.horizontal.tool.360°.to.form.an.annulus.of.deposition,.a.vertical.tube.(with.or.without.rotation.or.high-speed.rotation),.and.essentially.rectangular.arrangements.for.tapes.or.sheets.of.material..The.horizontal.tool. is. difficult. to. scale. over. a. wide. range. of. process. parameters. and. can. suffer. from.reaction-induced.depletion.of.precursors.which.can.in.turn.lead.to.compositional.changes..This.can.be.somewhat.overcome.by.process.variation,.relatively.high.gas.flows,.and.by.

MBE, PLD, PVD, or sublimation

MOCVD /ALD

Spin mist depositionLiquid flow

Eror accesvoles

CrackaRizer

FIGURE 3.1Comparison.of.film.deposition.approaches.


rotation.of. the.wafer(s)..Wrapping. the.horizontal. tube. into.an.annulus.does.not.solve.depletion-type. problems. but. does. increase. the. available. deposition. area.. The. vertical.tube.reactor.is.generally.defined.by.a.uniform.gas.flow.down.from.the.top.of.the.reactor.across.a.horizontal.disk.with.the.wafer.on.top.of.it..Depending.on.process.conditions.the.wafer.may.be.fixed.or.rotated.rapidly.in.order.to.employ.viscous.drag.to.pull.the.gas.down.and.radially.outward.to.counter.thermal.buoyancy..Thermal.buoyancy.is.an.issue.for. both. vertical. and. horizontal. tubes—heated. gas. rises. that. can. generate. significant.recirculating.thermal.cells—these.cells.can.bring.precursors.and.byproducts.together.–.generating.particles.and.wall.coatings..In.turn-coated.reactor.features.can.later.flake.and.deposit.particles.on. the.growing.films..Particles. formed. in. recirculating.cells. can.also.deposit.on.growing.films..Further,.such.cells.can.also.generate.variable.zones.of.greater.and.lesser.gas.phase.precursor.concentrations.and.hence.special.and.temporal.variations.in.the.growing.film.composition.Thermophoresis.generally.works.to.the.benefit.of.either.reactor. design. because. it. “pushes”. particles. away. from. the. hottest. surface,. which. is.

Heater

(a)

(c) (d)

(b)Heater

Cross-section Top view

Inlet SusceptorExhaust

FIGURE 3.2Comparison.of.MOCVD.deposition.reactors:.(a).horizontal.tool.reactors,.(b).vertical.reactors,.stationary.or.rotat-ing.susceptor,.(c).in-line.reactor.showing.sequential.top.and.bottom.coating,.and.(d).roll-to-roll.(R2R).tape.coat-ing.reactor.showing.sequential.top.and.bottom.coating.

TABLE 3.1

Process.Benefits.Comparison.for.ZnO.Production

Property Sputtering MOCVD

Conformality − ++Alloy/dopant.composition.tuning

−− ++

Purity + +Surface.damage − ++Deposition.temperature 0 +Deposition.rate + +Structure.control 0 ++Manufacturability + +Producibility.of.nanostructures/nanowires

0 +


generally.the.deposition.surface..The.horizontal.annulus.may.loosely.be.distinguished.from.the.vertical.tube.reactor.by.the.introduction.of.gas—generally.from.a.central.region.in.a.horizontal.reactor.wrapped.into.an.annulus.versus.gas.injection.being.introduced.generally.uniformly.across.the.top.of.a.vertical.tube.reactor..Rectangular.type.systems.must.balance.all.of.these.deposition.issues.and.generally.require.even.greater.control.of.flow.dynamics.for.long-term.clean.operation.

3.2.3 Chemistries Employed

ZnO. in. particular. is. a. very. interesting. material. with. respect. to. the. dynamic. range.of. precursors. available. for. growth. of. films. by. MOCVD.. The. general. CVD. process. is.reviewed. in. Figure. 3.3. and. Table. 3.2.. The. materials. listed. in. the. table. and. also. oth-ers. have. been. used. to. produce. ZnO. materials.. There. are. however. many. nuances. to.implementing.the.many.precursors.and.trade-offs.in.order.to.produce.ideal.films..For.example,.H.and.N.are.considered.p-type.or.at.least.compensating.materials;.however,.most.precursors.have.H.in.their.chemical.formula.and.N2.is.typically.thought.of.as.a.cheap.process.gas..Addition.of.NOx.appears.to.lead.to.etching.of.films.under.certain.

M1Rx + M2Ry + O2 M1M2Oz + CO2 + H2O + etcMetalorganicprecursor(s)

Metal oxide Gaseous reaction products

Precursors in vapor form are transported to the deposition planeHot deposition plane decomposes precursors to deposit filmElement partial pressure ratios in the gas phase control composition inthe film - depends on T, residence time and other factorsPrecursors are volatile compounds containing the element(s)desired tobe deposited and other elements that form gaseous reaction by-products, typically composed of C,H,O, and /or N

Physical form and vapor pressure determines best delivery method:– Gas ® cylinder (pressure ~ atm)– Liquid ® bubbler (vapor pressure ~ few to hundreds of Torr)– Solid ® Sublimator or flash evaporator (mTorr to several Torr)

FIGURE 3.3The.MOCVD.process.(ALD.is.a.specific.form.of.MOCVD.wherein.the.reactants.are.alternatively.presented.to.the.deposition.surface,.generally.with.a.pump/purge/pressure-change.step.between.them).

TABLE 3.2

Basic.ZnO.Precursor.Chemistries

Zn DMZn,.DEZn,.Zn(AcAC),.Zn(thd)Alloys.(Mg,.Cd,.Mn) CP2Mg,.CP2Mn,.CPdMn,.

Mn(thd)2,.DMCdp-type.dopant.(P,.As,.Li,.N,.K,.Na) Tert-butyl.phosphine,.Ph3,.

Tert-butyl.Arsine,.AsH3

Li.tert-butoxide,.N2,.N2O..NO..NH3,.wwK,.zzNa

n-type.dopant.(Ga,.Al,.In,.F,.B) TMGa,.TEGa,.TMAl,.TEAl,.TMIn,.HF,.TEB,.B2H6

Others TEOS,.SiH4,.GeH4,.xxCo,.yyFeOxidizer O2,.NO,.NO2,.H2O,.Alcohol,.O3


process.parameters..Water.and.alcohol.oxidizers.lead.to.lower.temperature.high.depo-sition. rate. growth.. Alloy. growth. is. fairly. straightforward;. however,. as. concentration.limits.are.exceeded.the.crystal.form.may.change—for.example,.the.MgZnO.structure.is.stable.in.the.ZnO.structure.only.up.to.~20%–30%.Mg.depending.on.process.parameters..Deposition.temperatures.generally.fall.into.two.ranges.at.viable.growth.rates—~200°C–400°C.with.H2O.or.alcohol.and.~300°C–600°C.with.oxygen..However,.some.processes.for.specific.material.properties.may.require.much.higher.growth.temperatures..Grown.film. structure. is. also. dependent. on. substrate. structure—ZnO. epitaxy. is. straightfor-ward. on. ZnO. whereas. only. highly. oriented. columnar. crystals. may. be. produced. on.glass.or.Si.substrates,.for.example.

3.2.4 Special p-Type

P-type.doping.of.ZnO.has. remained.elusive.despite.many.concentrated.efforts.on. the.issue..Doping.atom.size.and.ability.to.activate.are.a.significant.problem..Film.light.con-ductivity. is. also. a. problem—p-type. films. can. easily. appear. n-type. if. measured. while.illuminated. or. for. significant. periods. after. illumination.. The.p-type. doping. levels. and.compensation.levels.have.caused.many.a.researcher.significant.consternation.with.a.given.sample.repeatedly.measuring.p-type.in.one.measurement.and.n-type.in.another—even.after.attempts.of.thermally.relaxing.samples.in.the.dark..A.recent.patent,.by.LumenZ.(ref.their.patent.and.paper).has.shown.significant.progress.in.p-type.ZnP.development.using.K.as.a.dopant.

3.2.5 General Process Parameters

The.range.of.process.parameters.for.MOCVD.growth.of.ZnO.films,.as.mentioned.earlier,.can.vary.a.great.deal..Table.3.3.reviews.typical.process.conditions.for.films.deposited.with.oxygen.as.the.oxidizer.in.a.rotating.disc.reactor;.in.this.case.Al.doped.ZnO—the.resulting.films.would.be.conducting.in.the.10–100.ohm/sq..range.and.have.a.transmittance.of.~90%,.depending.on.details.of.the.specific.process.conditions..While.the.presented.parameters.focus.on.low.pressure.to.make.full.use.of.rotating.disc.reactor.dynamics.to.produce.a.uni-form.laminar.flow.other.systems.may.operate.through.atmospheric.pressure.and.achieve.largely.equivalent.results.

TABLE 3.3

Typical.Al.(n-type).Doped.ZnO.MOCVD.Deposition.Parameters.with.Oxygen.as.the.Oxidizer;.Water.Would.Use.a.Lower.Temperature

Process Parameter Range

Carrier.gas.flow.(Ar.or.N2) 4,000–10,000.sccmOxygen.gas.flow 200–5000.sccmSample.rotation.speed 750.rpmAr.flow.rate.through.DEZn.bubbler 35–[email protected]°C.and.350.TorrAr.flow.rate.through.TMAl.bubbler 0–[email protected]°C.and.350.TorrDeposition.temperature 350°C–600°CChamber.pressure 5–100.Torr


3.3 General System Design Considerations

In.designing.an.MOCVD.system.there.are.a.great.many.considerations,. including. the.chemistry.to.be.used,.the.application.for.which.the.grown.materials.are.to.be.used,.sub-strate. limitations,.cost,.supporting.infrastructure,.and.so.forth..For.review.throughout.this.section,.Figure.3.4.shows.a.complete.system.schematic.and.Figure.3.5.shows.a.sum-mary.legend.of.most.symbols.employed.in.the.schematic.drawings.in.this.chapter..Figure.3.6.shows.a.full.research.and.development.tool.with.the.features.highlighted..Figure.3.4.highlights. the.main.features.of.a.system.as.follows.(starting.top. left.of. the. image.and.proceeding.essentially.clockwise.around.the.periphery):.a.series.of.gas.sources.reactive.and.inert.(including.a.bubbler),.a.distribution.of.those.gases,.a.flash.evaporator.for.low.vapor.pressure.sources,.a.series.of.a.single.p-type.dopant,. fast.switching.manifold.for.sources,.alloy.and.n-type.dopant.bubbler.sources,.a.primary.exhaust.system,.and.scrub-ber.or.abatement.system..Moving.from.left.to.right.on.the.centerline,.the.figure.shows.the.load.lock.with.its.exhaust.and.then.the.process.chamber.with.associated.gas.inputs,.exhaust,.and.gauging.

Figure.3.6.shows.the.components.of.an.actual.research.system.and.Figure.3.7.provides.a.highlighted.model.of.key.components.of.the.reactor.assembly..The.top.left.shows.a.gas.and.bubbler.source.panel.(without.bubblers.or.baths).that.is.actually.mounted.on.the.reverse.side..Center.right.is.a.research.scale.rotating.disc.reactor;.below.the.deck.it.sits.on.are.the.rotation.assembly.and.to.the.left.are.the.pressure.controlling.throttle.valve,.cutoff.valves,.particle.filter,.and.pump..The.right.side.shows.the.electronics,.which.in.this.case.include.an.upstream.plasma.power.source,.a.pump.starter.controller,.a.multi-zone.temperature.readout.panel,.a.control.computer,.a.printer,.and.power.supply..Other.readout.electronics.and.interface.control.boxes.are.mounted.on.the.reverse.side..While.a.load.lock.is.not.on.the.pictured.system,.it.is.an.easy-to-add.option.

Figure.3.8a.shows.a.cross.section.of.a.typical.rotating.disc.reactor..The.right.side.of.the.figure. shows. more. detail. of. the. heating—rotation. assembly.. Figure. 3.8b. shows. the. top.view.of.a.showerhead.of.a.typical.RDR.

3.4 System Solutions

3.4.1 General System Schematic

Figure.3.4.depicts.a.most.complete.full.function.MOCVD.system.schematic.for.ZnO,.its.alloys. and. related. materials—including. capabilities. for. producing. doped/conducting,.LED,. sensing,. magnetic,. piezoelectric,. memristor,. and. related. material. applications..While.individual.components.have.been.described.previously,.the.show.system.features.the.following.items,.from.inlet.to.outlet:.gas.purification.modules.to.assure.highest.qual-ity. uncontaminated. material. growth,. gas. source. lines. for. delivering. precursor. and. car-rier.gases,.bubbler.sources.of.varying.configurations.and.in.this.case.with.separation.for.p-. and. n-type. sources,. water. bubblers,. options. for. ozone. sources,. sublimation. sources,.liquid. delivery. flash. evaporation. sources,. pressure. balanced. fast. switching. manifolds.(for.bubbler.and.gas.sources). for.growth.of.complex. thin.or. thick.multilayer.structures.(which.also.allows.the.system.to.be.operated.in.an.ALD.mode.as.needed),.a.multiple.injec-tion.point.showerhead.to.minimize.prereactions.in.the.transport.lines.with.provision.for.


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pulsed.or.continuous.plasma.enhancement,.a.temperature-controlled.chamber.and.heated.deposition.plane,.an.atmosphere-reactor.isolation.load.lock.and.work.product.transport.system,.a.process.exhaust.system.with.filters,.and.a.load.lock.exhaust.system..This.overall.system.design.is.suitable.for,.as.we.shall.review.below.in.the.next.sections,.horizontal.or.vertical.deposition.systems.

Mass flow controller Filter gasket Manual valve

Particle filter

Scroll pump

Bubbler

Pump

LDS

Temperaturecontrolled bath

Mini scrubberand filter

Liquid deliverysystem

Flash evaporator

PF

B

Normally open valve

High temp normallyclosed valve

Normally closed valve

PuriferGate valve

VCR cap

Pump ballastB

PYRO

Condensation trap

Leak detector port

PyrometerAir line regulator

1/4 leak detectorport

Pressure controller

Capacitance manometer

Wide range gauge

Throttle valve

Switch gauge

Checkvalve

Turbo pumpTurbo

SG

PS Mass transport monitor

Low differentialmass flow controller

FIGURE 3.5Definition.of.symbols.used.throughout.this.chapter.

MOCVD system-reactor-electronics-exhaust

100W microwavepower supply

Throttle valvepressure

controllerCenter/outer

temperature androtation controller

PC interfacescreen

Filament outerpower supply

UHV constructionFully automaticFeedback control

MOCVD system gas panel

Components of a MOCVD system

Pressure balanced precursor switching manifold

FIGURE 3.6Photograph.of.a.ZnO.MOCVD.research.and.development.tool.with.main.features.labeled.


3.4.2 Horizontal Systems

Horizontal.systems.are.of.the.easiest.type.to.enter.the.MOCVD.technology.arena;.however,.they.are.generally.limited.in.ultimate.scaling.and.range.of.process.parameters..Horizontal.tubes.are.often.best.suited.to.low.pressure.processes,.where.prereaction.detriments.and.laminar. flow. advantages. are. greatly. lessened.. Horizontal. tubes. are. easy. to. begin. with.because. R&D. systems. can. be. put. together. very. simply,. at. low. cost,. and. with. minimal.sophistication..As.material.requirements.and.deposition.areas. increase.the.systems.can.become.much.more.complex—for.example,.heating.may.be.zoned,.different.precursors.may.be.introduced.separately.at.different.locations.and.at.different.temperatures,.and.gas.injection.distribution.patterns.can.be.varied.greatly.to.promote.uniform.growth.free.of.prereaction.byproducts..Two.basic.considerations,.as.will.be.discussed. in.the.following.

Transfer arm

Gate valve

Load port

To turbopump and

rough pump

Reactorchamber

FIGURE 3.7Key.reactor.portion.of.the.system.with.major.components.identified.

Wafer carrier

(a) (b)

Outerfilament zone

Innerfilament zone

Heater coilrotating

shaft

FIGURE 3.8Cross.section.of.a.typical.rotating.disc.MOCVD.reactor.(a).and.close.up.isometric.view.of.a.heater.assembly.for.the.same.system.(b).


sections.are.whether.the.tube.wall.is.hot—essentially.the.same.temperature.as.the.growth.zone.or.significantly.cooler.than.the.growth.zone.

3.4.2.1 Modeling

As. with. all. systems,. modeling. is. best. used. to. establish. base. reactor. design. solutions..Primary.issues.are.gas.flow.and.thermal.distribution..It.is.well.known.that.in.a.horizontal.tube.reactor.where.the.wall.is.cold.and.the.deposition.zone.is.hot.that.thermal.buoyancy.generally. leads. to. the.generation.of.recirculation.cells.which.can.give.rise. to.significant.prereaction.rates.and.hence.formation.of.particles..The.general.solution.is.to.increase.the.gas.flow.to.dampen.the.formation.of.such.recirculation.cells..Another.issue,.as.sample.size.is. increased,. is. the. potential. decrease. in. precursor. concentration. from. the. introduction.side. to. the.exhaust.side;. that. is.across. the.wafer..Precursor.depletion.results. in.nonuni-form.growth—both.in.terms.of.thickness.and.composition.as.precursors.generally.deplete.at.different.rates..Alternatively,.the.reactor.walls.may.be.kept.hot.in.which.case.laminar.flow.is.easier.to.maintain.without.recirculation;.however.the.walls.are.subject.to.coating.since.they.are.exposed.to.precursor.gases.and.“drive”.reactions.at.approximately.the.same.speed.as.on.the.surface.(different.surface.materials.may.catalyze.reactions.differently.and.growing.materials.may.adhere.to.a.given.surface.differently.than.to.an.alternative.surface)..Coated.walls.can.generate.particles.or.lead.to.drifting.growth.conditions.as.heat.transfer.patterns.change.or.precursors.deplete.differently.

Figure.3.9.shows.an.example.of.the.refined.modeling.of.a.horizontal.tube.reactor..In.this.reactor.the.inlets.(left).and.outlets.(right).are.cold..An.expanding.inlet.flow.is.then.shaped.through.a.slot.and.allowed.to.again.expand..The.mid.lamp.heated.zone.is.the.hot.zone,.and.in.this.case.the.quartz.walls.are.allowed.to.come.to.an.intermediate.temperature.between.“cold”.and.the.deposition.temperature..The.result.is.seen.as.a.uniform.evenly.distributed.flow.over.the.growth.surface..In.this.case.the.exhaust.side.slot.is.used.to.channel.flow.and.

FIGURE 3.9Example.of.modeling.used.to.design.a.cold.wall.central.zone.heated.tube.reactor.


acts.as.a.heat.shield..Not.shown.is.a.wafer.transfer.arm.which.comes.in.from.the.exhaust.side.(presently.just.shown.as.an.open.tube).

3.4.2.2 Tool Design and Systems

Several.of.the.issues.of.concern.when.designing.a.horizontal.tube.were.mentioned.in.the.previous.section..In.practice.there.are.four.main.methods.of.heating.with.tube.reactors—furnace,. external. lamp,. internal. radiant. heating. whether. from. lamps. or. filaments,. and.induction..Furnace.heaters.are.generally.metal.coilings.surrounded.by.an.insulator..Such.a.heating.furnace.is.shown.in.Figure.3.10.wherein.a.small.horizontal.tube.is.located.within.a. conventional. tube. furnace.. Such. systems. may. be. contain. one. or. more. heating. zones..External.lamp.heaters.are.generally.augmented.by.surrounding.them.with.highly.reflec-tive.surfaces—typically.gold.coated..Figure.3.11.shows.such.a. lamp-heated.tube.reactor.(without.the.lamp.heating.assembly.in.place)..Note.that.this.system.also.includes.a.wafer.transfer. load. lock. system.. Both. systems. utilize. full. control. system. packages,. which. is.important.for.reproducibility.and.accurate.process.control.

Oxide.systems.present.challenges.for.high-temperature.heating..External.furnaces.with.alumina. tubes.can.heat.up. to.~1500°C..Such.systems.allow.the.use.of.all. ceramic.com-ponents,. so. the. oxide. environment. at. temperature. is. not. a. problem.. Induction. heating.systems.can.heat.much.higher.(we.have.shown.graphite.susceptors.in.an.inert.atmosphere.to.heat.through.2400°C);.however,. there.are.few.metals.capable.of.being.heated.to.such.temperatures. that.will. survive. in.an.oxidizing.atmosphere..Further,. such. temperatures.with.oxidizing.precursors.are.likely.to.lead.to.significant.prereaction.or.even.combustion.if.the.process.parameters.are.suitable..Internally.mounted.heating.assemblies.are.equally.limited.by.oxidation;.whether.by.filaments.or.lamps..The.lamps.have.contacts.which.can.fail.if.they.are.allowed.to.heat.sufficiently.to.oxidize..Mixed.metal.alloy.filaments.them-selves.can.be.heated.to.~1400°C–1500°C.before.catastrophic.failure;.however,.heat.losses.and.other. limitations. limit.achievable.wafer. temperatures. to.about.900°C–1000°C..Such.filaments.also.expand.and.warp.significantly..Boron.nitride–coated.filaments.of.graphite.can.also.achieve.the.~1000°C.temperature.range.but.at.the.high.end.are.subject.to.oxida-tion.of.the.encapsulated.carbon.filament.if.it.becomes.exposed.to.the.atmosphere..Another.filament.material. MoSi2. is. a. viable. solution. if. it. is. compatible. with. the. specific. process.

FIGURE 3.10Self-contained.furnace-heated.tube.reactor.system..Note.the.external.plasma.application.coil.on.the.right-side.vapor.inlet.side.


chemistry;.however,.in.this.case.the.geometries.available.have.limitations.and.so.it.may.be.difficult.to.achieve.good.temperature.uniformity.

Inlet.of.gases,.as.with.all.oxidizing.systems,.is.generally.configured.to.separate.the.met-alorganic.or.gas.precursors.from.the.oxidizer.until.introduction.into.the.reactor..In.hori-zontal.tube.systems.this.is.generally.done.by.introducing.the.precursors.through.quartz.“Ts”.with.distributed.nozzles.separate. from.an.oxidizer. injection.nozzle..A.uniform.or.point.injection.of.inert.gas.is.generally.done.at.the.inlet.flange.

3.4.3 Vertical Systems

Vertical.systems.with.horizontal.wafer.holding.disks.or.susceptors.(rotating.or.not).are.generally.more.complex.to.design.than.horizontal.systems..While.the.gases.and.precur-sors.are.introduced.flowing.down.from.the.top;.the.hot.susceptor.heats.these.gases.giving.rise.to.a.thermal.buoyancy–driven.counter.flow..The.counter.flow.gives.rise.to.convection.cells.generally.rising.in.the.center.and.falling.at.the.edges—generating.recirculation..To.counter.this.effect.the.gas.flow.must.be.made.very.high.to.push.down.the.convection.flow.and.thus.be.rather.inefficient.or.the.technique.of.high-speed.rotation.may.be.used..High-speed.rotation.produces.a.viscous.drag.at.the.surface,.sweeping.the.gas.outward.and.cre-ating.a.lower.pressure.zone.at.the.center..The.lower.pressure.zone.acts.as.a.pump.inducing.flow.in.the.direction.counter.to.the.direction.of.thermal.buoyancy–induced.flow..The.net.result,.when.balanced.properly,. is.a.highly.uniform.downward.sweeping. laminar.flow.

FIGURE 3.11Lamp-heated.200.mm.wafer.reactor.system.(as.shown.modeled.in.Figure.3.9)..Left-side.quartz.chamber,.middle.monitoring.and.exhaust.chamber,.then.gatevalved.load.lock.chamber,.and.then.transfer.arm..In.this.instance.the.pumps.are.mounted.in.frame.under.the.chamber.assembly.and.the.electronics.are.mounted.in.the.half.high.bays.at.the.right.


over.and.around.the.susceptor.out.to.the.exhaust..The.high-speed.rotation.effect.results.in.a.highly.efficient.reactor.performance.

High-speed.rotating.disc.reactors.(RDRs).can.operate.over.a.very.wide.range.of.process-ing.conditions,.from.low.pressure.to.atmospheric.pressures.and.with.a.wide.range.of.gases.and.flows..A.limitation.to.the.RDR.is.the.height.of.the.showerhead.above.the.deposition.plane..For.a.given.set.of.process.conditions.the.system.is.unable.to.produce.laminar.flow.and.or.unable.to.eliminate.recirculation—in.these.undesirable.cases.the.showerhead.often.becomes.coated.and.gas.phase.reactions.tend.to.increase—the.net.result.is.a.higher.prob-ability.of.particle.contamination.or.otherwise.defected.material.growth..Despite.the.afore-mentioned. complexities. of. gas. phase. dynamics,. the. vertical. style. reactors. also. provide.the.most.versatility.in.design.geometry.to.grow.materials.uniformly..Section.3.3.reviews.aspects.of.a.precursor.distributing.showerhead.whereat.the.precursors.are.separated.from.the.oxidizer..We.have.demonstrated.ZnO.growth.on.multiple.wafers.on.400.mm.diameter.platters—see.Figure.3.12a.and.b.on.200.mm.diameter.wafers.on.300.mm.platters.

Deposition.zones.in.static.or.rotating.disc.reactors.are.most.often.heated.by.induction.heaters.or.planar. radiant.heaters—such.systems.were. introduced. in.Section.3.3.and.an.example.of.a.hot.metal.filament.heater.is.shown.in.Figure.3.13..A.constant.issue.with.metal.heaters.is.thermal.expansion—the.large.400.mm.multi-zone.heater.shown.in.Figure.3.13.uses.patented.pinning. technology..Generally,.vertical.deposition. reactors.are. cold.wall;.however,.some.may.be.intermediately.heated.to.minimize.condensation.of.certain.species.while.(preferably).not.decomposing.and.depositing.materials.on.the.walls..Hot.wall.reac-tors.are.not.used.for.most.oxides.and.not.generally.for.ZnO.and.its.alloys.

3.4.3.1 Modeling

Modeling. in. vertical. reactors,. with. or. without. susceptor/wafer. carrier. rotation,. is. very.useful.in.designing.new.custom.tools.and.in.scaling.to.larger.sizes..In.scaling,.modeling.is.limited.not.only.to.just.flow.patterns.but.also.to.how.the.materials.of.construction.respond.to.changes. in. temperature..Both.are.very. important.with.oxides.where.gas.phase.reac-tions.can.be.very.high.and.where.the.materials.of.construction.tend.to.have.large.thermal.expansion.coefficients.and.are.subject.to.embrittlement.fractures.

Growing.complex.oxides. that.have.broad.range.of. chemical. reaction.pathways.and.a.broad.range.of.possible.process.parameters.require.great.care.in.how.reactive.species.are.

(a) (b)

FIGURE 3.12(a).400.mm.platter.for.uniform.growth.of.up.to.38.×.2.in..wafers..(b).Uniform.oxide.films.grown.on.200.mm.wafers.grown.in.a.300.mm.platter.system.


introduced.into.the.system..For.example,.ZnO.and.its.alloys.can.be.grown.using.DEZn.and.oxygen.at.a.few.hundred.degrees.centigrade.and.at.rates.below.<1.nm/s;.alternatively.the.same.alloys.can.be.grown.at.>5.nm/s.with.water.vapor..Significantly.different.reactions.and.hence.film.properties.can.be.selected.by.varying.the.chemistry.(chemicals.used.and.their.partial.pressures.in.the.reactor).and.growth.temperature..With.the.same.chemicals.but.under.different.process.parameters. (especially.different. temperatures).films.can.be.grown. amorphous,. polycrystalline,. nanotipped. or. as. nanowires,. and. epitxially.. Oxides.are.notorious.for.prereactions.and.as.such.care.should.be.taken.to.prevent.upstream.dif-fusion.of.oxygen.into.precursor.delivery.lines..Such.diffusion.is.sometimes.prevented.by.relatively.high.gas.flows;.however,.such.high.flows.within.a.uniform.background.flow.can.“push”.other.precursors.out.of.a.region.needed.for.uniform.deposition.

Figure.3.14.shows.examples.of.the.uniform.background.flow.used.to.push.all.gases.lami-narly. through. the. reaction. chamber,. the. effect. of. switching. process. gases. sequentially.into.the.reactor.(in.this.case.as.part.of.the.uniform.flow.for.easy.viewing),.and.the.flow.streamlines.from.two.symmetric.gas.injectors.(of.different.gas).as.would.be.used.to.tune.film.uniformity..In.practice.a.great.many.variations.of.gas.injection.schemes.exist..Some.gases.may.be.separately.preheated.or.plasma.activated.in.some.way..Others.may.need.to.be.cooled.until.the.last.minute.before.they.are.injected.into.the.reactor..Ultimately,.model-ing.of.the.chemistry.with.the.flows.and.temperatures.is.most.beneficial.in.reactor.design.


Over. the. years. we. have. modeled. and. built. a. great. many. custom. MOCVD. reactors. for.oxides.and.other.materials..They.include.small.centimeter.to.2.in..wafer.scale.reactors.as.

FIGURE 3.13Figure.3.13.Multi-zone.large.area.oxide.heater..In.some.versions.the.shaft.may.also.be.heated.for.temperature.tuning.


shown.in.Figure.3.15,.which.includes.separate.injectors,.repositionable.plasma.activation.(of.the.individual.or.merged.gas.streams),.lamp.(as.shown),.or.induction.or.other.radiant.heaters.to.work.horse.research.reactors.capable.of.depositing.films.on.several.2.in..wafers.simultaneously.(Figure.3.16).or.large.production-ready.tools.depositing.on.8.in..or.larger.wafers.(a.platter.of.which.is.shown.in.Figure.3.12).

Figure.3.15a.shows.the.flow.model.(A).of.a.static.susceptor.reactor.with.plasma.(B).and.thermal.(C).activation.and.Figure.3.15b.shows.the.lamp.heating.assembly.for.this.reactor..Figure. 3.16. shows. a. typical. mid-size. reactor. system,. showing. the. control. system,. the.

(a) (b) (c)

FIGURE 3.14Three.flow.pattern.images.in.a.high-speed.rotating.disc.reactor..(a).Uniform.flow.streamlines.and.temperature.distribution,.(b).alternating.flow.wavefronts.as.may.be.used.in.a.growth.with.rapidly.switching.gas.composi-tions.to.form.a.superlattice,.and.(c).the.flows.of.two.specific.injectors.within.a.uniform.flow.that.is.not.shown.

834.724834.542774.359714.176653.993593.81

473.445500.627

413.262350.079282.836

(A)

(a) (b)

(C)

(B)

Temperature (K)

FIGURE 3.15(a).shows.the.flow.model.(A).of.a.static.susceptor.reactor.with.thermal.heating.and.plasma-activated.(B).and.plasma-only.activation.(C).and.(b).shows.the.lamp.heating.assembly.for.this.reactor.in.standard.lighting.


reactor.and.load.lock.assembly,.the.gas.switching.manifold,.and.the.main.portion.of.the.exhaust.system..The.bubblers.and.main.portion.of.the.gas.delivery.system.are.mounted.in.the.back.of.the.reactor..To.the.right.of.Figure.3.16.is.shown.an.optional.height.adjust-able. showerhead. which. also. offers. a. planar. diode. plasma. igniting. showerhead.. Figure.3.17.shows.a.very.high.temperature.oxide.system.where.the.reactor.is.mounted.within.an.external.furnace..In.this.case.all.internal.elements.are.made.of.ceramic.for.thermal.and.chemical.stability.at.high.temperatures..In.this.system.samples.are.exchanged.through.a.lift.mechanism.at.the.bottom.of.the.reactor.assembly.

A. great. many. reactor. system. solutions. exist. or. can. be. made. to. work. for. ZnO. (and.many.other.oxides).according. to. the.specific.need. (temperature. range,.chemistries,.and.so.forth)..The.examples.presented.here.span.small.R&D.tools,.intermediate.tools.for.com-plex. research. and. development. programs,. pilot/manufacturing. prototypes,. and. more.advanced.parameter.processing.reactors.

3.4.4 In-Line and Roll-to-Roll Systems

In-line.and.roll-to-roll.(R2R).systems.were.shown.in.the.bottom.of.Figure.3.2..Such.sys-tems.are.most.used.where. large. sheets.or. continuous. tapes.are. required—most.often.in.the.glass.and.photovoltaic.industries..They.will.only.briefly.be.reviewed.in.this.sec-tion.and.are.grouped.together.because.of.their.generally.similar.overall.geometry.and.operating.principles..These.tools.are.designed.for.high.volume.throughput..Particular.attention.has.come.about.with.the.growth.of.photovoltaics.in.the.market.and.the.need.for.large.area.transparent.conducting.coatings..Glasses.often.present.a.thermal.limit.around.500°C.and.tapes.(or.their.other.coatings).may.present.thermal.limits.as.low.as.~150°C.

3.4.4.1 Modeling

The.flow.dynamics.of.in-line.and.tapes.present.a.challenge.of.how.to.uniformly.scale.film.deposition.over.large.areas.for.long.time.periods.without.generating.significant.coating.

(a) (b)

FIGURE 3.16(a).shows.a.typical.mid-size.reactor.system.and.(b).a.height.adjustable.plasma.oxide.showerhead.that.can.be.used.in.such.systems.


residue.(particle.generators).or.having.the.process.drift.as.a.result.of.even.thicker.coatings..For.example,.a.tool.coating.a.kilometer-long.tape.(or.1000.m.of.1.m.long.glass.sheets).with.a.1.m.long.deposition.area.can.be.expected.to.produce.coatings.on.adjacent.hardware.on.the.order.of.1.mm,.if.not.more,.in.the.deposition.region..Such.thick.coatings.can.be.the.source.of.processkilling.contamination.

Figure.3.18.shows.a.sequence.of.modelings.of.different.gas-injection-deposition.plane.configurations..Many.configurations.are.used.depending.on.the.process.parameters..The.simplest. designs. center. around. the. uniform. injection. of. precursors. downward. onto. a.moving.substrate.and.effectively.pumping.byproducts..As.the.substrate.moves.under.the.flow,.non-uniformities.in.the.flow.along.the.path.even.out..The.same.concept.can.be.used.perpendicular.to.the.path.if.the.flow.patterns.ultimately.even.out..Additional.modeling.considerations.dictate.a.high.degree.of.thermal.control.such.that.the.deposition.zone.is.highly.uniform.and.the.adjacent.hardware.neither.depletes.process.gases.nor.promotes.excess.deposition.on. their.surfaces,.handling.conditions.such.that. they.do.not.generate.

FIGURE 3.17A.system.with.very.high.temperature.reactor.in.a.furnace.is.shown..In.this.case.all.internal.elements.are.made.of. ceramic. for. thermal. and. chemical. stability. at. high. temperatures.. In. this. system. samples. are. exchanged.through.a.lift.mechanism.at.the.bottom.of.the.reactor.assembly.


particles,.and.significant.controls.instilled.such.that.the.substrate.or.tapes.are.consistently.transported.through.the.system.


After.modeling.significant.effort.is.required.for.proper.design.of.continuous.sheet.han-dling.or.tape.driving..For.sheet.substrates.a.variety.of.load.and.manipulation.approaches.exist.but.generally. center.on. replaceable. cassettes.of. the. sheets.and.a.manipulator. that.place.them.onto.continuous.belts..This.approach.requires.careful.isolation.of.both.coated.and.uncoated.product.from.the.coating.process.zone.itself.lest.contamination.affect.sub-sequent.performance..For.tapes.the.moving.belt.itself.is.generally.the.substrate—the.tap..Source.and.take.up.reels.are.most.often.used..Care.is.needed.to.account.for.the.changing.spool.diameters.and.the.associated.changing.reel.speeds,.tension.and.position.in.relation.to.the.deposition.plane..The.changing.spool.diameter.is.generally.accounted.for.by.using.tension-controlled.motors,.and.idling.and.tensioning.wheels..Care.must.also.be.given.to.loading,. threading,.and.unloading.of. tapes..Figures.3.19.and.3.20.show.a.research. tape.deposition.tool.that.can.also.approximate.in-line.depositions.and.a.gas.panel.schematic.of.a.production.system.

3.4.5 Atomic Layer Deposition (an MOCVD Subset Process)

Atomic.layer.deposition.(ALD).enables.thin.film.deposition.with.excellent.uniformity.and.thickness.control—it.is.essentially.a.subset.of.standard.CVD.processing.and.as.such.it.can.

(a)

(c)

(b)

(d)

FIGURE 3.18Example.flow.models.of.in-line/tape.systems..(a).An.essentially.uniform.flow.down.to.a.continuous.tape.perpen-dicular.to.the.flow,.(b).example.flow.from.one.injector.within.a.uniform.flow,.(c).an.image.angled.horizontal.flow,.and.(d).an.example.of.a.graded.laminar.flow.showerhead.of.potential.use.with.an.angled.horizontal.flow.system.


be.performed.with.either.a.standard.MOCVD.system.or.a.system.streamlined.to.capitalize.on.the.restricted.set.of.process.requirements..ALD.is.a.most.elegant.deposition.solution.when.the.chemistry.allows.it.to.work.as.desired.and.the.film.thickness.or.a.series.of.thin.films.requires.high.precision.and.sharp.interfaces..ALD.works.when.a.chemistry.is.devel-oped.such. that.one.element.of. the. to-be-grown.compound.can.be.deposited.nominally.in.one.layer,.excess.material.pumped.away,.and.the.next.reactive.element.exposed.to.the.surface—reaction.takes.place,.byproducts.are.then.pumped.or.flushed.away,.and.the.cycle.begins.anew.until.the.precise.number.of.layers.are.grown..ALD.also.often.allows.signifi-cantly.lower.growth.temperatures.than.CVD,.which.can.be.beneficial.when.working.with.temperature-sensitive.under-layers..Difficulties.arise.when.layer.coatings.are.incomplete,.when. low-temperature.reactivity. is.weak,.multiple.components.of.differing.chemistries.are. needed. to. form. a. multi-element. compound,. reactivity. of. different. compounds. are.driven.at.different.temperatures,.and.so.on..For.example,.thick.film.LEDs.with.an.active.multi-quantum. well. active. region. is. routinely. grown. by. MOCVD,. whereas. a. structure.needing.5.or.10.(nano).layers.of.Al2O3.is.best.made.by.ALD..ZnO.can.be.grown.by.ALD,.and.while.tools.are.available.for.ALD.growth,.it.is.open.to.where.ALD.will.benefit.ZnO.devices—possibly.in.transistor.devices.

An.ALD.chamber.typically.differs.from.standard.MOCVD.chambers.in.that.it.operates.at.lower.temperatures.and.generally.attempts.to.minimize.the.surface.area.and.volume—this.is.done.to.minimize.the.amount.of.time.required.for.a.minimally.sufficient.amount.of. precursor. to. be. introduced. to. the. deposition. zone. before. excess. material. is. purged.away..Figure.3.21.shows.a.model.of.a.rotating.disc.ALD.tool.which.has.some.benefit.to.the.mixing.of.pulsed.precursors;.however,.systems.in.general.do.not.utilized.platter.rotation..

FIGURE 3.19Photograph. of. a. simple. research. continuous. tape. tool. used. for. showerhead. and. process. tuning. simulating.either.continuous.tape.or.in-line.coating.apparatus..(Courtesy.of.Aventa.Systems,.Boston,.NA).Seen.in.the.image.are.the.tape.drive.and.idling.wheels,.a.broad.8.in..tape,.the.heater.manifold,.and.a.patent.pending.bottom.up.gas.delivery.system.


Figure.3.21.(right.side).also.shows.a.typical.precursor/purge.cycling.routine;.part.of.the.time,.however.brief,.may.be.allowed.for.a.given.precursor.to.“soak”.the.surface.in.order.to.assure.uniform.coverage.and.or.reaction.with.a.prior.layer..Figure.3.22.shows.a.physi-cal.modeling.of.a.static.wafer.ALD.reactor.and.the.resulting.physical.reactor—in.this.case.for.up.to.12.in..diameter.wafers.and.with.allowance.for.plasma-enhanced.ALD.modes.of.operation—an.important.feature.for.oxides..Figure.3.23.shows.a.gas.panel.schematic.of.an.

Removableshowerhead

Opticalmonitors

(Reel in)

LDS

Zn Dopant

Specialsources

Heater

(Reel out)

Blower Blower Scroll

ZnO- TCO Tape coating tool

H20

2 31

H2O

O2

Ar

H20

FIGURE 3.20Schematic.diagram.of.an.in-line.or.R2R.film.deposition.tool.highlighting.vapor.inlets,.reaction.chamber,.and.exhaust.system.

Time

Start Set range Clear Close

1600150015601540152015001480146014401420140013801360134013200

100

200

300

400

500

600

700

800

900

1000

Select/cutterCurrent values

ReactantReactant

ReactantReactant

Reactant

OxidizerOxidizerOxidizerOxidizer

Press10.0 0.0 0.0 0.0

None None None

Pres

sure

(a) (b)

FIGURE 3.21(a).Fluid.dynamic.model.of.a.small.volume.ALD.system.and.(b).image.of.a.typical.ALD.process.recipe.


ALD.tool..An.ALD.system.schematic.is.generally.much.simpler.than.an.MOCVD.system’s.gas.panel.schematic—most.notable. is. that.bubblers.may.be.simplified. to.vapor.sources.and.switching.manifolds.predominantly.rely.on.simple.“vacuum”.switching.and.do.not.rely. on. the. complex. gas. switching. technologies. employed. in. complex. nanolayer. struc-ture.MOCVD.growth.tools..The.trade-off.is.generally.deposition.time.for.highly.accurate.(“digital”).layer.growth—see.Figure.3.21.Another.important.feature.of.an.ALD.system.is.that.the.control.system.can.be.greatly.simplified,.such.systems.generally.have.far.fewer.valves.and.controllers. than.do.MOCVD. tools.and.as. such. their. control. systems.can.be.compacted.and.use.less.sophisticated.hardware;.an.example.is.that.an.ALD.system.can.be.controlled.with.robust.microprocessor.hardware.and.safely.run.with.a.laptop—Figure.3.24.

(a) (b)

FIGURE 3.22(a).Solid.Works™.physical.model.of.a.12.in..wafer.platter.and.(b).photograph.of.the.same..Note.this.reactor.also.has.a.plasma.enhancement.operation.capabilities.(this.reactor.is.configured.for.manual.or.automatic.transfer;.however,.the.transfer.system.is.not.shown).

Facility bulkhead Facility bulkhead

1/4”softstart

MO-4

PF

Exhaust

1/4”Overpressurebypass

MO-3MO-2MO-1

O2

H2O

Inert

SG

SG

FIGURE 3.23Schematic. of. a. dedicated. (streamlined). ALD. gas. panel;. note. the. far. fewer. components. than. in. a. standard.MOCVD.tool.and. that.a. standard.MOCVD.tool,. through.proper.valve.operation,. can.generally.provide. the.necessary.ALD.operations.


shows.such.a.controller..It.should.be.noted.that.while.a.well-designed.MOCVD.tool.can.generally.operate.in.an.ALD.mode.an.ALD.tool.needs.significant.rework.to.operate.in.an.MOCVD.mode.

3.4.6 MOCVD-Enhanced Processes

There. are. a. range. of. ways. to. enhance. the. MOCVD. process—they. include. plasma-enhanced.deposition.and.UV.or.otherwise.photon-enhanced.deposition,.among.other.enhancements..Figure.3.25.shows.three.versions.of.upstream.plasma.sources—first.an.upstream. microwave. plasma. source,. then. a. simple. coil. coupled. rf. plasma. interfaced.with.a.tube.reactor,.and.finally.a.small.vertical.tube.reactor.with.an.in.stream.rf.plasma..Figure.3.26.shows.two.in.reactor.plasma-enhanced.MOCVD.configurations.(left),.a.par-allel.plate.(diode),.and.right,.a.ring.coil.plasma..Figure.3.27.shows.a.UV.photon.enhance-ment.of.MOCVD.through.the.adaptation.of.a.high-intensity.UV.Hg.lamp.which.imparts.significant. energy. to. gas. phase. molecules. as. well. as. surface. atoms. to. enhance. reac-tive. and. surface. energies. during. deposition.. While. not. shown,. lasers. have. also. been.used. to. enhance. process. reactant. energies.. Hot. wire. filaments. may. be. used. in. some.instances.as.well.but.generally.present.a.problem.in.oxide.systems.because.of.oxidation.or.contamination.

FIGURE 3.24Example.of.a.compact.low-cost.ALD.controller.that.can.satisfy.research.or.production.needs.

(a) (b) (c)

FIGURE 3.25Three. upstream. microwave. plasma-enhanced. MOCVD. configurations:. (a). small. ~120. W. upstream. plasma.column,. (b). vertical. or. horizontal. upstream. induction. coil. low. frequency. rf. plasma. column,. and. (c). simple.vertical.tube.reactor.in.stream.rf.plasma.


3.5 Packing Fractions and COO

An. important.aspect. of.MOCVD. production. tools. is. the.operating. cost..While. the. tool.cost. represents. a. significant. upfront. cost,. ultimately. it. is. the. cost. of. operation. that. has.the.biggest.effect.on.routine.production.costs..One.continuing.factor. is. the.process.effi-ciency.and.the.relative.area.onto.which.reactants.are.deposited..Figure.3.28.illustrates.this.concept—one.large.wafer.covers.more.area.that.several.smaller.wafers.in.the.same.size.platter..For.example,.as.shown.in.Figure.3.29,.with.a.small.wafer.edge.exclusion,.1.×.8.in..wafer.on.a.9.5.in..platter.covers.57%.of.the.area,.1.×.12.in..on.a.13.75.in..platter.covers.67%,.whereas.50.×.2.in..on.a.16.in..platter.covers.only.50%.of.the.area.compared.to.1.×.16.in..on.an.18.in..platter.covering.73%.of.the.area..Alternatively,.15.×.4.in..wafers.on.a.14.in..×.22.in..plate.have.more.than.60%.packing.factor..One.can.argue.precise.edge.exclusions.and.exact.

(a) (b)

FIGURE 3.26Two.in.reactor.plasma-enhanced.MOCVD.configurations:.(a).parallel.plate.(diode).plasma-enhanced.MOCVD.reactor.and.(b).ring.coil-activated.plasma,.also.in.system.

FIGURE 3.27UV. photon–enhanced. MOCVD. through. adaptation. of. high-intensity. UV. Hg. lamp. to. enhance. reactive. and.surface.energies.during.deposition.


wafer-to-wafer.platter.dimensions;.however,.net.reactor.efficiency.generally.favors.single.wafers.closely.matched.to.the.platter.size—this.matches.the.experiences.gained.in.the.Si.world.over.the.past.decades..There.are.of.course.always.mitigating.factors.such.as.process.time.versus.preparation. time,.a. long.process. time.may.favor.batch.operations..Another.cost.factor.to.consider.is.the.cost.of.the.wafer.failure.at.a.given.stage.of.processing.which.depends.on.the.value.of.the.produced.product..Most.recently.the.GaN.market.has.seen.the.embracing.of.large.MOCVD.cluster.tools.as.mechanism.to.reduce.production.costs;.albeit.still.with.relatively.small.4.in..and.possibly.soon.6.in..wafers.It.will.be.very.interesting.to.see.if.compound.semiconductor.markets.fully.follow.Si.production.routes—that.is,.cluster.tool.style.production.is.presently.being.embraced.and.the.question.is.how.will.processing.evolve.if.high.value.large.diameter.(8.or.12.in.).wafers.become.the.standard.

To.carry.out.a.formal.cost.of.ownership,.at.minimum.several.factors,.as.follows,.should.be.considered:.depreciation—tool.and.facilities,.number.of.operational.hours.within.available.shifts,.working.days,.and.preplanned.offtimes,.specific.device.structure.and.a.layer-by-.layer.time.and.material.consumed.(including.bakeouts,.anneals,.cool.down,.and.other.process/cycle.steps).should.be.accounted.for..Analysis.must.be.performed—noting.each.layer.is.likely.to. be. of. a. different. composition. and. as. such. require. different. consumption. calculations..

1 – 8˝

38 – 2˝ 4˝ × 24pc΄sØ 9.5˝

Ø 16˝

FIGURE 3.28Comparison.of.wafer.layouts.on.different.susceptors—single.and.multiple.wafers.on.a.disc.susceptor.and.mul-tiple.wafers.on.a.rectangular.susceptor.

90%

80%

70%

60%

50%Are

a util

izat

ion

40%

30%0 5 10 15 20

Wafer size

FIGURE 3.29Estimation.of.practical.usable.area.of.wafers.at.different.platter.sizes—the.wafer.size.represents.wafers.in.the.following.arrays—disc.susceptors:.50.wafers.at.2.in.,.8.wafers.at.4.in.,.1.wafer.at.8.in.,.1.wafer.at.12.in.,.and.1.wafer.at.16.in..and.a.rectangular.platter.with.5.×.3.=.15.×.4.in..wafers.(61%.packing.fraction.versus.50%.for.a.disc).


Production.time.should.also.be.allocated.to.regular.maintenance.and.unscheduled.mainte-nance,.cost.of.associated.facilities.should.be.included,.and.net.manpower.should.be.included,.including.accounting.for.sick.and.vacation.time.and.whole.person.times.(quite.often.elegant.cost.of.ownership.calculations.are.carried.out.that.do.not.account.for.whole.people.or.often.in.small.operations.rules.that.require.always.having.at.least.two.people.present.can.some-times.double.the.number.of.personnel.required.compared.to.the.number.of.people.predicted.from.the.calculations.for.a.given.shift)..Other.accountable.factors.that.should.be.considered.are.quantities.of.destructive. testing.test.wafers.used.per.run,.source.chemical.exchanges.pre-total.consumption—that.is,.one.may.change.a.bubbler.or.gas.cylinder.when.it.reaches.90%.consumption..These.factors.and.others.should.be.diligently.and.accurately.assessed.in.order.to.determine.the.operational.costs.associated.with.a.given.tool;.however,.in.the.end,.it.is.the.reasonable.throughput.of.quality.material.that.is.most.important.

3.6 Example Application Results

MOCVD.of.ZnO.and.its.alloys.have.been.used.in.a.great.many.applications..This.section.highlights.just.a.few.of.those.many.applications.MOCVD.tools.can.be.applied.to.

3.6.1 Transport Contact Layers for GaN LEDs

GaN.LEDs.have.become.the.most.important.source.of.lighting..The.actual.diode.is.rapidly.approaching. theoretical. limits. of. performance. and. so. increasing. attention. is. being. paid.to.the.supporting.aspects.of.device.structures,.such.as.the.contact.layers.that.must.let.the.light.out..Initially,.thin.metal.contact.layers.were.used;.however,.layers.thin.enough.to.let.most.of.the.light.out.could.neither.support.high-power.currents.nor.effectively.uniformly.spread.the.current.across.the.whole.diode..Sputtered.indium.tin.oxide.(ITO).for.initial.fix;.however,.indium.is.an.increasingly.expensive.element.and.sputtering.while.versatile.for.a.contact.layer.does.not.offer.as.full.a.range.of.process.tuning.as.MOCVD.does..Figure.3.30.

(a) (b)

FIGURE 3.30Comparison. of. GaN. LED. light. output. when. standard. Ni/Au. contacts. (a). are. used. versus. ZnO. contacts. (b)..Light.output.from.Ni/Au.contacted.GaN.LED:.40.mA.current,.output.=.190.82.mcd.at.λ.=.509.08.nm;.(b).Light.output.from.ZnO-contacted.GaN.LED,.40.mA.current,.output.=.354.58.mcd.at.501.71.nm..(Courtesy.of.Podium.Photonics,.Corp.)


compares.Ni/Au.contacts.to.ZnO.contacts.on.like.diodes—the.difference.is.striking.both.in.terms.of.light.output.and.relative.lifetime..The.“burn-out”.test.for.the.shown.ZnO.con-tacted.diode.was.terminated.at.5×.the.average.burn-out.time.for.the.previously.standard.Ni/Au.contacts..In.comparison.of.ITO.to.ZnO.the.results.vary.greatly.with.processing.(not-ing.ZnO.and.ITO.have.different.tolerances.to.processing.environment.conditions,.LED.top.layer,.doping.levels,.annealing,.and.so.forth..It.is.safe.to.say.ZnO.has.demonstrated.clear.advantages.in.some.cases;.however,.little.manufacturing.data.is.available.to.date.regarding.ZnO.contacted.LEDs.

Another.appealing.LED.structure.made.possible.with.transparent.contacts.is.shown.in.Figures.3.31.and.3.32.whereat.LEDs.are.grown.and.then.lifted.off.their.substrates.to.make.free-standing.LEDs—with.both.sides.being.LED.contact.layers..In.the.shown.example.ZnO.is.applied.to.both.sides.as.the.contact..The.result.is.single.or.stackable.LEDs.as.shown.in.Figure.3.32.

3.6.2 ZnO as the Contact Layer for Photovoltaics

ZnO.is.emerging.as.a.contact. layer.of.choice.for.many.advanced.photovoltaic.materials—most.prominently.CIGS. (Solid.State.Technology,.Feb..2008,.p..53).and.related.compounds.where. it.appears. to.have.direct.benefits.over. ITO..ZnO.is.also.routinely.used. in.other.PV.material.systems;.including.Si.and.CdTe..Utilization.of.ZnO.is.expected.to.grow.as.costs.and.other.factors.continue.to.become.more.important..ZnO.by.CVD.has.the.advantage.over.sput-ter.sources.because.of.its.ability.to.provide.smooth,.rough,.amorphous,.polycrystalline,.and.single.crystal.films.depending.on.processing.conditions,.as.seen.in.Figure.3.33,.which.is.very.important.for.photovoltaics.whose.TCO.properties.are.designed.to.match.widely.varying.PV.cell.properties..Figure.3.34.shows.some.recent.PV-related.film.structures.where.ZnO.nanotips.were.grown.on.ZnO:Ga.films.as.a.contract.layer.with.enhanced.light-trapping.properties.

3.6.3 Nanowires as Photonic Nanosensors

The. use. of. nanowires. as. photon. (or. chemical). sensors. has. recently. come. under. great.scrutiny.because.of.their.high.sensitivity..Figure.3.35.shows.an.array.of.ZnO.nanowires.

Pictures of 2 mm × 2 mm diewith thick ZnO on both sides. pGaN/ barrier/ MQWs on

PlyGaNTM

TCO layers grown on both sidesenable ohmic contact viaphysical, thick film orevaporative means.

GoldeneyeInc

FIGURE 3.31Unique.transparent.LED.substrates.allow.advanced.contact.and.illumination.designs.


before.harvesting,.their.same.single.wire.photovoltaic.response,.and.the.inset.shows.a.harvested. single. nanowire. sensor.. Single. crystal. ZnO. nanowires. have. been. grown. by.MOCVD.on.several.substrates—but.most. importantly.on.Si.and.at.temperatures.com-patible. with. underlying. CMOS. elements.. The. responsivity. of. such. crystals. has. been.measured.to.be.in.the.extent.of.1000–8000A/W.at.the.rate.of.1.V..The.nanowires.demon-strated.bandgap.response.cut-off.(making.them.solar.blind).and.sensitizing.with.other.materials.has.also.been.shown..The.challenge.is.to.optimally.grow,.harvest,.and.package.

1PM 20 KV 03 011 15 . 0x v X26 .0K 1.58 nmS

500 nm

(a) (b) (c)

FIGURE 3.33SEM.images.of.different.ZnO.morphologies.fabricated.by.MOCVD:.smooth.polycrystalline.(a),.polycrystalline.highly.aligned.columnar.growth.with.a.surface.of.nanotips.(b),.and.nanowires.(c).

Stack them, break them into microchips, flip halfover and run off of AC directly.

GoldeneyeInc

Like OLEDs, EpiChipsTM can be mixed into a matrixand coated to make large area flexible films oflight.UnLike OLEDs, EpiChipsTM are non-toxic, robust, UVinsensitive, high brightness chunks of light.

FIGURE 3.32Unique.lighting.capabilities.demonstrated.with.MOCVD-deposited.transparent.and.conductive.ZnO.as.front.and.back.contacts—note.the.stacked.LEDs.that.are.only.contacted.by.probes.


x30,000#71,306512 × 480

1 μm 11 mm5 kvZnO n/TCO

GZO film

ZnO nanotips

01 . TIF

FIGURE 3.34Example.complex.photovoltaic.contact.layer.structure;.first.layer.smooth.and.continuous.to.form.a.good.cur-rent.spreading.layer.topped,.in.this.case,.with.light-absorbing.nanotips,.other.structures.would.be.smooth.and.planar.or.highly.faceted..(Courtesy.of.Rutgers.University,.New.Brunswick,.NJ.)

–1–10

AMRAY1 µm20.0 kv17.100× 14 AUG 09#0000 –8

–6

–4

Curr

ent (

nA)

–2

1.43 µm 0

2

4

6

8

–0.5 0 0.5

DarkLight

1Voltage (V)

(a)

(b)

FIGURE 3.35(a).Example.ZnO.nanowires.which.have.been.harvested.to.be.placed.across.contacts.and.generate.photocurrent.response.data.(b)..(Courtesy.of.Drexel.University,.Philadelphia,.PA.)


such.structures..Similar.structures.can.also.be.functionalized.for.enhanced.sensitivity.to.specific.molecules.or.proteins.and.so.for.chemical.or.biological.sensors.

3.6.4 Transistors

ZnO.transistors.have.shown.great.promise.as.transparent.transistors.for.display.as.well.as. for. intermediate.high-power.high-speed. low-cost.devices..There.are.many.publica-tions.on.the.anticipated.move.to.ZnO.transistors.in.place.of.silicon.transistors.in.displays.because. of. their. superior. dynamic. properties.. More. recently. ZnO. has. been. looked. at.as. a. potential. high-speed. device. and. potentially. as. an. intermediate. device.. ZnO. pos-sesses.properties.equal.or.superior.to.GaN.and.SiC.for.RF.power.that.include.high.break-down.field;.on/off.ratios.>10E10,.mobilities.~25.cm2/Vs;.low-cost.fabrication.and.silicon.integration—CMOS. compatibility;. complex. structures—bandgap. tuning. by. alloying;.low.toxicity,.environmentally.friendly.manufacturing.and.waste,.and.high.carrier.con-centration;.and.the.promise.of.enhancement.mode.operation.(zero.off)..The.applications.include.power.supplies,.PFC,.UPSs,.electric/hybrid.vehicles,. industrial.motor.control,.switching.power.amplifiers,.and.cellular.and.radar.RF.amplifiers..Perhaps.most.impor-tantly,.in.terms.of.materials.growth,.processing,.and.waste,.ZnO.is.likely.to.be.the.low-cost.device.material.next.to.Si..The.potential.here.is.to.make.extremely.low-cost.devices.(i.e.,.about.a.tenth.the.cost.because.of.Si.and.glass.compatibility).in.comparison.to.GaN.or. SiC.. The. state. of. the. art. is. showing. very. promising. results,. but. significant. work.remains. to.show.ZnO.to.be.a.competitive.alternative. to.Si,.SiC,.and.GaN..Figure.3.36.shows. a. high-speed. device. test. structure. and. Figure. 3.37. shows. performance. data. of.MOCVD-grown.ZnO.transistors.

3.6.5 ZnO as Phosphors and Electroluminescent Materials

ZnO.is.an.exceptionally.versatile.material—it.itself.is.often.used.as.a.phosphor—having.a.blue.white.spectrum.that.can.be.controlled.by.material.structure,.doping.and.alloying..Figure.3.38a.shows.the.spectrum.of.ZnO.under.various.degrees.of.laser.annealing.in.an.

ZnO deposited on thermallygrown 200A SiO2 film on a p+silicon substrate.

�e substrate acts as thedevice gate.

Similar depositionparameters to TCO films, butwithout the TMAI(Ga).

Metal contacts are Ti/Pt.

Structure is annealed.

S/D contact width = 5 μm

Drain contact - Metal2 μm

P+ silicon - gate

ZnO 500A

SiO2 200A

Mesa

S S

ZnO S/D (Ti/Au) SiO2

Gatep+ Si

S

DDD

LG = 2 μm LG = 5 μm LG = 10 μm

LG

Source contact - metal

WG = 400 μm

FIGURE 3.36ZnO.MOSFET.test.structure.


oxygen.environment—having.a.broad.visible.spectrum.emission.or.just.band.edge.emis-sion.spectra..Figure.3.38b.shows.the.electroluminescence.output.of.a.ZnSiO.film.doped.with.Mn.

3.6.6 Display Contacts

As.a.transparent.conductor,.ZnO.can.produce.highly.conductive.films.suitable.for.display.traces. over. large. distances.. Figure. 3.39. shows. a. ZnO-coated. simple. LCD. display. glass.and.Figure.3.40.shows.a.test.display.made.from.equivalently.ZnO.coated.glass..Such.dis-play.glasses.are.easy.to.coat.and.the.films.are.easy.to.wet.process.with.etchants.of.much.lower.acid.content.than.needed.for.IOT—thereby.reducing.material.costs.and.making.for.a.greener.display—less.toxic.waste.and.use.of.a.more.abundant.material:.Zn.versus.In.

1.E-01

1.E-03

1.E-05

1.E-07

1.E-09

1.E-11

1.E-13

1.E-15–2 0 2 4

Gate voltage (V)(a) (b)

Dra

in cu

rren

t (A

)

Dra

in cu

rren

t (m

A)

Drain voltage (V)6 8

Vd = 5 V

Vg = 0 VVg = 4 VVg = 6 VVg = 8 VVg = 10 VVg = 12 V

Vd = 2 VVd = 0.5 V

10 00

5

10

15

20

25

2 4 6 8 10 12

FIGURE 3.37Show. are. the. drain. current. as. a. function. on. (a). gate. voltage. and. (b). drain. voltage.. (Courtesy. of. AFOSR,.Arlington,.VA.)

Wavelength (nm)

PL Spectra of laser annealed ZnO/glass sample: CL07,PL excitation wavelength = 250 nm

3000.00E+00

5.00E+05

PL in

tens

ity (a

. u.)

1.00E+06

1.50E+06

2.00E+06

2.50E+06

3.00E+06

3.50E+06

SubCell 2Cell 4Cell 6 Cell 7

Cell 5Cell 3Cell 1

Cell 8

400 500 300

AC voltage (V)

EL intensity vs applied AC (1000 Hz) voltage for ZnSi2O4:Mn

2001000.0001

EL in

tens

ity (F

L)

0.001

0.01

0.1

1

10

100

400 500600 700 800

(a) (b)

FIGURE 3.38(a).Broad.photoluminescence.ZnO.spectra.under.various.degrees.of. laser.annealing.(b).electroluminescence.spectra.of.a.ZnSiO.sample.doped.with.Mn—inset.shows.actual.illuminated.film.


3.7 Conclusion

ZnO.and.its.alloys.will.continue.to.find.increasing.use.as.transparent.contacts.for.photo-voltaics.and.LEDs,.and.as.sensors,.and.potentially.as.LEDs,.phosphors,. transistors,.and.lasers.among.other.applications..MOCVD.is.well.suited.to.develop.and.manufacture.such.

FIGURE 3.39Display.glass.with.patterned.MOCVD.deposited.ZnO.contacts.before.processing.into.test.display.(shown.in.Figure.3.31).

FIGURE 3.40Test.display.using.MOCVD-deposited.ZnO.contacts.


materials.for.all.of.these.applications..MOCVD.has.been.shown.to.offer.great.range.and.versatility.in.producing.structures.of.ZnO.and.its.alloys..MOCVD.produces.films.of.high.quality,.with.good.uniformity.of.composition,.thickness,.structure,.and.doping.levels.over.a.broad.range.of.temperatures..MOCVD.tool.designs.have.been.shown.to.be.adaptable.to.horizontal,.vertical,.or.tape.styles,.serving.a.wide.range.of.applications.from.research.scale.to.full.production..We.conclude.that.MOCVD.will.contribute.to.many.ZnO-based.applica-tions.for.a.long.time.to.come.

References

Adekore,.B..T..and.Pierce,.J..(2009).Metalorganic.chemical.vapor.deposition.of.zinc.oxide..October.29,.2009,.Patent.application.number:.20090269879.

Pierce,.J..M.,.Wen,.H.,.Liu,.K.,.Kumrr,.M.,.Tresback,.J.,.Ali,.Y..S.,.Krahnert,.A.,.Adekore,.B..T..(2011).Growth.and.structural.characterization.of.intrinsic,.acceptor,.and.donor.doped.(Mg,Zn)O.epi-layers. via. metalorganic. vapor. phase. epitaxy. on. (1. 0. 1‾. 0). ZnO. substrates.. J. Cryst. Growth,.325(1),.20–26.

Stevie,.F..A.,.Maheshwari,.P.,.Pierce,.J..M.,.Adekore,.B..T..and.Griffis,.D..P..(2012).SIMS.analysis.of.zinc.oxide.LED.structures:.Quantification.and.analysis.issues..Surf. Interface Anal..doi:.10.1002/sia.4919.

81

4p-Type ZnO: Current Status and Perspective

Zhizhen Ye, Haiping He, Jianguo Lu, and Liping Zhu

4.1 Introduction

ZnO.has.been.regarded.as.a.promising.candidate.for.next.generation.ultraviolet.(UV).light-emitting.devices.(light-emitting.diode.[LED].and.laser.diode.[LD]),.due.to.its.wide.band.gap.(3.37.eV),.large.exciton.binding.energy.(60.meV),.and.small.exciton.Bohr.radius.(1.8.nm).[1]..There.are.also.other.advantages.of.ZnO.as.an.emitter,.such.as.the.availability.of.large-area.ZnO.substrate.(currently.3.in..ZnO.wafer.is.commercially.available).[2],.the.relatively.low.temperature.for.epitaxial.growth,.and.the.natural.abundance.of.Zn.and.Mg.elements..The.last.one.is.important.given.the.rapid.growing.concern.that.indium,.which.is.the.key.material.for.constructing.InGaN.LED,.is.expected.to.be.exhausted.in.10.years.. In.particular,. the.realization.of.optically.pumped.UV.lasing. in.ZnO.film.[3].triggered.extensive.studies.on.this.material..ZnO.is.expected.to.become.an.alternative.or. even. a. replacement. to. GaN.. Since. both. electron. and. hole. injection. are. mandatory.for. electrically. driven. light-emitting. devices. such. as. LED,.n-. and. p-type. doping. with.electron.and.hole.concentrations.well.in.excess.of.1017.cm−3.are.necessary..However,.as.a. wide. band. gap. semiconductor,. ZnO. suffers. from. the. doping. asymmetry. problem..

CONTENTS

4.1. Introduction........................................................................................................................... 814.2. p-Type.Doping.by.Group-V.Elements................................................................................ 82

4.2.1. Nitrogen.Doping....................................................................................................... 824.2.2. Phosphorus.Doping..................................................................................................844.2.3. Arsenic.Doping.........................................................................................................854.2.4. Antimony.Doping.....................................................................................................864.2.5. Mechanism.of.P,.As,.Sb.Doping.............................................................................. 87

4.3. p-Type.ZnO.by.Co-Doping..................................................................................................884.3.1. N-III.Co-Doped.p-Type.ZnO...................................................................................884.3.2. Dual-Doped.p-Type.ZnO......................................................................................... 89

4.4. p-Type.Doping.by.Group-I.Elements................................................................................. 894.4.1. Lithium.Doping........................................................................................................ 894.4.2. Sodium.Doping......................................................................................................... 924.4.3. Potassium.Doping..................................................................................................... 93

4.5. Other.Dopant.Elements....................................................................................................... 934.6. Problems.and.Outlook......................................................................................................... 94Acknowledgments......................................................................................................................... 97References........................................................................................................................................ 97


ZnO.is. intrinsically.n-type,.and.p-type.doping.has.proven.to.be.difficult. [4,5]..This. is.mainly.due.to.(1).the.severe.self-compensation.by.native.donor.defects,.(2).the.low.solu-bility.of.acceptor.dopants.in.ZnO.lattice,.and.(3).the.relatively.deep.acceptor.levels.of.the.potential.acceptors.[6].

The.success.in.p-type.doping.of.GaN,.a.material.very.similar.to.ZnO.in.crystal.structure.and.properties,.and.the.subsequent.commercial.utilization.of.GaN-LEDs.and.LDs.[7],.has.inspired.great.and.continuous.efforts. to.overcome.the.p-type.doping.of.ZnO..Since. the.early.report.on.p-type.ZnO.[8].in.1997,.numerous.experimental.and.theoretical.works.have.been.carried.out..Various.elements,.including.group-V.(N,.P,.As,.Sb),.group-I.(Li,.Na,.K),.and.group-IB.(Ag,.Cu),.have.been.tested.as.the.potential.acceptor.dopant..Moreover,.co-doping.of.dual.acceptors.or.donor.and.acceptor,.for.example,.Li-N.and.Al-N,.respectively,.have.also.been.proposed.as.a.possible.strategy..p-Type.conductivity.was.achieved.using.almost.all.of.aforementioned.dopants,.although.it.suffers.from.the.instability.problem..In.2005,.Kawasaki’s.group.[9].reported.the.room.temperature.(RT).electroluminescence.(EL).in.ZnO.p-i-n.LED.realized.by.N-doping..In.2006,.Xu.et.al..[10,11].reported.RT.EL.in.ZnO.p–n.homojunction.LED.fabricated.by.metalorganic.chemical.vapor.deposition.(MOCVD),.which.is.the.best.technique.for.industrial.manufacture.of.commercial.LED..In.2007,.Ryu.et.al..[12].reported.the.electrically.pumped.UV.lasing.in.ZnO.laser.diodes..So.far,.many.groups.have.reported.the.EL.in.ZnO-LEDs.[13–22]..Although.p-type.ZnO.has.even.raised.some.controversies,.for.example,.some.researchers.doubted.whether.it.indeed.exists.or.not,.these.reports.corroborate.the.existence.of.p-type.conductivity.in.ZnO..It.is.widely.accepted.that.the.most.pressing.challenge.right.now.is.the.stability.of.p-ZnO,.since.the.resistivities.of.some.of.the.p-type.ZnO.are.comparable.with.those.of.state-of-the-art.p-type.GaN,.say.1–10.Ω.cm.

In.this.review,.we.describe.the.experimental.efforts.made.by.various.acceptor.dopants.to.achieve.p-type.ZnO..Some.representative.theoretical.works.are.also.overviewed.as.inter-ludes..The.results.are.classified.with.respect.to.the.acceptor.dopants..We.start.with.group-V.dopants,.including.N,.P,.As,.and.Sb,.by.emphasizing.N-doping..We.then.move.on.to.the.co-doping.of.group-III.elements.with.N.as.well.as.dual-acceptor.doping.. In.Section.4.4,.p-type.doping.by.group-I.dopants,.mainly.Li.and.Na,.is.discussed..p-type.doping.by.other.dopants.is.addressed.in.Section.4.5..Finally,.we.give.an.outlook.of.p-type.ZnO,.by.pointing.out.the.remaining.problems.and.possible.solutions.

4.2 p-Type Doping by Group-V Elements

4.2.1 Nitrogen Doping

Among.all.the.group-V.elements,.N.used.to.be.considered.as.the.most.suitable.acceptor.dopant.for.ZnO,.and.was.investigated.a.lot.since.the.early.time.of.p-type.doping.of.ZnO,.because.the.ionic.radius.of.nitrogen.is.very.close.to.that.of.oxygen,.and.the.energy.level.of.nitrogen. in.ZnO.is.relatively.shallow.among.all. the.group-V.elements..However,. the.early. results. reported. were. not. so. exciting.. Sato. and. Futsuhara. et. al.. tried. to. fabricate.p-ZnO.films.using.N2.as.dopant.source.in.the.1990s..But.they.did.not.observe.the.p-type.transformation.[23,24]..Many.other.subsequent.investigations.using.N2.as.dopant.source.also.failed..The.earliest.p-type.ZnO.film.was.reported.by.Minegishi.et.al..[8].using.NH3.as.nitrogen.dopant.source.in.1997,.with.a.low.hole.concentration.(1.5.×.1016.cm−3)..After.that,.a.series.of.reports.on.fabricating.p-ZnO.appeared..At.the.same.time,.people.realized.that.


the.high.energy.of.N-N.bond.in.N2.which.would.not.break.under.usual.conditions.is.the.reason.they.could.not.get.p-ZnO.using.N2.as.dopant.source.

As.in.ZnO,.only.when.nitrogen.atoms.substitute.the.oxygen.sites,.they.act.as.acceptors..If.they.exist.in.other.forms,.such.as.(N2)O,.they.will.act.as.donors.compensating.the.accep-tors..In.2001,.Yan.et.al..[25].calculated.the.acceptor.formation.energy.of.N2,.N2O,.NO,.and.NO2,. respectively,.and.concluded.that.without.external.energy.assistance,.NO.and.NO2.should.be.the.most.suitable.dopant.source..Therefore,.NO.was.used.as.both.N.source.and.O.source.directly. to. fabricate.p-type.ZnO:N.by.CVD.[26]..Meanwhile,.people.sought. to.activate.N.atoms. in. the.dopant.source.with.external.energy,. for.example,.plasma.assis-tance.[10,11,27–29],.electron.cyclotron.resonance.[30],.ion.implantation.[31,32],.etc.,.in.order.to. increase.the.reaction.activity.and.the.doping.concentration.of.N.in.ZnO..Many.posi-tive.results.showed.that.introducing.external.energy.assistance.did.enhance.the.N.doping.concentration.

In. the. past. decade,. our. group. has. put. considerable. efforts. in. the. experimental. stud-ies.of.doping.ZnO.with.nitrogen.to.increase.the.doping.concentration..p-Type.conductiv-ity. was. realized. in. ZnO. thin. films. by. different. doping. methods,. such. as. N-doping. in.NH3-O2.atmosphere.by.direct.current.reactive.magnetron.sputtering,.co-doping.of.III-N.using.magnetron.sputtering,.and.MOCVD.using.NO.as.the.dopant.source.[33–35]..Upon.doping,.p-type.conductivity.with.hole.concentration.of.1.1.×.1017–2.0.×.1018.cm−3,.mobility.of.0.3–1.8.cm2/(V.s),.and.resistivity.of.about.3–100.Ω.cm.was.achieved.

Decent.electrical.properties.were.obtained.in.the.N-doped.ZnO.film.grown.by.MOCVD.in. NO-N2O. ambient,. which. are. comparable. to. those. of. Mg-doped. p-type. GaN. [36]..Diethylzinc.(DEZ).and.NO.were.used.as.zinc.precursor.and.N.dopant.source,.respectively..It.is.interesting.that.when.we.used.N2O.as.the.second.oxygen.source,.the.hole.concentra-tion.increases.largely.with.the.resistivity.decreasing,.consistent.with.the.hypothesis.that.moderate.N2O.can.decrease.the.hole-killer.centers.(e.g.,.Zni,.VO).[37,38]..The.flow.ratio.of.NO/N2O.plays.an.important.role.in.achieving.p-type.ZnO.film..Excessive.N2O.produces.oxygen-rich.environment,.which.leads.to.fewer.N.doping.into.the.films.[39],.and.simul-taneously. the. formation. of. undesirable. donor-like. defect. (N2)O. [40].. This. results. in. the.decrease.of.both.the.hole.concentration.and.mobility.

In.order.to.further.increase.the.solubility.of.N.in.ZnO.film,.a.valid.N-doping.method.has.been.developed.with.N.atom.activated.by.a.radio.frequency.source.using.MOCVD.technology..By.using.NO.plasma,.Xu.et.al..grow.p-type.ZnO.thin.films.on.n-type.bulk.ZnO. substrates. [10,11].. A. typical. ZnO. homojunction. shows. rectifying. behavior. with. a.turn-on. voltage. of. about. 2.3. V.. EL. at. RT. has. been. demonstrated. with. near. band. edge.emission.at.I.=.80.mA.and.defect-related.emissions.in.the.blue-yellow.spectrum.range,.as.shown.in.Figure.4.1.

For.the.N-doping.in.ZnO,.it.is.revealed.that.excess.zinc.and.interstitial.hydrogen.are.critical.for.the.doping.process..Owing.to.the.larger.electron.negativity.of.oxygen.than.that.of.nitrogen,.Zn.preferentially.combines.with.O.rather.than.with.N..Excess.Zn.was.required.to.incorporate.N.into.ZnO.films..Since.hydrogen.is.inevitable.in.many.experi-mental. conditions,. people. had. to. pay. attention. to. the. role. of. hydrogen. during. their.investigations. on. the. doping. mechanism.. The. presence. of. hydrogen. can. suppress. Zn.interstitials,.which.is.well.known.to.be.one.of.the.reasons.to.prevent.the.realization.of.p-type.for.ZnO..Moreover,.one.popular.viewpoint.is.about.the.passivation.effect.of.H.in.N.doping.process..Lu.et.al..[41].proposed.a.hydrogen-assisted.doping.model.and.consid-ered.that.H.would.enter.ZnO.with.N.at.the.same.time.and.form.a.NO-H.complex,.which.could.effectively.increase.the.doping.concentration.of.N..A.hydrogen.atom.locates.at.a.neighboring.site. to. the.N.atom.substituting.an.O.atom,.which.could.prevent. intrinsic.


donor.formation..H.can.be.driven.out.by.a.post-growth.annealing.and.the.N.dopants.activated..Such.a.model.was.approved.by.several.groups.both.theoretically.and.experi-mentally.[42–45].

In.terms.of.the.effects.of.N-doping,.several.reports.of.EL.from.LED.using.N-doped.ZnO.film.as.the.p-type.layer.seem.promising.[9–11,16,17,19]..However,.the.stability.of.p-ZnO:N.is.still.a.problem.that.needs.to.be.addressed.for.practical.application.of.ZnO-based.LED..In.addition,.very.recently.some.researchers.argued.that.N.is.a.deep.acceptor.in.ZnO.accord-ing.to.both.theoretical.[46].and.experimental.results.[47]..We.will.discuss.this.in.detail.in.Section.4.6.

4.2.2 Phosphorus Doping

While.most.efforts.on.p-type.doping.of.ZnO.were.focused.on.nitrogen.doping,.fewer.stud-ies.had.considered.other.group-V.elements..p-type.ZnO.films.have.also.been.obtained.by.using.phosphorus.dopant..Aoki.et.al..[48].reported.p-type.ZnO.films.created.by.excimer.laser.doping.from.Zn3P2..Afterward,.Kim.et.al.. [49].prepared.p-ZnO.thin.films.by.sput-tering.a.ZnO.target.doped.with.P2O5.at.high.temperatures.followed.by.a.thermal.anneal-ing.process..This.group.also.investigated.the.effects.of.phosphorus.doping.on.the.optical.properties.of.ZnO.thin.films,.obtaining.an.acceptor.energy.level.located.at.127.meV.above.the. valence. band. [50].. They. further. demonstrated. the. operation. of. a. UV-light-emitting.ZnO.homojunction.LED.by.growing.P-doped.p-type.ZnO.on.Ga-doped.n-type.ZnO..The.ZnO.LED.emitted.380.nm.UV.light.at.RT.and.showed.clear.rectification.with.a.threshold.voltage.of.3.2.V.[18].

Another.group.also.has.done.a.number.of.studies.on.P-doped.ZnO..Heo.et.al..[51].exam-ined. the. behavior. of. phosphorus. in. ZnO. thin. films. grown. by. pulsed. laser. deposition,.focusing.on.the.effects.of.annealing.on.doping..However,.they.have.not.obtained.p-type.ZnO.by.P-doping..Instead,.they.achieved.p-type.conduction.in.P-doped.ZnO.films.only.

Wavelength (nm)

Inte

nsity

(a. u

.)

350 400 450 500

I=20 mA

I=80 mA

550 600 650 700

FIGURE 4.1RT. EL. spectra. of. the. ZnO-LED.. (Reproduced. with. permission. from. Xu,. W.Z.,. Ye,. Z.Z.,. Zeng,. Y.J.,. Zhu,. L.P.,.Zhao,.B.H.,. Jiang,.L.,.Lu,. J.G.,.He,.H.P.,.and.Zhang,.S.B.,.ZnO.light-emitting.diode.grown.by.plasma-assisted.metal.organic.chemical.vapor.deposition,.Appl. Phys. Lett.,.88,.173506,.2006..Copyright.2006;.Xu,.W.Z.,.Ye,.Z.Z.,.Zeng,.Y.J.,.Zhu,.L.P.,.Zhao,.B.H.,.Jiang,.L.,.Lu,.J.G.,.He,.H.P.,.and.Zhang,.S.B.,.ZnO.light-emitting.diode.grown.by.plasma-assisted.metal.organic.chemical.vapor.deposition,.Appl. Phys. Lett.,.94,.169901,.2009.(88,.173506,.2006)..Copyright.2009,.American.Institute.of.Physics.)


when.a.dilute.concentration.of.Mg.was.added.to.enlarge.the.band.gap.and.increase.the.formation.energy.of.donors.[52]..Afterward,.there.have.been.a.series.of.reports.on.P-doped.ZnMgO.by.the.same.group.[53–56].

Recently,.Xiu.et.al..[57].reported.the.growth.by.molecular.beam.epitaxy.of.textured.p-type.films.with.relatively.high.hole.concentrations.(6.×.1018.cm−3).and.insisted.on.the.inevitable.competition. between. donors. and. acceptors. in. P-doped. ZnO. depending. on. the. growth.temperature.. Pan. et. al.. have. recently. developed. an. effective. route. to. prepare. P-doped.p-type. ZnO. thin. films. by. MOCVD. with. a. special. thermal. evaporator. [58].. Diethylzinc.(DEZn).and.P2O5.powder.(5N).were.used.as.the.zinc.and.phosphorus.sources,.respectively..Furthermore,.ZnO-based.p-n.homojunction.was.fabricated.by.deposition.of.an.Al-doped.n-type.ZnO.layer.on.a.P-doped.p-type.ZnO.layer.on.quartz.substrate..The.I-V.characteris-tics.of.the.ZnO.homojunction.exhibit.rectifying.behavior.

4.2.3 Arsenic Doping

There.are.also.a. few.reports.on.p-type.ZnO.by.As.doping..The.realization.of.As-doped.p-type.ZnO.film.was.demonstrated.by.Ryu.et.al..[59],.with.very.high.hole.concentration.of.1018–1021.cm−3..The.film.was.grown.on.GaAs.substrate.by.pulsed.laser.deposition.and.As.is.incorporated.by.in-diffusion.from.the.substrate..The.same.group.also.used.hybrid.beam. deposition. (HBD). to. grow.As-doped. p-type. ZnO. films. on. ZnO. substrate,. with. a.more.reasonable.hole.concentration.in.the.range.of.mid-1017.cm−3.and.a.high.hole.mobil-ity.of.35.cm2/(V.s).[60]..The.p-type.behavior.is.also.supported.by.the.performance.of.pn.photodiode. and. metal-semiconductor. field. effect. transistor. (MESFET). fabricated. using.the.As-doped.ZnO.films.[61]..In.addition,.they.integrated.the.As-doped.p-type.ZnO.and.ZnBeO.films.with.ZnO/ZnBeO.multiple.quantum.wells.to.fabricate.LED.[13].and.LD.[12],.from.which.near.band.edge.EL.and.electrically.pumped.UV.lasing.were.observed.at.RT..Figure.4.2.shows.the.EL.spectrum.of.ZnO-LED.based.on.As-doped.p-type.ZnO..The.device.exhibits.pure.near.band.edge.emission.and.almost.negligible.green.band..Purple.light.from.the.device.can.be.observed.by.naked.eyes,.as.shown.in.the.inset.

Wavelength (nm)

EL in

tens

ity (a

. u.)

200 300 400 500 600 700 800 900

FIGURE 4.2RT.EL.of.ZnO-LED.based.on.As-doped.p-type.ZnO..The.inset.shows.a.device.emitting.purple.light..(Reproduced.with.permission.from.Ryu,.Y.R.,.Lee,.T.S.,.Lubguban,.J.A.,.White,.H.W.,.Kim,.B.J.,.Park,.Y.S.,.and.Youn,.C.J.,.Next.generation.of.oxide.photonic.devices:.ZnO-based.ultraviolet.light.emitting.diodes,.Appl. Phys. Lett.,.88,.241108,.2006..Copyright.2006,.American.Institute.of.Physics.)


The. strategy. of. annealing. of. ZnO. film/GaAs. substrate. was. also. adopted. by. Botha.et.al..[62].and.Sun.et.al..[63].to.realize.p-type.conduction..Besides,.As-doped.p-type.ZnO.films.were.reported.by.other.groups.using.various.methods..Krtschil.et.al..[64].reported.As-doped.p-type.ZnO.films.by.metalorganic.vapor.phase.epitaxy.(MOVPE).using.AsH3.as.the.As.precursor..Wang.et.al..[65].reported.the.doping.of.As.by.annealing.the.MBE-grown.ZnO.films.along.with.a.GaAs.wafer.in.air..However,.the.window.of.annealing.tempera-ture.for.grow.p-type.films.in.their.experiments.is.extremely.narrow.(only.about.10°C)..Ion.implantation.and.subsequent.rapid.annealing.[66].were.also.applied.for.p-type.As.doping.

4.2.4 Antimony Doping

Sb-doped. ZnO. has. not. been. extensively. studied.. An. early. report. [67]. of. Sb. doping.for. p-type. ZnO. was. grown. by. laser-assisted. diffusion. method. between. a. Sb. layer.and. a. ZnO. film.. However,. the. reliability. of. the. conductivity. type. has. been. ques-tioned.. Recently,. Xiu. et. al.. [68]. fabricated. Sb-doped. p-type. ZnO. films. on. n-Si. (100). by.electron-cyclotron-resonance-assisted. molecular-beam. epitaxy.. Further. works. on.Sb-doped.ZnO.were.carried.out.by.this.group..Mandalapu.et.al..[69].fabricated.a.Ga-ZnO/Sb-ZnO/p-Si.homojunction.by.molecular.beam.epitaxy..The.turn-on.voltage.of.the.junc-tion.is.around.2.V..With.the.development.of.antimony.doping.techniques,.the.same.group.reported.the.fabrication.and.characterization.of.Sb-doped.p-type.ZnO/Ga-doped.n-type.ZnO.closely.packed.columnar.structure.LEDs.on.Si,.which.exhibited.dominant.UV.emis-sions.at.RT.[70]..There.has.been.inspiring.progress.that.electrically.pumped.ZnO.diode.lasers. on. Si. using. Sb-doped. ZnO. as. p-type. layer. and. Ga-doped. ZnO. as. n-type. layer.with.a.MgZnO/ZnO/MgZnO.quantum.well.in.between.were.realized.[71]..As.shown.in.Figure.4.3,.random.UV.lasing.at.around.380.nm.was.demonstrated.at.RT.with.very.low.lasing.threshold.current.density.of.10.A/cm2..Recently,.Guo.et.al..[72].reported.the.growth.and.properties.of.epitaxial.Sb-doped.p-type.ZnO.thin.films.by.pulsed.laser.deposition.(PLD)..p-type.conductivity.was.realized.with.hole.concentration.of.1.9.×.1017.cm−3,.resistiv-ity.of.4.2.Ω·cm,.and.Hall.mobility.of.7.7.cm2/(V·s).

Pan.et.al..[73].have.also.investigated.p-type.behavior.in.Sb-doped.ZnO.thin.films.depos-ited. by. oxygen. plasma-assisted. PLD. using. a.ZnO. target.mixed. with. Sb2O3.. In.order. to.reveal.the.formation.mechanism.of.the.acceptor,.light.doping.was.explored.to.avoid.the.

Lasing

(a)

Au/NiOAu/Ti

Sb:ZnO 350 nm

Ga:ZnO 350 nm

Si (100) substrateMgO/ZnO buffer

0.0660 mA50 mA40 mA30 mA

0.05

0.04

0.03

0.02

0.01

360 370 380 390Wavelength (nm)(b)

EL in

tens

ity (a

. u.)

400 410

FIGURE 4.3(a).Schematic.structure.and.(b).electrically.pumped.lasing.spectra.of.ZnO.laser.diode.by.Sb-doping..The.inset.shows.an.optical.microscope. image.of. lasing.device.driven.at.30.mA..Arrows. indicate. isolated. lasing.spots.on. the.diode.surface.. (Reproduced.with.permission. from.Chu,.S.,.Olmedo,.M.,.Yang,.Z.,.Kong,. J.Y.,.and.Liu,.J.L.,.Electrically.pumped.ultraviolet.ZnO.diode.lasers.on.Si,.Appl. Phys. Lett.,.93,.181106,.2008..Copyright.2008,.American.Institute.of.Physics.)


degradation. of. optical. properties.. Two. acceptor. states,. with. the. acceptor. levels. of. 161.and.336.meV,.were. identified.by.well-resolved.photoluminescence.spectra,.as.shown. in.Figure.4.4..The.shallow.acceptor.was.assigned.to.SbZn.–.2VZn.complex..The.deep.acceptor.was.assigned.to.VZn.formed.during.oxygen.plasma.growth.

4.2.5 Mechanism of P, As, Sb Doping

The.mechanism.of.p-type.doping.by.larger.size.group-V.elements.such.as.P,.As,.and.Sb.was.investigated.both.theoretically.and.experimentally..The.calculated.acceptor.level.for.As.sub-stituting.O.(AsO).is.very.deep.[39],.which.indicates.very.low.hole.concentration.at.RT..This.is.contradictory.to.the.experimental.findings..Limpijumnong.et.al..[74].proposed.that.large-size-mismatched.impurities.such.as.As.and.Sb.would.substitute.Zn.site.rather.than.O.site.and.form.AsZn.–.2VZn.complex,.which.acted.as.acceptor.with.relatively.low.ionization.energy.of.~150.meV.. Indeed,.acceptors.with.energy. levels.of.~120.and.~160.meV.were. identified.in. low. temperature.photoluminescence. spectra.of.As-doped.ZnO.films.by. several.groups.[60,62]..So.far,.there.is.still.no.compelling.evidence.for.the.existence.of.such.complexes.in.ZnO..Recent.measurements.based.on. the. channeling.emission. techniques.by.Wahl.et. al.. [75,76].demonstrated.the.As3+.substitution.for.Zn2+.in.As-implanted.ZnO.single.crystals.and.the.Sb3+.substitution.for.Zn2+.in.Sb-implanted.ZnO.bulk.crystals,.which.supports.the.validity.of.the.As(Sb)Zn.–.2VZn.complex.model..Very.recently,.however,.Limpijumnong.et.al..[77].performed.first-principles.calculation.on.P-doped.ZnO,.and.proposed.that.hole.accumulation.at.ZnO/Zn3P2.interface.is.a.plausible.explanation.for.the.observed.p-type.conductivity..They.suggest.that.this.explanation.may.also.be.applied.to.As-,.Sb-,.and.even.N-doped.ZnO..The.various.theoretical.models.indicate.that.further.efforts.are.required.to.clearly.understand.the.mecha-nism.of.p-type.ZnO.doped.with.group-V.elements.

Photon energy (eV)

PL in

tens

ity (a

. u.)

3.0

DAP2-LO

D0X-3LO

D0X-LO

D0X-4LO

D0X-2LO

DAP1-LO

103

104

105

106

107

108

3.1 3.2 3.3 3.4

DAP2

DAP1C

Undoped ZnO

2.9 3.0 3.1 3.2

FWHM=12 meV

3.357T=10 K

PL in

tens

ity (a

. u.)

Photon energy (eV)3.3 3.4

Sb-doped ZnO B

I9

Y

A

FX-2LOFX-LO

TES(D0X)

D0XD+X

FXn=1A

FXn=1B

FX n=2A

FIGURE 4.4Comparison. of. 10. K. PL. spectra. of. undoped. and. Sb-doped. p-type. ZnO. film.. (Reproduced. with. permission.from. Pan,. X.H.,. Guo,. W.,. Ye,. Z.Z.,. Liu,. B.,. Che,. Y.,. He,. H.P.,. and. Pan,. X.Q.,. Optical. properties. of. antimony-doped.p-type.ZnO.films.fabricated.by.pulsed.laser.deposition,.J. Appl. Phys.,.105,.113516,.2009..Copyright.2009,.American.Institute.of.Physics.)


4.3 p-Type ZnO by Co-Doping

4.3.1 N-III Co-Doped p-Type ZnO

In.1999,.Yamamoto.and.Yoshida.[78].proposed.the.co-doping.method.to.solve.the.unipolar-ity.in.ZnO.based.on.ab.initio.electronic.band-structure.calculations..The.Madelung.energy.decreases. with. the.n-type. doping. using. group-III. elements. (III.=.Al,. Ga,. or. In),. while. it.increases.with.the.p-doping.using.N.species,.indicating.the.localization.of.N.states..When.the.N-III.co-doping.approach.is.applied,.it.not.only.enhances.the.incorporation.of.N.but.also. gives. rise. to. a. shallower. acceptor. level. in. the. band. gap. in. p-type. ZnO.. The. N/III.ratio.is.predicated.to.be.2:1.in.co-doped.p-type.ZnO..Yan.et.al..[79].demonstrated.that.the.co-doping.approach.can.create.an. impurity.band.by.passive.donor–acceptor.complexes.in.ZnO.based.on.density-functional.theory.calculations..The.p-type.ZnO.is.achieved.by.the.effective.doping.of.the.impurity.band.if.excess.N.atoms.are.available,.rather.than.the.traditional.co-doping.concept.

In.2001,. Joseph.et.al.. [80].grew.p-type.ZnO.films.by.N-Ga.co-doping.via.a.PLD.sys-tem.combined.with.a.plasma.gas.source..The.p-type.conductivity.was.confirmed.using.Hall-effect.and.Seebeck.coefficient.measurements..The.N-Al.co-doping.method.has.been.studied.extensively.to.produce.p-type.ZnO..Wang.and.Zunger.[81].revealed.that.the.suc-cessful. p-type. doping. of. ZnO. largely. depends. on. engineering. a. stable. local. chemical.bonding. environment. based. on. first-principle. calculations.. Doping. with. N-Al. is. bet-ter.than.doping.with.N-Ga.and.N-In.since.the.corresponding.III-N.and.III-O.bonds.are.stronger.for.III.=.Al..In.2004,.Lu.et.al..identified.the.enhancement.of.N.solubility.in.ZnO.by.N-Al.co-doping.[82]..Figure.4.5.shows.the.SIMS.depth.profiles.of.the.main.elements.in.N-Al.co-doped.and.N.doped.ZnO.films.[82]..For.both.films,.nitrogen.is.well.detected..The.N.density.in.ZnO:(N,Al).is.much.higher.than.that.in.the.ZnO:N.film..This.enhancement.of.N.solubility.is.responsible.for.the.p-type.conductivity.of.the.doped.ZnO.film..The.typi-cal.resistivity.of.p-type.ZnO:(N,Al).films.was.57.3.Ω.cm,.with.a.Hall.mobility.of.0.43.cm2/(V.s).and.a.hole.concentration.of.2.52.×.1017.cm−3..Yuan.et.al..[83].grew.the.N-Al.co-doped.

Depth (nm)0

(a) (b)200 400

N

Inte

nsity

(cou

nts/

s)

AI

O

Zn

N-AI codoped ZnOfilm

600 800 1000100

101

102

103

104

105

106

107

Depth (nm)0 200 400

NInte

nsity

(cou

nts/

s) O

Zn

N doped ZnO film

600 800 1000100

101

102

103

104

105

106

107

FIGURE 4.5SIMS.depth.profiles.of.N-Al.co-doped.(a).and.N-doped.(b).ZnO.films..(Reproduced.with.permission.from.Lu,.J.G.,.Ye,.Z.Z.,.Zhuge,.F.,.Zeng,.Y.J.,.Zhao,.B.H.,.and.Zhu,.L.P.,.p-type.conduction.in.N-Al.co-doped.ZnO.thin.films,.Appl. Phys. Lett.,.85,.3134,.2004..Copyright.2004,.American.Institute.of.Physics.)


ZnO.films.using.NH3.as.the.nitrogen.source,.and.the.p-type.conductivity.was.obtained.in.a.temperature.range.of.380°C–480°C..Also,.N-Al.co-doped.p-type.ZnO.films.could.be.prepared. using. CH3COONH4. [84],. N2. [85,86],. and. AlN. [87,88]. as. the. nitrogen. sources..Using. the. N-Al. co-doped. ZnO. film. as. the. p-type. layer,. ZnO-based. p-n. homojunction.LEDs.were.fabricated.[89].

The. N-In. co-doping. method. was. also. applied. to. grow. p-type. ZnO.. Chen. et. al.. [90].obtained.the.N-In.co-doped.p-type.ZnO.films.by.magnetron.sputtering.in.a.N2O-Ar.ambi-ent.using.Zn-In.alloy. target..The. typical. resistivity.of.ZnO:(N,In).films.was.23.7.Ω. cm,.with.Hall.mobility.of.0.752.cm2/(V.s).and.hole.concentration.of.3.51.×.1017.cm−3..Besides.the.aforementioned.co-doping.methods,.the.N-Be.[91],.N-Zr.[92],.and.N-Ti.[93].co-doping.methods.were.also.used.to.prepare.p-type.ZnO.

4.3.2 Dual-Doped p-Type ZnO

The. dual-doping. method. means. that. two. acceptors. (e.g.,. Li-N. [94–97],. N-As. [64],. and.N-P. [98]). are. simultaneously. doped. into. ZnO,. with. emphasis. on. Li-N. dual-doping..Theoretically,.Li.and.N.are.the.two.best.candidates.for.p-type.doping.for.ZnO.with.regard.to.strain.effects.and.energy.levels.of.substitutional.LiZn.and.NO.acceptors.[39]..However,.Li. and. N. also. readily. induce. donor. levels. such. as. Li. interstitials. (Lii). in. ZnO:Li. and.N2-on-O.substitutions.(N2)O. in.ZnO:N.[40,99],. leading.to.self-compensations.for.p-type.doping.for.ZnO..In.contrast,.the.Li–N.dual-doping.can.result.in.stable.p-type.ZnO.with.low-resistivity.[94].

In.2006,.Lu.et.al..[94].firstly.proposed.the.Li-N.dual-doping.method.to.produce.p-type.ZnO..ZnO:(Li,N).films.were.prepared.by.PLD.using.the.ZnO–Li2O.ceramic.target..Ionized.N2O.was.used.as.the.N.source..The.lowest.resistivity.of.p-type.ZnO:(Li,N).films.was.0.93.Ω.cm,.with.Hall.mobility.of.0.75.cm2/(V.s).and.hole.concentration.of.8.92.×.1018.cm−3..The.p-type.conductivity.of.ZnO:(Li,N).was.reproducible.and.stable,.with.no.obvious.degrada-tion.observed.even.after.1.year.(Figure.4.6a)..The.acceptor.activation.energy.(EA).is.95.meV,.as.determined.from.temperature-dependent.Hall-effect.measurements,.which.is.close.to.the.predicted.level.of.Li.acceptor.

Also.in.2006,.Wang.et.al..[96].prepared.the.p-type.ZnO:(Li,N).films.by.magnetron.sputter-ing,.with.a.lowest.resistivity.of.119.Ω.cm,.Hall.mobility.of.1.74.cm2/(V.s),.and.hole.concen-tration.of.3.07.×.1016.cm−3..Recently,.Zhang.et.al..[97].investigated.the.formation.mechanism.of.Li-N.dual-doped.p-type.ZnO..Figure.4.6b.shows.the.total.density.of.states.of.undoped.ZnO.and.ZnO:(Li,N).systems..The.Lii–NO.complexes.can. form. in.ZnO:(Li,N),. suppress-ing.the.compensation.of.Lii.donor.for.LiZn.acceptor.and.(N2)O.donor.for.NO.acceptor..The.Lii–NO.complex.induces.an.impurity.band.above.the.valance.band.maximum,.resulting.in.a.decrease. in. the. ionization.energy.of. the.acceptor..They.may.be.responsible. for. the.enhancement.of.conductivity.and.stability.of.p-type.ZnO:(Li,N).

4.4 p-Type Doping by Group-I Elements

4.4.1 Lithium Doping

The.study.on.group-I.elements,.especially.Li.and.Na,.doping.in.ZnO.can.be.traced.back.to.the.1970s.[100–103]..As.the.common.impurities.in.hydrothermal-grown.ZnO.bulk.crystals,.Li.and.Na.were.found.to.be.very.deep.acceptors.(about.800.meV.for.Li.and.600.meV.for.Na),.


which.usually.result.in.high.resistivity.of.ZnO..Later.optical.experiments.[104].revealed.that.Li.and.Na.may.have.a.shallower.acceptor.level.~300.meV.in.ZnO..Moreover,.theoretical.calculations.predicted.that.group-I.elements.substituting.for.Zn,.such.as.LiZn,.can.have.even.shallower.acceptor. level. (e.g.,. 90.meV. for.Li. and.170.meV. for.Na). [39]..However,.more.detailed.theoretical.calculations.give.different.sentiment.toward.the.outlook.of.Li-doped.ZnO..On.a.positive.side,.Lee.et.al..proposed.[105].that.H.can.help.to.increase.the.solubility.of.Li.and.subsequent.H.removal.can.potentially.result.in.low-resistivity.p-type.ZnO..On.a.neg-ative.side,.Wardle.et.al..suggested.[99].that.p-type.doping.may.be.limited.by.the.formation.of.interstitials.and.complexes,.such.as.LiZn-Lii,.LiZn-H,.and.LiZn-AX..Experimentally,.Li.dop-ing.usually.increases.the.resistivity.of.otherwise.n-type.ZnO.[106].

Zeng.et.al..[107,108].reported.on.Li-doped.p-type.ZnO.thin.films.grown.by.dc.reactive.magnetron.sputtering..PL.analysis.demonstrated.that.LiZn.has.a.shallow.acceptor.level.

Energy (eV)

Preservation time (month)0

–1016

–1015

–10141014

1015

1016

1017

1018

1019

1020

1 2

n-type conductivity

p-type conductivity

High-resistivity region(ambiguous signal)

ZnO:N

ZnO:Li

ZnO:(Li,N) filmZnO:Li filmZnO:N film

3 4 5 6 7 8 9 10 11 12 13 14 15

Den

sity o

f sta

tes (

stat

es/ e

V ce

ll)Ca

rrie

r con

cent

ratio

n (c

m–3

)

ZnO ZnO:(Lii-No)

–6(b)

(a)

–4 –2 0 2 4

FIGURE 4.6(a).Carrier.concentrations.of.ZnO:N,.ZnO:Li.and.ZnO:(Li,N).films.as.a.function.of.the.preservation.period.after.deposition..(b).The.total.density.of.states.of.the.undoped.ZnO.and.ZnO:(Li,N).systems;.the.VBM.of.undoped.ZnO.and.ZnO:(Li,N).is.presented.with.dashed.line..(Reproduced.with.permission.from.Lu,.J.G.,.Zhang,.Y.Z.,.Ye,.Z.Z.,.Zhu,.L.P.,.Wang,.L.,.Zhao,.B.H.,.and.Liang,.Q.L.,.Low-resistivity,.stable.p-type.ZnO.thin.films.realized.using.a.Li-N.dual-acceptor.doping.method,.Appl. Phys. Lett.,.88,.222114,.2006..Copyright.2006;.Zhang,.B.Y.,.Yao,.B.,.Li,.Y.F.,.Zhang,.Z.Z.,.Li,.B.H.,.Shan,.C.X.,.Zhao,.D.X.,.and.Shen,.D.Z.,.Investigation.on.the.formation.mechanism.of.p-type.Li-N.dual-doped.ZnO,.Appl. Phys. Lett.,.97,.222101,.2010..Copyright.2010,.American.Institute.of.Physics.)


as. theoretically.predicted. [39],. independent.of. the.Li. concentration. in.ZnO..However,.another.deeper.acceptor.level,.which.is.probably.associated.with.complexes.containing.Li,. emerged. with. the. increase. of. Li. concentration. and. decreased. the. hole. concentra-tion.[108]..The.similar.dual.acceptor.states,.that.is,.shallow.level.and.deep.level,.for.Li.in.ZnO.were.also. revealed.by. theoretical. calculation. [109].and.electron.paramagnetic.resonance.and.photoluminescence.[110–112],.although.the.obtained.values.of.the.accep-tor.levels.were.somehow.different..The.electrical.properties.of.Li-doped.ZnO.thin.films.were.found.to.be.dependent.in.a.very.sensitive.way.on.the.Li.concentration..Afterward,.p-type.conduction.in.Li-doped.ZnO.was.also.confirmed.by.other.growth.methods,.such.as.pulsed.laser.deposition.[113–115],.thermal.treatment.of.Zn-Li.alloy.films.[116],.mag-netron.sputtering.[117],.solid-state.reaction.[118],.and.sol-gel.method.[119]..Particularly,.Lu.et.al..[113].revealed.that.p-type.Li-doped.ZnO.films.can.be.obtained.by.PLD.through.adjusting. the. growth. condition,. with. hole. concentration. of. 6.04.×.1017. cm−3. at. an. opti-mal.Li.content.of.0.6.at.%.under.ionized.oxygen.atmosphere,.whereas.the.doped.ZnO.films.exhibited.n-type.conductivity.with.ionization.power.off..Figure.4.7.shows.the.car-rier. concentration.of.ZnO.films.as.a. function.of.Li. content. in. the. range.of.0–1.8.at.%..Low.temperature.measurements.indicated.that.the.conductivity.of.Li-doped.ZnO.was.governed.by.Mott’s.variable. range.hopping.and. thermal-assisted.hopping.below.and.above. 40. K,. respectively. [114].. Furthermore,. p-type. conduction. was. demonstrated. in.ZnMgO.thin.film,.which.allows.band.gap.engineering.in.p-type.ZnO.[120]..The.experi-mental.observations.stimulated.more. theoretical.calculation. [121–123]..However,.most.results.pointed. to.a.complicated.doping.behavior. for.Li. in.ZnO,. rather. than.a. simple.substituting.acceptor.

More.recently,.RT.ferromagnetism.with.p-type.conductivity.was.reported.in.Li-doped.ZnO.thin.film.[114].as.well.as.nanorods.[118]..The.ferromagnetism.in.Li-doped.ZnO.was.attributed.to.magnetic.moments.of.cation.vacancies.mediated.by.holes.introduced.by.LiZn.and.VZn. [114]..The.positive. results. suggest. that.Li.provide.an.alternative.dopant. source.choice. for. formation.of.p-type. ZnO,.where.doping.mechanism,. however,. needs. further.investigation.

Li content (at. %)0.0

1015

1016

1017

1018

1015

1016

1017

1018

0.4 0.8

Elec

tron

conc

entr

atio

n (c

m–3

)

Hol

e con

cent

ratio

n (c

m–3

)

1.2 1.6 2.0

Semi-insulating region

p-type ZnO:Li(Ionization source is on)

n-type ZnO:Li(Ionization source is off)

2.4

FIGURE 4.7Carrier.concentration.as.a.function.of.Li.content.in.ZnO.films..When.the.ionization.source.is.on.(off),.ZnO.films.exhibit.p-type.(n-type).conductivity..(Reproduced.with.permission.from.Lu,.J.G.,.Zhang,.Y.Z.,.Ye,.Z.Z.,.Zeng,.Y.J.,.He,.H.P.,.Zhu,.L.P.,.Huang,.J.Y.,.Wang,.L.,.Yuan,.J.,.Zhao,.B.H.,.and.Li,.X.H.,.Control.of.p-.and.n-type.conductivi-ties.in.Li-doped.ZnO.thin.films,.Appl. Phys. Lett.,.89,.112113,.2006..Copyright.2006,.American.Institute.of.Physics.)


4.4.2 Sodium Doping

Competition.between.the.interstitial.and.substitutional.sites.is.essential.for.p-type.con-ductivity. in. group-I. doped. ZnO.. Theoretical. calculations. [39]. predict. that. Na. is. more.readily.occupying.the.substitutional.sites.than.Li,.although.the.acceptor.level.might.be.deeper.(the.predicted.170.meV.acceptor.level.is.very.close.to.Mg.in.GaN.and.reported.N.in.ZnO)..In.addition,.the.bonding.energy.of.Na-O.(256.kJ/mol).is.much.greater.than.that.of.Zn-O.(159.kJ/mol),.which.indicates.a.high.stability.of.the.NaZn.acceptors..These.advantages.stimulate.the.re-discovery.of.Na.as.a.promising.candidate.for.p-type.ZnO..The. first. attempt. on. doping. p-type. ZnO. films. with. Na. was. conducted. by. Yang. et. al..via. magnetron. sputtering. method. [124].. PLD. [21,125]. and. other. [126]. techniques. were.also.used.to.realize.Na.doping.in.ZnO.or.ZnMgO.films..Hole.concentration.typically.in.the.range.of.1016–1018.cm−3.can.be.achieved,.depending.on.the.Na.content.in.the.target,.the.growth.temperature,.the.post-growth.annealing.process,.etc..Besides.the.Hall-effect.data,.p-type.conductivity.in.ZnO:Na.films.was.further.demonstrated.through.electrical.and. optical. characterization. of. n-ZnO/p-ZnO:Na. homojunctions,. which. shows. a. well-resolved.rectifying.behavior.and.EL.under. forward.bias.voltage. [125]..Through. incor-poration.of.ZnMgO/ZnO.multi-quantum.wells. into. the.ZnO.p-n.homojunction.[22].or.fabrication.of.heterojunction.based.on.p-ZnMgO:Na/n-ZnO.[21],.convincing.RT.EL.spec-tra.and.even.white.light.emitting.under.naked.eyes.were.observed..Figure.4.8.shows.the.I–V. curve.of. the.ZnO.p-n. junctions. involving.eight.periods.of.ZnO/ZnMgO.quantum.wells,.which.is.schematically.shown.in.the.inset..It.also.reveals.RT.EL.spectrum.whose.intensity.is.enhanced.with.increasing.forward.current..While.the.multi.quantum.wells.can.be. incorporated. to.suppress. the.defect.emission,. the.EL.emission. intensity. is. still.too.low,.because.the.crystal.quality.of.p-type.ZnO.layer.is.not.satisfying.at.current.stage..This.evidence,.which.is.a.first.demonstration.of.EL.from.ZnO.homojunction.based.on.group-I.doped.p-type.ZnO,.should.draw.more.attention.of.the.ZnO.community,.although.the.output. light. is.still. too.weak.to.produce.any.commercial.photonic.applications.. In.addition,.further.evidence.of.p-type.conductivity.in.Na-doped.ZnO.comes.from.the.field.effect.transistor.(FET).performance.of.single.ZnO:Na.nanowire..Liu.et.al..[127].reported.Na-doped.p-type.ZnO.microwires.by.conventional.vapor.transport.method..In.a.recent.

Bias voltage (v)(a) (b)–40 –30 –20

–20

0

20Curr

ent (

mA

)

EL in

tens

ity (a

. u.)

40

60

P-ZnO:Na

MQWs

n-ZnO:AISilicon

P-Zn0.85Mg0.15O

n-Zn0.8Mg0.2O

80

ZnO

ZnMgO

100

–10 0 10 20 30 40

Room temperature

Background

160 mA (4 A/cm2)

20 mA (0.5 A/cm2)

1.6 2.0 2.4 2.8Photon energy (eV)

3.2 3.6 4.0

FIGURE 4.8(a). I–V. characteristics.of.ZnO-LED.constructed.by.p-ZnO:Na.and.eight-period.ZnO/ZnMgO.multi-quantum.wells,.showing.rectifying.behavior..(b).EL.spectrum.shows.the.UV.emission.is.enhanced.as.the.applied.current.increases.. (From.Ye,.Z.Z..et.al.,.Chin. J. Semicond.,.29,.1433,.2008..Reproduced.with.permission. from.Chinese.Institute.of.Electronics.)


study,.He.et.al.. [128].synthesized.Na-doped.p-ZnO.nanowires.through.combination.of.thin.film.and.nano.techniques.by.PLD..The.transfer.characteristics.of.single.nanowire.FET. confirms. the. p-type. conductivity. of. the. nanowire. which. is. relatively. stable. even.when.exposed.to.ambient.air.

Low-temperature.PL.and.PL.excitation.spectroscopy.are.both.sensitive.and.useful.tech-niques.for.exploring.the.acceptor.behavior.of.Na.in.ZnO..Although.the.resolution.is.still.limited.by.the.crystal.quality.of.polycrystalline.ZnO:Na.film,.an.acceptor-related.electron-to-neutral.acceptor. transition. is. resolvable. in. the.10.K.PL.spectrum,.where.an.acceptor.level.around.164.meV.can.be.deduced.[125]..This.level.is.also.observed.in.the.PL.excitation.spectrum..These.two.proofs.are.important.as.they.are.in.accordance.with.the.theoretical.expectation.of.NaZn.acceptor.level.in.ZnO.lattices..The.relatively.shallow.acceptor.level.of.NaZn.acceptor.promise.a.relatively.high.hole.concentration.in.p-type.ZnO.

Other.than.designing.or.optimizing.the.light.extractions.from.ZnO.LEDs,.deep.insights.into.the.p-type.mechanism.of.ZnO:Na.films.are.highly.desired..Based.on.a.series.of.experi-ments,.we.suggest. that. the.key.factors. influencing.the.conductivity.type.and.resistivity.of.ZnO:Na.films.can.be. (1). the.competition.between.NaZn.acceptors.and.Nai.donors,. (2).the.enhancement.of.NaZn.solubility.in.ZnO.by.H.co-doping,.and.(3).the.activation.of.NaZn.acceptor. which. depends. on. the. growth. and. post-annealing. temperature.. A. qualitative.model.was.proposed.[129].to.tentatively.interpret.the.experimental.observations,.and.can.be.viewed.as.an.experimental.road.sign.to.achieve.p-type.ZnO:Na.films..However,.more.detailed.studies,.including.electrical,.optical,.and.magnetic.measurements,.should.be.car-ried.out.in.the.future,.as.those.pioneering.and.recent.work.in.Na-doped.ZnO.bulk.crystals.

4.4.3 Potassium Doping

Theoretical.calculations.[39].predicted.that.potassium.(K).can.also.act.as.an.acceptor. in.ZnO,.although.its.energy.level.(0.32.eV).is.deep..Recently,.Tay.et.al..[130].reported.p-type.doping.of.ZnO.by.K.in.aqueous.solution.at.low.temperature..Through.annealing.the.films.below.300°C,.p-type.conductivity.with.hole.concentration.of.3.8.×.1017.cm−3.and.mobility.of.<0.1.cm2/(V.s).can.be.obtained..UV.EL.from.p-ZnO:K/n-GaN.heterojunction.was.observed.at.RT..Zhang.et.al..also.tried.K.as.the.p-type.dopant.for.ZnMgO.films.by.PLD.[131]..The.p-type.conductivity.is.confirmed.by.Hall-effect.measurements.with.a.hole.concentration.up.to.5.5.×.1018.cm−3.and.the.rectifying.behavior.of.a.ZnMgO:K/ZnO:Ga.p-n.heterojunction..Significantly,.p-type.conductivity.is.found.to.be.quite.stable.over.a.period.of.six.months.

4.5 Other Dopant Elements

Compared.with.group-IA.elements,.group-IB.elements.(Ag,.Cu,.and.Au).tend.to.occupy.the.substitutional.sites.than.interstitial.sites.because.the.formation.energy.of.these.group-IB.elements.at.interstitial.sites.is.much.greater.than.on.substitutional.sites.under.oxygen-rich.growth.conditions.[132]..Among.these.group-IB.elements,.Ag.is.considered.as.a.potential.p-type.dopant.for.ZnO.[133–136]..Using.an.electrochemical.process,.p-type.Ag-doped.ZnO.thin.films.were.obtained.with.a.binding.energy.of.117.meV.deduced.from.temperature-dependent.PL.spectra.[133]..Other.reported.carrier.concentration.values.varied.from.1016.to.1018.cm−3.[133–136]..However,.the.acceptor.levels.of.these.group-IB.elements.on.Zn.sites,.especially.for.Cu.and.Au,.are.rather.deep.[132]..This.drawback.impedes.their.applications.


in.light-emitting.devices,.though.a.p-n.hetero-junction.greenish.LED.had.been.fabricated.based.on.p-type.ZnO.doped.with.Cu.and.n-type.6H-SiC.[137].

Other.pathways.were.also.proposed.to.realize.p-type.doping.of.ZnO..By.tuning.intrin-sic. defects. in. nominally. undoped. ZnO. films,. p-type. conductivity. has. been. observed.by. several. groups. [138–140].. Based. on. first-principles. calculations,. Janotti. et. al.. [141].proposed.that.F.placed.at.interstitial.sites.in.ZnO.and.ZnMgO.acts.as.a.shallow.accep-tor,. which. possibly. results. in. p-type. conductivity.. However,. experimental. results. on.F-doping.in.ZnO.have.not.yet.been.reported.so.far..On.the.other.hand,.it.was.recently.suggested.that.p-type.doping.of.ZnO.can.be.improved.by.anion.substitution,.replacing.O.with.other.group-VI.elements,.such.as.S,.Se,.and.Te.[142]..This.method.is.based.on.the.fact.that.the.valence.band.maximum.of.zinc.chalcogenides.is.much.higher.than.that.of.ZnO,.which. lowers. the.acceptor.activation.energy..For. example,. the.acceptor. level.of.N.is.expected.to.be.only.5.meV.in.ZnO0.81S0.19.compared.to.~220.meV.in.ZnO.[143]..Very.recently,.experimental.efforts.on.this.strategy.have.been.carried.out.by.Pan.et.al..[144]..They.claimed.that.the.p-type.conductivity.of.Cu-doped.ZnO.films.is.improved.by.alloying.with.S.

4.6 Problems and Outlook

It. should. be. noted. that. although. p-type. conductivity. has. been. observed. in. ZnO. films.doped.with.various.acceptors,.it.is.difficult.to.figure.out.which.element.is.the.most.efficient.and.prospective.dopant.at.current.stage..For.example,.N.has.been.regarded.as.one.of.the.most.promising.acceptors.for.a.quite.long.period..However,.it.suffers.from.the.problems.of. solubility.and.stability..Recently,. theoretical.work.by.van.der.Waal’s.group. [46].even.strongly.questioned.the.availability.of.N.as.appropriate.acceptor.for.ZnO.due.to.the.very.deep.acceptor.level..Cui.and.Bruneval.[145].proposed.that.the.poor.stability.of.N-doped.p-type.ZnO.is.due.to.the.prevailing.compensation.centers,.N2.molecule.substituting.oxy-gen,.over.the.acceptors..But.subsequently,.Kawasaki’s.group.[19].reported.the.ZnO-LED.based.on.N-doping.with.output.power.up.to.70.μW,.which.is.very.exciting.(Figure.4.9)..They.also.demonstrated.the.availability.of.such.UV.LED.to.excite.green.phosphor,.convert-ing.a.part.of.UV.emission.into.green..The.UV.emission.of.ZnO.LED.will.make.it.possible.to.excite.many.existing.phosphors,.enabling.construction.of.white.LED.and.better.color.rendering..Another.example.is.the.large-size.mismatched.group-V.doping..Although.posi-tive.results.on.p-type.conductivity.and.light-emitting.devices.have.been.reported,.one.may.argue.[146].that.complex.acceptor.(P,.As,.Sb)Zn-2VZn.is.disadvantageous.to.the.crystal.qual-ity.and.p-type.stability,.because.it.contains.two.additional.defects.and.may.induce.large.local.strain.in.the.films.

Although.steady.progress.has.been.achieved. in.producing.p-type.conductivity.and.even.LED.devices,.the.crystal.quality.of.the.p-type.ZnO.films.needs.further.improve-ment.and.the.long.term.p-type.stability.is.still.a.challenge..There.is.a.trade-off.between.the.doping.and.the.film.quality..Due.to.the.severe.self-compensation.and.low.solubility.of. dopants. for. ZnO,. generally. one. needs. to. dope. the. films. under. severe. nonequilib-rium.conditions..This. inevitably.results. in.high.density.of.defects. in. the.films..So. far.most. of. the. reported. p-type. ZnO. films. have. been. grown. by. sputtering. or. laser. abla-tion.approaches,.which.may.lead.to.higher.hole.concentration.but.lower.crystal.quality.than.those.grown.by.molecular.beam.epitaxy.(MBE).or.MOCVD..Such.films.suffer.from.


the.low.hole.mobility..Moreover,.the.abundance.of.defects,.grain.boundaries,.and.local.strain.in.the.film.make.the.experimental.studies.on.the.electrical.properties.and.mecha-nism.of.p-type.doping.much.more.complex..For.example,.Krtschil.et.al..[64].used.scan-ning.capacitance.microscopy.to.study.the.spatial.distribution.of.the.conduction.type.in.As-doped. ZnO. films.. They. observed. local. p-type. conductivity. and. direct. correlation.between.the.n-type.areas.and.topographical. features. in. the.films.(Figure.4.10),.which.probably. indicate. selective. dopant. incorporation. effect. sensitive. to. the. growth. mode.and. structural. defects.. Therefore,. MOCVD. or. MBE. should. be. used. to. grow. “useful”.p-type. ZnO. films.. Accordingly,. doping. techniques. under. conditions. close. to. equilib-rium.should.be.developed.to.improve.the.electrical.properties.while.maintaining.rea-sonable.film.quality..We.suggest.that.doping.ZnO.with.group-I.acceptors.via.MOCVD.using.group-I.metalorganic.precursors.is.a.potential.solution.

It.should.be.emphasized.that.stable.p-type.ZnO.with.reasonable.hole.concentration.does.exist,. as. strongly.evidenced.by. the.observation.of.a.mixed-conduction. transition.show-ing. p-type. and. n-type. behavior. upon. blue/UV. illumination. and. subsequent. annealing.in. a. single. P-doped. sample. [147].. The. typical. reported. duration. of. p-type. conductivity.is.several.months,.after.which.a.complete.transformation.into.n-type.was.observed..The.mechanism. of. this. common. phenomenon. is. unclear. so. far.. The. development. of. donor.centers.seems.to.be.a.plausible.explanation..Barnes.et.al..[148].suggested.that.hydrogen-generated.donors.are.responsible.for.this.behavior,.while.Wang.et.al..[81].attributed.it.to.a.metastable.N-on-O.substitution,.which.serves.as.donors..On.the.other.hand,.persistent.photoconductivity.(PPC).or.noisy.Hall-effect.measurements.may.also.result.in.the.mixed-conduction.transition.[149]..According.to.our.results,.ZnO.doped.with.Li-N.dual.accep-tor.and.ZnMgO.doped.with.Na.have.better.p-type.stability..In.our.Li-N.co-doped.ZnO,.p-type.conductivity.can.be.traced.all.along.15.months,.with.small.fluctuations. in.terms.of. the.hole.concentration..Similar.results.were.obtained. in.our.ZnMgO:Na.films..These.

Wavelength (nm)350

0.00

(a) (b)20 40 60

Current density (Acm–2)80

0.5EL in

tens

ity (a

. u.)

1.0

Bare LED D1.5

400 450 500 550

With greenphosphor

LED C

InGaN LEDfor normalization

LED D

Inte

grat

ed o

utpu

t pow

er (μ

W)

LED ALED B

NO*

NH3

600 65010–2

10–1

100

101

102

103

104

FIGURE 4.9(a).Integrated.EL.intensity.for.four.p-ZnMgO:N/n-ZnO.heterojunction.LEDs..(b).EL.spectra.for.bare.and.with.a.green.phosphor.coating.for.LED.D.at.an.operation.current.of.40.mA..The.inset.is.a.picture.taken.under.standard.laboratory.illumination..Emission.from.the.phosphor.can.be.clearly.seen.as.indicated.by.an.arrow..(Reprinted.with.permission.from.Nakahara,.K.,.Akasaka,.S.,.Yuji,.H.,.Tamura,.K.,.Fujii,.T.,.Nishimoto,.Y.,.Takamizu,.D.,.Sasaki,. A.,. Tanabe,. T.,. Takasu,. H.,. Amaike,. H.,. Onuma,. T.,. Chichibu,. S.F.,. Tsukazaki,. A.,. Ohtomo,. A.,. and.Kawasaki,.M.,.Nitrogen.doped.MgxZn1−xO/ZnO.single.heterostructure.ultraviolet.light-emitting.diodes.on.ZnO.substrates,.Appl. Phys. Lett.,.97,.013501,.2010..Copyright.2010,.American.Institute.of.Physics.)


exciting. results. may. point. to. the. possible. solutions. for. the. stability. problem.. Based. on.theoretical.calculations,.Shen’s.group.recently.proposed.[97].that.a.LiI-NO.impurity.band.is.formed.above.the.valence.band.maximum,.which.reduces.the.compensation.of.LiZn.accep-tors.by.highly.mobile.LiI.donors..This.model.provides.a.possible.explanation.for.the.rela-tively.good.stability.of.Li-N.co-doped.p-type.ZnO..The.mechanism.for.the.stable.p-type.ZnMgO:Na.is.under.study.

On.the.other.hand,.there.are.also.attempts.to.improve.the.p-type.conductivity.by.engi-neering. the.acceptor. level..Some.theoretical.work.sheds. light.on.strategies. to. lower. the.acceptor.ionization.energy,.for.example,.by.replacing.Zn.by.isovalent.Mg.or.Be.[150],.or.by.producing.an.impurity.band.between.the.VBM.and.acceptor.level.via.donor–acceptor.co-doping. [79].. These. designs. seem. applicable. provided. the. concomitant. problems. can.be.carefully.addressed,.namely,.the.increase.of.band.gap.hence.the.ionization.energy.of.acceptors.when. replacing.Zn.with.Mg.or.Be,. and. the.degradation.of. crystal.quality.by.the.plenty.of.impurities.and.defects.induced.by.co-doping.method..It.is.noteworthy.that.acceptor.level.of.~100–200.meV,.as.claimed.by.most.of.the.experimental.works.on.group-V.and.group-I.dopants,.is.not.so.deep.for.achieving.reasonable.hole.concentration,.consider-ing.that.the.acceptor.level.of.Mg.in.GaN.is.170.meV..In.many.reports,.the.acceptor.level.was.derived.from.the.acceptor-related.PL.lines,.especially.from.the.neutral.acceptor.bound.exciton.(A0X).by.applying.Haynes’.Rule..However,.some.researchers.argued.that.so.far.no.decisive.experiments.on.A0X.have.been.presented.[151],.and.there.is.no.strong.evidence.that.Haynes’.Rule.applies.for.acceptors.in.ZnO.[27]..Considering.the.diversity.of.acceptor.

ZnO:N

ZnO:As

n-type

n-type

p-type

p-type

Depleted

0(a)

(b)

∆z=200 nm δC/δV=20 V0

0

15 µm 15 µm

0∆z=350 nm δC/δV=20 V

15 µm 15 µm

FIGURE 4.10Typical.(left).AFM.and.(right).SCM.images.of.ZnO.layers.which.were.conventionally.mono-doped.with.(a).nitro-gen.or.(b).arsenic..The.morphology.is.not.smooth.and.reveals.three-dimensional.islands..The.corresponding.SCM.images.proof.dominant.n-type.regions.despite.the.acceptor.doping..(Reproduced.with.permission.from.Krtschil,.A.,.Dadgar,.A.,.Oleynik,.N.,.Blasing,.J.,.Diez,.A.,.and.Krost,.A.,.Local.p-type.conductivity.in.zinc.oxide.dual-doped.with.nitrogen.and.arsenic,.Appl. Phys. Lett.,.87,.262105,.2005..Copyright.2005,.American.Institute.of.Physics.)


levels.for.certain.dopant.reported.in.the.literature,.one.may.conclude.that.definitive.deter-mination.of.acceptor.levels.in.ZnO.is.required.in.the.future.study.

Nevertheless,.the.continuous.reports.on.electrically.driven.luminescence.and.even.las-ing. in.p-type.ZnO-based.homo-structures. in.recent.years.are.very. inspiring..Moreover,.there.have.been.some.preliminary.results,.both.experimentally.and.theoretically,.on.the.improved.stability.of.p-type.ZnO..It.is.reasonable.to.believe.that.the.steady.progress.on.these.issues.achieved.by.worldwide.scientists.will.lead.ZnO.to.a.prospective.future.as.a.promising.light-emitting.material.

Acknowledgments

This. work. was. supported. by. the. Natural. Science. Foundation. of. China. (Nos.. 50532060,.90201038.and.60340460439),.the.“973”.Program.(Nos..2006CB604906.and.G2000068306),.the.Cultivation.Fund.of.the.Key.Scientific.and.Technical.Innovation.Project.(No..707035).and.the.Doctoral.Foundation.(Nos..20060335087.and.20070335010).of.Ministry.of.Education.of.China.

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.138.. Y..J..Zeng,.Z..Z..Ye,.W..Z..Xu,.J..G..Lu,.H..P..He,.L..P..Zhu,.B..H..Zhao,.Y..Che,.and.S..B..Zhang,.p-type.behavior.in.nominally.undoped.ZnO.thin.films.by.oxygen.plasma.growth,.Appl. Phys. Lett..88,.262103.(2006).

.139.. G..Z..Xing,.B..Yao,.C..X..Cong,.T..Yang,.Y..P..Xie,.B..H..Li,.and.D..Z..Shen,.Effect.of.annealing.on.conductivity.behavior.of.undoped.zinc.oxide.prepared.by.rf.magnetron.sputtering,.J. Alloys Compd..457,.36.(2008).


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.142.. O..Maksimov,.Recent.advances.and.novel.approaches.of.p-type.doping.of.zinc.oxide,.Rev. Adv. Mater. Sci..24,.26.(2010).

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Part II

ZnO Epitaxy

107

5ZnO Nanostructures and Thin Films Grown in Aqueous Solution: Growth, Defects, and Doping

S.J. Chua, C.B. Tay, and J. Tang

5.1 Introduction

Zinc.oxide. (ZnO). is. a. IIb-VI. compound.semiconductor.with.a. large.direct.band.gap.of.around. 3.37. eV. at. room. temperature. and. crystallizes. preferentially. in. the. hexagonal.wurtzite.structure..It.is.a.multifunctional.material.with.promising.optical,.piezoelectric,.and.ferromagnetic.properties.which.can.be.exploited. in.a.diverse.range.of.applications.such.as.solid-state.lighting.[1–3],.solar.cells.[4],.transparent.conductor.[5],.active.material.for.varistors.[6],.transparent.transistors.[7],.piezoelectric.power.generation.[8],.dilute.mag-netic.semiconductor.[9,10],.spintronics,.and.magneto-optical.switches.[11]..It.can.be.grown.with.a.wide.variety.of.techniques.as.described.in.the.other.parts.of.this.book..One.inter-esting.property.of.this.wide.gap.semiconductor.is.its.propensity.to.form.nanostructures.such.as.rods,.wires,.belts,.tubes,.and.brushes,.which.can.be.exploited.for.new.applications.

CONTENTS

5.1. Introduction......................................................................................................................... 1075.2. Motivations.for.ZnO.Growth.in.Aqueous.Solution....................................................... 1085.3. ZnO.Nanostructures.......................................................................................................... 1085.4. Patterned.Substrate.for.Growth.of.ZnO.Structures....................................................... 110

5.4.1. Electrodeposition.of.Porous.Film.Using.Polystyrene.Template....................... 1105.4.2. Vertically.Aligned.Nanorods................................................................................ 1105.4.3. Horizontally.Aligned.Nanorods.......................................................................... 111

5.5. ZnO.Thin.Films................................................................................................................... 1115.6. Understanding.the.Growth.Mechanism.of.ZnO.in.Aqueous.Solution...................... 111

5.6.1. Thermodynamics.of.Growth................................................................................. 1125.6.2. Interfacial.Energy................................................................................................... 1135.6.3. Solubility.of.Zinc..................................................................................................... 1155.6.4. Applications.of.Solubility.as.Growth.Control.Variable..................................... 119

5.7. Defects.in.Aqueous.Solution.Grown.ZnO...................................................................... 1235.8. Doping.in.Aqueous.Solution............................................................................................. 1285.9. n-Type.Doping..................................................................................................................... 1295.10. p-Type.Doping..................................................................................................................... 1295.11. Optoelectronic.Applications.of.Aqueous.Solution.Grown.ZnO.................................. 1315.12. Conclusions.......................................................................................................................... 134References...................................................................................................................................... 135


5.2 Motivations for ZnO Growth in Aqueous Solution

Growth.of.ZnO.in.aqueous.solution.is.not.new..It.has.been.practiced.since.the.early.1960s.when.ZnO.single.crystals.were.hydrothermally.grown.at.high.temperatures..Interest.in.growth.of.nanostructures.and.epitaxial.thin.films.began.in.the.1990s.based.on.the.possible.emerging.applications.such.as.in.optoelectronics,.radiation.hard.electronic.devices,.visible.blind.devices,.semiconductor.spintronics,.and.transparent.conducting.oxide.

Aqueous.solution.growth.is.attractive.because. it. is. inexpensive.and.has. low.environ-mental. impact..Although.the.quality.of. the.ZnO.is.generally.better.when.grown.in.gas.phase.at.high.temperatures.above.300°C,.growth.in.aqueous.solution.has.clear.advantages.over.gas.phase.in.terms.of.cost,.energy.usage,.and.simple.processing.steps,.particularly.for.low.temperature.processes.[12].

Gas-phase. methods. consume. a. large. amount. of. energy. to. convert. the. solid. state. Zn.source. to. free.Zn2+. ions. in.vapor.state.as.growth.precursors..Upon.condensation.of. the.solid. ZnO,. this. excess. energy. is. simply. discarded. into. the. environment.. Furthermore,.recycling.waste.of.material.is.uneconomical.because.the.exhaust.gases.are.emitted.in.large.diluted.volumes,.especially.when.high.vacuum.systems.are.used..In.contrast,.very.little.extra.energy.is.needed.in.aqueous.chemical.growth.methods.to.break.the.lattice.bonds.of.the.solid.Zn.source.to.form.free.Zn2+.ions..This.is.because.the.energy.needed.to.dissolve.the.zinc.salt.and.break.the.lattice.bonds.are.provided.by.the.hydration.energy.in.water.at.room.temperature..Upon.dissolution,.growth.proceeds.by.hydrolysis.and.condensation..The.aqueous.solution.growth.system.is.typically.a.closed.system.which.allows.easy.sepa-ration.and.recycling.of.materials,.thus.minimizing.the.wastage.of.energy.and.material.

In.solution.phase,.growth.precursors.have.higher.concentration.and.better.homogeneity.than.in.the.gas.phase,.especially.when.high.vacuum.growth.conditions.are.used..At.the.right.conditions,.these.lead.to.high.homogeneity.and.faster.growth.rates.than.that.of.the.gas.phase..However,.it.is.noted.that.due.to.much.lower.growth.temperatures,.typically.less.than.100°C.in.solution.methods,.growth.units.may.not.have.enough.kinetic.energy.to.dif-fuse.across.the.surface.to.obtain.a.smooth.film.layer.growth.and.to.occupy.the.appropri-ate.lattice.sites,.leading.to.vacancies.and.interstitials.and.incorporation.of.hydroxyl.ions..However,.these.defects.can.be.removed.by.post-growth.processing.

Finally,. growing. in. solution. is. a. low. cost,. safe,. and. simple. process. not. requiring.expensive.vacuum.equipment..Basic.equipment.consists.of.a.growth.vessel,.water.bath,.or.convection.oven..In.comparison,.gas-phase.methods.will.need.a.special.setup.in.order.to.operate.at.high.temperatures.and.vacuum.conditions..In.the.case.of.CVD.or.MOCVD,.the.growth.precursors.are.hazardous.and.additional.safety.systems.are.needed.

Reliable.p.and.n.doping.have.been.obtained.with.solution-grown.ZnO,.which.is.essential.for.any.LED.device,.will.be.described.in.this.chapter..Given.the.same.band.gap.energy.as.GaN,.ZnO.will.be.a.disruptive.technology.if.the.emission.efficiency.can.be.improved.given.the.much.lower.cost.of.manufacturing.

5.3 ZnO Nanostructures

ZnO.is.particularly.suited.for.growth.in.solution.as.evident.from.the.literature.document-ing.the.variety.of.ZnO.nanostructures.that.have.been.fabricated.in.chemical.solutions.

109ZnO Nanostructures and Thin Films Grown in Aqueous Solution

at. low. growth. temperatures. of. 60°C–90°C.. A. rich. variety. of. nanostructures. [13]. such.as.nanorods.[14–16],.nanowires.[17],.nanospears.[18],.nanocolumns.[19],.nanobelts.[20],.and.nanotubes.[21].has.been.reported..Among.these.diverse.selection.nanostructures,.nanorods.have.emerged.to.be.the.most.extensively.studied.because.of.their.potential.as.basic.building.blocks.for.other.structures.in.order.to.exploit.the.multifunctional.proper-ties.of.ZnO.

Growth.precursors.in.aqueous.solution.generally.consist.of.a.zinc.salt,.such.as.zinc.ace-tate.(Zn(CH3COO)2,.hereon.denoted.as.ZnAc2.for.brevity),.zinc.nitrate.(Zn(NO3)2),.or.zinc.chloride.(ZnCl2),.to.provide.the.zinc.ions,.and.a.base.such.as.sodium.hydroxide.(NaOH).and.aqueous.ammonia.(NH4OH).to.control.the.solubility.of.zinc..The.most.common.route.for.ZnO.nanorod.growth.consists.of.Zn(NO3)2.and.hexamethylenetetramine.(HMT).which.decomposes. into. formaldehyde. and. ammonia. upon. heating. and. provides. a. slow. con-trolled.supply.of.OH−,.fulfilling.the.role.of.the.base..The.rate.of.HMT.decomposition.is.dependent.on.pH:.the.reaction.half-life.increases.with.pH.from.1.6.h.at.pH.2.0.to.13.8.h.at.pH.5.8.[22]..By.further.adding.surfactants.such.as.polyethylenimine.(PEI).[4].and.trisodium.citrate.[23].into.the.growth.solution,.the.growth.habit.can.be.modified.to.give.rise.to.differ-ent.and.interesting.morphologies.

Homogeneous.growth.of.nanorods.in.aqueous.solution.was.reported.by.Andres-Verges.[24]..Using.the.same.approach,.Vayssieres.chemically.and.electrostatically.manipulated.the.interfacial.free.energy.to.heterogeneously.grow.arrays.of.ZnO.nanorods.on.various.sub-strates.without.any.templates.or.surfactants.[25]..The.diameter.of.the.rods.can.be.reduced.while.the.length.can.be.increased.by.reducing.the.equimolar.concentrations.of.Zn(NO3)2.and.HMT.in.the.growth.solution:.microrods.with.diameters.of.1–2.μm.at.0.1.M,.nanorods.with.diameters.of.100–200.nm.at.0.01.M,.and.nanowires.with.diameters.of.10–20.nm.at.0.001.M..The.direct.relationship.between.the.concentration.of.precursors.and.the.diam-eters.were.attributed.to.diffusion-limited.growths.

O’Brien. and. coworkers. [22]. performed. a. detailed. study. on. the. effect. of. complexing.ligand,.zinc. counter-ion,. ionic. strength,. supersaturation,. deposition. time,. and. substrate.on.the.growth.of.ZnO.nanorods..By.employing.a.wide.range.of.precursors,.the.authors.showed.that.the.choice.of.zinc.salts.and.ligands.were.important.in.determining.the.mor-phology.of. the.growth..The.best.approach.for.growing.oriented.rods. is.by. limiting. the.concentration.of.one.reactant.(either.Zn2+.or.OH−).in.the.presence.of.an.excess.of.the.second.component.in.order.to.establish.a.diffusion-limited.growth.regime.

Despite.the.large.volume.of.literature.on.low.temperature.aqueous.solution.growth,.a.unified.understanding.of.the.growth.mechanisms.has.not.been.achieved..Studies.of.growth.factors.are.often.based.on.empirical.observations.such.as.effect.of.reactant.concentrations,.pH,.ionic.strength,.counter-ions,.temperature,.and.growth.duration..Part.of.this.difficulty.is.caused.by.the.wide.variety.of.precursors.and.surfactants..Another.reason.is.that.most.of.the.identified.factors.are.secondary.variables.that.affect.the.degree.of.supersaturation.of.zinc.or.the.solubility.of.zinc.which.determines.the.driving.force.toward.the.growth.of.ZnO..When.the.experimental.results.are.complemented.with.thermodynamic.modeling,.as.shown.by.Vayssieres.who.had.successfully.applied.the.concept.of.thermodynamic.equi-librium.in.the.size.prediction.of.nanoparticles.and.addressed.issues.of.interfacial.energy.and.surface.charge.relative.to.the.pH.of.the.growth.solution.[26],.much.better.understand-ing. and. insight. can. be. derived.. Our. group. extended. this. thermodynamic. modeling. to.heterogeneous.growth.on.GaN.[16].and.Si.substrates.[27].seeded.with.ZnO.nanoparticles.for.the.growth.system.consisting.of.ZnAc2.and.NH4OH.[28]..In.particular,.in.the.pH.region.ranging.from.10.to.11.where.heterogeneous.growth.of.ZnO.typically.occurs,.it.was.shown.that.the.growth.morphology.is.mainly.governed.by.the.solubility.and.temperature.term..


Recently,.similar.thermodynamic.modeling.approach.has.also.been.applied.successfully.to.verify.their.experimental.results.using.a.continuous.circulation.reactor.by.Lange.et.al..[29,30]..More.details.on.the.thermodynamic.modeling.and.its.applications.will.be.provided.in.the.sections.that.follow.

5.4 Patterned Substrate for Growth of ZnO Structures

The.growth.of.ZnO.nanorods.on.ZnO.seed.layers.deposited.non-epitaxially.on.foreign.sub-strates,.are.usually.randomly-oriented..Well-aligned.and.ordered.growth.is.highly.desired.in. many. applications. such. as. photonic. crystals. and. piezoelectric. generators,. as. well. as.for.ease.and.consistency.of.device.fabrication..For.industrial.applications,.it.is.important.for. these.processes.to.be. large.scale;.have.high.throughput,. low.cost,.and.low.tempera-ture.for.application.to.a.wide.range.of.substrates;.and.be.catalyst-free.for.integration.with.existing.silicon-based. technologies..Several.groups.have.demonstrated. the. feasibility.of.various. approaches. toward. achieving. porous. films,. vertically. and. horizontally. aligned.ordered.patterns.using.either.top-down.lithography.methods.or.bottom-up.self-assembly.methods.

5.4.1 Electrodeposition of Porous Film Using Polystyrene Template

Liu.et.al..demonstrated.the.electrodeposition.of.robust.and.porous.ZnO.films.in.aqueous.solution.of.zinc.nitrate.on.ITO.substrates.that.had.been.templated.with.an.array.of.poly-styrene.particles.[31].

5.4.2 Vertically Aligned Nanorods

Wang.Zhong.Lin’s.group.at.Georgia.Tech.has.demonstrated.wafer-scale,.vertically.aligned.patterned.growth.of.ZnO.nanorods.on.Si.substrates.with.the.pattern.formed.by.using.laser.interference.lithography.[32].and.electron.beam.lithography.[33].with.the.smallest.pattern.size.of.60.nm.to.the.largest.a.few.microns..Growth.of.the.nanorods.is.performed.in.an.equimolar.solution.of.5.mM.Zn(NO3)2.and.HMT.for.24.h.

Multiple.nanorods.are.obtained.from.a.single.patterned.hole.when.the.hole.diameter.exposes.the.grain.boundary.or.dislocation.sites.of.the.underlying.seed.layer..Depending.on.the.type.of.seed.layer,.which.determines.the.grain.size,.the.limit.of.the.pattern.diameter.to.obtain.single.rod.growth.can.range.from.100.nm.for.magnetron.plasma.sputtered.seed.layers.to.1.μm.for.GaN.epilayer.substrates.

Selective.patterning.was.achieved.by.depositing.ZnO.seed.layer.onto.the.nanoimprinted.hydrophilic. patterns. consisting. of. self-assembled. monolayers. of. n-octadecyltrichlorosi-lane.on.SiO2,.followed.by.an.oxygen.plasma.treatment.to.prevent.subsequent.growth.on.the.other.sections.of.SiO2.[34].

Li.et.al..reported.on.the.electrodeposition.of.Zn.into.the.pores.of.an.AAO.template,.followed.by.thermal.treatment.in.air.at.300°C.for.35.h.to.fully.oxidize.the.Zn.nanorods.into.ZnO.[35]..An.alternative.approach.was.demonstrated.by.Wang.et.al..who.devel-oped. SiO2. patterns. on. GaN. substrates. through. inductively. coupled. plasma. etching.with.AAO.template.as.a.mask,.followed.by.the.growth.of.well-aligned.ZnO.nanowires.


arrays.with.an.average.diameter.of.65.nm.and.periodicity.of.110.nm.from.the.exposed.GaN.surface.[36].

5.4.3 Horizontally Aligned Nanorods

Horizontally.aligned.patterns.have.also.been.reported.using.aqueous.solution..By.cover-ing.the.top.surface.of.ZnO.seed.patterns.with.metal.layers.such.as.Cr.or.Sn,.horizontal.growth.of.ZnO.nanorods.was.initiated.from.the.open.sidewalls.[37]..In.another.approach.which.takes.advantage.of.the.propensity.for.growth.in.direction.of.the.c-axis,.dense.and.well-aligned.horizontal.nanorods.were.grown.on.the.patterned.a-plane.surface.of.a.single.crystal.ZnO.substrate.[38].

5.5 ZnO Thin Films

In.comparison.to.the.large.number.of.reports.on.nanostructures,.there.are.very.few.reports.on.the.epitaxial.growth.of.ZnO.using.solution.methods..These.reports.can.be.traced.to.Lange’s.group.at.UCSB,.United.States,. and. their. coworkers.who. reported.on. the.direct.growth. [39].as.well.as. lateral.epitaxial.growth. [40].of.ZnO.films.on.MgAl2O4. (111). sub-strates..Kim.et.al..extended.the.lateral.epitaxial.growth.method.to.grow.ZnO.thin.film.on.GaN.buffered.Al2O3.(0001).substrates.[19]..Although.Lange’s.group.and.Kim.et.al..reported.discontinuous.film.at.pH.7.5,.Sim.et.al..managed.to.obtain.a.continuous.epitaxial.film.in.a.single.step.by.growing.at.150°C.[41].

In.general,.the.current.state-of-the-art.for.growing.a.smooth.continuous.film.requires.a.seed.layer.to.be.grown.at.pH.7.5.and.a.subsequent.film.growth.at.pH.10.9.with.the.help.of.trisodium.citrate.(Na3C6H5O7).as.a.surfactant.[19,40]..Without.NaC3,.a.rough.surface.aris-ing.from.island.mode.growth.is.obtained.from.solution.at.both.pH.7.5.[41].and.10.9.[39]..Although. this.approach.succeeds. in.producing.a. smooth.and.continuous.film,. it. incor-porates.a.higher.concentration.of.native.defects.in.the.film.at.pH.10.9,.and.includes.large.citrate.ions.into.the.structure.during.growth.

An.alternative.route.was.proposed.in.which.the.initial.seed.layer.of.ZnO.is.grown.at.pH.10–11.so.that.a.high.density.of.nucleation.with.a.very.good.coverage.of.the.substrate.can.be.obtained.in.a.short.time..Although.these.nuclei.contain.a.high.density.of.defects.as.will.be.explained.later.in.the.mechanism.section,.by.minimizing.the.growth.time,.the.thickness.of.this.high.defect.density.layer.can.be.minimized.relative.to.the.subsequent.low.defect.density.layers..The.subsequent.film.growth.is.then.performed.at. low.pH..Although.the.growth.rate.is.slow,.it.produces.a.low.defect.density.and.a.flat-top.morphology.which.is.suitable.for.coalescence.into.a.smooth.film.as.shown.by.Sim.et.al..[41].

5.6 Understanding the Growth Mechanism of ZnO in Aqueous Solution

As. mentioned. in. the. earlier. section,. thermodynamic. modeling,. when. reconciled. with.experimental.observations,. is.a.powerful. tool. for.understanding.the.growth.of.ZnO..In.this.section,.a.general.model.is.presented.to.understand.the.growth.rate.morphology.from.a.basic.thermodynamics.point.of.view.


5.6.1 Thermodynamics of Growth

O’Brien. and. coworkers. [22]. had. shown. using. HR-TEM. images. of. ZnO. rods. that. the.initial.stages.of.growth.are.based.on.2D-nucleation.mechanism.in.which.polar.nanopar-ticles. undergo. oriented. aggregation. followed. by. dissolution-recrystallization. to. form.single.crystalline.material..In.the.later.growth.stage.at.low.supersaturation.levels,.the.2D-nucleation.mechanism.slows.down.while.the.dissolution-recrystallization.processes.become.more.dominant..This.is.in.agreement.with.the.work.of.Sun.and.coworkers.[42].who.transformed.ZnO.microrods.which.were.grown.using.ZnCl2.and.NH4OH.at.95°C.into.microtubes.through.an.extended.aging.process.at.lower.temperature.of.90°C.where.the. dissolution. process. of. the. unstable. Zn-rich. (0001). polar. surface. is. faster. than. the.non-polar.{−1010}.side.walls.

Therefore,.in.order.to.model.the.growth.thermodynamically,.we.begin.with.the.homo-geneous.nucleation.of.spherical.particles.in.aqueous.solution..It.is.well.known.that.the.first.stage.in.homogeneous.nucleation.is.the.formation.of.embryos.through.collision.between.individual.ions..These.embryos.are.unstable.thermodynamically.because.their.large.sur-face.area.to.volume.ratio.leads.to.a.high.surface.energy.resulting.in.many.of.these.embryos.dissolving.before.they.can.grow.into.stable.nuclei..The.thermodynamically.stable.size.of.an.embryo.is.determined.by.the.energy.balance.between.the.surface.energy.required.to.form.the.embryo.and.the.energy.released.due.to.a.phase.transformation.from.liquid.phase.to.solid.phase.when.a.spherical.particle.is.formed..Figure.5.1.shows.this.energy.balance.which.is.known.as.the.Gibbs.free.energy.of.nucleation.(ΔG).as.a.function.of.the.embryo.radius,.r..The.newly.formed.embryo.is.stable.if.its.radius,.r,.is.greater.than.r*.and.it.will.continue.to.grow.bigger.to.reduce.its.Gibbs.free.energy..On.the.other.hand,.when.r.<.r*,.the.embryo.will.preferentially.dissolve.into.the.solution.to.reduce.its.Gibbs.free.energy.

Surfaceenergyterm Critical point

for nucleation

r*

Total Gibbs freeenergy term

Embryo radius, r

Phasetransformation

term

Gib

bs fr

ee en

ergy

∆G

∆G*

FIGURE 5.1The.Gibbs.free.energy.of.nucleation.with.respect.to.embryo.radius..The.critical.radius.r*.and.energy.ΔG*.depend.on.the.balance.between.the.surface.and.volume.energy.of.the.growing.embryo.


Mathematically,.the.Gibbs.free.energy.can.be.expressed.[43].as

.∆ ∆G r r GV= +4

43

2 3π γ π . (5.1)

wherer.is.the.radius.of.the.nucleusγ.is.the.interfacial.energyΔGV.is.the.change.of.Gibbs.free.energy.per.unit.volume.of.the.solid.phase,.given.by.the.

equation

.∆G kT

VCS

V = −

lnZn

. (5.2)

whereV.is.the.atomic.volumek.is.the.Boltzmann.constantT.[K].is.the.growth.temperatureC.[mol/L].is.the.concentration.of.zinc.acetate.added.into.the.solutionSZn. [mol/L]. is. the. solubility. of. zinc,. defined. as. the. total. Zn. ion. concentration. in. the.

solution

By.setting.dΔG/dr.=.0,.the.critical.energy.for.nucleation.to.occur,.ΔG*.given.by

.

∆G Vk T C S

∗ = ⋅( )

163

2

2 2

3

2π γ

ln / Zn

. (5.3)

Although.Equation.5.3.was.written. for.homogeneous.nucleation,. it. can.also.be.applied.for.heterogeneous.nucleation..Heterogeneous.nucleation.differs.from.homogeneous.nucle-ation.in.the.sense.that.the.interface.energy.between.the.embryo.and.the.substrate.surface.is.usually.less.than.that.between.the.embryo.and.the.solution..This.is.because.the.contact.area.between. the.embryo.and. the.solution. is. reduced.upon.adsorption.of.embryo.on.a.substrate..Equation.5.3.shows.that.four.variables:.interfacial.tension,.concentration.of.zinc.solute,.solubility.of.zinc,.and.growth.temperature,.which.determine.the.free.energy.for.heterogeneous.nucleation.of.the.embryo,.can.be.manipulated.to.promote.nucleation.in.the.solution..While.the.concentration.of.zinc.solute,.C,.and.the.growth.temperature,.T,.are.con-trollable.experimental.variables,.the.values.of.interfacial.energy.γ.and.the.solubility.of.zinc.SZn.are.difficult.to.measure.and.control.directly..Both.of.these.variables.will.be.described.further.in.the.following.sections.

5.6.2 Interfacial Energy

The.interfacial.energy.is.a.function.of.the.choice.of.substrate,.surface.roughness,.pH.of.the.solution,.and.growth.temperature.

The. use. of. substrates. that. have. close. lattice-matching. with. ZnO. reduces. the. interfa-cial.energy.between.the.embryo.and.the.substrate.surface.and.lowers.the.free.energy.for.


nucleation..For.example,.the.lattice.constants,.a,.of.GaN.and.ZnO.are.3.1890.and.3.2495.Å,.respectively..The.corresponding.lattice.mismatch.works.out.to.be

.∆a

a aa

= − = − = − ≈ −substrate film

film

3 189 3 24953 2495

0 018618 1 9. .

.. . %%

where.the.negative.value.indicates.that.the.ZnO.film.is.compressed..Due.to.the.small.mis-match,.ZnO.readily.nucleates.and.grows.vertically.on.c-plane.GaN.substrates.in.solution.as.seen.in.Figure.5.2a..When.grown.on.a-plane.GaN,.off-normal.growth.with.well-defined.epitaxial.relationship.ZnO[0001]//GaN[10-10].has.also.been.reported.[44].

In.comparison,.Si.(100),.with.a.cubic.lattice.of.lattice.constant.of.5.43.Å,.does.not.pro-vide.lattice.conformation.for.the.wurtzite.structure.of.ZnO..As.a.result,.the.large.interface.energy.at.the.ZnO–Si.interface.limits.nucleation.on.Si.(100).substrates.surface.and.results.in.poor.growth.coverage.on. the.substrate.as.shown.in.Figure.5.2b.. It. is.believed. that. if.growth.occurs.on.the.Si.surface,.it.is.likely.to.originate.from.surface.defect.sites.on.the.Si.substrate.

The.interfacial.energy.of.substrates.with.large.lattice.mismatches.or.non-conformation.of.lattice.types.is.usually.reduced.by.applying.a.seed.layer.of.ZnO.from.which.heteroge-neous.growth.can.proceed.readily..These.seed.layers,.usually.consisting.of.ZnO.nanopar-ticles,.provide.a.rich.layer.of.dislocations.on.the.substrate.from.which.growth.of.nanorods.can. begin. [45].. By. applying. these. nanoparticles. uniformly. across. the. substrate,. a. good.coverage.of.nanorods.can.be.easily.obtained.in.wafer.scale.as.shown.in.Figure.5.2c,.albeit.not.uniformly.oriented.

Seed.layers.are.commonly.deposited.by.spin-coating.[46].or.dip-coating.[47].a.layer.of.ZnO.nanoparticles.or.a.zinc.acetate-based.gel.film..To.improve.particle.adhesion,.coalescence,.and.alignment.of.the.seed.layer,.a.thermal.annealing.step.ranging.from.200°C–1000°C.is.usually.applied.with.higher.annealing.temperatures,.leading.to.larger.grain.size.due.to.Oswald.ripening..The.optimum.thermal.annealing.step.for.a.gel.film.depends.on.both.the.thickness.of.the.gel.layer.and.the.solvent.that.is.used..Since.the.solvent.vaporization.and.decomposition.of.zinc.acetate.is.almost.simultaneously.followed.by.crystallization.of.the.zinc.oxide,.the.heating.rate.should.be.controlled.to.allow.enough.time.for.the.structure.to.relax.after.vaporization.and.decomposition.before.crystallization.in.order.to.obtain.high.quality.films.[47,48]..Greene.et.al..have.shown.that.optimum.vertical.alignment.for.zinc.acetate–based.gel.in.ethanol.is.obtained.after.annealing.at.350°C.[49].

(a) (b)1 µm 10 µm 1 µm

(c)

FIGURE 5.2SEM.image.of.ZnO.nanorods.grown.on.(a).GaN.substrate,. (b).Si. (100),.and.(c).Si. (100).pre-seeded.with.ZnO.nanoparticles.


Another.route.to.achieve.epitaxial.ZnO.nanorod.growth.on.Si.is.to.deposit.epitaxial.Au.films.on.Si.(111).as.demonstrated.by.Wang.et.al..[50].who.obtained.well-aligned.nanorods.along.the.c-axis.with.an.in-plane.epitaxial.relationship.(1.×.1).Au.(111).[−110]//(1.×.1).ZnO.(0001).[11–20].between.Au.and.ZnO.

The. interfacial. energy. is. also. a. function. of. the. pH. of. the. solution. [26]. as. shown. in.Figure.5.3.which.shows.the.calculated.values.of.interfacial.energy.at.various.pH.and.ionic.strength..The. interfacial.energy.drops.rapidly.as. the.pH.moves.away.from.the.point.of.zero.charge.(PZC).of.the.surface..In.fact,.in.the.pH.range.of.10–11,.where.ZnO.is.usually.grown,.the.interfacial.energy.for.a.particular.substrate.has.reduced.to.a.small.but.constant.value.and.does.not.significantly.affect. the.Gibbs.free.energy.expressed.in.Equation.5.1,.compared.to.the.solubility.of.zinc.[27].

5.6.3 Solubility of Zinc

The.solubility.of.zinc.in.aqueous.solution.can.be.calculated.from.the.temperature-depen-dent.ionic.equilibrium.of.the.solution..Such.a.model.will.account.for.all.the.possible.zinc.complex.species.and.is.very.useful.in.understanding.aqueous.solution.growth..One.such.model. for. the.growth.system.using.ZnAc2. and.NH4OH. is.described.here..When.ZnAc2.and.NH4OH.are.mixed,.the.main.species.consist.of.the.various.hydroxides,.ammines,.and.acetate.complex.species..The.reaction.equations.and.the.corresponding.rate.constants.at.298.K.are.given.in.the.following.

Hydroxide.complex.formation.[51]

.Zn OH Zn OH

Zn(OHZn OH

21 2

4 410+ − ++

+ −+ ↔ = =( )[ ) ]

[ ][ ].K . (5.4a)

10–20

9

PZC

–15

∆γ(m

J m-2

)

–10

–5

0

11 12

0.03 M0.10 M 0.01 M

pH

FIGURE 5.3Calculated.variations.of.interfacial.energy.as.a.function.of.pH.(From.Vayssieres,.L.,.Int. J. Nanotechnology,.2,.411,.2005).at.0.01,.0.03,.and.0.10.M.of.zinc.acetate.assuming.the.PZC.lies.at.pH.9.and.the.maximum.surface.charge.density.is.1.cm−2.


.Zn OH) OH Zn OH)

Zn OH OH( (

[ ( ) ][ ].+ −

+ −+ ↔ ↓ = =2 211 711

10K . (5.4b)

. Zn OH Zn OH Zn OH)( ) ( ) [ ( ] .2 2 3 2

4 510↓↔ = = −K . (5.4c)

.Zn OH) OH Zn OH

Zn OH)OH

( ( )[ ( ]

[ ].

2 3 43 1 7110↓ + ↔ = =− −−

−−K . (5.4d)

.Zn OH OH Zn OH)

Zn OHOH

( ) ([ ( ) ]

[ ].

2 42

542

20 612 10↓ + ↔ = =− −

−

−−K . (5.4e)

Acetate.complex.formation.[51]

.Zn Ac Zn Ac

Zn AcZn Ac

26 2

1 310+ − ++

+ −−+ ↔ = =( )

[ ( ) ][ ][ ]

.K . (5.5a)

.Zn Ac Ac Zn Ac

Zn AcZn Ac Ac

( ) ( )[ ( ) ]

[ ( ) ][ ].+ −

+ −−+ ↔ = =2 7

2 0 810K . (5.5b)

Ammine.complex.formation.[52]

.Zn NH Zn NH

Zn NHZn NH3

23

28

32

23

2 5910+ ++

++ ↔ = =( )[ ( ) ][ ][ ]

.K . (5.6a)

.Zn NH Zn NH

Zn NHZn NH

23 3 2

29

3 22

23

24 912 10+ +

+

++ ↔ = =( )[ ( ) ][ ][ ]

.K . (5.6b)

.Zn NH Zn NH

Zn NHZn NH

23 3 3

210

3 32

23

36 923 10+ +

+

++ ↔ = =( )[ ( ) ][ ][ ]

.K . (5.6c)

.Zn NH Zn NH

Zn NHZn NH

23 3 4

211

3 42

23

48 624 10+ +

+

++ ↔ = =( )[ ( ) ][ ][ ]

.K . (5.6d)

.NH OH NH H O

NHNH OH4 3 2 12

3

4

4 3910+ −+ −+ ↔ + = =K

[ ][ ][ ]

. . (5.6e)

The.ionic.equilibrium.for.the.aqueous.solution.can.be.obtained.by.simultaneously.solving.the.reaction.equations,.the.mass,.and.charge.balance..The.mass.balance.equation.can.be.written.as

. cAc Ac Zn Ac Zn Ac= + +− +[ ] [ ( ) ] [ ( ) ]2 2 . (5.7)


.c m m

m

N NH NH Zn NH= + + ⋅+ +

=∑[ ] [ ] [ ( ) ]4 3 3

2

1

4

. (5.8)

where.cAc.and.cN.are.the.total.concentrations.of.acetate.and.ammine.ions.The. charge. balance. equation. equates. the. positive. and. negative. charges. in. the. aqueous.solution.and.can.be.written.as

.

2 224 3

2

1

4

[ ] [ ( ) ] [ ( ) ] [ ] [ ( ) ]

[ (

Zn Zn OH Zn Ac NH Zn NH

Zn

+ + + + +

=

+ + + +

=

∑ m

m

OOH Zn OH Ac OH) ] [ ( ) ] [ ] [ ]3 422− − − −+ + + . (5.9)

The. temperature. dependence. [53]. of. the. rate. constants. in. the. reaction. Equations. 5.4a.through.e,.5.5a.and.b,.and.5.6a.through.e.is.estimated.using.the.following.equation:

.log

.KK

HR T T

T

T

r2

1

0

1 22 31 1

= −

∆. (5.10)

wherethe.ideal.gas.constant.R.=.8.314.×.10−3.kJ/mol/KKT1.and.KT2.are.rate.constants.at.temperature.T1.and.T2,.respectivelyΔrH0.is.the.standard.enthalpy.of.reaction.and.is.given.by

.

∆ ∆ ∆r i f j i f i

ij

H n H n H0 0 0= −∑∑ . (5.11)

whereΔf H0.is.the.standard.enthalpy.of.formationi.and.j.specify.reactants.and.products,.respectivelyni.and.nj.are.the.amounts.in.moles.of.each.substance.in.the.chemical.reaction

The. standard. enthalpy. values. for. the. product. and. reactants. for. reaction. Equations. 5.4.through.5.6.are.summarized.in.Table.5.1.

In.the.calculation.of.the.ionic.equilibrium,.it.is.assumed.that.the.equilibrium.concentra-tions.of.zinc.acetate.complex.species.are.very.small.compared.to.the.other.species..Thus,.in.the.range.of.0°C–150°C,.the.temperature.dependence.of.K6.and.K7.in.Equation.5.5a.and.b.is.neglected.

Using.the.model.described.earlier,. the. ionic.equilibrium.of. the.ZnAc2.and.NH4OH.in.an.aqueous.solution.can.be.calculated.for.various.ZnAc2.and.NH4OH.precursor.concen-trations.and.temperature..A.detailed.description.of. the.procedure.to.calculate. the. ionic.equilibrium.concentrations.has.been.published.[16]..Figure.5.4.shows.the.typical.plot.of.the.solubility.of.zinc.and.concentration.of.the.major.zinc.complexes.as.a.function.of.pH.at.300.K..The.pH.is.increased.by.adding.more.NH4OH.while.keeping.the.mass.of.ZnAc2.constant.at.0.016.M.


TABLE 5.1

List.of.Enthalpy.Values

Species ΔfH0 (kJ/moL)

Zn2+ −153.64Zn(OH)+ −388.35Zn(OH)2 −611.95Zn(OH)2.↓ −641.90Zn(OH)3

− −817.97Zn(OH)4

2− −1125.64Zn(NH3)2+ −244.81*Zn(NH3)2

2+ −338.07*Zn(NH3)3

2+ −434.68*Zn(NH3)4

2+ −536.72*H20 −285.83OH− −230NH3 −80.29NH4

+ −132.5

Source:. Smith,.R.M..and.Martell,.A.E.,.in.Critical Stability Constants,.Vol..4,.Plenum.Press,.New.York,.1976,.41;.Wagman,.D.D..et.al.,.J. Phys. Chem. Ref. Data,.11(2),.1982;.Wesolowski,.D.J..et. al.,. in. Aqueous Systems at Elevated Temperatures and Pressures. (eds:. Palmer,. D.A.,. Fernandez-Prini,. R.,. and.Harvey.A.H.),.Elsevier,.London,.U.K.,.2004,.535.

Note:. Enthalpy.values.with.an.asterisk.(*).denotes.calculated.val-ues. of. enthalpy. of. formation. from. tabulated. enthalpy. of.reaction.

pH7.0

10–7

(b)Zn2+

10–6

10–5

10–4

10–3

10–2

7.5 8.0

(a) Zinc acetatecomplexes

(d) Zinc hydroxidecomplexes

(e) Total zinc ions

Conc

entr

atio

n (m

ol/L

)

(ZnAc2) = 0.15 M, 300 K

(c) Zinc ammine complexes

8.5 9.0 9.5 10.0 10.5

FIGURE 5.4Equilibrium.complex.concentrations.and.solubility.of.zinc.as.a.function.of.pH.at.300.K..The.pH.is.increased.by.adding.more.NH4OH.while.keeping.the.mass.of.ZnAc2.constant.at.0.016.M..Curves.show.the.equilibrium.con-centrations.of.(a).zinc.acetate.complexes,.(b).Zn2+.ions,.(c).zinc.ammine.complexes,.(d).zinc.hydroxide.complexes,.and.(e).total.zinc.ion.concentration,.respectively.


Several.important.features.can.be.seen.from.Figure.5.4:

. 1..As. shown. by. curve. (a). in. Figure. 5.4,. the. concentrations. of. zinc. acetate. com-plexes.are.small.compared.to.the.other.zinc.complexes..This.justifies.the.omis-sion.of.temperature.effect.on.equilibrium.constants.K6.and.K7.in.Equation.5.5a.and.5.5b.

. 2..The.“true”.solubility.of.zinc.is.given.by.the.sum.of.all.its.zinc.complexes,.namely.the.hydroxides,.ammines,.and.acetates..The.solubility.shows.a.minimum.point.in.the.pH.range.of.8.5–9.5..Interestingly,.this.pH.range.coincides.with.the.condi-tion.where.the.net.charge.on.the.ZnO.surface.is.neutral,.referred.to.as.point.of.zero.charge.(PZC).

. 3..At.pH.values.greater.than.9.7,.the.increase.in.zinc.solubility.is.contributed.mainly.by.the.increasing.concentration.of.zinc.ammine.complexes..On.the.other.hand,.at.pH.values.lower.than.8,.the.increase.is.due.to.Zn2+.ions..The.different.dominant.species.at.various.pH.play.an.important.role.in.determining.the.growth.rate.and.the.structural.defects.of.ZnO.[27].

Proceeding.from.the.equilibrium.concentrations,.the.solubility.of.zinc,.or.the.total.concen-tration.of.zinc.ions.in.the.precursor.solution,.is.given.by.the.sum.of.all.the.zinc.species.in.the.solution:

.

S mm

n pp

p

Zn Zn Zn OH Zn NH Zn Ac= + + ++ − + + − +

=

[ ] [ ( ) ] [ ( ) ] [ ( ) ]( ) ( )2 23

2 2

1

22

1

4

1

4

∑∑∑== nm

. (5.12)

Although.solubility.is.the.main.driving.force.for.nucleation.and.growth.in.solution,.it.is.rarely.used.as.a.growth.control.variable.due.to.measurement.difficulties..Instead,.pH.is.the.dominant.growth.control.variable..This.is.reflected.in.the.reported.growth.procedures.where.a.final.adjustment.to.the.pH.range.of.10–11.is.usually.practiced.

5.6.4 Applications of Solubility as Growth Control Variable

With. this. thermodynamic.model,.many.aspects.of. the.growth.can.be.understood..One.example.is.the.growth.of.ZnO.nanorods.on.unintentionally.doped.GaN.epilayers.[14,16]..The.GaN.epilayer.is.about.3.μm.thick.and.is.grown.on.c-plane.sapphire.at.1020°C.using.an.EMCORE.D125.MOCVD.with.trimethylgallium.and.ammonia.as.the.Ga.and.N.source,.respectively..Growth.of.ZnO.was.carried.out.at.various.temperatures.ranging.from.60°C.to.150°C.and.with.various. precursor. concentrations.of. zinc.acetate.and.ammonia. for.a.fixed.duration.of.4.h..The.values.of.the.lengths,.L,.and.densities,.B,.of.the.nanorods.were.measured.experimentally.from.SEM.images.and.fitted.empirically.using.the.temperature.and.solubility.of.zinc.as.variables

.L A S

EkT

ALm aL

L= +

1 2Zn exp . (5.13)

.B A S

EkT

ABn aB

B= +

1 2Zn exp . (5.14)


where.m.=.1.81,.n.=.−5.22,.AL1.=.8.710.×.108,.AL2.=.−24.3,.AB1.=.1.047.×.10−8,.AB2.=.67.2,.k.=.8.62.×.10−5.eV.. K−1,. EaL.=.0.77. eV,. EaB.=.−2.11. eV.. The. units. for. B,. L. and. SZn. are. cm−2,. nm. and. mol/L,.respectively..These.equations.provide.a.convenient.measure.of. length.and.density.for.a.given.precursor.concentration.and.temperature.and.is.useful.for.prediction.of.the.length.and.density.of.ZnO.nanorods.for.various.growth.conditions..In.fact,.the.same.approach.can.be.applied.for.other.lattice-matched.substrates..It.is.worth.noting.that.the.constants.AB1.and.AL1.are.interface-related.constants..For.a.rougher.surface,.AB1.is.expected.to.be.larger.while.AL1.smaller,. leading.to.a.higher.density.and.shorter. length.of.nanorods..Valuable.insights.into.the.growth.can.be.obtained.from.the.contour.plots.of.log(B).and.L.at.373.K.are.shown.in.Figure.5.5a.and.b,.respectively.

. 1..The. maximum. achievable. density. for. the. ZnAc2-NH4OH. system. is. about. 1010.cm−2.with.a.corresponding.length.of.less.than.1.μm..Higher.densities.of.nanorods.can.be.obtained.by.growing.in.a.pH.less.than.9.7.at.the.expense.of.poor.substrate.coverage..It.is.also.possible.that.higher.densities.of.nanorods.are.likely.to.result.in.coalescence.of.rods.and.formation.of.a.film..One.way.to.enhance.the.density.is.to.use.surfactants.such.as.PEI.or.AlCl3.to.increase.the.aspect.ratio.of.the.rods.

. 2..The.maximum.length. is.about.8.μm.with.a.corresponding.density. less. than.107.cm−2..Longer. lengths.can.be.obtained.simply.by.extending. the.growth.duration.beyond.4.h.or.refreshing.the.growth.solution.after.4.h..Due.to.the.low.density.of.rods,.it.is.expected.that.several.growth.cycles.can.be.employed.to.extend.the.rod.lengths.

. 3.. It.is.clear.that.the.densities.and.length.cannot.be.maximized.simultaneously.in.a.single.growth.step..In.order.to.maximize.density.and.length,.a.two-step.process.may.be.required.where.the.substrate. is.grown.in.a.“high-density”.solution.and.then.transferred.to.a.second.solution.to.maximize.the.length.of.the.rods.

Another.example.of.the.application.of.solubility.is.the.prediction.of.growth.morphology.of.ZnO.nanorods.on.seeded.substrates.[27]..Seeding.was.performed.by.spin-coating.a.thin.layer.of.ZnO.nanoparticles.of.10–20.nm.in.diameter.

0

20

40

60pH 9.7

pH 10.6

c/s Zn= 60

c/sZn= 20

80

100

50

log [B(cm2)]

100 150Volume of NH4OH (×10–2 mL)

Mas

s of Z

n (A

c)2 (

×10–2

g)

200 250

7

8

1

2

4

8

99.5

0(b)(a)

20

40

60pH 9.7

pH 10.6

c/s Zn= 60

c/sZn= 20

80

100

50

Length (μm)

100 150Volume of NH4OH (×10–2 mL)

Mas

s of Z

n (A

c)2 (

×10–2

g)

200 250

FIGURE 5.5The.contour.plot.of.(a).log[B(cm−2)].=.7,.8,.9,.and.9.5,.and.(b).length.(μm).=.1,.2,.4,.and.8.for.various.concentrations.of.ZnAc2.and.NH4OH..The.values.of.density.and.length.are.shown.inside.black.circles..The.volume.of.water.used.is.40.mL..The.validity.boundaries.of.pH.between.9.7.and.10.6,.and.degree.of.supersaturation.of.zinc.(C/SZn).between.20.and.60.are.shown.as.thick.black.lines.


Table.5.2.summarizes.the.growth.over.a.wide.range.of.pH.as.shown.in.Figure.5.6..At.the.extreme.ends.of.the.pH.(<7.and.>11),.the.solubility.of.zinc.is.too.high.for.any.growth.to.take.place..In.the.middle.of.this.pH.range.8–10.2,.coinciding.with.the.PZC.of.ZnO,.uneven.coverage.of.substrate.surface.is.observed..At.the.PZC,.the.interfacial.energy.is.at.the.maxi-mum.and.the.high.free.energy.requirement.results.in.poor.growth..Moving.away.from.the.PZC,.the.interfacial.energy.drops.quickly.to.a.small.constant.value,.while.the.solubility.increases..This.trade-off.results.in.an.optimal.pH.point.of.10.6–10.8.where.dense.and.uni-form.rods.are.grown.

To. understand. how. pH. affects. the. Gibb’s. free. energy. of. nucleation,. we. will. need. to.examine.Equation.5.3.which.is.reproduced.here.for.the.convenience.of.the.discussion.

.

∆G Vk T C S

∗ = ⋅( )

163

2

2 2

3

2π γ

ln / Zn

. (5.15)

whereV.is.the.atomic.volumek.is.the.Boltzmann.constant.defined.as.1.38.×.10−23.J/KT.[K].is.the.growth.temperatureC.[mol/L].is.the.concentration.of.zinc.acetate.used.in.the.experimentSZn.[mol/L].is.the.solubility.of.zincγ.is.the.interfacial.energy

0.02 M(7) 0.04 M(7.5)

0.4 M(10.6)0.3 M(10.2)

0.1 M(9)

1.1 M(10.8)

(a) (b) (c)

(d) (e) (f )

FIGURE 5.6SEM.morphology.of.ZnO.nanorods.grown.on.Si.(100).substrates.with.a.pre-coat.of.ZnO.nanoparticles.using.growth.solutions.with.0.02.M.ZnAc2.and.(a).0.02.M,.(b).0.04.M,.(c).0.1.M,.(d).0.3.M,.(e).0.4.M,.and.(f).1.1.M.NH4OH..The.concentration.of.NH4OH.and.the.corresponding.initial.solution.pH.values.in.square.parentheses.are.indi-cated.on.the.top.left.corner..Scale.bar.shows.1.μm..(Reproduced.from.J. Cryst. Growth,.311,.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.1278..Copyright.2009,.with.permission.from.Elsevier.)

TABLE 5.2

Summary.of.Observed.Growth.Behavior.with.Solution.pH

pH Range <7 7–7.5 9–10.2 10.6–10.8 >11

Observed.growth None Slow.and.uniform Poor.coverage Fast.and.uniform None


For.constant.values.of.interfacial.energy,.γ,.ZnAc2.concentration.C.and.temperature.T,.we.can.rewrite.Equation.5.15.as

.

∆G Vk T C S

K

K S∗ = ⋅

( )=

( )163

2

2 2

3

21

22

π γln ln/ /Zn Zn

. (5.16)

where.K1.and.K2.are.constants.Equation.5.16.is. important.because.it.states.that.the.solubility.of.zinc.is.the.dominant.

factor.affecting.the.Gibbs.free.energy.in.the.pH.range.where.nucleation and uniform growth of ZnO nanorods occur..Since.the.initial.value.of.SZn.can.be.calculated.from.the.ionic.equilib-rium.of.the.growth.solution.using.thermodynamic.data,.a.correlation.between.SZn.and.the.morphology.of.the.nanorods.can.be.observed.as.shown.in.Figure.5.7.where.the.dominant.role.of.SZn.is.clearly.demonstrated.

First,.when.ZnAc2.is.kept.at.0.02.M.to.maintain.a.constant.value.of.C,.in.Equation.5.16,.while.NH4OH.is.increased.from.0.4.to.1.1.M.to.vary.the.pH.within.the.range.of.10.6–10.8,.the.density.of.rods.is.observed.to.reduce.with.higher.concentrations.of.NH4OH.as.shown.in.Figure.5.7d.through.f..A.higher.concentration.of.NH4OH.increases.the.solubility.of.zinc.SZn. and. thus,. from. Equation. 5.16,. leads. to. a. higher. Gibbs. free. energy. for. nucleation,. a.lower.rate.of.nucleation.and.a.lower.density.of.rods..The.same.behavior.can.be.observed.in.Figure.5.7a.through.c.as.well.as.Figure.5.7g.through.i.when.0.01.and.0.03.M.of.ZnAc2.are.used,.respectively..This.clearly.shows.the.dominant.role.of.solubility.over.interfacial.energy.in.the.pH.range.that.uniform.growth.of.ZnO.occurs.

It. can.also. be. seen. that. the.morphology.of. the. rods. can.be.predicted. from. the.value.of.SZn..There.are.three.distinct.regions.shown.in.Figure.5.7:

•. Region.I.where.SZn.is.less.than.0.88.mmol/L,.we.obtained.uniform.and.dense.array.of.ZnO.nanorods.with.good.surface.coverage.as.shown.in.Figure.5.7a,.d,.and.g.

•. Region.II.where.SZn.is.in.between.0.88.and.1.56.mmol/L,.a.transition.region.where.both.nanorods.and.large.clustered.rods.exist.in.Figure.5.7b,.e,.and.h.

•. Region. III.of.Figure.5.6,. in.which.SZn. is.greater. than.1.56.mmol/L,.shows. large.clustered.rods.with.poor.surface.coverage.that.are.obtained.as.shown.in.Figure.5.7c,.f,.and.i.

To.confirm.the.morphology.dependence.on.SZn,.and.not.on.the.concentration.of.NH4OH,.a.sample.is.grown.in.a.solution.of.0.006.M.ZnAc2.and.0.4.M.NH4OH..The.value.of.SZn.at.this.point.is.indicated.by.a.square.“○”.in.Figure.5.7..The.corresponding.SEM.image.in.Figure.5.8.shows.a.mixed.morphology.which.confirms.SZn.dependence.instead.of.[NH4OH].dependence.

As.mentioned.in.the.earlier.section,.pH.by.itself.is.not.a.good.predictor.for.morphology..This.point.is.illustrated.clearly.in.Figure.5.9.which.plots.SZn.against.the.pH.instead.of.the.concentration.of.NH4OH.as. in.Figure.5.7.. It.can.be.seen.clearly. that. the. three. types.of.morphologies.cannot.be.grouped.using.the.pH.variable.alone..For.example,.for.samples.a,.e,.and.i.which.are.grown.in.a.tight.range.of.pH.values.of.about.10.6,.several.types.of.morphology.of.the.ZnO.nanorods.can.be.observed..A.combination.of.pH.and.concentra-tion.of.ZnAc2. is.required.to.get.a.good.prediction.of. the.morphology..The.solubility.of.zinc.which.already.incorporates.the.pH.and.various.reactant.concentrations.does.a.much.better.job.in.predicting.the.nanorods.morphology.

The.dependence.of.these.morphology.trends.on.the.value.of.SZn.is.independent.of.substrate.such.as.glass,.ITO,.silicon.(111),.silicon.(100),.and.plastic..This.is.attributed.to.the.presence.of.a.seed.layer.of.ZnO.nanoparticles.on.the.substrate.which.provides.similar.interface.properties.


5.7 Defects in Aqueous Solution Grown ZnO

Thermodynamic.arguments.show.that.in.any.material.a.certain.proportion.of.defects.exist.in.equilibrium.at.any.non-zero. temperatures. [57]..Several.recent.reviews.on.the.defects.in.ZnO.[58–60].showed.a.lack.of.consensus.on.the.relationship.between.various.defects.

Concentration of NH4OH0.4 M

2.0

1.0

0.00.3 0.4 0.5 0.6 0.7 0.8

Concentration of ammonia (mol/L)

Solu

bilit

y of z

inc (

mm

ol/L

)

0.9 1.0 1.1 1.2 1.3

0.01 M

0.02 M

0.03 M

Concentration of ZnAc2

0.8 M

0.006 M ZnAc2

0.01 M ZnAc

0.02 M ZnAc2

0.03 M ZnAc2

a

d

g

bi

f

I

(a) (b) (c)

(d) (e) (f )

(g) (h) (i)

II

IIIc

e

h

1.1 M

FIGURE 5.7Plot.showing.the.solubility.of.zinc,.SZn,.against. the.concentration.of.NH4OH.for.0.006,.0.01,.0.02,.and.0.03.M.ZnAc2..The.SZn.data.points.which.are.labeled.(a–i).correspond.to.the.SEM.images.(a–i),.respectively..The.value.of.SZn.for.0.006.M.ZnAc2,.and.0.4.M.NH4OH.is.marked.with.a.white.circle.(○).and.the.corresponding.SEM.image.is.shown.in.Figure.5.8..Growth.in.region.I.produces.uniform.nanorods,.region.II.a.mixed.morphology.of.nanorods.and.large.rods.and.region.III.only.large.rods..(Reproduced.from.J. Cryst. Growth,.311,.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.1278..Copyright.2009,.with.permission.from.Elsevier.)


and.growth.methods..This.is.not.surprising.because.defects.depend.on.a.combination.of.growth.conditions,.subsequent.processing.and.storage.conditions.[61]..For.example,.ZnO.grown. in. aqueous. solution. commonly. results. in. a. defect-related. broad. yellow-orange.spectrum. in. the. photoluminescence. while. those. in. vapor. phase. usually. exhibit. green.defect-related.photoluminescence.[62]..For.samples.grown.with.a.combination.of.growth.methods.such.as.hydrothermally.grown.nanorods.from.sputtered.seed.layers,.complica-tions.can.arise.due. inter-diffusion.of.defects.after. thermal.annealing.between. the.seed.

pH9.5

0.0003

0.0008

0.0013

Solu

bilit

y of z

inc (

M)

0.0018

0.00230.02 M 0.01 M 0.006 M

c

b

III

II

I

f

e

a

h

dg

i

0.03 M

9.7 9.9 10.1 10.3 10.5 10.7 10.9 11.1 11.3

FIGURE 5.9Plot.of.solubility.of.zinc.against.pH.for.0.006.M,.0.01.M,.0.02.M,.and.0.03.M.of.ZnAc2..The.corresponding.SEM.images.from.samples.a,.e,.and.i.in.Figure.5.7.are.shown.here.for.ease.of.comparison.

30.0 kV ×11.0K 2.73 µm30.0 kV ×15.0K 2.00 µm

FIGURE 5.8SEM.image.showing.the.top.and.cross-sectional.view.of.a.sample.grown.in.0.006.M.ZnAc2.and.0.4.M.NH4OH..The.mixed.morphology.confirms.the.dependence.of.SZn.which.is.shown.in.Figure.5.7..(Reproduced.from.J. Cryst. Growth,.311,.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.1278..Copyright.2009,.with.permission.from.Elsevier.)


layer.and.the.rods..Also,.storage.in.ambient.condition.has.been.reported.to.recover.ther-mally.quenched.yellow.emission.[61].

Assignment.of.this.emission.to.the.defects.using.their.theoretically.calculated.values.is.tricky.for.a.particular.type.of.point.defect.because.there.can.be.several.reported.energy.levels.values.[63–65],.depending.on.the.method.of.calculation,.assumptions,.and.the.size.of.the.supercell.that.was.used..This.can.be.seen.in.the.tabulated.energy.levels.for.the.various.point.defects.in.a.review.by.Djurisic.et.al..[58]..Generally,.interactions.between.individual.defects.and.formation.of.defect.complexes.with.other.point.defects.or.impurity.dopants.are.usually.not.taken.into.account..Furthermore,.recent.experimental.results.suggest.that.defect.and.impurity.complexes.play.a.more.significant.role.than.previously.thought.[66].

Defects. in.ZnO.grown. in.aqueous.solution.have.been.studied.as.a. function.of.growth.pH.using.a.combination.of.photoluminescence.and.Raman.scattering.spectroscopy.[27]..By.increasing.the.pH.of.the.growth.solution.using.higher.concentrations.of.NH4OH,.the.solu-tion.consists.of.higher.concentrations.of.negatively.charged.majority.growth.units.and.OH−..These.attract.more.H+.ions.from.the.surface.of.ZnO.to.enter.into.the.solution,.leading.to.a.lower.concentration.of.hydrogen.defects,.and.freeing.up.more.sites.for.adsorption.of.zinc.ions..This.results.in.a.faster.growth.rate.due.to.the.attractive.force.between.the.positively.charged.Zn-face.and.the.negatively.charged.growth.species..However,.the.faster.growth.rate.prevents.complete.dehydration.of.excess.hydroxyl.groups,.which.leads.to.excess.oxygen..The.increased.incorporation.these.hydroxyl.groups.in.the.lattice.results.in.higher.concentrations.of.oxygen-related.defects.and.compressive.stress,.which.can.be.tracked.by.the.shift.in.the.E2H.peak.position.in.the.Raman.spectra.as.shown.in.the.inset.of.Figure.5.10.

Wavenumber (cm–1)

250 300

Nor

mal

ized

inte

nsity

(a. u

.)

0.4 M NH4OH

0.8 M NH4OH

1.1 M NH4OH

350 400 450 500 550 600

2E2L(M)

E1(TO)

E2H

A1(TO)

0.3439.5

440.0

E 24 pe

ak ce

nter

(cm

–1)

440.5

441.0

441.5

0.5 0.7 0.9Concentration of ammonia (M)

1.1

Defectmode

FIGURE 5.10The.Raman.spectra.measured.from.ZnO.samples.grown.with.0.4,.0.8,.and.1.1.M.NH4OH.on.a.ZnO.nanopar-ticle.seeded.glass.substrate..The.inset.shows.the.shift.of.the.E2H.peak.to.higher.frequencies.as.concentration.of.NH4OH.is.increased..When.compared.against.the.E2H.peak.of.unstrained.bulk.ZnO.at.439.cm−1,.the.increasing.frequency.indicates.progressively.higher.compressive.stresses.in.ZnO.nanorods.with.higher.concentration.of.NH4OH.in.the.growth.solution..(Reproduced.from.J. Cryst. Growth,.311,.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.1278..Copyright.2009,.with.permission.from.Elsevier.)


In.comparison,.when.growth.is.carried.out.when.the.pH.is.less.than.that.at.PZC.of.ZnO,.there.is.a.marked.improvement.of.the.ratio.between.the.UV.and.visible.emission.as.shown.in.Figure.5.11..In.this.pH.regime,.both.the.majority.growth.species.and.the.substrate.sur-face.are.positively.charged.resulting.in.a.much.slower.growth.rate.and.lesser.structural.defects..The.slow.growth.rate.allows.dehydration.to.proceed,.thus.significantly.reducing.the.concentration.of.oxygen.related.defects.and.thus.the.intensity.of.the.orange.emission..More.importantly,.in.this.regime,.the.H+.tend.to.remain.on.the.surface.and,.consequently,.lead.to.a.higher.concentration.of.hydrogen-related.defects.which.contribute.as.a.shallow.donor.[67].with.higher.UV.emission.intensity.

This.understanding.of.the.defect.formation.mechanism.is.useful.in.controlling.the.type.and.concentration.of.defects.during.the.growth.process..The.growth.regime.where.pH.is.less.than.that.for.PZC.is.of.particular.interest.because.it.not.only.reduces.the.concentration.of.native.defects.which.requires.a.high. temperature.post-growth.anneal. to. remove.but.also.has.a.high.concentration.of.H.defects.which.can.help.to.increase.solubility.of.p-type.dopants.such.as.Li,.Na,.and.K.[68–70].

The. changes. to. the. PL. spectra. with. respect. to. thermal. treatment. in. various. ambi-ent.conditions.provide.useful.clues.to.the.chemical.nature.of.various.defect.emissions..Figure.5.12.shows.one.such.systematic.study.in.air.and.nitrogen.ambient..Consistent.with.several. other. reports. [71,72],. yellow-orange. defect. emission. is. reduced. when. annealed.in.nitrogen.ambient.and.increased.in.air.or.O2,.leading.to.the.assignment.of.interstitial.oxygen.

Besides.the.orange.emission,.a.comparatively.weaker.green.component.at.about.500.nm.can. also. be. seen. in. Figure. 5.12.. Continual. reduction. of. green. emission. is. observed. for.both.air.and.nitrogen.ambient.as.annealing.temperature.is.increased..Considering.that.the.

Wavelength (nm)330

Inte

nsity

(a. u

.)

380

1.1 MpH 10.8

0.3 MpH 10.1

0.04 MpH 7.4

1.1 MpH 10.8

0.3 MpH 10.1

0.04 MpH 7.4

0.02 MpH 7.3

0.02 MpH 7.3

430 480 530 580 630

FIGURE 5.11Photoluminescence.spectra.recorded.from.samples.grown.in.0.02.M,.0.04.M,.0.3.M,.and.1.1.M.NH4OH.while.the.concentration.of.ZnAc2.is.kept.constant.at.0.02.M..The.visible.defect.emission.decreases.while.the.ultraviolet.band.edge.emission.increases.when.the.concentration.of.NH4OH.is.reduced..A.marked.improvement.of. the.ratio.between.the.UV.and.visible.emissions.can.be.seen.when.the.growth.solution.pH.is.less.than.that.at.PZC..(Reproduced.from.J. Cryst. Growth,.311,.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.1278..Copyright.2009,.with.permission.from.Elsevier.)


High pH sample annealed in air

10,000

1,000

100

600°C 400°C

200°C

400°C600°C

800°C

4400

150

300

450

600

750

900

540 640

200°C

As-grown

As-grown800°C

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.)

10

1350 370 390

Wavelength (nm) Wavelength (nm)410 430

(a)

Low pH sample annealed in air

600°C400°C

0

150

300

450

600200°C

As-grown200°C

400°C

600°C

800°C

As-grown800°C

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.)

10,000

1,000

100

10

1

Wavelength (nm)440 540 640

Wavelength (nm)350

(c)370 390 410 430

(b)

High pH sample annealed in nitrogen ambient10,000

1,000

100

600°C 400°C

200°C

400°C

600°C

800°C440

0

150

300

450

600

750

900

540 640

200°C800°C

As-grown

As-grown800°C

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.)

10

1350 370 390

Wavelength (nm) Wavelength (nm)410 430

(d)

Low pH sample annealed in nitrogen amblent

600°C400°C

0

150

300

450

600200°CAs-grown 200°C

400°C

600°C

800°C

As-grown

800°C

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.)

10,000

1,000

100

10

1


Wavelength (nm)350 370 390 410 430

FIGURE 5.12PL.spectra.of.sample.grown.in.high.pH.(10.7).after.annealing.at.various.temperatures.in.(a).air.and.(b).nitrogen.ambient,.as.well.as.for.low.pH.sample.(7).annealed.in.(c).air.and.(d).nitrogen.ambient..The.sharp.peak.at.650.nm.is.due.to.the.doubling.of.the.325.nm.laser.line.and.should.be.ignored..The.intensities.are.shown.in.logarithmic.scale..The.green.emission.is.located.at.500.nm..Comparing.(a).and.(b),.the.yellow-orange.defect.emission,.cor-related.to.excess.oxygen.as. interstitials,. is.reduced.when.annealed.in.nitrogen.ambient.but. increased.in.air..Comparing.(c).and.(d),.the.green.emission,.correlated.to.Zn.vacancies.and.interstitials,.is.reduced.by.annealing.in.air.and.nitrogen..(Reproduced.from.J. Cryst. Growth,.311,.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.1278..Copyright.2009,.with.permission.from.Elsevier.)


intensity.of.the.green.emission.is.reduced.by.annealing.in.both.air.and.nitrogen.ambient,.and.taking.into.account.the.formation.energy.of.native.defects.arising.from.growth.in.an.oxygen-rich.environment.[63],.the.likely.candidates.for.the.green.emission.are.zinc.vacan-cies.and.interstitial.zinc..The.low.migration.energy.barrier.of.interstitial.zinc.[63].allows.it.to.diffuse.into.zinc.vacancy.sites.and.thus.reduce.the.green.emission.intensity.under.various.annealing.conditions.

It.should.be.pointed.out.that.the.identification.of.the.green.emission.remains.inconclu-sive.despite.vigorous.investigations.involving.optical.spectroscopy.such.as.photolumines-cence.(PL).and.Raman.spectroscopy.[15],.electron.paramagnetic.resonance.(EPR),.positron.annihilation. spectroscopy. (PAS),. and.x-ray.photoelectron.spectroscopy. (XPS). [61,73].. So.far,.the.green.emission.has.been.attributed.to.singly.ionized.oxygen.vacancies.(VO

+).[71],.zinc.vacancies.(VZn),.surface.defect.complexes.involving.VZn.[62,66,73]..More.investigations.are.needed.to.identify.the.origins.of.the.green.emission.

5.8 Doping in Aqueous Solution

Undoped.ZnO.is.inherently.n-type.with.background.carrier.concentrations.attributed.to.the.presence.of.native.donor.defects.such.as.zinc.interstitials.and.oxygen.vacancies.[74].as.well.as.hydrogen-related.donors.[75].

Bulk.ZnO.substrates.that.are.hydrothermally.grown.at.350°C–400°C.have.an.intrinsic.electron.concentration.of. less. than.1014.cm−3.due. to.compensation.effect. from.Li.and/or.Na.incorporation.[60]..Epitaxial.films.that.are.grown.at.much.lower.temperatures.(about.80°C–90°C).in.aqueous.solution.have.been.reported.to.have.a.background.electron.con-centration.of.about.1016–1017.cm−3.for.pH.8.[70,76].and.1019.cm−3.at.pH.10.9.[76],.which.com-pare.well.with.gas-phase.methods.such.as.those.produced.using.MBE.which.are.around.1017.cm−3.[60].

In. the. case. of. nanorods,. the. determination. of. background. carrier. concentrations. is.challenging.because.Hall.effect.measurements.cannot.be.applied..Alternative.methods,.complementing.the.photoluminescence.spectra.are.FET.measurements.[77],.electrochem-ical. impedance. spectroscopy. [78,79],. and. capacitance-voltage. measurements. [80]. with.reported.background.carrier.concentrations.ranging.from.1017.to.1019.cm−3..The.interpreta-tion.of.these.measurement.results.is.complicated.by.the.reliability.of.contacts.to.the.indi-vidual.nanorod.[81],.non-uniformity.of.nanorods,.piezoelectric.effect.[82,83],.and.surface.effects.[84].

For.fabrication.of.optoelectronic.devices,.a.stable.p-.and.n-doping.above.1017.cm−3.have.to.be.achieved..To.date,.reports.of.successful.p.and.n-type.doping.have.been.dominated.by.gas.phase.growth.methods.such.as.magnetron.sputtering,.pulsed.laser.deposition,.and.MBE.[85]..The.best.values.of.electron.and.hole.concentrations.are.in.the.range.of.1020.and.1019.cm−3.[86].with.the.typical.mobility.for.low.n-.and.p-type.doping.around.200.and.5–50.cm2/(V.s).[85],.respectively.

In.contrast.to.gas-phase.methods,.aqueous.solution.methods.only.have.a.few.reports.for.n-type.doping.with.Al.[87],.In.[88,89].and.Ga.[90],.n-type.co-doping.with.Ga.and.In,.and.p-type.ZnO.with.K.[70]..This.is.surprising.because.aqueous.solution.growth.methods.offer.several.advantages.over. the.gas.phase.methods..First,.ZnO.films.grown.using.aqueous.chemical.growth.methods.are.no.less.bad.in.intrinsic.defect.density.compared.to.those.grown.by.gas-phase.methods..Second,.the.high.dopant.concentrations.in.aqueous.solution.


should.favor.a.more.homogeneous.and.a.higher.level.incorporation.of.dopants.during.the.growth.process..Third,.low.growth.temperatures.prevent.diffusion.of.dopants.and.forma-tion.of.dopant.metal.clusters.

5.9 n-Type Doping

The.n-type.doping.in.aqueous.solution.has.been.achieved.by.substituting.Zn.with.group.III.elements.such.as.Ga,.Al,.and.In..As.shown.in.Table.5.3,.the.doping.levels.of.n-ZnO.are.com-parable.to.the.best.values.reported.so.far.in.the.literature.which.are.in.the.range.of.1020.cm−3.

5.10 p-Type Doping

The.p-ZnO.levels.for.samples.grown.in.aqueous.solution.are.still.much.lower.than.the.best.reported.values.that.are.in.the.range.of.1019.cm−3.

Compared.to.n-type.doping,.p-type.doping.poses.a.much.more.difficult. task.because.of.the.high.densities.of.intrinsic.defects,.low.solubility.of.p-type.dopant.species.and.ten-dencies.of.dopants.to.form.deep.level.instead.of.shallow.level.acceptor.states.[92]..Due.to.the.low.solubility.of.p-type.dopants,.it.is.important.to.minimize.the.unintentional.donor.concentration..This.can.be.achieved.by.carrying.out.the.growth.process.at.pH.8,.as.shown.in.Table.5.3,.to.achieve.a.background.doping.of.about.1016–1017.cm−3.

There.are.two.groups.of.candidates.for.p-type.dopants:.Group.I.elements.which.substi-tute.Zn.atoms.and.Group.V.elements.which.substitute.O.atoms..Calculated.bond.lengths.and.defect.energy.levels.for.various.dopants.from.Group.I.and.V.show.that.in.terms.of.strain.energy,.Li.and.N.are.the.best.candidates.from.Group.I.and.V,.respectively,.while.in.terms.of.defect.energy.levels,.all.Group.I.elements.have.a.shallower.energy.level.com-pared.to.Group.V.elements..Therefore,. in.theory,. it.appears.that.Group.I.elements,.par-ticularly. Li,. will. be. promising. candidates. as. p-dopants.. However,. experimental. results.show.otherwise..Hydrothermally.grown.bulk.ZnO.crystals.are.typically.grown.in.a.high.

TABLE 5.3

Summary.of.Doping.Concentration,.Mobility,.and.Resistivity.for.Various.n-Doping.Reports.in.the.Literature.for.Solution.Growth.Method

Doping Type

Concentration (cm−3)

Mobility (cm2/Vs)

Resistivity (Ω cm) Dopant Substrate Ref.

n 1.4.×.1016 0.45 9.9.×.101 Uid Sapphire.(pH.7.5) [76]n 6.7.×.1017 12.8 7.3.×.10−1 Uid Spinel.(pH.8) [76]n 1.8.×.1019 13.9 2.5.×.10−2 Uid Spinel.(pH.10.9) [76]n 1.9.×.1021 16.8 2.0.×.10−4 Al Glass/quartz [87]n 3.2.×.1019 7.95 2.5.×.10−2 In Spinel [88]n 1.1.×.1019 >37.9 <1.5.×.10−2 In ZnO [89]n 3.1.×.1020 28 7.2.×.10−4 Ga Spinel [90]n 3.1.×.1020 42 4.8.×.10−4 Ga,.In Spinel [91]


concentration.of.NaOH,.KOH,.or.LiOH.bases.as.mineralizers..The.high.doping.concen-tration.of.Group.I.ions.did.not.give.good.p-type.conductivity.and.the.ZnO.crystals.were.highly.resistive,.instead.they.lead.to.a.high.level.on.incorporation.of.Li.and.Na.ions.in.the.crystal.lattice.as.substitutional.and.interstitial.sites..While.substitutional.sites.are.accep-tors,.the.interstitial.sites.act.as.a.donor.[93].and.compensate.the.acceptor.contributions.

In. comparison. to. hydrothermal. growth. which. typically. employs. 1–3. M. of. NaOH/KOH.to.increase.the.solubility.to.ZnO,.low.temperature.aqueous.solution.methods.per-mit.the.concentration.of.Li,.Na,.or.K.ions.to.be.adjusted.independently.of.the.OH−.ions,.thus.enabling.a.lower.concentration.of. interstitial.dopants.to.be.achieved..Furthermore,.by.choosing.K.as.a.dopant.which.has.a.much.larger.ionic.radius.than.Zn,.the.interstitial.occupation.can.be.made.energetically.unfavorable.

Following.this.approach,.stable.p-type.doping.with.K.doping.has.been.demonstrated..Growth.was.carried.out.in.aqueous.solution.at.90°C..The.film.growth.strategy,.beginning.with.a.short.growth.at.pH.10–11,.achieves.good.growth.coverage.short.nanorods.on.the.entire.substrate..This.is.followed.by.a.series.of.slow.growth.cycles.at.pH.7.5.without.any.surfactants.to.coalesce.the.nanorods.to.form.the.bulk.of.the.film.with.lower.native.defect.density.and.thus.a.lower.intrinsic.background.electron.concentration.

The.highest.doping.concentration.of.3.8.×.1017.cm−3.was.obtained.with.0.07.M.KAc.in.the.as-grown.film..As.shown.in.Figure.5.13,.the.activation.of.intrinsic.hydrogen.defects.through.thermal.annealing.at.temperatures.higher.than.400°C.can.over-compensate.the.p-type.dop-ing.and.convert.the.film.to.n-type.with.an.electron.concentration.of.1.×.1019.cm−3..This.sharp.increase.in.electron.concentration.on.the.order.of.1019.cm−3.is.observed.even.in.undoped.films..By.extending.the.annealing.time.beyond.30.min.above.700°C.to.drive.out.the.hydro-gen. ions,. the.electron. concentration. can.be. reduced.and. the. p-type. conductivity. can.be.recovered..The.ability.to.drive.out.hydrogen.defects.by.annealing.underlines.the.impor-tance.of.the.low.growth.temperatures.employed.in.aqueous.solution.growth..Low.growth.temperatures.below.100°C.not.only.prevents.the.activation.of.hydrogen.defects.in.as-grown.

01014

1015

1016

1017

1018

1019

20 40Anneal time at 800°C (min)

Conc

entr

atio

n (c

m–3

)

Mob

ility

(cm

2 /V s

)

60 80

0.24 M KAc

1000.1

1

10

100

FIGURE 5.13Effect.of.duration.of.anneal.at.800°C.at.nitrogen.ambient.on.the.carrier.concentration.and.mobility.for.ZnO.films.with.0.24.M.KAc..Data.points.for.as-grown.samples.were.represented.at.0.min..The.electron.concentrations.and.mobilities.are.marked.by.“∙”.and.“×”.respectively,.while.the.hole.concentration.and.mobility.by.“○”.and.“+,”.respectively..The.as-grown.sample.is.p-type..After.annealing.for.10.min,.the.sample.reverts.to.n-type..Further.annealing.at.20.min.and.beyond,.p-type.conductivity.is.recovered..The.hole.concentration.decreases.a.little.for.longer.duration.of.annealing.. (Reproduced.with.permission.from.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,. J. Phys. Chem. C,.114,.9981..Copyright.2010,.American.Chemical.Society.)


samples,.but.it.also.reduces.the.stability.of.incorporated.hydrogen.defects.and.thus.makes.it.easier.to.remove.these.defects.through.annealing.at.temperatures.above.400°C.

In.Figure.5.14,.variable.PL.temperature.measurements.show.the.presence.of.two.acceptor.energy.levels.in.K-doped.ZnO.films.at.137.and.182.meV.above.the.valence.band.maximum.(VBM)..The.137.meV.energy.level.is.present.in.both.undoped.and.doped.ZnO.films.and.is.attributed.to.zinc.vacancies.(VZn).while.the.182.meV.energy.level.is.assigned.to.potassium.in.the.zinc.substitutional.sites.(KZn)..These.levels.have.been.identified.and.confirmed.with.temperature.dependent.Hall.measurements..Haynes.factors.of.0.095–1.were.obtained.for.all.undoped.and.K-doped.ZnO.samples.

5.11 Optoelectronic Applications of Aqueous Solution Grown ZnO

ZnO.is.a.multifunctional.material.with.many.applications..In.this.section.we.focus.on.the.optoelectronic.applications.of.aqueous.solution.grown.ZnO..Although.ZnO.is.very.simi-lar.to.GaN,.it.has.some.additional.advantages:.larger.exciton.binding.energy,.availability.of. large.single.crystals.of.ZnO,. lower.environmental. impact.and. toxicity..Djurisic.et.al..recently.reviewed.the.applications.of.ZnO.nanostructures.in.LEDs,.laser.diodes,.photo-detectors,.and.photovoltaic.cells.[58].while.Park.et.al..presented.a.more.in-depth.review.ZnO-based.LED.[85]..Here,.some.of.the.reported.applications.of.ZnO.grown.in.aqueous.solution.are.presented..The.recent.successful.reports.of.p-.and.n-type.doping.of.ZnO.have.been.accompanied.by. fabrication.of.heterojunction.LEDs..A.p-ZnO:K.on.n-GaN.hetero-junction.LED.was.demonstrated.to.have.p-n.junction.characteristics.with.a.turn.on.voltage.of.about.2.45.V.and.a.low.reverse.bias.current.on.the.order.of.10−5.A.at.a.reverse.bias.of.5.V.as.shown.in.Figure.5.15.[70]..The.electroluminescence.at.20.mA,.shown.in.Figure.5.16,.

Inte

nsity

(a. u

.)

3.20(a) (b)

3.25

CBM

VBM

90 meV

137 meV

182 meV

2-LO1-LO

3.40Eg - 69 meV (D1X)

Eg - 73 meV (A1X)

Eg - 60 meV (FX)

Eg - 90 meV (D1, H)

Eg - 182 meV (e, A2)

Eg - 137 meV (e, A1)

3.35

3.30

3.25

Peak

ener

gy (e

V)

3.20

3.150 100 200

Temperature (K)300

15X25X35X45X55X65X75X85X100X120X140X160X180X200X230X260X290X

A1

A1X

D1X

A2

D1

3.30 3.35Energy (eV)

3.40 3.45

0.07 M KAc, 800°C 30 min vacuum

FIGURE 5.14(a).Variable.temperature.photoluminescence.spectra.for.p-type.ZnO.films.doped.with.K.from.15.K.to.290.K,.with.the.inset.showing.the.defect.energy.levels.in.ZnO:K.derived.from.the.shift.in.(b).the.component.peak.positions.with.respect.to.temperature.


0.00–1 1 3 5–3–5 –0.01

0.01

0.020–10

–9–8–7–6–5–4–3–2–1

50.03

0.04

0.05

0.06

0.07

0.08

0.09

Voltage (V) Cu

rren

t (A

)

log

[Cur

rent

(A)]

Voltage (V)

Sapphiren-GaN

p-ZnO

In/ZnTi/AI/Ni/Au

FIGURE 5.15I–V.characteristic.of.the.p-ZnO/n-GaN.diode..The.inset.shows.the.I–V.plotted.in.logarithmic.scale.and.the.sche-matic.diagram.of.the.device..(Reproduced.with.permission.from.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.J. Phys. Chem. C,.114,.9981..Copyright.2010,.American.Chemical.Society.)

Wavelength (nm)360

0

100

200

300Inte

nsity

(a. u

.)

400

500

600

700

800

410 460 510

70 mA

60 mA

50 mA

40 mA

30 mA

20 mA

560 610 660

FIGURE 5.16Electroluminescence. spectra.measured.at. room.temperature. in.continuous.current.mode.with. forward.bias.currents. from. 20. to. 70. mA.. The. EL. spectra. are. similar. to. the. PL. spectra. of. the. p-ZnO. layer.. The. UV. emis-sion.originates.from.bound.excitons.while.the.broad.yellow-orange.peak.originates.from.deep.level.defects..(Reproduced.with.permission.from.Tay,.C.B.,.Chua,.S.J.,.and.Loh,.K.P.,.J. Phys. Chem. C,.114,.9981..Copyright.2010,.American.Chemical.Society.)


consists.of.a.UV.peak.at.372.nm.and.a.shoulder.at.378.nm.which.are.attributed.to.bound.excitons,.and.a.broad.yellow-orange.emission.from.the.deep.level.defects.

Similarly,.a.heterojunction.p-GaN/n-ZnO:Ga.has.been.fabricated.as.shown.in.the.inset.of.Figure.5.17.[94]..The.I–V.characteristics.and.electroluminescence.spectra.of.the.device.with.undoped.and.Ga-doped.n-ZnO.layer.are.also.shown..The.electroluminescence.spec-tra.under.a.forward.bias.injection.current.of.20.mA.shows.a.UV.component.centered.at.393.nm.and.a.visible.component.centered.at.530.nm.

The.observed.turn-on.voltages.for.ZnO/GaN.p-n.junctions.range.from.2.5.to.4.V,.deviat-ing.from.the.expected.range.of.about.3.3.V..This.suggests.that.the.interface.between.the.heterojunctions.and/or.the.contacts.play.an.important.role.in.the.device.characteristics.

The.use.of.ZnO.as.a.transparent.conductor.has.seen.widespread.application.of.aqueous.solution.method..This.is.mainly.driven.by.the.indium.supply.shortage,.resulting.in.rising.indium.prices..The.viability.of.ZnO.as.a.transparent.conductive.oxide.has.been.evaluated.favorably.against.ITO.

One.example.is.the.growth.of.epitaxial.ZnO.on.the.p-GaN.surface.of.a.blue.GaN-based.LED,. replacing. the. traditional. thin. Ni. (5. nm)/Au. (10. nm). film. which. serves. as. both. a.current.spreading.layer.as.well.as.a.transparent.window.[95]..At.an.injection.current.of.20.mA,.the.ZnO.TCO.layer.demonstrated.an.external.quantum.efficiency.improvement.of.1.93.times.over.the.thin.metal.electrode,.with.a.linear.increase.in.output.power,.indicating.minimal.heating.effects.

Another.similar.application.for.LED.is.the.use.of.aqueous.solution.grown-ZnO.nanorods.to.roughen.the.top.surface.of.LEDs.to.enhance.light.extraction.[96]..For.blue.InGaN-based.LEDs.with.a.peak.emission.wavelength.centered.at.455.nm,.the.optimal.rod.dimensions.for.the.diameter,.length,.and.peak-to-peak.spacing.between.adjacent.nanorods.are.175.nm,.250.nm,.and.2.5.μm,.respectively..These.values.are.in.agreement.with.optical.simulation.results.reported.by.Kim.et.al..[97]..When.subjected.to.a.post-growth.thermal.annealing.treatment.at.475°C.in.N2.ambient.for.15.min,.these.rods.improve.the.light.extraction.by.50%.based.on.the.light.output.power.measurements..Furthermore,.evident.from.Figure.5.18,.at.100.mA,.

Undoped

Undoped

Voltage (V) Energy (eV)

2.48 at %Ga

2.48 at %Ga

–10–20

(a) (b)

0

20

40n-contact(Ti/Au)

0 10 20 1.5 2.0 2.5 3.0 3.5

Curr

ent (

mA

)

Inte

nsity

(a. u

.)

ZnO:GaP-contact(Ni/Au)

FIGURE 5.17(a).I–V.characteristics.of.the.n-ZnO/p-GaN.LED.with.and.without.Ga.doping,.after.thermal.treatment.and.mea-sured.at.room.temperature..The.inset.shows.the.schematic.structure.and.the.microscopic.top.view.of.the.hetero-junction.LED..(b).EL.spectra.from.the.heterojunction.LED.with.and.without.Ga.doping,.with.the.inset.showing.the.white.emission.from.the.Ga-doped.LED.at.a.forward.bias.of.20.V..(Reproduced.with.permission.from.Springer.Science+Business.Media:.Appl. Phys. B.100,.2010,.705,.Le,.H.Q.,.Lim,.S.K.,.Goh,.G.K.L.,.Chua,.S.J.,.Ang,.N.S.S.,.and.Liu,.W.)


the.higher.thermal.conductivity.of.these.nanorods.(1.47.W/cm.K.for.ZnO.bulk.versus.1.3.W/cm..K.for.GaN.film).shifts.the.light.output.saturation.point.to.higher.injection.currents.

5.12 Conclusions

Although.progress.has.been.made.in.the.growth.of.ZnO.in.aqueous.solution,.much.more.needs.to.be.done.to.understand.growth.and.defect.mechanisms..In.particular.control.of.defects.and.the.quality.of.interfaces.are.critical.in.achieving.good.and.stable.p-type.doping.and.stable.long-term.operation.of.devices.

Both. n-. and. p-type. dopings. have. been. demonstrated. using. aqueous. solution. growth.method,.successfully.demonstrating.the.potential.of.aqueous.solution.growth.methods.in.fabricating.devices.such.as.LEDs.at.low.temperatures..The.next.step.will.be.the.growth.of.ZnO-based.alloys.such.as.CdZnO.and.MgZnO.for.device.fabrication.to.cover.the.UV.and.visible.wavelengths.

By.exploiting.the.advantages.of.low.cost,.wafer.scale.production.and.low.growth.tem-peratures,. aqueous. solution. grown. ZnO. is. emerging. as. a. promising. growth. route. for.various. device. applications,. as. has. been. demonstrated. in. transparent. conductive. oxide.applications.

Injection current (mA)40

0.2

0.4

Ligh

t-out

put p

ower

(a. u

.)

0.6

0.8

1.0

1.2

1.4(a)

(b)

1μm 1μm 1μm

1 h 2 h 4 h

50 60 70 80

As-fabricated LEDsZnO nanorods grown for1 h, d~150 nm, L~1.0 μm2 h, d~175 nm, L~2.5 μm4 h, d~225 nm, L~5.0 μm

90 100

FIGURE 5.18(a).SEM.image.showing.the.ZnO.nanorods.grown.on.the.exposed.stripe-like.p-GaN.region.with.alternating.Ni/Au.contact.for.1,.2,.and.4.h..(b).Light.output.power.versus.the.injection.current.for.blue.LEDs.with.ZnO.nanorods.grown.for.1,.2,.and.4.h.with.the.corresponding.estimated.diameter.and.length.provided.in.the.leg-end.of.the.plot..It.is.seen.that.the.best.light.extraction.is.obtained.for.ZnO.nanorods.with.lengths.of.2.5.μm.and.diameter.about.175.nm..(Reproduced.from.J. Cryst. Growth,.312,.Soh,.C.B.,.Tay,.C.B.,.Chua,.S.J.,.Le,.H.Q.,.Ang,.N.S.S.,.and.Teng,.J.H.,.1848..Copyright.2010,.with.permission.from.Elsevier.)


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141

6Second Harmonic Generation and Related Studies on ZnO Films

Maria Cristina Larciprete and Mario Bertolotti

6.1 Introduction

Second. harmonic. generation. (SHG). has. been. the. most. investigated. nonlinear. opti-cal.process,.since. its.discovery.by.Franken.in.the.1960s.[1].. In.SHG.processes.the.fre-quency.of.an.incoming.beam,.ω,.is.doubled.due.to.second-order.optical.susceptibility.χijk

( )2 . (−2ω,ω,ω).of. the.nonlinear.material..Within.the.electric-dipole.approximation,.the.third-rank.tensor.χijk

( ) ,2 .or.equivalently.the.�d. tensor.components,.dijk ijk= ( ) ( )1 2 2/ χ ,.present.nonvanishing.terms.only.if.the.material.has.a.non-centrosymmetric.crystal.structure,.that.is,.it.belongs.to.a.group.symmetry.without.center.of.inversion,.thus.giving.rise.to.the. so-called. bulk. or. electric dipole induced. SHG.. Several. nonlinear. optical. techniques.have.been.developed,.in.order.to.allow.the.different.components.of.the.third-rank.ten-sor.χijk

( )2 .to.be.determined.with.reference.to.a.well-characterized.sample,.which.is.usually.α-quartz,.KDP,.or.BBO.

ZnO.bulk.crystal.structure,.that.is,.wurtzite,.belongs.to.the.noncentrosymmetric.point.group.symmetry.6mm.with.a.hexagonal.primary.cell..The.c-axis.ZnO.(0002).presents.five.nonvanishing.second.order.susceptibility.tensor.elements.χ χ χ χ χ χ113

21312

2232

2322

3112

3222( ) ( ) ( ) ( ) ( ) ( )= =; ; ; .

and. χ3332( ) . [2].. The. values. of. the. nonlinear. optical. coefficients. are. typically. measured.

in. pm/V. and,. as. a. general. rule,. they. depend. on. the. incoming. frequency. via. a. dis-persion. relation.. According. to. the. piezoelectric. contraction,. which. is. an. abbreviated.notation.usually.adopted.to.replace.the.last.two.subscripts.of.χijk

( )2 .by.a.single.subscript.running.from.1.to.6.(xx.=.1,.yy.=.2,.zz.=.3,.yz.=.zy.=.4,.xz.=.zx.=.5,.xy.=.yx.=.6),.they.become.

CONTENTS

6.1. Introduction......................................................................................................................... 1416.2. Measurements.of.the.Tensor.Components...................................................................... 1426.3. Thin.Films............................................................................................................................ 1476.4. Effect.of.the.Crystalline.Structure.................................................................................... 1486.5. Effect.of.Thickness.............................................................................................................. 1526.6. Surface.Contributions........................................................................................................ 1546.7. Doping.................................................................................................................................. 1586.8. Third.Harmonic.Generation............................................................................................. 1596.9. Conclusions.......................................................................................................................... 160References...................................................................................................................................... 162


χ χ χ χ152

242

312

322( ) ( ) ( ) ( ); ; ; .and.χ33

2( ),.or.equivalently.d15;.d24;.d31;.d32.and.d33..The.tensor.�d.is.therefore.written.as.follows:

.

�d

d

d

d d d

=

0 0 0 0 00 0 0 0 0

0 0 0

15

24

31 32 33

. (6.1)

Furthermore,.being.the.point.group.symmetry.6mm.invariant.for.rotation.about.the.z-axis,.the.number.of.independent.nonvanishing.coefficients.is.reduced.to.three,.namely,.d15.=.d24;.d31.=.d32,.and.d33:

.

�d

d

d

d d d

=

0 0 0 0 00 0 0 0 0

0 0 0

15

15

31 31 33

. (6.2)

A.possible.simplification,.in.the.spectral.ranges.far.from.absorption.resonances,.may.be.obtained.by.applying.to.the.crystal.Kleinman’s.symmetry.rules.[3].to.further.reduce.the.number.of. independent. components..The.perfect.wurtzite. crystalline. structure,. in. fact,.presents. internal. relations. between. structural. parameters,. and. to. a. first. approximation.allows.the.assumption.d31.=.d15.and.|d33|.=.2d31..As.a.consequence,.when.the.sample.is.sup-posed.to.be.highly.crystalline,.it.is.possible.to.limit.the.investigation.only.to.the.measure-ment.of.the.largest.component,.that.is,.d33..On.the.other.hand,.when.the.film.crystalline.quality.or.the.spectral.conditions.do.not.allow.this.assumption,.it.is.necessary.to.charac-terize.the.different.nonlinear.optical.tensor.elements.independently.

6.2 Measurements of the Tensor Components

Several.experimental.methods.have.been.developed,.for.the.determination.of.values.(and.signs).of.all.nonzero.components.of.the.nonlinear.susceptibility.tensor.in.crystals..Since.its.introduction,.the.Maker.fringes.technique.[4],.which.is.based.on.the.investigation.of.oscillations.of. the.SH. intensity.by.changing. the.crystal. thickness,.has.been.one.of. the.most.used.

Briefly,.this.technique.consists.in.measuring.the.SH.signal.transmitted.through.the.crys-tal.as.a.function.of.the.angle.of.incidence.of.the.fundamental.beam,.which.can.be.varied.by.placing.the.sample.onto.a.rotation.stage..The.polarization.states.of.fundamental.and.gen-erated.beams.are.selected.by.rotating.a.half-wave.plate.(polarizer).and.a.linear.polarizer.(analyzer),.placed.before.and.after.the.sample,.respectively..Usually,.in.order.to.minimize.the.influence.of.laser.energy.fluctuations,.a.fraction.of.the.fundamental.beam.is.split.to.a.reference.line.of.the.setup,.where.the.SH.signal.from.a.reference.crystal,.that.is,.a.quartz.plate,.is.simultaneously.measured.at.a.fixed.incidence.angle..On.the.measurement.line.the.second.harmonic.beam.is.detected.with.a.photomultiplier,.using.interference.filters.tuned.at.the.SH.wavelength.and.removing.the.residual.part.of.the.fundamental.beam.by.means.of.dichroic.filters.


A.full.expression.of.the.SH.power.W2ω,.as.a.function.of.the.incidence.angle,.α,.is.given.by.the.following.[5]:

. WA

t T Wn n

2

3

22

2512 sinω ω ω ω

ω ω

α π α αΨ α

α( ) =

⋅ ( )⋅ ( )⋅ ⋅( )( )

( ) −4

2222

2

αα

( ) ⋅ ( ) 2 effd . (6.3)

whereA.is.the.fundamental.beam.transverse.area.(here.assumed.to.be.uniform)Wω.represents.the.power.of.the.incident.fundamental.beamtω.is.the.fundamental.field.transmission.coefficientT2ω.is.the.SH.power.transmission.coefficient

Taking.into.account.material.dispersion,.nω.and.n2ω.are.the.refractive.indices.at.the.funda-mental.and.generated.frequency,.respectively..Finally,.Ψ(α).is.a.phase.factor.given.by.the.following:

.Ψ =

⋅

⋅ ⋅ ′ − ⋅ ′( )π

λα α α αω ω ω ω

Ln n

24

2 2( ) cos( ) ( ) cos( ) . (6.4)

whereL.is.sample.thicknessαω

’ .and.α ω2’ .are.the.internal.refraction.angles.corresponding.to.ω.and.2ω

The.effective.optical.nonlinearity,.deff(α),.is.determined.by.the.�d.tensor.components,.as.well.as.by.the.optical.axis.orientation.with.respect.to.the.sample.surface.and.the.fundamental.beam.characteristics..The.general.expression.for.deff(α).is.rather.complicated.being.depen-dent.on.the.polarization.state.of.both.first.and.second.harmonic.electric.fields.respectively.and,.of.course,.on.the.fundamental.beam.incidence.angle,.α..However,.it.can.be.signifi-cantly.simplified.for.particular.crystalline.symmetries..In.what.follows,.we.will.use.indif-ferently.dij.or.χij

( )2 .to.mention.the.nonlinear.optical.coefficients.Depending.on.the.polarization.state.of.both.fundamental.and.generated.beams,.it.is.pos-

sible.to.address.the.different.nonlinear.optical.tensor.components.separately.and.to.retrieve.their.absolute.values..As.shown.in.Figure.6.1,.the.p.polarization.corresponds.to.the.electric.field.oscillating.within.the.plane.of.incidence,.while.the.s.polarization.is.perpendicular.to.

s

x

α

z

y

p

FIGURE 6.1Sketch. of. the. sample. in. the. Maker. fringes. experimental. configuration.. Thick. arrows. identify. the. different.polarization.state.for.the.incident.beam.(⇢).


the.plane.of.incidence..Of.course,.in.general,.the.polarization.angle,.ϕ,.defined.with.respect.to.the.incidence.plane,.may.assume.all.values.ranging.between.0°.(p.polarization).and.90°.(s.polarization)..Considering.the.ZnO.group.symmetry,.the.final.expressions.for.deff(α).for.three.different.polarization.configurations.are.as.follows:

.

d d

d d

d d

s p

p

p p

eff

eff

eff

ω ω

ω ω

ω ω

α

α

α

ω

ω

ω

2

2

2

31 2

4515

15 2

=

=

=

sin

sin

cos siin cos sin sin sin2 312

2 332

2α α α α αω ω ω ω ω+ +d d

. (6.5)

where.the.apices.sω-p2ω,.45ω-p2ω.and.pω-p2ω.refer.to.three.different.polarization.combina-tions.of.fundamental.and.generated.beams..Equations.6.5.show.that,.from.the.experimen-tal.point.of.view,.the.term.d31.can.be.independently.evaluated.from.the.measurement.of.the.p-polarized.SH.signal.for.a.s-polarized.fundamental.beam;.d15.from.data.corresponding.to.p-polarized.SH.signal.for.a.45°-linearly.polarized.fundamental.beam..Finally,.the.largest.component,.d33,.is.addressed.when.both.SH.and.fundamental.beams.are.p-polarized..It.is.worth.noting.that.the.expression.of.deff. in.the.latter.case.also.includes.a.dependence.on.d31.and.d15.which.should.be.determined.separately.

If.Kleinman’s.symmetry.rules.can.be.applied,.by.fitting.only.the.pω-p2ω.measurements,.which.also.give.the.strongest.signal,.one.may.retrieve.the.value.of.d33..In.this.case,.in.fact,.it.is

. d d dp peff

ω ω α α α α αω ω ω ω ω2 1

22

1233 2 33 2

2 2= − + −

cos sin sin sin cos . (6.6)

Some.authors.therefore.limited.their.investigation.to.the.measurements.of.the.largest.com-ponent,.that.is,.d33.

Nevertheless,.in.some.circumstances.Kleinman’s.symmetry.is.broken.and.the.value.of.the.ratio.d33/d31.can.deviate.significantly.from.the.ideal.value,.that.is,.close.to.an.absorp-tion.resonance.for.instance..Thus.an.independent.evaluation.of.the.three.components,.or.at.least.two.of.them,.allows.obtaining.more.information.on.the.crystallites.properties.

In.Figure.6.2. the.Maker. fringe.patterns.measured. from.a.550.nm.thick.ZnO.film.are.shown..The.angularly.resolved.SHG.signals.arising.from.the.c-axis.oriented.ZnO.films.go.to.zero.at.normal.incidence.and.are.p-polarized.for.both.p-polarized.and.s-polarized.fundamental.beam..As.evidenced.in.the.experimental.curves,.p.polarization.of.the.funda-mental.beam.generally.results.in.higher.conversion.efficiency.

Alternatively.to.Maker.fringes.method,.the.reflective.second.harmonic.generation.(RSHG).scheme.involves.the.detection.of.the.SH.signal.at.a.fixed.incidence.angle.as.a.function.of.the.azimuthal.angle,.which.is.the.angle.between.the.incidence.plane.and.the.x.axis.on.the.sample.surface,.as.shown.in.Figure.6.3..Thus.in.this.experimental.configuration.the.sample.is.rotated.along.its.surface.normal,.that.is,.along.the.z-axis,.and.the.reflected.SH.signal.is.measured.

The.p-polarized.[6].and.s-polarized.[7].SH.signals.originating.from.the.group.symmetry.6mm.as.a.function.of.the.azimuthal.angle.ϕ.are,.respectively,.given.by.the.following:

.

P d E x d d E z

P d E z

p p

s p

bulk

bulk

ω ω

ω ω

2

2

15 02

31 33 02

31 02

12

= + +( )

=

ˆ ˆ

ˆ . (6.7)


. Pp sbulk

ω ω2 0=

. Ps sbulk

ω ω2 0=

First.of. all,.Equation.6.7. rules.out. the.dependence.of.p-polarized.SH.signal.on. the.azi-muthal.angle.if.the.direction.of.ZnO.(0002).is.the.z-axis.of.the.film.[6]..As.a.consequence,.apart.from.tilt.of.the.film.optical.axis,.the.pω-p2ω.SH.curves.are.centered.on.the.polar.(radar).diagram,.as.shown.in.Figure.6.4.[6].

Incidence angle (deg)

–500

0.2SHG

sign

al (a

. u.)

0.4

0.6

–40 –30 –20 –10 0 10 20 30 40 500

0.2

0.4

0.6

FIGURE 6.2Second. harmonic. intensity. as. a. function. of. the. fundamental. beam. incidence. angle,. measured. in. the.Maker.fringes.scheme.from.a.550.nm.thick.ZnO.film..Fundamental.beam,.tuned.at.1064.nm,.is.p-polarized.(closed.circles).and.s-polarized.(open.circles),.respectively..The.solid.curves.correspond.to.theoretical.fitting.curves.

s

z

p

xy

ϕ

FIGURE 6.3Sketch.of.the.sample.in.the.RSHG.experimental.configuration..Thick.arrows.identify.the.different.polarization.state.for.the.incident.beam.


Furthermore,. it. is. interesting. to. observe. that. for. the. group. symmetry. 6mm,. the. cor-responding.nonlinear.susceptibility.tensor.does.not.allow.bulk.electric.dipole.contribute.to.the.RSHG.in.the.sω-s2ω.configuration.[7]..Being.the.total.generated.SH.signal.composed.of.both.bulk.and.surface.contributions,. it. is. thus.possible.to.isolate.only.surface.related.SH. signal. via. RSHG. technique,. when. both. the. fundamental. and. generated. beams. are.s-polarized..In.fact,.the.occurrence.of.a.non-zero.signal.in.the.sω.s2ω.configuration.is.used.to.probe.the.nonvanishing.polarity.induced.by.surface.effects.

Recently,.a.noncollinear.scheme.was.successfully.employed.to.observe.SHG.from.ZnO.films.[8]..In.this.experimental.configuration,.the.fundamental.beam.is.split.into.two.beams.of.comparable.power,.while.the.temporal.overlap.of.the.incident.pulses.is.controlled.with.an.external.delay.line..The.polarization.of.both.fundamental.beams.is.systematically.var-ied.with.two.identical.rotating.half-wave.plates..The.sample.is.placed.onto.a.rotation.stage,.and.a.fixed.incidence.angle.is.opportunely.chosen..After.passing.through.two.collimating.

300270

24020

25(b)

120

90

60

30

0

330

300270

240

210

180

ZnO

tem

pera

ture

300

pp 150

25

20

15

10

5

0

5

10

15

20

25(c)

300270

240(a)

120

90

60

30

0

330210

180

ZnO

tem

pera

ture

200

pp 150

25

20

15

10

5

0

5

10

15

120

90

60

30

0

330210

180

ZnO

tem

pera

ture

100

pp 150

25

20

15

10

5

0

5

10

15

20

25

FIGURE 6.4RSHG.pattern.arising.from.ZnO.thin.film,.1.6.μm.thick,.at.the.conditions.of.Si.(111).substrate.temperature.of.(a).100°C,.(b).200°C,.and.(c).300°C..Both.fundamental.and.generated.beams.are.p-polarized..(Reprinted.from.J. Cryst. Growth,.290,.Lo,.K.-Y.,.Lo,.S.-C.,.Chu,.S.-Y.,.Chang,.R.-C.,.and.Yu,.C.-F.,.Analysis.of.the.growth.of.RF.sput-tered.ZnO.thin.films.using.the.optical.reflective.second.harmonic.generation,.532–538..Copyright.2006,.with.permission.from.Elsevier.)


lenses,. the.pump.beams.are.sent.to.intersect. in.the.focus.region,.forming.the.incidence.angles.α1.and.α2.with.respect.to.the.sample.normal,.as.pointed.out.in.Figure.6.5..The.inter-action.of.two.incident.beams.linearly.polarized,.tuned.at.ω1.and.ω2.(or.as.in.this.case.at.ω1.=.ω2.=.ω).with.a.noncentrosymmetric.material,.produces.a.nonlinear.polarization.oscil-lating.at.the.frequency.ω1.+.ω2.(or.2ω)..According.to.the.wave.vectors’.conservation.law,.the.generated.beam.is.emitted.nearly.along.the.bisector.of.the.aperture.angle.between.the.two.pump.beams..This.beam,.namely,. the.noncollinear.SH.signal,. is.collected.as.a. function.of.the.polarization.states.of.both.fundamental.beams,.while.an.analyzer.allows.selecting.the.desired.SH.polarization.state..The.resulting.“polarization.chart”.is.a.map.of.the.SH.signal.as.a.function.of.the.polarization.states.of.both.pump.beams,.and.it.is.characteristic.for. any. different. symmetry. structure.. The. investigation. of. this. chart. allows. retrieving.the.values.of. the.nonlinear.optical. coefficients.and,.as. shown. from.the. investigation.of.ion-beam-sputtered.ZnO.films,.it.is.also.a.useful.tool.to.control.the.orientation.of.the.opti-cal.axis.and.its.angular.tilt.with.respect.to.the.surface.normal.[8].

6.3 Thin Films

The.possibility.to.get.a.nonlinear.optical.response.from.thin.films.is.particularly.attractive,.especially.since.they.can.be.easily.used.for.integrated.nonlinear.optical.devices..Second-order. nonlinear. optical. response. has. been. obtained. from. ZnO. films. grown. by. pulsed.laser.ablation. [9],. spray.pyrolysis. [10,11],. laser.deposition. [12],. reactive.sputtering. [13].as.well. as. self-assembling. by. Laser-MBE. [14,15],. metalorganic. chemical. vapor. deposition.[16],.dual.ion.beam.sputtering.[17,18],.rf.magnetron.sputtering.[19–22],.metalorganic.aero-sol.deposition.[23],.gas.transport.reaction.technique.[24],.and.plasma-assisted.molecular.beam. epitaxy. [25].. Third. harmonic. generation. (THG). as. well. was. recently. observed. in.ZnO.nanocrystalline.films.[18,20,26,27].

It.must.be.pointed.out.that.the.generation.in.a.bulk.homogeneous.crystal.may.be.easily.treated.considering.the.two.separate.contributions.from.the.bulk.and.the.surface,.while.in.the.case.of.thin.films.the.SHG.process.is.much.more.complicate.

2ω

ω

ω α1α2

n

FIGURE 6.5Details.of.the.experimental.geometry.used.for.noncollinear.SHG.[8]..The.two.pump.beams.impinge.onto.the.sample.with.angles.α1.and.α2..The.noncollinear.SH.beam.is.directed.along.the.bisector.(dotted.line).


Thin.films.are.usually.in.a.polycrystalline.state.which.is.characterized.by.a.distribution.of.grains,.that.is,.spatial.domains.having.the.same.crystal.orientation..In.some.cases.the.orientation.distribution.within.the.grains.results.in.a.mean.direction.of.the.optical.axis,.thus.affecting.the.nonlinear.optical.tensor.components..The.size.of.the.grains.also.play.an.important.role.because.one.has.to.take.into.account.the.SH.production.due.to.grain.boundaries.or.to.defects.such.as.stacking.faults..This.situation.is.not.always.easily.predict-able.and.is.strongly.dependent.on.the.growing.method,.while.the.type.of.the.substrate.as.well.may.influence.the.second.harmonic.production..On.the.other.hand,.following.these.considerations,.second.harmonic.measurements.can.be.a.useful.tool.to.indirectly.assess.the.crystalline.quality.of.a.given.thin.film.

Besides.nonlinear.optical.characterization,.other.complementary.methods,.as.for.instance.the.x-ray.diffraction.(XRD).patterns,.scanning.electron.microscopy.(SEM),.or.atomic.force.microscopy. (AFM),. allow. characterizing. the. distribution. of. the. grain. size.. Within. this.frame,.a.comparison.with.the.SH.experimental.results.allows.a.better.understanding.of.the. mechanisms. behind. SHG.. By. comparison. with. other. conventional. characterization.methods,.in.fact,.nonlinear.optical.measurements.can.also.be.employed.as.a.suitable.and.sensitive.method.for.testing.the.textures.of.ZnO.films.fabricated.by.different.techniques.and.conditions.of.growth.[7].

Several.authors.investigated.how.the.systematical.change.of.a.given.parameter.during.film.growth.influences.the.resulting.nonlinear.optical.properties..Concerning.for.instance.the. dual. ion. beam. sputtering. technique,. the. influence. of. the. oxygen. content. over. the.second-order.nonlinear.optical.properties.has.been.studied.[17]..With.reference.to.mag-netron.sputtering. technique. the.effect.of.substrate. temperature.was.studied.and. it.was.shown.[7,19].that.the.second.harmonic.signal.measured.in.reflection.decreases.as.the.tem-perature.of.the.substrate.during.deposition.increases..The.influence.of.rf.power.on.SHG.from.ZnO.films.deposited.by.rf.sputtering.was.also.addressed.[22].

In. addition. to. the. different. deposition. techniques,. also. a. large. variety. of. materials.was.successfully.employed.as. substrates..The.most. commonly.used.are. sapphire. [8,13–16,23–25].and.glass.[10,11,13,17,18],.since.they.do.not.give.bulk.contribution.to.the.second.harmonic.signal..The.possibility,.explored.in.some.works,.to.get.second-order.nonlinear.optical.response.from.ZnO.films.grown.on.silicon.substrate.is.particularly.attractive.since.it. is. expected. to.offer. the. further. advantage. to. integrate. ZnO.with. the.Si-based. micro-electronics. devices. [7,19,21,22].. Finally,. considering. the. use. of. crystals. substrates,. as. for.instance.α-BBO.or.LiNbO3.[20],.it.must.be.pointed.out.that.the.use.of.noncentrosymmetric.structures.has.to.be.carefully.checked.because.they.may.also.produce.SH.and.therefore.can.significantly.influence.the.SHG.measurements.

6.4 Effect of the Crystalline Structure

In.the.works.of.different.authors,.SHG.has.been.related.to.several.structural.parameters.such.as.grain.size.and.shape.[11],.film.thickness.[11,13,15],.crystalline.structure.[9,11,14],.and.orien-tation.of.the.crystallites.[23]..Beyond.the.bulk.electric-dipole.contribution,.arising.from.the.non-zero.terms.of.the.nonlinear.optical.tensor,.scientists.agree.that.a.significant.part.of.the.SHG.signal.may.be.generated.also.at.grain.boundaries.and.interfaces..These.contributions.allow. explaining. the. strong. enhancement. of. the. overall. nonlinear. susceptibility,. experi-enced.in.some.experimental.works,.for.very.thin.films.with.respect.to.the.bulk.values.[14,15].


In. polycrystalline. films. both. electronic. and. optical. properties,. including. SHG,. can.be. affected. by. the. structure. of. grain. boundaries.. First. of. all,. boundaries. represent. a.break. in. the.crystalline.structure,. that. is,.a.break. in. the.symmetry.that. itself.can.be.a.source.of.SHG..Furthermore,.as.the.crystalline.order.near.the.grain.boundaries.encloses.the. different. neighboring. grains,. it. is. likely. that. grain. boundaries. represent. a. way. to.increase.SHG.

In.the.study.of.SHG.from.ZnO.films,.the.effect.of.grain.boundaries.emerged.from.the.experimental.evidence.that.film.with.lower.crystallinity.may.show.larger.second-order.nonlinear.optical.response,.with.respect.to.film.with.higher.crystallinity..This.effect.was.observed.for.the.first.time.by.Cao.et.al..[9].in.both.thin.(45.nm).and.thick.(235.nm).ZnO.films.deposited.by.pulsed.laser.ablation..SH.measurements.in.the.Maker.fringes.configu-ration. were. performed. from. two. couples. of. films. having. same. thickness. but. different.crystallinity. (verified. by. XRD),. and. the. absolute. values. of. the. nonlinear. optical. coeffi-cients.were.retrieved..The.experimental.SH.curves,.together.with.the.XRD.results.show.that.samples.with.lower.crystallinity.exhibit.more.efficient.SH.signals,.thus.evidencing.that.the.SH.signal.is.generated.not.only.in.the.crystallites.but.also.at.grain.boundaries..This.result.obtained.from.the.thicker.films.is.shown.in.Figure.6.6,.being.consistent.for.both.45.and.235.nm.thick.films..Furthermore,. it.was. found.that. the. thicker.films.have.lower. nonlinear. optical. coefficients.. Thus,. the. authors. infer. that. part. of. the. SH. signal.arises.from.interfaces.

The.results.obtained.in.this.work.represented.the.beginning.of.a.deeper,.more.compre-hensive,.investigation.of.the.mechanism.of.SHG.from.ZnO.films.

Incident angle (deg)

–600

20

40

SHG

inte

nsity

(a.u

.) 60

80

100

–40 –20 0 20 40 60

FIGURE 6.6Measured.SHG.intensities.from.two.ZnO.films,.235.nm.thick,.as.a.function.of.the.incident.angle.of.the.funda-mental.beam,.when.the.fundamental.beam.is.p-polarized..The.FWHM.of.XRD.patterns.of.those.two.films.are.0.1º.(crosses).and.0.26º.(closed.circles),.respectively..(Reprinted.with.permission.from.Cao,.H.,.Wu,.J.Y.,.Ong,.H.C.,.Dai,.J.Y.,.and.Change,.R.P.H.,.Second.harmonic.generation.in.laser.ablated.zinc.oxide.thin.films,.Appl. Phys. Lett., 73,.572–574,.1998..Copyright.1998,.American.Institute.of.Physics.)


In.order.to.put.in.evidence.a.correlation.between.SH.efficiency.and.the.grain.shape.in.polycrystalline.ZnO.films,.Neuman.et.al..[11].systematically.probed.a.set.of.70.specimens,.ranging.from.0.1.to.1.μm.in.thickness,.prepared.by.spray.pyrolysis.technique..SHG.was.investigated.as.a.function.of.both.thickness.and.grain.size,.while.a.140.μm.thick.ZnO.bulk.crystal.was.used. for.comparison..The.nonlinear. susceptibility. tensor.components.were.determined.from.the.angular.dependence.of. the.SH.signal,. that. is,.by.using.the.Maker.fringes.scheme..At.the.same.time,.SEM.images.allowed.to.perform.the.grain.size.statistics.of.sample.surfaces..Despite.the.high.number.of.samples.investigated.in.this.accurate.work,.neither.a.clear.dependence.on.thickness.nor.a.correlation.with.grain.size.emerged.from.experimental.results,.as.shown.in.Figure.6.7.

In.order.to.find.a.correlation.between.SH.and.structural.parameters,.interestingly,.the.ZnO.crystallites.were.then.treated.as.columns.of.height.h.and.square.cross.section.di2.and.a.new.structural.parameter.was.introduced.to.take.into.account.the.columns.height.as.well.as.the.aspect.ratio.of.the.ZnO.such.crystallite.columns:

.

σ =+( )∑

∑2 42

2

d d h

d

i ii

ii

. (6.8)

By.classifying.the.investigated.samples.with.the.given.parameter,.a.prevalence.of.a.par-ticular.aspect.ratio.which.may.imply.surface-related.effects.comes.out.from.experimental.data.. On. the. other. hand,. the. authors. admit. that. this. effect. can. be. merely. the. result. of.a.coincidence.of.growth.parameters.causing.both.a.particular.aspect. ratio.and.efficient.SHG.. More. significantly,. the. structure. of. the. nonlinear. susceptibility. tensor,. χ(2),. which.was.reconstructed.from.the.angular.dependence.of.SH.signals,.does.not.include.new.ten-sor.elements.that.can.be.typically.associated.to.surface.effects..Thus,.from.experimental.evidence,.the.authors.finally.conclude.that.the.films.showed.high.SHG.and.provided.an.even.more.efficient.SHG.than.the.bulk.crystal,.but.still.bulk.effects.govern.the.SH.intensity.in.films.some.hundreds.of.nanometers.thick.[11].

Grain. boundaries. effects. on. SHG. are. evidenced. in. Ref.. [7],. where. the. influence. of.substrate. temperature. (100°C–300°C). on. the. film. structural. and. optical. properties. is.

Film thickness (µm)0

(b)(a)

0 0

1 1

0.5 1.0Lc 0.1 0.2

SHG

sign

al (a

. u.)

SHG

sign

al (a

. u.)

0.3Grain size (µm)

FIGURE 6.7Relative.strength.of.SHG.signals.versus.(a).film.thickness.and.(b).average.crystallite.grain.size.at.43°.incident.angle..Data.for.pure.ZnO.films.(triangles).and.In-doped.films.(squares).are.shown..Filled.symbols.refer.to.crys-tallite.samples;.hollow.symbols.to.amorphous.ones..The.dashed.line.in.(a).shows.the.expected.dependence.of.signal.versus.film.thickness.as.estimated.from.ZnO.refractive.index.data..The.vertical.line.indicates.the.coher-ence.length.LC..(Reprinted.with.permission.from.Neumann,.U.,.Grunwaid,.R.,.Griebner,.U.,.Steinmeyer,.G.,.and.Seeber,.W.,.Second-harmonic.efficiency.of.ZnO.nanolayers,.Appl. Phys. Lett.,.84,.170–172,.2004..Copyright.2004,.American.Institute.of.Physics.)


investigated..ZnO.films,.1.06.μm.thick,.with.c-axis.orientation.were.grown.by.rf.magne-tron.sputtering.on.Si.(111).substrate..RSHG,.XRD,.and.SEM.have.been.used.to.analyze.the.quality.of.the.crystallite.structure..The.experimental.curves.obtained.for.the.pω-p2ω.signal.via.360°.azimuthal.scanning.show.that.the.RSHG.signal.intensity.from.ZnO.films,.cor-responding.to.the.curve.radius,.decreases.as.substrate.temperature.is.increased,.as.shown.in.the.previously.reported.Figure.6.4..Furthermore,.a.slight.misalignment.of.experimen-tal.SH.curves.from.the.center.of.the.polar.diagram.is.evidenced,.indicating.that.the.z-axis.of.the.ZnO.crystal.is.not.perfectly.normal.to.the.film.surface.but.is.slightly.tilted.

Considering.the.crystalline.point.of.view,.the.sample.showing.the.highest.SH.efficiency.(substrate.temperature.100°C). is.not.the.one.with.the.best.crystalline.structure,.as. indi-cated.by.the.XRD.analysis.testing.the.quality.of.the.layer’s.bulk..Nevertheless,.the.total.SH.signal.is.determined.by.both.contributes.from.the.film.surface.and.the.underlying.bulk.layer,.that.is,.the.surface.morphologies.and.the.quality.of.the.bulk.are.both.determinant.to.the.analysis.of.SH.results..The.distribution.of.grain.size,.retrieved.from.SEM.images,.indicates. that. the.grain.size. increases.with. increasing.substrate. temperature..When.the.experimental.results.are.compared.with.the.characteristic.grain.size.parameters,.the.cor-relation.between.the.RSHG.intensity.and.the.substrate.temperature.reveals.that.the.effect.of.the.grain.boundaries.is.the.dominant.part.of.the.RSHG.mechanism.

In.the.work.of.Liu.et.al..[16],.several.350.nm.thick.ZnO.films.grown.by.metalorganic.chemical.vapor.deposition.(MOCVD).on.sapphire.substrates.were.investigated..The.SHG.was.measured.in.the.Maker.fringes.configuration.[4,5].from.films.deposited.at.different.temperature,.ranging.from.200°C.to.500°C..Among.the.different.possible.growth.condi-tions,. in. fact,. temperature. is.one.of. the.most. important.parameters. that.determine. the.quality.of.epitaxial.film,.and.thus,.it.affects.the.resulting.SHG..The.experimental.inves-tigation.evidenced. the. strongest.SHG. intensity. corresponding. to. the. lowest. crystalline.quality,.for.deposition.temperature.250°C..This.enhancement.is.ascribed.both.to.the.poor.crystallinity.and.to.film.defects..The.explanation.of.experimental.results.in.terms.of.film.defects,.given.in.Ref..[16],.is.the.following..In.the.ideal.condition,.the.unit.cells.possess.the.same.in-plane.orientations.within.the.whole.film,.and.one.unit.cell.is.connected.with.the.neighboring.by.similar.atomic.bounds,. that. is,. these.bounds.are. in. their.“equilibrium”.positions..When.the.in-plane.orientations.of.two.adjacent.unit.cells.are.different,.as.in.the.real.films,.the.atomic.bounds.among.them.will.deviate.from.their.equilibrium.positions..This.may.favor.the.formation.of.dangling.bonds.at.the.interfaces.of.different.unit.cells,.which.in.turn.results.in.extra.carriers.or.film.defects..In.fact,. it.has.been.demonstrated.that.an.increase.in.carrier.density.and.defects.can.enhance.[28].the.optical.nonlinearity.in.crystals.

The. use. of. different. substrates. also. affects. the. crystalline. structures.. In. Ref.. [20],. for.instance,.ZnO.thin.films.were.deposited.by.rf.magnetron.sputtering.on.two.different.crys-tals,. α-BBO. (0012). and. LiNbO3. (0001),. respectively.. While. XRD. patterns. reveal. that. the.grain.size.of.the.ZnO.film.is.not.dependent.on.the.type.of.substrate,.the.linear.transmit-tance.spectra.indicate.that.the.film.deposited.on.lithium.niobate.possesses.better.struc-tural.quality.and.higher.degree.of. crystallinity..The.SHG.measurements,.performed. in.the.Maker.fringes.scheme,.provide.for.the.effective.values.of.the.nonlinear.optical.coef-ficients,.χeff

( )2 ,.extremely.different.values.for.the.ZnO.grown.on.the.two.types.of.substrate,.respectively..The.low.value.obtained.for.the.α-BBO.substrate,.χeff pm/V( ) .2 0 23= ,.is.entirely.ascribed.to.the.ZnO.film,.being.the.α-BBO,.a.centrosymmetric.crystal..The.much.higher.value.obtained.for.the.LiNbO3.substrate,. χeff pm/V( ) .2 12 9= ,.is.ascribed.to.the.little.lattice.misfit. between. ZnO. and. LiNbO3. (−8.44%). which. is. even. lower. than. for. ZnO/Sapphire.(−15.53%).[29]..Moreover,.it.must.be.pointed.out.that.the.Maker.fringes.curve.presented.[20].


for.the.latter.case.may.also.include.the.contribution.of.the.LiNbO3.substrate.[30],.which.is.a.good.nonlinear.material.and.may.itself.significantly.contribute.to.the.SHG.

The.important.role.of.a.high.density.of.interfaces.between.the.grains.is.also.addressed.in.Ref.. [24],.where.a.ZnO.film.with.110.nm.thickness.was.fabricated.on.(0001).sapphire.substrate.by.the.modified.chemical.vapor.deposition.method.and.its.SHG.response.was.compared.to.a.1.mm.thick.ZnO.bulk.crystal..The.SH.signal.is.measured.as.a.function.of.the.sample.rotation.azimuthal.angle.and.the.ratios.of.the.nonlinear.optical.components.r1.=.d31/d33.(or.d32/d33).and.r2.=.d24/d33.(or.d15/d33).are.retrieved..From.the.retrieved.ratios.val-ues,.the.theoretical.model.[24].allows.evaluation.of.the.sign.of.d33.and.to.find.indirectly.that.the.components.d33.and.d31.are.enhanced.in.the.film.with.respect.to.the.single.crystal,.thus.confirming.the.significant.contribution.of.grain.boundaries.on.SHG.efficiency.

As. well. as. for. the. linear. optical. susceptibility,. the. second-order. susceptibility. is. also.subject. to. the.effect.of.dispersion,. that. is,. the.values.of. the.nonlinear.optical.coefficients.are.frequency.dependent..Thus,.a.more.comprehensive.investigation.of.a.given.crystalline.structure.should.also.include.the.wavelength.dependence.of.the.second-order.susceptibil-ity..For.instance,.in.Ref..[18].SHG.measurements.were.carried.out.by.means.of.the.rotational.Maker.fringes.technique.in.the.transmission.scheme.for.three.different.fundamental.wave-lengths,.that.is,.1064,.1542,.and.1907.nm,.and.the.dispersion.of.the.second-order.nonlinear.optical.tensor.for.ZnO.films.grown.by.dual.ion.beam.sputtering.has.been.retrieved.

Practically.in.all.the.works.presented.up.till.now,.ZnO.films.with.the.crystallites.axis.oriented.about.perpendicular. to.the.substrate.surface.are.employed..One.has.to.ascribe.the.reason.for.that.to.ZnO.hexagonal.lattice,.which.shows.a.strong.tendency.to.crystallize.with.such.orientation,.thus.the.so-called.c-axis-oriented.films.can.be.easily.produced.on.either.polycrystalline.or.amorphous.substrates.by.different.techniques..As.a.consequence,.very.seldom.ZnO.films.with.a-axis.orientation.(112_0).have.been.grown,.that.is,.films.where.the.crystallites.axis,.and. thus. the.optical.axis,. is.aligned.parallel. rather. than.normal. to.the. substrate. surface.. When. grown.with. this. particular. orientation,. the. ZnO. films. dis-play.peculiar.Maker.fringes.patterns.with.respect.to.c-axis-oriented.films..In.particular,.a.maximum.in.the.SH.signal.appear.just.at.normal.incidence,.that.is,.where.it.is.completely.forbidden.by.symmetry.rules.for.c-axis-oriented.films..Furthermore,.it.was.experimentally.shown.that.the.total.conversion.efficiency.in.a-axis.ZnO.is.strongly.increased.with.respect.to.c-axis.ZnO.[23].

6.5 Effect of Thickness

As.already.mentioned,.from.experimental.evidence.it.becomes.evident.that.the.film.thick-ness.significantly.affects.the.second-order.susceptibilities.as.well.as.the.particular.deposi-tion.technique..There.were,.in.fact,.several.efforts.to.find.a.correlation.between.thickness.and.SHG,.and.many.systematic.studies.were.performed..Nevertheless,.a.clear-cut.depen-dence.is.yet.to.be.found,.mostly.because.thickness.is.of.course.not.the.unique.factor.affect-ing.SH.efficiency.

A.systematical.study.[13].on.films.whose.thickness.ranged.between.5.and.350.nm.was.performed.by.Wang.et.al..Films.were.prepared.by. two.different.methods,. that. is,. reac-tive. sputtering. and. plasma. enhanced. chemical. vapor. deposition.. Under. Kleinman’s.approximation. (i.e.,. χ χ χ χ χ15

2242

312

322

3320( ) ( ) ( ) ( ) ( ).= = = = 5 ),. it. was. found. that. the. absolute. values.

of.the.nonlinear.optical.components.decrease.with.the.film.thickness,.although.the.ratios.


between. the. two. non-zero. components. χ χ312

332( ) ( )/ . show. no. evidence. of. a. dependence. on.

the.film.thickness..For.thinner.films,.the.nonlinear.optical.coefficient.can.be.as.large.as.χ33

2 17 89( ) .= pm/V. (for. the. 44.2. nm. film). while. it. decreases. to. χ332 0( ) .=7 8 pm/V . for. the.

343.5.nm.film..In.Figure.6.8. the.χ332( ).values.obtained.from.films.of.different. thicknesses.

are.reported.[13]..The.observed.drop.in.the.second-order.susceptibilities. for. the.thicker.films.is.interpreted.according.to.a.model.in.which.the.polar.axis,.which.is.supposed.to.be.initially.aligned.with.respect.to.the.substrate,.flips.with.increase.in.the.film.thickness,.thus.deteriorating.the.nonlinear.optical.response.

A.peculiar.peak-like.effect,.in.thickness,.was.observed.in.Ref..[15].where.several.ZnO.films.grown.by.laser.molecular.beam.epitaxy.(at.500°).ranging.from.tenths.to.hundreds.of.nanometers.were.investigated..The.SH.signal,.measured.as.a.function.of.the.fundamental.beam. polarization. angle,. allowed. the. recovery. of. the. nonvanishing. components. of. the.nonlinear.optical.tensor..In.particular,.an.extraordinary.enhancement.of.the.optical.non-linearity.is.reported.for.a.44.4.nm.thick.sample,.since.the.values.of.the.nonlinear.optical.components. dramatically. peak. to. d33.=.−83.7. pm/V,. d31.=.14.7. pm/V. and. d15.=.15.2. pm/V,.exceeding.by.nearly.14.times.those.of.bulk.ZnO..Furthermore,.the.spectral.investigation.over.the.same.film.structure.reveal.that.these.values.considerably.increase.as.the.SH.fre-quency.approaches.the.material.bandgap.[14]..Outside.the.enhancement.range,.35–65.nm.in.thickness,.the.susceptibilities.values.are.small.and.almost.comparable.with.those.for.bulk.material.

This.strong.enhancement.is.explained.taking.into.account.the.microcrystallite.shape..Microstructural. investigations,. obtained. via. AFM,. scanning. tunneling. microscopy.(STM),. and. transmission. electron. microscopy. (TEM). images. demonstrate. that. a. thin.film.with.a. thickness. ranging.within.35.and.70.nm.consists.of.hexagonal-shaped.col-umns.with.lateral.nanocrystals.sizing.from.40.to.60.nm.separated.by.grain.boundaries.perpendicular. to. the.c-axis..As. the.film.thickness. is. increased,. the.crystallinity.of. the.film.increases.as.well,.leading.to.the.enhancement.of.the.second-order.nonlinear.effect..However,.also.the.average.grain.size.is.increased.together.with.the.film.thickness,.thus.

Thickness at center (nm)0.00

6

8

10

12

14

16

18

50.00 100.00 150.00 200.00 250.00

Sputtered filmsPECVD filmsFitting curve

X2 (pm

/ V

)

300.00 350.00

FIGURE 6.8Film.thickness.dependence.of.the.larger.component.of.the.susceptibility,.χ33

2( )..(Reprinted.with.permission.from.Wang,.G.,.Kiehne,.G.T.,.Wong,.G.K.,.Ketterson,.J.B.,.Liu,.X.,.Chang,.R.P.H.,.Large.second.harmonic.response.in.ZnO.thin.films,.Appl. Phys. Lett.,.80,.401–403,.2002..Copyright.2002,.American.Institute.of.Physics.)


reducing.the.surface.contribution.to.the.second-order.nonlinear.effect..These.two.con-trasting.tendencies.end.up.with.a.peak-like.structure.for.the.thickness.dependence.of.the.second-order.nonlinear.susceptibilities.and.give.an.explanation.to.the.experimental.results.

6.6 Surface Contributions

SHG. from. a. surface. was. observed. for. the. first. time. by. Terhune. and. coworkers. from. a.centrosymmetric.crystal.of.calcite.[31],.thought.at.that.time.it.was.still.believed.that.crys-tals.could.only.exhibit.SHG.if.the.crystal.was.non-centrosymmetric..In.1968,.Bloembergen.et.al..[32].pointed.out.that.the.second.harmonic.signal.was.generated.also.from.a.surface,.that.itself.represents.a.symmetry.break..Later.on.it.was.shown.that.surface.SHG.could.be.employed.for.single.monolayer.detection.[33].and.since.then,.surface.SHG.was.developed.as.a.useful.detection.method.of.molecular.adsorption.and.orientation.

From.the.point.of.view.of.SHG.process,.an.interface.represents.an.interesting.issue..At.a.crystalline.interface,.in.fact.half.of.the.atomic.forces.experienced.in.the.bulk.crystal.are.not.present,.thus.causing.deviations.in.the.atomic.and.electronic.structures..The.main.conse-quences.occurring.at.interface.is.the.variation.of.interplanar.distances.of.the.top.layers.as.well.as.the.redistribution.of.the.atoms.to.a.different.packing.structure..Even.if.the.symme-try.is.maintained.within.the.surface.planes,.the.out-of-plane.break.in.symmetry.modifies.the.second-order.susceptibility.tensor.χ(2),.giving.rise.to.a.novel.contribution.to.SHG.

Typically,.the.investigations.of.surface-related.SHG.are.performed.by.rotating.the.sam-ple.with.respect.to.the.incident.beam,.that.is,.about.the.z-axis.(see.Figure.6.3)..As.a.result,.the.second.harmonic.signal.will.vary.with. the.azimuth.angle.of. the.sample.due. to. the.symmetry.of.the.atomic.and.electronic.structure.

In.addition.to.surface.terms,.in.general.thin.films.as.well.as.bulk.crystal.may.contain.a.high.density.of.extended.defects,.such.as.stacking.faults,.dislocations,.symmetrical.tilt.grain.boundaries,.and.twin.boundaries..These.extended.defects.could.be.induced.in.ZnO.thin.films.by.residual.stress.due.to.the.lattice.mismatch.between.film.itself.and.the.sub-strate..Furthermore,.also.the.discrepancy.between.the.thermal.expansion.coefficients.may.play.a.role.in.the.process.of.defects.formation.

For.ZnO.films,.particular.attention.has.been.given.to.the.formation.of.twin.boundaries.defects.[8,21,22]..By.definition,.twin.boundary.stands.for.the.regular.growing.together.of.crystals.of. the.same.sort,. so. that.only.a.slight.misorientation.exists.between. them..The.so-called.twinned.crystals.share.some.of.the.same.crystal.lattice.with.a.mirror.symmetry.operation..Very.frequently.they.naturally.occur.within.crystal.growth.or.can.be.induced.through.mechanical.stress..Two.adjacent.crystals.share.a.single.composition.surface.often.appearing.as.mirror. images.across.the.boundary..As.a.consequence,. the.polarity. in.the.twin.boundary.exhibits.a.mirror.symmetry.across.the.boundary.plane.[34],.often.with.one.crystal.the.mirror.image.of.the.other,.giving.rise.to.a.source.of.nonlinear.optical.signal,.if.appropriately.stimulated.

In.Ref..[8],.the.effect.of.both.the.surface.contribution,.arising.from.the.polar.Zn–O.bond.on.the.top.layer,.and.the.additional.contribution.of.twin.boundaries.was.investigated.from.ZnO.(0002).830.and.550.nm.thick.prepared.by.MOCVD.on.sapphire..Surface.related.SHG.is.investigated.via.reflective.SHG.(RSHG).using.s-polarization.for.both.fundamental.and.generated.beams..This.polarization.configuration.was.chosen.because.the.resulting.SHG.


signal.for.6mm.symmetry.is.zero,.thus.the.generated.signal.under.these.conditions.is.a.signature.for.other.than.bulk.effects.

On.one.side,.at.the.film.surface.the.polar.Zn-O.bonds.break.the.bulk.symmetry,.that.is,.the.hexagonal.close.packed.structure.6mm.point.group.symmetry,.and.form.a.layer.hav-ing. 3m. symmetry. over. the. well-grown. 6mm. ZnO.. On. the. other. hand,. the. formation. of.twin.boundaries,. induced.by. the. lattice.mismatch.with. the.substrate,. is.also. responsible.for.a.nonvanishing.polarity.[34],.which.in.turn.exhibits.a.mirror.relation.across.the.bound-ary. plane.. Thus,. the. corresponding. second-order. nonlinear. optical. susceptibility. tensor.includes.six.independent.components..The.outcome.is.that.the.experimental.sω-s2ω-RSHG.patterns.evidence.the.3m.symmetrical.structure.on.the.surface.of.ZnO.(0002).and.also.reveal.the.additional.twin.boundary.contribution.with.mirrorlike.symmetry..In.other.words,.this.polarization.configuration,.where.both. fundamental.and.generated.beams.are.polarized.perpendicularly.to.the.plane.of.incidence,.is.most.sensitive.to.the.symmetry.of.the.surface.structure.because.it.only.includes.anisotropic.nonlinear.susceptibility.tensor.elements.The.following.fitting.equation.introduced.by.the.authors.[8]:

.I ae bss

i= ( ) + ( )ψ ϕ ϕsin sin 32. (6.9)

takes.into.account,.throughout.two.empirical.parameters,.both.the.effect.of.twin.boundar-ies.and.polar.surface,.since.a.represents.the.strength.of.nonlinearity.due.to.the.formation.of.twin.boundaries.and.b.the.polar.strength.of.Zn–O.bonding.with.3m.symmetry,.respec-tively..The.relative.phase.difference.between.the.SHG.field.arising.from.the.3m.symmetric.structure.of.Zn–O.bonding.and.from.twin.boundaries,.respectively,.is.represented.by.the.term.ψ..The. resulting.pattern.of. the.RSHG.azimuthal. scanning. is. shown. in.Figure.6.9,.

12090

603

2

1

30

0

330

300270

Azimuthal angle (deg)

S.H

. sig

nal (

a. u.

)

240

210

180

150

FIGURE 6.9Theoretical.simulation.of.RSHG.patterns.for.both.fundamental.and.generated.beams.s-polarized,.calculated.via.Equation.6.9..The.typical.sixfold.pattern.reflects.the.contribution.of.both.polar.surface.and.twin.boundaries.


where.the.peculiar.sixfold.profile.reflects.the.simultaneous.contributions.of.both.the.3m.group.symmetry.and.twin.boundaries..In.this.work.[8].the.crystallinity.of.the.two.samples.was.checked.also.via.XRD.patterns,.and.experimental.results.show.that.the.effect.of.twin.boundaries.is.similar.for.both.films,.despite.the.difference.in.crystallinity..On.the.other.hand,.the.nonlinear.source.generated.from.the.polar.surface.induced.by.the.Zn-O.bonding.is.found.to.be.stronger.from.the.surface.of.the.better.crystallized.ZnO.(0002).film.

The.mere.effect.of.twin.boundaries.was.about.isolated.[21].by.investigating.ZnO.thin.film.grown.on.Si.(111).substrate.by.magnetron.sputtering,.whose.resulting.surfaces.were.not.perfectly. smooth. in.order. to.eliminate. the.RSHG.contribution.of.3m.symmetry. [8]..Since.the.s-polarized.RSHG.excited.by.s-polarized.fundamental.light.(sω-s2ω).is.sensitive.to.the.anisotropic.contribution.dominated.by.the.symmetrical.structure,.this.polarization.configuration.is.employed.also.to.probe.the.nonvanishing.polarity.of.twin.boundaries.in.a.ZnO.film.. It.was.studied.that. the.overall.effect.of. the. lone.twin.boundaries.contribu-tion. on. the. strained. interface. corresponds. to. a. second. order. susceptibility. tensor. with.the.elements.indicated.as.χxxx

( ),2 m.and. χxyy( ),2 m,.being.the.x.direction.parallel.to.the.boundary.

plane.and.the.y.direction.perpendicular.to.the.boundary.plane..As.a.result,.the.perturbed.second-order.susceptibility.leads.to.a.nonvanishing.sω.s2ω-RSHG.intensity.which.can.be.expressed.explicating.the.nonlinear.susceptibility.tensor.elements.[21]:

.Iss xxx xyy= +( )cos cos sin( ) ( )ϕ χ ϕ χ ϕ2 2 2 2

22 . (6.10)

The.experimental.curves,.an.azimuthal.scan,.form.two-lobed.symmetric.patterns.indicat-ing.that.the.grain.boundaries.have.a.polar.configuration.across.the.boundary.plane.and.possess.a.directional.arrangement.

The.stress.degree.of.ZnO.(0002).thin.films,.which.is.thought.to.be.the.major.cause.of.twin. boundaries. formation,. can. be. estimated. from. XRD. patterns.. In. Ref.. [21],. this. was.found.to.decrease.with.increasing.substrate.temperature.

The.mirrorlike.symmetrical.RSHG.azimuthal.curves.given.by.Equation.6.10.result.in.a.two-lobed.pattern,.shown.in.Figure.6.10.in.which.there.is.no.additional.symmetrical.con-tribution.from.the.group.symmetry.3m..A.slight.asymmetry.that.may.appear.in.the.two-lobed.pattern,.in.disagreement.with.Equation.6.10,.would.denote.that.the.twin.boundary.does.not.have.a.mirror.symmetry,.that.is,.the.two.planes.shift.one.relatively.to.the.other.along.the.twin.boundary.[34].due.to.a.stress.gradient..As.a.result,.the.mirror.structure.with.forward-backward.symmetry.is.broken..This.effect.can.be.taken.into.account.by.modify-ing. the. second-order. susceptibility. by. the. stress. gradient,. which. is. properly. described.with.an.empirical.parameter.a.[21]:

.I ass xxx xyy= +( ) +cos cos sin( ) ( )ϕ χ ϕ χ ϕ2 2 2 2

22 . (6.11)

Despite.the.efforts.of.several.authors,.the.formation.mechanism.of.twin.boundaries.as.well.as.the.relationship.with.the.film.quality.under.various.film.growth.conditions.is.yet.to.be.completely.understood..In.Ref..[22],.ZnO.films.were.deposited.on.Si.(111).substrates.by.rf.sputtering.and.the.rf.power.was.systematically.changed.in.the.range.40–100.W..The.formation.and.ordering.of.ZnO.grain.boundaries.is.obviously.influenced.by.the.deposi-tion.rate.of.rf.sputtering..RSHG.was.then.employed.to.analyze.the.symmetric.structure.of.ZnO.grains,.whose.net.direction.changes.with.rf.power.


The.value.of.χxxx( ),2 m.depends.on.the.amount.of.symmetrical.polar.structures.at.the.bound-

aries..If.sputtered.atoms.have.suitable.migration.energy.and.deposition.time.to.form.better.quality.ZnO.film,.more. regular.boundary.planes.are. formed..Experimental. results. [22].show.that.the.value.of.χxxx

( ),2 m.reaches.a.maximum.at.the.rf.power.of.80.W,.in.correspondence.of. a. minimum. of. FWHM. XRD. peak,. which. results. in. the. best. crystallinity. among. the.investigated.samples,.while.the.value.of.χxxx

( ),2 m.is.not.affected.by.rf.power..Furthermore,.it.was.shown.that.the.twin.boundaries.are.formed.along.the.(110).direction.under.the.lowest.energy.condition. [34]..Consequently,.possible. shift.appearing. in. the. two-lobed.symme-try.patterns.would.indicate.that.the.twin.boundaries.are.formed.with.an.angle.tilt.with.respect.to.the.preferential.direction..This.angular.shift.can.be.included.in.the.theoretical.fitting.curve.[22].

More.recently.[25].the.effect.of.surface.roughness.on.SHG.was.systematically.studied..Polarity-controlled.ZnO.films.obtained.by.using.different.buffer. layers.were.grown.on.(0001).sapphire.substrates.by.plasma.assisted.molecular.beam.epitaxy..Selective.growths.of.Zn-polar.and.O-polar.ZnO.were.achieved.by.introducing.MgO.and.Cr.compound.buf-fer.layers,.respectively..At.the.end.of.these.processes,.four.types.on.ZnO.films.were.real-ized,.two.Zn-polar.ZnO.films.grown.on.7.nm.thick.MgO.and.CrN.buffer.layer,.and,.for.comparison,.two.O-polar.ZnO.films.grown.on.1.5.nm.thick.MgO.and.Cr2O3.buffer.layers.[25]..Film.thickness.ranged.from.270. to.450.nm..After.sample.preparation,. the.effective.second-order.NLO.coefficient.was.determined.by.using.the.Maker.fringes.technique..The.experimental.results.show.a.correlation.between.the.grain.size.and.thus.surface.rough-ness.with.the.values.of.deff..Specifically,.it.was.found.that.the.deff.of.the.ZnO.films.increased.with.decrease.in.the.grain.size.and.surface.roughness,.as.shown.in.Figure.6.11..This.effect.

0 60 120 180 240 300

0

0

1

2

3

4

0

1

2

3

4

0

1

2

3

4

60 120 180 240 300 0

0

060 120 180 240 300

60 120300°C

SHG

inte

nsity

(a. u

.)

100°C 200°C

180Rotational angle (j)

Rotational angle (j)

240 300

FIGURE 6.10The.RSHG.pattern.measured.from.a.ZnO.film.on.Si.(111).substrate.deposited.at.a.temperature.of.100°C,.200°C,.and.300°C.respectively..Both.fundamental.and.generated.beams.are.s-polarized..(Reprinted.with.permission.from.Lo,.K.-Y.,.Lo,.S.-C.,.Yu,.C.-F.,.Tite,.T.,.Huang,.J.-Y.,.Huang,.Y.-J.,.Chang,.R.-C.,.and.Chu,.S.-Y.,.Optical.second.harmonic.generation.from.the.twin.boundary.of.ZnO.thin.films.grown.on.silicon,.Appl. Phys. Lett.,.92,.091909–2,.2008..Copyright.2008,.American.Institute.of.Physics.)


can.be.explained.by. the. increase.of. surface. scattering.experienced.by. the. fundamental.beam.as.grain.size.and.surface.roughness.are.increased,.respectively.

6.7 Doping

Doping,.that.is,.the.intentional.addition.of.impurities.to.a.ZnO.thin.film.within.the.growth.process,.produces.remarkable.changes.in.the.corresponding.electrical,.morphological,.and.optical.properties..The.effect.of.the.dopant.element.depends,.of.course,.on.its.electronega-tivity.and.ionic.radius..In.the.case.of.ZnO.thin.films,.it.is.experienced.that.doping.with.donor. impurities. to.achieve.high.n-type. conductivity. is. less.difficult. than.doping.with.acceptor. impurities. to.achieve.p-type.conductivity.. It.was. shown,. in. fact,. that.ZnO. is.a.naturally.n-type.semiconductor.because.of.intrinsic.defects.such.as.Oxygen.vacancies.[35].

Several.reports.on.doping.of.ZnO.thin.films.are.available,.such.as.those.reporting.dop-ing.with.In.[36,37],.Ga.[38],.Li.[39],.and.F.[40].although.these.works.only.deal.with.struc-tural,.electrical.and/or.linear.optical.properties..The.effect.of.different.dopant.elements.such.as.Al,.In,.Cu,.Fe,.and.Sn.on.the.microstructure.of.ZnO.thin.films.has.been.investi-gated.in.detail.in.Ref..[41].

A. comprehensive. description. concerning. variations. in. structural,. morphological,. opti-cal,.as.well.as.nonlinear.optical.(NLO).properties.induced.by.F.as.dopant.element,.has.been.reported.in.Ref..[27]..Here,.the.F.atoms.are.incorporated.effectively.in.ZnO.films.deposited.by.a.chemical.deposition.technique..The.SHG.response.of.fluorine-doped.zinc.oxide.thin.films.(ZnO:F),.having.thickness.in.the.range.530–570.nm,.is.investigated.with.particular.attention.to. the.effect.of.substrate. temperature.on.the.morphological,.optical,.and.nonlinear.optical.(NLO).properties..Maker. fringes. technique. in. transmission.mode.was.employed. for.SHG.and.THG.measurements..From.the.point.of.view.of.crystalline.structure,.XRD.and.SEM.evi-denced.a.polycrystalline.structure.with.a.preferential.growth.along.the.ZnO.(0002).plane,.with.a.corresponding.average.crystal.size.of.less.than.32.nm.for.the.films..It.was.found.that.

Surface roughness (RMS = nm)

SH d

eff c

onst

ant (

pm/V

)

02.5

3.0

3.5

4.0

4.5

5.0

2 4 6 8 10

FIGURE 6.11The.effective.second-order.nonlinear.coefficient.as.a.function.of.surface.roughness.addressed.by.RMS..The.dotted. line. shows. the. Lorentzian. fitting. curves.. (Reprinted. with. permission. from. Park,. J.S.,. Yamazaki,. Y.,.Takahashi,. Y.,. Hong,. S.K.,. Chang,. J.H.,. Fujiwara,. T.,. and. Yao,. T.,. Origin. of. second-order. nonlinear. optical.response. of. polarity-controlled. ZnO. films,. Appl. Phys. Lett.,. 94,. 231118–3,. 2009.. Copyright. 2009,. American.Institute.of.Physics.)


the.substrate.temperature.affects.the.morphology.of.the.surfaces.and.some.optical.properties.as.well..The.films.deposited.at.low.temperature.(400°C).showed.a.uniform.surface.covered.with.needle-like.grains.as.well.as.efficient.SHG.and.THG.response..On.the.contrary,.for.the.films.deposited.at.525°C,.a.porous.surface.with.irregularly.shaped.grains.was.obtained.and.a.decrease.of. the.SHG.and.THG.response.was.observed..The.highest.values.of. the.evalu-ated.nonlinear.χeff

( )2 .coefficients.were.found.for.the.sample.deposited.at.lowest.temperatures,.χ31

2 0( ) .= 1 3 pm/V.and.χ332 26( ) .= 4 pm/V.(400°C).[27].and.χ31

2( ) .= 5 3 pm/V.and.χ332( ) .= 16 4 pm/V .

(425°C).[42],.that.is,.the.increase.of.substrate.temperature.negatively.affects.the.observed.SHG.intensity..This.circumstance.should.be.directly.connected.to.the.crystal.and.domain.sizes.achieved.by.the.deposition.temperature.as.well.as.to.the.efficiency.of.incorporation.of.F.atoms.within.the.film.texture..As.the.substrate.temperature.increases,.a.slight.increase.in.the.crys-tallite.size.of.the.films.occurs,.from.26.to.35.nm..Being.the.density.of.grain.boundaries.and.interfaces.higher.when.the.crystallite.size. is.smaller,.one.may. infer. that.grain.boundaries.effects.have.an.influence.on.the.SH.response..Furthermore,.authors.observed.that.the.incor-poration.efficiency.of.F.decreased.markedly.with.increasing.substrate.temperatures.due.to.vaporization,.thus.avoiding.an.optimal.inclusion.of.F.into.the.ZnO.film.structure.

In.Ref..[43].highly.epitaxial.Al-doped.ZnO,.grown.by.RF.magnetron.sputtering.on.sap-phire.is.investigated..Typically,.Al.dopants.are.introduced.to.achieve.n-type.ZnO.(Al:ZnO).[44]..By.using.the.femtosecond.Ti:Sapphire.laser.at.the.near-resonant.SH.wavelength,.in.reflection.geometry,.the.SHG.was.measured.at.two.different.incidence.angles.of.45.0°.and.20.4°.to.avoid.the.sapphire’s.contribution..Different.sets.of.measurements.were.performed..First.of.all,.SH.signal.was.detected.as.a. function.of.azimuthal.angle..Once. the.SH.was.found.to.be.isotropic.about.this.type.of.rotation.it.was.possible.to.exclude.spurious.effect.due.to.SH.from.the.substrate.and.to.confirm.that.the.film.is.composed.only.by.well.aligned.c-axis.domain..Further.SH.curves.were.taken.as.a.function.of.fundamental.beam.polariza-tion.angle..According.to.the.6mm.group.symmetry,.the.p-polarized.SH.fields.were.found.to.be.always.dominant.regardless.of.the.polarization.of.the.fundamental.beam,.and.the.strongest.signal.occurred.under.the.pω-p2ω.polarization.configuration.

The.ratios.between.the.nonlinear.coefficients.retrieved.from.the.experimental.results.indi-cate.that.Kleinman’s.symmetry.is.broken.due.to.the.absorption.at.SH.wavelength.(405.nm).and.the.dominant.component.of.the.nonlinear.susceptibility.tensor.is.d33..The.high.value.obtained.for.the.ratio.d33/d31.=.25.6.shows.that.d33.is.the.dominant.component.of.the.nonlinear.susceptibility.tensor,.while.the.value.d15/d31.=.2.72.indicates.the.deviation.from.Kleinman’s.symmetry.(d15.=.d31).in.the.Al:ZnO.films.at.the.fundamental.wavelength.of.810.nm.

Absolute. values. of. nonlinear. optical. components. were. found. to. be. d15.=.0.35. pm/V,.d31.=.0.13.pm/V,.and.d33.=.3.27.pm/V,.being.these.values.smaller.than.those.measured.by.Maker.fringes.technique.for.ZnO.film.on.sapphire.at.1064.nm.[9].

Finally,.in.Ref..[11],.several.of.the.specimens.giving.rise.to.efficient.SHG.response.were.doped.with.Indium..However,.authors.do.not.address.specific.effects.related.to.the.doping,.while.they.only.report.that.the.doped.samples.do.not.tend.to.break.the.general.trend.of.the.undoped.samples.

6.8 Third Harmonic Generation

Finally,.THG.is.responsible.for.the.frequency.tripling.of.the.incoming.beam.at.ω,.via.the.third-order.susceptibility,.χijkl

( )3 .(−3ω,ω,ω,ω).which.is.a.fourth-rank.tensor,.symmetry-allowed.


in.all.materials,.regardless.of.symmetry.rules..As.well.as.for.SHG,.from.the.Maker.fringes.of. the. generated. signal. at. 3ω,. the. components. of. the. fourth-rank. tensor. χijkl

( )3 ,. generally.expressed.in.m2/V2,.can.also.be.evaluated..Similarly,.the.different.non-zero.components.of.the.third-order.susceptibility.can.be.addressed.by.choosing.different.polarization.states.for.fundamental.and.generated.beam,.respectively.

If.compared.to.SHG,.a.relatively.low.number.of.works.have.been.published,.involving.the.generation.of.third.harmonic.from.ZnO.films.[18,20,26,27]..In.most.of.them,.THG.is.investigated.via.the.Maker.fringes.technique..Being.third-order.nonlinear.optical.phenom-ena.related.to.an.odd.powered.term.in.the.nonlinear.optical.polarization,.THG.may.occur.in.any.material,.including.air..As.a.consequence,.during.THG.measurements,.it.is.strongly.suggested.to.keep.the.sample.under.vacuum,.in.order.to.avoid.spurious.contributions.

A.strong. third-order.nonlinear.optical. coefficient,. χ(3). of. 1.4.×.10−12. esu,.was.measured.from.ZnO.films.grown.by.pulsed. laser.ablation,.allowing.the.generation.the. third.har-monic.of.ultrashort.pulses.with.the.efficiency.of.percent.[26].

The.fourth-rank.nonlinear.optical.tensor.χijkl( )3 .for.the.6mm.point.group.symmetry.presents.

two.nonvanishing.components,.namely,.χ(3)1111.and.χ(3)

3333..For.ZnO.films.deposited.along.the.(0002).planes,.that.is,.with.the.axis.mainly.normal.to.sample.surface,.it.is.possible.to.inves-tigate.the.χ3333

3( ) .by.selecting.the.pω-p3ω.polarization.configuration..In.Ref..[18].the.third-order.susceptibility.component. χ3333

3( ) .was.evaluated. for.ZnO.samples.grown.by.dual. ion.beam.sputtering..Maker.fringes.technique.was.applied,.at.a.fundamental.wavelength.of.1907.nm.and.a.value.for.χ3333

3( ) .of.1.32.×.10−12.esu,.corresponding.to.1.85.×.10−20.(m2/V2),.has.been.found.In. Ref.. [27],. the. highest. cubic. nonlinear. optical. coefficients. was. evaluated. via. Maker.

fringes.experiments.from.fluorine.doped.ZnO.films,.and.it.was.found.χ(3)ZnO:F.=.0.72·10−12.

esu,.that.is,.χ(3)ZnO:F.=.1.0·10−20.(m2/V2)..The.same.magnitude.order.was.measured.from.ZnO.

films.deposited.by.rf.sputtering.on.crystal.substrate,.χeff(3) .of.0.98.and.1.22.×.10−20.(m2/V2),.for.

α-BBO.and.LiNbO3.substrates,.respectively.[20].

6.9 Conclusions

As.a.final. remark,. the.wide.variety.of.works.on.SHG.from.ZnO.films,.prepared.under.different.deposition.techniques.and.conditions,.is.an.indication.of.the.growing.interest.in.this.nonlinear.optical.material..Furthermore,.due.to.the.large.SH.conversion.bandwidth.ZnO.is.a.promising.material.for.autocorrelation.studies..In.the.work.of.Griebner.et.al.[10],.a.0.85.μm.thick.ZnO.film,.grown.by.spray.pyrolysis.on.glass.substrate.was.successfully.employed.for.the.characterization.of.20.fs.pulses..Experimental.results.show.a.nonreso-nant,.instantaneous.nonlinear.response.of.the.polycrystalline.ZnO.films.as.well.as.neg-ligible.group.velocity.dispersion..As.a.further.example.in.Ref..[45],.a.220.nm.thick.ZnO.crystal.grown.by.molecular.beam.epitaxy.onto.sapphire.substrate.was.used.for.sub-10.fs.pulse.measurement.by.a.fringe-resolved.autocorrelation.method.

Before.concluding,.we.wish.to.point.out.that.the.wide.variety.of.deposition.techniques.and. resulting. film. structures,. as. well. as. the. different. possible. contributions. to. SHG.may. justify. the. spread. of. published. values. for. the. measured. nonlinear. optical. coeffi-cients. that. differ. by. more. than. one. order. of. magnitude,. as. well. as. the. discrepancy. on.the.sign.of.d33..For.the.sake.of.completeness,.all.the.values.reported.by.different.authors.are.resumed.in.Table.6.1,.in.terms.of.dij.coefficients,.together.with.some.other.significant.data,.as.the.deposition.technique,.the.parameters.of.the.fundamental.beam.laser.source.as.


TABLE 6.1

Measured.nonlinear.optical.coefficients.of.several.different.ZnO.films

Deposition Technique Substrate Laser Source Thickness dij Value Reference

Pulsed.laser.ablation

Sapphire λ.=.1064.nm 45.nm Highly.crystalline.film

[9]

τpulse.=.5.ns d33.=.5.4.pm/V

frep.=.10.Hz d31.=.1.pm/V

45.nm Low.crystalline.film

d33.=.6.7

d31.=.1.8.pm/V

235.nm Highly.crystalline.film

d33.=.2.05.pm/V

d31.=.0.5

235.nm Low.crystalline.film

d33.=.4.4.pm/V

d31.=.0.7.pm/VSpray.pyrolysis Glass λ.=.830.nm 850.nm deff.=.10.pm/V [10]

τpulse.=.20.fsfrep.=.88.MHz

Spray.pyrolysis Glass λ.=.830.nm 0.1–1.0.μm d31.=.d32.=.2.1.pm/V [11]

τpulse.=.20.fs d15.=.d24.=.2.6.pm/Vfrep.=.88.MHz d33.=.−7.3.pm/V

Plasma-enhancedCVD

Sapphire λ.=.1064.nm 44.2.nm d33.=.8.94.pm/V [13]Sapphire 343.5.nm d33.=.3.90.pm/V

Laser.molecular.beam.epitaxy

Sapphire λ.=.720–1100.nm

44.4.nm d33.=.−83.7.pm/V [14]

τpulse.=.35.ps d15.=.15.2.pm/Vfrep.=.10.Hz d31.=.14.7.pm/V

Laser.molecular.beam.epitaxy

Sapphire λ.=.1064.nm 44.4.nm d33.=.−83.7.pm/V [15]

τpulse.=.200.ns d31.=.14.7.pm/Vfrep.=.500.Hz d15.=.15.2.pm/V

MOCVD Sapphire λ.=.1064.nm 350.nm Deposited.at.200°C [16]

τpulse.=.7.ns d33.=.3.60.pm/Vfrep.=.10.Hz d31.=.1.12.pm/V

350.nm Deposited.at.250°Cd33.=.4.60.pm/Vd31.=.1.62.pm/V

350.nm Deposited.at.300°Cd33.=.1.60.pm/Vd31.=.0.47.pm/V

Dual.ion.beam.sputtering

Glass λ.=.1064.nm 370.nm |d33|.=.1.23.pm/V [17]

τpulse.=.5.nsfrep.=.14.Hz

(continued)


well.as.the.film.thickness..The.nonlinear.optical.coefficients.for.bulk.(0001).ZnO.platelet,.χ33

2( ) .= −14 31 pm/V.and.χ312( ) .= 1 36 pm/V.[46].and.for.the.(1010).platelet.χ33

2( ) .= −14 77 pm/V .and.χ31

2( ) .= 1 41 pm/V.[46].are.also.included.in.Table.6.1,.for.comparison.

References

. 1.. P..A..Franken,.A..E..Hill,.C..W..Peters,.and.G..Weinreich,.Generation.of.optical.harmonics,.Phys. Rev. Lett..7,.118–119.(1961).

TABLE 6.1 (continued)

Measured.nonlinear.optical.coefficients.of.several.different.ZnO.films

Deposition Technique Substrate Laser Source Thickness dij Value Reference

Dual.ion.beam.sputtering

Glass λ.=.1064.nm 550.nm d33.=.0.90.pm/V [18]


frep.=.14.Hz d31.=.0.31.pm/V

λ.=.1542.nm d33.=.0.25.pm/V


frep.=.14.Hz d31.=.0.10.pm/V

λ.=.1907.nm d33.=.0.16.pm/V


frep.=.14.Hz d31.=.0.08.pm/VRf.magnetron.sputtering

α-BBO.substrate

λ.=.1064.nm 1.2.μm deff 11 pm/V( ) .2 0= [20]

τpulse.=.16.psfrep.=.10.HzLiNbO3.

substrate1.2.μm deff 6 45 pm/V( ) .2 =

Metalorganic.aerosol.deposition

Sapphire λ.=.790.nm 150.nm d33.=.−12.pm/V [23]a-axis.oriented.films

τpulse.=.35.fs d31.=.d32.=.2.8.pm/Vfrep.=.1.kHz 300.nm d33.=.−15.pm/V

d31.=.d32.=.4.9.pm/VChemical.spray.technique

Glass λ.=.1064.nm 570.nm d33.=.13.2.pm/V [27]

τpulse.=.5.ns d31.=.5.15.pm/Vfrep.=.25.kHz

Chemical.spray.deposition

Glass λ.=.1064.nm 540.nm d33.=.8.20.pm/V [42]

τpulse.=.5.ns d31.=.2.65.pm/Vfrep.=.12.kHz

Commercial Sapphire λ.=.810.nm 285.nm d33.=.3.27.pm/V [43]

τpulse.=.300.fs d15.=.0.35.pm/Vfrep.=.82.MHz d31.=.0.13.pm/V

ZnO.single.crystal.platelets

(0001).orientation

1064.nm d33.=.−7.15.pm/V [46]d31.=.0.68.pm/V

(101_0).orientation

d33.=.−7.38.pm/Vd31.=.0.70.pm/V


. 2.. V.. G.. Dmitriev,. G.. G. Gurzadyan,. and. D.. M.. Nikogosyam,. Optics. of. nonlinear. crystals,. in.Handbook of Nonlinear Optical Crystals,. eds.. V.. G.. Dmitriev,. G.. G.. Gurzadyan,. and. D.. N..Nikogosyan,.Springer,.Berlin,.Germany,.pp..3–65,.1997.

. 3.. D..A..Kleinman,.Nonlinear.dielectric.polarization.in.optical.media,.Phys. Rev..126,.1977–1979.(1962).

. 4.. P..D..Maker,.R..W..Terhune,.M..Nisenoff,.and.C..M..Savage,.Effects.of.dispersion.and.focusing.on.the.production.of.optical.harmonics,.Phys. Rev. Lett..8,.21–22.(1962).

. 5.. J..Jerphagnon.and.S..K..Kurtz,.Maker.fringes:.A.detailed.comparison.of.theory.and.experiment.for.isotropic.and.uniaxial.crystals,.J. Appl. Phys..41,.1667–1681.(1970).

. 6.. K.-Y..Lo,.S.-C..Lo,.S.-Y..Chu,.R.-C..Chang,.and.C.-F..Yu,.Analysis.of.the.growth.of.RF.sputtered.ZnO.thin.films.using.the.optical.reflective.second.harmonic.generation,.J. Cryst. Growth.290,.532–538.(2006).

. 7.. K..Y..Lo,.Y..J..Huang,.J..Y..Huang,.Z..C..Feng,.W..E..Fenwick,.M..Pan,.and.I..T..Ferguson,.Reflective.second.harmonic.generation.from.ZnO.thin.films:.A.study.on.the.Zn–O.bonding,.Appl. Phys. Lett..90,.161904.(2007).

. 8.. F..A..Bovino,.M..C..Larciprete,.A..Belardini,.and.C..Sibilia,.Evaluation.of.the.optical.axis.tilt.of.zinc.oxide.films.via.noncollinear.second.harmonic.generation,.Appl. Phys. Lett..94,.251109.(2009).

. 9.. H..Cao,.J..Y..Wu,.H..C..Ong,.J..Y..Dai,.and.R..P..H..Change,.Second.harmonic.generation.in.laser.ablated.zinc.oxide.thin.films,.Appl. Phys. Lett..73,.572–574.(1998).

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167

7Optical Properties and Carrier Dynamics of ZnO and ZnO/ZnMgO Multiple Quantum Well Structures

Bong-Joon Kwon and Yong-Hoon Cho

7.1 Introduction

ZnO. is. a. direct. and. wide. bandgap. semiconductor. with. large. exciton. binding. energy.(60.meV),. which. shows. a. great. promise. for. the. application. of. high. efficiency. and. short.wavelength.light-emitting.devices..ZnO.has.been.investigated.for.many.decades..Its.lat-tice.parameters.have.been.known.since.1935,.whereas.detailed.optical.studies.have.been.carried.out.from.the.mid-1950s.until.now..But.it.has.recently.attracted.much.attention.for.optoelectronic.and.electronic.devices.due.to.the.amenability.to.wet.chemical.etching,.a.high-radiation.resistance,.relatively.low.material.costs,.and.the.availability.of.large.area.ZnO. substrates.. Moreover,. because. of. the. large. exciton. binding. energy,. excitonic. lumi-nescence. and. lasing. above. room. temperature. are. expected.. These. considerations. lead.

CONTENTS

7.1. Introduction......................................................................................................................... 1677.2. Optical.Properties.of.Undoped.ZnO.Thin.Films........................................................... 168

7.2.1. Fundamental.Optical.Properties.in.ZnO............................................................ 1687.2.2. Depth-Resolved.Optical.Properties.in.ZnO........................................................ 1737.2.3. Optical.Properties.of.ZnO.Grown.under.Various.Conditions......................... 176

7.3. Optical.Properties.of.p-Type.ZnO.................................................................................... 1807.3.1. N-Doped.ZnO.Epilayer.......................................................................................... 1807.3.2. As-Doped.ZnO.Epilayer........................................................................................ 1817.3.3. P-Doped.ZnO.Epilayer........................................................................................... 1827.3.4. Sb-Doped.ZnO.Epilayer......................................................................................... 184

7.4. Optical.Properties.of.ZnO.with.Different.Polarities..................................................... 1867.4.1. Polarity-Controlled.ZnO.Epilayer........................................................................ 1867.4.2. O-Face.and.Zn-Face.ZnO....................................................................................... 189

7.5. Optical.Properties.of.ZnO/ZnMgO.Multiple.Quantum.Wells.................................... 1907.5.1. Homoepitaxial.ZnO/ZnMgO.MQWs.................................................................. 1907.5.2. Heteroepitaxial.ZnO/ZnMgO.MQWs................................................................. 194

7.6. Summary.............................................................................................................................. 198Acknowledgments....................................................................................................................... 199References...................................................................................................................................... 199


researchers.to.actively.pursue.the.development.of.ZnO-based.optoelectronic.devices..Since.quantum.well.(QW).structures.have.been.widely.used.in.high-performance.semiconductor.optoelectronic.devices,.ZnO.QWs.have.been.actively.investigated.and.show.much.larger.optical.gain.compared.with.other.semiconductors..Despite.these.advantages,.applications.of.ZnO.are.hindered.by.the.lack.of.reproducible,.high-quality,.and.low-resistivity.p-type.ZnO.due.to.the.self-compensation.and.low.solubility.of.dopants.

The. optical. properties. of. ZnO-based. systems. are. associated. with. both. intrinsic. and.extrinsic.effects,.which.provide.the.basis.of.many.important.applications.such.as.lasers,.light-emitting. diodes,. and. photodetectors.. Photons. of. sufficient. energy. can. excite. elec-trons.from.the.filled.valence.bands.to.the.empty.conduction.bands..As.a.result,.the.optical.spectra.of.semiconductors.provide.a.rich.source.of.information.on.their.electronic.proper-ties.. In.many.semiconductor.materials,.photons.can.also. interact.with. lattice.vibrations.and.with.electrons. localized.on.defects,. thus.making.optical. techniques.also.useful. for.studying.these.interactions..Intrinsic.optical.transitions.take.place.between.the.electrons.in.the.conduction.band.and.holes.in.the.valence.band,.including.excitonic.effects.due.to.the. Coulomb. interaction.. Excitons. are. classified. into. free. and. bound. excitons.. In. high-quality. samples. with. low. impurity. concentrations,. the. free. exciton. can. exhibit. ground.state.and.excited.state.transitions..Extrinsic.properties.are.related.to.defects.or.dopants,.which.usually.create.discrete.electronic.states.in.the.bandgap,.and.therefore.influence.both.optical-absorption.and.emission.processes..In.theory,.excitons.could.be.bound.to.neutral.or.charged.donors.and.acceptors..The.electronic. states.of. these.bound.excitons.depend.strongly.on.the.semiconductor.material..Other.defect-related.transitions.could.be.observed.in.optical.spectra. [1,2]..Furthermore,. in.quantum.structure.based.on.ZnO,. the.excitonic.emissions.are.affected.by.the.quantum.confinement.effect.and.quantum-confined.stark.effect.due.to.well.width,.built-in.electric.fields,.etc..So,.the.optical.properties.of.ZnO.quan-tum.structure.must.be.understood.for.higher.performance.of.QW.devices.

This.chapter.reviews.the.intrinsic.and.extrinsic.optical.properties.and.carrier.dynam-ics.of.various.ZnO.epilayers.and.ZnO/ZnMgO.multiple.quantum.well.structures.includ-ing.donor-.and.acceptor-bound.excitons.together.with.a.discussion.of.longitudinal.optical.phonon.replicas.of.the.main.excitonic.emissions,.the.donor-acceptor.pair.transition,.and.the.deep-level.emissions.depending.on.various.growth.methods,.substrate,.buffer.layers,.p-type.dopants,.surface.polarities,.and.native.point.defects..The.chapter.is.organized.as.follows:.the.fundamental.optical.transitions.in.undoped.ZnO.including.excitonic.transi-tions,.two.electron.satellites.(TESs),.and.deep-level.emissions..Also,.the.depth.properties.of.ZnO.and.the.optical.properties.of.ZnO.grown.under.various.substrates.and.conditions.are.described.in.Section.7.2..Then,.recent.progress.in.the.growth.of.p-type.ZnO.and.optical.characteristics.is.described.in.Section.7.3..Section.7.4.discusses.the.optical.characteristics.of.polarity-controlled.ZnO.together.with.O-.and.Zn-face.ZnO..Section.7.5.describes. the.optical.properties.and.carrier.dynamics.of. the.homoepitaxial.and.heteroepitaxial.ZnO/ZnMgO.multiple.quantum.wells.(MQWs)..Finally,.Section.7.6.summarizes.the.main.points.

7.2 Optical Properties of Undoped ZnO Thin Films

7.2.1 Fundamental Optical Properties in ZnO

ZnO.is.a.direct.bandgap.semiconductor.with.a.room.temperature.(RT).bandgap.of.3.37.eV..Because. of. the. large. exciton. binding. energy. of. 60.meV,. the. optical. emission. spectrum.

169Optical Properties and Carrier Dynamics

of. ZnO. shows. sharp. excitonic. lines. that. dominate. even. above. RT.. The. PL. spectrum. of.ZnO.at.low.temperature.shows.typical.excitonic.transitions,.which.are.free.exciton.(FX),.donor-bound.exciton.(DBX),.acceptor-bound.exciton.(ABX),.and.their.phono-replicas..Cho.et. al.. [3]. reported. the.optical.properties. of. excitonic. and.phonon-assisted. transitions. in.ZnO. epilayers.. The. ZnO. samples. were. grown. by. plasma-assisted. molecular. beam. epi-taxy. (PAMBE). on. an. Al2O3. (0001). substrate. by. employing. a. thin. MgO. buffer. layer. [4]..Figure.7.1.shows.temperature-dependent.PL.spectra.taken.from.a.high-quality.ZnO.epi-layer..The.sharp.excitonic.features.associated.with.FX,.DBX,.ABX,.LO-phonon.replicas.of.FX.(FX-nLO),.and.LO-phonon.replicas.of.DBX.(DBX-nLO).can.be.clearly.observed.for.10.K.PL.spectrum..According.to.their.energy.values,.the.emissions.at.3.372,.3.357,.3.321,.3.308,.and.3.287.eV.(367.6,.369.2,.373.2,.374.7,.and.377.1.nm).at.10.K.are.attributed.to.FX,.DBX,.ABX,.FX-1LO,.and.DBX-1LO.transitions,.respectively..The.oscillatory.structure.of.the.PL.spec-trum.has.an.energy.periodicity.of.about.70.meV,.LO-phonon.energy.of.ZnO..At.T.<.70.K,.the.DBX.transition.is.predominant,.while.the.FX.transition.becomes.dominant.at.T.>.70.K..The.ABX.transition.is.observed.at.3.2.eV.up.to.30.K.and.then.emission.intensity.decreases.due.to.thermal.ionization.from.bounded.acceptors.with.temperature..FX-nLO.transitions.are.more.dominant.than.DBX-nLO.transitions.from.10.to.300.K,.and.DBX-nLO.transitions.can.be.observed.at.T.<.50.K..The.FX.and.FX-nLO.transitions.become.dominant.above.90.K,.and.then.the.FX.and.FX-nLO.emissions.start.to.merge.together.due.to.the.broader.spectral.shape.at.the.higher.energy.side.of.each.FX-nLO.transition..In.the.case.of.thermal.equilib-rium.in.the.exciton.system,.the.spectral.shape.of.the.FX-1LO.and.FX-2LO.emissions.can.be.described.by.the.formula

3.15 3.2090 K

300 K

250 K

200 K

175 K130 K90 K

50 K

10 K

70 K

50 K

30 K

20 K

10 KFX-3LO

FX-2LOFX-1LO

ABX

DBX DBXZnO Sub.

FX

FX

DBX-3LO

DBX-2LO

(a) (b)

395 390 385

Wavelength (nm) Wavelength (nm)

380 375 370 365 395 390 385 380 375 370 365

DBX-1LO


PL in

tens

ity (a

. u.)

PL in

tens

ity (a

. u.)

3.30 3.35 3.40 3.15 3.20 3.25Photon energy (eV)

3.30 3.35 3.40

FIGURE 7.1Temperature-dependent.PL.spectra.from.the.front.surface.of.the.ZnO.epilayer.taken.from.10.K.to.(a).90.K.and.(b).300.K..(Reproduced.with.permission.from.Cho,.Y.H.,.Kim,.J.Y.,.Kwack,.H.S.,.Kwon,.B.J.,.Dang,.L.S.,.Ko,.H.J.,.and.Yao,.T.,.Appl. Phys. Lett.,.89,.201903,.2006..Copyright.2006,.American.Institute.of.Physics.)


.I E E E E kTex KE ex KE( ) ∝ −( )

1 2 exp

and

.I E E E kTex ex KE( ) ∝ −( )

1 2 exp ,

respectively,.where.Eex1 2 .is.the.density.of.states.in.a.parabolic.exciton.band.of.a.direct.band-

gap.semiconductor,.and.EKE.is.the.kinetic.energy.of.the.exciton.with.respect.to.the.bottom.of.the.exciton.band.[5]..It.is.found.that.the.FX.emission.energy.is.∼3.3.eV.and.the.FX-1LO.emission.energy.is.∼3.24.eV.at.RT..The.slight.variation.in.the.energy.separation.between.FX.and.its.phonon.replicas.with.temperature.is.related.to.the.redistribution.of.excitons.in.the.band.(i.e.,.exciton.thermalization).with.increasing.temperature..From.the.sample,.three.orders.of.magnitude.lower.intensity.of.deep-level.emission.was.observed.at.around.2.3.eV.in.ZnO.than.the.excitonic.emissions.at.low.temperature..The.narrow.linewidth.of.excitonic.transitions. and. negligible. deep-level. emission. indicate. the. high. material. quality. of. the.high-quality.ZnO.sample.

Another.transition.of.an.exciton.bound.to.a.neutral.donor.can.be.observed.during.the.recombination. process,. because. the. final. state. of. the. donor. can. be. the. 1s. ground. state.(typical.D0X.transition).or.a.higher.excited.state.like.2s,.2p,.etc..The.latter.process.is.usu-ally.called.a.TES..The.donor-binding.energies.can.be.derived.from.the.energy.separation.between.D0X.and.TES,.since.the.energy.difference.between.n.=.1.and.n.=.2.states.would.be. equal. to. 3/4. of. the. donor-binding. energy. based. on. a. hydrogen-like. effective. mass.approach..Thonke.et.al..[6].observed.the.very.sharp.bound.exciton.(BE).lines.together.with.the.TES.transitions.at.low-temperature.PL.spectra.taken.from.the.ZnO.substrate.material.grown.by.seeded.chemical.vapor.transport..Figure.7.2.shows.the.PL.spectrum.of.the.BE.lines,.which.were.shifted.by.29.9.meV.compared.with.the.TES.lines..The.TES.lines.mirror.the.principal.BE.lines..They.pointed.out.that.the.upper.BE.line.is.a.DBE,.and.even.the.lower.energy.lines.around.3.360.eV.are.donor-.(and.not.acceptor-).related..The.energy.splitting.of.(29.9.±.0.2).meV.between.1s.and.2(s,p).of.the.lowest.donor.states.was.estimated.for.the.domi-nant.donor.in.the.sample.from.the.spacing.of.the.TES.lines.from.the.D0X.lines..They.found.that.the.ionization.energy.for.these.donors.would.be.4/3.×.29.9.meV.=.39.9.meV..The.band.at.≈.3.320.eV. is.possibly.also.a.TES.originating.from.a.second.donor.with.a.higher.bind-ing.of.4/3.×.(29.9.+.11.4).meV.=.55.meV..The.LO-phonon.replicas.show.roughly.two.orders.of.magnitude.less.intense.than.the.BE.lines..They.found.that.this.result.is.similar.to.the.case.of.GaN.and.II–VI.semiconductors.and.pointed.out.that.D0X.lines.commonly.couple.only.weakly.with.the.optical.phonons,.whereas.the.neutral-acceptor-bound.exciton.(A0X).lines.show.much.more.intense.replicas..From.a.donor-to-acceptor.pair.(DAP).transition.at.about.3.22.eV.and.free.electron-acceptor.transition.(FA).showing.at.T.>.40.K,.they.also.found.the.acceptor-binding.energy.as.about.195.meV.

The. understanding. of. point. defects. is. important. for. the. application. of. the. success-ful.optoelectronic.devices..Defects. in.ZnO.can. lead. to.an.emission.band.that. is. located.between.420.and.700.nm..Various.models.have.been.proposed.for.the.explanation.of.the.green.emission.in.ZnO,.such.as.the.emission.involves.O-vacancy.(VO),.interstitial.Zn.and.O,.Zn-vacancy.(VZn),.and.an.electronic.transition.from.an.interstitial.Zn.to.a.Zn-vacancy.[7–11]..The.energy.levels.of.intrinsic.and.extrinsic.defects.in.ZnO.have.also.been.reported.by. theoretical. investigations. [12].. Zhao. et. al.. [13]. investigated. the. ZnO. implanted. with.O.and.Zn.in.order.to.find.the.origin.of.the.green.emission..They.showed.that.the.VZn.is.


responsible.for.the.observed.deep-level.emission.by.comparing.the.PL.spectra.for.the.sam-ples.with.different.implantations..They.thermally.treated.the.samples.in.an.oxygen.gas.environment.after.the.implantation..They.showed.that.the.O.and.Zn.implantations.clearly.influence.on.the.deep-level.emission.by.comparing.the.PL.spectra.for.the.samples.with.different.implantations..From.these.results,.it.was.concluded.that.the.VZn.is.responsible.for.the.observed.deep-level.emission.(green.emission)..They.measured.the.PL.spectra.of.the.as-grown.sample.and.the.samples.annealed.in.O.gas.and.Ar-air.mixing.gas.as.shown.in.Figure.7.3..Almost.no.emission.between.400.and.700.nm.is.observed.in.the.PL.spectrum.from.the.as-grown.ZnO,.whereas.a.broadband.emission.appears.in.this.range.after.anneal-ing.in.O.and.Ar-air.mixing.gas..They.observed.that.the.PL.intensity.of.the.broadband.from.the.sample.annealed.in.Ar-air.mixing.gas.is.much.higher.than.from.the.sample.annealed.in.O.gas..From.these.results,.they.found.that.the.O.interstitial.(Oi).is.not.involved.in.this.emission.

Figure.7.4.shows.the.RT.PL.spectra.from.the.Zn-.and.O-implanted.samples.with.vari-ous.concentrations..There.are.only.the.FXs.and.the.broadband.emission.(i.e.,.white.light.emission.band.[WLEB]. in.Figure.7.4). in. the.RT.PL.spectra..The.PL.intensity.was.nor-malized.to.the.FX,.so.the.relative.intensity.of.the.broadband.emission.in.the.normal-ized.PL.spectra.provides.a. relative. indication.of. the.defect. concentration. involved. in.the.broad.emission..They.observed.that.the.broad.emission.is.significantly.suppressed.after.Zn.implantations.in.Figure.7.4a..In.contrast,.the.O.implantations.have.much.less.

3.310101

102

103

104

D0

(D0X)105

3.315 3.320

T = 4.5 K

29.9 meV

2(s,p)

3.340 3.345 3.350 3.355 3.360 3.365

1s

3.325

TES

Energy (eV)

Energy (eV)

PL in

tens

ity

3.330 3.335 3.340

FIGURE 7.2Details.of.the.BE.(upper.trace,.top.energy.scale).and.the.TES.regions.(lower.trace,.bottom.energy.scale).in.PL.in.comparison..The.major.features.of.the.BE.pattern.can.be.rediscovered.in.the.TES.spectrum,.and.similarly.repeated.in.the.next.peak,.which.is.11.4.meV.low.in.energy..Note.that.the.top.energy.axis.is.shifted.by.29.9.meV.relative.to.the.bottom.energy.axis..Inset:.The.excitation.of.the.D0.final.state.taking.place.in.the.recombination.process,.which.results.in.the.“TES”.line..(Reproduced.from.Physica B,.308–310,.Thonke,.K.,.Gruber,.T.,.Teofilov,.N.,.Schönfelder,.R.,.Waag,.A.,.and.Sauer,.R.,.945,.2001..Copyright.2001,.with.permission.from.Elsevier.)


4000

0.01

0.1

1

10

100

5000 6000

Wavelength (Å)

PL in

tens

ity (a

. u.)

7000 8000

Annealedin Ar-airmixing gas

Annealedin O-gasAs-grown

9000

RT

FIGURE 7.3PL.spectra.of.the.as-grown.and.the.annealed.ZnO.samples.in.Ar-air.mixing.gas.and.O.gas,.measured.at.RT.with.excitation.power.of.20.mW,.the.excitation.wavelength.is.350.nm..(Reproduced.with.permission.from.Zhao,.Q.X.,.Klason,.P.,.Willander,.M.,.Zhong,.H.M.,.Lu,.W.,.and.Yang,.J.H.,.Appl. Phys. Lett.,.87,.211912,.2005..Copyright.2005,.American.Institute.of.Physics.)

4000 5000(a)

6000 7000 8000

0.25

0.50

0.75

1.00

1.25T=RT WLEB Annealed

after Znimplantation(×1017/cm3)

PL in

tens

ity (a

. u.)

No151050100500

Wavelength (Å)4000

(b)

0.2

0.4

0.6

0.8

1.0

1.2

1.4 T=RT WLEB Annealedafter O

implantation(×1017/cm3)

5000Wavelength (Å)

PL in

tens

ity (a

. u.)

6000 7000

No151050100500

8000

FIGURE 7.4PL.spectra.of.the.Zn-.(a).and.O-implanted.(b).ZnO.samples.after.thermal.annealing.in.oxygen.gas,.measured.at.RT.with.excitation.power.of.20.mW,.the.excitation.wavelength.is.350.nm..(Reproduced.with.permission.from.Zhao,.Q.X.,.Klason,.P.,.Willander,.M.,.Zhong,.H.M.,.Lu,.W.,.and.Yang,. J.H.,.Appl. Phys. Lett.,. 87,. 211912,.2005..Copyright.2005,.American.Institute.of.Physics.)


effect.on.the.broad.emission..No.broadband.emission.is.observed.for.as-grown.sample.before.thermal.treatment.as.shown.in.Figure.7.3..They.expected.that.the.Zni.concentra-tion.increases.with.increasing.Zn.concentration.and.the.Oi.concentration.increases.in.O-implanted.samples.with.increasing.O.concentrations..Therefore,.they.concluded.that.the.broadband.emission.does.not.originate.from.both.Zni.and.Oi,.but.VO.or.VZn.are.the.only.two.possible.defects.that.are.involved.in.the.broadband.emission..As.seen.in.Figure.7.4,.they.observed.that.Zn.implantations.have.more.influence.on.the.broadband.emis-sion.than.O.implantations..These.results.indicated.that.during.annealing.the.implanted.Zn.atoms.in.the.samples.reduce.the.defects.involved.in.the.broadband.emission.more.effectively.than.implanted.O.atoms.

7.2.2 Depth-Resolved Optical Properties in ZnO

Several.groups.have.reported.optical.properties.of.various.ZnO.structures,.but.some.of.the.fundamental.information.of.ZnO.is.still.not.well.understood..The.dominant.emission.peak. energy. measured. at. RT. is. somewhat. varied. and. controversial,. and. the. dominant.emission.peak. energy.at.RT. is.often. found. to.be. lower. than. the.expected.RT-free.exci-ton.emission.peak.energy.of.∼3.3.eV.[14–17]..Cho.et.al..[3].reported.depth-resolved.optical.properties.of.excitonic.and.phonon-assisted.transitions.in.ZnO.epilayers.[4].by.cathodolu-minescence.(CL).and.PL..Figure.7.5a.and.b.shows.CL.spectra.of.a.ZnO.epilayer.for.Ve.of.5.and.20.kV.with.temperature.varying.from.5.to.300.K..Thus,.it.provides.depth-resolved.emission.properties.from.the.top.surface.and.from.the.interior.area.of.the.sample,.respec-tively..The.CL.spectra.taken.in.whole.temperature.range.at.Ve.of.5.kV.show.very.similar.spectral.change.to.the.PL.spectra.shown.in.Figure.7.1..However,.the.CL.spectra.taken.at.

Photon energy (eV)

CL in

tens

ity (a

. u.)

CL in

tens

ity (a

. u.)

3.1

300 K 300 K

250 K200 K

175 K

150 K

125 K

100 K

75 K

50 K30 K

5 K

250 K200 K

175 K

150 K

125 K

100 K75 K50 K30 K

5 K

400 400 390 380 370390 380Wavelength (nm) Wavelength (nm)

370

Ve = 5 kV Ve = 20 kV

(a) (b)3.2 3.3 3.4

Photon energy (eV)3.1 3.2 3.3 3.4

FIGURE 7.5Temperature-dependent.CL.spectra.of.the.ZnO.epilayer.excited.by.electron.beam.with.acceleration.voltages.of.(a).5.kV.and.(b).20.kV..(Reproduced.with.permission.from.Cho,.Y.H.,.Kim,.J.Y.,.Kwack,.H.S.,.Kwon,.B.J.,.Dang,.L.S.,.Ko,.H.J.,.and.Yao,.T.,.Appl. Phys. Lett.,.89,.201903,.2006..Copyright.2006,.American.Institute.of.Physics.)


Ve.of.20.kV.show.very.different.spectral.change.with.temperature..One.can.notice.that.the.emission.properties.near.the.top.surface.(5.kV).and.the.interior.(20.kV).of.the.sample.are.similar. only. at. low. temperatures.. However,. the. FX. emission. becomes. strongest. among.the.transitions.for.Ve.=.5.kV,.while.a.decrease.in.bound.exciton.(BX).emission.is.observed.for.the.case.of.Ve.=.20.kV.with.increasing.temperature..As.a.result,.the.RT.emission.peak.is.dominated.by.FX.emission.at.∼3.30.eV.for.Ve.=.5.kV,.whereas.it.is.dominated.by.FX-1LO.emission.at.∼3.24.eV.for.Ve.=.20.kV..One.can.still.see.both.the.FX.and.FX-1LO.transition.fea-tures.at.300.K.in.the.case.of.Ve.=.5.kV.

To.investigate.the.depth-resolved.optical.transitions,.the.electron.acceleration.voltage-dependent.CL.spectra.of.ZnO.were.taken.at.various.temperatures.of.5,.150,.and.300.K..At.5.K,.the.DBX.emission.is.dominant.and.the.DBX.peak.position.is.almost.the.same.in.all.the.cases.with.an.electron.voltage.varying.from.2.to.20.kV..At.150.K,.the.FX.emission.is.dominant.for.lower.electron.voltages.(Ve.<.10.kV),.while.the.emission.intensities.from.FX.and.FX-1LO.become.comparable.for.higher.electron.voltages.(Ve.>.10.kV)..Interestingly,.at.300.K,. the.FX.emission.is.dominant.for. lower.electron.voltages,.while.the.FX-1LO.emis-sion.becomes.predominant.for.higher.electron.voltages..As.a.result,.the.RT.emission.peak.can.be.either.∼3.30.eV.(when.FX.emission.is.dominant).or.∼3.24.eV.(when.FX-1LO.emis-sion.is.dominant),.depending.on.the.depth.of.the.sample.in.this.case..Moreover,.Ye.et.al..[18].showed.depth-resolved.CL.information.that.was.calculated.as.a.function.of.electron.penetration.depth.using.Monte.Carlo.simulations.of.electron-solid.interaction..Figure.7.6a.shows.simulated.total.energy-dose.profiles. for.a.500.nm.ZnO.epilayer.on.sapphire.sub-strate.calculated.using.a.number.of.beam.energies..Simulated.CL.generation.profiles.by.correcting.the.profiles.in.Figure.7.6a.for.internal.absorption.are.shown.in.Figure.7.6b.

Due.to.the.different.penetration.depths.by.varying.acceleration.voltage,.one.can.suspect.that.the.self-absorption.(reabsorption).process.of.FX.emission.from.deep.penetration.depth.can.happen.and.may.be.responsible.for.this.observation.in.CL.spectra..In.order.to.confirm.

Depth (nm)Depth (nm)

Ener

gy d

ose (

a. u.

)

CL g

ener

atio

n (a

. u.)

0 250 500

6 keV2 keV

(a) (b)

Internal absorptioncorrection

Eb (keV) Eb (keV)

9 keV12 keV14 keV16 keV

18 keV20 keV

6 keV2 keV

9 keV12 keV14 keV16 keV

18 keV20 keV

750 0 250 500 750

FIGURE 7.6(a).Simulated. total. energy-dose.profiles. for.a.500.nm.ZnO.epilayer.on.sapphire. substrate. calculated.using.a.number. of. beam. energies.. (b). Simulated. CL. generation. profiles. by. correcting. the. profiles. in. (a). for. internal.absorption..(Reproduced.with.permission.from.Ye,.J.D.,.Zhao,.H.,.Liu,.W.,.Gu,.S.L.,.Zhang,.R.,.Zheng,.Y.D.,.Tan,.S.T.,.Sun,.X.W.,.Lo,.G.Q.,.and.Teo,.K.L.,.Appl. Phys. Lett.,.92,.131914,.2008..Copyright.2008,.American.Institute.of.Physics.)


whether. the. self-absorption. is. responsible. for. the. observed. phenomenon,. they. directly.performed.PL.experiments.for.the.backside.of.the.same.ZnO.sample.by.an.excitation.wave-length.of.325.nm..Since.the.materials.at.the.backside.(i.e.,.sapphire.and.MgO).have.higher.bandgap.energy.than.the.excitation.energy.and.ZnO.bandgap,.the.luminescence.from.the.backside. is.only. from.ZnO.epilayer.and. is.not.affected.by. the. self-absorption. from.the.sample..Figure.7.7.exhibits.the.temperature-dependent.PL.spectra.taken.from.the.backside.of.ZnO.epilayer,.which.shows.very.similar.spectral.change.with.temperature.to.that.of.CL.spectra.taken.at.20.kV.as.seen.in.Figure.7.5b..Therefore,.the.possibility.of.self-absorption.can.be.ruled.out.in.the.observed.phenomenon.

It.was.noted.that.stronger.FX-1LO.emission.than.FX.emission.is.possible.at.the.back.surface.of.the.sample.at.elevated.temperatures.(T.>.150.K),.in.contrast.to.the.case.of.the.front.surface.of.the.sample.in.which.FX.emission.is.always.stronger.than.FX-1LO.emis-sion. for. all. measured. temperatures.. The. strong. FX-1LO. emission. may. be. associated.with.the.strong.exciton-LO-phonon.interaction.coupling.in.highly.polar.ZnO.material..However,.if.it.is.only.due.to.the.intrinsic.properties.of.ZnO,.the.intensity.ratio.of.FX.to.FX-1LO.emissions.should.be.almost.the.same.at.a.certain.temperature.even.though.their.intensities. can.be.varied,.depending.on. the. sample.depth..Therefore,.based.on. these.results,.it.may.not.be.solely.explained.by.the.intrinsic.nature.of.ZnO..Instead,.it.can.be.

3.0

410ZnO sub.

400 390 380 370 360

300 K

250 K200 K

175 K

150 K

125 K

100 K

75 K50 K

30 K10 K

3.1 3.2 3.3Photon energy (eV)

PL in

tens

ity (a

. u.)

Wavelength (nm)

3.4 3.5

FIGURE 7.7Temperature-dependent.PL.spectra.from.the.backside.of.the.ZnO.epilayer.measured.from.10.to.300.K..Above.150.K,.the.FX-1LO.becomes.dominant.over.FX..(Reproduced.with.permission.from.Cho,.Y.H.,.Kim,.J.Y.,.Kwack,.H.S.,.Kwon,.B.J.,.Dang,.L.S.,.Ko,.H.J.,.and.Yao,.T.,.Appl. Phys. Lett.,.89,.201903,.2006..Copyright.2006,.American.Institute.of.Physics.)


deeply.related.to.the.different.status.of.other.extrinsic.properties.of.ZnO.depending.on.the.sample.depth.due.to.the.status.of.strain,.defects,.or.impurities.that.can.be.different.at.the.sample.surface.and.in.the.interface.region..On.the.other.hand,.they.observed.a.higher. intensity.ratio.of. the.overall.excitonic.emissions.to.the.deep-level.emission.(at.∼2.3.eV).from.the.front.surface.rather.than.that.observed.the.from.back.surface,.indicat-ing.relatively.poor.material.quality.as.expected. for. the.ZnO/substrate. interface.area..Excitons.can.recombine.radiatively.at.the.exciton.band.minimum.of.wave.vector.K.=.0.(i.e.,.zero-phonon.FX.emission).or.with.emission.of.LO-phonons.at.|K|.>.0.(i.e.,.FX-nLO.emission).. On. the. other. hand,. excitons. can. also. recombine. nonradiatively. via. defect.states.or.traps..Supposing.there.exists.only.radiative.recombination,.the.relative.inten-sity.ratio.of.FX.to.FX-nLO.emissions.should.decrease.with.increasing.temperature.since.there.are.more.excitons.with.|K|.>.0.as.a.result.of.thermalization.(as.seen.in.all.cases)..If.the.nonradiative.processes.via.defects.or.traps.were.considered,.the.preceding.radia-tive.recombination.processes.of.excitons.will.be.reduced..However,. the.reduction. for.FX-1LO.emission.may.be.smaller.than.that.for.FX.emission.since.the.relaxation.time.of.the.LO-phonon.emission.is.very.fast.(∼10−13.s).and.defects.can.increase.the.1LO-phonon.scattering.[19]..Therefore,.it.was.concluded.that.a.strong.decrease.of.FX.emission.with.respect.to.FX-1LO.emission.can.be.expected.when.the.probed.volume.is.of.poor.mate-rial.quality,.which.is.the.case.for.CL.observed.at.20.kV.(Figure.7.5b).and.PL.observed.from.the.backside.(Figure.7.7).

7.2.3 Optical Properties of ZnO Grown under Various Conditions

It.is.well.known.that.nonradiative.recombination.centers.and.polarization-induced.electric.field.that.are.generated.along.the.growth.direction.due.to.spontaneous.and.piezoelectric.polarizations.reduce.the.internal.quantum.efficiency.and.influence.on.the.device.proper-ties..Therefore,.both.the.suppression.of.native.defect.formation.and.the.control.of.growth.plane.are.important.for.the.realization.of.optoelectronic.devices..The.recombination.mech-anisms.and.atomic.identity.of.nonradiative.recombination.centers.depending.on.the.influ-ence.of.various.substrates.must.be.understood.

Fujimoto.et.al..[20].showed.the.optical.characteristics.of.the.ZnO.films.grown.on.a-plane.sapphire. and. ScMgAlO4. (SCAM). substrates. by. metal-organic. chemical. vapor. deposition.(MOCVD).under.various.conditions.in.an.H2.ambient,.as.shown.in.Figure.7.8a..They.found.best.PL.properties.and.intense.electroluminescence.from.the.ZnO.film.grown.under.Zn-rich.conditions..The.strongest.D0X.peak.at.3.361.eV.exhibits.a.narrow.full.width.at.half.maxi-mum.(FWHM).of.2.3.meV..The.energy.separation.between.the.peaks.in.the.near-band-edge.emission.(NBE).and.the.TES.peaks.(a,.b,.and.c).were.34,.40,.and.48.meV,.respectively,.and.therefore. the.peaks. in. the.D0X.at.NBE.was.assigned.as. I4,. I6,.and. I9. [21]..Free.excitons.A.and.BωL.were.clearly.detected.at.3.376.eV.and.3.390.eV,.respectively.[22]..A.peak.detected.at.3.420.eV.is.assigned.as.a.FX.(n.=.2).line..They.found.that.this.peak.disappeared.at.tempera-tures.T.>.50.K,.as.shown.in.Figure.7.8b.and.it.is.agreed.with.the.result.reported.by.Tsukazaki.et.al..[23]..They.concluded.that.these.fine.excitonic.transitions.of.the.PL.spectra.indicate.the.good.crystallinity.of.the.ZnO.films.grown.by.MOCVD.under.Zn-rich.condition..They.also.measured.temperature-dependent.PL.in.order.to.investigate.the.effect.of.the.Zn-rich.condi-tions.(Sample.A)..With.increasing.the.temperature.from.8.to.300.K,.the.phonon.replicas.dis-appear.at.T.>.60.K.and.the.zero-phonon.FX.emission.peak.becomes.dominant,.which.agrees.well.with.the.results.in.the.literature.by.Cho.et.al..[3]..They.concluded.that.the.rich.excitonic.PL.structures.are.probably.due.to.the.reduction.of.Zni.and.VZn.by.using.N2O.gas.and.Zn-rich.


conditions..They.obtained.a.long.PL.lifetime.(τPL).of.2.6.ns.at.300.K.and.high.internal.quan-tum.efficiency.of.5.5%.for.the.ZnO.films,.which.are.comparable.to.the.best.values.[τPL.of.3.8.ns.(Ref..[25]).and.internal.quantum.efficiency.of.9.6%.(Ref..[26]).at.300.K].

Nam.et.al..investigated.a.nonpolar.(a-plane).ZnO.film.grown.on.r-plane.sapphire.using.a.PAMBE,.which.was.compared.with.a.polar.(c-plane).ZnO.film.on.the.c-plane.sapphire.[27]..They.measured.the.low-temperature.PL.spectra.of.the.polar.and.nonpolar.ZnO.epitaxial.films.as.shown.in.Figure.7.9..For.polar.ZnO.films.(Figure.7.9a),.the.peaks.at.3.375,.3.381,.and.3.419.eV.correspond.to.the.A,.B,.and.C.free.excitons.(FXA,.FXB,.and.FXC),.respectively,.as.reported.in.bulk.ZnO.[28]..The.transitions.detected.at.3.359.and.3.363.eV.are.attributed.to.A-excitons.bound.to.neutral.donors.(D0X).with.12–16.meV.of.donor.localization.energy.[29]..As.shown.in.the.PL.spectrum.of.a.nonpolar.ZnO.film.(Figure.7.9b),.the.dominant.D0X.transition. peak. at. 3.386.eV. is. higher. than. that. in. FX. in. unstrained. ZnO.. The. transition.at.3.326.eV.is.attributed.to.a. free-electron-to-bound-hole. (e-A0). transition,.which. is.often.observed.in.the.ZnO.system.at.3.314.eV.in.polar.(strain-free).ZnO.[30,31]..The.large.blue-shifts.of.the.transition.energies.as.compared.to.those.of.strain-free.ZnO.were.explained.by.the.compressive.strain.between.a-plane.ZnO.films.on.r-plane.sapphire.induced.by.lattice.and.thermal.expansion.mismatches.[32]..In.the.temperature-dependent.PL.spectra.of.polar.ZnO,.FXA.and.its.phonon.replicas.are.clearly.observed.at.T.>.55.K,.whereas.the.D0X.line.disappears.at.around.140.K..In.the.case.of.nonpolar.ZnO,.however,.there.is.no.signature.of.FX.even.at.RT..From.this.result,.they.pointed.out.that.the.localization.energy.of.the.D0X.in.nonpolar.ZnO.is.much.larger.than.that.in.polar.ZnO,.which.can.be.due.to.the.different.identity.of.the.donor.or.a.different.exciton.itself.

Reynolds.et.al..[33].measured.the.FX.emission.spectra.and.the.time-resolved.PL.(TRPL).for.an.unstrained.ZnO.sample.and.for.the.strained.sample.grown.by.a.hydrothermal.pro-cess..In.the.unstrained.sample,.only.the.Γ5.exciton.is.observed.since.the.Γ6.is.forbidden,.

D

C

B 50 K40 K30 K

20 K

10 K8 K

×103

(a) (b)

TES40 meV

48 meV

34 meV FB=3

.390

eV

FA(n

=2)=

3.42

1 eV

FA=3

.376

eV

bac

I6-1LO

I6

I4

I9

Sample A

Photon energy (eV)Photon energy (eV)

Log

PL in

tens

ity (a

. u.)

3.30

O2 gas ona-sapphire sub.

N2O gas ona-sapphire sub.

N2O gas onSCAM sub.

Zn-rich ona-sapphire sub.

3.35 3.40 3.45 3.38 3.40 3.42 3.44

FIGURE 7.8(a).PL.spectra.measured.at.8.K.for.ZnO.films.grown.under.various.conditions.(samples.A–D).by.H2.ambient.MOCVD.with.repeated.temperature.modulation..Sample.D.was.referred.to.Ref..[24]..(b).Temperature.depen-dence.of.higher-resolution.PL.spectrum.from.8. to.50.K. in. the.range.of. free-exciton.energy. for. the.ZnO.film.grown. under. Zn-rich. condition. (sample. A).. (Reproduced. with. permission. from. Fujimoto,. E.,. Watanabe,. K.,.Matsumoto,. Y.,. Koinuma,. H.,. and. Sumiya,. M.,. Appl. Phys. Lett.,. 97,. 131913,. 2010.. Copyright. 2010,. American.Institute.of.Physics.)


while.both.the.Γ5.and.Γ6.excitons.are.clearly.observed.and.are.shifted.to.lower.energies.by.∼0.0025.eV.due.to.a.tensile.strain.in.the.strained.sample.as.shown.in.Figure.7.10..They.mea-sured.the.lifetime.of.the.Γ5.exciton.in.the.strained.sample.and.in.the.unstrained.sample,.which.were.determined.to.be.259.and.322.ps,.respectively..The.lifetime.of.the.Γ6.exciton.was.245.ps..It.was.point.out.that.FX.lifetimes.are.determined.by.both.radiative.decay.and.nonradiative.decay.and.by.capture.processes.

Recently,. Takamizu. et. al.. made. a. direct. correlation. between. the. equivalent. internal.quantum.efficiency.(ηint

eq).at.300.K.(defined.as.the.ratio.of.integrated.PL.intensity.at.300.K.and.that.at. lowest.measured.temperature).and.the.TRPL.lifetime.(τPL).for.the.NBE.exci-tonic.PL.peak.in.ZnO.epilayers.grown.on.Zn-polar.ZnO.substrates.by.PAMBE.[26]..They.found.that.the.value.of.ηint

eq.for.the.NBE.free-excitonic.emission.peak.was.9.6%.(=.I300K/I13K).at.300.K,.which.is.higher.than.6.3%.[25].of.the.epilayers.grown.by.laser-assisted.molecular.beam.epitaxy.on.SCAM.substrates.using.the.high-temperature-annealed.self-buffer.layer.[34]..They.observed.that.the.fast.component.of.the.lifetime.(1.2.ns).is.longer.than.that.of.the.bulk.GaN.substrate.prepared.by.the.lateral.epitaxial.overgrowth.technique.(860.ps.[35]),.

Photon energy (eV)

PL in

tens

ity (a

. u.)

3.15 3.20 3.25 3.30

e-A0 3.326 DX 3.386

3.403.35

DAP

DX

DX-1LO

DX-2LO3.288

3.359

FXA 3.375

FXB 3.381

FXC 3.4193.3633.218

(b)

(a)

3.45 3.50

FIGURE 7.9PL.spectra.of.(a).polar.ZnO.on.c-plane.sapphire.(c-ZnO).and.(b).nonpolar.ZnO.on.r-plane.sapphire.(a-ZnO).at.12.K.on.a.logarithm.scale..(Reproduced.with.permission.from.Nam,.Y.S.,.Lee,.S.W.,.Baek,.K.S.,.Chang,.S.K.,.Song,.J.-H.,.Han.S.K.,.Hong,.S.-K.,.and.Yao,.T.,.Appl. Phys. Lett.,.92,.201907,.2008..Copyright.2008,.American.Institute.of.Physics.)


and.concluded.that.the.concentration.of.nonradiative.recombination.centers.was.consid-erably. reduced.. Figure. 7.11. exhibits. 13.K. PL. spectrum. of. their. best. sample. (Tg.=.800°C)..The.spectrum.exhibits.well-resolved.PL.peaks.between.3.376.and.3.394.eV,.which.originate.from. the. lower. and. upper. polariton. branches. of. A. and. B. excitons. [25].. They. observed.sharp.PL.peaks.due.to.the.recombination.of.D0Xs.(3.35.∼.3.374.eV),.and.the.TES.at.3.3332.eV..They.pointed.out.that.these.fine.excitonic.features.reflect.the.high.purity.with.low.defect.density.for.the.Zn-polar.homoepitaxial.ZnO.films.

Energy (eV)

Nor

mal

ized

PL

inte

nsity

3.3650

0.2

0.4

0.6

0.8

Strained ZnOUnstrained ZnO

1.0

3.370 3.375

6

5

5

3.380

FIGURE 7.10Free-exciton.emission.spectra.for.strained.and.unstrained.ZnO.samples..(Reproduced.with.permission.from.Reynolds,.D.C.,.Look,.D.C.,.Jogai,.B.,.Hoelscher,.J.E.,.Sherriff,.R.E.,.Harris,.M.T.,.and.Callahan,.M.J.,.J. Appl. Phys.,.88,.2152,.2000..Copyright.2000,.American.Institute.of.Physics.)

PL in

tens

ity (a

. u.)

3.0 3.1 3.2 3.3 3.4

13 K He-Cd 325 nm(5 W/cm2)

Photon energy (eV)

TES

In 3

.357

0H

3.3

640

AI 3

.360

8

A(n

=2) 3

.421

4

B ωL 3

.393

2BωT 3

.382

1

Aω T 3

.376

7A

ω L 3.3

785

3.35

13 K He-Cd 325 nm(5 W/cm2)

3.40 3.45(b)Photon energy (eV)(a)

FIGURE 7.11(a).PL.spectrum.measured.at.13.K.of.the.ZnO.epilayer.grown.on.a.Zn-polar.ZnO.substrate.(Tg.=.800°C)..(b).Near-band-edge.high-resolution.PL.spectrum.at.13.K..(Reproduced.with.permission.from.Takamizu,.D.,.Nishimoto,.Y.,.Akasaka,.S.,.Yuji,.H.,.Tamura,.K.,.Nakahara,.K.,.Onuma,.T.,.Tanabe,.T.,.Takasu,.H.,.Kawasaki,.M.,.and.Chichibu,.S.F.,.J. Appl. Phys.,.103,.063502,.2008..Copyright.2008,.American.Institute.of.Physics.)


7.3 Optical Properties of p-Type ZnO

The. internal.quantum.efficiency.of.ZnO-based.photonic.devices. is. strongly. influenced.by.the.luminescence.yield.of.each.constituent.epilayer..Hence,.p-type.ZnO.epilayer.with.good.crystal.and.optical.qualities. is.needed.for. the.devices.with.high.efficiency..Many.problems.remain.to.be.solved.in.the.growth.of.efficient.and.stable.p-type.ZnO.for.practi-cal.device.applications..Up.to.now,.considerable.efforts.have.been.made.to.produce.p-type.materials. theoretically.and.experimentally..Group-V.elements.of.N,.As,.P,.and.Sb.were.considered.as.p-type.dopants.and.had.produced.p-type.ZnO. thin.films. [36,37–42]. and.some. groups. accomplished. p-type. ZnO. by. controlling. partial. pressure. of. oxygen. [43]..The.investigations.of.optical.properties.of.p-type.ZnO.are.very.important.to.understand.the.nature.of.the.acceptors.and.DAP.transitions.formed.in.p-type.ZnO.by.various.doping.techniques.

7.3.1 N-Doped ZnO Epilayer

Look. et. al.. [36]. have. succeeded. in. the. growth. of. N-doped,. p-type. ZnO. by. molecular.beam. epitaxy. (MBE),. which. was. confirmed. by. the. Hall-effect. measurement.. Figure.7.12.exhibits.the.low-temperature.PL.results.of.N-doped.ZnO.together.with.the.case.of.undoped.ZnO.bulk.for.comparison..They.observed.a.series.of.sharp.peaks.dominated.in.high-quality.bulk.ZnO.probably.due.to.bound.excitons.associated.with.neutral.donors.[44].or.very.shallow.neutral.acceptors. [45]..The.weaker.nature.of. the.A0X. in.undoped.ZnO.is.probably.due.to.the.low.donor.and.acceptor.concentrations.in.this.material.[46]..The.most.obvious.feature. in.PL.properties.of. the.N-doped.ZnO.samples.compared.to.the.undoped.ZnO.samples.is.a.strong.emission.that.appeared.at.3.315.eV.(near.the.deep.A0X.line.at.3.318.eV.in.the.undoped.case),.and.a.relatively.smaller.emission.in.the.D0X.region..They.explained.that.the.3.315.eV.transition.is.attributed.to.A0X.transition.related.to.NO.(antisite.N.on.O.site).and.that.the.emission.intensity.of.the.A0X.is.dominant.over.

Energy (eV)

PL in

tens

ity (a

. u.)

3.20

400

0

800

1200

1600N-doped MBEUndoped bulk

×1300 ×16

TES

×1

A0X - 1LO,and D0A0

A0X D0X

3.25 3.30 3.35

FIGURE 7.12PL. spectra,. at. 2.K,. for. two. ZnO. samples,. an. undoped. bulk. sample,. and. an. N-doped,. MBE-grown. epitaxial.layer..(Reproduced.with.permission.from.Look,.D.C.,.Reynolds,.D.C.,.Litton,.C.W.,.Jones,.R.L.,.Eason,.D.B.,.and.Gantwell,.G.,.Appl. Phys. Lett.,.81,.1830,.2002..Copyright.2002,.American.Institute.of.Physics.)


the.D0X.because.of. the. large.number.of.NO.centers..The.broad.emission.at.3.238.eV. in.the.N-doped.sample.was.attributed.to.an.LO-phonon.replica.of.A0X.and.DAP.(or.D0A0).transitions..They.also.estimated.the.acceptor-binding.energy.(EA.=.0.17.∼.0.20.eV)..They.pointed.out.that.the.A0X.emission.is.much.stronger.than.the.DAP.recombination.in.heav-ily.N-doped.ZnO,.which.is.opposite.to.the.case.of.heavily.Mg-doped.GaN.[47]..From.the.results,.they.concluded.that.the.luminescence.in.ZnO-based.devices.is.associated.more.with.excitonic.processes.than.GaN-based.ones.

7.3.2 As-Doped ZnO Epilayer

Ryu.et.al.. [48].have.succeeded.in.the.growth.of.As-doped,.p-type.ZnO.by.hybrid.beam.deposition.[49]..PL.spectrum.from.p-type.ZnO:As.samples.shows.new.As-related.peaks..These. new. peaks. are. located. at. 3.359,. 3.322,. and. 3.273.eV. for. lightly. doped. ZnO:As. in.Figure.7.13b,.and.at.3.219.and.3.172.eV.for.a.heavily.doped.case.in.Figure.7.13c..The.peaks.at.3.359.eV.are.dominant.in.the.lightly.doped.sample,.while.the.peaks.at.3.219.and.3.172.eV.become. dominant. in. the. heavily. doped. sample.. As. the. As. concentration. increases,. the.

(a)

3.362 eV - (D°X)

Wavelength (nm)

Coun

ts (a

. u.)

360 365 370 375 380 385 390

(b)

3.273 eV - (FA)

3.204 eV - DAP

3.322 eV - (FA)

3.359 eV - (A°X)

Wavelength (nm)

Coun

ts (a

. u.)

360 365 370 375 380 385 390

Wavelength (nm)(c)

3.219 eV - DAP

3.172 eV - DAP

3.348 eV - (A°X)3.314 eV - (FA)

Coun

ts (a

. u.)

360 370 380 390 400 410

FIGURE 7.13PL.spectra.for.ZnO.samples.measured.at.12.K:.(a).undoped,.(b).lightly.doped.(NAs.∼.low.1018.cm−3),.and.(c).heavily.doped.(NAs.∼.low.1020.cm−3)..The.peaks.are.shifted.to.lower.energies.in.(c).as.the.As.concentration.is.increased..(Reproduced.with.permission.from.Ryu,.Y.R.,.Lee,.T.S.,.and.White,.H.W.,.Appl. Phys. Lett.,.83,.87,.2003..Copyright.2003,.American.Institute.of.Physics.)


peak. positions. are. shifted. to. lower. energies.. It. is. reasonable. to. attribute. peaks. located.at.3.359.eV.as.A0X.emissions.and.the.peaks.located.at.3.322.and.3.273.eV.as.the.emissions.between.free.electrons.and.acceptor.holes. (FA)..They.observed.DAP.transitions.at.3.219.and.3.172.eV..They.estimated.the.optical.binding.energy.(EA).of.As.acceptors.by.using.these.results.and.the.relation,.EFA. (3.322.or.3.273.eV).=.Eg. (3.437.eV)—EA.+.kBT/2..Neglecting.the.thermal.energy.term.in.the.equation,.the.derived.values.of.EA.were.about.115.and.164.meV.

7.3.3 P-Doped ZnO Epilayer

Kim.et.al..reported.that.phosphorus.(P)-doped,.p-type.ZnO.can.be.fabricated.with.good.reliability.on.c-plane.sapphire.substrates.by.radio.frequency.(rf).magnetron.sputtering.by.using.a.ZnO.target.mixed.with.P2O5.[42]..Since.the.as-grown.ZnO:P.layer.exhibits.semi-insulator.property,.phosphorus.dopants.need.to.be.thermally.activated.to.act.as.an.accep-tor. in. ZnO:P. layers.. At. rapid. thermal. annealing. activation. temperature. around. 800°C,.most.of.the.as-grown.semi-insulating.ZnO:P.is.converted.into.p-type.ZnO:P..Hwang.et.al..[50].performed.PL.experiments.for.the.P-doped,.p-type.ZnO.thin.films.with.varying.the.excitation.laser.power.densities.from.6.to.30.mW/cm2.at.10.K..They.observed.that.the.peak.position.at.3.310.eV.is.independent.of.the.laser.power.density.at.10.K,.whereas.the.peak.at.3.241.eV.shifts.to.the.lower.energy.side.with.decreasing.laser.power.density..This.obser-vation.strongly.suggests.that.the.origin.of.the.emission.peak.at.3.241.eV.is.from.the.DAP.transition.and.the.emission.peak.at.3.310.eV.can.be.attributed.to.the.conduction.band.to.acceptor.transition.[or.FA.transition.or.(ABX)]..They.estimated.the.acceptor.energy.of.the.phosphorus.dopant.from.the.FA.transition.at.3.310.eV.PL.spectra.of.p-type.ZnO:P,.which.was.located.at.127.meV.above.the.valence.band,.resulting.in.a.high.hole.concentration.in.the.p-type.ZnO:P.

Kwon. et. al.. [51]. have. investigated. the. influence. of. phosphorus. doping. on. optical.properties.of.ZnO.thin.films.grown.on.c-plane.sapphire.substrates.with.different.phos-phorus. concentrations.. A. comparison. of. 10.K. PL. spectra. of. the. ZnO. thin. films. with.different. phosphorus. doping. is. presented. in. Figure. 7.14a.. An. undoped. ZnO. sample.

Photon Energy (eV) Photon Energy (eV)

High-doped ZnO:P

Low-doped ZnO:P

Undoped ZnO

2.2 2.4 2.6 2.8 3.0 3.2 3.4

PL in

tens

ity (a

. u.)

PL in

tens

ity (a

. u.)

Wavelength (nm) Wavelength (nm)

2.9 3.0

High-doped ZnO:P

Low-doped ZnO:P

Undoped ZnO

DAP’

DAPDAP DAP

(a) (b)

420 400 380 360 600 560 520 480 440 400 360

-2LO-1LO

FA

FX

10 K 10 K

BX

BX

3.1 3.2 3.3 3.4 3.5

FIGURE 7.14(a).10.K.PL.spectra.of.undoped.ZnO,.low-doped.p-type.ZnO:P,.and.high-doped.p-type.ZnO:P.samples..(b).10.K.PL.spectra.with.wider.spectral.range..(Reproduced.with.permission.from.Kwon,.B.J.,.Kwack,.H.S.,.Lee,.S.K.,.Cho,.Y.H.,.Hwang,.D.K.,.and.Park,.S.J.,.Appl. Phys. Lett.,.91,.061903,.2007..Copyright.2007,.American.Institute.of.Physics.)


showed.a.dominant.DBX. transition.at. 3.356.eV.and.DAP. transition.at. ∼3.1.eV.with. its.LO-phonon.replicas..For.the.phosphorus-doped.p-type.ZnO.samples,.additional.lower.energy.emissions.than.DBX.transition.at.∼3.32.and.∼3.24.eV.were.observed.and.assigned.to.the.free.electrons.to.the.acceptor.transition.(or.ABX)..The.intensities.of.these.lower.energy.emissions.become.stronger.with. increasing.phosphorus.doping.concentration..Furthermore,.a.new.DAP.transition.with.different.emission.energies.from.DAP.transi-tion.in.undoped.ZnO.(which.will.be.denoted.by.DAP’.in.ZnO:P).is.observed.in.ZnO:P,.while.the.DAP.transition.at.∼3.1.eV.disappears.by.phosphorus.doping,.which.indicates.that.undoped.ZnO.possesses.deep.(native).acceptors..Since.p-type.ZnO.samples.have.a.large.amount.of.shallow.acceptor.due.to.phosphorus.doping,.the.DAP.transition.and.the. deep-level. emission. are. changed.. Figure. 7.14b. shows. 10.K. PL. spectra. of. the. three.ZnO. thin.films.with.a.wider. spectral. range..A.green-colored,.deep-level.broad.emis-sion.is.seen.at.∼2.25.eV.for.the.undoped.ZnO.sample,.whereas.a.greenish-blue-colored,.deep-level.emission.appears.at.∼2.4.eV.for.both.p-type.ZnO:P.samples..The.deep-level.emission.peak.energy.of.p-type.ZnO:P.samples.is.blueshifted.with.respect.to.that.of.the.undoped.ZnO.sample..It.is.well.known.that.the.deep-level.(green).emission.in.undoped.ZnO.is.related.to.a.variety.of.defects.such.as.donor.defect.Zn.interstitial.(Zni),.O-vacancy.(VO),.acceptor.defect.Zn.vacancy.(VZn),.and.antisite.defect.O.substitutional.Zn.(OZn).due.to.the.poor.stoichiometry.of.ZnO.[52]..This.result.indicates.that.p-type.ZnO:P.has.very.few.native.donor.and.acceptor.defects,.and.that.the.observed.DAP’.and.greenish-blue.deep-level.emissions. in.p-type.ZnO:P.are.not.due.to.the.native.defects.but.due.to.the.phosphorus. dopants. in. ZnO.. By. doping. phosphorus. in. ZnO,. they. observed. a. dra-matic.suppression.of.both.DAP.(at.∼3.1.eV).and.green.deep-level.(at.∼2.25.eV).emissions.observed.in.undoped.ZnO,.as.well.as.an.emergence.of.DAP’.(at.∼3.24.eV).and.greenish-blue.deep-level. (at.∼2.4.eV).emissions.. It.was.emphasized.that. the.peak.energy.differ-ence.between.the.green.and.greenish-blue.deep-level.emissions.is.well.agreed.with.that.between. DAP. and. DAP’. emissions. in. undoped. ZnO. and. p-type. ZnO:P,. respectively..This.agreement.strongly.suggests.that.phosphorus.doping.in.ZnO.mostly.reduces.the.native.deep.acceptor.states.present.in.undoped.ZnO.(which.are.responsible.for.DAP.and.green.deep-level.emissions),.and.generates.shallow.acceptor.states.(which.are.respon-sible.for.DAP’.and.greenish-blue.deep-level.emissions).in.ZnO.to.show.a.strong.p-type.conductivity.in.ZnO:P.

Kwon.et.al..[51].performed.PL.and.TRPL.experiments.as.a.function.of.temperature.vary-ing.from.10.to.300.K.for.the.undoped.ZnO.and.p-type.ZnO:P.samples..The.optical.transi-tion.related.to.DBX.is.predominant.at.low.temperatures,.while.the.FX.transition.becomes.dominant.with.increasing.temperature.for.all.the.ZnO.samples..They.found.that.the.inten-sity.ratio.of.300.to.10.K.PL.(I300.K/I10.K). for.excitonic.emission.has.much.improved.for.the.high-doped.ZnO:P.sample. (I300.K/I10.K.=.0.149),. compared. to. the. low-doped.ZnO:P.sample.(I300.K/I10.K.=.0.045),.whereas. the.deep-level.emission. intensity. ratio.has. increased.slightly.for. the. high-doped. ZnO:P. (I300.K/I10.K.=.0.359). sample. compared. to. the. low-doped. ZnO:P.(I300.K/I10.K.=.0.279)..Therefore,.luminescence.efficiency.of.ZnO.has.improved.with.increas-ing.phosphorus.doping.concentration.

The.measured.lifetimes.of.DBX.transition.at.10.K.were.about.24–26.ps.and.those.of.FX.transition. at. 300.K. were. about.<.16. ps. for. all. the. samples,. which. were. estimated. by. the.deconvolution.processes.with.instrument.response.function.(IRF)..The.measured.lifetimes.of.undoped.ZnO.are.much.shorter.than.the.exciton-radiative.lifetime.of.about.322.ps.in.bulk.ZnO.[33],.indicating.that.the.recombination.in.the.undoped.ZnO.samples.is.domi-nated.by.nonradiative.processes.even.at.low.temperatures..It.was.noted.that.there.was.not.much.change.in.lifetime.depending.on.the.phosphorus.doping.concentration.in.the.p-type.


ZnO.samples..In.doped.p-type.semiconductors.(p0.≫ n0),.a.radiative.lifetime.can.be.defined.for.the.minority.carriers.as

. τr sp 01/≈ B p ,

where.Bsp.is.the.radiative.recombination.coefficient.(transition.probability.of.spontaneous.emission)..Nonradiative.lifetime.also.depends.on.the.np.product,.but.since.it.occurs.via.mid-gap.levels.it.is.much.less.sensitive.to.the.majority.population,.p.in.this.case..Thus,.the.nonradiative.lifetime.of.electron.in.a.p-type.semiconductor.is.given.by

. τ σ νnr th t1/ N≈ n ,

whereσn.is.electron.capture.cross.sectionvth.is.carrier.thermal.velocityNt.is.trap.density.[53]

The. difference. in. hole. concentration. between. undoped. and. high-doped. ZnO. is. almost.two.orders.of.magnitude,.resulting.in.a.decrease.in.the.radiative.lifetime.for.high-doped.p-type.ZnO.about.two.orders.of.magnitude,.while.nonradiative.lifetime.will.not.be.much.influenced. by. changing. doping. concentration.. On. the. other. hand,. the. annealing. effect.may.lower.the.electron.capture.cross.section.and.trap.density.by.curing.native.defects,.leading.to.an.increase.in.nonradiative.lifetime.for.the.doped.ZnO.samples..That.is,.they.can.estimate.that.the.radiative.lifetime.of.doped.ZnO.decreases.(to.probably.the.similar.value.of.nonradiative.lifetime.in.undoped.ZnO),.while.the.nonradiative.lifetime.of.doped.ZnO.increases..This.explains.why.they.did.not.observe.much.change.in.measured.lifetime.depending.on.the.phosphorus.doping.concentration.in.ZnO.[54].

7.3.4 Sb-Doped ZnO Epilayer

The.optical.properties.of.Sb-doped,.p-type.ZnO.films.obtained.by.thermal.oxidation.of.the.Zn-Sb.starting.material.were.reported.by.Przeździecka.et.al..[55]..A.very.well.resolved.PL.spectrum.is.observed.at.10.K.for.a.Sb-doped.ZnO.film.with.the.hole.concentration.above. 1.×.1017. cm−3,. as. shown. in. Figure. 7.15.. Peaks. at. 3.353.eV. and. at. about. 3.3.eV. are.dominated..The.emission.located.at.3.353.eV.corresponds.probably.to.A0X.specifically.associated.with.antimony.acceptors.due.to.the.highly.doped.sample.with.Sb.atoms.[29]..However,. the.origin.of. this.peak.is.still.controversial.. [56,57]..They.observed.that. the.emission.located.at.about.3.3.eV.consists.of.two.individual.peaks,.which.are.located.at.3.311.and.3.300.eV..They.estimated.that. the.emission. located.at.about.3.311.eV.may.be.related.to.Sb.dopants..They.found.that.the.equivalent.peaks.corresponding.to.nitrogen.[58],.arsenic.[59],.and.phosphorus.[50].acceptors.in.ZnO.matrix.are.in.the.same.spectral.region;.therefore,.it.can.be.supposed.that.3.311.eV.may.also.be.an.A0X.emission.and.the.peak.at.3.300.eV.may.be.the.emission.associated.with.FA.[60],.as.shown.in.Figure.7.16..In.the.PL.spectrum,.they.also.observed.a.DAP.emission.located.at.3.230.eV.and.its.phonon.replicas. at. 3.159. and. 3.083.eV. due. to. a. blueshift. in. the. temperature. range. of. 10–50.K.[48]..They.obtained.an.acceptor-binding.energy.EA.=.137.meV.from.the.position.of. the.FA. peak,. which. corresponds. well. to. theoretical. predictions. for. SbZn—2VZn. acceptor.complex..[41]


Energy (eV)

T=10K

PL In

tens

ity (a

. u.)

3.08

3 eV

3.15

9 eV

3.23

0 eV

3.30

0 eV

3.31

1 eV

3.35

3 eV

2.1 2.2 2.3 2.4 2.5 2.6 2.7

2.3 eV

2.8 2.9 3.0 3.1 3.2 3.3 3.4

FIGURE 7.15PL.spectrum.at.10.K.for.the.Sb-doped.p-ZnO.sample.obtained.by.thermal.oxidation.of.the.Zn-Sb.starting.mate-rial. (500°C. O2. for. 10.min. +. 800°C. O2. or. N2. for. 2.min).. (Reproduced. with. permission. from. Przezdziecka,. E.,.Kaminska,.E.,.Pasternak,.I.,.Piotrowska,.A.,.and.Kossut,.J.,.Phys. Rev. B,.76,.193303,.2007..Copyright.2007,.American.Physical.Society.)

3.05 3.10

DAP-2LO

DAP-LO

Energy (eV)3.34

PL in

tens

ity (a

. u.)

PL in

tens

ity (a

. u.)

3.35 3.36 3.37 3.38

3.377 eV3.367 eV

FA A0X A0X3.356 eV

3.353 eV

3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

35 K

DAP

FX 136 K95 K65 K55 K55 K45 K35 K30 K28 K

10 K

T (K)

22 K

3.15 3.20 3.25Energy (eV)

3.30 3.35 3.40

FIGURE 7.16PL. spectra. of. ZnO:Sb. sample. obtained. by. thermal. oxidation. of. the. Zn-Sb. starting. material. (500°C. O2. for.10.min.+.800°C.O2.or.N2.for.2.min).measured.as.a.function.of.temperature..FX,.free.exciton;.A0X,.acceptor.bound.to.exciton;.FA,.free.electrons.and.acceptor.transition;.and.DAP,.donor-acceptor.pair..Inset:.Zoom.of.excitonic.peak.at.35.K,.with.the.individual.components.of.excitonic.peak.clearly.showing.(solid.line,.experimental.data;.dotted.lines,.Lorentzian.multiple.peaks)..(Reproduced.with.permission.from.Przezdziecka,.E.,.Kaminska,.E.,.Pasternak,.I.,.Piotrowska,.A.,.and.Kossut,.J.,.Phys. Rev. B,.76,.193303,.2007..Copyright.2007,.American.Physical.Society.)


7.4 Optical Properties of ZnO with Different Polarities

Studies.on.the.surface.polarity.of.ZnO.have.been.promoted.by.many.researchers.in.the.point.of.view.of.crystalline.quality,.electrical.property,.p-type.doping,.surface.morphol-ogy,. and. optical. property. [36,61–64].. Since. the. O-polar. (0001). or. Zn-polar. (0001̄). face. of.ZnO.single.crystal.have.different.surface.configuration,.the.surface.polarity.would.lead.to.differences.in.the.chemical.and.physical.properties.

7.4.1 Polarity-Controlled ZnO Epilayer

Park.et.al..have.succeeded.in.the.growth.of.O-polar.and.Zn-polar.ZnO.using.Cr-compound.intermediate.layers..The.polarity.of.ZnO.film.grown.on.CrN.(rocksalt).is.determined.to.be.Zn-polar,.while.O-polar.ZnO.films.are.grown.on.the.Cr2O3.(rhombohedral).layer.[65]..Kwon. et. al.. [66]. reported. on. optical. properties. and. carrier. dynamics. of. these. polarity-controlled.ZnO.films..For.comparison,.they.prepared.two.kinds.of.ZnO.samples.that.have.both.polarities.in.the.same.ZnO.sample:.(i).a.ZnO.single.crystal.grown.hydrothermally.(Tokyo.Denpa).with.O-face.and.Zn-face.surfaces.(i.e.,.front.and.back.side.of.the.single.crys-tal).and.(ii).a.periodically.polarity-inverted.(PPI).ZnO.sample.that.has.periodic.O-polar.and.Zn-polar.surface.areas.on.the.same.ZnO.sample..The.growth.and.structure.of.PPIZnO.were.reported.elsewhere.[65].

Figure.7.17.shows.the.PL.spectra.of.the.O-polar.ZnO,.Zn-polar.ZnO,.and.PPIZnO.thin.films.at.low.temperature.(12.K).together.with.those.of.O-face.and.Zn-face.of.a.ZnO.single.crystal.for.comparison..Broad.two.peaks.at.3.36.and.3.32.eV.are.observed.at.12.K.in.both.O-polar.and.Zn-polar.ZnO.samples..Comparing.with.ZnO.single.crystal,.the.PL.peaks.at.

Photon energy (eV)

PL In

tens

ity (a

. u.)

3.05

405 400 395 390Wavelength (nm)

385 380 375 370 365

3.10 3.15 3.20

Zn-face

O-face

O-polar ZnO

Zn-polar ZnO

PPIZnO

DBXABX

ZnO SC

ZnO SC

DBX-2LO DBX-1LO

FX-2LO FX-1LO

TES FX

71 meV

71 meV 71 meV

3.25 3.30 3.35 3.40

FIGURE 7.17PL.spectra.of.O-polar.ZnO,.Zn-polar.ZnO,.PPIZnO,.O-face.ZnO.single.crystal,.and.Zn-face.ZnO.single.crys-tal.at.12.K..The.PL.spectra.are.shifted.in.the.vertical.direction.for.clarity..(Reproduced.with.permission.from.Kwon,.B.J.,.Sun,.Y.,.Chung,.J.S.,.Cho,.Y.H.,.Park,.J.S.,.and.Yao,.T.,.Appl. Phys. Lett.,.94,.061918,.2009..Copyright.2009,.American.Institute.of.Physics.)


3.36.and.3.32.eV.are.attributed.to.DBX.(dashed.line).and.ABX.(dotted.line),.respectively..The.DBX.and.ABX.transition.energies.in.the.polar.ZnO.are.consistent.with.those.of.ZnO.epilay-ers.grown.on.Al2O3.(0001).substrates.by.employing.a.thin.MgO.buffer.layer.using.PAMBE.[4]..In.the.case.of.O-polar.ZnO,.they.observe.dominant.DBX.transition,.while.dominant.ABX.transition.is.observed.in.Zn-polar.ZnO..A.huge.line.in.ABX.and.a.small.line.in.DBX.for.Zn-polar.ZnO.grown.on.a.CrN.layer.are.very.similar.to.the.spectra.of.N-doped,.p-type.ZnO.grown.by.MBE.[36],.probably.due.to.the.influence.of.N.atoms.in.the.CrN.interme-diate. layer.. In.both. the.O-polar.and.Zn-polar.ZnO. thin.films,. they.observe.very.broad.peaks.at.around.3.24.eV,.which.is.attributed.to.DAP.transition..In.the.PPIZnO.sample,.FX.transition.at.3.376.eV,.a.different.level.of.main.DBX.at.3.365.eV,.and.broad.peaks.(∼3.31.and.∼3.24.eV).are.observed..In.the.PL.spectra.of.ZnO.samples,.the.oscillatory.structure.of.the.PL.spectrum.has.an.energy.periodicity.of.∼.71.meV,.which.is.LO-phonon.energy.of.ZnO.(dash-dotted.line).

Figure.7.18a.through.c.shows.PL.and.PL-excitation.(PLE).spectra.of.the.O-polar.ZnO,.Zn-polar.ZnO,.and.PPIZnO.samples.at.12.K..A.PLE.absorption.edge.is.observed.at.about.3.37.eV. for. O-polar. ZnO. and. PPIZnO. samples,. while. a. PLE. absorption. edge. together.

Photon energy (eV)

PPIZnO

Zn-polar ZnO

O-polar ZnO

Wavelength (nm)560 520 480 440 400 360

~ 71 mev

~ 71 mev

~ 71 mev

PL in

tens

ity (a

. u.)

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6(c)

(b)

(a)

FIGURE 7.18PL. and. PLE. spectra. of. (a). O-polar. ZnO,. (b). Zn-polar. ZnO,. and. (c). PPIZnO. thin. films. at. low. temperature..(Reproduced.with.permission.from.Kwon,.B.J.,.Sun,.Y.,.Chung,.J.S.,.Cho,.Y.H.,.Park,.J.S.,.and.Yao,.T.,.Appl. Phys. Lett.,.94,.061918,.2009..Copyright.2009,.American.Institute.of.Physics.)


with.absorption.tail.states.is.observed.at.about.3.37.and.3.33.eV.for.Zn-polar.ZnO..Note.that.the.PLE.spectrum.measured.at.∼2.6.eV.for.Zn-polar.ZnO.shows.a.broad.absorption.tail.state.near.the.ABX.transition.energy.in.Zn-polar.ZnO,.indicating.that.different.kinds.of.defects.were.formed.in.Zn-polar.ZnO.grown.on.a.CrN.intermediate.layer..The.PLE.spectra.measured.at.the.detection.energies.of.excitonic.transitions.(i.e.,.3.36.and.3.32.eV).clearly. show. a. phonon-assisted. absorption. feature. with. a. spacing. of. the. LO-phonon.energy.(∼71.meV).for.all.the.samples..The.O-polar.ZnO.shows.a.dominant.broad.deep-level. emission. centered. at. ∼2.2–2.4.eV,. and. Zn-polar. ZnO. thin. film. shows. deep-level.emission. at. ∼2.6–2.8.eV.. It. has. been. discussed. that. the. deep-level. green. luminescence.often.observed.in.ZnO.can.be.associated.with.native.defects.such.as.Zn-vacancies.[67]..So,.the.relative.change.in.deep-level.emission.properties.depending.on.the.samples.may.be.related.to.the.different.status.of.native.defects.and.also.influenced.by.acceptorlike.N.atoms.from.the.CrN.intermediate.layer.for.the.Zn-polar.ZnO.and.PPIZnO.thin.films.[36]..The.PPIZnO.sample,.which.encountered.different.processes.such.as.oxidation.to.form.Cr2O3.during.the.growth,.shows.a.slight.change.in.the.properties.of.DBX.and.the.deep-level.emission.

The.peak.shifts.of.FX,.DBX,.and.ABX.emissions.are.observed.with.increasing.tempera-ture.from.12.to.300.K..The.excitonic.emissions.in.the.range.of.3.29–3.31.eV.are.observed.for.all.samples..They.found.that.the.integrated.PL.intensity.ratios.from.300.to.12.K.(I300.K/I12.K).for.excitonic.emission.are.0.039,.0.073,.and.0.064.for.the.O-polar.ZnO,.Zn-polar.ZnO,.PPIZnO. samples,. respectively.. The. intensity. ratio. of. PPIZnO. shows. about. the. average.value.of.O-polar.and.Zn-polar.ZnO.as.they.alternatively.exist.with.the.same.width.in.the.PPIZnO.sample.

TRPL.decay.curves.were.taken.at.20.and.300.K.O-polar.ZnO,.Zn-polar.ZnO,.and.PPIZnO.thin.films..The.measured.lifetimes.of.excitonic.transitions.in.the.O-polar.ZnO,.Zn-polar.ZnO,.and.PPIZnO.thin.films.are.in.the.range.of.14–30,.20–50,.and.47–103.ps.at.20.K,.respec-tively..The.lifetimes.of.ABX.are.much.longer.than.those.of.FX.and.DBX.for.all.the.samples,.consistent.with. the. results. shown. in. the. literature. [68]..They.note. that. the.FX. lifetimes.measured.at.20.K.are.much.shorter.than.the.measured.ABX.lifetimes.in.this.work.and.the.FX.radiative.lifetimes.of.ZnO.reported.in.the.literature.(e.g.,.∼322.ps.in.Ref..[33]),.indicating.that.the.capture.processes.(by.acceptorlike.defects.or.impurities.to.form.ABX).and.the.non-radiative.decay.processes.(via.nonradiative.centers).are.dominated.for.all.the.samples.even.at.low.temperature.[69,70]..Nitrogen.atoms.from.the.CrN.intermediate.layer.can.introduce.acceptorlike.features.in.the.Zn-polar.and.PPIZnO.thin.films.[36]..As.a.result,.native.donor.states.may.be.reduced.or.compensated.as.they.observed.a.much.smaller.intensity.of.DBX.than.ABX.in.the.Zn-polar.ZnO.thin.film,.and.some.nonradiative.centers.may.decrease.as.they.observed.longer.excitonic.lifetimes.in.Zn-polar.ZnO.than.O-polar.ZnO..Regarding.the.longer.lifetimes.of.PPIZnO.than.other.polar.ZnO.samples,.they.need.to.consider.the.oxidation.procedure.of.the.CrN.layer.for.forming.Cr2O3.layer.before.O-polar.ZnO.growth..Because. the.CrN.surface.was.exposed. to. the.oxygen.plasma.at.650°C. [65],. the.material.quality.of.Zn-polar.ZnO.area.on.the.CrN.surface.may.be.improved.by.the.oxidation.in.the.PPIZnO.thin.film..The.oxidation.temperature.is.high.enough.to.show.an.annealing.effect.and.elevate.the.optical.properties.of.the.PPIZnO.thin.film.[71],.resulting.in.longer.lifetimes.for.PPIZnO.than.the.O-polar.and.Zn-polar.ZnO.samples.

A.short. lifetime.can.be.explained. that. the.measured. lifetimes.are.much.shorter. than.the.exciton-radiative.lifetime.of.about.322.ps.in.bulk.ZnO,.indicating.that.the.recombina-tion. in. the.ZnO.samples. is.dominated.by.nonradiative.processes. even.at. low. tempera-ture..Measurements.of.PL.lifetimes.in.ZnO.samples.have.yielded.a.wide.distribution.of.values.strongly.depending.on.sample.purity.and.preparation.techniques,.but.in.general,.


the.experimental.values.of.TRPL.have.always.been.smaller.than.the.theoretical.radiative.lifetime..This.has.been.attributed.to.the.presence.of.dominant.nonradiative.recombination.pathways,.since.1/τPL.=.1/τr.+.1/τnr..TPL.is.affected.by.a.small.lifetime.between.τr.and.τnr..If.measured.lifetime.is.short.at.low.temperature,.measured.lifetime.is.nonradiative.lifetime.because.radiative.lifetime.is.much.longer.than.nonradiative.lifetime.at.low.temperature,.which.will.not.affect.much.on.measured.lifetime.

7.4.2 O-Face and Zn-Face ZnO

Yamamoto.et.al..[72].studied.the.differences.in.PL.spectra.between.Zn-polar.and.O-polar.faces.in.single.crystal.ZnO.as.a.function.of.temperature..Figure.7.19.shows.the.temperature-dependent.PL.spectra.in.Zn-polar.and.O-polar.faces..The.PL.spectrum.at.low.temperature.exhibits.very.sharp.excitonic.features.associated.excitons.bound.to.neutral.donors.(D0X).[21,44]..With.increasing.temperature.from.50.K.to.RT,.the.FX.and.their.FX-1LO.and.FX-2LO.become.dominant.because.BXs.are.thermally.dissociated.into.FXs.[14]..The.PL.intensity.of.the.Zn-polar.face.is.found.to.be.larger.than.that.of.the.O-polar.face.at.all.the.measured.temperatures,.as.suggested.that.the.increased.FX.emission.is.caused.by.the.extra.excitons.generated.in.an.inversion.layer.near.the.surface.in.the.Zn-polar.ZnO.[62,73].

They.pointed.out.that.the.FX.emission.can.be.partly.reabsorbed.by.the.sample.because.of.a.large.absorption.coefficient.in.the.FX.energy.region.[74]..Therefore,.for.accurate.compari-son.of.exciton-phonon.coupling.strength,.the.PL.intensities.of.FX-1LO.and.FX-2LO.transi-tions.have.been.compared..Figure.7.20.shows.the.PL.spectra.measured.from.the.two.polar.faces.as.a. function.of. temperature,.which.were.normalized.by.the.FX-1LO.PL.intensity..

Photon energy (eV)

PL in

tens

ity (a

. u.)

3.0

50 K

75 K

100 K

150 K

200 K

250 K

RT ×50

Zn–faceO–face

×20

×10

×5

×4

×3XX–1LOX–2LO

(D0X)

3.1 3.2 3.3 3.4 3.5

FIGURE 7.19Temperature.dependence.of.PL.spectra.in.the.Zn-polar.(solid.line).and.O-polar.(dashed.line).faces..The.PL.peaks.due.to.the.excitons.bound.to.neutral.donors,.free.A-excitons,.and.their.mLO.phonon.replicas.are.denoted.as.D0X,.X,.and.X-mLO,.respectively..(Reproduced.with.permission.from.Yamamoto,.A.,.Moriwaki,.Y.,.Hattori,.K.,.and.Yanagi,.H.,.Appl. Phys. Lett.,.98,.061907,.2011..Copyright.2011,.American.Institute.of.Physics.)


They.found.that.the.normalized.PL.energies.and.PL.intensity.ratios.of.the.two.polar.faces.agree.well. in. the. longer.wavelength.region.than.the.FX-1LO.at.all. temperatures.. It.was.concluded.that.exciton-phonon.coupling.strengths.of.the.two.polar.faces.are.the.same.in.ZnO.single.crystal.since.the.PL.intensity.ratios.of.FX-1LO.to.FX-2LO.are.the.same.in.both.faces..They.suggested.that.the.differences.in.the.relative.PL.properties.can.be.caused.by.the.opposite.band.bending.effects.at.the.two.polar.faces.

7.5 Optical Properties of ZnO/ZnMgO Multiple Quantum Wells

Quantum. well. structures. have. been. widely. used. in. high-performance. semiconductor.optoelectronic.devices..Due.to.the.large.exciton.binding.energy.of.60.meV.in.ZnO,.ZnO/ZnMgO.MQWs.provides.higher.efficiency.of.emission.in.the.UV.range..Sapphire,.GaN,.Si,. ZnO,. and. SCAM. substrates. have. been. used. for. the. growth. of. ZnO/ZnMgO. MQWs.structures..The.optical.properties.of.MQWs.influenced.by.the.internal.strains,.Coulomb.interaction,. coupling. to. polar. phonons,. built-in. electric. fields,. quantum. confinement,.quantum-confined. Stark. effect. (QCSE),. etc.,. originated. between. MQWs,. and. substrates.need.to.be.understood.for.applications.in.optical.devices.

7.5.1 Homoepitaxial ZnO/ZnMgO MQWs

The. achievement. of. high-quality. ZnO/ZnMgO. MQWs. structures. has. been. somehow.limited.by.the.use.of.lattice-mismatched.substrates..Some.problems.caused.by.the.use.of.

Photon energy (eV)

Nor

mal

ized

PL

inte

nsity

3

75 K

50 K

100 K

150 K

200 K

250 K

RT

Zn–faceO–face

X–2LOX–1LO

3.1 3.2 3.3 3.4 3.5

FIGURE 7.20PL.spectra.normalized.at.the.X-1LO.band.as.a.function.of.temperature..The.solid.and.dashed.lines.correspond.to.the.Zn-polar.and.O-polar.faces,.respectively..(Reproduced.with.permission.from.Yamamoto,.A.,.Moriwaki,.Y.,.Hattori,.K.,.and.Yanagi,.H.,.Appl. Phys. Lett.,.98,.061907,.2011..Copyright.2011,.American.Institute.of.Physics.)


lattice-mismatched.substrate.such.as.rough.surface,. low.PL.efficiency,.and.strong. inter-nal.electric.field.can.be.solved.by.using.ZnO.substrate.for.the.homoepitaxial.growth.of.ZnO/ZnMgO. MQWs.. Li. et. al.. [75]. investigated. the. optical. properties. of. ZnO/ZnMgO.MQWs.structures.with.different.well.widths.grown.on.ZnO.substrates.by.MBE..The.ZnO/Zn0.9Mg0.1O.MQWs.were.grown.on.O-face.(0001).ZnO.substrates. (MAHK.Co.,. Japan).by.MBE..Details.of.the.growth.procedures.were.reported.elsewhere.[76]..The.thicknesses.of.well.layers.(Lw).were.set.to.2.(W2).and.5.nm.(W5).with.a.barrier.thickness.(Lb).of.7.nm.for.two.different.samples..For.comparison,.the.optical.properties.of.a.ZnO.substrate.were.also.investigated..Figure.7.21.shows.PL.spectra.of.samples.W2,.W5,.and.a.ZnO.substrate.mea-sured.at.10.K..The.PL.spectrum.of.the.ZnO.substrate.exhibits.very.dominant.sharp.peaks.at.3.361.and.3.356.eV.due.to.DBX.transitions..FX,.TES,.and.ABX.transitions.can.be.clearly.distinguished.at.3.377,.3.334,.and.3.323.eV,.respectively..LO-phonon.replicas.of.FX.(FX-nLO).and.LO-phonon.replicas.of.DBX.(DBX-nLO).can.be.seen. in. the.oscillatory.PL.spectrum.with.an.energy.periodicity.of.about.72.meV,.corresponding.to.the.LO-phonon.energy.of.ZnO..All. these.peaks.are.observable. in.samples.W2.and.W5,.except. for.ZnO.FX.transi-tion.that.is.overlapped.with.ZnO.MQWs.emissions..Two.additional.emissions.from.ZnO.MQWs.and.ZnMgO.barrier.layers.are.observed.in.samples.W2.and.W5..The.broad.weak.emissions.at.about.3.5.eV.come.from.the.ZnMgO.barrier.layers,.while.the.strong.emission.from.MQWs.in.W2.(W5).is.blueshifted.to.3.387.eV.(3.368.eV).compared.with.bulk.ZnO.exci-tonic.peak.(3.356.eV).due.to.the.QCSE..The.energy.of.well.emission.of.W2.is.well.consistent.with.the.reported.value.of.ZnO/Zn0.9Mg0.1O.(Lw.∼.1.7.nm).in.other.literature.[77]..From.the.PL.spectra,.they.found.that.the.intensity.ratio.of.the.well.to.barrier.emissions.(defined.by.Iwell/Ibarrier). for.W5.is.nearly.400,.which.is.much.larger.than.that.for.W2.(Iwell/Ibarrier.∼25),.indicating.more.effective.carrier.transfer.from.the.barriers.to.the.wells.for.sample.W5.

Figure.7.22.shows. temperature-dependent.PL.spectra.of.samples.W2,.W5,.and.a.ZnO.substrate..For.the.reference.ZnO.sample.(Figure.7.22c),.the.DBX.transition.is.predominant.below. 130.K,. while. the. FX. transition. becomes. dominant. with. temperature. higher. than.

Photon energy (eV)


Well

Well

340

PL in

tens

ity (a

. u.)

3.0 3.2 3.4 3.6 3.8

W5

ZnO Sub.

W2

BarrierDBX

ABX

TES

FX

72 meV

FX-1LO

DBX-1LODBX-2LO

DBX-3LO

Barrier

FIGURE 7.2110.K.PL.spectra.from.W2,.W5,.and.a.ZnO.substrate..PL.spectra.have.been.vertically.shifted.for.clarity..“Well”.and.“Barrier”.denote.emission.from.the.well.and.barrier,.respectively..(Reproduced.with.permission.from.Li,.S.M.,.Kwon,.B.J.,.Kwack,.H.S.,.Jin,.L.H.,.Cho,.Y.H.,.Park,.Y.S.,.Han,.M.S.,.and.Park,.Y.S.,.J. Appl. Phys.,.107,.033513,.2010..Copyright.2010,.American.Institute.of.Physics.)


130.K.. ABX. can. be. observed. up. to. 50.K. and. ABX-1LO. up. to. 30.K. and. then. both. disap-pear.due.to.thermal.ionization.from.bound.acceptors.[3]..TES.transition.is.negligible.with.increasing. temperature.. DBX-nLO. transitions. can. be. observed. below. 90.K. and. FX-nLO.transitions.become.dominant.above.110.K..Then.FX.and.FX-nLO.transitions.start.to.merge..The.narrow.linewidth.of.excitonic.transitions.indicates.the.high.quality.of.the.ZnO.sub-strate..For. samples.W2.and.W5,. the.emissions. from.ZnO.substrate.are.negligible.when.the.temperature.is.higher.than.130.K,.as.shown.in.Figure.7.22a.and.b..The.emission.from.ZnMgO.layer.can.be.resolved.for.temperature.below.200.K.in.both.samples..In.Figure.7.22a,.the.peak.energies.of.the.ZnO.MQW-related.emission.indicated.by.closed.circles.show.the.“S-shaped”.temperature.dependence.[78].as.the.temperature.increases.from.10.to.300.K,.which.can.be.attributed.to.the.exciton.localization.caused.by.well-width.variations.and/or.alloy-potential.inhomogeneities.of.ZnO/ZnMgO.MQWs.[79]..The.temperature-dependent.PL.spectra.of.W5.shown.in.Figure.7.22b.are.rather.similar.to.those.observed.in.typical.ZnO.epilayer..The.redshift.behavior.in.the.temperature.range.of.10–300.K.is.ascribed.to.band-gap.shrinkage.effect..These.results.indicate.that.two.kinds.of.MQWs.with.different.well.widths.exhibit.different.exciton.dynamics.due.to.different.well-width.(and.other.potential).fluctuations.

Excitation.power-dependent.PL.spectra.were.measured.for.samples.W2.and.W5.at.10.K.by.varying.excitation.power.over. three.orders.of.magnitude..They.observed.almost.no.shift. in.the.PL.peak.energy.for.both.samples.W2.and.W5..It.has.been.reported.that.the.transition.from.quantum.confinement.regime.to.QCSE.regime.occurs.in.ZnO/Zn0.9Mg0.1O.MQWs.grown.on.Al2O3.substrates.when.the.well.width.is.about.twice.the.exciton.Bohr.radius.(∼2.nm).of.ZnO.bulk.[77]..The. internal.electric.field. is.determined.from.the.sum.of.spontaneous.and.strain-induced.piezoelectric.polarizations.between.the.well.and.the.

Wavelength (nm)

Photon energy (eV)

PL in

tens

ity (a

. u.)

3.2

380 360 380 360 380 360

3.4 3.6 3.2 3.4 3.6 3.2 3.4 3.6

(a) (b) (c)

FIGURE 7.22PL.spectra.of.(a).W2,.(b).W5,.and.(c).a.ZnO.substrate.over.the.temperature.range.of.10–300.K..MQWs.emission.peaks.of.W2.and.W5.(indicated.by.closed.circles.and.closed.rectangles,.respectively),.FX.(inverted.open.trian-gles),.and.DBX.(open.circles).of.the.ZnO.substrate..Spectra.have.been.vertically.shifted.for.clarity..(Reproduced.with.permission.from.Li,.S.M.,.Kwon,.B.J.,.Kwack,.H.S.,.Jin,.L.H.,.Cho,.Y.H.,.Park,.Y.S.,.Han,.M.S.,.and.Park,.Y.S.,.J. Appl. Phys.,.107,.033513,.2010..Copyright.2010,.American.Institute.of.Physics.)


barrier.. In. case. of. ZnO/ZnMgO. QWs,. theoretical. studies. have. shown. negligible. QCSE.when.both.Mg.composition.and.Lw.are.small..The.negligible.internal.field.effect.can.be.explained.by.the.cancellation.of.spontaneous.and.piezoelectric.polarizations.between.the.well.and.the.barrier.in.the.ZnO/ZnMgO.QW.structures.[80]..From.the.power-dependent.PL.spectra,.they.observed.no.PL.peak.shift.of.MQWs.emission.in.both.samples.W2.and.W5,.indicating.a.negligible.built-in.electric.field.in.the.ZnO/Zn0.9Mg0.1O.MQWs.grown.on.ZnO.substrates.by.MBE.[79].

Figure.7.23.shows.TRPL.decay.curves.monitored.at. the.well.and.the.barrier.emission.peaks.for.samples.W2.and.W5,.together.with.the.decay.curve.detected.at.DBX.peak.of.a.ZnO.substrate.and.the.corresponding.IRF.profile..The.decay.time.of.DBX.transition.from.a.ZnO.substrate.is.about.180.ps..Insets.of.Figure.7.23.show.TRPL.decay.curves.of.the.well.emissions.for.samples.W2.and.W5.at.the.condition.of.direct.excitation.(i.e.,.excitation.below.the.barrier.bandgap.energy).and.indirect.excitation.(i.e.,.excitation.above.the.barrier.band-gap.energy)..For.both.samples,.the.temporal.profiles.of.the.luminescence.from.the.wells.exhibit. a. slower. rise. time. and. a. longer. decay. time. for. indirect. excitation. compared. to.direct.excitation..Especially.for.sample.W5,.they.observed.a.prolonged.rise.time.(∼63.ps).and.a.longer.decay.time.(∼169.ps).of.the.well.emission.under.the.indirect.excitation.condi-tion,.as.compared.to.the.direct.excitation.case.for.which.the.rise.time.and.the.decay.time.of.the.well.emission.are.∼38.and.∼160.ps,.respectively..Furthermore,.the.higher.intensity.ratio.of.the.QW.to.barrier.emissions.(as.shown.in.Figure.7.22).and.the.faster.decay.time.of.

Time (ps)

PL in

tens

ity (a

. u.)

DBX

Barrier

Indirect excitation

Indirect excitation

Direct excitation

Direct excitation

30

3020100

20100Time (ps)

Time (ps)Barrier

Well

Well

IRF

0 50 100 150 200

(a)

(b)

(c)

FIGURE 7.23Time-resolved.PL.decay.curves.of.(a).W2,.(b).W5,.and.(c).a.ZnO.substrate.under.the.indirect.excitation.condition.(together.with.IRF)..The.insets.show.measured.time-resolved.PL.decay.curves.from.the.well.emissions.of.W2.and.W5.under.the.direct.excitation.condition.(dotted.lines).and.the.indirect.excitation.condition.(solid.lines)..(Reproduced.with.permission.from.Li,.S.M.,.Kwon,.B.J.,.Kwack,.H.S.,.Jin,.L.H.,.Cho,.Y.H.,.Park,.Y.S.,.Han,.M.S.,.and.Park,.Y.S.,.J. Appl. Phys.,.107,.033513,.2010..Copyright.2010,.American.Institute.of.Physics.)


the.barrier.emission.(∼22.ps).are.simultaneously.observed.for.sample.W5..These.results.are.suggestive.of.the.efficient.carrier.transfer.process.from.the.barriers.to.the.wells.for.sample.W5..On.the.other.hand,.the.rise.time.difference.between.the.indirect.and.direct.excitation.conditions.for.sample.W2.is.∼13.ps,.which.is.much.smaller.than.the.case.of.sample.W5.(∼25.ps)..Together.with.the.relatively.smaller.intensity.ratio.of.the.QW.to.barrier.emissions.of.sample.W2,.they.can.conclude.that.the.carrier.transfer.efficiency.from.the.barriers.to.the.wells.is.smaller.for.sample.W2.than.W5..They.can.interpret.this.in.terms.of.the.saturation.of.the.localized.states.in.sample.W2.due.to.the.smaller.density.of.states.in.MQWs.with.nar-rower.well.width.than.W5.[81]..Although.the.different.barrier.decay.time.between.samples.W2.and.W5.(∼55.and.∼22.ps,.respectively).may.also.be.partly.influenced.by.different.mate-rial.qualities.and.small.Mg.composition.discrepancy.of.ZnMgO.alloys,.the.carrier.transfer.process.would.be.the.dominant.reason.for.the.shorter.barrier.decay.time.of.sample.W5..For.TRPL.analysis,.the.built-in.internal.field.effect.between.the.well.and.barrier.regions.[79].can.be.almost.negligible.as.observed.in.power-dependent.PL.experiments.

7.5.2 Heteroepitaxial ZnO/ZnMgO MQWs

Despite.the.drawbacks.due.to.lattice-mismatched.substrate,.the.heteroepitaxial.growth.of.ZnO/ZnMgO.MQWs.has.been.studied.for.better.device.fabrications..Furthermore,.their.optical.properties.such.as.quantum.confinement.and.internal.electric.field.must.be.under-stood.to.fabricate.the.high-performance.semiconductor.optoelectric.devices.

Morhain.et.al.. [82].demonstrated.the.presence.of.a. large.electric.field.along.the.(0001).growth.direction.in.wurtzite.ZnO/Zn0.78Mg0.22O.QWs.grown.by.MBE.on.the.c-plane.sap-phire.substrates.using.1-μm-thick.ZnO.templates..The.width,.LB,.of.the.Zn0.78Mg0.22O.bar-riers.was.quite.large.(200.nm).to.ensure.maximum.possible.value.of.the.electric.field,.as.the.field.in.the.well. is.known.to.be.approximately.proportional.to.the.LB/(LW.+.LB).[83]..Figure.7.24.shows.10.K.PL.spectra.for.various.ZnO/Zn0.78Mg0.22O.QW.samples.with.a.wide.range. of. well. widths. (LW.=.1.6,. 2.6,. 5.2,. 7.1,. and. 9.5.nm,. respectively).. The. presence. of. a.strong.electric.field.was.directly.evidenced.by.the.PL.peaks.of.the.three.wider.QWs.(5.2,.7.1,.and.9.5.nm),.which.are.located.below.the.ZnO.excitonic.transition..They.showed.that.the.strength.of.coupling.of.excitons.to.LO-phonons.would.be.enhanced.with.increasing.LW,.which.was.another.indication.of.the.electric.field.effect.as.shown.by.the.increase.in.the.relative.intensities.of.phonon.replicas.[84]..A.blueshift.of.∼80.meV.is.observed.when.the.excitation.power.density.is.increased.from.0.1.to.100.W/cm2.for.the.QWs.with.well.width.of.7.1.nm,.as.shown.in.Figure.7.25..This.result.was.originated.from.the.screening.of. the.internal.electric.field.by.high.densities.of.carriers,.which.is.the.further.evidence.for.large.built-in.electric.fields..It.can.be.very.large.in.the.wider.QWs.(5.2,.7.1,.and.9.5.nm),.while.this.effect.remains.small.in.the.narrower.QWs.(1.6.and.2.6.nm)..They.mentioned.that.the.full.screening.would.yield.PL.energies.above.the.ZnO.excitonic.gap,.which.would.permit.a.direct.estimation.of.the.QCSE.

Zhang.et.al..[77].studied.on.the.low-temperature.PL.properties.of.ZnO/Mg0.1Zn0.9O.QWs.with.graded.well.width.(Lw).grown.by.MOCVD..Mg0.1Zn0.9O/ZnO/Mg0.1Zn0.9O.QW.struc-ture.was.grown.on.an.Mg0.1Zn0.9O.buffer.layer,.which.was.formed.on.Al2O3.(112̄0).wafers.because.the.critical.thickness.for.ZnO.growth.is.about.ten.times.larger.than.that.on.Al2O3.(0001).substrate.[85,86]..The.emission.of.ZnO.QWs.is.changed.strongly.depending.on.the.sample.position.(i.e.,.Lw.=.1.8–6.0.nm),.as.shown.in.Figure.7.26..The.emission.peak.exhibits.a.blueshift.due.to.the.quantum.confinement.effect.with.decreasing.Lw..When.Lw.>.4.nm,.a.position-independent.small.emission.(denoted.as.B).appears.at.about.3.36.eV,.while.the.stronger. emission. (denoted. as. A). shows. further. redshift. with. increasing. Lw.. When. the.


exciton.is.located.in.a.narrower.well.than.exciton.Bohr.diameter.(2aB.∼.4.nm).of.ZnO.bulk.[87],.a.quantum.confinement.effect.is.dominant.due.to.the.limitation.of.the.exciton.move-ment.(Figure.7.27a)..On.the.other.hand,.when.the.well.width.becomes.wider.than.2aB,.its.energy.is.similar.to.the.bulk.case..However,.with.the.presence.of.the.internal.electric.field,.the.emission.energy.for.a.wider.well.can.be.even.smaller.due.to.the.QCSE.(Figure.7.27c)..They.mentioned.that.peak.B.is.attributed.to.D0X.in.ZnO,.which.is.different.from.the.emis-sion.of.localized.excitons.[88],.whereas.the.position-dependent.peak.A.is.attributed.to.the.emission.due.to.spatially.separated.localized.carriers.caused.by.QCSE..It.is.clear.that.the.transition.from.quantum.confinement.regime.to.QCSE.regime.occurs.at.Lw.∼ 2aB.(Figure.7.27b)..The.existence.of.the.electric.field.inside.the.well.layer.is.further.demonstrated.by.investigating. the. excitation. power. dependence. of. the. emission. spectra.. The. emission.energy.of.the.peak.is.found.to.exhibit.a.blueshift.with.increase.in.the.excitation.power..Therefore,.they.concluded.that.it.would.be.better.to.keep.the.well.size.to.be.less.than.2aB.to.utilize.the.advantages.of.low-dimensional.quantum.structures.

Makino.et.al..[89].reported.on.the.optical.properties.of.ten-period,.ZnO/MgZnO.MQWs.grown.on.lattice-matched.SCAM.substrates.fabricated.by.laser.MBE..PL.and.absorption.spec-tra.were.measured.at.5.K.for.the.ZnO/Mg0.12Zn0.88O.MQWs.with.Lw.of.17.5.and.6.91.Å,.together.with.a.500-Å-thick.ZnO.epilayer.on.SCAM.for.comparison.[90]..As.shown.in.Figure.7.28,.both.the.PL.and.absorption.peaks.shifted.toward.the.higher.energy.side.with.decreasing.Lw.due.to.the.quantum.confinement.effect..They.showed.the.temperature-dependent.PL.and.

Energy (eV)

PL in

tens

ity (a

. u.)

2.8 3.0 3.2 3.4

BarrierZnO

9.5 nm

1.6 nm

×20

2.6 nm5.2 nm

7.1 nmZno/Zn0.22Mg0.78O

Quantum wells

3.6 3.8 4.0

FIGURE 7.24Continuous-wave.PL.spectra.of.ZnO/Zn0.78Mg0.22O.QWs.of.various.widths,.as.indicated,.taken.at.T.=.10.K..The.pump-power.density.was.100.mW/cm2..The.PL.energies.from.the.Zn0.78Mg0.22O.barrier.layers.and.those.of.the.ZnO.buffer.layers.are.shown.by.dashed.lines..(Reproduced.with.permission.from.Morhain,.C.,.Bretagnon,.T.,.Lefebvre,.P.,.Tang,.X.,.Valvin,.P.,.Guillet,.T.,.Gil,.B.,.Taliercio,.T.,.Teisseire-Doninelli,.M.,.Vinter,.B.,.and.Deparis,.C.,.Phys. Rev. B,.72,.241305,.2005..Copyright.2005,.American.Physical.Society.)


Energy (eV)

PL In

tens

ity (a

. u.)

2.9 3.0

T=10 KLw=7.1 nm

3.1 3.2

0.1 mW/cm2

100 W/cm2

3.3

FIGURE 7.25Continuous-wave.PL.spectra.of.the.7.1.nm.QW,.for.various.pump-power.densities..The.highest.power.density.(top.spectrum).is.100.W/cm2..This.density.was.divided.by.a.factor.3.1.and.again.for.the.six.higher.energy.spec-tra..Then,.between.the.four.lower.energy.spectra.a.factor.of.10.was.applied..(Reproduced.with.permission.from.Morhain,.C.,.Bretagnon,.T.,.Lefebvre,.P.,.Tang,.X.,.Valvin,.P.,.Guillet,.T.,.Gil,.B.,.Taliercio,.T.,.Teisseire-Doninelli,.M.,.Vinter,.B.,.and.Deparis,.C.,.Phys. Rev. B,.72,.241305,.2005..Copyright.2005,.American.Physical.Society.)

3.25 3.30A B


Widthincrease

MgZnO

MgZnOZnO

PL in

tens

ity (a

. u.)

3.40 3.45 3.50

6.0

4.03.53.4

3.0

2.4

1.8Lw(nm)

FIGURE 7.26Low-temperature.PL.spectra.of.ZnO/Mg0.1Zn0.9O.QWs.measured.at.different.sample.positions.or.well.widths.(Lw)..Inset.shows.the.sample.structure..A.few.of.Lw.values.were.given.on.the.right..Peaks.B.and.A.are.emissions.due.to.ZnO.band.edge.and.the.localized.carriers,.respectively..(Reproduced.with.permission.from.Zhang,.B.P.,.Liu,.B.L.,.Yu,.J.Z.,.Liu,.C.Y.,.Liu,.Y.C.,.Segawa,.Y.,.and.Wang,.Q.M.,.Appl. Phys. Lett.,.90,.132113,.2007..Copyright.2007,.American.Institute.of.Physics.)


absorption.spectra.in.the.ZnO/Mg0.27Zn0.73O.MQWs.on.SCAM.substrates.with.Lw.of.23.5.Å,.as.shown.in.Figure.7.29..It.was.concluded.that.excitonic.PL.from.MQWs.persists.up.to.RT.since.the.PL.peak.is.located.near.the.absorption.band..TRPL.measurements.showed.smaller.nonradiative.decay.rates.of.excitons.in.MQWs.on.SCAM.substrates.than.those.in.samples.grown. on. sapphire,. which. is. attributed. to. the. high. crystal. quality. of. MQWs. on. lattice-matched.substrates..They.could.tune.the.PL.emission.energy.ranging.from.3.3.to.3.6.eV.by.

Photon energy (eV)

Nor

mal

ized

PL

inte

nsity

(a. u

.)

Abs

orpt

ion

3.2 3.4 3.6 3.8

Barrier abs.B+2LO

B+3LO

n=1

n= 1He-Cd5.0K

17.5A(3.5L)

Lw = 6.91A(1.5L)

ZnO(500A)A, BI6

B+LO

4.0

n≥

n≥

FIGURE 7.28PL.and.absorption.spectra.in.[ZnO(Lw)/Mg0.12Zn0.88O]10.MQWs.measured.at.5.K.for.well.widths.(Lw.=.17.5.and.6.91.Å)..Absorption.energy.of.barrier.layers.is.shown.by.a.horizontal.arrow..Spectra.in.a.500-Å-thick.ZnO.film.are.also.shown..“A,.B”. indicates.A-.and.B-exciton.absorption.bands,.“I6”.shows.PL.of.a.bound.exciton.state,.“B.+.LO,.B.+.2LO,.and.B.+.3LO”.correspond.to.exciton-phonon.complex.transitions,.“n.=.1”.shows.the.lowest.excitonic.absorption.of.the.well.layers,.and.“n.≥.2”.means.the.excited.states.of.the.exciton.or.higher.interband.(subband).transitions.. (Reproduced.with.permission.from.Makino,.T.,.Chia,.C.H.,.Nguen.T..Tuan,.Sun,.H.D.,.Segawa,.Y.,.Kawasaki,.M.,.Ohtomo,.A.,.Tamura,.K.,.and.Koinuma,.H.,.Appl. Phys. Lett.,.77,.975,.2000..Copyright.2000,.American.Institute.of.Physics.)

(a) Lw < 2aB

A AB B

EA < EB

(c) Lw > 2aB(b) Lw ~ 2aB

EA = EB

FIGURE 7.27Illustration.of.band.alignment.with.an.internal.electric.field..From.(a).to.(c),.the.well.width.(Lw).is.increased.grad-ually..Peaks.B.and.A.are.emissions.due.to.ZnO.band.edge.and.the.localized.carriers,.respectively..(Reproduced.with.permission.from.Zhang,.B.P.,.Liu,.B.L.,.Yu,.J.Z.,.Liu,.C.Y.,.Liu,.Y.C.,.Segawa,.Y.,.and.Wang,.Q.M.,.Appl. Phys. Lett.,.90,.132113,.2007..Copyright.2007,.American.Institute.of.Physics.)


changing.the.barrier.height.and.well.layer.thickness,.demonstrating.the.wide.tunability.on.RT.excitonic.emission.based.on.the.ZnO.quantum.structure.

7.6 Summary

We. have. presented. the. intrinsic. and. extrinsic. optical. properties. and. carrier. dynamics.of.various.ZnO.epilayers.and.ZnO/ZnMgO.multiple.quantum.well.structures..First,.we.described.the.fundamental.optical.transitions.in.undoped.ZnO.associated.with.free.exci-ton,. donor-. and. acceptor-bound. exciton,. phonon. replicas,. two. electron. satellite,. donor-acceptor. pair,. and. deep-level. emissions.. Also,. the. depth-resolved. properties. of. ZnO.epilayers.and.the.optical.properties.of.ZnO.grown.under.various.substrates.and.condi-tions.were.discussed..Second,.the.recent.progress.in.the.growth.and.optical.characteristics.of.various.p-type.ZnO.doped.with.N,.As,.P,.and.Sb.was.described..Third,.we.discussed.the.optical.properties.of.the.polarity-controlled.ZnO.films.and.the.differences.in.optical.char-acteristics.between.Zn-polar.and.O-polar.faces.in.single.crystal.ZnO..Finally,.the.optical.properties.and.carrier.dynamics.of.the.homoepitaxial.and.heteroepitaxial.ZnO/ZnMgO.MQWs.with.different.well.widths.were.described..From.this.overview,.we.described.the.various. optical. properties. of. ZnO-based. semiconductors.. It. is. obvious. that. ZnO-based.semiconductors. have. high. potentialities. for. future. optoelectronic. devices. in. the. blue-ultraviolet.region,.although.more.researches.are.still.required.to.achieve.p-type.ZnO.and.high-quality.active.region.with.stability.and.reliability.

Photon energy (eV)

Abs

orpt

ion

Nor

mal

ized

PL

inte

nsity

(a. u

.)

3.0 3.2 3.4 3.6

5 K

n=1

80 K(×13)

294 K(×333)

n=1

x = 0.27Lw = 23.5A

3.8

FIGURE 7.29Temperature.dependence.of.PL.and.absorption.spectra. in. [ZnO(23.5.Å)/Mg0.27Zn0.73O]10.on.SCAM.substrates..The.upper.two.spectra.were.multiplied.by.the.indicated.values.for.normalization..(Reproduced.with.permis-sion.from.Makino,.T.,.Chia,.C.H.,.Nguen.T..Tuan,.Sun,.H.D.,.Segawa,.Y.,.Kawasaki,.M.,.Ohtomo,.A.,.Tamura,.K.,.and.Koinuma,.H.,.Appl. Phys. Lett.,.77,.975,.2000..Copyright.2000,.American.Institute.of.Physics.)


Acknowledgments

We.acknowledge.support.by.the.WCU.Program.(No..R31-2008-000-1071-0).of.the.Ministry.of.Education,.Science.and.Technology.

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205

8p-Type ZnO-N Films: Preparation and Characterization by Synchrotron Radiation

C.W. Zou and W. Gao

8.1 Introduction

ZnO. materials. have. attracted. considerable. attention. worldwide. for. its. optoelectronic.devices.and.other.applications.due.to.its.wide.band.gap.of.3.37.eV.and.large.exciton.bind-ing. energy. of. 60. meV. at. room. temperature. [1–6].. Compared. with. traditional. electronic.devices.based.on.GaN.or.other.semiconductor.materials,.ZnO-based.materials.show.clear.advantages. such. as. low. cost,. easy. nano-structurization,. special. piezoelectric. property,.and.compatibility.with.wet-chemical.etching.treatment..All.of. these.are.promising.and.suitable.for.fabrication.of.electronic.devices.such.as.UV.light.emission.diode.(LED),.laser.diode,.and.various.UV.sensor.devices.[7–11].

Normally,. the.electronic.devices.made.by.ZnO.materials.are.based.on.ZnO.p-n. junc-tions,.which.contain.two.different.conducting.modes:.one.side.shows.n-type.conducting.property.while.the.other.side.shows.p-type.mode..It.is.known.that.the.n-type.conducting.ZnO.film.or.nanostructures.can.be.prepared.easily.with.good.electric.characteristics.and.sufficient.stability,.by.doping.the.group.III.atoms.such.as.Al.and.Ga.into.the.ZnO.host.or.even.by.the.self-doping.procedure.[12–15].

However,.the.realization.of.stable.p-type.ZnO.films.is.still.a.bottleneck.for.the.application.of.ZnO-based.devices.due.to.the.intrinsic.point.defects.such.as.O.vacancies.and.Zn.inter-stitials,. which. shows. distinct. self-compensation. effect. for. the. doping. acceptors. [16–22]..The.p-type.ZnO.films.or.nanostructure.preparation.has.been.studied.for.a.quite.long.time.by.doping.with.different.dopants/atoms.[18,23–28],. including.from.group.I.(Li,.Na.etc.),.group.V.(N,.P,.As,.etc.),.or.some.other.atoms.such.as.Ag.and.Cu,.while.no.big.breakthrough.

CONTENTS

8.1. Introduction......................................................................................................................... 2058.2. Preparation.Methods.for.Nitrogen-Doping.ZnO.Films................................................ 206

8.2.1. Nitrogen-Doping.during.the.ZnO.Film.Growth.Process................................. 2078.2.2. Nitrogen-Doping.by.Post-Growth.Treatments................................................... 209

8.3. Synchrotron.Radiation–Related.Studies......................................................................... 2108.3.1. Synchrotron.Radiation–Based.Characterization.Methods............................... 2108.3.2. Doping.Mechanism.Studied.by.Synchrotron.Radiation.................................. 211

8.4. Summary.and.Outlook...................................................................................................... 215Acknowledgments....................................................................................................................... 217References...................................................................................................................................... 218


has.been.achieved.in.this.field.till.now.to.terminate.various.claims.for.p-type.ZnO.fabrica-tion.and.provide.a.final.conclusion.for.high-quality.p-type.ZnO.with.sufficient.hole.carrier.concentration.and.mobility.as.well.as.high.stability.[3,8,18,29].

In.fact,.the.difficulty.for.the.p-type.ZnO.realization.lies.not.only.in.the.self-compensation.effect.by.the.intrinsic.point.defects.in.ZnO.crystal,.but.also.in.the.low.solubility.of.dopants.in.the.ZnO.host.lattice.and.the.relatively.deep.acceptor.energy.level,.since.the.deep.accep-tor.energy.level.allows.the.holes.to.have.a.low.probability.to.be.thermally.excited.for.the.real.p-type.conductivity.[17,18,30].

To.find.a.reliable.way.to.achieve.p-type.ZnO.doping,.many.theoretical.calculations.have.been. conducted.. According. to. these. theoretical. calculation. results. [19,31–34],. nitrogen.atom.has.been.considered.to.be.the.most.promising.candidate.for.the.p-type.doping.for.ZnO.materials,.which.is.not.only.due.to.its.similar.atom.radius.as.oxygen,.but.also.to.the.relatively.shallow.acceptor.energy.level.induced.by.N.dopants.in.ZnO.host.when.nitrogen.atoms.occupy.the.oxygen.sites.[20,34,35]..People.consider.that.the.N.atoms.replace.the.O.atoms.sites-(N)o.to.act.as.the.shallow.acceptors,.which.respond.to.the.hole-carriers.forma-tion..However,.many.experimental.results. indicated.that.the.nitrogen-doped.ZnO.films.always.show.unstable.p-type.conductivity.and.the.p-type.property.is.also.very.sensitive.to.the.preparation.parameters.such.as.the.growth.temperature.or.post-annealing.treatment.[36–38]..Furthermore,.it.is.a.fact.that.the.hole.concentration.is.not.high.enough.although.the.N.atoms.concentration.can.be.quite.high.in.ZnO:N.samples..Obviously,.the.nitrogen-doping.mechanism.in.ZnO.is.still.not.well.understood.. In.order. to.clarify. the.nitrogen.doping.effects.in.ZnO.host,.the.best.way.is.to.examine.the.chemical.states.and.the.related.local.structures.of.nitrogen.atoms.directly.

It.is.known.that.synchrotron.radiation.is.a.powerful.light.source,.which.covers.the.elec-tromagnetic. spectrum.ranging. from. infrared/far-infrared. to.hard.x-ray.with.very.high.intensity.and.brightness..The.photoelectron.spectroscopy.(PES).and.x-ray.absorption.spec-troscopy.(XAS).based.on.synchrotron.radiation.are.useful.tools.to.investigate.the.specific.chemical.states,.local.atoms.arrangement,.and.the.related.electronic.states,.thus.obtaining.the.information.about.the.site.and.neighborhood.of.the.doping.atoms..Obviously,. these.synchrotron.radiation.based.methods.are.suitable.for.study.of.the.nitrogen-doping.mecha-nism.in.ZnO.material..In.fact,.the.spectroscopy.method.based.on.synchrotron.radiation.has.been.successfully.applied.to.investigate.arsenic.(As).atoms.doping.mechanism.in.ZnO.film,.and.support.the.AsZn-2VZn.doping.model.directly.[34,39].

This.review.chapter.will.focus.on.the.preparation.of.p-type.nitrogen-doped.ZnO.films.and.the.related.doping.mechanism.investigation.by.synchrotron.radiation–based.meth-ods,.such.as.XAS.and.PES..In.this.chapter.we.intend.to.find.out.the.current.research.sta-tus.and.to.give.a.general.statement.for.the.synchrotron.radiation.and.related.studies.of.nitrogen-doped.ZnO.films.

8.2 Preparation Methods for Nitrogen-Doping ZnO Films

Among.the.acceptor.impurities.that.substitute.for.oxygen.in.ZnO,.nitrogen.is.considered.to.be.the.most.suitable.p-type.dopant.due.to.the.considerations.of.both.atomic-size.and.elec-tronic-structure..The.nitrogen.atom.has.the.closest.atomic.size.to.oxygen,.and.therefore,.it.is.expected.to.result.in.minimum.lattice.strain.in.ZnO..The.energy.of.the.valence.2p.states.and. the. electro-negativity. of. nitrogen. are. also. the. closest. to. those. of. the. oxygen. atom,.


particularly.when.it.is.compared.with.other.group-V.dopants..Thus.preparation.of.nitro-gen-doped.ZnO.film.for.p-type.conductivity.has.attracted.many.researchers.worldwide;.and.various.nitrogen-doped.ZnO.films.with.p-type.conductivity.have.been.prepared.by.both.chemical.and.physical.routes.[16,40–43].

8.2.1 Nitrogen-Doping during the ZnO Film Growth Process

Different.physics.or.chemical.techniques.have.been.adopted.for.the.nitrogen-doping.ZnO.films.preparation..The.typical.physical.preparation.includes.molecule.beam.epitaxy.(MBE),.pulsed.laser.deposition.(PLD),.magnetron.sputtering,.or.other.physical.vapor.deposition.methods.[16,25,37,44,45]..By.chemical.routes,.ZnO.films.preparation.and.the.related.doping.can.be.achieved.by.metalorganic.chemical.vapor.deposition.(MOCVD).[46,47],.ultrasonic.spray.pyrolysis.[48,49],.sol-gel.method.[50,51],.and.so.on.

For.those.ZnO:N.films.preparation.with.physical.techniques,.the.nitrogen.atoms.doping.procedure.was.achieved.by.introducing.the.nitrogen-related.gas.source.such.as.N2,.NH3,.NO,.NO2,.and.N2O.during.the.growth.process.of.the.films..Look.et.al..prepared.ZnO:N.film.by.MBE.method.with.rf.plasma.source.[52]..For.the.N-doped.ZnO.film.preparation,.a.flux.of.N2.gas.was.added.to.the.O2.gas.flow.in.the.rf.plasma.source..The.obtained.ZnO:N.films.demonstrated.obvious.p-type.conductivity.with.the.resistivity.of.4.×.101.Ω.cm,.hole.mobil-ity.of.2.cm2/(V.s).and.hole.concentration.of.9.×.1016.cm−3..Photoluminescence.measurements.in.this.N-doped.layer.also.show.a.strong.peak.near.3.32.eV,.which.comes.from.the.neutral.acceptor-bound.excitons.

Tsukazaki.et.al..reported.the.realization.of.a.ZnO-based.light.emission.diode.by.laser.MBE. on. insulating. ScAlMgO4. substrate. [53].. The. p-type. N-doped. layer. was. grown. by.a. repeated. temperature. modulation. technique. in. which. high-N. concentration. layers. of.15. nm. thick. were. grown. at. low. temperatures,. then. annealed. at. higher. temperatures,.followed. by. the. growth. of. 1. nm. low-N. concentration. layers. at. high. temperatures.. The.whole.process.was. intended. to.obtain.an.overall.high.N. incorporation. in. the.N-doped.layer..They.reported.the.related.hole.mobility.and.acceptor.activation.energy.of.5–8.cm2/(V.s).and.100.meV,. respectively..Following. this.procedure,. they.observed. the.blue. light.emission.from.a.p–n.ZnO.homo-junction.using.N.as.the.acceptor.in.the.p-type.layer.

Ye’s.group.[54].also.deposited.the.N-doped.ZnO.film.by.magnetron.sputter.using.the.NH3.as.the.nitrogen.source,.which.showed.the.p-type.ZnO.film.with.the.hole.concentration.of.3.2.×.1017.cm−1.and.a.resistivity.of.35.Ω.cm..Their.results.indicated.that.the.extra.Zn.atoms.and.the.interstitials.H.atoms.played.an.important.role.for.the.formation.of.p-type.conductivity.

Guo.etc.. [55].also.use.the.PLD.method.to.prepare.nitrogen-doped.ZnO.film.by.using.metallic.Zn.as.the.target.and.N2O.gas.as.the.working.gas..During.the.deposition.process,.they.used. the.electron.cyclotron.resonance. (ECR).plasma. to.active. the.nitrogen.source,.which.enhanced.the.nitrogen-doping.concentration.significantly..The.final.p-type.ZnO:N.film.show.distinct.p-type.characters.with. the.hole.concentration.of.3~6.×.1018. cm−3,.hole.mobility.of.0.1~0.4.cm2/(V.s).and.resistivity.of.2–5.Ω.cm..However,.the.p-type.conductiv-ity.of.those.ZnO:N.film.shows.an.obviously.low.stability,.which.seriously.affects.the.real.applications.based.on.ZnO.p-n.junction.

Nitrogen-doped.ZnO.film.can.also.be.prepared.by.chemical.techniques.such.as.chemi-cal.vapor.deposition,.ultrasonic.spray.pyrolysis,.and.sol-gel.methods..Minegishi.et.al..[56].reported.p-type.doping.of.ZnO.films.grown.on.sapphire.(0001).by.chemical.vapor.deposi-tion,.using.NH3.as.nitrogen.source..They.reported.a.carrier.concentration.of.1.5.×.1016.cm−3.with.estimated.ionization.energy.of.100.meV.and.Hall.mobility.of.12.cm2/(V.s)..They.also.indicated.that.hydrogen.may.play.some.roles.in.the.nitrogen.incorporation,.and.that.the.


appropriate.growth/annealing.conditions.for.obtaining.p-type.material.are. limited.to.a.narrow.range.that.control.turns.out.to.be.very.difficult.

Coutts.and.coworkers.[57].prepared.the.p-type.ZnO:N.films.by.MOCVD.with.the.pre-cursors.of.diethylzinc.(DEZn).and.nitric.oxide.(NO)..In.their.experiment,.DEZn.is.used.as.a.Zn.source,.and.NO.gas. is.used.to.supply.both.O.and.N.to.form.an.N-doped.ZnO.film..With.these.precursors,.they.have.routinely.reached.an.N.concentration.in.the.ZnO.films.of.about.1–3.at.%..When.the.N.concentration. level. is.higher. than.2.at.%,. the.films.demonstrate.p-type.characteristics.with.the.hole.concentration.of.1.0.×.1018.cm−3.and.hole.mobility.of.~10−1.cm2/(V.s)..Recently,.Cao.et.al. reported.the.ZnO:N.film.preparation.by.simple.sol-gel.spin.coating.method.[58],.and.p-ZnO.film.with.low.resistivity.was.obtained..In.their.experiment,.zinc.acetate.2-hydrate.[Zn(CH3COO)2·2H2O].was.firstly.dissolved.into.2-methoxyethanol.solution.with.the.addition.of.sol.stabilizers.monoethanolamine.(MEA)..Ammonia.acetate.solution.is.used.as.the.nitrogen.sources..For.nitrogen-doped.ZnO.film.preparation,.the.atomic.ratio.of.Zn/N.was.1:3.and.the.respective.sols.were.spin.coated.on.glass.substrates.at.3000.rpm..The.precursor.films.were.preheated.at.280°C.for.10.min.in.air.in.order.to.remove.the.volatile.materials..The.finally.obtained.ZnO:N.film.shows.good.p-type.conductivity.with.the.hole.concentration.of.7.5.×.1017.cm−3,.hole.mobility.of.1.3.cm2/(V.s),.and.the.resistivity.of.0.35.Ω.cm.

More.recently,.Lautenschlaeger.et.al..investigated.the.nitrogen.incorporation.in.homo-epitaxial. ZnO. layers. grown. by. chemical. vapor. deposition,. using. NH3. as. nitrogen. pre-cursor. [59].. They. explored. the. growth. on. both. Zn-polar. and. O-polar. faces,. and. found.the.nitrogen. incorporation. to.be.more. favorable.on. the.Zn-polar. face..Based.on.Raman.and.photoluminescence.spectroscopy.measurements,. they.concluded.that.relatively.low.growth.temperatures.and.Zn-polar.single-crystal.substrates.are.essential.for.N.incorpora-tion.in.homo-epitaxy.by.using.chemical.vapor.deposition.techniques.

Comparing.with.the.physical.methods,.the.chemical.routes.for.nitrogen-doped.ZnO.film.fabrication.show.lots.of.advantages.such.as.facile.and.low.cost..However,.chemical.method.will.easily.introduce.some.unwanted.impurities.into.ZnO.film.such.as.carbon.or.hydro-gen,.which.will. compensate. the.doping.effect.of. the.acceptor.and.degraded. the.p-type.conductivity.to.some.extent.

To.further.improve.the.p-type.conductivity.including.the.hole.concentration,.mobility.and.the.p-type.stability,.researchers.have.also.tried.the.co-doping.technique.for.ZnO.films.[26,60–63]..The.term.of.co-doping.means.that.along.with.the.acceptors.that.are.incorpo-rated.to.produce.holes,.donors.are.also.incorporated.during.the.growth..Obviously,.this.would.lead.to.the.compensation.effect.in.the.ZnO.host..However,.compensation.during.the.film.growth.is.actually.quite.desirable.[64,65],.since.it.will.shift.the.Fermi.level.away.from.the.valence.band.maximum.(VBM).toward.the.middle.of.the.gap..Through.co-doping.method,.it.will.lower.the.formation.energy.of.acceptors,.increasing.the.acceptor.solubility.and.finally.resulting.in.higher.hole.concentrations.due.to.an.enhancement.of.the.dopant.concentration.and.lowering.of.the.ionization.energy.

The.main.co-doping.methods.for.ZnO:N.film.include.N-Ga,.N-Al,.and.N-In.co-doping.route.[61,63,66–68]..Theoretical.calculation.also.proved.that.by.introducing.the.group.III.atoms.such.as.Ga,.Al.and.In.into.ZnO:N.film,.it.will.greatly.increase.the.nitrogen.solubil-ity,.thus.obtained.the.p-ZnO.with.enough.hole.concentration.as.well.as.acceptable.stability.[65]..Actually,.various.experimental.results.have.confirmed.the.improvement.of.the.p-type.conductivity.for.ZnO:N.film.and.the.related.stability.

Joseph.et.al..[69].have.reported.p-type.ZnO.through.codoping.using.N.(in.the.form.of.N2O).with.Ga.as.a.co-dopant..They.reported.quite.high.hole.concentrations.of.4.×.1019.cm−3.and. low. room. temperature. resistivity. of. 2. Ω. cm.. High-quality. p-type. ZnO:N. film. by.


(In,N).co-doping.route.with.ultrasonic.spray.pyrolysis.method.[70].shows.the.hole.con-centration. of. 2.4.×.1018. cm−3. and. the. hole. mobility. of. 155. cm2/(V. s);. the. resistivity. was.measured.to.be.as.low.as.1.7.×.10−2.Ω.cm..The.stability.of.the.p-type.characteristic.was.also.obviously.improved,.indicating.that.the.co-doping.method.should.be.the.suitable.way.for.p-type.ZnO.film.preparation.and.realization.of.the.ZnO-based.p-n. junction.in.the.future.

8.2.2 Nitrogen-Doping by Post-Growth Treatments

The.nitrogen.doping.process.for.ZnO.materials.can.also.be.operated.by.some.post-growth.treatments. such. as. post-annealing. under. nitrogen-containing. atmosphere. or. direct.nitrogen. ion-implantation. [71–73].. ZnO:N. film. can. also. be. prepared. by. annealing. zinc.oxynitride/nitride.precursor.films.in.oxygen.ambience.

For.effective.nitrogen.doping,. it. is.essential. to.substitute.oxygen.by.nitrogen.atom.in.ZnO. host. lattice.. Accordingly,. the. use. of. source. species. that. contain. only. one. nitrogen.atom.per.entity.(NH3,.NO,.N,.NO2).should.be.more.amenable.to.acceptor-state.formation.because.of.the.large.dissociation.energy.of.N2.(~9.9.eV)..By.using.the.gas.of.NH3.as.the.nitrogen.source,.Kim.et.al..[74].obtained.p-type.ZnO:N.film.by.thermal.annealing.the.as-prepared.pure.ZnO.film.in.NH3.ambient..They.prepared.the.pure.ZnO.film.with.the.thick-ness.of.260.nm.and.then.annealed.the.film.in.a.horizontal.furnace.under.NH3.atmosphere.at.600°C–700°C..Then.the.activation.annealing.was.carried.out.in.N2.ambient.at.800°C.for.30.min..After.the.aforementioned.annealing.treatment,.the.incorporation.of.nitrogen.atoms.into.the.ZnO.host.was.confirmed.by.secondary.ion.mass.spectrometry.(SIMS)..The.final.ZnO.film.showed.p-type.with.the.hole.concentration.of.1.06.×.1016.cm−3,.the.hole.mobility.of.15.8.cm2/(V.s),.and.the.resistivity.of.40.18.Ω.cm..The.results.show.that.the.nitrogen.atoms.can.be.doped.into.the.ZnO.host.by.thermal.diffusion..In.addition,.the.activation.annealing.treatment.plays.an.important.role.in.the.nitrogen-doping.process,.and.the.p-type.conver-sion.was.achieved.by.enhancing.the.activation.acceptors.in.the.films.by.using.a.thermal.annealing.process.

It. is.known.that. ion. implantation. is.widely.used. in. the.microelectronics. industry. for.selective.area.doping.and.device.isolation..Thus,.nitrogen-doped.ZnO.films.can.also.be.obtained.by.N+.implantation..However,.the.N+.implantation.with.large.kinetic.energy.will.degrade.the.quality.of.the.prepared.ZnO.film.to.certain.extent,.thus.the.followed.anneal-ing.treatment.is.necessary.and.important.for.the.re-crystallization.of.ZnO.film,.which.will.remove.the.accumulated.damages.and.achieve.the.selective.area.doping.

Tsai.et.al..prepared.p-type.ZnO.films.using.rf.reactive.magnetron.sputtering.following.by.N+.ions.implantation.and.subsequent.annealing.in.a.vacuum.to.achieve.low.resistivity.conductive.thin.films.[75]..The.obtained.ZnO:N.film.shows.distinct.p-type.conductivity.with.low.resistivity.varied.from.1.05.×.10−1.to.9.80.×.10−1.Ω.cm..In.addition,.the.ZnO:N.film.showed.quite.stable.hole.conductivity.and.kept.p-type.character.without.any.obvious.deg-radation.of.electric.conduction.after.tens.of.days.

Ion.implantation.technique.is.convenient.for.nitrogen.doped.ZnO.film.since.the.nitrogen.ions’.energy.and.the.implantation.dose.could.be.precisely.controlled..It.is.also.suitable.for.selective.area.doping;. thus,. this.method.can.be.used.to.dope.ZnO.nanostructures.such.as.nanowire/nanorods.to.fabricate.the.nano.p-n.junctions.[55,76]..However,.the.extremely.large.concentration.of.lattice.defects.produced.by.N+.implantation.will.typically.lead.to.remarkable.degradation.of.the.electronic.properties.(e.g.,.decreasing.the.carrier.mobility.and.creating.non-radiative.recombination.centers).that.may.lead.to.significant.barriers.for.practical.usage.of.such.techniques.


Since.the.solubility.of.nitrogen.in.ZnO.host.is.quite.low,.it.is.essential.to.improve.the.con-centration.of.N.dopants.for.better.p-type.film.fabrication..Thus.some.researchers.proposed.to.prepare.the.ZnO:N.film.by.annealing.zinc.oxynitride/nitride.precursor.films.in.oxygen.gas,.which.was.considered.to.overcome.the.problem.of.low.solubility.of.nitrogen.in.ZnO.film.and.further.to.improve.the.hole.concentration.[77–80]..This.method.was.reported.to.be.effective. for.nitrogen-doped.ZnO.film.preparation..Several.groups.also.claimed.that.they.obtained.good.p-type.ZnO:N.films.by.thermal.oxidation.of.a.Zn3N2.precursors.in.oxy-gen.ambience..The.p-type.characteristic.of.ZnO:N.films.prepared.by.this.method.is.quite.sensitive.to.the.annealing.temperature..If.the.annealing.temperature.is.low,.the.N.atoms.will. not. be. activated. to. act. as. the. effective. acceptors,. while. the. N. atoms. concentration.will.decrease.dramatically.and.a. large.amount.of.point.defects.will.appear. if.annealed.with.much.higher.temperature..Thus.thermal.oxidation.temperature.should.be.crucial.for.p-type.ZnO:N.film.preparation.by.annealing.zinc.oxynitride/nitride.precursor.films. in.oxygen.gas.

8.3 Synchrotron Radiation–Related Studies

8.3.1 Synchrotron Radiation–Based Characterization Methods

Synchrotron.radiation.is.exactly.a.kind.of.light.source,.which.covers.almost.all.wavelengths.of.the.electromagnetic.spectrum.with.much.higher.intensity.than.those.of.conventional.x-ray.tubes.used.for.crystallography.or.x-ray.diffraction.instruments.[81]..For.producing.synchrotron.radiation,.electrons.from.a.linear.accelerator.with.a.speed.close.to.that.of.light.are.injected.into.a.storage.ring.under.high.vacuum..The.storage.ring.consists.of.curved.sections.joined.with.straight.parts..Magnetic.fields,.from.strong.bending.magnets.around.the.ring,.force.the.accelerated.electrons.to.follow.the.ring.in.the.curved.sections..When.the.high-energy.particles.hit.the.curved.parts,.they.lose.part.of.their.energy.as.synchrotron.radiation,.which.is.emitted.tangential.to.these.curved.sections..The.wavelength.of.the.syn-chrotron.radiation.can.be.tuned.by.changing.the.magnetic.field,.for.example,.with.wigglers.or.undulators.consisting.of.an.array.of.dipole.magnets,.giving.a.continuous.energy.range.from.infrared.to.hard.x-rays..Thus.the.synchrotron.radiation.has.many.advantages.such.as.high.intensity.and.brightness,.broad.wavelength,.high.collimation,.and.good.polarization,.which.makes.it.an.excellent.light.source.for.all.kinds.of.research.projects.

Synchrotron.radiation–based.characterization.methods.could.be.used. to.examine. the.chemical.states.of.dopants.in.details.and.thus.can.satisfy.the.doping.mechanism.inves-tigation..Among.those.synchrotron.radiation.methods,.x-ray.absorption.near-edge.spec-troscopy. (XANES). and. PES. are. regarded. as. powerful. tools. for. investigating. the. local.environment.around.atoms,.providing.element-specific.information.about.chemistry,.site.occupancy.and.the.neighboring.environment.[82–84].

The.synchrotron.radiation–based.spectroscopy.method.was.based.on.the.light-atomic.interaction.process..The.brief.scheme.is.shown.in.the.following.picture.(Figure.8.1)..When.x-rays.from.synchrotron.source.hit.a.sample,.the.oscillating.electric.field.of.the.electromag-netic.radiation.interacts.with.the.electrons.bound.in.an.atom..The.radiation.will.be.either.scattered.by.these.electrons.or.absorbed.and.excite.the.electrons..If.the.sample.is.not.such.thick,.part.of. the.x-ray.will. transmit. the.sample.as.well..When.the.x-ray.was.absorbed,.the.related.core-level.electrons.will.be.excited. to. those.unoccupied. levels.or.even.go. to.the.vacuum.as.photoelectrons..Sometimes.the.Auger.electrons.will.also.be.produced.in.


this.process..Part.of.the.excited.electrons.also.have.the.possibility.to.go.back.to.the.basic.states. with. the. fluorescence. light. emission.. Accordingly,. at. certain. energy. levels. where.the.absorption.increases.drastically.and.gives.rise.to.an.absorption.edge..Each.such.edge.occurs.when.the.energy.of.the.incident.photons.is.just.sufficient.to.cause.excitation.of.a.core.electron.of.the.absorbing.atom.to.a.continuum.state,.that.is,.to.produce.a.photoelectron..Thus,.the.energies.of.the.absorbed.radiation.at.these.edges.correspond.to.the.binding.ener-gies.of.electrons.in.the.K,.L,.M,.etc.,.shells.of.the.absorbing.elements.

8.3.2 Doping Mechanism Studied by Synchrotron Radiation

Among.the.promising.elements.from.group.V,.nitrogen.appears.to.be.the.best.candidate.for.p-doping.in.ZnO..As.discussed.earlier,.the.ionic.radii.of.nitrogen.and.oxygen.are.of.com-parable.size,.N.has.the.lowest.ionization.energy.of.all.possible.group-V.elements,.and.it.does.not.form.the.N-on-Zn.antisite.(NZn)..However,.till.now,.no.reproducible.p-type.ZnO:N.film.with.stable.conductivity.can.be.achieved,.though.different.doping.methods.and.treat-ments.have.been.attempted..Obviously,.the.most.important.step.should.be.conducted.to.investigate.the.chemical.nature.of.nitrogen.dopants.in.ZnO.host,.and.to.understand.the.essential.doping.mechanism.for.ZnO:N.film.

XAS,.especially.the.XANES,.is.quite.sensitive.to.the.coordination.and.oxidation.state.of.the.absorbing.atoms,.while.PES.can.directly.reflect.the.chemical.states.of.elements..Thus.combining.these.two.characterization.methods,.the.nitrogen.dopants.in.ZnO.host.can.be.clearly.revealed.with.the.help.of.advanced.synchrotron.source.

Paul.Fons.prepared.the.nitrogen-doped.ZnO.film.by.MBE.method.and.systematically.examined.the.nitrogen.location.in.ZnO.lattice.by.XANES.[85,86]..They.recorded.the.N.K-edge.and.compared.them.with.the.first-principles.calculations.in.Figure.8.2,.showing.that.nitrogen,.in.fact,.incorporates.substitutionally.at.O.sites.where.it.is.expected.to.act.as.an.acceptor..After.annealing,.the.(N2)o

−.molecules.are.detected,.leading.to.compensa-tion.rather. than.p-type.doping..They.suggest. that. the. incorporation.of.N.atoms.as.an.acceptor.is.metastable;.and.effective.p-type.doping.of.ZnO.with.N.may.be.possible.only.for. low-temperature.growth.processes..This.conclusion. is.quite.contrary. to. the. recent.report,.which.shows.that.high.annealing.temperature.will.activate.more.acceptors.and.enhance.the.p-type.property..Obviously,.the.nitrogen-doping.mechanism.in.ZnO.host.is.still.not.clear.

Samples

Transmitted x-rays

Scattered x-rays

Incident x-rays

Fluorescence light

Photoelectrons Auger electrons

FIGURE 8.1Scheme.for.the.interaction.between.x-ray.and.solid-state.sample.


Yano.et.al.. [87]. systematically.studied. the.nitrogen.species.and. the.related.chemical.states.of.N.atoms.in.ZnO:N.samples.prepared.by.N2

+.implantation.method.with.photo-emission.spectroscopy. (PES).and.near-edge.x-ray.absorption.fine.structure. (NEXAFS)..In. the. N2

+. implanted. samples,. N–O,. Zn–N. or. Zn–N–O. bonds. are. formed,. which. can.be.characterized.by.specific.chemical.shifts. in.PES.or.absorption.peaks.in.NEXAFS.as.shown.in.Figure.8.3..The.results.indicate.that.low-energy.nitrogen.bombardment.of.ZnO.may.break.Zn–O.bonds.at.the.surface.and.produce.nitrogen.species.of.different.stabili-ties,.including.several.nitrogen.oxides.and/or.oxynitrides.and.molecular.nitrogen..The.existence.of.molecular.nitrogen.in.ZnO.may.have.an.important.implication.on.the.type.of.conductivity.in.ZnO.

Recently,.Hoffmann.et.al..[88].also.studied.the.chemical.nature.of.N-ions.incorporated.into.epitaxial.ZnO.films.by.PES.using.synchrotron.radiation.and.near-edge.x-ray.absorp-tion.spectroscopy.(NEXAFS).techniques..Three.main.N1s-PES.components.were.assigned.to.molecular.N2,.N–O.bonds.and.N–Zn.bonds.combined.with.the.help.of.NEXAFS.data..In.addition,.the.thermal.stability.of.the.nitrogen.compounds.was.investigated.and.the.N–O.bonds.have.the.highest.stability:.heating.to.800°C.reduces.the.N–O.bonds.drastically,.but.also.reduces.the.N–Zn.bonds..In.contrast,.the.amount.of.N2.will.increase.by.heating..Their.results.will.lead.to.an.optimization.of.the.nitrogen.implantation.process.for.a.better.doping.efficiency.

Thermal.annealing.was.usually.employed.to.treat.ZnO:N.sample.since.it.is.an.effective.technique.to.improve.the.crystalline.quality.as.well.as.activate.acceptor.impurities.in.ZnO..However,.thermal.annealing.will. inevitably.result. in.decomposition.of.nitrogen.and.oxy-gen.from.the.sample,.which.can.severely.deteriorated.the.electrical.properties.of.the.ZnO:N.sample..To.solve.this.dilemma.and.exploit.the.advantage.of.nitrogen.dopant,.Sun.et.al..[89].

(a)

(b)

(c)

400 420Photon energy (eV)

Abs

orpt

ion

(a. u

.)

440 460

(d)

(e)

(f)

FIGURE 8.2Experimental.(a).and.simulated.x-ray.absorption.spectra.for.(b).N.on.an.O.site,.(c).N2.on.an.O.site,.(d).N.in.a.tetrahedral.interstitial,.(e).N.in.a.Zn.site,.and.(f).N.in.an.octahedral.interstitial..(Reproduced.with.permission.from. Fons,. P.,. Tampo,. H.,. Kolobov,. A.V.,. Ohkubo,. M.,. Niki,. S.,. Tominaga,. J.,. Carboni,. R.,. Boscherini,. F.,. and.Friedrich,.S.,.Phys. Rev. Lett.,.96,.045504,.2006..Copyright.2006,.the.American.Physical.Society.)


developed.a.novel. technique. to.realize.effective.p-type.ZnO..The.effective.p-type.ZnO:N.film.was.achieved.with.ammonia.as.N.doping.source. followed.by. thermal.annealing. in.N2O.plasma.protective.ambient..N2O.plasma.can.provide.both.nitrogen.ion.and.oxygen.ion,.which.will.be.helpful.for.restrain.the.decompose.of.nitrogen.and.oxygen.from.ZnO:N.sample.during.thermal.annealing.process.under.high.temperature..Based.on.the.aforementioned.method,.a.stable.and.reproducible.p-type.ZnO:N.film.with.hole.concentration.of.~1017.cm−3.has.been.achieved..While.if.annealing.the.sample.in.pure.oxygen.ambient,.only.weak.p-type.ZnO:N.film.with.remarkably.lower.hole.concentration.of.~1015.cm−3.could.be.obtained.

To.explore.the.mechanism.of.the.p-type.doping.behavior.of.ZnO:N.film,.soft.x-ray.absorp-tion.near-edge.spectroscopy.(XANES).measurements.with.synchrotron.radiation.has.been.applied.to.investigate.the.local.electronic.structure.and.chemical.states.of.nitrogen.atoms.in.ZnO:N.films.[90]..The.distinct.dependence.on.the.annealing.ambient.was.observed.in.the.normalized.O.K-edge.XANES.spectra,.as.shown.in.Figure.8.4a..For.comparison,.the.un-doped.ZnO.sample.was.also.measured..The.spectra.show.four.peaks.marked.as.fea-ture.A,.B,.C,.and.D.with.the.energies.of.528.4,.530.8,.535.3,.541.3.eV,.respectively..According.to.previous.reports,.the.O.K-edge.XANES.spectra.reveals.the.unoccupied.O.2p.states,.and.the. features.are.assigned. to. the. following.hybridized.states:. feature.B–D.are.attributed.to.electron.transitions.from.O.1s.to.O.2pσ.(along.the.bilayer).and.O.2pπ.(along.the.c-axis).states.[91,92],.which.have.been.generally.observed.in.standard.ZnO.samples.

The.origin.of.these.features.has.been.well.understood.based.on.the.comparison.between.the.first.principle.multi-scattering.simulation.and. the.experimental. results..The. feature.A. centered. at. 528.4. eV. appeared. only. in. annealed. ZnO:N. samples,. which. is. primarily.assigned.to.O.2p–N.1sp.hybridized.states,.and.is.suggested.to.arise.from.N.atoms.at.O.sites.(N)o..These.(N)o.are.regarded.as.the.acceptors,.and.responsible.for.the.p-type.characteris-tic.of.N-doped.ZnO.films..It.can.also.be.found.that.the.feature.A.intensity.for.the.sample.

Binding energy (eV) Photon energy (eV)

Nor

mal

ized

inte

nsity

(a. u

.)

Nor

mal

ized

inte

nsity

(a. u

.)390 400 410

(a) (b)395 400 405 410 415

DE

P2P1 P4

As-grown

P3

CB

A

×100

0.3 keV N2*0.3 keV N2*

2 keV N2*

2 keV N2*

2 keV N2* +annealing

2 keV N2* +annealing

420

FIGURE 8.3(a).N.1s.core-level.photoemission.spectra.(full.circles).obtained.from.ZnO.surfaces.bombarded.with.0.3.and.2.keV.N2

+.ions.for.30.min.and.after.annealing.at.400°C.for.1.h;.(b).the.related.N.K-edge.NEXAFS.spectra.from.ZnO.surfaces..(Reproduced.from.Surf. Sci.,.600,.Petravic,.M.,.Deenapanray,.P.N.K.,.Coleman,.V.A.,.Jagadish,.C.,.Kim,.K.J.,.Kim,.B.,.Koike,.K.,.Sasa,.S.,.Inoue,.M.,.and.Yano,.M.,.L81..Copyright.2006,.with.permission.from.Elsevier.)


Photon energy (eV)520

0.60.70.80.91.01.1

0.60.70.80.91.01.1

Inte

nsity

(a. u

.)

0.60.70.80.91.01.1

0.60.7

A

BC

D

ZnO:N annealed in N2Oplasma protective ambient

ZnO:N annealed in O2 ambient

0.80.91.01.1

525(a)

530 535 540 545 550 555

As-deposited ZnO:N

Undoped ZnO

560 565

(b)

p-type ZnO : NHoles

CB

VBPeak C

Peak B

Peak A

Ols binding energy ~530 eV

Ols

Ef

FIGURE 8.4(a).Normalized.O.K-edge.XANES.spectra.for.ZnO:N.samples.on.c-plane.sapphire.annealed.at.800°C.under.dif-ferent.ambient..Note.that.significant.changes.in.spectra.features.were.observed.among.the.samples.annealed.under.different.ambient..(b).The.transitions.for.the.peaks.A~C.in.the.XANES.spectra..(Reproduced.from.Appl. Surf. Sci.,.257,.Li,.Q.W.,.Bian,.J.M.,.Sun,.J.C.,.Liang,.H.W.,.Zou,.C.W.,.Sun,.Y.L.,.and.Luo,.Y.M.,.1634..Copyright.2010,.with.permission.from.Elsevier.)


annealed.in.N2O.plasma.is.much.higher.than.that.annealed.in.O2,.suggesting.that.the.den-sity.of.N.atoms.at.O.sites.(N)o.is.much.higher.when.annealed.in.N2O.plasma..Therefore,.the.results.from.XANES.measurements.are.consistent.with.the.electric.conductivity.testing.

Furthermore,.the.Fermi.level.can.be.roughly.estimated.from.its.correlation.with.the.rel-evant.XANES.spectra.in.Figure.8.4..Normally,.the.binding.energy.of.O.1s.is.~530.eV.refer-ring.to.the.Fermi.level..In.Figure.8.4,.peaks.B.and.C.with.the.photon.energies.of.531.and.535.eV.are.regarded.to.be.associated.with.the.electron.transitions.from.O.1s.state.to.O.2pσ.and.O.2pπ.unoccupied.states..These.unoccupied.states.are.both.above.the.Fermi.level..The.peak.A.with.the.photon.energy.of.~529.eV.should.relate.to.the.electron.transitions.from.O.1s.state.to.the.unoccupied.states.located.at.the.valence.band.(VB),.which.are.related.to.the.holes.at.the.VB..Thus,.the.peak.A.assigned.in.Figure.8.4b.directly.shows.the.existence.of.unoccupied.states.or.hole.states.in.the.p-type.ZnO:N.film.[90].

For. better. understanding. the. mechanism. of. the. nitrogen. doping. for. p-type. ZnO. film.preparation,.Zou,.Gao.and.their.group.systematically.investigated.the.chemical.states.of.nitrogen.dopants.in.ZnO.film.as.the.function.of.annealing.temperature.combining.with.the. related. electric. conductivity. testing. [68,93].. The. ZnO:N. film. was. prepared. by. ther-mal.annealing. the.Zn3N2.precursor.with.different. temperature.under.oxygen.ambience..The. conductivity.measurements. for. the.ZnO:N.films. shows. that.p-type.doping.of.ZnO.with.N.dopants.can.only.be.achieved.with.a.suitable.annealing.temperature..To.clearly.see.the.situation.of.nitrogen.atoms.in.ZnO.lattice.after.annealing.at.different.temperatures,.synchrotron.radiation.based.PES.and.XANES.techniques.were.conducted.for. the. inves-tigation.as.shown.in.Figure.8.5..From.the.PES.results. in.Figure.8.5a,. it.can.be.observed.that. with. higher. annealing. temperatures,. the. peak. associated. with. nitrogen. molecules.(N2)o.at.oxygen.sites.decreased.markedly.and.a.new.peak.appeared,.which.is.most.likely.associated.with.nitrogen.atoms.(N)o.at.oxygen.sites..These.nitrogen.atoms.are.regarded.as.the.acceptors,.and.responsible.for.the.p-type.characteristics.of.nitrogen-doped.ZnO.films.

XANES.measurements.in.Figure.8.5b.around.the.nitrogen.K-edge.produced.spectra.with.seven.main.peaks.associated.with.1s.to.2p.π*.and.other.electronic.transitions.in.the.N-Zn.bond..The.spectra.indicate.a.greatly.reduced.nitrogen.molecule.contribution.for.an.annealing.temperature.of.550°C,.consistent.with.the.PES.results..Experimental.results.for.the.550°C.and.600°C.samples.agree.with.the.theoretical.simulation,.indicating.that.the.residual.nitrogen.atoms.occupy.the.oxygen.sites.and.become.(N)o.acceptors.at.these.annealing.temperatures.

Overall,.the.results.indicate.that.ZnO:N.films.show.p-type.behavior.after.annealing.at.500°C.and.550°C,.and.come.back.to.n-type.behavior.after.annealing.at.600°C..A.mecha-nism.to.explain.this.behavior.was.proposed.as.shown.in.Figure.8.6..It.is.concluded.that.the. nitrogen. atoms. that. behave. as. acceptors. are. metastable. and. sensitive. to. annealing.temperature..The.p-type.doping.of.ZnO.with.nitrogen.dopants.can.only.be.realized.after.annealing.within.a.suitable.temperature.window.[93,94].

8.4 Summary and Outlook

Nitrogen.is.considered.to.be.the.most.suitable.dopant.for.the.p-type.ZnO.film.realization.due.to.the.similar.radius.as.O.atoms.and.the.shallow.acceptor.level,.which.have.attracted.tre-mendous.attention.from.people.working.in.this.field.since.the.achievement.of.good.p-type.ZnO:N.film.will.be.a.breakthrough.for.ZnO-based.electronic.and.optical.devices.fabrication..While.the.current.nitrogen-doped.ZnO.films.are.still.not.satisfactory.for.the.real.devices.


Binding energy (eV)(a)

390 400

C΄

AInte

nsity

(a. u

.)

C

600°C

550°C

450°C

Zn3N2

D

B

Nls

410 420

395 400 405 410

P1

P1

P1

P3

P3Inte

nsity

(a. u

.)

P2

450°C

550°C

600°C

(×10)

(×15)

By theory cal.

N K-edge XANES

P4P5

P6

P6

P7

415Photon energy (eV)

420 425 430(b)

FIGURE 8.5(a).N.1s.peaks.for.ZnO:N.samples.with.different.annealing.temperatures..The.exciting.photon.energy.is.500.eV..(b).Nitrogen.K-edge.NEXAFS.spectra.for.samples.with.different.annealing.temperatures..The.calculated.curve.is.listed.for.comparison..(Reproduced.with.permission.from.Zou,.C.W.,.Yan,.X.D.,.Han,.J.,.Chen,.R.Q.,.Gao,.W.,.and.Metson,.J.,.Appl. Phys. Lett.,.94,.171903,.2009..Copyright.2009,.American.Institute.of.Physics.)


application.due.to.the.stability.and.reproducibility,.researchers.are.making.good.progress.on.making.p-type.ZnO.films..The.essential.problem.is.the.doping.mechanism.of.nitrogen.in.ZnO.lattice.is.not.fully.understood..Fortunately,.synchrotron.radiation–based.spectroscopy.techniques.can.be.suitable.and.powerful.tools.for.this.doping.mechanism.study.

In.this.chapter,.we.have.systematically.reviewed.the.p-type.ZnO:N.preparation.processes.and.the.doping.mechanism.revealed.by.synchrotron.radiation.studies..With.the.help.of.the.synchrotron-based.spectroscopy.techniques,.the.nitrogen.species.in.ZnO.host,.the.chemi-cal.states.and.the.related.occupation.site.of.N.atoms.can.be.investigated.in.detail,.which.can. directly. reveal. the. nitrogen-doping. mechanism.. The. experimental. results. indicate.that.the.nitrogen.atoms.that.behave.as.acceptors.are.metastable.and.sensitive.to.anneal-ing.temperature..Accordingly,.p-type.doping.of.ZnO.with.nitrogen.dopants.can.only.be.realized.after.annealing.within.a.suitable.temperature.window..This.conclusion.is.based.on.synchrotron.radiation.studies,.and.will.provide.a.good.direction.for.p-type.ZnO:N.film.preparation.in.the.future.

Currently,.p-type.ZnO.film.preparation.is.still.one.of.the.hot.research.topics.though.it.has.been.studied.for.more.than.10.years..The.achievement.of.stable.and.producible.p-type.ZnO.film.with.high.quality.has.not.been.confirmed.yet.by.any.research.group.till.now..Despite.the. difficulties. in. achieving. p-type. conductivity,. ZnO. remains. a. promising. material. for.electronic.and.optoelectronic.device.applications..Thus.the.related.doping.process.for.ZnO.film.for.p-type.conductivity.should.be.continued..The.key.point.of.the.continuing.research.should.be.on.the.doping.mechanism.investigation.for.the.dopants. in.ZnO.host..Only.if.the. mechanism. of. p-type. doping. process. is. clearly. understood,. the. high-quality. p-type.ZnO.film.for.real.application.could.be.realized..From.this.point.of.view,.the.synchrotron.radiation.and.related.techniques.should.be.the.promising.methods.for.the.doping.mecha-nism.investigation.in.the.future.studies.

Acknowledgments

The.authors.would.like.to.pay.thanks.to.the.Research.Center.for.Surface.and.Materials.Science.at.the.University.of.Auckland.for.the.support.for.writing.this.book..This.work.was.partially.supported.by.the.Startup.Funding.for.New.Faculty.at.the.University.of.Science.and.Technology.of.China.(USTC).and.the.Youth.Innovation.Funding.from.USTC.

Annealing temperatures T (°C)

n-type n-type

Defects Donors

(e)

p-type

500°C

Zn3N2

Zn3N2 ZnO: N

(N)o–H (C)(not actived)

(N)o species(holes)

(N)o species(holes)

600°C

(N2)o exists

ZnO (N2)o (N)o(N)o + O2 (O)o + N2

FIGURE 8.6Proposed. doping. mechanism. for. ZnO:N. films.. (Reproduced. with. permission. from. Zou,. C.W.,. Yan,. X.D.,.Han,.J.,.Chen,.R.Q.,.Gao,.W.,.and.Metson,.J.,.Appl. Phys. Lett.,.94,.171903,.2009..Copyright.2009,.American.Institute.of.Physics.)


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. 72.. G..Braunstein,.A..Muraviev,.H..Saxena,.N..Dhere,.V..Richter,.and.R..Kalish,.p-type.doping.of.zinc.oxide.by.arsenic.ion.implantation,.Applied Physics Letters.87.(2005).192103.

. 73.. Y..F..Mei,.G..G..Siu,.R..K..Y..Fu,.K..W..Wong,.P..K..Chu,.C..W..Lai,.and.H..C..Ong,.Determination.of. nitrogen-related. defects. in. N-implanted. ZnO. films. by. dynamic. cathodoluminescence,.Nuclear Instruments and Methods in Physics Research Section B—Beam Interactions with Materials and Atoms.237.(2005).307.

. 74.. S..J..Jung,.Y..Nakamura,.A..Kishimoto,.and.H..Yanagida,.Modified.CuO.surface.appropriate.for.selective.CO.gas.sensing.at.CuO/ZnO.heterocontact,.Journal of the Ceramic Society of Japan.104.(1996).415.

. 75.. S..Y..Tsai,.Y..M..Lu,.and.M..H..Hon,.Fabrication.of. low.resistivity.p-type.ZnO.thin.films.by.implanting. N+. ions,. Proceedings of the 17th International Vacuum Congress/13th International Conference on Surface Science/International Conference on Nanoscience and Technology. 100. (2008).042037.

. 76.. D.. Weissenberger,. M.. Duerrschnabel,. D.. Gerthsen,. F.. Perez-Willard,.A.. Reiser,. G.. M.. Prinz,.M..Feneberg,.K..Thonke,.and.R..Sauer,.Conductivity.of.single.ZnO.nanorods.after.Ga.implanta-tion.in.a.focused-ion-beam.system,.Applied Physics Letters.91.(2007).132110.

. 77.. D..Klaitabtim,.S..Pratontep,.and. J..Nukeaw,.Growth.and.characterization.of.zinc.oxynitride.thin.films.by.reactive.gas-timing.RF.magnetron.sputtering,.Japanese Journal of Applied Physics.47.(2008).653.

. 78.. C..W..Zou,.R..Q..Chen,.and.W..Gao,.The.microstructures.and.the.electrical.and.optical.properties.of.ZnO:N.films.prepared.by.thermal.oxidation.of.Zn3N2.precursor,.Solid State Communications.149.(2009).2085.

. 79.. J..Zhang.and.L..X..Shao,.p-type.ZnO.nano-thin.films.prepared.by.oxidation.of.Zn3N2.deposited.by. rf. magnetron. sputtering,. Optoelectronics and Advanced Materials—Rapid Communications. 3.(2009).676.

. 80.. D..Wang,.Y..C..Liu,.R..Mu,.J..Y..Zhang,.Y..M..Lu,.D..Z..Shen,.and.X..W..Fan,.The.photolumines-cence.properties.of.ZnO:N.films.fabricated.by.thermally.oxidizing.Zn3N2.films.using.plasma-assisted. metal-organic. chemical. vapour. deposition,. Journal of Physics—Condensed Matter. 16.(2004).4635.

. 81.. D..H..Bilderback,.P..Elleaume,.and.E..Weckert,.Review.of.third.and.next.generation.synchrotron.light.sources,.Journal of Physics B—Atomic Molecular and Optical Physics.38.(2005).S773.

. 82.. H..Wende,.Recent.advances.in.x-ray.absorption.spectroscopy,.Reports on Progress in Physics.67.(2004).2105.

. 83.. F..de.Groot,.High.resolution.x-ray.emission.and.x-ray.absorption.spectroscopy,.Chemical Reviews.101.(2001).1779.


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. 85.. P..Fons,.H..Tampo,.A..V..Kolobov,.M..Ohkubo,.S..Niki,.J..Tominaga,.R..Carboni,.F..Boscherini,.and. S.. Friedrich,. Direct. observation. of. nitrogen. location. in. molecular. beam. epitaxy. grown.nitrogen-doped.ZnO,.Physical Review Letters.96.(2006).045504.

. 86.. P..Fons,.H..Tampo,.S..Niki,.A..V..Kolobov,.M..Ohkubo,.J..Tominaga,.S..Friedrich,.R..Carboni,.and. F.. Boscherini,. Soft. x-ray. XANES. of. N. in. ZnO:N—Why. is. doping. so. difficult?,. Nuclear Instruments and Methods in Physics Research Section B—Beam Interactions with Materials and Atoms.246.(2006).75.

. 87.. M..Petravic,.P..N..K..Deenapanray,.V..A..Coleman,.C..Jagadish,.K..J..Kim,.B..Kim,.K..Koike,.S..Sasa,.M..Inoue,.and.M..Yano,.Chemical.states.of.nitrogen.in.ZnO.studied.by.near-edge.X-ray.absorption. fine. structure. and. core-level. photoemission. spectroscopies,. Surface Science. 600.(2006).L81.

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. 89.. J..C..Sun,.H..W..Liang,.J..Z..Zhao,.J..M..Bian,.Q..J..Feng,.L..Z..Hu,.H..Q..Zhang,.X..P..Liang,.Y..M..Luo,.and.G..T..Du,.Ultraviolet.electroluminescence.from.n-ZnO:Ga/p-ZnO:N.homojunction.device.on.sapphire.substrate.with.p-type.ZnO:N. layer. formed.by.annealing. in.N2O.plasma.ambient,.Chemical Physics Letters.460.(2008).548.

. 90.. Q..W..Li,.J..M..Bian,.J..C..Sun,.H..W..Liang,.C..W..Zou,.Y..L..Sun,.and.Y..M..Luo,.Effects.of.anneal-ing.ambience.on.ZnO:N.films.grown.by.MOCVD.and.the.p-type.doping.mechanism.of.ZnO:N.films.investigated.by.XANES,.Applied Surface Science.257.(2010).1634.

. 91.. J..W..Chiou,.K..P..K..Kumar,. J..C.. Jan,.H..M..Tsai,.C..W..Bao,.W..F..Pong,.F..Z..Chien.et. al.,.Diameter.dependence.of.the.electronic.structure.of.ZnO.nanorods.determined.by.x-ray.absorp-tion. spectroscopy. and. scanning. photoelectron. microscopy,. Applied Physics Letters. 85. (2004).3220.

. 92.. J..W..Chiou,.J..C..Jan,.H..M..Tsai,.C..W..Bao,.W..F..Pong,.M..H..Tsai,.I..H..Hong.et.al.,.Electronic.structure. of. ZnO. nanorods. studied. by. angle-dependent. x-ray. absorption. spectroscopy. and.scanning.photoelectron.microscopy,.Applied Physics Letters.84.(2004).3462.

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223

9Optical Properties of MgZnO/ZnO Heterostructures Grown on Sapphire Substrates by Plasma-Assisted Molecular Beam Epitaxy

Y.M. Lu, P.J. Cao, W.J. Liu, D.L. Zhu, X.C. Ma, D.Z. Shen, and X.W. Fan

9.1 Introduction

Recently,.ZnO-based.semiconductors.have.been.attracting.increasing.attention.as.promis-ing. candidates. for. optoelectronic. applications. in. ultraviolet. (UV). regions. [1,2].. Because.ZnO.has.a.very.large.exciton.binding.energy.(60.meV),.it.allows.efficient.excitonic.emis-sion.at.high.temperature.[3]..As.is.well.known,.an.exciton-related.lasing.process.can.easily.achieve.higher.gain.and.lower.threshold,.ZnO.desirable.to.develop.optoelectronic.devices.based.on.excitonic.effect..In.addition,.in.order.to.obtain.high-performance.light-emitting.diode.(LED).devices,.one.of.the.key.techniques.is.to.construct.a.heterojunction.to.realize.double.confinement.actions.for.electrons.and.photons.in.optoelectronic.devices..Because.the.ionic.radius.of.Mg2+.(0.57.Å).is.close.to.that.of.Zn2+.(0.6.Å),.MgxZn1−xO.alloy.is.consid-ered.to.be.a.suitable.potential.barrier.material.by.doping.Mg.as.a.substitute.for.Zn2+.ion.in.ZnO. [4,5]..However,.ZnO.and.MgO.belong. to.wurtzite. structure.and. rocksalt. struc-ture,.respectively..The.difference.between.ZnO.and.MgO.with.regard.to.their.thermally.stable. structure. results. in.phase. separation.of.MgxZn1−xO.at.high.Mg.composition.. It. is.suggested.that.the.Mg.content.of.x.=.0.33.is.the.solubility.limit.of.MgxZn1−xO.thin.films.for.the.wurtzite.structure.[7]..Hence,.the.main.researches.are.concentrated.on.the.growth.and.

CONTENTS

9.1. Introduction.........................................................................................................................2239.2. Experimental....................................................................................................................... 226

9.2.1. Materials.Growth.................................................................................................... 2269.2.2. Characterization.Techniques................................................................................227

9.3. Results.and.Discussion......................................................................................................2279.3.1. Optical.Properties.of.MgZnO.Thin.Films.with.the.Wurtzite.Structure.........2279.3.2. Structural.Characterization.and.Optical.Properties.

of.ZnO/MgZnO.Heterostructure.........................................................................2339.3.3. Band.Diagram.of.ZnO/MgZnO.Quantum.Well.Structure.............................. 2379.3.4. Stimulated.Emission.of.ZnO/MgZnO.Quantum.Well.Structure.................... 242

9.4. Summary.............................................................................................................................. 249Acknowledgments.......................................................................................................................250References......................................................................................................................................250


structural.characterization.of.MgxZn1−xO.alloy.thin.films.[6–10].Various.techniques.have.been.employed.for.fabricating.MgxZn1−xO.thin.films.with.the.wurtzite.structure,.such.as.pulsed.laser.deposition.(PLD)[6],.laser.ablation.molecular-beam.epitaxy.(L-MBE)[7],.metal.organic.chemical.vapor.deposition.(MOCVD)[8],.and.plasma-assisted.molecular.beam.epi-taxy.(P-MBE)[9,10]..All.the.results.show.that.single.wurtzite.phase.MgZnO.layers.with.high.Mg.content.have.been.successfully.fabricated,.in.particular,.Ref..[8].given.that.maximum.Mg.concentration.is.up.to.49.at.%..In.these.reported.works,.the.control.of.Mg.composition.was.realized.by.adjusting.the.ratio.of.Zn/Mg.in.MgZnO.thin.films..However,.because.Zn.and.Mg.atoms.are.very.active,.a.little.variance.in.the.source.temperatures.will.cause.more.change.in.Zn.and.Mg.partial.pressures..Therefore,.it.is.difficult.to.control.the.composition.by.changing.the.metal.source..In.our.recent.work.[10],.MgZnO.thin.films.with.different.Mg.contents.were.grown.at.a.large.range.of.the.oxygen.flux.by.P-MBE..By.controlling.the.oxy-gen.flow.rate,.two-dimensional.(2D).growth.of.the.MgZnO.thin.films.with.single.hexago-nal.phase.was.realized..The.composition.of.MgZnO.thin.films.was.controlled.by.adjusting.the.oxygen.flow.rate..Although.many.groups.have.successfully. reported. the.growth.of.MgZnO.thin.films.with.single.wurtzite.phase.structure.[6–9],.only.a.few.works.dealt.with.the.origin.of.photoluminescence.for.MgxZn1−xO.thin.films.[5,8]..And.this.is.very.important.to.characterize.and.improve.the.quality.of.ZnMgO.alloy.films.

Besides. the. fabrication.of.wurtzite.phase.ZnMgO.alloy.films,.ZnO/MgZnO.quantum.well.structures.for.optoelectric.applications.in.UV.region.also.receive.considerable.atten-tion..Because.the.emission.process.with.exciton.recombination.can.be.further.enhanced.if.such.low-dimensional.structures.as.single.quantum.well.(SQW).and.multiple.quantum.well.(MQW).structures.are.constructed,.the.quantum-well.approach.is.effective.toward.the.goal.of.a.current-injection.laser..In.fact,.ZnO/MgZnO.heterostructures.had.been.recently.applied.in.fabricating.LEDs.to.obtain.highly.effective.UV.emission.in.the.electrolumines-cence.(EL).[11]..So.far,.there.are.two.interesting.aspects.concerning.the.researches.of.ZnO/MgZnO.quantum.well.structures..One.aspect.is.fabricating.and.confirming.the.quantum.well.structure.based.on.the.band.gap.engineering,.taking.into.account.the.quantum.size.effect..The.other.one.is.the.excitonic.transition.properties.in.ZnO/MgZnO.quantum.well.heterostructures..In.particular,.the.excitonic.spontaneous.emission.and.stimulated.emis-sion.properties.have.been.extensively.studied.[12–39].

In. the. early. works,. ZnO/MgZnO. quantum. well. structures. were. firstly. fabricated. on.Al2O3.substrates.by.MOCVD.and.PMBE.[12–18]..However,.the.results.show.that.the.sam-ples.which.were.grown.directly.on.sapphire.substrates.are.found.in.rough.surfaces.and.interfaces.due.to.a. large.lattice.mismatch.between.ZnO.and.sapphire.(18%).[13,18]..This.drawback.results. in. that.controllability.of. layer. thickness. is.not.sufficient. to.realize. the.proper.quantum.confinement.effect,.and.PL.efficiency.is.not.high.enough.to.enable.obser-vation.of.free.exciton.emission.at.RT.[18]..Much.effort.has.been.made.to.improve.the.inter-face.quality.between.ZnO.and.sapphire..For.example,.ZnO/MgZnO.SQWs.were.grown.by.utilizing.GaN.templates.[14].or.employing.ZnO.[15].or.MgZnO.[12].as.a.buffer.layer,.in.which.a.strong.UV.emission.and.obvious.carrier.confinement.were.observed.at.RT.[14,15]..We.have.also.reported.on.the.optical.properties.of.ZnO/.Mg0.1Zn0.9O.SQWs.with.ZnO.buf-fer.layer.fabricated.on.c-plane.Al2O3.substrates.by.P-MBE..Efficient.excitonic.emission.from.these.SQWs.can.be.observed.up.to.RT.[16]..Recently,.Brandt.et.al..[17].studied.the.prop-erties.of.ZnO/MgZnO.single.heterostructures.and.MgZnO/ZnO.quantum.wells.grown.by.pulsed-laser.deposition.on.sapphire.and.ZnO.(0001)..The.superiority.of.the.structural.and.morphological.properties.in.the.homoepitaxial.MgZnO/ZnO.quantum.wells.has.been.clearly.observed..To.further.enhance.the.efficiency.of.photoemission.at.RT,.some.research-ers. successfully. fabricated. ZnO/MgZnO. MQWs. on. lattice-matched. ScAlMgO4. (SCAM).

225Optical Properties of MgZnO/ZnO Heterostructures Grown on Sapphire Substrates

substrates. [19,21]..As.a.result,.a.bright.photoluminescence.(PL).of. free.excitons.could.be.observed.at.RT.due.to.the.improvement.of.the.interface..However,.the.SCAM.substrates.are.scarce.and.very.expensive.compared.to.abundantly.available.sapphire.substrates,.and.therefore.one.of.the.challenges.is.to.achieve.high-quality.ZnO.MQWs.with.RT.PL.on.sap-phire.substrates,.making.them.versatile.for.practical.applications.

On. the.other.hand,.a.number.of. researches.are. concerning. the.excitonic.properties.of.ZnO/MgZnO.quantum.well.structures..In.addition,.some.studies.related.to.the.subband.energy.levels.in.low.dimension.structures.were.conducted..Ohtomo.et.al..[18].reported.exci-tonic. luminescence.accompanied.by.the.quantum.confinement.effect.at.4.2.K..They.used.the.ratio.of.ΔEC/ΔEV.as.a.fitting.parameter.and.obtained.the.best.fit.when.ΔEC/ΔEV.=.9.that.is,.ΔEC.=.414.meV.and.ΔEV.=.46.meV..The.quantum.subband.level.of.n.=.1.was.estimated.for.various.well.layer.thicknesses..Subsequently,.Makino.et.al..[19].observed.RT.excitonic.emis-sion. in.ZnO/MgZnO.MQWs.on.SCAM.substrates..They.calculated.the. transition.energy.corresponding.to.the.lowest.subband.levels.of.an.electron.and.a.hole.confined.in.a.QW,.but.the.results.of.their.calculations.were.about.40–80.meV.higher.than.the.experimental.data..Giuliano.Coli.et.al..[20].studied.the.excitonic.transition.energies.in.ZnO/ZnMgO.MQWs.by.taking.into.account.the.effects.of.the.exciton–phonon.interaction..They.suggested.a.value.for.the.heavy-hole.band.mass.of.0.78m0.and.a.conduction-valence.band.ratio.in.the.range.60/40–70/30..The.result.shows.that.the.calculated.excitonic.transition.energies.agree.very.well.with.the.published.experimental.data..Sun.et.al..[21].systematically.investigated.the.well.width.dependence.of.exciton.binding.energies.in.ZnO/ZnMgO.multiquantum.wells..The.remark-able.reduction.in.coupling.strength.between.excitons.and.longitudinal.optical.phonons.is.closely.correlated.with.the.enhancement.of.the.exciton.binding.energy,.indicating.that.the.stability.of.excitons.is.greatly.increased.by.the.enhancement.of.exciton.binding.energy.in.quantum.wells..Makino.et.al..[22].reported.the.thermal.quenching.mechanisms.of.local-ized.excitons.and.free.excitons. in.ZnO/MgZnO.quantum.wells..Misra.et.al.. [23].studied..temperature-dependent.PL.from.ZnO/MgZnO.MQWs.of.different.well. layer.thicknesses.in.the.range.~1–4.nm.grown.on.(0001).sapphire.with.a.ZnO.buffer.layer..They.observed.for.the.first. time.excitonic.emissions. from.different.wells.at.RT,. showing. the.expected.size-dependent.quantum.confinement.effects..Zippel.et.al..[39].investigated.the.electronic.cou-pling.in.ZnO/MgZnO.double-quantum-well.structures..The.shift.of.the.transition.energy.of.the.free.exciton.at.RT.to.lower.energies.due.to.electron.coupling,.with.decreasing.barrier.width,.is.in.good.agreement.with.effective.mass.theory..They.estimated.the.exciton.diffu-sion.length.in.the.MgZnO.barrier,.which.is.about.135.nm.at.10.K.and.below.75.nm.at.RT.

Motivated. by. these. advantages. the. stimulated. emission. properties. in. ZnO/MgZnO.MQWs. are. extensively. reported.. Since. 1997. stimulated. emission. and. optically. pumped.laser. action. from. ZnO. thin. film. were. observed. at. RT. [1,2].. A. notable. progress. in. las-ing. or. stimulated. emission. has. been. achieved. from. a. variety. of. low. dimensional. ZnO.structures,.such.as.microcavity.[24],.nanowires.[25–27],.nanorods.[28,29],.nanoribbons.[30],.and.quantum.well. structures. [32,33],. for. the. realization.of. low-threshold. lasers.. In.par-ticular,.because.the.quantum-well.structure.is.an.important.approach.to.further.improve.the. efficiency. in. semiconductor. lasing. [31],. the. study. of. stimulated. emission. in. ZnO-based.quantum.well.structures.has.attracted.considerable.attention.[32–35]..Ohtomo.and.co-workers. [32].have.also. reported.RT.stimulated.emission. in.ZnO/Mg0.12Zn0.88O.MQW.heterostructures.on.SCAM.substrates,.and.measured.thresholds.below.22.kW/cm2.for.well.widths.in.the.range.7–47.Å,.with.a.minimum.of.11.kW/cm2.for.the.47.Å.thick.QW..In.addi-tion,.stimulated.emission.induced.by.exciton–exciton.scattering.in.their.MQWs.has.been.demonstrated. [33]..Although.Ohtomo.et.al..have. realized. the. lasing.with. lower. thresh-old. power. density. in. ZnO/MgZnO. MQW,. it. was. obtained. by. using. a. scarce. substrate.


ScAlMgO4..Therefore,.it.is.expected.that,.in.quantum.wells,.the.observation.of.this.phe-nomenon.can.be.realized.by.using.a.common.substrate.Al2O3..In.fact,.blue–violet.electro-luminescence,.in.our.progress.to.ZnO-based.LEDs.and.LDs,.has.been.observed.from.the.homojunction.LEDs.fabricated.on.Al2O3.substrate.[36,37]..In.addition,.the.low-temperature.(5.K).stimulated.emission.caused.by.exciton–exciton.scattering.in.ZnO/Mg0.1Zn0.9O.single-quantum.well.has.been.observed.[34]..Significantly,.we.reported.RT.stimulated.emission.in.the.ZnO/Mg0.2Zn0.8O.MQWs.grown.on.Al2O3,.in.which.the.exciton.binding.energy.was.determined.to.be.122.meV,.but.the.threshold.is.as.high.as.about.200.kW/cm2.at.RT.[35]..In.order.to.further.decrease.the.threshold.of. the.stimulated.emission,.we.fabricated.ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells.(ADQWs).[38]..For.ADQWs,.which.consist.of.wells.of.two.different.widths,.a.wide.well.(WW).and.a.narrow.well.(NW),.coupled.by.a.thin.barrier,.the.excitonic.processes.and.luminescence.also.attract.great.attention.related.to. the.coupling.between. the. two.wells. [39,40]..The.exciton.recombination. in.ADQWs. is.modulated.by. the.exciton. tunneling.and. influences. the.processes.of. the.exciton. forma-tion,. relaxation,. and. recombination. [40,41].. The. research. on.ADQWs. has.been.proceed-ing.for.a.long.time.[39–44]..However,.only.very.few.studies.are.conducted.on.ZnO-based.ADQW. [38,39],. in. particular,. there. are. no. reports. about. the. stimulated. emission. of. the.ZnO/ZnMgO.ADQWs..In.the.cited.paper.[39],.authors.reported.the.optical.properties.of.MgZnO/ZnO.ADQWs.grown.by.pulsed-laser.deposition..The.result.shows.that.two.lumi-nescence.peaks.can.be.identified.as.excitonic.transition.from.the.quantum.wells.WW.and.NW,.respectively..An.additional.recombination.peak.is.attributed.to.a.spatially. indirect.excitonic.transition.involving.the.electron.subband.of.the.narrow.well.and.the.heavy.hole.subband.of.the.wide.well..We.observed.the.carrier.increasing.effect.in.WW.due.to.exciton.tunneling.from.NW.to.WW.in.ZnO/Zn0.85Mg0.15O.ADQWs..The.stimulated.emission.with.low.threshold.(64.kW/cm2).at.66.K.was.reported.for.the.first.time.[38].

9.2 Experimental

9.2.1 Materials Growth

The.growth.was.carried.out.using.a.V80H.molecular-beam.epitaxy.(MBE).system.equipped.with.Knudsen-cells.for.a.Zn.solid.source.(99.9999%).and.an.Mg.solid.source.(99.999%).and.radio. frequency. (rf)-plasma. source. for.oxygen.. The.background.vacuum.of. the.growth.chamber.was.about.1.×.10−10.mbar.with.a.liquid.nitrogen.supply..The.oxygen.flow.rate.is.controlled.by.a.leak.valve..The.rf.power.of.oxygen.plasma.was.300.W..To.obtain.MgZnO.alloy.films.with.different.Mg.contents,.the.growth.condition.was.changed.by.adjusting.the.oxygen.flow.rate.and.beam.fluxes.of.Mg..The.c-plane.sapphire.was.used.as.substrate..In.order.to.obtain.a.clean.fresh.surface,.the.substrates.were.chemically.etched.in.a.hot.solution.of.H2SO4:H3PO4.=.3:1.at.160°C.for.15.min..Before.growth,.the.substrates.were.thermally.pre-treated.at.800°C.for.30.min.and.exposed.to.an.oxygen.plasma.at.650°C.for.another.30.min,.which.was.expected.to.remove.surface.contaminant.and.obtain.oxygen.terminated.Al2O3.(0001).surface..The.films.were.grown.at.550°C–650°C.in.the.growth.pressure.of.5.0.×.10−5.mbar..The.growth.process.was.monitored.by.in situ.RHEED.

First,.MgZnO.thin.films.with.the.wurtzite.structure.were.grown.on.c-plane.Al2O3.by.PMBE..During.growth,.the.oxygen.flow.rate.was.kept.at.0.2–2.0.sccm,.and.beam.fluxes.of.Zn.and.Mg.were.3.0.×.10−5.and.1.5–3.0.×.10−7.mbar,.respectively..The.films.were.grown.


at.550°C..It.denotes.that.the.composition.of.the.MgZnO.thin.films.may.be.controlled.by.adjusting.the.ratio.of.Zn/Mg.or.the.oxygen.flow.rate..The.MgZnO.thin.films.with.dif-ferent.Mg.contents,.used.in.the.study.of.Section.1.3.1,.were.grown.by.adjusting.the.ratio.of.Zn/Mg..Actually,.our.recent.work.shows.that.it.is.more.easy.to.control.the.oxygen.flow.rate.[10].

For.the.SQW.heterostructure,.a.high-quality.ZnO.buffer.layer.about.50.nm.is.grown.on.sapphire.substrate.at.800°C..Then.a.ZnO.layer.was.sandwiched.between.a.60–500.nm.thick.MgZnO.barrier.layer.and.a.30–60.nm.thick.MgZnO.capping.layer..Here,.the.thickness.of.ZnO.layer.varied. from.1. to.20.nm..The.films.were.grown.at.550°C–650°C..The.detailed.structure.is.given.in.the.Ssections.9.3.2.and.9.3.3.

The.ZnO.MQWs.used.in.the.study.were.grown.on.c-plane.Al2O3.by.P-MBE,.following.the.deposition.of.a.100.nm.thick.MgZnO.buffer.layer..The.structures.consist.of.10.MQWs.with. 1.5.nm. thick. ZnO. wells. and. 10.nm. thick. MgZnO. barriers.. The. film. was. grown. at.550°C.before.exposure.to.oxygen.plasma.for.30.min..The.samples.of.ZnO/MgZnO.ADQWs.were.grown.on.an.m-plane.Al2O3.substrate.by.P-MBE.at.650°C..The.structure.consists.of.a.50.nm.ZnMgO.buffer.layer.followed.by.five.periods.of.ZnO/MgZnO.ADQWs.and.then.a.50.nm.MgZnO.cap.layer..Each.period.of.ZnO/MgZnO.ADQWs.includes.one.narrow.ZnO.well,.one.thin.MgZnO.barrier.and.one.wide.ZnO.well,.the.thin.barrier.and.the.wide.well,.respectively.. Each. period. of. the. ADQW. was. separated. by. a. 40.nm. MgZnO. barrier.. For.details,.see.Section.9.3.4.

9.2.2 Characterization Techniques

The. quality. of. the. grown. samples. was. characterized. by. the. Rigaku. Company. O/max-RA.x-ray.system,.D/Max.2400.double.crystal.diffractometer.at.40.kV.and.98.mA,.and.VG.ESCALABMK.II.XPS.system..The.surface.morphology.was.analyzed.by.the.measurement.of.atomic.force.microscope.(AFM)..The.optical.properties.of.all.samples.were.studied.by.absorption.and.photoluminescence. (PL).spectra..UV.absorption.spectra.were.measured.using.the.SHIMADZU.UV-3101.PC.spectrophotometer..A.JY63.Micro.Raman.spectrometer.was.employed.for.PL.measurement..The.luminescence.was.detected.by.a.charged-coupled.device.(CCD).detector..The.325.nm.line.of.a.He-Cd.laser.was.used.as.the.excitation.source..The.liquid-nitrogen.cooling.system.was.used.in.conjunction.with.the.sample.stage.to.cool.a.sample.down.to.80.K..The.temperature.was.controlled.by.the.TMS94.from.80.K.to.RT.

9.3 Results and Discussion

9.3.1 Optical Properties of MgZnO Thin Films with the Wurtzite Structure

In. this. section,. MgxZn1−xO. thin. films. were. grown. on. (001). c-plane. sapphire. substrates.by.P-MBE.(the.growth.conditions.see.Table.9.1)..The.MgxZn1−xO.alloy.thin.films.keep.the.wurtzite. crystal. structure. with. x. values. changing. from. 0. to. 0.2.. Optical. properties. of.MgxZn1−xO.alloy.thin.films.will.be.discussed.

The.XRD.spectra.of.the.MgxZn1−xO.(0.≤.x.≤.0.2).alloy.thin.films.are.shown.in.Figure.9.1a..Three.peaks.can.be.observed.in.the.XRD.spectra..The.peak.at.41.68°.is.the.(006).diffraction.peak.of.Al2O3,.and.the.other.two.peaks.are.attributed.to.the.(002).and.(004).diffraction.peaks.of. MgxZn1−xO,. respectively.. No. signal. of. the. cubic. phase. is. observed.. This. indicates. that.MgxZn1−xO.alloy.films.with.single.wurtzite.structure.were.obtained.in.the.Mg.content.range.


of.x.=.0–0.2..The.inset.gives.the.enlarged.line.shapes.of.MgxZn1−xO.(002).diffraction.peaks..It.should.be.noticed.that.the.(002).peak.shifts.from.34.44°.to.34.67°.with.increasing.Mg.content.from.0. to.0.20..According. to.Bragg.diffraction.equation,. the.calculated. lattice.constant.of.c-axis.orientation.decreases.from.5.205.to.5.185.Å..This.indicates.that.Zn2+.ions.in.the.ZnO.lat-tice.were.replaced.partly.by.Mg2+.ions,.resulting.in.the.decrease.of.the.lattice.constant.with.Mg.content.increase..Figure.9.1b.shows.x-ray.rocking.curves.of.the.(002).diffraction.peaks.for.MgxZn1−xO.alloy.films..The.appearance.of.double.crystal.diffraction.peak.confirms.that.the.grown.samples.are.single-crystal.structure..With.increasing.x.value,.the.full.width.at.half.maximum.(FWHM).of.the.rocking.curve.is.broadened.from.0.249°.to.0.708°..This.means.that.the.crystal.quality.of.the.MgxZn1−xO.films.worsens.due.to.the.increase.of.the.lattice.disorder.in.high.Mg.composition..The.detailed.result.is.listed.in.Table.9.1.

TABLE 9.1

Deposition.Conditions.and.XRD.Results.of.MgZnO.Thin.Films.with.Different.Mg.Contents

NoMg Content

(%)Beam Fluxes of Mg (Pa)

c Axis Lattice Constant (Å)

FWHM of (0002) Peak(°)

1 0 0 5.205 0.2492 0.06 5.0.×.10−5 5.196 0.3093 0.12 8.9.×.10−5 5.190 0.3864 0.20 1.2.×.10−5 5.185 0.708

Source:. Reproduced. from.Wu,.C..X..et.al.,.Chin. J. Luminesc.,. 25,.277,.2004..With.permission.

(a) (b)

x=0

x=0.06

x=0.20

x=0

x=0.06

x=0.12

x=0.20

34 350.249

0.309

0.386

0.708

FWHM

(004)2θ

30 40 50 60 70 –2 –1 0 1 22θ ∆θ (deg)

x=0.12

Mg x

Zn1–

xO (0

02) MgxZn1–xO (002)

AL 2

O3(

006)

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.)

FIGURE 9.1The.x-ray.diffraction.spectra.(a).and.x-ray.rocking.curve.(b).of.the.MgxZn1−xO.alloy.thin.films.with.x.value:.x.=.0,.0.06,.0.12,.0.20..The.inset.shows.enlarged.lineshape.of.MgxZn1−xO.(002).diffraction.peak..(Reproduced.from.Wu,.C..X..et.al.,.Chin. J. Luminesc.,.25,.277,.2004..With.permission.)


Figure.9.2.shows.the.photoluminescence.PL.spectra.of.the.MgxZn1−xO.alloy.thin.films.at.RT..For.all.samples,.only.one.intense.UV.emission.which.shifts.to.high-energy.side.with.increasing.x.value.is.observed,.and.there.is.no.deep.level.emission.related.to.impurities.or.defects..This.indicates.that.the.MgxZn1−xO.alloy.thin.films.have.high.quality..The.inset.of.Figure.9.2.gives.the.PL.peak.position.and.FWHM.as.functions.of.x.values..With.increasing.Mg.composition.from.x.=.0–0.2,.the.UV.peak.energy.increases.from.3.29.to.3.47.eV,.and.the.FWHM.of.emission.peak.changes.from.62.to.89.meV..The.broadening.of.the.PL.band.with.increasing.Mg.content.is.consistent.with.the.XRD.result,.revealing.the.effect.of.the.compo-sition.disorder.on.the.crystalline.quality.

Figure.9.3.shows.the.transmittance.spectra.of.the.MgxZn1−xO.alloy.thin.films.at.RT..All.the.samples.are.highly. transparent. in. the.visible. region. from.400. to.800.nm.and.have.a.sharp.absorption.edge.in.the.UV.region..The.absorption.edge.shifts.to.high-energy.side.with.increasing.Mg.content.in.the.films,.which.agrees.well.with.the.change.of.the.PL.peak.position..For.evaluating.the.band.gap.(Eg),.α2.∝.(hν.−.Eg).relationship.was.employed.by.fitting.the.data,.where.α.is.the.absorption.coefficient.and.hv.is.the.photon.energy.[45]..The.calculated.band.gap.as.a.function.of.x.value.is.shown.in.the.inset.of.Figure.9.3..As.can.be.seen,.the.band.gap.Eg.increases.from.3.30.to.3.68.eV.with.changing.x.value.from.x.=.0–0.2.

In.order.to.study.the.origin.of.UV.emission.of.the.MgxZn1−xO.thin.films,.PL.spectra.were.measured.at.different.temperatures.for.a.typical.sample.with.Mg.content.of.x.=.0.12,.as.shown.in.Figure.9.4..A.strong.UV.luminescence.peak.without.deep.center.emission.can.be.observed.in.the.whole.temperature.range..At.temperatures.lower.than.162.K,.there.is.no.noticeable.change. for. the.PL.peak.position..With. increasing. temperature. from.162.K. to.RT,. the.UV.band.shifts.to.low.energy.side.obviously.with.the.increasing.temperature,.and.a.shoulder.appears.at.high-energy.side.of.the.UV.peak..The.inset.of.Figure.9.5.shows.the.temperature.dependence.of.the.PL.band.FWHM.for.the.Mg0.12Zn0.88O.film..At.low.temperatures,.the.PL.band.broadened.significantly.with.the.increasing.temperature..At.162.K,.the.FWHM.of.the.PL.band.reaches.the.maximum.value.of.67.5.meV..As.the.temperature.increases.further,.the.

Photon energy (eV)

Inte

nsity

(a. u

.)

0.00

60

80

FWH

M (m

eV)

Peak

pos

ition

(eV

)

100

0.05 0.10X value

0.15 0.203.20

3.25

3.30

3.35

3.403.453.50

2.0 2.4 2.8

X=0.20

X=0.12

X=0.06

X=0

3.2 3.6

FIGURE 9.2The.PL.spectra.of. the.MgxZn1−xO.alloy. thin.films.at.RT..The. inset. shows. the.PL.peak.energy.positions.and.FWHMs.of.MgxZn1−xO.films.as.a.function.of.x.values..(Reproduced.from.Wu,.C..X..et.al.,.Chin. J. Luminesc.,.25,.277,.2004..With.permission.)


FWHM.of.the.PL.band.shows.an.abnormal.decrease..This.indicates.that.the.PL.band.is.not.from.the.same.origin..In.order.to.confirm.further.the.UV.emission.mechanism,.the.UV.band.is.fitted.by.using.Gaussian.line.shape..The.inset.of.Figure.9.4.shows.the.fitted.result.of.the.spectra.at.102,.162,.246,.and.294.K,.respectively..As.can.be.seen,.the.UV.emission.band.with.a.shoulder.can.be.fitted.into.two.Gaussian.peaks..At.low.temperatures,.the.emission.peak.at.low.energy.side.dominates.in.the.PL.spectra..With.the.increasing.temperature,.the.contribu-tion.of.the.high-energy.peak.becomes.increasingly.important.

Inte

nsity

(a. u

.)

1.8 2.0366 K294 K246 K210 K198 K186 K

174 K138 K/1.5114 K/2

Photon energy (eV)

Photon energy (eV)

Inte

nsity

(a. u

.)

91 K/23.4 3.6

294 K

246 K

162 K

102 K

3.461 eV

3.500 eV

3.485 eV3.445 eV

3.445 eV3.426 eV

3.429 eV

2.2 2.4 2.6 3.4 3.6

FIGURE 9.4PL.spectra.measured.at.temperature.ranging.from.81.to.366.K.for.Mg0.12Zn0.88O.thin.film..The.inset.exhibits.the.fitting.result.of.Gaussian.lineshape.of.the.PL.peak.at.102,.162,.246,.and.294.K..(Reproduced.from.Wu,.C..X..et.al.,.Chin. Phys. Lett.,.22,.2655,.2005..With.permission.)

Photon energy (eV)2.0

0

20

40

60Tr

ansm

ittan

ce (%

)

80

100

2.5 3.0 3.5 4.0

x=0

x=0.06x=0.12

x=0.20

0.003.3

3.4

3.5

Band

gap

(eV)

3.6

3.7

0.05 0.10X value

0.15 0.20

FIGURE 9.3The. absorbance. spectra. of. the. MgxZn1−xO. alloy. thin. films. at. RT.. The. inset. shows. the. band. gap. energy. of.MgxZn1−xO.films.as.a.function.of.x.values..(Reproduced.from.Wu,.C..X..et.al.,.Chin. J. Luminesc.,.25,.277,.2004..With.permission.)


The. temperature. dependence. of. the. PL. integrated. intensity. (○). for. the. Mg0.12Zn0.88O.alloy.film.is.shown.in.Figure.9.5..Although.the.intensity.of.UV.emission.decreases.monoto-nously.with.the.increasing.temperature.from.81.to.366.K,.two.thermal.quenching.processes.were.found.in.different.temperature.ranges..Therefore,.the.temperature.dependence.of.the.intensity.is.studied.by.using.two-step.dissociation.model.described.by

.

I TI

AEk T

AEk TB B

( )exp exp

,=+ −

+ −

0

11

221

. (9.1)

whereI0,.A1.and.A2.are.constantsKB.is.Boltzmann.constantE1.and.E2.are.the.activation.energies

Bimberg. et. al.[46]. have. reported. the. dissociation. processes. of. the. excitons. bound. to.acceptors.or.donors,.where.two-step.dissociation.processes.were.described.well.by.using.Equation.9.1..Makino.et.al.[22].also.studied.the.thermal.quenching.mechanisms.of.local-ized.excitons.and.free.excitons.in.ZnO/MgZnO.quantum.wells.by.using.this.method..In. this. study,. curve. 1. (the. solid. lines). in. Figure. 9.5. expresses. the. results. fitted. with.Equation. 9.1.. The. two. thermal. activation. energies. of. E1.=.28.1.meV. and. E2.=.57.6.meV.are.obtained..With.the.increasing.temperature,.there.are.two.different.dissociation.pro-cesses. with. two. activation. energies:. (a). dissociation. resulting. in. one. free. exciton,. (b).dissociation.resulting.in.one.free.electron.and.one.free.hole.from.the.free.exciton..For.the.two.dissociation.processes,.the.activation.energies.of.E1.=.28.1.meV.and.E2.=.57.6.meV.are.

1000/T (K–1)2 4

PL in

tegr

ate i

nten

sity (

a. u.

)

6 8 10

100

55

162 K3

1

2

Experimental data

IoI(T)=E1

kBT1+A1exp(– — )

E2

kBT+ A2exp(– —)

Theoretical fit

60

65

70

200 300Temperature (K)

FWH

M (m

eV)

400

12

FIGURE 9.5The.PL.peak.integrated.intensities.of.Mg0.12Zn0.88O.as.a.function.of.temperature.ranging.from.81.to.366.K..The.theoretical.simulation.(solid.line).to.the.experimental.data.points.(solid.circle).from.Equation.9.1..The.inset.shows.the.FWHM.of.PL.peak.as.a.function.of.temperature.ranging.from.91.to.342.K..(Reproduced.from.Wu,.C..X..et.al.,.Chin. Phys. Lett.,.22,.2655,.2005..With.permission.)


attributed.to.the.activation.energies.corresponding.to.bound.exciton.to.free.exciton.(EB),.and. the. binding. energy. of. the. free. exciton. (Ex),. respectively.. Therefore,. the. observed.UV.PL.spectra.are.a.superposition.of.the.free.exciton.emission.band.on.the.high-energy.side.and.the.bound.exciton.emission.band.on.the.low-energy.side..In.the.temperature.range.81–162.K,.the.UV.emission.spectra.are.dominated.by.the.recombination.of.neutral.donor-bound. exciton. (D0,. X),. and. the. thermal. quenching. is. dominated. by. the. disso-ciation.from.the.neutral.(D0,.X).to.free.exciton..In.this.process,.the.contribution.of.free.exciton. emission. gradually. increases. due. to. the. increase. of. free. exciton. density,. and.the.UV.band.exhibits.a.significant.broadening,.as.shown.in.Figure.9.4..This.broaden-ing. includes.two.parts:. (1). the.broadening.from.the. interaction.between.excitons.and.phonons,.and.(2).increase.of.the.contribution.of.free.exciton.emission..When.the.tem-perature.increases.to.162.K,.emission.intensity.of.free.exciton.is.comparable.to.that.of.bound.exciton,.and.the.FWHM.of.PL.band.reaches.the.maximum.value.of.67.5.meV..At.temperatures.higher.than.162.K,.the.bound.exciton.dissociates.more.rapidly,.and.the.PL.spectra.are.dominated.by. the. recombination.of. the. free.exciton..Because. the.effect.of.this.process.on.the.FWHM.is.larger.than.that.of.increasing.temperature,.the.UV.band.exhibits.an. interesting.narrowing..When. the. temperature. is.up. to.RT,. the.PL.band. is.dominated.by.the.free.exciton.recombination,.and.the.dissociation.is.mainly.from.the.thermal. activation. from. free. exciton. to. free. electron. and. free. hole.. From. the. results.mentioned.earlier,.we.have.clearly.suggested.that.the.decrease.of.the.emission.intensity.in.the.temperature.range.81–162.K.is.due.to.the.dissociation.of.bound.excitons.with.an.activation.energy.of.28.meV,.and.the.quick.quenching.of.UV.emission.above.162.K.is.due.to.the.dissociation.of.free.excitons.with.a.thermal.activation.energy.of.~58.meV..To.fur-ther.confirm.the.two.steps.dissociation.processes.of.exciton.complexes,.the.temperature.dependences.of.the.UV.PL.spectra.in.lower.and.higher.temperature.regions.are.fitted.by.Equation.(9.2),.respectively,

.

I TI

AEk TB

( )exp

=+ −

0

1. (9.2)

As.shown.in.Figure.9.5,.curves.2.and.3.are.the.fitted.results,.where.the.activation.energy.of. E1

’ .=.29.7.meV.in.lower.temperature.range.and.the.activation.energy.of.E2’ .=.58.4.meV.in.

higher. temperature. range. are. obtained,. respectively.. The. values. of. E1’ . and. E2

’ . are. close.to. the.fitting.results.of.E1.=.28.1.meV.and.E2.=.57.6.meV.from.curve.1,. indicating. the.exis-tence.of.two-step-dissociation.processes..In.addition,.the.free.exciton.binding.energy.of.Ex.=.57.6.meV.is.consistent.with.the.result.of.Schmidt.et.al.. in.Ref..[47],. in.which.the.free.exciton.binding.energy.of.MgZnO.alloy.films.was.considered.to.be.slightly.smaller.than.the.value.of.ZnO.bulk.materials.(∼60.meV).

For. the. sample. with. Mg. content. of. x.=.0.2,. it. is. found. that. the. photon. energy. of. PL.spectra.exhibits.a.great.redshift.contrasting.with.the.band.gap.energy.evaluated.by.the.absorption.spectra..We.suggested.that.the.UV.emission.at.RT.of.the.sample.mainly.origi-nates.from.the.excitons.bound.to. localized.states.by.the. influence.of.Mg.composition..The. excitons. bound. to. localized. states. in. alloy. semiconductors. had. been. reported. by.many.groups.before.[7,48],.in.which.the.luminescence.peaks.showed.a.large.stoke.shift.to.the.lower.energy.side.of.the.absorption.edge..In.high.Mg.content,.the.emission.pro-cess.with.the.localized.exciton.recombination.can.be.enhanced,.resulting.in.an.obvious.redshift.in.PL.spectrum.at.RT.


9.3.2 Structural Characterization and Optical Properties of ZnO/MgZnO Heterostructure

The.Mg0.12Zn0.88O/ZnO.heterostructures.were.grown.on.(001).c-plane.sapphire.substrate.by.P-MBE..After.the.epitaxy.of.a.100.nm.thick.Mg0.1Zn0.9O.barrier.layer,.a.variety.of.ZnO.well. layers,.with. the. thickness.varying. from.1. to.20.nm,.were.sandwiched.between.the.buffer.layer.and.a.60.nm.Mg0.12Zn0.88O.capping.layer..The.quality.of.the.grown.samples.was.characterized. by. measuring. XRD,. reflection. high-energy. electron. diffraction. (RHEED).and.optical.properties.

Figure.9.6.shows.the.XRD.spectra.of.the.grown.ZnO.thin.film,.Mg0.12Zn0.88O.alloy.film,.and.Mg0.12Zn0.88O/ZnO.heterostructure..In.Figure.9.6a,.three.peaks.are.observed.in.XRD.pattern.of.the.ZnO.thin.film..The.peak.at.41.681°.is.the.(006).diffraction.peak.of.Al2O3,.and.the.other.two.peaks.located.at.34.441°.and.72.721°.are.attributed.to.the.(002).and.(004).dif-fraction.peaks.of.ZnO,.respectively..For.the.Mg0.12Zn0.88Oalloy.film.(Figure.9.6b),.XRD.spec-trum.shows.the.same.three.peaks.without.another.cubic.phase,.indicating.the.formation.of.Mg0.12Zn0.88O.alloy.film.with.wurtzite.structure..Because.Zn2+. ions.in.the.ZnO.lattice.were.replaced.partly.by.Mg2+.ions.with.smaller.radium,.we.noted.that.the.(002).peak.of.Mg0.12Zn0.88O.shifts.to.34.571°.due.to.the.decrease.of.the.lattice.constant.along.the.c-plane..Figure.9.6c.gives.the.XRD.result.of.Mg0.12Zn0.88O/ZnO.heterostructure.with.the.thickness.of.20.nm.ZnO.layer..Compared. to.Mg0.12Zn0.88O.alloy.film,. the. (002)-oriented.diffraction.peak.shows.a.broadening.from.0.161°.to.0.211°..This.broadening.is.considered.to.be.due.to.the.superposition.of.ZnO.(002).and.Mg0.12Zn0.88O.(002).diffraction.peaks,.identifying.the.

2θ (deg)30 40

(a)

(b)

(c)

34.55

34.56

Inte

nsity

(a. u

.)

34.46

Peak

(002)

(004)ZnO

FWHM

0.20

0.15

0.12

50

Mg0.12Zn0.88O/ZnO

Mg0.12Zn0.88O

60 70

AI2O3(006)

FIGURE 9.6The. x-ray. diffraction. spectra. of. the. grown. ZnO. thin. film. (a),. the. Mg0.12Zn0.88O. alloy. thin. film. (b). and. the.Mg0.12Zn0.88O/ZnO. heterostructure. (c).. (Reproduced. from. J. Cryst. Growth,. 278,. Lu,. Y.M.,. Wu,. C.X.,. Wei,. Z.P.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.Liu,.Y.C.,.Shen,.D.Z.,.and.Fan,.X.W.,.Characterization.of.ZnO/Mg0.12Zn0.88O.heterostructure. grown. by. plasma-assisted. molecular. beam. epitaxy,. 299,. 2005.. Copyright. 2005;. J. Luminesc.,.119–120,.Wei,.Z.P.,.Lu,.Y.M.,.Shen,.D.Z.,.Wu,.C.X.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.and.Fan,.X.W.,.Effect.of.interface.on.luminescence.properties.in.ZnO/MgZnO.heterostructures,.551,.2006..Copyright.2006,.with.permis-sion.from.Elsevier.)


formation.of.the.MgZnO/ZnO.heterostructure..Surface.morphology.and.crystal.quality.of.the.samples.have.been.investigated.by.RHEED.

Figure.9.7.shows.RHEED.patterns.along. [1210]. the.direction.of. the.substrate.exposed.to.O-plasma.at.650°C.for.30.min.and.the.grown.ZnO.and.Mg0.12Zn0.88O.thin.films..After.plasma. treatment,. the. substrate. gives. a. streaky. pattern. (Figure. 9.7a),. which. indicates. a.well-ordered.and.flat.Al2O3.surface..For.the.grown.samples,.a.change.of.the.RHEED.pat-terns.from.sharp.streaky.to.discontinuous.streaky.is.observed,.as.shown.in.Figure.9.7b.and. c.. A. sharp. streaky. pattern. shows. the. formation. of. smooth. and. flat. surface. for. the.grown.ZnO.sample,.whereas.discontinuous.streaky.pattern.indicates.that.the.surface.of.the. Mg0.12Zn0.88O. alloy. becomes. rough.. This. morphology. evolution. reveals. the. effect. of.lattice.disorder.due.to.the.introduction.of.Mg.atoms.

Figure.9.8. shows. the.normalized.PL. spectra.and.absorption. spectra.of.Mg0.12Zn0.88O/ZnO.heterostructures.with.different.well.widths.at.300.K..For.the.absorption.spectrum.of.sample.with.20.nm.well.width.(A20),.a.high-energy.absorption.edge.and.a.low-energy.step.(arrow.in.the.figure).are.observed,.which.are.attributed.to.the.absorption.from.MgZnO.and.ZnO.layer,.respectively..With.decreasing.well.width,.the.absorption.from.ZnO.layer.weakens..When.the.ZnO.layer.thickness. is.4.nm.(sample.A4),. the.absorption.is.a.gentle.curve..For.sample.A2.with.2.nm.well.width,.the.absorption.become.sharper..For.the.RT.PL.spectra,.two.PL.bands.corresponding.to.the.two.absorptions.are.observed,.respectively..The. stronger. emission. at. about. 3.3.eV. (a). and. a. weaker. emission. at. about. 3.5.eV. (b). are.attributed.to.the.recombination.from.the.MgZnO.and.ZnO.layer,.respectively..The.absorp-tion.spectra.and.PL.spectra.confirm.the.formation.of.ZnO/MgZnO.heterostructure..The.PL.peak. (a).at. low-energy.side,.which. is. from. the. radiative. recombination.of.ZnO.well.layer,.has.a.large.blueshift.(∼40.meV).with.decreasing.well.thickness.from.20.to.2.nm..This.phenomenon.is.attributed.to.the.quantum.confinement.effect..The.observed.broadening.of.the.linewidth.for.all.samples.should.originate.from.the.existence.of.interface-localized.exciton.emission..With.decreasing.well.width,.the.influence.of.the.interface.becomes.weak,.resulting.in.the.disappearance.of.the.localized.exciton.emission.

(a) (b)

(c)

FIGURE 9.7The.RHEED.patterns.of.Al2O3.substrate. treated.by.Oplasma. (a),. the.ZnO.thin.film.(b).and. the.Mg0.12Zn0.88O.alloy.thin.film.(c)..(Reproduced.from.J. Cryst. Growth,.278,.Lu,.Y.M.,.Wu,.C.X.,.Wei,.Z.P.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.Liu,.Y.C.,.Shen,.D.Z.,.and.Fan,.X.W.,.Characterization.of.ZnO/Mg0.12Zn0.88O.heterostructure.grown.by.plasma-assisted.molecular.beam.epitaxy,.299,.2005..Copyright.2005;.J. Luminesc.,.119–120,.Wei,.Z.P.,.Lu,.Y.M.,.Shen,.D.Z.,.Wu,.C.X.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.and.Fan,.X.W.,.Effect.of. interface.on.luminescence.properties.in.ZnO/MgZnO.heterostructures,.551,.2006..Copyright.2006,.with.permission.from.Elsevier.)


In.order.to.further.study.the.influence.of.the.interface-localized.exciton,.we.measured.the.temperature-depended.PL.spectra.of.A20,.as.shown.in.Figure.9.9..From.the.spectra,.we.find.that.the.intensity.of.peak.(b).is.quenching.rapidly.with.the.increase.in.temperature.while.the.intensity.of.the.peak.(a).is.quenching.slower..The.peak.(a).position.has.a.blue-shift.at.the.same.time..It.is.considered.that.the.localized.exciton.of.MgZnO.obtains.enough.active.energy.to.delocalize.and.relax.into.the.ZnO.layer.with.increasing.temperature..For.peak.(a),. the.blueshift.of.peak.position.is.due.to.the.localized.exciton.of.ZnO.well. layer.delocalized.to.free.exciton..It.can.be.seen.in.Figure.9.9,.as.ZnO.layer.thickness.is.decreas-ing,.the.luminescence.from.MgZnO.layer.becomes.weaker.and.weaker,.which.is.attributed.to.the.interface.state,.and.reduces.with.the.decrease.of.ZnO.well.layer.thickness.[49]..The.PL.from.localized.exciton.of.MgZnO.and.ZnO.weakens.

Figure.9.10.shows.the.time-decay.curves.of.the.luminescence.from.ZnO.well.layer.at.RT..The.time-decay.curves.can.be.well.described.by.a.biexponential.decay.for.all.samples..For.sample.A20,.a.bi-exponential.function.fits.the.decay.profile.well.with.a.reduced.chi-square.value,.w2r,.of.1.024.compared.with.a.single-exponential.fit.with.w2r.of.2..Time.constants.from.the.fit.are.0.58.and.3.82.ns..Similarly,.time.constants.of.sample.A4,.A2.are.0.43,.0.14.ns.for.fast.process;.and.3.43,.1.83.ns.for.slow.process..The.short.lifetime.of.0.58.ns.from.sample.A20.is.comparable.to.the.recombination.lifetimes.of.0.1.ns.at.293.K.for.free.excitons.in.ZnO.films.[49]..Such.a.long.lifetime.indicates.very.low.density.of.non-radiative.defects.in.well.intra-layers..The.slow.lifetime.observed.in.our.quantum.well.is.about.several.times.of.the.fast.lifetime.that.is.contributed.to.free.exciton..This.clearly.shows.that.the.slow.decay.pro-cesses.are.not.from.the.free.state.exciton..The.slowdown.in.the.exciton.relaxation.must.be.due.to.the.trapping.of.exciton.in.potential.fluctuation.in.the.interface..Sugawara.[50].has.shown.theoretically.that.lifetimes.of.such.excitons.are.several.times.that.of.the.free.exci-tons..This.kind.of.decay.may.suggest.two.different.recombination.channels..We.prefer.the.

1.8 2.0 2.2 2.4

A2

A4

A20PL

inte

nsity

(a. u

.)

a

b

2.6 2.8 3.0Photon energy (eV)

3.2 3.4 3.6 3.8 4.0

FIGURE 9.8The.PL.and.absorption.spectra.of.the.samples.with.different.well.thicknesses.at.RT..(Reproduced.from.J. Cryst. Growth,.278,.Lu,.Y.M.,.Wu,.C.X.,.Wei,.Z.P.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.Liu,.Y.C.,.Shen,.D.Z.,.and.Fan,.X.W.,.Characterization. of. ZnO/Mg0.12Zn0.88O. heterostructure. grown. by. plasma-assisted. molecular. beam. epitaxy,.299,.2005..Copyright.2005;. J. Luminesc.,. 119–120,.Wei,.Z.P.,.Lu,.Y.M.,.Shen,.D.Z.,.Wu,.C.X.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.and.Fan,.X.W.,.Effect.of.interface.on.luminescence.properties.in.ZnO/MgZnO.heterostructures,.551,.2006..Copyright.2006,.with.permission.from.Elsevier.)


Time delay (ns)10

System

PL in

tens

ity (a

. u.)

A4

A20

Well thickness

Life

time (

ns)

A8

20

20.1

0.2

0.30.4

0.5

0.6

4 6 8 10 12 14 16 18 20 22

30

FIGURE 9.10Time-decay.curve.spectra.of.samples.with.different.well. thicknesses. (A20.(°),.A4.(∇),.and.A2.(∆))..The.solid.curves.are. the. least-squares.fit.of.data.with. two.exponential.decay,. I(t).=.I1exp(−t/τ1).+. I2exp(−t/τ2)..The. inset.shows.the.lifetime.vs..well.thickness.for.the.three.samples..(Reproduced.from.J. Cryst. Growth,.278,.Lu,.Y.M.,.Wu,.C.X.,.Wei,.Z.P.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.Liu,.Y.C.,.Shen,.D.Z.,.and.Fan,.X.W.,.Characterization.of.ZnO/Mg0.12Zn0.88O.heterostructure.grown.by.plasma-assisted.molecular.beam.epitaxy,.299,.2005..Copyright.2005;.J. Luminesc.,.119–120,.Wei,.Z.P.,.Lu,.Y.M.,.Shen,.D.Z.,.Wu,.C.X.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.and.Fan,.X.W.,. Effect. of. interface. on. luminescence. properties. in. ZnO/MgZnO. heterostructures,. 551,. 2006.. Copyright.2006,.with.permission.from.Elsevier.)

Energy (eV)2.0 2.2

300 K

285 K

255 K

225 K

195 K

PL in

tens

ity (a

. u.)

165 K

135 K

105 K

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

FIGURE 9.9The.temperature-depended.PL.spectra.of.A20.(105–300.K)..(Reproduced.from.J. Cryst. Growth,.278,.Lu,.Y.M.,.Wu,.C.X.,.Wei,.Z.P.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.Liu,.Y.C.,.Shen,.D.Z.,.and.Fan,.X.W.,.Characterization.of.ZnO/Mg0.12Zn0.88O. heterostructure. grown. by. plasma-assisted. molecular. beam. epitaxy,. 299,. 2005.. Copyright. 2005;.J. Luminesc.,.119–120,.Wei,.Z.P.,.Lu,.Y.M.,.Shen,.D.Z.,.Wu,.C.X.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.and.Fan,.X.W.,.Effect.of.interface.on.luminescence.properties.in.ZnO/MgZnO.heterostructures,.551,.2006..Copyright.2006,.with.permission.from.Elsevier.)


following.explanation:.the.well.layer.PL.is.from.two.components..The.fast.decay.is.assigned.to.the.free.exciton.recombination..The.slow.decay.is.assigned.to.the.localized.exciton.recom-bination..These.phenomena.have.been.seen.in.GaAs.SQWs.at.20.K.[51]..The.reduction.of.interface.state.with.the.decrease.of.the.well.layer.thickness,.as.have.been.described.earlier,.can.also.be.confirmed.with.TRPL..The.slow.components.from.the.interface-localized.exci-ton.contribute.69.1%,.60.4%,.33.4%.of.the.PL.signal.for.sample.A20,.A4,.A2,.respectively..The.inset.of.Figure.9.10.shows.the.luminescence.lifetime.as.a.function.of.well.width..The.shorter.lifetime.with.decreasing.well.thickness.can.be.explained.by.the.effect.of.the.strong.quantum.confinement.in.thin.well.structure..To.further.clarify.the.origin.of.recombination,.the.time-decay.spectra.were.measured..Figure.9.11.shows.TRPL.spectra.monitored.at.vari-ous.times.after.pulsed.excitation.of.sample.A2..The.whole.spectra.are.integrated.during.the.same.time.interval,.and.are.normalized.in.intensity..From.the.figure,.with.the.increase.in.the.decay.time,.the.PL.peak.position.shifts.toward.the.low-energy.side..The.energy.dif-ference.of.the.peak.between.(a).0.ns.and.(d).1.5.ns.is.about.30.meV..It.confirms.that.the.PL.spectrum.consists.of.two.parts.(free.exciton.at.high-energy.side.and.localized.excitons.at.low-energy.side).as.explained.earlier..The.recombination.of.free.exciton.has.a.faster.process.than.that.of.the.localized.exciton,.with.increasing.time.the.free.exciton.recombination,.the.slow.process.of.localized.exciton.dominant..This.phenomenon.existing.at.RT.is.attributed.to.the.ZnO.with.high-exciton.binding.energy.(60.meV),.for.localized.excitons.the.binding.energy.is.about.30.meV,.and.the.exciton.and.localized.exciton.existed.even.at.RT.

9.3.3 Band Diagram of ZnO/MgZnO Quantum Well Structure

Although.MgZnO/ZnO.quantum.wells.and.supperlattice.structures.have.been.frequently.reported,. there. has. been. little. information. on. the. conduction. and. valence. band. offsets.

Photon energy (eV)3.0 3.1 3.2

(d) 1 .5 ns

(c) 1 .0 ns

(b) 0 .5 ns

Tim

e-re

solv

ed in

tens

ity (a

. u.)

(a) 0 ns

30 meV 300 K

3.3 3.4 3.5 3.6

FIGURE 9.11Time-resolved. PL. spectra. of. sample. A2. at. RT.. (Reproduced. from. J. Cryst. Growth,. 278,. Lu,. Y.M.,. Wu,. C.X.,.Wei,.Z.P.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.Liu,.Y.C.,.Shen,.D.Z.,.and.Fan,.X.W.,.Characterization.of.ZnO/Mg0.12Zn0.88O. heterostructure. grown. by. plasma-assisted. molecular. beam. epitaxy,. 299,. 205.. Copyright. 2005;.J. Luminesc.,.119–120,.Wei,.Z.P.,.Lu,.Y.M.,.Shen,.D.Z.,.Wu,.C.X.,.Zhang,.Z.Z.,.Zhao,.D.X.,.Zhang,.J.Y.,.and.Fan,.X.W.,.Effect.of.interface.on.luminescence.properties.in.ZnO/MgZnO.heterostructures,.551,.2006..Copyright.2006,.with.permission.from.Elsevier.)


between. ZnO. and. ZnMgO. [18,20].. Moreover,. experimental. data. on. band. alignment. of.ZnO/MgZnO. heterojunctions. are. still. less.. X-ray. photoelectron. spectroscopy. (XPS). has.been.demonstrated.to.be.a.direct.and.powerful.tool.for.measuring.the.band.discontinui-ties.of.heterojunctions.[52,53],.which.was.usually.used.to.characterize.the.band.configu-ration.at.the.interface.of.heterostructures..Here,.we.report.an.XPS.study.on.the.valence.band.offset.ΔEV.at.a.Mg0.15Zn0.85O/ZnO.heterojunction.[54]..A.500.nm.thick.ZnO,.500.nm.Mg0.15Zn0.85O,. and. 5.nm. ZnO/500.nm. Mg0.15Zn0.85O. grown. on. c-sapphire. substrates. by.P-MBE.were.used.as.the.samples..The.XPS.was.performed.by.an.ESCALAB.250.XPS.instru-ment.with.Al.Kα(hν.=.1486.6.eV).as.the.radiation.source,.which.is.competent.for.precisely.calibrating.work.function.and.Fermi.energy.level..All.XPS.spectra.were.calibrated.by.the.C.1s.peak.(284.6.eV)..The.air.absorption.and.contaminations.on.surfaces.may.influence.the.preciseness.of.measurements..To.get.rid.of.the.contamination.effect,.all.the.samples.were.subjected.to.a.surface.cleaning.procedure.by.Ar+.bombardment..After.the.bombardment,.peaks.related.to.impurities.were.greatly.reduced.

Based.on.the.method.of.a.core-level.(CL).photo.emission,.XPS.may.be.employed.in.deter-mining. the.ΔEV.at.heterojunction. interfaces..Appropriate.shallow.core-level.peaks.were.referenced.to.the.top.of.the.valence.band.for.the.ZnO.and.MgZnO.thick.films,.using.a.lin-ear.extrapolation.method.to.determine.the.valence.band.maximum.(VBM)..The.resulting.binding.energy.differences.between.the.core.peaks.and.valence.band.minimum.for.the.single.layer.were.then.combined.with.core-level.binding.energy.differences.for.the.het-erojunction.sample.to.obtain.ΔEV..This.standard.method.can.be.depicted.by.the.formula.as. shown.on. the.ZnO.and.Mg0.15.Zn0.85O.band.diagram. in.Figures.9.12.and.9.13..ΔEV. is.obtained.by.the.following.expression:

.∆ ∆E E E E E EV p p= −( ) − −( ) +Zn

ZnOVBMZnO

ZnMgZnO

VBMMgZnO

CL2 2 . (9.3)

whereE EpZn

ZnOVBMZnO

2 −( ) .is.the.energy.difference.between.Zn.2p.and.VBM.in.ZnO.film

E EpZnMgZnO

VBMMgZnO

2 −( ).is.the.energy.difference.between.Zn.2p.and.VBM.in.MgZnO.film

∆E E Ep pCL ZnZnO

ZnMgZnO= −( )2 2 . is.the.energy.difference.between.Zn.2p.core.levels.(CLs).in.the.

ZnO/MgZnO.heterojunction,.respectively

The.CL.spectra.of.Zn.2p3/2.recorded.on.the.ZnO.sample.are.shown.in.Figure.9.12a,.which.are.quite.symmetric,.indicating.the.uniform.bonding.state..The.only.peak.located.at.1021.55.eV.corresponds.to.the.Zn–O.bond..Similarly,.the.CL.of.Zn.2p3/2.in.MgZnO.is.determined.to.be.1021.50.eV.in.Figure.9.12b..The.Zn.2p3/2.of.the.ZnO/MgZnO.heterojunction.is.shown.in. Figure. 9.12c.. The. FWHM. of. Zn. 2p3/2. peak. of. ZnO/MgZnO. (1.61.eV). does. not. differ.obviously. from.that.of. the.Zn.2p3/2. in. the.ZnO.(1.60.eV).and.MgZnO.(1.59.eV).samples..Therefore,. it.can.be.considered.that. the.Zn.2p. from.the.ZnO.layer.and.MgZnO.layer. in.the.ZnO/MgZnO.heterojunction.coincides.in.the.spectra,.and. ∆E E Ep pCL Zn

ZnOZnMgZnO= −( ) =2 2 0..

Figure.9.12d.is.the.Mg.2p.of.the.ZnO/MgZnO.heterojunction,.which.also.supports.that.the.Zn.2p.of.ZnO/MgZnO.is.composed.of.Zn.2p.of.ZnO.and.MgZnO..The.valence.band.(VB).spectra.recorded.on.ZnO.and.MgZnO.samples.are.shown.in.Figure.9.13..This.linear.method.has.already.been.widely.used.to.determine.the.VBM.of.semiconductors.[55,56]..Figure.9.13a.shows.the.VB.XPS.spectra.of.the.ZnO.sample..A.VBM.value.of.1.96.eV.is.deduced.from.the.VB.spectra.by.linear.fitting.as.depicted.earlier..The.VB.XPS.spectra.recorded.on.MgZnO.in.


Figure.9.13b.show.a.VBM.of.2.04.eV.by.the.same.method..Table.9.2.shows.a.summary.of.the.band.offset.results..The.ΔEV.of.ZnO/Zn0.85Mg0.15O.heterojunction.is.calculated.to.be.0.13.eV.by.placing.those.experimental.values.into.Equation.9.3.

Figure.9.14.shows.the.schematic.diagram.of.the.energy.band.lineups.in.the.ZnO/MgZnO.heterostructure.with.all.of.the.energy.scales.included..The.conduction-band.offset.ΔEC.can.be.calculated.by.∆ ∆E E E EC g g V= − −ZnO ZnMgO ..The.band.gaps.of.ZnO.and.Mg0.1Zn0.85O.are.3.37.and.3.68.eV.at.RT,.respectively..So.ΔEC.is.estimated.to.be.−0.18.eV,.indicating.a.type-I.align-ment.for.ZnO/Mg0.15.Zn0.85O.heterojunction..The.band.gap.difference.of.0.31.eV.between.Mg0.15Zn0.85O.and.ZnO.has.an.almost.3:2.ratio.between.ΔEC.and.ΔEV..Coli.and.Bajaj.have.demonstrated.that.ΔEC/ΔEV.is.in.the.range.3/2–7/3.in

ZnO/MgZnO. superlattice. [20],. which. supports. the. rationality. of. our. results.. In. the.ΔEV.measurements,. strain. is.an. important. impact. factor..The.critical. thickness.of.ZnO/MgO.is.reported.to.be.5.5.nm.[57]..The.lattice.mismatch.between.ZnO/MgO.is.larger.than.that.between.ZnO.and.Mg0.15Zn0.85O..For.the.ZnO.film.grown.on.Mg0.15Zn0.85O.nm.is.far.below.the.critical.thickness..Therefore,.the.ZnO.on.Zn0.85Mg0.15O.suffers.tensile.strain.[58]..Fortunately,.the.strain.is.so.small.that.it.can.be.neglected.in.our.work..For.example,.the.c-axis.lattice.constant.of.Mg0.15Zn0.85O.(namely.ZnO.c-axis.lattice.constant).is.0.5178.nm.as.determined.by.XRD,.which.shows.a.0.48%.mismatch.with.ZnO..According.to.the.biaxial.relaxation.coefficient.RB.=.−εzz/εxx,.εxx.and.εzz.are.the.strains.perpendicular.and.parallel.to.c-axis.direction,.respectively,.RB.is.1.035.for.ZnO.[59],.the.a-axis.strain.is.about.0.46%..In.ZnO/MgO. heterostructure,. a. 8.3%. mismatch. brings. a. shift. of. 220.meV. on. ΔEV. [60].. By.

Binding energy (eV)1017 1020 1023 1026

1017 1020

Inte

nsity

(a.u

.)

1023

ZnO/Zn0.85Mg0.15O Zn0.85Mg0.15O

Zn0.85Mg0.15O

1026 1017 1020 1023 1026

46 48 50

48.93 eV

52 54

Zn 2p

Zn 2pZn 2p

FWHM=1.61

FWHM=1.60 FWHM=1.59

Mg 2p

ZnO

FIGURE 9.12CL.Zn.2p3/2.spectra.of.ZnO.(a).and.Mg0.15.Zn0.85O.(b).and.ZnO/ZnMgO.(c).samples,.Mg.2p.spectra.of.ZnO/.Mg0.15Zn0.85O.(d)sample..(Reproduced.with.permission.from.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Li,.B.H.,.Shen,.D.Z.,.Yao,.B.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.Valence.band.offset.of.ZnO/Zn0.85Mg0.15O.heterojunction.measured.by.x-ray.photoelectron.spectroscopy,.Appl. Phys. Lett.,. 93,.082108,.2008..Copyright.2008,.American.Institute.of.Physics.)


linear.extrapolation.method,.the.energy.shift.due.to.the.strain.in.ZnO/Mg0.15Zn0.85O.is.less.than.12.meV,.the.error.from.which.is.acceptable.in.this.work.

According.to. the.earlier.analysis,. the.band.offset.of. the.ZnO/MgZnO.heterostructure.shows.a.type-I.band.alignment,.and.the.ratio.of.conduction.band.offset.and.the.valence.band.offset.is.about.3:2..We.calculate.the.energy.level.of.ZnO/Mg0.12Zn0.88O.SQW.by.using.this.result..Figure.9.15.shows.a.schematic.band.diagram.for.the.wells.and.barriers.in.ZnO/

Binding energy (eV)0 1 2 3

2.04 eV

1.96 eVIn

tens

ity (a

. u.)

ZnO VBM

(a)

(b)4 5

1 2 3 4 5

Zn0.85Mg0.15O VBM

FIGURE 9.13The.VB.spectra.for.ZnO.(a).and.Mg0.15.Zn0.85O.(b).samples..The.VBM.values.are.determined.by.linear.extrapola-tion.of.the.leading.edge.to.the.base.line..(Reproduced.with.permission.from.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Li,.B.H.,.Shen,.D.Z.,.Yao,.B.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.Valence.band.offset.of.ZnO/Zn0.85Mg0.15O.heterojunction.measured.by.x-ray.photoelectron.spectroscopy,.Appl. Phys. Lett.,.93,.082108,.2008..Copyright.2008,.American.Institute.of.Physics.)

TABLE 9.2

Values.of.Band.Offsets.Determined.in.Our.Experiment

ZnO ZnO Zn 2p3-ZnO ZnMgO ZnMgO Zn 2p3-ZnMgO ZnO/ZnMgOValance Band

Offset

Zn.2p3 VBM VBM Zn.2p3 VBM VBM Zn.2p3-.Zn.2p3 ΔEV

1021.55 1.96 1019.59 1021.50 2.04 1019.46 0 0.13(eV) (eV) (eV) (eV) (eV) (eV) (eV) (eV)

Source:. Reproduced. with. permission. from. Su,. S.C.,. Lu,.Y.M.,. Zhang,. Z.Z.,. Shan,. C.X.,. Li,. B.H.,. Shen,. D.Z.,.Yao,.B.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.Valence.band.offset.of.ZnO/Zn0.85Mg0.15O.heterojunction.measured. by. x-ray. photoelectron. spectroscopy,. Appl. Phys. Lett.,. 93,. 082108,. 2008.. Copyright. 2008,.American.Institute.of.Physics.


Mg0.12Zn0.88O.SQW.(Lw.=.3.nm)..The.electron.and.hole.effective.masses.are.0.28m0.and.1.8m0.(m0. is. the. free. electron. mass). [18],. respectively.. Here. we. assume. that. the. electron/hole.effective.mass. is. the.same. for.ZnO.well. layer.and.Mg0.12Zn0.88O.barrier. layer..The.band.discontinuity.ΔEC/ΔEV.=.3/2.is.determined.by.XPS.[54],.and.applying.the.Kronig-Penney.model. the. first. subband. energies. in. the. conduction. and. valance. band. are. calculated. to.be.49.and.11.meV,.respectively..The.transition.energy.of.the.localized.exciton.in.an.SQW.structure.is.given.by.[61],

Zn 2p

1019.59 eV1019.46 eV

∆EV=0.13 eV

∆EC=0.18 eV

EVZnO

ECZnO

ECZnMgO

EgZnO = 3.37 eV

EgZnMgO = 3.68 eV

EVZnMgo

Zn 2p

FIGURE 9.14Energy.band.diagram.of.thin.ZnO/.Mg0.15.Zn0.85O.heterojunction.interface..A.type-I.heterojunction.is.formed..(Reproduced.with.permission.from.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Li,.B.H.,.Shen,.D.Z.,.Yao,.B.,.Zhang,.J.Y.,. Zhao,. D.X.,. and. Fan,. X.W.,. Valence. band. offset. of. ZnO/Zn0.85Mg0.15O. heterojunction. measured. by. x-ray.photoelectron.spectroscopy,.Appl. Phys. Lett.,.93,.082108,.2008..Copyright.2008,.American.Institute.of.Physics.)

EV1 = 11 meV ∆EV = 80 meV

∆EC = 120 meVEC1 = 49 meV

Eg =3.42 eV

FIGURE 9.15Diagram.of.conduction.and.valence.bands.between.barrier.and.well.layers.in.ZnO/Zn0.9.Mg0.1O.SQW.with.well.width.of.Lw.=.3.nm..The.calculated.quantum.subband.level.is.given..(Reproduced.from.Superlatt. Microstruct.,.48,.Su,.S.C.,.Lu,.Y.M.,.Xing,.G.Z.,.and.Wu,.T.,.Spontaneous.and.stimulated.emission.of.ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells,.485–490,.2010..Copyright.2010;.Appl. Surf. Sci.,.254,.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Yao,.B.,.Li,.B.H.,.Shen,.D.Z.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.The.optical.properties.of.ZnO/ZnMgO.single.quantum.well.grown.by.P-MBE,.7303–7305,.2008..Copyright.2008,.with.permission.from.Elsevier.)


. E E E E En m g n m n mB

, ,= + + − . (9.4)

whereEg.is.the.band.gap.of.ZnOEn.is.the.energy.of.the.nth.subband.in.conduction.bandEm.is.the.energy.of.mth.subband.in.valence.band

En mB

, .is.the.binding.energy.of.the.exciton.localized.in.the.nth.and.the.mth.subbands.in.the.SQWs..Assuming.that.Eg.at.low.temperature.is.3.420.eV.and.EB

1 1, .in.our.samples.is.75.meV.similar. to. that. in. the. ZnO/MgZnO. MQWs. grown. on. ScAlMgO4. substrates. [19],. E1,1. for.ZnO/Mg0.12Zn0.88O.SQW.(3.nm).is.estimated.to.be.3.405.eV..This.calculated.value.is.in.good.agreement.with.the.experimental.data.for.the.same.structure..It.is.observed.that.the.peak.from.ZnO.well.layer.is.located.at.3.407.eV.in.PL.spectra.[16]..This.means.that.the.ratio.of.ΔEC/ΔEV.=.3/2.obtained.by.XPS.measurement.is.reasonable.

9.3.4 Stimulated Emission of ZnO/MgZnO Quantum Well Structure

In.our.progress.to.ZnO-based.LEDs.and.LDs,.blue–violet.electro-luminescence.has.been.observed.from.the.homojunction.LEDs.fabricated.on.a.common.substrate.Al2O3. [36,37]..To.further.enhance.the.device.performance,.it.is.very.necessary.to.fabricate.and.study.the.ZnO/MgZnO.quantum.well.structure.grown.on.Al2O3..In.our.early.work,.we.had.obtained.stimulated.emission.in.ZnO/MgZnO.SQWs.on.Al2O3,.but.the.excitation.threshold.of.the.P.band.is.as.high.as.76.kW/cm2.at.5.K.[35]..Subsequently,.we.reported.the.property.of.stimu-lated.emission.at.RT.by.optical.pumping.of.ZnO/MgZnO.MQWs.on.Al2O3,.the.detailed.result.will.be.given.as.follows.

The.quantum.well.structure.was.grown.on.c-plane.Al2O3.substrate.by.P-MBE,.following.the.deposition.of.a.100.nm.thick.Mg0.2Zn0.8O.buffer.layer..The.structures.consist.of.10.MQWs.with.1.5.nm.thick.ZnO.wells.and.10.nm.thick.Mg0.2Zn0.8O.barriers..The.film.was.grown.at.550°C.before.exposure.to.oxygen.plasma.for.30.min..Figure.9.16.shows.RT.PL.and.absorp-tion.spectra.in.ZnO/Mg0.2Zn0.8O.MQWs..The.PL.spectrum.is.dominated.by.the.near.band-edge.emission.at.3.330.eV.with.weak.deep-level.emission,. indicating.high.optical.quality..Compared.with.the.ZnO.PL.peak.of.3.290.eV.at.RT.[13],.the.luminescence.in.ZnO/Mg0.2Zn0.8O.MQWs.shows.an.obvious.blueshift.of.about.40.meV.due.to.the.quantum.confinement.effect.[13]..Also.as.shown.in.Figure.9.16,.the.absorption.spectrum.is.located.on.the.higher.energy.side.of.the.PL.peak..In.Section.1.3.2,.it.has.been.demonstrated.that.the.RT.luminescences.in.ZnO/MgZnO.heterostructures.were.composed.of.localized.exciton.emissions.at.the.lower.energy.side.and.the.free.exciton.emissions.at.the.higher.energy.side.by.the.time-resolved.PL.measurements.[13]..For.ZnO/MgZnO.MQWs,.the.RT.spontaneous.emission.was.studied.by.the.temperature-dependent.PL.measurement..The.PL.spectrum.in.ZnO/Mg0.2Zn0.8OMQWs.is.dominated.by.localized.exciton.emission.at.low.temperatures.(below.163.K).while.the.free.exciton.transition.gradually.dominates.the.spectrum.at.higher.temperatures.up.to.RT.[34]..Here,.we.emphasize.on.describing.the.stimulated.emission.of.ZnO/MgZnO.MQWs.

The. stimulated. emission. experiments. were. performed. using. the. pulse. laser. output.(350.nm). from. an. optical. parametric. amplifier. (OPA). in. an. active. passive. mode. locked.femtosecond.Ti:.sapphire.laser.operating.at.a.repetition.rate.of.1.kHz..The.excitation.light.was.focused.on.the.sample.surface.using.a.cylindrical.lens..Emission.from.the.sample.edge.was.collected.into.a.spectrometer.(the.spectral.resolution.was.approximately.0.5.nm).and.detected.by.an.electrically.cooled.charge-coupled.device..Figure.9.17.shows.the.stimulated.emission.spectra.of.ZnO/Mg0.2Zn0.8OMQWs,.measured.under.strong-pulsed.laser.output.


of. the.OPA.at.RT..At. the.excitation.density.of.200.kW/cm2,.a.new.peak. (P2). is.observed.at.3.200.eV.with.a.FWHM.of.135.meV,.which. is. lower. than. the. spontaneous.PL.peak. in.Figure.9.17.by.about.130.meV..As.the.excitation.density.increases.further.to.230.kW/cm2,.a.sharp.peak.(P).at.3.170.eV.with.an.FWHM.of.25.meV.emerges.rapidly.from.the.lower.energy.side. of. the. P2. peak. and. dominates. the. spectrum.. When. the. excitation. density. reaches.350.kW/cm2,. the. intensity.of. the.P.peak.increases.further.with.no.change.in.the.energy.position.and.the.P2.peak.is.thoroughly.suppressed..As.shown.in.Figure.9.18,.the.intensity.(L).of.the.P.band.increases.superlinearly.with.the.excitation.density.(IEX)..The.dependence.of.L.on.IEX.can.be.fitted.by.L IEX∝ 5 5. ..Thus,.the.superlinear.increase.of.the.P.peak.intensity,.as.well.as.the.narrow.linewidth.with.the.total.suppression.of.the.other.emissions,.clearly.indicates.that.stimulated.emissions.have.occurred.in.our.sample..Compared.with.the.pre-vious.reports.on.stimulated.emission.from.ZnO.MQWs.at.RT.[32–34],.the.positions.of.the.P.and.P2.peaks.are.in.good.agreement.with.that.expected.from.an.inelastic.collision.between.excitons,.in.which.one.of.the.two.excitons.obtains.energy.from.the.other.and.scatters.into.a.higher.exciton.state.with.a.quantum.number.n.>.1,.while.the.other.recombines.radiatively..The.photons.emitted.in.this.process.have.the.energies.of.Pn.given.by.[62]

.p E E

nkT nn ex bex= − −

− = ∞( )1

1 32

2 32 , � . (9.5)

where.Eex,Ebex,.and.kT.are.the.free.exciton.energy,.the.exciton.binding.energy,.and.thermal.energy,.respectively..If.the.value.of.Eex.is.estimated.to.be.3.330.eV.by.the.spontaneous.emis-sion.peak. in.Figure.9.16,. the.exciton.binding.energy. is.determined.to.be.about.122.meV.by.Equation.9.4.using.Pn.=.3.170.eV,.which. is.very.close. to. the.exciton.binding.energy.of.115.meV. in. ZnO/MgZnO. MQWs. reported. by. Sun. et. al.. [21]. and. much. lower. than. that.

Energy (eV)

Nor

mal

ized

inte

nsity

2.1 2.4

PLAbsorption

RT

3.330 eV

2.7 3.0 3.3 3.6

FIGURE 9.16The. RT. absorption. spectrum. and. the. PL. spectrum. excited. by. a. He–Cd. laser. for. ZnO/Mg0.2Zn0.8O. MQWs..(Reproduced.with.permission.from.Sun,.J.W.,.Lu,.Y.M.,.Liu,.Y.C.,.Shen,.D.Z.,.Zhang,.Z.Z.,.Li,.B.H.,.Zhang,.J.Y.,.Yao,.B.,.Zhao,.D.X.,.and.Fan,.X.W.,.Room.temperature.excitonic.spontaneous.and.stimulated.emission.properties.in.ZnO/MgZnO.multiple.quantum.wells.grown.on.sapphire.substrate,.J. Phys. D Appl. Phys.,.40,.6541–6544,.2007..Copyright.2007,.Institute.of.Physics.)


obtained.in.ZnO/BeZnO.MQWs.[62]..Then,.the.energy.difference.between.P∞.and.P2.is.cal-culated.to.be.about.31.meV.by.Equation.9.4.using.Ebex.=.122.meV..Fortunately,.this.calculated.value.is.in.good.agreement.with.the.observed.peak.shift.between.the.P.and.P2.emissions.in.Figure.9.18..Thus,.the.mechanism.of.the.stimulated.emission.in.our.ZnO/Mg0.2Zn0.8O.MQWs.can.be.reasonably.attributed.to.inelastic.exciton–exciton.scattering..Moreover,.the.large.exciton.binding.energy.of.122.meV.further.confirms.the.excitonic.nature.of.the.RT.spontaneous.emission.in.the.MQWs.discussed.earlier.

Although.we.obtained.strong.RT.stimulated.emission. in.ZnO/Mg0.2Zn0.8O.MQWs.on.Al2O3.substrate,.the.excitation.threshold.of.the.P.band.is.rather.high.(230.kW/cm2).in.com-parison.with.the.reported.result.by.Ohtomo.et.al.,. in.which.they.realized.a. low.thresh-old. (about. 11.kW/cm2). in. 3.nm. ZnO/MgZnO. MQWs. on. ScMgAlO4. [32].. In. particular,.the.stimulated.emission.with.the.threshold.of.76.kW/cm2.could.but.be.realized.at.5.K.for.ZnO/MgZnO.SQWs.on.Al2O3.[35]..Taking.into.account.the.effect.of.carrier.concentration,.

Energy (eV)

PL in

tens

ity (a

. u.)

2.4 2.6

×1

×1/3

×1/16

×1/25

RTP

P2

2.8

200 kW/cm–2

230 kW/cm–2

300 kW/cm–2

350 kW/cm–2

3.0 3.2

FIGURE 9.17The.evolution.of.the.RT.PL.spectra.in.the.ZnO/Mg0.2Zn0.8O.MQWs.as.the.excitation.density.increases.from.200.to.350.kW/cm2.by.a.pulse.laser.output.of.the.OPA..(Reproduced.with.permission.from.Sun,.J.W.,.Lu,.Y.M.,.Liu,.Y.C.,.Shen,.D.Z.,.Zhang,.Z.Z.,.Li,.B.H.,.Zhang,. J.Y.,.Yao,.B.,.Zhao,.D.X.,.and.Fan,.X.W.,.Room.temperature.excitonic.spontaneous.and.stimulated.emission.properties.in.ZnO/MgZnO.multiple.quantum.wells.grown.on.sapphire.substrate,.J. Phys. D Appl. Phys.,.40,.6541–6544,.2007..Copyright.2007,.Institute.of.Physics.)


we. design. one. kind. special. quantum. well. structure-asymmetric. double. quantum. wells.(ADQWs).[38]..It.is.proposed.to.decrease.lasing.threshold.by.enhancing.the.carrier.con-centration.due.to.the.excitonic.tunneling.from.the.narrow.wells.(NWs).to.the.wide.wells.(WWs).of.the.ADQWs.[41,42].

The.ZnO/Mg0.15Zn0.85O.ADQW.samples.were.grown.on.an.m-plane.Al2O3.substrate.by.P-MBE.at.650°C..Elemental.Zn.(6N).and.Mg.(5N).were.evaporated.using.conventional.effu-sion.cells..Pure.oxygen.(5N).was.used.as.the.oxygen.source.and.oxygen.plasma.was.gener-ated.through.a.radio.frequency.(rf).activated.radical.cell..The.rf.power.of.the.oxygen.plasma.was.300.W..Before.growth,.the.substrates.were.inserted.into.an.ultrahigh.vacuum.chamber.and.annealed.at.800°C. for.30.min,.which.was.expected. to. remove. the. surface. contami-nants..The.structure.consists.of.a.50.nm.Mg0.15Zn0.85O.buffer.layer.followed.by.five.periods.of. ZnO/Mg0.15Zn0.85O. ADQWs. and. then. a. 50.nm. Mg0.15Zn0.85O. cap. layer.. Each. period. of.ZnO/.Mg0.15Zn0.85O.ADQWs.includes.one.narrow.ZnO.well,.one.thin.Mg0.15Zn0.85O.barrier.and.one.wide.ZnO.well,.which.will.be.denoted.later.as.Ln/Lb/Lw,.where.Ln,.Lb,.and.Lw.are.the.widths.of.the.narrow.well,.the.thin.barrier.and.the.wide.well,.respectively..Each.period.of.the.ADQW.was.separated.by.a.40.nm.Mg0.15Zn0.85O.barrier.

Photoluminescence.(PL).spectra.were.excited.by.the.325.nm.line.of.a.He–Cd.laser.with.output.power.50.mW..Energy.dispersive.spectroscopy.(EDS).was.used.to.determine.the.Mg.contents.in.the.MgZnO.barrier..The.stimulated.emission.experiments.were.performed.using.the.pulse.laser.output.(325.nm).from.an.OPA.in.an.active–passive.mode.locked.femtosecond.Ti:sapphire. laser. operating. at. a. repetition. rate. of. 1.kHz.. Emission. from. the. sample. edge.was.collected.into.a.spectrometer.(the.spectral.resolution.was.approximately.0.5.nm).and.detected.using.an.electrically.cooled.charge-coupled.device..The.structure.of.a.single-period.ADQW.that.consists.of.two.wells.of.different.widths.coupled.by.a.thin.barrier.is.shown.in.Figure.9.19a.and.b.shows.the.reflection.high.energy.electron.diffraction.(RHEED).spectra.of.

Excitation density (kW/cm–2)

100

PL in

tens

ity (a

. u.)

102

103

104

200 400300 600500

L ~ I 5.5EX

FIGURE 9.18The.integrated.intensity.of.the.stimulated.emission.as.a.function.of.the.excitation.density..(Reproduced.with.permission.from.Sun,.J.W.,.Lu,.Y.M.,.Liu,.Y.C.,.Shen,.D.Z.,.Zhang,.Z.Z.,.Li,.B.H.,.Zhang,.J.Y.,.Yao,.B.,.Zhao,.D.X.,.and.Fan,.X.W.,.Room.temperature.excitonic.spontaneous.and.stimulated.emission.properties.in.ZnO/MgZnO.multiple.quantum.wells.grown.on.sapphire.substrate,.J. Phys. D Appl. Phys.,.40,.6541–6544,.2007..Copyright.2007,.Institute.of.Physics.)


the.ZnO.well.layers..The.ZnO.well.layers.are.produced.by.two-dimensional.growth..The.RHEED.reveals.the.high.quality.of.the.ZnO/Mg0.15Zn0.85O.ADQWs..To.provide.confirmation.of.the.structure.of.the.ADQWs.further,.86.K.PL.spectra.of.the.ZnO/.Mg0.15Zn0.85O.MQWs.with.3,.6.nm.well.widths.and.3.nm/6.nm/6.nm.ZnO/Mg0.15Zn0.85O.ADQWs.are.shown.in.Figure.9.20..The.emission.peaks.of.the.3,.6.nm.MQWs.are.consistent.with.the.NWs.(3.nm).and.WWs.(6.nm).of.the.ADQWs,.respectively..Comparison.of.the.PL.spectra.of.MQWs.and.ADQWs.indicated.that.the.ZnO/Mg0.15Zn0.85O.ADQW.structure.was.obtained..In.order.to.further.the.understanding.of.the.PL.properties.of.the.ADQWs,.the.temperature-dependent.PL.spectra.of. the.3.nm/6.nm/6.nm.ADQWs.are.displayed. in.Figure.9.21..The.weak.emission.peak.at.3.60.eV. is.attributed. to. the. luminescence.of. the.Mg0.15Zn0.85O.barrier. layers..The.emission.peaks.at.3.398.and.3.440.eV.are.attributed.to.the.6.nm.WWs.and.3.nm.NWs.at.66.K,.respec-tively.. It. is.obvious. that. the.emission. from.the.WWs.dominates. the.spectrum..The.main.cause.for.the.difference.between.the.emission.intensities.of.NWs.and.WWs.is.the.exciton.tunneling.from.the.NWs.to.the.WWs..Most.of.the.excitons.excited.in.an.NW.tunnel.through.

ZnO wide well

ZnMgO coupling barrier

ZnMgO wide barrier

ZnO thin well

(a)

(b)

FIGURE 9.19(a).The.structure.of.single-period.ZnO/.Mg0.15.Zn0.85O.ADQWs..(b).The.RHEED.spectra.of.the.ZnO.well.layer.of.the.ZnO/.Mg0.15.Zn0.85O.ADQWs..(Reproduced.from.Superlatt. Microstruct.,.48,.Su,.S.C.,.Lu,.Y.M.,.Xing,.G.Z.,.and.Wu,.T.,.Spontaneous.and.stimulated.emission.of.ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells,.485–490,.2010..Copyright.2010;.Appl. Surf. Sci.,.254,.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Yao,.B.,.Li,.B.H.,.Shen,.D.Z.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.The.optical.properties.of.ZnO/ZnMgO.single.quantum.well.grown.by.P-MBE,.7303–7305,.2008..Copyright.2008,.with.permission.from.Elsevier.)


Photon energy (eV)2.6 2.8

317 K

269 K

Nor

mal

ized

inte

nsity

(a. u

.)

241 K

210 K

187 K

150 K114 K

3.60 eV

3.428 eV3.380 eV

86 K

3.0 3.2 3.4 3.6

FIGURE 9.21The. temperature-dependent. PL. spectra. of. the. 3.nm/6.nm/6.nm. ADQWs.. (Reproduced. from. Superlatt. Microstruct.,. 48,. Su,. S.C.,. Lu,. Y.M.,. Xing,. G.Z.,. and. Wu,. T.,. Spontaneous. and. stimulated. emission. of. ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells,.485–490,.2010..Copyright.2010;.Appl. Surf. Sci.,.254,.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Yao,.B.,.Li,.B.H.,.Shen,.D.Z.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.The.opti-cal.properties.of.ZnO/ZnMgO.single.quantum.well.grown.by.P-MBE,.7303–7305,.2008..Copyright.2008,.with.permission.from.Elsevier.)


86 K6 nm MQW

3 nm MQW3/6/6 ADQW

Nor

mal

ized

inte

nsity

(a. u

.)

3.6

FIGURE 9.20The.86.K.PL.spectra.of. the.ZnO/.Mg0.15.Zn0.85O.MQWs.with.3,.6.nm.well.widths.and.3.nm/6.nm/6.nm.ZnO/.Mg0.15.Zn0.85O.ADQWs..(Reproduced.from.Superlatt. Microstruct.,.48,.Su,.S.C.,.Lu,.Y.M.,.Xing,.G.Z.,.and.Wu,.T.,.Spontaneous.and.stimulated.emission.of.ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells,.485–490,.2010..Copyright.2010;.Appl. Surf. Sci.,.254,.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Yao,.B.,.Li,.B.H.,.Shen,.D.Z.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.The.optical.properties.of.ZnO/ZnMgO.single.quantum.well.grown.by.P-MBE,.7303–7305,.2008..Copyright.2008,.with.permission.from.Elsevier.)


the.thin.barrier.to.a.WW,.which.induces.the.difference.between.the.exciton.distributions.in.the.NW.and.WW..The.PL.peak.of.the.NW.disappeared.with.the.increase.of.temperature..The.reason.is.that.increasing.the.temperature.would.enhance.the.exciton.tunneling.rate..On.the.other.hand,.temperature.also.influences.the.stability.of.excitons..The.thermal.dissocia-tion.of.excitons.will.reduce.the.PL.intensity.of.the.NW..Since.the.ZnO.has.a.high.exciton.binding.energy.(60.meV),.the.thermal.dissociation.of.excitons.can.be.neglected..The.exciton.tunneling.from.the.NWs.to.WWs.leads.to.the.carrier.concentration.increasing.in.the.WWs.of.the.ADQWs..It.is.possible.to.realize.a.low.threshold-.stimulated.emission.in.WW..Figure.9.22.shows.the.stimulated.emission.spectra.of.3.nm/6.nm/6.nm.ADQWs,.measured.under. the.pulsed.laser.output.of.the.OPA.at.66.K..At.low.pumping.intensities,.the.spontaneous.emis-sion.bands.were.observed.at.3.380.eV..At.the.excitation.density.of.64.kW/cm2,.a.new.peak.(P2).is.observed.at.3.320.eV,.which.is.lower.than.the.spontaneous.PL.peak..As.the.excitation.density.increases.further.to.90.kW/cm2,.a.sharp.peak.(P).at.3.303.eV.emerges.rapidly.from.the.lower.energy.side.of.the.P2.peak.and.dominates.the.spectrum.

Figure.9.23.gives.the.integrated.emission.intensity.(L).versus.the.excitation.density.(IEX)..The.interdependence.of.L.and.IEX.can.be.fitted.with.L Iex∝ 2 9. ..Thus,.the.superlinear.increase.of.the.P.peak.intensity,.as.well.as.the.narrow.linewidth.with.the.total.suppression.of.the.other.emissions,.clearly.indicates.that.stimulated.emissions.have.occurred.in.this.sample..The. threshold. is.about.64.kW/cm2..Comparing.with. the.previous. reports.on.stimulated.emission.from.ZnO.MQWs.[3,9],.the.positions.of.the.P.and.P2.peaks.are.in.good.agreement.with.what. is.expected.from.an.inelastic.collision.between.excitons,. in.which.one.of. the.two.excitons.obtains.energy.from.the.other.and.scatters.into.a.higher.exciton.state.with.a.quantum.number.n.>.1,.while.the.other.recombines.radiatively..The.photons.emitted.in.this.process.have.the.energies.of.Pn.given.by.Equation.9.5..If.the.value.of.Eex.is.estimated.to.be.3.380.eV.from.the.spontaneous.emission.peak,.the.exciton.binding.energy.is.determined.as.about.69.meV.using.Equation.9.4.with.Pn.=.3.303.eV..Then,.the.energy.difference.between.

Photon energy (eV)

Inte

nsity

(a. u

.)

Exci

tatio

n in

tens

ity

3.1 3.2

P23.320 eV

P3.303 eV

3.3 3.4 3.5 3.6

FIGURE 9.22The. evolution. of. the. P. band. emission. as. the. excitation. intensity. increases. from. 26. to. 90.kW/cm2. at. 66.K..(Reproduced. from. Superlatt. Microstruct.,. 48,. Su,. S.C.,. Lu,. Y.M.,. Xing,. G.Z.,. and. Wu,. T.,. Spontaneous. and.stimulated.emission.of.ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells,.485–490,.2010..Copyright.2010;.Appl. Surf. Sci.,.254,.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Yao,.B.,.Li,.B.H.,.Shen,.D.Z.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.The.optical.properties.of.ZnO/ZnMgO.single.quantum.well.grown.by.P-MBE,.7303–7305,.2008..Copyright.2008,.with.permission.from.Elsevier.)


P∞.and.P2.is.calculated.to.be.about.17.meV.using.Equation.9.4.with.Eexb .=.69.meV..This.cal-

culated.value. is. in.good.agreement.with.the.observed.peak.shift.between.the.P.and.P2.emissions.in.Figure.9.22..Thus,.the.mechanism.of.the.stimulated.emission.studied.in.this.work.can.be.reasonably.attributed.as.inelastic.exciton–exciton.scattering..The.threshold.is.64.kW/cm2.at.66.K.in.ADQW.structures..The.most.likely.reason.for.this.is.that.the.ADQWs.have. special. structures. in. comparison. with. the. SQW,. in. which. the. threshold. is. about.76.kW/cm2.at.5.K.[35]..The.exciton.tunneling.from.the.NWs.to.WWs.of.the.ADQWs.leads.to.the.increasing.carrier.concentration.in.WWs.of.the.ADQWs,.and.realizes.low.threshold.stimulated.emission..The.large.internal.electric.field.expected.in.ZnO/MgZnO.QWs.may.provide.other.mechanisms.for.lowering.thresholds.in.ADQWs.

9.4 Summary

In. this. chapter,. we. have. summarized. some. recent. researches. on. optical. properties. of.MgZnO/ZnO.heterostructures.grown.on.sapphire.substrates.by.P-MBE.

The.MgxZn1−xO.alloy.thin.films.with.the.wurtzite.crystal.structure.of.x.ranging.from.0.to.0.2.were.obtained..Absorption.edges.of.the.films.shift.to.high-energy.side.with.increas-ing.x.value.in.the.absorption.spectra.at.RT..PL.spectra.at.RT.show.an.intense.UV.emission,.whose.peak.energy.could.be.tuned.from.3.29.to.3.47.eV.depending.on.the.Mg.content.in.the.films,.and.no.the.deep.level.emission.was.observed..The.origin.of.UV.emission.at.RT.was.attributed.to.the.free.exciton.emission.by.the.temperature-dependent.PL.spectra.

Photon energy (eV)

Inte

nsity

(a. u

.)

2.8 3.0 3.2 3.4

3.38 eV

3.303 eV(a)

(b)

325 nmHe-Cd laser

Pulse laser92 kW/cm–2

3.6

FIGURE 9.23The.integrated.intensity.of.the.stimulated.emission.as.a.function.of.the.excitation.density..(Reproduced.from.Superlatt. Microstruct.,.48,.Su,.S.C.,.Lu,.Y.M.,.Xing,.G.Z.,.and.Wu,.T.,.Spontaneous.and.stimulated.emission.of.ZnO/Zn0.85Mg0.15O.asymmetric.double.quantum.wells,.485–490,.2010..Copyright.2010;.Appl. Surf. Sci.,.254,.Su,.S.C.,.Lu,.Y.M.,.Zhang,.Z.Z.,.Shan,.C.X.,.Yao,.B.,.Li,.B.H.,.Shen,.D.Z.,.Zhang,.J.Y.,.Zhao,.D.X.,.and.Fan,.X.W.,.The.optical.properties.of.ZnO/ZnMgO.single.quantum.well.grown.by.P-MBE,.7303–7305,.2008..Copyright.2008,.with.permission.from.Elsevier.)


Optical.properties.of.ZnO/MgxZn1−xO.heterostructures.with.different.well.widths.were.reported.in.detail..All.samples.show.that.a.strong.UV.emission.band.from.the.ZnO.layer,.which.has.a.large.blueshift.(∼40.meV).with.decreasing.well.thickness.from.20.to.2.nm..This.phenomenon.is.attributed.to.the.quantum.confinement.effect..In.addition,.the.existence.of.the.interface.effect.between.ZnO.and.MgZnO.layers.leads.to.the.observation.of.the.recom-bination.from.MgZnO.potential.barrier..The.time-decay.curves.can.be.well.described.by.a.biexponential.decay.for.all.samples..The.fast.process.is.from.the.recombination.of.the.free.exciton,.while.the.slow.process.is.attributed.to.the.recombination.of.the.localized.exciton.in. the. interface. of. ZnO. and. MgZnO.. With. decreasing. well. thickness,. the. fast. process.gradually.increases.and.dominantly.contributes.to.the.PL.spectrum.due.to.the.interface.improvement.

XPS.determinations.of.the.ΔEV.of.ZnO/Mg0.15Zn0.85O.heterojunctions.were.performed,.which.shows.a.type-I.band.alignment.with.ΔEV.=.0.13.eV.and.ΔEC.=.0.18.eV..The.ΔEC/ΔEV.in.ZnO/.MgZnO.heterojunction.was.estimated.to.be.3:2..In.this.case,.the.quantum.energy.levels.in.the.conduction.and.valance.band.of.the.ZnO/Mg0.1Zn0.9O.SQW.with.well.width.of.3.nm.are.calculated.by.using.the.Kronig-Penney.model..The.first.subband.energies.in.the.conduction.and.valence.band.are.49.and.11.meV,.respectively..The.transition.energy.from.n.=.1.electron.subband.to.n.=.1.hole.subband.is.estimated.to.be.3.405.eV.for.ZnO/MgZnO.SQW.with.well.width.of.3.nm..It.is.noticed.that.the.experiment.data.is.in.good.agreement.with.theoretical.calculation.

RT. stimulated. emission. caused. by. inelastic. exciton–exciton. scattering. was. observed.in.the.ZnO/Mg0.2Zn0.8O.MQWs.grown.on.Al2O3..The.exciton.binding.energy.was.deter-mined.to.be.122.meV..But.the.threshold.is.as.high.as.about.200.kW/cm2.at.RT..Subsequently,.we. first. reported. the. stimulated. emission. of. ZnO/Mg0.15Zn0.85O. ADQWs. fabricated. on.Al2O3.substrate.by.P-MBE..The.exciton. tunneling.properties. from.NWs.to.WWs.of. the.ADQWs.were.observed,.which.leads.to.the.carrier.concentration.increasing.in.WWs.of.the.ADQWs..The.threshold.is.decreased.to.the.values.as.low.as.of.64.kW/cm2.in.ADQW.structures..The.origin.of.the.stimulated.emission.is.attributed.to.exciton–exciton.scatter-ing.in.the.WWs.of.ADQWs..This.work.is.considered.to.be.very.important.for.the.design.of.semiconductor.lasers.

Acknowledgments

The.authors.would.like.to.express.their.thanks.to.Drs..S.C..Su,.Z.P..Wei,.C.X..Wu,.and.J.W..Sun.for.their.valuable.works..These.works.were.supported.by.the.“863”.High.Technology.Research. Program. in. China,. under. Grant. No. 2001AA31112,. the. Innovation. Project. of.Chinese.Academy.of.Sciences,.the.“973”.program.under.Grant.Nos..2006CB604906,.and.the.National.Natural.Science.Foundation.of.China.under.Grant.No.10674133.and.No..60976036.

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. 41.. S..Ten,.F..Henneberger,.M..Rabe,.and.N..Peyghambarian,.Femtosecond.study.of.exciton.tunnel-ing.in.(Zn,Cd)Se/ZnSe.asymmetric.double.quantum.wells,.Phys. Rev..B.53,.12637.(1996).

. 42.. N..Sawaki,.R..A..Hopfel,.E..Gornik,.and.H..Kano,.Time-resolved.measurement.of. tunneling.and.energy.relaxation.of.hot.electrons.in.GaAs/AlGaAs.double.quantum.well.structures,.Appl. Phys. Lett..55,.1996.(1989).

. 43.. D..Y..Oberl,.J..Shap,.T..C..Damen,.J..M..Kuo,.J..E..Henry,.and.S..M..Goodnick,.Optical.phonon-assisted.tunneling.in.double.quantum.well.structures,.Appl. Phys. Lett..56,.1239.(1990).

. 44.. A.. P.. Heberle,. W.. W.. Ruhle,. M.. G.. W.. Alexander,. and. K.. Kohler,. Resonances. in. tunnelling.between. quantum. wells,. Semicond. Sci. Technol.. 7,. B421. (1992);. A.. P.. Heberle,. X.. Q.. Zhou,.A..Takeuchi,.W..W..Ruhle,.and.K..Kohler,.Dependence.of.resonant.electron.and.hole.tunnelling.times.between.quantum.wells.on.barrier.thickness,.Semicond. Sci. Technol..9,.519.(1994).

. 45.. J..Tauc,.R..Grigorovichi,.and.A..Vancu,.Optical.properties.and.electronic.structure.of.amorphous.germanium,.Phys. Status Solidi..15,.627–637.(1966).

. 46.. D..Bimberg,.M..Sondergeld,.and.E..Grobe..Thermal.dissociation.of.excitons.bounds.to.neutral.acceptors.in.high-purity.GaAs,.Phys. Rev. B.4,.3451–3455.(1971).

. 47.. R..Schmidt,.B..Rheinländer,.M..Schubert,.D..Spemann,.T..Butz,.J..Lenzner,.E..M..Kaidashev,.M..Lorenz,.A..Rahm,.H..C..Semmelhack,.and.M..Grundmann..Dielectric. functions(1.to.5.eV).of.wurtzite.MgxZn1−xO.(x.=.0.29.thin.films),.Appl. Phys. Lett..82,.2260–2262.(2003).

. 48.. R..Zimmermann,.Theory.of.exciton.linewidth.in.II–VI.semiconductor.mixed.crystals,.J. Cryst. Growth.101,.346.(1990).

. 49.. T..Koida,.A..Uedomo,.A..Tsukazaki,.T..Sota,.M..Kawasaki,.and.S..F..Chichibu,.Direct.comparison.of.photoluminescence.lifetime.and.defect.densities.in.ZnO.epilayers.studied.by.time-resolved.photoluminescence. and. slow. positron. annihilation. techniques,. Phys. Stat. Solidi (a).. 201(12),.2841.(2004).

. 50.. M.. Sugawara,. Theory. of. spontaneous-emission. lifetime. of. Wannier. excitons. in. mesoscopic.semiconductor.quantum.disks,.Phys. Rev. B.51,.10743.(1995).

. 51.. A..Satake,.T..Ikemoto,.K..Fujiwara,.L..Schrottke,.R..Hey,.and.H..T..Grahn,.Dynamical.transport.of.photoexcited.carriers.between.a.narrow.and.a.wide.quantum.well.embedded.in.a.GaAs/AlAs.superlattice,.Physica E.13,.711.(2002).

. 52.. J.-J..Chen,.B..P..Gila,.M..Hlad,.A..Gerger,.F..Ren,.C..R..Abernathy,.and.S..J..Pearton,.Determination.of.MgO/GaN.heterojunction.band.offsets.by.x-ray.photoelectron.spectroscopy,.Appl. Phys. Lett..88,.042113.(2006).

. 53.. Y..Lu,.J..C..Le.Breton,.P..Turban,.B..Lepine,.P..Schieffer,.and.G..Jezequel,.Measurement.of.the.valence-band. offset. at. the. epitaxial. MgO-GaAs(001). heterojunction. by. x-ray. photoelectron.spectroscopy,.Appl. Phys. Lett..88,.042108.(2006).

. 54.. S..C..Su,.Y..M..Lu,.Z..Z..Zhang,.C..X..Shan,.B..H..Li,.D..Z..Shen,.B..Yao,.J..Y..Zhang,.D..X..Zhao,.and.X..W..Fan,.Valence.band.offset.of.ZnO/Zn0.85Mg0.15O.heterojunction.measured.by.x-ray.photoelectron.spectroscopy,.Appl. Phys. Lett..93,.082108.(2008).

. 55.. J..R..Waldrop.and.R..W..Grant,.Measurement.of.AlN/GaN.(0001).heterojunction.band.offsets.by.x-ray.photoemission.spectroscopy,.Appl. Phys. Lett..68,.2879.(1996).

. 56.. E.. A.. Kraut,. R.. W.. Grant,. J.. R.. Waldrop,. and. S.. P.. Kowalczyk,. Semiconductor. core-level.to. valence-band. maximum. binding-energy. differences:. Precise. determination. by. x-ray.photoelectron.spectroscopy,.Phys. Rev. B.28,.1965.(1983).


. 57.. S..H..Park,.T..Hanada,.D..C..Oh,.G..Fujimoto,.J..S..Park,.J..H..Chang,.M..W..Cho,.and.T..Yao,.Lattice. relaxation. mechanism. of. ZnO. thin. films. grown. on. C-Al2O3. substrates. by. plasma-assisted.molecular-beam.epitaxy,.Appl. Phys. Lett..91,.231904.(2007).

. 58.. K..Zitouni.and.A..Kadri,.Effects.of.lattice-mismatch.induced.built-in.strain.on.the.valence.band.properties. of. wurtzite. ZnO/Zn1−xMgxO. quantum. well. heterostructures,. Phys. Status. Solidi. C.4,.208.(2007).

. 59.. Y..F..Li,.B..Yao,.Y..M..Lu,.Z..Z..Zhang,.B..H..Li,.D..Z..Shen,.and.X..W..Fan,.Characterization.of.biaxial.stress.and.its.effect.on.optical.properties.of.ZnO.thin.films,.Appl. Phys. Lett..91,.021915.(2007).

. 60.. Y..F..Li,.B..Yao,.Y..M..Lu,.Z..Z..Zhang,.D..X..Zhao,.J..Y..Zhang,.D..Z..Shen,.and.X..W..Fan,.Valence-band. offset. of. epitaxial. ZnO/MgO(111). heterojunction. determined. by. x-ray. photoelectron.spectroscopy,.Appl. Phys. Lett..92,.192116.(2008).

. 61.. C..Parks,.A..K..Ramdas,.M..R..Mellich,.and.L..R..Mohan,.Piezomodulated-reflectivity.study.of.minibands.in.AlxGa1−xAs/GaAs.superlattices,.Phys. Rev. B.48,.5413.(1993).

. 62.. Y..R..Ryu,.J..A..Lubguban,.T..S..Lee,.H..W..White,.T..S..Jeong,.C..J..Youn,.and.B..J..Kim,.Excitonic.ultraviolet.lasing.in.ZnO-based.light.emitting.devices,.Appl. Phys. Lett..90,.131115.(2007).

Part III

ZnO Alloys

257

10The (Mg,Zn)O Alloy

Holger von Wenckstern, Rüdiger Schmidt-Grund, Carsten Bundesmann, Alexander Müller, Christof P. Dietrich, Marko Stölzel, Martin Lange, and Marius Grundmann

10.1 Introduction

The.functionality.of.many.modern.solid.state.devices.is.governed.by.appropriate.design.of.heterojunctions..Semiconductor.heterojunctions.are.essential. in. state.of. the.art. light-emitting.diodes,.solid.state.lasers,.or.high.electron.mobility.field.effect.transistors.[1]..The.properties.of.the.heterojunction.itself.depend.on.differences.of.material.parameters.(e.g.,.electron.affinity,.effective.mass,.and.dielectric.constant)..Ternary.semiconductor.systems.offer. the.possibility. to. tune.these.parameters.continuously. in.a.certain.range.without.a.

CONTENTS

10.1. Introduction......................................................................................................................... 25710.2. Structural.Properties.......................................................................................................... 260

10.2.1. Wurtzite–Rocksalt.Structural.Phase.Transition................................................. 26010.2.2. Lattice.Constants..................................................................................................... 261

10.3. .Infrared-Vacuum.Ultraviolet.Dielectric.Function.and.Absorption.Coefficient.........26410.3.1. Phonons.................................................................................................................... 26510.3.2. Dielectric.Constants............................................................................................... 27010.3.3. Refractive.Index...................................................................................................... 27110.3.4. Fundamental.Bandgap.and.Excitons................................................................... 272

10.3.4.1. Temperature.Dependence.......................................................................27710.3.4.2. Pressure.Dependence.............................................................................. 279

10.3.5. Higher.Band-to-Band.Transitions........................................................................ 27910.4. Doping.of.(Mg,Zn)O...........................................................................................................280

10.4.1. Aluminum-Doping.of.(Mg,Zn)O.......................................................................... 28110.4.2. Gallium-.and.Indium-Doping.of.(Mg,Zn)O.......................................................28410.4.3. p-Doping.of.(Mg,Zn)O........................................................................................... 286

10.5. Exciton.Recombination...................................................................................................... 28910.5.1. Alloy.Broadening.................................................................................................... 28910.5.2. Origin.of.the.Near-Band-Edge.Luminescence................................................... 29110.5.3. Temperature-Dependent.Localization.Effects.................................................... 294

10.6. (Mg,Zn)O.Heterostructures.............................................................................................. 29710.7. (Mg,Zn)O.Nanostructures.................................................................................................302

10.7.1. (Mg,Zn)O.Nanowires.............................................................................................30310.7.2. Core/Shell-Heterostructures.with.a.ZnO.Core..................................................305

References...................................................................................................................................... 307


change.of.the.crystallographic.structure.which.is.important.for.the.efficiency.of.devices..The.principles.of.heterostructures.and.bandgap.engineering.developed.for.III–V.semicon-ductors.including.nitrides.also.apply.to.oxide.semiconductors.[2]..In.the.case.of.ZnO.[3],.the.bandgap.is.commonly.engineered.by.cationic.substitution.with.Be.or.Mg.resulting.in.an.increase.of.the.fundamental.bandgap.or.with.Cd.causing.a.bandgap.decrease.[4,5]..Anionic.substitution.by.S,.Se,.and.Te,.respectively,.also.leads.to.a.decrease.of.the.bandgap..In.Table.10.1,.we.have.compiled.material.properties.of.the.binaries.used.in.solid.state.solution.with.ZnO.for.bandgap.engineering..It.is.evident.that.only.BeO.and.ZnS.have.a.wurtzite.modifi-cation.under.ambient.conditions.that.is.favorable.for.exploiting.the.complete.composition.range..However,.BeO.is.highly.toxic.and.most.scientists.avoid.its.use.in.laboratory.

In.Figure.10.1,.the.dependence.of.the.energy.gap.is.sketched.for.solid.state.solutions.of.ZnO.and.the.oxides.BeO,.MgO,.and.CdO,.and.the.zinc.compounds.ZnS,.ZnSe,.and.ZnTe..Since. BeO. and. ZnS. have. wurtzite. crystal. structure,. these. compounds. form. solid. state.

TABLE 10.1

Properties.(at.Room.Temperature).of.ZnO,.Oxides,.and.Zinc.Compounds.Forming.Ternary.Alloys.with.Zinc.Oxide

Crystal Structure

Bandgap a c

Miscibility Gap

Abundance Earth Crust(ppm) [6](eV) (Å) (Å)

ZnO w 3.37 3.2501.[7] 5.2062.[7] — 75BeO w 10.585.[8] 2.6979.[9] 4.3772.[9] —[10]MgO rs 7.674.[11,12] 4.212.[13] — 0.55–0.68 950CdO rs 2.42.[14] 4.6942.[15] — — 0.11ZnS w 3.74.[16] 3.8140.[17] 6.2576.[17] — 260

zb 3.723.[18] 5.405.[19] — —ZnSe zb 2.720.[20] 5.667.[21] — 0.1–0.9 0.05ZnTe zb 2.26.[22] 6.0882.[19] — 0.3.[3]–0.988.[23] 0.005

W,.wurtzite;.rs,.rocksalt;.zb,.zincblende.

Alloy composition0.0

2

4

E g(e

V)

6

8

10

0.2 0.4 0.6 0.8 1.0

Zn(O,S)Zn(O,Se)Zn(O,Te)(Cd,Zn)O (w)(Mg,Zn)O (w)(Mg,Zn)O (c)(Be,Zn)O

FIGURE 10.1Dependence.of.the.bandgap.of.solid.ternary.state.solutions.with.ZnO.as.one.of.the.binary.components..A.mis-cibility.gap.of.the.solid.state.solution.is.indicated.by.dashed.lines,.solid.lines.indicate.that.a.solid.state.solution.was.reported.for.the.corresponding.alloy.composition..The.black,.horizontal,.dotted.line.corresponds.to.the.energy.gap.of.binary.ZnO.at.room.temperature.


solutions.with.ZnO.in.any.mixture..For.all.other.solutions.a.miscibility.gap.exists.as.indi-cated.by.the.dashed.lines.in.Figure.10.1.and.as.given.in.Table.10.1.

Putting. the. focus. on. cationic. substitution. [3],. the. bandgap. of. the. binary. oxides. BeO,.MgO,.CdO,.and.ZnO.is.visualized.in.Figure.10.2a..The.experimentally.observed.composi-tion.spread.within.the.wurtzite.modification.is.indicated.as.well..Only.for.(Be,Zn)O,.the.whole.composition.range.can.be.used.to.realize.single.phase.thin.films..An.important.dis-advantage.of.(Be,Zn)O.alloys.compared.to.wurtzite.(Mg,Zn)O.alloys.is.the.comparatively.strong.dependence.of. the.c-lattice.constant.on.alloy.composition.of. (Be,Zn)O..Favorable.growth.directions.for.thin.film.heterostructures.to.be.used.in.optoelectronic.devices.are.nonpolar.directions.in.order.to.suppress.discontinuities.of.the.spontaneous.polarization.at.heterostructure.interfaces..For.growth.along.nonpolar.directions,.the.c-lattice.constant.lies.in.plane.and.should.therefore.change.only.little.with.increasing.Mg/Be.content..This.is. only. the. case. for. (Mg,Zn)O. as. the. experimental. data. of. Figure. 10.2b. clearly. shows..

Mg, Be content02.5

0

2

4

6

8

10

12

3(a) (b)

(c)

ZnO

E g(e

V)

E g(e

V)

CdO (c)

BeZnO

MgO (c)

(Mg,Zn)O(Be,Zn)O

(Mg,Zn)O (hexagonal)BeO

3.5a (Å)

c (Å

)

4 4.5 5

4.4

3.0

3.5

4.0

4.5

5.0

5.5

6.09 9

11

3.0

3.5

4.0

4.5

5.0

5.5

6.0

11

4.6

4.8

5.0

5.2

0.25 0.5 0.75 1

Mg, Be content0 0.25 0.5 0.75 1

FIGURE 10.2(a).Sketch. of. the. change.of. the.bandgap.energy.versus. the.a-lattice. constant. for.BeO,. MgO,. CdO,. and.ZnO..The. dashed,. red. line. indicates. alloy. compositions. experimentally. realized. within. wurtzite. modification..(b).Dependence.of.the.c-lattice.constant.on.composition.for.(Mg,Zn)O.and.BeZnO..(c).Experimental.data.of.the.bandgap-dependence.on.alloy.composition.for.wurtzite.(Mg,Zn)O.and.BeZnO..For.(b).and.(c).data.for.(Mg,Zn)O.from.Refs..[24–28].for.(Be,Zn)O.from.Ref..[10].were.considered.


Within.the.wurtzite.modification,.the.dependence.of.the.bandgap.of.(Be,Zn)O.and.(Mg,Zn)O. on. the. Be-. and. Mg-content,. respectively,. is. in. principle. the. same. and. is. depicted. in.Figure. 10.2..Hence,. much. larger. bandgap. variation. can. be. obtained. for. (Mg,Zn)O. with.respect.to.BeZnO.for.a.given.change.of.the.c-lattice.constant,.making.(Mg,Zn)O.the.barrier.material.of.choice.for.realization.of.nonpolar.optoelectronic.devices.

10.2 Structural Properties

ZnO.crystallizes.in.the.wurtzite.structure,.MgO.in.the.rocksalt.structure,.and.MgxZn1−xO.in. the. wurtzite. or. the. rocksalt. structure,. depending. on. the. Mg-content. x.. The. wurtzite.structure.with.the.lattice.constants.a.and.c.belongs.to.the.dihexagonal-pyramidal.crystal.class.within.the.hexagonal.crystal.system.(space.group.C v6

4 .in.the.Schoenflies.and.P63mc.in.the.short.standard.notation;.point.group.C6v.and.6mm,.respectively)..The.two.atom.spe-cies.that.occupy.the.positions.of.a.close-packed.hexagonal.lattice.are.located.at.(0,0,0).and.(0,0,u)a..Each.of.the.atoms.is.fourfold.coordinated,.that.is,.each.atom.has.four.next.nearest.neighbors..The.cell-internal.structure.parameter.u.is.defined.as

.u

ac

= + .13

14

2

2 . (10.1)

For.the.ideal.wurtzite.structure,.the.ratio.c a/ = /8 3 .and.u0.=.3/8.holds..The.rocksalt.struc-ture.belongs.to.the.hexakis.octrahedral.crystal.class.within.the.cubic.crystal.system.(space.group. Oh

1. in. the. Schoenflies. and. Pm3m. in. the. short. standard. notation;. point. group. Oh.

and.m3m,.respectively)..The.two.atom.species.occupy.two.face-centered.cubic.(fcc).sublat-tices.with.the.lattice.constant.a..The.diatomic.base.is.represented.by.the.positions.(0,0,0).and.(1/2,1/2,1/2)a.with.the.distance. 3 2a/ ..Each.of.the.atoms.is.sixfold.coordinated,.that.is,.each.atom.has.six.next.neighbors..The.volumes.Vw.and.Vrs.of.the.primitive.unit.cells.(one-atomic).of.the.wurtzite.and.rocksalt.lattice,.respectively,.are.given.by

.V a c V aw rs= =3

214

2 3, . . (10.2)

10.2.1 Wurtzite–Rocksalt Structural Phase Transition

For.the.MgxZn1−xO.mixed.crystal.thin.films,.a.phase.transition.between.the.wurtzite.struc-ture.and.the.rocksalt.structure.occurs..At.ambient.pressure,.the.wurtzite-rocksalt.struc-ture.phase.transition.is.found.between.x.=.0.3.and.0.8,.depending.on.the.growth.method,.the.substrates,.and.the.actual.structural.properties.of.the.thin.films..Sans.et.al..[29].inves-tigated.the.dependence.of.the.phase.transition.on.the.hydrostatic.pressure.and.found.a.hysteresis-like. behavior,. that. is,. when. increasing. the. pressure,. the. wurtzite. to. rocksalt.structure.phase.transition.occurs.at.quite.higher.pressure.than.the.backward.process..The.x.dependence.of.the.transition.pressure.is.described.by.[29]

. P P p xT = −0 ( ), . (10.3)

where.P0.=.9.5.(3.0).GPa.and.p.=.20.(23).GPa.for.the.pressure.up-.(down-).stroke.[29]..The.extrapolation.to.ambient.pressure.yields,.that.MgxZn1−xO.is.metastable.and.can.occur.in.


both.structures.in.a.very.large.composition.range.of.0.13.<.x.<.0.47.[29]..This.range.could.be.responsible.for.the.different.x.values.of.the.phase.transition.reported.in.literature,.since.the.actual.phase.within.this.range.is.expected.to.depend.very.sensitively.on.the.growth.method.and.conditions.

10.2.2 Lattice Constants

Lattice.constants.reported.for.MgxZn1−xO.thin.films.grown.by.different.methods.on.differ-ent.substrates.are.shown.for.the.wurtzite.structure.phase.(c.and.a,.Refs..[24,25,28,30–45].and.this.work). in.Figure.10.3.and.for. the.rocksalt.structure.phase.(a,.Refs.. [10,35,46–48].and.this.work).in.Figure.10.4..For.wurtzite.structure,.c.decreases.by.≈.0.07.Å.with.increas-ing.Mg-content.up.to.the.largest.wurtzite.structure.Mg-content.with.x.≈.0.6..For.both,.the.wurtzite.and.the.rocksalt.structure.phase,.a.increases.from.the.binary.end.components.to.their.largest.respective.lowest.x.values.by.≈.0.03.and.≈.0.02.Å,.respectively,.showing.a.much.weaker.x.dependence.as.c..Despite.of.the.large.variation.of.values.reported.(in.the.range.of.≈.0.04.Å.for.c),.the.x.dependencies.of.the.lattice.constants.are.more.or.less.similar.for.all.samples.grown.by.different.methods,.except.of. those.grown.by.pulsed-laser.deposition.(PLD).at.very.high.or.low.oxygen.partial.pressure.(this.work)..The.lattice.constants.and.related.quantities.are.found.to.depend.almost.linearly.on.x,.defined.as

. l l x l x l x= = + + ,0 1 220( ) . (10.4)

where.li.stands.for.a.(wurtzite.or.rocksalt.structure),.c,.Vw,.Vrs,.c/a,.and.u,.and.the.bow-ing.parameters.l2.are.small..Parameters.of.Equation.10.4.are.listed.in.Tables.10.2.and.10.3..Upon.the.phase.transition.from.the.wurtzite.to.the.rocksalt.structure,.the.lattice.constants.undergo.a.remarkable.step.in.the.order.of.1.Å..The.rocksalt.structure.a-axis.is.almost.the.mean.value.of.the.wurtzite.structure.a-.and.c-axes..As.this.step.is.accompanied.by.a.change.from.fourfold.to.sixfold.coordination,.strong.discontinuities.of.physical.properties.upon.the.phase.transition.are.expected..Figure.10.5.shows.the.volume.of.the.primitive.unit.cell.

Mg content x0.0

(a) (b)

5.14

5.16

5.18

5.20

5.22

5.24

5.26

3.22

3.24

3.26 a (Å

)

c (Å

) 3.28

3.30

3.32

3.34

0.1 0.2 0.3 0.4 0.5 0.6Mg content x

0.0 0.1 0.2 0.3 0.4 0.5 0.6

FIGURE 10.3Lattice.constants.of.wurtzite.structure.MgxZn1−xO.thin.films.grown.by.various.methods..(a).c.(Refs..[24,25,28,30–45].and.this.work).and.(b).a.(Refs..[24,30,35,38,39,40,43].and.this.work)..For.a,.the.data.with.higher.values.(dashed.line).compared.to.the.whole.bunch.(solid.line,.averaging.all.data.by.a.linear.x.dependence,.cf..Table.10.2).are.obtained.for.samples.grown.by.PLD.with.very.high.and.very.low.oxygen.partial.pressure.(this.work)..Please.note.the.same.Å-span.for.the.c-.and.a-scale.


(Equation. 10.2).. The. volume. increases. with. increasing. x. within. the. wurtzite. structure.phase.and.decreases.within.the.rocksalt.structure.phase,.corresponding.to.the.behavior.of.a.and.c..Parameters.for.the.x.dependence.can.be.found.in.Table.10.3.

For.wurtzite.structure.materials,.the.ratio.c/a.and.u.(Equation.10.1).are.important.param-eters.that.are.a.measure.of.the.“hexagonality”.and.thus.of.the.electronic.properties,.such.

TABLE 10.2

Parameters.of.Equation.10.4.for.the.Lattice.Constants

Data Set x-Range

l0(x = 0) l1 l2

(Å) (Å) (Å)

a.(w).(Figure.10.3) 0–0.6 3.247 +0.053 0c.(w).(Figure.10.3) 0–0.6 5.205 −0.107 0a.(rs).(Figure.10.4) 0.6–1 4.266 −0.056 0a.(w).Ref..[39] 0–0.34 3.2491 0.047 0c.(w).Ref..[39] 0–0.34 5.2048 0.072 0c.(w).Ref..[45] 0–0.3 5.207 +0.023 −0.005

W,.wurtzite;.rs,.rocksalt.

Mg content x0.6

4.16

4.18

4.20

4.22

4.24

4.26

4.28

0.7 0.8 0.9 1.0

a (Å

)

FIGURE 10.4a-lattice.constants.of.rocksalt.structure.MgxZn1−xO.thin.films.grown.by.various.methods.(Refs..[10,35,46–48].and.this.work)..Please.note.that.the.a-scale.covers.the.same.Δa-range.as.in.Figure.10.3..The.line.corresponds.to.the.averaging.of.all.data.by.a.linear.x.dependence,.cf..Table.10.2.

TABLE 10.3

Parameters.of.Equation.10.4

Data Set x-Range l0(x = 0) l1 l2

Vw.(Figure.10.5) 0–0.6 47.58.Å3 +0.54.Å3 0Vrs.(Figure.10.5) 0.6–1 19.40.Å3 −0.75.Å3 0c/a.(Figure.10.6) 0–0.6 1.6043 −0.0598 0u.(Figure.10.6) 0–0.6 0.3795 +0.0100 0


as.the.ionicity,.the.difference.in.the.bond.length,.the.static.polarizability,.and.parameters.of.the.electronic.band.structure.like.crystal.field.interaction.energy..As.can.be.seen.in.Figure.10.6,. c/a. shrinks. and. u. increases. with. x. variation. toward. the. rocksalt. structure. phase.transition.and.becomes.significantly.smaller.and.respectively.larger.as.the.ideal.wurtzite.structure.values.(red.lines.in.Figure.10.6),.indicating.a.strong.ionicity.and.a.strong.c-axis.displacement.of.the.cation..The.static.polarization.of.ZnO.and.Mg0.15Zn0.85O.was.calculated.to.be.−5.6.and.−4.8.μC/cm2,.respectively,.reflecting.the.healing.of.the.strong.lattice.distor-tion.of.ZnO.by.incorporating.Mg.in.the.lattice.[38]..Parameters.for.the.x.dependence.of.c/a.and.u.can.be.found.in.Table.10.3.

The.pressure.(P).dependence.of.c,.a,.Vw,.and.c/a.was.investigated.in.Ref..[49].for.x.=.0.and.0.15.for.compression.up.to.11.GPa..As.expected,.a.reduction.of.the.cell.volume.by.≈.3.Å3.was.found.for.both.x.values,.whereas.pure.ZnO.is.less.compressed.than.(Mg,Zn)O.[49]..Furthermore,.a.distortion.of.the.unit.cell.by.applying.pressure.was.found,.expressed.by.c/a.=.1.6043.−.6.4P(GPa).

Mg content x Mg content x0.0

47.5

48.0

48.5

49.0

0.1 0.2 0.3 0.4 0.5 0.6 0.6 0.7 0.8 0.9 1.018.5

19.0

19.5

20.0

20.5V w

(Å3 )

V rs (

Å3 )

FIGURE 10.5 Volume.of.the.primitive.unit.cell.of.wurtzite.(Vw,.a).and.rocksalt.(Vrs,.b).structure.MgxZn1−xO,.calculated.from.the.data.in.Figures.10.3.and.10.4.by.means.of.the.Equation.10.2.and.measured.in.Ref..[38]..Please.note.the.same.Å3-span.for.Vw.and.Vrs..The.data.highlighted.by.the.red.line.corresponds.to.those.highlighted.by.the.red.line.in.Figure.10.3..The.black.lines.correspond.to.the.averaging.of.all.data.by.linear.x.dependence,.c.f..Table.10.3.

Mg content x0.0

(a) (b)

1.56

1.57

1.58

1.59

1.60

1.61

1.62

1.63

1.64

0.1 0.2 0.3 0.4 0.5 0.6Mg content x

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0.376

0.378

0.380

0.382

c/a u

0.384

0.386

0.388

FIGURE 10.6c/a.-ratio.(a).for.wurtzite.structure.MgxZn1−xO.and.cell-internal.structure.parameter.u.(b).calculated.from.the.data.in.Figure.10.3.by.means.of.Equation.10.1.and.from.Refs..[38–40]..The.dashed.lines.correspond.to.the.ideal.values.of.c/a.=. 3/8and.u0.=.3/8.for.the.wurtzite.structure..The.solid.lines.correspond.to.the.averaging.of.all.data.by.a.linear.x.dependence,.cf..Table.10.3.


for.ZnO.and.c/a.=.1.5973.−.9.1P(GPa).for.Mg0.15Zn0.85O..The.stronger.distortion.of.the.(Mg,Zn)O.lattice.reflects.the.finding.that.the.mixed.crystals.with.large.x.values,.but.wurtzite.structure.phase,.are.metastable.even.at.relatively.low.pressure.(cf..Section.10.2.1.and.Ref..[29]).

10.3 Infrared-Vacuum Ultraviolet Dielectric Function and Absorption Coefficient

The.complex.dielectric.function.�ε.of.a.material.represents.the.interaction.of.a.propagating.electromagnetic. wave. with. electrically. polarizable. oscillators. within. the. material. in.energy.transfer.(imaginary.part,.ε2).and.phase.drift,.or.rather.velocity.modification.(real.part,. ε1).. In. semiconductors,. considering. only. the. high-frequency. spectral. range,. such.polarizable.oscillators.are.usually.the.free.electron.plasma.(far.infrared.(IR).up.to.visible.(VIS).spectral.range,.not.discussed.here),.polar.phonons.(far.IR.to.near.IR.(NIR).spectral.range,.Section.10.3.1),.free.and.bound.excitons.(BXs).(VIS.to.near.ultraviolet.(UV).spectral.range,.Section.10.3.4),.electronic.band-to-band.transitions.(IR.to.vacuum.UV.(VUV).spec-tral.range,.Section.10.3.5),.and.electronic.core.level.excitations.(VUV.up.to.extended.UV.or.x-ray.spectral.range,.not.discussed.here)..Closely.related.to.the.dielectric.function.is.the.

absorption.coefficient,.α ω ω ε ω ε ω ε ω( ) ( ) ( ) ( ) ,= + −

/2

1 2

12

22

1i .which.is.a.direct.measure.of.the.energy.transfer.from.the.electromagnetic.wave.to.the.material.

In.general,.�ε.(as.well.as.α).is.a.tensor.(�̂ε).with,.in.its.principle.axis.representation,.one.to.three.independent.components,.depending.on.the.symmetry.of.the.crystal.structure.and.on.additional.physical.fields.like.strain.or.external.or.built-in.electric.fields..For.wurtzite.structure.(Mg,Zn)O,.whose.crystal.structure.belongs.to.the.hexagonal.system,.�̂ε.has.two.independent.components. ||�ε .and. ⊥�ε ,.where.∥.and.⊥.represent.the.orientation.of.the.electric.field.of.the.wave.with.respect.to.the.optical.axis.that.coincides.with.the.crystal.structure.c-axis..For.the.rocksalt.structure.(Mg,Zn)O,.whose.crystal.structure.belongs.to.the.cubic.system,.�̂ε.is.a.scalar.(�ε)..In.the.following,.we.omit.the.complex.notation.

Experimental. spectra. of. the. dielectric. function. of. MgxZn1−xO,. obtained. by. means. of.spectroscopic.ellipsometry.[50–54],.are.reported.for.the.IR.spectral.range.in.Refs..[48,55,56],.for.the.NIR-VIS-UV.spectral.range.in.Refs..[11,57–61],.and.for.the.VUV.spectral.range.in.Ref..[62]..In.Ref..[61],.ε̂.is.shown.for.temperatures.between.10.and.470.K..Spectra.of.the.dielectric.function.calculated.from.first.principles.are.presented.in.Ref..[63]..Reflectivity.spectra.are.shown.in.Ref..[64]..Experimental.spectra.of.the.absorption.coefficient.are.usually.obtained.by.transmission.measurements..In.Refs..[36,44,65–76],.α-spectra.in.the.near.bandgap.spec-tral. range.are.shown.for.room.temperature..Temperature-dependent.data.are.shown.in.Refs.. [43,77].. α-spectra. calculated. from. the. experimental. dielectric. function. spectra. are.presented.in.Ref..[78].and.calculated.from.first.principles.in.Ref..[63].

Figure.10.7.summarizes.typical.spectra.of.ε1.and.ε2.for.MgxZn1−xO.thin.films.obtained.by.spectroscopic.ellipsometry.in.the.IR-UV.(a-oriented,.x.=.0.075.[56,61].and.VUV.(c-oriented,.x.=.0.17.[62]).spectral.range..Contributions.of.the.phonon.modes.(Section.10.3.1).and.the.elec-tronic.band-to-band.transitions.as.well.as.the.related.excitonic.polarizabilities.(Sections.10.3.4.and.10.3.5).are.clearly.visible..In.the.spectral.range.between.the.phonon.mode.energies.and.the.energies.of.the.lowest.excitonic.polarizability,.ε2.is.almost.zero.and.the.material.is.trans-parent.. In. this.spectral.range,. the.refractive. indices.can.be.derived.(Section.10.3.3)..While.the.dielectric.functions.(DFs).of.MgxZn1−xO.in.the.IR.exhibit.considerable.anisotropy,.in.the.below-bandgap.spectral.range.and.above.the.bandgap.ε∥.and.ε⊥.show.only.small.differences.


10.3.1 Phonons

Phonon.mode.parameters,.such.as.frequencies,.broadening.parameters,.or.(relative).inten-sities,.are.related.to.thin.film.properties,.for.instance,.crystal.structure,.composition,.strain,.crystal. quality,. defects,. or. exciton. localization.. Hence,. the. knowledge. of. phonon. mode.properties.can.help.to.probe.thin.film.properties..Phonon.mode.properties.of.MgxZn1−xO.films.have.been.studied.by.several.groups..They.have.used.mainly.Raman.scattering,.both.nonresonant.[44,55,56,79–85].and.resonant.[81,86–90],.but.also.IR.techniques..The.IR.tech-niques. include. Fourier-transform-infrared. (FTIR). transmission. and. reflection. measure-ments.[74,91–93],.and.IR.spectroscopic.ellipsometry.(IRSE).[35,48,55,56].

Group.theory.predicts.characteristic.phonon.modes.for.both.the.wurtzite.and.the.rock-salt.crystal.structure..A.crystal.with.wurtzite.structure.contains.four.atoms.per.primitive.unit.cell,.two.of.each.atom.species..Consequently,.12.phonon.branches.exist,.3.acoustical.and.9.optical..The.optical.phonons.at.the.Γ-point.of.the.Brillouin.zone.belong.to.the.follow-ing.irreducible.representation.[94]:

. Γopt = + + +1 2 1 21 1 1 2A B E E . . (10.5)

The.branches.with.E1-.and.E2-symmetry.are.twofold.degenerated..Both.A1-.and.E1-modes.are.polar,.and.split.into.transverse.(TO).and.longitudinal.optical.(LO).phonons.with.different.frequencies.ωTO.and.ωLO..For.the.lattice.vibrations.with.A1-.and.E1-symmetry,.the.atoms.move.parallel.and.perpendicular.to.the.c-axis,.respectively..Both.A1-.and.E1-modes.are.Raman.and.IR.active,.that.is,.can.be.probed.by.Raman.scattering.and.by.IR.techniques..The.two.nonpolar.E2-modes. (E2

(1),. E2(2)). are. Raman. active. only.. The. B1-modes. are. IR. and. Raman. inactive,.

so-called.silent.modes..Table.10.4.summarizes.phonon.mode.frequencies.of.ZnO.


–50–25

0255075

255075

100125

0.06 0.08 1

A1(LO1)

A1(TO1)

A1(TO2)E1(LO1)

E1(TO2)

E1(TO1)

2 3 4 5

EA

EA–Exb

E3 E5

6 7 8 9

3

4

5

6

0

1

2

3300 500 700

a-Mg0.075Zn0.925O c-Mg0.17Zn0.83O

20 000 40 000 60 000

ε 2ε 1

ε 2ε 1

ω (cm–1)

FIGURE 10.7Real.(ε1).and.imaginary.part.(ε2).of.the.DFs.of.a-oriented.Mg0.075Zn0.925O.(IR-UV.spectral.range).and.c-oriented.Mg0.17Zn0.83O.(VUV.spectral.range).at.RT.for.polarization.perpendicular.(solid.lines).and.parallel.(dashed.lines).to. the.c-axis.determined.by.spectroscopic.ellipsometry.[56,61,62]..Optical.phonon.modes.(Section.10.3.1).and.transition.energies.(Sections.10.3.4.and.10.3.5).are.marked..Note.the.different.scales.for.the.IR-to-NIR.and.the.VIS-to-VUV.spectral.regions.


The.Γ-point.optical.phonons.of.a.crystal.with.rocksalt.structure.belong.to.the.following.irreducible.representation.[97]:

. Γopt1u= F . . (10.6)

The.F1u-mode.is.polar.and.splits.into.TO-.and.LO-modes..The.F1u-mode.is.IR.active.and.Raman.inactive..Table.10.5.summarizes.phonon.mode.frequencies.of.MgO.

Most.groups.studied.hexagonal.MgxZn1−xO.films..The.investigations.revealed.phonon.modes.that.are.characteristic.for.the.wurtzite.crystal.structure..The.phonon.mode.frequen-cies.were. found. to.be.shifting.with.x..Figure.10.8.summarizes. the.phonon.modes.with.A1-.and.E1-symmetry.versus.x..A.one-mode.behavior.with.an.additional. impurity-type.mode.(IM).was.found.for.the.phonon.modes.with.A1-symmetry,.and.a.two-mode.behavior.was.found.for.the.phonon.modes.with.E1-symmetry.[55,56].

Chen.and.Shen.[91].suggested.to.employ.the.modified.random-element-isodisplacement.(MREI).model.by.Chang.and.Mitra. [99]. to.describe. the.phonon.mode. frequency.varia-tion.with.x..A.good.agreement.with.experimental.data.was.reported.[91]..However,. the.calculation.according.to.the.MREI.model.is.not.straightforward,.because.the.impact.of.the.different.crystal.structures.of.ZnO.and.MgO.on.certain.parameters,.such.as.phonon.mode.frequencies. or. dielectric. constants,. is. yet. not. fully. understood.. For. instance,. the. MREI.model.requires.the.knowledge.of.the.phonon.mode.frequencies.and.dielectric.constants.of.the.binary.components.ZnO.and.MgO,.but.it.is.not.clear.if.the.parameters.of.cubic.MgO.can.be.used.to.model.the.properties.of.hexagonal.MgxZn1−xO.films.

Alternatively,.Ye.et.al..[86].suggested.to.use.a.quadratic.expression.to.model.the.variation.of.the.phonon.mode.frequencies.with.x:

. ω ω ω( ) ( ) ( ).x x x bx x= − + − −1 21 1 . (10.7)

TABLE 10.5

Phonon.Mode.Frequencies.of.MgO

Sample F1u (TO) F1u (LO) Reference

Bulk 401 718 [98]Film 396 727 [48]

All.values.are.given.in.units.of.cm−1.

TABLE 10.4

Phonon.Mode.Frequencies.of.ZnO

Sample E2(1) A1 (TO) E1 (TO) E2

(2) A1 (LO) E1 (LO)

Bulk 99 379 410 439 575 587Films — 377 409 437 573 585

Sources:. Ashkenov,.N..et.al.,.J. Appl. Phys.,.93,.126,.2003;.Bundesmann,.C..et.al.,.Optical.properties.of.ZnO.and.related.compounds,.in.Transparent Conductive Zinc Oxide,.Springer.Series.in.Materials.Science,. Vol.. 104,. K.. Ellmer,. A.. Klein,. and. B.. Rech. (eds.),.Springer-Verlag,.Berlin,.Germany,.2008.and.references.therein.

All. values. are. averaged. numbers. from. multiple. references. given. in.units.of.cm−1.


ω1.and.ω2.address.the.phonon.mode.frequencies.of.hexagonal.ZnO.and.MgO,.respectively,.and.b.is.a.bowing.parameter.that.is.likely.to.be.affected.by.alloy-induced.disorder.or.strain.[86]..A.good.agreement.with.experimental.data.was.reported.[86].and.can.be.seen.in.Figure.10.8..Table.10.6.summarizes.the.best-fit.model.parameters.for.the.model.curves.plotted.in.Figure.10.8..The.variation.of.the.phonon.mode.frequencies.with.x.was.often.explained.by.

Mg content x

0.0

400

500

ω (c

m–1

)ω

(cm

–1)

600

700TO/LO Bundesmann et al. (Ref. 55)

TO/LO Bundesmann et al. (Ref. 55)

Hexagonal MgxZn1–xOA1-symmetry

IM Bundesmann et al. (Ref. 55)LO Lautenschläger et al. (Ref. 84)LO Wu et al. (Ref. 90)

LO Sonawane et al. (Ref. 93)

400

(a)

(b)

500

600

700

0.1 0.2 0.3

E1-symmetry

0.4 0.5 0.6

FIGURE 10.8Phonon.mode.frequencies.of.hexagonal.MgxZn1−xO.films.[55,84,90,93]..The.solid.lines.are.model.curves.accord-ing.to.Equation.10.7.with.the.parameters.in.Table.10.6.

TABLE 10.6

Best-Fit.Parameters.of.the.Model.Curves.in.Figure.10.8.according.to.Equation.10.7

Mode A1 (TO) A1 (IM) A1 (LO) E1 (TO) E1 (LO) E1 (TO) E1 (LO)

ω1 376.(3) 562.(4) 577.(2) 406.(3) 517.(3) 512.(4) 584.(5)

ω2 367.(18) 577.(43) 618.(12) 408.(16) 515.(37) 539.(22) 688.(38)b −77.(36) −184.(30) −192.(21) −67.(73) 77.(58) −35.(45) −129.(74)

All.values.are.given.in.units.of.cm−1..The.numbers.in.parentheses.are.error.bars.of.the.curve.fitting.procedure.


alloy-induced.effects..However,.the.scattering.of.the.data.points.in.Figure.10.8.suggests.that.the.phonon.frequencies.are.impacted.by.other.film.properties,.possibly.strain.or.defects.

Some.groups.reported.also.on.the.variation.of.the.E2(1)-.and.E2

(2)-mode.frequency.with.x.[40,44,85]..For.MgxZn1−xO.films,.the.E2

(1)-.and.E2(2)-mode.were.reported.to.shift.slightly.

to.smaller. frequencies.with. increasing.x. [44,85].. In.contrast. to. that,. the.E2(1)-mode.shifts.

slightly.to.higher.frequencies.for.MgxZn1−xO.powder.samples.[40]..Again,.strain.in.addition.to.alloy-induced.effects.might.influence.the.phonon.mode.frequencies..All.groups.found.that.the.linewidth.of.E2

(1)-.and.E2(2)-mode.increases.with.increasing.x.due.to.alloy-induced.

disorder.The.appearance.of.additional.modes.was.reported,.which.are.separated.from.the.host.

lattice. modes. [55,56,85,92].. Indeed,. incorporation. of. Mg. in. ZnO. can. cause. additional.vibrational. modes.. If. the. number. of. impurity. atoms. is. small. compared. to. the. number.of.host.lattice.atoms.and.the.mode.amplitude.is.concentrated.to.a.few.atoms.around.the.impurity,. the. induced. mode. is. “localized”. (so-called. local. vibrational. mode. (LVM)).. In.Ref.. [100],. a. simple. model. for. the. calculation. of. LVM. frequencies. in. three-dimensional.(3D).crystals.was.introduced..Table.10.7.summarizes.the.calculated.local.mode.frequencies.with.A1-.and.E1-symmetry.for.the.case.that.Mg.is.incorporated.in.ZnO.[56].

In.contrast.to.the.above.described.case,.no.LVM.is.predicted.if.the.heaviest.host.lattice.atom. is. substituted. by. an. even. heavier. atom,. for. instance,. if. Zn. replaces. Mg. in. MgO.[100,101].. Additional. modes. for. hexagonal. MgxZn1−xO. films. were. mainly. reported. with.frequencies.ranging.from.512.to.535.cm−1.[55,56,85,92]..This.frequency.range.agrees.quite.well.with. the. local.mode.frequency.ωloc,1. in.Table.10.7..Hence,. the.additional.modes.are.likely.to.be.mixed.modes.that.originate.from.the.local.mode.of.Mg.in.ZnO.

Some. groups. employed. resonant. Raman. scattering. (RRS). to. study. the. properties. of.LO-modes..The.intensity.of.the.LO-modes.is.considerably.enhanced.in.RRS.because.the.photon.energy.of. the. incident. laser.beam. is. close. to. the.energy.of.an.electronic. transi-tion,.which.leads.to.a.resonance.effect..Multiple.peak.structures.corresponding.to.1LO-,.2LO-,.3LO-,.and.up.to.6LO-mode.scattering.were.experimentally.observed..First.of.all,.RRS.was.used. to.determine.phonon.mode. frequencies.of. the.LO-modes.of.MgxZn1−xO.films.[81,87–90]..However,.a.lineshape.analysis.provided.even.more.information..Refs..[86,88,89].described.that.exciton.localization.leads.to.a.relative.enhancement.of.second-order.scat-tering.due.to.an.enhanced.Fröhlich.contribution..Thus,.the.intensity.of.the.2LO-mode.is.increased.relative.to.the.intensity.of.the.1LO-mode.when.exciton.localization.is.present.

Kong.et.al.. [83]. reported.also.on. temperature-dependent.Raman.scattering.studies.of.hexagonal.MgxZn1−xO.films.with.0.≤.x.≤.0.323..The.frequencies.and.linewidth.of.the.A1(LO)-.and.E1(LO)-modes.in.the.temperature.range.from.83.to.578.K.were.determined.and.a.model.that.describes.the.temperature.dependence.was.presented.

TABLE 10.7

Local.Mode.Frequencies.of.Mg.in.ZnO

Symmetry

Mg Replaces Zn Mg Replaces O

ωloc,1 ωloc,2 ωloc,3 ωloc,4

E1 518 251 636a 375A1 492 238 604a 357

Source:. Bundesmann,.C..et.al.,.J. Appl. Phys.,.99,.113504,.2006.All.values.are.given.in.units.of.cm−1.a. According.to.[100,101].no.local.mode.is.predicted.


The.number.of.publications.on.phonon.mode.properties.of.cubic.MgxZn1−xO.films.is.lim-ited.[48,56,79,91],.mainly.due.to.the.fact.that,.according.to.group-theoretical.predictions,.cubic.MgxZn1−xO.films.should.not.reveal.film-related.Raman.modes..Hence,.Raman.scattering.can.be.used.only.to.verify.the.rocksalt.crystal.structure.of.cubic.MgxZn1−xO.films.by.showing.that.no.film-related.Raman.modes.are.detectable.[48,56,79,90]..In.order.to.study.the.phonon.mode.properties.of.cubic.MgxZn1−xO.films,.IR.techniques.have.to.be.employed.

Published.phonon.mode. frequencies.of.cubic.MgxZn1−xO.films.are.depicted. in.Figure.10.9..In.Refs..[48,56],.IRSE.was.used.to.study.cubic.MgxZn1−xO.films.with.0.69.≤.x.≤.1..A.one-mode.behavior.was.found..As.predicted.by.the.local.mode.model.mentioned.above,.no.additional.mode.originating.from.a.local.mode.of.Zn.in.MgO.was.observed..The.phonon.mode.frequencies.of.the.cubic.MgO.thin.film.agreed.well.with.values.of.MgO.single.crys-tals.(see.Table.10.5)..Starting.from.MgO,.a.systematic.decrease.of.the.TO-.and.LO-mode.frequencies.with.decreasing.x.was.observed..The.decrease.was.found.to.be.linear.and.can.be.modeled.by.the.following.equation:

. ω ω ω( ) ( )x x x= − +1 21 . (10.8)

Again,.ω1.and.ω2.address.the.phonon.mode.frequencies.of.cubic.ZnO.and.MgO,.respectively..Table.10.8.summarizes.the.best-fit.model.parameters.

Mg content x0.5

400

500

600

700

0.6 0.7 0.8 0.9 1.0

ω (c

m–1

)

TO/LO Jasperse et al. (Ref. 98)TO/LO Chen and Shen et al. (Ref. 91)TO/LO Bundemann et al. (Refs. 48,56)

Cubic MgxZn1–xOF1u

FIGURE 10.9Phonon.mode.frequencies.of.cubic.MgxZn1−xO.films.[48,56,91].and.of.a.MgO.bulk.sample.[98]..The.dashed.lines.are.linear.approximations.of.the.data.from.Ref..[48,56].according.to.Equation.10.8.

TABLE 10.8

Best-Fit.Parameters.of.the.Model.Curves.in.Figure.10.9.according.to.Equation.10.8

Mode F1u(TO) F1u(LO)

ω1 298.(6) 582.(6)

ω2 396.(2) 727.(2)

All. values. are. given. in. units. of. cm−1..The.numbers.in.parentheses.are.error.bars.of.the.curve.fitting.procedure.


Furthermore,.Refs..[48,56].describe.a.systematic.variation.of.the.phonon.mode.broadening.parameter. of. cubic. MgxZn1−xO. films.. Starting. from. MgO,. the. broadening. parameter.increases.with.decreasing.x.due.to.alloy-induced.disorder..However,.it.was.also.reported.that.growth.conditions.seem.to.have.an.impact.on.the.phonon.mode.broadening.parameter,.which.might.be.related.to.crystal.quality.

Chen.et.al..[91].described.IR.reflection.studies.of.cubic.MgxZn1−xO.films.with.0.49.≤.x.≤0.60..In.contrast.to.Refs..[48,56],.a.two-mode.behavior.with.an.additional.IM.was.reported..The.phonon. mode. frequencies. in. Ref.. [91]. do. not. fit. those. reported. in. Refs.. [48,56]. for. cubic.MgxZn1−xO.films,.but.are.similar.to.those.reported.in.Refs..[55,56].for.hexagonal.MgxZn1−xO.films..The.discrepancies. in. the.phonon.mode. frequencies.were. suggested. to.be.due. to.a.complex.phase.transition.behavior.from.wurtzite.to.rocksalt.crystal.structure.[56].

10.3.2 Dielectric Constants

The.static.as.well.as.the.high-frequency.dielectric.constants,.εstat.and.ε∞,.respectively,.have.a.large.impact.on.many-particle.effects.such.as.the.exciton.formation.and.electron–phonon.interaction.. The. exciton. binding. energy. depends. on. εstat. (Exb. ∝. εstat

−2).. The. longitudinal-transversal. splitting. of. IR-active. optical. phonons. is. determined. by. both,. εstat. and. ε∞.. For.undoped.semiconductors.εstat.and.ε∞.are.connected.with.the.n.longitudinal.and.transversal.phonon.mode.frequencies.ωLO,i.and.ω TO,i,.respectively,.via.the.Lyddane–Sachs–Teller.relation.ε ε ω ωstat LO, TO,= ∞

=

−∏ i

n

i i1

2 2 .. The. static. dielectric. constants. can. be. determined. from. the. IR.dielectric.function.spectra.with.sufficient.spectral.data.coverage.below.and.above.the.rest-strahlen.regions..For.photon.energies.far.above.the.phonon.resonances.but.still.sufficiently.below.the.electronic.band-to-band.transitions,.the.DFs.converge.to.ε∞,.which.is.equal.to.the.square.of.the.Cauchy.parameter.A.(cf..Section.10.3.3)..ε∞.measures.the.sum.of.all.linear.elec-tronic.polarizabilities.for.all.photon.energies.within.and.above.the.fundamental.band-to-band.transition.energy.until.the.shortest.end.of.the.electromagnetic.spectrum.

Figures.10.10.and.10.11.summarize.ε∞.and.εstat.of.MgxZn1−xO.thin.films.obtained.from.the.analysis.of.IR.ellipsometry.data.[48,55,56,96].and.calculated.from.the.refractive.index.

Mg content x

0.6 0.8

3.0

3.5

4.0

0.0 0.2 0.4 1.0

ε ∞

FIGURE 10.10ε∞.of.MgxZn1−xO.thin.films.obtained.from.the.analysis.of.IR.ellipsometry.data.[48,55,56,96].(open.symbols).and.calcu-lated.from.the.refractive.index.dispersion.[11,58].(filled.symbols).(cf..Table.10.9)..Dashed.lines.are.guide.to.the.eyes.


dispersion.via.the.Lyddane–Sachs–Teller.relation.[11,58]..Besides.the.disappearance.of.the.anisotropy,.both.ε∞.and.εstat.change.abruptly.upon.the.phase.transition.from.wurtzite.to.rocksalt.structure.due.to.the.coordination.number.change.(from.4.to.6).and.the.associated.change. of. bond. polarizability. (increase. in. splitting. between. TO. and. LO. phonon. mode.frequencies,. Section. 10.3.1),. and. critical-point. (CP). characteristics. of. the. two. polytypes.(Section.10.3.4)..The.general.trend.for.εstat.to.decrease.with.increasing.x.is.also.reflected.in.the.general.trend.of.the.exciton.binding.energy.to.increase.with.x.(cf..Section.10.3.4).

10.3.3 Refractive Index

The. dielectric. function. in. the. spectral. range. between. the. phonon. mode. frequencies. and.the.lowest.energy.of.the.excitonic.polarizabilities.near.the.fundamental.bandgap.is.charac-terized.by.a.vanishing.imaginary.part..Therefore,.the.real.part.of.the.dielectric.function.is.directly.connected.to.the.refractive.index..Considering.that.the.wurtzite.structure.MgxZn1−xO.is.optically.uniaxial,.the.main.refractive.indices.for.light.polarization.parallel.and.perpen-dicular.to.the.optical.axis.read.n|| ||= ε .and.n⊥ ⊥= ε ,.respectively,.and.the.birefringence.is.defined.by.Δn.=.n∥.−.n⊥..For.the.rocksalt.structure.phase,.n.is.isotropic..The.dispersion,.that.is,.the.photon.energy.dependence,.of.the.refractive.index.follows.from.the.low-energy.tail.of.the.real.part.of.the.electronic.excitations.at.higher.photon.energies.and.can.be.described.either.by.a.series.expansion.of.a.harmonic.oscillator.resulting.in.the.so-called.Cauchy.approximation.(Equation.10.9).or.with.single.oscillator.models.like.the.Sellmeier.(Equation.10.10).dispersion.relation..While.Equation.10.9.holds.only.for.energies.well.below.the.bandgap.energy,.that.is,.where.the.absorption.coefficient.becomes.negligible.[11],.the.single.oscillator.models.applies.for.energies.close.to.the.lowest.resonance.energy..Equations.10.9.and.10.10.read

.n x A x

B x C xi

k

ik k i

k kik k

( ) ( )( ) ( )

, = + +

,

=∑λ

λ λ0

2

2 4 . (10.9)

Mg content x0.6 0.80.0

6

8

10

12

0.2 0.4 1.0

ε sta

t

FIGURE 10.11εstat.of.MgxZn1−xO.thin.films.obtained. from.the.analysis.of. IR.ellipsometry.data. [48,55,56,96]. (open.symbols).and.calculated.from.ε∞.presented.in.Figure.10.10.via.the.Lyddane–Sachs–Teller.relation.using.the.phonon.mode.energies.shown.in.Section.10.3.1.(filled.symbols)..Dashed.lines.are.guide.to.the.eyes.


.n

Di

i

i( )λ λ

λ λ= +

−,

,1

2

202 . (10.10)

where.λ.denotes.the.vacuum.wavelength.and.i.indicates.the.two.polarization.directions.parallel. and. perpendicular. to. the. wurtzite-lattice. c-axis.. The. parameter. x. describes. the.composition. dependencies. of. the. Cauchy. parameters. for. the. indices. of. refraction. of.MgxZn1−xO.compounds.by.a.kth.order.polynom.[11]..λ0.denotes.the.resonance.wavelength.of.the.effective.oscillator.and.A,.B,.C,.and.D.are.parameters..A.is.connected.to.the.high-frequency.dielectric.constant.by.A = ∞ε .(cf..Section.10.3.2),.while.D.can.directly.be.corre-lated.to.the.electronic.structure.of.the.material.[36]..Parameters.of.Equations.10.9.and.10.10.for.MgxZn1−xO.are.listed.in.Table.10.9.

The.birefringence.Δn.of.wurtzite.structure.MgxZn1−xO.is.discussed.in.Refs..[58,65].only..Two.contrary.results.for.the.x.evolution.of.Δn were.found..While.in.Ref..[65].Δn.remains.positive,.that.is,.n∥.>.n⊥.for.all.x-values,.in.Ref..[58].a.change.of.the.sign.of.Δn.was.found.from.positive.for.ZnO.to.negative.for.all.x.≥.0.1..The.origin.of.this.discrepancy.remains.unclear,.but.different.structural.properties.cannot.be.excluded.to.be.responsible.for.these.findings..In.both.Refs.,.hints.for.an.increase.of.|Δn|.are.obvious.

10.3.4 Fundamental Bandgap and Excitons

In.the.photon.energy.range.of.the.electronic.band-to-band.transitions,.typical.CP.structures.in.the.dielectric.function.occur..They.are.connected.to.Van-Hove.singularities.within.the.density.of. states. in. the.electronic.energy-momentum.band.diagram. in.zero,.one,. two,.or.three.dimensions.(0D,.1D,.2D,.or.3D).commonly.abbreviated.by.M0-,.M1-,.M2-,.and,.M3-type.CPs,.respectively..With.increasing.photon.energy.starting.within.the.bandgap.of.a.semicon-ductor,.the.dielectric.function.reveals.the.fundamental.absorption.edge,.which.is.typically.of.the.3D-M0-CP.type.for.direct-gap.materials.like.MgxZn1−xO..At.higher.energies.CP.structures.occur,.which.are.often.described.as.3D-M1-,.or.equivalently.as.2D-M0-.or.M2-type.singu-larities.(cf..Section.10.3.5)..The.degeneracy.of.the.topmost.valence.bands.is.lifted.for.crystals.with.cubic.symmetry,.like.rocksalt.structure.MgxZn1−xO,.caused.by.the.spin–orbit.interac-tion.energy.and.therefore.split. twofold..For.reduced.crystal.symmetry.like.for.hexagonal.MgxZn1−xO,.the.so-called.crystal.field.is.introduced.that.lifts.the.degeneracy.further.and.the.topmost.valence.bands.are.split.threefold..Therefore,.at.the.fundamental.absorption.edge,.transitions.between.these.two-.respective.three-valence.bands.and.the.conduction.band.take.place.along.with.the.related.excitonic.polarizabilities..For.the.wurtzite.structure,.the.occur-rence.of.these.transitions.is.determined.by.symmetry.and.differs.for.polarization.of.the.light.parallel.and.perpendicular.to.the.optical.axis.[11,58,104].

Energetically.close.below.the.fundamental.absorption.edge,.the.dielectric.function.is.deter-mined.by.the.excitonic.polarizabilities..For.wide.gap.oxides,.the.ground-.and.excited.state.excitonic.contributions.to.the.dielectric.function.are.typically.more.pronounced.than.those.of.the.electronic.single-particle.band-to-band.transitions..For.MgxZn1−xO.mixed.crystals,.these.features.are.hard.to.resolve.in.linear.spectroscopic.techniques.like.ellipsometry.or.reflection.measurements.at.room.temperature.due.to.the.large.inhomogeneous.broadening.induced.by.compositional.disorder.and.the.large.thermal.(homogeneous).broadening.caused.by.the.cou-pling.to.the.lattice..Here,.modeling.of.the.experimental.data.by.lineshape.function.approaches.is. necessary. for. the. determination. of. the. dielectric. function. spectra. and. the. individual.contributions.due.to.excitons.and.band-to-band.transitions.[11,58]..By.doing.this.carefully,.


parameters.of.these.contributions.like.resonance.energy,.broadening,.and.amplitude.can.be.obtained..A.further,.widely.used.technique.for.the.determination.of.the.fundamental.band-gap.properties.are.absorption.measurements..The.problem.here.is.the.correct.interpretation.of.the.low-energy.tails.in.the.absorption.spectra,.which.can.be.impacted.by.structural.prop-erties.like.surface.roughness.or.disorder-induced.tails.in.the.absorption.coefficient.(Urbach.tail)..Furthermore,.the.correct.consideration.of.excitonic.features.is.mandatory,.which.reveal.distinct.shapes.of.the.absorption.coefficient.compared.to.the.fundamental.band-to-band.tran-sition..Especially.if.they.are.very.broad.like.in.MgxZn1−xO.mixed.crystals,.they.are.often.not.considered. in. the.analysis.of. the.absorption.coefficient. that. leads. to.widely.spread.values.of.the.bandgap.energy.reported.in.the.literature..Spectra.of.the.absorption.coefficient.in.the.

TABLE 10.9

Parameters.of.the.Refractive.Index.Dispersion.for.Equations.10.9.and.10.10

Reference x Ai0 Ai

1 Ai2

Bi0 Bi

1 Bi2 Ci

0 Ci1 Ci

2λ0,i

(nm) Di(10−2 μm2) (10−3 μm3)

[65] 0 n∥ 214.3 2.66[65] 0 n⊥ 211.4 2.60[65] 0.24 n∥ 181.5 2.4365] 0.24 n⊥ 179.3 2.37[65] 0.36 n∥ 180.1 2.32[65] 0.36 n⊥ 168.6 2.27[65]a 0–0.36 n∥ 1.915 −0.26 2.92 −2.9 1.7 −3.0[65]a 0–0.36 n⊥ 1.899 −0.25 2.85 −3.2 1.6 −3.1[58] 0 n∥ 1.966 1.81 3.6[58] 0 n⊥ 1.916 1.76 3.9[58] 0.1–0.37 n∥ 1.844 −0.78 1.81 −4.5 3.6 −4.9[58] 0.1–0.37 n⊥ 1.916 −0.57 1.76 −4.5 3.9 −4.9[103] 0.57 n 154.5 2.18[103] 0.70 n 144.1 2.07[103] 0.83 n 132.8 1.95[103] 0.87 n 131.7 1.91[103] 1 n 100.4 1.86[103]a 0.57–1 n 2.016 −0.485 0.160 0.7 1.7 −1.8 1.9 −2.5 0.7[11] 0.68–1 n 2.146 −0.508 0.083 1.6 −2.5 1.3 1.38 −1.48 0.36[36]b 0 n 197.7 2.62[36]b 0.16 n 181.3 2.37[36]b 0.20 n 173.2 2.54[36]b 0.42 n 149.6 2.24[36]b 0.56 n 137.9 2.06[69]b 0 n 199 2.57[69]b 0.16 n 202 2.34[69]b 0.23 n 181 2.29[69]b 0.30 n 193 2.17

a. Adapted.to.Equation.10.9.b. Anisotropy.is.not.discussed.


bandgap-near.spectral.range.are.reported.in.Refs..[29,30,36,43,44,63,65–76,105,146].and.of.the.dielectric.function.in.Refs..[11,57–61,63,78]..Reflectivity.spectra.are.reported.in.Refs..[39,64].

Experimental. values. for. the. energy. Eg. [24–29,33,34,36,37,39,41,42,44,45,57,59,65,.66,68–78,105–117]. of. the. fundamental. absorption. edge,. mainly. determined. by. absorp-tion. measurements,. and. of. the. lowest. band-to-band. transition. energies. EA,. EB,. and. EC.[11,39,58,61,96],.obtained.by.spectroscopic.ellipsometry,.reflectivity,.and.absorption.mea-surements,.of.MgxZn1−xO.are.shown.in.Figure.10.12.together.with.theoretically.obtained.bandgap.energy.data.[63,118–120]..The.Eg.data.cover.a.wide.energy.range,.especially.for.the.wurtzite.structure.phase.of.MgxZn1−xO.with.high.Mg-content..This.can.be.attributed.to.the.large.alloy-induced.broadening.of.the.absorption.edge.(cf..Figure.10.14),.which.pos-sibly.would.cause.a.misinterpretation.of.the.absorption.data..The.composition.evolution.of.the.energies.Ei.of.the.fundamental.absorption.edge.respective.the.band-to-band.transitions.can.be.well.described.by.Vegard’s.law.including.a.bowing.parameter.b:

. E x xE x x E x bx xi i i( ) ( ) ( ) ( ) ( )= = + − = − − .1 1 0 1 . (10.11)

Parameters.obtained.by.adapting.Equation.10.11.to.the.EA.and.Eg.data.(black.lines.in.Figure.10.12).and.to.the.EA.data.only.(blue.lines.in.Figure.10.12).are.collected.in.Table.10.10.together.with.data.from.the.literature..Even.if.the.data.scatters.a.lot,.it.seems.that.the.bowing.of.the.composition.dependence.of.the.fundamental.bandgap.is.larger.for.the.rocksalt.than.for. the.wurtzite. phase..Furthermore,. for. the.extrapolated.bandgap.of. the.ZnO.rocksalt.phase,.different.values.are.obtained.only.if.values.available.for.EA.are.considered.in.the.modeling.or.both.Eg.and.EA.data..Calculations.[121].suggest.the.indirect.gap.(Eind.∼.5.4.eV).high-pressure.rocksalt.structure.phase.of.ZnO.with.a.Γ-point.bandgap.energy.EA.∼.6.54.eV,.

Mg content x

Ener

gy (e

V)

0.6 0.80.0 0.2 0.4 1.03

4

Room temp.Low temp.

Experiment:

Theory, EA:

5

6

7

8Eg Eg,A average

EA averageEA

EB

EC

FIGURE 10.12Experimentally.obtained.energies.[24–29,33,34,36,37,39,41,42,44,45,57,59,65,66,68–78,105–117].of.the.fundamental.absorption.edge.Eg.(squares),.mainly.determined.by.absorption.measurements,.and.of.the.lowest.band-to-band.transition.energies. EA. (upright. triangles),. EB. (downright. triangles),. and.EC. (diamonds). (in. the.order.of. their.spectral.appearance).[11,39,58,61,96,146]..Filled.symbols.represent.data.for.the.wurtzite.phase.and.open.symbols.those.for.the.rocksalt.phase.of.the.ternary.MgxZn1−xO.system..The.dashed.lines.represent.calculated.bandgap.energy.data.[63,118–120]..The.solid.lines.represent.an.adaption.of.Equation.10.11.to.all.EA.and.Eg.data.(black).and.to.the.Eg.data.only.


which. is.closer. to. the.value.obtained.by.extrapolating.the.rocksalt.EA.data. to.x.=.0..The.large.variation.of.the.bandgap.of.MgxZn1−xO.(compared.to.III–V.semiconductors).is.related.to. the.so-called.bandgap.anomaly.caused. in. the.strong.contribution.of.d-orbitals. to. the.valence.band.states.in.the.Zn-rich.x-range,.which.vanishes.for.x.=.1.[29].

The. properties. of. the. fundamental. absorption. edge. of. MgxZn1−xO. are. mainly. gov-erned. by. the. excitonic. polarizabilities,. due. to. their. large. binding. energy. Exb. even. at.room. temperature.. For. the. MgxZn1−xO. system,. it. is. found. that. Exb. increases. from. the.value.of.∼.60.meV,.exhibiting.a.drop.with.increasing.x.for.small.values.of.the.Mg-content.(Figure.10.13).[11,28,57–59,61,77]..This.bowing.can.be.attributed.to.structural.disorder.in.mixed.crystals.[122]..This.disorder.is.also.reflected.in.the.broadening.parameter.Γexc.of.the.excitonic.polarizabilities,.which.increases.from.values.of.some.tens.of.meV.for.ZnO.

Mg content x0.0

0.05

0.06

E xb(

eV) 0.07

300 K4 K0.08

0.2 0.4 0.6 0.8 1.0

FIGURE 10.13Exciton.binding.energy.Exb.of.MgxZn11−xO.[11,58,59,77]..Filled.symbols.represent.data.for.the.wurtzite.phase.and.open.symbols.those.for.the.rocksalt.phase.

TABLE 10.10

Parameters.of.Equation.10.11.and.Values.for.the.Bowing.Parameter.b

Data Set x-RangeEi (x = 0) Ei(x = 1) b

(eV) (eV) (eV)

EA.and.Eg.(Figure.10.12) 0–0.8 3.339 6.470 1.89EA.and.Eg.(Figure.10.12) 0.45–1 3.679 7.675 2.48EA.(Figure.10.12) 0–0.51 3.415 6.658 1.57EA.(Figure.10.12) 0.67–1 7.316 7.687 6.60Ref..[11] 0.67–1 7.6 7.6 7Ref..[39]a 0–0.3 −4.86Ref..[118]b,c 0.62–1 3.4 7.8 0.88…4.34Ref..[119]c 0–0.34 1.34Ref..[119]c 0–1 0.56

a. Including.the.influence.of.the.spontaneous.polarization.b. Composition.dependent.c. Theory.


and.MgO.to.values.of.more.than.100.meV.for.x.values.in.the.range.of.0.5.(at.room.tem-perature,.Figure.10.14)..Furthermore,.the.absorption.edge.smears.out.with.an.exponential.low-energy.tail,.characterized.by.the.Urbach.energy.EU..As.can.be.seen.in.Figure.10.14,.EU.is.found.at.room.temperature.to.be.in.the.same.order.as.Γexc..For.lower.temperature,.the.carrier.localization.due.to.potential.fluctuations.is.more.pronounced,.leading.to.a.strong.low-energy.shift.of.Eg.as.found.from.transmission.measurements.applied.to.MgxZn1−xO.films.with.x.=.0.55.and.0.77.(Figure.10.15).[116]..The.temperature.evolution.of.Γexc.further.

Temperature (K)10

0.00

0.05Eg

EA

EB

EC

0.10

∆E (e

V)

0.15

0.20

100

x = 0.074

x = 0.77

x = 0.55

x = 0.18x = 0

E U

FIGURE 10.15Temperature.dependence.of.Eg.(squares).[77,116],.EA.(upright.triangles).[61],.EB.(downright.triangles).[61],.and.EC.(diamonds).[61].for.different.values.of.x..The.solid.lines.for.x.=.0,.0.074,.and.0.18.represent.the.temperature.evolu-tion.according.to.the.model.of.Equation.10.12.while.the.dashed.lines.for.x.=.0.55.and.0.77.are.guides.for.the.eye.only..The.actual.values.of.the.energies.for.the.different.x.values.are.obtained.by.E0.+.ΔE.(0.≤.x.≤.0.55).and.E0.+.2ΔE.(x.=.0.77)..For.the.different.x.values,.E0.is.equal.to.3.265.eV.(x.=.0),.3.4565.eV.(x.=.0.074),.3.649.eV.(x.=.0.18),.5.078.eV.(x.=.0.55),.and.6.056.eV.(x.=.0.77)..The.arrows.for.x.=.0.55.and.0.77.represent.the.energy.shift.due.to.the.Urbach.tail.

Mg content x0.6 0.8

300 K4/10 K

0.01

0.1

0.01

0.1

0.0 0.2 0.4 1.0

Г exc

(eV)

E U(e

V)

FIGURE 10.14FWHM.data.of.the.excitonic.polarizabilities.[11,28,57–61,77].(Γexc,.left.axis.and.squares).and.of.the.Urbach.band-tail.energy.[91,116].(EU,.right.axis.and.circles).in.dependence.on.the.Mg-content.x..Filled.symbols.represent.data.for.the.wurtzite.phase.while.open.symbols.those.for.the.rocksalt.phase..The.lines.are.guides.to.the.eye.


implies.that.with.increasing.Mg-content.the.inhomogeneous.part.of.the.exciton.broad-ening.strongly.increases,.reflecting.the.mixed.crystal.induced.disorder.(cf..Figure.10.16,.Equation.10.15,.Table.10.11).

10.3.4.1 Temperature Dependence

From.the.temperature.dependence.of.the.bandgap.energies.as.well.as.of.the.broadening.parameters,.conclusions.can.be.drawn.to.the.coupling.properties.of.the.electronic.system.to.the.phononic.system..The.shift.to.lower.energy.with.increasing.temperature.is.mainly.determined.by.the.polaron.coupling..Here,.usually.the.coupling.of.the.electronic.system.to.the.n.branches.of.the.phonon.band.structure.is.approximated.by.the.coupling.to.an.effec-tive.low-.and.high-energy.branch,.that.is,.n.=.2,.in.the.so-called.two-oscillator.model.[123]:

.E T E

wT

i i

i

ni i

i( ) ( )

exp /= − ( ) −

.=

∑01

1

K α θθ

. (10.12)

Temperature (K)10

Г exc

(eV)

0.01x=0

x=0.074

x=0.18

0.02

0.03

0.04

0.05

0.06

100

FIGURE 10.16FWHM.data.of.the.excitonic.polarizabilities.(Γexc).in.dependence.on.the.temperature.for.different.values.of.x.[61,77])..The.lines.represent.the.model.according.to.Equation.10.15.

TABLE 10.11

Left.Part:.Parameters.of.Equation.10.12.for.the.Temperature.Dependence.of.Ei,.Displayed.in.Figure.10.15..Right.Part:.Parameters.of.Equation.10.15.for.the.Temperature.Dependence.of.Γexc,.Displayed.in.Figure.10.16

Ei(T), Equation 10.12 Γexc(T), Equation 10.15

xEg(0 K) EA(0 K) α θ1 = θeff Γinhom Γhom(0 K) 1/β

(eV) (eV) (10−4 eV/K) (K) (meV) (meV) (10−4 eV/K)0 — 3.429 3.4 360 9 5 5360.075 — 3.620 5.1 450 22 16 3710.18 3.812 — 5.3 515 29 20 255

Please.note.that.the.used.one-oscillator.model.does.not.yield.a.real.phonon.temperature.θ1.for.both.Ei(T).and.Γexc(T),.but.is.equal.to.θeff..w1.is.identical.to.1.


Here,. the. weighting. factors. of. the. coupling. efficiencies. to. the. ith. branch. with. the. pho-non. energy. or. rather. the. equivalent. temperature. θi. have. to. fulfill.

i

n

iw=∑ =

11.. The. high-.

temperature.slope.parameter.α.is.related.to.the.spectral.integrated.phonon.density.of.states.[124,125].and.therefore.reflects.the.polaron.coupling.strength..The.spectral.distribution.of.the.polaron.coupling.strength.is.reflected.in.the.weighting.factor.relation.between.high-.or.low-energy.phonons..This.can.be.expressed.by.an.effective.phonon.temperature.θeff:

.θ θeff =

=∑wi i

i

n

1

, . (10.13)

which.accounts.for.the.spectral.distribution.of.the.density.of.states..Now,.the.dispersion.coefficient.θdisp

.

θθ

θ θdispeff

eff= −=

∏1

1

i

i

n

n . (10.14)

can.be.defined,.ranging.from.0.to.1,.where.values.above.0.5.reflect.the.strong.and.those.below.the.week.low-energy.polaron.coupling.regime.[123].

Figure.10.15.displays.the.temperature.dependence.of.Eg.[77,116].and.EA—EC.[61].for.some.Mg-contents.in.a.logarithmic.temperature.scale..For.the.low-x.data,.it.is.obvious.that.the.temperature.of.the.inflection.point.(between.100.and.200.K).of.the.temperature.evolution.of. Ei. coincides. nearly. for. all. three. Mg-contents,. reflecting. the. week. Mg. dependence. of.the.phonon.energies.for.such.a.low.Mg-content.(cf..Figure.10.8)..But.the.high-temperature.evolution. differs. significantly:. the. high-energy. slope. becomes. steeper. for. increasing. x..Parameters. for. the.approximation.of.Equation.10.12,.where,. caused.by. the. limited. tem-perature.range.where.data.are.available,.only.one.effective.phonon.branch.(n.=.1).was.con-sidered,.are. listed. in.Table.10.11..Considering.θ1.as. the.effective.phonon.energy.θeff.and.assuming.that.the.phonon.energies.θi.are.independent.of.x.(cf..Figure.10.8),.one.finds.that.with.increasing.x.the.dispersion.coefficient.θdisp.decreases,.reflecting,.even.if.the.overall.oscillator.strength.(∝.α).seems.to.increase,.a.reduced.coupling.strength.of.the.low-energy.polarons.in.the.mixed.crystals.due.to.lattice.disorder.(cf..Equations.10.13.and.10.14).

The.exciton.broadening.parameter.Γexc. reflects. the.exciton. lifetime.and. is.a.mixture.of.the.homogeneous.decay.rate.caused.in.the.electronic.dephasing.by.coupling.to.the.phonon.bath.and.of.the.inhomogeneous.decay.rate.caused.in.exciton.scattering.at.lattice.disorder.or.impurities..Both.parts.yield.a.different.temperature.dependence..While.the.inhomogeneous.rate.can.be.assumed.to.be.almost.temperature.independent,.the.homogeneous.rate.strongly.increases.with.temperature,.reflecting.the.electron–phonon.interaction.similar.to.the.reduc-tion.of. the.electronic.bandgap..Therefore,.an.expression.similar. to.Equation.10.12.can.be.assumed..Considering.the.coupling.with.one.effective.phonon.mode.(θeff).only,.it.reads

.

Γ Γ Γexcinhom hom eff

effK( ) ( )

exp /T

T= + + ( ) −

−2 1 2

01

β θθ

1 2/

. . (10.15)

with.the.parameters.having.the.same.meaning.but.different.prefactors.as.for.Equation.10.12..Γinhom.represents.the.inhomogeneous.decay.rate.that.dominates.Γexc.at.low.temperatures.


while. the. second. term. represents. the. homogeneous. decay. rate.. Please. note. the. use. of.the.reciprocal.effective.phonon.temperature.in.Equation.10.15,.which.reflects.the.weaker.increase.of.Γexc.with.T.if.the.coupling.strength.is.reduced.

Figure.10.16.displays.the.temperature.dependence.of.Γexc. [61,77].for.some.Mg-contents.in.a.logarithmic.temperature.scale..For.low.temperatures,.the.strong.increase.of.the.inho-mogeneous.decay.rate.with.increasing.x.is.obvious..At.the.temperature.that.corresponds.to.the.energy.of.the.lower.effective.phonon.branch.(between.100.and.200.K),.the.broaden-ing.starts.to.increase.due.to.the.coupling.to.the.phonon.bath..It.can.be.seen.clearly.that.the.high-temperature.evolution.differs.significantly:.the.high-energy.slope.becomes.more.flat.for.increasing.x,.reflecting.a.restricted.polaron.coupling.in.the.mixed.crystals.due.to.lattice.disorder..Parameters.for.the.approximation.of.Equation.10.15,.where,.caused.in.the.limited. temperature. range. where. data. are. available,. only. one. effective. phonon. branch.(θ1.=.θeff).was.considered.and.taken.from.the.approximation.of.Equation.10.12.to.the.tem-perature.evolution.of.the.Ei.data,.are.listed.in.Table.10.11..Besides.the.strong.increase.of.Γinhom,.the.homogeneous.low-temperature.decay.rate.Γhom(0.K).increases.with.x..One.expla-nation.could.be.the.raise.of.the.integrated.phonon.density.of.states.that.is.indicated.by.the.increase.of.α.and.β.as.well.

10.3.4.2 Pressure Dependence

The.pressure.dependence.of.Eg.in.wurtzite.structure.(at.ambient.pressure).MgxZn11−xO.has.been.studied.by.Sans.et.al.. [29].. It.was.found.that.Eg.depends.linearly.on.the.hydrostatic.pressure.P.below.the.wurtzite-rocksalt.phase.transition.(cf..Section.10.2.1).with.dE/dP.=.22.4,.22.1,.24.1,.24.9,.27.4.meV/GPa.for.x.=.0,.0.035,.0.065,.0.09,.0.13,.respectively.[29]..It.is.obvious.that.with.increasing.bandgap.the.pressure.coefficient.increases,.which.is.related.to.the.dimin-ishing.influence.of.the.d-orbitals.to.the.valence.band.with.increasing.Mg-content.[29].

10.3.5 Higher Band-to-Band Transitions

For.photon.energies.well.above.the.fundamental.bandgap.at.the.Γ-point,.the.dielectric.func-tion.is.determined.by.band-to-band.transitions.between.lower.respective.higher.valence.and.conduction.bands..These.transitions.typically.occur.at.less.symmetric.critical.points.in.the.Brillouin.zone.or.are.distributed.over.an.extended.range.in.the.momentum.space..These.transitions.are.mostly.spectrally.broadened,.but.are.very.sensitive.to.structural.or.alloy-induced.disorders.

Reports.on.the.properties.of.band-to-band.transitions.at.energies.higher.than.the.funda-mental.bandgap.are.rare..Only.Refs..[62,126].report.two.distinct.band-to-band.transitions.at.photon.energies.of.approximately.7.5.and.9.eV. in. the.photon.energy.range.above. the.bandgap.energy.and.9.5.eV.(Figure.10.17),.determined.by.means.of.spectroscopic.ellipsom-etry..There.are.considerably.less.transitions.than.in.ZnO,.where.seven.could.be.identified.in.this.energy.range.[127]..The.dielectric.function.of.these.two.broad.features.is.2D.M0-.and.3D.M2-CP.like,.respectively..These.two.transitions.are.characterized.by.a.rising.broadening.with.increasing.x,.reflecting.the.compositional.disorder..For.larger.Mg-contents,.the.low-energy.structure.disappears.and.the.high-energy.transition.further.broadens.and.becomes.almost.indistinguishable.for.x.=.1..Surprisingly,.E3.seems.to.be.almost.independent.of.x.for.the.wurtzite.structure.alloys,.as.can.be.seen.in.Figure.10.17,.and.increases.if.more.Mg.is.incorporated.in.the.mixed.crystals.in.the.rocksalt.structure.phase..From.the.independence.of.E3.of.x,.for.the.wurtzite.structure.phase.of.the.alloy.system.it.can.be.assumed.that.the.related.bands.in.the.band.structure.are.not.influenced.by.the.Mg.constituent..The.transition.


with. the. energy. E5. increases. within. the. wurtzite. and. the. rocksalt. structure. phase,. but.seems.to.become.constant.for.high.Mg-content..For.tiny.amounts.of.Zn.within.the.crystal,.this.transition.seems.either.to.become.forbidden.or.the.respective.bands.vanish.

In.Ref..[127],.the.band-to-band.transitions.at.E3.are.assigned.for.hexagonal.ZnO.to.take.place.at.the.A-point.of.the.Brilloin.zone.and.those.at.E5.at.Σ-line.between.the.M-.and.the.Γ-point,.which.have.no.counterparts.in.the.rocksalt.band.structure..This.could.explain.the.(almost).disappearance.of.both.transitions.for.x.close.to.one..Maybe,.E3.is.forbidden.in.the.rocksalt.structure.phase.and.the.observed.transition.should.rather.be.assigned.to.the.E4.transition..For.the.hexagonal.ZnO,.E2.and.E4.transitions.are.weak.and.were.found.at.the.Γ-.and.the.L-point,.respectively.[127]..From.symmetry,.these.transitions.are.expected.to.take.place.also.in.the.rocksalt.structure.MgxZn1−xO.[120].but.seem.to.be.masked.due.to.the.larger.broadening.of.the.transitions.in.the.mixed.crystals..The.transitions.in.ZnO.with.E6.and.E7.are.at.the.H-point.[127],.which.has.no.counterpart.in.the.rocksalt.band.structure,.and.are.expected.to.vanish.with.increasing.x..But.this.is.not.confirmed.due.to.the.limited.spectral.range.where.data.are.currently.available.[62,126].

10.4 Doping of (Mg,Zn)O

Besides.the.engineering.of.the.bandgap,.the.control.of.the.density.of.free.carriers.and.their. type.via.doping.is.essential. to.realize.efficient.optoelectronic.devices..Similar. to.the.case.of.ZnO,.the.group.III.elements.are.the.first.choice.for.n-type.doping.and.group.I.and.group.V.elements,.especially.nitrogen,.are.the.natural.choice.for.acceptor.dopants..However,.hydrogen.is.likely.incorporated.in.the.(Mg,Zn)O.lattice.during,.for.example,.metal-organic. vapor. phase. epitaxy. (MOVPE).. We. will. first. shortly. review. the. role. of.hydrogen.in.ZnO.

Hydrogen.can.be.incorporated.at.interstitial.site.forming.a.bond.with.lattice.oxygen.in.ZnO..The.exact.binding.site.is.still.controversially.discussed,.due.to.the.observation.of.two.different.IR.absorption.lines.at.3326.cm−1.[128],.assigned.to.the.antibonding.(AB).site.of.H,.and.at.3611.cm−1.[129].assigned.to.H.at.the.bond.center.(BC).site..While.the.association.of.the.3611.cm−1.line.with.the.local.vibration.mode.of.the.hydrogen.bound.to.oxygen.at.BC.

Mg content x0.0

7.0

7.5

8.0Ener

gy (e

V)

8.5

9.0

9.5

10.0

0.2 0.4

E5

E3

0.6 0.8 1.0

FIGURE 10.17Energies.of.the.E3.and.E5.higher.band-to-band.transitions.in.dependence.on.x.[62,126]..Filled.symbols.represent.data.for.the.wurtzite.phase.while.open.symbols.those.for.the.rocksalt.phase..The.two.energies.are.named.E3.and.E5.because.of.the.spectral.coincidence.(for.low.x).with.the.energies.equally.named.for.ZnO.[126,127].


site.is.widely.accepted,.the.3326.cm−1.still.posed.questions:.Why.can.this.line.dominate.IR.absorption.spectra.even.though.the.formation.energy.of.H.on.the.AB.site.is.0.2.eV.higher.than.that.of.H.on.BC.site.(HBC).[129,130]?.Two.alternative.models.for.the.3326.cm−1.were.developed.showing.that.hydrogen.either.passivates.a.zinc.vacancy.or.that.the.formation.energy.of.hydrogen.on.AB.site.is.lowered.in.the.vicinity.of.CaZn.[131]..However,.neither.of.the.models.was.capable.of.explain.the.thermal.stability.of.the.hydrogen-related.lumines-cence.line.at.3.3628.eV.labeled.I4.in.the.literature.[132].up.to.500°C..Janotti.et.al..proposed.hydrogen.multicentre.bonds.to.Zn.by.replacing.oxygen.HO.[133]..In.this.configuration.HO.is.a.shallow.donor.and.contributes.to.n-type.conductivity..The.multicentre.bond.model.is.capable.to.explain.the.thermal.stability.and.the.oxygen-partial-pressure.dependence.of.the.unintentional.n-type.conduction.of.ZnO..Later,.Lavrov.et.al..induced.besides.HBC.a.hydrogen-related.defect.in.ZnO.that.they.attributed.to.HO.and.which.causes.the.I4.recom-bination.line.[134]..The.recombination.line.of.HBC.was.found.at.3.3601.eV..The.ionization.energy.of.HBC.and.HO.are.determined.to.be.53.and.47.meV,.respectively.[134].

The.group.III.elements,.aluminum,.gallium,.and.indium,.substitute.zinc,.act.as.single.donors,.and.can.be.well.described.within.effective.mass.theory..In.binary.ZnO,.the.ioniza-tion.energy.of. these.donors. increases.with. increasing.atomic.number:.Ed(AlZn).=.53.meV,.Ed(GaZn).=.54.6.meV,.Ed(InZn).=.63.2.meV.[135]..Hence,.one.should.expect.that.the.resistivity.of.Ga-.but.especially.In-doped.(Mg,Zn)O.is.higher.than.that.of.Al-doped.(Mg,Zn)O.

The.binary.ZnO.is.a.potential.candidate.to.substitute.indium.tin.oxide.(ITO).as.trans-parent.conducting.oxide.used.as.an.electrode.in.flat.panel.displays.or.thin.film.solar.cells..Even. though. the.conductivity.and. the.process. temperature. in.which.high.conductivity.material.can.be.realized.favor.the.usage.of.ITO,.the.increasing.indium.price.demands.to.seek.an.alternative..Here,.ZnO.has.entered.the.market.in.the.last.years.and.especially.as.front-electrode.of.solar.cells.it.has.replaced.ITO.to.a.large.extent..Highly.conducting.ZnO.is. mostly. realized. by. doping. with. aluminum. (ZnO:Al,. often. referred. to. as. AZO). and/or.doping.by.hydrogen..Since.the.bandgap.of.ITO.is.with.3.75.eV,.about.450.meV.higher.than.that.of.ZnO,.ITO.transmits.more.light.in.the.UV..Hence,.there.are.efforts.to.increase.the.bandgap.of.ZnO.by.alloying.with.Mg.in.order.to.extend.the.bandgap.of.ZnO.and.to.improve.the.external.quantum.efficiency.of.thin.film.solar.cells.in.the.UV.

We. collected. key. results. of. reported. doping. attempts. of. (Mg,Zn)O. thin. films. and.discussed.their.influence.on.the.electrical.properties.in.the.subsequent.paragraphs.

10.4.1 Aluminum-Doping of (Mg,Zn)O

Electrical.properties.of.MgxZn1−xO:Al.thin.films.are.reported.for.0.≤.x.≤.0.6.and.Al-doping.concentrations.up.to.3.at.%..In.general,.the.resistivity.increases.for.increasing.Mg-content.at.fixed.Al-doping.level..The.origin.of.this.behavior.is.attributed.to.an.increase.of.the.effec-tive.electron.mass.me.and.an.increase.of.the.ionization.energy.Ed.of.the.AlZn.donor.with.increasing.Mg-content.

The. electrical. properties. of. (Mg,Zn)O:Al. grown. by. PLD. on. silica. glass. substrates. at.200°C.were.investigated.by.Matsubara.et.al..at.room.temperature.in.dependence.on.the.magnesium.content.ranging.from.0.to.0.42.[67]..Doping.levels.of.1.and.2.3.at.%.were.real-ized,.respectively..For.both.cases,.the.resistivity.of.the.thin.films.decreased.with.increasing.Mg-content..Further,. the.mobility.decreased.systematically.with. increasing.Mg-content..For. binary. ZnO,. the. resistivity. was. about. 3.×.10−4. Ω. cm. for. 1. at.%.Al. and. only. slightly.lower. for.2.3.at.%.Al,.and.the.Hall.effect.mobility.was.about.50.and.35.cm2/Vs,.respec-tively..The.alloying.caused.a.strong.decrease.of.the.mobility.resulting.in.values.of.about.10.cm2/Vs.and.lower.for.x.≥.0.24..For.x.=.0.42,.the.mobility.decreased.for.a.doping.level.of.


2.3.at.%.Al.to.about.6.cm2/Vs,.and.the.resistivity.increased.to.7.×.10−3.Ω.cm..The.free.elec-tron.concentration.n.decreased.only.slightly.from.7.×.1020.to.4.×.1020.cm−3.for.x.∼.0.23.for.the.higher.Al-doping.level..If.the.magnesium.content.is.increased.above.0.23,.the.free.electron.concentration.decreases.more.significantly.and.is.about.1.5.×.1020.cm−3.for.x.=.0.42.

Cohen.et.al..realized.epitaxial.(Mg,Zn)O:Al.thin.films.on.c-plane.sapphire.by.direct.cur-rent.magnetron.sputtering.at.substrate.temperatures.ranging.from.150°C.to.500°C.[136]..For.magnesium.contents.of.x.=.0.05,.0.1,.and.0.2,.the.structural.quality.and.the.electrical.properties.were.investigated.in.dependence.on.the.aluminum.concentration..In.general,.the.c-lattice.constant.decreases.with.increasing.Mg-content.for.a.given.Al-doping.con-centration,.whereas.the.c-lattice.constant.increases.with.increasing.Al.concentration.for.a.given.alloy.composition..Samples.with.low.resistivity.were.obtained.after.an.annealing.step.at.415°C.in.the.presence.of.indium..During.the.annealing,.indium.is.incorporated.into. the. thin.films.and.acts.as.an.additional.donor..This.was.monitored.by.evaluating.the. resistivity. for. different. annealing. times..A. rapid. decrease. of. the. resistivity. occurs.within. the.first.5.h.after.which. it. slowly.decreases..The.results. indicate. that. indium.is.indeed.incorporated,.acts.as.a.donor,.and.reduces.the.resistivity..For.annealed.thin.films,.the.resistivity,.Hall.mobility,.and.free.electron.concentration.exhibit.clear.dependencies.on.the.alloy.composition.and.the.Al-doping.concentration..Highest.mobilities.of.about.30.cm2/Vs.are.obtained.for.undoped,.nonalloyed.thin.films..Both,.an.increased.Al.con-centration.and.a.higher.Mg-content,.respectively,.result.in.a.decrease.of.the.room.tem-perature.Hall.mobility..The.free.electron.concentration.increases.with.the.Al-doping.level.and.is.expectedly.highest.for.the.lowest.Mg-content..The.resistivity,.being.the.product.of. mobility. and. carrier. density,. does. not. change. systematically. with. the. doping. con-centration.due.to.the.opposing.variations.of.the.mobility.and.the.carrier.concentration..Lowest.resistivity.is.achieved.for.low.magnesium.content.and.an.aluminum.concentra-tion.between.1.and.2.at.%.

Lu.et.al..used.chemical.vapor.deposition.to.grow.MgxZn1−xO:Al.thin.films.(x.=.0–0.21).with.Al-doping.concentration.up.to.7.at.%..A.growth.temperature.of.450°C.was.used,.and.the. thin.films.were.deposited.on.glass. substrates. [137]..For.an.Al-content.of.4.at.%,. the.resistivity.increases.from.about.10−3.Ω.cm.for.x.=.0.to.about.2.×.10−2.Ω.cm.for.the.highest.Mg-content. of. x.=.0.21.. In. this. range. of. alloy. composition,. the. mobility. drops. from. 6. to.2.cm2/Vs.and.the.free.electron.concentration.from.1021.cm−3.to.2.×.1020.cm−3.

The.decrease.of.the.Hall.mobility.with.Mg-content.for.a.given.Al-doping.concentration.is.on.the.one.hand.caused.by.an.increased.scattering.rate.due.to.alloy.scattering,.and.one.the.other.hand.a.change.of.the.effective.mass.results.in.changes.in.mobility..Matsubara.et. al..provided.first. evidence. that. the.effective.mass. in.MgxZn1−xO. increases.with. x. for.x.≤.0.23.by.comparing.the.free.carrier.absorption.of.ZnO.and.Mg0.23Zn0.77O.having.a.free.electron. concentration. of. 4.5.×.1020. cm−3. [67].. The. authors. observed. that. the. plasma. fre-quency.ωp.given.by

.ω

ε εpe

= ,∞

nem

2

0

. (10.16)

where.ε0.is.the.vacuum.permittivity.and.ε∞.is.the.high-frequency.dielectric.constant.of.the.investigated.material,.shifts.to.lower.frequency.for.the.alloyed.sample.and.suggest.that.the.increase.of.the.effective.mass.is.the.cause.

Lu.et.al..and.Cohen.et.al.. investigated. the.UV.absorption.of.MgxZn1−xO.thin.films. in.dependence.on.the.free.electron.concentration.n.[136,137]..For.electron.densities.exceeding.


the.density.of.states.at.the.conduction.band.minimum.states.of.higher.energy.are.filled.resulting. in.a.blueshift.of. the.absorption.edge.known.as. the.Burstein–Moss.effect..The.increase.of.the.absorption.edge.ΔE.due.to.the.Burstein–Moss.shift.is.given.by:

.∆ =

,

//E

hm

n2 3 2

2 338π r

. (10.17)

with.mr.being.the.reduced.mass.(1/mr.=.1/me.+.1/mh).If.the.free.electron.concentration.n.is.in.the.order.of.the.volume.of.excitons.aB,X.(n a∼ B,X

−3 ).or.a.higher.bandgap.renormalization.has.to.be.considered.and.causes.a.more.complicated.dependence.of.the.absorption.edge.on.free.carrier.density..For.binary.ZnO,.the.exciton.Bohr.radius.is.about.1.85.nm.(using.me.=.0.3.m0.and.mh.=.0.79.m0),.and.bandgap.renormalization.is.expected.to.influence.the.absorption.edge.for.n.>.2.×.1020.cm−3..Thus,.a.range.of.free.electron.concentrations.of.almost.two.orders.of.magnitude.can.be.used.to.determine.the.reduced.mass.mr.for.a.given.Mg-content.by.means.of.Equation.10.17..In.Figure.10.18,.experimental.results.of. the.dependence.of. the.absorption.edge.on.the.free.carrier.density.of.Lu.et.al..and.Cohen.et.al..are.depicted..The.shift.of.the.absorption.edge.for.a.given.Mg-content.is.much.stronger.for.the.samples.investigated.by.Lu.et.al.,.and.according.to.Equation.10.17.the.reduced.mass.determined.by.Lu.et.al..is.considerably.lower.than.values.published.by.Cohen.et.al..The.values.are.summarized.in.Table.10.12.together.with.values.obtained.by.

n2/3 (cm–2)0.1013 1.1013 2.1013 3.1013 4.1013

0.03

0.06

∆E

0.09

0.12

0.15

x = 0.05–0.06

x = 0.1–0.11

x = 0.2–0.21

0.18

0.21

0.24

FIGURE 10.18Dependence.of.the.absorption.edge.of.(Mg,Zn)O.thin.films.on.the.free.electron.concentration..The.lines.are.fit.to.the.data.of.Cohen.et.al..and.Lu.et.al..by.means.of.Equation.10.17.

TABLE 10.12

Reduced.Mass.of.(Mg,Zn)O.Thin.Films.as.Given.in.the.Literature.and.Values.Obtained.Considering.Data.of.Cohen.et.al..and.Lu.et.al

mr x = 0.05 − 0.06 x = 0.1 − 0.11 x = 0.2 − 0.21

Ref..[136] 0.462.m0 0.7338.m0 1.5586.m0

Ref..[137] 0.34.m0 0.4.m0 0.49.m0

Fits.of.Figure.10.18 0.51.m0 0.83.m0 1.14.m0


jointly.fitting.the.data.from.Lu.et.al..and.Cohen.et.al..In.principle,.the.dependence.of.me.and.mh.on.the.Mg-content.can.be.determined..For.that,.the.electron.effective.mass.must.be.determined.from.the.plasma.frequency.accessible.from.IR.absorption.measurements..This.value.can.subsequently.be.used.to.calculate.the.effective.hole.mass.from.the.reduced.mass.obtained.from.the.dependence.of.the.absorption.edge.on.free.electron.concentration..Up.to.know,.it.was.only.shown.that.the.plasma.frequency.decreases.for.a.given.free.electron.concentration.with.increasing.Mg-content.[67].implying.that.the.effective.electron.mass.me.increases.due.to.alloying..The.reduced.mass.mr.increases.with.Mg-content.(cf..Table.10.12)..The.spread.of.experimental.data.obtained.by.different.groups.is.large.so.far;.more.results.are.needed.to.conclude.on.me.(x).and.mh.(x).and.additional.support.by.theory.is.demanded.

Another.question.to.be.answered.is.the.dependence.of.the.ionization.energy.of.the.AlZn.donor. on. the. Mg-content.. Here,. the. position. of. the. I6a. recombination. line. being. due. to.the. recombination. of. excitons. bound. to. neutral. AlZn. [132]. can. be. used. to. calculate. the.binding. energy. EB. of. AlZn. by. using. Hayne’s. rule. [138].. In. the. study. of. Dietrich. et. al.,.MgxZn1−xO.thin.films.grown.by.PLD.were. investigated.by. low-temperature.photolumi-nescence.(PL).(cf..Section.5.3)..The.highest.Mg-content.for.which.the.I6.recombination.line.was.identified.is.0.06..For.higher.Mg-contents,.alloy.broadening.prevents.an.unambigu-ous. attribution. of. donor-BX. lines.. The. localization. energy. of. the. I6. transition. increases.with.increasing.Mg-content,.see.Table.10.13.(cf..Table.10.16)..By.using.Haynes’.rule.[139].for.recombination.of.excitons.bound.to.neutral.donors.in.ZnO.[135],.the.ionization.energy.of.the.AlZn.donor.in.MgxZn1−xO.can.be.estimated.in.dependence.of.x.and.is.depicted.in.Figure.10.19.. The. ionization. energy. increases. from. about. 53.meV. for. x.=.0.005. to. about. 90.meV.for.x.=.0.058..The. increase. in.Ed. is. in. this.range.of.alloy.composition. linear.and.follows:.Ed(x).=.51.meV.+.643.meV.×.x.

10.4.2 Gallium- and Indium-Doping of (Mg,Zn)O

The.available.literature.concerning.the.electrical.properties.of.Ga-.and.In-doped.(Mg,Zn)O,.respectively,.is.scarce.and.will.be.discussed.jointly.in.this.section.

Similar. to. the. case. of. (Mg,Zn)O:Al,. the. resistivity. of. Mgx. Zn1−xO:Ga. increases. with.increasing.Mg-content.and.is.also.due.to.a.decrease.of.the.Hall.mobility.and.the.free.elec-tron.concentration. [140,141]..The. free.electron.concentration.n. and. the.Hall.mobility.are.depicted.in.Figure.10.20.in.dependence.on.the.Ga-content.of.samples.grown.by.PLD.having.

TABLE 10.13

Localization.Energy.of.the.I6a.Transition.due.to.Recombination.of.Excitons.Bound.to.the.Neutral.Aluminum.Donor.in.MgxZn1−xO.PLD.Thin.Films

xLocalization Energy of Al0 X Transition (meV)

Ionization Energy of AlZn Donor (meV)

0.005 15 53.20.008 16 560.014 18 61.30.031 21 69.50.058 28 88.4

Source:. Meyer,.B.K..et.al.,.Phys. Rev. B,.76,.184120,.2007.Note:. The.ionization.energy.Ed.of.the.AlZn.donor.was.calcu-

lated.using.Hayne’s.rule.for.ZnO.Eloc.=.0.37Ed—4.2.meV.


three.different.alloy.compositions.[141].. In.all.cases,.n. increases.first.with.increasing.Ga.doping.level..If.the.Ga-content.is.increased.above.a.critical.value.that.is.1.and.0.5.at.%.for.Mg-contents.up.to.and.above.10.at.%,.respectively,.the.free.electron.concentration.decreases.with. increasing.Ga.doping. level..The.highest.values.realized.are.1.1.×.1020,.9.2.×.1019,.and.3.×.1019.cm−3.for.Mg-contents.of.5,.10,.and.15.at.%,.respectively..As.the.Ga-content.in.thin.films.with.5.and.10.at.%.Mg,.respectively,.exceeds.3.at.%,.the.free.electron.concentration.increases.again.slightly..For.these.doping.levels.a.ZnGa2O4.spinel.phase.was.observed.[141].

The.Hall.mobility.shows.similar.trend.for.all.alloy.compositions..First.it.increases.with.increasing.Ga-content.up.to.0.5.at.%..The.maximal.value.of.34.cm2/Vs.for.x.=.5.at.%.and.22.cm2/Vs.for.x.=.10.at.%.and.x.=.15.at.%.as.well.

The. influence. of. the. Ga-content. on. the. resistivity. and. crystallinity. of. Mg0.1Zn0.9O:Ga.thin.films.grown.at.450°C.by.molecular.beam.epitaxy.(MBE).on.sapphire.substrates.was.

0.02 0.03x

0.04 0.05 0.060.0140

50

60

E B(m

eV)

70

80

90

100

FIGURE 10.19Increase.of.the.AlZn.binding.energy.of.(Mg,Zn)O.thin.films.on.the.Mg.concentration.

Ga-content (%)

0.01

(a) (b)

1018

1019n(cm

–3)

1020

0.1 1 10

Mg0.05Zn0.95O:GaMg0.10Zn0.90O:GaMg0.15Zn0.85O:Ga

Ga-content (%)

0.01

5

10

15

20

Hal

l mob

ility

(cm

–2/V

s)

25

30

35

40

0.1 1 10

Mg0.05Zn0.95O:GaMg0.10Zn0.90O:GaMg0.15Zn0.85O:Ga

FIGURE 10.20Dependence.of.(a).the.free.electron.concentration.n.and.(b).Hall.mobility.on.Ga-content.of.MgxZn1−xO.thin.films..(Adapted.from.Wei,.W..et.al.,.J. Appl. Phys.,.107,.013510,.2010.)


investigated.by.Harada.et.al..[142]..For.Ga.concentrations.of.1018.cm−3.or.lower,.single.phase.layers.are.obtained..In.this.regime,.the.free.carrier.concentration.increases.with.increas-ing.Ga.concentration.and.leads.to.a.decrease.of.the.thin.film.resistivity..For.Ga.concen-trations.between.1020.cm−3.and.two.phases.of.binary.ZnO.and.Mg0.2Zn0.8O.are.observed..In.this.regime,.the.resistivity.decreases.rapidly.with.increasing.Ga.concentration.due.to.the.increasing.free.electron.concentration.[143]..For.Ga.concentrations.above.1020.cm−3,.the.films.are.polycrystalline,.and.the.resistivity.increases.with.increasing.Ga.concentration.

An. influence. of. the. dopant. concentration. on. the. crystallinity. was. also. observed. for.(Mg,Zn)O:In. thin.films.grown.by.spray.pyrolysis.on.glass.substrate.at.500°C. [144]..The.preferential.c-axis.orientation.observed.for.undoped.layers.is.lost.for.high.indium.concen-trations,.diffraction.peaks.corresponding.to.(1011̄).and.(101̄1).crystal.orientation.increase.in.intensity.with.increasing.In.concentration.

In.general,.structural.disorder.introduces.tail.states.in.the.bandgap.that.cause.the.absorp-tion.of.light.with.subbandgap.energy..These.tail.states.are.referred.to.as.Urbach.tails.in.the.literature..We.also.note.that.thermal.disorder.introduces.such.tail.states.whose.density.increases.with.increasing.temperature..The.absorption.coefficient.α.can.be.expressed.by.the.Urbach.form.[145].for.energies.slightly.lower.than.the.fundamental.bandgap:

.α α( ) exp

( )E T

E EE

, =−

,

0

0

g

T X. (10.18)

where.Eg.denotes.the.energy.of.the.fundamental.bandgap,.α0.a.material.parameter,.and.E0(T,X). the. Urbach. energy. representing. the. width. of. the. exponential. tail.. The. Urbach.energy.contains.contribution.from.thermal.(T).and.structural.disorder.(X).[145]:

. E K U U02 2( ) ( )T X T X, = ⟨ ⟩ + ⟨ ⟩ . . (10.19)

K.is.a.constant.and.U.is.the.displacement.of.atoms.from.their.equilibrium.positions..In.the.following,.the.Urbach.energy.of.MgxZn1−xO.[146].and.Mg0.05Zn0.95O:Ga.[141].layers.will.be.compared.for.room.temperature.for.which.the.contribution.due.to.thermal.disorder.is.the.same.and.differences.observed.can.be.attributed.to.structural.disorder.

In.Figure.10.21a,.the.Urbach.energy.of.MgxZn1−xO.[146].is.plotted.for.Mg-contents.up.to.about.34%..The.Urbach.energy.increases.from.about.30.meV.for.x.=.0.to.about.110.meV.for.x.=.0.34..The.increase.of.E0.is.within.the.error.bars.of.the.experiment.linear.for.the.whole.composition.range.and.this. is. indicated.by.the.solid. line. in.Figure.10.21a. following.the.functional.dependence:.E0(x).=.(34.31.+.1.96.×.x)meV..In.Figure.10.21b,.the.Urbach.energy.of.Mg0.05Zn0.95O:Ga.[141].is.plotted.versus.the.Ga-content..Similar.to.Figure.10.21a,.the.Urbach.energy. increases. linearly. with. increasing. dopant. concentration.. The. Urbach. energy. of.nominally.undoped.Mg0.05Zn0.95O.as.read.off.from.the.linear.fit.in.Figure.10.21a.is.roughly.42.meV..Introducing.additionally.0.05%.Ga.increases.the.Urbach.energy.to.about.100.meV.for.this.alloy.composition.

10.4.3 p-Doping of (Mg,Zn)O

The.utilization.of.ZnO-based.heterostructures.for.optoelectronic.applications. like. light-emitting.diodes.requires.control.not.only.of.carrier.density.in.the.active.ZnO.channel.but.also.in.the.surrounding.barrier.material.for.which.(Mg,Zn)O.is.one.promising.candidate..It.is.essential.that.the.barrier.material.is.ambipolar,.which.presents.a.major.challenge.for.


the.case.of.p-type.(Mg,Zn)O..In.the.following,.we.will.shortly.summarize.recent.results.on.p-doing.of.(Mg,Zn)O.hetero-.and.homoepitaxial.thin.films.

Most. experimental. results. published. deal. with. properties. of. p-type. (Mg,Zn)O. thin.films.deposited.on.amorphous.substrates.[147–154,159],.and.much.less.data.concerning.the.growth.of.p-type.(Mg,Zn)O.on.bulk.ZnO.wafers.[158,160],.on.sapphire.[156,157,161],.or.uncommon.substrate.material.for.(Mg,Zn)O.growth.like.GaAs.[155].are.available..In.any.case,.the.annealing.of.the.acceptor-doped.(Mg,Zn)O.thin.films.is.of.high.importance.for. obtaining. p-type. layers. [147,152,155–157,159–161].. Mostly,. acceptor-doped. (Mg,Zn)O.layers. are. n-conducting. in. their. as-grown. state.. The. incorporated. acceptors. can. then.be. activated. by. thermal. annealing.. In. Table. 10.14,. publications. on. p-type. (Mg,Zn)O.are. summarized.. Successful. p-doping. attempts. are. mostly. achieved. by. using. amor-phous.substrates..Structural.imperfections.like.dislocation.play.an.important.role.since.they. promote. the. formation. of. zinc. vacancies,. gettering. of. interstitial. zinc,. solubility.of.acceptor.dopants,.and,. in.case.of.P,.As,.or.Sb,.the.creation.of.complexes.of.the.kind.2VZn–XZn(X.=.P,As,Sb).[162].that.act.as.singly.chargeable.acceptors.[163,164]..A.post-growth.annealing.additionally.drives.out.hydrogen.and.leads.again.to.a.gettering.of.interstitial.zinc.and.other.metal.impurities.in.dislocations.and.grain.boundaries..The.annealing.is.predominantly.done.in.an.oxygen-bearing.ambient.to.suppress.the.formation.of.oxygen.vacancies.being.a.double.donor.in.ZnO.[165].

In.order.to.elucidate.the. importance.of.structural.defects.for.the.realization.of.p-type.(Mg,Zn)O,.we.will.discuss.properties.of.(Mg,Zn)O:P.thin.films.deposited.by.PLD.on.bulk.ZnO.substrates.[166–168]..In.the.as-grown.state,.all.thin.films.exhibit.n-type.conductivity,.which. clearly. exceeds. that. of. the. underlying. substrate.. Hall. effect. measurements. were.used.to.determine.the.free.electron.concentration.and.the.Hall.mobility.at.room.tempera-ture..The.data.is.compiled.in.Figure.10.22.for.four.series.of.(Mg,Zn)O:P.thin.films.labeled.A,.B,.C,.and.D,.respectively.(cf..Table.10.15)..For.each.series,.the.oxygen.partial.pressure.p(O2).was.varied.during.growth.between.3.×.10−4.and.0.1.m.bar..The.nominal.composition.is.given.in.Table.10.15.for.each.series.

The. free. electron. concentration. is. for. each. series. except. for. D. minimal. at.p(O2).=.0.016.m.bar..For.lower.as.well.as.for.higher.p(O2),.the.growth.conditions.are.more.Zn-rich..The.reason.for.that.is.twofold..First.of.all,.the.number.of.scattering.events.particles.undergo.from.substrate.to.target.increases.with.increasing.growth.pressure,.and.lighter.

Ga-content (%)Mg content x00.00

0

20

40

Urb

ach

ener

gy (m

eV)

Urb

ach

ener

gy (m

eV)

60

80

MgxZn1–xO Mg0.05Zn0.95O:Ga100

120

0.10(a) (b)

0.20 0.30 0.4080

100

120

140

160

180

200

2 4 6 8

FIGURE 10.21Dependence.of.the.room.temperature.Urbach.energy.on.(a).Mg-content.of.nominally.undoped.MgxZn1−xO.and.on.(b).Ga-content.for.Mg0.05Zn0.95O..(Data.are.taken.from.Wei,.W..et.al.,.J. Appl. Phys.,.107,.013510,.2010;.Jin,.C..and.Narayan,.R.J.,.J. Electron. Mater.,.35,.869,.2006.)


target.constituents.(like.oxygen).will.be.more.likely.scattered.to.higher.angles.than.the.more.heavy.target.constituents.(like.zinc)..This.results.in.Zn-rich.growth.conditions.for.high.p(O2)..At.low.p(O2),.the.desorption.of.oxygen.has.to.be.considered.and.that.results.again.in.Zn-rich.growth.conditions..For.p(O2).=.0.016.m.bar,.it.is.evident.that.(i).the.free.carrier.concentration.is.lower.for.higher.Mg-content.and.(ii).that.for.a.given.Mg-content.

TABLE 10.14

Survey.of.Literature.on.p-Type.(Mg,Zn)O

SubstrateGrowth Method

Mg-Content (at.%)

Dopant Concentration

(at.%)

Annealing Temperature

(°C)Annealing Ambient

pmax (cm−3) Reference

Glass PLD 10 2.(P) 600 O2 ∼.1016 [147]Glass PLD 11–28 0.3.(Li) — — 2.×.1018 [148]Glass PLD 10 0.4.(Li) — — 2.2.×.1017 [149]Glass Sputtering 10 ?.(N) — — 1.5.×.1014 [150]Glass Sputtering 10 ?.(Al+N) — — 3.4.×.1018 [150]Quartz Sputtering 6 ?.(Al+N) — — 1017 [151]Glass Sputtering 6 ?.(Al+N) — — 1017 [151]Glass Sputtering 4–16 0.1.(Li) 430–560 O2 1.5.×.1017 [152]Quartz USP ? ?.(Al+N) — — 8.3.×.1018 [153]SiO2 Sputtering 0.5 ?.(As) — — 2.×.1017 [154]p-GaAs MOCVD 14 ?.(As) 610 O2 2.×.1017 [155]c-Sapphire PA-MBE ? ?.(N) 600 O2 6.1.×.1017 [156]c-Sapphire PA-MBE ? ?.(N) 600 O2 6.1.×.1017 [157]n-ZnO PLD 10 2.(P) 600 O2 2.×.1018.* [158]

PLD,.pulsed-laser.deposition;.USP,.ultrasonic.spray.pyrolysis;.MOCVD,.metal-organic.chemical.vapor.deposi-tion;.PA-MBE,.plasma-enhanced.molecular.beam.epitaxy;.pmax,.highest.concentration.of.free.holes.reported;*,.net.doping.concentration.Na–Nd;.?,.alloying.or.doping.by.the.given.elements.is.stated.in.reference,.however,.doping.level.is.not.provided.

Hal

l mob

ility

(cm

2 \Vs)

10–4 10–3 10–2 10–110–41014

1015

1016

1017

1018

1019

1020

10–3 10–2 10–1

Series ASeries BSeries CSeries D

(a) (b)

n (c

m–3

)

0

25

50

75

100

125

150

175

200Series ASeries BSeries CSeries D

p(O2) (m bar) p(O2) (m bar)

FIGURE 10.22Dependence.of.the.(a).room.temperature.free.electron.concentration.and.(b).room.temperature.Hall.mobility.of.(Mg,Zn)O:P.thin.films.grown.on.bulk.ZnO.on.oxygen.partial.pressure.p(O2).applied.during.growth.

TABLE 10.15

Nominal.Composition.of.(Mg,Zn)O:P.Series.Grown.on.Bulk.ZnO.by.PLD

Series A B C D

Composition Mg0.04Zn0.96O:P0.001 Mg0.04Zn0.96O:P0.01 Mg0.06Zn0.94O:P0.001 Mg0.06Zn0.94O:P0.01


higher.free.carrier.concentrations.are.observed.for.higher.phosphorous.doping.levels..This.implicates.that.incorporation.of.phosphorous.preferentially.creates.shallow.donors.rather.than.acceptors.

The. Hall. mobility. is. highest. for. intermediate. p(O2).. It. is. higher. for. lower. Mg-content.and.for.higher.phosphorous.doping.level..The.effective.mass.of.electrons.increases.with.increasing.Mg-content.and.explains. the.differences.observed. for. series.with. lower. and.higher. Mg-content.. The. higher. mobility. observed. for. the. higher. phosphorous. doping.level. is. likely. connected. to. the. higher. free. carrier. concentration. in. those. samples. that.reduce.the.screening.length.and.with.that.reduce.the.scattering.rate.of.charged.defects.like. dislocations,. ionized. impurities,. or. grain. boundaries.. For. p(O2).=.0.1.mbar,. the. Hall.mobility. is. for. each. series. comparatively. low.. The. reason. is. the. existence. of. a. metallic.surface-conducting.layer.for.p(O2).=.0.1mbar.causing.that.the.measured.Hall.mobility.and.free.carrier.concentration.are.lower.and.higher,.respectively,.than.the.bulk.values.[166].

10.5 Exciton Recombination

10.5.1 Alloy Broadening

PL.spectroscopy.is.a.powerful.tool.to.investigate.the.carrier.recombination.and.its.dynam-ics. within. MgxZn1−xO. alloys.. Low-temperature. PL. spectra. of. (Mg,Zn)O. thin. films. are.shown.in.Figure.10.23..On.the.one.hand,.a.strong.blueshift.can.be.observed.with.increas-ing.Mg.concentration.x.due.to.the.increase.of.the.bandgap.(see.Section.10.1)..On.the.other.hand,. the. fine. structure. is. masked. by. the. alloy. broadening.. While. in. binary. ZnO,. the.different.recombination.lines.are.very.narrow.(partially.w.<.1.meV).at.2.K,.the.linewidth.w.strongly.increases.in.the.ternary.alloy.MgxZn1−xO.so.that.the.different.emissions.cannot.be.spectrally.separated.any.more.

The.PL.lineshape.of.alloys.is.strongly.influenced.by.the.random.atom.distribution.within.the.host.lattice..In.an.ideal.MgxZn1−xO.alloy,.Mg2+.cations.are.randomly.placed.with.a.prob-ability.x.on.Zn2+.host.lattice.sites,.leading.to.local.potential.variations..Within.the.theory.of.alloy.broadening,.it.is.expected.that.due.to.the.high.exciton.binding.energy.of.ZnO.[169].and.its.alloys.[58],.the.shape.of.the.exciton.is.not.strongly.influenced.by.the.random.alloy.

Photon energy (eV)3.1 3.2 3.3 3.4 3.5 3.6

x = 0.0

FX

BX

PL in

tens

ity (a

rb. u

nits

)

x = 0.015

x = 0.08

x = 0.22x = 0.33

x = 0.18

3.7 3.8 3.9 4.0 4.1 4.2 4.3

FIGURE 10.23Low-temperature.PL.spectra.of.MgxZn1−xO.thin.films.grown.by.pulsed-laser.deposition.with.x.ranging.from.0.0.to.0.33..Emission.lines.of.bound.excitons.(BXs).and.free.excitons.(FX).are.labeled.for.x.=.0.0.and.0.015..Spectra.are.shifted.for.clarity.


potential..Accordingly,.the.relative.movement.of.electron.and.hole.can.approximately.be.described.by.a.hydrogen-like.wave.function.[170,171]..The.exciton.averages.the.local.Mg.concentration.over.its.volume.V aexc B= 10 3π ,.leading.to.a.smoothly.fluctuating.potential.on.the.scale.of.the.exciton.Bohr.radius.aB..If.the.average.spans.over.a.very.large.number.of.cation.sites,.the.potential.is.approximately.Gaussian.distributed.with.a.standard.deviation:

.σ0

1=∂∂

−Ex

x xcV

g

exc

( ) . (10.20)

with. c. being. the. cation. concentration. within. the. alloy. and. ∂Eg/∂x. being. the. change. of.the.bandgap.Eg.with.Mg.concentration.x..For.low.temperatures,.the.emission.linewidth.w.should.be.approximately.given.by. w = 2 2 2 0ln σ ..However,.it.is.well.known.that.for.many.materials.Equation.10.20.severely.underestimates.the.spectral.linewidth.of.the.emission.peak.[172–175]..The.full.width.at.half.maximum.w.versus.the.Mg.concentration.x.of.vari-ous.MgxZn1−xO.thin.films.is.depicted.in.Figure.10.24.in.the.wurtzite.regime..It.can.be.seen.that.the.experimental.values.exceed.theory.from.Equation.10.20.in.the.whole.concentration.range..In.MgxZn1−xO,.this.deviation.may.be.attributed.to.a.possible.clustering.of.the.Mg2+.ions.[176].

Equation.10.20.can.be.modified.to.include.the.clustering.of.Mg.atoms.[176]:

.σcl g

exc

g

exc≈

∂∂

/ − / =∂∂

−n

Ex

x n x ncV

Ex

x n xcV

( ) ( )1 . (10.21)

Here,.n.is.the.mean.number.of.Mg.atoms.within.a.cluster..Using.∂Eg/∂x.=.1.35.+.4.8x.eV.[77],.ε(x).=.7.46.+.2.24x.[55,175,180],.and.n.=.5,.the.PL.linewidths.can.be.well.described.for.small.

Mg concentration0.0

0

20

40

W (m

eV)

60

80

100

Takagi et al. (MBE)Heitsch et al. (PLD)Kubota et al. (L-MBE)Mller et al. (PLD)Zippel et al. (PLD)Grundmann et al. (PLD)Lorenz et al. (PLD)�eoryWith clustering

120

140

0.1 0.2 0.3 0.4 0.5

FIGURE 10.24Low-temperature.PL.linewidth.of.selected.samples.found.in.literature.[27,138,168,175,177–179].in.dependence.on.the.Mg.concentration..The.corresponding.growth.method.is.given.in.the.legend..Similar.to.other.alloys,.the.PL.linewidth.is.much.larger.than.expected.from.theory.Equation.10.20..Taking.into.account.a.possible.clustering.of.the.Mg.atoms,.a.very.good.agreement.with.the.best.data.can.be.found.for.x.<.0.2..However,.for.larger.Mg.concen-trations.only.a.few.reports.can.be.found.in.literature.showing.low-temperature.(T.<.15.K).PL.data.


Mg-contents.. However,. supercell. density. functional. theory. (DFT). calculations. [181,182].indicate.that.this.is.indeed.energetically.disadvantageous..MgnO.clusters.may.be.directly.transferred. to. the. substrate. during. the. PLD. growth. process,. for. example.. However,. no.further.evidence.has.been.found.up.to.now.with.regard.to.clustering.

Another. possible. explanation,. also. applicable. to. other. semiconductor. alloys,. is. the.overestimation.of. the.exciton.radius..Within. the.currently.established. theory,. the.exci-ton.is.often.interpreted.as.a.hydrogen-like.quasiparticle.with.a.radius.aB.around.its.cen-ter-of-mass.(COM)..However,.this.ansatz.neglects.the.finite.masses.of.electron.and.hole..While.in.a.hydrogen.atom.the.nearly.resting.proton.is.much.heavier.than.the.electron,.the.effective.hole.mass.in.ZnO.differs.by.less.than.a.factor.of.4.from.the.effective.mass.of.an.electron.[135],.leading.to.the.conclusion.that.electron.and.hole.together.move.around.the.COM,.each.sampling.over.a.smaller.volume.than.expected.from.conventional.theory..The.resulting.smaller.exciton.volume.may.contribute.to.the.significant.underestimation.of.the.linewidths.

Nevertheless,.for.high.Mg.concentrations.(x.>.0.2).no.agreement.with.theory.(with.or.without.taking.into.account.clustering.phenomena).is.found..This.can.partially.be.attrib-uted.to.a.superposition.of.the.main.luminescence.peak.with.its.phonon.replicas,.which.can.be.found.on.the.low-energy.peak.side.separated.by.about.72.meV.from.each.other.[95]..The.peaks.strongly.overlap.and.cannot.be.spectrally.resolved.for.the.samples.with.high.Mg-contents,.leading.to.the.apparently.higher.PL.linewidths..However,.additional.effects. such. as. significant. degradation. in. structural. quality. (as. the. Mg. concentration.approaches.the.phase.separation.limit,.see.Section.10.2.1).may.also.contribute.to.the.large.broadening.

10.5.2 Origin of the Near-Band-Edge Luminescence

At.low.temperatures,.a.large.fraction.of.the.excitons.becomes.localized.within.disorder-induced. potential. minima.. Besides,. excitons. are. also. subject. to. binding. to. neutral. (or.charged).shallow.point.defects.[183]..In.ZnO,.excitons.bound.to.neutral.donors.or.accep-tors.are.visible.in.PL.spectra.by.additional.spectral.recombination.lines..These.can.be.seen.in. the. leftmost.spectrum.in.Figure.10.23.. In. the. ternary.alloy.MgxZn1−xO,.the.fine.struc-ture.of.the.different.BX.emissions.is.masked.by.the.alloy.broadening..However,.for.small.Mg-contents. the. main. emission. processes. in. MgxZn1−xO. can. still. be. identified. as. (D0,X).[138],.mainly.being.attributed.to.neutral.aluminum.donors.(AlZn).[132,184]..For.Mg.con-centrations.x.≤.0.03,.the.transition.of.free.excitons.(FXs).can.also.be.observed.as.a.shoulder.on. the.high-energy.side.of. the.spectrum.[138,185],.but. it. is.masked.by. the.alloy.broad-ening.for.larger.Mg-contents,.still.permitting.to.observe.the.phonon.replica.of.the.main.emission.band.[24]..As.can.be.seen.on.the.right.side.of.Figure.10.23,.only.a.single.broad,.unstructured.emission.band.can.be.observed.for.highest.Mg-contents.

Despite.the.fact.that.the.fine.structure.of.the.UV.emission.band.cannot.be.spectrally.resolved. for. higher. Mg-contents. within. the. thin. films. using. time-integrated. PL,. the.emission.from.FXs.and.BXs.can.still.be.revealed.for.Mg.concentrations.x.<.0.1.by.time-resolved.PL..In.ZnO,.FXs.exhibit.at. low.temperatures.a.very.fast.decay.mainly.due.to.nonradiative.decay.and.capture.processes.[186]..A.similar.behavior.can.be.observed.in.MgxZn1−xO,.also.explaining.the.small.contribution.to.the.time-integrated.PL.spectra..The.FXs.are.visible.at.0.ns.as.a.fast.initial.decay.(τ.<.10.ps).on.the.high-energy.side.of.the.time-delayed.spectrum.of.a.Mg0.04Zn0.96O.thin.film.(see.Figure.10.25)..The.(D0,X).peak,.which.is.the.dominant.emission.process.here,.exhibits.a.decay.time.τ.=.100.ps.similar.to.typical.decay.times.of.donor-BXs.in.ZnO.[187].


A. significant. redshift. of. the. (D0,X). emission. maximum. over. time. is. visible. in. Figure.10.25..As. the. spectra. were. measured. at. low. temperatures,. a. transfer. between. different.impurities.due.to.phonon-assisted.hopping.is.not.expected..However,.the.main.emission.band.consists.of.a.superposition.of.different.impurity.types..In.addition,.the.localization.energy.for.a.single.defect.depends.on.the.local.composition.of.the.mixed.crystal.and.the.corresponding.alloy.potential..According. to.Rashba.and.Gurgenishvili. [188],. the. transi-tion.matrix.element. for. the.BX.decreases.with. increasing. localization.energy,. leading.to.an.increasing.decay.time..This.manifests.in.the.observed.redshift.of.the.time-dependent.emission.maximum..Additionally,.tunneling.transfer.between.neighboring.impurities.may.contribute.to.the.observed.behavior.[176].

Luminescence.transients.from.several.samples.with.different.Mg-contents.are.depicted.in.Figure.10.26..While.MgxZn1−xO.samples.with.low.Mg-contents,.similarly.to.ZnO,.gener-ally.exhibit.a.very.fast.luminescence.decay.at.low.temperatures,.a.slow.decay.component.can.be.observed.for.x.≥.0.06..It.was.already.observed.by.Kubota.et.al..[177].and.later.identi-fied.by.Müller.et.al..[178].to.be.attributed.to.excitons.localized.within.alloy.potential.fluc-tuations..In.pure.ZnO,.a.radiative.recombination.of.FXs.is.only.possible.for.nearly.resting.quasiparticles.due.to.momentum.conservation..The.scattering.at. impurities.and.defects.weakens.this.transition.rule,.leading.to.the.short.decay.times.

In.alloys,. the.coupled.electron-hole.wave. function. is. strongly. influenced.by. the.alloy.potential. fluctuations.. The. momentum. conservation. can. be. fulfilled. for. a. significantly.larger.number.of.states.in.comparison.to.pure.ZnO..On.the.one.hand,.this.leads.to.the.alloy.broadening..On.the.other.hand,.this.also.results.in.a.reduction.of.the.exciton.coherence.volume. (either. the.wave. functions.are. strongly.confined.within. local.potential.minima.or,. in. the. case. of. delocalized. excitons,. incoherently. oscillating). and. electron-hole. over-lap,. leading. to. the. significantly. increasing. radiative. lifetimes.. For. small. Mg. concentra-tions,.the.barrier.heights.separating.different.minima.within.the.alloy.potential.are.too.small. to. allow. the. formation. of. strongly. localized. states.. Therefore,. excitons. may. still.move.nearly.freely.within.the.crystal.and.are.captured.at.impurities.or.reach.nonradiative.


2.0 ns

1.5 ns

1.0 ns

0.5 ns

Inte

nsity

(a. u

.)0.1 ns

0.0 ns

3.38 3.39 3.40 3.41 3.42

FIGURE 10.25Time-delayed.PL.spectra.of.a.Mg0.04Zn0.96O.thin.film.recorded.at.5.K..While.the.fine.structure.of.the.low-temper-ature.emission.cannot.be.observed.by.time-integrated.PL.as.it.is.masked.by.alloy.broadening,.the.fast.decay.of.the.free.excitons.is.visible.at.small.times.after.excitation.using.time-resolved.PL.


recombination.centers..As.the.mean.barrier.heights.grow.for.increasing.Mg-contents,.the.contribution.of.alloy-localized.states.strongly.increases.

In.Figure.10.27,.typical.time-delayed.PL.spectra.for.a.Mg0.18Zn0.82O.thin.film.on.a-sapphire.substrate. are. depicted.. For. small. delay. times,. the. spectrum. is. dominated. by. the. fast.process.attributed.to.BXs.with.its.emission.maximum.at.3.694.eV,.while.the.second,.slow.process.due.to.alloy-localized.states.becomes.visible.on.the.high-energy.side.after.a.few.

Time (ns)0 10

Inte

nsity

(a. u

.)

20

x=0.04

x = 0.08

x = 0.18

x = 0.33

30 40 50

FIGURE 10.26Low-temperature. PL. transients. of. different. MgxZn1−xO. samples. with. x.=.0.04,. 0.08,. 0.18,. and. 0.33.. While. for.x.=.0.04,.only.the.fast.luminescence.decay.mainly.attributed.to.donor-bound.excitons.can.be.observed,.the.sam-ple.with.x.=.0.08.already.shows.a.pronounced.slow.contribution.due.to.excitons.localized.within.alloy.potential.fluctuations.


170 ns

80 ns

30 ns

12 ns

5 ns

2 ns1 ns

0 ns

3.64 3.68 3.72 3.76

Inte

nsity

(a. u

.)

FIGURE 10.27Time-delayed.spectra.of.a.Mg0.18Zn0.82O.sample..The.fast.decay.on.the.low-energy.side.of.the.spectrum.is.due.to. donor-bound. excitons. (denoted. by. squares),. and. the. slow. decay. is. attributed. to. alloy-localized. excitons.(denoted.by.circles).


nanoseconds.and.subsequently.dominates.the.time-delayed.spectra..Despite.the.relatively.high.Mg.concentration.of.0.18,.donors.induce.low-energy-BX.states.

The. non-exponential. decay. characteristics. allow. the. differentiation. between. the. two.emission.bands. [178]..While. the. fast.component. from.the.donor-BXs.can.be.empirically.fitted.by.a.stretched.exponential.decay.[189,190],

. f t a ts( ) exp( ( ) )= − / ,τ α . (10.22)

with.the.initial.decay.time.τ.and.the.exponent.α.indicating.the.deviation.from.a.monoex-ponential.decay,.the.slow.decay.process.can.be.described.by.a.power.law.function.[191,192]:

. f t a t tp( ) ( )= / + /1 0β . (10.23)

with. the. time.constant. t0.and. the.exponent.β..Extracting. the. intensity.ratios.of. the. two.contributions.as.a.function.of.the.photon.energy,.it.is.possible.to.determine.the.spectral.maxima.of.both.processes..For.this.sample,.the.energetic.distance.is.15.meV,.similar.to.the.exciton.localization.energy.on.aluminum.donors.in.ZnO.[132].

10.5.3 Temperature-Dependent Localization Effects

The.PL.peak.energy.of.MgxZn1−xO.alloys.exhibits.an.S-shape.dependence.for.decreasing.temperature..First,.a.blueshift.of.the.FX.peak.energy.EFX.occurs.due.to.the.increase.of.the.bandgap..Further,.a.redshift.appears.that.is.caused.by.the.thermalization.of.excitons.in.the.disordered.system..Finally,.another.blueshift.occurs.due.to.the.frustration.of.the.thermal-ization..We.note.that,.generally,.exciton.localization.plays.a.role.only.for.temperatures.at.which.excitons.are.stable..For.MgxZn1−xO,.exciton.dissociation.takes.place.well.above.room.temperature.[58].

A. respective. theory. of. exciton. localization. in. disordered. systems. was. published. by.Runge.[193]..In.general,.a.so-called.Stokes.shift.S.of.the.luminescence.(compared.to.absorp-tion).occurs.in.an.inhomogeneously.broadened.system.[194]:

. S T kT= γ( ) , . (10.24)

whereT.is.the.temperaturek.the.Boltzmann.constant

Assuming.a.Boltzmann.population.of.a.density.of.states.with.Gaussian.broadening,.Equation.10.24.can.be.simplified.to.S0.=.σ2/kT.[195].and.thus.γ0.=.(σ./.kT)2..Generally,.S.≤.S0.and.0.≤.γ.≤.γ0.[176]..The.Stokes.shift.and.therein.γ(T).can.be.determined.by.numerically.solving.[196]

.γ γ β γ γ ε

exp ( )exp= − −

0

�

kT. (10.25)

with. �ε .being.the.exciton.transport.barrier.and.β.being.the.ratio.of.carrier.recombination.and.carrier.transport.time.


Although.this.model.can.explain.the.S-shaped.shift.of.the.PL.maximum.in.many.alloys.and.heterostructures,.it.does.not.take.into.account.the.exciton.binding.at.shallow.defects,.which.can.be.observed.in.(Mg,Zn)O.(see.Section.10.5.2).

For. smallest. x,. FX. and. BX. peak. can. be. followed. individually. over. large. temperature.ranges.. The. peak. energy. of. the. BXs. EBX. is. given. as. EBX.=.EFX−Eloc. with. Eloc. being. the.localization.energy..Eloc.is.directly.connected.to.the.impurity.ionization.energy.via.Hayne’s.rule.[132,139].and.displays.the.energy.necessary.to.remove.the.exciton.from.the.impurity.[197]..For.x.>.0.03,.the.FX.emission.is.masked.by.the.alloy.broadening.and.the.dominant.recombination. line. appears. as. superposition. of. FX. and. BX. emission.. In. this. case,. tem-perature-dependent.PL.spectra.exhibit.an.S-shape.dependence.of.the.main.luminescence.energy.similar.to.the.one.induced.by.disorder.[138]..The.peak.energy.shifts.to.lower.ener-gies.with.increasing.temperature.due.to.the.shrinkage.of.the.bandgap,.but.undergoes.an.additional.blueshift.in.an.intermediate.temperature.regime.(T.=.50.K.in.Figure.10.28).due.to.exciton.ionization.from.the.point.defects.[176].

Subsequently,. one. can. deduce. that. both. disorder. and. shallow. point. defects. can. be.responsible. for. a. temperature-dependent. S-shape. of. the. peak. energy.. They. represent.competing.exciton.localization.effects..Only.PL.spectra.of.MgxZn1−xO.thin.films.having.a.very.limited.range.of.Mg-contents.(x.≤.0.03).clearly.show.both.phenomena.separately.[138],.as.can.be.seen.in.Figure.10.28.for.x.=.0.03..There,.both.BX.and.FX.emission.can.be.spectrally.resolved..Additionally,.a.slight.S-shape.can.be.seen.for.the.BX.emission.due.to.disorder..In.all.other.cases.(x.≥.0.04),.both.channels.cannot.be.observed.spectrally.separated.due.to.large.broadening.w.>.Eloc.and.a.complex.dependence.EPL(T).develops.

The. intensity.of. recombination. lines. from.donor-BXs. IBX(T). is. found. to.decrease.with.increasing.temperature.due.to.ionization.of.excitons.from.the.impurities.[197]..Assuming.a.Boltzmann.factor.between.BX.and.FX.densities,.one.can.derive.[176]

.I T

nn

CNN

EkT

BXBX

X

D

D( ) ’ exp

’= = + −

−

1 0

1

. (10.26)

Energy (eV)(a) (b) Temperature (K)

PL in

tens

ity (a

. u.)

3.24 3.27

x = 0.005 x = 0.005 x = 0.03 x = 0.063.30 3.33 3.36 3.39 0

–50

–40

–30

–20

–10

0

10

20

100 200 300 0 100 200 300 0 100 200 3005102030

BX

BX

∆E (m

eV)

BX

BX

FX

FX

FX

FX

T (K)

40

5060708090100110125140160180200230260290

FIGURE 10.28(a).Temperature-dependent.PL.spectra.between.5.and.290.K.of.a.Mg0.005Zn0.995O.thin.film..Spectra.are.shifted.vertically.for.clarity..Transitions.FX.and.BX.are.labeled.for.5.K..(b).Temperature.dependence.of.FX.(triangles.up).and.BX.(triangles.down).emission.energy.in.MgxZn1−xO.alloys..The.energy.position.is.given.relative.to.low-temperature.maximum..Solid.lines.are.guide.to.the.eyes.


with.nBX.and.nX.being.the.density.of.the.BXs.and.the.total.density.of.excitons.due.to.exter-nal.excitation,.respectively..C′.represents.the.ratio.of.degeneracies.of.the.involved.levels.(C′.=.gFX/gBX). and. E′. is. the. activation. energy. of. the. BX. decay.. E′. is. found. to. be. close. to.Eloc.[197]..The.ratio.of.total.donor.and.neutral.donor.concentration.N ND D/ 0 .can.be.modeled.within.standard.theory.[1]:

.

NN

gNN

DkT

D

D

D

C0

1 2 1

1 2 1 1 4= − + +

−

ˆ exp/

. (10.27)

where.ĝ.is.the.degeneracy.factor,.D.is.the.donor.activation.energy,.and.NC.is.the.conduction.band.density.of.states.approximately.given.as

.N

m kTC

e=

22 2

3 2* /

π�. (10.28)

with.effective.electron.mass.me∗..A.respective.model.for.the.intensity.of.the.FX.recombina-

tion.IFX(T).is.given.by.[176]

.I T

nn

T Tm

FXBX

X

FX

FXnr

/( )

( )= −

+ +

−

1 110

0

1

ττ

. (10.29)

with.τFX0 .and.τFX

nr .being.the.low-temperature.radiative.and.the.nonradiative.carrier.lifetime.of.FXs,.respectively..Values.T0.and.m.are.chosen.empirically..A.fit.of.the.individual.tem-perature-dependent.intensities.of.free.and.BX.recombination.is.only.possible.for.samples.showing.a.clear.separation.between.both.channels.and.can.be.seen.in.Figure.10.29a.for.

1000/T (1/K)10 100

1000/T (1/K)

FX

(a) (b)

FX

BX

BX

MgxZn1–xO

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.)

10

X = 0.03

MgxZn1–xO

X = 0.06

10–3

10–2

10–1

100

10–3

10–2

10–1

100

100

FIGURE 10.29Temperature-dependent.PL.intensity.from.MgxZn1−xO.alloys.with.(a).x.=.0.03.and.(b).0.06..(a).BX.(triangles.down).and.FX.(triangles.up).contributions.can.be.observed.individually.and.fitted.by.Equations.10.26.and.10.29.(solid.lines)..(b).The.solid.line.represents.a.fit.of.the.intensity.decay.of.the.single.broad.emission.band,.and.the.dashed.lines.represent.a.fit.considering.the.contributions.of.BX.and.FX.recombination.according.to.Equations.10.26.and.10.29.


x.=.0.03.. In. contrast,. for. larger. Mg-contents. only. one. broad. band. can. be. observed. (see.Figure.10.29b)..The.derived.fit.parameters.together.with.respective.localization.energies.can.be.found.in.Table.10.16.

An.increase.of.the.donor.localization.energy.Eloc.and.the.thermal.activation.energy.ET.can. be. observed. with. increasing. Mg-content.. This. indicates. an. increase. of. the. binding.energy.of.the.involved.donor.and.therefore.amplifies.the.observed.S-shape.for.samples.with.Mg-contents.above.x.=.0.058,.as.the.blueshift.attributed.to.the.transfer.of.excitons.from.impurity-bound.to.free.states.spans.a.larger.energy.range.for.larger.Mg-contents.[138].

As.mentioned.above,.the.energy.difference.between.BX.peak.and.the.emission.maximum.of.excitons.localized.within.alloy.potential.fluctuations.has.been.found.to.be.similar.to.AlZn.localization.energy.in.ZnO.[178].(see.Section.10.5.2)..Indeed,.this.does.not.contradict.the.significant.increase.of.Eloc.observed.for.samples.with.small.Mg.concentrations:.Due.to.the.large.potential.fluctuations.within.the.samples.with.x.>.0.06,.at.low.temperatures.the.hot.excitons.relax.nearly.immediately.after.their.creation.into.local.potential.minima.attrib-uted.to.regions.of.low.Mg-content..This.leads.to.a.redshift.of.the.emission.peak.compared.to.the.donor.levels.that.are.randomly.distributed.within.the.crystal.potential.landscape.

10.6 (Mg,Zn)O Heterostructures

The.large.exciton.binding.energy.in.ZnO.allows.the.fabrication.of.highly.efficient.excitonic.devices.operating.at.room.temperature..Light-emitting.diodes.(LEDs).[198].and.lasing.from.ZnO.structures.[199].were.reported..The.internal.quantum.efficiency.of.ZnO-based.devices.can.be.improved.by.the.formation.of.heterostructures.with.its.ternary.alloys.[10,200]..Both.group-VI.and.group-II.elements.can.be.replaced..Oxygen.atoms.were.successfully.substi-tuted.by.sulfur.[201].and.selen.[202],.substituting.Zn,.the.materials.(Mg,Zn)O.[203],.(Cd,Zn)O.[204],.and.(Be,Zn)O.[10].were.proposed.in.literature,.and.even.quaternary.(Be,Mg,Zn)O.[205].and.(Mg,Cd,Zn)O.[30].were.considered..(Mg,Zn)O.and.(Be,Zn)O.exhibit.a.larger.bandgap.than.pure.ZnO..The.bandgap.in.(Cd,Zn)O.is.reduced.in.comparison.to.ZnO,.but.the.fabrication.is.problematic.because.of.the.low.solubility.of.Cd.in.ZnO.[30].

In.this.chapter,.the.focus.will.lie.on.the.ZnO/MgxZn1−xO.material.system..It.allows.the.fabrication.of.type-I.heterostructures.[206]..For.both.electrons.and.holes,.the.potential.energy.is.lower.in.ZnO.than.in.the.(Mg,Zn)O.layers..If.ZnO.is.grown.between.two.(Mg,Zn)O.layers.

TABLE 10.16

Parameters.from.Fitting.the.Temperature-Dependent.BX.Emission.Intensities.of.MgxZn1−xO.Thin.Films.Using.Equation.10.26

xEloc

(meV)ET

(meV)E′

(meV) C′D

(meV)ND

(cm−3)

0.005 15 16.5 15 — — —0.03 21 22 18 340 52 1.×.1016

0.06 28 27 28 1200 80 4.×.1016

Sources:. Grundmann,.M..and.Dietrich,.C.P.,. J. Appl. Phys.,. 106,.123521,.2009;.Dietrich,.C.P..et.al.,.New J. Phys.,.12,.033030,.2010.


and.its.thickness.is.sufficiently.small.so.that.quantization.effects.occur,.such.a.structure.can.be.regarded.as.a.quantum.well.(QW).with.finite.barriers.leading.to.an.increase.of.the.recombination.efficiency.

Both.ZnO.and.MgxZn1−xO.crystallize. in. the.wurtzite. structure. for.Mg-contents.up. to.x.<.0.5.[27]..Therefore,.both.show.a.spontaneous.polarization.parallel.to.the.crystallographic.c-axis.into.the.direction.of.the.O-face.determined.side.as.indicated.in.Figure.10.30a.

High-quality. heterostructures. grow. pseudomorphic. and. therefore. at. least. one. of. the.layers.is.strained..The.polarization.necessarily.contains.a.piezoelectric.component,.which.is.also.discontinuous.at.the.heterointerface..Additionally,.the.spontaneous.polarization.in.MgxZn1−xO.is.a.function.of.the.Mg-content.x,.leading.to.an.additional.discontinuity.at.the.heterointerface..If.the.layer.sequence.is.organized.in.a.fashion.that.the.heterointerfaces.are.perpendicular.to.the.crystallographic.c-axis,.an.electric.field.forms.along.the.growth.direc-tion.due.to.the.discontinuity.of.the.polarization..Such.a.stacking.sequence.is.very.common.due.to.the.preferential.growth.of.ZnO.and.its.compounds.along.the.c-axis.[95,207–209]..All.samples.discussed.in.this.part.have.their.growth.direction.parallel.to.the.crystallographic.c-axis..The.piezoelectric.polarization.counteracts.the.change.in.spontaneous.polarization.in.c-axis-oriented.growth.and.might.in.principle.cancel. its. influence.completely..Such.a.behavior.is.suggested.in.some.studies.for.small.Mg-contents.[210]..However,.for.x.>.0.15,.a. finite. change. in. the. polarization. at. the. heterointerface. is. evident. from. most. studies.[210–213]..The.change.in.polarization.will.produce.an.electric.field.across.the.QW,.leading.to.the.occurrence.of.the.quantum-confined.Stark.effect.(QCSE).

The.electric.field.adds.a.linear.change.in.the.potential.parallel.to.the.growth.direction,.modifying. the.potential. landscape. into.a. triangular. shape..Electrons.and.holes.will.be.separated.by. this.built-in.electric.field,. leading. to.a.decrease. in.electron.and.hole.wave.function.overlap..Therefore,.the.oscillator.strength.decreases.and.the.radiative.lumines-cence.decay.time.increases.[214–216]..Additionally,.the.transition.energy.decreases.due.to.the.lowering.of.the.electron.energy.level.and.rise.of.the.hole.energy.level..With.increasing.well.width,.the.luminescence.maximum.will.shift.even.beneath.the.recombination.energy.of.FXs.in.bulk.ZnO.[213–215,217]..Also,.the.separation.of.the.electrons.and.holes.leads.to.an.increasing.exciton.Bohr.radius,.resulting.in.a.decrease.of.the.exciton.binding.energy.via.the.Coulomb.interaction..In.experiments,.this.manifests.itself.in.an.increase.of.the.exciton.phonon.coupling.and.increasing.Huang–Rhys.factor.[217–220].

All. these.considerations.hold.true. for.single.excitons.. In.a. typical.PL.experiment,. the.optical. excitation. will. generate. an. electron-hole. plasma.. Electrons. and. holes. thermally.relax.into.the.respective.minima,.accompanied.by.exciton.formation..During.this.process,.however,. additional. radiative. recombinations. can. take. place,. resulting. in. photons. with.

MgZnOZnO

MgZnOZnO

ZnOMgZnO

a-sapphire(a) (b)

PP

P

P

a-sapphire

–σ–σ +σ+σ

ZnO substrate

MgZnO

(000

–1)

FIGURE 10.30Illustrations. of. (a). ZnO/MgxZn1−xO. single. heterostructures. grown. on. sapphire. substrate. and. (b). a. wedge-shaped.double.heterostructure.grown.on.ZnO.(0001̄)..Indicated.are.the.surface.polarities.differing.in.both.cases..Sheet.charges.σ.appear.at.the.interfaces.due.to.a.difference.in.the.polarization.


higher.energy.than.the.characteristic.luminescence..Additionally,.the.high.density.of.free.carriers. leads.to.a.partial.screening.of.the.polarization-induced.interface.charges.in.the.system.and.therefore.a.reduction.of.the.electric.field.in.the.QW..The.QCSE.will.therefore.be.diminished.and.a.blueshift.of.the.luminescence.is.expected.with.increasing.excitation.density,.which.indeed.has.been.observed.in.the.ZnO/(Mg,Zn)O.system.[215,221].

As.the.QCSE.strongly.depends.on.the.Mg.concentration.in.the.(Mg,Zn)O.layers,.an.idea.for.wedge-shaped.samples. [222].as. indicated. in.Figure.10.30b. is.noteworthy.. In.spite.of.having.multiple.samples.of.a.single.thickness.each,.wedge-shaped.samples.feature.a.thick-ness.gradient.resulting.in.different.QW.widths.within.one.sample..This.has.the.advantage.of.an.equal.composition.of.the.(Mg,Zn)O.barrier.layers.for.all.QW.widths.

In.literature,.the.QCSE.in.ZnO/(Mg,Zn)O.QW.structures.has.been.observed.in.samples.grown.by.metal-organic.chemical.vapor.deposition.(MOCVD).[223],.MBE.[213–215,224–226],.laser-MBE.[217–220],.and.recently. for.PLD.[227]-grown.samples..However,. there.are.also.reports.on.ZnO/(Mg,Zn)O.QWs.not. showing.any.effects.of.QCSE.despite. the. large.well.widths. attained. in. these. studies,. for. example,. samples. grown. with. MBE. [27,228],. laser-MBE.[229],.and.PLD.[179,230–232]..The.absence.of. the.QCSE.can.be.understood.in.terms.of.interface.abruptness.[214,224,227,233]..As.shown.by.Davis.et.al..[214],.the.QCSE.can.be.diminished. by. ion. implantation. of. MBE-grown. ZnO/(Mg,Zn)O. QW. samples.. They. con-cluded.that.intermixing.the.interfaces.smoothens.the.triangular.well.potential,.resulting.in.an.increased.wave.function.overlap.of.electron.and.hole.inside.the.well.and.an.increased.transition.energy.

Looking.at.the.PLD.process,.for.example,.the.energy.of.the.fast.ionic.species.in.the.PLD.plasma.can.be.as.high.as.hundreds.of.eV.[234,235]..The.topmost.layers.of.a.growing.film.are. therefore.subject. to.a.continuous. ion.erosion. [234,236],. leading. to.an. intermixing.of.adjacent. layers. and. softening. of. the. otherwise. abrupt. discontinuity. of. conduction. and.valence.band.at.the.QW.interfaces..The.intermixing.in.the.PLD.process.can.be.strongly.reduced.by.a.reduction.of.the.laser.fluence.applied.to.the.target.[227]..The.QCSE.is.there-fore.a.sensitive.tool.to.judge.the.interface.abruptness.in.polar.QW.structures.

In.the.following,.we.will.present.data.of.low-temperature.(T.=.2.K).time-integrated.and.time-resolved.PL.of.ZnO/(Mg,Zn)O.QW.structures..Performing.such.measurements.on.QWs,.one.has.to.carefully.consider.the.wavelength.of.the.excitation.laser..While.the.time-integrated.PL.spectra.are.nearly.independent.of.the.excitation.wavelength,.a.change.in.the.carrier.dynamics.of.the.excitons.inside.the.QWs.is.visible.when.the.excitation.energy.is.tuned.below.or.above.the.bandgap.of.the.barrier.layers.[179],.as.can.be.seen.in.Figure.10.31.for.a.1.5.nm.thick.QW.

In.the.case.of.excitation.below.the.bandgap.of.the.(Mg,Zn)O.barriers,.a.single.expo-nential.decay.is.visible,.while.in.the.case.of.excitation.above.the.bandgap.of.the.barrier.layers.an.additional.process.is.visible.in.the.transient..The.excitons.in.the.(Mg,Zn)O.are.trapped.in.potential.fluctuations.caused.by.alloy.disorder.and.therefore.exhibit.a.slow.non-exponential.decay.[178]..This.decay.superimposes.the.dynamics.of.the.QW.due.to.the.capture.of.excitons.out.of.the.barrier.layers.(delayed.feeding).and.the.reabsorption.of.the.(Mg,Zn)O.luminescence.[179]..By.measuring.the.decay.of.the.barrier.luminescence,.it.is.possible.to.separate.the.influence.of.the.barrier.layer.from.the.dynamics.of.the.QW.as.shown.in.the.inset.in.Figure.10.31b.and.therewith.to.determine.the.decay.time.of.the.QW.excitons.

By.investigating.QW.samples.with.the.QCSE.differing.in.well.width.and.Mg-content.x.in.the.barriers.by.PL.experiments,.it.is.possible.to.determine.the.change.in.polarization.at.the.interface.between.(Mg,Zn)O.and.ZnO..In.Figure.10.32,.time-integrated.PL.spectra.are.depicted.for.QWs.with.different.well.widths.grown.by.PLD..Clearly.visible.is.a.redshift.


of.the.QW.luminescence.with.increasing.well.width.far.beneath.the.transition.energy.of.FXs.in.bulk.ZnO,.indicating.the.presence.of.an.electric.field.inside.the.QWs.and.the.QCSE,.respectively..QWs.with.well.widths.≤3.nm.emit.above.the.free.bulk.exciton.transition,.as.in.these.cases.the.confinement.effects.are.larger.than.the.effects.caused.by.the.QCSE..The.spectra.in.Figure.10.32.show.pronounced.phonon.replica.increasing.in.intensity.compared.to.the.zero.phonon.line.with.increasing.well.width,.indicating.a.growing.exciton.phonon.coupling.in.the.QW..Therefore,.the.Huang–Rhys.parameter.increases.[217–220].

In. addition,. time-resolved. measurements. were. performed. to. determine. the. kinetics.of.excitons.inside.the.QW..Transients.from.the.luminescence.maxima.of.four.points.on.a.wedge-shaped.sample.are.shown.in.Figure.10.32b..Visible.is.the.increasing.decay.time.with.increasing.well.width.as.well.as.the.non-exponential.behavior.of.the.transients..An.average.decay.time.[190,227].can.be.determined.from.the.transients.using.a.stretched.exponential.function. [189]. (Equation. 10.22). as. depicted. in. Figure. 10.32b.. In. literature. [214,225,237],.

Time (ns)(a) (b)Time (ns)0 1

QW luminescence decay

0 1 2

Contribution of the barrier

Time (ns)3

QW luminescence decaySingle exponential fit

Inte

nsity

(a. u

.)

Inte

nsity

(a. u

.) Inte

nsity

(a. u

.)

Fit

2 3 40 1 2 3 4

Single exponential fitQW luminescence decay

FIGURE 10.31Transients.and.fits.of.the.QW.luminescence.for.excitation.(a).beneath.and.(b).above.the.bandgap.of.the.barrier.layers..The.inset. in.(b).shows.the.two.components.of.the.model.to.fit.the.QW.luminescence.transient.for.an.excitation.above.the.bandgap.of.the.barrier.layers..The.contribution.of.the.barrier.is.the.transient.taken.at.the.spectral.luminescence.maximum.of.the.barrier.

Time (ns)

Fits

Energy (eV)

Energy (eV)(a) (b)

Inte

nsity

(a. u

.)

3.25 3.30

4.3 nm3.9 nm3.6 nm3.2 nm

3 nm3.6 nm4.3 nm5.2 nm6.9 nm

3.35T=2K, Eexc=3.55 eV

Inte

nsity

(a. u

)

Inte

nsity

(a. u

.)

0 202.9 3.0 3.1 3.2 3.3

XAZnO

3.4 40 60 80 100 120 140

FIGURE 10.32(a).Normalized.PL.spectra.of.ZnO/MgxZn1−xO.QWs.differing.in.well.width..The.dashed.line.indicates.the.transi-tion.energy.of.free.excitons.in.bulk.ZnO..(b).Normalized.transients.measured.at.the.peak.maxima.of.four.points.on.a.wedge-shaped.sample.ranging.from.3.2.to.4.3.nm.well.width..The.corresponding.spectra.are.shown.in.the.inset..The.transients.have.been.modeled.by.a.stretched.exponential.Equation.10.2.decay.as.indicated.


mostly.a.single.exponential.decay.is.fitted.to.the.initial.fast.part.of.the.transient,.underes-timating.the.average.luminescence.decay.times..Transients.of.the.QW.luminescence.with.thin.well.widths.exhibit.a.nearly.single.exponential.decay.[179],.implying.that.the.decay.stems.from.a.single.transition.with.typical.decay.times.of.400.ps..In.contrast,.the.carrier.dynamics.of.thick.QWs.is.severely.influenced.by.the.electric.field..The.observed..stretched.exponential.decay.arises.from.the.fact.that.the.decay.time.of.the.luminescence.at.a.fixed.emission.energy.E.increases.with.time.due.to.a.decrease.of.screening.of.the.internal.elec-tric.field.after.the.exciting.laser.pulse.due.to.recombination.and..diffusion.of.the.charged.carriers.inside.the.QW..The.decay.time.of.excitons.inside.the.QW.can.reach.values.up.to.microseconds.for.large.well.widths.[215].

The.role.of.the.screening.of.the.internal.electric.field.can.be.revealed.by.varying.the.exci-tation.power,.leading.to.a.shift.of.the.QW.luminescence.peak.[215,227],.due.to.screening.of.the.sheet.charges.at.the.interfaces.by.photo-excited.carriers..Additionally,.the.effects.of.the.screening.can.be.directly.seen.by.obtaining.the.dependence.of.the.decay.time.on.the.emission.energy.for.one.QW.as.depicted.in.Figure.10.33.

A.systematic.increase.of.the.decay.time.is.observed.for.the.QW.luminescence.toward.lower.energies..A.maximum.is.reached.clearly.beneath.the.QW.peak..At.lower.energies,.the.faster.decay.processes.from.the.high-energy.flank.of.the.first.phonon.replica.dominate.the. luminescence..Therefore,. it.can.again.be.concluded.that. the.QW.luminescence.does.not.only.consist.of.one.single.transition.but.represents.a.dynamic.relaxation.of.the.excited.carriers.[227],.also.explaining.the.relatively.large.full.width.at.half.maximum.(FWHM).of.the.QW.emission..At.lower.energies,.the.luminescence.decay.is.dominated.by.the.phonon.replicas,.which.reproduce.the.dependence.of.the.zero.phonon.peak.

With.the.dependence.of.the.PL.peak.energy.and.decay.time.on.the.QW.width,.it.is.pos-sible. to. determine. the. internal. electric. field. in. conjunction. with. numerical. calculations.[227,237],.obtaining.the.transition.energies.and.a.quantity.proportional.to.the.inverse.oscil-lator.strength..Such.a.calculated.dependence.is.shown.in.Figure.10.34a.for.a.wedge-shaped.sample.grown.by.PLD.[227].

The.quantity.proportional.to.the.inverse.oscillator.strength.has.been.normalized.to.decay.times.of.QWs.with.the.lowest.width,.as.for.these.samples.the.influence.of.nonradiative.recom-bination.is.the.smallest,.assuming.a.thickness.independent.nonradiative.recombination.rate.

Photon energy (eV)

3.0 3.1 3.2 3.3 3.4

10

Ave

rage

dec

ay ti

me (

ns)

PL-in

tens

ity (a

. u.)

100

FIGURE 10.33Time-integrated.PL.intensity.(left.scale).and.average.decay.time.(right.scale).as.a.function.of.the.luminescence.energy.


In.addition,.values.obtained.for.QWs.grown.with.a.high.laser.fluence.in.PLD.[179].are.shown.in.Figure.10.34a..These.samples.do.not.show.significant.effects.of.the.QCSE..The.dependence.of.the.peak.energy.on.the.well.width.was.modeled.by.a.finite.well.model.with-out.the.presence.of.an.electric.field.[230],.as.indicated.by.the.long.dashed.line..The.decay.times.for.such.samples.vary.between.200.and.500.ps.[179,229],.clearly.indicating.that.even.for.the.largest.well.widths.electrons.and.holes.are.not.separated.by.an.internal.electric.field.

Figure.10.34b.shows.the.determined.average.decay.times.as.a.function.of.the.lumines-cence.energy.for.samples.with.different.Mg-contents.grown.by.PLD.[227].and.MBE.[215]..For.MBE-grown.samples,.values.up.to.0.9.MV/cm.are.reported.[215,225],.leading.to.polar-ization.differences.of.up.to.0.024.C/m2.between.the.MgxZn1−xO.layers.and.the.QW.[213,238].

The.experimental.data.from.the.PLD-grown.samples.are.best.described.for.field.strengths.between.0.31.and.0.52.MV/cm.[227],.as.indicated.by.the.solid.lines..Taking.into.account.the.Mg-content.x.in.the.individual.samples,.a.change.x.×.(0.012.±.0.003).C/m2.in.the.polariza-tion.of.was.obtained.[227]..Further,.screening.by.photo-generated.free.carriers.affects.the.determination.of.the.QW.emission.energy.already.for.low.excitation.densities.and.leads.to.an.underestimation.of.the.electric.field.strength.and.with.that.of.the.QCSE.

The.results,.however,. indicate.that.a.wide.range.of.field.strengths.can.be.obtained.by.adequate.growth.conditions.allowing.the.control.of.the.internal.electric.fields.through.the.control.of.interface.abruptness.

10.7 (Mg,Zn)O Nanostructures

Semiconductor. nanostructures,. particularly. nanowires. (NWs),. are. of. large. interest. due.to. their. applicability. as. building. blocks. for. nanoscale. electronic. and. photonic. devices..

QW thickness (nm) Energy (eV)

Ave

rage

dec

ay ti

me (

s)

Ave

rage

dec

ay ti

me (

ns)

QW

emiss

ion

ener

gy (e

V)

02.8

2.8

3.0

3.0

PLDSamples grown by

E = 0.52 MV/cm

E = 0.31 MV/cm

MBE

PLD low fluence

PLD low fluence

PLD high fluencePLD high fluence

TheoryTheory

Theory w/o QCSEMBE

MBE

3.2

3.2 3.4 3.6

3.4

3.6

2 4 6 8 1010–10 10–2

100

102

104

106

10–8

10–6

10–4

XAZnO

FIGURE 10.34(a).Dependence.of.the.QW.peak.maximum.(left.scale).and.average.decay.time.(right.scale).on.the.well.width..Full.symbols.indicate.data.obtained.for.a.wedge-shaped.sample.grown.with.a.low.laser.fluence.in.PLD,.there-fore.showing.a.pronounced.QCSE.[227],.while.open.symbols.are.taken.from.Ref..[225].for.samples.grown.with.MBE..The.solid.lines.give.the.expected.dependence.according.to.variational.calculations..Bar.symbols.indicate.data.obtained.on.QWs.grown.with.a.high.PLD.laser.fluence.[179,230]..The.long.dashed.line.is.the.respective.fit,.assuming.absence.of.an.internal.electric.field..(b).Dependence.of.the.average.decay.time.at.the.QW.lumines-cence.peak.on.the.luminescence.energy.for.various.samples.(x.=.0.15–0.4).grown.by.PLD.[227].and.MBE.[215].with.x.=.0.22..The.solid.lines.give.the.calculated.dependence.[227].for.the.minimal.and.maximal.electric.field.strength.in.the.QWs.for.the.PLD-grown.samples..The.short.dashed.lines.in.both.pictures.indicate.the.transition.energy.of.free.excitons.in.bulk.ZnO.


They.offer.outstanding.optical.and.electrical.properties.that.can.carefully.be.modified.in.a.desired.way.[239]..In.this.course,.wide.bandgap.semiconductor.nanostructures.as.ZnO.NWs.or.GaN.NWs.are.at.the.core.of.current.technology..However,.the.rising.complexity.of.modern.devices.is.based.on.the.usage.of.semiconductor.heterostructures..Therefore,.in.particular,.NW.heterostructures.gain.much.scientific.interest.

The.bandgap.of.ZnO.can.be.easily.adjusted.by.the.incorporation.of.Mg.atoms.(enhance-ment). or. Cd. atoms. (reduction). into. the. ZnO. crystal. matrix.. Whereas. MgxZn1−xO. thin.films.with.high.Mg-contents.are.well.established.(see.Section.10.2.),.the.incorporation.of.a.large.amount.of.Mg.into.ZnO.nanostructures.in.combination.with.good.crystalline.qual-ity.is.much.harder.to.obtain..For.CdyZn1−yO,.several.publications.exits.that.demonstrate.the.successful.growth.of.thin.films.(see.Section.10.1.)..Nevertheless,.no.CdyZn1−yO-based.nanostuctures.have.been.reported.up.to.now.

An.approach.to.enhance.the.emission.properties.of.NWs.is.the.incorporation.of.QWs..Besides.offering.a.tunable.bandgap,.QW.heterostructures.allow.a.better.performance.of.optical.devices. in.comparison.with.active.bulk. layers..Therefore,. they.are.an.important.step.in.order.to.realize.photonic.NW.devices.[240–242]..Here,.we.focus.on.NW.heterostruc-tures.with.MgxZn1−xO.barriers.and.ZnO.QWs,.in.which.the.MgxZn1−xO.barriers.play.a.very.important.role..The.highly.efficient.performance.of.ZnO-based.nanostructures.is.strongly.connected.to.challenges.in.NW.growth..On.the.one.hand,.MgxZn1−xO.barriers.should.con-tain.large.Mg-contents.and.therefore.high.barrier.emission.energies.because.they.restrict.the.QW.emission.energy.as.to.an.upper.limit..On.the.other.hand,.exciton.diffusion.and.carrier.transport.from.the.barrier.layer.into.the.QW.has.to.be.efficient..Both.are.influenced.by.the.strength.of.disorder-induced.potential.fluctuations.in.the.alloy..Hence,.only.a.small.range.of.layer.thicknesses.and.Mg-contents.is.reasonable.for.the.successful.establishment.of.MgxZn1−xO-based.NW.heterostructures..Note.that.also.a.homogeneous.distribution.of.Mg.atoms.over.the.whole.surface.of.the.NW.has.to.be.achieved.in.order.to.ensure.a.uni-form.QW.emission.

For.realizing.homogeneous.core/shell-heterostructures.with.growth.techniques.based.on.highly.directional.beams.of.atoms.or.ions.like.MBE.or.PLD,.one.has.to.keep.in.mind.that.the.density.of.NWs.has.to.be.small.enough.to.avoid.shadowing.effect.due.to.neighbor-ing.objects,.for.example,.other.NWs.[243]..If.shadowing.effects.occur,.the.thickness.of.QWs.and.therefore.also.their.emission.energies.would.fluctuate.along.the.NWs.and.between.neighboring.NWs.

10.7.1 (Mg,Zn)O Nanowires

MgxZn1−xO.NWs.emit.at.energies.larger.than.that.of.ZnO.and.can.be.utilized.as.starting.structure. for. the. growth. of. NW. QW. heterostructures.. In. fact,. several. approaches. have.been. developed. to. grow. MgxZn1−xO. NWs. with. large. Mg-contents. and. high. crystalline.quality..The.most.important.results.concerning.MgxZn1−xO.NW.growth.are.summarized.in.Table.10.17..First.reports.were.given.by.Heo.et.al..[244].using.MBE..They.obtained.a.sig-nificant.blueshift.of.60.meV.for.the.wire.luminescence..Subsequently,.aligned.MgxZn1−xO.NWs.with.even.larger.Mg-contents.and.higher.luminescence.energies.were.obtained.by.using.MOCVD.[245–248].or.high-pressure.PLD.(hp-PLD).[174]..Figure.10.35a.shows.a.scan-ning. electron. microscopy. (SEM). image. of. aligned. MgxZn1−xO. NWs. and. Figure. 10.35b.cathodoluminescence.(CL).spectra.recorded.at.T.=.10.K.of.hp-PLD-grown.NWs.[174]..With.hp-PLD.emission.energies.of.up.to.3.63.eV.can.be.achieved.corresponding.to.a.Mg-content.of.up.to.∼17%..Using.MOCVD,.the.transition.from.MgxZn1−xO.NWs.to.nanowalls.could.be.manipulated.and.studied.[249].


Even. larger. Mg-contents. can. be. obtained. for. samples. fabricated. by. using. the. vapor.phase.transport. (VPT).method.[250,251,255–258]..Lu.et.al..present. luminescence.spectra.of. NWs. showing. emission. energies. of. up. to. 4.07.eV. having. NW. densities. of. 1–10.μm−2..However,.the.VPT-grown.NWs.suffer.from.the.fact.that.only.a.quasi.alignment.of.NWs.can.be.reached..Besides,.Lee.et.al..followed.up.a.low-temperature.hydrothermal.approach.

Energy (eV)

CL-in

tens

ity (a

. u.)

3.4(b)(a)

1 µm

3.5 3.6

FIGURE 10.35(a).SEM.image.of.well-aligned.MgxZn1−xO.NWs.grown.by.hp-PLD..(b).CL.spectra.of.the.NWs.recorded.at.T.=.10.K..(Adapted.from.Lorenz,.M..et.al.,.Appl. Phys. Lett.,.86,.143113,.2005.)

TABLE 10.17

Overview.of.MgxZn1−xO.NW.Growth.Fabricated.by.Various.Growth.Techniques

Growth Method Reference Results

MBE [244] Catalyst-assisted.(Au).growth.of.thin.MgxZn1−xO.NWsStrong.Mg-signal.in.energy.dispersive.x-ray.spectroscopy.(EDX)Blueshift.of.luminescence.of.∼60.meV

MOCVD [246] Aligned.MgxZn1−xO.NWs.on.Si.(001)Maximum.Mg-contents.up.to.0.17Luminescence.energies.up.to.3.575.eV.(ensemble.of.NWs)

[248] MgxZn1−xO.NWs.with.high.aspect.ratios.and.Mg-contents.up.to.0.03[249] Transition.from.MgxZn1−xO.NWs.to.ultrathin.nanowalls.studied

Mg-contents.up.to.10%.but.small.blueshift.observedhp-PLD [174] Aligned.MgxZn1−xO.NWs.on.a-sapphire.(Au.as.catalyst)

Luminescence.energies.up.to.3.63.eV.are.obtainedVPT [250,251] Quasi-aligned.MgxZn1−xO.NWs.grown.using.pure.Zn.and.Mg.powders.and.

Au-nanoparticles.as.seed.layers.on.Si.(100)Mg-contents.up.to.0.32.determined.with.EDXLuminescence.energies.up.to.4.07.eV.are.obtained

[252] MgxZn1−xO.NWs.using.Au/Si(100)-templatesMg-contents.up.to.0.05.and.luminescence.energies.up.to.3.41.eV

In-diffusion [253] ZnO/MgO.core/shell.structures.grown.by.VPT.and.annealedEmission.energies.up.to.3.48.eV.are.obtained

[254] ZnO.NWs.grown.by.VPT.were.spin.coated.by.acetates.of.Mg.and.sintered.at.1000°CLuminescence.energies.up.to.3.45.eV.achieved

Note:. Growth.details,.Mg-content.and.luminescence.energies.are.given.if.available.in.the.referenced.source.


and.demonstrated.a.blueshift.of.150.meV.of.the.optical.bandgap.[259]..Further,.Hsu.et.al..fabricated.MgxZn1−xO.NWs.with.emission.energies.up.to.3.475.eV.[253,254].via.in-diffusion.of.Mg.into.ZnO.by.using.ZnO.cores.and.MgO.shells..Note.that.the.ZnO/MgO.core/shell.samples.were.fabricated.by.the.VPT.process.

Up.to.now,.no.core/shell-heterostructures.have.been.reported.that.use.a.MgxZn1−xO.NW.core..VPT.processes.that.offer.the.incorporation.of.large.Mg-contents.are.usually.carried.out.in.a.tube.furnace.without.the.possibility.to.change.the.source.material.during.growth..Subsequently,. fabricating. heterostructures. remains. challenging. since. different. process.runs. are. necessary.. Further,. directed. growth. methods. like. MBE. or. PLD. are. limited. to.minimal.MgxZn1−xO.NW.densities.of.1–10.μm−2,.but.densities.of.<.0.1.μm−2.are.necessary.for.homogeneous.core/shell-heterostructures.in.order.to.avoid.shadowing.effects..In.this.context,.new.basic.approaches.are.necessary.for.the.growth.of.QW.heterostructures.using.a.MgxZn1−xO.NW.core.

10.7.2 Core/Shell-Heterostructures with a ZnO Core

Due. to. the. lack. of. possibilities. for. growing. core/shell. NW. heterostructures. with. a.MgxZn1−xO. NW. core,. efforts. have. been. made. using. ZnO. NW. cores.. ZnO. NWs. benefit.from.the.fact.that.they.can.be.grown.in.a.low.lateral.density.[243].and.that.they.show.a.high.crystalline.quality..In.most.cases,.the.ZnO.NWs.are.surrounded.by.MgxZn1−xO/ZnO/MgxZn1−xO.QWs..For.some.device.applications,.it.can.be.an.important.issue.to.grow.NW.heterostructures.with.a.modulated.composition.along.defined.directions,.either.the.radial.and/or.the.axial.direction.[260].

ZnO. NW. core/MgxZn1−xO. shell-heterostructures. have. been. investigated. by. sev-eral. groups.. Radial. QW. NW. heterostructures. were. grown. and. optically. investigated.by. the. group. of. G.C.. Yi. [261,262].. The. QWs. were. grown. around. the. ZnO. NWs. using.MOVPE..The.optical.properties.and.growth.details.are.summarized.in.Table.10.18.and.are.compared.with.other.kinds.of.heterostructures..High-quality.radial.NW.QWs.can.be.obtained.by.this.vapor.phase.growth.method,.even.in.high.density,.because.the.reactant.molecules.can.homogeneously.cover.the.major.part.of. the.NW..From.the.same.group,.heterostructures.with.a.ZnO.core.and.a.MgxZn1−xO.shell.were.also.studied.electrically.as.oxide.electronic.nanodevices.[263]..A.higher.mobility.and.a.smaller.subthreshold.swing.

TABLE 10.18

.Overview.over.Important.Results.Achieved.for.Quantum.Well.Nanowire.Heterostructures

Type of NW Heterostructure

Reference and Growth Method

Energetic Range (eV)

Mg-Content/ EMgZnO (eV) Comments

Radial.QW [261,262]MOCVD

3.382.−.3.467 ∼0.2/3.55 Density.of.NWs:.∼102–103.μm−2

Random.orientation.of.NWsAxial.QW [264.–.267]

MOCVD[268,242]MBE

3.375.–.3.515 ∼0.2/3.58 Density.of.NWs:.∼102–103.μm−2

NWs.with.parallel.orientation3.42 ∼0.1/3.49 Density.of.NWs:.∼102.μm−2

Well-aligned.NWs.on.SiC/sapphireCore/shell

Axial.QWRadial.QW

[243]PLD

3.41a

3.42.−.3.47∼0.1/3.525 Density.of.NWs:.below.0.1.μm−2.by.

introducing.a.ZnO.buffer.layerNWs.well.aligned.(perp..to.surface)

Note:. Growth.method,.spectral.range.of.QW.emission,.Mg-content.and.comments.are.given.a. QW-peak.energy.in.axial.direction.not.given.for.all.samples.


was.found.for.the.field.effect.transistors.made.of.the.NW.heterostructure.in.comparison.to.devices.made.of.pure.ZnO.

Axial.QW.NW.heterostructures.were.also.grown.and.studied.in.the.group.of.G.C..Yi.[264–267].. Well-aligned. ZnO. NWs. with. a. high. density. served. as. core.. The. QWs. were.grown.on.the.NWs’.tips.in.axial.direction..Due.to.the.parallel.orientation.and.homoge-neous.length.of.the.NWs,.the.QWs.were.solely.grown.on.the.tip.of.the.NWs..The.proper-ties.of.these.heterostructures.are.also.summarized.in.Table.10.18..Bakin.et.al..reported.the.growth.of.axial.QW.NW.heterostructures.using.MBE.as.a.growth.technique.[242,268].

With.a.two-step.thermal.evaporation.process,.ZnO-core/MgxZn1−xO-shell-heterostruc-tures.were.successfully.grown.[269,270]..A.blueshift.of.the.MgxZn1−xO-related.luminescence.of.134.meV.compared.to.ZnO.was.observed.[270]..PLD-grown.core/shell.NW.heterostruc-tures.that.exhibit.a.QW.in.axial.and.radial.direction.were.demonstrated.by.Cao.et.al..[243]..The.NWs.were.grown.in.very.low.lateral.density.(<.0.1.μm−2).to.obtain.homogeneous.QWs..Thereby,.they.found.out.that.the.growth.rates.in.axial.and.radial.direction.using.a.direc-tional.technique.as.PLD.can.differ.by.a.factor.of.3–5.[243].

However,. as. one. can. extract. from. Table. 10.18. the. maximum. QW. emission. energy. is.relatively.small..A.maximum.blueshift.of.only.0.14.eV.compared.to.ZnO.luminescence.is.reported..This.is.caused.by.the.small.Mg-content.in.the.barrier.layers..The.thicknesses.of.the.respective.QWs.were.already.very.small.(down.to.1.1.nm),.so.that.a.further.reduction.is.difficult..Hence,.the.only.possibility.to.obtain.larger.energies.is.to.use.larger.Mg-contents.in.the.MgxZn1−xO.barriers..Recently,.core/shell.NW.heterostructures.with.large.confine-ment.energies.were.reported.by.Lange.et.al..[271]..They.deduced.Mg-contents.in.the.barrier.from.CL.spectra.exceeding.values.of.25%..The.QW.emission.energy.could.be.tuned.in.a.much.larger.spectral.range.(from.3.42.to.3.68.eV).

Lange.et.al..were.able.to.clearly.distinguish.between.QWs.in.axial.and.radial.direction.by.scanning.CL.measurements..The.spectra.for.the.different.QW.thicknesses.are.given.in.Figure.10.36.together.with.an.idealized.growth.scheme.

Energy (eV)3.0

(a) (b)Substrate

Axial QWRadial QW

ZnO coreMgZnO

MgZnOZnO-QW

Ev

Ev

Ec

Lw,a

Lw,r

Ec

3.5

Radial ZnO QW

CL-

inte

nsity

(a. u

.)

ZnOcore

Axial ZnO QW

MgZnOshell

Roomtemperature

320p

160p

120p

80p

160p

120p

80p

4.0

FIGURE 10.36(a).Idealized.growth.scheme.of.the.MgxZn1−xO/ZnO.quantum.well.nanowire.heterostructure.grown.on.a.verti-cal.freestanding.ZnO.NW..(b).CL.spectra.of.the.axial.and.radial.QWs.with.different.QW-thicknesses.measured.at.room.temperature..The.baselines.are.shifted.for.clarity..(Adapted.from.Lange,.M..et.al.,.J. Vac. Sci. Technol. A,.29,.03A104,.2011.)


The.large.difference.in.the.emission.energies.of.the.QWs.in.the.radial.and.axial.direc-tion.is.a.result.of.the.directed.growth.process,.which.causes.a.factor.of.2.in.the.respective.growth.rates,.which.is.below.the.value.observed.by.Cao.et.al..[243].

Another. important.aspect. is. the.homogeneity.of. the.QW.emission.energy.. It. is.deter-mined. by. the. QW. thickness. and. the. bandgap. difference. between. barrier. and. active.layer..The.emission.energy.along.the.growth.direction.of.the.NW.was.investigated.[271]..A.significant.increase.of.the.QW.emission.energy.near.the.NW.tip.was.observed.that.was.ascribed.to. locally.reduced.growth.rates.at. the.NW.side. facets.with.a. lateral.spreading.of.500.μm..The.local.fluctuations.of.the.MgxZn1−xO.bandgap.were.typically.below.20.meV.within.their.samples..Calculations.reveal.that.the.magnitude.of.QW.energy.fluctuations.is. only. 1/5. of. the. magnitude. of. barrier. bandgap. fluctuations. assuming. a. constant. QW.thickness..Therefore,.the.maximal.energetic.difference.caused.by.fluctuations.of.the.bar-rier.bandgap.is.only.±.4.meV,.a.value.much.smaller.than.the.linewidth.of.the.luminescence.band..In.this.relation,.the.growth.of.the.QW.can.be.assumed.to.be.homogeneous.in.energy.over.the.whole.NW.

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321

11Growth and Characterizations of Zn(Mg,Cd)O Alloys and Heterostructures Using Remote-Plasma-Enhanced MOCVD

Kenji Yamamoto and Jiro Temmyo

11.1 Introduction

Zinc.oxide.(ZnO).is.a.wide,.direct.bandgap.semiconductor.(Eg.=.3.3.eV.at.room.tempera-ture),. attracting. much. interest. in. optoelectronic. devices,. such. as. light-emitting. diodes,.laser. diodes,. and.photodetectors,.due. to.a. large.exciton-binding. energy.of. 60.meV. [1,2]..An.important.step.in.designing.optoelectronic.devices.is.the.bandgap.engineering.to.pro-duce.barrier.layers.and.quantum.wells.(QWs).in.heterostructures.as.well.as.p-type.doping.under.intensive.study.[3]..Modulation.of.the.bandgap.has.been.demonstrated.by.the.devel-opment.of.Zn1−xCdxO.and.MgyZn1−yO.systems.[4,5]..However,.there.has.been.a.big.issue.on.structural.phase.transition.from.wurtzite.to.rock.salt.with.increasing.alloy.content.in.contrast.to.the.Ga(In,Al)N.ternary.system..While,. in.particular,.wurtzite.Zn1−xCdxO.has.been.examined.by. laser.and/or.conventional.molecular.beam.epitaxy. (MBE). [5–7],.and.conventional.metal-organic.chemical.vapor.deposition.(MOCVD).[8–10],.it.is.well.known.that.it.is.difficult.for.growing.wurtzite.Zn1−xCdxO.system.with.high.Cd.content..In.order.to.overcome.this.problem,.we.have.developed.a.remote-plasma-enhanced.metal-organic.

CONTENTS

11.1. Introduction......................................................................................................................... 32111.2. Growth.System.and.Peripherals....................................................................................... 322

11.2.1. Remote-Plasma-Enhanced.Metal-Organic.Chemical.Vapor.Deposition........ 32211.2.2. Characterizations.................................................................................................... 324

11.3. Characteristics.of.Zn(Mg,Cd)O.Alloys............................................................................ 32411.3.1. Structural.Properties.............................................................................................. 32511.3.2. Optical.Bandgap.and.PL.Energy.......................................................................... 32611.3.3. PL.Alloy.Broadening.............................................................................................. 328

11.4. Characteristics.of.Zn0.85Cd0.15O/ZnO.MQWs..................................................................33311.4.1. Structural.Properties..............................................................................................33411.4.2. Optical.Properties...................................................................................................335

11.5. Light-Emitting.Devices......................................................................................................33811.6. Summary.and.Outlook......................................................................................................343Acknowledgments.......................................................................................................................345References......................................................................................................................................345


chemical. vapor. deposition. system. (RPE-MOCVD). and. successfully. grown. wurtzite.Zn1−xCdxO.with.higher.cadmium.content.of.up.to.0.6.[11]..MgyZn1−yO.alloys.also.have.been.grown.by.MBE.[4,12–16],.MOCVD.[17–19],.and.RPE-MOCVD.[20].as.well.as.radio-frequency.magnetron.sputtering.[21]..In.this.system,.the.solid.solution.films.of.MgyZn1−yO.can.be.syn-thesized.with.a.Mg.content.of.up.to.approximately.3%.from.ZnO-MgO.quasi-binary.phase.diagram. under. thermal. equivalent. conditions. [22]. and,. however,. it. has. been. reported.that.typical.Mg.content.up.to.33%.is.available,.providing.the.corresponding.bandgap.of.3.99.eV.[4]..In.addition.to.this.course,.some.feasibility.studies.on.ZnO-based.light-emitting.diodes.have.been.achieved,.showing.a.potential.application.of.the.ZnO-alloy.system.to.the.UV-visible.range.optical.devices.[23–27]..The.visible.EL.emission.from.Zn1−xCdxO-based.heterojunction.diodes.was.demonstrated,.indicating.the.feasibility.of.the.semiconductor.material.system.for.high-performance.optical.device.applications,.while.the.full.width.at.half-maximum.(FWHM).was.relatively.large.[25,28]..In.order.to.overcome.these.issues,.we.have.evaluated. the. fundamental.properties.such.as.structural.and.optical.properties.of.alloys.and.fabricated.heterostructures.including.nanostructures.

11.2 Growth System and Peripherals

11.2.1 Remote-Plasma-Enhanced Metal-Organic Chemical Vapor Deposition

A.synthesis.of.wurtzite.ZnO.binary,.and.Zn1−xCdxO.and.MgyZn1−yO.ternary.alloys.with.high.alloy.content.and.its.heterostructures.is.possible.using.the.RPE-MOCVD.technique.[29].. The. schematic. diagram. of. RPE-MOCVD. is. illustrated. in. Figure. 11.1.. Diethyl. zinc.(DEZn),.dimethyl.cadmium.(DMCd),.and.bis-ethylcyclopentadienyl.magnesium.(EtCp2Mg).were.used.as.group-II.metal-organic.(MO).sources..These.MO.sources.were. introduced.into.the.reactive.region.by.H2.carrier.gas..Oxygen.radical.was.used.as.group-VI.source,.which.was.generated.by.rf.of.13.56.MHz.from.oxygen.gas..The.typical.chamber.pressure.for. the.growth.is.between.1.and.10.Pa. in.order. to.generate.plasma..The.pressure. is.one.order.of.magnitude.lower.than.the.pressures.in.the.general.atmospheric-.and.low-pressure.MOCVDs..Since.the.radicals.help.to.decompose.MO.sources,.substrate.temperature.dur-ing.growth.is.relatively.low,.and.between.300°C.and.650°C..A-plane.(11–20).sapphire.was.used.as.the.substrate.for.the.growth.of.wurtzite.(0001).Zn1−xCdxO.and.MgyZn1−yO.alloys.and.the.heterostructures.

As. the. features. in. RPE-MOCVD. technique,. here. we. describe. radicals. in. the. reactive.chamber.for.growth..Figure.11.2.shows.a.typical.plasma.spectrum.for.O2.influx.under.the.condition.with.H2.carrier.gas..The.radicals.are.essential.for.ZnO.growth..On.O,.H,.and.OH.radical,.a.rf.power.dependence.is.illustrated.in.Figure.11.3..When.rf.power.increases,.the.increment.of.concentration.of.all.radicals.can.be.seen..Dependence.of.the.alloy.content.in.Zn1−xCdxO.[11].and.MgyZn1−yO.[20].on.rf.power.has.been.investigated..Especially,.for.the.growth.of.Zn1−xCdxO.with.high.Cd.content,.it.has.been.found.that.the.growth.under.the.high.nonthermal.equilibrium.condition.with.radicals.is.needed.[11].

The. RPE-MOCVD. utilizing. high. nonthermal. equilibrium. condition. is. the. powerful.growth.technique.for.p-.and.n-type.ZnO..The.control.of.the.conductivity.has.been.achieved.by.utilizing.(Mg)ZnO:N.[32].and.ZnO:Cu.[33].for.p-type.and.MgyZn1−yO:In.[34].for.n-type.

The.quality.of.RPE-MOCVD-grown.ZnO.alloy.has.improved,.and.the.MgyZn1−yO-based.Schottky.diodes.have.been.fabricated.[35].

323Growth and Characterizations of Zn(Mg,Cd)O Alloys and Heterostructures

Thermocouple

O radical

O2Plasma

generator

EtCp2Mg,DEZn,DMCd,

H2H2

rf supply13.56 MHz

Matchingcircuit

Exhaust

Substrate holder,heater

Substrate

FIGURE 11.1Schematic. diagram. of. the. reaction. chamber. of. remote-plasma-enhanced. MOCVD. (RPE-MOCVD). system..(From.Yamamoto,.K.,.Characterization.of.film.and.hetero-structure.light-emitting.diode.based.on.Zn(Mg,Cd)O,.PhD.thesis,.2010.)

Wavelength (nm)

Inte

nsity

(a. u

.)

200 400

OH

O

HO2

600 800

FIGURE 11.2A.typical.plasma.emission.spectrum.under.oxygen.influx..(From.Gangil,.S.,.Study.on.p-type.doping.and.char-acterization.of.ZnO-based.for.optical.devices,.PhD.thesis,.2008.)


11.2.2 Characterizations

Alloy. contents. such. as. Cd. content. x. in. Zn1−xCdxO. ternary. alloy. and. Mg. content. y. in.MgyZn1−yO. ternary. alloy. were. precisely. analyzed. by. Electron. Probe. Micro. Analysis.(EMPA),. Atomic. Absorption. Spectroscopy. (ASS),. and. Rutherford. Backscattering.Spectrometry. (RBS)..The.phase.and.crystallinity.was.characterized.by.x-ray.diffraction.(XRD).. The. transmittance. spectra. were. recorded. on. an. ultraviolet-visible-near-infrared.scanning.spectrometer.over.a.wavelength.range. from.300. to.800.nm..The.optical.band-gap.energy.was.determined.from.a.plot.of.α2(hν).as.a.function.of.photon.energy.(hν)..The.emission.properties.such.as.peak.energy.and.FWHM.were.measured.by.the.steady-state.photoluminescence.(SSPL).and.the.time-resolved.PL.(TRPL).spectroscopy..A.He-Cd.laser.was.used.as.an.excitation.light.source.for.steady-state.PL.measurements..TRPL.measure-ments.using.a.streak.camera.system.with.the..frequency-doubled.beam.of.a.mode-locked.Ti:sapphire.laser.with.a.pulse.width.of.1.5.ps.and.a.repetition.rate.from.4.to.0.8.MHz.were.performed.at.low.temperature..The.excitation.energy.and.power.density.were.3.49.eV.and.1.6.μJ/cm2,.respectively.

11.3 Characteristics of Zn(Mg,Cd)O Alloys

In. order. to. design. high-performance. ZnO-based. heterojunctions. precisely,. we. need.detailed. information.on.optical.bandgap,.PL.peak.energy,.and. its.FWHM.in.ZnO-alloy.systems..In.general,.it.is.known.that.the.bandgap.bowing.and.the.broadening.of.PL.spectra.are.caused.by.the.exciton.localization.in.compound.semiconductors.[36]..The.exciton.in.oxide.semiconductor.seems.to.be.easily.localized.by.the.alloy.fluctuation,.because.a.ZnO-based.semiconductor.has.strong.electronegativities.and.the.small.Bohr.radius.of.exciton..The.PL.FWHM.of.various.ternary.alloy.II–VI.and.III–V.semiconductors.on.alloy.content.

RF power (W)

Emiss

ion

inte

nsity

(a. u

.)

010

102

103

104

20 40 60

H

OH

O

80 100

FIGURE 11.3RF.power.dependence.for.O,.H,.and.OH.radicals..(From.Gangil,.S.,.Study.on.p-type.doping.and.characterization.of.ZnO-based.for.optical.devices,.PhD.thesis,.2008.)


has.been.analyzed.by.theoretical.model.based.on.the.statistical.alloy.fluctuation.[36–39]..The.localization.of.the.exciton.in.oxide.semiconductors.seems.to.occur.at.the.minima.of.the.potential.fluctuation.caused.by.alloy.fluctuation,.showing.larger.PL.FWHM.compared.with.a.typical.AlGaN-based.system.[39].and,.however,.very.few.reports.on.the.detail.of.luminescence.broadening.has.been.reported.for.just.MgyZn1−yO.system.[40].yet.

We. have. systematically. characterized. the. optical. properties,. such. as. optical. band-gap.and.PL.FWHM,.of.Zn(Mg,Cd)O.alloy.system.grown.by.RPE-MOCVD..We.describe.precisely. the.dependency.of.emission.energy.and.FWHM.of.Zn1−xCdxO.on.Cd.content.as.well.as.MgyZn1−yO.system..Comparing.with.the.experimental.results.and.theoretical.calculations.based.on.the.exciton.model,.the.broadening.of.PL.FWHM.is.clarified.to.be.caused.by.alloy.fluctuation.in.Zn(Mg,Cd)O..As.another.way.to.confirm.alloy.broadening,.we.also.have.carried.out.TRPL.measurements.and.derived.the.localized.depth.of.the.exci-ton.in.ZnO-based.systems,.indicating.a.good.agreement.with.the.tendency.of.PL.FWHM.broadening.[41].

11.3.1 Structural Properties

Figure.11.4.shows.the.dependency.of.c-axis.length.of.wurtzite.Zn(Mg,Cd)O.films.determined.by.XRD.on.alloy.content.(x,.y)..Wurtzite.Zn1−xCdxO.(0.≤.x.≤.0.6).and.MgyZn1−yO.(0.≤.y.≤.0.3).have. been. obtained.. The. c-axis. of. ZnO. is. 0.5207.nm.. The. c-axis. length. of. Zn1−xCdxO.increases.up.to.0.5416.nm.with.increasing.up.to.Cd.content.of.0.55,.indicating.the.substitu-tion.between.Cd2+.(0.78.Å).and.Zn2+.(0.60.Å)..In.contrast,. the.c-axis. length.of.MgyZn1−yO.decreases. down. to. 0.5143.nm. with. increasing. up. to. Mg. content. of. 0.29,. indicating. the.

y in MgyZn1–yO x in Zn1–xCdxO1.0

0.510

0.515

0.520

0.525c-ax

is le

ngth

(nm

)

0.530

RPE-MOCVD RPE-MOCVDPLD (Ohtomo et al.) MBE (Sadofev et al.)

0.535

0.540

0.545

0.550

0.8 0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0

FIGURE 11.4Dependency.of.c-axis.length.of.Zn(Mg,Cd)O.films.on.alloy.content.(x,.y)..The.solid.circle.and.the.solid.square.are.RPE-MOCVD-grown.Zn1−xCdxO.and.MgyZn1−yO,.respectively..Solid.curves.are.fitted.by.quadratic.function.of.alloy.content..Open.circle.and.dashed.line.show.MBE-grown.Zn1−xCdxO.(Ref..7).and.PLD-grown.MgyZn1−yO.(Ref..[4]),.respectively..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural.and.optical.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permission.from.Elsevier.)


substitution.between.Mg2+.(0.57.Å).and.Zn2+.(0.60.Å)..The.c-axis.length.c(x).(c(y)).in.Zn1−

xCdxO.(MgyZn1−yO).as.a.function.of.the.Cd.content.x.(Mg.content.y).is.represented.by.the.following.equation:

. c x x x( ) = + − ≤ ≤( )0 5207 0 056 0 029 2. . . , 0 0.6x . (11.1)

. c y y y( ) = − − ≤ ≤( )0 5207 0 023 0 005 2. . . . 0 y 0.3 . (11.2)

The.variation.of.c-axis.of.Zn(Mg,Cd)O.on.alloy.content.include.the.quadratic.term.with.small. factor,.because.Zn(Mg,Cd)O.consists.of.ZnO.of.Wurtzite.and.CdO.(MgO).of.rock.salt..Sadofev.et.al..have.recently.reported.that.Zn1−xCdxO.(0.≤.x.<.0.32).was.synthesized.by.MBE.[7]..The.c-axis.of.Zn1−xCdxO.increases.along.the.results.in.this.work.with.Cd.content..Ohtomo.et.al.. reported. that. in. the.early. research. for.MgyZn1−yO,. the.variation.of. c-axis.length.of.MgyZn1−yO.decreases.down.to.0.517.nm.with.increasing.up.to.Mg.content.of.0.33.[4]..The.variation.of.c-axis.on.Mg.content.in.this.work.is.slightly.smaller.than.that.of.the.reported.results.

11.3.2 Optical Bandgap and PL Energy

Transmittance.and.photoluminescence.(PL).spectra.of.wurtzite.Zn(Mg,Cd)O.films.at.room.temperature.is.shown.in.Figure.11.5..The.optical.bandgap.energy.was.determined.from.a.plot.of.α2(hν).as.a.function.of.photon.energy.(hν)..A.He-Cd.laser.was.used.as.an.excitation.light.source.for.steady-state.PL.measurements..PL.peak.of.Zn1−xCdxO.shifts.toward.a.lower.energy.from.3.28.eV.of.ZnO.to.1.8.eV.of.Zn0.47Cd0.53O.along.lower-energy-shift.of.absorption.edge.with.increasing.Cd.content..PL.peak.of.MgyZn1−yO.increases.to.3.4.eV.of.Mg0.18Zn0.82O.along.higher-energy-shift.of.absorption.edge.with.increasing.Mg.content.

The.bandgap.energy.and.PL.peak.energy.of.Zn(Mg,Cd)O.films.at.room.temperature.as.a.function.of.alloy.content.is.presented.in.Figure.11.6a..The.bandgap.energy.of.Zn1−xCdxO.decreases.down.to.1.8.eV.and.increases.up.to.Cd.content.of.0.6,.which.covers.the.entire.visible.range..Makino.et.al..have.synthesized.Zn1−xCdxO.films.with.Cd.content.up.to.0.07.by.pulsed.laser.deposition.(PLD).[5]..In.contrast,.the.bandgap.of.MgyZn1−yO.increases.up.to.3.75.eV.with.increasing.Mg.content.up.to.0.3..Ohtomo.et.al..reported.that.the.bandgap.of.MgyZn1−yO.with.y.of.up.to.0.33.grown.by.PLD.increases.up.to.3.99.eV.[4]..The.energy-shift.of.the.optical.bandgap.and.PL.peak.of.MgyZn1−yO.alloys.on.alloy.content.are.in.good.agreement.with.these.previous.results..The.bandgap.energy.Eg(x).of.the.ternary.alloy.is.determined.by.the.following.equation.including.bowing.parameter.b:

.

E x E x E x bx x

x x

g,ZnCdO g,ZnO g,CdO( ) = −( ) + − −( )

= −( ) + −

1 1

3 28 1 2 3 3 0. . . 44 1x x−( ) , . (11.3)

.

E y E y E y by y

y y

g,MgZnO g,ZnO g,MgO( ) = −( ) + − −( )= −( ) + −

1 1

3 28 1 7 8 3 4. . . 77 1y y−( ). . (11.4)

Here,.the.bandgap.of.ZnO.(Eg,ZnO),.CdO.(Eg,CdO),.and.MgO.(Eg,MgO).are.3.28,.2.3.[42],.and.7.8.eV.[43],.respectively..By.fitting.this.function.to.the.experimental.results,.we.found.that.the.


bowing.parameter.b.for.Zn1−xCdxO.and.MgyZn1−yO.is.3.0.and.3.5.eV,.respectively..The.large.b.for.Zn(Mg,Cd)O.is.due.to.the.large.difference.in.electron.negativities.between.the.bina-ries.of.ZnO.and.CdO.(MgO)..Dependence.of.PL.peak.energy.of.Zn(Mg,Cd)O.films.at.room.temperature.on.alloy.content.is.also.shown.in.Figure.11.6a..PL.peak.energy.of.Zn1−xCdxO.decreases. down. to. 1.8.eV. with. increasing. Cd. content.. In. contrast,. PL. peak. energy. of.MgyZn1−yO.increases.up.to.3.4.eV.with.increasing.Mg.content.up.to.0.2..PL.peak.of.metal-organic.vapor.phase.epitaxy.(MOVPE)-grown.Zn1−xCdxO.(x.<.0.085).[9].shifts.toward.lower.energy.with.increasing.Cd.content.in.accordance.with.the.tendency.in.this.work..Stokes’.shift.of.Zn(Mg,Cd)O,.defined.as.the.difference.of.energy.between.optical.bandgap.(open.circle).and.PL.peak.(solid.circle),.is.shown.in.Figure.11.6b..The.Stokes’.shift.for.Zn1−xCdxO.increases.with.increasing.Cd.content,.and.reaches.a.maximum.of.400.meV.at.x.of.around.0.2..When.Cd.content.is.over.0.2,.Stokes’.shift.gradually.deceases.with.an.increment.of.Cd.content..This.result.indicates.that.the.degree.of.the.alloy.fluctuation.reaches.the.maximum.at.x.=.0.2..Similarly,.Stokes’.shift.of.MgyZn1−yO.(0.<.y.<.0.2).also.increases.to.50.meV.with.an.increase.up.to.Mg.content.of.0.1..Stokes’.shift.of.MgyZn1−yO.drastically.increases.around.200.meV.at.Mg.content.of.0.2.

Photon energy (eV)

Tran

smitt

ance

(%)

Nor

mal

ized

PL

inte

nsity

1.5

0

50

(e)Zn0.47Cd0.53O

(d)Zn0.70Cd0.30O

(c)Zn0.86Cd0.14O

(a)Mg0.18Zn0.82O

(b)ZnO

1000

50

1000

50

100

50

1000

RT

50

100

0

2.0 2.5 3.0 3.5

FIGURE 11.5Transmittance.and.photoluminescence.spectra.Zn(Mg,Cd)O.films.for.(a).Mg0.18Zn0.82O,.(b).ZnO,.(c).Zn0.86Cd0.14O,.(d).Zn0.70Cd0.30O,.and.(e).Zn0.47Cd0.53O..These.spectra.are.taken.at.room.temperature..(Reproduced.from.J. Cryst. Growth,. 312,. Yamamoto,. K.,. Tsuboi,. T.,. Ohashi,. T.,. Tawara,. T.,. Gotoh,. H.,. Nakamura,. A.,. and. Temmyo,. J.,.Structural.and.optical.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permission.from.Elsevier.)


11.3.3 PL Alloy Broadening

Dependency.of.PL.peak.energy.of.Zn(Mg,Cd)O.films.at.20.K.on.alloy.content.is.shown.in.Figure.11.7a..The.PL.peak.energy.of.ZnO.is.3.36.eV..The.PL.peak.energy.of.Zn(Mg,Cd)O.films.is.fitted.by.the.quadratic.function.of.alloy.content.(x,.y).as.in.the.following.equations:

. E x x xPLZnCdO,20K( ) . . . ,= − +3 36 4 49 4 18 2 . (11.5)

. E y y yPLMgZnO,20K( ) . . . .= + +3 36 0 75 3 71 2 . (11.6)

The.PL.FWHM.of.Zn(Mg,Cd)O.on.alloy.content.is.shown.in.Figure.11.7b..PL.FWHM.of.ZnO.is.36.meV..In.Zn1−xCdxO.under.Cd.content.x.of.0.2,.PL.FWHM.increases.with.increas-ing.x,.and.reaches.a.maximum.of.280.meV.at.x.of.0.2..When.x.is.over.0.2,.PL.FWHM.gradu-ally.deceases.with.increasing.x..FWHM.of.MgyZn1−yO.also.increases.up.to.140.meV.with.an.increase.up.to.Mg.content.of.0.2..Here.the.broadening.tendency.of.PL.of.Zn(Mg,Cd)O.films.is.in.agreement.with.the.increment.tendency.of.Stokes’.shift.shown.in.Figure.11.6b..These.parameters.represent.alloy.fluctuations.in.Zn(Mg,Cd)O.films.

The.alloy.broadening.has.been.observed.for.other. ternary.III–V.and.II–VI.compound.alloys.such.as.AlzGa1−zAs.[44],.AlzGa1−zN.[39],.Zn1−zCdzS.[38],.Zn1−zCdzSe.[38],.and.CdSzSe1−z.[37].due.to.their.small.exciton.Bohr.radius..Here.we.compare.with.the.theoretical.results.based.on.statistical.disorder.of.random.alloy..The.alloy.broadening.of.PL.in.A1−xBx.alloy.due.to.this.effect.is.given.by.the.following.equation.[38,44]:


0100200300400500

(b)

Stok

es’sh

ift (m

eV)

0.8 0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

(a)

Room temperature

Optical band gapPL peakOptical band gap, PLD

Ener

gy (e

V)

FIGURE 11.6(a).Energy.of.optical.bandgap.and.PL.peak.of.Zn(Mg,Cd)O.films.at.room.temperature.as.a.function.of.alloy.con-tent..The.dashed.lines.show.PLD-grown.Zn1−xCdxO.(Ref..5).and.MgyZn1−yO.(Ref..4)..(b).Stokes’.shift.of.Zn(Mg,Cd)O.films.as.a.function.of.alloy.content..The.solid.line.shows.the.tendency.as.guide.to.the.eyes..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural.and.optical.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permission.from.Elsevier.)


.∆ x

dE xdx

x xV xV x

( ) = ( )

−( ) ( )( )2 2 2 1 0ln ,ex

ex. (11.7)

if.a.Gaussian.line.shape.is.assumed,.where.Eex(x).is.the.exciton.transition.energy.that.is.dependent.on.alloy.content.x. V0(x).is.the.volume.of.the.elementary.cell,.and.Vex(x).is.the.volume.of. the.exciton..Schubert.et.al..have.derived.the.relevant.exciton.volume.using.a.statistical.theory.expressed.by

.V x r xex B( ) = ( )

43

3π, . (11.8)

where.rB(x).denotes.the.Bohr.radius.of.exciton..The.volume.of.elementary.cell.of.wurtzite.Zn(Mg,Cd)O.is.estimated.by

.V x

Na x c x0

21 3 32

( ) = ( ) ( ) , . (11.9)

where.N.=.6.is.the.number.of.cations.in.the.unit.cell.for.wurtzite.structure.and.a(x).and.c(x). are. lattice. constants.of.Zn(Mg,Cd)O.alloys.. In. this. calculation,.we.assume. that. the.crystal.structure.of.CdO.and.MgO.is.wurtzite,.while.the.rock.salt.structure.is.stable.for.CdO.and.MgO..PL.FWHM.Δ(x).values.have.been.calculated.for.Zn(Mg,Cd)O.alloys.using.rB.values.of.ZnO,.CdO,.and.MgO.shown.in.Table.11.1..Since.rB.of.CdO.is.not.available,.we.have.assumed.that.rB.in.CdO.is.1.5.times.to.rB.in.ZnO.using.the.ratio.of.exciton.Bohr.

1.5

2.0

2.5

3.0

3.5

4.0

4.5

(a)

20 K

PL p

eak

ener

gy (e

V)


0100200300400500

(b)

PL F

WH

M (m

eV)

0.8 0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0

FIGURE 11.7(a).PL.peak.energy.of.Zn(Mg,Cd)O.films.at.20.K.as.a.function.of.alloy.content.(x,.y)..Solid.curves.are.fitted.by.quadric.of.alloy.content.(x,.y)..(b).PL.FWHM.of.Zn(Mg,Cd)O.films.on.alloy.content..The.dashed.lines.show.the.alloy.broadening.calculated.using.Equation.11.7..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,. Ohashi,. T.,. Tawara,. T.,. Gotoh,. H.,. Nakamura,. A.,. and. Temmyo,. J.,. Structural. and. optical. properties. of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permis-sion.from.Elsevier.)


radius.of.CdSe.(4.34.nm)/ZnSe.(3.3.nm).and.CdS.(3.0.nm)/ZnS.(2.16.nm).that.is.around.1.3..For.the.dependence.of.rB.on.alloy.content,.a.linear.variation.with.alloy.content.has.been.assumed..As.shown.in.Figure.11.7b,.PL.FWHM.of.Zn1−xCdxO.and.MgyZn1−yO.are.maxima.at.Cd.content.0.15.and.at.Mg.content.0.85,.respectively..The.tendency.of.the.experimental.PL.FWHM.of.Zn(Mg,Cd)O.on.alloy.content.is.in.good.agreement.with.the.theoretical.esti-mated.results..However,.the.maximum.PL.FWHM.(270.meV).of.Zn1−xCdxO.is.three.times.higher.than.that.(90.meV).of.the.estimation..O’Donnell.et.al..have.reported.that.the.large.PL.FWHM.of.InGaN.alloys.showing.PL.peak.energy.of.2.5.eV.at.15.K.is.200.meV.[45]..It.seems.that.large.PL.FWHM.of.Zn(Mg,Cd)O.is.affected.not.only.by.the.statistical.alloy.fluc-tuation,.but.also.the.localization.of.exciton.as.observed.in.the.InGaN-system.[46]..Alloy.broadening.of.PL.of.Zn(Mg,Cd)O.have.been.previously.reported.only.on.MgyZn1−yO.by.Heitsch.et.al..[40]..The.report.also.shows.that.the.PL.FWHM.of.MgyZn1−yO.compare.with.theoretical.alloy.broadening.by.Zimmermann’s.and.Shubert’s.model,.and.the.increment.tendency.of.PL.FWHM.is.the.same.as.the.alloy.broadening.

In.order.to.evaluate.the.localized.depth.of.exciton.in.Zn(Mg,Cd)O.films,.we.have.mea-sured.the.TRPL.spectroscopy.at.8.K..Mg0.03Zn0.97O.is.measured.because.of.the.limit.of.the.excitation. energy. of. 3.49.eV.. First. we. have. analyzed. the. PL. lifetime. of. Zn(Mg,Cd)O.. PL.decay.curves.of.Zn(Mg,Cd)O.at.8.K.are.shown.in.Figure.11.8..Figure.11.8a.shows.decay.curves.of.ZnO,.Zn0.89Cd0.11O,.and.Mg0.03Zn0.97O,.and.PL.decay.curves.of.Zn1−xCdxO.with.high.Cd.content.(0.30,.0.55).are.shown.in.Figure.11.8b..PL.decay.curves.are.well.fitted.by.double.exponential.function.I(t).=.I1exp(−t/τ1).+.I2exp(−t/τ2).and.triple.exponential.function.I(t).=.I1exp(−t/τ1).+.I2exp(−t/τ2).+.I3exp(−t/τ3),.respectively..PL.lifetime.(τ1,.τ2,.τ3).of.Zn(Mg,Cd)O.on.alloy.content. is.shown.in.Figure.11.9..PL. lifetime.τ2.of.ZnO.is.125.ps..The.PL. life-time.gradually.increases.with.increasing.Cd.content.under.0.19,.showing.a.typical.slow.PL.lifetime.τ2.of.125.ps..At.Cd.content.over.0.3,.it.drastically.increases.up.to.54.ns.and.is.two.orders.longer.than.that.in.ZnO..The.PL.lifetime.in.semiconductors.with.indirect.transition.is.known.to.be.longer.than.that.in.a.semiconductor.with.direct.transition..We.are.able.to.speculate. that. the. critical. ZnCdO. possibly. has. an. indirect. transition. element,. while. no.rock.salt.type.CdO.(111).was.observed.by.XRD..PL.lifetime.of.Mg0.03Zn0.97O.is.measured.because.of. the. limit.of. the.excitation.energy.for.TRPL.measurement,.and.is. three.times.larger.than.that.of.ZnO.

TABLE 11.1

Parameters.for.ZnO,.CdO,.and.MgO.Used.in.Equations.11.7.through.11.9

Structure rB (nm) Vex (nm3) a (nm) c (nm) V0 (nm3)

ZnO Wurtzite 1.8 147 0.3250 0.5205 0.143CdO Wurtzitea 2.5 393 0.3546b 0.5733b 0.183MgO Wurtzitea 0.8c 13 0.3281 0.5033 0.141

Source:. Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural. and. optical. properties. of. Zn(Mg,Cd)O. alloy. films.grown. by. remote-plasma-enhanced. MOCVD,. 1703,. 2010..Copyright.2010,.with.permission.from.Elsevier.

a. In.the.calculation,.we.assume.that.the.crystal.structure.of.CdO.and.MgO.is.wurtzite,.while.the.rock.salt.structure.is.stable.for.CdO.and.MgO.

b. Ref..[47].c. Ref..[43].


Figure.11.10a.and.b.shows.the.normalized.time-integrated.PL.(TIPL).spectra.at.8.K.and.a.summary.of.slow.component.PL.lifetime.τslow. (τ2,.τ3).dispersion.to.photon.energy..We.discuss. slow. components.τ2. and. τ3. because. the. fast. decay. component. τ1. most. probably.represents.the.nonradiative.recombination..The.τ2.and.τ3.values.increase.with.decreasing.photon.energy,.showing.the.characteristic.of.the.localized.system,.where.the.decay.of.exci-ton.consists.of.both.radiative.recombination.and.the.transfer.process.to.the.tail..The.depth.of.the.localization.was.evaluated.by.assuming.the.exponential.distribution.of.the.density.

Decay time (ns)

PL in

tens

ity (a

. u.)

(a)0.20.0 0.4 0.6 0.8 1.0

Laser

8 K

x = 0.11

y = 0.03

x = 0 (ZnO)

Decay time (ns)

PL in

tens

ity (a

. u.)

(b)0 100

Laser

200 300

x = 0.30

x = 0.55

400 500

FIGURE 11.8PL. decay. curves. of. Zn(Mg,Cd)O. films. at. 8.K.. (a). Mg0.03Zn0.97O,. ZnO,. and. Zn0.89Cd0.11O.. (b). Zn0.70Cd0.30O. and.Zn0.45Cd0.55O..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural.and.optical.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permission.from.Elsevier.)

in MgyZn1–yO in Zn1–xCdxO

0.1101

PL li

fetim

e τ (p

s)

102

103

104

105

106

0.0 0.1 0.2 0.3xy

0.4 0.5 0.6

τ3

τ2

τ1

FIGURE 11.9PL.lifetime.(τ1,.τ2,.τ3).of.Zn(Mg,Cd)O.films.on.alloy.content.(x,.y).at.8.K..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural.and.opti-cal.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permission.from.Elsevier.)


of.the.tail.states.and.by.fitting.the.photon.energy.dependence.of.τslow.values.using.the.fol-lowing.equation.given.by.Gourdon.and.Lavallard.[48]:

.τ τ

EE E E

( ) =+ −( )

r

me1 0exp, . (11.10)

whereτr.is.the.radiative.lifetimeEme.is.mobility-edge.energyE0.is.the.localized.depth.(potential.fluctuation)

The.localized.depth.E0.in.Mg(Zn,Cd)O.alloys.are.plotted.as.a.function.of.alloy.content.(x,.y).in.Figure.11.11a..E0.in.ZnO.is.8.meV..E0.in.Zn1−xCdxO.drastically.increases.up.to.160.meV.with.an.increase.up.to.Cd.content.of.0.2..Over.Cd.content.of.0.2,.E0.decreases.with.the.Cd.content..The.PL.FWHM.of.MBE-grown.Zn1−xCdxO.shown.as.the.comparison.is.the.same.as.the.tendency.in.this.work.[49].

Photon energy (eV)(b)

(a)

8 Kx=0.55 x=0.30 x=0.19 x=0.18 x=0.11 x=0 y=0.03

Life

time τ

slow

(ps)

Nor

mal

ized

TIP

L in

tens

ity

2.0101

102

103

104

105

106

2.5 3.0 3.5

τ3

τ2

E 0.19E 0.30E 0.55E 0.18 E 0.11

E x=0E y=0.03

me me me me meme

me

FIGURE 11.10(a). Time-integrated. PL. (TIPL). spectra. of. Zn(Mg,Cd)O. films. at. 8.K. and. (b). slow. component. PL. lifetime. τslow.(τ2.and.τ3).dispersion.to.photon.energy..The.open.circle.and.the.open.triangle.show.τ2.and.τ3,.respectively..The.solid.curves.are.results.fitted.by.using.Equation.11.10..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural.and.optical.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permis-sion.from.Elsevier.)


The.increment.of.E0.is.in.agreement.with.the.tendency.of.TIPL.shown.in.Figure.11.11b..This. means. the. localization. of. exciton. in. Zn1−xCdxO. strongly. affects. PL. FWHM.. E0. of.Mg0.03Zn0.97O. grown. by. RPE-MOCVD. is. slightly. larger. than. that. of. ZnO.. PLD-grown.Mg0.08Zn0.92O.[50].and.MBE-grown.Mg0.15Zn0.85O.[51].are.shown..When.considering.these.data.for.MgyZn1−yO,.it.is.found.that.the.increment.of.E0.with.alloying.in.MgyZn1−yO.agrees.well.with.that.of.TIPL.FWHM.

11.4 Characteristics of Zn0.85Cd0.15O/ZnO MQWs

ZnCdO-based.heterostructures.have.been.fabricated.and.characterized.on.PL.properties.[52]..In.order.to.improve.internal.quantum.efficiency.due.to.low-dimensional.quantum-confinement.effects,.the.formation.of.the.quantum.structures.is.needed..Various.nanostruc-tures.such.as.QWs.[53–55],.nanorods.[56],.and.dots.[57,58].have.been.investigated..While.the.growth.and.optical.properties.of.Zn1−xCdxO-based.multiple.quantum.wells.(MQWs).by.MBE.have.been.recently.reported.in.terms.of.radiative.and.nonradiative.recombina-tion.processes.[53],.hydrogen.effects.on.radiative.efficiency.[54],.and.polarization-induced.electric.field.effects.on.dramatic. increase.of. the. lifetime. [55],.very. few.data. seem. to.be.


0

50

100

150

200

(b)

TIPL

FW

HM

(meV

)Lo

caliz

ed d

epth

, E0 (

meV

)

(a) 0

50

100

150

200 RPE-MOCVD RPE-MOCVD

8 K

MBE-(Buyanova et al.)PLD (Muller et al.)MBE (Shibata et al.)

0.8 0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0

FIGURE 11.11(a). The. localized. depth,. E0,. of. Zn(Mg,Cd)O. films. at. 8.K. and. (b). TIPL. FWHM. as. a. function. of. alloy. content.(x,. y).. Solid. circles. and. solid. squares. show. the. data. from. RPE-MOCVD-grown. Zn1−xCdxO. and. MgyZn1−yO,.respectively..The.open.circle.shows.MBE-grown.Zn1−xCdxO.(Ref..49)..The.open.square.and.open.triangle.show.PLD-grown.(Ref..50).and.MBE-grown.MgyZn1−yO.(Ref..51),.respectively..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Tsuboi,.T.,.Ohashi,.T.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Structural.and.opti-cal.properties.of.Zn(Mg,Cd)O.alloy.films.grown.by.remote-plasma-enhanced.MOCVD,.1703,.2010..Copyright.2010,.with.permission.from.Elsevier.)


available.. We. describe. the. synthesis. of. Zn0.85Cd0.15O/ZnO. MQWs. by. RPE-MOCVD. and.discuss.the.optical.properties.by.using.TRPL.spectroscopy,.focusing.on.the.dependency.of.the.PL.lifetime.and.the.associated.oscillator.strength.of.the.exciton.[59].

11.4.1 Structural Properties

The. wurtzite. (0001). Zn0.85Cd0.15O/ZnO. MQWs. were. grown. on. a-plane. (11–20). sapphire.substrate.by.RPE-MOCVD..The.10-period.MQW.consisting.of.Zn0.85Cd0.15O.well.(Lw)/ZnO.barrier.(LB.=.10.nm).is.sandwiched.between.ZnO.buffer.layer.(100.nm.thick).and.ZnO.cap-ping.layer.(30.nm.thick)..The.Zn0.85Cd0.15O.well.width.(Lw).varied.from.2.to.21.nm..(0001).Zn0.85Cd0.15O.bulk.grown.on.(11–20).sapphire.substrate.was.used.as.the.reference.of.MQWs..The.thickness.of.each.layer.in.MQWs.was.controlled.by.tuning.the.growth.time.accord-ing.to.the.growth.rate..The.growth.rate.of.Zn0.85Cd0.15O.and.ZnO.is.estimated.from.these.films.grown.on.(11–20).sapphire.substrate,.and.is.2.37.and.1.70.nm/min,.respectively..We.have. confirmed. that. the.film. thickness. linearly. increases.with. increasing.growth. time..The.periodicity.between.Zn0.85Cd0.15O.well.layer.and.ZnO.barrier.layer.in.MQWs.was.con-firmed.by.the.XRD.satellite.pattern..Figure.11.12a.shows.a.typical.XRD.satellite.pattern.of.(0002).diffraction.from.Zn0.85Cd0.15O/ZnO.MQWs.having.Lw.of.2.nm.and.LB.of.10.nm..The.expected.periodic.length.(Lw.+.LB).of.the.MQWs.is.12.nm..The.XRD.satellite.peaks.repre-sented.the.existence.of.the.periodicity.in.MQWs..The.satellite.peaks.were.observed.up.to.the.first.order.while.the.XRD.pattern.contains.the.peak.of.Al2O3.(11–20).substrate.Cu.Kβ..The.intensity.of.the.satellite.peak.is.low..It.seems.that.the.interdiffusion.of.Cd.occurs.at.Zn0.85Cd0.15O/ZnO.interface.in.MQWs.during.growth,.caused.by.the.high.vapor.pressure.of.CdO..The.sin(2θ/2).at.the.satellite.peak.angle.as.a.function.of.the.order.of.the.satellite.peak.is.shown.in.Figure.11.12b..The.line.shows.the.designed.periodicity..It.is.found.that.the.periodic.length.of.MQWs.is.close.to.the.designed.value.

2θ (degree)

Inte

nsity

(cou

nts)

sin(2

θ/2)

Al 2O

3(11–

20) C

uKβ

ZnCd

/ZnO

(000

2)

Order of satellite peak, n

0.27

0.28

0.29

0.30

0.31

0.32

(b)(a)

0

–2–33635343332100

101

102

103

104

–1 0 1 2 3

–1

FIGURE 11.12(a).XRD.satellite.pattern. for. (0002).diffraction. from. the.Zn0.85Cd0.15O/ZnO.MQWs. (Lw.=.2.nm).. (b). sin(2θ/2).as.a. function. of. the. order. of. the. satellite. peak.. Solid. circles. and. dashed. line. show. the. experimental. data. and.the.design.value,.respectively..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Adachi,.M.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,.J.,.Synthesis.and.characterization.of.ZnCdO/ZnO.multiple.quantum.wells.by.remote-plasma-enhanced.MOCVD,.1496,.2010..Copyright.2010,.with.permission.from.Elsevier.)


11.4.2 Optical Properties

SSPL. spectra. from. Zn0.85Cd0.15O/ZnO. MQWs. at. 20.K. are. shown. in. Figure. 11.13a.. SSPL.measurement.was.investigated.by.employing.a.cw.He-Cd.laser,.as.excitation.source,.with.the.power.density.of.1.2.W/cm2..PL.peak.energy.as.a.function.of.well.width.Lw.in.MQWs.is.shown.in.Figure.11.13b..PL.peak.energy.of.140.nm.thick.Zn0.85Cd0.15O.bulk.is.2.77.eV..PL.peak.energy.of.MQWs.shows.a.blueshift.from.2.77.to.2.97.eV.with.decreasing.Lw.from.21.down.to.2.nm..This.emission.seems.to.have.radiative.transitions.between.quantum.energy.levels..We.can.estimate.the.quantum.energy.levels.using.the.finite-square-potential.model..In.the.calculation,.we.need.the.conduction.to.valence.band.offset.ratio.(ΔEc/ΔEv).and.the.effective.masses.in.Zn0.85Cd0.15O..X-ray.photoelectron.spectroscopy.has.provided.the.ΔEc/ΔEv.of.64/36.for.Zn0.95Cd0.05O/ZnO.heterostructure.[60]..When.using.ΔEc/ΔEv.of.64/36.and.the.effective.masses.in.ZnO.(me.=.0.28.m0,.mhh.=.0.59.m0).[55],.we.obtain.a.good.fit..Here,.m0.is.the.free.electron.mass..ΔEc.and.ΔEv.are.250.and.140.meV,.respectively..The.energy-shift.well.agrees.with.the.calculation..No.quantum-confinement.Stark.effect.(QCSE).was.observed,.while. QCSE. in. polar. c-oriented. ZnO-based. QWs. due. to. the. internal. electric. field. was.observed.[55]..FWHM.of.PL.as.a.function.of.Lw.is.shown.in.Figure.11.13c..PL.FWHM.from.ZnCdO.bulk.is.around.200.meV..PL.FWHM.from.MQWs.slightly.increases.with.decreasing.Lw.under.4.nm.due.to.the.disordered.interface.between.well.and.barrier.

TRPL.measurement.using.a.streak.camera.system.with.frequency-doubled.beam.of.a.mode-locked.Ti:sapphire.laser.with.a.pulse.width.of.1.5.ps.and.repetition.rate.of.4.MHz.

Lw=2 nm

Zn0.85 Cd0.15O bulk140 nm

11 nm

8 nm

4 nm

(a)

ZnCdO 20 KZnO


Nor

mal

ized

PL

inte

nsity

Well width, Lw (nm)

(b)

(c)

ExperimentCalculation

00

50

100

PL F

WH

M (m

eV)

PL p

eak

ener

gy (e

V)150

200

250

300

2.75

2.80

2.85

2.90

2.95

3.00

5 10 15 20 25 Bulk

FIGURE 11.13(a).Normalized.PL.spectra.from.Zn0.85Cd0.15O/ZnO.MQWs.at.20.K..The.peaks.with.a.triangle.indicate.PL.compo-nent.from.Zn0.85Cd0.15O.well.layer..(b).Dependence.of.PL.peak.energy.of.MQWs.on.well.width,.Lw..Solid.circles.and.dashed.line.show.the.experimental.and.the.calculated.result,.respectively..(c).Dependence.of.PL.FWHM.on.Lw..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Adachi,.M.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,. J.,.Synthesis.and.characterization.of.ZnCdO/ZnO.multiple.quantum.wells.by. remote-plasma-enhanced.MOCVD,.1496,.2010..Copyright.2010,.with.permission.from.Elsevier.)


were.performed.at.low.temperature..The.excitation.energy.and.power.density.were.3.49.eV.and.1.6.μJ/cm2,.respectively..Figure.11.14.shows.PL.decay.curves.from.(a).Zn0.85Cd0.15O.bulk.and.(b).MQWs.having.Lw.of.2.nm.recorded.at.8.K..The.PL.decay.curves.were.well.described.by.a.bi-exponential.decay.function.I(t).=.I1.exp(−t/τ1).+.I2.exp(−t/τ2)..The.solid.lines.show.the.fitted.curves.in.Figure.11.14.

The. τ1. and. τ2. in. Zn0.85Cd0.15O. bulk. are. 16. and. 80.ps,. respectively.. By. utilizing. the.Gourdon’s.formula.based.on.the.localized.exciton.model.[48],.we.have.evaluated.that.the.depth.of.localization.E0.and.mobility-edge.Eme.of.reference.bulk.system.are.200.meV.and.2.88.eV,.respectively..The.PL.lifetime.τ1.and.τ2.in.MQWs.are.13.and.54.ps,.respectively..We.discuss.the.slow.component.τ2.because.the.fast.component.τ1.most.probably.represents.the.nonradiative. recombination..As. the. temperature. increases,.τ1.decreases.and.τ1.becomes.dominant,.which.is.generally.accepted.to.be.due.to.the.enhanced.nonradiative.process.at.higher.temperature,.as.reported.in.Ref..[61].

The. τ1. and. τ2. in. MQWs. as. a. function. of. Zn0.85Cd0.15O. well. width,. Lw,. is. shown. in.Figure.11.15a..PL.lifetime.τ2.in.MQWs.gradually.decreases.from.70.to.54.ps.with.decreasing.Lw.from.8.to.2.nm,.indicating.the.enhancement.of.the.exciton.recombination.in.Zn0.85Cd0.15O.well.layer..We.discuss.this.decreasing.tendency.in.the.viewpoints.of.the.enhancement.of.

Decay time (ps)(b)

(a)

8 K

PL in

tens

ity (a

. u.)

PL in

tens

ity (a

. u.)

2000 400 600 800

τ1=13 ps

τ1=16 ps

τ2=80 ps

MQWs, Lw=2nm

Zn0.85Cd0.15O bulk layer

τ2=54 ps

FIGURE 11.14PL.decay.curves.from.(a).Zn0.85Cd0.15O.bulk.and.(b).Zn0.85Cd0.15O/ZnO.MQWs.having.Lw.of.2.nm,.recorded.at.8.K..The.solid.lines.show.the.decay.curves.fitted.by.the.double.exponential.function.of.I(t).=.I1.exp(−t/τ1).+.I2.exp.(−t/τ2)..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Adachi,.M.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,. J.,.Synthesis.and.characterization.of.ZnCdO/ZnO.multiple.quantum.wells.by. remote-plasma-enhanced.MOCVD,.1496,.2010..Copyright.2010,.with.permission.from.Elsevier.)


exciton.recombination.model.in.accordance.with.the.treatment.by.Ploog.et.al..[62]..It.is.well.known.that.GaAs/AlzGa1−zAs.MQW.system.shows.the.tendency.of.a.decrease.of.the.PL.lifetime.with.thinner.well.widths,.indicating.the.enhanced.recombination.due.to.the.local-ization.of.the.carrier.in.the.MQWs.[62,63]..This.tendency.fairly.corresponds.to.our.results.on.the.ZnO-based.MQWs.discussed..Here,.we.describe.the.oscillator.strength.of.the.exci-ton.in.our.system..The.oscillator.strength. f.of. the.exciton.in.Zn0.85Cd0.15O/ZnO.MQW.is.estimated.from.the.measured.PL.lifetime.τ2.by.using.the.equation.below.[63]:

.f

m cne R

= 2 0 03

2 2

πεω τ�

, . (11.11)

whereñ.is.the.refractive.index.of.ZnO.(ñ.=.2.2)ε0.is.the.dielectric.constant.of.vacuumm0.ισ.the.electron.rest.massc.is.τηε.speed.of.light.in.vacuume.is.τηε.electric.chargeω.is.the.angular.frequency.at.PL.peak.energyτR.is.the.radiative.lifetime

Well width, Lw (nm)

Osc

illat

or st

reng

thPL

life

time,

τ 1, τ2 (p

s)

00.0

0.5

1.0

1.5

0

20

40

60

80

100

(a)

(b)

8 K

5 10 15 20 25 Bulk

τ2

τ1

FIGURE 11.15(a). Dependence. of. PL. lifetime. (τ1,. τ2). in. Zn0.85Cd0.15O/ZnO. MQWs. at. 8.K. on. Lw.. (b). Dependency. of. oscillator.strength.estimated.using.τ2.in.Equation.11.11.on.Lw..The.oscillator.strength.in.MQWs.is.normalized.by.bulk’s.value..(Reproduced.from.J. Cryst. Growth,.312,.Yamamoto,.K.,.Adachi,.M.,.Tawara,.T.,.Gotoh,.H.,.Nakamura,.A.,.and.Temmyo,. J.,.Synthesis.and.characterization.of.ZnCdO/ZnO.multiple.quantum.wells.by. remote-plasma-enhanced.MOCVD,.1496,.2010..Copyright.2010,.with.permission.from.Elsevier.)


We.utilize.τ2.measured.at.a.low.temperature.of.16.K.as.the.radiative.lifetime.τR..Here,.the.agreement.of. f.values.between. the.experimental.and.the.calculated.results.seems.to.be.unclear.even.in.GaAs/AlzGa1−zAs.MQWs.[64]..The.dependency.of.the.oscillator.strength.on.well.width.is.shown.in.Figure.11.15b..The.oscillator.strength.in.MQWs.is.normalized.by.the.value.in.the.Zn0.85Cd0.15O.bulk..The.oscillator.strength.in.Zn0.85Cd0.15O/ZnO.MQWs.increases.up.to.1.28.(Lw.of.2.nm).with.decreasing.well.width..The.results.clearly.show.that.the.exciton.recombination.is.enhanced.in.the.MQWs.

11.5 Light-Emitting Devices

For.optoelectronic.devices,.overcoming.technical.issues.on.heterostructure.formation.and.doping.is.required..ZnO.easily.change.to.n-type.conductivity.due.to.donor-related.defects.such.as.oxygen.vacancy.and/or.interstitial.zinc..In.addition,.due.to.self-compensation,.it.is.difficult.to.obtain.a.high.hole.concentration.of.around.1018.cm−3..While.ZnO.homo-junction.diodes.have.been.fabricated.[24],.the.EL.came.via.the.defect-related.levels.[3]..Therefore,.many. ZnO-based. heterojunctions. with. various. p-type. semiconductors. as. hole. supplier.have.been.fabricated,.and.EL.emissions.have.been.demonstrated..We.have.utilized.p-type.4H-SiC.as.hole.supplier.and.growth.substrate..Three.points.that.show.the.advantages.of.hole.supplier.for.n-ZnO.are.as.follows:.(1).High.hole.concentration.of.1018.cm−3.in.p-type.4H-SiC.is.commercially.available;.(2).the.bandgap.of.4H-SiC.is.3.26.eV,.which.is.very.close.to.ZnO.(3.28.eV);.and.(3).4H-SiC.has.a.hexagonal-type.crystal.structure.(a-axis.lattice.constant;.a4H-SiC.=.0.3076.nm,.aZnO.=.0.3240.nm).and.has.a.relatively.good.lattice.matching.to.ZnO,.with.a.small.lattice.mismatch.of.5.3%..A.realization.of.high-quality.ZnO.optoelectronic.devices.due.to.these.advantages.is.expected..In.this.case,.the.device.configuration.is.p-side.down.while.the.general.devices.are.n-side.down..High.thermal.conductivity.of.4.9.W.cm−1K−1.in.4H-SiC.is.able.to.take.p-side.down..We.have.demonstrated.visible.(blue,.green,.and.red).EL.emissions.from.Zn1−xCdxO-based.heterojunctions.utilizing.p-4H-SiC.substrates.[25,28]..When.a.shorter.wavelength.emission.n-Zn(Mg,Cd)O/p-4H-SiC.heterojunction.is.designed,.the. band. alignment. may. become. type-II. according. to. a. relationship. between. electron.affinity,.χ,.and.bandgap.energy,.Eg,.in.each.semiconductor..However,.since.an.uncertainty.of.electron.affinity.in.4H-SiC.is.large,.it.is.difficult.to.determine.the.band.alignment,.type-I.or.-II,.only.from.the.relationship.between.χ.and.Eg..Here,.the.band.lineup.in.the.hetero-junctions.is.experimentally.determined.by.evaluating.the.EL.properties.of.the.heterojunc-tions.consisting.of.n-Mg0.19Zn0.81O.barrier/n-Zn(Mg,Cd)O.active/p-4H-SiC.substrates.with.various.n-Zn(Mg,Cd)O.with.different.bandgaps..A.hole. injection.toward.n-Zn(Mg,Cd)O.region.and.recombination.processes.are.discussed,.taking.into.account.band.offsets.at.the.heterointerface.

A.schematic.diagram.of.the.heterojunction.with.n-Mg0.19Zn0.81O/n-Zn(Mg,Cd)O.on.p-4H-SiC.substrate. fabricated.by.RPE-MOCVD. is. shown. in.Figure.11.16a..The.heterojunction.consisted. of. 200.nm. thick. non-intentionally. n-doped. Zn(Mg,Cd)O. layer. on. p-4H-SiC:Al.(0001). substrate,.and.400.nm.thick.non-intentionally.n-doped.Mg0.19Zn0.81O.barrier. layer,.and.non-intentionally.n-doped.ZnO.contact.layer..Indium.was.used.as.an.ohmic.contact.to. the. non-intentionally. n-doped. ZnO. contact. layer.. The. backside. contact. was. formed.with. an. aluminum. layer.. The. non-intentionally. n-doped. Zn(Mg,Cd)O. was. grown. for.Zn0.87Cd0.13O.(Eg.=.2.80.eV),.Zn0.92Cd0.08O.(3.00.eV),.Zn0.95Cd0.05O.(3.11.eV),.ZnO.(3.28.eV),.and.Mg0.14Zn0.86O.(3.52.eV).as.shown.in.Figure.11.16b..Commercial.p-4H-SiC:Al.substrate.had.


a.hole.concentration.of.2.×.1018.cm−3..The.electron.concentrations.of.the.non-intentionally.n-doped.Mg0.19Zn0.81O.barrier.layer.and.the.non-intentionally.n-doped.ZnO.contact.layer.were.6.×.1019.cm−3.and.7.×.1018.cm−3,.respectively.

Figure. 11.17a. shows. typical. EL. spectra. from. n-Zn(Mg,Cd)O/p-4H-SiC. heterojunctions.at.room.temperature..The.n-Zn0.87Cd0.13O/p-4H-SiC.heterojunction.had.EL.peak.at.2.4.eV.and.the.n-Zn0.93Cd0.07O/.p-4H-SiC.heterojunction.had.EL.peak.at.2.8.eV..On.the.other.hand,.the. heterojunctions. with. n-ZnO/p-4H-SiC. and. with. n-Mg0.14Zn0.86O/p-4H-SiC:Al. had.main. EL.peak. at. 2.8.eV.with. a. shoulder. peak. at. around.2.5.eV..Figure. 11.17b. shows. the.dependence.of.the.alloy.content.of.n-Zn(Mg,Cd)O.on.EL.peak.emission.energy..Dashed.line. shows. PL. peak. emission. energy. of. Zn1−xCdxO. film. and. MgyZn1−yO. film. grown. on.sapphire.substrate.. It.was.found.that.decreasing.the.Cd.content.x.down.to.around.0.07,.the.EL.emission.energy.increased.according.to.the.increasing.tendency.of.the.PL.emis-sion.energy..However,.above.the.optical.bandgap.of.n-Zn0.95Cd0.05O.(∼3.1.eV),.EL.emission.energy.acquired.a.constant.value.of.2.8.eV,.in.spite.of.their.larger.PL.emission.energy.of.the.corresponding.Zn(Mg,Cd)O.films..Figure.11.17c.shows.EL.FWHM.versus.the.alloy.content.of.n-Zn(Mg,Cd)O..The.FWHM.increased.from.0.6.to.1.eV.with.decreasing.the.Cd.content.x.down.to.around.0.05..However,.FWHM.was.saturated.at.1.eV.above.Cd.content.in.the.region.where.EL.emission.energy.became.constant..By.applying.the.Anderson.model.for.these.heterojunctions,.it.was.considered.that.these.ELs.were.coming.from.the.n-Zn(Mg,Cd)O.and/or.p-4H-SiC.substrate.via.different.radiative.recombination.processes.depending.on.the.alloy.content.of.n-Zn(Mg,Cd)O.

Figure.11.18a.shows.the.EL.spectrum.from.n-ZnO/p-4H-SiC.heterojunction.at.room.tem-perature.under.the.injection.current.of.200.mA..As.compared.with.PL.from.p-4H-SiC:Al.substrate,. broad. EL. emission. at. around. 2.8.eV. from. n-ZnO/p-4H-SiC:Al. heterojunction.originated.from.PL.emissions.at.2.9,.2.5,.and.1.9.eV..Since.the.density.of.the.excited.carri-ers.in.p-4H-SiC:Al.under.the.EL.condition.was.higher.than.that.under.the.PL.condition,.

Mg0.19Zn0.81O

Mg0.14Zn0.86O

Zn0.95Cd0.05O4H-SiC

Zn0.92Cd0.08O

Zn0.87Cd0.13O

Cd content, xin Zn1–xCdxO(b)

Band

gap

(eV)

0.62.0

2.5

3.0

3.5

4.0

0.5 0.4 0.3 0.2 0.1 0.0 0.1 0.2 0.3Mg content, yin MgyZn1–yOp-electrode Al

n-Zn(Mg,Cd)O active layer (200 nm)

n-Mg0.19Zn0.81O barrier (400 nm)

(a)

n-electrode inZnO contact (30 nm)

p-4H-SiC:Al (0001) substratep=2×1018 cm-3

FIGURE 11.16(a).Schematic.diagram.of.the.heterojunctions.with.n-Mg0.18Zn0.82O/n-Zn(Mg,Cd)O/p-4H-SiC.substrates..(b).The.bandgaps.of.Zn(Mg,Cd)O.used.as.an.active.layer.


it.was.considered.that.the.excited.carriers.were.trapped.by.the.radiative.transition.levels.at. 2.5. and. 2.9.eV. (observed. at. low. temperature. PL). as. well. as. 1.9.eV. (observed. at. room.temperature.PL)..From.this. result,.EL.emission.energy.of.2.8.eV.shown. in.Figure.11.18a.was. expected. to. be. related. with. the. recombination. process. via. these. transition. levels.in. p-4H-SiC:Al. substrate. under. the. strong. current. injection. condition.. PL. spectra. from.p-4H-SiC:Al.substrate.as.taken.from.a.low.temperature.to.room.temperature.are.shown.in.Figure.11.18b..Up.to.80.K,.PL.emission.energy.of.p-4H-SiC.was.2.9.eV..PL.emission.energy.was.2.5.eV.between.80.and.200.K,.and.was.1.9.eV.between.200.K.and.room.temperature..It.was.found.that.p-4H-SiC:Al.substrate.has.some.radiative.recombination.levels.caused.by.doped.aluminum..Therefore,.three.radiative.transition.levels.existed.in.p-4H-SiC:Al.sub-strate.as.shown.by.the.temperature-dependent.PL.spectra..Figure.11.19.shows.the.injection.current.dependence.of.EL.intensity.at.room.temperature..A.maximum.value.of.peak.at.2.9.eV.in.each.EL.spectrum.was.taken.as.the.EL.intensity..Under.all.injection,.current.EL.spectra.were.dominant.at.2.9.eV..EL.intensity.of.n-Zn0.93Cd0.07O/p-4H-SiC:Al.heterojunction.linearly.increased.with.the.injection.current,.indicating.hole.injection.to.n-Zn0.95Cd0.05O..In.the.case.of.the.heterojunctions.utilizing.n-ZnO.and.n-Mg0.14Zn0.86O,.under.lower.injection.current,.the.slope.k.(EL.intensity.∝.Currentk).was.estimated.as.0.16.and.0.36,.respectively..We.speculated.that.a.small.k.was.due.to.low.emission.efficiency.because.of.the.spatially.separated. transitions.at. the.heterointerface.of. the. type-II.heterojunctions..Under.higher.injection.current,.EL.intensity.linearly.increased.with.the.injection.current..In.addition,.EL.

Photon energy (eV)(a)

×2

×4Nor

mal

ized

EL

inte

nsity

1.5 2.0 2.5 3.0 3.5

n-Mg0.14Zn0.86O/p-4H-SiC

n-Zn0.95Cd0.05O/p-4H-SiC


n-ZnO/p-4H-SiC

Optical bandgap (eV)2.4 2.6 2.8 3.0 3.2 3.4 3.6

0.20.10.00.10.20.30.0

0.5

1.0

1.5

(c)

FWH

M (e

V)

y in MgyZn1–yOx in Zn1–xCdxO

FIGURE 11.17(a).EL.spectra.from.heterojunctions.at.room.temperature.under.injection.current.of.200.mA..Triangle.and.circle.show.the.main.peak.and.the.shoulder.peak,.respectively..(b).EL.peak.energy.and.(c).FWHM.of.heterojunctions.as.a.function.of.the.alloy.content.(x,.y).in.the.n-Zn(Mg,Cd)O.active.layer.


spectra.had.the.shoulder.peak.at.around.2.5.eV.(as.seen.in.Figure.11.18a)..These.dependen-cies.indicate.electron.injection.to.p-4H-SiC:Al.due.to.small.ΔEC.

Figure.11.20.shows.the.energy-band.diagrams.of.the.n-ZnO/p-4H-SiC:Al.heterojunction.under. (a). thermal. equilibrium. state. and. (b). forward. bias. voltage. of. 2.5.V,. based. on. the.Anderson.model.[65]..The.electron.affinities.χ.of.ZnO.and.4H-SiC.were.taken.as.4.05.eV.[66].and.3.5.±.0.3.eV.[67],.respectively..Acceptor.level.energy.of.Al.in.4H-SiC.was.191.meV.[68].. The. energetic. barrier. ΔEC. for. electrons. is. ΔEC.=.χZnO.−.χ4H-SiC.=.(4.05.−.3.5). eV.=.0.55.eV,.while.the.energetic.barrier.ΔEV.for.hole.is.ΔEV.=.ΔEg,ZnO.−.ΔEg,4H-SiC.+.ΔEC.=.(3.28.−.3.26.+.0.55).eV.=.0.57.eV.. Under. the. forward. bias. voltage. of. 2.5.V,. it. was. found. that. electrons. in. the.n-ZnO.region.might.be.injected.to.the.p-4H-SiC.region.and.holes.in.p-4H-SiC.region.might.be.blocked.by.the.barrier.of.ΔEV.at.heterointerface..As.seen.from.this.band.diagram,.two.radiative.recombination.processes.can.be.considered.to.cause.the.constant.EL.energy.of.2.8.eV..One.was.the.transition.of.carriers.in.the.p-4H-SiC.region.and.another.was.the.transi-tion.of.the.spatially.separated.electrons.and.holes.at.heterointerface.

Figure. 11.21. shows. the. alloy. content. dependence. of. ΔEC. and. ΔEV. for. n-Zn(Mg,Cd)O/p-4H-SiC..The.electron.affinities.of.CdO.and.MgO.were.4.35.and.1.37.eV.[66],.respec-tively..The.electron.affinity.of.Zn1−xCdxO.is.assumed.to.be.linearly.dependent.on.x.and.lies.between.4.05.eV.(ZnO).and.4.35.eV.(CdO)..In.the.case.of.MgyZn1−yO,.the.electron.affinity.is.also.assumed.to.be.linearly.dependent.on.y.and.lies.between.4.05.eV.(ZnO).and.1.37.eV.(MgO).. The. bandgaps. of. Zn1−xCdxO. (0.<.x.<.0.6),. ZnO,. and. MgyZn1−yO. (0.<.y.<.0.3). were.taken.from.our.experimental.results..In.the.case.of.heterojunctions.utilizing.n-Zn1−xCdxO.

Photon energy (eV)

Nor

mal

ized

PL in

tens

ityEL

inte

nsity

(a. u

.)

1.5 2.0

290 K×700

90 K×700

20 K

E(AI)

2.5(b)

(a)

3.0 3.5

PL p-4H-SiC

EL @ 200 mAn-ZnO/p-4H-SiC

FIGURE 11.18Injection.current.dependence.of.EL.intensity.of.the.heterojunctions.at.room.temperature..The.slope.k.is.power.value.in.the.equation.of.EL.intensity.∝.Currentk.


(0.05.<.x.<.0.6),. holes. in. p-4H-SiC. region. were. possibly. blocked. at. heterointerface. due. to.type-I.band.alignment..Therefore,.EL.spectra.have. the. intense.peak.. In. contrast,. in. the.case.of.the.heterojunction.utilizing.n-ZnxCd1−xO.(0.≤.x.<.0.05).and.n-MgyZn1−yO,.electrons.in.n-Zn(Mg,Cd)O.region.may.be.injected.to.p-4H-SiC.region.because.the.heterojunctions.have. a. type-II. band. alignment.. In. the. case. of. the. heterojunction. utilizing. n-ZnO. and.n-Mgy0.14Zn0.86O,.it.is.considered.that.electrons.in.n-Zn(Mg,Cd)O.can.easily.be.injected.to.p-4H-SiC.region.

For. EL. below. 2.8.eV. shown. in. Figure. 11.17b,. the. band. alignment. is. type-I,. and. the.recombination.process.due.to.band-to-band.transition.in.the.n-Zn(Mg,Cd)O.region.was.dominant..In.addition,.since.FWHM.was.large,.some.recombination.centers.and/or.transi-tions.at.heterointerface.also.contributed.to.the.EL..On.the.other.hand,.EL.at.2.8.eV.from.the.heterojunction.was.due.to.the.other.recombination.processes..We.speculated.for.the.transitions.in.p-4H-SiC:Al.substrate.and.the.transitions.of.spatially.separated.carriers.at.heterointerface.

Injection current (mA)

EL in

tens

ity (a

. u.)

100 101

k=0.36

(c)

k=0.16

k=0.96

k=0.97

k=0.97

102 103

n-Mg0.14Zn0.86O/p-4H-SiC

(a)


(b)

n-ZnO/p-4H-SiC

FIGURE 11.19Anderson.model.energy-band.diagram.of.n-ZnO/p-4H-SiC.heterojunction.under.(a).thermal.equilibrium.and..(b).a.forward.bias.voltage.of.2.5.V.


It. has. been. clarified. that. if. n-Zn1−xCdxO. (0.05.<.x.<.0.6). is. utilized. as. an. active. layer.to. obtain. visible. (blue,. green,. and. red). EL,. the. heterojunction. has. type-I. band. align-ment.. However,. if. n-Zn1−xCdxO. (0.≤.x.<.0.05). and. n-MgyZn1−yO. (0.<.y). are. utilized. for.shorter. wavelength. (violet,. UV). EL. emission,. the. band. alignment. in. the. heterojunc-tion.becomes.type-II..From.the.analysis.of.the.energy-band.diagram,.type-I.heterojunc-tions.for.shorter.wavelength.EL.emission.need.to.insert.p-MgyZn1−yO.barrier.between.an.active.layer.and.p-4H-SiC.substrate..We.have.fabricated.the.heterojunctions.consisting.of.n-Mg0.04Zn0.96O/n-Zn0.96Cd0.04O/p-Mg0.04Zn0.96O:N.on.p-4H-SiC.substrates.using.RPE-MOCVD..The.use.of.p-MgZnO.in.the.heterojunction.resulted.in.a.narrower.EL.spectra,.indicating.a.reduction.of.an.injection.of.electrons.toward.p-4H-SiC.region.due.to.type-I.heterojunctions.[26].

Recently,.in.order.to.improve.the.internal.quantum.efficiency,.we.have.fabricated.het-erojunctions.composed.of.five-stacked.Zn0.80Cd0.20O(7.nm)/Zn0.92Cd0.08O(10.nm).MQWs.on.p-4H-SiC.substrates.using.RPE-MOCVD.and.demonstrated.band-edge.green.EL.emission,.which.is.in.agreement.with.that.of.PL.from.the.MQWs.[69].

11.6 Summary and Outlook

The.growth.and.characteristics.of.wurtzite.Zn(Mg,Cd)O.alloys.and. its.heterostructures.using.RPE-MOCVD.were.described..We.systematically.characterized.the.dependence.of.the.structural.and.the.optical.properties.of.wurtzite.Zn(Mg,Cd)O.on.alloy.content..The.bandgap’s.bowing.parameter.is.3.0.for.Zn1−xCdxO.and.3.5.for.MgyZn1−yO.due.to.the.large.

n-ZnO (b)(a) n-ZnO

∆Ev=0.27 eV

∆Ec=0.25 eV

Eg,4H-SiC=3.26 eV

4HSiC = 3.9 eV

ZnO=4.05 eV

∆EA(AI)=0.19 eV

∆EA(AI)=0.19 eV

Eg,Zno=3.28 eV

Ev

EFEC EA

Ev

EC EcEF

EC

(3.5±0.3 eV)

Vacuumlevel

Vacuumlevel

Bais+2.5 V

3.07 eVEFEV

EAEFEV

p-4H-SiC p-4H-SiC

Zno=4.05 eV

FIGURE 11.20(a).EL.spectrum.from.the.n-Zn(Mg,Cd)O/p-4H-SiC.heterojunctions.at.room.temperature.under.an.injection.cur-rent.of.200.mA..(b).PL.spectra.from.the.p-4H-SiC:Al.substrate.taken.at.various.temperatures.


difference.of.electronegativity.between.ZnO.and.CdO.(MgO)..We.found.the.alloy.broaden-ing.of.PL.by.applying.the.theory.based.on.the.statistical.alloy.fluctuation.and.the.localiza-tion.of.exciton..We.have.clarified.that.the.alloy.broadening.of.steady-state.PL.FWHM.of.MgyZn1−yO.is.in.good.agreement.with.the.increment.tendency.of.PL.caused.by.the.statis-tical.alloy.fluctuation.on.alloy.content..PL.FWHM.of.Zn1−xCdxO.reaches.a.maximum.of.270.meV.at.Cd.content.of.0.2,.and.is,.however,.three.times.larger.than.the.calculated.value..To.confirm.the.alloy.broadening.via.another.method,.we.derived.the.localized.depth.of.exciton.in.Zn(Mg,Cd)O.alloys.at.8.K.by.applying.the.Gourdon.and.Lavallard.model.to.the.dispersion.of.PL.lifetime.on.photon.energy..The.increment.tendency.of.the.localized.depth.is.also.in.good.agreement.with.the.PL.FWHM..The.localization.of.exciton.in.Zn(Mg,Cd)O.alloys.strongly.affects.PL.FWHM.due.to.the.small.Bohr.radius.of.exciton.

In.10-period.Zn0.85Cd0.15O/ZnO.MQWs,.the.XRD.satellite.peaks.represented.the.existence.of.the.periodicity.in.MQWs..The.quantum.energy.levels.were.observed.by.the.blueshift.of.steady-state.PL.peak.at.low.temperature..Moreover,.PL.lifetime.τ2.decreases.with.decreas-ing.Zn0.85Cd0.15O.well.width,.indicating.the.enhancement.of.the.exciton.recombination.in.the.well.layer.

The.band.offset.of.the.heterojunctions.consisting.of.n-Zn(Mg,Cd)O/p-4H-SiC.has.been.analyzed. from. the. dependence. of. EL. emission. properties. on. alloy. content.. In. the. case.of.n-Zn1−xCdxO.(0.05.<.x.<.0.6).for.visible.EL.emission.in.the.range.between.blue.and.red,.the. band. alignment. of. the. heterojunctions. is. type-I.. EL. emission. comes. from. exciton.

y in MgyZn1–yOx in Zn1–xCdxO0.6

–1.0

–0.5

–1.0

–0.5

0.0

0.5

0.0

Ener

gy (e

V)En

ergy

(eV)

0.5

0.5 0.4

(a)

(b)

Type-IIType-I

0.3 0.2 0.1 0.0 0.1 0.2 0.3

FIGURE 11.21Band.offset,.(a).ΔEc.and.(b).ΔEv,.of.the.heterojunction.interface.of.n-Zn(Mg,Cd)O/p-4H-SiC.as.a.function.of.alloy.content.(x,.y).in.the.n-Zn(Mg,Cd)O.active.layer.


recombination.in.the.band.edge.of.n-Zn1−xCdxO.layer..In.contrast,.in.the.case.of.n-Zn1−xCdxO.(0.≤.x.<.0.05).and.n-MgyZn1−yO.(0.<.y.<.0.2).for.violet.and.UV.EL.emission,.the.band.align-ment.is.type-II..The.carrier.recombination.occurred.at.the.heterointerface.and.the.4H-SiC..A.need.of.the.introduction.of.p-MgyZn1−yO.barrier.into.the.devices.was.derived.in.order.to.obtain.type-I.heterojunctions.for.a.shorter.wavelength.EL.emission.

We.showed.the. feasibility.of. the.optoelectronic.devices.based.on.ZnO-based.systems.from.the.preceding.results.

Acknowledgments

The. authors. thank. Mr.. S.. Shigemori,. Mr.. J.. Ishihara,. Ms.. T.. Tsuboi,. Dr.. A.. Nakamura,.Mr.. T.. Ohashi,. Mr.. M.. Adachi,. Mr.. K.. Enomoto,. Dr.. G.. Zhang,. and. Dr.. S.. Gangil,. who.are.affiliated.with.RIE,.Shizuoka.University,.and.Dr..H..Gotoh.and.Dr..T..Tawara.of.Basic.Research.Laboratories,.NTT.Corporation,.Dr..T..Fujii.of.ROHM.Co.,.Ltd.,.for.enormous.con-tribution. in. the.measurement.study.and.fruitful.discussions..Dr..T..Fujii.of.ROHM.Co.,.Ltd..“as.below:”.The.authors.thank.Mr..S..Shigemori,.Mr..J..Ishihara,.Ms..T..Tsuboi,.Dr..A..Nakamura,.Mr..T..Ohashi,.Mr..M..Adachi,.Mr..K..Enomoto,.Dr..G..Zhang,.and.Dr..S..Gangil,.who.are.affiliated.with.RIE,.Shizuoka.University,.and.Dr..H..Gotoh.and.Dr..T..Tawara.of.Basic.Research.Laboratories,.NTT.Corporation,.Dr..T..Fujii.of.ROHM.Co.,.Ltd.,. for.enor-mous.contribution.in.the.measurement.study.and.fruitful.discussions.

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351

12Structural and Optical Properties of Zn1−xCuxO Thin Films

Ram S. Katiyar and Kousik Samanta

12.1 Introduction

The.field.of.Spintronics.is.multidisciplinary.in.nature;.the.core.concept.of.this.field.is.to.utilize.and.control.the.electron’s.charge.as.well.as.spin.degrees.of.freedom.in.the.semi-conducting. system.. This. emerging. field. has. led. to. an. extensive. search. for. materials. in.which.semiconducting.properties.can.be.integrated.with.magnetic.properties.in.order.to.realize.the.objective.of.fabrication.of.spin-based.devices.[1,2]..The.most.crucial.step.to.fab-ricate.a.practical.Spintronics.device.is.the.injection.of.sufficient.spin-polarized.carriers.into.the.semiconducting.system..The.prediction.of.magnetic.impurity-doped.semiconductors.(DMSs).can.provide.an.enabling.breakthrough.in.achieving.high.spin-injection.efficiency.[3,4]..The.3d.transition.metal.(TM)-doped.ZnO.can.be.the.most.promising.dilute.magnetic.semiconductor.(DMS).at.room.temperature.for.Spintronics.applications.[3,5]..Early.theoret-ical.studies.by.Dietl.et.al..[5].predicted.that.the.TM-doped.p-type.ZnO.might.display.Curie.temperatures.above.room.temperature..In.fact,.room.temperature.ferromagnetism.has.been.reported.for.TM-doped.ZnO.system,.although.these.materials.were.n-type.[6–9]..However,.there.exist.controversies.whether.the.observed.ferromagnetism.is.intrinsic.property.of.the.DMS.thin.films.or.it.arises.from.the.clustering.or.impurities.[10–12]..In.this.context,.recent.reports.about.the.observation.of.room.temperature.ferromagnetism.in.copper-doped.ZnO.have.been.taken.with.great.interest.by.the.scientific.community..This.is.mostly.because.of.the.fact.that.the.metallic.copper.(Cu),.as.well.as.all.possible.Cu-based.secondary.phases,.

CONTENTS

12.1. Introduction......................................................................................................................... 35112.2. Structural.Analysis............................................................................................................. 352

12.2.1. Raman.Scattering.Studies...................................................................................... 35212.2.2. Local.Structure.Analysis.of.Zn1−xCuxO.Thin.Films........................................... 355

12.2.2.1. Detection.of.Cu-Related.Secondary.Oxides.in.Zn1−xCuxO.System...... 35512.2.2.2. Effect.of.Cu.Substitution.in.ZnO.Lattice.............................................. 356

12.3. Optical.Properties.of.Cu-Doped.ZnO.Thin.Films......................................................... 36212.3.1. Low-Temperature.Photoluminescence.of.ZnO................................................... 36212.3.2. Photoluminescence.of.Zn1−xCuxO.Thin.Films....................................................365

Acknowledgments....................................................................................................................... 369References...................................................................................................................................... 369


are.nonferromagnetic.[13]..So,. if.any.ferromagnetism.is.observed.in.a.Cu-based.system,.then.it.will.undoubtedly.be.the.intrinsic.property.of.the.material..Despite.the.above.inter-est,.the.Cu-doped.ZnO.system.is.not.beyond.controversies..There.are.several.contradicting.reports.where.some.authors.have.confirmed.[14,15].the.occurrence.of.FM.in.this.system.while.others.have.ruled.it.out.[16]..Even.in.studies.where.room.temperature.ferromagne-tism.is.reported,.the.effect.of.carrier.type.on.the.ferromagnetic.properties.is.unclear.[17]..Buchholz.et.al..found.that.p-type.carriers.are.essential.for.realizing.ferromagnetism.in.the.ZnO:Cu.system.but.nonferromagnetic.in.n-type.system.[17]..In.sharp.contrast.to.this,.Hou.et.al..reported.ferromagnetism.in.n-type.ZnCuO.films.[18]..Another.problem.is.that.it.is.difficult.to.fabricate.high-quality.samples.with.controlled.dopant.concentrations.because.of.the.poor.solubility.of.Cu.in.ZnO.[12]..In.the.case.of.Cu-doped.ZnO,.the.empty.Cu.3d.states.are.located.in.the.gap.region.without.the.hybridization.with.the.Zn.4s.conduction.band,.and.thus.Cu.3d.states.are.strongly.localized.and.that.is.the.reason.why.the.Cu.solu-bility.in.ZnO.is.so.low:.∼3%.at.most.

12.2 Structural Analysis

12.2.1 Raman Scattering Studies

Raman.scattering.study.is.one.of.the.most.powerful.nondestructive.technique.to.charac-terize.the.structural.analysis,.phase.formation,.and.precipitations.of.other.phases.

The. Wurtzite. structure. of. ZnO. has. the. space. group. C6v4. with. two. formula. units. per.

primitive.cell.with.all.atoms.occupying.C3v.sites..Each.Zn.atom.is. tetrahedrally.coordi-nated.to.four.O.atoms.and.vice.versa..The.numbers.of.optical.symmetry.modes.for.the.C6v

4.are.given.by.[19].Γ.=.A1.+.2B1.+.E1.+.2E2..The.B1.modes.are.silent. in.Raman.scattering.whereas. the.A1. and.E1.modes.are.polar.and.hence.exhibit.different. frequencies. for. the.transverse-optical. (TO). and. longitudinal-optical. (LO). phonons,. because. of. the. macro-scopic.electric.field.associated.with. the.LO.phonons..The.nonpolar.E2.modes.have. two.frequencies,.namely,.E2

high.and.E2low,.associated.with.the.motion.of.oxygen.(O).atoms.and.

zinc.(Zn).sub-lattice,.respectively.[19].The.substitution.effects,.such.as.optical.phonon.confinement,.activation.of.second-order.

modes,. and. the. presence. of. secondary. phases. in. Cu. substituted. (Zn. site). ZnO,. can. be.detected. by. Raman. scattering. analysis.. First-order. Raman. spectrum. of. Zn1−xCuxO. thin.films.are.shown.in.Figure.12.1.[20]..Besides.the.strong.Al2O3.substrate.peaks,.two.charac-teristic.optical.modes.of.Wurtzite.ZnO.at.98.5.and.439.cm−1.corresponding.to.the.E2

low.and.E2

high.modes.were.observed.[19].According.to.the.Raman.selection.rule,.only.E2.and.A1.(LO).modes.can.be.observed.in.the.

backscattering.geometry.of.highly.c-axis-oriented.thin.films..The.absence.of.low-intensity.A1.(LO).signal.in.our.spectrum.around.574.cm−1.may.be.due.to.the.dominating.Al2O3.sig-nal.at.576.cm−1..The.oxygen.sub-lattice.vibrational.mode.(E2

high).was.found.to.shift.(2.6.cm−1).toward.the.lower.frequency.side.compared.to.the.ZnO.thin.film..This.shift.is.due.to.the.confinement.of.optical.phonon.in.a.finite.region..When.Cu.substitutes.Zn.in.the.ZnO.host.lattice,.it.forms.the.ternary.alloy.of.Zn1−xCuxO.and.the.allowed.region.for.the.optical.pho-non.becomes.finite.compared.to.the.infinite.region.of.pure.ZnO..The.atomic.substitution.in.a.host.lattice.induces.structural.disorder..This.disorder.breaks.the.translational.symmetry.of.the.allowed.phonons.of.the.host.lattice.and.leads.to.the.contribution.of.k.≠.0.phonons.to.


the.Raman.line.shape,.corresponding.to.the.finite.size.effect..The.disorder-induced.effects.(lower.frequency.shift.and.broadening).in.Zn1−xCuxO.can.be.explained.by.alloy.potential.fluctuations.(APF).using.a.spatial.correlation.(SC).model.[21–24]..In.an.ideal.crystal,. the.region.over.which.the.spatial.correlation.function.of.the.phonon.extends.is.infinity..When.the.crystal.is.alloying,.the.spatial.correlation.region.of.the.phonon.becomes.finite.owing.to.the.potential.fluctuation.of.the.alloying.disorder,.which.gives.rise.to.the.relaxation.of.k.=.0.selection.rule.in.Raman.scattering.[25]..The.assumption.of.a.Gaussian.attenuation.factor.exp(−2r2/L2),.where.L.is.the.diameter.of.the.correlation.region,.leads.to.an.average.over.q.with.a.similar.weighting.factor.exp(−k2L2/4).upon.Furrier.transformation..It.successfully.accounted.for.the.q.vector.relaxation.related.to.the.finite.size.effect.[21].and.the.structural.disorder.[22]..Assuming.a.finite.spatial.correlation.region.in.the.alloying.material.and.then.the.Raman.intensity.at.a.frequency.ω.can.be.written.as.[26]

.

Ik k L dk

k( )

exp( )[ ( )] [ ]

ωω ω

≅ −− +∫ 4 4

2

2 2 2

20

2

0

1π /

/Γ

wherek.has.the.unit.of.2π/a.(k = k’.±.2nπ/a)“a”.is.the.lattice.constantn.is.the.reflective.index.of.the.materialΓ0.is.the.FWHM.of.E2

high.mode.of.undoped.ZnO.Raman.line

Assuming.one-dimensional.linear.chain.model,.the.dispersion.relation.for.Wurtzite.ZnO.structure.can.be.written.as.follows.by.assuming.the.analytical.mode.relationship.[21,26],.ω(k).=.A.+.B.cos(πk),.where,.A.and.B.are.constants..The.estimated.L.values.corresponding.

Raman shift (cm–1)100 200 300

Inte

nsity

(a. u

.)

400 500 600

ZnO

1% Cu

3% Cu

5% CuAg

Zn1–xCoxO thin films

S

S

E2high

E2low

700

*M

FIGURE 12.1Raman.spectrum.of.Zn1−xCuxO.thin.films.on.Al2O3.substrate,.the.CuO-related.Ag.mode.at.296.8.cm−1.observed.in.Zn0.95Cu0.05O.thin.film..(Reproduced.with.permission.from.Sudakar,.C.,.Thakur,.J.S.,.Lawes,.G.,.Naik,.R.,.and.Naik,.V.M.,.Ferromagnetism.induced.by.planar.nanoscale.CuO.inclusions.in.Cu-doped.ZnO.thin.films,.Phys. Rev. B,.75,.054423,.2007..Copyright.2007,.American.Institute.of.Physics.)


to.1%,.3%,.and.5%.Cu-doped.ZnO.are.12.33,.9.39,.and.8.38.nm,.respectively.[20]..The.red.shift.of.the.E2

high.mode.in.Zn1−xCuxO.thin.films.confirms.the.substitution.of.Cu2+. ion.in.the.Zn2+.site.in.ZnO.host.lattice.[20]..Sudhakar.et.al..[12].have.used.the.Raman.scattering.technique.to.investigate.the.presence.of.nanoscale.size.local.structure.of.Zn1−xCuxO.thin.films.deposited.by.the.reactive.magnetron.co-sputtering.technique..Figure.12.2.shows.the.Raman.spectra.of.ZnO,.ZnO:xCu,.and.CuO.sputtered.films.on.the.sapphire.substrate..At.low.concentrations.(x.<.1%).of.Cu,.the.Raman.spectrum.shows.a.broad.asymmetric.mode.at.∼580.cm−1.

This.mode.is.attributed.to.the.E1(LO).mode.of.ZnO..Although.this.mode.is.not.allowed.in.the.scattering.geometry.used,.its.observation.shows.breakdown.in.the.Raman.selection.rules.due.to.the.incorporation.of.Cu.into.the.ZnO.crystal.lattice..The.broadening.of.the.E1(LO). mode. is. due. to. defects. associated. with. the. oxygen. sub-lattice. or. cation. intersti-tials.[27]..At.higher.concentrations.(>3%),.this.band.evolves.into.two.distinct.peaks:.broad.peaks.at.580.and.634.cm−1,.which.are.attributed. to.E1(LO).of.ZnO.and.Bg.mode.of.CuO.[28]..Distortions.in.the.oxygen.sub-lattice.result.when.CuO.nanophase.is.incorporated.into.the. (0001). cationic. planes. of. ZnO. at. Zn. atom. positions,. with. the. oxygen. atom. situated.in.the.adjacent.anion.planes.[29]..The.presence.of.uniformly.distributed.small.nanoscale.amounts.of.Cu–O.at.concentrations.less.than.2.at..%.does.not.seem.to.induce.observable.distortions.in.the.structure.of.ZnO.as.determined.from.HRTEM.[29]..However;.the.Raman.spectra. show. evidence. for. the. presence. of. CuO. nanophase. in. ZnO. even. at. such. small.

Raman shift (cm–1)200 400

Inte

nsity

(a. u

.)

CuO

ZnO:8.3 Cu

E 2(low

)

E 2(hig

h)

ZnO:4.3 Cu

ZnO:2 Cu

ZnO:0.9 Cu

x = 0.9

x = 2

x = 4.3

x = 8.3

ZnO

Ag

Sapphiresubstrate

600 800500 600 700

1000

Bg(1)

Bg(2)

FIGURE 12.2Raman. spectra. of. Cu-doped. ZnO. (x.=.0%–8.3%). thin. films. on. sapphire. substrate. along. with. CuO. and.the. bare. sapphire. substrate.. The. peaks. from. sapphire. are. shown. by*.. The. inset. shows. the. deconvo-luted. curves.. (Reproduced. with. permission. from. Sudakar,. C.,. Thakur,. J.S.,. Lawes,. G.,. Naik,. R.,. and. Naik,.V.M.,.Ferromagnetism.induced.by.planar.nanoscale.CuO.inclusions.in.Cu-doped.ZnO.thin.films,.Phys. Rev. B,.75,.054423,.2007..Copyright.2007,.American.Physical.Society.)


dopant. concentrations.. At. 8.3%. of. Cu,. very. strong. bands. are. observed. at. 296,. 345,. and.634.cm−1,.which.coincide.with.the.Raman.spectrum.of.CuO.[30].

12.2.2 Local Structure Analysis of Zn1−xCuxO Thin Films

The.high-Tc. ferromagnetism.in.TM-doped.ZnO.thin.films.are.still.a.controversial. issue;.this. is.often.due.either.to.magnetic.contamination.or.magnetic.secondary-phase.forma-tion..Thorough.material.characterization.is.required.to.clarify.the.bonding.and.magnetic.properties. of. the. dopant. in. the. host. semiconductor.. The. x-ray. absorption. spectroscopy.(XAS).and.near.edge.x-ray.absorption.fine.structure.(NEXAFS).are.the.most.useful.and.positive.characterization.techniques.for.the.high-Tc.DMS.materials.to.get.insights.of.the.bonding.environment,.formal.charge.state,.and.atom-specific.magnetic.properties.

The. XANES. characterization. of. a. series. of. pulsed. laser. deposition. (PLD)-grown.Cu-doped.ZnO.thin.films.were.carried.out.by.Ma.et.al..[31]..The.x-ray-absorption.spectra.around.the.Cu.K.edge.were.measured.with.the.radiation.polarization.vector.ε.quasiparal-lel.(∥).and.quasiperpendicular.(⊥).to.the.sample.surface,.respectively,.for.which.a.grazing.incident.angle.of.2°–5°.is.used..The.Zn.K.edge.data.were.measured.only.in.the.⊥ε.geom-etry..The.Cu.Kα.and.Zn.Kα.emission.intensities.were.recorded.by.a.13.element.Ge.detector.at.90°.to.the.x-ray.beam.path..The.x-ray.pulses.were.processed.with.a.1.μs.peaking.time.using.the.electronics.from.X-ray.Instrumentation.Associate.(XIA)..In.an.x-ray-absorption.process,.the.initial-state.core.electron.(ψi).is.excited.into.a.final.state.(ψf)..For.a.1s.core.elec-tron,.the.intensity.of.such.a.transition.is.determined.by.the.cross.section.[32]:

.µ ω ε δ ω ψ ε ψ ψ ε ε ψ( , , ) ( ) . . , . ,� �

� �k E E r r kf i f i f i= − − ⟨ ⟩ + ⟨ ⟩

2 214

whereħω.is.the.photon.energyε.is.the.polarization.vectorr̂.is.the.unit.vector.along.the.interaction.length.Rk.is.the.photoelectron.momentum

The.first.and.second.terms.are.the.dipole.and.quadrupole.contributions,.respectively..For.the.K.absorption.edge.of.copper,. the.dipole. transition. involves.1s.→ 4p..The.quadrupole.transition.1s → 3d.is.also.nonzero.if.the.inversion.symmetry.is.lifted.either.vibrationally.or.structurally,.such.as.the.case.for.the.3d9.Cu2+.ion..By.sweeping.the.x-ray.energy.across.the. absorption. edge,. the. excited. electrons. will. probe. all. of. the. selection-rule-allowed.empty.states.in.the.band.structure,.which.gives.x-ray.absorption.near.edge.structure.(x-ray.absorption.near.edge.structure,.XANES).

12.2.2.1 Detection of Cu-Related Secondary Oxides in Zn1−xCuxO System

Figure.12.3.shows.the.XANES.spectra.and.their.derivatives.for.two.films.with.different.Cu-doping. levels,.as.well.as.reference.bulk.CuO.and.Cu2O.spectra..The.sample.spectra.were.all.collected.in.the.∥ε.geometry..The.spectra.are.referenced.to.the.Cu.K.edge.of.copper.metal..A.pre-edge.feature.located.near.−2.eV.exists.for.all.spectra.except.for.Cu2O,.better.seen.by.the.first.derivatives.in.the.right.panel.of.Figure.12.3.

This. feature. is. due. to. the. quadrupole. 1s.→.3d. transition. that. is. only. allowed. when. the.empty.d.states.are.available,.such.as.in.the.3d9.electronic.configuration.of.the.Cu2+.ion,.and.the.


structural.unit.around.the.absorbing.atoms.lacks.centrosymmetry..The.3d10.electronic.con-figuration.of.the.Cu1+.ion.prohibits.such.a.transition..Clearly.a.Cu2+.component.exists.in.both.samples,.and.the.6%.Cu-doping.sample.may.consist.essentially.of.the.CuO.entities.since.its.spectrum.is.very.similar.to.that.of.CuO..The.rising.edge.of.the.1%.sample.coincides.roughly.with.that.of.bulk.Cu2O,.and.it.is.tempting.to.assign.it.to.the.Cu1+.state..However,.the.large.dif-ference.in.the.rest.of.the.spectrum.from.that.of.the.bulk.Cu2O.may.suggest.otherwise..The.mixing.of.the.spectra.of.CuO.and.Cu2O.in.any.proportion.does.not.reproduce.the.spectrum..Together.with.the.existing.3d.peak,.it.is.very.likely.that.the.rising.edge.of.the.Cu.1%.sample.is.in.fact.the.“shakedown”.feature.of.the.1s.→.4p.transition.in.the.Cu2+.ion.[31]..The.shakedown.feature. is.due. to. the.final-state. screening. (or.many-body.effect).of. the.Cu.1s. core.hole.by.the.ligand.metal.charge.transfer.(L →.M).due.to.orbital.mixing.(2p–3d.mixing.here).[33]..It.is.shifted.downward.by.3.eV.when.compared.with.that.of.the.bulk.CuO,.which.would.suggest.a.stronger.screening.effect.and.possibly.a.larger.reduction.in.S0

2.as.well.[34]..The.CuO.in.the.Cu.6%.sample.are.likely.in.a.form.of.larger.clusters..The.electronic.structure.is.nearly.identical.to.that.of.bulk.CuO..The.electronic.structure.of.small.Cu.clusters.may.depart.significantly.from.that.of.the.bulk.material.due.to.the.change.in.the.bonding.property..The.prominent.down-ward.shift.of.the.shakedown.feature.is.the.result.of.increased.covalence.of.the.Cu–O.bond.

12.2.2.2 Effect of Cu Substitution in ZnO Lattice

The.Zn.K.edge.XANES.spectra.of.the.copper-doped.films.prepared.at.8.cm.(t-s).and.a.ZnO.film.measured.at.⊥ε.compared.with.those.of.a.ZnO.powder.is.shown.in.Figure.12.4.[31]..

Energy (eV)0 40

(a)

(b)(a)

(b)

3d

CuO

CuO

0 20

Cu2O(x½)Cu2O

FIGURE 12.3Cu.K.edge.XANES.spectra.(left).and.the.first.derivatives.(right).of.two.N2O-prepared.films.and.bulk.CuO.and.Cu2O..(a).Cu.1%.and.(b).Cu.6%..The.arrows.indicate.the.shakedown.feature.due.to.ligand.metal.charge.transfer..The.zero.energy.is.set.to.the.Cu.K.edge.of.metal..(Reproduced.with.permission.from.Ma,.Q.,.Buchholz,.D.B.,.and.Chang,.R.P.H.,.Local.structures.of.copper-doped.ZnO.films,.Phys. Rev. B,.78,.214429,.2008..Copyright.2008,.American.Physical.Society.)


The.spectrum.of.the.ZnO.film.is.typical.for.a.c-axis.oriented.ZnO.film.measured.in.this.geometry.[35].

The.arrow.indicates.a.characteristic.peak.that. is.similar.to.the.one.appearing.on.the.Cu.K.edge.XANES.of.copper-doped.films,.when.the.Cu/Zn.substitution.occurs..For.the.Cu-doped. films. in. Figure. 12.4,. changes. in. the. XANES. are. readily. observable.. These.changes.are.likely.the.result.of.Cu.ions,.or.at.least.a.fraction.of.them,.occupying.the.Zn.sites..When.copper.exists.as.clusters.in.the.films,.the.Zn.K.edge.XANES.remains.the.same.as. that. of. the. ZnO. film.. Thus,. the. clusters. have. a. minimum. effect. on. the. band. struc-ture.of.the.ZnO.host..Figure.12.5.displays.the.Cu.K.edge.XANES.spectra.and.their.first.derivatives.for.the.O2-prepared.films..For.comparison,.the.data.of.the.bulk.CuO.are.also.presented.

Weak.polarization.dependence.is.seen.in.the.spectra.that,.however,.do.not.deviate.dras-tically.from.that.of.the.bulk.CuO,.especially.the.position.of.the.L →.M.feature..The.3d.peaks.located.at.1.eV.below.that.of.the.bulk.CuO..The.energy.difference.ΔE(L–M)-3d.between.the.3d.and.L →.M.features.is.7.5.eV..The.shoulder.at.∼21.eV.in.the.⊥ε.data.corresponds.well.to.that.observed.on.the.Zn.K.edge.spectra.(Figure.12.4).and.is.likely.resulted.from.the.same.ori-gin..Thus,.the.band.structure.near.the.doping.atoms.is.regulated.by.the.lattice.structure..The.shoulder.weakens.with.increasing.doping.level;.more.CuO.clusters.form.and.obscure.the.CuZn.substitution.effect.

The. NEXAFS. and. resonant. inelastic. x-ray. scattering. (RIXS). techniques. are. the. most.powerful.to.identify.the.valence.states.of.ions.and.to.differentiate.between.the.localized.ions.(substitution.or.interstitial).and.metallic.clusters.in.the.complex.materials.system..The.element-specific.NEXAFS.and.RIXS.spectroscopy.is.capable.of.probing.the.partial.density.of.unoccupied.and.occupied.states.of.constituent.elements,.respectively,.and.thus.are.pow-erful.tools.for.understanding.local.electronic.structure.around.target.atoms.

Recently,.Thakur.et.al..[36].have.carried.out.extensive.studies.on.NEXAFS.to.understand.the.role.of.oxygen.O.2p.states.on.the.electronic.structure.of.ZnO:Cu.system.as.a.function.

Photon energy (eV)9640

0.0ZnO film

ZnO powder

Cu 1%, N2O

Cu 3%, O2

0.5

1.0

1.5N

orm

aliz

ed am

plitu

des

2.0

2.5

3.0

9660 9680 9700

FIGURE 12.4Zn.K.edge.XANES.spectra.of. the.copper-doped.films.and.a.ZnO.film.compared.with. those.of.a.nano-ZnO.powder.. (Reproduced. with. permission. from. Ma,. Q.,. Buchholz,. D.B.,. and. Chang,. R.P.H.,. Local. structures. of.copper-doped.ZnO.films,.Phys. Rev. B,.78,.214429,.2008..Copyright.2008,.American.Physical.Society.)


of.Cu-doping..The.NEXAFS.experiments.at.the.O.K.edge.were.carried.out.to.probe.the.orbital.character.of.the.spectral.features.of.the.O.2p.unoccupied.states.in.the.conduction.band.and.its.hybridization.with.different.Cu.and.Zn.orbitals..Figure.12.6a.and.b.shows.the.normalized.spectra.of.ZnO:Cu.(x.=.0,.0.03,.0.05,.0.07,.and.0.10).thin.films.acquired.at.room.temperature. in.both.total.electron.yield.(TEY).and.total.fluorescence.yield.(TFY).mode,.ensuring.both.surface.and.bulk.sensitivities,.respectively..The.assignment.of.the.observed.spectral.features.as.follows.[37,38]:.region.between.528–539.eV.(A1–C1).mainly.attributed.to.O.2p.hybridization.with.highly.dispersive.Zn.3d4s/Cu.3d.states.that.form.the.bottom.of.the.conduction.band.with.peak.C1.at.538.eV.due.to.the.transitions.to.nondispersive.O.2p.states..The.region.between.539–550.eV.(D1).is.assigned.as.the.O.2p–Zn.4p/Cu.4sp.hybrid-ized.state,.and.the.one.above.550.eV.spectrum.arises.due.to.the.O.2p.states.that.extend.to.Zn/Cu.higher.orbitals.

The.two.pre-edge.peaks,.namely,.A1.(530.eV).and.B1.(532.5.eV),.evolve.with.Cu-doping.in.ZnO.and.their.intensities.increase.monotonically.with.Cu.concentrations,.suggesting.a.strong.hybridization.of.O.2p.orbitals.with.Cu.3d.states..These.pre-edge.features.carry.a.substantial.amount.of. information.and.can.be.attributed.to.the.unoccupied.bands.of.primary.O.2p–Cu.3d. character..A.continuous.evolution.of. these. spectral. features.with.Cu-doping.indicate.more.unoccupied.states.at.the.Cu.3d.levels.and.therefore.reflects.the.presence.of.more.charge.carriers,.electrons,.or.holes.[36]..The.significant.enhancement.of.pre-edge.spectral.features.reveals.that.the.effective.doping.of.Cu.in.ZnO.matrix.induces.a.strong.hybridization.of.s–p–d.orbitals..The.spectral.features.above.537.eV.are.quite.similar.and.nearly.independent.of.Cu.concentrations.and.dominated.by.the.contributions.from.

Photon energy (eV)0

0.0

0.5

(a)

(a)(b)

(b)(c)

(c)CuO

CuO

4.5 eV

L->M

3d

4p

1.0

1.5

2.0

2.5

20 0 20

FIGURE 12.5Cu.K.edge.XANES.spectra.and.the.first.derivatives.of.O2-prepared.films.measured.at.two.sample.orientations:.∥ε.(solid.line).and.⊥ε.(dotted.line)..(a).Cu.3%,.(b).Cu.7%,.and.(c).Cu.14%.compared.to.bulk.CuO..The.zero.energy.is.set.to.the.Cu.K.edge.of.metal..(Reproduced.with.permission.from.Ma,.Q.,.Buchholz,.D.B.,.and.Chang,.R.P.H.,.Local.structures.of.copper-doped.ZnO.films,.Phys. Rev. B,.78,.214429,.2008..Copyright.2008,.American.Physical.Society.)


multiple.scattering.effects..So,.it.is.reasonable.to.conclude.that.Cu.ions.are.incorporated.in.the.system.and.responsible.for.changes.in.the.local.electronic.structure.of.this.mate-rial.[36]..The.probing.depth.of.TFY.mode.spectra.is.larger.by.roughly.an.order.of.magni-tude.and.it.is.therefore.more.representative.of.the.“bulk”.composition.of.the.films..The.line.shape.of. the.TFY.mode.spectra.bears.close.resemblance. to.TEY.mode.spectra;. the.only.difference.is.the.broad.B1.peak.profile,.which.continuously.increases.in.weight.with.Cu-doping..This.can.be.attributed.to.the.saturation.effects.that.are.dominating.in.the.TFY.mode.and/or.the.stoichiometry.of.O.ions.is.different.on.the.surface.of.the.films.than.that.of.the.bulk.[36].

The.normalized.Cu.L3,2.edge.spectra.of.ZnO:Cu.thin.films.of.TEY.mode.and.TFY.mode.are.presented.in.Figure.12.7a.and.b,.respectively..The.CuO.and.Cu2O.spectra.have.also.been.collected.under.the.same.experimental.conditions.and.are.chosen.as.reference.spec-tra.of.Cu2+.and.Cu1+,.respectively..All.of.the.spectra.stem.from.excitations.of.a.core.electron.in.the.2p1/2.or.2p3/2.manifold.to.the.unoccupied.3d.state,.that.is,.transitions.from.a.ground.state. of. 2p63dn. to. an. excited. electronic. configuration. of. 2p53dn+1. with. different. multiple.excitations..As.a.result.of.spin-orbit.coupling.in.the.2p.state,.the.spectra.display.two.prom-inent.features.in.the.energy.range.of.928–940.and.949–955.eV,.respectively,.corresponding.to.the.L3.(2p3/2.→.3d).and.L2.(2p1/2.→.3d).absorptions..In.the.case.of.Cu2O.sample,.it.exhib-its.asymmetric.L3,2.edges,.while.CuO.shows.sharp.and.very.symmetric.lines..The.small.

Photon energy (eV)525 530 535 540 545

TFY mode

TFY mode

(a)

(b)

Inte

nsity

(a. u

.)

O K edge

D1

D1

C1

C1

B1

B1

A1

A1

x=0

x=0

x=0.03

x=0.03

x=0.05

x=0.05

x=0.07

x=0.07

x=0.1

x=0.1

Z nl–2Cu 2O

550 555 560 565

FIGURE 12.6O.K.edge.NEXAFS.spectra.of.Zn1−xCuxO.(x.=.0,.0.03,.0.05,.0.07,.and.0.10).thin.films.collected.at.10.K..(a).Surface.sensitive.TEY.mode..(b).Bulk.sensitive.TFY.mode..(Reproduced.with.permission.from.Thakur,.P.,.Bisogni,.V.,.Cezar,.J.C.,.Brookes,.N.B.,.Ghiringhelli,.G.,.Gautam,.S.,.Chae,.K.H.,.Subramanian,.M.,.Jayavel,.R.,.and.Asokan,.K.,.Electronic.structure.of.Cu-doped.ZnO.thin.films.by.x-ray.absorption,.magnetic.circular.dichroism,.and.reso-nant.inelastic.x-ray.scattering,.J. Appl. Phys.,.107,.103915,.2010..Copyright.2010,.American.Institute.of.Physics.)


peak.at.932.2.eV.in.the.Cu2O.Cu.L3.spectrum.(TEY.mode).is.due.to.absorption.by.small.amount. of. CuO. present. at. the. surface. of. the. sample. as. a. consequence. of. unavoidable.surface.oxidation..The.presence.of.Cu.in.CuO.is.in.the.nominal.Cu2+.ionization.state,.cor-responding.to.a.3d9.ground.state.with.a.small.contribution.from.3d10L′,.where.L′.denotes.a.hole.in.the.O.valence.band.[39,40]..The.L3,2.NEXAFS.final.state.is.thus.dominated.by.the.2p53d10.configuration..The.extra.electron.that.fills.the.well-localized.d.shell.provides.a.very.effective.screening.of.the.core-hole.potential..In.this.way,.the.2p63d9.→.2p53d10.transitions.give.rise.to.a.sharp.resonance.at.a.photon.energy.much.lower.than.the.ones.for.which.the.weak.2p63d9.→.2p53d94sp.transitions.are.excited,.and.the.intermixing.between.2p53d10.and.2p53d94sp1.final.states.is.negligible..In.Cu2O,.Cu.appears.as.Cu1+,.with.an.almost.completely.full.3d.shell..The.ground.state.is.a.combination.of.3d10,.3d94sp1,.and.3d104sp1L′,.where.the.second.configuration.gives.the.largest.contribution.to.the.Cu.L3,2.absorption.spectrum.at.threshold.[40]..The.NEXAFS.final.state.is.then.mostly.2p53d104sp1.and.the.photon.energy.of. the.L3.edge.is. larger.than.for.CuO.since.some.extra.energy.is.needed.to.promote.an.electron. into. the.4sp.band..The.NEXAFS.spectra.of.ZnO:Cu. thin.films,. specially.at. the.Cu.L3.region,.exhibit.multiple.absorption.peaks..Thakur.et.al..[36].identified.three.domi-nant.peaks..The.first.peak,.labeled.as.A2.(930.eV),.corresponds.to.the.transitions.from.the.Cu.(2p3/2,1/2)3d9O.2p6.ground.states.into.the.Cu.(2p3/2,1/2)−13d10O.2p6.excited.states,.in.which.(2p3/2,1/2)−1.represents.a.2p3/2.or.2p1/2.hole..The.second.peak.B2.(931.4.eV).is.either.due.to.Cu.3d8.states.or.originating.from.the.Cu.(2p3/2,1/2)3d9L′.into.the.Cu(2p3/2,1/2)−13d10L′.excited.states..

Photon energy (eV)

Inte

nsity

(a. u

.)

925 930 935 940 945 950 955 960 965

TFY mode

TFY mode

(b)

(a)

C2

C2

B2

B2

A2

A2

x=0.03 CuO

Cu 2O

CuO

Cu 2O

x=0.05x=0.07

x=0.10

x=0.03x=0.05

x=0.07

x=0.10

Z nl–2Cu 1O

Cu L 3–2edge

FIGURE 12.7(a).Cu.L3,2.edge.NEXAFS.spectra.of.Zn1−xCuxO.thin.films.for.various.concentrations.collected.at.10.K..(a).Surface.sensitive. TEY. mode.. (b). Bulk. sensitive. TFY. mode.. Spectra. of. reference. samples. of. Cu2O. and. CuO. are. also.shown.for.comparison.. (Reproduced.with.permission.from.Thakur,.P.,.Bisogni,.V.,.Cezar,. J.C.,.Brookes,.N.B.,.Ghiringhelli,.G.,.Gautam,.S.,.Chae,.K.H.,.Subramanian,.M.,.Jayavel,.R.,.and.Asokan,.K.,.Electronic.structure.of.Cu-doped.ZnO.thin.films.by.x-ray.absorption,.magnetic.circular.dichroism,.and.resonant.inelastic.x-ray.scat-tering,.J. Appl. Phys.,.107,.103915,.2010..Copyright.2010,.American.Institute.of.Physics.)


The.third.peak.C2.(934.5.eV).belongs.to.the.presence.of.copper.ions.Cu1+.and.can.also.be.associated.to.the.oxygen.deficiency,.that.is,.2p63d10–2p53d104s1.transition.in.the.Cu(I).sites.of.the.oxygen-deficient.layer..It.is.clear.from.the.spectrum.that.the.gain.of.intensity.of.B2.is.with.a.relative.decrease.of.peak.A2.and.C2.with.Cu.concentration..Both.TEY.and.TFY.spectra.of.Cu.L3,2.edge.display.similar.spectral.profile.except. for. the.peak.C2,.which. is.more.prominent.at.the.surface.of.the.samples..A.continuous.increase.in.the.intensity.of.peak.B2.at.the.expense.of.peaks.A2.and.C2.indicates.that.Cu3+.components.are.increasing.with.Cu.concentrations..These.behaviors.obviously.reflect.that.Cu.L3,2.NEXAFS.spectra.exhibit.divalent.Cu.apart. from.mixed-valence.Cu3+/Cu1+.states.and.Cu.valence.changes.with.increase.in.Cu.concentrations.

The.room.temperature.RIXS.spectra.of.ZnO:Cu.thin.films.are.displayed.in.the.bottom.panels.of.Figure.12.8..The.RIXS.spectra.are.plotted.as.a.function.of.the.energy.loss,.that.is,.the.incident.energy.position.is.set.to.be.0.eV.of.RIXS.with.a.combined.energy.resolution.of.0.5.eV..As.evident,.RIXS.spectra.exhibit.multiple.spectral.features.when.excitation.energy.is.tuned.at.different.regions.of.Cu.L3-edge.NEXAFS..RIXS.at.A2,.which.is.an.indicative.of.Cu2+.ions.at.substitutional.site.in.ZnO.matrix,.consists.of.two.main.features.

Energy loss (eV)

–60

80Experimental resolution

(FWHM-0.5eV)

RLXS at Cu L3 edge160

x = 0.10

x = 0.03

x = 0.05x = 0.03

0

300

0

20

40

928 930 932Photon energy (eV)

Inte

nsity

(a. u

.)

934 936

–5 –4 –3 –2 –1 0 1

A2

A2

B2

B2

C2

C2

Znl–3Cu2O

FIGURE 12.8Measured.Cu.L3.edge.RIXS.spectra.of.Zn1−xCuxO.(x.=.0.03,.0.05,.and.0.10).thin.films.at.room.temperature.(RT),.plotted.on.an.energy-loss.scale..(Reproduced.with.permission.from.Thakur,.P.,.Bisogni,.V.,.Cezar,.J.C.,.Brookes,.N.B.,.Ghiringhelli,.G.,.Gautam,.S.,.Chae,.K.H.,.Subramanian,.M.,.Jayavel,.R.,.and.Asokan,.K.,.Electronic.structure.of.Cu-doped.ZnO.thin.films.by.x-ray.absorption,.magnetic.circular.dichroism,.and.resonant.inelastic.x-ray.scat-tering,.J. Appl. Phys.,.107,.103915,.2010..Copyright.2010,.American.Institute.of.Physics.)


The.shoulder-like.peak.located.at.∼0.5.eV.below.the.elastic.peak.(at.0.eV).can.be.associ-ated.with.the.low.energy.loss.d–d.excitations.due.to.crystal.field.splitting.or.spin.flip.excita-tions.[41]..The.main.peak.located.at.∼1.8.eV.is.due.to.the.local.Cu.d–d.excitation.loss.feature.and.corresponds.to.transitions.to.3d9.multiple.states..It.is.also.noted.that.this.feature.gains.its.spectral.weight.with.Cu.concentrations.indicating.a.strong.correlation.between.mag-netic.and.electronic.properties.of.this.DMS..RIXS.at.B2.is.mainly.dominated.by.an.intense.spectral.feature.at.∼2.2.eV..Since.this.feature.represents.the.d–d.excitations.from.Cu3+.states.(real.Cu3+.or.actually.Cu2+.plus.an.oxygen.p.hole.L′),.the.unchanged.peak.intensity.with.Cu.concentrations.is.very.intriguing..As.shown.in.the.preceding.text,.the.NEXAFS.intensity.corresponding.to.this.feature.varies.considerably.with.Cu-doping..Moreover,.the.relative.intensity.of.the.d–d.excitations.compared.to.the.elastic.peak.is.almost.opposite.to.that.of.the.excitation.energy.A2..The. small. chemical. shift.of. center.of.mass. toward. the.higher.energy.loss.side.is.due.to.the.different.d-d.excitations.than.those.of.Cu2+.states..Hence,.the.RIXS.at.B2.is.a.combined.contribution.from.d8.(real.Cu3+).and.d9L′.(∼.formally.Cu3+).states,.hybridized.with.O.2p6.orbitals..In.other.words,.indirectly.d-d.excitations.at.∼2.2.eV.reflects.the.hole.states.at.oxygen.site,.confirming.the.O.K.edge.absorption,.which.reveal.doping-induced.lower-lying.O.2p.“impurity”.states..Hence,.the.overall.RIXS.energy-loss.features.exhibit.strong.d–d.excitations,.which.are.at.variance.with.the.excitation.energy.and.thus.confirm.the.mixed-valence.states.of.the.localized.Cu.ions.

12.3 Optical Properties of Cu-Doped ZnO Thin Films

High. electronic. conductivity,. optical. transparency,. and. piezoelectric. properties. of. ZnO.make.it.the.most.promising.multifunctional.semiconducting.material.for.applications.in.optoelectronics.devices.operating.in.blue.and.ultraviolet.(UV).regions.[3,42,43]..Therefore,.it. is.necessary. to.choose.a.suitable.dopant. to.ensure. that. it.would. leave.emission.spec-trum.and.transparency.of.ZnO.intact..The.group-I.element.Cu.is.a.very.good.dopant.in.ZnO,. because. it. simultaneously. comprises. of. an. electron. donor. state. of. Cu2+. (2T2). and.corresponding.acceptor.Cu+.(3d10). level.within.the.forbidden.energy.region.in.ZnO.[44]..Furthermore,.Cu.acts.as.a.luminescence.activator,.and.as.a.compensator.of.n-type.materi-als.it.is.of.considerable.significance.for.the.II–VI.compound.semiconductor.in.general.[44]..The.emission.spectra.can.be.extended.from.the.UV.to.infraned.(IR).region.depending.on.the.concentration.of.Cu,.defects. in.ZnO,.and.excitation.conditions.[45,46]..Moreover,.Cu.impurity.in.ZnO.is.well.known.to.give.the.green.emission.at.room.temperature.

12.3.1 Low-Temperature Photoluminescence of ZnO

ZnO.is.a.direct.bandgap.semiconductor.that.crystallizes.in.the.wurtzite.symmetry..The.valence.band.is.split.by.crystal.field.and.spin-orbit.interaction.into.three.states.named.A,.B,.and.C.(Figure.12.9).[47].

The.low-temperature.PL.spectra.of.ZnO.thin.films.have.been.studied.by.many.research.groups..Figure.12.10.shows.the.luminescence.from.high-quality.ZnO.substrate.extending.from.the.band.edge.to.the.blue/violet.spectral.range.[48].

Thonke.et.al.. [48].have.assigned.the.dominating.lines. in.the.spectra.originating.from.bound.exciton.(BE).recombinations,.such.as.neutral.donor.donors.(D0X).and/or.acceptors.(A0X)..These.transitions.are.followed.by.the.LO.phonon.replicas.with.an.energy.separation.


of.72.meV..The.donor-acceptor-pair.(D0A0).transition.is.also.found.at.3.22.eV,.again.followed.by.phonon.replicas. [48]..The.4.2.K.PL.spectra.are.dominated.by.several. close-lying.BEs.(D0X).in.the.range.from.3.3595.to.3.364.eV.(Figure.12.10.and.inset)..The.most.intense.emis-sion.at.3.3628.eV.is.assigned.as.a.donor-bound.exciton.(I4);.whereas,.a.slightly.weaker.line.at. 3.3597.eV. is. closer. to. the. location. of. the. acceptor-bound. exciton. “I8.”. These. lines. are.

Energy (eV)

λvac (nm)

PL si

gnal

(log

.)

PL si

gnal

(lin

ear)

Energy (eV)

2.8

440 420 400 380 360

101

102

103

104

105

2.9

3.358

369.2 369.0 368.8 368.6

3.360 3.362 3.364

3.0 3.1

- LO- LO

I1

I0/I

I4

- LO

TES

(D0.XA)(D0_X)

(D0.A0)

(A0.X) ???(A0.XA)

3.2 3.3 3.4

FIGURE 12.10PL. spectrum. of. ZnO. substrate. showing. bound. exciton,. donor. acceptor. pair,. and. two. electron. .satellite.(TES). .transition;. inset. shows. the. PL. spectrum. of. bound. exciton. region.. (Reproduced. from. Donor–.acceptor.pair. transitions. in.ZnO.substrate.material,.Physica B,.308,.945,.Thonke,.K.,.Gruber,.Th.,.Teofilov,.N.,.Schonfelder,.R.,.Waag,.A.,.and.Sauer,.R..Copyright.2001,.with.permission.from.Elsevier.)

Both S-O

J = 1/2

J = 1/2

Eg(B)–Eg(A) = 4.9 meV

Eg(C)–Eg(B) = 43.7 meV

CB

VB

CF

Eo EoEg(A)

Eg(C)

Г5Г7

Eg(B)

Г1Г7

Г9∆cf

∆so

FIGURE 12.9Schematic.representation.of.valance.band.structure.and.symmetries.of.hexagonal.ZnO..The.splitting.into.three.valence.bands.(A,.B,.C).is.caused.by.crystal.field.and.spin-orbit.splitting.


replicated.by.a.separation.of.29.9.meV.at.the.lower.energy.region.labeled.as.“two.electron.satellite”.(TES)..A.TES.related.to.a.donor-bound.exciton.(D0X).occurs.in.the.spectrum.if.in.the.BE.recombination.process.the.neutral.donor.(i.e.,.the.final.state.of.the.recombination.process).is.excited.from.the.1s.ground.state.to.an.excited.state.like.2s,.2p,.etc..[48]..The.free.exciton.(FXA).transition.is.also.observed.at.3.375.eV.[47].

In.Figure.12.11,.the.PL.spectrum.of.the.D0X.is.shifted.by.29.9.meV.and.compared.with.the.TES.part..Apart.from.a.line.broadening,.the.TES.spectrum.is.identical.with.the.princi-pal.BE.spectrum..This.strongly.suggests.that.not.only.the.upper.BE.line.is.a.donor-bound.exciton,.but.even. the. lower.energy. lines.around.3.360.eV.are.donor. related..The.energy.splitting.of.1s–2s/p.can.be.estimated.directly.from.the.spacing.between.TES.lines.and.D0X.lines..This.energy.separation.is. in.good.agreement.with.the.3/4th.of.the.donor-binding.energy.in.a.hydrogenic.effective.mass.approach.(EMA).[48,49].

Liu.et.al..have.studied.the.low-temperature.PL.of.metalorganic.chemical.vapor.deposi-tion.(MOCVD)-grown.ZnO.(Figure.12.12).and.Cu-doped.ZnO.thin.films.[50]..The.spectrum.at.11.4.K.exhibits.a.strong.near.band.edge.(NBE).UV.emission.and.negligible.visible.emis-sion..In.the.case.of.a.ZnO.thin.film,.the.NBE.consists.of.two.peaks.at.369.0.nm.(3.357.eV).and.368.3.nm.(3.364.eV)..These.two.peaks.are.commonly.ascribed.to.the.emission.of.exci-tons.bound.to.neutral.donors..However,.the.identification.of.this.transition.line.at.3.357.eV.is.rather.complicated..Meyer.et.al..assigned.this.peak.as.the.donor-bound.exciton.caused.by.the.unintentional.indium.(In).impurities.in.ZnO.[47],.while.Hwang.et.al..observed.the.same.I9.(3.355.eV).peak.at.10.K.PL.of.p-type.ZnO:P.and.3.352.eV.in.the.case.of.pure.ZnO.thin.films.prepared.by.RF.Magnetron.Sputtering. [51]..They.have. identified. this.peak.as. the.acceptor-bound.exciton.(A0X).[51]..The.weak.shoulder.at.367.0.nm.(3.378.eV).is.attributed.to.FXA.emission.[52]..The.emission.at.373.6.nm.(3.318.eV).is.peculiar.to.ZnO.films.grown.on.

Energy (eV)

Energy (eV)

PL in

tens

ity

3.310

3.340 3.345 3.350 3.355 3.360 3.365

101

102

103

104

105

3.315 3.320 3.325 3.330

TES

T = 4.5K

D0

(D0.X)

29.9 meV

2(s.p)

Is

3.335 3.340

FIGURE 12.11Comparison. of. the. bound. exciton. (upper. trace,. top. energy. scale). and. the. TES. regions. (lower. trace,. bottom.energy.scale). is. in. the.4.2.K.PL.spectrum.. Inset.shows.the.excitation.of. the.D0.final.state. taking.place. in. the.recombination.process,.which. results. in. the.TES. line.. (Reproduced. from.Donor–acceptor.pair. transitions. in.ZnO.substrate.material,.Physica B,.308,.945,.Thonke,.K.,.Gruber,.Th.,.Teofilov,.N.,.Schonfelder,.R.,.Waag,.A.,.and.Sauer,.R..Copyright.2001,.with.permission.from.Elsevier.)


the.Si.(111).substrate..It.is.probably.due.to.an.excitonic.transition.[53–55].bound.to.the.Si.impurity.or.rotation.domain.structure-induced.localized.state-bound.exciton.transition..In.order.to.identify.the.fine.structure.of.the.visible.emission.in.the.range.of.420–600.nm,.the.exposure.time.of.the.charge-coupled.device.(CCD).detector.was.increased..A.broad.peek.in.the.range.of.470–540.nm.can.be.detected,.which.is.induced.by.native.defects,.such.as.oxygen.vacancy.(Vo).and.interstitial.zinc.(Zni)..The.origin.of.the.deep.level.emission.in.undoped.ZnO.films.is.not.yet.clearly.understood.but.it.does.not.have.a.characteristic.fine.structure,.and.the.intensity.from.the.undoped.ZnO.film.is.extremely.weak.

12.3.2 Photoluminescence of Zn1−xCuxO Thin Films

The.PL.spectra.change.significantly.when.Cu.is.doped.in.ZnO.thin.films.(Figure.12.13).[50]..The.UV.band.shifts.to.the.shorter.wavelength.region.(369.0–368.7.nm,.368.3–366.9.nm,.

Wavelength (nm)

Inte

nsity

(a. u

.)

360 380

367.0nm

373.6 nm

369.0 nm

×10

368.3nm

400 420 440 460 480 500 520 540 560 580 600

FIGURE 12.12PL.Spectra.of.ZnO.thin.film.on.(111).Si.substrate;.data.recorded.at.11.4.K..(Reproduced.with.permission.from.Liu,.Y.,.Liang,.H.,.Xu,.L.,.Zhao,.J.,.Bian,.J.,.Luo,.Y.,.Liu,.Y.,.Li,.W.,.Wu,.G.,.and.Du,.G.,.Cu.related.doublets.green.band.emission.in.ZnO:Cu.thin.films,.J. Appl. Phys.,.108,.113507,.2010..Copyright.2010,.American.Institute.of.Physics.)

Wavelength (nm)

Inte

nsity

(a. u

.)

360 380

366.3nm

366.9nm

368.7nm

400 420

×20×10

2a

3a3b

4a4b

5a5b

6a6b

1b1a

2b

440 460 480 500 520 540 560 580 600

FIGURE 12.13PL.spectra.of.ZnO:Cu.thin.film;.data.recorder.at.11.4.K..(Reproduced.with.permission.from.Liu,.Y.,.Liang,.H.,.Xu,.L.,.Zhao,.J.,.Bian,.J.,.Luo,.Y.,.Liu,.Y.,.Li,.W.,.Wu,.G.,.and.Du,.G.,.Cu.related.doublets.green.band.emission.in.ZnO:Cu.thin.films,.J. Appl. Phys.,.108,.113507,.2010..Copyright.2010,.American.Institute.of.Physics.)


367.0–366.3.nm).compared.with.the.undoped.ZnO.film,.and.the.emission.related.to.Si.impu-rity.disappears..Meanwhile,.a.strong.emission.of.around.510.nm.(green.band). is.clearly.observed..It.has.the.characteristic.fine.structure.with.clear.doublet.features.of.Cu-doped.ZnO.first.reported.by.Dingle.[45]..The.dashed.line.in.Figure.12.13.shows.the.zero-phonon.lines.associated.with.the.structured.emission.

The.energies.of.the.peak.positions.of.the.zero-phonon.lines.and.their.replicas.are.shown.in.Table.12.1.[50].

The.energy.spacing.between.any.two.adjacent.lines.in.the.fine.structure.of.either.series.is.about.70.meV.(close.to.the.LO.phonon.energy.of.ZnO).

In.every.doublet.region,.there.are.two.lines.separated.by.∼27.meV..The.spectrum.includes.two.zero-photon-lines.denoted.by.1a.and.1b..However,.the.zero-photon-lines.are.not.the.most.intense.peak.in.the.spectrum.because.of.the.overlap.between.the.emission.related.to.Cu.and.the.emission.related.to.lattice.defects.that.are.enhanced.when.Cu.impurity.is.doped.in.the.lattice..The.existence.of.two.sets.of.fine.structures.with.an.energy.interval.of.∼27.meV.postulates.that.two.kinds.of.transitions.coupled.with.the.lattice.vibration.exist.[56].. Compared. to. the. unintentionally. doped. film,. it. is. reasonable. to. conclude. that. Cu.is. incorporated. in.ZnO. lattice.and. responsible. for. the. structured.green.band.emission..Earlier,. Reynolds. et. al.. [57]. observed. very. similar. emission. from. a. vapor. phase-grown.ZnO.sample..They.suggested.a.two-donor,.deep.acceptor.vibronic.model.to.quantitatively.explain.the.spectra.by.a.method.of.Gaussian.function.fitting..In.that.case,.Cu.has.not.been.conceded.as.a.participant;.however,.the.Cu.impurity.was.present.in.their.sample.as.mea-sured.by.glow-discharge.mass.spectroscopy.

The.nature.of.green.emission.∼2.5.eV.remained.controversial. for.decades.. In.the.early.studies,.it.is.because.of.the.Cu.impurities.in.ZnO,.but.later.it.was.interpreted.due.to.oxygen.vacancies. that.created.a.deep.donor. level. responsible. for. the.green.emission..However,.the.transition.from.donor.state.to.valance.band.in.n-type.semiconductors.is.very.unlikely..The.GL.band.with.its.characteristic.fine.structures.is.more.convenient.related.to.the.Cu.impurities.in.ZnO;.whereas.the.structureless.GL.band.near.the.same.position.and.width.is.related.to.the.transition.from.the.conduction.band.or.shallow.donor.level.to.the.native.defects,.like.Zn.vacancy.

Dingle. [45]. has. studied. the. luminescent. transitions. in. n-type. ZnO. crystals. con-taining. copper. (4.±.2.ppm),. aluminum. (<2.ppm),. iron,. magnesium,. silicon,. boron,. and.indium..The.experiment.was.conducted.from.1.6.K.to.20.K,.the.PL.spectrum.dominated.by.a.broad.GL.band.at.∼2.45.eV.with.the.characteristic.fine.structures.as.shown.in.the.Figure.12.14.

At.low.temperature,.the.band.decays.exponentially.with.a.decay.time.of.440.± ions..The.time.resolved.spectroscopy.confirmed.that. the.whole.system.decays.with. the.same. life.time. [45].. In. the.zero-phonon. region,. there.are. two.sharp. lines. (A.and.B). separated.by.0.1.meV.as.shown.in.Figure.12.15.

TABLE 12.1

The.Energies.of.the.Zero-Phonon.Line.and.Its.Replicas

Line number 1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b

Wavelength.(nm)

434.1 438.9 446.0 449.8 457.2 461.9 469.0 474.5 482.2 487.2 495.1 499.7

Energy.(eV) 2.8559 2.8245 2.7798 2.7565 2.7118 2.6840 2.6436 2.6218 2.571 2.5446 2.5042 2.4808

Source:. Liu,.Y..et.al.,.J. Appl. Phys.,.108,.113507,.2010.


A4000 4500

+

LO20,500 eV 2LO

5000

Arb

itrar

y em

issio

n in

tens

ity

5500 6000 6500

FIGURE 12.14The.broad.green.emission.from.ZnO.at.1.6.K;.the.enlarged.portion.shows.the.fine.structure.with.the.no-phonon.line.at.2.859.eV..(Reproduced.with.permission.from.Dingle,.R.,.Luminescent.transitions.associated.with.diva-lent.copper.impurities.and.the.green.emission.from.semiconducting.zinc.oxide,.Phys. Rev. Lett.,.23,.579,.1969..Copyright.1969,.American.Physical.Society.)

A/B = 2.25

2.85897 eV

1.6°K

A

B

0.11 meV

Slit

0.05 meV

12°K(20.4°K)

2.85908 eV

FIGURE 12.15High-resolution.spectra.in.the.region.of.A.and.B.zero-phonon.lines.near.2.859.eV..(Reproduced.with.permission.from.Dingle,.R.,.Luminescent.transitions.associated.with.divalent.copper.impurities.and.the.green.emission.from.semiconducting.zinc.oxide,.Phys. Rev. Lett.,.23,.579,.1969..Copyright.1969,.American.Physical.Society.)


The. intensity.ratio.A/B.=.2.25.±.0.05.does.not.vary. in. the. temperature.range.1.6–20.4.K.and.with.the.independent.crystal.origin..Dingle.[45].has.also.studied.the.behavior.of.these.lines.and.the.band.as.a.function.of.temperature,.uniaxial.stress,.applied.magnetic.field,.and.polarization.of.the.excitation.source..The.defining.information.for.the.identification.of.GL.band.has.been.obtained.from.the.splitting.of.A.and.B.lines.under.the.applied.mag-netic.field..Each.component.of.zero-phonon.lines.splits.into.four.symmetrically.disposed.components.under.the.applied.magnetic.field.parallel.to.the.crystal.c-axis..No.evidence.of.interactions.between.the.components.of.A.and.B.lines.was.found.and.the.splitting.patterns.are.linear.with.the.applied.field.strength..The.g.factor.obtained.from.the.angle-dependent.splitting.of.the.applied.magnetic.field.(g∥.=.0.73.±.0.05.and.g⊥.=.1.48.±.0.05).is.nearly.identi-cal.with.the.values.obtained.from.the.electron.paramagnetic.resonance.(EPR).studies.of.ground.state.of.divalent.copper.(Cu2+).on.a.Zn.site.in.copper-doped.ZnO.(g∥.=.0.7383.±.0.0003.and.g⊥.=.1.5237.±.0.003).[58]..The.absence.of.thermalization.between.A.and.B.lines.in.zero.magnetic. field,. invariance. of. A/B. intensity. ratio,. and. identical. of. Zeeman. patterns. for.A.and.B.provide.the.evidence.for.the.origin.of.GL.with.the.characteristic.fine.structures.belonging.to.the.Cu2+.(CuZn).in.Zn.site.of.the.ZnO.lattice.[45].

The.ground.state.of.Cu2+.in.ZnO.(CuZn.acceptor).has.been.established.by.infrared.absorp-tion.and.EPR.studies.[58]..It.appears.that.the.copper.substitutes.for.zinc,.entering.a.slightly.distorted.tetrahedral.site..When.the.Fermi.level.is.low.enough,.the.copper.is.in.divalent.(Cu2+).state,.having.a.d.hole.that.in.its.ground.state.is.in.a.triply.degenerated.t2.orbital..The.states.of.d.hole.in.a.trigonally.disordered.tetrahedral.field.are.shown.in.Figure.12.16..The.free.ion.term.2D.of.Cu2+.ion.is.split.by.tetrahedral.crystal.field.into.the.2E(D).and.2T2(D).states.that.in.turn.are.split.by.the.combination.of.week.spin-orbit.coupling.and.trigonal.crystal.field.[58].

Another.interesting.observation.in.Cu-doped.ZnO.is.the.behavior.of.the.zero-phonon.line.due.to.the.isotope.shift..At.low.temperature,.intracenter.transitions.from.the.lowest.sub-lattice.of.the.2T2.state.to.tow.sublevels.of. 2E.state.are.responsible.for.the.absorption.line.at.717.and.722.meV..For.the.intracenter.Cu2+.(2E.−.2T2).transition,.two.zero-phonon.lines.(ZPLs).are.observed.in.absorption.[49,50].and.are.attributed.to.the.Γ4(2T2).to.Γ5,6(2E).and.the.Γ4(2T2). to.Γ4(2E). transitions..A.highly. resolved. transmission. spectrum.of. the. low-energy.(∼717.meV).ZPL.is.shown.in.Figure.12.17.[46].

a

2T2

2E

2D

+Td

Г4,Г5

Г4,Г5

b c

+ S.O. + C3V

d

e

+H

f g h

ГA

Г0

Г6

FIGURE 12.16Energy.level.diagram.of.Cu2+.in.ZnO..(Reproduced.with.permission.from.Dietz,.R.E.,.Kamimura,.H.,.Sturge,.M.D.,.and.Yariv,.A.,.Electronic.structure.of.copper.impurities.in.ZnO,.Phys. Rev.,.132,.1559,.1963..Copyright.1963,.American.Physical.Society.)


The.two.components.of.the.line.~.717.meV.(split.by.~.0.1.meV).are.due.to.the.two.isotopes.(63/65).of.Cu.in.ZnO.[46]..This.was.also.confirmed.by.their.transformation.into.a.single.line.in.the.crystals.doped.with.only.63Cu.isotope.[59]..Dietz.et.al..[58].concluded.that.the.Cu2+.t2.wave.function.is.radially.expanded.relative.to.the.d.wave.function.of.Cu.ion,.and.the.t2.hole.spends.about.60%.of.its.time.on.the.Cu2+.ion,.while.it.spends.the.rest.of.the.time.in.the.oxygen.sp3.orbitals.

Acknowledgments

Kousik.Samanta.acknowledges.Dr..A..K..Arora,.and.Dr..C..S..Sundar,.and.the.director.of.IGCAR.for.encouragement..Ram.S..Katiyar.acknowledges.partial.financial.support.from.the.DOE-EPSCoR.Grant.#DE-FG02-08ER46526.and.IFN-NSF.Grant.#EPS-1002410.

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Energy (eV)

0.7164

Inte

nsity

0.7166

ZnO: Cu

(2T2–2E)

0.7168

122 µeV

65 63

0.7170

T=1.8 KE c

0.7172

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373

13Structural and Magnetic Properties of ZnO Alloy Films with Cu, Cr, and Fe by RF Magnetron Sputtering Technique

X.M. Wu, L.J. Zhuge, Z.F. Wu, and C.G. Jin

CONTENTS

13.1. Introduction......................................................................................................................... 37413.2. Experiment........................................................................................................................... 375

13.2.1. Materials.Growth.................................................................................................... 37513.2.2. Characterization.Techniques................................................................................ 378

13.2.2.1. X-Ray.Diffraction..................................................................................... 37813.2.2.2. Transmission.Electron.Microscopy.(TEM):.Atomic.Resolution.

Characterization....................................................................................... 37913.2.2.3. Scanning.Electron.Microscopy.(SEM)..................................................38013.2.2.4. Raman.Spectroscopy............................................................................... 38113.2.2.5. Chemical.Characterization:.X-Ray.Photoelectron.Spectroscopy...... 38113.2.2.6. Magnetic.Characterization.Using.SQUID............................................383

13.3. Zn1−xCuxO-.Based.Diluted.Magnetic.Semiconductors..................................................38413.3.1. Introduction.............................................................................................................38413.3.2. Experiment...............................................................................................................38413.3.3. Results.and.Discussion..........................................................................................385

13.3.3.1. X-Ray.Diffraction.(XRD)..........................................................................38513.3.3.2. Scanning.Electron.Microscopy.(SEM)..................................................38513.3.3.3. Transmission.Electron.Microscopy.(TEM)...........................................38513.3.3.4. X-Ray.Photoelectron.Spectroscopy.(XPS)............................................. 38713.3.3.5. Raman.Spectra.......................................................................................... 38713.3.3.6. Magnetic.Properties.................................................................................388

13.3.4. Conclusion............................................................................................................... 38913.4. Zn1−xCrxO-.Based.Diluted.Magnetic.Semiconductors................................................... 390

13.4.1. Introduction............................................................................................................. 39013.4.2. Experiment............................................................................................................... 39013.4.3. Results.and.Discussion.......................................................................................... 391

13.4.3.1. X-Ray.Diffraction.(XRD).......................................................................... 39113.4.3.2. Transmission.Electron.Microscopy.(TEM)........................................... 39213.4.3.3. X-Ray.Photoelectron.Spectroscopy.(XPS)............................................. 39213.4.3.4. Magnetic.Properties................................................................................. 393

13.4.4. Conclusion............................................................................................................... 39813.5. Zn1−xFexO-.Based.Diluted.Magnetic.Semiconductors................................................... 398

13.5.1. Introduction............................................................................................................. 39813.5.2. Experiment............................................................................................................... 399


13.1 Introduction

The.fast.growing.field.of.microelectronics.and.information.processing.has.been.based.on.fabricating. devices. out. of. conductors. and. doped. semiconductors,. where. the. charge. of.the.electron.is.exploited..Generation.to.generation.improvements. in.the.performance.of.microelectronic.devices.are.currently.accomplished.by.shrinking.the.size.of.the.elements.in. the. chips..However,. this. approach.will. soon.hit. the.barrier.due. to. the. issues. related.to.fabrication.of.shallow.channel.junctions.and.heat.management.[1]..Similarly,.informa-tion. storage,. the. second. vital. piece. of. the. microelectronic. revolution,. depends. on. mag-netic.storage.devices.that.exploit.the.spin.component.of.the.electron.[2,3]..With.the.goal.of.realizing.spintronic.devices.such.as.spin.valves,.spin.light-emitting.diodes.(LEDs),.non-volatile.memory,.spin.transistors,.and.ultra-fast.optical.switches,.a.significant.amount.of.research.thrust.has.been.focused.on.discovering.materials.suitable.for.these.applications.[4,5]..One.of.the.ingenious.ways.to.combine.the.spin.and.charge.of.the.carriers.in.mate-rial.and.achieve.these.effects,. is.synthesizing.a.material.with.semiconducting.as.well.as.magnetic.properties..Since.conventionally.used.semiconductors.such.as.Si.and.GaAs.are.diamagnetic.and.possess.a.very.small.g-factor.(measure.of.the.interaction.strength.with.an.applied.magnetic.field),.there.must.be.a.way.to.integrate.the.semiconducting.and.magnetic.properties.by.some.other.means.

These.were.the.ideas.that.prompted.the.evolution.of.a.new.class.of.materials.known.as. “Diluted. Magnetic. Semiconductors”. (or. DMSs,. in. short),. where. a. fraction. of. a. host.semiconductor.material’s.atoms.are.substituted.by.ferromagnetic.atoms..DMSs.have.been.attracting. much. interest. of. almost. a. decade. now. due. to. their. potential. to. manipulate.charge.and.spin.degrees.of.freedom.in.a.single.material..Dietl.et.al..predicted.high-tem-perature. ferromagnetism. in. transition-metal. (TM)-doped. wide-band-gap. semiconduc-tors.particularly. in.ZnO.and.GaN..This. fact.has.motivated.many. researchers. to. study.the.properties.of.TM-doped.semiconductors..Recent.reports.on.the.observation.of.room-temperature. (RT). ferromagnetism. in. TM-doped. ZnO. have. been. welcomed. with. great.enthusiasm.by.the.scientific.community..ZnO-based.DMSs.have.some.advantages.over.others. because. of. their. unique. characteristics.. There. has. been. a. great. deal. of. interest.in.ZnO.semiconductor.materials. lately,.as.seen.from.the.surge.of.a.relevant.number.of.publications..The.interest.in.ZnO.is.fueled.and.fanned.by.its.prospects.in.optoelectronic.applications. owing. to. its. direct. wide. band. gap. (Eg.∼.3.3.eV. at. 300.K).. Some. optoelec-tronic.applications.of.ZnO.overlap.with.those.of.GaN,.another.wide-gap.semiconductor.(Eg.∼.3.4.eV.at.300.K).which.is.widely.used.for.the.production.of.green,.blue–ultraviolet,.and.white.light-emitting.devices..However,.ZnO.has.some.advantages.over.GaN.among.

13.5.3. Results.and.Discussion..........................................................................................40013.5.3.1. Optical.Microscopy.(OM).......................................................................40013.5.3.2. X-Ray.Diffraction.(XRD)..........................................................................40213.5.3.3. X-Ray.Photoelectron.Spectroscopy.(XPS).............................................40413.5.3.4. Scanning.Electron.Microscopy.(SEM)..................................................40513.5.3.5. Magnetic.Properties.................................................................................406

13.5.4. Conclusion............................................................................................................... 41213.6. Conclusion........................................................................................................................... 412References...................................................................................................................................... 413

375Structural and Magnetic Properties

which.are.the.availability.of.fairly.high-quality.ZnO.bulk.single.crystals.and.a.large.exci-ton.binding.energy. (∼60.meV)..ZnO.also.has.much.simpler. crystal-growth. technology,.resulting. in. a. potentially. lower. cost. for. ZnO-based. devices.. This. thesis. is. focused. on.the.hotspots.and.challenges.in.the.field.of.ZnO.material.research..Based.on.mean.field.theory,.it.has.been.predicted.that.ZnO.is.a.promising.candidate.for.DMS.material.to.real-ize.a.Curie.temperature.above.room.temperature.[6,7]..There.are.several.reports.on.fer-romagnetic.properties.in.Zn1−xFexO,.Zn1−xCuxO,.and.Zn1−xCoxO.thin.films.[8–14],.and.the.origin.of.ferromagnetism.is.controversial.and.often.attributed.to.the.fabrication.artifact.of. these.alloy.systems..Moreover,.TM-doped.ZnO.(ZnTMO).nanoparticles.have.several.practical.applications. in.diversified.fields. [15–19]..However,. the.origin.of.observed. fer-romagnetism.still.remains.controversial,.and.the.origin.of.ferromagnetism.varies.in.dif-ferent.TM-doped.ZnO.films.

In.this.chapter,.ZnO-based.DMS.materials.will.be.dealt.with.in.detail.with.different.dopants. such. as. Cu,. Cr,. and. Fe.. Thin. films. of. these. materials. have. been. synthesized.by. RF. magnetron. sputtering. method. and. its. microstructure. and. magnetic. properties.were.investigated..The.structure.properties.of.the.samples.are.discussed.in.order.to.lay.a. strong. foundation. for. discussing. the. origins. of. the. ferromagnetism. which. we. have.observed.

13.2 Experiment

13.2.1 Materials Growth

Sputtering.is.an.atomistic.sandblasting.process,.in.which.the.argon.atoms.are.used.for.a.physical.bombardment.etching.process.on. target.material..This. reaction.causes. the.erosion.of.the.target.surface.and.deposition.as.a.film..When.the.incident.particles.strike.the.material.surface.with.enough.energy,. the.atoms.of. the.material.surface.are.being.dislocated. from. the. surface.. Because. this. reaction. can. occur. between. any. molecules.within. the. chamber,. it. is. required. to. keep. low. atmospheric. pressure. to. prevent. any.undesired. species. like.atoms,. ions,. electrons,.photons,.neutrons,. and.even.molecules.from.being.involved.in.the.sputtering.operation..The.mainly.used.species.in.sputtering.are.gas.ions.like.Ar+.

As.depicted.in.Figure.13.1,.the.incident.atoms.are.accelerated.toward.the.target.surface.so. that. surface. atom. structure. is. partially. etched. and. finally. surface. atoms. are. being.sputtered. from. the. surface.. The. physical. meaning. of. this. process. can. be. described. in.Equation.13.1.

.Y = numberof emittedparticles

numberof incident particles. (13.1)

where.Y.is.sputter.yield.rate.[20,21]..There.is.another.formula.called.Sigmund’s.formula-tion.[22].

.S K

MM

M M

EU

M Mi t

i t Ot i=

+( )

( )2 α . (13.2)


whereK.is.a.constant.of.range.0.1–0.3Mi.and.Mt.are.the.masses.of.incident.ion.and.target.atom,.respectivelyE.is.the.field.that.accelerates.ionsU.is.the.binding.energy.of.the.target.atomsα(Mt/Mi).is.a.monotonically.increasing.function.of.(Mt/Mi)

This.formulation.considers.the.sputter.yield.more.physically.and.has.been.correlated.with.existing.yield.measurements.or.used.to.estimate.values.of.elements.[22,23].

Sputtering.yield. is.a. function.of.number.of.emitted.versus.number.of. incident.atoms.as.described.in.Equations.13.1.and.13.2;.so.this.yield.depends.on.the.amount.of.energy.applied.to.incident.atoms..Incident.atoms.have.about.40–1000.eV,.which.is.high.enough.to.dislodge.numerous.surface.atoms..The.first.incident.atom.is.a.key.of.total.sputtering.opera-tion..Once.it.hits.the.surface,.atoms.on.the.surface.hit.other.atoms,.and.then.more.and.more.collision.can.occur;.a.total.surface.bonding.starts.being.collapsed..Other.incident.atoms.finally.accelerate.a.total.collision.

A.widely.used,.basic.sputtering.method.is.magnetron.sputtering..The.supportive.spe-cies,.incident.ion,.in.the.chamber.is.an.Ar.gas..Ar.atoms.are.ionized.by.the.applied.elec-tric. field. in. the. glow. discharge. system,. attracted. toward. the. target. material. [24].. Then,.Ar. atoms. strike. the. target. with. sufficient. energy. to. dislocate. the. surface. atoms. by. the.momentum.transfer..The.major.energy.source.for.this.operation.is.the.plasma.generated.by.applying.a.DC.potential.between.a.substrate.and.a.target.[24].

Originally,.the.term.“magnetron”.is.used.in.microwave-related.applications..In.this.case,.this.mean.microwave.plasma.is.a.static.microwave.field.configured.at. the.cathode..The.magnetic.field.is.built.up.parallel.to.the.surface.of.the.cathode.and.electric.field.is.set.up.perpendicularly.to.the.magnetic.field..Ion.bombardment.on.the.cathode.surface.leads.to.the.emission.of.electrons.from.the.cathode..Then,.these.are.constrained.by.electron.and.magnetic.flux—E × B.drift.[25].

Sputtered atom Surfaceatoms

Sputtered atom

Incidentation

FIGURE 13.1A.Schematic.of.sputtering.operation.


This.E × B.drift,.which.is.called.Hall.Effect,.causes.the.electrons.to.move.parallel.to.the.cathode.surface.and.form.a.closed.current.loop.[25]..Because.the.magnetic.field.is.restrict-ing.the.primary.electrons.near.the.cathode.region,.there.is.less.possibility.that.the.primary.electrons.can.directly.hit.the.anode.without.any.collision..The.electrons.on.a.closed.cur-rent.loop.are.collided.with.ionized.gas.atoms.and.other.electrons..This.collision.results.in.a.very.dense.plasma.activity.near.the.cathode.region..Then,.this.closed.loop.ring.becomes.an.etch.track.that.causes.the.erosion.of.the.cathode.surface..Finally,.the.sputtered.atoms.from.the.target.surface.are.deposited.onto.the.substrate.

RF. discharge. can. be. seen. schematically. in. Figure. 13.2.. In. addition,. a. magnet. can.be. added. to. these. two. setups. in. order. to. enhance. the. deposition. rates.. RF. Magnetron.Sputtering.is.a.reliable.technique.used.to.deposit.many.different.types.of.films,.including.electrically.insulating.samples.[26]..A.high-voltage.RF.source.at.a.frequency.of.13.56.MHz.is.used.to.ionize.a.sputtering.gas.which.produces.the.plasma..The.ionized.gas.then.bom-bards.the.target.where.multiple.collisions.take.place,.releasing.atoms.of.the.target.mate-rial.into.the.plasma..These.atoms.condense.upon.the.substrate.which.is.placed.in.front.of.the.target..A.blocking.capacitor.is.placed.in.the.circuit,.and.a.matching.network.is.used.to.optimize.the.power.transfer..A.permanent.magnet.is.added.to.the.sputtering.gun.in.order.to.enhance.the.deposition.rate..This.is.done.by.the.trapping.of.electrons,.from.a.Hall.effect.near.the.target.surface.[27]..This.magnet.creates.lines.of.magnetic.flux.that.are.per-pendicular.to.the.electric.field,.or.parallel.to.the.target.surface..This.static.magnetic.field.retains.secondary.electrons.(SE).in.that.region.which.drift.in.a.cycloidal.path.on.the.target.and.increase.the.number.of.collisions.that.occur..RF.Magnetron.sputtering.was.chosen.for.this.particular.application.for.a.variety.of.reasons..First,.DC.sputtering.of.thiogermanate.materials.is.not.possible.because.they.are.electrically.insulating..Next,.it.is.known.that.RF.sputtering.produces.thin.films.that.have.a.high.density.and.are.relatively.smooth.when.compared.with.films.produced.from.PLD.processes.[26]..In.addition,.RF.deposition.offers.good.compositional.control.so.that.the.resulting.thin.films.have.compositions.similar.to.that.of.the.target.material.

A.RF.sputtering.system.was.used.for.ZnO.deposition.on.various.substrates.due.to.its.low.cost,.simplicity,.and.low.deposition.temperature..In.the.sputtering.system,.mechani-cal.pump.and.molecular.pump.were.used.for.roughing.and.high-vacuum,.respectively..

Work gaspuffing

Puffing out

RF power: 13.56 MHz

1

56

7

2

38

40

9 10

11

FIGURE 13.2Schematic.diagram.of.the.experimental.setup.of.RF.magnetron.sputtering..(1..Magnetic.pole;.2..shield;.3..sub-strate;.4..substrate.heating.device;.5..sputtering.target;.6..magnetic.lines.of.force;.7..electric.field;.8..baffle.plate;.9..matching.network;.10..power.source;.11..RF.generator.)


A.composite.target.of.single.crystalline.ZnO.(60.mm.in.diameter).containing.various.Cu,.Cr.pieces,.and.Fe.pieces,.respectively.was.used.for.deposition,.which.is.located.around.75.mm.from.the.heated.substrate..Before.deposition,.the.substrates.were.dipped.in.ace-tone. to. remove. the. surface. contamination,. rinsed. with. a. large. amount. of. de-ionized.water,.dried.in.a.flux.of.N2,.and.placed.into.the.chamber..The.substrate.was.heated.in.the.temperature.range.200°C–600°C.to.get.better.crystallinity..Sputtering.was.achieved.by.bombarding.the.ZnO.target.with.energetic.ions.of.Ar+..Usually.Ar.gas.was.introduced.at.1–2.Pa,.with.the.base.pressure.of.1.×.10−4.Pa.in.chamber..After.gases.were.introduced.into.the.chamber,.the.pressure.was.held.at.1–2.Pa.during.sputtering.deposition..In.order.to.supply.energy.to.the.argon.atoms,.an.RF.power.of.100–200.W.was.applied..ZnO.atoms.at.the.surface.are.knocked.loose.and.condense.on.the.substrates,.where.the.ZnO.film.is.formed.with.average.50–400.A/min.deposition.rate.

13.2.2 Characterization Techniques

The. thin.films. in. this. study.which.are. synthesized.by.RF.magnetron. technique.are.all.characterized.for.the.following.properties:

. 1..Structural.properties.by.x-ray.diffraction

. 2..Microstructural.and.nanostructural.properties.by.transmission.electron.micros-copy.(TEM)

. 3..Morphology.by.scanning.electron.microscopy.(SEM)

. 4..Structural.properties.by.Raman.Spectroscopy

. 5..Chemical.analysis.by.x-ray.photoelectron.spectroscopy.(XPS)

. 6..Magnetic.properties.by.superconducting.quantum.interference.device.(SQUID)

13.2.2.1 X-Ray Diffraction

X-ray.diffraction.(XRD).[29].is.a.non-destructive,.convenient.way.to.check.the.crystal.struc-ture,. lattice.parameter,. preferred. orientation. of. thin.films,. stress. in.a.material,. etc..The.basic. principle. is. that. when. a. monochromatic. beam. of. x-rays. interacts. with. a. periodic.crystal,. there.will.be.peak. intensities. in. the. reflected/diffracted.beam.when. the.planes.satisfy.the.Bragg’s.law:

. n dλ θ= 2 sin . (13.3)

whereλ.is.the.wavelength.of.the.x-ray.beamd.is.the.interplanar.spacing.of.the.material.in.the.crystalθ.is.the.angle.of.incidence

Thus.the.peaks.are.characteristic.to.the.material,.as.the.crystal.structure.is.the.one,.which.determines.the.diffraction.of.the.incident.beam,.and.the.relative.intensities.between.the.different.reflected.peaks..The.relative.intensity.of.the.diffracted.beam.can.be.written.as

.I F phki= −

22

2

1 cossin cos

θθ θ

. (13.4)


In.this.equation,.Fhkl.is.the.structure.factor,.which.is.an.extension.of.the.atomic.scattering.factor,.given.by

.F f i hx ky lzhkl n n n n

n

N

= + +−

∑ exp{ ( )}21

π

wherefn.is.the.atomic.scattering.factor(hkl).are.the.plane.indices

In.Equation.13.2,.p.is.the.multiplicity.factor.and.the.term.in.the.parenthesis.is.the.Lorentz.Polarization. factor..The. structural.qualities.of. the.ZnO.films.were. investigated.by.XRD.(Rigaku.D/max-3C).using.Cu.Kα.radiation..The.x-ray.intensity.is.plotted.as.a.function.of.2θ.to.reveal.the.diffraction.pattern.for.a.sample..Phase.identification.is.possible.by.com-paring.the.diffraction.peaks.to.known.material.standards.in.the.JCPDS.catalog..With.this.method,.recognition.of.any.dopant-induced.secondary.phases.within.the.semiconductor.host.was.determined.

13.2.2.2 Transmission Electron Microscopy (TEM): Atomic Resolution Characterization

Due.to.the.disadvantages.of.XRD,.to.find.material. in.small.quantities.or.to.identify.the.local.structure,.one.has.to.resort.to.TEM.[30]..It.is.by.far.the.most.effective.and.direct.tech-nique.available.to.fully.characterize.a.material.in.the.atomic.scale..Moreover,.while.XRD.can.reveal.the.texture.perpendicular.to.the.substrate.plane,.to.get.the.inplane.orientation.of.an.epitaxial.thin.film.on.the.substrate,.TEM.is.the.preferred.technique..These.aspects.are. important. considerations. for. a. complete. analysis. of. DMS.. In. this. dissertation. TEM.analysis.was.done.using.a.TecnaiG220.TEM..This.is.the.most.effective.and.direct.technique.available.for.full.atomic.scale.characterization.of.a.material.

A.TEM.is.an.instrument.that.relies.on.the.fact.that.we.can.probe.the.atomic.nature.of.a.material.because.the.resolution.obtainable.is.comparable.to.the.wavelength.of.the.prob-ing.beam..In.the.case.of.the.electron.beam.accelerated.with.a.voltage.V,.the.wavelength.is.given.by

.λ = h

m2 0eV. (13.5)

For.a.200.kilo.electron.volts.(keV).electron.beam,.the.wavelength.can.be.as.small.as.0.03.Å..The.actual.resolution.obtainable.in.a.TEM.is.about.0.14.nm.in.high-resolution.Z-contrast.mode.and.0.24.nm.in.high-resolution.TEM.mode..This.is.due.to.the.spherical.aberrations.and.stigmatism..Essentially,.in.a.TEM.electrons.are.generated.from.an.electron.gun.and.then.accelerated.by.a.high.voltage.(200.kV),.and.focused.using.multiple.electromagnetic.lenses.. This. high-energy. beam. interacts. with. the. material. of. the. TEM. sample. and. the.resulting.image.can.be.captured.on.a.film.or.a.CCD.camera..In.the.TecnaiG220.TEM.is.used.for.analysis.in.this.dissertation..It.is.equipped.with.capabilities.to.obtain.images.on.both.the.film.and.the.CCD.camera.

Cross. section. TEM. sample. preparation:. All. cross. section. TEM. samples. in. this. work.were.prepared.by.first.sticking.two.pieces.together,.with.film.sides.facing.each.other.using.M-bond.glue..The. sample. is. then.ground. to. level. the. top. surface..A. series. of.diamond.


polishes.with.decreasing.grain.sizes.is.then.employed.to.smooth.out.the.first.side.of.the.sample..The.sample.is.then.flipped.over.and.ground.to.a.thickness.of.800.μ,.beyond.which.again,.diamond.polishing. is.done.to.reduce.the.thickness. to.about.120.μ..The.sample. is.then.dimpled.to.the.point.where.the.dimpled.region.is.about.15–20.μ.thick..After.this,.a.precision.ion.polishing.system.is.used.to.create.a.small.hole.near.the.interface.of.film.and.substrate,.within.the.dimpled.region.around.which.there.is.very.thin.area.(10–100.nm.in.thickness).which.can.then.be.viewed.in.the.TEM.

13.2.2.3 Scanning Electron Microscopy (SEM)

The.SEM.used.in.these.studies.was.a.Hitachi.S-4700.field.emission.SEM..The.SEM.consists.of. four. systems:. the. illuminating/imaging. system,. the. information. system,. the. display.system,.and.vacuum.system..The.display.system.consists.of.a.cathode-ray.tube.for.observ-ing.and.photographing.the.surface.of.interest..The.vacuum.system.removes.gases.from.the.microscope.column.to.reduce.the.gas.interference.with.high-resolution.imaging..The.SEM.system.is.shown.in.Figure.13.3.

Electrons.emitted.by.the.electron.gun.are.accelerated.through.an.electric.field.and.the.change.in.voltage.as.they.pass.from.the.gun.to.the.specimen.determines.the.energy.of.the.electrons.at.the.point.that.they.start.to.interact.with.the.specimen..The.SEM.images.used.in.this.work.were.taken.with.an.accelerating.voltage.ranging.from.about.1.0.to.3.0.kV..When.an.electron.enters.the.specimen,.it.can.interact.in.two.different.ways.with.the.atoms.in.the.specimen..If.it.comes.close.enough.to.an.electron.that.is.already.in.the.specimen.(usually.as.part.of.one.of.the.atoms.in.the.specimen),.then.it.can.transfer.some.of.its.energy.to.that.electron.in.what.is.called.an.inelastic.interaction.(as.when.one.billiard.ball.hits.another.bil-liard.ball)..If.an.electron.comes.close.enough.to.the.relatively.massive.nucleus.of.an.atom.in.the.specimen,.then.it.will.be.deflected.with.almost.no.loss.in.energy.in.what.is.called.an.elastic.interaction.(as.when.a.billiard.ball.hits.a.bowling.ball)..Typically,.a.high-energy.elec-tron.moving.through.the.specimen.is.gradually.slowed.down.through.numerous.inelastic.interactions,.very.few.of.which.change.its.direction.significantly..Elastic.interactions.occur.much. less.often.but.are. responsible. for.most.of. the.changes. in.direction.. If.an.electron.

Electric andmagnetic fieldsfor acceleration

and focusing

Center of projection Detector

Detectorsignal

Display

Display rastersynchronized to

beam scanBackscattered and

secondary electrons

Elec

tron

sour

ce

Scanningbeam

Sample

FIGURE 13.3A.schematic.of.an.SEM.


from.the.electron.gun.is.deflected.and.comes.out.of.the.specimen,.it.is.known.as.a.back-scattered.electron.(BSE)..Electrons.that.are.part.of.the.specimen.but.are.kicked.out.through.inelastic.interactions.are.known.as.secondary.electrons.(SE)..Because.the.origin.of.a.par-ticular.scattered.electron.is.not.directly.measured,.scattered.electrons.are.most.commonly.classified.by.their.energy,.which.can.be.measured..The.energy.is.typically.expressed.in.units.of.electron.volts.(eV),.the.kinetic.energy.acquires.an.electron.as.it.traverses.a.change.in.a.potential.of.1.V..If.the.electron.gun.is.at.a.potential.of.−1000.V.relative.to.ground.and.the.specimen.is.ground,.each.electron.in.the.beam.will.hit.the.specimen.with.1000.eV.or.1.keV..Electrons.escaping. the.specimen. typically.have.energy.somewhat. lower. than. the.energy.of.the.incident.electrons.

The.detected.electrons.are.accelerated.toward.a.scintillator.by.a.potential.difference.of.10–15.keV.and.are.converted.into.a.proportional.number.of.photons.by.the.scintillator..The.light.signal.is.amplified.and.converted.back.into.an.electrical.pulse.by.a.photomultiplier.tube..The.electrical.pulse.(photocurrent.signal).modulates.the.CRT.screen.brightness,.and.an.image.is.produced.

13.2.2.4 Raman Spectroscopy

The.structural.qualities.of.the.films.were.investigated.by.confocal.micro-Raman.spectros-copy.(JY-H800)..Raman.spectra.were.recorded.using.514.nm.excitation.line.in.backscatter-ing.geometry..Raman.spectroscopy. is.a.nondestructive. technique. that. is.used. to.probe.the.local.structure.of.a.material..For.this,.it.uses.scattered.radiation.of.sample.from.a.laser.beam..This.scattered.radiation.comes.from.the.energy.transfer.between.the.incident.radia-tion.and.the.scattering.system..Two.types.of.scattering.can.occur:.Stokes.and.anti-Stokes..Stokes.scattering.occurs.when.the.system.is.placed.into.an.excited.state,.the.resulting.scat-tered. radiation. is. at. a. lower. frequency. than. the. incident. radiation.. In. anti-Stokes. scat-tering,.an.incident.photon.is.destroyed.while.another.photon.is.created.simultaneously,.which.gives.scattered.radiation.that.has.a.higher.frequency.than.the.incident.radiation..This.can.be.represented.mathematically.as

. � � �v v vs o e= ± . (13.6)

where. the. subscripts. s,. o,. and. e. refer. to. the. scattered,. incident,. and. excited. radiation.frequencies.

Raman.spectra.were.collected.using.a.Renishaw.in.Via.spectrometer.using.a.514.nm.line.from.an.Ar+.ion.laser..All.samples.were.placed.into.holders.which.were.covered.with.a.transparent.tape.to.prevent.exposure.to.atmospheric.moisture.and.oxygen.

13.2.2.5 Chemical Characterization: X-Ray Photoelectron Spectroscopy

X-ray.photoelectron.spectroscopy.(XPS). is.a.quantitative.spectroscopic.surface.chemical.analysis.technique.also.known.as.Electron.Spectroscopy.for.Chemical.Analysis.(ESCA)..It.can.be.used.to.perform.the.following.surface.(up.to.10.nm).analysis:

. 1..Detection.of.elements.(except.H.and.He).with.concentrations.>0.1%–1%

. 2..Quantitative.analysis.of.sample.composition.within.a.few.%.(error.<10%)

. 3..The.chemical.or.oxidation.state.identification.of.one.or.more.of.the.elements.in.the.sample


. 4..Measurement. of. the. uniformity. of. elemental. composition. as. a. function. of. ion.beam.etching.(depth.profiling):

. a.. Non-destructive.<10.nm

. b.. Destructive.∼.several.hundred.nanometers.by.Ar+.ion.sputtering

. 5..Measurement.of.the.uniformity.of.elemental.composition.across.the.top.surface.(line.profiling.or.mapping).with.a.lateral.resolution.of.few.μm–mm

In.this.dissertation.XPS.was.done.with.a.PHI-550.x-ray.photoelectron.spectrometer.(the.source.was.Al.Kα,.x-ray,.10.kV,.30.mA).after.Ar+.sputtering.(10−5.Pa,.4.kV,.15.mA,.2.nm/min).of.5.min..It.has.been.used.mainly.to.determine.the.composition.of.the.DMS.films,.chemical.state.of.the.dopants,.and.bonding.characteristics.of.the.various.atoms..A.determination.of.the.oxidation.state.of.Cu,.Cr,.and.Fe.in.the.ZnO.was.very.essential.in.this.study.to.deter-mine.whether.it.would.provide.localized.3D.electron.spins.needed.to.produce.ferromag-netism.in.the.ZnO.DMS.films..Also,.the.analysis.of.bonding.state.of.the.oxygen.atoms.in.both.the.as.deposited.and.annealed.films.gave.insight.about.the.relative.concentration.of.native.defects.like.oxygen.vacancies,.which.appear.to.play.an.important.role.in.determin-ing.the.ferromagnetic.properties,.in.the.film.

In.XPS,.x-ray.irradiation.of.a.material.under.ultra-high.vacuum.(UHV).leads.to.the.anal-ysis.of.emission.of.electrons.from.the.core.orbitals.of.the.surface.elements.present.in.the.top.10.nm.of.the.material..The.binding.energy.of.the.emitted.photoelectron.can.be.deter-mined.from.the.following.equation:

. hν = + +KE BE sΦ . (13.7)

wherehν.is.the.energy.of.the.incident.radiationKE.is.the.kinetic.energy.of.the.photoelectron.as.measured.by.the.instrumentBE.is.the.binding.energy.of.the.orbital.from.which.the.electron.is.ejectedΦs.is.the.work.function.of.the.instrument

The. binding. energy. of. the. electrons. reflects. the. local. environment. of. the. specific. sur-face.elements.from.which.they.are.ejected..Thus,.any.shift.observed.in.the.peak.position.can.be.correlated.with.a.change.in.oxidation.state,.bonding.strengths,.etc..The.number.of.electrons.reflects.the.proportion.of.the.specific.elements.on.the.surface..To.count.the.number.of.electrons.at.each.KE.value,.with.the.minimum.error,.XPS.must.be.performed.under.UHV.conditions.because.electron.counting.detectors.in.XPS.instruments.are.typi-cally.1.m.away.from.the.material.irradiated.with.x-rays..The.emitted.photoelectrons.are.energy.separated.and.detected.by.a.complex.energy.analyzer..The.photoelectrons.pass-ing. through. the. energy. analyzer. are. deflected. by. the. electrostatic. field. and. leave. the.analyzer.at.a.specific.position.depending.on.its.energy..A.hemispherical.energy.analyzer,.consisting.of.two.concentric.hemispheres.with.a.steady.voltage.(ΔV).applied.across.them,.is.one.of.the.most.commonly.used.analyzers.[31]..This.potential.is.such.that.the.potential.at.the.centerline.of.the.two.hemispheres.is.constant..This.potential.is.referred.to.as.the.pass.energy..The.resolution.is.found.to.be.better.for.low.pass.energy,.but.this.condition.results.in.weaker.signal.strength.of.the.photoelectrons..As.most.of.the.experiments.are.carried.out.at.a.constant.pass.energy,. the.resolution.of. the.analyzer.also.remains.con-stant..In.this.work,.a.pass.energy.of.100.eV.has.been.used.for.the.survey.spectrum.giving.a.resolution.∼2.eV..For.the.high-resolution.scans.of.the.peaks.of.the.various.constituent.


elements.a.pass.energy.of.20.eV.has.been.used.which.gave.a.resolution.of.1.eV.or.better..The. electrons. exiting. the. hemispherical. analyzer. are. then. detected. by. a. multichannel.detector,.also.called.the.electron.multiplier.

13.2.2.6 Magnetic Characterization Using SQUID

The.measurements.were. done. using. a. Quantum. Design. MPMS. SQUID. magnetometer..Currently,. SQUIDs. provide. the. most. sensitive. resolutions. for. magnetic. field. measure-ments..A.SQUID.is.a.Josephson.junction.consisting.of.a.superconducting.ring.having.a.gap.filled.with.a.very.thin.insulting.material..A.superconducting.current.can.tunnel.through.this.thin.insulating.layer..Using.this.technique,.very.tiny.magnetic.fields.of.the.order.of.10−14.T.(10−10.G).can.be.measured..Figure.13.4.shows.a.schematic.of.a.Josephson.junction.used.in.a.SQUID.magnetometer.

Magnetization. versus. field. loops. were. taken. at. various. temperatures.. Ferromagnetic.materials.will.produce.a.finite.hysteresis. in. these. loops..Therefore,. loop.hysteresis.was.used.to.verify.ferromagnetism.in.our.samples..The.samples.were.mounted.inside.a.plastic.drinking.straw.(which.has.no.ferromagnetic.components).and.placed.perpendicular.to.the.applied.field.

Additionally,. Field. Cooled/Zero. Field. Cooled. (FC/ZFC). measurements. were. used. to.track. the.magnetic. response.as.a. function.of. temperature..FC.measurements.were.per-formed.by.measuring.the.magnetization.as.the.sample.is.cooled.down.to.5.K.in.a.small.applied.field..ZFC.measurements.were.performed.by.cooling.the.sample.to.5.K.in.zero.field.and.then.applying.a.small.field.as.the.sample.was.heated.back.to.room.temperature.while.measuring.the.magnetization.

The. diamagnetic. responses. of. the. sample. and. substrate. were. subtracted. from. the. M.versus.H.data..This.was.performed.for.each.measurement..The.magnet.was.swept.to.3.T.to.reveal.the.diamagnetic.and.paramagnetic.response.of.the.sample.and.substrate..The.slope.of.the.high.field.response.is.the.background.magnetic.susceptibility,.χ.=.M/H..The.suscep-tibility.multiplied.by.the.applied.field.is.then.subtracted.from.each.data.point.

Magnetic field

Superconductor

Biasing current Biasing current

Voltage variationfor steadily increasing

magnetic flux

Josephsonjunction

FIGURE 13.4An.schematic.of.a.Josephson.junction.in.a.SQUID.


13.3 Zn1−xCuxO- Based Diluted Magnetic Semiconductors

13.3.1 Introduction

Searching. for. spintronic. materials. that. combine. semiconducting. and. ferromagnetic.properties,.DMSs,.is.currently.the.most.topical.field.in.magnetism..DMS.materials.have.attracted.a.great.deal.of.interest.in.recent.years.as.they.promise.to.integrate.the.advantage.of.semiconductor.band.gap.engineering.with.controllable.magnetic.properties.and.they.can.be.realized. in.a.single.semiconductor.device. [32]..Following. the.prediction.of.Dietl.et.al..[7].that.ZnO.should.have.a.high.Tc.with.hole.doping,.there.were.flurry.of.experimen-tal. reports.on.TM-doped. ZnO.systems.proving.or.disproving. high.Tc. ferromagnetism..There.are.still.debates.on.whether.the.magnetic.behavior. in.DMSs.is.an.intrinsic.prop-erty.or.due.to.the.nanoclusters.of.a.magnetic.phase.or.both..Therefore,.despite.numerous.experimental.and.theoretical.reports.in.the.literature,.the.origin.of.DMS.is.still.unclear.

In.this.context,.recent.reports.about.the.observation.of.RT.ferromagnetism.in.copper-doped. ZnO. have. been. taken. with. great. interest. by. the. scientific. community.. This. is.mostly.because.of.the.fact.that.the.metallic.copper.(Cu),.as.well.as.all.possible.Cu-based.secondary. phases,. is. nonferromagnetic. [33].. So,. if. any. ferromagnetism. is. observed. in.a.Cu-based.system,. then. it.will.undoubtedly.be. the. intrinsic.property.of. the.material..Despite.the.abovementioned.interest,.the.Cu-doped.ZnO.system.is.not.beyond.contro-versies.. There. are. several. contradicting. reports. where. some. authors. have. confirmed.[13,34–37]. the. occurrence. of. FM. in. this. system,. while. others. have. ruled. it. out. [14].Moreover,. the. ferromagnetic.exchange. interaction. in.ZnO:Cu.system.remains.unclear..Another.problem.in.this.system.is.the.difficulty.to.fabricate.high-quality.samples.with.controlled.dopant.concentrations.because.of.the.poor.solubility.of.Cu.in.ZnO.[38]..In.this.paper,.one.kind.of.Cu-doped.ZnO.nanostructures,.well-aligned.Zn0.92Cu0.08O.nanorod.arrays,.were.synthesized.by.RF.plasma.deposition.method.that.exhibited.room-temper-ature.ferromagnetism.

13.3.2 Experiment

Zn0.92Cu0.08O. films. were. grown. on. Si. substrates. using. an. RF. plasma. deposition. with. a.composite. target. of. a. ZnO. (60.mm. in. diameter). ceramic. containing. several. Cu. pieces.(2.mm.×.2.mm). on. the. surface.. The. deposition. chamber. was. evacuated. by. a. molecular.pump.to.a.base.pressure.below.6.×.10−4.Pa..Pure.argon.gas.was.introduced.into.the.cham-ber.as.the.working.gas.using.a.mass.flow.controller,.the.flow.rate.was.regulated.at.30.sccm.(SCCM.denotes.cubic.centimeter.per.minute.at.standard.temperature.and.pressure.[STP]).and.the.chamber.pressure.was.fixed.at.2.0.Pa..Prior.to.deposition,.a.presputtering.cleaning.was.performed.for.10.min.to.eliminate.possible.contaminants.from.the.target..Deposition.was.then.conducted.at.a.RF.power.of.150.W.at.room.temperature.. In.addition,.no.extra.catalysts.or.additives.appear.in.this.approach.

The.structural.qualities.of. the.nanorods.were. investigated.by.XRD.(Rigaku.D/max-3C).using.Cu.Kα. radiation.and.confocal.micro-Raman.spectroscopy.(JY-H800)..Raman.spectra. were. recorded. using. 514.nm. excitation. line. in. backscattering. geometry.. The.cross-sectional.morphology.was.examined.by.using.a.field-emission.scanning.electron.microscope. (FE. SEM). (Hitachi. S-4700). and. transmission. electron. microscope. (TEM).(TecnaiG200)..The.Cu.concentration.in.the.samples.was.evaluated.by.energy.dispersive.x-ray.spectroscopy.(EDXS).(Hitachi.S-4700).analysis..Magnetic.measurements.were.per-formed.using.a.quantum.design.SQUID.(MPMSXL).magnetometer.with.magnetic.field.


parallel. to. the. film. surface.. Electrical. resistivity. measurements. were. made. using. the.Four-Point.Probe.system.with.1.mA.current.

13.3.3 Results and Discussion

13.3.3.1 X-Ray Diffraction (XRD)

XRD.patterns.of.the.samples.are.shown.in.Figure.13.5..It.clearly.shows.c-axis.orientations.with.(002).peaks..No.peaks.corresponding.to.copper/copper.oxide.or.any.other.secondary.phases.are.discerned.in.the.XRD.patterns.for.the.Zn0.92Cu0.08O.nanorods..This. indicates.that,. within. the. x-ray. detection. limits,. the. Zn0.92Cu0.08O. nanorods. are. monophasic.. The.inset.of.Figure.13.5.compares.the.peaks.for.Zn0.92Cu0.08O.nanorods.and.pure.ZnO.films..Interestingly,.a.comparison.of.the.full.width.at.half.maximum.(FWHM).value.of.(002).peak.of.Cu-doped.and.the.pure.ZnO.samples.shows.an.improvement.in.crystallinity.with.dop-ing.by.a.0.1°.decrease.in.the.FWHM.value..Also.visible.in.the.inset.is.the.relative.small.shift.of.the.(002).peak.for.the.Zn0.92Cu0.08O.nanorods,.which.may.be.due.to.the.incorporation.of.Cu.into.ZnO.lattice.[40].or.the.variation.of.residual.stress.in.the.samples.[41].


The.cross-sectional.morphologies.of.the.films.are.shown.in.Figure.13.6..Aligned.nanorods.are.found.in.the.SEM.images.for.the.Zn0.92Cu0.08O.films..These.Cu-doped.ZnO.nanorod.arrays.were.aligned.perpendicular.to.the.Si.substrate..In.contrast,.no.significant.aligned.nanorods.are.discerned.in.the.pure.ZnO.films..This.result.indicates.that.the.Cu.doping.may.play.an.important.role.in.the.growth.process.of.our.Zn0.92Cu0.08O.nanorod.array.

13.3.3.3 Transmission Electron Microscopy (TEM)

In. order. to. further. measure. the. structure. of. ZnO. nanorod. array,. higher. magnification.image. for. the. Zn0.92Cu0.08O. nanorods. in. Figure. 13.7. shows. that. the. diameter. of. each.

ZnO(002)*

Nanorods

20 30 40 50 60

33.5 34.02θ°

2θ°

34.5

Inte

nsity

(a. u

.)

Inte

nsity

(a.u

.)ZnO(002)* Zn0.92Cu0.08O

Zn0.92Cu0.08O

Pure ZnO thin films

Pure ZnO

FIGURE 13.5XRD.patterns.for.pure.ZnO.films.and.Zn0.92Cu0.08O.nanorods..(Reproduced.from.Wu,.Z.F..et.al.,.Appl. Phys. Lett.,.93,.023103,.2008..With.permission.)


(a)

(b)

15.0 kV

15.0 kV

×30.0 k

×20.0 k

500 nm

500 nm

FIGURE 13.6Cross-sectional.SEM.images.for.the.samples,.(a).Pure.ZnO.films,.(b).Zn0.92Cu0.08O.nanorods..(Reproduced.from.Wu,.Z.F..et.al.,.Appl. Phys. Lett.,.93,.023103,.2008..With.permission.)

200 nm

60 nm

FIGURE 13.7Cross-sectional.TEM.images.for.Zn0.92Cu0.08O.nanorods..(Reproduced.from.Wu,.Z.F..et.al.,.Appl. Phys. Lett.,.93,.023103,.2008..With.permission.)


nanorod.has.little.variation.from.bottom.to.top.and.the.average.diameter.is.around.60.nm..The.length.of.the.synthesized.nanorods.is.approximately.700.nm..In.addition,.they.are.in.high.density.and.are.uniformly.distributed.over.the.substrate..Hence,.we.suggest.that.the.method.in.our.experiment.can.be.applied.to.large-scale.manufacture.of.aligned.Cu-doped.ZnO.nanorods.array.

13.3.3.4 X-Ray Photoelectron Spectroscopy (XPS)

Further. characterization.was.done.using.XPS. to. investigate. the.bonding.characteristics.and.identify.the.oxidation.states.of.the.copper.in.the.film..A.detailed.high-resolution.scan.of.the.peaks.(Figure.13.8).indicates.the.nature.of.the.bonding.of.oxygen.atoms.to.the.cop-per.atoms.in.the.film..The.scan.shows.a.Cu.2p3/2.peak.at.933.5.eV.and.a.Cu.2p1/2.peak.at.953.5.eV,.which.corresponds.to.a.oxidation.state.of.+2,.which.is.believed.to.be.facilitated.by.the.presence.of.point.defects.(PDs).in.the.system.[42]..Thus,.both.the.high-resolution.XRD.and.XPS.measurements.suggest.that.most.of.the.Cu.atoms.have.been.substituted.directly.into.the.ZnO.lattice.

13.3.3.5 Raman Spectra

Figure.13.9.shows.the.Raman.spectra.of.ZnO.film.and.Zn0.92Cu0.08O.nanorods.deposited.in.the.Si.substrate..The.spectra.are.divided.into.two.regions.(100.∼.500,.550.∼.1800.cm−1).for.avoiding. the.high. intensity.peak.at.520.cm−1. from.the.Si.substrate..No.evidence. for.the. presence. of. CuO. phase. in. Zn0.92Cu0.08O. nanorods. is. shown. in. the. Raman. spectra..The.peaks.at.303.and.1000.cm−1.are.due.to.the.scattering.from.the.silicon.substrate..The.peak.at.about.437.cm−1.is.E2.(high).of.ZnO.signals..At.Zn0.92Cu0.08O.nanorods,.the.Raman.spectrum.shows.a.broad.asymmetric.mode.at.∼580.cm−1..This.mode.is.attributed.to.the.E1.(LO).mode.of.ZnO..The.broadening.of.the.E1.(LO).mode.is.due.to.the.defects.associated.

Measured spectrumFit sum

Cu 2p3/2Cu 2p1/2

933.5 eV

920 930 940Binding energy (eV)

Phot

on in

tens

ity (a

. u.)

950 960 970

953.5 eV

FIGURE 13.8XPS.spectrum.showing.Cr.2p.core.level.of.Zn0.92Cu0.08O.film.(after.Ar.ion.sputtering.for.2.min).


with.the.oxygen.vacancy.or.cation.interstitials.[43]..For.deposition.chamber.was.evacu-ated.to.a.base.pressure.below.6.×.10−4.Pa.and.no.additional.oxygen.was.introduced.in.the.deposition.process,.the.samples.in.this.study.are.definitely.oxygen.deficient..There.is.a.general.agreement.that.oxygen.vacancies.will.be.present.in.oxygen-deficient.ZnO.[44,45]..Based.on.the.earlier.analysis,.considerable.oxygen.vacancies.can.exist.in.our.Zn0.92Cu0.08O.nanorods.

13.3.3.6 Magnetic Properties

Figure.13.10.shows.the.magnetization.versus.field.curves.of.Zn0.92Cu0.08O.nanorods.recorded.at.300.K..The.diamagnetic.contribution. from.Si.substrate.has.been.subtracted.here..The.sample.shows.clear.room.temperature.ferromagnetism.behavior.with.a.saturation.magne-tization.(Ms).about.0.03.μB/Cu.atom..This.value.is.comparable.with.that.reported.by.Wang.et.al..[46],.and.the.result.is.significant.to.show.that.the.Curie.temperature.of.our.sample.is.higher.than.300.K..ZnO.is.nonmagnetic,.and.neither.metallic.copper.nor.copper.oxides.are.ferromagnetic..The.Cu-doped.ZnO.nanorods.are.therefore.expected.to.be.free.of.fer-romagnetic.precipitates.and.the.ferromagnetism.observed.in.our.samples.is.expected.to.be.intrinsic.to.the.Cu-ZnO.matrix.[13].

Until.now,. the.origin.of. ferromagnetism. in.oxide.DMS.remains.a.very.controversial.topic.. On. addressing. the. origin. of. ferromagnetic. in. the. Zn0.92Cu0.08O. nanorods,. elec-trical. resistance. of. the. samples. was. measured.. The. resistivities. of. undoped. ZnO. film.and. Zn0.92Cu0.08O. nanorods. are. about. 1.8.×.105. and. 2.1.×.105. Ω. cm. at. room. temperature,.respectively..Therefore,.our.results.show.that.the.doping.of.copper.increases.the.resis-tivity.value. in. the. system..This. is.understood. to.arise.because.of. the. incorporation.of.holes. [13]. or. the. implantation. of. additional. electron. trap. centers. in. the. materials. due.to.copper.doping..This.high.resistivity.value.of.Zn0.92Cu0.08O.nanorods.unambiguously.rules.out.the.possible.carrier-mediated.exchange.[47],.such.as.Ruderman-Kittel-Kasuya-Yosida.mechanism..Moreover,.conventional.superexchange.interactions.cannot.produce.

1500

1000

E1 (LO)

Si

SiSl

Pure ZnO films

Zn0.92Cu0.03O film

E2 (high)

500

200 400 600 800 1000 1200 1400 1600 1800

Inte

nsity

(a. u

.)


0

FIGURE 13.9The.Raman.spectra.of.pure.ZnO.film.and.Zn0.92Cu0.08O.nanorods.on.Si.substrate..(Reproduced.from.Wu,.Z.F..et.al.,.Appl. Phys. Lett.,.93,.023103,.2008..With.permission.)


long-range. magnetic. order. at. concentrations. of. magnetic. cations. of. a. few. percent,. i.e.,.(8.at.%).Cu-doped.ZnO.[48,49].

Based.on.the.earlier.analysis,.we.propose.that.the.BMP.model.[48,49].may.be.legitimate.in.our.samples..The.oxygen.vacancy.in.the.samples.may.be.act.both.as.electron.donors,.and.as.electron.traps.which.can.bind.the.electrons.[50–52]..In.addition,.the.higher.resistiv-ity.value.for.Zn0.92Cu0.08O.nanorods.indicates.that.additional.electron.trap.centers.related.to.the.Cu.doping.may.be.also.influential.in.our.samples..Each.trapped.electron.couples.the. local.moments.of. several.nearby. transition.metal.atoms..The.radius.of. this. trapped.electron.orbital.in.ZnO.is.estimated.to.be.0.5.nm,.which.is.enough.to.contain.a.couple.of.dopant.atoms.in.the.8%.Cu-doped.ZnO.sample.[52]..The.trapped.electron.will.align.in.an.antiparallel.configuration.with.the.individual.dopant.Cu.ion.spins..This.leads.to.an.effec-tive.ferromagnetic.coupling.between.coupled.dopant.atoms.

13.3.4 Conclusion

In.summary,.one.kind.of.Cu-doped.ZnO.nanostructures,.well-aligned.Zn0.92Cu0.08O.nano-rod.arrays,.were.synthesized.by.RF.plasma.deposition.method..The.Zn0.92Cu0.08O.nano-rod.arrays.were.aligned.perpendicular. to. the.Si. substrate..No.secondary.phases. in. the.sample. are. found. within. the. XRD. and. Raman. detection. limits.. XPS. result. shows. that.Cu.in.the.films.exists.mainly.in.the.form.of.Cu2+..By.studying.the.structural.property.of.TM-substituted.polycrystalline.ZnO.samples,.we.find.that.Cu2+.ions.successfully.substi-tute.for.the.Zn2+.ions.bonded.in.Cu–O.in.the.ZnO.lattice.and.do.not.change.the.structure.of.ZnO..The.Zn0.92Cu0.08O.nanorods.exhibit.obvious.room.temperature.ferromagnetic.order-ing.which.was.ascribed.to.be.originated.from.BMP.model.

–0.03

–1000 –2000 2000 40000H (Oe)

–0.02

–0.01Mag

netiz

atio

n (µ

B/C

u)

0.00

0.01

0.02

0.03

0.04

FIGURE 13.10Magnetization.loop.of.the.Zn0.92Cu0.08O.nanorods.measured.at.300.K..(Reproduced.from.Wu,.Z.F..et.al.,.Appl. Phys. Lett.,.93,.023103,.2008..With.permission.)


13.4 Zn1−xCrxO- Based Diluted Magnetic Semiconductors

13.4.1 Introduction

Recent. developments. in. the. field. of. spintronics. (spin-based. electronics). have. led. to. an.extensive.search.for.materials.where.the.semiconducting.properties.can.be.integrated.with.new.magnetic.properties.to.realize.novel.spin-based.devices.[4,5]..Dietl.et.al..[7].predicted.high-temperature.ferromagnetism.in.transition-metal.(TM).doped.wide-band-gap.semi-conductors.particularly.in.ZnO,.GaN,.GaAs,.and.ZnTe..Throughout.the.last.decade,.ZnO.doped.with.minute.amounts.of.TM.elements.has.been.intensively.investigated.as.a.prom-ising.DMS.for.potential.spintronic.device.applications.[53,54]..ZnO.is.wurtzite.structure,.in.which.each.atom.of.zinc.is.surrounded.by.four.oxygen.atoms.in.tetrahedral.coordination..The.high.conductivity.achieved.by.doping.can.be.applied.in.surface.acoustic.wave.devices.and.transparent.conducting.electrodes.

Sato.et.al..predicted.from.band.calculations.that.ZnO.doped.with.V,.Cr,.Fe,.Co,.and.Ni.could.be.ferromagnetic.[55,56]..Of.these.TMs,.Cr.is.particularly.attractive..Cr.was.chosen.as.the.preferred.TM.dopant.by.several.research.groups.because.(1).theoretical.research.on.Cr-based. ferromagnetic. semiconductors. supports. the. prospect. of. producing. FM;. (2). Cr.metal. is.antiferromagnetic,.thus.eliminating.any.role.of.Cr.precipitates.in.yielding.spu-rious. FM;. and. (3). the. only. ferromagnetic. oxide. of. Cr,. CrO2. with. a. Tc. of. 386.K,. is. very.unlikely.to.form.under.the.low.oxygen.pressure.conditions.usually.employed.in.vacuum.deposition.techniques.[57]..However,.compared.with.the.widely.studied.Co-.or.Mn-doped.ZnO.systems,.both.theoretical.and.experimental.researches.on.Cr-doped.ZnO.are.scarce..Moreover,.the.experimental.results.on.the.studies.of.Cr-doped.ZnO.are.in.conflict.with.each.other..Extensive.searches.carried.out.by.Venkatesan.et.al.. [58].and.Ueda.et.al.. [59].did.not.provide.any.observation.of.FM.in.Cr-doped.ZnO..Meanwhile,.some.other.studies.indicate.that.Cr-doped.ZnO.films.are.ferromagnetic.at.room.temperature.[60,61]..The.cause.of.FM.in.doped.oxide.materials.is.still.undetermined;.there.are.several.candidate.mecha-nisms.including.double.exchange,.carrier-mediated.RKKY.type.coupling,.super.exchange,.and.F-center.exchange.[62]..Recently,.several.groups.have.found.that.PDs.play.crucial.roles.in.the.FM.of.ZnO-based.DMSs.[60,63],.though.the.detailed.mechanism.is.not.yet.clear..Liu.et.al..[60].and.Kittilstved.et.al..[64].ascribe.the.FM.they.observed.in.Cr–ZnO.and.Co–ZnO.films.to.interstitial.zinc.(Zni)..Kittilstved.et.al..[64].have.found.that.the.magnetization.in.their.Cr–ZnO.films.is.correlated.to.oxygen.vacancies.(Vo).

In.order.to.gain.a.deeper.understanding.of. the.magnetic.origin. in.ZnO-based.DMSs,.Zn1−xCrxO.films.are.deposited.by.RF.magnetron.sputtering.technique.on.different.work-ing.parameters..The.structures.of.the.samples.are.discussed.in.great.detail.in.order.to.lay.a.strong.foundation.for.discussing.the.origins.of.the.magnetism.

13.4.2 Experiment

Experimental. condition. 1:. Zn1−xCrxO. (x.=.0,. 0.013,. 0.032,. 0.045,. and. 0.098). films. were.deposited.on.quartz.glass.substrates.using.an.RF-magnetron.sputtering.technique.with.a.composite.target.of.a.ceramic.polycrystalline.ZnO.containing.several.Cr.pieces.on.the.surface..The.amount.of.Cr.pieces.can.be.changed.to.adjust.the.content.of.Cr.in.the.films..Before.deposition,.the.substrates.were.dipped.in.acetone.to.remove.the.surface.contami-nation,.dried.in.a.flux.of.N2,.and.placed.into.the.chamber..After.vacuum.pumping,.the.sputtering.was.performed.with.an.Ar.pressure.of.2.0.Pa.and.an.RF.power.of.100.W.in.


the.chamber.evacuated.to.5.×.10−4.Pa.before.an.Ar.gas.flow.of.20.sccm.was.introduced.through.a.mass.flow.controller..The.temperature.of.the.substrate.was.20°C,.which.was.monitored.using.a.thermocouple.located.under.the.substrate.holder..Experimental.con-dition. 2:. Zn1−xCrxO. (x.=.0,. 0.03,. and. 0.07). films. were. grown. on. Al2O3. (001). substrates.using.an.RF.magnetron.sputtering.technique.with.a.composite.target.of.single.crystal-line.ZnO.(60.mm.in.diameter).containing.various.Cr.pieces,.respectively..The.amount.of.Cr.pieces.can.be.changed.to.adjust.the.content.of.Cr.in.the.films..Before.deposition,.the.substrates.were.dipped.in.acetone.to.remove.the.surface.contamination,.rinsed.with.a.large.amount.of.de-ionized.water,.dried.in.a.flux.of.N2,.and.placed.into.the.chamber..After.vacuum.pumping,. the.sputtering.was.performed.with.an.Ar.pressure.of.1.5.Pa.and. an. RF. power. of. 150.W. in. the. chamber. evacuated. to. 5.×.10−4. Pa. before. an. Ar. gas.flow. of. 20. sccm. was. introduced. through. a. mass. flow. controller.. The. temperature. of.the.substrate.was.20°C,.which.was.monitored.using.a.thermocouple.located.under.the.substrate.holder.

The.films’.structures.were.examined.by.HRTEM.using.an.FEI.Tecnai.G220.and.by.XRD.using.a.Rigaku.diffractometer.with.Cu.Kα.radiation..Thin.specimens.for.TEM.investiga-tions.were.obtained.by.mechanically.polishing,.dimple.grinding,.and.ion.milling..(XPS.spectra.were.measured.with.a.PHI-550.photoelectric.spectrometer.after.Ar+.sputtering.for.2.min.to.determine.the.chemical.states.of.Cr.atom.in.the.films..Magnetic.properties.were.measured.using.a.commercial.SQUID.magnetometer.(Quantum.Design,.MPMS-5.S)..The.photoluminescence. (PL). spectra.were.carried.out.by.using.280.nm.excitation. light. from.an.Xe.lamp.(JASCO.FP-6500).and.a.monochromator.with.a.resolution.of.2.nm..The.thick-nesses.of. the.films.were.nominally.400.and.600.nm.for.experimental.condition.1.and.2,.respectively,.which.were.measured.by.a.surface.profilometer.(Kosaka.ET-350)..Electrical.resistivity.measurements.were.made.using.the.four-point.probe.system.with.a.1.mA.cur-rent..All.experimental.results.shown.in.this.paper.are.for.samples.Zn1−xCrxO.whose.com-positions.were.checked.by.energy.dispersive.spectroscopy..All. the.measurements.were.conducted.at.room.temperature.



The.crystal.structure.and.film.orientation.of.the.as-grown.films.were.determined.from.θ-2θ.scans.of.XRD..The.XRD.patterns.of.Zn1−xCrxO.films.(x.=.0,.0.013,.0.032,.0.045,.and.0.098,.respectively).formed.on.squartz.glass.substrates.in.condition.1.are.shown.in.Figure.13.11a..All.peak.positions.of.the.films.correspond.to.the.standard.diffraction.pattern.of.wurtzite.hexagonal.ZnO.except.Zn0.902Cr0.098O.film..The.undoped.ZnO.sample.showed.polycrystal-line.growth.along.(100),.(002),.and.(101).directions..However,.with.increasing.Cr.concen-tration,.growth.along.the.(002).direction.is.strongly.favored.at.the.expense.of.the.other.peaks.indicating.the.influence.of.Cr.incorporation..Interestingly,.when.Cr.concentration.was. further. increased.up. to.9.8.at.%,. the.film.did.not. show.any.ZnO.phase. formation.and.instead.resulted.in.peaks.of.ZnCr2O4..The.Cr.dopant.concentration.dependency.of.the.d.(002).value.is.shown.in.the.inset.of.Figure.13.11a..The.d.(002).value.decreases.with.increasing.Cr.doping.at.lower.x,.which.is.within.the.expectation.due.to.the.substitutional.incorporation.of.smaller.ionic.radii.of.Cr.than.that.of.Zn..With.x.further.increasing.up.to.0.045,.an.increase.of.the.d.(002).value.is.observed,.which.suggests.that.some.redundant.Cr.enters.into.the.interstitial.site.of.ZnO..Therefore,.the.doping.limit.for.Cr.in.ZnO.here.is.about.3%,.which.is.also.within.the.scope.presented.by.Jin.et.al..[65]..Considering.the.


fact.that.the.XRD.system.employed.can.only.detect.phases.that.are.>1.5%,.it.cannot.be.confirmed.from.XRD.alone.if.any.nanoscale.impurities.are.present.in.the.films.with.con-centrations.below.this.detection.limit.

In.condition.2,.seven.peaks.appear.at.2θ.=.31.7°,.34.44°,.36.22°,.47.66°,.56.48°,.62.88°,.and.72.34°,.which.correspond.to.ZnO.(100),.(002),.(101),.(102),.(110),.(103),.and.(004),.respectively,.without. any. secondary. phase. up. to. the. detection. limit. to. the. instrument. as. shown. in.Figure.13.11b..However,.the.intensity.of.(002).peak.is.higher.than.that.of.others,.indicating.the.c-axis.orientation.of.Zn1−xCrxO.(0.≤.x.≤.0.07).films..Nevertheless,. the.position.of. (002).peak. is.at.2θ.=.34.44,.34.32,.and.34.24°.respectively. for.sample.a,.b,.c,. indicating. that. the.position.of.(002).peak.shifts.to.lower.angles.with.increasing.Cr.concentration..This.pre-sumably.results.from.the.substitution.of.Zn2+.by.larger.ionic.radii.of.Cr..Due.to.Cr.incorpo-ration,.the.c-axis.constant.of.Zn1−xCrxO.films.increased.from.5.198.Å.for.x.=.0.to.5.221.Å.for.x.=.0.03.and.5.224.Å.for.x.=.0.07.as.determined.by.plotting.the.(00l).diffraction.peak.values.as.a.function.of.cos2θ/sinθ.and.extrapolating.to.θ.=.90°.[68].

13.4.3.2 Transmission Electron Microscopy (TEM)

High-resolution.TEM.(HRTEM).was.performed.to.determine.the.state.of.Cr.atoms.which.could.not.be.detected.by.XRD..It.demonstrates.the.insight.into.the.detailed.atomic.structure.of.Zn1−xCrxO.films..Presented.in.Figure.13.12.is.the.typical.HRTEM.image.and.selected.area.electron.diffraction.(SAED).pattern.of.the.Zn0.968Cr0.032O.film.in.condition.1..The.average.grain.diameters.are.estimated.to.be.∼10.nm,.and.the.diffraction.rings.can.be.indexed.into.the.polycrystalline.wurtzite.hexagonal.ZnO..There.are.no.detectable.traces.of.Cr-related.secondary.phases.in.the.overall.of.Zn0.968Cr0.032O.thin.film.


Further.characterization.was.done.using.XPS.to.investigate.the.bonding.characteristics.and.oxidation.states.of.Cr.in.the.film..The.Zn1−xCrxO.(x.=.0.07).samples.have.been.characterized.

300

(100)(101)

40 502θ (°)

60 70

(004)

(002) a:x = 0b:x = 0 .03c:x = 0 .07

a1000

Inte

nsity

(a. u

.)

2000

3000

4000

5000

6000

7000

b

c

(b)30 40 50

2θ (°)(a)60 70

00.2610

0.2615

0.2620

1 2Cr dopant (at. %)

3 4 5

x = 0

0.013

0.032

0.045

0.098

(004

)

(101

)(0

02) d

(002

) nm

Inte

nsity

(a. u

.)

(100

FIGURE 13.11(a).XRD.patterns.of.Zn1−xCrxO.films.(x.=.0,.0.013,.0.032,.0.045,.and.0.098,.respectively).grown.on.quartz.glass.sub-strates..The.insert.illustrates.the.change.in.the.d.(002).value.of.ZnO.with.different.Cr.concentrations..“*”.indi-cates.the.peaks.of.the.impurity.phase.of.ZnCr2O4..(Reproduced.with.permission.from.Zhuge,.L.J..et.al.,.Scripta Mater.,.60,.214,.2009.);.(b).XRD.spectra.of.the.second.type.of.Zn1−xCrxO.(x.=.0,.0.03,.and.0.07).films..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)


with.regard.to.the.chemical.state.of.Cr.element.by.XPS..The.XPS.survey.spectrum.of.the.film.was.recorded.after.Ar.ions.sputtering.of.2.min.(to.remove.any.adsorbed.surface.con-taminants),.as.shown.in.Figure.13.13..For.the.present.samples,.the.peaks.of.Cr2p.are.located.at.576.1.eV.for.Cr2p3/2.and.584.9.eV.for.Cr2p1/2,.respectively..This.is.very.close.to.Cr2+.in.CrO.[69–71]..It.can.be.concluded.that.Cr.element.in.the.films.exists.mainly.in.the.form.of.Cr2+..This.means.that.there.is.no.Cr.metal.or.CrO2.formed.in.the.sample.


Magnetic.measurements.on.Zn1−xCrxO.films.were.performed.using.SQUID.magnetometer..All.the.measurements.were.corrected.for.substrate.effects..Magnetization.versus.magnetic.field. (M-H). loops. for.Zn1−xCrxO.films. (x.=.0.013,.0.032,.and.0.045,. respectively).measured.at.300.K.in.condition.1.are.shown.in.Figure.13.14..All.three.loops.are.found.to.be.hyster-etic,.indicating.ferromagnetism.at.room.temperature..The.moment.per.Cr.atom.at.300.K.decreased.with.increasing.Cr.concentration..The.1.3%.Cr.sample.possesses.a.net.moment.of.0.79.μB/Cr.atom,.the.3.2%.Cr.sample.has.a.moment.of.0.67.μB/Cr.atom,.and.the.4.5%.Cr.sample.has.a.moment.of.0.55.μB/Cr.atom..The.coercive.forces.(Hc).of.the.1.3%,.3.2%,.and.4.5%.Cr-doped.films.are.about.61,.48,.and.59.Oe,.respectively..It.was.worth.to.note.that.the.sample.that.showed.clear.ZnCr2O4.in.XRD,.prepared.at.9.8%.Cr-doped.ZnO.film,.did.not.show.any.FM.behavior,.thus.ruling.out.any.role.of.the.secondary.phases.in.the.observed.magnetic.behavior..The.calculated.average.magnetic.moment.of.Cr.atom.is.about.3.7.μB/Cr.[72]..This.value.is.much.larger.than.our.experimental.data..Usually.this.phenomenon.is.ascribed.to.an.increasing.occurrence.of.antiferromagnetic.coupling.between.neighboring.magnetic.ions.due.to.superexchange.interaction.[9].

5 nm

(100)(002)(101)(110)(200)(112)

FIGURE 13.12Typical.TEM.image.and.selected.area.electron.diffraction.pattern.of.Zn1−xCrxO.film.with.x.=.0.032..(Reproduced.from.Zhuge,.L.J..et.al.,.Scripta Mater.,.60,.214,.2009..With.permission.)


Now,.we.discuss. the.origin.of.FM. in.Zn1−xCrxO.films.. Let.us. consider. the.possibility.of.formation.of.Cr-related.secondary.phases..The.secondary.phases.are.a.concern.in.any.diluted.magnetic.system.as.a.source.of.spurious.magnetic.signal..ZnCr2O4.is.an.inverse.spinel.with.antiferromagnetic.properties.(TN.≈.12.5.K)..Among.the.impurity.phases.related.to.Cr:ZnO.systems,.CrO2. is. the.only. ferromagnetic.phase. (Tc.∼.386.K)..However,.neither.CrO2. nor. other. phases. (antiferromagnetic. Cr. metal,. Cr2O3,. and. Cr3O4). are. detected. via.XRD,.TEM,.and.XPS..The.stronger.FM.behavior.in.samples.with.lower.concentrations.of.

Binding energy (eV)

Inte

nsity

(a. u

.)

599.3 594.3 589.3

584.9

576.1

584.3 579.3 574.3 --

FIGURE 13.13Cr.2p.core-level.spectra.of.the.second.type.of.Zn1−xCrxO.(x.=.0.07).film..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)

–1.0

c: x = 0.45 / Hc=59 Oe

b: x = 0.032 / Hc=48 Oe

a: x = 0.013 / Hc=61 Oe

Zn1–xCrxO / 300 k

H (Oe)

–0.8

–0.6

–0.4M (µ

B / Cr

atom

)

–0.2

–10,000 0

0.0

abc

0.4

0.2

0.6

0.8

1.0

–30,000 –20,000 30,00010,000 20,000

FIGURE 13.14M.versus.H.of.Zn1−xCrxO.films.(x.=.0.013,.0.032,.and.0.045,.respectively).at.300.K..(Reproduced.from.Zhuge,.L.J..et.al.,.Scripta Mater.,.60,.214,.2009..With.permission.)


Cr.and.its.apparent.weakening.and.subsequent.disappearance.at.higher.doping.concen-trations.also.confirmed.the.absence.of. ferromagnetic.CrO2.. In.fact,. it. is.very.difficult. to.obtain.the.pure.CrO2.phase.by.conventional.sputtering.methods.[73]..On.the.other.hand,.Cr.metal,.Cr2O3,.Cr3O4,.and.ZnCr2O4.are.antiferromagnetic.materials..Even.if.these.phases.can.be.present.in.small.quantities,.the.FM.should.not.be.related.to.these.phases..As.dis-cussed.above,. the.observed.FM. is.not. from. the.Cr-related. secondary.phases..The.most.probable.reason.for.magnetic.properties.is.ferromagnetic.coupling.of.Cr.atoms.dissolved.in.the.ZnO.matrix.

Recently,. it.was. found.that. the.PDs.play.crucial. roles. in. the.FM.in.ZnO-based.DMSs.[60,63].. PL. is. a. useful. tool. for. the. investigation. of. intrinsic. PDs. in. ZnO,. including. zinc.vacancy.(VZn),.interstitial.oxygen.(Oi),.interstitial.zinc.(Zni),.singly.negatively.changed.Zn.vacancy.(VZn

− ),.and.oxygen.vacancy.(Vo)..As.there.are.still.no.definite.agreements.on.the.origin. and. positions. of. the. emission. of. ZnO,. the. donor. levels. of. PDs. used. in. here. are.based.on.the.recent.reports.of.ZnO.[60,74]..Figure.13.15.shows.room.temperature.PL.spec-tra.of.Zn1−xCrxO.films.(x.=.0.013,.0.032,.and.0.045,.respectively)..All.the.PL.curves.show.three.peaks..The.broad.UV.emission.(∼3.13.eV).has.been.frequently.observed.in.the.TM-doped.ZnO.[74],.and.can.be.attributed.to.the.near.band.edge.emission..A.broad.violet.emission.around.2.99.eV.is.related.to.VZn.and.Oi..Another.broad.blue.emission.around.2.65.eV.may.be. attributed. to. the. singly. negatively. charged. Zn. vacancy. (VZn

− ). [60].. If. specific. defect.among.them.is.the.main.contribution.to.the.FM,.the.related.PL.intensity.should.have.the.same.decreasing.trend.with.the.increase.of.Cr.concentration.as.the.magnetization.of.films..In.Figure.13.15,.the.relative.PL.intensities.of.VZn

− .decrease.steadily.with.the.increase.of.Cr.concentration,.which.resemble.the.decreasing.trend.of.magnetization..Therefore,.it.can.be.concluded.that.the.observed.FM.is.an.intrinsic.property.of.Cr-doped.ZnO.films.and.the.FM.should.relate.mainly.to.both.Cr.doping.and.VZn

− .Until.now,.the.origin.of.FM.in.DMS.materials.remains.a.very.controversial.topic..On.

addressing.the.origin.of.FM.in.Zn1−xCrxO.films,.electrical.resistance.of.the.samples.was.measured..The.resistivity.of.undoped.ZnO.is.about.0.2–0.3.Ω.cm.and.that.of.the.Zn1−xCrxO.

2.5 2.7 2.9Photon energies (eV)

PL in

tens

ity (a

. u.)

3.1 3.3

x = 0.013

x = 0.032

x = 0.045

3.5

FIGURE 13.15RT.PL.spectra.of.Zn1−xCrxO.films.(x.=.0.013,.0.032,.and.0.045,.respectively)..(Reproduced.from.Zhuge,.L.J..et.al.,.Scripta Mater.,.60,.214,.2009..With.permission.)


films.is.of.the.order.of.104.Ω.cm..The.presence.of.Cr.ions.in.ZnO.increases.the.resistivity.by.four.orders..This.seems.to.argue.against.a.free.carrier.mediated.mechanism.(such.as.the.Ruderman-Kittel-Kasuya-Yoshida-type.model).for.FM.in.Zn1−xCrxO.films.[14]..Based.on. the. basic. framework. of. Coey. et. al.. [75],. Kittilstved. et. al.. [76]. suggest. that. shallow.donors. (VZn

− .here). are. energetically.aligned.with. the.TM+/2+. level. (Cr2+.here). leading. to.effective. dopant-defect. hybridization,. which. would. induce. FM. in. TM-doped. ZnO.. By.implicating.defect-bound.carriers,. the.FM.in.Cr-doped.ZnO.films.can.be.described.by.BMP.models.[64].

Recently,.Dietl.et.al..[77,78].have.reported.that.FM.for.similar.materials,.such.as.(Zn,.Co)O. and. (Zn,. Cr)Te,. may. originate. from. uncompensated. spins. at. the. nanocrystal. surface.with.large.density.of.magnetic.ions..Obviously,.the.origins.of.FM.in.DMS.are.very.compli-cate.d..More.detailed.works.are.essential.to.understand.the.magnetic.behaviors.of.these.materials.

Figure.13.16a.shows.the.magnetization.versus.the.magnetic.field.measured.at.low.tem-perature. (5.K).by.using.SQUID. for. the.Zn1−xCrxO.(x.=.0.07). sample.. It. shows.distinctly.a.hysteresis.loop.at.low.temperature,.which.indicates.that.the.sample.has.ferromagnetism..The. coercive.field. (Hc). is. about. 200.Oe.and. the. Ms. of. the.film. is. estimated. to.be. 2.2.μB.per.Cr. ion. from.the.M-H.curve..Figure.13.16b.shows. the.magnetization.curve.at.300.K..Clearly.elongated.hysteresis.loops.indicate.that.a.ferromagnetic.structure.is.present.in.the.sample.c..The.Ms.of.the.film.is.decreased.to.be.0.69.μB.per.Cr.ion.from.the.M-H.curve..More.importantly,.sample.c.shows.coercive.field.of.80.Oe.as.shown.in.the.inset.of.Figure.13.16.

Figure. 13.17. shows. the. zero-field. cooled. (ZFC). and. field. cooled. (FC). magnetization.curves.in.a.1000.Oe.field.for.the.same.sample..The.separation.between.FC.and.ZFC.curves.indicates.that.a.hysteresis.behavior.which.is.consistent.with.our.previous.observation.from.Figure.13.16.and.that.the.ferromagnetic.behavior.is.persistent.up.to.300.K..The.result.indi-cates.that.the.Zn1−xCrxO.films.are.ferromagnetic.with.Tc.above.room.temperature..There.are.three.possible.origins.of.the.ferromagnetism.in.Zn1−xCrxO.that.we.should.consider..The.first.one.is.the.weak.ferromagnetism.of.possible.magnetic.phases.in.Cr:ZnO.films.such.as.Cr-related.oxide..The.second.one.is.a.less.likely.cause.that.the.quantity.of.metallic.Cr.

20,0000 40,000–20,000–40,000H (Oe)

–1

–2

0

1

2 a 5 K

a 5 K

80

200

0.10

0.05

0.00

–0.05

–0.10–300 –200 –100 0 100 200 300

b 300 K

b 300 K

M (

µ B / Cr

)

M(µ

B/Cr)

H (Oe)

FIGURE 13.16The.magnetic.hysteresis.loops.(M-H.curve).of.Zn1−xCrxO.(x.=.0.07).films.(a).at.5.K,.(b).at.300.K,.respectively..The.inset.shows.the.coercive.field.of.(a).at.5.K,.(b).at.300.K,.respectively..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)


might.be.too.small.to.be.detected.for.XRD..The.metallic.Cr.is.a.well-known.paramagnetic.material,.so.it.does.not.contribute.to.the.ferromagnetism.of.the.sample..The.third.is.the.carrier-induced.ferromagnetism.

Figure. 13.18. shows. saturated. magnetization. at. 5.K. versus. various. Cr. concentrations..Increasing.the.Cr.content.from.around.3.to.5.at.%.resulted.in.a.decrease.in.the.relative.mag-netization.response..This.is.similar.to.the.phenomenon.of.the.Zn1−xFexO.films.mentioned.earlier,.which.indicates.that.most.of.magnetization.is.not.due.to.any.precipitating.second-ary.phase,.but.may.be.due.to.carrier-mediated.ferromagnetism..The.strong.concave.M–T.

0

0.8

1.2

1.6

T (K)

10050 150 200 250 300

ZFCFC

M (µ

B / Cr)

FIGURE 13.17The.temperature.dependence.of.magnetization.at.field.cooled.(FC).and.zero-field.cooled.(ZFC).condition.at.a.magnetic.field.of.1000.Oe.of.Zn1−xCrxO.(x.=.0.07).films..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)

1.20.03 0.04

Cr concentration (X)0.05 0.06 0.07

1.4

1.6

1.8

2.0

2.2

2.4

M (µ

B / Cr

)

FIGURE 13.18Saturated.magnetization.at.5.K.versus.various.Cr.concentrations..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)


behavior.is.often.observed.in.carrier-localized.regime,.which.is.strikingly.different.from.the.usual.Weiss.mean-field.prediction.[79,80]..This.is.in.agreement.with.the.above.results..From.the.XPS.results.in.the.preceding.text,.Cr.ions.in.the.Zn0.93Cr0.07O.film.exist.mainly.in.the.form.of.Cr2+,.which.has.a.moment.of.4.μB.as.theoretical.value.in.ZnO..Our.finding.of.per.Cr.ion.is.below.the.theoretical.value..However,.taking.into.account.the.absence.of.second-ary.phases.suggested.by.XRD.results,.we.assume.that.ferromagnetism.of.Zn0.93Cr0.07O.film.may.be.due.to.the.exchange.interaction.between.free.delocalized.carriers.(hole.or.electron.from.the.valence.band).and.the.localized.d.spins.on.the.Cr.ions..Carrier-medicated.ferro-magnetism.is.the.possible.origin.of.the.ferromagnetism.

Cr. cluster. is. paramagnetic. in. Zn1−xTMxO. systems.. However. their. magnetic. behav-iors.that. increasing.the.TM.content.resulted.in.a.decrease. in.the.relative.magnetization.response,.whereas,.that.the.magnetic.moments.increase.with.further.increasing.of.the.TM.concentration.are.identical..It.means.that.the.origin.of.the.main.ferromagnetism.may.due.to.carriers.that.mediate.the.interaction.of.the.magnetic.ions.rather.than.secondary.phase.contribution.

13.4.4 Conclusion

In.summary,.in.condition.1,.RT.FM.was.observed.in.polycrystalline.Zn1−xCrxO.(0.013.≤.x.≤.0.045).films.prepared.by.RF.magnetron.sputtering..Structural.characterization.using.XRD,.TEM,.and.XPS.was.done.to.provide.evidence.for.the.absence.of.any.secondary.phase.or.nanoclusters.in.Zn1−xCrxO.films.with.x.≤.0.045..The.saturated.magnetization.is.∼0.79.μB/Cr.atom.at.x.=.0.013.and.decreases.with.increasing.Cr.dopant..The.experimental.results.show.that. VZn

− ,.together.with.Cr.dopant.plays.an.important.role.in.the.ferromagnetic.origin.in.Cr:ZnO..The.FM.in.films.can.be.described.by.BMP.models.with.respect.to.defect-bound.carriers.

In. condition. 2,.Zn1−xCrxO.films.were. prepared. by. the.RF.magnetron. sputtering. tech-nique.on.Al2O3. (001). substrates..By.studying. the.structural.properties.of.Cr-substituted.polycrystalline.ZnO.samples,.we.have.found.that.Cr2+.ions.have.successfully.substituted.for.the.Zn2+.ions.bonded.in.Cr–O.in.the.ZnO.lattice.and.do.not.change.the.structure.of.ZnO..The.result.of.magnetic.measurement.shows.that.the.Zn1−xCrxO.films.were.ferromagnetic.respectively..The.main.magnetization.is.not.due.to.any.precipitating.secondary.phase,.but.may.be.due.to.carrier-mediated.ferromagnetism.

13.5 Zn1−xFexO- Based Diluted Magnetic Semiconductors

13.5.1 Introduction

The. exiting. possibility. of. utilizing. both. the. charge. and. spin. character. of. an. electron.to.develop.novel. spintronic.devices. like.Spin.FETs.and.Spin.LEDs. [4,5].has. led. to.an.extensive. search. for. materials. in. which. semiconducting. properties. can. be. integrated.with.magnetic.properties..There.is.a.growing.interest.in.DMSs,.where.magnetic.ions.are.doped.into.the.semiconductor.hosts,.due.to.the.possibility.of.utilizing.both.charge.and.spin.degrees.of.freedom.in.the.same.materials,.allowing.us.to.design.a.new.generation.spin.electronic.device.with.enhanced. functionalities..Theoretical.studies.on. the.basis.of. Zener’s. p-d. exchange. model. have. shown. that. wide-gap. semiconductors. such. as.ZnO. doped. with. transition. metal. are. promising. candidates. for. room. temperature.


ferromagnetic. DMSs. [7].. First-principle. calculations. by. Sato. and. Katayama-Yoshida.[81]. have. also. predicted. that. ZnO-based. DMSs. exhibit. ferromagnetism. using. local-spin-density.approximation. (LSDA). calculation..As.one.of. the.wide-band-gap.oxides,.ZnO. has. attracted. a. great. deal. of. attention. due. to. a. wide. range. of. its. technological.applications.. Due. to. its. large. band. gap. (3.3.eV). and. exciton. binding. energy. (60.meV).in.the.wurtzite.structure.at.room.temperature,.ZnO.is.a.promising.material.for.many.optoelectronic.applications.

A.number.of.recent.studies.of.magnetism.in.semiconductor.oxide.thin.films.doped.with.TMs.have.found.evidence.for.RTFM..In.particular,.semiconductor.oxide.thin.films.doped.with.nonmagnetic. elements.were. also. found. to.exhibit.RTFM..Subsequently,. a.number.of.experiments.on.ZnO-based.DMSs.in.bulk,.thin.film,.and.nanoparticle.forms.revealed.ferromagnetic.properties. [8–14],.and.among. them.ZnO-based.DMSs.nanoparticles.have.attracted.much.attention..Current.interest.in.such.magnetic.nanoparticle.systems.is.moti-vated. by. unique. phenomena. such. as. superparamagnetism. [82],. quantum. tunneling. of.magnetization. [83],. and,. particularly,. magnetism. induced. by. surface. effects. [84].. In. the.nanoparticle.form,.the.structural.and.electronic.properties.are.modified.at.the.surface.as.a.result.of.the.broken.translational.symmetry.of.the.lattice.or.dangling.bond.formation,.giving.rise.to.weakened.exchange.coupling,.site-specific.surface.anisotropy,.and.surface.spin.disorder.[85,86]..That.is,.the.modification.of.the.electronic.structure.at.the.surface.of.the.nanoparticles.plays.a.crucial.role.in.the.magnetism.of.this.system..Recently,.Karmakar.et. al.. [87]. have. reported. room. temperature. ferromagnetism. in. Fe-doped. ZnO. (ZnO:Fe).nanoparticles.in.the.proposed.core/shell.structure,.where.Fe2+.ions.are.situated.mostly.in.the.core.and.Fe3+.ions.in.the.surface.region..However,.LSDA+U.calculation.[88].has.indi-cated.the.insulating.antiferromagnetic.state.to.be.more.stable.than.the.ferromagnetic.state.for.ZnO:Fe.system..Also,.there.has.been.a.considerable.recent.work.[89].suggesting.that.structurally.perfect.ZnO-based.DMSs.do.not.exhibit.ferromagnetic.order..These.reports.[88,89].imply.that.not.only.the.magnetic.dopants.themselves.but.also.the.defects.are.neces-sary.for.ferromagnetism..However,.the.origin.of.observed.ferromagnetism.still.remains.controversial,.and.the.origin.of.ferromagnetism.varies.in.different.TM-doped.ZnO.films..The.cause.of.FM. in.doped.oxide.materials. is. still.undetermined;. there.are. several. can-didate. mechanisms. including. double. exchange,. carrier-mediated. RKKY. type. coupling,.super.exchange,.and.F-center.exchange.[62].

In.this.report,.Zn1−xFexO.films.are.deposited.by.RF.magnetron.sputtering.technique.on.different.working.parameters..We.investigate.here.the.growth.process.of.the.fractal.and.calculate.the.dimensionality.of.the.fractal.in.the.ZnO.film.doped.with.Fe..The.structures.of.the.samples.are.discussed.in.great.detail.in.order.to.lay.a.strong.foundation.for.discussing.the.origins.of.the.magnetism.

13.5.2 Experiment

The.films.were. grown.on.oriented. Al2O3. (001). substrates.using.an.RF-magnetron. sput-tering. technique.with.a.composite. target.of.single.crystalline.ZnO.(60.mm.in.diameter).containing.various.Fe.pieces.(9.mm.×.9.mm).on.the.surface..The.amount.of.Fe.pieces.can.be.changed.to.adjust.the.content.of.Fe.in.the.films..Before.deposition,.the.substrates.were.dipped. in.acetone. to. remove. the. surface.contamination,. rinsed.with.a. large.amount.of.de-ionized. water,. dried. in. a. flux. of. N2,. and. placed. into. the. chamber.. The. first. type. of.ZnO1−xFexO.(x.=.0,.0.052,.0.103,.0.157,.0.212.marked.as.sample.A,.B,.C,.D,.E.respectively).films.were. deposited. at. 400°C. and. the. deposition. time. was. 45.min.. After. vacuum. pumping,.the.sputtering.was.performed.with.an.Ar.pressure.of.2.0.Pa.and.an.RF.power.of.100.W.in.


the.chamber.evacuated.to.5.×.10−4.Pa..The.thickness.of.the.films.is.nominally.400.nm..The.second.type.of.Fe:ZnO.films.were.deposited.at.room.temperature.and.the.deposition.time.was.30.min..The.sputtering.was.performed.with.an.Ar.pressure.of.0.5.Pa.and.an.RF.power.of.150.W.in.the.chamber.evacuated.to.8.5.×.10−4.Pa.before.a.flow.of.argon.gas.of.30.sccm.was.introduced.through.a.mass.flow.controller..In.order.to.obtain.the.deposition.rate.of.the.films,.a.film.was.deposited.at.room.temperature.for.a.deposition.time.of.30.min.giving.a.film.thickness.of.about.150.nm,.as.measured.with.an.ET350.surface.profilometer.(Kosaka.Laboratory.Ltd.),.the.deposition.rate.being.about.0.083.nm.s−1..The.thickness.of.the.films.used.for.experiments.was.thus.estimated.to.be.about.10.nm.by.controlling.the.sputtering.time.to.about.2.min.under.the.same.experimental.conditions..The.third.type.of.ZnO1−xFexO.(x.=.0,.0.08,.0.15.marked.as.sample.a,.b,.c.respectively).films.were.deposited.at.room.temper-ature.on.Si.substrates.and.the.deposition.time.was.30.min..The.sputtering.was.performed.with.an.Ar.pressure.of.2.Pa.and.an.RF.power.of.100.W.in.the.chamber.evacuated.to.4.×.10−3.Pa.before.argon.gas.in.a.flow.of.40.scmm.was.introduced.through.a.mass.flow.controller..The.thickness.of.the.films.is.nominally.320.nm.

Optical.microscopy.using.a.Leica.DM4000M.was.used.for.the.observation.of.film.sur-face..XRD.measurements.were.carried.out.with.a.Rigaku.D/Max-2500PC.diffractometer.(Cu.Kα,.X-ray,.λ.=.0.15406.nm)..XPS.spectra.were.measured.with.a.PHI-550.photoelectric.spectrometer.(Mg.Kα,.x-ray).after.Ar+.sputtering.of.5.min..They.have.been.used.for.deter-mining.the.composition.and.the.chemical.states.of.the.films..The.concentration.of.Fe.in.the.Zn1−xFexO.films.was.also.calculated.based.on.the.XPS.data..A.quantum.design.super-conducting. quantum. interference. device. (SQUID,. MPMSXL). magnetometer. was. used.to. investigate. the. magnetic. properties.. All. the. measurements. were. conducted. at. room.temperature.


13.5.3.1 Optical Microscopy (OM)

The.surface.image.of.the.second.type.of.the.ZnO.film.undoped.with.Fe.is.shown.Figure.13.19..It.shows.that.the.distribution.of.particles.is.very.uniform.and.the.boundary.particles.have.aggregated.to.some.small.islands.because.of.the.shielding.effect..However,.there.are.no.fractal.aggregates.in.the.growth.processes.of.the.ZnO.film.undoped.with.Fe.

When. Fe. is. doped. in. the. ZnO. film,. a. bright. field. image. of. a. fractal. aggregation. is.observed,.which.is.shown.in.Figure.13.20..The.result. indicates. that.many.fractals.and.other.small.clusters.are.still. in. the.aggregating.stage..From.Figure.13.20,.one.can.also.see.that.a.fractal.aggregate.consists.of.many.small.particles.and.some.clusters.of.par-ticles.are.not.connected.with.the.main.aggregate..So,.we.suppose.that.the.fractal.aggre-gates.were.the.result.of.cluster.diffusion-limited.aggregation.(CDLA).of.particles.on.the.surface.of.the.film.

To. compare. the. observed. structures. with. the. CDLA. model,. we. measured. the. fractal.dimension.(D).of.a.main.branch.in.the.film.by.Sandbox.method.[91]..Figure.13.21a.is.the.closer.view.of.the.grain-aggregate.shown.in.the.right.region.of.Figure.13.20..We.measured.D.=.1.47.as.shown.in.Figure.13.21b..The.fractal.dimension.of.the.main.branch.was.smaller.than.that.expected.by.the.CDLA.model.(D.=.1.72).

We.will.now.discuss.the.growth.mechanism.of. the.observed.fractal.aggregates..EDS.indicates.that.both.the.fractal.aggregates.and.the.clusters.contained.some.Fe..From.the.XPS.results,.Fe.element. in. the.films.exists.mainly. in. the.form.of.Fe2+..With.the.ratio.of.atomic. consistency. of. Fe,. the. amount. of. Fe. doped. in. the. film. is. about. 10. at.%.. In. our.


experiment,.the.contents.of.Fe.can.be.adjusted.by.changing.the.surface.area.ratio.of.Fe.to.the.ZnO.target.

FeO.shows.a.ferromagnetic.behavior,.which.has.been.studied.[92]..The.strong.magnetic.interactions.among.the.Fe2+.particles.then.came.into.play.and.influenced.further.aggrega-tion,.resulting.in.a.smaller.mass.dimension.(1.47).of.the.final.aggregates..Similar.experiments.were.also.performed.in.Fe-Cu.binary-alloy.system.and.Ag-Co.thin.films.[93]..The.fractal.

0.1 mm

FIGURE 13.20The.distribution.of.the.fractal.aggregates.in.the.second.type.of.the.ZnO.film.doped.with.Fe..(Reproduced.from.Cuia,.M.L..et.al.,.Vacuum,.82,.613,.2008..With.permission.)

0.1 mm

FIGURE 13.19The.surface.image.of.the.second.type.of.the.ZnO.film.undoped.with.Fe..(Reproduced.from.Cuia,.M.L..et.al.,.Vacuum,.82,.613,.2008..With.permission.)


dimension.of.two.experiments.is.1.42.±.0.02.and.1.47.±.0.02,.respectively..The.difference.can.be.probably.attributed.to.the.difference.in.magnetic.interactions.involved.in.Fe.and.Co.cases.

In.addition,.nanoparticle.aggregation.is.a.major.phenomenon.observed.in.many.differ-ent.fields.of.activity.[94,95].. It.may.also. influence.the.further.aggregation.of. the.surface.particles,.resulting.in.the.appearance.of.the.fractal.

Experimentally.obtained.patterns.depend.strongly.on.the.surface.tension.which.is.not.incorporated.into.the.original.version.of.CDLA..Correspondingly,. the.anisotropy.of.the.surface.tension.is.not.considered.in.CDLA.and.it. is.probably.the.lack.of.the.anisotropy.that.leads.to.the.fractal.growth.[96]..So,.the.change.of.the.surface.tension.may.be.the.main.reason.resulting.in.the.appearance.of.the.fractal.


The.XRD.patterns.of.the.first.type.of.Zn1−xFexO.films.(x.=.0,.0.052,.0.103,.0.157,.and.0.212,.respectively). are. shown. in. Figure. 13.22,. and. Zn1−xFexO. films. have. a. preferential. c-axis.orientation. in. evidence.. For. x.=.0,. 0.052,. 0.103,. and. 0.157,. only. Zn1−xFexO. (002). and. (004).peaks.were.observed,.which.indicated.that.Fe-doped.did.not.change.the.wurtzite.struc-ture.of.ZnO;.however,.for.x.=.0.212,.Zn1−xFexO.(004).peak.disappeared.and.two.peaks.were.observed.at.2θ.=.36°.and.2θ.=.44.46°.corresponding.to.ZnO.(101).and.Fe.(110).peak.respec-tively..The.position.of.(002).peak.was.at.2θ.=.34.3,.34.24,.34.08,.33.5,.and.33.14°for.x.=.0,.0.052,.0.103,.0.157,.and.0.212,.respectively,.which.indicated.that.the.position.of.(002).peak.shifted.to. lower.angles.with. increasing.Zn1−xMnxO.films.[98,99]..This.results. from.the.Fe2+. ions.substituting.for.a.part.of.Zn2+.ions.in.the.ZnO.lattice..Fe.element.has.two.major.ions—Fe2+.and.Fe3+,.the.radius.of.which.is.0.074.nm.and.0.064.nm,.respectively..However,.the.radius.of.Zn2+.is.0.072.nm.and.oxygen.vacancies.were.produced.in.the.progress.of.sputtering,.so.we.think.Fe.element.in.the.Zn1−xFexO.films.was.in.the.form.of.Fe2+.

XRD. patterns. of. the. third. type. of. the. Zn1−xFexO. films. were. measured,. as. shown. in.Figure.13.23..The.peak.appears.at.2θ.=.34.34°,.which.corresponds.to.(002).directions.of.the.

(a)

2.0

(b)1.5 2.0 2.5

InL

In N

(L)

3.0 3.5 4.0

2.5

3.0

3.5

4.0

4.5

5.0

D= 1.47

5.5

FIGURE 13.21The.second.type.of.the.films:.(a).The.closer.view.of.the.branch.shown.in.the.right.region.of.Figure.13.20..(b)The. branch. has. fractal. dimension. D.=.1.47.. (Reproduced. from. Cuia,. M.L.. et. al.,. Vacuum,. 82,. 613,. 2008.. With.permission.)


ZnO.hexagonal.wurtzite.structure..It.is.easy.to.find.out.that.all.the.films.are.single.phase.with.c-axis.preferred.orientation.and.that.no.evidence.of.any.other.secondary.phases.and.impurities.of.Fe.element.is.detected..Meanwhile,.as.Fe.concentration.increases,.the.position.of.the.Zn1−xFexO.peak.moves.to.lower.angels,.which.is.consistent.with.that.of.other.transi-tion.metal.(TM).atoms.doped.in.the.ZnO.films,.such.as.Co-doped.ZnO.[59,100],.Mn-doped.ZnO.[101,102],.and.similar.variation. tendency.has.also.been.observed.and.explained. in.greater.detail.in.Ref..[65].

The.XRD.patterns.of. the.second.type.of. the.ZnO.films.undoped.and.doped.with.Fe.are. shown. in. Figure. 13.24,. respectively.. It. shows. that. ZnO. film. undoped. Fe. has. (101),.(103),.(112),.and.(004).diffraction.peaks,.while.Fe-doped.ZnO.film.only.has.(002).and.(004).

25,000ZnO (002)

ZnO (101)Fe (110)

ZnO (004)

A:x=0B:x=0.052C:x=0.103D:x=0.157E:x=0.212

20,000

15,000

10,000

5,000

030 40 50

2θ (°)

Inte

nsity

(a. u

.)

60

A

B

CD

E

70

FIGURE 13.22XRD.spectra.of.the.first.type.of.the.Zn1−xFexO.(x.=.0,.0.052,.0.103,.0.157,.and.0.212).films..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)

35

34.34

2θ (°)

Nor

mal

ized

inte

nsity

(a. u

.)

4030

c(x=0.15)

b(x=0.08)

a(x=0)

FIGURE 13.23XRD.spectra.of.the.third.type.of.the.Zn1−xFexO.(x.=.0,.0.08,.and.0.15).films..(Reproduced.from.Chen,.A.J..et.al.,.J. Phys. D Appl. Phys.,.39,.4762,.2006..With.permission.)


diffraction.peaks.and.has.no.diffraction.peak.corresponding.to.Fe..However,.Fe.doped.in.the.ZnO.film.makes.c-axis.orientation.of.ZnO.film.increase.obviously..From.Figure.13.24,.we.can.also.know.that.the.2θ.angles.of.the.(002).diffraction.peak.positions.of.the.ZnO.films.undoped.and.doped.with.Fe.are.34.24°.and.33.98°,.respectively..According.to.the.Joint.Committee.on.Powder.Diffraction.Standards.(JCPDS),.for.the.powder.ZnO.without.stress,.2θ.is.equal.to.34.43°,.while.the.2θ.angles.for.the.(002).peak.of.the.films.undoped.and.doped.with.Fe.are.both.smaller.than.34.43°..The.difference.between.the.samples.of.the. film. and. powder. as. we. have. discussed. earlier. indicates. that. compressive. stress. is.formed.within.the.crystal.lattice.of.ZnO.films..Intensity.of.the.stress.can.be.calculated.by.the.formula.[103]:

. σ = 4.5 10 ( )/ N/m110 0

2× −C C C . (13.8)

where.σ,.C,.and.C0.are.the.mean.stress.in.the.ZnO.film,.lattice.constant.of.ZnO.film,.and.lattice.constant.of.ZnO.powder.(standard,.C0.=.0.5206.nm)..The.crystal.lattice.constants.of.the.film.samples.can.be.calculated.as.0.5233.and.0.5272.nm.for.undoped.and.doped.with.Fe,.respectively..On.calculation,.the.compressive.stress.of.ZnO.film.undoped.with.Fe.is.equal. to. 5.168.×.108. N/m2,which. is. obviously. less. than. 1.268.×.109. N/m2. of. the. Fe-doped.ZnO.film..So,.the.fractal.pattern.may.result.from.the.increase.of.the.stress.of.the.film.after.doping.with.Fe.


The.chemical.bonding.state.of.Fe.element.in.the.first.type.of.films.is.shown.in.Figure.13.25..For.the.present.samples,.the.peaks.of.Fe.2p.are.located.at.709.4.eV.for.Fe.2p3/2.and.722.4.eV.for.Fe.2p1/2,.respectively..This.is.very.close.to.Fe2+.in.FeO.(709.30.eV.for.Fe.2p3/2,.722.30.eV.for.Fe.2p1/2).according.to.the.handbook.of.XPS..The.Fe.2p3/2.photopeak.(at.709.30.eV).is.also.found.to.be.in.good.agreement.with.the.literature.[104]..Moreover,.from.the.XPS.handbook.we.also.get.that.the.peaks.of.Fe.2p3/2.and.Fe.2p1/2.for.metal.Fe.are.located.at.706.75.eV.and.

30 40 502θ (°)

Inte

nsity

(a. u

.)

60 70

(004)

(112) (004)

ZnO (002)

ZnO (103)

33.88

34.24

b

a (101)

FIGURE 13.24XRD.spectra.of.the.second.type.of.(a).ZnO.film.undoped.with.Fe.(b).Fe-doped.ZnO.film..(Reproduced.from.Cuia,.M.L..et.al.,.Vacuum,.82,.613,.2008..With.permission.)


719.95.eV,.and.for.Fe3+.in.the.Fe2O3.at.710.70.and.724.30.eV,.respectively..Besides,.XPS.data.shows.that.no.Fe.clusters.exist.in.the.Zn1−xFexO.films.except.sample.E..Now.a.conclusion.can.be.made.that.Fe.element.in.the.films.exists.mainly.in.the.form.of.Fe2+,.which.is.consis-tent.with.the.result.of.XRD.

Meanwhile,.the.chemical.bonding.state.of.Fe.element.in.the.second.and.third.types.of.the.Fe:ZnO.films.is.the.same.as.the.one.of.the.first.type..That.is.to.say,.Fe.element.in.all.the.films.exists.mainly.in.the.form.of.Fe2+.

From.the.earlier.analysis.and.the.XPS.and.XRD.results.in.the.second.type.of.the.films,.we.suggest.that.Fe2+.ions.have.successfully.substituted.for.the.Zn2+.ions.in.the.ZnO.lattice.and.free.from.clusters.such.as.FeO.and.Fe2O3..Moreover,.as.Fe.concentration.increases.in.the.films,.the.full.width.at.half-maximum.(FWHM).of.the.(002).diffraction.peak.is.found.to.be.broadened.and.the.intensity.decreases.gradually..This.presumably.is.due.to.the.fact.that.the.ionic.radius.of.Fe2+.is.known.to.be.larger.than.that.of.Zn2+.by.about.5%.[65].and.that.the.lattice.disorder.and.strain.are.induced.by.Fe2+.ion.substitution.[99,105].

From.the.earlier.results,.we.find.out.that.Fe.ions.with.different.chemical.states.have.suc-cessfully.substituted.for.the.Zn2+.ions,.which.induce.the.changes.of.crystal.lattice.constant.of.crystalline.ZnO..However,.Fe-doped.did.not.change.the.wurtzite.structure.of.ZnO..At.the.same. time,. the.crystalline.quality.of. the.films.gets.worse.and. the.particle. size.also.gets.smaller.because.of.the.Fe-doped.in.all.types.of.films..Thus.we.can.modulate.different.chemical.states.of.Fe.ions.and.different.concentrations.of.Fe.ions.in.the.films.using.differ-ent.working.parameters,.so.as.to.lay.a.strong.foundation.for.obtaining.ferromagnetic.upon.room.temperature.later.


Now.we.can.get.a.conclusion.that.the.crystalline.quality.of.the.first.type.of.the.films.get.worse.and.the.particle.size.also.gets.smaller.because.of.the.Fe-doping.which.can.be.dem-onstrated. by. the. SEM. surface. micrographs,. as. shown. in. Figure. 13.26.. The. four. images.revealed.that.the.average.size.of.the.particle.decreased.with.increasing.Fe.concentration..

705 710 715

13.4 eV

709.4 eV

722.8 eV

Binding energy (eV)

Inte

nsity

(a. u

.)

720 725

FIGURE 13.25Fe.2p.core-level.spectra.of.the.first.type.of.Zn1−xFexO.(x.=.0.052).film..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)


It.may.be.because.Fe.ions.could.disturb.the.ZnO.crystal.lattice.and.obstruct.the.crystal.growth..The.phenomena.was.consistent.with.that.of.Wang..et.al..[106].

From.the.SEM.results.of.the.third.type.of.films,. it. is.obvious.that.the.particle.size.of.the.films.decreases.with.the.increase.of.Fe.concentration..The.pure.ZnO.particles.appear.mainly.in.shape.like.slice,.as.shown.in.Figure.13.27a..The.amount.of.slices.gradually.dis-appears.with.the.increase.of.Fe.concentration,.as.shown.in.Figure.13.27b.and.c,.respec-tively..Meanwhile,.the.amount.of.granular.particles.increases.with.increasing.Fe.dopant.in.ZnO.films.


The.first.type.of.the.Zn1−xFexO.films.were.measured.the.magnetic.properties.using.a.SQUID..Figure.13.28a.shows.the.magnetization.curve.at.5.K.for.the.Zn1−xFexO.(x.=.0.212).film.with.the.magnetic.field.applied.parallel.to.the.film..Clearly.elongated.hysteresis.loops.indicate.that.a.ferromagnetic.structure.was.present.in.the.sample.E..The.diamagnetic.background.of.the.substrate.was.subtracted.from.the.data..More.importantly,.sample.E.shows.coercive.

Sample b

15.0 kV 11.4 mm×50.0 k 1.00μm

(a) (b)

Sample c

15.0 kV 11.6 mm×50.0 k 1.00μm

(c)

Sample d

15.0 kV 11.6 mm×50.0 k 1.00μm

(d)

Sample e

15.0 kV 11.8 mm×50.0 k 1.00μm

FIGURE 13.26(a–d).The.SEM.images.of.the.first.type.of.the.Zn1−xFexO.(x.=.0.052,.0.103,.0.157,.and.0.2120).films..(Reproduced.from.Chen,.Z.C..et.al.,.Thin Solid Films,.515,.5462,.2007..With.permission.)


field.of.1560.Oe.as.shown.in.the.inset.of.Figure.13.28..The.result.indicates.that.sample.E.is.ferromagnetic..The.magnetization.value.is.66.6.emu/cm3.(4.51.μB/Fe).under.15.kOe.exter-nal.magnetic.field,.as.for.sample.E..We.also.performed.magnetic.measurement.at.300.K.for.the.Zn1−xFexO.(x.=.0.212).film.as.shown.in.Figure.13.28b..However,.no.coercive.field.is.observed,.indicating.that.there.is.no.ferromagnetism.in.the.sample.E.

Based.on.the.experiments,.we.can.rule.out.the.possible.magnetic.contamination.and.the.secondary.phase.of.metallic.Fe,.Fe3O4,.or.Fe2O3.clusters.as.the.origins.of.the.ferromagne-tism.of.the.Zn1−xFexO.films..Figure.13.29.shows.saturated.magnetization.at.5.K.versus.vari-ous.Fe.concentrations..Increasing.the.Fe.content.from.around.10.3.to.15.7.at.%.resulted.in.a.decrease.in.the.relative.magnetization.response,.and.further.increasing.the.Fe.content.up.to.21.2.at.%.induced.an.increasing.magnetization.inversely..This.provides.strong.evidence.that.most.of.the.magnetization.is.not.due.to.any.precipitating.secondary.phase,.that.is,.Fe.cluster,.Fe3O4,.or.Fe2O3..If.the.formation.of.a.secondary.Fe-related.phase.is.responsible.for.the.ferromagnetic.behavior,.an.increase.in.Fe.concentration.would.presumably.increase.the. secondary.phase.volume. fraction.and. related.magnetization. signature.. Instead,. the.opposite.behavior.is.observed.

Otherwise,. although. a. secondary. phase. Fe-related. Oxide. is. not. observed. by. XRD,. it.might.exist..However,. it.would.thus.not.obscure.the.DMS.ferromagnetism.according.to.

Sample a

15.0 kV 11.5 mm×50.0 k 1.00 μm

Sample b

15.0 kV 11.5 mm×50.0 k 1.00 μm

Sample c

15.0 kV 11.5 mm×50.0 k 1.00 μm

(a) (b)

(c)

FIGURE 13.27(a–c).SEM.pictures.of.the.third.type.of.the.Zn1−xFexO.(x.=.0,.0.08,.and.0.15).films..(Reproduced.from.Chen,.A.J..et.al.,.J. Phys. D Appl. Phys.,.39,.4762,.2006..With.permission.)


the.earlier.analysis..According.to.the.results.of.XRD.and.XPS,.we.find.out.that.there.are.Fe.clusters. in. the.sample.E,.and.all. the.Fe. ions.are.Fe2+. ions. in. the.films..However,. the.magnetization.values.of.metallic.Fe.and.Fe2+. ion.are.2.2.and.6.μB/Fe.respectively,.so.we.think.that.both.Fe.clusters.and.Fe2+. ions.contribute. to. the.ferromagnetism.of.sample.E..The. carrier-induced. ferromagnetism. (RKKY. or. double. exchange. mechanism). is. often.reported. for. the. III–V.semiconductors. [107,108]..The. indirect.exchange. interaction.using.the.carrier-mediated.model. [109].may.be.applied. to.our.results. to.explain. the.observed.

–4

–6

–8

–8

–4

0

8

4–2

0

0

2

4

6

8

a 5 Kb 300 K

–10,000 10,000 20,000–20,000

–10000 0H (Oe)

–20000 10000 20000

1560

a 5 Kb 300 KM

(em

u/cm

3 )

M (e

mu/

cm3 )

H (Oe)

FIGURE 13.28The.magnetic.hysteresis.loops.(M-H.curve).of.the.first.type.of.the.Zn1−xFexO.(x.=.0.212).films.(a).at.5.K,.(b).at.300.K,.respectively..The.inset.shows.the.coercive.field.of.(a).at.5.K,.(b).at.300.K,.respectively..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)

0.120

1

2

3

4

5

0.16 0.20Fe concentration (X)

M (µ

B / Fe

)

FIGURE 13.29Saturated.magnetization.at.5.K.versus.various.Fe.concentrations..(Reproduced.from.Jin,.C.G..et.al.,.Thin Solid Films,.518,.2152,.2010..With.permission.)


ferromagnetism. in. the.films..As.Fe. concentration. increases,.oxygen.vacancies. and.zinc.interstitials. increase,. which. induce. that. the. concentration. of. free. electrons. increases.. It.has.been.discussed. in.our.past.work.as.shown. in.Ref.. [110]..Therefore,.a. ferromagnetic.phase.may.be. favored.at.higher. electron. concentration.because. their.mean. free.path. is.reduced,.limiting.the.interaction.to.the.first.few.relatively.stronger.and.ferromagnetically.ordered.nearest.neighbors..Although.these.films.do.not.show.ferromagnetic.behaviors,.we.maybe.acquire.Tc.above.room.temperature.by.modulating.the.parameter.of.preparation.and.changing.concentration.of.Fe.

In.our.recent.experiment,.the.ferromagnetism.of.Zn1−xTMxO.(TM.=.Mn.and.Co).films.is.also.observed..The.structural.analysis.using.XRD.and.Raman.spectroscopy.indicated.that.Mn.incorporates.successfully.into.the.ZnO.lattice.and.does.not.change.the.wurtzite.struc-ture.of.the.ZnO..XPS.data.showed.that.no.Fe.clusters.existed.in.the.Zn1−xMnxO.films,.and.existed.mainly.in.the.form.of.Mn2+..No.secondary.phases.in.all.the.samples.were.found.within.the.XRD.and.Raman.detection.limits..A.point.worth.emphasizing.is.that.nanoscale.columnar.grain.arrays.were.found.in.the.cross-sectional.SEM.images.of.Mn-doped.ZnO.films..As.the.magnetron.sputtering.method.can.produce.economically.feasible.large.area.films.with.well-controlled. composition,.we. suggest. that. the.method. in.our. experiment.may.be.applied.to.future.large-scale.manufacturing.of.aligned.Mn-doped.ZnO.nanoscale.columnar.grains.and.nanorod.arrays..The.origin.of.magnetic.properties.of. the.films. is.discussed.in.detail.later.

Figure. 13.30. shows. the. magnetization. versus. field. curves. of. the. Zn1−xMnxO. (x.=.0.017,.0.029,.and.0.067).films.recorded.at.300.K..The.diamagnetic.contribution.from.Si.substrate.has. been. subtracted. here.. The. sample. shows. clear. room. temperature. ferromagnetism.behavior.with.an.Ms.about.0.36.and.0.16.μB/Mn.atom.for.Zn1−xMnxO.(x.=.0.029.and.0.067).films,.respectively..The.result.is.significant.to.show.that.the.Curie.temperature.of.our.sam-ple.is.higher.than.300.K..ZnO.is.nonmagnetic,.and.neither.metallic.manganese.nor.manga-nese.oxides.are.ferromagnetic..The.Mn-doped.ZnO.films.are.therefore.expected.to.be.free.of.ferromagnetic.precipitates.and.the.ferromagnetism.observed.in.our.samples.is.expected.to.be.intrinsic.to.the.Mn-ZnO.matrix.[111].

H (Oe)

Mag

netiz

atio

n (µ

B/Mn)

–4000

–0.4

–0.2

0.0

0.2

0.4

–2000 0 2000 4000

Zn0.983Mn0.017OZn0.971Mn0.029OZn0.933Mn0.067O

FIGURE 13.30Magnetization.loop.of.the.Zn1−xMnxO.(x.=.0.017,.0.029,.and.0.067).films.measured.at.300.K.


Until.now,.the.origin.of.ferromagnetism.in.oxide.DMS.remains.a.very.controversial.topic..On.addressing.the.origin.of.ferromagnetic.in.the.Zn1−xMnxO.films,.electrical.resistance.of.the.samples.was.measured..The.resistivities.of.Zn1−xMnxO.films.are.on.the.level.of.104.Ω.cm.at.room.temperature..Therefore,.our.results.show.that.the.doping.of.manganese.does.not.change.the.resistivity.value.in.the.system..This.high.resistivity.value.of.Zn1−xMnxO.films.unambiguously.rules.out.the.possible.carrier-mediated.exchange.[47],.such.as.Ruderman-Kittel-Kasuya-Yosida. mechanism.. Moreover,. conventional. superexchange. interactions.cannot.produce.long-range.magnetic.order.at.concentrations.of.magnetic.cations.of.a.few.percent,.i.e.,.(2.9.at.%).Mn-doped.ZnO.[48,49].

Based.on.the.earlier.analysis,.we.propose.that.the.BMP.model.[48,49].may.be.legitimate.in.our.samples..The.oxygen.vacancy.in.the.samples.may.be.act.both.as.electron.donors,.and.as.electron.traps.which.can.bind.the.electrons.[50–52]..In.addition,.the.higher.resis-tivity.value.for.Zn1−xMnxO.films.indicates.that.additional.electron.trap.centers.related.to.the.Mn.doping.may.be.also.influential.in.our.samples..Each.trapped.electron.couples.the.local.moments.of.several.nearby.transition.metal.atoms..The.radius.of.this.trapped.elec-tron.orbital.in.ZnO.is.estimate.to.be.0.5.nm,.which.is.enough.to.contain.a.couple.of.dopant.atoms.in.the.2.9%.Mn-doped.ZnO.sample.[52]..The.trapped.electron.will.align.in.an.anti-parallel.configuration.with.the.individual.dopant.Mn.ion.spins..This.leads.to.an.effective.ferromagnetic.coupling.between.coupled.dopant.atoms.

The.structural.analysis.using.XRD.and.Raman.spectroscopy.indicated.that.all.Zn1−xCoxO.films.have.a.similar.ZnO.hexagonal.wurtzite.phase.without.Co-related.secondary.phases.in.the.samples.with.Co.concentrations.below.10.5.at.%,.other.than.the.Co.doping.concen-tration.up.to.31.3.at.%,.Co3O4.phase.can.also.be.observed..Co.incorporates.successfully.into.the.ZnO.lattice.and.does.not.change.the.wurtzite.structure.of.the.ZnO..Co.doping.was.seen.to.suppress.near.band.edge.and.violet.emissions.due.to.the.degradation.of.material.quality.and.the.increase.of.nonradiative.centers..The.O2−→Co2+.charge-transfer.process.in.Co3O4.clusters.embedded.in.Zn1−xCoxO.films.with.x.up.to.0.313.

We.observed.distinct.ferromagnetic.behavior.at.5.K.in.the.doped.samples.only..Despite.the.presence.of.some.intrinsic.defects.in.undoped.ZnO.films,.no.trace.of.FM.was.observed.in. the. undoped. and. milled. ZnO. sample. that. was. first. tested. under. similar. conditions.using.the.SQUID..This.confirms.that.defects.alone.in.ZnO.films.cannot.account.for.the.observed.FM.in.the.doped.films..Figure.13.31.shows.the.magnetization.versus.field.curves.of.the.Zn1−xCoxO.(x.=.0.067,.0.158,.and.0.313).films.recorded.at.5.K..The.diamagnetic.contri-bution.from.Si.substrate.has.been.subtracted.here..All.the.films.show.clear.hysteresis,.and.one.might.draw.the.conclusion.that. these.films.possess.ferromagnetism..The.Ms.of. the.samples.varied.in.the.range.of.5.7–12.emu/g.for.the.sample.of.different.initial.particle.size.and.different.doping.concentration..The.Ms.increases.with.the.increase.of.doping.concen-tration..Higher.Ms.in.lower.size.starting.nanopowder.is.likely.to.be.caused.by.enhanced.doping.and.higher.FM.ordering.in.nanometer-sized.ZnO.particles.

The.origin.of.observed.FM.at.5.K.in.these.films.could.arise.from.a.number.of.possibili-ties,.such.as.the.intrinsic.property.of.the.doped.films,.extended.defects.in.the.ZnO.films,.formation. of. some. nanoscale. Co-related. secondary. phase,. Co. precipitation,. and. CoO..However,.CoO.phase.can.be.easily.ruled.out,.since.CoO.is.antiferromagnetic.with.a.Neel.temperature.of.293.K..Second,.metallic.Co.is.also.an.unlikely.source.of.this.FM,.as.XRD.and.HRTEM.results.show.no.metallic.Co.clusters.in.the.films..Undoped.ZnO.does.not.exhibit.any.measurable.magnetization..Hence,.defects.alone.cannot.make.the.observed.high.mag-netic.moment.observed.in.the.doped.ZnO.films..Thus,.TMs.essentially.play.the.key.role.in.the.observed.FM..PL.spectra.showed.a.band.gap.modification.that.suggests.Co2+.ions.were.successfully.incorporated.into.the.wurtzite.lattice.at.the.Zn2+.sites..Therefore.FM.is.


expected.to.arise.from.the.intrinsic.exchange.interaction.of.magnetic.moments.mediated.by.the.defects.in.doped.films.

There. are. several. mechanisms. proposed. in. the. literature. regarding. the. origin. of. FM.in.DMSs..The.exact.mechanism.of.intrinsic.FM.in.TM-doped.oxides.is.still.under.debate..A. diversity. of. theories. has. been. proposed,. such. as. RKKY. interaction,. superexchange,.double-exchange.between.the.d.states.of.TMs,.free.carrier-mediated.exchange,.and.sp-d.exchange.mechanism,.etc.. [6]..The.RKKY.interaction.is.based.on.free.electrons.but.ZnO.cannot.transform.into.a.metal.at.such.a.low.doping..The.resistivities.of.Zn1−xCoxO.films.are.on.the.level.of.104.Ω.cm.at.room.temperature..Therefore,.our.results.show.that.the.doping.of.manganese.does.not.change.the.resistivity.value.in.the.system..This.high.resistivity.value.of.Zn1−xMnxO.films.unambiguously.rules.out.the.possible.carrier-mediated.exchange.[47],.such.as.Ruderman-Kittel-Kasuya-Yosida.mechanism..Direct. interactions.such.as.double-exchange.or.superexchange.cannot.be.responsible. for. the.FM.because.the.magnetic.cat-ions.are.dilute. in.our.samples..All. these.proposed.theories.cannot.well.accord.with.the.experimental.results. in.DMSs.[6]..According.to. the. literature,.magnetic.cations,.carriers,.and. defects. can. make. up. BMPs. that. may. be. responsible. for. the. FM.. In. addition. to. the.magnetic.doping.effect,.Vo.defects.have.been.suggested.to.play.an.important.role.in.the.magnetic.origin.for.oxide.DMSs.[112]..The.theoretical.studies.suggest.that.Vo.can.cause.an.obvious.change.in.the.band.structure.of.host.oxides.and.makes.a.significant.contribution.to.the.FM.[75,113]..The.formation.of.BMPs,.which.include.electrons.locally.trapped.by.oxygen.vacancy,.with.the.trapped.electron.occupying.an.orbital.overlapping.with.the.d.shells.of.TM.neighbors,.has.also.been.proposed.to.explain.the.origin.of.FM.[75]..Oxygen.vacancies.are.inherently.present.in.as-grown.ZnO.films.due.to.the.stabilization.of.structure..On.the.basis.of.observed.strong.D.band.emission.in.PL.and.intense.defect.modes.seen.in.Raman.spectra.(no.shown),.we.presume.that.oxygen.vacancies.play.a.key.role.in.the.observed.FM.at.5.K..We.notice.that.the.increase.of.Co.concentration.shows.increase.in.Ms.owing.to.the.increase.in.O-vacancy.or.vacancy.clusters.that.may.help.to.create.more.BMPs.and.their.percolation.[114]..Our.systematic.study.shows.that.oxygen-vacancy.defect.constituted.BMPs.are.one.of.

H (Oe)

Mag

netiz

atio

n (e

mu/

g)

–15,000 –10,000

Zn0.933Co0.067OZn0.842Co0.158OZn0.687Co0.313O

–15

–10

–5

0

5

10

15

–5,000

At 5K

0 5,000 10,000 15,000

FIGURE 13.31Magnetization.loop.of.the.Zn1−xCoxO.(x.=.0.067,.0.158,.and.0.313).films.measured.at.5.K.


the.promising.candidate.for.the.origin.of.FM.in.this.system..Within.the.BMP.model,.the.greater.density.of.Vo.and.more.doping.help.to.produce.more.BMPs,.which.yields.a.greater.overall.volume.occupied.by.BMPs,.leading.to.the.overlap.of.BMPs.and.enhancement.of.FM..This.evolution.is.observed.in.our.case,.increase.in.magnetization.with.the.Co.concentration.indicating.that.the.FM.in.our.samples.may.be.due.to.percolation.of.BMPs.

13.5.4 Conclusion

ZnO1−xFexO.films.were.prepared.by.the.RF.magnetron.sputtering.technique.on.Al2O3.(001).substrates.on.different.working.parameters..According.to.the.results.of.XRD.and.XPS,.the.first.type.of.ZnO1−xFexO.films.had.a.preferential.c-axis.orientation.and.the.position.of.(002).diffraction.peak.shifts.to.the.lower.degree.side.with.increasing.Fe.component..Fe.element.in.the.films.exists.mainly.in.the.form.of.Fe2+,.and.the.Fe2+.ions.have.successfully.substituted.for.the.Zn2+.ions.in.the.ZnO.lattice.and.free.from.clusters..However,.in.the.second.type.of.ZnO1−xFexO.films,.the.position.of.(002).diffraction.peak.shifts.to.the.higher.degree.side.with.increasing.Fe.component..Fe.element.in.the.films.exists.mainly.in.the.form.of.Fe3+,.and.the.Fe3+.ions.have.successfully.substituted.for.the.Zn2+.ions.in.the.ZnO.lattice.and.free.from.clusters..XPS.study.of.the.Zn1−xFexO.films.exhibit.that.Fe.exists.mainly.in.the.form.of.Fe2+.in.the.third.type.of.ZnO1−xFexO.films..XRD.patterns.of.the.samples.show.that.the.diffraction.peak.shifts.to.the.lower.degree.side.with.increasing.Fe.component..Meanwhile,.the.Fe2+.ions.have.successfully.substituted.for.the.Zn2+.ions.in.the.ZnO.lattice.and.free.from.clusters.

In.all.the.films,.the.crystalline.quality.of.the.films.gets.worse.and.the.particle.size.also.gets.smaller.because.of.the.Fe-doped..The.Fe.ions.with.different.chemical.states.have.suc-cessfully.substituted.for.the.Zn2+.ions,.which.induce.the.changes.of.crystal.lattice.constant.of.crystalline.ZnO..Now.we.can.come.to.a.conclusion.that.we.can.modulate.the.different.chemical.states.of.Fe.ions.and.the.different.concentrations.of.Fe.ions.in.the.films.using.dif-ferent.working.parameters,.so.as.to.lay.a.strong.foundation.for.obtaining.ferromagnetism.

By. studying. structural. property. of. Fe-substituted. polycrystalline. ZnO. samples,. the.result. of. magnetic. measurement. shows. that. the. Zn1−xFexO. films. were. ferromagnetic.respectively..Subsequently,.we.discussed. the.origin.of. the. ferromagnetism. in. the.films..The.main.magnetization.is.not.due.to.any.precipitating.secondary.phase,.but.may.be.due.to.carrier-mediated.ferromagnetism.

13.6 Conclusion

Diluted.magnetic.semiconductors.(DMSs).have.been.attracting.much.interest.of.almost.a.decade.now.due.to.their.potential.to.manipulate.charge.and.spin.degrees.of.freedom.in.a.single.material..Dietl.et.al..predicted.high-temperature. ferromagnetism. in.TM.doped.wide-band-gap. semiconductors. particularly. in. ZnO. and. GaN.. This. fact. has. motivated.many.researchers.to.study.the.properties.of.TM-doped.semiconductors..Recent.reports.on.the.observation.of.room-temperature.(RT).ferromagnetism.in.TM-doped.ZnO.have.been.welcomed.with.great.enthusiasm.by.the.scientific.community..ZnO-based.DMS.have.some.advantages.over.others.because.of.their.unique.characteristics,.such.as.having.a.large.band.gap.(∼3.4.eV),.large.exciton.binding.energy.at.RT.(∼60.meV),.high.optical.gain.(300.cm−1),.and.very.short.luminescence.lifetime,.which.are.required.for.various.optoelectronic.and.magneto-optical.devices.


This.thesis.is.focused.on.the.hotspots.and.challenges.in.the.field.of.ZnO.material.research..The.purpose.of.this.thesis.is.to.investigate.the.room.ferromagnetic.behavior.and.the.origin.of.ferromagnetism.in.ZnO-based.DMS.films..TM.(Here.Cu,.Cr,.and.Fe).doped.ZnO.thin.films.were.synthesized.via.magnetron.sputtering.method.and.its.microstructure,.optical.properties,.and.magnetic.properties.were.investigated..The.following.are.the.major.results:

. 1..Structural.analyses.suggest.that.Cu,.Cr,.and.Fe.occupied.the.Zn.sites.successfully.and.did.not.change.the.wurtzite.structure.of.ZnO.at.low.doping.content..However,.a.small.amount.of.ZnCr2O4.clusters.could.be.found.in.Zn1−xCrxO.films.when.the.doping.level.x.reached.0.098..A.point.worth.emphasizing.is.that.nanoscale.colum-nar.grain.arrays.were.found.in.the.cross-sectional.images.of.Cu-doped.ZnO.films..As.the.magnetron.sputtering.method.can.produce.economically.feasible.large.area.films.with.well-controlled.composition,.we.suggest.that.the.method.in.our.experi-ment.may.be.applied. to. future. large-scale.manufacturing.of.aligned.Cu-doped.ZnO.nanoscale.columnar.grains.and.nanorod.arrays.

. 2..The.ZnO.films.doped.with.moderate.TM.(Cu,.Mn,.and.Co).exhibit.obvious.ferro-magnetic.ordering,.which.originates.from.TM-ZnO.matrix.rather.than.impurities.and.can.be.ascribed.to.originate.from.the.bound.magnetic.polarons.(BMP).model..The. ferromagnetism. of. these. TM-doped. ZnO. films. also. suggests. their. potential.applications.for.future.spintronics..In.addition,.the.magnetic.moment.per.TM.atom.decreased.as.the.TM.concentration.further.increased..It.is.proposed.that.the.decrease.in.magnetic.moment.per.TM.atom.as.the.TM.concentration.increases.is.due.to.an.increase.in.the.number.of.TM.atoms.that.occupy.adjacent.cation.lattice.positions.with.an.attendant.increase.in.antiferromagnetic.interaction.between.those.TM.atoms.

. 3..The.ZnO.films.doped.with.moderate.Cr.exhibit.obvious.ferromagnetic.ordering.upon.the.room.temperature..The.saturated.magnetization.is.∼0.79.μB/Cr.atom.at.x.=.0.013.and.decreases.with.increasing.Cr.dopant..The.experimental.results.show.that.VZn

− ,.together.with.Cr.dopant.plays.an.important.role.in.the.ferromagnetic.ori-gin.in.Cr:ZnO..The.FM.in.films.can.be.described.by.BMP.models.with.respect.to.defect-bound.carriers..FM.may.originate.from.uncompensated.spins.at.the.nano-crystal.surface.with.large.density.of.magnetic.ions..Obviously,.the.origins.of.FM.in.DMS.are.very.complicated..More.detailed.works.are.essential.to.understand.the.magnetic.behaviors.of.these.materials.

. 4..The. ZnO. films. doped. with. moderate. Fe. exhibit. obvious. ferromagnetic. order-ing.below.the.room.temperature..The.magnetization.value.is.4.51.μB/Fe.under.15.kOe.external.magnetic.field.at.5.K,.as.for.the.Zn1−xFexO.(x.=.0.212).film..The.main.magnetization.is.not.due.to.any.precipitating.secondary.phase,.but.may.be.due.to.carrier-mediated.ferromagnetism..Until.now,.the.origin.of.FM.in.DMS.materials.remains.a.very.controversial.topic.

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Related Materials


Volume One

Materials

Edited by

Zhe Chuan Feng

Electronic Materials and Devices Series Feng

Handbook of Zinc O

xide and Related Materials

Volume One

Series Editors: Yongbing Xu and Jean-Pierre Leburton

Handbook of Zinc Oxide and Related Materials

Materials

Volume One

ISBN: 978-1-4398-5570-6

9 781439 855706

90000

K12599

Through their application in energy-efficient and environmentally friendly devices, zinc oxide (ZnO) and related classes of wide gap semiconductors, including GaN and SiC, are revolutionizing numerous areas, from lighting, energy conversion, photovoltaics, and communications to biotechnology, imaging, and medicine. With an emphasis on engineering and materials science, Handbook of Zinc Oxide and Related Materials provides a comprehensive, up-to-date review of various technological aspects of ZnO.

Volume One presents fundamental knowledge on ZnO-based materials and technologies. It covers the basic physics and chemistry of ZnO and related compound semiconductors and alloys. The first part of this volume discusses preparation methods, modeling, and doping strategies. It then describes epitaxial methods used to create thin films and functional materials. The book concludes with a review of alloys and related materials, exploring their preparation, bulk properties, and applications.

Covering key properties and important technologies of ZnO-based devices and nano-engineering, the handbook highlights the potential of this wide gap semiconductor. It also illustrates the remaining challenging issues in nanomaterial preparation and device fabrication for R&D in the twenty-first century.

Features• Presents the essentials on ZnO-based materials and technologies • Describes the key properties of ZnO and its alloys • Emphasizes the growth and characterization of novel nanostructures• Highlights the remaining issues in nanomaterial preparation for future

R&D

Materials Science

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znobased materials

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