electronics engineering series w687...

Edited by

Vincent Consonni

Guy Feuillet

Wide Band Gap Semiconductor Nanowires 2

Over the last ten years, GaN and ZnO nanowires have emerged as potentialbuilding blocks for the next generation of optoelectronic devices includingLEDs, lasers, UV photodetectors and solar cells. This has led to an increasingnumber of publications in the field, which now outnumbers 10,000. In viewof this wealth of information, this two-volume series is intended to give adetailed status of the research topic dedicated to GaN and ZnO nanowires.In particular, dealing with these two different but closely relatedsemiconductors yields valuable comparisons and benefits the generalunderstanding of this subject, helping promote the development of relatedoptoelectronic applications.

The comprehensive books gather review articles written by pioneering andworld-leading scientists at the forefront of basic and applied research,covering all aspects from low-dimensionality effects to optoelectronicdevices through to nanowire growth and their related heterostructures.

This second volume is devoted to the formation and characterization ofheterostructures made from GaN and ZnO nanowires. It also addresses theadvanced fabrication of optoelectronic devices such as LEDs, lasers, UVphotodetectors, and solar cells, on the basis of physical properties andgrowth processes presented in the first volume.

This book is of interest not only to physicists, chemists or materialsscientists interested in the topic of one-dimensional nanostructures and theiroptoelectronic applications, but also to semiconductor scientists already inthe field but looking for an extended overview.

Vincent Consonni is Associate Research Scientist at CNRS (French Centerfor National Research) in France. His research has focused on the chemistryand physics of crystal growth and of condensed matter for micro- andnanostructures involving GaN and ZnO nanowires.

Guy Feuillet is Senior Research Scientist at CEA (French Atomic andAlternative Energy Commission) in France. He has initiated and coordinatedmany internal R&D programs (GaN and ZnO nanostructures, X-ray detection,solid state lighting) during his work at CEA. He is a permanent member ofthe scientific advisory board at CEA/LETI.

Wide Band GapSemiconductorNanowires 2

Heterostructures andOptoelectronic Devices

www.iste.co.uk Z(7ib8e8-CBGIHH(

Edited byVincent Consonni and Guy Feuillet

ELECTRONICS ENGINEERING SERIES

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Wide Band Gap Semiconductor Nanowires 2

Series EditorRobert Baptist

Wide Band GapSemiconductor Nanowires 2

Heterostructures andOptoelectronic Devices

Edited by

Vincent ConsonniGuy Feuillet

First published 2014 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, aspermitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced,stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers,or in the case of reprographic reproduction in accordance with the terms and licenses issued by theCLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at theundermentioned address:

ISTE Ltd John Wiley & Sons, Inc.27-37 St George’s Road 111 River StreetLondon SW19 4EU Hoboken, NJ 07030UK USA

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© ISTE Ltd 2014The rights of Vincent Consonni and Guy Feuillet to be identified as the authors of this work have beenasserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2014941789

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-687-7

Printed and bound in Great Britain by CPI Group (UK) Ltd., Croydon, Surrey CR0 4YY

Contents

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

PART 1. GaN AND ZnONANOWIREHETEROSTRUCTURES . . . . . . . . . . . 1

CHAPTER 1. AlGaN/GaNNANOWIREHETEROSTRUCTURES . . . . . . . . . . 3Jörg TEUBERT, Jordi ARBIOL and Martin EICKHOFF

1.1. A model system for AlGaN/GaN heterostructures . . . . . . . . . . . . . 31.2. Axial AlGaN/GaN nanowire heterostructures . . . . . . . . . . . . . . . 41.2.1. Structural properties of axial AlGaN/GaN nanowireheterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.2. Optical properties of axial AlGaN/GaN nanowireheterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.3. Lateral internal electric fields. . . . . . . . . . . . . . . . . . . . . . . 121.2.4. Axial internal electric fields. . . . . . . . . . . . . . . . . . . . . . . . 141.2.5. Optical characterization of single-AlGaN/GaNnanowires containing GaN nanodisks. . . . . . . . . . . . . . . . . . . . . . 151.2.6. Electrical transport properties. . . . . . . . . . . . . . . . . . . . . . . 18

1.3. AlGaN/GaN core–shell nanowire heterostructures. . . . . . . . . . . . . 191.3.1. Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.3.2. Optical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 231.3.3. Electronic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241.3.4. True one-dimensional GaN quantum wiresecond-order self-assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1.4. Application examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291.4.1. AlGaN/GaN nanowire heterostructure optochemical gas sensors . 301.4.2. AlGaN/GaN nanowire heterostructure resonanttunneling diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

vi Wide Band Gap Semiconductor Nanowires 2

1.5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341.6. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

CHAPTER 2. InGaNNANOWIREHETEROSTRUCTURES . . . . . . . . . . . . . . 41Bruno DAUDIN

2.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.2. Self-assembled InGaN nanowires. . . . . . . . . . . . . . . . . . . . . . . 432.3. X-ray characterization of InGaN nanowires. . . . . . . . . . . . . . . . . 462.4. InGaN nanodisks and nanoislands in GaN nanowires . . . . . . . . . . . 492.5. Selective area growth (SAG) of InGaN nanowires. . . . . . . . . . . . . 522.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552.7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

CHAPTER 3. ZnO-BASEDNANOWIREHETEROSTRUCTURES . . . . . . . . . . . 61Guy FEUILLET and Pierre FERRET

3.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.2. Designing ZnO-based nanowire heterostructures . . . . . . . . . . . . . 633.3. Growth of ZnxMg1-xO/ZnO core–shellheterostructures by metal-organic vapor phase epitaxy. . . . . . . . . . . . . 663.4. Misfit relaxation processes in Znx Mg1-xO/ZnOcore–shell structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703.5. Optical efficiency of core–shell oxide-based nanowire heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . 733.6. Axial nanowire heterostructures. . . . . . . . . . . . . . . . . . . . . . . . 763.7. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . 803.8. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

CHAPTER 4. ZnO ANDGa NANOWIRE-BASED TYPE IIHETEROSTRUCTURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Yong ZHANG

4.1. Semiconductor heterostructures . . . . . . . . . . . . . . . . . . . . . . . . 854.2. Type II heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.3. Optimal device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 884.4. Electronic structure of type II core–shell nanowires . . . . . . . . . . . . 914.5. Synthesis of the type II core–shell nanowires and their signatures . . . 944.6. Demonstration of type II effects in ZnO–ZnSecore–shell nanowires and photovoltaic devices . . . . . . . . . . . . . . . . . 964.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024.9. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

N

Contents vii

PART 2. INTEGRATION OFGaN AND ZnONANOWIRES INOPTOELECTRONICDEVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

CHAPTER 5. AXIALGaNNANOWIRE-BASEDLEDS . . . . . . . . . . . . . . . . 107Qi WANG, Hieu N’GUYEN, Songrui ZHAO and Zetian MI

5.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075.2. Top-down GaN-based axial nanowire LEDs . . . . . . . . . . . . . . . . 1085.2.1. Fabrication of top-down GaN-based axial nanowires. . . . . . . . . 1085.2.2. Device fabrication of axial nanowire LEDs . . . . . . . . . . . . . . 1105.2.3. Performance characteristics of top-down axialnanowire LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5.3. Bottom-up GaN-based axial nanowire LEDs . . . . . . . . . . . . . . . . 1125.3.1. Growth techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.3.2. Doping, polarity and surface charge properties . . . . . . . . . . . . 1135.3.3. Design and typical performance of bottom-upaxial nanowire LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.4. Carrier loss processes of axial nanowire LEDs . . . . . . . . . . . . . . . 1215.4.1. Auger recombination. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215.4.2. Electron overflow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1225.4.3. Surface recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

5.5. Controlling carrier loss of GaN-based nanowire LEDs . . . . . . . . . . 1245.5.1. p-type modulation doping and AlGaNelectron blocking layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245.5.2. InGaN/GaN/AlGaN core–shell dot-in-a-wirephosphor-free white LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

5.6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1275.7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

CHAPTER 6. RADIALGaNNANOWIRE-BASEDLEDS . . . . . . . . . . . . . . . 135Shunfeng LI

6.1. Radial GaN nanowire-based LED: an emerging device . . . . . . . . . . 1356.2. Growth of GaN nanowires and radial nanowire-based devices . . . . . 1386.3. Radial GaN nanowire-based LED structure . . . . . . . . . . . . . . . . . 1456.4. Characteristics of radial NW devices. . . . . . . . . . . . . . . . . . . . . 1506.5. Further work and perspectives. . . . . . . . . . . . . . . . . . . . . . . . . 1526.6. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

CHAPTER 7. GaNNANOWIRE-BASEDLASERS. . . . . . . . . . . . . . . . . . . . 161Xiang ZHOU, Jordan Paul CHESIN and Silvija GRADEČAK

7.1. Introduction to nanowire lasers . . . . . . . . . . . . . . . . . . . . . . . . 1617.2. Theoretical considerations and simulations . . . . . . . . . . . . . . . . . 163

viii Wide Band Gap Semiconductor Nanowires 2

7.3. The first experimental observations of lasing in nanowires. . . . . . . . 1657.4. GaN nanowire-based lasers . . . . . . . . . . . . . . . . . . . . . . . . . . 1667.5. Toward wavelength tunability: nanowire lasersbased on GaN/InxGa1-xN heterostructures. . . . . . . . . . . . . . . . . . . . . 1697.6. GaN nanowire lasers coupled with hybrid structures . . . . . . . . . . . 1717.7. Challenges and opportunities . . . . . . . . . . . . . . . . . . . . . . . . . 1737.8. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

CHAPTER 8. GaNNANOWIRE-BASEDULTRAVIOLETPHOTODETECTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179Lorenzo RIGUTTI and Maria TCHERNYCHEVA

8.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1798.2. Growth and fabrication techniques . . . . . . . . . . . . . . . . . . . . . . 1808.3. GaN nanowire photoconductive detectors . . . . . . . . . . . . . . . . . . 1838.4. p–i–n junction-based GaN nanowire detectors . . . . . . . . . . . . . . . 1878.5. Single-wire GaN/AlN multiple quantum disk photodetectors . . . . . . 1908.6. Single-wire InGaN/GaN core–shell photodetectors . . . . . . . . . . . . 1938.7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1978.8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1978.9. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

CHAPTER 9. ZnONANOWIRE-BASEDLEDS . . . . . . . . . . . . . . . . . . . . . 203Magnus WILLANDER and Omer NOUR

9.1. Outline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2039.2. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2039.3. Growth of ZnO nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . 2059.4. White light emission from ZnO nanowires . . . . . . . . . . . . . . . . . 2099.5. ZnO NW white LEDs on solid crystalline substrates . . . . . . . . . . . 2129.6. ZnO NWs white LEDs on flexible substrates . . . . . . . . . . . . . . . . 2149.7. Enhancing the emission of ZnO nanowire-based LEDs . . . . . . . . . . 2209.8. Conclusion and future prospective . . . . . . . . . . . . . . . . . . . . . . 2229.9. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

CHAPTER 10. ZnONANOWIRE-BASED SOLARCELLS . . . . . . . . . . . . . . . 227Jason B. BAXTER

10.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22710.1.1. Solar energy conversion and nanostructured solar cells . . . . . . 22710.1.2. Use of ZnO in solar cells. . . . . . . . . . . . . . . . . . . . . . . . . 228

10.2. ZnO nanowire dye-sensitized solar cells . . . . . . . . . . . . . . . . . . 22910.3. Quantum dot-sensitized nanowire solar cells . . . . . . . . . . . . . . . 235

Contents ix

10.4. Extremely thin absorber solar cells . . . . . . . . . . . . . . . . . . . . . 23710.5. Nanowire arrays completely filled with inorganic absorbers . . . . . . 23910.6. ZnO nanorod – organic hybrid solar cells . . . . . . . . . . . . . . . . . 24110.7. ZnO nanowire arrays for photoelectrochemical water splitting. . . . . 24410.8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24510.9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24710.10. Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

LIST OFAUTHORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

Preface

This book is devoted to the specific case of wires obtained from a given kind ofsemiconductors, namely the semiconducting materials with a direct and wide bandgap (WBG). In short, semiconductors are considered as WBG semiconductingmaterials if their band gap energy is typically above 1.5/1.6 eV. The interest of thesematerials for optoelectronic devices lies in the fact that they are well-adapted foremission, detection or absorption processes in most of the visible range, and part ofthe UV range as well. From the more basic point of view, the large refractive indexand high exciton binding energy as well as the strong photon/exciton interactionsgive rise to long sought effects such as polariton lasing at room temperature forinstance. The two main materials composing the family of WBG semiconductingmaterials are GaN and ZnO. They have close band gap energy in the near UV region(i.e., around 3.3/3.4 eV), and have in common that their cationic alloys span thevisible as well as the UV range (and also part of the near IR region for In-richGaInN alloys). More importantly, they both crystallize, in standard conditions, in thestrongly anisotropic wurtzite crystalline phase, leading to a large number of similarphysical quantities such as lattice parameters and piezoelectric constants and ofsimilar physical processes related for instance to polarity.

GaN and its alloys are now well-mastered and used in a flurry of industrialapplications as optoelectronic devices. On the other hand, ZnO is less advanced interms of industrial applications and its development is mainly hampered by thedifficulty for controlling p-type doping. However, ZnO has a stronger excitonbinding energy than GaN (60 meV vs. 25 meV) and also a stronger oscillatorstrength. GaN and related alloys are generally heteroepitaxially grown on foreignsubstrates since low-cost nitride substrates with large dimensions are still notavailable. In contrast, ZnO and related alloys can homoepitaxially be grown ontoZnO substrates with excellent structural properties but still with limited availabilityand sizes. Therefore, epitaxial growth is mostly carried out heteroepitaxially for both

xii Wide Band Gap Semiconductor Nanowires 2

kinds of materials, typically yielding epitaxial planar layers with a high density ofstructural defects. If such WBG semiconducting materials with a rather poorstructural quality are actually used for some optoelectronic devices such ascommercial LEDs for the moment, the improvement of their overall structure wouldcertainly be beneficial for additional potential optoelectronic devices but also for theunderstanding of the physical processes at stake in these devices.

The need for WBG semiconducting materials with better structural quality is oneof the main reasons that propelled (nano)wires to their present day status in the fieldof semiconductor research: when grown onto foreign substrates, and as for the caseof planar layers, wires can relax the elastic strain energy originating from largelattice mismatch by forming misfit dislocations. But these lie in the basal plane orbend towards the nearby lateral surfaces of wires, thus leaving defect-free materialsin their core. This process whereby dislocations can bend towards the lateral growthfront had been demonstrated beforehand in epitaxial lateral overgrowth (ELO).

The second reason behind the development of WBG semiconductor wires –considered for a long time as the unwanted result of wrong growth conditions whentrying to synthesize 2-dimensional (2D) epitaxial layers– is related to the increasinginterest for low-dimensionality objects, typically of sub-micron or nanometer size.The specific structural, optical, and electronic properties of these low-dimensionalityobjects open new opportunities for nanoscale optoelectronic devices, especially tofully exploit the strong photon/exciton interactions. As an example, wires allow for afull confinement of light in their section with free propagation along their axis. Suchphysics and the related optoelectronic applications are nonetheless limited by thelarge developed surfaces of the wires, for which surface passivation is for instancerequired in order to prevent light diffusion. Because of the presence of surfacestates, Fermi level pinning also leads to band bending affecting the carrier mobilityalong the wires and resulting in possible carrier trapping. In return, this specificproperty makes wires very invaluable objects to investigate surface effects in WBGsemiconductors and can also be beneficial in photodetection applications.

Looking back in time, the first demonstration of semiconductor wire growth wasachieved by the pioneering work of Wagner and Ellis in 1964 according to thevapor-liquid-solid (VLS) mechanism [WAG 64]. In the field of WBGsemiconducting materials for optoelectronic devices, which are the materials that weare interested in in this book, one of the first “nano”objects that were looked intowere dots, named quantum dots when the typical dimensions are smaller than the DeBroglie’s wavelength, inserted as they were in 2D epitaxial layers. For instance, thedots can be grown according to the so-called Stransky-Krastanov mode owing to theelastic stress relaxation processes at play in lattice mismatched heteroepitaxialsystems. This is nevertheless limited somehow to heteroepitaxial layers in a state of

Preface xiii

compressive strain, and of medium lattice mismatch range (typically a few percent).For one heteroepitaxial system, such dots have once and for all a fixed size given bythe nature of the involved materials. Thus, one had to think of other possibilities formaking sub-micron or nano objects with an easier control over their sizes andshapes. Instead of playing for instance with strain to form dots, the easier way togrow low-dimensionality structures is to try and depart from the 2D growthconditions, thereby changing the atomic diffusion and incorporation processes,hence using growth modes different from the usual 2D mode. This time, this leads tothe controlled formation of 1D objects, now referred to as nanowires, microwires ormore generally wires, depending on their lateral dimensions, or also asnanocolumns, nanorods or microrods.

Interestingly, in terms of growth conditions, while most of the semiconductor(i.e., Si, Ge, arsenides, phosphides, …) wires can exclusively be grown by VLS orvapor-solid-solid mechanisms in the bottom-up approach, one of the most amazingproperties of GaN and ZnO is their ability to grow in the form of wires followingcatalyst-free approaches (i.e., self-induced growth, spontaneous growth, …). Thesecatalyst-free approaches are expected to reduce potential contamination into thewires and, more importantly, offer new valuable growth modes with greatpotentiality for optoelectronic devices. The first demonstrations of GaN and ZnOwire growth were shown in 1998 by molecular beam epitaxy [YOS 97, SAN 98] andin 2001 by vapor phase transport [HUA 01, PAN 01] and in solution [VAY 01],respectively. Basically, GaN wires can mainly be grown by molecular beam epitaxyand metal-organic chemical vapor deposition. In contrast, ZnO wires canadditionally be deposited by vapor phase transport, pulsed-laser deposition or morespecifically in solution via the low-cost and low-temperature chemical bathdeposition technique for instance.

As discussed above, growing wires with dedicated properties in a reproducibleway requires a good control of the growth conditions. When it comes to radial aswell as axial heterostructures grown around or on top of the wires, things aresomehow more complicated, since growth conditions very often have to be movedfrom the initial 1D case in order to stack the layers on top of each other. As in thecase of any kind of heterostructures, managing the lattice mismatch issue may alsobe essential. This does depend upon the sizes involved and may potentially lead tothe generation of misfit dislocations at the interfaces between the constituting layers.Moreover, owing to the specific geometry of the wires, other types of defects mayalso be introduced, such as stacking faults or inversion domain boundaries forinstance, the origin of which has to be identified in order to better limit theiroccurrence. In return, identifying the right conditions for growing heterostructureswith a good structural quality opens up a flurry of applications in the field ofoptoelectronics. These will benefit not only from the wave guiding properties of the

xiv Wide Band Gap Semiconductor Nanowires 2

wires (i.e., specific optical modes) but also from the control over the density ofdefects into the wires, leading to a decrease in the number of non-radiativerecombination centers. These applications also take advantage of the larger surfaceto volume ratio at low-scale dimensions, leading for instance to much largeremitting or absorbing surfaces than in 2D layers or to efficient photodetectors.

The book has been organized along the lines of these introductory remarks.

Accordingly, it is the aim of the first part of volume 1 to focus on the specificproperties of WBG semiconductor wires, in order to point out what differentiatesthese objects from their 2D counterparts. This appears as a necessary step in order topoint out what these specificities could bring for the physics and applications ofWBG semiconductors in the field of optoelectronics. It is nonetheless also the aim ofthis first part to try and pin-point the present day limitations associated with the useof WBG semiconductor wires, in order to draw possible solutions for a thorough useof these 1D objects. As for the second part of volume 1, it is dedicated to thedifferent growth methods for the deposition of GaN and ZnO wires, stressing themechanisms at play for the nucleation and growth of these 1D objects. The mostinteresting growth methods are discussed in detail with a special emphasis on thenecessary ingredients to spontaneously grow GaN and ZnO wires. In volume 2, thefirst part aims at reviewing the different axial or radial heterostructures that can beintegrated into GaN and ZnO wires. This is done to address relevant potentialoptoelectronic applications including LEDs, lasers, UV photodetectors and solarcells, which are presented and discussed in the second part of volume 2.

As revealed by the very numerous publications, the subject is far from beingclosed and new results emerge at a quick pace. With this in mind, this book isintended to give the reader a detailed overview of the current status of research inthe field of WBG semiconductor wires for optoelectronic devices. As announced inthe very title of this book, the choice was deliberately made to intermix chaptersdevoted to GaN and ZnO wires: the two materials have a lot in common, and thetwo communities will gain from mutual exchanges.

We hope that the reviews presented here by pioneering and world-leadingscientists in the field, the discussion on the chemistry, physics, and applications ofWBG semiconductor wires, together with the comparison between the two kindsof materials and between the different growth methods will be a useful source ofinformation not only for the new comers in the field, but also for the alreadyinvolved engineers and scientists who seek a detailed overview of the subject to givetheir work a new impulse.

Preface xv

Finally, we would like to warmly thank all our friends and colleagues who tookpart in this book project to create a lively, fruitful and high level place on the hottopic of WBG semiconductor wires.

Vincent CONSONNIGuy FEUILLET

June 2014

Bibliography

[WAG 64] WAGNER R.S., ELLISW.C., Appl. Phys. Lett., 4, 89 (1964).

[YOS 97] YOSHIZAWA M., KIKUCHI A., MORI M., et al., Japanese J. Appl. Phys., 36, L459(1997).

[SAN 98] SANCHEZ-GARCIAM.A., CALLEJA E., MONROY E., et al., J. Cryst. Growth, 183, 23(1998).

[HUA 01] HUANGM.H., MAO S., FEICK H., et al., Science, 292, 1897 (2001).

[PAN 01] PAN Z.W., DAI Z.R., WANG Z.L., Science, 291, 1947 (2001).

[VAY 01] VAYSSIERES L., KEIS K., LINDQUIST S.E., et al., J. Phys. Chem., B 105, 3350(2001).

PART 1

GaN and ZnO Nanowire Heterostructures

1

AlGaN/GaN Nanowire Heterostructures

1.1. A model system for AlGaN/GaN heterostructures

In order to address real (optoelectronic) device applications based on GaNnanowires (NWs), the control of carrier confinement and of optical transitionenergies by alternating the optical band gap, either parallel or perpendicular to thegrowth direction, is of major importance. Within the group III-nitride (III-N) materialsystem, this can be achieved either by the realization of AlGaN/GaN nanowireheterostructures (NWHs) that expand the energies of the involved optical transitionsto the ultraviolet regime or by the realization of InGaN/GaN NWHs that open theway to the blue and green spectral region. Both types of NWHs impose differentrequirements in terms of growth conditions and – mainly due to the different growthtemperature – exhibit different structural and morphological properties because thesurface mobility of adatoms and the crystalline phase stability are strongly affectedby the applied growth conditions. Therefore, AlGaN/GaN NWHs will be addressedin this chapter, while InGaN/GaN NWHs will be addressed in section 2.1.

The self-organized growth of GaN NWs results in nanostructures that, due to thepossibility of lateral strain–relaxation during growth, exhibit very low densities ofstructural defects despite a large lattice mismatch with respect to the underlyingsubstrate. Hence, they represent a perfect starting point for the growth of NWHs withoptical properties that are only weakly influenced by recombination related tostructural defects. Therefore, AlGaN/GaN NWHs are an ideal model system for theinvestigation of basic material properties because – although two-dimensional (2D)AlGaN/GaN heterostructures are well understood – the relations between structuralcharacteristics on the one hand and optical as well as electrical characteristics on theother hand are still a topic of current research.

Chapter written by Jörg TEUBERT, Jordi ARBIOL and Martin EICKHOFF.

4 Wide Band Gap Semiconductor Nanowires 2

In the following, the growth, structural, optical and electrical properties ofdifferent types of AlGaN/GaN NWHs are discussed. Here, we focus on those NWHssynthesized by a self-assembled bottom-up growth process. Resembling thechronology of the research work in the past two decades, we start the discussion withaxial AlGaN/GaN NWHs grown along the polar growth direction (section 1.2) byplasma-assisted molecular beam epitaxy (PA-MBE). Here, the structuralcharacteristics and the strain distribution of NWHs as a consequence of the differentlattice parameters of AlGaN and GaN are discussed. Furthermore, the opticalproperties of GaN nanodisks (NDs) embedded in AlGaN/GaN NWHs as well as theirdependence on the structural characteristics are summarized. The role of axial andlateral internal electric fields is discussed and the benefits ofmicro-photoluminescence spectroscopy (μ-PL) for the analysis of single NWs andeven single NDs are demonstrated.

In section 1.3 radial or core–shell AlGaN/GaN NWHs are addressed. Also in thiscase, we start the discussion with a review of growth issues and structural propertiesbefore we summarize recent results of their optical characterization. Depending onthe applied growth technique, core–shell NWHs can be grown with polar, semi-polarand non-polar side facets. In the first case, this gives rise to carrier accumulation atlateral hetero-interfaces and hence a higher degree of freedom for the design ofelectronic properties compared to PA-MBE grown axial NWHs. Such concepts aresummarized in detail. The discussion in this section also includes the realization ofone-dimensional (1D) GaN quantum wires (QWRs) realized by selective nucleationof GaN on the edges of AlN/GaN core–shell NWHs.

In section 1.4 we summarize two complementary application concepts forAlGaN/GaN NWHs, namely an optochemical sensor for the detection of oxygen andhydrogen based on GaN NDs in axial AlGaN/GaN NWHs and a resonant tunnelingdiode realized on the non-polar lateral surface of AlN/GaN double-barrier core–shellheterostructures. (The applications of NWHs as LEDs, lasers and UV photodetectorsare addressed in Chapters 5–8). Finally, conclusions are presented in section 1.5.

1.2. Axial AlGaN/GaN nanowire heterostructures

With typical diameters of several 10–100 nm, GaN NWs cannot be considered 1Dnanostructures and additional quantum effects due to a transition from quantum wells(QWs) to their counterparts embedded in NWHs, i.e. a further reduction indimensionality, are not expected. Thus, the properties of embedded axial quantumstructures, i.e. NDs, resemble the properties of 2D QWs rather than trulyzero-dimensional (0D) behavior. Still, the morphological and optical properties ofaxial quantum structures embedded in NWHs are governed by theirthree-dimensional (3D) geometry that provides, e.g., the possibility for strainrelaxation on the free lateral surface or strain management due to the presence of

AlGaN/GaN Nanowire Heterostructures 5

lateral shells. Due to the specific growth kinetics, PA-MBE has become the techniqueof choice for the growth of axial NWHs. The discussion in this section thereforefocuses on this synthesis method. A detailed discussion of the self-induced growth byMBE can be found in Chapter 8 of Vol. 1 [CON 14].

1.2.1. Structural properties of axial AlGaN/GaN nanowire heterostructures

Homogeneous GaN NWs are widely considered as nanostructures exhibiting anextremely low density of structural defects. The possibility for strain relaxation onthe lateral surfaces results in the absence of misfit dislocations when the NWdiameter is small enough [YOS 97, CAL 00]. For the same reason it was found thathomogeneously doped GaN NWs are free of strain for Si and Mg as dopants[FUR 08, RIC 08]. Whereas the incorporation of Si does not enhance the formationof structural defects even in high concentrations [FUR 08], it was reportedin [ARB 09] that doping with Mg results in the formation of triple-twin domainswhich, in high concentrations, cause the formation of zinc blend atomic cells in thewurtzite stacking.

In axial AlGaN/GaN NWHs, the situation is different as the formation of coherentinterfaces, due to the change in alloy composition along the growth axis, results in thegeneration of compressive (tensile) strain in the GaN NDs (the AlGaN barrier). In the2D case, the deposition of a compressively strained GaN layer on an AlN substrateleads to the formation of islands which is an efficient mechanism for strain relaxationwhen a critical layer thickness of typically several monolayers (MLs) is exceeded.This relaxation mechanism is the driving force for the Stranski–Krastanov growth ofGaN quantum dots (QDs) on AlN and AlGaN that has been intensely studied in recentyears [DAU 08]. It remains an interesting question as to how the limited diameterof NWs and the corresponding possibility for strain relaxation influence the criticalthickness for GaN grown on AlN embedded in NWs. In [GLA 06] strained layerson top of free-standing NWs were considered in a theoretical model showing thatthe critical layer thickness depends on the NW radius. It was further estimated thatthere exists a critical value of the radius, below which arbitrarily thick coherent layersshould be obtainable (for a more profound discussion of stress relaxation see Chapter2 in Vol. 1).

Regarding the general structure of GaN NDs, they have been found to appear asflat disks with sharp interfaces to AlN barriers [FUR 11]. ML fluctuations of the NDheight have also been observed, particularly in the case of AlGaN barriers [RIG 10b,PIE 13]. It has been observed by several groups that the ND side walls are facetedand that the NDs are slightly bent downward on the outer edges [FUR 11, BOU 10,RIS 05a]. In [FUR 11], it was reported that this structure originates in faceting ofthe GaN base top surface, where {1103}-planes form the outer edges, rather than instrain relaxation of the embedded NDs. The top surface faceting is most probably a


consequence of the N-face polarity of the NWs [MAT 12] in combination with the N-rich growth conditions. Subsequent overgrowth with ND and barrier material resultsin the typical shape of the ND stack.

It has also been observed by different groups that, while the GaN base and theGaN NDs grow in a 1D growth mode, the growth of the AlGaN barrier often exhibitsa lateral growth rate that results in the formation of a lateral shell that increases inthickness during growth of each barrier [RIS 05a, TCH 08, BOU 10, ZAG 11]. Thelateral growth rate of the barrier material, which depends on the growth temperatureand the total metal flux during barrier growth, strongly impacts the morphology andthe resulting strain distribution of the GaN/AlGaN ND stack. In [LAN 10], no lateralgrowth of the AlN barrier was found (see Figure 1.1(a)), while in [BOU 10] thepresence of a lateral shell was mentioned (see Figure 1.4). Also in [RIS 05a], thepresence of a lateral AlGaN shell is reported. In [TCH 08], a lateral growth rate forAlN of 35% compared to the axial growth rate was found, and in [FUR 11], it wasfound to be 11% for AlN barriers and linearly decreasing with the Al concentrationin the barriers [Al]bar (see Figure 1.2).

Figure 1.1. a) High-resolution transmission electron microscopy (HRTEM) image of aGaN/AlN NWH. Five periods of AlN/GaN grown on a GaN base are visible. The arrowindicates the growth direction; b) high-angle annular dark field (HAADF) image showing thatno significant inter-diffusion occurs between AlN and GaN; and c) profile of the c lattice-parameter, along the growth axis taken in the central part of an NW, obtained from thegeometrical phase analysis of the HRTEM image. The arrow indicates the growth direction.For convenience, the x-axis origin has been taken as the top of the GaN NW base before thegrowth of the first AlN layer. (Reprinted with permission from [LAN 10]. Copyright © 2010,American Physical Society)

In a first systematic experimental study on elastic strain relaxation in AlN/GaNNW super-lattices, Landré et al. found by in situ high-resolution X-ray diffractionexperiments that AlN/GaN (2.3 nm/2 nm) super-lattices (Figure 1.1a, b) on a GaN NWbase are in elastic equilibrium, i.e. the strain is distributed between the GaN and AlNlayers according to their layer thickness and an averaged in-plane lattice parameter


corresponding to an Al0.55Ga0.45N alloy is adopted. By HRTEM analysis, an increasein the c-parameter in the GaN NDs compared to the GaN NW base was found thatcorresponds to a decrease in the in-plane lattice parameter according to the transitionfrom relaxed GaN to that of Al0.55Ga0.45N (Figure 1.1(c)). The c-parameter in the AlNbarriers (lower value in Figure 1.1(c)) corresponds to the value that is obtained whenthe in-plane lattice parameter for Al0.55Ga0.45N and the Poisson ratio for AlN areconsidered. No evidence for strain relaxation due to the presence of misfit dislocationswas found [LAN 10].

A different situation arises when the growth parameters are changed and anenhanced lateral growth rate during the growth of the AlN barriers is obtained.In [FUR 11], a lateral growth rate of 11% from the axial growth rate was observedduring barrier growth in a nine-fold AlN/GaN ND structure. As is depicted inFigures 1.2(a) and (b), this results in an increasing ND diameter and a decrease in theshell thickness along the growth direction as well as full encapsulation of the GaNNDs with AlN. Hence, free relaxation of NDs at the NW periphery can no longer beassumed and compressive stress along the c-direction is also exerted in that region.This effect remains but becomes less pronounced with decreasing Al-concentrationin the barriers [Al]bar since a linear dependence of the lateral growth rate on [Al]barwas observed.

As a consequence of these boundary conditions, the strain state of the NDs isaltered compared to that described in [LAN 10] (Figure 1.1). In particular, a differentstrain state of each individual ND within a multi-ND structure has to be considered,which complicates the discussion of optical properties. In [FUR 11], such AlN/GaNND structures with different ND heights were investigated by high-angle annular darkfield STEM with respect to the formation of misfit dislocations.

While for thin NDs with a height of dND = 1.2 nm, no dislocations were found(Figure 1.3a), NDs with dND = 2.5 nm and dND = 3.5 nm (Figure 1.3b) showed theoccurrence of dislocations, often compensated by an inverse dislocation in closevicinity. In such structures, a low number of single dislocations are also found(Figure 1.3b). Here, it should be noted that for an ND diameter of 50 nm, thepresence of a single dislocation already corresponds to an areal density of1.2 × 1010 cm−2 and provides almost complete relaxation of the lattice mismatch[FUR 11, LAN 10].

Strain relaxation due to the formation of dislocations in AlN/GaN ND structureshas also been found by HRTEM analysis reported in [BOU 10]. Here, the authorsfound the formation of dislocations at the interface between the GaN ND and theAlN barrier (depicted in Figure 1.4). The presence of dislocations could explain theobserved deviation of results from geometric phase analysis (GPA) and strainsimulations based on a valence-force model [KEA 66]. Using the latter, a strain


maximum in the ND center and in the edges of the basal plane was estimated but notobserved in GPA.

a)20 nm 10 nm b) 30 nm c)

Figure 1.2. HAADF-STEM image of NWs with 1.7 nm thick GaN NDs surrounded by AlNbarriers. The GaN appears in bright contrast, the surrounding AlN in dark; and (b) HRTEMimage (bright field) of the same sample. The increase in ND diameter along the growth directionis visible. HAADF-STEM of an NW with [Al]bar = 0.41. Lateral growth is observed hereas well, however, with a reduced rate compared to AlN barriers. The dotted line marks theincrease in GaN diameter along the growth axis. (Reprinted with permission from [FUR 11].Copyright © 2011, American Physical Society). For a color version of this figure, seewww.iste.co.uk/consonni/nanowires2.zip

Whereas the composition of AlGaN barriers has mainly been regarded as beingconstant throughout the barrier, a detailed analysis by Pierret et al. has revealed that,depending on the intentional Al-concentration, significant fluctuations of theAl-concentrations can occur in AlGaN NWs grown on a GaN base if the growthtemperature is too high [PIE 13]. This might also result in the destabilization ofAlGaN/GaN interfaces of GaN NDs and influence the emission properties of GaNNDs [RIG 10b].

In summary, the structural properties of GaN NDs embedded in AlGaN/GaNNWHs strongly depend on the impact of mechanical strain by the AlGaN barriersand due to the presence of a lateral AlGaN shell. Both factors influence the possibleformation of dislocations on the one hand and the radial and axial strain distributionon the other hand. They are also expected to affect the optical characteristics ofNWHs via polarization-induced internal electric fields.

1.2.2. Optical properties of axial AlGaN/GaN nanowire heterostructures

After the first reports on the self-assembled catalyst-free synthesis of GaN NWsand particularly of their intriguing optical characteristics [YOS 97, CAL 00], only afew reports dealt with an in-depth optical analysis of such structures[RIS 03, CAL 00]. In 2005, Ristic et al. extended this field of optical properties ofNWs to those of NWHs, realized by embedding GaN NDs between AlxGa1−xNbarriers in a GaN NW [RIS 05a].


a)

10 nmdND = 1.2 nm5 nm

dND = 3.5 nm

b)

10 nm 5 nm

Figure 1.3. HRTEM, frequency filtering on the [1101] lateral planes parallel to the growthaxis and dislocation analysis of GaN/AlN NWH samples with an ND height of dND = 1.2and 3.5 nm, see a) and b), respectively. NDs have been marked with arrows. The presenceof misfit dislocations has been marked with gray bars. Most dislocations appear pair wisethus they are successfully compensated by an inverse dislocation as marked by white dottedellipsoids. Single dislocations are observed with a lower density. (Reprinted with permissionfrom [FUR 11]. Copyright © 2011, American Physical Society). For a color version of thisfigure, see www.iste.co.uk/consonni/nanowires2.zip

Figure 1.4. a) HRTEM image showing a dislocation at the AlN/GaN interface for threesuccessive inclusions; and b) enlargement showing the insertion of an extra (0002)-planein AlN. ([BOU 10]. Copyright © 2010, IOP Publishing. Reproduced by permission of IOPPublishing. All rights reserved)


In contrast to 2D systems, where the lateral extension of the QWs can be regardedas infinite, the NW geometry imposes specific geometric boundary conditions withzero stress along the lateral surface, which results in a strain gradient along the NWdiameter. Hence, to calculate the potential profile along the NW diameter, these strainvariations and the corresponding interplay between strain-dependent contributions ofthe deformation potential and the piezoelectric polarization have to be analyzed. Thismechanism gives rise to the so-called “strain confinement” that was first mentionedin [RIS 05a, RIS 05b] and systematically elaborated in [RIV 07]. For compressivelystrained GaN NDs between AlGaN barriers, the resulting strain-induced piezoelectricpolarization fields cause a carrier confinement in the NW center, whereas thedeformation potential leads to a confinement of carriers at the NW surface, asschematically shown in Figure 1.5 I, II. As the deformation potentials for conductionband and valence band differ for GaN [VUR 03], situations with the confinement forholes (electrons) determined by piezoelectric polarization and that for electrons(holes) by the deformation potential can occur, depending on the Al concentration inthe barriers, [Al]bar, and the ND thickness, dND (see Figure 1.5 III (IV)). Frommechanical considerations it was concluded that for thick (thin) NDs, piezoelectric(I) (deformation potential (II)) confinement prevails, while for intermediatethicknesses the cases III and IV can occur.

By calculation of the corresponding wave functions, it was demonstrated that thespatial separation of electrons and holes can be controlled by the extrinsic parameters[Al]bar and dND of the NWH. The results obtained applying this “strain confinementmodel” were used to explain the observed decrease in PL intensity with decreasing NDthickness as a reduced influence of the piezoelectric polarization and a correspondingspatial separation of electrons (confined in the ND center) and holes (confined at theperiphery) leading to a reduced oscillator strength for radiative recombination and anenhanced non-radiative recombination via surface states [RIV 07]. Thus, the presenceof a strained NW core and the strain relaxation at the lateral surfaces gave rise tointeresting physics in AlGaN/GaN NWHs and stimulated further research activities inthis direction.

In contrast to the ND material, i.e. GaN, the barrier material, AlGaN or AlN, alsoexhibits a lateral growth rate under some growth conditions and hence leads to theformation of a lateral shell consisting of the barrier material [TCH 08, FUR 11]. Inthat case, PA-MBE growth of a multi-ND NW heterostructure results in a 3Dconfinement due to the presence of a lateral AlGaN or AlN shell that decreases inthickness along the growth direction as shown in Figure 1.2(a). The presence of thislateral shell was demonstrated to severely impact the optical emission properties ofAlGaN/GaN NWHs in various respects: the compressive stress on the ND lateralsurface exerted by the AlN shell can significantly alter the band profile in the NDand, depending on its thickness, cause transitions from case (I) to case (II) ofFigure 1.4 along a single multi-ND NWH shown in Figure 1.2a.

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