Springer Series in Materials Science 227

Michael KneisslJens Rass Editors

III-Nitride Ultraviolet EmittersTechnology and Applications

Springer Series in Materials Science

Volume 227

Series editors

Robert Hull, Charlottesville, USAChennupati Jagadish, Canberra, AustraliaRichard M. Osgood, New York, USAJürgen Parisi, Oldenburg, GermanyTae-Yeon Seong, Seoul, Korea, Republic of (South Korea)Shin-ichi Uchida, Tokyo, JapanZhiming M. Wang, Chengdu, ChinaYoshiyuki Kawazoe, Sendai, Japan

The Springer Series in Materials Science covers the complete spectrum ofmaterials physics, including fundamental principles, physical properties, materialstheory and design. Recognizing the increasing importance of materials science infuture device technologies, the book titles in this series reflect the state-of-the-artin understanding and controlling the structure and properties of all importantclasses of materials.

More information about this series at http://www.springer.com/series/856

Michael Kneissl • Jens RassEditors

III-Nitride UltravioletEmittersTechnology and Applications

123

EditorsMichael KneisslInstitute of Solid State PhysicsTechnische Universität BerlinBerlinGermany

and

Ferdinand-Braun-InstitutLeibniz-Institut für HöchstfrequenztechnikBerlinGermany

Jens RassInstitute of Solid State PhysicsTechnische Universität BerlinBerlinGermany

and

Ferdinand-Braun-InstitutLeibniz-Institut für HöchstfrequenztechnikBerlinGermany

ISSN 0933-033X ISSN 2196-2812 (electronic)Springer Series in Materials ScienceISBN 978-3-319-24098-5 ISBN 978-3-319-24100-5 (eBook)DOI 10.1007/978-3-319-24100-5

Library of Congress Control Number: 2015952531

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Preface

In the past two decades, group III-nitride-based ultraviolet light-emitting diodes(UV-LEDs) and their applications have undergone a progressively acceleratingdevelopment. This can be demonstrated by many metrics. For example, the numberof published articles in the area of UV-LEDs is steadily increasing and has reachednearly 1.000 journal articles per year in 2014 (see Fig. 1). However, we have foundthat the fast progress in this field makes it difficult to obtain or maintain a com-prehensive overview of all these very rapidly developing research areas. Manytimes when researchers in the field of semiconductor materials and optoelectronicsdevices describe the applications of UV emitters, large gaps in information arerevealed. On the other hand, developers and engineers who are working in variousareas of applications of UV emitters and detectors often do not comprehend thecomplexities in materials and device development. In order to put all thesedevelopments into a context, various chapters in this book aim to provide a com-prehensive examination of the state of the art in group III-nitride-based materials,ultraviolet emitters, and their applications. It is intended for researchers andgraduate-level students in the area of electrical engineering, material science, andphysics as well as scientists, developers, and engineers in various application fieldsof UV emitters and detectors. The book provides an overview of group III-nitridematerials including their structural, optical, and electronic properties as well as key

Fig. 1 Journal articles ineach year for publicationsunder the keywords “ultravi-olet” and “light emittingdiode” (Source Web ofScience. Retrieved 17 July2015 from apps.webofknowledge.com)

v

features of various optoelectronic components, like UV-LEDs, ultraviolet lasers,and photodetectors. It also offers an introduction to a number of key applications forUV emitters and detectors, including water purification, phototherapy, gas sensing,fluorescence excitation, plant growth lighting, and UV curing. Although eachchapter stands on its own and can be understood without the knowledge of theothers, the organization of the chapters has been deliberately chosen to start withchapters focusing on basic materials properties, followed by chapters on ultravioletdevices, and to conclude with several chapters describing key applications for UVemitters and detectors. In the first chapter, Michael Kneissl provides an introductionto group III-nitride UV emitter technologies and their applications. This is followedby Matthias Bickermann’s review of the growth and structural properties of bulkAlN substrates. In the third chapter, Eberhard Richter, Sylvia Hagedorn, ArneKnauer, and Markus Weyers review the use of sapphire as a substrate fornitride-based light emitters in the UV range, especially the growth of low defectdensity AlGaN templates by hydride vapor phase epitaxy. In Chap. 4 HidekiHirayama discusses crystal growth techniques for low defect density AlN andAlGaN layers on sapphire and presents state-of-the-art performance characteristicof UVC-LEDs on sapphire. In Chap. 5 Shigefusa F. Chichibu, Hideto Miyake,Kazumasa Hiramatsu, and Akira Uedono provide an in-depth discussion of theeffects of dislocations and point defects on the internal quantum efficiency of thenear-band-edge emission in AlGaN-based DUV light-emitting materials.Understanding the role of defects on the IQE of UV-LEDs is critical for improvingthe efficiency and output power of UV-LEDs. In the area of devices, the opticalpolarization and light extraction from UV-LEDs is reviewed by Jens Rass andNeysha Lobo-Ploch. The homoepitaxial growth of UVC-LEDs on bulk AlN sub-strates and their application in water disinfection is examined by James R.Grandusky, Rajul V. Randive, Therese C. Jordan, and Leo J. Schowalter in Chap. 7.Noble Johnson, John Northrup, and Thomas Wunderer discuss optical gain inAlGaN quantum well laser heterostructures and present the state of the art in thedevelopment of AlGaN-based UV laser diodes in Chap. 8, and in Chap. 9 solar andvisible blind UV photodetectors are reviewed by Moritz Brendel, Enrico Pertzsch,Vera Abrosimova, and Torsten Trenkler. In Chap. 10 Marlene Lange, Tim Kolbe,and Martin Jekel examine the application of UVC-LEDs for water purification andin Chap. 11 Uwe Wollina, Bernd Seme, Armin Scheibe, and Emmanuel Gutmanndescribe the application of UV emitters in dermatological phototherapy. In Chap.12 Hartmut Ewald and Martin Degner review the role of UV emitters in gas-sensingapplications and in Chap. 13 Emmanuel Gutmann, Florian Erfurth, Anke Drewitz,Armin Scheibe, and Martina Meinke discuss UV fluorescence detection andspectroscopy systems for applications in chemistry and life sciences. In Chap. 14Monika Schreiner, Inga Mewis, Susanne Neugart, Rita Zrenner, Melanie Wiesner,Johannes Glaab, and M.A.K. Jansen examine the application of UV-LEDs for plantgrowth lighting, especially the triggering of the secondary plant metabolism withUVB light. In the final chapter, the application of LEDs for UV curing is reviewedby Carsten Dreyer and Franziska Mildner.

vi Preface

We like to thank all the authors of the various chapters for their timely andwell-prepared contributions. This book would not have been possible without theircommitment, hard work, and perseverance. We would especially like to thankClaus Ascheron at Springer Science for presenting us with the opportunity to editthis book and for his continued support during the process.

Berlin, Germany Michael KneisslJens Rass

Preface vii

Contents

1 A Brief Review of III-Nitride UV Emitter Technologiesand Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Michael Kneissl1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 UV Light Emitters and Their Applications . . . . . . . . . . . . . . . 31.3 UV-LEDs—State of the Art and the Challenges Ahead . . . . . . 41.4 UV-LEDs—Key Parameters and Device Performance . . . . . . . 71.5 The Role of Defects on the IQE of UV-LEDs . . . . . . . . . . . . 101.6 Current-Injection Efficiency and Operating Voltages

of UV-LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.7 Light Extraction from UV-LEDs . . . . . . . . . . . . . . . . . . . . . . 131.8 Thermal Management and Degradation of UV-LED . . . . . . . . 141.9 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2 Growth and Properties of Bulk AlN Substrates . . . . . . . . . . . . . . 27Matthias Bickermann2.1 Properties and History of AlN Crystals . . . . . . . . . . . . . . . . . 272.2 AlN Bulk Growth by the PVT Method: Theory . . . . . . . . . . . 292.3 AlN Bulk Growth by the PVT Method: Technology . . . . . . . . 312.4 Seeded Growth and Crystal Enlargement . . . . . . . . . . . . . . . . 342.5 Structural Defects in PVT-Grown AlN Bulk Crystals. . . . . . . . 362.6 Impurities and Resulting Properties of AlN Substrates . . . . . . . 382.7 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . 41References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

ix

3 Vapor Phase Epitaxy of AlGaN Base Layers on SapphireSubstrates for Nitride-Based UV-Light Emitters. . . . . . . . . . . . . . 47Eberhard Richter, Sylvia Hagedorn, Arne Knauerand Markus Weyers3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.2 Growth of Al(Ga)N Buffer Layers by MOVPE. . . . . . . . . . . . 493.3 Techniques for MOVPE of Al(Ga)N Base Layers

with Reduced TDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.4 Growth of AlGaN Layers by HVPE . . . . . . . . . . . . . . . . . . . 54

3.4.1 Fundamentals of the HVPE Technique. . . . . . . . . . . . 543.4.2 The Choice of Substrate. . . . . . . . . . . . . . . . . . . . . . 583.4.3 Selected Results from Growth of AlGaN

Layers by HVPE. . . . . . . . . . . . . . . . . . . . . . . . . . . 583.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4 Growth Techniques of AlN/AlGaN and Developmentof High-Efficiency Deep-Ultraviolet Light-Emitting Diodes . . . . . . 75Hideki Hirayama4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.2 Research Background of DUV LEDs. . . . . . . . . . . . . . . . . . . 764.3 Growth Techniques of High-Quality AlN on Sapphire

Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.4 Marked Increase in Internal Quantum Efficiency (IQE) . . . . . . 854.5 222–351 nm AlGaN and InAlGaN DUV LEDs . . . . . . . . . . . 904.6 Increase in Electron Injection Efficiency (EIE) by MQB . . . . . 974.7 Future LED Design for Obtaining High Light

Extraction Efficiency (LEE) . . . . . . . . . . . . . . . . . . . . . . . . . 1044.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5 Impacts of Dislocations and Point Defects on the InternalQuantum Efficiency of the Near-Band-Edge Emissionin AlGaN-Based DUV Light-Emitting Materials . . . . . . . . . . . . . . 115Shigefusa F. Chichibu, Hideto Miyake, Kazumasa Hiramtsuand Akira Uedono5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1185.3 Impacts of Impurities and Point Defects

on the Near-Band-Edge Luminescence Dynamics of AlN. . . . . 1205.4 Effective Radiative Lifetime of the Near-Band-Edge

Emission in AlxGa1−xN Films . . . . . . . . . . . . . . . . . . . . . . . . 125

x Contents

5.5 Impacts of Si-Doping and Resultant Cation VacancyFormation on the Luminescence Dynamicsof the Near-Band-Edge Emission in Al0.6Ga0.4N FilmsGrown on AlN Templates . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

6 Optical Polarization and Light Extraction from UV LEDs . . . . . . 137Jens Rass and Neysha Lobo-Ploch6.1 Light Extraction from UV LEDs . . . . . . . . . . . . . . . . . . . . . . 1386.2 Optical Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

6.2.1 Factors Influencing the Light PolarizationSwitching in AlGaN Layers . . . . . . . . . . . . . . . . . . . 143

6.2.2 Optical Polarization Dependence on SubstrateOrientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

6.2.3 Influence of the Optical Polarization on the LightExtraction Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 149

6.3 Concepts for Improved Light Extraction. . . . . . . . . . . . . . . . . 1526.3.1 Contact Materials and Design . . . . . . . . . . . . . . . . . . 1526.3.2 Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . 1576.3.3 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

7 Fabrication of High Performance UVC LEDson Aluminum-Nitride Semiconductor Substrates and TheirPotential Application in Point-of-Use Water DisinfectionSystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171James R. Grandusky, Rajul V. Randive, Therese C. Jordanand Leo J. Schowalter7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

7.1.1 Types of UVC Light Sources . . . . . . . . . . . . . . . . . . 1727.1.2 What Is UVC Light? . . . . . . . . . . . . . . . . . . . . . . . . 1737.1.3 How Does Germicidal UV Work . . . . . . . . . . . . . . . 174

7.2 Fabrication of UVC LEDs on AlN Substrates . . . . . . . . . . . . . 1757.3 Leveraging Performance Gains in UVC LEDs

for POU Water Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . 1837.3.1 Effect of UVT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1837.3.2 Design Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . 1867.3.3 Modeling Flow Cells . . . . . . . . . . . . . . . . . . . . . . . . 1867.3.4 Example Flow Analysis . . . . . . . . . . . . . . . . . . . . . . 1877.3.5 Working with UVC Light . . . . . . . . . . . . . . . . . . . . 189

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

Contents xi

8 AlGaN-Based Ultraviolet Laser Diodes . . . . . . . . . . . . . . . . . . . . 193Thomas Wunderer, John E. Northrup and Noble M. Johnson8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1948.2 Growth on Bulk AlN for Highest Material Quality . . . . . . . . . 196

8.2.1 Bulk AlN Substrate . . . . . . . . . . . . . . . . . . . . . . . . . 1968.2.2 Homoepitaxial AlN . . . . . . . . . . . . . . . . . . . . . . . . . 1978.2.3 AlGaN Laser Heterostructure . . . . . . . . . . . . . . . . . . 1988.2.4 Multiple Quantum Well Active Zone . . . . . . . . . . . . . 199

8.3 High Current Capability in High Band Gap AlGaNMaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

8.4 High Injection Efficiency at High Current Levels . . . . . . . . . . 2058.5 Optically Pumped UV Lasers . . . . . . . . . . . . . . . . . . . . . . . . 2088.6 Alternative Concepts for Compact Deep-UV III-N Lasers . . . . 211

8.6.1 Electron-Beam Pumped Laser . . . . . . . . . . . . . . . . . . 2128.6.2 InGaN-Based VECSEL + Second Harmonic

Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2138.7 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 214References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

9 Solar- and Visible-Blind AlGaN Photodetectors . . . . . . . . . . . . . . 219Moritz Brendel, Enrico Pertzsch, Vera Abrosimova,Torsten Trenkler and Markus Weyers9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2199.2 Basics of Photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

9.2.1 Characteristic Parameters and Phenomena . . . . . . . . . 2229.2.2 Various Types of Semiconductor Photodetectors . . . . . 232

9.3 III-Nitrides for Solid-State UV Photodetection . . . . . . . . . . . . 2429.3.1 AlGaN-Based Photoconductor. . . . . . . . . . . . . . . . . . 2449.3.2 AlGaN-Based MSM Photodetector . . . . . . . . . . . . . . 2459.3.3 AlGaN-Based Schottky Barrier Photodiode. . . . . . . . . 2479.3.4 AlGaN-Based p-i-n Photodiode. . . . . . . . . . . . . . . . . 2489.3.5 AlGaN-Based Avalanche Photodetector . . . . . . . . . . . 2509.3.6 AlGaN-Based Photocathode . . . . . . . . . . . . . . . . . . . 2529.3.7 III-Nitride-Based Devices of High Integration

Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2549.4 Present Status of Wide Bandgap Photodetectors . . . . . . . . . . . 2559.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 257References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

10 Ultraviolet Light-Emitting Diodes for Water Disinfection . . . . . . . 267Marlene A. Lange, Tim Kolbe and Martin Jekel10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26710.2 Basic Principles of UV Disinfection . . . . . . . . . . . . . . . . . . . 268

10.2.1 Factors Influencing UV Fluence . . . . . . . . . . . . . . . . 271

xii Contents

10.2.2 Modeling and Validation of UV ReactorPerformance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

10.3 Case Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27410.3.1 Proposal for an Experimental Setup

to Test UV LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . 27510.3.2 Test Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 27810.3.3 Results of Tests Conducted with UV LEDs . . . . . . . . 282

10.4 Potential of UV LEDs for Water Disinfection . . . . . . . . . . . . . 287References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

11 Application of UV Emitters in Dermatological Phototherapy. . . . . 293Uwe Wollina, Bernd Seme, Armin Scheibeand Emmanuel Gutmann11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29311.2 Sources for UV Phototherapy . . . . . . . . . . . . . . . . . . . . . . . . 294

11.2.1 Natural Sunlight . . . . . . . . . . . . . . . . . . . . . . . . . . . 29511.2.2 Gas Discharge Lamps . . . . . . . . . . . . . . . . . . . . . . . 29611.2.3 Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29811.2.4 UV-LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

11.3 Variants of Dermatological UV Phototherapy . . . . . . . . . . . . . 30011.3.1 Psoralen Plus UVA (PUVA) Therapy . . . . . . . . . . . . 30011.3.2 Broadband UVB (BB-UVB) Therapy . . . . . . . . . . . . 30211.3.3 Narrowband UVB (NB-UVB) Therapy . . . . . . . . . . . 30211.3.4 UVA-1 Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . 30311.3.5 Targeted UV Phototherapy . . . . . . . . . . . . . . . . . . . . 30311.3.6 Extracorporeal Photochemotherapy (ECP) . . . . . . . . . 304

11.4 Mechanisms of Action for Major DermatologicalIndications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30611.4.1 Psoriasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30711.4.2 Atopic Dermatitis . . . . . . . . . . . . . . . . . . . . . . . . . . 30711.4.3 Vitiligo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30811.4.4 Cutaneous T-Cell Lymphomas . . . . . . . . . . . . . . . . . 30811.4.5 Lichen Planus and Alopecia Areata . . . . . . . . . . . . . . 30911.4.6 Systemic Sclerosis and Morphoea . . . . . . . . . . . . . . . 30911.4.7 Graft-Versus-Host Disease . . . . . . . . . . . . . . . . . . . . 30911.4.8 Polymorphic Light Eruption . . . . . . . . . . . . . . . . . . . 310

11.5 Clinical Studies with Novel UV Emitters . . . . . . . . . . . . . . . . 31011.5.1 Study with an Electrodeless Excimer Lamp . . . . . . . . 31011.5.2 Study with UV-LEDs . . . . . . . . . . . . . . . . . . . . . . . 312

11.6 Summary and Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

Contents xiii

12 UV Emitters in Gas Sensing Applications . . . . . . . . . . . . . . . . . . 321Martin Degner and Hartmut Ewald12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32112.2 Light Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 32412.3 Absorption Spectroscopic Systems . . . . . . . . . . . . . . . . . . . . 32912.4 Light Sources for UV Spectroscopy. . . . . . . . . . . . . . . . . . . . 33312.5 Optical and Electrical Properties of the LEDs

for Spectroscopy Application . . . . . . . . . . . . . . . . . . . . . . . . 33712.6 Application of UV-LED Absorption Spectroscopy. . . . . . . . . . 342

12.6.1 Ozone Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34212.6.2 Ozone Sensor Design . . . . . . . . . . . . . . . . . . . . . . . 34212.6.3 Measurement Arrangement . . . . . . . . . . . . . . . . . . . . 34312.6.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34312.6.5 SO2 and NO2 Sensor . . . . . . . . . . . . . . . . . . . . . . . . 34312.6.6 Sensor Design for SO2/NO2 Exhaust Gas Sensor . . . . 34412.6.7 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . 345

12.7 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . 348References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

13 UV Fluorescence Detection and Spectroscopy in Chemistryand Life Sciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351Emmanuel Gutmann, Florian Erfurth, Anke Drewitz,Armin Scheibe and Martina C. Meinke13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35113.2 Fundamentals and Apparative Aspects of Fluorescence

Detection and Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 35313.3 Fluorescence in Lab-Based Instrumental Analysis . . . . . . . . . . 35913.4 Fluorescence Chemical Sensing for Environmental

Monitoring and Bioanalytics. . . . . . . . . . . . . . . . . . . . . . . . . 36113.5 Detection of Microorganisms Using Autofluorescence . . . . . . . 36913.6 Fluorescence in Medical Diagnosis of Skin Diseases . . . . . . . . 37413.7 Summary and Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

14 UV-B Elicitation of Secondary Plant Metabolites . . . . . . . . . . . . . 387Monika Schreiner, Inga Mewis, Susanne Neugart, Rita Zrenner,Johannes Glaab, Melanie Wiesner and Marcel A.K. Jansen14.1 Nature and Occurrence of Secondary Plant Metabolites . . . . . . 38814.2 Nutritional Physiology of Secondary Plant Metabolites . . . . . . 38914.3 Association Between Fruit and Vegetable Consumption

and Chronic Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39014.4 Secondary Plant Metabolites Within the Plant–Environment

Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39114.4.1 UV-B Perception and Signaling in the Plant . . . . . . . . 391

xiv Contents

14.4.2 UV-B as Stressor and Plant Regulator . . . . . . . . . . . . 39314.5 Structure-Differentiated Response to UV-B . . . . . . . . . . . . . . 394

14.5.1 Flavonoids and Other Phenolics . . . . . . . . . . . . . . . . 39514.5.2 Glucosinolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

14.6 Tailor-Made UV-B Induction of Secondary PlantMetabolites by UV-B LEDs . . . . . . . . . . . . . . . . . . . . . . . . . 40214.6.1 Current State of Research: UV-B Light-Emitting

Diodes for Plant Lightning . . . . . . . . . . . . . . . . . . . . 40214.6.2 Advantages of UV-B LEDs for Targeted Triggering

of Plant Properties. . . . . . . . . . . . . . . . . . . . . . . . . . 40214.6.3 Experimental Setup for Targeted Triggering

of Plant Properties by UV-B LEDs . . . . . . . . . . . . . . 40414.7 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406

15 Application of LEDs for UV-Curing . . . . . . . . . . . . . . . . . . . . . . 415Christian Dreyer and Franziska Mildner15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41515.2 Light Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41715.3 Chemistry and Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 41915.4 Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42215.5 Medical Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42415.6 Coatings, Inks, and Printing . . . . . . . . . . . . . . . . . . . . . . . . . 42715.7 Stereolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43015.8 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . 431References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432

Erratum to: Ultraviolet Light-Emitting Diodesfor Water Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E1Marlene A. Lange, Tim Kolbe and Martin Jekel

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435

.

Contents xv

Contributors

Vera Abrosimova JENOPTIK Polymer Systems GmbH, Berlin, Germany

Matthias Bickermann Leibniz Institute for Crystal Growth (IKZ), Berlin,Germany; Institute for Chemistry, Technische Universität, Berlin, Germany

Moritz Brendel Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany

Shigefusa F. Chichibu Institute of Multidisciplinary Research for AdvancedMaterials, Tohoku University, Sendai, Japan

Martin Degner Institute of General Electrical Engineering, University of Rostock,Rostock, Germany

Anke Drewitz Department Photonics and Sensorics, Gesellschaft zur Förderungvon Medizin-, Bio- und Umwelttechnologien e.V. (GMBU), Jena, Germany

Christian Dreyer Fraunhofer-Einrichtung für Polymermaterialien und CompositePYCO, Teltow, Germany

Florian Erfurth Department Photonics and Sensorics, Gesellschaft zur Förderungvon Medizin-, Bio- und Umwelttechnologien e.V. (GMBU), Jena, Germany

Hartmut Ewald Institute of General Electrical Engineering, University ofRostock, Rostock, Germany

Johannes Glaab Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany

James R. Grandusky Crystal IS, Green Island, NY, USA

Emmanuel Gutmann Department Photonics and Sensorics, Gesellschaft zurFörderung von Medizin-, Bio- und Umwelttechnologien e.V. (GMBU), Jena,Germany

xvii

Sylvia Hagedorn Ferdinand-Braun-Institut, Leibniz-Institut fürHöchstfrequenztechnik,Berlin, Germany

Kazumasa Hiramtsu Department of Electrical and Electronic Engineering, MieUniversity, Tsu, Japan

Hideki Hirayama RIKEN, Quantum Optodevice Laboratory, Saitama, Japan

Marcel A.K. Jansen School of Biological, Environmental and Earth Sciences,University College Cork, Distillery Field, Cork, Ireland

Martin Jekel Technische Universität Berlin, Fachgebiet Wasserreinhaltung,Berlin, Germany

Noble M. Johnson PARC Palo Alto Research Center, Inc., Palo Alto, CA, USA

Therese C. Jordan Crystal IS, Green Island, NY, USA

Arne Knauer Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany

Michael Kneissl Institute of Solid State Physics, Technische Universität, Berlin,Germany; Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany

Tim Kolbe Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany

Marlene A. Lange Technische Universität Berlin, Fachgebiet Wasserreinhaltung,Berlin, Germany

Neysha Lobo-Ploch Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany

Martina C. Meinke Center of Experimental and Applied Cutaneous Physiology(CCP), Department of Dermatology, Venerology and Allergology, Charité –

Universitätsmedizin Berlin, Berlin, Germany

Inga Mewis Department Plant Quality, Leibniz-Institute of Vegetable andOrnamental Crops Großbeeren and Erfurt e. V., Großbeeren, Germany

Franziska Mildner Fraunhofer-Einrichtung für Polymermaterialien und CompositePYCO, Teltow, Germany

Hideto Miyake Department of Electrical and Electronic Engineering, MieUniversity, Tsu, Japan

Susanne Neugart Department Plant Quality, Leibniz-Institute of Vegetable andOrnamental Crops Großbeeren and Erfurt e. V., Großbeeren, Germany

John E. Northrup PARC Palo Alto Research Center, Inc., Palo Alto, CA, USA

Enrico Pertzsch JENOPTIK Polymer Systems GmbH, Berlin, Germany

xviii Contributors

Rajul V. Randive Crystal IS, Green Island, NY, USA

Jens Rass Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany; Institute of Solid State Physics, Technische Universität Berlin,Berlin, Germany

Eberhard Richter Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany

Armin Scheibe Department Photonics and Sensorics, Gesellschaft zur Förderungvon Medizin-, Bio- und Umwelttechnologien e.V. (GMBU), Jena, Germany

Leo J. Schowalter Crystal IS, Green Island, NY, USA

Monika Schreiner Department Plant Quality, Leibniz-Institute of Vegetable andOrnamental Crops Großbeeren and Erfurt e. V., Großbeeren, Germany

Bernd Seme Department Photonics and Sensorics, Gesellschaft zur Förderung vonMedizin-, Bio- und Umwelttechnologien e.V. (GMBU), Jena, Germany

Torsten Trenkler JENOPTIK Polymer Systems GmbH, Berlin, Germany

Akira Uedono Division of Applied Physics, Faculty of Pure and Applied Science,University of Tsukuba, Tsukuba, Ibaraki, Japan

Markus Weyers Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik,Berlin, Germany

Melanie Wiesner Department Plant Quality, Leibniz-Institute of Vegetable andOrnamental Crops Großbeeren and Erfurt e. V., Großbeeren, Germany

Uwe Wollina Department of Dermatology and Allergology, Hospital Dresden-Friedrichstadt, Academic Teaching Hospital of the Technical University ofDresden, Dresden, Germany

Thomas Wunderer PARC Palo Alto Research Center, Inc., Palo Alto, CA, USA

Rita Zrenner Department Plant Quality, Leibniz-Institute of Vegetable andOrnamental Crops Großbeeren and Erfurt e. V., Großbeeren, Germany

Contributors xix

Chapter 1A Brief Review of III-Nitride UV EmitterTechnologies and Their Applications

Michael Kneissl

Abstract This chapter provides a brief introduction to group III-nitride ultravioletlight emitting diode (LED) technologies and an overview of a number of keyapplication areas for UV-LEDs. It covers the state of the art of UV-LEDs as well asurvey of novel approaches for the development of high performance UV lightemitters.

1.1 Background

More than two decades have passed since a series of fundamental breakthroughs inthe area of gallium nitride (GaN) semiconductor materials led to the first demon-stration of high efficiency and high brightness blue LEDs [1–4]. Today, GaN-basedblue and white LEDs have achieved efficiencies surpassing that of any conventionallight source and billions of LEDs are fabricated every week. This tremendousprogress has been accompanied by increasing penetration of blue and white LEDsinto new applications and larger markets, starting with backlighting of LCD dis-plays in mobile phones, computer screens, and TVs, followed by automotive andstreet lighting, and finally conquering lighting applications in all places. Therefore,it is only fitting that in 2014 the Royal Swedish Academy of Sciences awarded theNobel Prize for Physics to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura“for the invention of efficient blue light emitting diodes which has enabled brightand energy-saving white light sources” [5]. This example shows how breakthroughsin development of new semiconductor materials and device technologies can lead toa paradigm shift of a complete industry, in this case solid-state lighting. The RoyalSwedish Academy of Sciences stated in their press release “in the spirit of Alfred

M. Kneissl (&)Institute of Solid State Physics, Technische Universität, Berlin, Germanye-mail: kneissl@physik.tu-berlin.de

M. KneisslFerdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany

© Springer International Publishing Switzerland 2016M. Kneissl and J. Rass (eds.), III-Nitride Ultraviolet Emitters,Springer Series in Materials Science 227, DOI 10.1007/978-3-319-24100-5_1

1

Nobel the Prize rewards an invention of greatest benefit to mankind; using blueLEDs, white light can be created in a new way” [5]. Despite these tremendousachievements, we have to date only utilized a very narrow sliver of the emissionspectrum that gallium nitride devices are capable of generating. By adding alu-minum nitride (AlN) to the GaN alloy system, the emission wavelength ofAlGaN-based LEDs can be tuned over almost the entire UVA (400–320 nm), UVB(320–280 nm), and UVC (280–200 nm) spectral range with emission wavelength asshort as 210 nm. Although the efficiencies and power levels of AlGaN-basedUV-LEDs today are still modest (see Fig. 1.1) compared to their visible wavelengthcounterparts, another paradigm shift in the area of semiconductor-based UV sourcesis just around the corner. Without doubt the efficiencies and output powers ofUV-LEDs will continue to improve over the coming years and at the same time thecosts per milliwatt of UV light from LEDs will drop significantly. For manyhigh-end applications, e.g., in the area of medical diagnostics, phototherapy, andsensing, UV-LEDs are already competitive since they enable major advances insystem design and performance and only contribute a small fraction of the overallcost. As the UV-LED performance improves over time it is clear that many moreapplication areas will follow.

In order to put all these developments into a context, this chapter will provide acomprehensive overview of the state of the art in group III-nitride-based materials,

Fig. 1.1 Reported external quantum efficiencies for AlGaN, InAlGaN-, and InGaN quantum wellLEDs emitting in the UV spectral range [6–32]

2 M. Kneissl

ultraviolet emitters, and their applications. Several key applications for UV emittersand detectors will be discussed, including water purification, phototherapy, gassensing, fluorescence excitation, plant growth lighting, and UV curing. In addition,the optical, electronic, and structural properties of group III-nitride materials as wellas the design and key performance parameters of UV-LEDs will be reviewed.Furthermore, the most important technological challenges for the realization of highefficiency, high power UV light emitters will be examined and a number ofapproaches to overcome these hurdles will be presented.

1.2 UV Light Emitters and Their Applications

Compared to conventional UV sources, such as low and medium pressure mercurylamps [33], ultraviolet LEDs offer a number of advantages. UV-LEDs are extremelyrobust, compact, environmentally friendly, and can exhibit very long lifetimes. Theydo not require any warm-up times and can be switched on and off within a few tensof nanoseconds or even faster. These unique properties designate UV-LEDs as a keyenabling component for a number of new applications that cannot be realized withconventional UV sources. For example, the ability to rapidly turn UV-LEDs on andoff gives rise to advanced measuring detection algorithms and improved baselinecalibrations that can significantly enhance the sensitivity of the system. By closelyspacing different UV-LEDs, it is also possible to realize multi-wavelength modules,which may be able to identify specific gases, biomolecules, or organisms. UV-LEDsare operated at moderate DC voltages, which make them ideally suited for batteryoperation or solar cell powering. They are also easily electronically dimmable,which can be an important energy saving feature, e.g., in water purification appli-cations, where the required UV radiation dose strongly depends on the volume of thewater flow. But most importantly, their emission can be tuned to cover any wave-length in the UVA (400–320 nm), UVB (320–280 nm), and UVC (280–200 nm)spectral range. A compilation of different applications for UV-LEDs is shown inFig. 1.2. Important applications in the UVA spectral range include UV curing ofinks, paints, coatings, resins, polymers, and adhesives as well as 3D-printing forrapid prototyping and lightweight construction. Additional applications can be foundin the area of sensing, e.g., whitening agents or so-called blankophores, detection ofsecurity features, e.g., in ID cards and banknotes, and medical applications likeblood gas analysis. Key applications in the UV-B are phototherapy [34, 35], espe-cially the treatment of psoriasis and vitiligo, as well as plant growth lighting, e.g., forthe targeted triggering of secondary plant metabolites [36]. High volume applica-tions in the UVC are water purification [37–42], e.g., point-of-use systems,wastewater treatment, and recycling as well as disinfection of medical equipmentand food. There are also a number of sensing applications [42–44] for UVB- andUVC-LEDs since many gases (e.g., SO2, NOx, NH3) and biomolecules exhibitabsorption bands in these spectral regions, including tryptophan, NADH, tyrosine,DNA, and RNA. UVC-LEDs can also be used for non-line-of-sight communication

1 A Brief Review of III-Nitride UV Emitter Technologies … 3

[45] and are also of interest for basic science experiments in the area of gravitationalsensors, e.g., for enabling the charge management systems in the ESA/NASA laserinterferometer space antenna (LISA) mission [46]. The foundation of all theseapplications, however, is the development of high-efficiency and high powerAlGaN-based LEDs emitting in the UVA, UV-B, and UVC spectral range.

Thus, the economic and societal benefits resulting from the development of awide range of applications that require UV-LED technologies are perfectly obvious.Latest market studies predict rapid technological advances in the development ofsemiconductor-based UV-LED sources. For the world market of UV-LED com-ponents alone an annual growth rate of more than 28 % is being forecasted by YoleDéveloppement, reaching a total volume of US$ 520 million by 2019 [47].

1.3 UV-LEDs—State of the Art and the Challenges Ahead

A large number of research groups have reported AlGaN-based light emittingdiodes in the ultraviolet spectral range [6–32] and several companies have started tocommercialize UV-LED devices. In the 365–400 nm spectral range, many

Fig. 1.2 Applications of UVA (400–320 nm), UVB (320–280 nm), and UVC (280–200 nm)LEDs

4 M. Kneissl

companies are already offering UVA-LEDs with excellent performance. Companiesinclude for example, Nichia (Japan), Nitride Semiconductors (Japan), Epitex(Japan), UVET Electronics (China), Tekcore (Taiwan), Seoul Opto Device (Korea),SemiLEDs (USA), Luminus Devices (USA), Lumex (USA), and LED Engine(USA). Access to commercial UVB-LEDs is much sparser with companies SETi(USA), Dowa (Japan), Nikkiso (Japan), and UVphotonics (Germany) offeringdevices in the 320–280 nm range. Similarly in the UVC, only a few companies arecurrently selling LED devices, including SETi (USA), Nitek (USA),Crystal-IS/Asahi-Kasei (USA/Japan), Hexatech (USA), UVphotonics (Germany),Nikkiso (Japan), and LG Electronics (Korea). Whereas the performance forUVA-LEDs, especially in the wavelength range 365–400 nm is already suitable formany applications, the external quantum efficiencies (EQE) of most UVB and UVCemitters are still in the single digit percentage range. Currently, most UVB- andUVC-LEDs provide only a couple of milliwatts of output power and lifetimes areoften limited to less than a thousand hours [30, 41]. There are multiple reasonswhich limit the performance of group III-nitride-based deep UV-LEDs and nearlyevery layer in the heterostructure poses a different challenge as illustrated inFig. 1.3.

Most UV-LED heterostructures are grown on (0001) oriented c-plane sapphiresubstrates. Sapphire substrate are readily available in various sizes with diametersranging from 2 to 8-in. for commercial wafers and 12-in. diameter sapphire sub-strates already being demonstrated as R&D samples [49]. Due to the large volumesof sapphire substrates that are being used for blue LED production, sapphire wafershave become very inexpensive. Most importantly, sapphire is fully transparentacross the entire UVA, UVB, and UVC spectral range due its large bandgap energyof 8.8 eV. Most AlGaN-based LED heterostructures are grown by metalorganic

Fig. 1.3 Schematic of an (In)AlGaN MQW UV-LED heterostructure [48]

1 A Brief Review of III-Nitride UV Emitter Technologies … 5

vapor phase epitaxy (MOVPE) using trimethylgallium (TMGa), triethylgallium(TEGa), trimethyaluminum (TMAl), and trimethylindium (TMIn) as group-IIIprecursor, as well as ammonia (NH3) for the group-V element. Silane (SiH4) and di(cyclopentadienyl)magnesium (Cp2Mg) are used as n- and p-doping sources andhydrogen or nitrogen as carrier gas. Typical growth temperatures for the depositionof AlGaN layers are in the range of 1000–1200 °C, but can be as high as 1500 °Cfor the deposition of the AlN base layer [50–52]. Since the sapphire substrate iselectrically insulating an Si-doped n-AlGaN current spreading layer is subsequentlydeposited to enable uniform lateral current-spreading and injection of electrons intothe AlGaN multiple quantum well (MQW) active region. In order to accommodatethe difference in the a-lattice constants of AlGaN and AlN (see Fig. 1.6), an AlxGa1−xN transition layer is inserted between the AlN base and the AlGaN currentspreading layer for strain management. The light emitting active region typicallycomprises a few nanometer thick AlGaN or InAlGaN quantum wells(QWs) separated by (In)AlGaN quantum barriers. The emission wavelength of theLEDs is primarily determined by the aluminum and indium mole fractions in theAlGaN or InAlGaN quantum wells as can be seen in Fig. 1.4a.

Additional contributions to the transition energies and consequently the emissionwavelength arise from the electron and hole confinement energies in the quantumwell and depend on the QW width as well as the quantum barrier height, i.e., thebarrier composition. Figure 1.4b shows the emission wavelength of Al0.72Ga0.28Ntriple quantum well LEDs with Al0.82Ga0.18N barriers and different QW widths.Since almost all UV-LEDs are grown on the polar c-plane of the wurtzite crystal,strong spontaneous and piezoelectric polarization charges arise at AlGaNheterostructure interfaces, leading to polarization fields in the QWs and conse-quently to a redshift in emission wavelength. This is effect is commonly known as

Fig. 1.4 a Emission spectra of AlGaN, InAlGaN, and InGaN quantum well LEDs emitting in theUVA, UVB, and UVC spectral range. b Emission wavelength of Al0.72Ga0.28N quantum wellLEDs for different QW width

6 M. Kneissl

the quantum confined Stark effect (QCSE) [53]. The magnitude of the polarizationfield will largely depend on the difference in the aluminum mole fractions betweenthe AlGaN quantum wells and AlGaN barriers as well as the strain state with theQW stack.

The active region is capped with an Mg-doped p-AlGaN electron blocking layer(EBL), followed by an Mg-doped p-AlGaN short period superlattice layer (SPSL)and a highly Mg-doped p-GaN ohmic contact layer. The function of the p-AlGaNelectron blocking layer (EBL) is to facilitate efficient hole injection into the QWactive region while preventing electron leakage from the AlGaN QW active regioninto the p-layers. Ideally, the Mg-doped p-AlGaN SPSL is UV-transparent in orderto prevent the absorption of UV photons. Looking at the different implementationsof UV-LEDs in the literature, of course, there are many deviations from this basicUV-LED structure. For example, instead of a p-AlGaN short period superlatticelayer, an Mg-doped bulk p-AlGaN layer can also be used. Some LEDheterostructure designs utilize a gradient of the aluminum mole fraction in thep-AlGaN layer in order to enhance the hole carrier concentrations through polar-ization doping [54]. Nevertheless, the key elements described above and in Fig. 1.3can be found in most UV-LEDs. While metalorganic vapor phase epitaxy(MOVPE) is the dominant growth technique for the realization of UV-LEDs,AlGaN-based UV light emitters have also been demonstrated using molecular beamepitaxy (MBE) [55] or hydride vapor phase epitaxy (HVPE) [56].

1.4 UV-LEDs—Key Parameters and Device Performance

There are several key parameters that characterize the performance characteristicsof UV-LEDs. The conversion efficiency by which the electrical input power isconverted into UV light output is described by the so-called wall plug efficiency(WPE) or power conversion efficiency (PCE). In general, the wall plug efficiency oflight emitting diodes is defined as the ratio of the total UV output power to the inputelectrical power, i.e., the drive current times the operating voltage of the device.This can be described by the following relationships:

WPE ¼ Pout

I � V ¼ gEQE�hxe � V ;

where Pout denotes the output power of the UV emission, I the drive current of theLED, V the operating voltage for the LED, hω the photon energy, and ηEQE theexternal quantum efficiency (EQE) of the LED. The external quantum efficiency ofthe UV-LED can be easily determined by measuring the total UV output power Pout

and dividing this by the drive current I and the photon energy hω according to thefollowing equation:

1 A Brief Review of III-Nitride UV Emitter Technologies … 7

gEQE ¼ e � Pout

I � �hxIn other words, the EQE can also be described as the ratio of the number of UV

photons emitted from the LED to the number of charge carriers injected into thedevice. The external quantum efficiency itself can be described as a product of theinjection efficiency ηinj, the radiative recombination efficiency ηrad, and the lightextraction efficiency ηext as also shown in the following formula:

gEQE ¼ ginj � grad � gext ¼ gIQE � gext

Therefore, the injection efficiency ηinj describes the ratio of charge carriers, i.e.,electrons and holes that reach the QW active region versus the total current beinginjected into the device. The radiative recombination efficiency ηrad is the fractionof all electron–hole pairs that recombine radiatively in the QW active region, i.e.,producing UV photons. The internal quantum efficiency ηIQE (or IQE) can becalculated as the product of the radiative recombination efficiency ηrad and theinjection efficiency ηinj. It is possible to determine the IQE by temperature andexcitation power-dependent photoluminescence measurements [57], although greatcare has to be taken in the interpretation of the data. The light extraction efficiencyηext is defined as the fraction of UV photons that can be extracted from the LEDcompared to all the UV photons that are generated in the active region. Theradiative recombination efficiency ηrad can be expressed as the ratio of the radiativerecombination rate Rsp versus the sum of radiative and nonradiative recombinationRnr rates and is described by the following equation:

grad ¼ Rsp

Rsp þ Rnr

The nonradiative recombination rate can be expressed as the sum of theShockley–Read–Hall recombination term and the Auger recombination term asfollows:

Rnr ¼ A � nþC � n3;

where A represents the Shockley–Read–Hall (SRH) recombination coefficient,C the Auger recombination coefficient, and the parameter n the charge carrierdensity in QW. The Shockley–Read–Hall recombination coefficient A is inverseproportional to the SRH recombination lifetime, which strongly depends on thedefect density in the materials. Nonradiative recombination lifetimes as short as afew tens of ps have been measured for highly defective AlGaN layers. In contrast,nonradiative recombination lifetimes in the ns range have been demonstrated forAlGaN quantum well emitters on low defect density bulk AlN substrates [58, 59].The magnitude of the Auger recombination coefficient C for III-nitride materials isstill being heavily debated [60–65] and values for the C coefficient for blue-violet

8 M. Kneissl

LEDs range between 1 × 10−31 and 2 × 10−30 cm6 s−1. Measurements to determinethe C coefficient for UV emitter are even more rudimentary [66]; also theoreticalmodels suggest that the Auger coefficient should become smaller and smaller forshorter emission wavelength [67].

Finally, the radiative recombination or spontaneous recombination rate Rsp canbe expressed as

Rsp ¼ B � n2;

with B representing the bimolecular recombination coefficient and n the chargecarrier density in QW. The B coefficient will strongly depend on the design of theactive region, e.g., the quantum well width, quantum barrier height, the strain stateof the AlGaN QWs, and the magnitude of the polarization field in the QW. Typicalvalues for the B coefficient are in the range of 2 × 10−11 cm3 s−1 [66, 68].

Today, the best InGaN-based blue LEDs boast external quantum efficiencies(EQE) of 84 % and wall plug efficiencies (WPE) of 81 % [69] and commercial blueLEDs exhibit EQEs of 69 % and WPEs of 55 %, respectively [70]. Theseextraordinary values can only be achieved by perfecting all constituents that con-tribute to the WPE and EQE. Due the development of thin film LED technologies,highly reflective metal contacts, and advanced packaging methods, light extractionefficiencies in modern blue LEDs exceed 85 %. Furthermore, low defect densityGaN/sapphire templates and highly efficient InGaN quantum well active regionslead to internal quantum efficiencies near 90 % [70]. Mg-doped GaN AlGaNelectron blocking layers and low resistance n-GaN current spreading layers lead toinjection efficiencies well beyond 90 % [70]. And finally, refined Mg- andSi-doping profiles and optimized ohmic contacts guarantee very low operatingvoltages, which translate into high WPE. However, the extraordinary characteristicsof blue LEDs do not translate into similar performance for shorter wavelengthLEDs. Whereas UVA-LEDs near 365 nm still exhibit EQEs beyond 30 %, at leastin research prototypes, typical LEDs in the UV-B and UVC spectral range exhibitexternal quantum efficiencies of 1–3 %, as can be seen in Fig. 1.1. A number offactors contribute to this overall small value, including poor radiative recombinationefficiencies and modest injection efficiencies, resulting in low internal quantumefficiencies as well as inferior light extraction and high operating voltages. From theanalysis of state-of-the-art UVC-LED one can estimate the light extraction effi-ciency to be below 10 %, and the radiative and injection efficiencies to be some-where around 50 %, respectively. In the following section the main causescontributing to these behaviors will be explained and different options to overcomethese deficiencies will be discussed.

1 A Brief Review of III-Nitride UV Emitter Technologies … 9

1.5 The Role of Defects on the IQE of UV-LEDs

A significant contribution to the low IQE in UV emitters can be attributed to therelatively high defect densities in AlN and AlGaN materials. For example, growthof AlN layers on (0001) sapphire substrates typically yields dislocation densities inthe range of 1010 cm−2 [50–52]. These threading dislocations form nonradiativerecombination pathways for the injected carriers leading to significantly reducedinternal (IQE) and external quantum efficiencies (EQE) [57, 70, 71]. Differentapproaches to reduce the dislocation densities in AlGaN and AlN layers on sapphiresubstrates have been demonstrated, including epitaxial lateral overgrowth(ELO) yielding defect densities in the mid 108 cm−2 range [72–76]. Otherapproaches to reduce the defect density include the growth on nano-patternedsapphire substrates [29, 77], the use of short period AlGaN superlattices [78] aswell as low temperature AlN or SiNx interlayers [18, 79]. To estimate the effect ofdislocations on the internal quantum efficiency of UV-LEDs the IQE was simulatedbased on a model initially published by Karpov et al. [71]. In order to compute theIQE, the radiative and noradiative recombination rates are calculated as a functionof the threading dislocation density and injection current density [23]. The modelassumes that threading dislocations form deep levels within the band gap that act asnonradiative recombination centers for electrons and holes. There are a number ofpotential defects in AlGaN wurtzite semiconductor materials that form deep levels,e.g., screw, edge, or mixed type threading dislocations as well as N- orGa-vacancies [80–83]. Although the nature of the dislocation and its electronicproperties is not part of the model, the parameter S in the simulation characterizesthe fraction of electrically active sites on the dislocation core. In addition, the IQE isgoverned by the electron and hole mobilities, nonequilibrium carrier concentrations,and consequently the minority carrier diffusion lengths. As can be seen from theresults of the simulation in Fig. 1.5a, dislocation densities in the range of 107 cm−2

are required in order to provide internal quantum efficiencies close to unity. Byutilizing advanced dislocation filtering techniques, one could expect to realizedislocation densities in this range even for heteroepitaxial growth on sapphiresubstrates. As can also be seen from Fig. 1.5b, for a fixed threading dislocationdensity, the IQE also strongly depends on the electron mobility and current density.This dependency is most pronounced for dislocation densities ranging between mid108 and 1010 cm−2, when the carrier diffusion lengths and the spacing of thedislocations are approximately the same size (Fig. 1.6).

Even lower defect densities can be achieved by homoepitaxial growth on bulkAlN substrates. For example, bulk AlN substrates grown by sublimation recon-densation have yielded dislocation densities as low as 104 cm−2 [84–87], althoughthe availability of these substrates is still limited and currently typical substratessizes are 1-in. diameter or even smaller. In addition bulk AlN substrates exhibitstrong absorption bands in the UVC spectral range [88, 89], which poses a seriouschallenge for the light extraction, since this is normally achieved by extracting theUV light through the substrate. As various studies show, the origin of these UVC

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absorption bands can be traced back to carbon impurities and the formation ofcomplexes [88–90]. Therefore these UVC absorption bands appear to be not afundamental problem, but a materials research challenge that can be solved byoptimizing the purity of the sources in order to reduce the carbon impurity con-centrations either by clever co-doping schemes, or by inserting a thick hydridevapor epitaxy grown AlN base layer [91]. UVC-LED heterostructures pseudo-morphically grown on bulk AlN substrate have been demonstrated by two groups[22, 92] with external quantum efficiencies of more than 5 % at emission wave-length near 270 nm and first steps to commercialize these devices have beenundertaken.

Fig. 1.5 Simulated internal quantum efficiencies (IQE) for an AlGaN MQW LED emitting near265 nm. a IQE versus threading dislocation density in the active region for different currentdensities. b IQE versus the electron mobility for different current densities (simulations arecourtesy of Martin Guttmann and Christoph Reich, Institute of Solid State Physics, TU Berlin)

Fig. 1.6 Bandgap energiesand emission wavelength ofInN, GaN, AlN and otherIII–V and II–VI compoundsemiconductor materialsplotted versus their latticespacing

1 A Brief Review of III-Nitride UV Emitter Technologies … 11

1.6 Current-Injection Efficiency and Operating Voltagesof UV-LEDs

Very often UV-LEDs suffer from poor injection efficiencies and high operationvoltages that originate from the relatively low conductivities of Mg-doped p-AlGaNlayers. This is due to the fact that the ionisation energy for the Mg acceptorincreases steadily with the aluminum mole fraction in the Mg-doped p-AlGaN layer[93, 94]. For Mg-doped AlN layers the acceptor level has been determined to beabout 510 meV above the valence band edge [70]. Therefore, in Mg-doped bulkAlGaN layers only a small fraction of the Mg acceptors are ionized at room tem-perature, resulting in very low free hole concentrations. But even the silicon dopingon n-AlGaN layers becomes increasingly more challenging at high aluminum molefractions [95–97]. Both n- and p-AlGaN layer resistances as well as the ohmiccontact resistances of the n- and p-metals contribute to the relatively high operatingvoltages of UV light emitting devices. Various approaches to improve the con-ductivity of p-AlGaN layers have been explored, e.g., utilizing short period AlxGa1−xN/AlyGa1−yN superlattices [98, 99], polarization doping of graded AlxGa1−xNlayers [100], and alternative p-layer materials like Mg-doped boron nitride(hBN) [101]. Also, the limits of silicon doping of n-AlGaN layers at very highaluminum mole fractions have been explored and record low resistivity AlGaN: Sicurrent-spreading layers in the composition range between 60 and 96 % have beendemonstrated [102, 103]. A number of studies have looked at ohmic metal contactformation to n-AlGaN layers and lowering the contact resistances [104–108] withvery promising results in the medium aluminum composition range. Since sapphireand AlN substrates are electrically insulating, electrons are normally laterallyinjected through the n-AlGaN layer. In order to reduce the n-layer series resistanceand to facilitate homogeneous current injection often interdigitated finger contactgeometries are employed in the chip design. Figure 1.7 shows a photograph of an

Fig. 1.7 Light-output versuscurrent (L–I) characteristic ofa flip-chip mountedUVB-LED on a ceramicpackage. The inset shows aphotograph of the LED chipwith interdigitated fingercontacts before mounting

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