COMPOUND SEMICONDUCTORS STRAINED LAYERS AND DEVICES

ELECTRONIC MATERIALS SERIES

This series is devoted to electronic materials subjects of active research interest and provides coverage of basic scientific concepts, as well as, relating the subjects to the electronic applications and providing details of the electronic systems, circuits or devices in which the materials are used.

The Electronic Materials Series is a useful reference source for senior undergraduate and graduate level students, as well as, for research workers in industrial laboratories who wish to broaden their knowledge into a new field.

Series Editors:

Professor A.F.W. Willoughhy Dept. of Engineering Materials University of Southampton UK

Professor R. Hull Dept. of Material Science & Engineering University of Virginia USA

Series Advisor:

Dr. Peter Capper GEC-Marconi Infra-Red Ltd. Southampton UK

Other Titles Available:

1. Widegap /I-VI Compounds for Opto-electronic Applications Edited by E. Ruda

2. High Temperature Electronics Edited by M. Willander and H.L. Hartnagel

3. Narrow-gap /I-VI Compoundsfor Optoelectronic and Electromagnetic Applications Edited by Peter Capper

4. Theory of Transport Properties of Semiconductor Nanostructures Edited by Eckehard Scholl

5. Physical Models of Semiconductor Quantum Devices Ying Fu; Magnus Willander

6. Quantum Effects in Semiconductor Materials and Devices, Edited by T. P. Pearsall

COMPOUNDSEMICONDUCTORS STRAINED LAYERS AND DEVICES

by

S. Jain IMECvzw

Leuven, Belgium

M. Willander Gothenburg University Gothenburg, Sweden

R. Van Overstraeten IMECvzw

Leuven, Belgium

SPRINGER SCIENCE+BUSINESS MEDIA, LLC

Library of Congress Cataloging-in-Publication Data

Jain, S.C. (Suresh C.), 1926-Compound semiconductors strained layers and devices 1 by S. Jain, M. Willander, R. Van Overstraeten

p. cm. -- (Electronic materials series) Includes bibliographical references and index. ISBN 978-0-7923-7769-6 ISBN 978-1-4615-4441-8 (eBook) DOI 10.1007/978-1-4615-4441-8

1. Compound semiconductors. 2. Layer structure (Solids) 1. Willander, M. IL Overstraeten, R. van. III Title. IV. Series.

TK7871.99.C65 135 2000 621.3815'2--dc21

Copyright ® 2000 by Springer Science+Business Media New York Originally published by Kluwer Academic Publishers in 2000 Softcover reprint of the hardcover lst edition 2000

99-089333

Ali rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, mechanical, photo­copying, recording, or otherwise, without the prior written permission of the publisher, Springer Science+Business Media, LLC.

Printed on acid-free paper.

Contents

Preface

1 Introduction 1.1 Evolution of strained layers ....... . 1.2 Conventional III -V-based heterostructures

1.2.1 Unstrained heterostructures 1.2.2 Strained heterostructures

1.3 III-Nitrides ......... . 1.3.1 Properties ........ . 1.3.2 Historical perspective ..

1.4 Wide bandgap II-VI semiconductors 1.4.1 Crystal structure, phase diagram and growth 1.4.2 Historical perspective ..... .

1.5 Material parameters .......... . 1.5.1 Lattice constants and bandgaps . 1.5.2 Effective masses and mobilities 1.5.3 Band alignments . . . . . . .

1.6 Scope and organization of this book

xi

1 1 4 4 5 6 6

8 10 10 13 14 14 15 15 18

2 Characterization and growth 19 2.1 Methods of characterization. . . . . . . . . . . . . 19

2.1.1 Electron Microscopy and X-ray diffraction. 19 2.1.2 Characterization by RHEED technique 21 2.1.3 Optical and magnetic methods . . . . . . . 22

2.2 Epitaxial growth methods . . . . . . . . . . . . . . 24

2.3

2.2.1 Substrates for the growth of II-VI and III-V semiconductors 24 2.2.2 MBE............... 25 2.2.3 MOVPE............. 27 2.2.4 MOMBE and related methods 2.2.5 ALE, MEMBE, and MMBE .. Growth of conventional III-V semiconductors 2.3.1 Growth by MBE, MOVPE and other techniques 2.3.2 Highly mismatched layers ... 2.3.3 Growth in nitrogen carrier gas ......... .

30

32 33 33 35 35

vi

2.4 Growth of II-VI semiconductors .. 2.4.1 Growth of ZnSe ...... . 2.4.2 Growth of CdTe and ZnTe 2.4.3 Growth of ZnS ...... .

CONTENTS

38 38 40 41

2.4.4 Photo-assisted growth of II-VI semiconductors 43 2.4.5 Interfaces of II-VI heterostructures . . . 44

2.5 Growth of III-nitride epilayers. . . . . . . . . . . . . . 47 2.5.1 Substrates for the growth of III-Nitrides. . . . 47 2.5.2 Laterally Epitaxially Overgrown (LEO)- and Pendeo (PE)-

epitaxial layers . . . . . . . . . . . . . . . . . . . . . .. 48 2.5.3 Growth of GaN and A1N by MOVPE ........... 50 2.5.4 Growth of GaN and A1N by MBE and related methods . 51 2.5.5 Comparison of III-Nitride layers grown on different sub-

strates and by different methods . . 53 2.5.6 Growth of InGaN and A1GaN alloys 54

3 Strain and critical thickness 59 59 59

3.1 Strain and energies of epilayers 3.1.1 Misfit strain ...... . 3.1.2 Dislocation and strain energies: Periodic arrays of dislo-

cations. . . . . . . . . . . . . . . . . 60 3.1.3 Non-periodic arrays of dislocations . 63

3.2 Processes involved in dislocation generation 65 3.2.1 Propagation of dislocations . . . . . 65

3.2.1.1 Excess stress . . . . . . . . 66 3.2.1.2 The kink model of dislocation propagation 67 3.2.1.3 Measurements of dislocation velocity. 69

3.2.2 Nucleation, multiplication and blocking 72 3.2.2.1 Nucleation.. 72 3.2.2.2 Multiplication 74 3.2.2.3 Blocking ... 75

3.3 Critical thickness . . . . . . . . 77 3.3.1 Theory of critical thickness 77 3.3.2 Strain and critical thickness of superlattices 78 3.3.3 Symmetrically strained superlattices . . . 79 3.3.4 Experimental values of critical thickness . 79 3.3.5 Critical thickness determined by islanding 84

4 Strain relaxation and defects 89 4.1 Strain in GeSi layers . . . . . . . . . 89 4.2 Strain in III-V semiconductor layers 91 4.3 Strain in II-VI layers. . . . . . . . . 94

4.3.1 Relaxation of strain in low mismatched layers 94 4.3.1.1 Relaxation in CdZnSe and in the initial stages

in ZnSe layers ................ 94 4.3.1.2 Relaxation with increase of layer thickness . .. 95

CONTENTS vii

4.3.1.3 Strain relaxation by electron irradiation. . .. 97 4.3.2 Relaxation of strain in highly mismatched layers . . .. 99

4.3.2.1 Strain relaxation in the early stages of growth 99 4.3.2.2 Periodic distribution of dislocations 100 4.3.2.3 Strain in magnetic superlattices 102

4.3.3 Strain oscillations .......... 104 4.4 Strain and defects in III-Nitride layers . . . 107

4.4.1 Strain distribution and dislocations. 107 4.4.2 Structural defects. . . . . . . . . . . 109 4.4.3 Effect of defect clusters on the minority carriers in GaN 109 4.4.4 Interfaces in III-Nitride epilayers . . . . . . . . . . . .. 110

5 Band structure and optical properties 5.1 Band structure .......... .

5.1.1 Zinc-blende semiconductors 5.1.2 Wurtzite (WZ) III-Nitrides

5.2 Band offsets . . . . . . . . . . . . . 5.2.1 General remarks ..... . 5.2.2 Band offsets: III-V heterostructures 5.2.3 Band offsets: II-VI heterostructures 5.2.4 Band offsets: III-Nitride heterostructures

115 115 115 118 121 121 121 122 125

5.3 Optical properties of III-V semiconductors . . . . 126 5.4 Optical properties of II-VI semiconductors . . . . 127

5.4.1 Reflectance, absorption, and luminescence 127 5.4.2 Effect of strain on the optical properties of II-VI semicon-

ductors ................. 129 5.4.3 II-VI quantum wells and superlattices . 133

5.4.3.1 Quantum confinement. . . . . 133 5.4.3.2 ZnSe/ZnSxSel_x superlattices 134 5.4.3.3 ZnSe/Znl_xCdxSe quantum wells 136 5.4.3.4 CdTe/ZnTe superlattices . . . . . 140 5.4.3.5 Raman studies of strained CdTe/ZnTe superlat-

tices . . . . . . . . . . . . . . . 142 5.4.3.6 Effect of hydrostatic pressure. 144

5.5 Optical properties of III-Nitrides . . . . . . . . 145 5.5.1 Intrinsic luminescence . . . . . . . . . . 145

5.5.1.1 PL and splitting of the valence band. 145 5.5.1.2 Binding energies and lifetimes of the excitons. 147

5.5.2 Yellow and DAP bands and persistent photoconductivity 147 5.5.3 Effect of strain on III-Nitride layers ........ 148 5.5.4 Temperature dependence of the optical transitions 150 5.5.5 Optical properties of InGaN alloys . . 151

5.5.5.1 Bandgap and PL . . . . . . . . . . . 151 5.5.5.2 Bowing parameter of InGaN . . . . 152 5.5.5.3 Nonuniform distribution of indium . 153

viii CONTENTS

5.5.6 AlGaN alloys: Optical absorption, bandgap and bowing parameter .................. .

5.5.7 III-Nitride quantum wells and superlattices 154 156

6 Electrical and magnetic properties 159 6.1 Electrical properties of II-VI semiconductors. 159

6.1.1 n-type doping. . . . . . . . . 159 6.1.2 p-type doping. . . . . . . . . 160

6.2 Electrical properties of n-type GaN . 162 6.2.1 Undoped GaN ........ 162 6.2.2 Si and Ge doped GaN: Carrier concentration 163 6.2.3 Electron mobility in n-type GaN . . . . . . . 165

6.2.3.1 Measurements of electron mobility in n-doped GaN ......................... 165

6.2.3.2 Monte Carlo simulations of electron mobility and velocity . . . . . . . . . . . . . . . 169

6.3 Electrical properties of p-type III-Nitrides . . . . . 170 6.3.1 Resistivity, hole concentration and mobility 170 6.3.2 Activation energy of Mg acceptors in GaN . 172

6.4 Electrical properties AlN, InN and alloys. 174 6.4.1 AlN and AlGaN ........ 174 6.4.2 InN ................ . 176

6.5 Schottky barriers and ohmic contacts. . . 176 6.5.1 Contacts on II-VI heterostructures 176 6.5.2 Contacts on III-Nitrides . . 179

6.5.2.1 Schottky barriers. 179 6.5.2.2 Ohmic contacts . 182

6.6 Effect of applied electric field . . . 185 6.6.1 Franz-Keldysh and Quantum Confined Stark Effects 185 6.6.2 Experimental results: II-VI quantum wells. . 187

6.6.2.1 CdTe/Cd1-xMnx Te quantum wells 187 6.6.2.2 Znl_xCdxSe/ZnSe quantum wells 188 6.6.2.3 CdZnSSe multiple quantum wells. 190

6.6.3 Effect of electric field on GaN . . . . . . . . 192 6.7 Piezoelectric effect . . . . . . . . . . . . . . . . . . 192

6.7.1 Piezoelectric effect in zinc blende and wurtzite semicon-ductors ................... . . . . . . . . . 192

6.7.2 Experiments on piezoelectric effect in II-VI quantum wells and superlattices . . . . . . . . . 194 6.7.2.1 CdS/CdSe superlattices . 194 6.7.2.2 CdS/ZnSe superlattices . 195

6.7.3 Piezoelectric effects in III-Nitrides 196 6.8 Effect of magnetic field on semiconductors 199

6.8.1 Magnetic polarons in diluted magnetic II-VI semiconductors 199 6.8.2 Transport properties . . . . . . . 200 6.8.3 Magnetic and Optical properties ............... 201

CONTENTS ix

6.8.3.1 Bulk crystals 201 6.8.3.2 ZnTe and ZnSe epilayers 202 6.8.3.3 Quantum wells and super lattices of diluted mag-

netic alloys 202 6.8.3.4 Digital alloys . 205 6.8.3.5 Effect of magnetic field on the properties of Quan-

tum dots 205 6.8.3.6 EMP bifurcation and asymmetric quantum wells 206 6.8.3.7 Cyclotron resonance measurements 206

7 Strained layer optoelectronic devices 207 7.1 Conventional-III-V semiconductor lasers . . . . . . . . . . . .. 207

7.1.1 Suppression of Auger recombination by strain in semicon­ductor lasers . . . . . . . . . . . . . . . . . . . . . . . . . 207

7.1.2 Pump-probe and other methods for measurement of Auger lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

7.1.3 PL of superlattices containing Sb in the active layers ... 211 7.1.4 InAso.gSbo.t/lno.8sAlo.lsAs strained layer superlattice mid-

IR lasers. . . . . . . . . . . . . . . . 212 7.1.5 Summary of this section ...... .

7.2 ZnSe-based light emitters and other devices 7.2.1 Light Emitting Diodes ....... . 7.2.2 Photo-pumped lasers ........ . 7.2.3 ZnSe-based electron-beam pumped lasers 7.2.4 ZnSe-based laser diodes ...... .

7.2.4.1 Structure and performance 7.2.4.2 Degradation and reliability

7.3 Other II-VI semiconductor applications 7.3.1 Modulators and switches .. 7.3.2 Optical detectors ...... .

7.4 III-Nitride Light Emitting Diodes .. 7.4.1 Fabrication and performance 7.4.2 Comparison with other LEDs

7.5 GaN based Lasers ......... . 7.5.1 General remarks ...... . 7.5.2 Optically pumped III-Nitride lasers. 7.5.3 III-Nitride laser diodes ....... . 7.5.4 7.5.5 7.5.6

8 Transistors

Mechanism of laser emission. . . . . Comparison of ZnSe- and GaN-based lasers Applications of LEDs and LDs . . . . . . .

8.1 InGaAs transistors 8.1.1 Early work 8.1.2 Recent work.

8.2 II-VI semiconductor transistors

213 214 214 216 219 220 220 223 228 228 229 231 231 235 236 236 237 238 241 242 243

245 245 245 247 251

x

8.3 III-Nitride based transistors ............. . 8.3.1 GaNjSiC HBT ................ . 8.3.2 Field Effect Transistors on sapphire substrate 8.3.3 FETs on SiC substrate.

8.4 Device Processing. . . . . . . 8.4.1 Etching ....... .

8.4.1.1 Wet etching 8.4.1.2 Dry etching.

8.4.2 Si ion implantation . . 8.4.3 Implantation for isolation

9 Summary and conclusions 9.1 Growth, defects and strain ....... . 9.2 Band structure and electronic properties

9.2.1 III-V semiconductors .... 9.2.2 II-VI semiconductors . . . . 9.2.3 III-Nitride semiconductors.

9.3 Applications and future work 9.3.1 Applications 9.3.2 Future work ..... .

Appendix A

Bibliography

Index

CONTENTS

253 253 254 258 260 261 261 262 262 264

265 265 267 267 267 269 271 271 272

275

281

333

Preface

During the last 25 years (after the growth of the first pseudomorphic GeSi strained layers on Si by Erich Kasper in Germany) we have seen a steady accu­mulation of new materials and devices with enhanced performance made pos­sible by strain. 1989-1999 have been very good years for the strained-Iayer­devices. Several breakthroughs were made in the growth and doping technology of strained layers. New devices were fabricated as a results of these break­throughs. Before the advent of strain layer epitaxy short wavelength (violet to green) and mid-IR (2 to 5 f.Lm) regions of the spectrum were not accessi­ble to the photonic devices. Short wavelength Light Emitting Diodes (LEDs) and Laser Diodes (LDs) have now been developed using III-Nitride and II-VI strained layers. Auger recombination increases rapidly as the bandgap narrows and temperature increases. Therefore it was difficult to develop mid-IR (2 to 5 f.Lm range) lasers. The effect of strain in modifying the band-structure and suppressing the Auger recombination has been most spectacular. It is due to the strain mediated band-structure engineering that mid-IR lasers with good per­formance have been fabricated in several laboratories around the world. Many devices based on strained layers have reached the market place. This book de­scribes recent work on the growth, characterization and properties o(compound semiconductors strained layers and devices fabricated using them.

The work that has been done on strain, dislocations, and mechanical prop­erties of strained layers is very extensive. It is not possible to describe all this work in a monograph of this size. However work is treated in sufficient details to cover essential aspects of recent developments. References to reviews and books published on the subject are quoted. The discussion of this work should be useful to engineers and material scientists concerned with effects of strain on the mechanical properties of crystalline layers of any material. Though the book is devoted to compound semiconductors, work on GeSi strained layers is included either for comparison with the results of compound semiconductors or to clarify physics and prove results of theoretical calculations using experiments performed on GeSi layers. The effects of strain on band structure, transport, and optical properties of both the zinc blende and the wurtzite compound semi­conductors are discussed. Piezoelectric Effects and Quantum Confined Stark Effects are included. Magnetic polarons in diluted II-VI magnetic polarons are also discussed. Among the applications, blue and green LEDs and LDs and mid-IR LDs are discussed. One whole chapter is devoted to these devices. An-

xii Preface

other chapter is devoted to Transistors based on conventional III-V, II-VI and III-Nitride semiconductors. The subject matter is treated at a level appropriate for students and senior researchers interested in material science, and in de­signing and modelling semiconductor devices. Several thousand papers of high quality have appeared on strained layers and devices in the last ten years. We have quoted some six hundred papers in the bibliography that are most relevant for a coherent discussion of the subject. To make the bibliography more useful, titles of the papers have been included.

Unfortunately Professor R. Van Overstraeten, one of the co-authors of the book, died on April 29, 1999. He was ill during the year and a half before this book was completed. He contributed significantly to the parts of the book written before he became ill. He did not see the manuscript in the final form.

We have benefited from interaction and collaboration with a large number of students and colleagues. It is impossible to mention them individually. Discus­sion with Dr. Uma Jain on strain and strain relaxation in lattice mismatched epilayers were very useful. Dr. K. van der Zanden and Dr. H. Hardtdegen read critically some parts of the book and made constructive criticism. We would like to thank sincerely Professor R. Mertens for useful discussions and for supporting this project. Madhulika Jain helped considerably in mechanical preparation of the manuscript. We are grateful to IMEC library, particularly to Greet Vanhoof and her colleague Griet Op de Beeck for the excellent work they did in providing bibliography and obtaining papers, books and Conference Proceedings from libraries abroad. Without their help it would have been diffi­cult to write a monograph on a modern topic where the literature is scattered in numerous publications world wide.

Finally, we must thank sincerely our wives for their unfailing support and help during the preparation of this book.

S. C. Jain IMEC, Kapeldreef 75 3001 Leuven, Belgium

June 26, 1999

M. Willander Gothenberg University jChalmers University of Technology Department of Physics S-41296 Goteborg, Sweden

June 26, 1999