FABRICATION OF GaAs DEVICES by Albert G. Baca and Carol I. H. Ashby Sandia National Laboratories Albuquerque, NM, USA
CONTENTS Acknowledgment Abbreviations xiii 1 Introduction to GaAs devices 1 1.1 Scope of this book 1 1.2 GaAs materials 2 1.3 Types of GaAs devices 4 1.3.1 Electronic devices 4 1.3.2 Photonic devices 5 1.4 A brief history of GaAs devices 6 1.4.1 History of GaAs electronic devices 6 1.4.2 History of GaAs photonic devices 9 1.5 Applications of GaAs devices 10 1.5.1 Photonic device applications 11 1.5.2 Electronic device applications 15 References 19 2 Semiconductor properties, growth, characterisation and processing techniques 21 2.1 Chapter scope 21 2.2 Semiconductor properties 21 2.2.1 Energy levels and band structure in semiconductors 22 2.2.2 Charged carriers in semiconductors 27 2.2.3 Carrier transport and continuity equations 32 2.3 Bulk crystal growth 33 2.3.1 Methods of crystal growth 34 2.3.2 Substrate properties and device requirements 36 2.4 Epitaxy 38 2.4.1 Molecular beam epitaxy 40 2.4.2 Metal-organic chemical vapour deposition 43 2.5 Material characterisation 46 2.5.1 Light-based techniques 47 2.5.2 Electron beam techniques 49 vii xv
2.5.3 Ionic techniques 53 2.5.4 Electrical characterisation 56 2.6 Processing techniques 60 2.7 Back end processing and analysis 60 2.7.1 Backside processing, die Separation and packaging 61 2.7.2 Reliability 64 2.7.3 Failure analysis 66 2.8 Conclusion 66 References 67 Cleaning and passivation of GaAs and related alloys 69 3.1 Chapterscope 69 3.2 Cleaning and native oxide removal 69 3.2.1 Removal of organic and metal ion contaminants 69 3.2.2 Removal of native oxide 71 3.2.3 Regrowth of native oxide 72 3.3 Passivation of GaAs 74 3.3.1 Electronic properties of the GaAs surface 75 3.3.2 Chalcogenide passivation: S and Se 91 3.3.3 Passivation for improved semiconductor regrowth 101 3.3.4 Passivation for improved contact metallisation 102 3.3.5 Special oxide passivations 106 3.3.6 Dielectric passivations: PECVD and ECR SiN x and SiO x N y 109 3.4 Conclusion 114 References 114 Wet etching and photolithography of GaAs and related alloys 117 4.1 Chapterscope 117 4.2 Mechanism of wet etch processes 118 4.3 Rates and profiles 118 4.3.1 Diffusion control 121 4.3.2 Reaction-rate control 123 4.3.3 Aging of etching Solution 124 4.4 Practical wet etching 125 4.4.1 Photoresist issues 127 4.4.2 Basic wet etches 132 4.4.3 Acidic wet etches 133 4.4.4 Metal etches 135 vin
4.5 Compositional selectivity 136 4.5.1 GaAs versus AlGaAs selectivity 137 4.5.2 Selectivity versus other materials 139 4.6 Effects of doping type 141 4.7 Electrolytic effects in wet etching 142 4.8 Effects of defects and damage 144 4.9 Conclusion 144 References 144 5 Dry etching of GaAs and related alloys 147 5.1 Chapter scope 147 5.2 Comparison of wet and dry etching 147 5.3 Overview of dry etching processes 149 5.4 Ion-beam etching (IBE) and ion effects in other plasma processes 150 5.5 Chemical dry etching 151 5.6 Plasma etching at very low ion energies 152 5.7 Conventional reactive ion etching (RIE) 153 5.7.1 Halogen-based plasmas for RIE 155 5.7.2 Alkane-based plasmas for RIE 159 5.8 High-density plasma etching (HDPE) 160 5.9 Reactive-ion-beam etching (RIBE) and chemically assisted ion-beam etching (CAIBE) 163 5.10 General issues for dry etching 167 5.10.1 Etch uniformity 167 5.10.2 Damage from dry etching 168 5.10.3 Resists and their behaviour in dry etching processes 170 5.10.4 Advantages of Ar addition 174 5.10.5 Methods for end-point determination 174 5.10.6 Plasma diagnostics for trouble-shooting 176 5.10.7 Chamber cleaning issues 176 5.10.8 Effect of Chamber materials 177 5.11 Conclusion 177 References 178 6 Ohmic contacts 179 6.1 Chapter scope 179 6.2 Principles of ohmic contacts 179 6.2.1 Definitions 180 6.2.2 Physics of ohmic contact formation 181 6.2.3 Metallurgy of ohmic contact formation 183 6.3 Fabrication and testing of ohmic contacts 186 6.3.1 Fabrication of ohmic contacts 186 6.3.2 Measurements of ohmic contacts 191 ix
6.4 Ohmic contacts to n-type GaAs 195 6.4.1 GeAuNi ohmic contacts 196 6.4.2 Limited Au contacts for improved thermal stability 197 6.4.3 Ohmic contacts to heavily doped surfaces 199 6.4.4 Refractory metals and contacts based on reducing the surface bandgap 199 6.5 Ohmic contacts to p-type GaAs 200 6.6 Conclusion 201 References 202 7 Schottky contacts 205 7.1 Chapterscope 205 7.2 Physics and characterisation of Schottky contacts 205 7.2.1 Physics of Schottky contacts 205 7.2.2 Interfacial properties of Schottky contacts 210 7.3 Fabrication of Schottky contacts 214 7.3.1 Basic recessed gate fabrication 214 7.3.2 Self-aligned Schottky gates 217 7.3.3 Schottky gate structures 219 7.4 Electrical characteristics of GaAs Schottky contacts 223 7.5 Reliability of GaAs Schottky contacts 225 7.6 Conclusion 227 References 227 8 Field effect transistors 229 8.1 Chapterscope 229 8.2 Field effect transistor basics 229 8.2.1 Field effect transistor tutorial 230 8.2.2 Field effect transistor Performance and reliability issues 236 8.2.3 Field effect transistor structures and materials 240 8.2.4 Overview of field effect transistor fabrication 244 8.3 Doping FETs 247 8.4 Isolation of FETs 251 8.5 Source and drain ohmic contacts 253 8.6 Gate metal contacts 254 8.7 Passivation 254 8.8 Degradation of FETs 256 8.8.1 Definition and characterisation of hot electrons 257 x
8.8.2 Hot electron degradation 260 8.8.3 Other types of degradation 264 8.9 Conclusion 265 References 265 9 Heterojunction bipolar transistors 267 9.1 Chapter scope 267 9.2 HBT basics 267 9.2.1 Bipolar transistor tutorial 268 9.2.2 Other GaAs HBT Performance and reliability issues 272 9.2.3 HBT device structure and material issues 275 9.2.4 Overview of HBT fabrication 279 9.3 MESA etching for GaAs-based HBTs 281 9.3.1 Emitter mesa etch 281 9.3.2 Base mesa etch 285 9.3.3 Collector mesa etch 286 9.4 Ohmic contacts for GaAs-based HBTs 286 9.4.1 Emitter metal ohmic contacts 286 9.4.2 Base metal ohmic contacts 287 9.4.3 Collector metal ohmic contacts 289 9.5 Passivation of GaAs-based HBTs 289 9.5.1 Ledge passivation 290 9.5.2 Dielectric passivation 291 9.5.3 Sulphur passivation 294 9.6 Variations on HBT processing 294 9.7 Reliability of HBTs 299 9.8 Conclusions 303 References 303 10 Wet oxidation for optoelectronic and MIS GaAs devices 305 10.1 Chapter scope 305 10.2 Mechanism of wet oxidation processes 305 10.2.1 Chemistry of wet and dry oxidation of AlGaAs 306 10.2.2 Electronic consequences of oxidation processes 307 10.3 Rates and profile evolution 308 10.3.1 Al-mole-fraction effects 310 10.3.2 Layer thickness effects 313 10.3.3 Proximity enhancement effect 314 10.3.4 Wet oxidation of other materials 315 10.3.5 Miscellaneous observations 317 10.4 Practical wet oxidation 319 xi
10.5 Applications in optoelectronic devices 321 10.5.1 Structural issues for oxide VCSELs 321 10.5.2 Defect-related issues for optoelectronic devices 324 10.6 Applications in electronic GaAs devices 325 10.6.1 Problems with wet and dry oxidation for MIS devices 325 10.6.2 GaAs-on-insulator applications 326 10.7 Conclusion 326 References 327 Glossary 329 Index 347 xn