Table Of ContentFabrication of
GaAs Devices
Albert G. Baca and Carol I.H. Ashby
Sandia National Laboratories, New Mexico, USA
The Institution of Engineering and Technology
Published by The lnstitution of Engineering and Technology, London, United Kingdom
First edition O 2005 The lnstitution of Electrical Engineers
Paperback edition O 2009 The lnstitution of Engineering and Technology
First published 2005 (0 86341 353 6)
Paperback edition 2009
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British Library Cataloguing in Publication Data
Baca, A.G.
Fabrication of CaAs devices. - (EMIS processing series 6)
1. Gallium arsenide semiconductors
I. Title II. Ashby, Carol Ill. lnstitution of Electrical Engineers
621.3'815 2
ISBN 978-0-86341-353-7 (hardback)
ISBN 978-1-84919-067-1 (paperback)
Typeset in India by Newgen Imaging Systems (P) Ltd, Chennai
First printed in the UK by MPC Books Ltd, Bodmin, Cornwall
Reprinted in the UK by Lightning Source UK Ltd, Milton Keynes
CONTENTS
Acknowledgment xiii
Abbreviations xv
1 IntroductiontoGaAsdevices 1
1.1 Scopeofthisbook 1
1.2 GaAsmaterials 2
1.3 TypesofGaAsdevices 4
1.3.1 Electronicdevices 4
1.3.2 Photonicdevices 5
1.4 AbriefhistoryofGaAsdevices 6
1.4.1 HistoryofGaAselectronicdevices 6
1.4.2 HistoryofGaAsphotonicdevices 9
1.5 ApplicationsofGaAsdevices 10
1.5.1 Photonicdeviceapplications 11
1.5.2 Electronicdeviceapplications 15
References 19
2 Semiconductorproperties,growth,
characterisationandprocessingtechniques 21
2.1 Chapterscope 21
2.2 Semiconductorproperties 21
2.2.1 Energylevelsandbandstructurein
semiconductors 22
2.2.2 Chargedcarriersinsemiconductors 27
2.2.3 Carriertransportandcontinuity
equations 32
2.3 Bulkcrystalgrowth 33
2.3.1 Methodsofcrystalgrowth 34
2.3.2 Substratepropertiesanddevice
requirements 36
2.4 Epitaxy 38
2.4.1 Molecularbeamepitaxy 40
2.4.2 Metal-organicchemicalvapourdeposition 43
2.5 Materialcharacterisation 46
2.5.1 Light-basedtechniques 47
2.5.2 Electronbeamtechniques 49
vii
Contents
2.5.3 Ionictechniques 53
2.5.4 Electricalcharacterisation 56
2.6 Processingtechniques 60
2.7 Backendprocessingandanalysis 60
2.7.1 Backsideprocessing,dieseparationand
packaging 61
2.7.2 Reliability 64
2.7.3 Failureanalysis 66
2.8 Conclusion 66
References 67
3 CleaningandpassivationofGaAsandrelated
alloys 69
3.1 Chapterscope 69
3.2 Cleaningandnativeoxideremoval 69
3.2.1 Removaloforganicandmetalion
contaminants 69
3.2.2 Removalofnativeoxide 71
3.2.3 Regrowthofnativeoxide 72
3.3 PassivationofGaAs 74
3.3.1 ElectronicpropertiesoftheGaAs
surface 75
3.3.2 Chalcogenidepassivation:SandSe 91
3.3.3 Passivationforimprovedsemiconductor
regrowth 101
3.3.4 Passivationforimprovedcontact
metallisation 102
3.3.5 Specialoxidepassivations 106
3.3.6 Dielectricpassivations:PECVDand
ECRSiN andSiO N 109
x x y
3.4 Conclusion 114
References 114
4 WetetchingandphotolithographyofGaAsand
relatedalloys 117
4.1 Chapterscope 117
4.2 Mechanismofwetetchprocesses 118
4.3 Ratesandprofiles 118
4.3.1 Diffusioncontrol 121
4.3.2 Reaction-ratecontrol 123
4.3.3 Agingofetchingsolution 124
4.4 Practicalwetetching 125
4.4.1 Photoresistissues 127
4.4.2 Basicwetetches 132
4.4.3 Acidicwetetches 133
4.4.4 Metaletches 135
viii
Contents
4.5 Compositionalselectivity 136
4.5.1 GaAsversusAlGaAsselectivity 137
4.5.2 Selectivityversusothermaterials 139
4.6 Effectsofdopingtype 141
4.7 Electrolyticeffectsinwetetching 142
4.8 Effectsofdefectsanddamage 144
4.9 Conclusion 144
References 144
5 DryetchingofGaAsandrelatedalloys 147
5.1 Chapterscope 147
5.2 Comparisonofwetanddryetching 147
5.3 Overviewofdryetchingprocesses 149
5.4 Ion-beametching(IBE)andioneffectsin
otherplasmaprocesses 150
5.5 Chemicaldryetching 151
5.6 Plasmaetchingatverylowionenergies 152
5.7 Conventionalreactiveionetching(RIE) 153
5.7.1 Halogen-basedplasmasforRIE 155
5.7.2 Alkane-basedplasmasforRIE 159
5.8 High-densityplasmaetching(HDPE) 160
5.9 Reactive-ion-beametching(RIBE)
andchemicallyassistedion-beametching
(CAIBE) 163
5.10 Generalissuesfordryetching 167
5.10.1 Etchuniformity 167
5.10.2 Damagefromdryetching 168
5.10.3 Resistsandtheirbehaviourindry
etchingprocesses 170
5.10.4 AdvantagesofAraddition 174
5.10.5 Methodsforend-pointdetermination 174
5.10.6 Plasmadiagnosticsfortrouble-shooting 176
5.10.7 Chambercleaningissues 176
5.10.8 Effectofchambermaterials 177
5.11 Conclusion 177
References 178
6 Ohmiccontacts 179
6.1 Chapterscope 179
6.2 Principlesofohmiccontacts 179
6.2.1 Definitions 180
6.2.2 Physicsofohmiccontactformation 181
6.2.3 Metallurgyofohmiccontactformation 183
6.3 Fabricationandtestingofohmiccontacts 186
6.3.1 Fabricationofohmiccontacts 186
6.3.2 Measurementsofohmiccontacts 191
ix
Contents
6.4 Ohmiccontactston-typeGaAs 195
6.4.1 GeAuNiohmiccontacts 196
6.4.2 LimitedAucontactsforimproved
thermalstability 197
6.4.3 Ohmiccontactstoheavilydopedsurfaces 199
6.4.4 Refractorymetalsandcontactsbased
onreducingthesurfacebandgap 199
6.5 Ohmiccontactstop-typeGaAs 200
6.6 Conclusion 201
References 202
7 Schottkycontacts 205
7.1 Chapterscope 205
7.2 PhysicsandcharacterisationofSchottky
contacts 205
7.2.1 PhysicsofSchottkycontacts 205
7.2.2 InterfacialpropertiesofSchottky
contacts 210
7.3 FabricationofSchottkycontacts 214
7.3.1 Basicrecessedgatefabrication 214
7.3.2 Self-alignedSchottkygates 217
7.3.3 Schottkygatestructures 219
7.4 ElectricalcharacteristicsofGaAsSchottky
contacts 223
7.5 ReliabilityofGaAsSchottkycontacts 225
7.6 Conclusion 227
References 227
8 Fieldeffecttransistors 229
8.1 Chapterscope 229
8.2 Fieldeffecttransistorbasics 229
8.2.1 Fieldeffecttransistortutorial 230
8.2.2 Fieldeffecttransistorperformanceand
reliabilityissues 236
8.2.3 Fieldeffecttransistorstructuresand
materials 240
8.2.4 Overviewoffieldeffecttransistor
fabrication 244
8.3 DopingFETs 247
8.4 IsolationofFETs 251
8.5 Sourceanddrainohmiccontacts 253
8.6 Gatemetalcontacts 254
8.7 Passivation 254
8.8 DegradationofFETs 256
8.8.1 Definitionandcharacterisationofhot
electrons 257
x
Contents
8.8.2 Hotelectrondegradation 260
8.8.3 Othertypesofdegradation 264
8.9 Conclusion 265
References 265
9 Heterojunctionbipolartransistors 267
9.1 Chapterscope 267
9.2 HBTbasics 267
9.2.1 Bipolartransistortutorial 268
9.2.2 OtherGaAsHBTperformanceand
reliabilityissues 272
9.2.3 HBTdevicestructureandmaterial
issues 275
9.2.4 OverviewofHBTfabrication 279
9.3 MESAetchingforGaAs-basedHBTs 281
9.3.1 Emittermesaetch 281
9.3.2 Basemesaetch 285
9.3.3 Collectormesaetch 286
9.4 OhmiccontactsforGaAs-basedHBTs 286
9.4.1 Emittermetalohmiccontacts 286
9.4.2 Basemetalohmiccontacts 287
9.4.3 Collectormetalohmiccontacts 289
9.5 PassivationofGaAs-basedHBTs 289
9.5.1 Ledgepassivation 290
9.5.2 Dielectricpassivation 291
9.5.3 Sulphurpassivation 294
9.6 VariationsonHBTprocessing 294
9.7 ReliabilityofHBTs 299
9.8 Conclusions 303
References 303
10 WetoxidationforoptoelectronicandMISGaAs
devices 305
10.1 Chapterscope 305
10.2 Mechanismofwetoxidationprocesses 305
10.2.1 Chemistryofwetanddryoxidationof
AlGaAs 306
10.2.2 Electronicconsequencesofoxidation
processes 307
10.3 Ratesandprofileevolution 308
10.3.1 Al-mole-fractioneffects 310
10.3.2 Layerthicknesseffects 313
10.3.3 Proximityenhancementeffect 314
10.3.4 Wetoxidationofothermaterials 315
10.3.5 Miscellaneousobservations 317
10.4 Practicalwetoxidation 319
xi
Contents
10.5 Applicationsinoptoelectronicdevices 321
10.5.1 StructuralissuesforoxideVCSELs 321
10.5.2 Defect-relatedissuesforoptoelectronic
devices 324
10.6 ApplicationsinelectronicGaAsdevices 325
10.6.1 Problemswithwetanddryoxidation
forMISdevices 325
10.6.2 GaAs-on-insulatorapplications 326
10.7 Conclusion 326
References 327
Glossary 329
Index 347
xii
Chapter 1
Introduction to GaAs devices
1.1 SCOPEOFTHISBOOK
OnemightaskwhyanotherbookaboutGaAsprocessingisneeded. Scopeofthisbook p.1
Ineveryfield,therearetwobodiesofknowledgerequiredfortrue
GaAsmaterials p.2
mastery. This is especially true in the field of device enginee-
ring. The first is related to understanding the ideal behaviour of TypesofGaAsdevices p.4
devicespredictedbyfundamentallawsofphysics.Thisisthesub- Electronicdevices p.4
ject of many textbooks and courses and is readily accessible to Photonicdevices p.5
everyengineer.However,aswesoondiscoveraswefabricatereal- AbriefhistoryofGaAsdevices p.6
worlddevices,behaviouroftendeviatesfromthatideal.Whenthat HistoryofGaAselectronic
occurs,thequestions“Why?”and“HowcanIfixitoravoiditnext devices p.6
time?” must be answered or the same problems will continue to HistoryofGaAsphotonicdevices p.9
plagueourdevices. ApplicationsofGaAsdevices p.10
The second body of knowledge helps us answer these chal- Photonicdeviceapplications p.11
lenging questions. It is that vast body of practical, even anec- Electronicdeviceapplications p.15
dotal, knowledge gradually accumulated by a practising engineer
References p.19
through everyday experiences with successes and failures in
designing and fabricating devices. It is some of this second body
ofknowledgethatwehopetoprovideinwritingthisbook.While
even the 30-year veteran will not have fully mastered this second
body of knowledge, the aim of this book is to share many of the
insights and solutions that we have learned during our combined
yearsworkingwithdevicesmadeofGaAsandrelatedmaterials.
In addition, many books on GaAs devices contain sections or
chapters on processing methods. However, most books provide
only a part of the knowledge required to actually make high-
qualitydevicesandalmostallofthepreviousbooksneglectsome
aspects of processing as specifically related to device operation
andreliability.
Most books on GaAs devices are mainly concerned with a
logical and educational presentation of the device operation,
naturally focusing on the physics of the devices and the deriv-
ation of equations that describe device operation. Some good
books on advanced processing techniques in general [1–5] and
evenonmodernGaAsprocessing[6]areavailable.TheGaAspro-
cessingbookbyWilliamsfocusesonprocesstechniquesforGaAs
1
IntroductiontoGaAsdevices
electronic technology. The device-oriented books describe ideal
operationanddonottreattheimpactofspecificprocessingchoices
on device operation [7–17]. The process-oriented books address
some impacts of processing choices but do not adequately relate
thesetosubtletiesindeviceperformanceandreliability.Especially
lackinginanyofthesebooksisanadequatetreatmentofthecrit-
ical importance of surfaces and interfaces in GaAs devices and
theircorrelationwithdeviceperformance.
This book attempts to address all aspects of GaAs pro-
cessing that deal with GaAs free surfaces and with interfaces
between GaAs and metal contacts or dielectrics. These subjects
are intimately involved in virtually all aspects of device pro-
cessing.Wehaveattemptedtoprovidebothfundamentalperspect-
ive and practical advice in the areas of cleaning and passivation
(Chapter 3), wet etching and photolithography (Chapter 4), dry
etching (Chapter 5) and GaAs-metal contacts (Chapters 6 and 7).
Chapters8and9discussdeviceperformanceforHBTsandFETs
and how this relates to processing choices. Chapter 10 deals with
specialprocessingissuessuchaswetoxidationthatareespecially
important in optoelectronic devices. Chapter 2 provides a variety
of background material about physical concepts, semiconductor
materials and characterisation techniques so that the student does
not have to consult multiple texts to gain perspective while using
thisbook.
Our main goal is to provide both fundamental and practical
informationtoaidboththebeginningandthepractisingengineerin
relating device performance, degradation and non-idealities with
processing choices made during device fabrication. Above all,
we have attempted to make this an “apprenticeship in a book”
by presenting as much practical information as possible that we
have learned through many years of processing of GaAs devices
ourselves. To the extent that some of these goals are achieved,
anotherbookonGaAsprocessingisworthwhile.
1.2 GaAsMATERIALS
Soon after the discovery of the transistor in 1947, gallium arsen-
ide and many other semiconductors were assessed as candidate
materials for electronic devices. GaAs became recognised as a
material with a large electron velocity that might be suitable for
high-speedelectronics.Ittookseveraldecadesbeforethis“mater-
ialofthefuture”madeasignificantcommercialentryinthe1970s,
much to the frustration and delayed ambitions of many of its
practitioners. Now, GaAs is the basis of a several billion dollar
worldwideindustryforhigh-frequencyandhigh-speedelectronics,
2
