Series Preface xxi
Preface xxiii
Foreword xxvii
List of Contributors xxxi
Part One - Growth 1
1 Bulk Growth of Mercury Cadmium Telluride (MCT) 3
P. Capper
1.1 Introduction 3
1.2 Phase equilibria 4
1.3 Crystal growth 5
1.3.1 Solid state recrystallization (SSR) 6
1.3.2 Traveling heater method (THM) 9
1.3.3 Bridgman 12
1.3.4 Accelerated crucible rotation technique (ACRT) 13
1.4 Conclusions 18
References 19
2 Bulk Growth of CdZnTe/CdTe Crystals 21
A. Noda, H. Kurita and R. Hirano
2.1 Introduction 21
2.2 High-purity Cd and Te 22
2.2.1 Cadmium 22
2.2.2 Tellurium 23
2.3 Crystal growth 23
2.3.1 Polycrystal growth 23
2.3.2 VGF single-crystal growth 24
2.4 Wafer processing 41
2.4.1 Process flow 42
2.4.2 Characteristics 44
2.5 Summary 48
Acknowledgements 48
References 49
3 Properties of Cd(Zn)Te Relevant to Use as Substrates 51
S. Adachi
3.1 Introduction 52
3.2 Structural properties 52
3.2.1 Ionicity 52
3.2.2 Lattice constant and crystal density 53
3.2.3 Spontaneous ordering 54
3.2.4 Structural phase transition 55
3.3 Thermal properties 55
3.3.1 Phase diagram 55
3.3.2 Specific heat and Debye temperature 56
3.3.3 Thermal expansion coefficient 57
3.3.4 Thermal conductivity and diffusivity 57
3.4 Mechanical and lattice vibronic properties 58
3.4.1 Elastic constant and related parameters 58
3.4.2 Microhardness 58
3.4.3 Optical phonon frequency and phonon deformation potential 59
3.5 Collective effects and some response characteristics 61
3.5.1 Piezoelectric constant 61
3.5.2 Fröhlich coupling constant 61
3.6 Electronic energy-band structure 62
3.6.1 Bandgap energy 62
3.6.2 Electron and hole effective masses 64
3.6.3 Electronic deformation potential 65
3.6.4 Heterojunction band offset 66
3.7 Optical properties 67
3.7.1 The reststrahlen region 67
3.7.2 The interband transition region 68
3.7.3 Near or below the fundamental absorption edge 69
3.8 Carrier transport properties 70
3.8.1 Low-field mobility 70
3.8.2 Minority-carrier transport 71
References 71
4 Substrates for the Epitaxial Growth of MCT 75
J. Garland and R. Sporken
4.1 Introduction 76
4.2 Substrate orientation 77
4.3 CZT substrates 78
4.3.1 Effects of poor thermal conductivity on MCT growth 78
4.3.2 Effects of substrate crystalline defects on MCT growth 79
4.3.3 Effects of substrate impurities 80
4.3.4 Effects of nonuniform substrate composition and substrate roughness 80
4.3.5 Effects of surface nonstoichiometry and contaminants 81
4.3.6 Characterization and screening of CZT substrates 81
4.3.7 Use of buffer layers on CZT substrates 82
4.4 Si-based substrates 82
4.4.1 Nucleation and growth of CdTe on Si 83
4.4.2 The effects of As and Te monolayers 84
4.4.3 Advantages of CdTe/Si substrates 85
4.4.4 Disadvantages of CdTe/Si substrates 86
4.4.5 Reduction of the dislocation density 87
4.4.6 Passivation of dislocations 88
4.5 Other substrates 89
4.6 Summary and conclusions 90
References 90
5 Liquid Phase Epitaxy of MCT 95
P. Capper
5.1 Introduction 95
5.2 Growth 96
5.2.1 Introduction 96
5.2.2 Phase diagram and defect chemistry 98
5.2.3 LPE growth techniques 98
5.3 Material characteristics 103
5.3.1 Composition and thickness 103
5.3.2 Crystal quality and surface morphology 105
5.3.3 Impurity doping and electrical properties 106
5.4 Device status 108
5.5 Summary and future developments 108
References 110
6 Metal-Organic Vapor Phase Epitaxy (MOVPE) Growth 113
C. D. Maxey
6.1 Requirement for epitaxy 113
6.2 History 114
6.3 Substrate choices 115
6.3.1 Orientation 115
6.3.2 Material 116
6.4 Reactor design 117
6.5 Process parameters 118
6.6 Metal-organic sources 119
6.7 Uniformity 120
6.8 Reproducibility 120
6.9 Doping 123
6.10 Defects 125
6.11 Annealing 127
6.12 In situ monitoring 127
6.13 Conclusions 128
References 128
7 MBE Growth of Mercury Cadmium Telluride 131
J. Garland
7.1 Introduction 131
7.1.1 The MBE growth technique 132
7.2 MBE Growth theory and growth modes 132
7.2.1 Growth modes 133
7.2.2 Quasiequilibrium theories 133
7.2.3 Kinetic theories 134
7.3 Substrate mounting 135
7.4 In situ characterization tools 135
7.4.1 Reflection high-energy electron diffraction 135
7.4.2 Spectroscopic ellipsometry 136
7.4.3 Other in situ characterization tools 139
7.5 MCT nucleation and growth 139
7.6 Dopants and dopant activation 141
7.7 Properties of MCT epilayers grown by MBE 143
7.7.1 Electrical properties 143
7.7.2 Optically measurable characteristics 144
7.7.3 Structural properties 144
7.7.4 Surface defects 145
7.8 Conclusions 146
References 147
Part Two - Properties 151
8 Mechanical and Thermal Properties 153
M. Martyniuk, J. M. Dell and L. Faraone
8.1 Density of MCT 154
8.1.1 Introduction 154
8.1.2 Variation of Density with X 154
8.1.3 Variation of density with temperature 155
8.1.4 Conclusion 158
8.2 Lattice parameter of MCT 158
8.2.1 Introduction 158
8.2.2 Variation of Lattice Parameter with X 158
8.2.3 Variation with temperature 160
8.2.4 Conclusion 162
8.3 Coefficient of thermal expansion of MCT 162
8.3.1 Introduction 162
8.3.2 Variation with X 162
8.3.3 Variation with temperature 163
8.3.4 Conclusion 166
8.4 Elastic parameters of MCT 166
8.4.1 Introduction 166
8.4.2 Elastic parameter values 167
8.4.3 Conclusion 170
8.5 Hardness and deformation characteristics of MCT 170
8.5.1 Introduction 170
8.5.2 Hardness 170
8.5.3 Deformation characteristics of MCT 174
8.5.4 Photoplastic effect 180
8.5.5 Conclusion 180
8.6 Phase diagrams of MCT 181
8.6.1 Introduction 181
8.6.2 Binary systems 181
8.6.3 Solid phases 181
8.6.4 Quasibinary systems 183
8.6.5 Liquidus, solidus, and solvus surfaces 185
8.6.6 Thermodynamics 186
8.6.7 Conclusion 187
8.7 Viscosity of the MCT melt 187
8.7.1 Introduction 187
8.7.2 Temperature variation of kinematic viscosity of the MCT melt 187
8.7.3 Conclusion 189
8.8 Thermal properties of MCT 189
8.8.1 Introduction 189
8.8.2 Specific heat (C p)
189
8.8.3 Thermal diffusivity (D ¿) 192
8.8.4 Thermal conductivity (K ¿) 194
8.8.5 Conclusion 197
References 197
9 Optical Properties of MCT 205
J. Chu and Y. Chang
9.1 Introduction 205
9.2 Optical constants and the dielectric function 206
9.3 Theory of band to band optical transition 206
9.4 Near band gap absorption 207
9.5 Analytic expressions and empirical formulas for intrinsic absorption and Urbach tail 209
9.6 Dispersion of the refractive index 216
9.7 Optical constants and related van Hover singularities above the energy gap 217
9.8 Reflection spectra and dielectric function 220
9.9 Multimode model of lattice vibration 221
9.10 Phonon absorption 222
9.11 Raman scattering 225
9.12 Photoluminescence spectroscopy 227
References 231
10 Diffusion in MCT 239
D. Shaw
10.1 Introduction 239
10.2 Self-diffusion 240
10.2.1 Hg self-diffusion 241
10.2.2 Cd self-diffusion 241
10.2.3 Te self-diffusion 241
10.2.4 Self-diffusion in doped material 242
10.2.5 Conclusions 242
10.3 Chemical self-diffusion 243
10.3.1 Composition: X Cd ~ 0.2 243
10.3.2 Composition: 0.198 ¿ X Cd ¿ 0.51 245
10.3.3 Cadmium telluride (CdTe) 245
10.3.4 Conclusions 246
10.4 Compositional interdiffusion 247
10.4.1 ¿d From Cid Profiles of X Cd Versus X 248
10.4.2 Conclusions 252
10.5 Impurity diffusion 253
10.5.1 Group 1 impurities 254
10.5.2 Group 3 and 5 impurities 256
10.5.3 Group 6 and 7 impurities 258
References 260
11 Defects in HgCdTe - Fundamental 263
M. A. Berding
11.1 Introduction 263
11.2 Native point defects in zincblende semiconductor 264
11.3 Measurement of native defect properties and density 266
11.4 Ab initio calculations 268
11.4.1 Defect formation energies 268
11.4.2 Electronic excitation energies 269
11.4.3 Defect free energies 270
11.4.4 Prediction of native point defect densities in HgCdgTe 270
11.5 Future challenges 272
References 272
12 Band Structure and Related Properties of HgCdTe 275
C. R. Becker and S. Krishnamurthy
12.1 Introduction 275
12.2 Parameters 277
12.2.1 Optical bandgap 277
12.2.2 Valence band offset 277
12.2.3 Electron effective mass 279
12.3 Electronic band structure 279
12.3.1 k·p theory 279
12.3.2 Hybrid pseudopotential tight-binding method 281
12.4 Comparison with experiment 288
12.4.1 Optical absorption 288
12.4.2 Auger recombination 289
Acknowledgements 293
References 293
13 Conductivity Type Conversion 297
D. Shaw and P. Capper
13.1 Introduction 297
13.2 Native defects in undoped MCT 298
13.3 Native defects in doped MCT 301
13.4 Defect concentrations during cool down 302
13.5 Change of conductivity type 304
13.5.1 CTC by thermal annealing 304
13.5.2 CTC by dry etching 307
13.6 Dry etching by IBM 307
13.6.1 IBM of vacancy-doped MCT 307
13.6.2 Modeling of IBM 309
13.6.3 IBM of impurity-doped MCT 311
13.6.4 Stability (relaxation) of CTC layers with respect to time and temperature after IBM 311
13.7 Plasma etching 313
13.7.1 CTC with Ar and Hg plasmas 313
13.7.2 CTC with H 2 /CH 4 plasmas
313
13.8 Summary 314
References 315
14 Extrinsic Doping 317
D. Shaw and P. Capper
14.1 Introduction 318
14.2 Impurity activity 319
14.2.1 Group I impurities 320
14.2.2 Group II impurities 320
14.2.3 Group III impurities 321
14.2.4 Group IV impurities 321
14.2.5 Group V impurities 321
14.2.6 Group VI impurities 321
14.2.7 Group VII impurities 322
14.2.8 Group VIII impurities 322
14.3 Thermal ionization energies of impurities 322
14.3.1 CdTe 322
14.3.2 LWIR and MWIR MCT 323
14.4 Segregation properties of impurities 324
14.4.1 Segregation in CdTe 325
14.4.2 Segregation in LWIR and MWIR MCT 326
14.5 Traps and recombination centers 327
14.5.1 Minority carrier lifetime in MCT 328
14.5.2 Reducing the concentrations of SRH centers 328
14.6 Donor and acceptor doping in LWIR and MWIR MCT 330
14.6.1 In 330
14.6.2 Iodine 331
14.6.3 Au 332
14.6.4 As 332
14.7 Residual defects 334
14.8 Conclusions 335
References 335
15 Structure and Electrical Characteristics of Metal/MCT Interfaces 339
R. J. Westerhout, R. H. Sewell, J. M. Dell, L. Faraone and C. A. Musca
15.1 Introduction 340
15.2 Reactive/intermediately reactive/nonreactive categories 341
15.2.1 Au/MCT interface 341
15.2.2 In/MCT interface 341
15.2.3 Ag/MCT interface 342
15.2.4 Cu/MCT interface 343
15.2.5 Sb/MCT interface 343
15.2.6 Cr/MCT interface 343
15.3 Ultrareactive/reactive categories 344
15.3.1 Al/MCT interface 344
15.3.2 Pt/MCT interface 345
15.3.3 Sm/MCT interface 345
15.3.4 Ti/MCT interface 345
15.3.5 Pd/MCT interface 346
15.3.6 Sn/MCT interface 346
15.3.7 Conclusion 347
15.4 Passivation of MCT 347
15.4.1 Introduction 347
15.4.2 Device design and passivation requirements 347
15.4.3 Criteria for good passivation 348
15.4.4 Properties for non CdTe passivant films on MCT 348
15.4.5 Passivation of MCT with CdTe 348
15.4.6 Conclusion 354
15.5 Contacts to MCT 354
15.5.1 Introduction 354
15.5.2 Metal/MCT contacts 354
15.5.3 Schottky barrier contacts 355
15.5.4 Ohmic contacts 356
15.5.5 Conclusions 356
15.6 Surface Effects on MCT 356
15.6.1 Introduction 356
15.6.2 Surface recombination velocity 357
15.6.3 Recombination velocity at heterointerfaces 357
15.6.4 Gated photoconductors 358
15.6.5 Gated photodiodes 358
15.6.6 Conclusions 359
15.7 Surface Structure of CdTe and MCT 359
15.7.1 Introduction 359
15.7.2 Surface structure and epitaxial growth 360
15.7.3 RHEED analysis of the (211) surface 361
15.7.4 Reconstruction of the (110) surface 363
15.7.5 Reconstruction of the (100) surface 365
15.7.6 Reconstruction of (111) surfaces 367
15.7.7 Conclusion 370
References 370
16 MCT Superlattices for VLWIR Detectors and Focal Plane Arrays 375
J. Garland
16.1 Introduction 376
16.2 Why HgTe-based superlattices 377
16.2.1 Advantages of HgTe/CdTe superlattices over MCT alloys 378
16.2.2 Problems with the use of HgTe/CdTe superlattices in VLWIR detectors and FPAs 381
16.2.3 Use of HgTe/CdTe superlattices as buffer layers on CdZnTe before MCT growth 382
16.2.4 Use of MCT-based superlattices as thermoelectric coolers for MCT detectors 383
16.2.5 HgTe/ZnTe superlattices 383
16.3 Calculated properties 384
16.3.1 Normal electronic band structure: band structures and optical absorptivities 384
16.3.2 Inverted electronic band structure: band structure and optical absorptivity 385
16.4 Growth 386
16.4.1 Substrate orientation 387
16.4.2 Doping 388
16.5 Interdiffusion 389
16.5.1 Effect of interdiffusion on the bandgap and optical absorption spectra 390
16.5.2 Measuring interdiffusion by X-ray diffraction 391
16.5.3 Measuring interdiffusion by STEM 393
16.6 Conclusions 395
Acknowledgements 396
References 396
17 Dry Plasma Processing of Mercury Cadmium Telluride and Related II-VIs 399
A. J. Stoltz
17.1 Introduction 400
17.2 Effects of plasma gases on MCT 401
17.3 Plasma parameters 403
17.3.1 Physics of plasmas 403
17.3.2 Hydrogen variations 405
17.3.3 Plasma parameters-effects on II-VI semiconductors 408
17.3.4 Plasma parameter change ECR to ICP 410
17.4 Characterization -surfaces of plasma-processed MCT 411
17.4.1 Surface chemical analysis 411
17.4.2 In vacuo crystallographic surface analysis 413
17.4.3 Ex vacuo atomic force microscopy 413
17.5 Manufacturing issues and solutions 416
17.5.1 Etch lag and lateral photoresist etching-ion angular distribution (microloading, RIE lag) 416
17.5.2 Macroloading 418
17.6 Plasma processes in the production of II-VI materials 420
17.6.1 Trench delineation 421
17.6.2 Type conversion 422
17.6.3 Via formation substitutionally doped MCT 422
17.6.4 Microlenses and antireflective structures 422
17.6.5 Cleaning 424
17.7 Conclusions and future efforts 424
References 425
18 MCT Photoconductive Infrared Detectors 429
I. M. Baker
18.1 Introduction 429
18.1.1 Historical perspective and early detectors 430
18.1.2 Introduction to MCT 431
18.1.3 MCT photoconductive arrays 431
18.2 Applications and sensor design 432
18.3 Photoconductive detectors in MCT and related alloys 434
18.3.1 Introduction to the technology of photoconductor arrays 435
18.3.2 Theoretical fundamentals for LW arrays 436
18.3.3 Special case of MW arrays 439
18.3.4 Nonequilibrium effects in photoconductors 439
18.4 SPRITE detectors 440
18.4.1 Introduction to the SPRITE detector 440
18.4.2 SPRITE operation and performance 441
18.4.3 Detector design and systems applications 444
18.5 Conclusions on photoconductive MCT detectors 444
Acknowledgements 445
References 445
Part Three - Applications 447
19 HgCdTe Photovoltaic Infrared Detectors 449
I. M. Baker
19.1 Introduction 450
19.2 Advantages of the photovoltaic device in MCT 450
19.3 Applications 450
19.4 Fundamentals of MCT photodiodes 451
19.4.1 Ideal photovoltaic devices 451
19.4.2 Nonideal behavior in MCT diodes 452
19.5 Theoretical foundations for MCT array technology 454
19.5.1 Thermal diffusion currents in MCT 454
19.5.2 Thermal generation through traps in the depletion region 455
19.5.3 Interband tunnelling 455
19.5.4 Trap-assisted tunnelling 456
19.5.5 Impact ionization 456
19.5.6 Photocurrent and quantum efficiency 457
19.5.7 Excess noise sources in MCT diodes 457
19.6 Manufacturing technology for MCT arrays 457
19.6.1 Junction forming techniques 458
19.6.2 Via-hole technologies using LPE 458
19.6.3 Planar device structures using LPE 459
19.6.4 Double layer heterojunction devices (DLHJ) 460
19.6.5 Wafer-scale processes using vapor phase epitaxy on low-cost substrates 461
19.6.6 MCT 2D arrays for the 3-5 ¿m (MW) band 463
19.6.7 MCT 2D arrays for the 8-12 ¿m (LW) band 463
19.7 Towards GEN III detectors 463
19.7.1 Two-color array technology 463
19.7.2 Higher operating temperature (HOT) device structures 464
19.8 Conclusions and future trends for photovoltaic MCT arrays 465
References 465
20 Nonequilibrium, Dual-Band and Emission Devices 469
C. Jones and N. Gordon
20.1 Introduction 469
20.2 Nonequilibrium devices 470
20.2.1 Introduction and theory 470
20.2.2 Nonequilibrium detectors 473
20.2.3 Emitters and other uses 476
20.3 Dual-band devices 476
20.3.1 Introduction 476
20.3.2 Mesa diodes 477
20.3.3 Planar diodes 482
20.3.4 Stacked loophole 483
20.4 Emission devices 484
20.5 Conclusions 489
References 489
21 HgCdTe Electron Avalanche Photodiodes (EAPDs) 493
I. Baker and M. Kinch
21.1 Introduction and applications 493
21.2 The avalanche multiplication effect 494
21.3 Physics of MCT EAPDs 495
21.3.1 Phenomenological model for EAPDs 496
21.3.2 Energy dispersion factor, ¿(E) 497
21.3.3 Impact ionization threshold energy 499
21.3.4 EAPD diodes at room temperature 501
21.3.5 MCT EAPD dark currents 503
21.3.6 MCT EAPD excess noise 504
21.4 Technology of MCT EAPDs 504
21.4.1 Theoretical foundations for the EAPD device technology 504
21.4.2 Via-hole technology 505
21.4.3 Planar and advanced structures 506
21.5 Reported performance of arrays of MCT EAPDs 506
21.5.1 Avalanche gain 506
21.5.2 Noise figure 507
21.5.3 Dark current 507
21.6 LGI as a practical example of MCT EAPDs 510
21.7 Conclusions and future developments 511
References 511
22 Room Temperature IR Photodetectors 513
J. Piotrowski and A. Piotrowski
22.1 Introduction 513
22.2 Performance of room temperature infrared photodetectors 514
22.2.1 Generalized model 514
22.2.2 Reduced volume devices 517
22.2.3 Design of high temperature photodetectors 518
22.3 HgCdTe as a material for room temperature photodetectors 519
22.3.1 Ultimate performance of HgCdTe devices 519
22.3.2 Non-equilibrium devices 521
22.3.3 3D high-temperature photodetector concept 522
22.4 Photoconductive devices 522
22.5 PEM, magnetoconcentration, and Dember IR detectors 524
22.5.1 PEM detectors 524
22.5.2 Magnetoconcentration detectors 525
22.5.3 Dember detectors 526
22.6 Photodiodes 526
22.6.1 Dark current and resistance of near room temperature photodiodes 527
22.6.2 Practical HgCdTe photodiodes 527
22.7 Conclusions 535
References 535
Index 539
