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Mercury Cadmium Telluride
Growth, Properties and Applications
Peter Capper (Edited by), James Garland (Edited by), Safa O. Kasap (Series edited by), Arthur Willoughby (Series edited by)
9780470697061, Wiley
Hardback, published 22 October 2010
608 pages
25.2 x 17.3 x 3.6 cm, 1.157 kg
Mercury cadmium telluride (MCT) is the third most well-regarded semiconductor after silicon and gallium arsenide and is the material of choice for use in infrared sensing and imaging. The reason for this is that MCT can be ‘tuned’ to the desired IR wavelength by varying the cadmium concentration. Mercury Cadmium Telluride: Growth, Properties and Applications provides both an introduction for newcomers, and a comprehensive review of this fascinating material. Part One discusses the history and current status of both bulk and epitaxial growth techniques, Part Two is concerned with the wide range of properties of MCT, and Part Three covers the various device types that have been developed using MCT. Each chapter opens with some historical background and theory before presenting current research. Coverage includes:
Series Preface xxi Preface xxiii Foreword xxvii List of Contributors xxxi Part One - Growth 1 1 Bulk Growth of Mercury Cadmium Telluride (MCT) 3 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 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 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 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 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 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 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 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 θ) 192 8.8.4 Thermal conductivity (K θ) 194 8.8.5 Conclusion 197 References 197 9 Optical Properties of MCT 205 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 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 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 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 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 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 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 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 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 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 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 μm (MW) band 463 19.6.7 MCT 2D arrays for the 8–12 μm (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 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 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, α(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 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
P. Capper
A. Noda, H. Kurita and R. Hirano
S. Adachi
J. Garland and R. Sporken
P. Capper
C. D. Maxey
J. Garland
M. Martyniuk, J. M. Dell and L. Faraone
J. Chu and Y. Chang
D. Shaw
M. A. Berding
C. R. Becker and S. Krishnamurthy
D. Shaw and P. Capper
D. Shaw and P. Capper
R. J. Westerhout, R. H. Sewell, J. M. Dell, L. Faraone and C. A. Musca
J. Garland
A. J. Stoltz
I. M. Baker
I. M. Baker
C. Jones and N. Gordon
I. Baker and M. Kinch
J. Piotrowski and A. Piotrowski
Subject Areas: Mechanical engineering & materials [TG]
