Mercury Cadmium Telluride

Dr. Peter Capper is a Materials Team Leader at BAE Systems Infrared Ltd., in Southampton, UK.James Garland is the editor of Mercury Cadmium Telluride: Growth, Properties and Applications, published by Wiley.

• Series Preface xxiPreface xxiiiForeword xxviiList of Contributors xxxiPart One - Growth 11 Bulk Growth of Mercury Cadmium Telluride (MCT) 3P. Capper1.1 Introduction 31.2 Phase equilibria 41.3 Crystal growth 51.3.1 Solid state recrystallization (SSR) 61.3.2 Traveling heater method (THM) 91.3.3 Bridgman 121.3.4 Accelerated crucible rotation technique (ACRT) 131.4 Conclusions 18References 192 Bulk Growth of CdZnTe/CdTe Crystals 21A. Noda, H. Kurita and R. Hirano2.1 Introduction 212.2 High-purity Cd and Te 222.2.1 Cadmium 222.2.2 Tellurium 232.3 Crystal growth 232.3.1 Polycrystal growth 232.3.2 VGF single-crystal growth 242.4 Wafer processing 412.4.1 Process flow 422.4.2 Characteristics 442.5 Summary 48Acknowledgements 48References 493 Properties of Cd(Zn)Te Relevant to Use as Substrates 51S. Adachi3.1 Introduction 523.2 Structural properties 523.2.1 Ionicity 523.2.2 Lattice constant and crystal density 533.2.3 Spontaneous ordering 543.2.4 Structural phase transition 553.3 Thermal properties 553.3.1 Phase diagram 553.3.2 Specific heat and Debye temperature 563.3.3 Thermal expansion coefficient 573.3.4 Thermal conductivity and diffusivity 573.4 Mechanical and lattice vibronic properties 583.4.1 Elastic constant and related parameters 583.4.2 Microhardness 583.4.3 Optical phonon frequency and phonon deformation potential 593.5 Collective effects and some response characteristics 613.5.1 Piezoelectric constant 613.5.2 Fröhlich coupling constant 613.6 Electronic energy-band structure 623.6.1 Bandgap energy 623.6.2 Electron and hole effective masses 643.6.3 Electronic deformation potential 653.6.4 Heterojunction band offset 663.7 Optical properties 673.7.1 The reststrahlen region 673.7.2 The interband transition region 683.7.3 Near or below the fundamental absorption edge 693.8 Carrier transport properties 703.8.1 Low-field mobility 703.8.2 Minority-carrier transport 71References 714 Substrates for the Epitaxial Growth of MCT 75J. Garland and R. Sporken4.1 Introduction 764.2 Substrate orientation 774.3 CZT substrates 784.3.1 Effects of poor thermal conductivity on MCT growth 784.3.2 Effects of substrate crystalline defects on MCT growth 794.3.3 Effects of substrate impurities 804.3.4 Effects of nonuniform substrate composition and substrate roughness 804.3.5 Effects of surface nonstoichiometry and contaminants 814.3.6 Characterization and screening of CZT substrates 814.3.7 Use of buffer layers on CZT substrates 824.4 Si-based substrates 824.4.1 Nucleation and growth of CdTe on Si 834.4.2 The effects of As and Te monolayers 844.4.3 Advantages of CdTe/Si substrates 854.4.4 Disadvantages of CdTe/Si substrates 864.4.5 Reduction of the dislocation density 874.4.6 Passivation of dislocations 884.5 Other substrates 894.6 Summary and conclusions 90References 905 Liquid Phase Epitaxy of MCT 95P. Capper5.1 Introduction 955.2 Growth 965.2.1 Introduction 965.2.2 Phase diagram and defect chemistry 985.2.3 LPE growth techniques 985.3 Material characteristics 1035.3.1 Composition and thickness 1035.3.2 Crystal quality and surface morphology 1055.3.3 Impurity doping and electrical properties 1065.4 Device status 1085.5 Summary and future developments 108References 1106 Metal-Organic Vapor Phase Epitaxy (MOVPE) Growth 113C. D. Maxey6.1 Requirement for epitaxy 1136.2 History 1146.3 Substrate choices 1156.3.1 Orientation 1156.3.2 Material 1166.4 Reactor design 1176.5 Process parameters 1186.6 Metal-organic sources 1196.7 Uniformity 1206.8 Reproducibility 1206.9 Doping 1236.10 Defects 1256.11 Annealing 1276.12 In situ monitoring 1276.13 Conclusions 128References 1287 MBE Growth of Mercury Cadmium Telluride 131J. Garland7.1 Introduction 1317.1.1 The MBE growth technique 1327.2 MBE Growth theory and growth modes 1327.2.1 Growth modes 1337.2.2 Quasiequilibrium theories 1337.2.3 Kinetic theories 1347.3 Substrate mounting 1357.4 In situ characterization tools 1357.4.1 Reflection high-energy electron diffraction 1357.4.2 Spectroscopic ellipsometry 1367.4.3 Other in situ characterization tools 1397.5 MCT nucleation and growth 1397.6 Dopants and dopant activation 1417.7 Properties of MCT epilayers grown by MBE 1437.7.1 Electrical properties 1437.7.2 Optically measurable characteristics 1447.7.3 Structural properties 1447.7.4 Surface defects 1457.8 Conclusions 146References 147Part Two - Properties 1518 Mechanical and Thermal Properties 153M. Martyniuk, J. M. Dell and L. Faraone8.1 Density of MCT 1548.1.1 Introduction 1548.1.2 Variation of Density with X 1548.1.3 Variation of density with temperature 1558.1.4 Conclusion 1588.2 Lattice parameter of MCT 1588.2.1 Introduction 1588.2.2 Variation of Lattice Parameter with X 1588.2.3 Variation with temperature 1608.2.4 Conclusion 1628.3 Coefficient of thermal expansion of MCT 1628.3.1 Introduction 1628.3.2 Variation with X 1628.3.3 Variation with temperature 1638.3.4 Conclusion 1668.4 Elastic parameters of MCT 1668.4.1 Introduction 1668.4.2 Elastic parameter values 1678.4.3 Conclusion 1708.5 Hardness and deformation characteristics of MCT 1708.5.1 Introduction 1708.5.2 Hardness 1708.5.3 Deformation characteristics of MCT 1748.5.4 Photoplastic effect 1808.5.5 Conclusion 1808.6 Phase diagrams of MCT 1818.6.1 Introduction 1818.6.2 Binary systems 1818.6.3 Solid phases 1818.6.4 Quasibinary systems 1838.6.5 Liquidus, solidus, and solvus surfaces 1858.6.6 Thermodynamics 1868.6.7 Conclusion 1878.7 Viscosity of the MCT melt 1878.7.1 Introduction 1878.7.2 Temperature variation of kinematic viscosity of the MCT melt 1878.7.3 Conclusion 1898.8 Thermal properties of MCT 1898.8.1 Introduction 1898.8.2 Specific heat (C p)1898.8.3 Thermal diffusivity (D θ) 1928.8.4 Thermal conductivity (K θ) 1948.8.5 Conclusion 197References 1979 Optical Properties of MCT 205J. Chu and Y. Chang9.1 Introduction 2059.2 Optical constants and the dielectric function 2069.3 Theory of band to band optical transition 2069.4 Near band gap absorption 2079.5 Analytic expressions and empirical formulas for intrinsic absorption and Urbach tail 2099.6 Dispersion of the refractive index 2169.7 Optical constants and related van Hover singularities above the energy gap 2179.8 Reflection spectra and dielectric function 2209.9 Multimode model of lattice vibration 2219.10 Phonon absorption 2229.11 Raman scattering 2259.12 Photoluminescence spectroscopy 227References 23110 Diffusion in MCT 239D. Shaw10.1 Introduction 23910.2 Self-diffusion 24010.2.1 Hg self-diffusion 24110.2.2 Cd self-diffusion 24110.2.3 Te self-diffusion 24110.2.4 Self-diffusion in doped material 24210.2.5 Conclusions 24210.3 Chemical self-diffusion 24310.3.1 Composition: X Cd ∼ 0.2 24310.3.2 Composition: 0.198 ≤ X Cd ≤ 0.51 24510.3.3 Cadmium telluride (CdTe) 24510.3.4 Conclusions 24610.4 Compositional interdiffusion 24710.4.1 ˜d From Cid Profiles of X Cd Versus X 24810.4.2 Conclusions 25210.5 Impurity diffusion 25310.5.1 Group 1 impurities 25410.5.2 Group 3 and 5 impurities 25610.5.3 Group 6 and 7 impurities 258References 26011 Defects in HgCdTe – Fundamental 263M. A. Berding11.1 Introduction 26311.2 Native point defects in zincblende semiconductor 26411.3 Measurement of native defect properties and density 26611.4 Ab initio calculations 26811.4.1 Defect formation energies 26811.4.2 Electronic excitation energies 26911.4.3 Defect free energies 27011.4.4 Prediction of native point defect densities in HgCdgTe 27011.5 Future challenges 272References 27212 Band Structure and Related Properties of HgCdTe 275C. R. Becker and S. Krishnamurthy12.1 Introduction 27512.2 Parameters 27712.2.1 Optical bandgap 27712.2.2 Valence band offset 27712.2.3 Electron effective mass 27912.3 Electronic band structure 27912.3.1 k·p theory 27912.3.2 Hybrid pseudopotential tight-binding method 28112.4 Comparison with experiment 28812.4.1 Optical absorption 28812.4.2 Auger recombination 289Acknowledgements 293References 29313 Conductivity Type Conversion 297D. Shaw and P. Capper13.1 Introduction 29713.2 Native defects in undoped MCT 29813.3 Native defects in doped MCT 30113.4 Defect concentrations during cool down 30213.5 Change of conductivity type 30413.5.1 CTC by thermal annealing 30413.5.2 CTC by dry etching 30713.6 Dry etching by IBM 30713.6.1 IBM of vacancy-doped MCT 30713.6.2 Modeling of IBM 30913.6.3 IBM of impurity-doped MCT 31113.6.4 Stability (relaxation) of CTC layers with respect to time and temperature after IBM 31113.7 Plasma etching 31313.7.1 CTC with Ar and Hg plasmas 31313.7.2 CTC with H 2 /CH 4 plasmas31313.8 Summary 314References 31514 Extrinsic Doping 317D. Shaw and P. Capper14.1 Introduction 31814.2 Impurity activity 31914.2.1 Group I impurities 32014.2.2 Group II impurities 32014.2.3 Group III impurities 32114.2.4 Group IV impurities 32114.2.5 Group V impurities 32114.2.6 Group VI impurities 32114.2.7 Group VII impurities 32214.2.8 Group VIII impurities 32214.3 Thermal ionization energies of impurities 32214.3.1 CdTe 32214.3.2 LWIR and MWIR MCT 32314.4 Segregation properties of impurities 32414.4.1 Segregation in CdTe 32514.4.2 Segregation in LWIR and MWIR MCT 32614.5 Traps and recombination centers 32714.5.1 Minority carrier lifetime in MCT 32814.5.2 Reducing the concentrations of SRH centers 32814.6 Donor and acceptor doping in LWIR and MWIR MCT 33014.6.1 In 33014.6.2 Iodine 33114.6.3 Au 33214.6.4 As 33214.7 Residual defects 33414.8 Conclusions 335References 33515 Structure and Electrical Characteristics of Metal/MCT Interfaces 339R. J. Westerhout, R. H. Sewell, J. M. Dell, L. Faraone and C. A. Musca15.1 Introduction 34015.2 Reactive/intermediately reactive/nonreactive categories 34115.2.1 Au/MCT interface 34115.2.2 In/MCT interface 34115.2.3 Ag/MCT interface 34215.2.4 Cu/MCT interface 34315.2.5 Sb/MCT interface 34315.2.6 Cr/MCT interface 34315.3 Ultrareactive/reactive categories 34415.3.1 Al/MCT interface 34415.3.2 Pt/MCT interface 34515.3.3 Sm/MCT interface 34515.3.4 Ti/MCT interface 34515.3.5 Pd/MCT interface 34615.3.6 Sn/MCT interface 34615.3.7 Conclusion 34715.4 Passivation of MCT 34715.4.1 Introduction 34715.4.2 Device design and passivation requirements 34715.4.3 Criteria for good passivation 34815.4.4 Properties for non CdTe passivant films on MCT 34815.4.5 Passivation of MCT with CdTe 34815.4.6 Conclusion 35415.5 Contacts to MCT 35415.5.1 Introduction 35415.5.2 Metal/MCT contacts 35415.5.3 Schottky barrier contacts 35515.5.4 Ohmic contacts 35615.5.5 Conclusions 35615.6 Surface Effects on MCT 35615.6.1 Introduction 35615.6.2 Surface recombination velocity 35715.6.3 Recombination velocity at heterointerfaces 35715.6.4 Gated photoconductors 35815.6.5 Gated photodiodes 35815.6.6 Conclusions 35915.7 Surface Structure of CdTe and MCT 35915.7.1 Introduction 35915.7.2 Surface structure and epitaxial growth 36015.7.3 RHEED analysis of the (211) surface 36115.7.4 Reconstruction of the (110) surface 36315.7.5 Reconstruction of the (100) surface 36515.7.6 Reconstruction of (111) surfaces 36715.7.7 Conclusion 370References 37016 MCT Superlattices for VLWIR Detectors and Focal Plane Arrays 375J. Garland16.1 Introduction 37616.2 Why HgTe-based superlattices 37716.2.1 Advantages of HgTe/CdTe superlattices over MCT alloys 37816.2.2 Problems with the use of HgTe/CdTe superlattices in VLWIR detectors and FPAs 38116.2.3 Use of HgTe/CdTe superlattices as buffer layers on CdZnTe before MCT growth 38216.2.4 Use of MCT-based superlattices as thermoelectric coolers for MCT detectors 38316.2.5 HgTe/ZnTe superlattices 38316.3 Calculated properties 38416.3.1 Normal electronic band structure: band structures and optical absorptivities 38416.3.2 Inverted electronic band structure: band structure and optical absorptivity 38516.4 Growth 38616.4.1 Substrate orientation 38716.4.2 Doping 38816.5 Interdiffusion 38916.5.1 Effect of interdiffusion on the bandgap and optical absorption spectra 39016.5.2 Measuring interdiffusion by X-ray diffraction 39116.5.3 Measuring interdiffusion by STEM 39316.6 Conclusions 395Acknowledgements 396References 39617 Dry Plasma Processing of Mercury Cadmium Telluride and Related II–VIs 399A. J. Stoltz17.1 Introduction 40017.2 Effects of plasma gases on MCT 40117.3 Plasma parameters 40317.3.1 Physics of plasmas 40317.3.2 Hydrogen variations 40517.3.3 Plasma parameters–effects on II–VI semiconductors 40817.3.4 Plasma parameter change ECR to ICP 41017.4 Characterization –surfaces of plasma-processed MCT 41117.4.1 Surface chemical analysis 41117.4.2 In vacuo crystallographic surface analysis 41317.4.3 Ex vacuo atomic force microscopy 41317.5 Manufacturing issues and solutions 41617.5.1 Etch lag and lateral photoresist etching–ion angular distribution (microloading, RIE lag) 41617.5.2 Macroloading 41817.6 Plasma processes in the production of II–VI materials 42017.6.1 Trench delineation 42117.6.2 Type conversion 42217.6.3 Via formation substitutionally doped MCT 42217.6.4 Microlenses and antireflective structures 42217.6.5 Cleaning 42417.7 Conclusions and future efforts 424References 42518 MCT Photoconductive Infrared Detectors 429I. M. Baker18.1 Introduction 42918.1.1 Historical perspective and early detectors 43018.1.2 Introduction to MCT 43118.1.3 MCT photoconductive arrays 43118.2 Applications and sensor design 43218.3 Photoconductive detectors in MCT and related alloys 43418.3.1 Introduction to the technology of photoconductor arrays 43518.3.2 Theoretical fundamentals for LW arrays 43618.3.3 Special case of MW arrays 43918.3.4 Nonequilibrium effects in photoconductors 43918.4 SPRITE detectors 44018.4.1 Introduction to the SPRITE detector 44018.4.2 SPRITE operation and performance 44118.4.3 Detector design and systems applications 44418.5 Conclusions on photoconductive MCT detectors 444Acknowledgements 445References 445Part Three - Applications 44719 HgCdTe Photovoltaic Infrared Detectors 449I. M. Baker19.1 Introduction 45019.2 Advantages of the photovoltaic device in MCT 45019.3 Applications 45019.4 Fundamentals of MCT photodiodes 45119.4.1 Ideal photovoltaic devices 45119.4.2 Nonideal behavior in MCT diodes 45219.5 Theoretical foundations for MCT array technology 45419.5.1 Thermal diffusion currents in MCT 45419.5.2 Thermal generation through traps in the depletion region 45519.5.3 Interband tunnelling 45519.5.4 Trap-assisted tunnelling 45619.5.5 Impact ionization 45619.5.6 Photocurrent and quantum efficiency 45719.5.7 Excess noise sources in MCT diodes 45719.6 Manufacturing technology for MCT arrays 45719.6.1 Junction forming techniques 45819.6.2 Via-hole technologies using LPE 45819.6.3 Planar device structures using LPE 45919.6.4 Double layer heterojunction devices (DLHJ) 46019.6.5 Wafer-scale processes using vapor phase epitaxy on low-cost substrates 46119.6.6 MCT 2D arrays for the 3–5 μm (MW) band 46319.6.7 MCT 2D arrays for the 8–12 μm (LW) band 46319.7 Towards GEN III detectors 46319.7.1 Two-color array technology 46319.7.2 Higher operating temperature (HOT) device structures 46419.8 Conclusions and future trends for photovoltaic MCT arrays 465References 46520 Nonequilibrium, Dual-Band and Emission Devices 469C. Jones and N. Gordon20.1 Introduction 46920.2 Nonequilibrium devices 47020.2.1 Introduction and theory 47020.2.2 Nonequilibrium detectors 47320.2.3 Emitters and other uses 47620.3 Dual-band devices 47620.3.1 Introduction 47620.3.2 Mesa diodes 47720.3.3 Planar diodes 48220.3.4 Stacked loophole 48320.4 Emission devices 48420.5 Conclusions 489References 48921 HgCdTe Electron Avalanche Photodiodes (EAPDs) 493I. Baker and M. Kinch21.1 Introduction and applications 49321.2 The avalanche multiplication effect 49421.3 Physics of MCT EAPDs 49521.3.1 Phenomenological model for EAPDs 49621.3.2 Energy dispersion factor, α(E) 49721.3.3 Impact ionization threshold energy 49921.3.4 EAPD diodes at room temperature 50121.3.5 MCT EAPD dark currents 50321.3.6 MCT EAPD excess noise 50421.4 Technology of MCT EAPDs 50421.4.1 Theoretical foundations for the EAPD device technology 50421.4.2 Via-hole technology 50521.4.3 Planar and advanced structures 50621.5 Reported performance of arrays of MCT EAPDs 50621.5.1 Avalanche gain 50621.5.2 Noise figure 50721.5.3 Dark current 50721.6 LGI as a practical example of MCT EAPDs 51021.7 Conclusions and future developments 511References 51122 Room Temperature IR Photodetectors 513J. Piotrowski and A. Piotrowski22.1 Introduction 51322.2 Performance of room temperature infrared photodetectors 51422.2.1 Generalized model 51422.2.2 Reduced volume devices 51722.2.3 Design of high temperature photodetectors 51822.3 HgCdTe as a material for room temperature photodetectors 51922.3.1 Ultimate performance of HgCdTe devices 51922.3.2 Non-equilibrium devices 52122.3.3 3D high-temperature photodetector concept 52222.4 Photoconductive devices 52222.5 PEM, magnetoconcentration, and Dember IR detectors 52422.5.1 PEM detectors 52422.5.2 Magnetoconcentration detectors 52522.5.3 Dember detectors 52622.6 Photodiodes 52622.6.1 Dark current and resistance of near room temperature photodiodes 52722.6.2 Practical HgCdTe photodiodes 52722.7 Conclusions 535References 535Index 539