II―VI Semiconductor Compounds

The following sections are included:

• Dedication

• INTRODUCTION

• CONTENTS

The following sections are included:

• INTRODUCTION

• EXPERIMENTAL TECHNIQUES
  • Back-Reflection Laue Method
  • Double-Crystal Diffraction Technique
  • Lang Topography
  • Extremely Asymmetric Bragg Reflection Topography

• CHARACTERISATION OF MICROSTRUCTURE
  • Anisotropic Mosaic Structure
    • Rocking curves of anisotropic structure
    • Topographs of anisotropic mosaic structure
    • Model of anisotropic mosaic structure
  • Twins
    • Twinned sample
    • Laue photograph of twins
    • Maps of twins
    • Topographs of twins
  • Dislocations
    • Extinction of dislocations
    • Determination of dislocations
  • Characterisation of Near-Surface Regions
    • Two diffraction geometries of GIABD
    • Penetration depth of EABRT
    • Example of EABRT
  • Integrated Intensities

• FOUR-CIRCLE DIFFRACTOMETRY
  • Instrument and Preliminary Studies
  • Integrated Intensities

• SUMMARY

• ACKNOWLEDGEMENTS

• REFERENCES

The development of new precursors has led to advances in MOVPE growth of both narrow and wide band gap II-VI's. Adducts such as Me₂ZnNEt₃ have been used successfully to eliminate the formerly severe problem of pre-reaction during the growth of ZnSe and ZnS. A similar approach has led to the in-situ control of pre-reaction in CdSe and CdS growth. Advances made in the chemistry of cadmium and tellurium precursors have resulted in the growth of high quality CdHgTe at lower temperatures. These recent developments are critically reviewed and possible growth mechanisms are discussed.

The following sections are included:

• Introduction

• Crystal Growth

• Far Infrared Spectroscopy
  • Reflection
  • Scattering Mechanisms

• Raman Scattering
  • Polarization Dependence
  • Effective Dielectric Constant
  • Transverse Effective Charge
  • Photon Energy Dependence of LO Phonon

• Ellipsometry
  • Dielectric Function
  • Critical Point Energy
  • Broadening Parameter

• Acknowledgments

• References

The II-VI semiconductors ZnX^VI, CdX^VI, and HgX^VI are known to have a metal d band inside the main valence band. Using all-electron self-consistent electronic structure techniques, we study their effects on valence properties. For II-VI semiconductors, we find that p-d repulsion and hybridization (i) lower the band gaps, (ii) alter the sign of the crystal-field splitting, (iii) reduce the spin-orbit splitting, (iv) change the valence band offset between common-anion semiconductors, and (v) increase the equilibrium lattice parameters, p-d repulsion is also shown to be responsible for the anomalously small band gaps in chalcopyrites, and for the negative exchange splitting in MnTe. We also study the electronic structure of ordered and random II-VI substitutional alloys and identify the mechanism for their band gap narrowing. The random alloys are represented by the “special quasirandom structures.” We show how chemical and structural perturbations lead to (i) distinct A-like and B-like features in the density of states and (ii) different C-like features associated with fluctuations in the local environments around the common sublattice.

The following sections are included:

• Introduction

• Experimental Observations

• Configuration-Interaction Picture
  • Valence, Charge and Oxidation States
  • Weak Hybridization Limit
  • Effects of Hybridization
  • Model Hamiltonian and Wavefuntions
  • Model Parameters

• Interpretation of Experimental Data
  • d → d Optical Transitions
  • Ionization Energies and Multiple Charge State
  • Photoemission spectroscopy

• Ligand-Field Theory

• One-Electron Theory

• Conclusion

• Acknowledgements

• REFERENCES

The following sections are included:

• Introduction

• General properties of rare earth ions

• RE doping of II-VI compounds

• Nature of emitting RE³⁺ centers

• Inter center energy transfer processes

• Impact excitation processes

• Impact ionization processes
  • Carrier trapping via RE excited state
  • RE bound exciton state formation

• Conclusions

• References

The following sections are included:

• Introduction
  • Band structure of doped Semiconductors
    • Shallow donors at low concentrations
    • Shallow donors at intermediate concentrations
    • Heavily doped semiconductors

• Linear optical properties of doped CdS
  • Not intentionally doped crystals
  • Intentionally doped crystals

• Optical nonlinearities of doped CdS

• Summary

• Acknowledgements

• References

The following sections are included:

• Introduction

• Sample Preparation and Structure Characterization

• Optical Properties of (Zn, Mn) and (Cd, Mn) Chalcogenide Quantum Wells and Superlattices
  • Exciton and donor-acceptor-pair transitions
  • Exciton States in Asymmetric Double-Quantum Wells
  • Phonons

• Magneto-Optical Properties Due to s,p - d Exchange Interaction
  • Magnetooptical Anisotropy
  • Magnetic Polaron Effects
  • Magnetic Phase Transition

• Concluding Remarks

• References

The following sections are included:

• Introduction

• The Bypassing Mechanism

• Energy Transfer Processes
  • One-Center Energy Transfer (Auger Recombination) Processes
  • Two-Center Energy Transfer (Auger Recombination) Processes
  • Three-Center Energy Transfer (Auger Recombination) Processes
    • Energy transfer from DAP to a deep impurity
    • Energy transfer from DAP to another donor (acceptor) impurity

• Conclusions

• References

The following sections are included:

• INTRODUCTION

• THE CRYSTALS

• ELECTRONIC BAND STRUCTURE

• OPTICAL PROPERTIES

• MAGNETIC PROPERTIES
  • Co-Co Pairs
  • Specific heat
  • Susceptibility
  • Magnetization
  • The nature of d-d exchange interaction
  • The model for the paramagnetic phase

• CONCLUSIONS

• References

The following sections are included:

• Introduction

• Confinement of charge carriers

• Strain
  • Structural Effects of Strain
  • Band structure

• Band lineups
  • ZnSe-ZnS
  • CdSe-ZnSe
  • CdS-ZnS
  • CdTe-ZnTe
  • ZnTe-ZnSe
  • CdSe-CdS
  • ZnSe-CdS
  • ZnTe-CdSe
  • Band offsets- Summary

• Optical assessment of II-VI superlattices
  • Interface Stability
  • Critical Thickness
  • Well width uniformity

• Electric field effects
  • Extrinsic fields
  • Intrinsic fields

• Optical Properties and Disorder
  • Jim measurement of exciton binding energies
  • Quantitative aspects of Disorder
    • Stokes' shifts
    • Fluorescence line narrowing
    • Photoluminescence decay of II-VI superlattices

• Devices
  • Blue electroluminescence
  • Blue-green lasers
  • Other devices

• Prospects

• Acknowledgements

• References

The following sections are included:

• Introduction

• Growth of ZnSe-ZnTe Superlattices
  • Lattice Mismatch and Buffer Layer
  • Surface Reconstruction
  • Crystal Growth Methods

• Interface of ZnSe-ZnTe Superlattice Structures
  • Interdiffusion
  • X-ray, Raman and RBS Measurements

• Photoluminescence of ZnSe-ZnTe Superlattices
  • Band Structure by Kronig-Penig Model
  • Proposed Band Structure by band discontinuity
  • Band Structure by k·P Theory
  • Interband Transitions
  • Excitons bound at isoelectronic Te Atoms and Clusters

• Doping Characteristics of ZnSe-ZnTe Superlattices

• Raman Scattering of ZnSe-ZnTe Superlattices
  • Raman Scattering Spectra
  • Thickness Ratio Dependence
  • Phonon Folding
  • Resonant Raman Scattering

• Summary

• Acknowledgments

• References

The following sections are included:

• INTRODUCTION

• ELECTRONIC PROCESSES AT ZERO MAGNETIC FIELD
  • Band Structure (B = 0)
  • Experimental Observations – Electron and Hole Transport

• MAGNETIC FIELD EFFECTS
  • Band Structure (B > 0)
  • Experimental Observations – Magneto-Optical Phenomena

• SUMMARY

• REFERENCES

II-VI compounds are widely used in todays materials science as constituents of superlattices and heterostructures. In most of the cases these material systems are strained macro-scopically, because of lattice misfit and/or differences in the thermal expansion coefficients. In addition, -strong piezoelectric fields may be present in the constituents which, being of the non-centrosymmetric cubic crystal class T_d, are piezoelectric as well. In the presence of a hydrostatic pressure these elastic and piezoelectric fields exhibit a linear change in the first order. Combined Raman scattering measurements under pressure from heterostructures grown along [001], [111] and [110], can lead to information about the phonon frequency shifts due to strains and piezoelectric fields. Such information, in the form of the so-called phonon deformation potentials, is used for strain characterization and also for the evaluation of useful crystal parameters, like the electrostriction coefficients. These aspects of strain, pressure and piezoelectric effects are reviewed for the most general case of II-VI constituents grown along an arbitrary direction. Specific applications are worked out in the case of ZnSe/GaAs, ZnSe/ZnS, and ZnTe/ZnSe systems.

Crystal structures and electronic structures of Zinc-compound strained superlattices are calculated by the first-principles pseudopotential method. It is shown that strain causes large level splitting and that the strain-induced light-hole gap appears in some superlattices. However, strained superlattices become quantum well systems of good quality for the states around the fundamental gap. With varying the strain environment, band offset drastically changes due to the energy-level shift by strain and charge transfer at the interface, mechanism of which is analyzed in order to predict the general feature of the variation of offset in strained superlattices.

A review on the applications of II-VI compounds for light emitters, detectors, linear and nonlinear optical devices, semiconductor electronics and other devices is given. Recent advances made possible by heterojunction and quantum structures are presented. Advantages of using II-VI compounds in these applications are discussed and competing technologies are pointed out. Operation principles and structures of the devices are described. Device possibilities for future applications are indicated.

The preparation methods and electrical properties of polycrystalline thin film CdS/CdTe solar cells and of other solar cells based on II-VI compound semiconductors are reviewed. The problems for achieving high efficiency of the CdS/CdTe solar cells are discussed.

This chapter provides an overview of the blue-green light emitting devices with an emphasis on laser diodes made of ZnSe-based materials. The physics and technological status of epitaxial layer growth, doping, and ohmic contacts are presented. The theoretical model of the strained-layer quantum well laser is reviewed for ZnCdSe/ZnSSe heterostructures. The current state and future prospects of blue-green quantum well lasers are discussed.

In this chapter, the organometallic vapor phase epitaxy (OMVPE) of HgCdTe by the direct alloy growth process (DAG) is described. The issues of uniformity, extrinsic doping, low temperature growth, choice of substrates and suitability of the materials for device applications are addressed. These results suggest that OMVPE will be a viable growth method for HgCdTe for future infrared device fabrication.

HgCdTe, a preferred semiconductor material for infrared (IR) detectors, is tunable over a broad spectral range. However, the HgCdTe devices in use today consist of rather primitive p-n homojunctions or heterojunctions. Recent developments in molecular beam epitaxy (MBE) HgCdTe growth and doping are giving access to device structures that were previously unachievable. This paper presents results on MBE HgCdTe growth, in situ doping and device performance of conventional p-on-n photodiode detectors and advanced HgCdTe heterostructures (p-i-n photodetectors, n-p-n dual-band photodiodes and double-heterostructure injection lasers) used for electro-optical IR applications.

The following sections are included:

• INTRODUCTION

• MATERIAL AND THIN FILM PREPARATION

• EXPERIMENTAL RESULTS
  • Pb_1-xHg_xS films
  • Pb_1-xHg_xSe films
  • Pb_1-xHg_xTe films

• DISCUSSION

• CONCLUSION

• REFERENCES

The following sections are included:

• INTRODUCTION

• TRANSPORT PROPERTIES IN POLYCRYSTALLINE SEMICONDUCTORS
  • Electrons and Holes in Semiconductors
  • Surface Scattering Phenomena
  • Mobility of Charge Carriers
  • Experimental Results

• REFERENCES

The following sections are included:

• SUBJECT INDEX