E. Corbin

Auger recombination in strained SixGe1−x/Si superlattices

Abstract A full scale microscopic calculation of Auger rates is performed on Si x Ge 1− x /Si superlattice structures with widely different parameters. We show that Auger rates are reasonably independent of superlattice parameters with Auger rates of some two orders of magnitude larger than that of bulk silicon. Bandstructure is found to be the dominant factor in the calculation and a simple geometrical model is devised to investigate different types of structure.

Quantitative theory of scattering in antimonide-based heterostructures with imperfect interfaces

We report quantitative calculations of carrier lifetimes in imperfect GaxIn1−xSb/InAs superlattice structures. A microscopic description of imperfections including substitutional anions and interface islands is obtained through a novel strain-dependent empirical pseudopotential calculation. The T matrix of scattering theory is used to take our calculations of scattering lifetimes beyond the Born approximation, including multiple scattering events. Carrier lifetimes are related to the microscopic nature of the defects, their proximity to the interfaces, and the size and shape of interface islands. Anomalous effects due to lattice relaxation are seen to alter hole lifetimes, and their dependence upon position. For isolated isovalent anion defects we predict electron and hole lifetimes as low as 0.2 and 0.8 μs, respectively, for typical defect concentrations.

Structural parameters governing properties of GaInSb/InAs infra-red detectors

A series of GaInSb/InAs heterostructures for infra-red detector devices are studied using a strain-dependent empirical pseudopotential scheme. The effect of a number of structural design parameters upon key properties of the structures for detector applications is examined, through the calculation of dynamical characteristics in the presence of commonly occurring defects and through optical absorption spectra. Large changes in the scattering cross-sections of particular defects are related to variations in the superlattice layer widths, enabling the wave-function engineering of optimised detector structures with regard to lifetimes.

Microscopic description of electronic structure and scattering in disordered antimonide-based heterostructures

Quantitative theoretical predictions of the carrier lifetimes in a number of imperfect GaxIn1−xSb–InAs superlattices are presented. Strain-dependent empirical pseudopotentials are used to provide a microscopic description of the stationary states in the structures and scattering theory is employed to extract lifetime information. The effect of interface islands is examined, and lifetimes are found to depend upon the detailed size, shape, and composition of the islands. The effect of higher order multiple scattering events is seen to be significant. For isolated isovalent Sb substitutional defects in the InAs layers, a lifetime of ≈0.4 μs is found to be typical. This is shown to be an order of magnitude shorter than in the case of As defects in the alloy layers.

InAs/ and InAs/ superlattices for infrared applications

We report a full-scale pseudopotential study of the optical properties of InAs/ and InAs/ superlattices, with particular emphasis on the infrared range of wavelengths. For both structures we examine the detailed origin of the absorption response and how cutoff wavelength varies with the period of the superlattice and with the alloy concentration. This entails a discussion of how wavefunction localization, band mixing and energy band dispersion can affect the absorption coefficient. Particular attention is paid to structures with cutoff wavelength in the ranges 2-5 m and 10-13 m. Calculated absorption spectra are compared with examples obtained experimentally. Although agreement between the spectra is good, it is found that neither the sharp features nor the absolute magnitude is reproduced adequately by the electronic structure obtained from idealized systems. Comparison of the bandgap with the gap between the highest two valence states allows structures where certain Auger recombination processes may be inhibited to be indicated. The effects of alloy scattering in the InAs/ system has also been investigated. A second-order perturbation theory calculation of the linewidth associated with the alloy potential suggests that the effects of alloy scattering are too large to be modelled as a perturbation of the virtual crystal case. A full-scale treatment is required to quantify this effect.

Optical spectra and recombination in Si–Ge heterostructures

Abstract We have studied optical spectra in p-type Si/SiGe heterostructures designed for applications in the range of 1–20 μm wavelengths. Our results have been obtained in full-scale microscopic calculations and we present optimized device structures for thermal imaging devices. We also studied in detail the effects of substrate and barrier doping on the Fermi level and the resultant thermal offsets, an important parameter for determining the magnitude of the dark current in any device. Auger rates are presented for band-to-band and intra-valence band processes and compared with data for bulk materials.

Structure and doping optimization of SiGe heterojunction internal photoemission detectors for mid-infrared applications

We present full-scale calculations of highly p-type doped siGe heterojunction internal photoemission (Hip) detectors that operate in the mid-infrared (3- to 5-?m) range of wavelength. We explore the effects of including undoped spacer layers within the highly doped siGe wells, for which a systematic body of experimental data is available to us, and which demonstrates a substantial reduction in the dark current produced by these devices. We model the doping explicitly by means of a screened Coulomb potential, leading to a full description of the resultant impurity band, and compare our calculated optical line shapes with recent state-of-the-art molecular-beam epitaxy experiments. The variation of the optical response with well width, germanium concentration, spacer width and position, and doping concentration are all considered.

Absorption and emission spectra of InAs/Ga1−xInxSb/AlSb nanostructures for infrared applications

Abstract We report an empirical pseudopotential study of the optical properties of several InAs/GaSb superlattice structures, some of these additionally containing Ga 1− x In x Sb and AlSb layers, designed for infrared (IR) applications. The optical absorption and emission spectra of these nanostructures are modelled, with a view to establishing a quantitative link with the microscopic signature of the interface. The emission spectra of the structures having laser applications are investigated for various population inversions. We gauge the role of atomic disorder and determine the degree of alloy layer disorder for which the virtual crystal (VC) approximation provides an adequate description of the overall lineshape. An analogous study was carried out for structures having applications as detectors. We find good agreement between the experimental absorption spectra and our computer model, and investigate the effects of Auger recombination on device performance.

Auger-free SiSiGe quantum well structures for infra-red detection at 10 μm

Abstract We have identified SiSiGe quantum well structures which strongly absorb light in the 10–15 μm range and whose band structure is engineered so that the Auger recombination processes are inhibited, thus enhancing the efficiency of such a device. We also present a comparison of our calculated line-shapes with recent experimental results.

GaAsAlAs and SiSiGe quantum well structures for applications in nonlinear optics

Abstract In this paper we present a full scale evaluation of optical spectra of GaAsAlAs and SiSiGe quantum well structures in which the linear and nonlinear responses arise due to virtual optical transitions between valence minibands. We aim at structures which could operate in the 10–15 μm range. We present both the magnitude and the frequency dependence of the second order susceptibility. We begin with a brief outline of our results concerning the first order response (absorption) in SiSiGe quantum well structures. We show that the valence band structure offers several advantages compared to the conduction band and that it is an excellent material for infrared detector manufacture. We find that—contrary to commonly held views—the strongest contributions to the second order response originate from regions lying farther from the zone centre. Also, the frequency of the peak response does not correspond to that expected from simple models.