Optical anisotropy of InAs/GaSb broken-gap quantum wells

We investigate in detail the optical anisotropy of absorption of linearly polarized light in InAs/GaSb quantum wells grown on GaSb along the [001] direction, which can be used as an active region of different laser structures. The energy level positions, the wave functions, the optical matrix elements, and the absorption coefficients are calculated using the eight-band k center dot p model and the Burt-Foreman envelope function theory. In these calculations, the Schrodinger and Poisson equations are solved self-consistently taking the lattice-mismatched strain into account. We find that a realistic Hamiltonian, which has the C (2v) symmetry, results in considerable anisotropy of optical matrix elements for different directions of light polarization and different directions of the initial-state in-plane wave vector, including low-symmetry directions. We trace how the optical matrix elements and absorption are modified when spin-orbit interaction and important symmetry breaking mechanisms are taken into account (structural inversion asymmetry, bulk inversion asymmetry, and interface Hamiltonian). These mechanisms result in an almost 100% anisotropy of the absorption coefficients as the light polarization vector rotates in the plane of the structure and in a plane normal to the interfaces

Scattering-assisted electric current in semiconductor superlattices in the Wannier-Stark regime

We have used the Monte Carlo technique to investigate the mechanism of scattering-assisted charge transport in semiconductor superlattices under a strong applied electric field in the Wannier-Stark (WS) regime. The distribution function of quasi-two-dimensional carriers localized in each WS level is calculated, and the contributions of different scattering mechanisms to the total scattering probability are analyzed. Based on these results, the drift velocity is derived as a function of the applied electric field. Due to the LO-phonon-induced resonant transfer of electrons between different spatially localized WS states, our calculated I-V characteristics oscillates with clear negative differential velocity behavior. At the electric field strength such that the Bloch oscillation energy is equal to an integer multiple of the LO phonon energy, a peak appears in the I-V curve. Our theoretical result agrees with the experimental data which was obtained from analyzing the terahertz response of superlattices to picosecond optical pulse excitation

Influence of band state mixing on interband magnetotunnelling in broken-gap heterostructures

We investigate in detail the effect of electron and light- and heavy-hole state mixing on the transmission coefficients and current-voltage characteristics of interband tunnelling broken-gap heterostructures in the presence of a quantizing magnetic field perpendicular to the interfaces. Double-barrier and barrierless resonant tunnelling structures made from InAs, A1Sb, and GaSb are considered. The multiband Burt envelope function theory and the cylindrical approximation are used to obtain analytical solutions for bulk dispersions, and wavefunctions in heterostructures are derived with the transfer matrix method. Taking into account the mixing of the states of different Landau-level indices at the interfaces due to the spin-orbit interaction, we have calculated the transmission coefficients for the coherent tunnelling transitions between the states of various Landau levels, as well as the corresponding tunnelling current components. We have shown that the mixing with the heavy-hole states results in additional peaks of the current density. The tunnelling processes in which the Landau-level index is not conserved may contribute significantly to these additional peaks

Energy transport in one-dimensional thermoelectric systems

The efficiency of thermoelectric power generators and the coefficient of performance of thermoelectric refrigerators increase rapidly in the region of small ZT, and then level off to a flat curve in the region of large ZT, where ZT is the figure of merit. Therefore, simply because one-dimensional thermoelectric materials have high ZT predicted theoretically does not imply that efficient thermoelectric devices can be built with such one-dimensional systems. Our numerical analysis, based on the fundamental thermodynamics which is independent of material systems, with emphasis on energy transport has confirmed this conjecture. (c) 2006 Elsevier Ltd. All rights reserved

Optoelectric spin injection in semiconductor heterostructures without a ferromagnet

We have shown that electron-spin density can be generated by a dc current flowing across a pn junction with an embedded asymmetric quantum well. Spin polarization is created in the quantum well by radiative electron-hole recombination when the conduction electron momentum distribution is shifted with respect to the momentum distribution of holes in the spin-split valence subbands. Spin current appears when the spin polarization is injected from the quantum well into the n-doped region of the pn junction. The accompanied emission of circularly polarized light from the quantum well can serve as a spin polarization detector

Effect of resonant impurity scattering on the carrier dynamics in Si/SiGe quantum wells

We have performed Monte Carlo simulations of the electron drift velocity in a delta-doped Si/SiGe quantum well, for high and low temperatures as well as strong and weak electric field. All scattering matrix elements of intervalley phonons, acoustic phonons, interface roughness, and impurity ions are calculated from the electron wave functions. Special attention was paid to the resonant state scattering which is far from understood both theoretically and experimentally. When the position of the delta-doped donor layer moves from the center of the quantum well to deep inside the barrier, we found for the first time the dramatic effect of the resonant state scattering on electron drift velocity. This effect is dominated by the resonant level broadening, which depends on the position of the delta-doped donor layer. Relative relaxation time of various scattering mechanisms was also derived from the Monte Carlo simulation

Spin-related phenomena in InAs/GaSb quantum wells

We have studied theoretically the influence of symmetry breaking mechanisms: structural inversion asymmetry, bulk inversion asymmetry, relativistic and non-relativistic interface Hamiltonian and warping on spin split of levels Delta E and optical absorption of linearly polarized light in asymmetrical quantum wells made from zincblende materials grown on [001] direction. The AlSb/InAs/GaSb/AlSb broken-gap quantum wells with hybridized electron-hole states sandwiched by the AlSb barriers have been considered. We have obtained substantial contributions of these effects into the absolute values of spin split of electron and hole states and spinflip optical transitions for the initial state in-plane wave vectors along low symmetry directions such as [12]

Optical absorption of polarized light in InAs/GaSb quantum wells

Using an eight-band k . p model Hamiltonian with the Burt-Foreman envelope function theory, we have investigated the optical absorption of both linearly and circularly polarized light, as well as related phenomena in InAs/GaSb broken-gap quantum wells grown along the [0 0 1] direction, with emphasis on the effects of electron-hole hybridization and the various symmetry-breaking mechanisms such as structural inversion asymmetry, bulk inversion asymmetry and interface Hamiltonian. The optical matrix elements exhibit unusual angular dependence in close connection with the spin-flip transitions which are originally forbidden. The spin split of the 2e subband results in two profound absorption peaks for the 1hh-2e transition for both linearly polarized and circularly polarized light. A large lateral optical anisotropy appears in the absorption coefficient of linearly polarized light, which can reach almost 100% with a reducing thickness of the quantum well. For the absorption of circularly polarized light, we found a large enhancement of electron spin polarization in the upper 2e subband, which was generally considered as forbidden if the polarization is along the direction perpendicular to the plane-of-light incidence

Landau level structures and semimetal-semiconductor transition in strained InAs/GaSb quantum wells

Using Burt's envelope function theory and the scattering matrix method, we investigate the hybridized electron-hole Landau levels in strained InAs/GaSb quantum wells sandwiched between wide-gap AlSb barrier layers under electric and a quantizing magnetic fields applied perpendicular to interfaces. At zero magnetic field, in the structures studied here, the lowest electron level in the InAs layer lies below the highest heavy-hole level in the GaSb layer. With increasing magnetic field, the electron levels move up and the heavy-hole levels move down, producing anticrossings and gaps in the Landau level structures. We have found that the Landau level structures depend strongly on the lattice-mismatched strain and the applied voltage. As a result, in the region before anticrossings, the g factor of the lowest electron Landau level has a larger value for the quantum well structure grown on GaSb than that for the structure grown on InAs, while in the region after anticrossings the situation reverses for the g factor. Under low magnetic field, the difference between the electron g factors for the structures grown on different substrates is found to be as large as 10 for zero bias and decreases significantly with increasing bias. When all electron levels become higher than hole levels at high magnetic fields, the semimetal-semiconductor transition occurs. The critical magnetic field B-c for the phase transition in structures grown on InAs is found to be lower than that in structures grown on GaSb. It is also obtained that a positive voltage biased across the InAs/GaSb well essentially decreases B-c. Therefore, for a fixed magnetic field, the semimetal-semiconductor transition can be controlled by a bias