Lowest Electron Subband Research Articles

Band hybridization and spin-splitting in InAs/AlSb/GaSb type II and broken-gap quantum wells

We present a detailed theoretical study on the features of band hybridization and zero-field spin-splitting in InAs/AlSb/GaSb quantum wells (QWs). An eight-band k⋅p approach is developed to calculate the electronic subband structure in such structures. In the absence of the AlSb layer, the hybridized energy gaps can be observed at the anticrossing points between the lowest electron subband and the highest heavy-hole subband in the InAs and GaSb layers respectively. In such a case, the position and magnitude of the gaps are spin-dependent. When a thin AlSb layer is inserted between the InAs and GaSb layers, we find that the lowest electron subband in the InAs layer is only hybridized with the highest light-hole subband which is also hybridized with the highest heavy-hole subband in the GaSb layer. The hybridized energy gaps and spin-splitting in the InAs/AlSb/GaSb QWs are reduced significantly. These results can be used to understand why electrons and holes can be well separated and why relatively high mobilities for electrons and holes can be achieved in InAs/AlSb/GaSb type II and broken-gap QWs. The present study is relevant to the applications of InAs/GaSb based QW structures as new generation of high-density and high-mobility electronic devices.

Spin splitting in the electron subband of asymmetricGaAs/AlxGa1−xAsquantum wells: The multiband envelope function approach

The dependence on carrier concentration of the anisotropic spin splitting of the lowest electron subband in asymmetrically doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells is determined. We employ the multiband envelope function approach based on 8\ifmmode\times\else\texttimes\fi{}8 and $14\ifmmode\times\else\texttimes\fi{}14\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonians. Our self-consistent calculations yield results in quantitative agreement with experimental data obtained from inelastic light scattering.

Coherent Control of Light Absorption and Carrier Dynamics in Semiconductor Nanostructures

We present two examples of coherent control of inter(sub)band transitions in a semiconductor double well by coherent light sources. Accounting for the upper hole subband and two lowest electron subbands, a microscopic theoretical analysis shows that electron-hole pair generation by a sub-picosecond pump pulse can be controlled by the intensity and the phase of a dc microwave field which resonantly couples the two electron subbands. Light absorption can be either enhanced or reduced. Secondly, it is shown that proper combination of two pulsed laser fields allows control of electron inter(sub)band transitions and final-state population, i.e., the formation of indirect versus direct excitons.

Effects of quantum confinement and strain in Zn1-xCdxSe/ZnSe strained-layer superlattices

The effects of strain and quantum confinement in Zn1-xCdxSe/ZnSe strained-layer superlattices grown by molecular beam epitaxy on the GaAs(100) substrates were studied using photoluminescence (PL) at 4.4 K. The sample with 30 periods and total thickness of 3600 AA was directly grown on the substrate without a buffer layer. The PL spectrum shows a single peak that is attributed to the free excitons between the lowest electron subband and ground heavy-hole subband of the Zn1-xCdxSe wells. The blue shifts of the excitonic peaks in the PL spectra induced by the effects of strain and quantum confinements were calculated on the basis of deformation potential theory and Bastard's method, respectively. In addition, the temperature dependence of the PL features of the sample was studied both theoretically and experimentally in detail. The experimental results coincide with the theoretical predictions very well.

Electron relaxation times due to the deformation-potential interaction of electrons with confined acoustic phonons in a free-standing quantum well.

The confined acoustic phonons in free-standing quantum wells are considered in detail. The Hamiltonian describing interactions of the confined acoustic phonons with electrons in the approximation of the deformation potential and the corresponding electron transition probability density are derived. They are used to analyze the electron scattering times (inverse scattering rate, momentum relaxation time, and the energy relaxation time) in the test-particle approximation as well as in the kinetic approximation. It is shown that the first dilatational mode makes the main contribution to electron scattering in the lowest electron subband. The contribution of the zeroth mode and the second mode are also essential while the modes of higher order are insignificant. Our analysis is performed for both nondegenerate and degenerate electron gases. It is shown that electron scattering by confined acoustic phonons interacting through the deformation potential is substantially suppressed up to the electron energies corresponding to the energy of the first dilatational mode.

Optical characterization of ZnSe/ZnSxSe1−x superlattices pseudomorphically grown on GaAs(100) substrates by molecular beam epitaxy

This article presents optical characteristics of ZnSe/ZnS0.12Se0.88 strained-layer superlattices (SLs) with ZnS0.06Se0.94 buffers pseudomorphically grown on GaAs(100) substrates by molecular beam epitaxy. The SL samples exhibit strong blue luminescence. The main emission peaks in photoluminescence spectra can be attributed to the free exciton transitions between lowest electron subband and ground heavy-hole subband of ZnSe wells. The temperature dependence of PL was investigated in detail. The experimental results of temperature dependence of peak positions and linewidths (FWHM) were fitted to the theoretical calculations, based on Varshni’s formula and broadening model. The activation energies of the samples were derived from the temperature dependence of PL intensities. The effects of strain and quantum confinement in ZnSe/ZnS0.12Se0.88 strained-layer SLs with different well thicknesses of 30, 60, and 120 Å were studied by experiments and theoretical calculations. Theoretical calculations and experimental observations are in reasonable agreement.

Fermi-edge singularities in the optical absorption and emission of doped indirect quantum wires.

Fermi-edge singularities in the optical spectra of doped indirect quantum wires are theoretically analyzed by using a screened Coulomb interaction. In the extreme quantum limit in which the Fermi level lies in the lowest electron subband, strong singularities appear when three requisites are fulfilled: (i) infinite hole mass, (ii) Fermi energy slightly smaller than the intersubband spacing for electrons, and (iii) a transverse separation of the order of 100 nm exists between the electron and the hole. In such a case the optical singularity associated with the bottom of the electron subband is negligible with respect to the one at the Fermi level. These results allow a clarifying discussion on the available experimental information.

Photoluminescence and magneto-transport of wide InGaAsInP modulation doped quantum wells

A photoluminescence (PL) study of 700Å wide modulation doped InGaAsInP quantum wells in magnetic fields up to 10 Tesla, is reported. PL from the lowest electron sub-bands, localised in the space charge potentials at the two sides of the QW, and from higher delocalised sub-bands extending across the whole width of the QW, is observed. The performance of simultaneous Quantum Hall Effect (QHE), Shubnikov-de-Haas and PL measurements is found to be crucial to obtain a reliable understanding of the behaviour of the PL spectra in magnetic field. In particular the QHE provides direct information on the number of filled sub-bands and Landau levels in a complicated system of total carrier density ∼10 12 cm −2, containing several populated electronic sub-bands.

Intersubband relaxation of photoexcited hot carriers in quantum wells

We have probed the dynamics of hot electrons in undoped GaAs/(GaAl)As quantum wells using time-resolved Raman techniques. By tuning the photon energy of a time-delayed weak probe, we study the n=1 to n=2 or n=2 to n=3 intersubban transitions by electronic Raman scattering. The lifetime and the intersubband scattering time of the electrons photoexcited on the lowest electronic subbands are determined separately. Calculation of the rate of intersubband scattering by longitudinal acoustical phonons successfully accounts for the observation of relatively long lived electrons on the second subband of a 215Å MQWS. When the well is narrower, so that the n=2 to n=1 transition can occur with emission of a longitudinal op phonon, we find that the lifetime of carriers in the n=2 band is too short to measure with our technique.

Time-resolved Raman scattering in GaAs quantum wells.

We employ a picosecond--time-resolved Raman-scattering technique to investigate the dynamics of two-dimensional electron-hole plasma confined to a multiple--quantum-well structure composed of layers of GaAs and (GaAl)As. The lifetime and the intersubband scattering time of the electrons photoexcited on the lowest electronic subbands are determined separately. Calculation of the rate of intersubband scattering by longitudinal-acoustical phonons successfully accounts for the observation of relatively long-lived electrons on the second subband of a 215-A\r{} multiple--quantum-well structure.

Energy levels of a hydrogenic donor in GaAs-GaAlAs quantum well structures in a magnetic field

Binding energies of the ground state (1 s) and of excited states (2 p ±) of a hydrogenic donor associated with the lowest electron subband in a GaAs quantum well sandwiched between two semi-infinite layers of Ga 1− x Al x As, are calculated as a function of the size of the well in the presence of an arbitrary magnetic field. A variational approach is followed and these energy levels are calculated for a finite value of the height of the potential barrier. The positively charged ion of the donor atom is assumed to be located at the center of the well and the applied magnetic field is taken to be parallel to the axis of growth of the quantum well structure. As expected, for a given value of the magnetic field, the binding energies are found to be larger than their values in a zero magnetic field. The contribution to the binding energy of a given state due to the magnetic field, at a given field, increases slowly as the well size is reduced.

Observation of double cyclotoron resonance and interband transitions in InAs-GaSb multi-heterojunctions

Abstract Magneto-absorption experiments are performed in an InAs-GaSb multiheterojunction where both electrons and holes are simultaneously confined at the interfaces. Oscillatory characteristics are observed and interpreted in terms of interband transitions from the single hole subband to the two lowest electron subbands, and of cyclotron resonances associated with these subbands. Two electron masses are obtained, arising directly as a consequence of the heterojunction potential.

Space-charge layers on Ge surfaces. I. dc conductivity and Shubnikov—de Haas effect

We have prepared metal-insulator-semiconductor ($M\ensuremath{-}I\ensuremath{-}S$) structure for major symmetry planes of Ge. The samples are examined and characterized systematically by capacitance-voltage ($C\ensuremath{-}V$) and conductivity measurements [$\ensuremath{\sigma}(V)$ and $\frac{d\ensuremath{\sigma}}{d{V}_{g}}$]. Lacquer-coated specimens of (111) surfaces prepared in this work give a new period in the magneto-oscillation spectrum and thus resolve the question of missing electrons raised in the experiments of Weber et al. We determine the energy splitting of the two lowest electron subbands on (111) Ge and predict the occupancy of the lowest level of the threefold degenerate valleys for ${N}_{s}$ above \ensuremath{\sim}1.0 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}2}$.