AlSb Quantum Wells Research Articles

Intersubband light absorption by holes in InAsSb/AlSb quantum well heterostructures

The absorption coefficients of intersubband optical transitions in the valence band of the AlSb/InAs0.84Sb0.16/AlSb quantum wells in framework of the four-band Kane model. It is established that the light absorption by holes may lead to the laser generation breakdown. It is shown that we need to decrease the quantum well width to range a<6 nm to neutralize the negative influence of this effect.

Effect of nonparabolicity of the electron and light-hole energy spectrum on the optical properties of heterostructures with deep AlSb/InAs0.86Sb0.14/AlSb quantum wells

The optical characteristics of heterostructures with deep quantum wells are studied using the AlSb/InAs0.86Sb0.14/AlSb structure within the framework of the four-band Kane model with regard to the nonparabolicity of the carrier energy spectrum. It is demonstrated that consideration of the nonparabolicity increases the number of size-quantization levels in the conduction band. At a quantum-well width of 100 A, the investigated heterostructure contains three size-quantization levels within the parabolic model and six levels within the Kane model. This is due to the fact that the effective mass of high-energy electrons is found to be larger than the mass of electrons at the bottom of the conduction band by a factor of four. It is shown that account for the nonparabolicity only slightly affects the overlap integral for the s and p states, but significantly increases the density of states in the conduction band, which causes considerable growth in the radiation-absorption coefficient.

Intersubband optical transitions in InAs/GaSb quantum wells

The effects of electron-hole hybridization, structural asymmetry with respect to spatial inversion, bulk asymmetry, and the interface Hamiltonian upon the optical absorption of linearly polarized light in broken-gap heterostructures are treated with the use of the eight-band Burt-Foreman envelope function theory and the self-consistent solution of the Schrodinger equation and the Poisson equation. The broken-gap heterostructures, specifically, the AlSb/InAs/GaSb/AlSb quantum wells, grown along the [001] direction offer promise for the fabrication of various devices. The anisotropy induced by the above-listed effects in the dispersion relations of size-quantization subbands and in optical matrix elements is established. The bulk asymmetry and the interface Hamiltonian modify the selection rules for intersubband transitions on the exposure of the structures to linearly polarized light. As a result, the initially forbidden spin-flip transitions are allowed. This brings about a large number of peaks in the dependence of the absorption coefficient on the photon energy, if the light polarization vector is directed along the axis of growth of the structure. If the light polarization vector is in the plane of the structure, the bulk asymmetry and the interface Hamiltonian induce strong longitudinal anisotropy of the absorption due to the hybridization of states with oppositely oriented spins. These effects are comprehensively studied for optical transitions involving hybridized electron-hole states in quantum wells grown on InAs.

Threshold characteristics of an IR laser based on deep InAsSb/AlSb quantum well

The basic threshold characteristics of a semiconductor IR laser based on a heterostructure with deep InAs0.84Sb0.16/AlSb quantum wells (QWs) have been studied. The threshold carrier densities and threshold current densities of radiative and Auger recombination (AR) were found. It is shown that at certain QW parameters the AR rate is strongly (by several orders of magnitude) suppressed. In this case, the emission wavelength falls within the interval 2–3.5 μm, which corresponds to the mid-IR spectral range. The internal quantum efficiency of emission at the lasing threshold was calculated and its dependence on the QW width within the AR suppression range was demonstrated. The laser structure was optimized with respect to the number of QWs.

Theoretical study of auger recombination processes in deep quantum wells

The basic processes and mechanisms of Auger recombination of nonequilibrium carriers in a semiconductor heterostructure with deep InAs0.84Sb0.16/AlSb quantum wells (QWs) are analyzed. It is shown that a zero-threshold Auger recombination process involving two heavy holes predominates in sufficiently narrow QWs, and a resonant process involving two electrons is dominant in wide QWs. The range of QW widths at which the Auger recombination is suppressed in a given structure to the greatest extent (suppression region) is determined. In this case, the threshold process involving two electrons remains the basic nonradiative recombination process, with its probability being several orders of magnitude lower than those for the zero-threshold and resonant mechanisms. In turn, the zero-threshold mechanism involving two electrons is totally impossible in the heterostructure under study because of the large conduction-band offset (which markedly exceeds the energy gap). Also, the range of emission wavelengths that corresponds to the suppression region is estimated. It is shown that the interval calculated belongs to the mid-IR range.

Measurement of the GaSb surface band bending potential from the magnetotransport characteristics of GaSb–InAs–AlSb quantum wells

Low-temperature magnetotransport measurements on GaSb∕InAs∕AlSb coupled quantum well structures with a GaSb cap layer and self-consistent calculations of their electronic structure have led to the determination of the Fermi level at the surface, EFS, of undoped molecular-beam-epitaxy-grown GaSb. EFS is pinned around 0.2eV above the top of the GaSb valence band when the GaSb cap layer width is greater than around 900Å. For smaller GaSb cap widths, EFS decreases with the GaSb width. The undoped GaSb∕InAs∕AlSb heterostructure’s Fermi level is determined by bulk donor defects in the AlSb layer adjacent to the InAs quantum well.

Room-temperature electroluminescence of AlSb∕InAsSb single quantum wells grown by metal organic vapor phase epitaxy

Intense mid-infrared (λ∼2μm) room temperature electroluminescence from metal organic vapor phase epitaxy (MOVPE) grown type-I single AlSb∕InAsSb∕AlSb quantum wells (QWs) is reported. The spectral position of the electroluminescent peaks is in good agreement with k∙p envelope function calculation in the frame of four-band Kane’s model taking into account the intermixing of s and p states in the deep quantum well. A four times increase of the emission intensity with temperature increasing from 77to300K can be explained by highly efficient radiative recombination of the electrons injected into the narrow AlSb∕InAsSb∕AlSb QW due to its specific design, leading to Auger process suppression.

Large negative persistent photoconductivity in InAs∕AlSb quantum wells

Negative persistent photoconductivity in backgated InAs∕AlSb quantum wells under red and blue light emitting diode illumination is observed to depend on both illumination energy and intensity. During these experiments the electron sheet density is varied by one order of magnitude from 5.6×1011 to 5.8×1010cm−2. This behavior is attributed to the role of optically excited holes in the AlSb barrier layers near the InAs quantum well. Furthermore, a long relaxation time in the electron sheet density is observed and attributed to the slow movement of electrons from the InAs quantum well to ionized deep levels in the AlSb barriers.

The Landé‐ g factor in heterostructures

We have studied the behavior of the Shubnikov-de Haas oscillations in magnetoresistance under applied magnetic fields with an arbitrary orientation. Different procedures to measure the Landé-g factor have been used in the literature. We propose four unambiguous methods and formulas for the evaluation of this quantity. The Landé-g factor obtained using these formulas compares within the 5% with the reported results of Brosig et al. [1] in InAs – AlSb quantum wells, Bompadre et al. [2] for Bismuth crystals, Kobayashi et al. [3] measuring magnetoresistance as a function of the gate Voltage and F. Fang et al. [4] for InAs – GaSb heterostructure. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Large blueshift of the band gap of GaAsSb∕AlSb quantum wells with ion implantation

We describe studies of intermixing in GaAsSb∕AlSb quantum wells with 18.8% and 31% arsenic, grown on GaSb substrates. Samples were implanted with a 1×1013cm−2 dose of 330keV boron ions and annealed. The band gap for the samples was determined from photoluminescence. The maximum blueshift of the band gap was 141nm (86meV) for the quantum wells with 18.8% arsenic and 198nm (124meV) for the quantum wells with 31% arsenic. The blueshifts are attributed to interdiffusion of both group-III and group-V constituents. Photoluminescence strength generally increased with annealing temperature.

An efficient multiband envelope function approximation method for spintronics

We have developed an eight-band finite-difference envelope functionapproximation model capable of reproducing in almost all situations the true D2d orC2vsymmetry of [001] grown zinc-blende heterostructures. We have used our model tostudy the relative contributions of the bulk inversion asymmetry (BIA) andstructural inversion asymmetry to the spin splitting in the conduction band ofasymmetric AlSb/GaSb/InAs/AlSb quantum wells, and clarify apparentlycontradictory statements about the relative magnitude of the two contributions.We show that, in the system under study, the inclusion of BIA effects changesconsiderably the angular dependence and the magnitude of the splitting. We alsoinvestigate how BIA changes the transmission properties of a resonant tunnellingstructure.

Scattering mechanisms in InAs–AlSb quantum wells

High-quality InAs–AlSb quantum wells have been grown by molecular beam epitaxy. The Drude scattering time τ D as well as the quantum scattering times τ Q have been evaluated from low-temperature magnetotransport experiments. While the τ D increases monotonically with carrier density, the quantum scattering time fluctuates as a function of carrier density depending on the cool-down cycle. For high-mobility samples we find that the ratio τ D/ τ Q can be as high as 88 indicating long-range potential fluctuations. If hydrostatic pressure is applied to the sample the carrier density is reduced by about 1×10 11 cm −2 kbar −1 at liquid He temperatures.

Landau damped intersubband plasmons in [formula omitted] quantum wells

We present a combined theoretical and Raman scattering study of coupled intersubband plasmon-LO phonon modes in InAs AlSb quantum wells, highlighting the importance of the InAs conduction band nonparabolicity in such systems. We demonstrate that under the illumination conditions of the Raman measurements, electron densities can be as high as 4 × 10 12 cm −2 and give evidence of Landau damping of intersubband plasmons.

Cyclotron resonance and subband-Landau level coupling in 2D electron and hole gases

We discuss the far-infrared absorption in quantum wells in the presence of a quantizing magnetic field. Our theory is applicable both to electrons and holes. Band structure effects are fully taken into account and our approach allows us to consider different sample geometries and potential profiles. As an example we discuss the spin-split cyclotron resonance in the non-parabolic 2D electron gas in InAs AlSb quantum wells. Our parameter-free calculations are in excellent agreement with recent experiments. The complex valence band structure results in anticrossings between Landau levels of different hole subbands.

Infrared electroabsorption modulation at normal incidence in asymmetrically stepped AlSb/InAs/GaSb/AlSb quantum wells

A normal-incidence modulation mechanism is proposed which uses the Stark effect to induce Γ-L transitions in asymmetrically stepped AlSb/InAs/GaSb/AlSb quantum wells (QWs). A significant feature of this structure is the unusual band alignments which localize two deep wells for the Γ and L bands in adjacent layers, i.e., the Γ-valley minimum is in the InAs while the L-valley minimum is in the GaSb. In contrast to a square QW, where the Stark shifts for both Γ and L subbands are in the same direction, the two step wells for Γ and L valleys in the proposed structure are oppositely biased in the presence of an electric field. Therefore, the first Γ and L subbands move toward each other, making the Γ-L crossover occur more efficiently. Near this point, most of the Γ electrons transfer to the L valleys, where they are allowed to make intersubband transitions under normally incident radiation. As a result, the device switches from being transparent to normal-incidence light to strongly absorbing it. The calculations indicate that excellent on/off ratios can be achieved in this structure operating at T≤150 K with electric fields on the order of ∼100 kV/cm for any infrared wavelength within the range of 3–20 μm.

Surface-layer modulation of electron concentrations in InAs–AlSb quantum wells

A strong dependence of the electron concentration in not-intentionally doped InAs–AlSb quantum wells on the nature of the surface layer is demonstrated. We find that quantum wells capped with a GaSb layer result in much higher electron concentrations than wells capped with an InAs layer. The difference in concentration increases as the top barrier thickness of the well decreases. These changes are due to a shift in the pinning position of the Fermi level at the surface. For InAs surface layers, the Fermi-level pinning position also depends sensitively on the presence of residual Sb, which is found to move the pinning position further into the conduction band of InAs. The Fermi level is found to be pinned above the conduction band edge of bulk InAs by 80 meV for as-grown InAs surface layers, and 300 meV for etch-exposed InAs surface layers.

Effects of barrier thicknesses on the electron concentration in not-intentionally doped InAs–AlSb quantum wells

We present the results of a study of the dependence of electron concentration in molecular-beam-epitaxy-grown not-intentionally doped InAs–AlSb quantum-well structures on the top barrier thickness in single-well structures and on the internal barrier thickness in multiwell structures. A contribution from surface donors was found to be dominant in single wells with thin top barriers. The bulk donor concentration in the AlSb barriers varied from 2 × 1015 to 1016 cm−3. Interface donors at the InAs–AlSb heterointerface were also found, with an energetic location below the Fermi level in the wells. They are primarily responsible for the increase of the carrier concentration with increasing temperature.

Carrier densities in InAs–Ga(Al)Sb(As) quantum wells

With the aim to study the influence of substitutional species on the band schemes and charge transfers in type-II heterostructures, the InAs–GaSb based quantum well structures have been prepared by molecular-beam epitaxy with a replacement of GaSb by GaSbAs and GaAlSb. Galvanomagnetic measurements show a similar behavior of decreasing hole density with increasing alloy composition in both cases, suggesting a transfer process insensitive to the cation–anion nature of the alloying element. That the high electron mobility also decreases with the composition is primarily governed by the degree of lattice mismatch.