GaAs-AlGaAs Quantum Wells Research Articles

Theoretical analysis of band structures and lasing characteristics in strained quantum wire lasers

The lasing characteristics of strained quantum wire lasers are analyzed using a tight binding calculation, and improved dynamic and spectral properties are predicted. An InGaAs quantum wire embedded in a plane of InAlGaAs with the same lattice constant and sandwiched by two-dimensional barrier layers of AlGaAs is considered. In this structure the biaxial strain effect appears together with the two-dimensional confinement effect. The results of the tight binding calculation indicate that the band mixing effect, which inherently exists in the valence bands of GaAs-AlGaAs quantum wires, is suppressed. In addition, the difference between the effective masses of the first conduction band and the first valence band is reduced as compared to the GaAlAs bulk and to GaAs-AlGaAs quantum wells and wires. Thus the advantages of both the quantum wire effect and the strain effect are incorporated in this structure. Calculation of the modulation dynamics and the spectral properties of the strained quantum wire lasers show significant enhancements. >

The theory of the photoabsorption in n-type Si-SiGe quantum well infrared photodetectors

The theory of multiple quantum well n-type Si-SiGe infrared detectors is presented. The coefficient of photoabsorption, quantum efficiency, and responsivity of Si-SiGe quantum well (QW) detectors are calculated taking in consideration the effects of depolarization and electron-electron exchange interaction. We show that the Si-SiGe quantum well detectors possess lower dark current, lower tunneling rates, and better photoabsorption characteristics including responsivity when compared to GaAs-AlGaAs photodetectors. These imply that Si-SiGe QW detector performance characteristics are superior to GaAs-AlGaAs QW infrared photodetectors aside from its advantage of compatibility to Si-readout circuity.

Photoabsorption in n-type Si-SiGe quantum-well infrared photodetectors

The calculation of the photoabsorption spectra and quantum efficiency of Si-SiGe quantum-well (QW) detectors are presented, with the effects of depolarization and electron-electron exchange interaction taken into consideration. We show that the Si-SiGe QW detectors possess lower dark current, lower tunneling rates, and better photoabsorption characteristics when compared to GaAs-AlGaAs photodetectors. These imply that Si-SiGe QW detector performance characteristics are superior to GaAs-AlGaAs QW infrared photodetectors in addition to its advantage of compatibility with Si-readout circuity.

Exciton binding energies and absorption in intermixed GaAs-AlGaAs quantum wells

The optical properties of excitons in layer-intermixed GaAs-AlGaAs quantum wells are studied theoretically. The electronic dispersion is obtained using the 6×6 Luttinger–Kohn Hamiltonian for the valence bands, and an accurate expression for the conduction band dispersion which includes the effects of nonparabolicity and warping to fourth order in k. The HH1-CB1 (1s) and LH1-CB1(1s) exciton binding energies are calculated as a function of diffusion time. The absorption for both TE and TM polarization is obtained at several wavelengths, and is seen to decrease significantly with increased intermixing. The decrease in absorption is larger for narrow wells, where the effects of intermixing are more pronounced for a given diffusion time.

Excitons in GaAsAlGaAs quantum wells: Effects of substrate orientation

The binding energy of excitons in GaAsAlGaAs quantum wells is studied theoretically as a function of the crystallographic growth direction. The electronic dispersion is obtained using the 4×4 Luttinger Hamiltonian for the valence bands, and an accurate expression for the conduction band dispersion which includes the effects of non-parabolicity and warping to fourth order in k. The exciton binding energies are obtained for six growth directions. The different subband mixing for each orientation is seen to strongly influence the exciton properties. The coupling of different exciton states is also dependent on the subband order and energy level spacing, which differ from each quantum well orientation.

Observation of quantum dot levels produced by strain modulation of GaAs–AlGaAs quantum wells

We present the first observation of resolved quantum dot levels produced by strain modulation of a semiconductor. Excitons are confined in lateral potential wells of up to 60 meV, the largest yet reported for strain-induced quantum wire or dot structures. The dependence of the quantum dot level separation on dot size is in agreement with calculations of the strain-induced band edge modulation. For 200 nm wide carbon dot stressors the level separations are approximately 2 meV. Frustration of acceptor recombination by three-dimensional localization of carriers in the strain-induced potential wells is clearly observed. Finally, we detect luminescence from a single quantum dot at low excitation intensity, a result of the frustration of nonradiative recombination by localization.

Hole-acoustic phonon interaction in quantum wells

The relaxation of holes in GaAs-AlGaAs quantum wells by emission of acoustic phonons via deformation potential coupling is calculated. Using the 4*4 Luttinger Hamiltonian and the Bir-Pikus Hamiltonian to describe the hole dispersion and the coupling to acoustic phonons the authors take into account the coupling of light and heavy holes. They find a distinct angular dependence of the hole relaxation which is mainly due to the anisotropy of the phonon dispersion and the coupling mechanism. The results confirm the predominance of parity-conserving processes and incomplete hole-spin relaxation derived from luminescence experiments.

Effects of impurity induced disorder on the index of refraction in GaAsAlGaAs quantum wells

A theoretical model for the refractive index in impurity induced disordered (IID) GaAsAlGaAs quantum wells is presented. The electron and hole dispersion is obtained from the Luttinger-Kohn Hamiltonian and a bulk conduction band Hamiltonian which includes the effects of nonparabolicity. We solve the Hamiltonians using a finite difference method which is flexible enough to accommodate the potential profile in disordered quantum wells. Both TE and TM refractive indices are calculated, and our results are in excellent agreement with the published experimental data available. The inter-diffusion coefficient of Al and Ga is obtained by comparing our calculated subband level results, including the effects of excitons, to the published experimental data. The theoretical model allows accurate calculation of the index of refraction, allowing the refractive properties of impurity induced disordered wells to be accurately tailored experimentally.

Orientation dependence of subband structure and optical properties in GaAsAlGaAs quantum wells: [001], [111], [110] and [310] growth directions

We calculate the valence subband dispersion in GaAsAl xGa 1−xAs quantum wells, with growth axes along the [001], [111], [110] and [310] directions, by solving the multiband effective mass equations for the four-component envelope function. Boundary conditions for conservation of probability current are given for each growth direction. The conduction band dispersion is obtained from an accurate expression for the bulk dispersion which includes the effects of anisotropy and nonparabolicity. We use the calculated dispersion to examine the dependence of optical interband transitions on both polarization and valence-subband mixing.

The shallow Si donor confined in a [formula omitted] quantum well

A spectroscopic study of a shallow donor and its bound exciton (BE) in narrow GaAs AlGaAs quantum wells (QWs) has been performed. The electronic structure has been investigated by means of selective photoluminescence (SPL) and PL excitation (PLE) spectroscopy. The excited states of the confined Si donor have been monitored via two electron transitions (TETs) of the confined donor BE, observed in SPL and PLE spectra. The energy position of the observed TET satellite yields a value on the 1s – 2s energy separation of 10.6 meV for a donor in the center of a 100 Å wide QW, which is in excellent agreement with recent theoretical predictions. Similar measurements have also been performed in a magnetic field. While the donor BE shifts towards higher energy with increasing field, with a diamagnetic shift similar to the free exciton, the TET satellite is found to shift considerably towards lower energies, i.e. the 1s – 2s energy separation increases with increasing magnetic field. To the best of our knowledge, this SPL study constitutes the first investigation of the magnetic field dependence of s-like excited states of the confined donor, since only transitions between the ground state and p-like excited donor states, observed in optical absorption, have been experimentally studied up to now.

Coherent transient spectroscopy of excitons in GaAs-AlGaAs quantum wells

Coherent transient optical spectroscopy techniques provide a means of studying the relaxation of electronic excitations in solids. Using these techniques the authors examine the relaxation of excitons in GaAs-AlGaAs multiple quantum wells at low temperature. Localization of excitons due to interface disorder results in inhomogeneous broadening of the absorption spectrum and relaxation due to migration between localization sites. The temperature dependence and energy dependence of the relaxation indicate the presence of complex scattering mechanisms. The dependence of the optical response on the polarization of the incident excitation fields is shown to result in a dramatic change of the dephasing and broadening characteristics of the optical response. This response is interpreted in terms of the presence of both localized and delocalized excitons. Mechanisms for the polarization dependence are discussed. >

Low-temperature-grown GaAs quantum wells: Femtosecond nonlinear optical and parallel-field transport studies

We find that GaAs-AlGaAs quantum wells grown at low temperature (300 °C) by molecular beam epitaxy are nearly semi-insulating and exhibit a broadened excitonic resonance. Studies of the femtosecond time-resolved nonlinear optical saturation response and parallel-field transport properties indicate that a subpicosecond nonradiative decay dominates the carrier recombination of this material. Annealing of low-temperature-grown quantum wells causes the arsenic point defects to form arsenic metallic precipitates within the quantum wells and up-shifts the absorption edge.

Energy spectrum and barrier localization of quantum well electrons in parallel magnetic fields

The magnetically tunable energy spectrum and the charge density distributions of electrons in GaAs-AlGaAs quantum wells with magnetic fields parallel to the interfaces are calculated both analytically and numerically. For states below the barrier edge and magnetic fields up to 15-20 T both methods yield nearly identical results. There are two types of localized states above the barrier. They originate from shallow depressions of the effective potential and from interferences.

Transient development of negative absolute minority electron mobility in a GaAsAlGaAs quantum well

We analyze the time dependent transient current response for a coupled electron-hole system in a GaAsAlGaAs quantum well. In this, we take account of the electron-hole interaction, electron-phonon and hole-phonon interactions as well as treating impurity scattering effects. Furthermore, the electron-electron and hole-hole Coulomb repulsions and associated dynamic screening effects are also incorporated. We examine the electron and hole mobilities in the ballistic regime, followed by their overshoot phenomena and finally saturation. Special attention is given to electron-hole drag which slows down the minority electrons, and at low temperature we trace out in detail the time development of negative absolute minority electron mobility for this system.

Post growth tailoring of the optical properties of GaAs-AlGaAs multiple quantum wells

The post-growth bandgap engineering of a fifty period multiple quantum well is reported. A controlled blue shift of the band edge is achieved through the deposition of a SiO/sub x/ encapsulant followed by rapid thermal annealing giving shifts of up to 16 meV. The expected depth dependence of this process is not observed and thus the blue shifted samples retain clearly resolved room temperature excitonic features and preserve the quantum confined Stark effect.

Coupling of quantum dots on GaAs.

With far-infrared spectroscopy, coupling between electron quantum dots becomes visible in the electronic excitation spectrum. We employ gated GaAs-AlGaAs quantum wells that enable field-effect tuning of the coupling between adjacent dots. For noninteracting quantum dots in a magnetic field we observe the characteristic edge- and bulk-mode spectrum.. The coupling of dots is reflected by a branching of the bulk mode into a cyclotron-resonance-like and a magnetoplasmonlike mode and a splitting of the edge mode. The latter is caused by formation of new edge orbits embracing two adjacent dots.

Optical properties of quantum wires produced by strain patterning of GaAsAlGaAs quantum wells

We have confined excitons to wires within continuous GaAsAlGaAs quantum wells. The confinement is produced by inhomogeneous strain created by patterning and etching a compressively stressed overlayer of amorphous carbon. Potential wells for excitons beneath 400 nm wide wires are 31 meV, as measured by the red-shift of the exciton emission. We compare our results to expectations based upon finite-element calculations of the strain tensor, and discuss the complicated effect of the inhomogeneous strain on valence-band structure.

Carrier decay in GaAs quantum wells

Carrier decays following pulsed excitation in GaAs-AlGaAs quantum wells have in the past been attributed to an excitonic recombination path. We show here that such an interpretation is inconsistent with the experimental evidence, and that the decays are controlled by recombination through electronic states at the GaAs-AlGaAs interfaces. We discuss the expected magnitude of the decay times and the influence of carrier trapping on the decay process.

A photoconductivity study of the excitons in doped and undoped [formula omitted] quantum wells

Photoconductivity (PC) measurements have been performed on narrow GaAs AlGaAs quantum wells (QWs) using dye-laser excitation in a limited energy range: the vicinity of the excitons. The derived high resolution PC spectra are then compared with photoluminescence excitation (PLE) spectra in the same energy range for the same samples. In these PC spectra several new features appear, which to the best of our knowledge have not been reported earlier: In an undoped QW the excited 2s states in addition to the normally observed 1s ground states to each of the heavy hole- (hh) and the light hole- (lh) states of the FE can be observed. Similarly, in a doped QW a peak interpreted as the bound exciton is observed on the low energy side of the FE. The same peaks can be observed also in PLE for the samples investigated, which is a strong support for the interpretation of the observed peaks.

Time-resolved raman scattering measurement of electron-optical phonon intersubband relaxation in GaAs quantum wells

We have measured the lifetime of electronic intersubband excitations by emission of longitudinal optical phonons in GaAsAlGaAs quantum wells using time-resolved anti-Stokes Raman scattering. We find the lifetime of electrons in the n = 2 level of a 146Å well to be ⩽ 1ps which is in excellent agreement with theoretical calculations of two-dimensional electron-photon interactions.