GaAs AlxGa1 Research Articles

Observation of optically detected magnetic resonance signals in AlxGa1−xAs

Optically detected magnetic resonance (ODMR) techniques which utilize weak spin effects on deep photoluminescence have been applied to MBE AlxGa1−xAs. ODMR [unlike conventional electron paramagnetic resonance (EPR)] experiments is ideally suited for thin epitaxial samples. Using this technique we have obtained results on molecular-beam epitaxy (MBE) AlxGa1−xAs grown at substrate temperatures of 570 and 620 °C. The four-line ODMR spectrum observed from AlxGa1−xAs at low x values is attributed to Ga atoms not on their normal sites. The physical defect we have ascribed to this signal is a Ga interstitial. To our knowledge this is the first observation of a self-interstitial in the GaAs–AlxGa1−xAs system. We suggest that the level ME6 observed in deep-level transient spectroscopy (DLTS) experiments on MBE grown under similar conditions is the Ga interstitial observed in the ODMR experiment. For x>0.3 the Ga spectrum is still observed but an additional single unsplit magnetic resonance line appears which might be due to oxygen.

Band and branch inversion phenomena in GaAs–AlxGa1–xAs superlattices

AbstractA detailed analysis of the symmetry properties of the Bloch envelope functions in GaAs–AlxGa1–xAs superlattices (SL) for finite values of k⟂ is made. It is pointed out that in the (E, k, k⟂) space, where k is the SL wave vector, the energy spectrum is built up of independent branches. Each one of these branches contains two curves of definite parity placed at the planes k = 0 and k = π/d, where d is the SL period. It is shown that with increasing magnitude of k⟂, an interchange of symmetries between branches (similar to the well known band inversion phenomenon) may occur at the plane k = 0 or k = π/d. Furthermore, it is also shown that a magnetic field may induce interchange of symmetries between contiguous bands.

Anomalous I–V characteristics of semiconductor heterojunction diodes due to transmission resonance

As sizes of semiconductor devices are reduced, quantum mechanical properties of electrons play an important role. For small heterojunction diodes including a potential barrier, the influence of transmission resonance on current–voltage characteristics is discussed. The diodes have been prepared using a GaAs–AlxGa1−xAs system by MBE. A preliminary result shows the anomalous I–V curve which is presumed to be due to transmission resonance.

The influence of nonparabolieity on self‐consistent treatment for GaAsAlxGa1–xAs heterojunction

AbstractThe self‐consistent determination is analysed of the relevant parameters of a GaAsAlxGa1–xAs heterojunction in the presence of nonparabolicity of bands. Starting from Kane's theory, a suitable form of the Schrödinger equation for electrons is given. It is shown that there are, generally, two different energy spectra. The notion of an extended effective mass is introduced, which are depending on the potential, the energy gap, and subband index. The numerical analysis of results is performed on GaAsAl0.4Ga0.6As heterojunction. These numerical results confirm theoretical conclusions exposed in the first part of the paper.

Screening of the electron–phonon interaction in GaAs heterostructures

AbstractThe effect of the screening of the electron‐phonon interaction on the polaron binding energy (E) and the polaron mass (m*) is investigated within second order perturbation theory and for different approximations to the two‐dimensional electron gas response (random‐phase approximation, Hartree‐Fock and Thomas‐Fermi approximation). For the electron–phonon contribution to the polaron ground state energy a dynamical screening is applied which takes into account the dielectric response at all frequencies. The influence of the finite width of the electron layer on E and m* is investigated. The results are applied to GaAsAlxGa1–xAs heterostructures and it is found that the mass renormalization for ne = 1011 cm−2 is 0.4% which is a factor of eight smaller than the effective mass of the unscreened ideal 2D polaron.

A parametric study of interband absorption in GaAs–AlxGa1−xAs quantum wells

Calculations of the optical transitions in GaAs–AlxGa1−xAs heterostructures are undertaken, with an eight‐band k ⋅ p method. The sensitivity of the resulting spectrum to structural, as well as to material, parameters is studied. For a rectangular quantum well, we find that the available data do not allow a unique determination of the conduction band offset Qe. By accounting for experimental uncertainties, we find that any value in the range 0.5–0.9 is compatible. For parabolic wells, however, we can narrow this range to 0.5–0.65.

Studies of exciton localization in quantum-well structures by nonlinear-optical techniques

An exciton moving in a random potential is a promising model system for the study of localization effects, since its energy spectrum can be measured directly, and there are no complications resulting from Coulomb interaction. This paper reviews our work on the use of nonlinear techniques, such as hole burning and four-wave mixing, to detect the motion of two-dimensional excitons in thin GaAs–AlxGa1−x As heterostructures, in which the random potential comes from fluctuations in layer width. A clear distinction is found between the behavior of excitons below and above the absorption line center. Below the line center, hole burning is easy, and both spectral and spatial diffusion are slow, i.e., the excitons behave as if they are localized; above it the reverse is true. This is strong evidence for a mobility edge at the line center, which is the position predicted classically.

Morphology of GaAs and AlxGa1−xAs grown by molecular beam epitaxy

In this paper two major morphological defects that occur in the growth of GaAs and AlxGa1−xAs by molecular beam epitaxy (MBE) are discussed. A uniform, fine scale (<1000 Å period) roughness found on AlxGa1−xAs wafers grown at certain temperatures is shown to result from the presence of a thin Ga layer that has segregated on the surface during growth. The surface roughness leads to high interfacial recombination velocities at GaAs–AlxGa1−xAs heterojunctions and strongly degrades heterostructure device performance, especially lasers. Larger (<20 μm) but localized (density ∼103–105 cm−2) oval defects have been proposed to result from spitting of globules of material from the group III element effusion cells. Here, evidence will be presented in support of this hypothesis. The shape of these oval defects as well as the AlxGa1−xAs surface roughness are shown to be related to the anisotropy of the interfacial energy of the group III elements on GaAs and AlxGa1−xAs.

Optical properties of superlattice and MQW laser diode

MBE grown GaAs–AlxGa1−xAs superlattice structure was investigated in terms of crystallography (x‐ray diffraction and direct lattice‐image observation by TEM) and optical properties (controllability of Eg, oppoiste Burstein shift, structure dependent refractive index and room‐temperature exciton). New device properties are described for an MQW laser diode/waveguide. These include anisotropic gain, low absorption loss, and stable longitudinal mode oscillation under high‐speed modulation. An MQW laser diode in the long wavelength region is presented, which is made using the GaSb–AlxGa1−xSb material system. Future prospects are also described.

Mobility of the two-dimensional electron gas in GaAs-AlxGa1−xAs heterostructures

The mobility of the two-dimensional electron gas in GaAs-Alx Ga1−xAs heterostructures was studied systematically in ten samples with density from 1.33×1011 to 7.8×1011 cm−2 in the temperature range T=4.2–300 K. A theoretical calculation using the variational wave function was carried out for scattering by screened impurity ions, optical phonons, and acoustic phonons through deformation and piezoelectric couplings. Good quantitative agreement between theory and experiment was obtained with no adjustable parameters.

Binding energies of acceptors in GaAs–AlxGa1−xAs quantum wells

We have used the variational method to calculate the acceptor binding energies in GaAs–AlxGa1−xAs quantum wells. The calculation includes the coupling of the top four valence bands of both materials in the multiband effective mass approximation. To ensure the convergence of the calculation, a large number of basis functions which are made up of the s-like or d-like spatial states multiplied by j=3/2 spinors are used for the expansion of the acceptor wave function. Because the quantum well potential reduces the symmetry from Td to D2d, the bulk Γ8 acceptor ground state splits into Γ6 and Γ7 states. The Γ6 state is predominantly derived from the heavy-hole subband and the Γ7 state is predominantly derived from the light-hole subband. We have calculated the binding energies of the Γ6 state (measured from the top of the heavy-hole subband) and the Γ7 state (measured from the top of the light-hole subband) for both center doped and edged doped quantum wells for various barrier heights as functions of well width. Except for well widths smaller than ≊40 Å, the Γ7 binding energy is greater than the Γ6 binding energy. In recent studies, the photoluminescence resulting from the acceptor levels to conduction band transition in MBE grown GaAs–AlGaAs superlattices has been measured. Our theoretical results are in excellent agreement with these experimental data.

Summary Abstract: Fractional quantum effect in transport along the GaAs–AlxGa1−xAs interface

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Structural and optical properties of GaAs–AlxGa1−xAs quantum wells

We report structural and optical data on single and multiple GaAs–AlxGa1−x As quantum well structures. The layers were grown in a fully automated system incorporating a high speed rotation stage of our own design. All layers were grown in the substrate temperature range 680–700 °C using an As2 source. Growth rates were normally about 1 μm/h for GaAs. The substrate rotation stage is capable of operation at up to 200 rpm compared to commercial equipment operating limits of about 10 rpm. This enables rotation of the substrate once during the deposition of each monolayer, thus avoiding the modulation of alloy composition found at lower rotation speeds. We present data showing the excellent layer uniformity obtained with this device. We have carried out TEM and photoluminescence studies on single quantum well structures grown with and without a prelayer of AlAs. The well with the prelayer shows a peak assigned to the n=1(e−hh) free exciton and a considerable decrease in impurity related luminescence at 6 K. We relate these results to improvements in structural properties seen in TEM. Multiple quantum well structures have been grown showing strong photoluminescence up to 300 K. Peaks assigned to the n=1(e−hh) and n=1(e−lh) transitions have been observed at room temperature and the possible excitonic nature of these peaks is discussed.

Transient capacitance spectroscopy on large quantum well heterostructures

We report transient capacitance measurements on AlxGa1−xAs–GaAs–AlxGa1−xAs (x∼0.35) double heterojunctions with a large quantum well active region (Lz∼800 Å). It is suggested that the thin GaAs layer acts as a ‘‘giant’’ artificial deep level. It follows then that the band edge discontinuity ΔEc determines the electron emission rates (from the thin layer), thus making it possible for ΔEc to be determined by transient capacitance measurements.

Effects of band bending on real space transfer in GaAs–AlxGa1−xAs layered heterostructures

A program for solving Poisson’s equation in a single-particle Monte Carlo simulation has been used to obtain the transverse electric field distribution in GaAs–AlxGa1−xAs layered heterostructures with an externally applied electric field parallel to the layer interfaces. The effect of transverse electric field on real space transfer in these heterostructures are described. Transverse field amplitudes are equal to or greater than threshold fields for real space transfer, and their effect on real space transfer can be significant. In general, the presence of a transverse field in a real space transfer device reduces the peak velocity and the peak-to-valley ratio. Also, the threshold field can either be increased or decreased by the transverse field. Saturation velocities appear to be higher if a transverse field is present, although this conclusion is somewhat tentative.

Far infrared emission from 2D electrons at the GaAs AlxGa1- xAs interface

We report the first observation of far infrared emission resulting from radiative decay of intersubband excitations and cyclotron excitations of the two-dimensional electrons in GaAs Al xGa 1-x As heterostructures.

Low-current-threshold and high-lasing uniformity GaAs–AlxGa1−xAs double-heterostructure lasers grown by molecular beam epitaxy

GaAs–AlxGa1−xAs double-heterostructure (DH) lasers that have current-threshold densities at least as low as similar-geometry DH lasers prepared by liquid-phase epitaxy have been prepared by molecular beam epitaxy for the first time. These broad-area lasers exhibited excellent stability and uniformity in lasing-filament distribution across the entire junction plane and throughout the entire current injection range up to catastrophic damage. The burn-off powers were typically about 14 W/mm. The differential quantum efficiencies were about 40%.

Double-crystal spectrometer measurements of lattice parameters and X-ray topography on heterojunctions GaAs–AlxGa1−xAs

Heterojunctions GaAs–AlxGa1−xAs involved in the elaboration of IR laser diodes have been studied. The difference in lattice parameter between the GaAs substrate and the aluminum-substituted epitaxic layer AlxGa1−xAs has been measured accurately on a double-crystal spectrometer for a series of compositions. These data coupled with radius of curvature determination have permitted calculation of the stress in the layer and the bulk lattice parameter of AlxGa1−xAs. characterization of the defects introduced during the liquid-phase epitaxy has been performed by X-ray topography.

Extremely high frequency hybrid integration—basis of progress in extremely high frequency device technology : R. Kuhn. Nachrichtentechnik: Elektronik25 (12), 445 (1975). (In German)

We report an experimental project to incorporate double-barrier tunnel structures into three-terminal devices. These devices have the negative-differential-resistance (NDR) features of the double barrier, and the added flexibility of a third controlling electrode. One way to make such a device involves the series combination of a double-barrier tunnel structure with a field-effect transistor. We have realized this concept in two types of devices, using samples grown by metalorganic chemical vapor deposition. The devices consist of a GaAsAlxGa1−xAs double-barrier tunneling heterostructure, the current through which is controlled by either an integrated vertical field-effect transistor or a planar metal-semiconductor field effect transistor. The voltage location and peak-to-valley current ratio of the NDR present in the source-drain circuit can be modulated with gate voltage. Experimental results for four samples are presented.

Index-profile determination of heterostructure GaAs planar waveguides from mode-angle measurements at 106 μm wavelength

Heterostructure GaAs–AlxGa1−xAs–GaAs waveguides for 10.6 μm wavelength grown by liquid-phase epitaxy have a linear variation of dielectric constant in their substrates. On the basis of their mode equation, a computer program was devised for obtaining optimum values of the six parameters that define the index profiles, given a sufficient number and distribution of mode-angle measurements. The optimized parameters agreed well with the measured values for a variety of waveguides, illustrating the generality of the parameter-optimization procedure. Two types of leaky modes were observed, whose effective index β/k is less than the seed index, (1) Lummer–Gehrcke modes that are incompletely reflected at the substrate–seed interface and (2) low-loss totally reflected modes whose evanescent fields radiate into high-index seeds.