Structure of heterointerfaces and photoluminescence properties of GaAs/AlAs superlattices grown on (311)A and (311)B surfaces: Comparative analysis

The photoluminescence properties of type II GaAs/AlAs superlattices grown on the (311) surface are determined by their polarity. Previous HRTEM investigations demonstrated a corrugation (with height of 1 nm and period of 3.2 nm) of both GaAs/AlAs and AlAs/GaAs interfaces in samples grown on the (311)A surface. In the present study, a lateral periodicity of 3.2 nm is also revealed in HRTEM images of a superlattice grown on the (311)B surface and in their Fourier transforms. However, this periodicity is poorly pronounced, which is due to fuzzy corrugation and the presence of a long-wavelength (>10 nm) disorder. Photoluminescence spectra of the GaAs/AlAs superlattice on the (311)A surface are strongly polarized relative to the direction of interface corrugation, in contrast to the (311)B superlattice, in which the corrugation is weakly pronounced. It was found that the strong mixing between the Θ and X minima of the conduction band, occurring only in sublattices with strongly corrugated interfaces, allows generation of bright red luminescence at 650 nm up to room temperature. The distinctions revealed between the superlattices grown on the (311)A and (311)B surfaces confirm that it is precisely the interface corrugation, and not crystallographic orientation, that governs the optical properties of (311) superlattices.     

Optical properties of GaAs/AlAs superlattices grown on (311)A GaAs surfaces

We present a temperature-dependence photoluminescence of (GaAs)₅/(AlAs)₅ superlattice grown on (311)A-oriented semi-insulating substrate by molecular beam epitaxy. The temperature dependence reveals an anomalous decrease of the PL width, which is explained in terms of phonon-assisted thermal activation of localized excitons.     

Impedance switching effects in GaAs/AlAs barrier structures

AlAs tunnel barriers in MBE-grown GaAs layers have been studied using pulsed and continuous I/V and swept temperature/capacitance measurements. Such layers demonstrate an impedance switching phenomenon which is persistent and repeatable. This impedance switching is manifested by the device as two distinct impedance modes: a high-impedance mode (of the order of 10k? at DC for a 0.3 mm dot), and a low-impedance mode (of the order of 10?). Such a phenomenon may restrict the operation of some devices, but may lead to other novel applications.     

Transverse optical phonon splitting in GaAs/AlAs superlattices grown on the GaAs(311) surface studied by the method of Raman light scattering

GaAs_n/AlAs_m superlattices grown on the GaAs (311)A and (311)B surfaces by molecular-beam epitaxy were studied by Raman light scattering. The form of the Raman scattering tensor allowed the TO _y and TO _x modes to be separately observed using various scattering geometries (the y and x axes correspond to atomic displacements along and across facets formed on the (311)A surface, respectively). The TO1_y and TO1_x modes exhibited splitting in superlattices grown on a faceted GaAs(311)A surface. The degree of splitting increased for superlattices with an average GaAs layer thickness of 6 monoatomic layers and less. No splitting was observed for superlattices grown under the same conditions on the (311)B surface, which indicates that the splitting effect is probably due to the formation of GaAs quantum wires on the faceted (311)A surface.     

Thermal stability of C-doped GaAs/AlAs DBR structures

Thermal stability of heavily carbon-doped and undoped DBRs has been investigated by reflectivity measurements and Raman spectroscopy. These analytical techniques are used to study the effect of heavy C-doping on Al–Ga interdiffusion during subsequent high-temperature anneals. Reflectivity spectra around the DBR stop-band wavelength clearly show that the growth-rate is reduced due to etching associated with the CBr₄ precursor used, but they also indicate that no Al–Ga interdiffusion that could significantly degrade the DBR performance takes place for any samples during annealing. The results are supported by Raman spectra, which indicate the positions of the LO and LOPC modes do not change when the DBRs are annealed, whether the DBRs are doped or not. Simulations of Al–Ga interdiffusion at GaAs/AlAs DBR interfaces indicates that intermixing up to ～15 nm on either side of each interface will not affect the reflectivity of the DBR stack significantly. The observed small changes in the stop-band central wavelength and peak reflectivity due to annealing is most likely a consequence of increased surface roughness resulted from annealing.     

Experimental study of microwave reflection gain of AlAs/GaAs/AlAs quantum well structures

Reports on microwave reflection gain measurements on single quantum well AlAs/GaAs/AlAs resonant tunnelling structures. Reflection gain against frequency measurements in the 0.050-26.5 GHz range were performed. The equivalent negative high-frequency resistance and capacitance as a function of frequency were determined for a diode of area 49 mu m/sup 2/. The point at which the reflection gain exceeds unity (0 dB) is noted to be the onset of possible oscillations, and thus yields a negative resistance.<>     

Resonance tunneling of X-electrons in AlAs/GaAs(111) structures: Pseudopotential calculations and models

The X-electron resonance tunneling in AlAs/GaAs(111) heterostructures with AlAs electrodes was considered. Calculations of the model with potential discontinuities at the boundaries were carried out using the scattering matrix method. The complex band structure was calculated by the empirical pseudopotential method. Resonance peaks in the transmission coefficients, related to X-states in AlAs and L-states in GaAs, were found. A model describing these processes is suggested.     

Raman study of GaAs quantum wires grown with partial filling of corrugated (311)A AlAs surfaces

Raman spectroscopy powered by theoretical modeling of vibrational modes was shown to be an effective tool to examine interface structure of superlattices (SLs). In this work we studied GaAs_n/AlAs_m (n=1–10 monolayers) corrugated superlattices (CSLs) grown on (311)A GaAs substrates using Raman spectroscopy and high-resolution transmission electron microscopy. The strongest modification of the calculated Raman spectra is found for the finite length of the GaAs-domain in the CSL in the case of partial (<1 nm) GaAs filling of the AlAs surface. This theoretical result is in a very good agreement with the sharp transformation of the experimental Raman spectra observed for this thickness range. Thus, calculated and experimental Raman spectra demonstrated a good agreement for both complete (≥1 nm) and partial (<1 nm) GaAs filling of the AlAs surface.     

Free engineering of buried oxide patterns in GaAs/AlAs epitaxial structures

The formation of a buried aluminium oxide from the surface of a GaAs/AlAs epitaxial structure is presented and validated through the localised electroluminescence in a light emitting diode device. This oxidation method relies on photolithography, plasma treatment and wet thermal oxidation. This enables localised buried oxide areas to be formed by a vertical oxidation process applied from the surface. Because of this new technology, the shape of the oxidised areas can be freely designed, unlike when using the standard lateral oxidation technology.     

Mixing of hole states in GaAs/AlAs(110) heterostructures

States in the AlAs/GaAs(12)/AlAs(110) quantum well and AlAs(6)/GaAs(12)(110) superlattice are considered. For analysis of mixing of light- and heavy-hole states in these structures, a parameter-dependent set of basis functions is suggested and the values of the parameter at which one of the functions describes basically heavy-hole states and the other light-hole states are determined. For the energy region considered in the study, four energy levels are determined in the AlAs/GaAs(12)/AlAs(110) quantum well and, correspondingly, four minibands in the AlAs(6)/GaAs(12)(110) superlattice. Analysis shows that the first and fourth levels in the quantum well and the first and fourth minibands in the superlattice are related basically to heavy-hole states. The other two states in the quantum well and the second and third minibands in the super-lattice are structurally more complex: in these states and minibands, the hole states are noticeably mixed. In these minibands, states are substantially separated in space and in spin.     

AlAs oxidation process in GaAs/AlGaAs/AlAs heterostructures grown by molecular beam epitaxy on GaAs (n11)A substrates

The lateral oxidation of AlAs layers grown on GaAs (100), (110) and (n11)A-oriented substrates (n=1, 2, 3, 4) was studied. The temperature dependence of the oxidation rate was measured between 390°C and 450°C. The oxidation rate is highly anisotropic and the anisotropy is related to the symmetry of the crystal structure. The oxidation process has an activation energy that depends on substrate orientation. The oxidation front line becomes irregular for temperatures higher than 450°C and the surface of the samples was degraded when the temperature exceeded 540°C. The time dependence of the oxidation rate was found to be similar to the Si oxidation process.     

Characteristics of the formation of (Al, Ga)Sb/InAs heterointerfaces in molecular-beam epitaxy

A thermodynamic model is given for the molecular-beam epitaxy formation of InSb, GaAs, and AlAs heterointerfaces in (Al, Ga)Sb/InAs heterostructures. The maximum critical temperature of formation of a planar InSb-type heterointerface on an (Al, Ga)Sb layer, T≈390 °C, is determined from a comparison of the pressure of Sb₄ molecules in the external flux with their equilibrium value above a stressed monolayer on a heterointerface and is found to be in good agreement with existing experimental data. In contrast, the critical temperature of formation of a heterointerface of the AlAs (GaAs) type, corresponding to the onset of rapid reevaporation of As, is much higher than the growth temperatures normally used in molecular-beam epitaxy (350–550 °C). Fiz. Tekh. Poluprovodn. 31, 1242–1245 (October 1997)     

Influence of MBE growth temperature on GaAs/AlAs resonant tunneling structures

Double barrier GaAs/AlAs tunneling structures with typical 2.5:1 room temperature peak-to-valley current ratios are examined using Deep Level Transient Spectroscopy. Deep level trap concentrations are found to be much higher in samples grown at 550° C compared to those grown at 650° C. For devices grown at 550° C, an impedance switch-ing effect due to a high concentration of deep levels is observed. The peak-to-valley ratio of the tunneling devices is largely unaffected by the growth temperatures in this range, indicating that higher growth temperatures can be employed to grow resonant tunnel-ing diodes than previously suggested in the literature.     

Lateral ordering of quantum dots and wires in the (In,Ga)As/GaAs(100) multilayer structures

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the \([0\bar 11]\) direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.     

Scattering of hot electrons by neutral acceptors in GaAs/AlAs quantum well structures

The optical alignment of hot electrons and its destruction in a magnetic field under conditions when electron scattering by neutral acceptors plays a large role is investigated. This makes it possible to determine the probability of the scattering of hot electrons from an initial photoexcited state, as well as the times characterizing the energy and momentum relaxation of hot electrons on scattering by neutral acceptors. The experimental results are compared with calculations. Fiz. Tekh. Poluprovodn. 32, 866–875 (July 1998)     

InGaAs/GaAs vertical-cavity surface-emitting lasers on (311)B GaAssubstrate

InGaAs/GaAs vertical cavity top-emitting dielectric-mirror surface-emitting lasers have been fabricated on a (311)B GaAs substrate. The threshold current was 25 mA at a lasing wavelength of 0.97 μm under pulsed operation at room temperature. The output was linearly polarised in the [2¯33] direction     

Novel multilayer modulation doped (Al,Ga)As/GaAs structures for self-aligned gate FETs

The fabrication of self-aligned gate by ion implantation modulation doped (Al,Ga)As/GaAs field effect transistors (MODFETs) utilising a novel multilayer structure capable of withstanding the high-temperature furnace anneals required for Si implant activation is reported. Typical measured extrinsic transconductances of 175 mS/mm at 300 K and 290 mS/mm at 77 K were achieved on 1.1 ?m-gate-length devices. Values of the two-dimensional electron gas saturation velocity of 1.9×107 cm/s at 300 K and 2.7×107 cm/s at 77 K were obtained from an analysis of the FET drain current/voltage characteristics using the charge-control model.     

Magnetic field effects in the luminescence spectra of type II GaAs/AlAs double layer structures

We report on low temperature luminescence studies of type II GaAs/AlAs heterostructures in magnetic fields up to 20 T. Application of magnetic fields perpendicular to the two-dimensional planes lead to a tremendous quenching of the indirect luminescence transitions. At higher excitation intensities, when the luminescence transitions are of a more Γ-Γ character within the GaAs layer, the application of magnetic field leads to an increase of the luminescence intensity. These effects are attributed to field induced lateral localization of photocreated carriers.