Resonant tunneling between transverse X states in GaAs/AlAs double-barrier structures under elevated hydrostatic pressure

We present a model to describe the mechanisms involved in tunneling between the quasiconfined X sub-bands of the AlAs layers in GaAs/AlAs double-barrier structures at pressures up to the type-II transition. The model involves self-consistent Schrödinger-Poisson calculation of the potential profiles within the device for a given relative alignment between the two X-like quantum wells, and thus allows prediction of the bias positions at which certain resonant tunneling processes will occur. By systematic variation of the parameters involved, in particular the charge distribution between the two AlAs layers, these predictions have been fitted to the measured vertical transport characteristics of a series of samples of different AlAs layer width. In the cases of those samples with X ground states that are transverse in nature, very good agreement has been obtained. The level of insight afforded by the model opens up alternative methods for the determination of important band-structure parameters, such as the light effective mass of the X minima in AlAs, and the ΓGaAs-XAlAs conduction-band offset. It also proves to be an extremely sensitive probe of the degree of symmetry between the AlAs layer widths of near-symmetric devices, and we are thus able to measure an asymmetry of around one monolayer in each of our devices. This explains entirely the asymmetry exhibited by the low bias Xt(1) → Xt(1) resonances of some nominally symmetric double-barrier structures, which has been a source of some debate in recent years

Magnetotunneling spectroscopy with the field perpendicular to the tunneling direction of the transverse X electrons in GaAs/AlAs double-barrier structures under hydrostatic pressure

We have measured the effect of in-plane magnetic field on tunneling resonances between transverse X states in GaAs/AlAs double-barrier structures under high hydrostatic pressure. Current-voltage and conductance-voltage measurements performed at pressures just beyond the type-II transition, and at fields up to 15 T, reveal clear field dependences of resonances originating from the Xt(1)→Xt(1) and Xt(1)→Xt(2) tunneling processes. Their behavior is consistent with a Lorenz force analysis, and therefore probes the in-plane electron dispersion around the X minima. Differences between measurements with the magnetic field oriented parallel to the [100] and [110] crystal axes reflect the anisotropy of the X minima, a first analysis indicating that the field dependence is dominated by the two Xt minima with large wave vectors perpendicular to the magnetic-field direction. In support of this, Schrödinger-Poisson modeling of the shift in bias position of the Xt(1) →Xt(2) resonance provides a value for the effective mass parallel to the Lorenz in-plane momentum vector which is consistent with the heavy principal effective mass of the X minima

Observation of inter-Landau-level transitions in resonant tunneling between transverse X states in GaAs/AlAs double-barrier structures under hydrostatic pressure

We have carried out sensitive measurement of the vertical transport characteristics of several GaAs/AlAs "double-barrier" heterostructures pressurized just beyond the type-II transition, and in the presence of longitudinal magnetic fields of up to 15 T. Towards the upper field limit, clear periodic structure is observed in the second derivative current-voltage characteristic of the resonance attributed to the process Xt(1)→Xt(1) + TOAlAs, where Xt(1) indicates the lowest quasiconfined subband associated with the transverse X minima in AlAs, and TOAlAs is a zone center transverse optical phonon. The periodic structure is interpreted as a series of transitions to collector states of increasing Landau index, with the requirement for conservation of in-plane momentum being satisfied for any interlevel transition by the phonon emission. Quantitative analysis of the data yields a value for the Landau-level separation, and thus also a value for the two-dimensional geometric effective mass of the transverse X minima in AlAs

GaAs high temperature optical constants and application to optical monitoring within the MOVPE environment

The real and imaginary components of the GaAs refractive index at temperatures between 20-700 degrees C have been obtained. Measurements were made by comparing the variable angle reflectivity of p-polarized and s-polarized 633 nm wavelength light from a deoxidized GaAs surface. By using these temperature-dependent optical constants for GaAs, modeling has allowed the behavior of surface photoabsorption (SPA) signals with temperature and oxide layers present to be predicted for different angles of incidence. The experimentally observed SPA signals during deoxidization of GaAs show strong qualitative agreement with these calculations at each of the angles of incidence considered. The measurement of data and application to modeling provides a platform for the measurement of temperature-dependent optical data for other III-V materials and for the investigation of deoxidation mechanisms

Enhancement of 2D → 2D tunneling by Γ-XZ mixing in GaAs/AlAs resonant tunneling structures at high pressure

We have investigated the 2D → 2D resonant tunneling in coupled X-band double quantum well structures, which can be achieved in GaAs/AlAs heterostructures by using high hydrostatic pressure. Until recently, there has been clear observation of 2D → 2D resonant tunneling between confined transverse XX,Y states but not between longitudinal XZ states. In this paper, we demonstrate the existence of such resonant tunneling in samples with very thin well and barrier layers. The existence of detectable 2D → 2D resonant tunneling between XZ states, even in a structure with a barrier thickness of 40 Å, is striking. However, by modelling the transport in terms of quantum beats between symmetric and anti-symmetric double well states, we show that Γ-XZ mixing can produce enhancements of up to ≈102

Enhancement of 2D -> 2D tunneling by Gamma-X-Z mixing in GaAs/AlAs resonant tunneling structures at high pressure

We have investigated the 2D --> 2D resonant tunneling in coupled X-band double quantum well structures, which can be achieved in GaAs/AlAs heterostructures by using high hydrostatic pressure. Until recently, there has been clear observation of 2D --> 2D resonant tunneling between confined transverse X-X,X-Y states but not between longitudinal X-Z states. In this paper, we demonstrate the existence of such resonant tunneling in samples with very thin well and barrier layers. The existence of detectable 2D --> 2D resonant tunneling between X-Z states, even in a structure with a barrier thickness of 40 Angstrom, is striking. However, by modelling the transport in terms of quantum beats between symmetric and anti-symmetric double well states, we show that Gamma-X-Z mixing can produce enhancements of up to approximate to 10(2)