D. Hilton

Optimisation of (Al,Ga)As/GaAs two-dimensional electron gas structures for low carrier densities and ultrahigh mobilities at low temperatures

The authors have grown (Al,Ga)As/GaAs two-dimensional electron gas (2DEG) structures with lightly doped regions of (Al,Ga)As and superlattices in the undoped GaAs. Using this technique, they have obtained ultralow density (2*1010 cm-2), high-mobility samples where phonon scattering at 4 K is the dominant factor in limiting the mobility; reducing the measurement temperature to below 1.5 K gives mobilities of up to 107 cm2 V-1 s-1.

Acoustic phonon scattering in ultra-high mobility, low carrier density GaAs/(Al,Ga)As heterojunctions

Abstract We have studied the low temperature electrical properties of a range of GaAs/(Al,Ga)As two-dimensional electron gas (2DEG) structures including a series of layers with low carrier densities (ns=(0.2−2)×1011cm−2) and extremely high mobilities; in one of these samples, the mobility at 1.5 K exceeded 107cm2V−1s−1. Because of the very low ionised impurity scattering rates in these layers, a detailed assessment of the acoustic phonon scattering process could be made. From the dependence of mobility on temperature and carrier density, the contributions to the phonon scattering rate from the piezoelectric and deformation potential interactions have been separated and compared with theoretical calculations. These results suggest that, in a 2DEG, the value of the piezoelectric coupling constant, h14, is ∼ 50% greater than in bulk GaAs; this enhancement is similar to that already noted for the deformation potential coupling constant, Ξ.

A comparison of AlAs/GaAs multiple quantum wells grown by molecular beam and migration-enhanced epitaxy

The authors report on comparative measurements of the optical properties of undoped AlAs/GaAs multiple quantum well (MQW) structures grown by conventional molecular beam epitaxy (MBE) and by migration-enhanced epitaxy (MEE). The structures were grown in pairs, one by MBE and one by MEE, at the same growth temperature which was varied between 530 degrees C and 680 degrees C. Growth conditions were monitored in all cases by observing RHEED oscillation behaviour and care taken to adjust the MEE conditions so that as nearly as possible each cycle of growth consisted of exactly one monolayer of the group III element. The growth structures were monitored by X-ray diffraction (XRD) and by low-temperature photoluminescence measurements. Analysis of the XRD results shows that the widths of the quantum wells are close to the expected value (75 AA) for the MBE samples for all growth temperatures but tend to be slightly high for the MEE samples. The AlAs/GaAs interfaces are broadened by about 2 monolayers for one interface and 3/4 monolayers for the other. For the MEE samples grown at 630 degrees C and 680 degrees C the XRD spectra show significant deterioration in quality, with large variations in QW period. Analysis of PL spectra shows good agreement of well widths with the XRD measurements but reveals an important difference between the MBE and MEE samples. The latter spectra are dominated by the presence of extrinsic luminescence whereas the former show only intrinsic exciton lines. The exciton linewidths are close to 3 meV for all the MBE samples but are about a factor two larger for MEE samples grown at 530 degrees C and 580 degrees C (the exciton transitions being completely swamped by extrinsic emission for higher growth temperatures). The luminescent efficiencies of the exciton lines in the MEE samples are about an order of magnitude lower than those in the MBE samples. Comparison of excitation and emission spectra from the MBE samples shows no measurable Stokes shift and, taken with the almost constant linewidths, suggests that the interfaces in these samples are all in the pseudo-smooth regime where the terrace lengths are smaller than the exciton diameter.

A comparison of photoconduction effects in (Al, Ga)As and GaAs/(Al, Ga)As heterostructures

The authors have studied the temperature, exposure time and wavelength dependence of the photo-conduction processes in thick Si-doped Al0.33Ga0.67As layers, and also in 2DEG samples with the same (Al, Ga)As composition, with and without a GaAs capping layer. They have deduced the spectral dependence of the photo-ionisation cross section for the Si-related DX centre in the (Al, Ga)As, and used this to model the expected persistent photo-conduction (PPC) behaviour of the 2DEG samples. Although good agreement has been obtained for a restricted range of experimental conditions, significant differences have also been found between the predicted and observed PPC effects, and also between the behaviour of capped and uncapped 2DEG samples. Other inconsistencies have been found, in the decay rate of the PPC on warming the sample and in the number of carriers that can ultimately transfer to the 2DEG channel, which suggest that the ionisation and capture processes for the DX centres in a 2DEG structure are modified from those in bulk (Al, Ga)As.

Interface effects on phonons in superlattices

Abstract A new modification of the familiar linear chain model is used to describe the propagation of longitudinal phonons along the axis of (GaAs)n1(AlAs)n2 superlattices. The model is able to describe the effects of broadening or alloying of the interfaces by using a local mode description of the susceptibility of each layer normal to the superlattice axis. The model is applied to Raman scattering measurements on short period superlattices, n=n1=n2=2…6; it gives a good description of frequency data in both the GaAs and AlAs LO phonon regions, and provides a quantitative assessment of the effective interface width that is in close agreement with X-ray measurements.

Slope resistance characteristics of GaAs‐(Al,Ga)As‐GaAs single barrier structures

Measurements of the incremental slope resistance of a GaAs‐(Al,Ga)As‐GaAs single barrier structure have been made at temperatures between 70 and 230 K. In contrast with other work we deliberately concentrate on the region close to zero applied bias. The deficiencies in the often‐used Wentzel–Kramers–Brillouin analysis of the electrical characteristics are exposed in this regime, and an exact, Airy function approach is found to be essential to describe our observations.

Far-Infrared and Raman Studies of Semiconductor Superlattices

The experimental techniques of Raman spectroscopy and far-infrared (FIR) Fourier-transform spectroscopy have been applied to a range of semiconductor superlattice specimens. Both resonant and non-resonant Raman scattering are available, and the Fourier-transform techniques include normal-incidence dispersive Fourier-transform spectroscopy (DFTS), attenuated total-reflection (ATR) spectroscopy, and oblique-incidence power Fourier-transform spectroscopy (FTS).

Effect of undoped GaAs spacers on the characteristics of GaAs‐(Al,Ga)As‐GaAs single barrier structures

The effects of undoped spacer layers on the electrical properties of single barrier heterostructures (both alloy and superlattice) have been investigated by measuring incremental slope resistance over the bias range −400 to +400 mV and at convenient temperature intervals between 70 and 290 K. The zero bias slope resistance,R s (0), and the effective barrier heights increase with spacer thickness. Also, the low‐temperature slope resistances,R s (V), decrease exponentially with the magnitude of the bias, V, while the effective barrier heights, deduced from high‐temperature measurements, decrease approximately linearly. This suggests that the decrease in R s (V) with bias is due simply to the voltage‐induced decrease in effective barrier height. R s (0) varies exponentially with zero bias effective barrier height for both alloy and superlattice barriers and this is consistent with the Γ electrons dominating the current transport through the barriers. All of our R s (V) curves are asymmetric and, using Airy function calculations, we have modeled curves similar to the experimental ones by assuming different doping levels for the two doped GaAs layers on either side of the barriers. This is possibly due to Si migration into the ‘‘undoped’’ barrier or the spacer layer closest to the substrate.

Raman spectroscopy of GaAs-AlAs superlattices : a study of interface roughness

Abstract The roughness of the interfaces in a series of (AlAs) n 1 (GaAs) n 2 (AlAs) n 3 (GaAs) n 4 superlattices has been measured by comparing the Raman frequencies of the confined GaAs-like LO phonons with results from a linear chain model modified to incorporate a description of the roughness. A new model is developed which shows that the interface roughness parameter W of a layer of average thickness n monolayers can be described by the expression W 2 = α nn s /(α n + n s ), with α = 1 and n s = 2 and 5 for GaAs and AlAs, respectively.