Shubnikov-de Haas and persistent photoconductivity measurements have been performed as a function of hydrostatic pressure to study the saturation of the free electron concentration and the mobilities of the individual subbands at high doping densities in very thin sheets (2. 5. 10 nm) of silicon donors in MBE GaAs. The samples were grown at very low temperature (400 "C) in order to limit dopant diffusion, and silicon concentrations were close to the solubility limit at this temperature. As has been shown previously with spike-doped GaAs(Si1, the relative occupancies and the mobilities of the lower subbands are very sensitive to the spreading of the dopant distribution. A routine was developed for the analysis of the Fourier transforms of the complex pattern of Shubnikov-de Haas peaks in order to provide quantitative values for the mobilities of the individual subbands. The results of this analysis are compared with values deduced from the magnetic fieid dependence of the resistivity and Hall effect. On applying hydrostatic pressures of the order of 15 kbar in the dark, a decrease of the free electron concentration of a factor of two was observed. This was accompanied by an

The cyclotron resonance (CR) of the two-dimensional electron gas (2D EG) in GaAs-(Ga, A1)As heterojunctions has been studied in the resonant-polaron regime for 2D carrier densities N, in the range (0.8-5.4) && 10" cm . A reflectivity technique has allowed the CR to be recorded at energies up to 35.63 meV, within the GaAs reststrahlen band, and a calculation of the dielectric response of the complete heterostructure has enabled the effective masses to be reliably evaluated from the line shapes and positions of the resonances. The results indicate that the 2D electrons are coupling to the LO phonon (36.7 meV), in agreement with theoretical predictions. At low carrier densities, the resonant-polaron contribution to the effective mass becomes apparent at cyclotron energies above 25 meV, and increases in size as the LO-phonon energy is approached: however" this mass enhancement is removed rapidly as N, is increased, indicating the importance of Landau-level occupancy and screening in the 2D EG. Close to the LO-phonon energy, large shifts in the resonance position, which are several times the linewidth in size, are produced by varying N, . This large N, dependence explains previous convicting reports of "enhanced" or "reduced" polaron effects in the 2D electron gas. A comparison of the experimental results with existing memory-function calculations of the polaron contribution to the effective mass indicates that the greater part of the N, dependence can be ascribed to Landau-level occupancy effects.

Abstract. Misfit dislocation formation in lattice-mismatched III-V heterostructures both under tensile and under compressive stress has been studied. Layers of GaAs under tensile stress have been grown by metal-organic vapour phase epitaxy on ln*Gai_xAs substrates with indium concentrations between 0.1 and 1.1%, Compressively strained In^Ga^xAs layers with indium concentrations between 0.5 and 2.5% have been grown on GaAs substrates. For the layers under tensile stress an asymmetrical dislocation pattern has been observed, whereas the compressively strained layers show a symmetrical dislocation pattern. A model describing the relaxation process by the formation of dissociated hexagonally shaped half-loop dislocations is proposed. A difference in the mobilities of the two possible misfit dislocation types is found to be the origin of asymmetrical strain relief at low growth temperatures. In layers under tensile stress the cross slipping of screw dislocations is counteracted by the shear stress, leading to relaxation in only one direction. In layers under compressive stress the nucleated misfit dislocations can undergo cross slipping, resulting in a cross hatched pattern at the surface. At higher growth temperatures the dislocation patterns become more symmetrical due to the higher dislocation mobilities. Growth hillocks are formed on the surfaces of the layers grown under tensile stress, due to local accumulation of dislocations. This hillock growth is prevented at higher growth temperatures by the higher mobility of the dislocations. It is also shown that the (mis)orientation of the substrate is revealed by non-parallel groups of dislocation lines observed at the surface of a relaxed epilayer.

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Results from on-wafer electroluminescence (EL) microscopy on AlGaN/GaN high-electron-mobility transistors with leakage currents varying over four orders of magnitude are presented. In the off-state region the integrated EL intensity is proportional to the leakage current and independent of gate width for the devices under study. The slope of the integrated EL-leakage current dependence is determined by the electrical field in the source-drain direction. The influence of the GaN cap thickness is small or even negligible for higher drain bias. Stress during accelerated aging results in enhanced degradation for areas of enhanced leakage current and/or electric field values.