A GaAs layer grown by MBE at a substrate temperature of 520 "C and containing three &doped planes with Si concentrations of 0.4, 1 and 4 x 1013 atoms/cm2 has been post-growth annealed in a furnace, up to temperatures of 648 "C. Secondary ion mass spectroscopy (SIMS) and capacitance voltage (CV)measurements have been carried out to measure profile-broadening. The most lightly doped plane gave a near-Gaussian diffusion profile with a diffusion coefficient comparable with literature values for simple diffusion of isolated Si,. atoms. The more heavily doped planes exhibit a complex profile shape with two components, a proportion of the atoms being confined to the original plane, together with an almost square-shaped profile of fast-diffusing atoms. Comparison of the CV and SIMS data suggests that formation of Si islands is taking place during deposition of the &doped plane, giving electrically inactive atoms which can subsequently diffuse into the surrounding GaAs during heat-treatment. This model is supported by preliminary local vibrational mode measurements which have been made on a set of multipleplane samples.

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The properties of deep donor states (DX centers) in III-V alloys are discussed in relation to their influence on device characteristics and performance. The techniques to avoid or minimize such deleterious effects in AlGaAs-based devices are discussed, along with their physical basis, and some guidelines for improved III-V device design are established. New results about the benefits of proper donor selection, the role of In alloying, the advantage of δ doping in layers and in modulation-doped devices, and the use of AlInAs and InGaP as alternative wide band-gap III-V alloys are presented.

Deep levels have been characterized in Si-doped and undoped InAlAs layers lattice matched to InP. At ambient pressure and low temperature. the Schottky junction capacitance shows a small persistent photocapacitance effect, related to photoionization thresholds at 0.6 and 1.1 eV. Deep-level translent spectroscopy spectra show a dominant, low-density electron trap at around 320 K, with an emission energy very dependent on the bias conditions and t h e hydrostatic pressure (0.5 to 0.9 eV). This is attributed to an interaction of a high density of interface states with the trap emission process. Under hydrostatic pressure, neither the PPC per cent nor the OLTS spectra shape changes. Photoluminescence spectra in both Sidoped and undoped InAlAs layers show two weak peaks at 0.80 and 0.95 eV, that we suggest are due to a native defect involving an As antisite. W e conclude that the detected traps have no relation with the DX centres.

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