The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5–300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (Te<20 K), the energy relaxation is via acoustic phonon emission. The magnitude and temperature dependence of the energy relaxation are found to be in good agreement with theoretical calculations using appropriate values of the deformation potential and piezoelectric coupling constants and ignoring screening. For Te⩾70 K, the dominant mechanism of energy loss is optic phonon emission. For the several samples studied, consistent values of the optic phonon energy and electron-optic phonon relaxation time, 90±4 meV and 5–10 fs, respectively, are measured. The energy agrees well with values obtained by other methods and the relaxation time is consistent with theoretical calculations of the Fröhlich interaction and indicate that hot phonon effects are absent up to 10−8 W/electron dissipation.

Far-infrared absorption due to Lyman transitions of the Be acceptor in GaAs has been measured in bulk molecular-beam-epitaxy-grown samples at various temperatures and under different illumination conditions. Sharp lines of energy ͑at 1.9 K͒ of 134.42Ϯ0.06, 166.76Ϯ0.05, and 182.30Ϯ0.05 cm Ϫ1 are observed, being the G, D, and C lines, respectively. Fiorentini's recent calculation ͓Phys. Rev. B 51, 10 161 ͑1995͔͒ of the acceptor energy spectrum in GaAs is in good agreement with these results. At 1.9 K the G, D, and C linewidths are 1.06Ϯ0.14, 1.16Ϯ0.04, and 1.86Ϯ0.05 cm Ϫ1 , respectively, and the integrated intensities are in the ratio 11.5Ϯ5.6:100:57.6Ϯ9.8. External illumination during cooling or measurement had no effect on the observed spectra.

The incorporation of magnesium and carbon in GaN grown by molecular beam epitaxy (MBE) has been investigated by secondary ion mass spectroscopy (SIMS) and other techniques. We have grown Mg:GaN in a wide range of chemical concentrations 1 × 10 17 -1 × 10 19 cm −3 . Low temperature photoluminescence of Mg:GaN is dominated by the donor-acceptor transitions associated with Mg at ∼3.253 eV. Carrier concentration for Mg:GaN in the range 1 × 10 17 -2 × 10 18 cm −3 with mobilities <10 cm 2 V −1 s −1 were measured by the Hall effect technique. In the C:GaN layers, it was found that carbon can be uniformly incorporated into the layer at a concentration ∼2 × 10 20 cm −3 . However, at this high concentration there is a tendency for carbon to diffuse into the undoped GaN buffer layer.

In this paper we demonstrate the feasibility of growing III-N semiconductors on novel lattice-matched oxide substrates. Although the growth parameters are not yet optimal, acceptable GaN layers have been grown.

We describe measurements of the electrical and luminescence properties of Mg-doped GaN films grown by plasma-enhanced molecular beam epitaxy on sapphire and GaAs substrates. Secondary ion mass spectroscopy measurements were used to determine the total Mg concentration in each of the films and showed the Mg profiles to be flat throughout the films. At low Mg beam fluxes, there is a linear relationship between the Mg concentration in the film [Mg] and the Mg flux but, for fluxes above about 10 −2 ML s −1 , [Mg] saturates at 2 × 10 19 cm −3 (at a growth temperature of 750 • C). We outline a simple model of Mg incorporation which explains the experimental data. Hall effect measurements reveal that p-type conductivity is obtained only in films grown under slightly nitrogen-rich conditions and demonstrate that, in thin films (< 1 µm), conductivity is dominated by high densities of donor-type defects. This is also borne out by photoluminescence (PL) results. Most of the samples showed bound exciton emission at low temperatures, in some cases involving neutral donors, in others neutral acceptors, the D 0 X emission suggesting that the incorporation of Mg results in a lowering of the GaN band gap. All samples show donor-acceptor recombination and detailed analysis suggests that Mg doping suffers from self-compensation. Room-temperature PL is dominated by a free electron-bound hole emission line at 3.2 eV in p-type samples but by deep emission in samples grown under Ga-rich conditions.