We report new lines in the photoluminescence (PL) spectrum of lightly Be-doped GaN. The low-temperature PL spectrum of the lightly doped sample is dominated by a transition at 3.385 eV with first and second LO phonon replicas. Power-resolved PL measurements showed that the peak at 3.385 eV narrowed in width and shifted to higher energies with increasing excitation intensity. Thus the transition is attributed to donor-to-acceptor recombination, involving a Be acceptor of optical ionization energy of between 90 and 100 meV. This is much shallower than the acceptor level of 250 meV induced by Mg doping. Increasing the doping, however, resulted in a quenching of the band-edge luminescence and the appearance of a broad transition centred around 2.4 eV which we assign to a complex involving Be. Undulations on the peak were consistent with interference effects. On increasing the doping level even further all luminescence was quenched.

We report the results of low-temperature photoluminescence measurements on GaN films grown by molecular beam epitaxy on (0001) sapphire substrates. Samples were either nominally undoped or doped with Si. The spectra are generally dominated by a sharp peak at 3.47 eV which is attributed to excitons bound to neutral donors. A much weaker peak (or shoulder) near 3.45 eV probably arises from excitons bound to neutral acceptors. On raising the temperature to 50 K, in some samples free exciton peaks can be partially resolved on the high-energy side of the main line. In others we believe that these free excitons are recaptured onto neutral acceptors, thus enhancing the low-energy side of the line. A broader emission line appears in many samples at an energy near 3.42 eV which shows significant variation in position between samples. Our data show that it represents a free-to-bound, probably a free hole-to-donor, transition. This donor has previously been associated with oxygen. Of particular interest is the fact that some samples show a second sharp peak at 3.27 eV, together with a second broader peak at about 3.17 eV (also variable in energy). The sharp peak is energetically consistent with its being either a donor-acceptor or a free electron-to-bound hole transition, but subsidiary measurements rule out both these possibilities. We suggest that it may represent an exciton bound to a deep donor or a shallow donor-bound exciton in zinc blende GaN inclusions contained within the mainly wurtzite material. We tentatively interpret the 3.17 eV line as a phonon replica of this zinc blende line, the phonon energy being perturbed by the small size of the inclusions and by strain effects within these inclusions.

We report the results of photoluminescence measurements on a number of GaN thin films grown by MBE on GaAs (111)B substrates. In particular, we draw attention to a new observation of a line at approximately 3.40eV which is accompanied by complex fine structure and interpret it as due to a donor-acceptor (DA) transition. Assuming a donor energy of 30meV, we derive an acceptor binding energy of approximately 80meV which is very much smaller than the accepted value of 250meV for the well established Mg acceptor. However, our result is in agreement with a recent estimate of the hydrogenic acceptor energy as being 85meV.

P-type doping of molecular beam epitaxy grown GaN has been investigated using beryllium, magnesium and carbon, the sample being characterized by luminescence under optical excitation by He-Cd laser light of energy 3.815 eV. Doping resulted in a strong reduction of the band-edge luminescence in some samples and the appearance of deep level bands at around 2.3 and 2.5 eV. Photoluminescence spectra in the region of the band gap using high gain revealed the presence of narrow lines at positions 3.541, 3.449, 3.357 and 3.266 eV in a wide variety of these samples. These lines were constant in position with respect to temperature over the range of 5 K to 300 K and it was noted that they are separated by the GaN longitudinal optical phonon energy of 92 meV. It is believed that these lines are caused by resonant Raman scattered replicas appearing in the photoluminescence spectra.

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.