We report on the growth of GaN with a zinc-blende, wurtzite, or a mixed phase structure on (001)GaP and (001)GaAs substrates by a low-temperature modified molecular beam epitaxy technique. By systematically varying the incident arsenic overpressure, films grown at a moderate substrate temperature of ≊620 °C show predominately wurtzite α-GaN, zinc-blende β-GaN, or a mixed phase of the two. Films containing only the metastable phase β-GaN were achieved by using a relatively high growth temperature of ≊700 °C and with an arsenic overpressure of ≊2.4×10−5 Torr. X-ray diffraction measurements indicate an improved crystalline quality for the layers grown at ≊700 °C compared to those grown at ≊620 °C as evident by a narrower full width at half-maximum of 35 min for β-GaN, which is among the narrowest reported to date.

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.