Optical emission bands in cubic GaN grown by MBE

We examine the proposition that mid-gap broad cathodoluminescence emission spectra of cubic and wurtzite GaN centered at 1.9 eV (red) and 2.2 eV (yellow) respectively are of similar origin. At high excitation levels, the red band shows a small blue shift comparable with the shallow-donor/deep-acceptor pair (DAP) recombination path proposed for the yellow band. The temperature dependence of peak energy supports this view, while band width and symmetry properties suggest that at least one of the defects in the DAP emission is strongly coupled to the lattice. Photoluminescence data point to donor and acceptor ionisation energies of about 15 meV and 100 meV respectively in the cubic phase. Energy dependence of emission decay times and peak energy migration are also indicative of DAP process. Finally we observe differences in surface micromorphology, flat and tesselated in cubic GaN and granulated in the wurtzite layers. Excitonic emission is suppressed at grain boundaries in both cases, while the respective red and yellow emission maps show little correlation with morphology.