Donor acceptor pair in molecular beam epitaxy grown GaN

We have recently found evidence of new donor acceptor pair (DAP) luminescence in molecular beam epitaxy (MBE) grown films. A variety of nominally undoped samples have been studied by photoluminescence (PL) over a temperature range of 5–300 K. The samples show intensive luminescence at energies of 3.404–3.413 eV varying with different sample at 5 K, as well as a fairly strong D °X line at low temperature. We attribute the line at 3.404–3.413 eV to DAP recombination which is over 0.1 eV different from the well known DAP caused by Mg-doping in GaN. The DAP line shows fine structure, it even predominates in one particular sample. The peak position shifts to higher energy with temperature increasing from 5 up to 70 K, and as the excitation laser intensity increases. The data are consistent with DAP luminescence involving an acceptor level of about 90 meV (presumably carbon) above the valence band edge in GaN. It is much shallower than the acceptor level of 250 meV produced by the p-type dopant Mg which is commonly used at present.