Different character of the donor-acceptor pair-related 3.27 eV band and blue photoluminescence in Mg-doped GaN. Hydrostatic pressure studies

High-pressure measurements were employed to study mechanisms of (i) the 3.2 7 eV emission band (and phonon replicas) and (ii) 2.9-3.1 eV blue photolumi nescence in different types of GaN:Mg samples. GaN layers with low Mg dopin g, grown on bulk GaN crystals and on sapphire substrates, were used to stud y the 3.27 eV uv band. This band results from donor-acceptor pair (DAP) rec ombination (with both impurities of shallow character). For high Mg doping resulting in blue light emission at about 3 eV, a bulk GaN crystal and a la yer grown on sapphire were employed. For the all samples the pressure coeff icient of the corresponding luminescence bands was measured at 77 K by mean s of the diamond anvil technique. The energy shift of the 3.27 eV band unde r hydrostatic pressure is used to determine the linear pressure coefficient characteristic for (shallow) DAP recombination. It has a value of about 35 meV/GPa, close to the pressure shift of the GaN band gap (about 40 meV/GPa ). The 2.9-3.1 eV blue luminescence band exhibits much lower pressure coeff icients of about 23 meV/GPa. It shows that the initial state of the blue em ission has localized character in both types of GaN material and supplies a strong argument against models of radiative recombination in highly-Mg-dop ed/compensated GaN material that postulate an involvement of delocalized (s hallow) electronic states as the initial states.