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

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Abstract
High-pressure measurements were employed to study mechanisms of (i) the 3.27 eV emission band (and phonon replicas) and (ii) 2.9–3.1 eV blue photoluminescence in different types of GaN:Mg samples. GaN layers with low Mg doping, grown on bulk GaN crystals and on sapphire substrates, were used to study the 3.27 eV uv band. This band results from donor-acceptor pair (DAP) recombination (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 layer grown on sapphire were employed. For the all samples the pressure coefficient of the corresponding luminescence bands was measured at 77 K by means of the diamond anvil technique. The energy shift of the 3.27 eV band under 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 coefficients of about 23 meV/GPa. It shows that the initial state of the blue emission has localized character in both types of GaN material and supplies a strong argument against models of radiative recombination in highly-Mg-doped/compensated GaN material that postulate an involvement of delocalized (shallow) electronic states as the initial states.