The low temperature photoluminescence (PL) of Mg-doped GaN co-doped with oxygen is investigated. Two PL bands are observed at low temperature at 2.5 and 2.8 eV in the p-type oxygen co-doped samples. Both the 2.5 eV and 2.8 eV bands are attributed to donor-acceptor (DA) transitions involving deep donors of different origin and a shallow Mg_(Ga) acceptor. The integrated intensity of the 2.8 eV band in p-type GaN decreases as the oxygen partial pressure is increased. The decrease is attributed to reduced compensation by deep native donors. The two bands are not formed in n-type (n > 10~(19) cm~(-3)) GaN:Mg over-doped with oxygen indicating that the deep donors responsible for these transitions have high formation energies in n-type material.
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