The magnesium acceptor states in GaN: an investigation by optically-detected magnetic resonance

We observe a systematic dependence of the g -values of magnesium acceptors on the photoluminescence energy that is used for the optical detection of magnetic resonance signals. This effect leads to a model which assumes that, in the magnesium acceptors, the hole is located in an atomic orbital of p -character, presumed to be on a nitrogen atom, and which, in the wurtzite crystal field, is an orbital doublet. The remaining degeneracy is then removed by a reduction in local symmetry associated with the relative positions of the magnesium dopant and the nitrogen atom upon which the hole is localized. Further changes in the local crystal field are caused by the presence of nearby defects or by strain and are accompanied by changes in acceptor depth. The model leads to the correct g -values and the correct correlation between the g -values and acceptor depth and is consistent with a range of acceptor states of different depths being formed by simple substitution of a magnesium ion at a gallium site