Positron lifetime spectroscopy revealed Ga vacancies and negative ions in bulk crystals. The concentration of Ga vacancies decreased with increasing Mg doping; as expected from the behaviour of the VGa formation energy as a function of the Fermi level. The concentrations of the negative ions were related to those of Mg impurities. The negative ions were therefore attributed to MgGa-. The negative charge on Mg suggested that Mg-doping converted n-type material to semi-insulating material; due mainly to the electrical compensation of ON+ donors by MgGa- acceptors.

Observation of Ga Vacancies and Negative Ions in Undoped and Mg-Doped GaN Bulk Crystals K.Saarinen, J.Nissilá, J.Oila, V.Ranki, M.Hakala, M.J.Puska, P.Hautojärvi, J.Likonen, T.Suski, I.Grzegory, B.Lucznik, S.Porowski: Physica B, 1999, 273-274, 33-8