Positron lifetime spectroscopy was used to study Ga vacancies and negative ions in bulk crystals. It was found that the concentration of Ga vacancies decreased with increasing Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. The concentration of negative ions was related to that of Mg impurities, as determined using secondary ion mass spectrometry. The negative ions were therefore attributed to MgGa-. The negative charge of Mg suggested that Mg doping converted n-type material to semi-insulating material. This was due mainly to the electrical compensation of ON+ donors by MgGa- acceptors.

The Influence of Mg Doping on the Formation of Ga Vacancies and Negative Ions in GaN Bulk Crystals K.Saarinen, J.Nissilä, P.Hautojärvi, J.Likonen, T.Suski, I.Grzegory, B.Lucznik, S.Porowski: Applied Physics Letters, 1999, 75[16], 2441-3