Observation of Ga vacancies and negative ions in undoped and Mg-doped GaN bulk crystals

Gallium vacancies and negative ions are observed in GaN bulk crystals by ap plying positron lifetime spectroscopy. The concentration of Ga vacancies de creases with increasing Mg doping, as expected from the behavior of the V-G a formation energy as a function of the Fermi level. The concentration of n egative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to Mg-Ga(-). The negative charge of Mg suggests that Mg doping converts n-type GaN to semi- insulating mainly due to the electrical compensation of O-N(+) donors by Mg -Ga(-) accepters. (C) 1999 Elsevier Science B.V. All rights reserved.