Vacancies as compensating centers in bulk GaN: doping effects

Gallium vacancy complexes have been identified in n-type bulk GaN by applying positron annihilation spectroscopy. Their formation is suppressed when the material becomes resistive by Mg doping, as expected from the behavior of the V-Ga formation energy as a function of the Fermi level. In Be-doped GaN vacancies are observed even in resistive material. The positron lifetimes show that their open volume is larger than expected for the N vacancy. A possible identification is a V-N - Be-Ga complex, where the atoms neighboring the N vacancy are strongly relaxed outwards, thus increasing the open volume. (C) 2002 Elsevier Science B.V. All rights reserved.