Defect studies in HVPE GaN by positron annihilation spectroscopy

We have used positron annihilation spectroscopy to study GaN grown by hydride vapor epitaxy. Our results imply that Ga vacancies in GaN are the dominant intrinsic acceptor defects in n-type nominally undoped pure material. Our experiments also show the universal role of the Ga vacancy as the most important compensating acceptor over four orders of magnitude of intentional oxygen doping. We have been able to identify the in-grown Ga vacancy-related defects as VGa-ON pairs and to determine both the binding energy of the pair as well as the formation energy of the isolated Ga vacancy. The experiments performed in polar homo-epitaxial GaN and non-polar hetero-epitaxial GaN give support for a growth-surface-dependent defect incorporation model. On the other hand, the incorporation of impurities in hetero-epitaxial Ga-polar GaN seems to be dominated by effects related to extended defects such as dislocations. The preliminary results obtained in electron irradiated GaN show that negatively charged isolated Ga vacancies are produced together with non-open volume negative ion-type defects when 2-MeV electrons are used, while neutral N vacancy related defects seem to trap positrons in the material irradiated with 0.45 MeV electrons.