Invited talk abstract

Positron annihilation spectroscopy can be used to identify vacancy defects in bulk semiconductor crystals and epitaxial layers. It yields quantitative information on vacancy concentrations in the range 10^15-10^20 cm-3. Positron localization into the hydrogenic states around negative centers can be applied to study also acceptors that have no open volume. Positron experiments detect Ga vacancies as dominant intrinsic acceptor defects in n-type GaN bulk crystals and in layers grown by MOCVD, MBE and HVPE. Doping with Si impurities instead of oxygen decreases the concentration of Ga vacancies by an order of magnitude. The concentration of V_Ga decreases with increasing Mg doping and no Ga vacancies are observed in p-type or semi-insulating layers doped with Mg. These trends agree well with the theoretical calculations, which predict that the formation energy of Ga vacancy is high in p-type and semi-insulating material, but greatly reduced in n-type GaN, and even further reduced due to the formation of V_Ga-O_N complexes. Thermal stability of Ga vacancies was studied in GaN bulk crystals after 2 MeV electron irradiation at 300 K. The isolated Ga vacancies recover in long-range migration processes at 500 - 600 K with an estimated migration energy of 1.5(2) eV. Since the native Ga vacancies in as-grown GaN survive up to much higher temperatures (1300 - 1500 K), we conclude that they are stabilized by forming V_Ga-O_N complexes. The estimated binding energy of 2.2(4) eV of such complexes is in good agreement with the results of theoretical calculations. In addition to doping, the presence of open-volume defects in GaN layers depends on the growth conditions. The concentrations of Ga vacancies increases strongly when more N rich stoichiometry is applied in the MOCVD growth. On the other hand, the lattice mismatch and associated dislocation density seem to have less influence on the formation of point defects than doping and stoichiometry - at least at distances >0.5 um from the layer/substrate interface. This suggests that the formation of point defects in both epitaxial layers and bulk crystals follows mainly the trends expected for defects in thermal equilibrium.