V. Ranki

Ga Vacancies as Dominant Intrinsic Acceptors in GaN Grown by Hydride Vapor Phase Epitaxy

Positron annihilation measurements show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than 1019 to below 1016 cm−3, as the distance from the interface region increases from 1 to 300 μm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of VGa, suggesting that the Ga vacancies are complexed with the oxygen impurities.

Vacancies as compensating centers in bulk GaN: doping effects

Abstract 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 VGa 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 VN–BeGa complex, where the atoms neighboring the N vacancy are strongly relaxed outwards, thus increasing the open volume.

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

Abstract Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the V Ga formation energy as a function of the Fermi level. The concentration of negative 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 − acceptors.

Target chamber for a slow positron beam: optimization of count rate and minimization of backscattering effects

Positrons, which scatter back from the target and annihilate in chamber walls near the detectors, may cause a significant error in annihilation parameters. We have constructed a new UHV target chamber for slow positron beam studies. In our design special care has been taken to reduce the effect of backscattered positrons. Detector wells are designed for two-detector coincidence measurements and they are situated on both sides of the target. The distance of the wells from the target can be adjusted by simple manipulators. This enables optimization regarding the count rate and the rate of backscattered positrons hitting the detector wells. The magnetic field in front of the target is increased by permanent magnets situated behind the target. The increased magnetic field guides the backscattered positrons effectively away from the detectors. The increased magnetic field also focuses the beam spot strongly.