Vacancy complexes in GaN single crystals

The paper presents a systematic investigation of point defects in a series of high-quality GaN crystals grown through halide vapor phase epitaxy (HVPE) and ammonothermal synthesis (AM). The GaN crystals were characterized using positron annihilation spectroscopy (PAS), along with measurements of photoluminescence (PL) and ab-initio modelling of various point defect configurations and calculations of positron lifetimes. The concentration of impurities in each GaN crystal was determined using secondary ion mass spectrometry. Our study revealed that the concentration of gallium vacancies (V Ga ) in GaN crystals is influenced by the levels of hydrogen and oxygen impurities. Most HVPE GaN crystals exhibit low concentrations of hydrogen and oxygen, with no defects detectable by PAS. Only one HVPE crystal showed an enhanced concentration of oxygen, which is associated with V Ga , forming negatively charged V Ga +O N complexes, where one nitrogen nearest neighbour of the vacancy is substituted by oxygen. In contrast, AM GaN crystals contain at least two orders of magnitude higher concentrations of hydrogen and oxygen impurities. This results in a high concentration (≥ 10 18 cm⁻³) of V Ga complexes with H and/or O. The configuration of these complexes is determined by the ratio of hydrogen to oxygen concentration within the crystal. The undoped n-type AM GaN crystal contains V Ga +2H+O N complexes, while the manganese-doped semi-insulating AM crystal contains V Ga +3H complexes, both complexes being in neutral charge state. The PL results provide evidence that the V Ga +2H+O N and V Ga +3H complexes are the source of the green emission bands at 2.34 eV and 2.37 eV, respectively, in AM crystals.