Oxygen and zinc vacancies were unambiguously shown to be formed in as-grown ZnO bulk crystals grown from the melt without being subjected to irradiation, from electron paramagnetic resonance and optically detected magnetic resonance studies. Concentrations of the defects in their paramagnetic charge states VO+ and VZn- were estimated to be about 2 x 1014 and 1015/cm3, respectively. The VZn- defect was concluded to act as a deep acceptor and to exhibit large Jahn–Teller distortion by 0.8eV. The energy level of the defect corresponding to the (2–/–) transition was Ev + 1.0eV. The isolated Zn vacancy was found to be an important recombination centre and was concluded to be responsible for the red luminescence centred at around 1.6eV. On the other hand, the oxygen vacancy seems to be less important in carrier recombination as it could be detected only in EPR but not in optically detected magnetic resonance measurements. Neither isolated VZn- nor VO+ centres participate in the so-called 'green' emission. It was also shown that whereas the concentrations of both defects could be reduced by post-growth annealing, the Zn vacancy exhibited higher thermal stability. The important role of residual contaminants such as Li in the annealing process was underlined.

Oxygen and Zinc Vacancies in As-Grown ZnO Single Crystals. Wang, X.J., Vlasenko, L.S., Pearton, S.J., Chen, W.M., Buyanova, I.A.: Journal of Physics D, 2009, 42[17], 175411