As-grown bulk crystals, and crystals annealed in vacuum, O, or Zn phase were characterized by using electrical, optical and magnetic resonance spectroscopic techniques. The experiments showed that the residual carrier concentration was caused by residual H, Al, Ga and O vacancies (VO) in the material. Annealing in O2 at about 1000C (2atm, 20h) reduced the H and VO donor concentrations by about an order of magnitude. Photoluminescence and deep-level transient spectroscopy results suggested that there was a correlation between a broad unstructured emission at 2.45eV (so-called green band) and a donor level which was 0.53eV below the conduction band and was attributed to the VO0/++ transition. By using deep level transient spectroscopy with optical excitation it was possible to observe a metastable level which was 0.14eV below the conduction band and which was attributed to VO2+/+ re-charging. The results provided evidence of negative-U properties for O vacancy defects; as predicted by recent theoretical calculations.

Properties of the Oxygen Vacancy in ZnO. Hofmann, D.M., Pfisterer, D., Sann, J., Meyer, B.K., Tena-Zaera, R., Munoz-Sanjose, V., Frank, T., Pensl, G.: Applied Physics A, 2007, 88[1], 147-51