The nature of the native defects and their roles in ZnO were studied using positron annihilation and optical transmission measurements. It was shown that O vacancies were the origin of the shift in the optical absorption band that caused the red or orange coloration. It also revealed that the donor nature of the O vacancy was ~0.7eV. In addition, it was shown the Zn interstitial was not the donor in as-grown ZnO and supported recent calculations which predicted that H in an O vacancy formed multi-center bonds and acted as a shallow donor.

Nature of Native Defects in ZnO. Selim, F.A., Weber, M.H., Solodovnikov, D., Lynn, K.G.: Physical Review Letters, 2007, 99[8], 085502