It was recalled that, although it had long been known that group-III dopants (such as Al) were efficient donors, the roles of impurities such as H and N were only now being investigated. This was also true of the simplest point defects, such as Zn and O vacancies and interstitials. Theory had shown that H was always a donor (not amphoteric), that the O vacancy was a deep donor (not shallow) and that the Zn interstitial was a shallow donor in agreement with electron irradiation experiments. Recent irradiation studies had shown that significant defect annihilation took place, even at low temperatures; thus showing why ZnO was so resistant to radiation.

Electrical and Optical Properties of Defects and Impurities in ZnO. Look, D.C., Coşkun, C., Claflin, B., Farlow, G.C.: Physica B, 2003, 340-342, 32-8