The photosensitive ESR signals of singly negatively charged zinc vacancies (VZn-, (VZn-)2- and interstitial oxygen ions in a tetrahedral surrounding (Oitetr-) were detected in undoped ZnO ceramic and their optical excitation and quenching behaviour was studied using the photo-ESR method. The investigations showed that all these defects were filled by electrons and have a concentration comparable to that of the oxygen vacancies. The photo-ESR experiments allow to arrange both defects in the energy-band diagram energetically. In contrast to electron irradiated ZnO single crystals the neutral zinc vacancy (VZnI could be detected only under intense optical illumination at 7K.

ESR and Photo-ESR Investigations of Zinc Vacancies and Interstitial Oxygen Ions in Undoped ZnO Ceramics. Pöppl, A., Völkel, G.: Physica Status Solidi A, 1991, 125[2], 571–81