It was shown that the intensity of the O vacancy (VO) related emission in ZnO at 2.45eV was related to the concentration of the donor level E4. The latter was located at 530meV below the conduction band and was attributed to VO0/++ re-charging. Deep level transient spectroscopy, with optical excitation, located the VO2+/+ level position at 140meV below the conduction band and provided evidence of negative-U properties of the O vacancies in ZnO.

Negative U-Properties of the Oxygen-Vacancy in ZnO. D.Pfisterer, J.Sann, D.M.Hofmann, B.Meyer, T.Frank, G.Pensl, R.Tena-Zaera, J.Zúñiga-Pérez, C.Martinez-Tomas, V.Muñoz-Sanjosé: Physica Status Solidi C, 2006, 3[4], 997-1000