Electron paramagnetic resonance was used to monitor O vacancies and Zn vacancies in a ZnO crystal irradiated near room temperature with 1.5MeV electrons. Out-of-phase detection at 30K greatly enhances the electron paramagnetic resonance signals from these vacancies. Following the electron irradiation, but before illumination, Fe3+ ions and non-axial singly ionized Zn vacancies were observed. Illumination with 325nm laser light at low temperature eliminated the Fe3+ signal while producing spectra from singly ionized O vacancies, neutral Zn vacancies, and axial singly ionized Zn vacancies. This light also produces electron paramagnetic resonance spectra from Zn vacancies having a OH- ion at an adjacent O site. The low-temperature response of the irradiated crystal to illumination wavelengths between 350 and 750nm was described. Wavelengths shorter than 600nm convert Fe3+ ions to Fe2+ ions and convert neutral O vacancies to singly ionized O vacancies. Neutral Zn vacancies were formed by wavelengths shorter than 500nm as electrons were removed from isolated singly ionized Zn vacancies. Warming above 120K in the dark reverses the effect of the illuminations. These wavelength-dependence results suggested that the ground state of the neutral O vacancy was deep, approximately 1.3eV above the valence band, and that the ground state of the singly ionized Zn vacancy was also deep, about 0.9eV above the valence band.

Further Characterization of Oxygen Vacancies and Zinc Vacancies in Electron-Irradiated ZnO. Evans, S.M., Giles, N.C., Halliburton, L.E., Kappers, L.A.: Journal of Applied Physics, 2008, 103[4], 043710