Further characterization of oxygen vacancies and zinc vacancies in electron-irradiated ZnO

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