EPR and optical study of oxygen and zinc vacancies in electron-irradiated ZnO

A ZnO crystal, grown by the seeded chemical vapor transport method, was irradiated near room temperature with 1.5 MeV electrons. The resulting oxygen and zinc vacancies were characterized using optical absorption, photoluminescence (PL), and electron paramagnetic resonance (EPR). A broad absorption band peaking near 409 nm gives the irradiated crystal a yellow color and is assigned to neutral oxygen vacancies. A broad emission band peaking near 700 nm in the irradiated crystal is assigned to donor–acceptor pair recombination involving zinc-vacancy acceptors. EPR spectra from Fe3+ ions and singly ionized nonaxial zinc vacancies are observed at 30 K after the irradiation, but before illumination. Illumination with 325 nm light at low temperature eliminates the Fe3+ signal while producing spectra from singly ionized oxygen vacancies, neutral zinc vacancies, and singly ionized axial zinc vacancies. The light also produces EPR spectra from zinc vacancies with an OH− ion occupying an adjacent oxygen site. No correlation was found between the green emission and the presence of oxygen and/or zinc vacancies.