Optical detection of electron paramagnetic resonance in room-temperature electron-irradiated ZnO

The dominant defect observed in the photoluminescence (PL) of room-temperature electron-irradiated ZnO by optical detection of electron paramagnetic resonance (ODEPR) is determined to be the positively charged oxygen vacancy $\left(V_{\mathrm{O}}^{+}\right)$. Its spectrum, labeled L3, was previously observed in a $4.2\mathrm{K}$ in situ irradiation study [Yu. V. Gorelkinskii and G. D. Watkins, Phys. Rev. B 69, 115212 (2004)], but it was thought there not to be stable at room temperature and was not identified. Here it is found to be stable to $400°\mathrm{C}$, where it disappears. It is observed as a competing process (negative signal) to the dominant PL band produced by the irradiation at $\sim 700\mathrm{nm}$, but is positive in a weaker band at $\sim 600\mathrm{nm}$. Models are presented for its electrical level position in the gap to explain the results. Two other ODEPR signals are also detected, one of which is tentatively identified as also associated with the oxygen vacancy.