Residual Native Shallow Donor in ZnO

High-energy electron irradiation in ZnO produces shallow donors at about $E_C-30\mathrm{}\mathrm{meV}$. Because the production rate is much higher for Zn-face (0001) than O-face $\left(000\overline{1}\right)$ irradiation, the donor is identified as a Zn-sublattice defect, most likely the interstitial ${\mathrm{Zn}}_I$ or a ${\mathrm{Zn}}_I$-related complex. The donor energy is quite close to that of the unirradiated sample, and of other samples discussed in the literature, strongly suggesting that ${\mathrm{Zn}}_I$ (and not $V_{\mathrm{O}}$) is the dominant native shallow donor in ZnO. An exceptionally high displacement threshold energy $\left(\sim 1.6\mathrm{MeV}\right)$ is quantitatively explained in terms of a multiple-displacement model.