Light-induced metastability in the wide-gap ZnO and CuGaSe$_{2}$ caused by anion vacancies

First-principles electronic structure calculations [1] show that anion vacancies in II-VI and chalcopyrite I-III-VI$_{2}$ semiconductors are a class of intrinsic defects that can produce metastable behavior and persistent photoconductivity (PPC), arising from a pronounced coupling between electronic and structural degrees of freedom. In ZnO, V$_{O}^{0}$ has a deep localized donor state in the gap, while V$_{O}^{2+}$ has a shallow level near the CBM. Illumination excites V$_{O}^{0}$ to V$_{O}^{+}$+e and to V$_{O}^{2+}$+2e, and this transition is accompanied by large lattice relaxation. The latter state is metastable and acts as a shallow donor, leading to persistent \textit{electron} photoconductivity ($n$-type PPC), which persists until it is thermally activated into the deep V$_{O}^{0} $. Comparing the behavior of the anion vacancy in the wide-gap chalcopyrite CuGaSe$_{2}$ to that in ZnO, we find an interesting asymmetry: V$_{Se}$ produces persistent \textit{hole} photoconductivity in $p$-CuGaSe$_{2}$, constituting the unusual case where a donor-like defect creates $p$-type PPC. \par [1] Stephan Lany and Alex Zunger, Phys. Rev. Lett. \textbf{93}, 156404 (2004).