Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors

Using first-principles electronic structure calculations we identify the anion vacancies in II-VI and chalcopyrite $\mathrm{Cu}\text{−}\mathrm{III}\text{−}{\mathrm{VI}}_2$ semiconductors as a class of intrinsic defects that can exhibit metastable behavior. Specifically, we predict persistent electron photoconductivity ($n$-type PPC) caused by the oxygen vacancy $V_{\mathrm{O}}$ in $n$-ZnO, originating from a metastable shallow donor state of $V_{\mathrm{O}}$. In contrast, we predict persistent hole photoconductivity ($p$-type PPC) caused by the Se vacancy $V_{\mathrm{Se}}$ in $p\text{−}{\mathrm{CuInSe}}_2$ and $p\text{−}{\mathrm{CuGaSe}}_2$. We find that $V_{\mathrm{Se}}$ in the chalcopyrite materials is amphoteric having two “negative-$U$”-like transitions, i.e., a double-donor transition $\epsilon (2+∕0)$ close to the valence band and a double-acceptor transition $\epsilon (0∕2-)$ closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects: type $\alpha$, which have a defect-localized-state (DLS) in the band gap, and type $\beta$, which have a resonant DLS within the host bands (e.g., the conduction band for donors). In the latter case, the introduced carriers (e.g., electrons) relax to the band edge where they can occupy a perturbed-host state. Type $\alpha$ is nonconducting, whereas type $\beta$ is conducting. We identify the neutral anion vacancy as type $\alpha$ and the doubly positively charged vacancy as type $\beta$. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between $\alpha$- and $\beta$-type behavior, resulting in metastability and PPC. In ${\mathrm{CuInSe}}_2$, the metastable behavior of $V_{\mathrm{Se}}$ is carried over to the $\left(V_{\mathrm{Se}}\text{−}{V}_{\mathrm{Cu}}\right)$ complex, which we identify as the physical origin of PPC observed experimentally. We explain previous puzzling experimental results in ZnO and ${\mathrm{CuInSe}}_2$ in the light of this model.