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 $mathrm{Cu}text{ensuremath{-}}mathrm{III}text{ensuremath{-}}{mathrm{VI}}_{2}$ 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 $ptext{ensuremath{-}}{mathrm{CuInSe}}_{2}$ and $ptext{ensuremath{-}}{mathrm{CuGaSe}}_{2}$. We find that ${V}_{mathrm{Se}}$ in the chalcopyrite materials is amphoteric having two “negative-$U$”-like transitions, i.e., a double-donor transition $ensuremath{epsilon}(2+∕0)$ close to the valence band and a double-acceptor transition $ensuremath{epsilon}(0∕2ensuremath{-})$ closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects: type $ensuremath{alpha}$, which have a defect-localized-state (DLS) in the band gap, and type $ensuremath{beta}$, 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 $ensuremath{alpha}$ is nonconducting, whereas type $ensuremath{beta}$ is conducting. We identify the neutral anion vacancy as type $ensuremath{alpha}$ and the doubly positively charged vacancy as type $ensuremath{beta}$. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between $ensuremath{alpha}$- and $ensuremath{beta}$-type behavior, resulting in metastability and PPC. In ${mathrm{CuInSe}}_{2}$, the metastable behavior of ${V}_{mathrm{Se}}$ is carried over to the $({V}_{mathrm{Se}}text{ensuremath{-}}{V}_{mathrm{Cu}})$ 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.