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Using first-principles electronic structure calculations we identify theanion vacancies in II-VI and chalcopyrite Cu-III-VI2 semiconductors as a classof intrinsic defects that can exhibit metastable behavior. Specifically, wepredict persistent electron photoconductivity (n-type PPC) caused by the oxygenvacancy VO in n-ZnO, and persistent hole photoconductivity (p-type PPC) causedby the Se vacancy VSe in p-CuInSe2 and p-CuGaSe2. We find that VSe in thechalcopyrite materials is amphoteric having two "negative-U" like transitions,i.e. a double-donor transition e(2+/0) close to the valence band and adouble-acceptor transition e(0/2-) closer to the conduction band. We introducea classification scheme that distinguishes two types of defects (e.g., donors):type-alpha, which have a defect-localized-state (DLS) in the gap, andtype-beta, which have a resonant DLS within the host bands (e.g., conductionband). In the latter case, the introduced carriers (e.g., electrons) relax tothe band edge where they can occupy a perturbed-host-state (PHS). Type alpha isnon-conducting, whereas type beta is conducting. We identify the neutral anionvacancy as type-alpha and the doubly positively charged vacancy as type-beta.We suggest that illumination changes the charge state of the anion vacancy andleads to a crossover between alpha- and beta-type behavior, resulting inmetastability and PPC. In CuInSe2, the metastable behavior of VSe is carriedover to the (VSe-VCu) complex, which we identify as the physical origin of PPCobserved experimentally. We explain previous puzzling experimental results inZnO and CuInSe2 in the light of this model.Comment: submitted to Phys. Rev.

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