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 Cu-III-VI₂ 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₎ in n-ZnO, originating from a metastable shallow donor state of V₎. In contrast, we predict persistent hole photoconductivity (p-type PPC) caused by the Se vacancy Vₒ₄ in p-CuInSe₂ and p-CuGaSe₂. We find that Vₒ₄ in the chalcopyrite materials is amphoteric having two ``negative-U''-like transitions, i. e. , a double-donor transition (2+∕0) close to the valence band and a double-acceptor transition (0∕2-) closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects: type, which have a defect-localized-state (DLS) in the band gap, and type, 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 is nonconducting, whereas type is conducting. We identify the neutral anion vacancy as type and the doubly positively charged vacancy as type. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between - and -type behavior, resulting in metastability and PPC. In CuInSe₂, the metastable behavior of Vₒ₄ is carried over to the (Vₒ₄-V₂ₔ) complex, which we identify as the physical origin of PPC observed experimentally. We explain previous puzzling experimental results in ZnO and CuInSe₂ in the light of this model.