Using first-principles electronic structure calculations the anion vacancies in II-VI and chalcopyrite Cu-III-VI2 semiconductors were identified as being a class of intrinsic defects that could exhibit metastable behavior. Specifically, persistent electron photoconductivity (n-type PPC) caused by the O vacancy VO in n-ZnO was predicted; originating from a metastable shallow donor state of VO. In contrast, persistent hole photoconductivity (p-type PPC) caused by the Se vacancy VSe in p-CuInSe2 and p-CuGaSe2 was predicted. It was found that VSe in the chalcopyrite materials was amphoteric; having 2 so-called negative-U like transitions, i.e., a double-donor transition ε(2+/0) close to the valence band and a double-acceptor transition ε(0/2–) closer to the conduction band. A classification scheme was introduced which distinguished 2 types of defect: type α, which had a defect-localized-state in the band gap, and type β, which had a resonant defect-localized-state within the host bands (e.g., the conduction band for donors). In the latter case, the introduced carriers (e.g., electrons) relaxed to the band-edge where they could occupy a perturbed-host state. Type-α was non-conducting, whereas type-β was conducting. The neutral anion vacancy was identified as being type-α, and the doubly-positively charged vacancy as being type-β. It was suggested that illumination changed the charge state of the anion vacancy and led to a cross-over between α- and β-type behavior, resulting in metastability and persistent photoconductivity. In CuInSe2, the metastable behavior of VSe was carried over to the (VSe-VCu) complex, which was identified as being the origin of the experimentally observed persistent photoconductivity. Puzzling experimental results for ZnO and CuInSe2 could be explained by this model.

Anion Vacancies as a Source of Persistent Photoconductivity in II-VI and Chalcopyrite Semiconductors. S.Lany, A.Zunger: Physical Review B, 2005, 72[3], 035215 (13pp)