Anion Vacancies as a Source of Persistent Photoconductivity in II-VI and Chalcopyrite Semiconductors

Stephan Lany, Alex Zunger

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581 Scopus Citations

Abstract

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 VO in n-ZnO, originating from a metastable shallow donor state of VO. In contrast, we predict persistent hole photoconductivity (p-type PPC) caused by the Se vacancy VSe in p-CuInSe2 and p-CuGaSe2. We find that VSe in the chalcopyrite materials is amphoteric having two "negative-U"-like transitions, i.e., a double-donor transition ε(2+ 0) close to the valence band and a double-acceptor transition ε(02-) closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects: type α, which have a defect-localized-state (DLS) in the band gap, and type β, 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 α is nonconducting, whereas type β is conducting. We identify the neutral anion vacancy as type α and the doubly positively charged vacancy as type β. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between α- and β-type behavior, resulting in metastability and PPC. In CuInSe2, the metastable behavior of VSe is carried over to the (VSe-VCu) 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.

Original languageAmerican English
Article number035215
Number of pages13
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume72
Issue number3
DOIs
StatePublished - 15 Jul 2005

NREL Publication Number

  • NREL/JA-590-38115

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