Metal-dimer atomic reconstruction leading to deep donor states of the anion vacancy in II-VI and chalcopyrite semiconductors

First-principles total-energy calculations reveal a novel local atomic reconstruction mode around anion vacancies in II-VI and chalcopyrite compounds resulting from the formation of metal dimers. As a consequence, the neutral Se vacancy has an unexpected low symmetry in ZnSe and becomes a deep donor in both ZnSe and CuGaSe2, contrary to the common belief regarding chalcopyrites. The calculated optical transition energies explain the hitherto puzzling absorption bands observed in the classic experiments of the color center in ZnS.