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 $\mathrm{C}\mathrm{u}\mathrm{G}\mathrm{a}\mathrm{S}{\mathrm{e}}_2$, 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.