Defect physics of theCuInSe2chalcopyrite semiconductor

Abstract

We studied the defect physics in ${\mathrm{CuInSe}}_2,$ a prototype chalcopyrite semiconductor. We showed that (i) it takes much less energy to form a Cu vacancy in ${\mathrm{CuInSe}}_2$ than to form cation vacancies in II-VI compounds (ii) defect formation energies vary considerably both with the Fermi energy and with the chemical potential of the atomic species, and (iii) the defect pairs such as $\left({2\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{-}}{+\mathrm{I}\mathrm{n}}_{\mathrm{Cu}}^{2+}\right)$ and $\left({2\mathrm{C}\mathrm{u}}_{\mathrm{In}}^{2\mathrm{-}}{+\mathrm{I}\mathrm{n}}_{\mathrm{Cu}}^{2+}\right)$ have particularly low formation energies (under certain conditions, even exothermic). Using (i)–(iii), we (a) explain the existence of unusual ordered compounds ${\mathrm{CuIn}}_5{\mathrm{Se}}_8,$ ${\mathrm{CuIn}}_3{\mathrm{Se}}_5,$ ${\mathrm{Cu}}_2{\mathrm{In}}_4{\mathrm{Se}}_7,$ and ${\mathrm{Cu}}_3{\mathrm{In}}_5{\mathrm{Se}}_9$ as a repeat of a single unit of $\left({2\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{-}}{+\mathrm{I}\mathrm{n}}_{\mathrm{Cu}}^{2+}\right)$ pairs for each $n=4,$ 5, 7, and 9 units, respectively, of ${\mathrm{CuInSe}}_2;$ (b) attribute the very efficient $p$-type self-doping ability of ${\mathrm{CuInSe}}_2$ to the exceptionally low formation energy of the shallow defect Cu vacancies; (c) explained in terms of an electronic passivation of the ${\mathrm{In}}_{\mathrm{Cu}}^{2+}$ by ${2\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{-}}$ the electrically benign character of the large defect population in ${\mathrm{CuInSe}}_2.$ Our calculation leads to a set of new assignment of the observed defect transition energy levels in the band gap. The calculated level positions agree rather well with available experimental data.