Ab initio study of intrinsic point defects in germanium sulfide

Abstract

The energetic and electronic properties of intrinsic point defects in germanium sulfide (GeS) were studied using first-principles methods. Point defects including single-site (e.g., vacancies, interstitials, and anitisites) and double-site defects (e.g., Schottky defects, Frenkel pairs) were considered. It was found that the lowest formation energy for neutral defects is associated with the Schottky dimer (SD), independent of chemical potentials of the species, and not with vacancies, as previously reported for similar materials like GeSe, SnS and SnSe. Furthermore, SD were studied in these similar materials and found to be energetically more stable than neutral vacancies for GeSe and similarly stable to Sn vacancy defects in SnS and SnSe. Charged states of the defects were considered and found to be energetically preferred over neutral defects. On allowing the defects to charge, Ge2- vacancy (VGe−2) defects were found to be the most stable defects in both Ge-rich and Ge-poor environments; consistent with the experimentally reported nonstoichiometric nature of GeS. Negative formation energies were obtained for Ge vacancies, in both environmental conditions, and thus they are expected to form spontaneously. The electronic structure was affected by the incorporation of point defects. For Ge vacancies, the Fermi level shifted below the valence band maxima (VBM), indicating p-type conductivity in agreement with experimental observations. S vacancies introduced occupied defect states and hybridized with VBM, and the fundamental bandgap was retained, indicating no preferential conductivity. The possibility of doping GeS to obtain n-type conductivity was explored.