Point defects in lead sulfide: A first-principles study

Abstract

The energetic and electronic properties of point defects in lead sulfide (PbS) were studied using first-principles methods. In particular, intrinsic defects including single-site and double-site defects (e.g. Schottky dimers and Frenkel pairs) were considered as well as extrinsic oxygen containing defects. A novel, stable and energetically preferred interstitial site was identified. Convergence of the calculations with supercell size was examined and found to be well-converged for most, but not all, defects in 250 atom supercells. For intrinsic defects, it was found that, after accounting for the chemical potentials of Pb and S in the environment, the lowest formation energies are associated with lead vacancies in S-rich conditions and sulfur vacancies in Pb-rich conditions and not with Schottky defects, as previously reported. Interstitials, Frenkel pairs, and antisite defects were all found to have much larger defect formation energies and are therefore unlikely to be found. The electronic band structure was affected by the presence of intrinsic defects. In particular, for lead vacancies, the Fermi level was shifted below valence band maximum, indicating p-type conductivity; and for lead interstitials, it was shifted above the conduction band minimum, indicating n-type conductivity. In contrast, sulfur vacancies were found to introduce levels deep in the band gap, which may affect the electronic properties significantly. The charged states of the point defects were examined and found to be preferred over the neutral states. The formation energies of oxygen defects were found to be highly competitive energetically with those of intrinsic defects, and therefore, oxygen point defects are expected to play a significant role in determining material properties. In particular, in Pb-rich conditions, oxygen is expected to be drawn into the material to occupy S vacancies.