Unusual Defect Chemistry of Thorium Doping of PbS

The unusual defect chemistry of thorium doping in the PbS system was investigated computationally to answer several open questions arising from the experimental observations. These include finding Th in a +4 oxidation state in contrast to Pb, attracting more than two oxygen atoms on average per thorium and affecting the growth morphology of PbS and its electronic properties. We find Th to be energetically stable at the lead lattice position in PbS and to attract 2-3 oxygens, including in the adjacent interstitial position, which binds strongly to Th. This adjacent interstitial atom allows the +4 oxidation state of Th in PbS as observed experimentally. Furthermore, the bandgap of the ideal material increased due to Th incorporation, in agreement with experimental observations. Finally, we calculated the surface energies of the (100), (110), and (111) surfaces for the systems with and without thorium incorporation. Surfaces (100) and (110) were found to have negative surface energies; however, (111) surface energy was positive and, thus, preferred for the growth of Th-doped PbS thin films. These results correlate well with the experimentally observed surface topography change for PbS thin film growth from the (100) to the (111) surfaces with addition of Th.