Defect energy levels and electronic behavior of Ni-, Co-, and As-doped synthetic pyrite (FeS2)

This work investigates the properties of Ni, Co, and As dopants in pyrite. Optical transmission spectroscopy and temperature-dependent Hall measurements were performed on doped pyrite crystals grown by chemical-vapor transport (CVT). The energy position(s) of the defect levels within the bandgap were determined from the optical spectrum. These values were then used to infer the concentration and occupancy of the defect levels from a statistical model fit to the temperature-dependent Hall concentration results. Doping pyrite with Ni atoms introduces partly filled, NiFe donor levels at 0.37 and 0.42 eV below the conduction band minimum (CBM). Doping with Co introduces a partially filled CoFe donor level at 0.09 eV below the CBM. Doping with As modifies the valence bands, which are of Fe d character. It reduces the gap slightly, and adds a hole. The Ni-doped, As-doped and undoped pyrite all have an absorption peak at 0.13 eV. Self-consistent GW electronic-structure calculations provide reliable conduction and valence band DOS for interpreting the optical spectra and fitting the data with the statistical model.