PHOTOCONDUCTION OF SYNTHETIC PYRITE FES2 SINGLE-CRYSTALS

The photoconduction of synthetic pyrite FeS2 single crystals with low impurity concentration prepared by chemical vapor transport method is reported over a temperature range from 15 K to 300 K. The energy gap is determined from the photoconductive spectral response by the Moss rule. At room temperature the indirect energy gap is found to be 0.83 +/- 0.02 eV. According to the experimental evidence for a sharp antibonding p-like state above the Fermi level of FeS2 reported by Folkerts et al. [J. Phys. C 20, 4135 (1987)], the indirect gap is assigned as the transition between Fe 3d t2g and S 3p sigma* states. The temperature variation of the indirect band gap shows a linear dependence between 100 K and 300 K with a negative temperature coefficience equal to 1.0 +/- 0.1 meV/K. A distinct feature at higher energy side of the spectrum is observed and attributed to the direct band-gap transition. At temperature lower than 60 K, a very sharp peak around 0.9 eV emerged below the absorption edge. This feature might be associated with the excess carriers induced by the photothermal ionization between the impurity levels and conduction band.