Characterization of near band-edge properties of synthetic p-FeS2 iron pyrite from electrical and photoconductivity measurements

Single crystals of pyrite FeS2 were grown by chemical vapor transport (CVT) using ICl3 as a transport agent. Optimum conditions were given for growing large single crystals. Band-edge properties of the synthetic crystals were characterized by temperature dependent electrical and photoconductivity measurements. The Hall-effect measurement confirmed p-type semiconducting behavior of the crystal at low temperature. Temperature dependent Hall-effect measurements showed that electrical mixed conduction of electrons and holes begins to dominate at temperatures higher than 60 K. The mixed-conduction behavior is due to the ionization effect of p- and n-type imperfections in synthetic FeS2. The p-type state (acceptor) may come from a chalcogen-deficiency defect in synthetic FeS2, while the n-type level (donor) is caused by an unwanted nickel impurity involved inside the crystal. Optical properties of the synthetic p-FeS2 are characterized using photoconductivity (PC) measurements in the temperature range between 10 and 300 K. The PC spectra at different temperatures can be decomposed into two transition features. The possible origins of the transition features A and B are assigned. Temperature dependences of the transition energies of the features are analyzed. The parameters that describe temperature variations of the transition energies in p-FeS2 are evaluated and discussed. Based on the experimental analyses of the electrical and optical measurements, a probable band-structure scheme for describing the near-band-edge characteristic of synthetic FeS2 is constructed.