Influence of temperature on thermoelectric properties of FexCo1 - xS2 thin films: A semiconductor to semimetal conversion

In this work, we investigate the thermoelectric properties of p and n-type thin films obtained by cobalt doping of FeS2. Films were synthesized by direct sulfuration of Co-Fe thin bilayers at 300 degrees C. It is found that at room temperature (RT), the Seebeck coefficient is reduced from 80 mu V/K to - 70 mu V/K when Co concentration is increased and the electrical resistivity of the films is decreased two orders of magnitude. X-ray diffraction and Raman measurements point out that Co is replacing Fe into the pyrite lattice and, subsequently is promoting a semiconductor to semimetal conversion. The influence of temperature on transport properties of different FexCo1-xS2 films has been investigated. Whereas the Seebeck coefficient is hardly modified, the film resistivity is drastically decreased when temperature increases what has been attributed to the thermal activation of electrical carriers. The influence of Co doping on the band scheme of FeS2 is shown. To this aim, donor and acceptor states are included into its forbidden gap. Whereas the band scheme of FeS2 exhibits an acceptor level with an E = 0.11 +/- 0.03 eV above the top of the valence band due to iron vacancies, a wide donor level close to the bottom of the conduction band (E = 0.08 +/- 0.05 eV) is created by the progressive replacement of iron by cobalt into the FeS2 lattice. (C) 2016 Elsevier B.V. All rights reserved.