Semiconducting Cu3BiS3 thin films formed by the solid-state reaction of CuS and bismuth thin films

Semiconducting thin films of Cu3BiS3, with structure and composition corresponding to that of the mineral wittichenite, have been prepared by heating at 300 degreesC in nitrogen atmosphere a chemically deposited CuS thin film on which a thin film of bismuth was thermally evaporated. This follows a stoichiometric reaction: CuS (300 nm) + Bi (103 nm) giving Cu3BiS3 (397 nm). The optical absorption coefficient in the visible region for this material is > 10(5) cm(-1). This optical absorption is classified as due to direct (forbidden) transitions, with an optical bandgap of 1.2 +/- 0.1 eV. Weak optical absorption related to an indirect (forbidden) gap of 0.65 eV is also identified. The Cu3BiS3 thin film has a p-type conductivity of 0.03 Ohm(-1) cm(-1) and is photoconductive. The mobility-lifetime product of photogenerated carriers is estimated to be 10(-6) cm(2) V-1.