Conversion of chemically deposited CuS thin films to Cu1.8S and Cu1.96S by annealing

Thin films of copper sulphide with thickness up to 0.5 mu m were deposited at 70 degrees C on glass substrates from a solution containing copper(II) chloride, sodium thiosulphate and dimethylthiourea. As prepared and after annealing at 200 degrees C in N-2 (100 millitorr), these films showed x-ray diffraction patterns matching that of the mineral covellite (CuS). Annealing the films for 1 h each at 300 degrees C and 400 degrees C in nitrogen resulted in their conversion to Cu1.8S (digenite) and Cu1.96S (chalcocite), respectively. The reduction in sulphur content of the films is evident in the x-ray florescence spectra. The sheet resistance of the films varied with annealing temperature. For a film of 0.5 mu m thickness, the observed sheet resistance values are: 180 Ohm/square (as prepared), 6 Ohm/square (200 degrees C), 17 Ohm/square (300 degrees C) and 30 Ohm/square (400 degrees C). The low sheet resistance (and thus the high conductivity, 10(3) Ohm(-1) cm(-1)) leads to a high near-infrared reflectance for the films, 65% (CuS) and 40% (Cu1.96S), at a wavelength of 2500 nm. Analyses of the optical band gap of the films indicate an indirect gap of 1.55 eV for CuS and Cu1.8S and 1.4 eV for Cu1.96S.