Abstract
The optical absorption behaviour of thin films of copper sulphide ( Cu x S) formed by a vacuum deposition process at substrate temperatures of 27, 125 and 200°C was studied with a view to establishing a correlation with their structure. From the reflection and transmission data, the optical constants and the absorption coefficient were computed. Both the direct edge at 1.8 eV and the indirect edge at 0.855 eV, 1.075 eV and 1.105 eV respectively for these films were observed. These transitions explain the observed dependence of the absorption coefficient on the photon energy. The unusually high values of the absorption coefficient for the film deposited at room temperature were due to scattering-absorption and are explained on the basis of Maxwell Garnett theory. For Cu x S compositions with 1.96 < x < 2.0, no changes in the direct edge were observed. The indirect edge shifts upwards by 0.22 eV when disordering of the copper sublattice occurs in the Cu 2S phase. The indirect transitions are associated with the changes in lattice structure and stoichiometry.
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