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The article reports the structural and optical properties of vacuum-evaporated cadmium sulfide (CdS) films with different thicknesses at room temperature. The structural investigations performed by means of X-ray diffraction (XRD) technique have showed that all the films have the zinc-blende structure, a face-centered cubic form with lattice constants a=b=c=5.82 Å and point group F43m. Crystallite sizes calculated from Scherrer relation are in the range of 173-345 Å. So far, because the optical parameters of the metastable cubic CdS have not been so well known, we apply spectroscopic ellipsometry to determine the thickness, optical constants and energy band gap of CdS thin film deposited by thermal evaporation onto opaque gold substrate, a perfect reflectivity and inert metal. As shown the measured spectral behavior of the optical constants and the band gap value of CdS thin film are in agreement with those obtained by the reflectance and transmittance methods. The energy band gap of CdS thin film determined from the spectral behavior of the absorption coefficient is about 2.46 eV.

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