Spectroscopic ellipsometry investigations of the optical constants of nanocrystalline SnS thin films

Different thicknesses of tin sulfide (SnS) thin films were deposited onto highly cleaned glass substrates by the thermal evaporation technique. Their structural characteristics were studied by x-ray diffraction (XRD). The XRD investigation shows that SnS films are polycrystalline with an orthorhombic-type structure. The microstructure parameters, e.g. crystallite size and microstrain, were calculated. It is observed that the crystallite size increases and the microstrain decreases with increasing the film thickness. The optical constants (n, k) and film thicknesses of SnS thin films were obtained by fitting the ellipsometric parameters (ψ and Δ) data using three-layer model systems in the wavelength range of 400-1800nm. It is found that the refractive index n increases with an increase of the film thickness. The possible optical transition in these films is found to be direct and indirect transitions. Both direct and indirect energy gaps increase with increasing the film thickness.