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Optical constants of vacuum evaporated thin films in the Se90−xTe10Snx (x = 0, 2.5, 5 and 7.5 at %) system were calculated from reflectance and transmittance data in the wavelength range of 300–2500 nm using Murmann’s equations. The maximum value of refractive index increased and suffered a shift towards the short wavelength as the Sn content increases this behavior can be attributed to increasing the values of cross-linking chains density and decrease tailing. The variations in the real and the imaginary parts of the dielectric constant, the dissipation factor tan(δ), the optical conductivity, the volume, and surface energy loss functions with photon energy have also been reported. The normal dispersion of the refractive index is discussed regarding the single oscillator Wemple-DiDomenico model from which the dispersion parameters were determined. The single oscillator energy, the dispersion energy, the high-frequency dielectric constant, the ratio of free charge carrier concentration to the effective mass, plasma frequency, single oscillator strength and its position are then estimated as a function of alloy composition. It is interesting to note that the increase of Sn content on the parent Se90Te10 is connected to the decreasing covalent nature of the structure. Besides, the presence of such a sharp absorption edge in transmission spectra recommends Se90−xTe10Snx thin films as a good optical filter material.

The Effect of Sn addition in the structural and optical properties of Se–Te chalcogenide alloy at the expense of the Se content has been studied. Stoichiometric bulk ingot materials of the Se90−xTe10Snx (x=0, 2.5, 5 and 7.5 at%) were prepared by a melt-quench technique. X-ray diffraction (XRD) studies indicated that the investigated alloys confirmed its amorphous nature. These results were confirmed by scanning electron microscopy (SEM) investigations and correlated to the rigidity percolation threshold of the lattice. Stoichiometric thermally evaporated thin film of the obtained compositions were carefully characterized to establish the interdependence between their chemical composition and some physical parameters, such as the average heat of atomization (Hs), the cohesive energy CE, the average coordination number 〈Z〉 and the optical band gap (Eg). It has been found that all the parameters varied linearly when Sn content was increased. The variation in the optical band gap (Eg) with Sn addition was discussed in terms of the width of localized states (Ee) and a chemical bond approach model (CBA).