Optical properties of Ge–Se–Te wedge-shaped films by using only transmission spectra

Amorphous Ge10Se90−xTex (with x = 0, 5, 10 and 15 at.%) thin films were prepared by thermal evaporation method. The optical transmission spectra of these films were measured in the wavelength range of 500–2500 nm in order to drive the refractive index and the absorption coefficient of these films. Applying the analytical expressions proposed by Swanepoel, enabling the calculations of optical constants of thin films with non-uniform thickness with high accuracy. Furthermore, the dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. It was found that, the mechanism of the optical absorption follows the rule of the allowed non-direct transition. The optical band gab, Eg, and the oscillator energy, Eo, decrease while the dispersion energy, Ed, increases by increasing Te content. The relationship between the obtained results and the chemical compositions of the Ge10Se90−xTex thin films were discussed in terms of the chemical bond approach, the excess of Se–Se homopolar bonds and the cohesive energy (CE).