Influence of composition on optical and dispersion parameters of thermally evaporated non-crystalline Cd50S50-xSex thin films

Non-crystalline thin films of chalcogenide Cd50S50-xSex system (30 & x 50) were obtained by thermal evaporation technique onto a pre-cleaned glass substrate at a vacuum of 8.2 × 10-4 Pa. The deposition rate and film thickness were kept constant at about 8 nm/s and 200 nm, respectively. Amorphous/crystalline nature and chemical composition of films have been checked using X-ray diffraction and energy dispersive X-ray spectroscopy (EDX). Optical properties of thin films were investigated and studied using the corrected transmittance, T(λ) and corrected reflectance, R(λ) measurements. Obtained data reveal that, the indirect optical energy gap (Eg) was decreased from 2.21 to 1.57 eV. On the contrary, Urbach energy (band tail width), EU was found to be increased from 0.29 to 0.45 eV. This behavior is believed to be associated with the increase of Se-content instead of S-content in the thin films of Cd50S50-xSex system. Chemical bond approach model, CBA was used to analyze the obtained values of Eg and EU. Optical density, skin depth, extinction coefficient, refractive index and optical conductivity of chalcogenide CdSSe thin films were discussed as functions of Se-content. Using Wemple-DiDomenico single oscillator model, the refractive index dispersion and energy parameters and their dependence on Se content were studied.