Determination of optical bandgap energy and optical characteristics of Cd30Se50S20 thin film at various thicknesses

At room temperature, the Cd30Se50S20 ternary thin films with thicknesses ranging from 100 to 400 nm were synthesized using a vacuum evaporation process in an adequate atmosphere of pure nitrogen. X-ray and scanning electron microscopy techniques were used to validate the amorphous nature of the investigated thin films. Changing the thickness of the pristine thin film had a significant effect on controlling the linear and nonlinear optical properties. The optical absorption coefficient was determined using the optical measurements (the transmittance, the reflectance spectra and the film's thickness) in the range of (300–2500 nm) and the thickness of the thin layer. The optical parameters were calculated in the linear path: the band tail energy and the optical bandgap energy determined in this work by several methods, including the first derivative of the transmittance, reflectance, and optical absorption coefficient curves as a function of wavelength, as well as the second derivative of the transmittance curves. The optical bandgap energy was also computed as a function of photon energy using Tauc formula, the relaxation time, and the portions of the optical conductivity curves. It turns out that the bandgap energy and the tail energy were affected by the increase in thickness so that the first decreases and the other increases in a harmonious behavior. The linear optical constants (the extinction coefficient, the refractive index), the linear dispersion parameters (the single oscillator energy, the dispersion energy), and the linear dielectric parameters were all affected by the increase in thickness of the thin layer. The phase velocity, the group velocity, and Kirchhoff functions were determined, as well as the linear optical dispersion coefficient. The Boling formula, on the other hand, was used to compute the nonlinear refractive index. Finally, the spectral distribution of molar parameters was discussed.