Characterization of optical constants of Se30Te70 thin film: Effect of the thickness

The optical measurements of Se30Te70 alloy thin film with different thicknesses (150, 450, 900 nm) are carried out in order to characterize their electronic structure and their optical constants using optical spectroscopy measurements. The optical absorption data point to an allowed indirect inter-band transition near the absorption edge. The estimated indirect Eg values are 1.32, 1.19 and 0.854 eV for the studied film thicknesses. The observed decrease in the optical energy gap with thickness is attributed to the increasing the band tailing with increasing film thickness (Urbach tail effect). The optical constants such as the absorption coefficient, refractive index and extinction coefficient of the films are calculated from the measured transmission and reflection of these films. The results show the dependence of these optical constants on the film thickness. The X-ray diffraction reveals that the deposited films are poly-crystalline in nature with preferential (113) orientation.