Compositional dependence of optical parameters in Se–Bi–Te–Ag thin films

Compositional dependence of optical parameters in thermally evaporated amorphous Se80.5Bi1.5Te18 − yAgy (for y = 0, 1.0, 1.5 and 2.0 at.%) quaternary thin films has been studied using well established Swanepoel method. The optical properties like, refractive index (n), extinction coefficient (k), absorption coefficient (α) and optical band gap (Eg) have been determined from the transmission spectra in the spectral range from 500 to 2500 nm. The optical band gap (Eg) has been estimated by using Taucʹs extrapolation method and is found to increase with an increase in the Ag concentration. Present study shows that the refractive index, extinction coefficient and optical band gap increase with the increasing Ag content which is in agreement with the earlier studies. While the increase in the refractive index with Ag content over the entire spectral range can be attributed to the increased polarizability of larger Ag atomic radius (153 pm) compared to the Te atomic radius (135 pm), the increase in the optical band gap with increasing Ag concentration is correlated to an increase in the cohesive energy and decrease in the electronegativity of the films under study. The dielectric constant and optical conductivity (σ) of the thin films under study are also found to increase with the Ag concentration.