Optical Properties of Bi Doped Se-te Thin Films

Abstract

Compositional dependence of optical constants of thermally evaporated amorphous Se82-x BixTe18 (x = 0, 1.5, 2.5, 3.5 and 4.5 at %) thin films has been studied using well established Swanepoel method. The optical parameters like, refractive index (n), extinction coefficient (k), film thickness (d), absorption coefficient (α) and optical band gap (Eg) etc. are determined from the transmission spectra in the spectral range from 500 to 2500 nm. Wemple-DiDomenico (WDD) single oscillator approach is used to study the dispersion parameters such as single-oscillator energy (E0), dispersive energy (Ed) and static refractive index (n0). Present results show that the refractive index increases with the increasing concentration of Bi which is in agreement with the earlier studies. The increase in refractive index with Bi content may be ascribed to the increase in mean polarizability per atom of the system. The optical energy gap has been estimated by using Tauc's extrapolation method and is found to decrease with an increase in Bi concentration. The decrease in optical band gap can be understood in terms of decrease in cohesive energy and increase in electronegativity of the films under study.