Optical parameters of ternary Te15(Se100-xBix)85 thin films deposited by thermal evaporation

Thin films of Te-15(Se100-xBix)(85) (x = 0, 1, 2, 3, 4 and 5 at.%) glassy alloys were deposited by thermal evaporation (at 10(-4) Pa) from bulk samples. Optical characterization of the films was done by analysing their transmission spectra taken in the spectral range 400-2300 nm. Swanepoel's method was used to calculate the refractive index (n) and extinction coefficient (k). It was found that the refractive index increases with an increase in Bi content. The Wemple-DiDomenico single-oscillator approach was used to calculate the average band gap energy (E-o), dispersion energy (E-d) and static refractive index (n(o)). The absorption coefficient (alpha) and film thickness were calculated from the transmission spectra of the films. The optical band gap (E-g) was estimated using Tauc's extrapolation and was found to decrease from 1.37 to 1.21 eV with Bi addition from 0 to 5 at.% in glassy alloys. The decrease in optical band gap is explained on the basis of the decrease in cohesive energy of the samples and the difference of electronegativity of the atoms involved. The real (epsilon(r)) and imaginary parts (epsilon(i)) of the dielectric constant for the films were also calculated and reported.