Compositional dependence of the optical properties of amorphous Se80 − xTe20Bix thin films using transmittance and reflectance measurements

Optical properties of amorphous Se80 − xTe20Bix thin films with different compositions (x = 0, 2, 4, 6 and 8 at.%) deposited by evaporation technique have been investigated by optical spectrophotometry measurements in the wavelength range of 400-2500 nm. An optical characterization method for uniform films based on Swanepoel's ideas (envelope method) has been employed to extract the refractive index n and film thickness d, with accuracies better than 1%. It was found that the refractive index increases with increasing the concentration of Bi at the expense of Se content. The increasing in refractive index n is interpreted in terms of polarizability and the mean coordination number. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple-DiDomenico model. The fundamental parameters for the investigated semiconducting thin films Se80 − xTe20Bix e.g. absorption coefficient and band gap are calculated in the strong absorption region of transmittance and reflectance spectra. The possible optical transition in these films is found to be allowed indirect transition with energy gap Egoptdecreases from 1.467 to 1.035 eV with increasing Bi content at expense of Se. Finally, the decrease in the optical band gap with increasing Bi content in Se80 − xTe20Bix has been explained in terms of the chemical bond approach.