Compositional dependence of the optical properties of amorphous antimony selenide thin films using transmission measurements

Abstract

Compositional dependencies of the optical properties of as-deposited amorphous antimony selenide Sb x Se (1− x) (with 5 ≤ x ≤ 20 at.%) prepared by thermal evaporation have been studied. The straightforward analysis proposed by Swanepoel has been successfully employed, and it has allowed us to determine the average thickness d¯, and the refractive index, n, of the films, with high accuracy. The refractive index, n and the average thickness d¯ has been determined from the upper and lower envelopes of the transmission spectra measured at normal incidence, in the spectral range from 400 to 2500 nm. The absorption coefficient α, therefore extinction coefficient k, have been determined from the transmission spectra at the strong absorption region. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model, and the optical absorption edge is described using the ‘non-direct transition’ model proposed by Tauc. Likewise, the optical energy gap is derived from Tauc's extrapolation. Finally, the chemical bond approach has been applied to interpret the decrease of the glass optical gap with increasing Sb content.