Refractive-index dispersion and the optical-absorption edge of wedge-shaped thin films of metal - chalcogenide glasses

Abstract

The optical constants and average thickness of metal - chalcogenide glass films of chemical composition , with x = 5, 10 and 15 at.%, are very accurately determined by a novel method, based only on the transmission spectra at normal incidence, measured over the 400 - 2200 nm spectral range. This useful optical method takes into consideration the non-uniform thickness of the thermally evaporated films (if not corrected, this may lead to too-large calculated values of the absorption coefficient, as well as serious errors in the values of the refractive index and film thickness). The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. The optical-absorption edges are described using the non-direct transition model proposed by Tauc and the optical energy gaps are calculated by Tauc's extrapolation. It has been found that the value of the refractive index increases clearly with copper content, whereas the optical band gap decreases from 1.80 to 1.64 eV.