Optical band gap and refractive index dispersion parameters of As x Se70Te30-x (0aparts per thousandcurrency signxaparts per thousandcurrency sign30 at.%) amorphous films

Amorphous As (x) Se70Te30-x thin films with (0a parts per thousand currency signxa parts per thousand currency sign30 at.%) were deposited onto glass substrates by using thermal evaporation method. The transmission spectra T(lambda) of the films at normal incidence were measured in the wavelength range 400-2500 nm. A straightforward analysis proposed by Swanepoel based on the use of the maxima and minima of the interference fringes has been used to drive the film thickness, d, the complex index of refraction, n, and the extinction coefficient, k. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model (WDD). Increasing As content is found to affect the refractive index and the extinction coefficient of the As (x) Se70Te30-x films. With increasing As content the optical band gap increases while the refractive index decreases. The optical absorption is due to allowed indirect transition. The chemical bond approach has been applied successfully to interpret the increase of the optical gap with increasing As content.