DETERMINATION OF THE REFRACTIVE-INDEX AND OPTICAL-ABSORPTION COEFFICIENT OF VAPOR-DEPOSITED AMORPHOUS AS-S FILMS FROM TRANSMITTANCE MEASUREMENTS

The optical transmission spectra of amorphous As-S films of composition As32S68 prepared by thermal evaporation are measured over the 0.3 to 2.0 mum spectral region. A simple straightforward procedure suggested by Swanepoel is applied for determining the index of refraction and layer thickness; the absorption coefficient is derived using the Connell and Lewis formulae based on the interference-free transmittance. Furthermore, thickness measurements made by a surface profiling stylus are also carried out to cross-check the results corresponding to the fringe pattern method. The dispersion of n is discussed in terms of the single-oscillator Wemple and DiDomenico model and the absorption edge is described using the "non-direct transition" model proposed by Tauc. Finally, the compositional trends of the refractive index, absorption coefficient, and optical band gap in the As-S chalcogenide glass system are analyzed in detail.