Calculation and analysis of the complex refractive index of uniform films of the As–S–Se glassy alloy deposited by thermal evaporation

Abstract

Abstract Optical reflection spectra, at normal incidence, of amorphous semiconductor thin films of chemical composition, As 40 S 40 Se 20 , deposited by thermal evaporation, have been obtained in the 0.56–3.10 eV spectral region. The optical constants of this glassy alloy have been determined by use of an optical characterization method, which is based only on the upper and lower envelopes of the reflection spectra. Such a procedure allows accurate determination of the real and imaginary parts of the complex refractive index and the thickness of the films. Thickness measurements using a surface-profiling stylus have been carried out to cross-check the results obtained by the envelope method. The dispersion of the refractive index has been analyzed according to a new model recently proposed by Solomon. This optical dispersion model takes into consideration the width of the valence and the conduction bands, introducing a correction to the model that is based on the single oscillator. Finally, the optical-absorption edge of the a -As 40 S 40 Se 20 thin films is described in terms of the non-direct transition model proposed by Tauc, in the strong-absorption region, and in the medium-absorption region, according to Urbach's rule.