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

Optical reflection spectra, at normal incidence, of amorphous semiconductor thin films of chemical composition, As40S40Se20, deposited by thermal evap oration, have been obtained in the 0.56-3.10 eV spectral region. The optica l constants of this glassy alloy have been determined by use of an optical characterization method, which is based only on the upper and lower envelop es of the reflection spectra. Such a procedure allows accurate determinatio n of the real and imaginary parts of the complex refractive index and the t hickness of the films. Thickness measurements using a surface-profiling sty lus have been carried out to cross-check the results obtained by the envelo pe method. The dispersion of the refractive index has been analyzed accordi ng to a new model recently proposed by Solomon. This optical dispersion mod el takes into consideration the width of the valence and the conduction ban ds, introducing a correction to the model that is based on the single oscil lator. Finally, the optical-absorption edge of the alpha-As40S40Se20 thin f ilms is described in terms of the non-direct transition model proposed by T auc, in the strong-absorption region, and in the medium-absorption region, according to Urbach's rule. (C) 1999 Elsevier Science S.A. All rights reser ved.