Optical properties of amorphous Ge20Se80 and Ag6(Ge0.20Se0.80)94 thin films

Thin films of chemical composition Ge20Se80 and Ag6(Ge0.20Se0.80)94 are prepared by thermal evaporation technique. The optical properties of these thin films are determined by a method, based only on the transmission spectra at normal incidence, measured over the 400–2000 nm spectral range. This useful optical method takes into consideration the non-uniform thickness of thermally evaporated thin films. The dispersion of refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. The optical absorption edge is described using the non-direct transition model proposed by Tauc and the optical band gap (Egopt) is calculated from the absorption coefficient (α) by Tauc’s extrapolation procedure. It has been found that the value of refractive index (n) and oscillator strength (Ed) increase while average energy gap (E0) and (Egopt) decrease after Ag incorporation. The decrease of (Egopt) has been explained on the basis of difference on the binding energies of different bonds which are arising due to the incorporation of Ag into a-Ge20Se80 thin films.