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

Thin films of chemical composition Ge20Se80 and Ag-6(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 (E-g(opt)) is calculated from the absorption coefficient (alpha) by Tauc's extrapolation procedure. It has been found that the value of refractive index (n) and oscillator strength (E-d) increase while average energy gap (E-0) and (E-g(opt)) decrease after Ag incorporation. The decrease of (E-g(opt)) 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. (c) 2007 Elsevier B.V. All rights reserved.