Optical properties of amorphous Ge_(20)Se_(80) and Ag_6(Ge_(0.20)Se_(0.80))_(94) thin films

摘要

Thin films of chemical composition Ge_(20)Se_(80) and Ag_6(Ge_(0.20)Se_(0.80))_(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 (α) 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-Ge_(20)Se_(80) thin films.