Optical Properties of $As_{36}Te_{42}Ge_{10}Si_{12}$ Thin Films

Thermally evaporated $As_{36}Te_{42}Ge_{10}Si_{12}$ amorphous chalcogenide films were prepared in a vacuum of $10^{-5}$ Torr on to glass substrates hold at about 300 K during the deposition process. Measurements of the optical properties have been made. The optical transmittance and reflectance spectra of films in the thickness range 155-395 nm were measured in the wavelength λ range 500-2500 nm. The refractive index n, the extinction coefficient k and the absorption coefficient α were calculated for the studied films. It is found that both n and k are independent on the film thickness. The refractive index n has anomalous behavior for the wavelength λ range 500-1500 nm, while it has normal dispersion for the wavelength greater than 1500 nm. The optical energy gap was estimated from absorption coefficient. The allowed optical transitions were found to be nondirect transitions with optical gap of 1.08 eV for the sample under test. The effect of annealing on the obtained optical parameters was also investigated.