Optical properties of As34Te44Ge10Si12 amorphous films

Amorphous thin films of As34Te44Ge10Si12 were prepared by thermal evaporation technique from its bulk glass onto glass substrates. The reflectance R and transmittance T have been measured in the spectral range 400-2500 nm. The values of the refractive index n, the extinction coefficient k and the absorption coefficient α were calculated for the studied thin films in the thickness range 137.5-201 nm. It is found that both n and k are independent of the film thickness. The refractive index n shows an anomalous behavior for the wavelength λ range 400-1100 nm, while it has normal dispersion for the wavelength greater than 1100 nm. The values of the dispersion energy Ed, the oscillator energy Eo, the lattice dielectric constant εL and the high frequency dielectric constant ε∞ have been determined. The value of εL is greater than ε∞ by 1.112, which indicates that there is free carrier contribution in polarization with very small concentration. The optical energy gap Egopt is calculated from the absorption coefficient α by Tauc's extrapolation procedure and the type of transitions were found to be allowed indirect transitions with optical gap of 0.95 eV for the sample under test.