Effect of film thickness on the optical parameters and electrical conductivity of Te10Ge10Se77Sb3 chalcogenide glass

Several thin films of Te10Ge10Se77Sb3 chalcogenide glass of different thicknesses (250 nm to 400 nm) were prepared by thermal evaporation under vacuum of 133 x 10(-6) Pa (10(-6) torr). X-ray diffraction analysis showed the amorphicity of the prepared films which become partially crystalline by annealing. Transmittance and reflectance measurements in the spectral range of 200 nm to 2500 nm have been carried out at normal incidence. The analysis of the absorption coefficient data showed the existence of indirect transition for the photon energy E in the range 1similar to3 eV and direct transition for E > 3 eV. From the determination of the optical constants (n, k), the dispersion of the refractive index has anomalous behaviour in the region of the fundamental absorption edge, and followed by the single- effective oscillator approach. The investigated optical parameters such as the optical energy gap E-opt, the high frequency dielectric constant epsilon(oo), the oscillator position lambda(o), and the oscillator strength So, were significantly affected by the film, thickness. The characteristic energy gap obtained from the conductivity measurements is nearly half the value of that obtained from the optical data as in the case of thickness 400 nm. The activation energy is 0.65 eV and the indirect optical gap is 1.32 eV.