Optical properties of thermally vacuum evaporated thin films

Abstract

Silver antimony diselenide thin films were prepared by a thermal vacuum evaporation technique onto quartz and glass substrates kept at room temperature . The as-deposited films were amorphous and transformed to a face centred cubic (FCC) polycrystalline nature with the lattice constant on post-deposition annealing above 423 K for one hour in argon atmosphere. The optical constants (the refractive index n, and the absorption index k) of the films were determined for several samples of different thickness (180 nm-270 nm), using spectrophotometric measurements of the transmittance T and reflectance R at normal incidence in the spectral range 500-2500 nm. These constants were also determined for preannealed films, (polycrystalline). The obtained values of both n and k were independent of the film thickness within the above-mentioned thickness range. The refractive index data fitted a single-oscillator model with high-frequency dielectric constants increasing from 13 for the amorphous films to 15 for the crystalline films. It was found that the high-frequency dielectric constant has the same values as the lattice dielectric constant . The analysis of the spectral behaviour of the absorption coefficient in the intrinsic absorption region revealed the existence of an indirect allowed optical transition with energy gap 1.2 eV for amorphous films and 1.03 eV for the crystalline films, respectively.