OPTICAL-PROPERTIES OF THERMALLY VACUUM-EVAPORATED AGSBSE2 THIN-FILMS

Silver antimony diselenide (AgSbSe2) thin films were prepared by a the rmal vacuum evaporation technique onto quartz and glass substrates kep t at room temperature (similar to 300 K). The as-deposited films were amorphous and transformed to a face centred cubic (FCC) polycrystallin e nature with the lattice constant a = 5.797+/-0.003 Angstrom 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 thickn ess (180 nm-270 nm), using spectrophotometric measurements of the tran smittance T and reflectance R at normal incidence in the spectral rang e 500-2500 nm. These constants were also determined for preannealed fi lms, T-A = 423 K (polycrystalline). The obtained values of both n and k were independent of the film thickness within the above-mentioned th ickness range. The refractive index data fitted a single-oscillator mo del with high-frequency dielectric constants increasing from 13 for th e amorphous films to 15 for the crystalline films. It was found that t he high-frequency dielectric constant epsilon(infinity) has the same v alues as the lattice dielectric constant epsilon(L). The analysis of t he spectral behaviour of the absorption coefficient in the intrinsic a bsorption 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.