STRUCTURAL AND OPTICAL-PROPERTIES OF THIN-FILMS OF CUGAS2

Thin films of CuGaS2 of various thicknesses (94-400 nm) were prepared either on glass or on quartz substrates. X-ray diffractograms show tha t CuGaS2 in a powder form or annealed samples in thin film form have a single-phase chalcopyrite tetragonal structure with lattice parameter s a = 0.535 nm and c = 1.048 nm with c/a = 1.959 and distortion parame ter x = 0.041. However, the as-deposited CuGaS2 films may have a polyc rystalline nature with very fine crystallites. The optical constants ( the refractive index n, the absorption index k and the absorption coef ficient alpha) of thin films of CuGaS2 either as deposited or after be ing annealed for 2 h at 673 K were determined using the transmission T and reflection R at normal incidence of light in the wavelength range 400-2500 nm. The refractive index in both cases las deposited and aft er being annealed) exhibits anomalous dispersion in the wavelength ran ge 400-600 nm. The permittivity epsilon(infinity) was found to be 6.12 and 6.02 for CuGaS2 as deposited and after being annealed respectivel y. A plot of (alpha h nu)(2) = f(h nu) shows that these films, whether as deposited or after being annealed, have two direct allowed optical transitions which are attributable to the splitting of the valence ba nd.