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.