Effect of heat treatment on the structural and optical properties of amorphous Sb2Se3 and Sb2Se2S thin films

Nearly stoichiometric thin films of Sb2Se3 and Sb2Se2S were deposited, by conventional thermal evaporation of the presynthesized materials, on glass substrates held at room temperature. X-ray diffraction studies on the annealed films, at Ta=373, 423, and 473 K, revealed an amorphous-to-crystalline phase transition at Ta=423 and 473 K for Sb2Se3 and Sb2Se2S, respectively. The crystal structure of the polycrystalline films of Sb2Se3 and Sb2Se2S is single-phase of an orthorhombic type structure, with space group Pbnm (62) and Z=4, as that of their source materials. The optical constants (n,k) and the thickness (t) of the investigated films were determined from optical transmittance data, in the spectral range 600–2500 nm using the Swanepoel method. The dispersion parameters of the indicated compositions were determined from the analysis of the refractive index. The analysis of the optical absorption spectra revealed that the polycrystalline films exhibited both direct and indirect energy gaps, on the contrary to the amorphous films which showed only a non-direct energy gap with a relatively higher Urbach's tail.