The effect of non-stoichiometry in Sb2Se3 films on their phonon and electronic properties in the infrared range

Due to the noticeable part of Sb₂Se₃ films in solar cell technology, the deposition of Sb₂Se₃ films with the desirable content and improved crystalline perfection is of crucial importance. Therefore, extensive experimental and theoretical studies of the electronic and phonon properties of these materials are of interest. In this work, the stoichiometry and morphology of antimony selenide films are correlated to their optical properties in the far-infrared regime and compared to theoretically calculated properties for an ideal crystal. For this study, the reflectance of a series of 1.4-1.7 μm thick Sb_xSe_y layers, produced on soda-lime glass substrates by molecular beam chemical deposition, was measured in the spectral range of 25-5000 cm⁻¹ using an FT-IR spectrometer and synchrotron radiation. The near-surface and bulk crystallographic structure and phase composition were previously determined using grazing incidence X-ray diffraction (GIXRD) under varying incident angles. The chemical composition and morphology were investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). It was shown that the spectral positions of numerous maxima of the reflectance of Sb_xSe_y layers for various x/y indices in the phonon excitation range of 25-230 cm⁻¹ are consistent with the positions of the maxima of the imaginary part of the dielectric function of Sb₂Se₃ crystals, calculated within the framework of density functional theory. A significant increase in the reflectance values was observed for Sb_xSe_y layers containing the largest relative amount of antimony x/y, which may be due to the increased content of the metallic form of antimony compared to the amount of the pure Sb₂Se₃ semiconducting phase. A correlation was found between the technological parameters of production (temperature) and the final chemical composition and morphology of the Sb_xSe_y layers. To explain the observed relationships, calculations of the cohesive energy of Sb₂Se₃ and Sb crystals and the binding energy of the residual antimony atom inside and on the surface of the antimony selenide supercell were performed. The main conclusions of the theoretical calculations are consistent with the results of EDX and XRD measurements of the Sb_xSe_y layers.