The effect of annealing in N2 atmosphere on the physical properties of SnSb4S7 thin films

Abstract

SnSb4S7 thin films were grown on glass substrates by means of the vacuum thermal evaporation technique. The annealing process was performed under nitrogen (N2) atmosphere at different temperatures (Ta) during one hour. The influence of annealing in nitrogen atmosphere on the structural, morphological and optical properties of SnSb4S7 thin films was studied by X-ray diffraction (XRD), Raman spectroscopy, Atomic Force Microscopic (AFM) and UV–Vis spectroscopy, respectively. XRD analysis confirmed the polycrystalline nature of SnSb4S7 films with the monoclinic crystalline phase. We also observed a change in the preferential orientation of the films from (4¯01) to the (2¯11) plane by increasing annealing temperature. We also found that the average crystallite size first decreases from 25.3 to 11.3 nm by increasing Ta up to 200 °C and then increases to 49.5 nm for Ta = 300 °C. The Raman spectra of the samples showed several peaks corresponding to the SnSb4S7 phase. The Raman spectroscopy confirmed the results obtained by XRD analysis. Atomic force microscopy studies showed that the change in the film morphology depends on the annealing in N2 atmosphere. Indeed, AFM images indicated the increase of roughness by increasing the annealing temperature above 200 °C. The effects of annealing on the optical properties and dispersion parameters of the films were investigated. It was found that annealing in N2 atmosphere decreases the transmittance of films and affects the absorption coefficient and the optical band gap of the samples. Indeed, the optical energy gap first increases from 1.80 to 1.90 eV up to 200 °C and then decreases to 1.45 eV for Ta = 300 °C. Wemple–DiDomenico and Spitzer–Fan models were used for the analysis of the dispersion of the refractive index and the determination of the optical and dielectric constants.