Thin films of tin sulfides: structure, composition and optoelectronic properties

Tin monosulfide (SnS) thin films prepared by chemical bath deposition (CBD) were annealed in a tubular furnace at different temperatures (350, 400 and 450 °C (1 h)), in the presence of elemental sulfur (S) resulted in formation of tin sulfides SnS₂ and Sn₂S₃. The as prepared and sulfur annealed SnS samples were characterized for their structure, morphology, composition, chemical state, optical and electrical properties. X-ray diffraction (XRD) analysis of the samples sulfurized at 400 °C (15 mg of S) showed phase pure SnS₂ (berndtite) with an indirect band gap of 1.8 eV. The formation of SnS₂ was further established by Raman analysis with an intense peak at 312 cm^-1 characteristic of this phase and also by high resolution XPS analysis of Sn3d peak where the binding energy (486.4 eV) was associated with the presence of Sn(IV). At a sulfurization temperature of 450 °C with 10 and 15 mg of S, pure Sn₂S₃ phase was observed by XRD analysis; the presence of the phase was confirmed by Raman spectra where an intense peak at 305 cm^-1 and another weak one at 232 cm^-1 were associated to this phase. The bandgap calculated for Sn₂S₃ was 0.9 eV (direct forbidden). Structure, morphology, composition, optical and electrical properties of the SnS, SnS₂ and Sn₂S₃ thin films are analyzed in detail and the results show that annealing of SnS thin films under different sulfur contents and temperatures, is an effective method to obtain other tin sulfide phases with adequate optoelectronic properties for various applications.