Influence of grain size on the band-gap of annealed SnS thin films

• Fabrication of SnS thin films which were vacuum annealed post deposition.
• Films were polycrystalline with the unit cell size independent of thickness.
• Unit cell size was also independent of annealing temperature.
• Only the grain size was found to vary.
• The band-gap and refractive index were found to be dependent of the grain size.

Thin films of SnS were fabricated at room temperature on glass substrates by thermal evaporation. X-ray photo-electron spectroscopy confirmed that the films were stoichiometric in nature even after vacuum annealing. X-ray diffraction, Raman analysis and electron microscopy showed that the films were polycrystalline in nature with ortho-rhombic structure and preferred orientation. The only variation due to film thickness and annealing temperature was the grain size. Thus, this provided a chance to investigate the variation in optical properties as a function of the grain size. The energy band-gap and refractive index were estimated using UV-visible absorption spectra. The variation in band-gap with grain size showed blue-shift typical of electron quantum confinement. The effect of crystal ordering on refractive index was also evident by the linear increase of the refractive index with grain size.