Formation of metastable cubic phase in SnS thin films fabricated by thermal evaporation

Metastable cubic SnS phase (π-SnS) is an emerging material having attractive properties for solar cell applications. In this study, we have investigated the thermal evaporation process of SnS with substrate temperatures of 250-350°C, and have observed the formation of π-SnS together with the stable orthorhombic phase (α-SnS) for the first time by using only SnS as a starting material. X-ray diffraction analysis shows that a single-phase α-SnS film is obtained at 350°C, and the fraction of π-SnS increases with decreasing the substrate temperature to 250°C. The grain shape of the SnS film grown at 250°C observed by scanning electron microscopy was more isotropic than 350°C, consistent with the cubic lattice of π-SnS. Electrical characterization by Hall measurement reveals the existence of the maximum point of carrier density as a function of substrate temperature, which is understood by the competition between the increases of crystal defects and the fraction of wider-gap π-SnS than α-SnS with decreasing substrate temperature. The formation of π-SnS brings about lowering of absorption coefficients below the absorption edge of π-SnS, which are analyzed from transmittance and reflectance spectra. The reason of π-SnS formation is discussed by comparing the conditions of film deposition with previous reports.