Sulfurization temperature dependence of the structural transition in Cu₂FeSnS₄-based thin films

Cu₂FeSnS₄ (CFTS)-based thin films have been prepared on Mo-coated glass substrates by magnetron sputtering technique combined with a post-sulfurization processing. Energy dispersive X-ray measurement indicates that the Sn content decreases with the increasing sulfurization temperature, and all samples are in Cu-poor and Fe-rich states. X-ray photoelectron spectroscopy and X-ray diffraction show that all phases belong to the CFTS and no impure phase, e.g. Cu₂SnS₃, can be discovered in thin films. However, samples exhibit a phase transition behavior from rhodostannite to stannite structure with the increasing sulfurization temperature. The frequency of A₁ mode of CFTS with rhodostannite structure is estimated to be 323.8 cm^-1 by using the empirical model, which coincides with the experimental value as measured by Raman spectra analysis. These results are helpful to understand the properties of CFTS-based thin films.