Copper tin sulfide (Cu x SnS y ) thin films evaporated with x = 3,4 atomic ratios: Influence of the substrate temperature and the subsequent annealing in sulfur

Cu x SnS y thin films with thicknesses of 1.5 μm and x atomic ratios ranging between 3 and 4 have been prepared by co-evaporation at substrate temperatures varied from 150 °C to 450 °C and at evaporated copper rates of 10⿿16 nm/min. The evolution of the structural, chemical, optical and electrical properties as a function of the deposition conditions has been analyzed for the various samples as-grown and after annealing at 500 °C in sulfur atmosphere. The layers prepared at T sus  ⿤ 350 °C have shown a mixture of hexagonal CuS and cubic Cu 2 SnS 3 phases, which reacted by the annealing to give orthorhombic Cu 3 SnS 4 (for r Cu  = 10 nm/min) or orthorhombic Cu 4 SnS 4 (for r Cu  = 16 nm/min). The increment of the substrate temperature to 450 °C allowed also the crystallization of orthorhombic Cu 3 SnS 4 , coexisting with a secondary CuS phase for r Cu  = 16 nm/min. The application of a chemical treatment in KCN solution has been effective to remove the copper sulfide excess in the samples. The orthorhombic Cu 3 SnS 4 has evidenced a gap energy of 1.4 eV and an electrical resistivity of 7.9 y 10 ⿿4  Ω cm, whereas the orthorhombic Cu 4 SnS 4 has shown a lower gap energy of 1.2 eV and a higher electrical resistivity of 8.2 y 10 ⿿2  Ω cm.