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

CuxSnSy thin films with thicknesses of 1.5 mu m and x atomic ratios ranging between 3 and 4 have been prepared by co-evaporation at substrate temperatures varied from 150 degrees C to 450 degrees 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 degrees C in sulfur atmosphere. The layers prepared at T-sus <= 350 degrees C have shown a mixture of hexagonal CuS and cubic Cu2SnS3 phases, which reacted by the annealing to give orthorhombic Cu3SnS4 (for r(cu) = 10 nm/min) or orthorhombic Cu4SnS4 (for r(cu)= 16 nm/min). The increment of the substrate temperature to 450 degrees C allowed also the crystallization of orthorhombic Cu3SnS4, 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 Cu3SnS4 has evidenced a gap energy of 1.4 eV and an electrical resistivity of 7.9 x 10(-4) Omega cm, whereas the orthorhombic Cu4SnS4 has shown a lower gap energy of 1.2 eV and a higher electrical resistivity of 8.2 x 10(-2) Omega cm. (C) 2016 Elsevier Ltd. All rights reserved.