Synthesis of Cu2SnS3, Cu3SnS4, and Cu4SnS4 thin films by sulfurization of SnS-Cu layers at a selected temperature and /or Cu layers thickness

Abstract

Three phases of the ternary Cu-Sn-S system were synthesized using thin films of chemical bath deposited tin sulfide (SnS) and thermally evaporated copper. Cu layers with different thicknesses (125–175 ​nm) were deposited on SnS thin films (250 ​nm) and the SnS: Cu stacks were sulfurized at temperatures in the range of 400–600 ​°C. Three different ternaries of Cu-Sn-S were obtained. Cu2SnS3 (CTS) thin films were formed for 125 ​nm copper on SnS(250 nm), sulfurized at 500 ​°C. Tetragonal crystalline structure was determined for CTS thin films by X-ray diffraction (XRD) analysis and confirmed by two characteristic Raman peaks at 296 and 352 ​cm-1. Optical properties showed a direct bandgap close to 1 ​eV and electrical conductivity of 1.9 (Ω ​cm)-1. Increasing the Cu thickness to 175 ​nm and the temperature of sulfurization to 550 ​°C, the Cu3SnS4 phase (orthorhombic) was formed which was confirmed by the high intense Raman peak at 316 ​cm-1. A direct bandgap of 1.75 ​eV and electrical conductivity in the order of 103 (Ω ​cm)-1 was obtained for this phase. Further increase in sulfurization temperature to 600 ​°C for the stack with the same Cu thickness (175 ​nm), Cu4SnS4 phase (orthorhombic) was achieved exhibiting a Raman vibrational peak at 317/322 ​cm-1, a direct bandgap of 1 ​eV, and electrical conductivity of 10 (Ω ​cm)-1. The morphological analysis of these three CTS phases by scanning electron microscopy presented distinct morphologies for each phase. X-ray photoelectron spectroscopy (XPS) analysis of the samples was done for their elemental composition and chemical state information. This is the first report to synthesize all the three stable phases of the Cu-Sn-S system viz. Cu2SnS3, Cu3SnS4, and Cu4SnS4 by selecting the sulfurization temperature and copper layer thickness in similar SnS: Cu layer thin films.