A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Abstract

Thin films of Cu₂SnS₃ and Cu₃SnS₄ were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S₂ atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu₂SnS₃ phase changes. For a temperature of 350 °C the films were composed of tetragonal (I-42m) Cu₂SnS₃. The films sulfurized at a maximum temperature of 400 °C presented a cubic (F-43m) Cu₂SnS₃ phase. On increasing the temperature up to 520 °C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu₃SnS₄ was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm⁻¹ for tetragonal Cu₂SnS₃, 303 and 355 cm⁻¹ for cubic Cu₂SnS₃, and 318, 348 and 295 cm⁻¹ for the Cu₃SnS₄ phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 10⁴ cm⁻¹. The estimated band gap energy was 1.35 eV for tetragonal (I-42m) Cu₂SnS₃, 0.96 eV for cubic (F-43m) Cu₂SnS₃ and 1.60 eV for orthorhombic (Pmn21) Cu₃SnS₄. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu₂SnS₃, cubic Cu₂SnS₃ and orthorhombic (Pmn21) Cu₃SnS₄ are 4.59 × 10⁻² Ω cm, 1.26 × 10⁻² Ω cm, 7.40 × 10⁻⁴ Ω cm, respectively.