Fabrication, structural and optical characterizations of thermally evaporated Cu₂SnS₃ thin films

Abstract

The bi-layer of metallic Cu-Sn precursors was thermally evaporated sequentially on microscopic glass substrates at the controlled thickness of 100nm, 500nm and 1000nm and at different substrate temperatures of 27⁰C, 100⁰C and 200⁰C. The bi-layer was subsequently sulphurized in a custom-built reactor for 1hour at 400⁰C to form Cu₂SnS₃ ternary films. The deposited Cu₂SnS₃ thin film samples were placed in the Scanning Electron Microscope (SEM) and the embedded SMARTSEM software produced the micrographs of each film. The micrographs from SEM show that the grain size increases with increasing substrate temperature (about 1μm)  from 27⁰C to 200⁰C. Elemental Cu, Sn, S and glass substrate constituents such as Na, Si, Mg and O were identified through EDS. The surface profile shows that the deposited Cu2SnS3 films are rough. The XRD spectra identified the crystal  structure, phases and lattices as Monoclinic Cu₂SnS₃ [-1 3 1], Anorthic Cu₂SnS₃ [-2 0 0 ] and Cubic Cu₂SnS₃ [1 1 1]. The UV-Vis Spectrophotometer measured the transmittance and reflectance of deposited Cu2SnS3 thin films in the range of  250nm – 900nm. The transmittance and reflectance data were used to calculate the optical absorption coefficient, α. The extrapolation of graph of (αhv)2 against hv  intercept hv axis which correspond to energy gap of the film. An Energy band gap, Eg, of the deposited Cu2SnS3 film is 1.65eV. Subsequently, refractive index n, extinction coefficient, k and optical conductivity σ0 were inferred. Refractive Index, n is 1.14, Extinction Coefficient, K, is 5.27 x 109 and Optical Conductivity , σ0 is 6.74708E+16 Ω-1cm-1.

Key words: Thermal Evaporation, Sulphurization, Energy band gap, grain sizes, Optical Conductivity, crystal structure.