Thickness effect on the optical properties of Bi/As2S3 bilayer thin films

• Optical properties of Bi/As2S3 bilayer thin films show a strong dependence on the Bi layer thickness.
• The increase in optical band gap is attributed to the decreased tailing of the band tails in the gap.
• The obtained lower values of Urbach energy indicate more ordered films can be produced as the Bi layer thickness increases.

The optical properties of bilayer thin films of Bi/As2S3 with various thicknesses which were prepared from Bi and As2S3 by thermal evaporation technique under high vacuum were characterized by Fourier Transformed Infrared Spectroscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The prepared bilayer films of 905 nm, 910 nm and 915 nm thickness have As2S3 as bottom layer (900 nm) and Bi as top layer (5,10,15 nm). X-ray diffraction studies confirm the amorphous nature of the prepared films. The optical properties show a strong dependence on the film thickness. It was found that the optical band gap increases with film thickness. The obtained lower values of Urbach energy indicate that as thickness increases, more ordered films can be produced. The reduction in disorder in bonding network is amply supported by the way of increase in band gap, increase in Tauc parameter (B1/2) and reduction in Urbach energy from the analysis of transmittance spectra. The change in XPS core level spectra and Raman spectra also shows the changes due to thickness.