Optical characterization of Sb2S3 vacuum annealed films by UV-VIS-NIR spectroscopy and spectroscopic ellipsometry: Determining the refractive index and the optical constants

Using UV-VIS-NIR spectroscopy and spectroscopic ellipsometry (SE) techniques, we studied the optical characteristics of Sb2S3 thin films produced by a one-step thermal evaporation process and annealed for 2 h at various temperatures under vacuum atmosphere. The X-ray diffraction study shows that a structural transition from amorphous to polycrystalline nature occurred at vacuum annealing temperature of 200 degrees C. According to UV-VIS-NIR spectroscopic measurements, the Sb2S3 films showed a high absorption coefficient in the visible region (alpha > 10(4) cm(-1)) and a direct band gap energy that decreases from 2.00 eV to 1.63 eV as the annealing temperatures increase. Furthermore, we calculate the optical constants of the elaborated Sb2S3 films, such as refractive index n and extinction coefficient k, by fitting the ellipsometric measurements of the pseudo-refractive index < n > and the pseudo-extinction coefficient < k >. It was observed that the obtained values for n at 800 nm are 2.7, 2.9, 3.5, and 4.0 for as-deposited and annealed films, respectively, at 150, 200, and 250 degrees C. The dielectric properties were also determined using the Wemple DiDomenico (WDD) and Spitzer-Fan models.