Optical parameters of both As$_2$S$_3$ and As$_2$S$_3$ thin films from ultraviolet to the near-infrared via variable-angle spectroscopic ellipsometer

In the UV-visible–near infrared regions from 245 to 1000 nm, variable-angle spectroscopic ellipsometer (VASE) was used to investigate optical functions of As$_2$S$_3$ and As$_2$S$_3$ thin films. In the entire measured spectral range, data were analyzed by assembly from several dispersion models. These assemblies comprise individual Tauc–Lorentz supplemented by several Lorentz (TL-group) or single Cody–Lorentz with several Lorentz (CL-group) models. For As$_2$S$_3$ and As$_2$S$_3$ thin films, the optical parameters were quantified. The model parameters, such as the Lorentz amplitude, resonance frequency, oscillator width, extinction coefficients, refractive indices, and Urbach and optical band energy of both films were obtained. The band gap energy $E_g$ was experimentally determined using the obtained data of CL-group from $(\alpha hv)^{1/2}$ vs $hv$ plots. It is found that the band gap energies of As$_2$S$_3$ and As$_2$S$_3$ were 1.796 and 2.349 eV, respectively. The $E_g$ values for the films were theoretically investigated by the bond statistics of the random covalent network model (CRNM) with the aid of Manca’s relation. Plausible agreement between the experimental and calculated $E_g$ values for both samples was obtained.