Nanocrystallization and optical properties of quaternary Sn-Se-Bi-Te chalcogenide thin films

Thin films of the pre-melt quenched bulk samples of (SnSe4)(95-x)(Bi2Te3)(5)(+x) (x = 0, 5, 15) system have been prepared by thermal evaporation technique. The structural and optical properties of these thin films have been investigated utilizing normal transmittance spectra. Various parameters like linear refractive index (n) determined using Swanepoel method, dielectric constants (epsilon(r) and epsilon(i)), optical conductivity (sigma(opt)) determined using the linear refractive index, extinction coefficient (k), and absorption coefficient (alpha) have been evaluated. The optical bandgap (E-g(opt)), direct and indirect (E-g(dir) and E-g(ind)), has been determined using the Tauc extrapolation method, and the bandgap values are found to decrease from 1.81 to 1.67 eV and 1.02 to 0.89 eV, respectively. E-g(opt) values show a similar trend to the theoretically calculated bandgap. The observed change in the optical parameters has been explained based on the decreasing defect states in the system. The nonlinear properties of the aforementioned system have also been reported. The results revealed that with the addition of metallic elements Bi and Te there is an increase in the semiconducting behavior of the system.