Investigation of amorphous-crystalline transformation induced optical and electronic properties change in annealed As₅₀Se₅₀ thin films

The present work reports the amorphous-crystalline phase transformation in thermally evaporated As₅₀Se₅₀ thin films upon annealing at below T_g (423 K) and above T_g (523 K). The structural transition was probed by XRD, Raman and X-ray photoelectron spectroscopy. The composition and surface morphology were probed by EDS and FESEM techniques. The transmittance and reflectance spectra over the wavelength range 500 nm–1200 nm were used to deduce the optical parameters. The various optical parameters of the as-prepared and annealed As₅₀Se₅₀ thin films were estimated and discussed in terms of density of defect states and disorders. The indirect optical energy gap decreased for the 423 K annealed film and abruptly increased for 523 K annealed film as compared to the as-prepared film. The Swanepoel envelope method, WDD model, and Sellemeire postulates were employed for the analysis of refractive index, static refractive index, oscillator energy, dispersion energy, oscillator wavelength and dielectric constant. The changes in the linear and nonlinear properties showed opposite behavior for the two annealed films. The non-linear refractive index and 3rd order susceptibility were found to be increased for 423 K annealed film and decreased for the 523 K annealed film. The optical as well as the electrical conductivity changed with annealing and the electrical susceptibility increased for 523 K annealed film. The tunable optical properties can be applied for several optoelectronic applications. Graphical abstract: The amorphous to crystalline structural transformation occurred at annealing above Tg. The 3rd order susceptibility was found to be more at 423 K annealing and less at 523 K annealing. [Figure not available: see fulltext.]