Thermal annealing induced changes in structural, linear and nonlinear optical properties of Bi7In28Se65 films for nonlinear applications

Abstract

This work reports the effect of thermal annealing (350 °C and 450 °C) on structural, linear and non-linear optical properties of thermally evaporated Bi7In28Se65 chalcogenide films of ∼800 nm thickness. The surface structure and elemental concentration were analyzed by the Field emission scanning electron microscope (FESEM) and Energy dispersive X-ray analysis (EDX). Based on the structural analysis, the 450 °C annealed film showed complete crystallinity whereas 350 °C annealed film showed less crystalline nature without much structural alterations from the as-prepared film which is mostly rhombohedral in nature. The Raman study showed the alteration on bonding in 450 °C annealed film than the as-prepared and 350 °C annealed films. The optical analysis by UV–Vis spectroscopy showed the decrease in transmittance and an increase in absorbance property with annealing. The direct bandgap and Tauc parameter decreased with annealing. Correspondingly, the refractive index increased along with other optical parameters such as lattice dielectric constant (εL), optical density (OD), skin depth (δ), real and imaginary dielectric constant (εr and εi), loss factor, volume energy loss function (VELF) and surface energy loss function (SELF) with annealing temperatures. The static linear refractive index (n0) for the studied films showed an incremental behavior with annealing and satisfied Moss's rule. Such annealing induced higher absorption coefficient (α) and bandgap (Eg) along with other optical constants of the annealed films showed the possibility to be useful for solar cell, opto-electronic and nonlinear device applications.