Effect ofX-ray Irradiation on Structural and Optical Properties of Topological Insulator Bismuth Telluride Nano-Structure Thin Film

Abstract

Bismuth Telluride (Bi2Te3) thin films were grown ontooptical flat fused quartz and chronic glass by using a thermal evaporation technique for optical and structural investigations, respectively.The prepared films were divided into three groups; the first group was as-grown films, and the second and third groups were irradiated with X-ray radiation with energies 6 and 15 MeV.The crystal structure and morphology of the grown Bi2Te3thin films were identified by X-ray diffraction (XRD) and scan electron microscopy (SEM) before and after exposed to X-ray irradiation. The optical constants(Refractive index, n, and absorption index, k) of as- grown and irradiated Bi2Te3 thin films were estimated and calculated in the wavelength range from 200 to 2500 nm by using spectrophotometric measurements of transmittance and reflectance at normal incidence. The estimated onset optical gap Egfor as-grown equal to 0.35 eV and this value was found to be decreased corresponding to the increasing in X-ray radiation energy.The variation of the refractive index of normal dispersion is well described by the single oscillator model. The dielectric constant at infinite frequency (e∞), the lattice dielectric constant (eL) and the ratio of free carrier concentration of the effective mass (N/m*) are calculated. Finally, the nonlinear optical parameters are calculated using some empirical relations.