Characterization of thin Bi2Te3-based films and effects of heat treatment on their optical properties

Abstract

Thin Bi2Te3 and based films were thermally evaporated onto highly cleaned glass substrate at high vacuum. Source materials are bulk alloys prepared by the standard melting method. X-ray diffraction analysis of the as-deposited films asserted a polycrystalline nature with a hexagonal type structure. Surface roughness and morphology were investigated by means of AFM and SEM respectively. Nano-crystallinity of the concerned materials was asserted by TEM investigations. Optical characteristics were defined for the synthesized films before and after annealing in air atmosphere for 3 h at 300 °C. Transmittance and reflectance spectra of the as prepared and annealed films were measured in the wavelength range 350–2700 nm using a double beam spectrophotometer. Absorption coefficient, refractive index and energy band gap values were derived based on the measured spectra and the film's thickness. It was found that Bi2Te3-based materials are highly refractive indexed material. Index of refraction was used for calculation and determination of optical dispersion energies, in terms of the single oscillator constants using the Wemple–DiDomenico method. The allowed optical transitions were observed as direct optical transitions with energy gap in the range of 0.56–0.82 eV. Real and imaginary parts of the optical dielectric function were estimated before and after annealing treatment, exhibiting very high values. Further increase was detected after annealing. The influence of thermal annealing on the optical properties of the Bi2Te3 and Se containing films is investigated and compared to the as-prepared corresponding samples in the present work.