Structural and optical properties of thermally evaporated Bi2Te3 films

Bi2Te3 films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 °C were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67-162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5-10 μm for Bi2Te3 films with different thicknesses (25-99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi2Te3 is a high refractive index material (n has values of 4.7-8.8 in the wavelength range 2.5-10 μm). The allowed optical transitions were found to be direct optical transitions with energy gap Egd=0.21 eV. The optical conductivities σ1 = ƒ(hν) and σ2 = f(hν) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions.