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 8C 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 mm 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 mm). The allowed optical transitions were found to be direct optical transitions with energy gap Ed g ¼ 0:21 eV. The optical conductivities s1 = ƒ(hn) and s2 = f(hn) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions.