Characterization of Bismuth Telluride thin films — Flash evaporation method

Abstract

Semiconducting Bi 2Te 3 thin films were grown by a flash evaporation method. The optical characteristics of Bi 2Te 3 samples have been analyzed using a spectrophotometer in the wavelength range of 400–800 nm. The transmittance falls steeply with decreasing wavelength. It reveals that Bi 2Te 3 films have considerable absorption throughout the wavelength region. The effect of thickness on the fundamental optical parameters like refractive index and extinction coefficient are studied. The optical band gap energy decreases typically from 1.4 to 1.0 eV with increasing thickness, which may be due to the increased grain size of the films at higher thicknesses. Bi 2Te 3 thin films with a thickness of 120–300 Å exhibit semiconducting behavior with an exponential decrease in resistivity with increasing temperature ranging from 303 to 483 K. To examine the temperature degeneracy of the resistivity in more detail, log ρ Vs 1/ T graphs are drawn and the activation energy calculated from these graphs decreases with increasing thickness in the range 0.16–0.06 eV. This decrease in activation energy with thickness of the film may be due to the change in barrier height. The change in barrier height is due to the change of crystallite size in the films.