Electrical resistivity of thermally evaporated bismuth telluride thin films

Abstract

Semiconducting chalcogenide thin films have been receiving considerable attention in the recent years because of their wide applications in the various fields of science and technology. The studies of the electronic properties of semiconductors have been largely stimulated by attractive micro-electronic device applications. Among the various V–VI compounds, Bismuth Telluride (Bi2Te3) is an established low-temperature thermo electric material and is widely employed in thermoelectric generators and coolers. The present work deals with the structural and the electrical characterization of Bi2Te3 thin films vacuum deposited on well-cleaned glass substrates. A constant rate of deposition was maintained through out the process. To obtain uniform and homogeneous film thickness through out on all the substrates a rotary drive was employed. Quartz crystal thickness monitor was used to measure the thickness of the samples. From the X-ray diffractogram the Bi2Te3 films are found to be amorphous at lower thicknesses and posses hexagonal polycrystalline structure at higher thickness, having lattice parameters a=4.44 Å and c=29.40 Å. The grain size of the Bi2Te3 thin films before annealing and after annealing are found to be 100 and 160 Å, respectively. The micro-strain and the dislocation density are found to decrease after annealing. The thermogravimetry–differential thermal analysis (TG–DTA) studies revealed that the Bi2Te3 films are non-decomposable. Electrical resistivity, TCR measurements have been carried out as a function of varying temperatures in the range 303–453 K are found to show the size effect. Analyzing the size dependence of electrical resistivity it is found that the electrical resistivity is a linear function of the reciprocal of thickness of the film. The energy gap of Bi2Te3 thin film was calculated from the graph lnρ vs. 1/T and it is found that the energy gap decreases with increasing thickness. From the negative values of TCR, it is inferred that Bi2Te3 films exhibit semiconducting behavior.