Highly oriented crystal growth of nanocrystalline bismuth telluride thin films with anisotropic thermoelectric properties using two-step treatment

Abstract

We investigated the anisotropic thermoelectric properties of highly oriented nanocrystalline bismuth telluride (Bi2Te3) thin films. The thin films were prepared by radio-frequency magnetron sputtering, followed by a two-step treatment combining homogeneous electron beam (EB) irradiation and thermal annealing. In this treatment, the EB irradiation dose was varied from 0 to 1.07 MGy and the annealing temperature was maintained at 300 °C. The obtained thin films had a crystallite size of approximately 80 nm, with considerably high crystal orientation along the c-axis direction, when the two-step treatment was employed with EB irradiation dose in the range of 0.22–0.87 MGy. To estimate the anisotropic thermoelectric properties of the films, we used a simple model that accounts for the crystal orientation of single- and poly-crystalline Bi2Te3 bulk materials. The thin films exhibited a high electrical conductivity in the in-plane direction, whereas the Seebeck coefficient was nearly isotropic. The in-plane and cross-plane power factors of the thin films reached maximum values of 17.1 and 4.7 μW/(cm·K2), respectively. This study provides a two-step treatment that can be used for enhancing the thermoelectric performance of materials with anisotropic structures.