Anisotropic Analysis of Nanocrystalline Bismuth Telluride Thin Films Treated by Homogeneous Electron Beam Irradiation

The in-plane and cross-plane transport properties of nanocrystalline bismuth telluride (Bi2Te3) thin films were evaluated to analyze their anisotropic behavior. Bi2Te3 thin films were prepared via radio frequency (RF) magnetron sputtering, followed by a subsequent treatment of thermal annealing and homogeneous electron beam (EB) irradiation at various EB doses. The crystallographic properties of the thin films were determined by X-ray diffraction (XRD) analysis. It was determined that the crystal orientation (Lotgering factor; F value) of Bi2Te3 thin films can be controlled by homogeneous EB irradiation treatments, without resulting in crystal growth. The electrical conductivity and Seebeck coefficient were measured in the in-plane direction of the films, and the thermal conductivity was measured in the cross-plane direction using the 3 omega method. The anisotropic analysis was performed by combining the F value of the thin films with a simple model based on the transport properties of the basal and lateral planes of single-and poly-crystal Bi2Te3. The electrical and thermal conductivities of the in-plane and cross-plane directions of the EB-irradiated thin films clearly differed; however, there was no significant difference between the Seebeck coefficient values of the two planes. Finally, we determined that the figure of merit, ZT, was enhanced by the homogeneous EB irradiation treatment, and the in-plane ZT value was 25% greater than that of the cross-plane direction.