Effects of annealing temperature on the structural, mechanical and electrical properties of flexible bismuth telluride thin films prepared by high-pressure RF magnetron sputtering

In this work BixTey thin films were deposited on polyimide substrate by a high-pressure RF magnetron sputtering technique. The deposited condition was maintained using a high pressure of 1.3 x 10(-2) mbar. The as-deposited films show Bi2Te3 structure with Te excess phase (Te-rich Bi2Te3). After that, as-deposited films were annealed in the vacuum chamber under the N-2 flow at temperatures from 250 to 400 degrees C for one hour. The microstructure, cross-section, [Bi]:[Te] content, and the mechanical, electrical and thermoelectric properties of as-deposited and different annealed films were investigated. It was found that the annealing temperature enhanced the crystallinity and film density for the temperature range 250-300 degrees C. However, the crystal structure of Bi2Te3 almost changed to the BiTe structure after annealing the films above 350 degrees C, due to the re-evaporation of Te. Nano-indentation results and cross-section images indicated that the hardness of the films related to the film density. The maximum hardness of 2.30 GPa was observed by annealing the films at 300 degrees C. As a result of an improvement in crystallinity and phase changes, the highest power factor of 11.45 x 10(-4) W m(-1)K(-2) at 300 degrees C with the carrier concentration and mobility of 6.15 x 10(20) cm(-3) and 34.03 cm2 V-1 s(-1), respectively, was achieved for the films annealed at 400 degrees C.