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

Kamolmad Singkaselit, Aparporn Sakulkalavek and Rachsak Sakdanuphab

Abstract

In this work Bi _x Te _y 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  ×  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 N₂ flow at temperatures from 250 to 400 °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 °C. However, the crystal structure of Bi₂Te₃ almost changed to the BiTe structure after annealing the films above 350 °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 °C. As a result of an improvement in crystallinity and phase changes, the highest power factor of 11.45  ×  10⁻⁴ W m⁻¹K⁻² at 300 °C with the carrier concentration and mobility of 6.15  ×  10²⁰ cm⁻³ and 34.03 cm² V⁻¹ s⁻¹, respectively, was achieved for the films annealed at 400 °C