Effect of vacuum annealing on structural, electrical and thermal properties of e-beam evaporated Bi 2 Te 3 thin films

Nanocrystalline thin films of a V-VI compound Bi 2 Te 3 are fabricated with uniform thickness by e-beam evaporation at room temperature. The as-deposited films are stoichiometric, monophasic, highly strained and polycrystalline. We studied the effect of vacuum annealing (at a pressure of ~ 3 × 10 − 6 mbar) on composition, structure, optical and electrical properties of these films. It is observed that, as the annealing temperature increases (from 100 °C to 300 °C), the crystallites grow with a preferential orientation along (110) planes with slight increase in the crystallite size from ~ 14 nm to 30 nm. This is associated with the breaking of quintuple layers and rearrangement of crystallographic planes in the crystallites with Te rich surface emerging on vacuum annealing as evidenced from the XRD, Raman and high-resolution TEM studies. The direct bandgap (0.116 eV) of as-deposited Bi 2 Te 3 changes from 0.092 eV to 0.113 eV on annealing at 100 °C to 300 °C, respectively. Interestingly, we observe a gradual change from a semiconductor to metallic behavior on annealing the samples from 100 °C to 300 °C. Such a transition from negative temperature coefficient (NTC) to positive temperature coefficient (PTC) is seen mainly due to the percolation of Te - rich crystallite surfaces, which evolve as the annealing temperature increases. While the films annealed at 200 °C and 250 °C shows a broad semiconductor to metallic transition at ~ 150 K and 200 K respectively, the thin films annealed at 300 °C are found to exhibit complete metallic behavior below room temperature. The electrical property and Seebeck coefficient studies with power factors in the range of ~ 4 to 12 × 10 − 4 W/K 2 m for films annealed above 200 °C suggest that the vacuum annealed Bi 2 Te 3 thin films are favorable for thermoelectric applications.