Effect of vacuum annealing on structural, electrical and thermal properties of e-beam evaporated Bi2Te3 thin films

Nanocrystalline thin films of a V-VI compound Bi2Te3 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 similar to 3 x 10(-6) mbar) on composition, structure, optical and electrical properties of these films. It is observed that, as the annealing temperature increases (from 100 degrees C to 300 degrees C), the crystallites grow with a preferential orientation along (110) planes with slight increase in the crystallite size from similar to 14 nm to 30 nm. This is associatedwith 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 Bi2Te3 changes from0.092 eV to 0.113 eV on annealing at 100 degrees C to 300 degrees C, respectively. Interestingly, we observe a gradual change from a semiconductor to metallic behavior on annealing the samples from 100 degrees C to 300 degrees 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 degrees C and 250 degrees C shows a broad semiconductor to metallic transition at similar to 150 K and 200 K respectively, the thin films annealed at 300 degrees C are found to exhibit complete metallic behavior belowroomtemperature. The electrical property and Seebeck coefficient studieswith power factors in the range of similar to 4 to 12 x 10(-4) W/K-2 m for films annealed above 200 degrees C suggest that the vacuum annealed Bi2Te3 thin films are favorable for thermoelectric applications. (C) 2017 Elsevier B.V. All rights reserved.