Effects of annealing on the thermoelectric properties of nanocrystalline Bi_(1.2)Sb_(0.8)Te_3 thin films prepared by thermal evaporation

摘要

Nanocrystalline films of thermoelectric compound Bi1.2Sb0.8Te3 were deposited on the glass substrates at room temperature, under a vacuum of similar to 2x10(-5) mbar by thermal evaporation. These films were annealed in vacuum of 2x10(-5) mbar at 120 degrees C, 150 degrees C and 180 degrees C. The analysis of XRD profile revealed that as-deposited film was made up of very fine crystallites of size of similar to 14nm. Raman spectra of as-deposited, 150 degrees C and 180 degrees C annealed films were analyzed to study the changes in the atomic bonding which indicated a variation of microstructure. This fact was supported by the emergence of diffraction lines corresponding to (110) and (00l) direction in XRD profile of the film annealed at 180 degrees C. FESEM analysis of the As-deposited and annealed films was conducted. It was found that the film formed consisted of nanosized clusters(lumps) along with pores between them. Magnified view depicted that each cluster was made up smaller crystallites that were packed together in a non-uniform fashion. The results obtained from XRD and FESEM prove that the films were nanocrystalline. Lattice parameters (c and a) were determined and overall c/a ratio was found to increase with annealing that indicated increase unit cell volume. This complied well with reduced defects, resulting strains and also indicated the change of crystal orientations in the film. Increase in the value of seebeck coefficient, by a factor of 6, was observed for film annealed at 180 degrees C (BST-180). This resulted in the 19 times increase in power factor of BST-180 as compared to as-deposited film.