Structural changes in nanocrystalline Bi2Te3/ Bi2Se3 multilayer thin films caused by thermal annealing

To assess the performance of thermoelectric devices with nanostructured materials at high operating temperatures, we investigated the effects of structural changes on the thermoelectric properties of nanocrystalline bismuth telluride (Bi2Te3)/bismuth selenide (Bi2Se3) multilayer thin films caused by thermal annealing. Multilayer thin films with 12 and 48 layers were fabricated by radio-frequency magnetron sputtering. These thin films were then thermally annealed at temperatures ranging from 250 to 350 degrees C. As the annealing temperature increased, flake-like nanocrystals were grown in the 12-and 48-layer thin films. X-ray diffraction peaks from three alloys, which were determined to be Bi2Te3, Bi2Se3, and (Bi2Te3) 0.4(Bi2Se3) 0.6, were observed in the thin films. This indicates that Bi2Te3 and Bi2Se3 layers were not completely diffused mutually in this range of annealing temperature. The 12-and 48-layer thin films exhibited increases in both the electrical conductivity and the absolute value of the Seebeck coefficient at the annealing temperature of 300 degrees C. One possible explanation for this improvement is that the band structure is tuned by inducing strain during the variation of atomic composition in the multilayer thin films. As a result, the power factor was significantly improved by the thermal annealing. In particular, the maximum power factor reached 13.7 mu W/(cm K-2) in the 12-layer thin film at the annealing temperature of 350 degrees C. Therefore, we may conclude that if the multilayer thin films undergo structural changes at higher operating temperature (approximate to 350 degrees C), thermoelectric devices composed of multilayer thin films are expected to exhibit suitable thermoelectric performance.