Atomic composition changes in bismuth telluride thin films by thermal annealing and estimation of their thermoelectric properties using experimental analyses and first-principles calculations

Abstract

Abstract We investigated the structural and thermoelectric properties of bismuth telluride films when the atomic composition is changed by evaporation of Te atoms from the films during thermal annealing. The bismuth telluride films were deposited using a magnetron radio frequency sputtering method without a substrate heating. Thereafter, the films were thermally annealed up to 450 °C in an electric furnace with Ar(95%)–H2(5%) gas mixture. The atomic composition of the as-deposited film was approximately that of Bi2Te3. By increasing the annealing temperature, the Te atoms were evaporated from the films and the composition approached that of Bi1Te1, as discovered from electron probe microanalysis. Furthermore, a change in crystalline phase was observed through X-ray diffraction patterns. When the thermoelectric properties of the films were measured at room temperature, the carrier concentration was significantly affected by the change in atomic composition. We confirmed this finding using energy band structures using first-principles calculations. The highest power factor of 16.9 μW/(cm∙K2) was achieved at an annealing temperature of 300 °C because the film achieved a high crystallinity and crystal orientation with relatively low defects owing to less evaporation of Te atoms.