Effects of post-annealing on thermoelectric properties of bismuth–tellurium thin films deposited by co-sputtering

This paper reports the microstructure evolution of Bi–Te thermoelectric films upon post-annealing and its effects on the thermoelectric properties. Bi–Te films with the composition of around 61 at.% Te and the thickness of 300 nm were deposited onto SiO2-coated Si substrates by using bismuth and tellurium targets in a radio frequency (RF) magnetron sputtering system. We annealed the films at different temperatures (100, 150 and 200 °C) under N2 ambient for 8 h, and characterized the crystallinity and morphology of the Bi–Te films. Microstructure characterization using X-ray diffraction and scanning electron microscopy disclosed that the post-annealing treatment entailed a drastic microstructural evolution by inducing the development of a strong texture of grains with their c-axis oriented normal to the substrate. In addition, we measured the electrical transport and thermoelectric properties of the films to reveal their close link with the microstructure changes. The electron mobility and Seebeck coefficient increase significantly, leading to a remarkable improvement in the power factor from 3.3 μW/K2 cm for the as-deposited sample to 24.1 μW/K2 cm for the 200 °C-annealed sample.