Study on thermoelectric property optimization of mixed-phase bismuth telluride thin films deposited by co-evaporation process

The development of Bi2Te3 thin films has huge potential in the pursuit of efficient thermoelectric micro/nanodevices due to their high Seebeck coefficient, high electrical conductivity and low thermal conductivity. The optimization of experimental parameters of BiTe thin films produced by co-evaporation will be investigated in this study. Co-evaporation is a low cost, easy-to-control process which can be used for high throughput and is scalable. We found that an optimal Te/Bi ratio of 1.5 with good thermoelectric properties can be directly synthesized by Bi and Bi2Te3 co-evaporation. Compared to the conventional Bi/Te co-evaporation process, high temperature annealing or substrate heating is not necessary for the process mentioned in this paper, which is a desirable feature when using polymer-based substrates, organic/inorganic hybrid thermoelectric generators, and flexible devices since they have relatively low tolerance to heat. The optimized Bi2Te3 thin films, which are mixed phases of Bi2Te3, Bi3Te4 and Te, possess high carrier concentration (6.65 × 1020 cm−3), low electrical resistivity (3.17 × 10−3 Ωcm), and extremely low thermal conductivity (0.59 W/mK) at room temperature on a smooth surface (roughness <5.5 nm) and are achieved by adjusting the deposition rate of Bi and Bi2Te3. The correlation between the structures of mixed phases, electrical and thermal properties will be discussed in detail.