Abstract
Bismuth telluride-based compounds have superior thermoelectric properties at room temperature regime. The thermoelectric thin films deposited at room temperature usually have many crystal defect. A post thermal treatment is required to eliminate the defects and improve the thermoelectric properties of the thermoelectric thin films. However, Te atoms have high vapor pressure and easily to evaporate during the annealing process, resulting in the variation of composition. In this study, a Bi0.4Sb1.6Te3/Te multilayered structure has been deposited on a polyimide substrate by RF/DC magnetron sputtering. The extra Te layers inserted in between the Bi0.4Sb1.6Te3 films is used to compensate the Te loss during thermal annealing. First, the effects of sputtering parameters and polyimide substrate on the characteristics of Bi0.4Sb1.6Te3 were investigated. The results show that a high process pressure reduces the deposition rate of the Bi0.4Sb1.6Te3 thin film. The composition will vary by changing the sputtering power due to different elemental sputter yield. Moreover, the polyimide substrate of high coefficient of thermal expansion possesses a high tensile thermal stress at annealing temperature and is susceptible to the formation of micro voids in the Bi0.4Sb1.6Te3 thin films. As for the microstructure and thermoelectric properties, we observed that some Sb-rich phases precipitate at the film surface after the specimen annealed at the temperature higher than 350 C. Interestingly, the Bi0.4Sb1.6Te3/Te films exhibits less and smaller Sb-rich precipitates than the Bi0.4Sb1.6Te3 films. The Bi0.4Sb1.6Te3/Te films were found to have a higher carrier concentration, a lower electrical resistivity and better crystallinity than the Bi0.4Sb1.6Te3 films. The discrepancy appears to increase with the increasing number of Te layers in the sample. It is suggested that the excess Te layers in the Bi0.4Sb1.6Te3/Te films are able to compensate the Te loss and suppress the formation of Te vacancies VTe during the high temperature annealing. A high thermoelectric power factor of 9.2×10-3 W/mK2 is obtained for the Bi0.4Sb1.6Te3/Te samples with 15 Te intersection layers after annealed at 400 C.