Abstract
Bismuth telluride has been considered as a promising candidate for thermoelectric devices due to its superior thermoelectric properties at room temperature. By developing thermoelectric devices using thin films and flexible substrates, we can fabricate light, wearable and high performance thermoelectric devices which may contribute to the development of self-powered wearable mobile electronics. However, during fabrication and operation of such devices, a different degree of stress will be inevitably introduced, therefore, the understanding of mechanical and electrical properties of thermoelectric thin films on flexible substrates is crucial for device performance and reliability considerations. In this study, p type Bi-Sb-Te thin films were deposited on polyimide substrates by RF magnetron sputtering. After deposition, thin films were subject to conventional thermal annealing or current-assisted annealing and the electrical resistivity of Bi-Sb-Te thin films was in-situ measured during cyclic tensile stressing. The results show that the mechanical stress induced at room temperature has no significant effect on Seebeck coefficient and carrier concentration of Bi-Sb-Te thin films, however, current-assisted annealing is beneficial for suppressing the stress-induced increase in electrical resistivity of Bi-Sb-Te films compared with conventional thermal annealing. To investigate this phenomenon, this study aimed at the observation of microstructure of Bi-Sb-Te thin films and found the stress-induced resistivity change at room temperature is associated with the degree of microcrack-close-up. Both crack width and crack depth of Bi-Sb-Te films with current-assisted annealing is smaller than those with conventional thermal annealing after stress is released. The current-assisted annealing is able to mitigate the degradation of carrier mobility induced by cyclic stressing for the Bi-Sb-Te films.