Abstract
Recently, environmental crisis spurs the researches on alternative energy and hence thermoelectric materials, which can convert heat into electricity, or alternatively, convert electricity into cooling, attract renewed attention. Due to the advantages of high cooling power density and short response time, thin-film thermoelectric coolers (TECs) have been considered as the plausible solution to the thermal management problem of microelectronics. Bismuth telluride based compounds are known to be the best thermoelectric materials near room temperature regime to date and are the potential candidates for thin-film TECs. A post-deposition thermal treatment is usually used to reduce the structural imperfections and improve thermoelectric properties of bismuth telluride based thin films prepared by physical vapor deposition and electrochemical deposition methods. Recently, spark plasma sintering (SPS) technique, using electric energy as the heat source, was demonstrated to be very effective in the rapid densification of bismuth telluride based powder materials. The SPS-prepared bismuth telluride based compounds have dense, textured and polycrystalline microstructures, and most important of all, excellent thermoelectric properties. Nevertheless, the details of how an electric current interacts with bismuth telluride compounds are still not fully understood. In this study, p-type Bi-Sb-Te and n-type Bi-Se-Te nanocrystalline films were prepared by a r.f. magnetron sputtering method followed by a electrical annealing process. An electric current of density ~ A/cm2 was introduced through films during thermal annealing. The electrically stressed p- and n-type films were found to have lower carrier concentration but much higher mobility than those thermally annealed at the same temperatures. A model based on electromigration-induced preferential Sb and Te diffusion is proposed to explain the observed Sb-rich and Te precipitation as well as the enhancement of Seebeck coefficient and electrical conductivity of the electrically stressed thin films. Owing to anisotropic diffusion and electrical transport properties, charged lattice defects in the (00l) grains of bismuth telluride crystal are preferentially eliminated under electric current stressing. The presented current-assisted annealing approach can be an efficient post-deposition treatment in a short cycle time and low temperature that prevents from unfavorable gross grain growth and evaporation of volatile constituents in Bi-Te based nanocrystalline thin films during high-temperature annealing process.