Thermoelectric Micro-Cooler of Bismuth Telluride Thin Films

In this study, we fabricated bismuth-telluride thin films and their in-plane thermoelectric micro-coolers (4mm×4mm) by using the flash evaporation method. We prepared fine powders of Bi_2.0Te_2.7Se_0.3 (n-type) and Bi_0.4Te_3.0Sb_1.6 (p-type). The thermoelectric properties of as-grown thin films are lower than those of bulk materials. Therefore the as-grown thin films were annealed in hydrogen at atmospheric pressure for 1 hour in a temperature range of 200 to 400°C. By optimizing the annealing temperature, thin films with high thermoelectric power factors of 8.8 μW/(cm·K²) in n-type and 13.8 μW/(cm·K²) in p-type are obtained. To evaluate the figure of merit of the thin film, the thermal conductivity of the n-type thin film is measured by the 3ω method. The thin film annealed at 200 °C exhibited a cross-plane thermal conductivity of 1.2 W/(m·K). Micro-coolers of flash-evaporated bismuth-telluride thin films are fabricated using three shadow masks. The shadow masks are prepared by standard micro-fabrication processes such as nitridation of Si, dry etching, and wet etching. Thermoelectric power of the as-grown thin film devices with 16 pairs of p-n legs are measured by YAG laser heating at the center of the devices. The thermoelectric power of thermoelectric legs is evaluated to be 180μV/K per one p-n leg pair. According to the Kelvin’s law, it corresponds to 54mV Peltier coefficient per p-n pair.