Optimization of Sb₂Te₃ and Bi₂Te₃ Thermoelectric Films for Infrared Detection and Energy Harvesting

We are developing antenna-coupled thermoelectric junctions for infrared detection and energy harvesting. COMSOL Multiphysics simulations identify Sb₂Te₃ and Bi₂Te₃ thermoelectric materials as ideal for the thermocouple junction at the antenna feed. Joule heating at the antenna feed from currents induced by antenna-collected radiation generates thermoelectric voltage and current for detection and energy conversion. In this work, we optimized the properties of RF sputtered telluride films for these applications. We performed a two-level full factorial optimization experiment with three factors: Argon gas pressure, substrate temperature, and RF power. Their low and high values were 7 and 8 mTorr, 175 and 225 deg C, and 15 and 25 W, respectively. The resulting eight depositions were performed using glass substrates in randomized order. A mid-point deposition at 7.5 mTorr, 200 deg C, and 20 W RF power was performed to reveal curvature in the main effects. We found for Sb₂Te₃ a maximum Seebeck coefficient of 149 μV/K, minimum resistivity 88 μΩ-m, and a maximum power factor (squared Seebeck coefficient over resistivity) of 1 μW/cm-K². Main effect plots, which are the response (power factor) averaged over two of the factors plotted vs the third, reveal that the power factor was most sensitive to changes in substrate temperature and least sensitive to variations in RF power. All main-effect plots show strong negative curvature. The mid-point response was significantly higher than those of the end points. The maximum power factor was achieved on the low-side of the midpoint. Interaction-effect plots are the response averaged over one of the factors plotted vs a second factor, for each value of the third factor. These reveal no significant interaction between gas pressure and RF power and the most significant interaction between RF power and substrate temperature. SEM images reveal a microstructure comprising ellipsoidal grains with half-micron major axis dimension. Similar results for Bi₂Te₃ will be presented. Photolithographically patterned bowtie-antenna-coupled thermocouples demonstrate successful deposition and lift-off. Response of this device to mm-wave radiation from a backward wave oscillator is consistent with COMSOL predictions.