Optimized Annealing and Patterning of Sb₂Te₃ and Bi₂Te₃ Thin Films for Antenna-Coupled Thermoelectric THz and mm-Wave Detectors

Antenna-coupled thermoelectric junctions hold potential for THz and mm-wave detection and energy harvesting. These devices require patterned deposition of thermoelectric materials with a junction at the antenna feed, which is heated by radiation-induced currents. Sputtered Sb₂Te₃ and Bi₂Te₃ thin films, ideal for this application, must be deposited over patterned photoresist on a suitable substrate followed by lift-off. Films deposited on room temperature substrates have high resistivity and low Seebeck coefficient. Films deposited on substrates at 175 ^oC have optimal thermoelectric properties, but such temperatures reflow and carbonize patterned photoresist, which complicates lift-off. This problem is avoided by deposition and lift-off at room temperature followed by an activation anneal. We describe a two-level full-factorial experiment to optimize the annealing conditions. We find that the optimized annealed films have Seebeck coefficients comparable to those obtained by heating the substrate during deposition. Additionally, annealing dramatically reduces the resistivity. Corresponding changes in the electrodynamic properties of the films are revealed by far-infrared spectroscopy. Simulation predicts a mm-wave responsivity of 4V/W for a single Sb₂Te₃-Be₂Te₃ junction coupled to a Ti bowtie antenna on glass substrate. Results of characterization using a backward wave oscillator on fabricated detector devices will be presented.