Ultrahigh Power Factor of Sputtered Nanocrystalline N-Type Bi₂Te₃ Thin Film via Vacancy Defect Modulation and Ti...

Magnetron-sputtered thermoelectric thin films have the potential for reproducibility and scalability. However, lattice mismatch during sputtering can lead to increased defects in the epitaxial layer, which poses a significant challenge to improving their thermoelectric performance. In this work, nanocrystalline n-type Bi₂Te₃ thin films with an average grain size of ≈110 nm are prepared using high-temperature sputtering and post-annealing. Herein, it is demonstrated that high-temperature treatment exacerbates Te evaporation, creating Te vacancies and electron-like effects. Annealing improves crystallinity, increases grain size, and reduces defects, which significantly increases carrier mobility. Furthermore, the pre-deposited Ti additives are ionized at high temperatures and partially diffused into Bi₂Te₃, resulting in a Ti doping effect that increases the carrier concentration. Overall, the 1 µm thick n-type Bi₂Te₃ thin film exhibits a room temperature resistivity as low as 3.56 × 10^-6 Ω∙m. Notably, a 5 µm thick Bi₂Te₃ thin film achieves a record power factor of 6.66 mW mK^-2 at room temperature, which is the highest value reported to date for n-type Bi₂Te₃ thin films using magnetron sputtering. This work demonstrates the potential for large-scale of high-quality Bi₂Te₃-based thin films and devices for room-temperature TE applications.