Thermoelectric and structural characterization of Al-doped ZnO/Y₂O₃ multilayers

The influence of Y₂O₃ nanolayers on thermoelectric performance and structure of 2% Al-doped ZnO (AZO) thin films has been studied. Multilayers based on five 50 nm thick AZO layers alternated with few nanometers thick Y₂O₃ layers were prepared by pulsed laser deposition on Al₂O₃ single crystals by alternate ablation of AZO target and Y₂O₃ target. The number of laser shots on Y₂O₃ target was maintained very low (5, 10 and 15 pulses in three separate experiments. The main phase (AZO) presents polycrystalline orientation and typical columnar growth not affected by the presence of Y2O3 nanolayers. The multilayer with 15 laser shots of Y₂O₃ showed best thermoelectric performance with electrical conductivity σ = 48 S/cm and Seebeck coefficient S = -82 μV/K, which estimate power factor (S²·σ) about 0.03 × 10^-3 W m^-1 K^-2 at 600 K. The value of thermal conductivity (κ) was found 10.03 W m^-1 K^-1 at 300 K, which is one third of typical value previously reported for bulk AZO. The figure of merit, ZT = S²·σ·T/κ, is calculated 9.6 × 10^-4 at 600 K. These results demonstrated the feasibility of nanoengineered defects insertion for the depression of thermal conductivity.