Bidirectional energy filtering for electronic and phonon transport in Al2O3/ZnO superlattice films with anisotropy

Abstract

Directional anisotropy transport in particular in nanostructured superlattice films could be crucial to understanding their thermoelectric properties, including figure-of-merit (ZT). However, few experimental studies have considered anisotropic properties in superlattice films. Therefore, this study investigated electronic and phonon transport anisotropy for atomic layer deposition Al2O3 (AO)/ZnO superlattice films on SiO2/Si substrates at 77–500 K using the four-point-probe 3-ω method and in-house Seebeck measurement system as a first step toward understanding corresponding superlattice film properties. In-plane and out-of-plane ZT values for AO/ZnO superlattice films were determined as 0.00017–0.19 and 0.00035–0.44, respectively, at 77–500 K. Higher ZT for both directions can be explained by synergistic combination of enhanced phonon scattering and bidirectional energy filtering in phonon and electronic transport compared to undoped ZnO films over the temperature range. We also found that out-of-plane thermal conductivity suppression was mostly attributable to increased ZT anisotropy in the out-of-plane direction rather than enhanced AO/ZnO superlattice film Seebeck and power factors. © 2019 Elsevier B.V.