Anisotropic carrier transport properties in layered cobaltate epitaxial films grown by reactive solid-phase epitaxy

Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 epitaxial film grown on the m-plane of an α-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroepitaxially grown with the CoO2 conducting planes inclined by ∼43° against the α-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was ∼1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.