Resistivity anisotropy of layered cobaltite Na x CoO2 thin films

Abstract

Layered cobaltite NaxCoO2 thin films with tilted c-axis have been grown on vicinal cut SrTiO3 (100) substrates by pulse laser deposition. The single phase and the epitaxial growth aligned to the c-axis of vicinal cut substrates have been studied by X-ray diffraction analysis. The resistivity measurements along and perpendicular to the substrate tilted direction in the plane, and the physical model that current in the inclined direction is composed of the in-plane and out-of-plane parts are employed to attain the resistivity anisotropy ρc/ρab. In NaxCoO2, ρc is larger than ρab, which is consistent with its structure that insulating Na+ layers stack along c-axis. ρc/ρab decreases with increasing Na concentration due to the enhancement of inter-plane electron hopping for large x. The charge order insulator Na0.5+δCoO2 has a sharp rise of ρc/ρab when temperature below 50 K. These observations can improve the present understanding of the layered cobaltite thermoelectric oxides.