Anisotropic thermoelectric properties in layered cobaltite AxCoO2 (A=Sr and Ca) thin films

We have fabricated epitaxial thin films of layered cobaltite AxCoO2 (A=Sr and Ca) on sapphire substrates by rf magnetron sputtering. The grown phase in the AxCoO2 films was found to be a monoclinic β-phase of primitive layered cobaltites and the epitaxial orientation of the film could be controlled by the surface plane of the substrates. The resistivity parallel to the CoO2 layers ρ‖ for the β-SrxCoO2 is as low as 2mΩcm at room temperature and shows metallic behavior. The ratio of perpendicular resistivity ρ⊥ to ρ‖ increases from 20 at room temperature to 90 at 3K. More isotropic nature was observed in the Seebeck coefficient. Parallel Seebeck coefficients S‖ of AxCoO2 are approximately 60μV∕K at room temperature and the perpendicular S⊥ are about half of S‖.