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

We have fabricated epitaxial thin films of layered cobaltite A(x)CoO(2) (A=Sr and Ca) on sapphire substrates by rf magnetron sputtering. The grown phase in the A(x)CoO(2) films was found to be a monoclinic beta-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 rho(parallel to) for the beta-SrxCoO2 is as low as 2 mOmega cm at room temperature and shows metallic behavior. The ratio of perpendicular resistivity rho(perpendicular to) to rho(parallel to) increases from 20 at room temperature to 90 at 3 K. More isotropic nature was observed in the Seebeck coefficient. Parallel Seebeck coefficients S-parallel to of A(x)CoO(2) are approximately 60 muV/K at room temperature and the perpendicular S-perpendicular to are about half of S-parallel to. (C) 2004 American Institute of Physics.