  We have fabricated Na_xCoO_2 thin films via lateral diffusion of sodium into
Co_3O_4 (111) epitaxial films (reactive solid-phase epitaxy: Ref. 4). The
environment of thermal diffusion is key to the control of the sodium content in
thin films. From the results of x-ray diffraction and in-plane resistivity, the
epitaxial growth and the sodium contents of these films were identified. The
thermoelectric measurements show a large thermoelectric power similar to that
observed in single crystals. The quasiparticle scattering rate is found to
approach zero at low temperatures, consistent with the small residual
resistivity, indicating high quality of the Na_xCoO_2 thin films.
