Preparation and anisotropic thermoelectric properties in misfit cobaltite thin films

We have investigated the control of crystal orientation in misfit layered cobaltite A₃Co₄O₉ (A=Ca, Sr) thin films and succeeded in growing epitaxial films with c-axis orientation perpendicular and parallel to the substrate surface by rf-planar magnetron sputtering. Anisotropic transport properties (parallel to the CoO₂ layers (||) and perpendicular to the CoO₂ layers (⊥)) were measured using the epitaxial films with c-axis parallel to the film plane. The resistivity for Sr₃Co₄O₉ parallel to the CoO₂ layers (ρ_||) is about 2.5 mΩ cm at room temperature while that perpendicular to the CoO₂ layers (ρ_⊥) is about 150 mΩ cm. Sr₃Co₄O₉ has lower parallel and perpendicular resistivity than Ca₃Co₄O₉. As for thermoelectric properties of Sr₃Co₄O₉, Seebeck coefficient parallel to the CoO₂ layers (S_||) is 90 μV/K and that perpendicular to the CoO₂ layers (S_⊥) is 30 μV/K at room temperature. The power factor parallel to the CoO₂ layers is calculated to be 3×10^-4 W/mK², about two times higher than that for Ca₃Co₄O₉. The anisotropic value of resistivity for Ca₃Co₄O₉ and Sr₃Co₄O₉ is about 40 and 60, respectively, and that of Seebeck Coefficient is similarly about 3.