Preparation and anisotropic thermoelectric properties in misfit cobaltite thin films

Abstract

We have investigated the control of crystal orientation in misfit layered cobaltite A/sub 3/Co/sub 4/O/sub 9/ (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 CoO/sub 2/ layers (/spl par/) and perpendicular to the CoO/sub 2/ layers (/spl perp/)) were measured using the epitaxial films with c-axis parallel to the film plane. The resistivity for Sr/sub 3/Co/sub 4/O/sub 9/ parallel to the CoO/sub 2/ layers (/spl rho//sub /spl par//) is about 2.5 m/spl Omega/ cm at room temperature while that perpendicular to the CoO/sub 2/ layers (/spl rho//sub /spl perp//) is about 150 m/spl Omega/ cm. Sr/sub 3/Co/sub 4/O/sub 9/ has lower parallel and perpendicular resistivity than Ca/sub 3/Co/sub 4/O/sub 9/. As for thermoelectric properties of Sr/sub 3/Co/sub 4/O/sub 9/, Seebeck coefficient parallel to the CoO/sub 2/ layers (S/sub /spl par//) is 90 /spl mu/V/K and that perpendicular to the CoO/sub 2/ layers (S/sub /spl perp//) is 30 /spl mu/V/K at room temperature. The power factor parallel to the CoO/sub 2/ layers is calculated to be 3/spl times/10/sup -4/ W/mK/sup 2/, about two times higher than that for Ca/sub 3/Co/sub 4/O/sub 9/. The anisotropic value of resistivity for Ca/sub 3/Co/sub 4/O/sub 9/ and Sr/sub 3/Co/sub 4/O/sub 9/ is about 40 and 60, respectively, and that of Seebeck Coefficient is similarly about 3.