Transport and Thermoelectric Properties of Ca$_3$Co$_4$O$_9$ Thin Films

It has been discovered recently that cobaltates have very large thermoelectric power, which shows that cobaltates hold great promise to be potential integrated heating spreading solution, such as thermal management of microprocessors. Among the cobaltates, Ca$_{3}$Co$_{4}$O$_{9}$ and Ca$_{2}$Co$_{2}$O$_{5}$ are exhibiting best thermoelectric properties. The \textit{ZT} value for these calcium-cobaltates reaches as high as 2.7 at $T \quad \ge $ 873 K, which clearly challenges the best conventional thermoelectric materials found in intermetallic compounds, such as Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ alloys. The purpose of this work is to study the transport and thermoelectric properties of Ca$_{3}$Co$_{4}$O$_{9}$ thin films. We have successfully grown the Ca$_{3}$Co$_{4}$O$_{9}$ c-axis orientated thin films using Pulsed Laser Deposition (PLD) technique on various substrates, including Si, LaAlO$_{3}$, Al$_{2}$O$_{3}$. The resistivity and thermoelectric power measurements show that these films have superior thermoelectric properties, similar to that found in the bulk samples. The detailed transport and thermoelectric properties of Ca$_{3}$Co$_{4}$O$_{9}$ thin films will be discussed. This work was supported by the U. S. Dept. of Energy, Office of Basic Energy Science, under contract No. DE-AC-02-98CH10886.