Growth, structural, and transport properties of epitaxial NaxCoO$_{2}$ thin films

Layered cobaltate NaxCoO$_{2}$ has attracted much attention recently due to its superconductivity and exceptionally high thermoelectric power. Here we report structural, electrical, and thermopower properties of epitaxial and topotaxial Na$_{x}$CoO$_{2}$ thin films on (0001) sapphire substrate. Topotaxial Na$_{x}$CoO$_{2}$ films were prepared by converting an epitaxial Co$_{3}$O$_{4}$ film to Na$_{x}$CoO$_{2 }$by annealing in Na vapor and epitaxial Na$_{x}$CoO$_{2 }$films were obtained by pulsed laser deposition. Structural analysis showed the films are $c$-axis oriented. For topotaxial films, annealing in different Na vapor pressures resulted in films with different Na concentrations, which showed distinct transport properties. For directly deposited epitaxial films by pulsed laser deposition, deposition parameters are found to control the Na concentration and hence the film properties. The largest thermoelectric power of the samples made by different methods is found to be similar in the range of 70-100 $\mu $V/K at room temperature.