Effect of substrate on the atomic structure and physical properties of thermoelectric Ca_3Co_4O_9 thin films

摘要

The incommensurately layered cobalt oxide Ca_3Co _4O_9 exhibits an unusually high Seebeck coefficient as a polycrystalline bulk material, making it ideally suited for many high temperature thermoelectric applications. In this paper, we investigate properties of Ca_3Co_4O_9 thin films grown on cubic perovskite SrTiO_3, LaAlO_3, and (La _(0.3)Sr_(0.7))(Al_(0.65)Ta_(0.35_)O _3 substrates and on hexagonal Al_2O_3 (sapphire) substrates using the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy analysis indicate strain-free growth of films, irrespective of the substrate. However, depending on the lattice and symmetry mismatch, defect-free growth of the hexagonal CoO_2 layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca_2CoO_3 buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO_2 stacking faults is observed, possibly due to weak interlayer interaction in this layered material. We propose that these stacking faults have a significant impact on the Seebeck coefficient and we report higher values in thinner Ca_3Co_4O_9 films due to additional phonon scattering sites, necessary for improved thermoelectric properties