Effect of substrate on the atomic structure and physical properties of thermoelectric Ca3Co4O9 thin films

The incommensurately layered cobalt oxide Ca₃Co ₄O₉ 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 Ca₃Co₄O₉ thin films grown on cubic perovskite SrTiO₃, LaAlO₃, and (La _0.3Sr_0.7)(Al_0.65Ta_0.35)O ₃ substrates and on hexagonal Al₂O₃ (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 CoO₂ layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca₂CoO₃ buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO₂ 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 Ca₃Co₄O₉ films due to additional phonon scattering sites, necessary for improved thermoelectric properties.