Mechanism of Formation of the Thermoelectric Layered Cobaltate Ca₃Co₄O₉ by Annealing of CaO–CoO Thin Films

The layered cobaltate Ca₃Co₄O₉ is of interest for energy-harvesting and heat-conversion applications because of its good thermoelectric properties and the fact that the raw materials Ca and Co are nontoxic, abundantly available, and inexpensive. While single-crystalline Ca₃Co₄O₉ exhibits high Seebeck coefficient and low resistivity, its widespread use is hampered by the fact that single crystals are too small and expensive. A promising alternative approach is the growth of highly textured and/or epitaxial Ca₃Co₄O₉ thin films with correspondingly anisotropic properties. Here, we present a two-step sputtering/annealing method for the formation of highly textured virtually phase-pure Ca₃Co₄O₉ thin films by reactive cosputtering from Ca and Co targets followed by an annealing process at 730 °C under O₂-gas flow. The thermally induced phase transformation mechanism is investigated by in situ time-resolved annealing experiments using synchrotron-based 2D X-ray diffraction (XRD) as well as ex situ annealing experiments and standard lab-based XRD. By tuning the proportion of initial CaO and CoO phases during film deposition, the method enables synthesis of Ca₃Co₄O₉ thin films as well as Ca_xCoO₂. With this method, we demonstrate production of epitaxial Ca₃Co₄O₉ thin films with in-plane electrical resistivity of 6.44 mΩ cm and a Seebeck coefficient of 118 μV K⁻¹ at 300 K.