Influence of pulsed laser deposition rate on the microstructure and thermoelectric properties of Ca₃Co₄O₉ thin films

The effects of deposition rate on the microstructure and thermoelectric (TE) properties of Ca₃Co₄O₉ thin films fabricated by pulsed laser deposition (PLD) technique were investigated. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) revealed that a fast deposition rate resulted in not only low crystallinity but also the existence of the Ca_xCoO₂ secondary phase. Formation of Ca_xCoO₂ was inevitable during the thin film growth, and this was discussed from both structural and compositional point of view. With longer deposition interval or with sufficient oxygen at a lower deposition rate, the Ca_xCoO₂ phase was able to transit into the desired Ca₃Co₄O₉ phase during the coalescence process. The quality of the thin films was further analyzed by electrical properties measurements. The Ca₃Co₄O₉ thin film fabricated at a slower deposition rate was found to exhibit a low electrical resistivity of 9.4 mΩ cm and high Seebeck coefficient of 240 μV/K at about 700 °C, indicating a good quality film.