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 CaxCoO2 secondary phase. Formation of CaxCoO2 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 CaxCoO2 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.