An investigation was made of  properties of Ca3Co4O9 thin films grown onto cubic perovskite SrTiO3, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates and onto hexagonal Al2O3 (sapphire) substrates using the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy analysis indicated strain-free growth of films, regardless of the substrate. However, depending upon the lattice and symmetry mismatch, defect-free growth of the hexagonal CoO2 layer was stabilized only after a critical thickness and, in general, the formation of a stable Ca2CoO3 buffer layer was observed near to the substrate/film interface. Beyond this critical thickness, a large concentration of CoO2 stacking faults was observed, possibly due to weak interlayer interaction in this layered material. It was proposed that these stacking faults had a significant impact on the Seebeck coefficient and higher values were found in thinner Ca3Co4O9 films due to additional phonon scattering sites, necessary for improved thermoelectric properties.

Effect of Substrate on the Atomic Structure and Physical Properties of Thermoelectric Ca3Co4O9 Thin Films. Q.Qiao, A.Gulec, T.Paulauskas, S.Kolesnik, B.Dabrowski, M.Ozdemir, C.Boyraz, D.Mazumdar, A.Gupta, R.F.Klie: Journal of Physics - Condensed Matter, 2011, 23[30], 305005