Kinetically controlled thin-film growth of layered β - and γ-Nax Co O2 cobaltate

We report growth characteristics of epitaxial Β- Na0.6 Co O2 and γ- Na0.7 Co O2 thin films on (001) sapphire substrates grown by pulsed-laser deposition. Reduction of the deposition rate could change the structure of the Nax Co O2 thin film from a Β phase with an island growth mode to a γ phase with a layer-by-layer growth mode. The γ- Na0.7 Co O2 thin film exhibits spiral surface growth with multiterraced islands and highly crystallized texture compared to that of the Β- Na0.6 Co O2 thin film. This heterogeneous epitaxial film growth can give an example of the strain effect of physical properties and growth dynamics of Nax Co O2 as well as the subtle nature of structural change.