Kinetically-controlled thin-film growth of layered $β$- and $γ-$Na$_{x}$CoO$_{2}$ cobaltate

Abstract

We report growth characteristics of epitaxial $β$-Na$_{0.6}$CoO$_{2}$ and $γ$-Na$_{0.7}$CoO$_{2}$ thin films on (001) sapphire substrates grown by pulsed-laser deposition. Reduction of deposition rate could change structure of Na$_{x}$CoO$_{2}$ thin film from $β$-phase with island growth mode to $γ$-phase with layer-by-layer growth mode. The $γ$-Na$_{0.7}$CoO$_{2}$ thin film exhibits spiral surface growth with multiterraced islands and highly crystallized texture compared to that of the $β$-Na$_{0.6}$CoO$_{2}$ thin film. This heterogeneous epitaxial film growth can give opportunity of strain effect of physical properties and growth dynamics of Na$_{x}$CoO$_{2}$ as well as subtle nature of structural change.