Kinetically-controlled thin-film growth of layered $\beta$- and $\gamma-$Na$_{x}$CoO$_{2}$ cobaltate

TL;DR

This study investigates how controlling the kinetic parameters during pulsed-laser deposition influences the phase, growth mode, and surface morphology of epitaxial Na$_x$CoO$_2$ thin films, revealing phase-dependent growth behaviors.

Contribution

It demonstrates that adjusting the deposition rate can switch the film structure between $eta$ and $\gamma$ phases and affects surface morphology, providing insights into strain effects and growth dynamics.

Findings

Reduction of deposition rate changes structure from $eta$ to $\gamma$ phase.

$\gamma$-Na$_{0.7}$CoO$_{2}$ exhibits spiral surface growth.

Heterogeneous epitaxial growth influences physical properties and structural transitions.

Abstract

We report growth characteristics of epitaxial $\beta$-Na$_{0.6}$CoO$_{2}$ and $\gamma$-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 $\beta$-phase with island growth mode to $\gamma$-phase with layer-by-layer growth mode. The $\gamma$-Na$_{0.7}$CoO$_{2}$ thin film exhibits spiral surface growth with multiterraced islands and highly crystallized texture compared to that of the $\beta$-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.