Nanoscale abnormal grain growth in (001) epitaxial ceria

Abstract

X-ray reciprocal-space mapping and atomic force microscopy (AFM) are used to study kinetics and mechanisms of lateral grain growth in epitaxial (001) ceria $({\text{CeO}}_{2})$ deposited by pulsed laser deposition on (001) yttria-stabilized zirconia (YSZ) and $(1\underset{}{2}10)$ ($r$-cut) sapphire. Rate and character of the grain growth during postannealing at $1050\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ are found to be strongly dependent on the type of the epitaxial substrate. Films deposited on YSZ exhibit signatures of normal grain growth, which stagnated after the lateral grain size reaches 40 nm, consistent with the grain-boundary pinning by the thermal grooving. In contrast, when $r$-cut sapphire substrate was used, abnormal (secondary) grain growth is observed. A small population of grains grow to well over 100 nm consuming smaller, $l10\text{ }\text{nm}$, grains, thus forming well-defined $g100\text{ }\text{nm}$ large (001) terminations and rendering the sample single-crystalline quality. The grain growth is accompanied by reduction in lateral rms strain, resulting in a universal grain size---rms strain dependence. Analysis of the AFM and x-ray diffraction data leads to the conclusion that bimodal initial grain population consisting of grains with very different sizes is responsible for initiation of the abnormal growth in (001) ${\text{CeO}}_{2}$ films on $r$-cut sapphire. Due to different surface chemistry, when a YSZ substrate is used, the initial grain distribution is monomodal, therefore only normal growth is active. We demonstrate that a $2.2\ifmmode^\circ\else\textdegree\fi{}$ miscut of the sapphire substrate eliminates the large-grain population, thus suppressing abnormal grain growth. It is concluded that utilization of abnormal grain growth is a promising way for synthesis of large (001) ceria terminations.