Thickness-dependent intergrowth of Ruddlesden–Popper impurity structures in solid-phase epitaxial growth of Ca₂RuO₄ thin films

In thin epitaxial films of Ruddlesden–Popper oxides [An+1BnO3n+1 (n = 1, 2, …)], the formation of intergrowth phases with varying n values is commonly observed as lattice impurity defects. These intergrown impurities often exert a pronounced influence on film properties, prompting extensive investigations into formation mechanisms for better control. In this study, we demonstrate that the intergrowth of an A3B2O7-structured impurity with n = 2 is notably influenced by the thickness of epitaxial thin-film growth of a Ruddlesden–Popper oxide Ca2RuO4 with n = 1 in solid-phase epitaxy. Employing structural analyses via X-ray diffraction and scanning transmission electron microscopy, we discovered an unusual intensification of A3B2O7 intergrowth within Ca2RuO4 thin films with reduced thicknesses of ≤25 nm. We further validated the significant impact of this atypical intergrowth on the electrical transport properties of Ca2RuO4 thin films through resistivity–temperature measurements. The source of the anomalous thickness-dependent behavior in A3B2O7 intergrowth was discussed based on the insights from the structural characterizations.