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

Abstract

In thin epitaxial films of Ruddlesden–Popper oxides [A_n+1B_nO_3n+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.