Microstructural accommodation of excess Ru in epitaxial SrRuO₃ films

The microstructures of Ru-excess SrRuO₃ films, which were grown epitaxially on SrTiO₃(001) substrates by ion-beam sputtering, were studied by transmission electron microscopy. The excess Ru can be accommodated by forming extended defects on the {100} planes, faulted dislocation loops, by making RuO₂ double layers. However, the most stable crystalline phase of the excess Ru in SrRuO₃ film was metallic Ru with a hexagonal structure. The orientation relationship between the Ru precipitates, the SrTiO₃ substrate, and the SrRuO₃ film can be described as follows: (011)_Ru//(002)_STO//(002)_SRO and [100]_Ru//[110]_STO//[110]_SRO where the subscripts STO and SRO indicate SrTiO₃ and SrRuO₃ respectively. Owing to the difference between the crystal symmetries of Ru and SrTiO₃, the precipitates showed different in-plane alignments along two perpendicular directions on the substrate and accordingly different anisotropic growth morphologies. The precipitates degrade the film surface by making deep trenches and act as sources for defect generation.