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.