Atomically resolved EELS mapping of the interfacial structure of epitaxially strained LaNiO3/LaAlO3 superlattices

The interfacial atomic structure of a metallic LaNiO3/LaAlO3 superlattice grown on a LaSrAlO4 substrate was investigated using a combination of atomically resolved electron energy loss spectroscopy (EELS) at the Al K, Al L-2,L-3, Sr L-2,L-3, Ni L-2,L-3, La M-4,M-5, and O K edges as well as hybridization mapping of selected features of the O K-edge fine structure. We observe an additional La1-xSrxAl1-yNiyO3 layer at the substrate-superlattice interface, possibly linked to diffusion of Al and Sr into the growing film or a surface reconstruction due to Sr segregation. The roughness of the LaNiO3/LaAlO3 interfaces is found to be on average around one pseudocubic unit cell. The O K-edge EELS spectra revealed reduced spectral weight of the prepeak derived from Ni-O hybridized states in the LaNiO3 layers. We rule out oxygen nonstoichiometry of the LaNiO3 layers and discuss changes in the Ni-O hybridization due to heterostructuring as possible origin.