Electron energy-loss study of the electronic structure of atomic scale SrTiO3-SrMnO3-LaMnO3 superlattices

SrTiO(3) (STO), SrMnO(3) (SMO), and LaMnO(3) (LMO) ultrathin films with layer thicknesses of 1-4 perovskite unit cells were grown on a STO substrate. The physical and electronic structures of ultrathin films and interfaces were characterized with nanoarea electron diffraction, scanning transmission electron microscopy (STEM), and electron energy-loss spectroscopy (EELS). The diffraction patterns show that the epitaxial growth of LMO, SMO, and STO with strain is closely lattice matched to the STO substrate. High resolution Z-contrast STEM images clearly show the superlattice structure with atomically sharp interfaces. EELS shows changes in the density of unoccupied states of oxygen, which is present in all films, and of titanium at the STO-SMO interface. We find that the O K edge in layered LMO shifts depending on the thickness of the film. No peak shifts of Mn in LaMnO(3) and SrMnO(3) are observed within our instrument resolution, indicating that the valence of the Mn ion is approximately constant across the interface while the electron density of oxygen atoms accepts the local charge transfer.