Oxygen deficiency provides abundant space for manipulating electronic and magnetic properties of epitaxial functional complex oxide thin films.(1-2) During epitaxial growth of these thin films, oxygen vacancies may originate from lattice-mismatch-induced strain by lowering their formation energy.(3) The presence of crystal defects, e.g. misfit dislocations, will however partially release the misfit strain and, therefore, affect the materials’ behavior by consequently adapting and altering the oxygen vacancy profile. A comprehensive understanding of the formation of oxygen vacancies at interfaces and their implication for the materials’ structural and physical properties is rather crucial.
Here, by using an aberration-corrected scanning transmission electron microscope (STEM) equipped with an electron energy-loss spectrometer (EELS), we performed atomic-scale investigations of the microstructure and electronic structure at the hetero-interface between SrTiO3 (STO) and SrMnO3 (SMO). Atomic-column-resolved high-angle annular dark-field (HAADF) and annular bright-field (ABF) images, which were simultaneously acquired, were used to evaluate the local lattice distortions and oxygen sub-lattice at the interface. The structural analysis yields a significant variation of the strontium-strontium distance near the hetero-interface. Using STEM-EELS line scans from the STO substrate across the hetero-interface to the surface of the SMO thin film, we observe an asymmetric cationic intermixing at the interface between STO and SMO. In order to estimate the influence of electron beam spreading, a simulation of the STEM-EELS spectrum image based on the Quantum Excitation of Phonons (QEP) model was performed at the hetero-interface demonstrating only weak beam-spreading effects and confirming the asymmetric cationic intermixing. The energy-loss near-edge structure (ELNES) of the Mn-L2, 3 edges close to and at the hetero-interface unveils a variation of the manganese oxidation state suggesting the formation of oxygen vacancies accompanied by strain variation. This work paves the way for understanding and controlling the interfacial structural and physical properties of manganite hetero-interfaces.
Keywords: aberration-corrected scanning transmission electron microscope, energy-loss near-edge structure, oxygen deficiency