Elemental intermixing within an ultrathin SrRuO₃ electrode layer in epitaxial heterostructure BaTiO₃/SrRuO₃/SrTiO₃

Aberration corrected scanning transmission electron microscopy is used to directly observe atom columns in an epitaxial BaTiO₃ thin film deposited on a 3.6 nm-thick SrRuO₃ electrode layer above an SrTiO₃ (001) substrate. Compositional gradients across the heterointerfaces were examined using electron energy-loss spectroscopy techniques. It was found that a small amount of Ba and Ti had diffused into the SrRuO₃ layer, and that this layer contained a non-negligible concentration of oxygen vacancies. Such point defects are expected to degrade the electrode's electronic conductivity drastically, resulting in a much longer screening length. This may explain the discrepancy between experimental measurements and theoretical estimates of the ferroelectric critical thickness of a BaTiO₃ ferroelectric barrier sandwiched between metallic SrRuO₃ electrodes, since theoretical calculations generally assume ideal (stoichiometric) perovskite SrRuO₃.