Resolving nanograin morphology and sub-grain nanodomains via scanning probe acoustic microscopy in high energy density BaTiO3 ferroelectric films

Energy storage property of a dielectric is closely tied with its nanostructure. In this study, we aim to achieve a deep understanding of this relationship in high energy density ferroelectric ceramic films, by probing into the nanograin and sub-grain nanostructures in polycrystalline BaTiO₃ films integrated on Si. Through scanning probe acoustic microscopy analyses, it is revealed that the BaTiO₃ films directly grown on Pt/Ti/Si mostly consist of large discontinuous columnar nanograins, while those grown on LaNiO₃-buffered Pt/Ti/Si substrates have a dominant microstructure of continuous columnar nanograins. Furthermore, ultrafine ferroelastic domains of ∼10 nm wide are revealed inside the grains of the buffered BaTiO₃ films, while those unbuffered films show about ∼50% increase in the domain width. The dielectric properties of the BaTiO₃ films are well correlated with their characteristic nanostructures. Under an increasing electric field, the LaNiO₃-buffered films display a slower decline in its dielectric constant and a later saturation of its electric polarization, leading to an improved energy storage performance. Device-level charge-discharge tests have verified not only the delayed polarization saturation and high energy density of the LaNiO₃-buffered BaTiO₃ film capacitors, but also a high power density in the same order as those of the ferroelectric ceramics.