Dielectric ultracapacitors based on columnar nano-grained ferroelectric oxide films with gradient phases along the growth direction

In this work, dielectric ultracapacitors were designed and fabricated using the combination of the phase boundary and nanograin strategies. Such ultracapacitors are based on submicron-thick Ba(Zr_0.2Ti_0.8)O₃ ferroelectric films sputter-deposited on Si at 500 ℃. With a composition near a PPB, a compressive strain and a high nucleation rate due to the lowered deposition temperature, these films exhibit a columnar nanograined microstructure with gradient phases along the growth direction. Such a micro-structure presents three-dimensional polarization inhomogeneities on a nanoscale, thereby significantly delays the saturation of the overall electric polarization. Consequently, a pseudo-linear, ultra-slim polarization-electric field hysteresis loop was obtained, featuring a high maximum applicable electric field (~5.7 MV/cm), a low remnant polarization (~5.2 μC/cm²) and a high maximum polarization (~92.1 μC/cm²). Such a P-E loop corresponds to a high recyclable energy density (W_rec~208 J/cm³) and charge-discharge efficiency (~88%). An in-depth electron microscopy study revealed that the gradient ferroelectric phases consist of tetragonal (T) and rhombohedral (R) polymorphs along the growth direction of the film. The T-rich phase is abundant near the bottom of the film and gradually transformed into the R-rich one near the surface. These films also exhibited a high Curie temperature of ~460 ℃ and a stable capacitive energy storage up to 200 ℃. These results suggest a feasible pathway for design and fabrication of high-performance dielectric film capacitors.