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Page 2 of 10 Ouyang et al. Microstructures 2023;3:2023027 https://dx.doi.org/10.20517/microstructures.2023.22

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reached 46.6 J/cm and 48.8 J/cm at an applied electric field of 2.31 MV/cm (300 V on 1.3 μm film) and 1.77
MV/cm (460 V on 2.6 μm film), respectively. This ramp-up in energy density can be attributed to increased
polarizability with a growing grain size in thicker polycrystalline films and is desirable in high pulse power
applications.

Keywords: Energy storage, ferroelectric, grain engineering, BaTiO , film capacitors, Si
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INTRODUCTION
With the development of various energy-harvesting technologies and associated applications, devices that
can provide both high-density storage and rapid electricity supply have become increasingly important.
Dielectric capacitors using ferroelectric (FE) ceramics with a high relative dielectric permittivity have
[1-5]
demonstrated such a potential . However, there are compromises that hinder the further development of
FE ceramic capacitors for energy storage applications. These compromises include complex compositions,
high composition sensitivity, and high processing temperatures, which are especially challenging for
integrated film capacitors . Therefore, the development of simple composition FE film capacitors with a
[6-8]
high energy density, such as BaTiO , has become an emergent challenge for the future of electric energy
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storage using dielectric capacitors.

Once a manufacturable composition was selected, the performance of FE film capacitors was mostly
[10]
[9]
affected by the substrate, which imposes a clamping effect and a residual strain , the deposition
chemistry , and the crystalline or grain characteristics [12,13] . These factors are usually intertwined, and hence
[11]
it is difficult to separate out their contributions to the energy storage performance of the film. In this work,
we chose the prototype FE of BaTiO for its simple composition and broad application in electro ceramics,
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especially ceramic capacitors. Established deposition chemistry, i.e., sputtering deposition in an oxygen-rich
atmosphere, was used for the preparation of BaTiO film capacitors on a Si substrate . Therefore, in this
[13]
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work, we were able to focus on the grain effects on the energy storage performance of the FE film capacitors,
including the effects of grain orientation and grain morphology.
Our previous investigations on preparing high energy density BaTiO -based film capacitors [13-15] have
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focused on a columnar nanograin design via a low-temperature process (350-400 °C) and a buffer-layer
technique. This approach resulted in a densely-packed nanograin array, which shows a (001)-texture, an in-
plane grain diameter down to 10-20 nm, and a grain length extending through the thickness of the film. Due
to periodic gradients in the electric polarization and relative dielectric permittivity, the total dielectric
displacement of such a film is confined and experiences a significantly delayed saturation under an external
field. Consequently, such a grain structure is endowed with a high breakdown field and a large energy
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storage density. For example, a recyclable energy density of ~130 J/cm at ~650 MV/m and ~229 J/cm at
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[13]
875 MV/m was achieved in columnar-grained BaTiO films of ~350 nm and ~160 nm thick, respectively.
[15]
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However, these high energy densities were achieved under very large electric fields, corresponding to a
much-reduced relative dielectric permittivity. This is not always the optimum solution for the storage and/
or supply of electric energy, especially in cases where high pulsed power is demanded under middle-to-low
electric fields.
In this work, we aimed to manipulate the grain characteristics, including grain orientation and morphology,
to achieve a high energy density at a reduced electric field in BaTiO film capacitors. To achieve this goal, a
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higher growth temperature at 500 °C was used, and a compromise was made between improved
crystallinity, which enhances the dielectric response, and increased average grain size, which reduces the