Ferroelectric oxide films with a large field-induced polarization can be used in dielectric capacitors for charge or energy storage in microelectronic systems and hence have attracted intense research interest. A high processing temperature is usually required to produce a well-crystallized polar phase and hence a large polarization in the film, corresponding to a high charge or energy density. However, high processing temperature not only reduces the charge‒discharge efficiency by producing a sizable remnant polarization but is also incompatible with the integration process. In this study, we address this problem by creating a large field-induced polarization (~55.8 μC/cm²) in BaTiO₃ films sputter-deposited on Si at 200 °C via a buffer-layer technique. This large polarization led to a high energy density and efficiency (W_rec ≈ 94.7 J/cm³, η ≈ 78.2%@4 MV/cm). The thickness of LaNiO₃ buffer layer was revealed to be the key factor determining the electric polarization (remnant and field-induced ones). A 50 nm LaNiO₃ thickness, corresponding to the aforementioned polarization and energy storage performance, not only ensures proper crystallization in the BaTiO₃ film, but also leads to an optimal combination of polycrystalline grains with a high dielectric constant. The latter accounts for the majority of the field-induced polarization. Our results revealed the key role of a buffer layer in tuning the microstructure of a low-temperature deposited ferroelectric oxide film. Furthermore, the excellent charge/energy storage performance of these 200 °C-deposited BaTiO₃ films has provided many opportunities for this simple dielectric in microelectronics.

In this work, thick BiFeO₃ films (~1 μm) were prepared on LaNiO₃-buffered (111)Pt/Ti/SiO₂/(100)Si substrates via radio-frequency magnetron sputtering without post-growth annealing. The effects of the substrate temperature on the film’s crystallinity, defect chemistry, and associated electrical properties were investigated. In contrast to the poorly crystallized BiFeO₃ film deposited at 300 °C and the randomly-oriented and (111)-textured films deposited at 500 and 650 °C, respectively, a (001)-preferred orientation was achieved in the BiFeO₃ film deposited at 350 °C. This film not only showed a dense, fine-grained morphology but also displayed enhanced electrical properties due to the (001) texture and improved defect chemistry. These properties include a reduced leakage current (J ≈ 2.4×10⁻⁵ A/cm²@200 kV/cm), a small dielectric constant (ε_r ≈ 243–217) with a low loss (tanδ ≤ 0.086) measured from 100 Hz to 1 MHz, and a nearly intrinsic remnant polarization (P_r) of ~60 μC/cm². A detailed TEM analysis confirmed the R3c symmetry of the BFO films and hence ensured good stability of their electrical properties. In particular, single-beam cantilevers fabricated from BiFeO₃/LaNiO₃/Pt/Ti/SiO₂/Si heterostructures showed excellent electromechanical performance, including a large transverse piezoelectric coefficient (e_31,f) of ~−2.8 C/m², a high figure of merit (FOM) parameter of ~4.0 GPa, and a large signal-to-noise ratio of ~1.5 C/m². An in-depth analysis revealed the intrinsic nature of the e_31,f piezoelectric coefficient, which is well fitted along a straight line of e_31,f ratio = (ε_rP_r) ratio with the reported representative results. These high-quality lead-free piezoelectric films processed with a reduced thermal budget can open many possibilities for the integration of piezoelectricity into Si-based micro-electro–mechanical systems (MEMSs).

In this work, dielectric ultracapacitors were designed and fabricated using a combination of phase boundary and nanograin strategies. These ultracapacitors are based on submicron-thick Ba(Zr_0.2Ti_0.8)O₃ ferroelectric films sputter-deposited on Si at 500 °C. With a composition near a polymorphic phase boundary (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 microstructure presents three-dimensional polarization inhomogeneities on the nanoscale, thereby significantly delaying the saturation of the overall electric polarization. Consequently, a pseudolinear, ultraslim polarization (P)–electric field (E) hysteresis loop was obtained, featuring a high maximum applicable electric field (~5.7 MV/cm), low remnant polarization (~5.2 μC/cm²) and high maximum polarization (~92.1 μC/cm²). This 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 consisted 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 transforms into the R-rich phase near the surface. These films also exhibited a high Curie temperature of ~460 °C and stable capacitive energy storage up to 200 °C. These results suggest a feasible pathway for the design and fabrication of high-performance dielectric film capacitors.

Dielectric capacitors with ultrahigh power density and ultra-fast charge/discharge rate are highly desired in pulse power fields. Environmental-friendly AgNbO₃ family have been actively studied for its large polarization and antiferroelectric nature, which greatly boost the electric energy storage performance. However, high-quality AgNbO₃-based films are difficult to fabricate, leading to a low breakdown field E_b (<1.2 MV/cm) and consequently arising inferior energy storage performance. In this work, we propose an interface engineering strategy to mitigate the breakdown field issue. A Ag(Nb,Ta)O₃/BaTiO₃ bilayer film is proposed, where the BaTiO₃ layer acts as a p-type semiconductor while Ag(Nb,Ta)O₃ layer is n-type, together with the n-type LaNiO₃ buffer layer on the substrate, forming an n-p-n heterostructure. The n-p-n heterostructure elevates the potential barriers for charge transport, greatly reducing the leakage current. An extremely large breakdown field E_b~4.3 MV/cm is achieved, being the highest value up to date in the niobate system. A high recoverable energy density W_rec~62.3 J/cm³ and a decent efficiency η~72.3% are obtained, much superior to that of the Ag(Nb,Ta)O₃ monolayer film (W_rec~46.4 J/cm³ and η~80.3% at E_b~3.3 MV/cm). Our results indicate that interface engineering is an effective method to boost energy storage performance of dielectric film capacitors.

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.

To meet the expectation set by Moore’s law on transistors, the search for thickness-scalable high dielectric constant (k) gate layers has become an emergent research frontier. Previous investigations have failed to solve the “polarizability–scalability–insulation robustness” trilemma. In this work, we show that this trilemma can be solved by using a gate layer of a high k ferroelectric oxide in its superparaelectric (SPE) state. In the SPE, its polar order becomes local and is dispersed in an amorphous matrix with a crystalline size down to a few nanometers, leading to an excellent dimensional scalability and a good field-stability of the k value. As an example, a stable high k value (37±3) is shown in ultrathin SPE films of (Ba_0.95,Sr_0.05)(Zr_0.2,Ti_0.8)O₃ deposited on LaNiO₃-buffered Pt/Ti/SiO₂/(100)Si down to a 4 nm thickness, leading to a small equivalent oxide thickness of ~0.46 nm. The aforementioned characteristic microstructure endows the SPE film a high breakdown strength (~10.5 MV·cm⁻¹ for the 4 nm film), and hence ensures a low leakage current for the operation of the complementary metal oxide semiconductor (CMOS) gate. Lastly, a high electrical fatigue resistance is displayed by the SPE films. These results reveal a great potential of superparaelectric materials as gate dielectrics in the next-generation microelectronics.

In the research field of energy storage dielectrics, the “responsivity” parameter, defined as the recyclable/recoverable energy density per unit electric field, has become critically important for a comprehensive evaluation of the energy storage capability of a dielectric. In this work, high recyclable energy density and responsivity, i.e., W_rec = 161.1 J·cm^–3 and ξ = 373.8 J·(kV·m²)^–1, have been simultaneously achieved in a prototype perovskite dielectric, BaTiO₃, which is integrated on Si at 500 ℃ in the form of a submicron thick film. This ferroelectric film features a multi-scale polar structure consisting of ferroelectric grains with different orientations and inner-grain ferroelastic domains. A LaNiO₃ buffer layer is used to induce a {001} textured, columnar nanograin microstructure, while an elevated deposition temperature promotes lateral growth of the nanograins (in-plane diameter increases from ~10–20 nm at lower temperatures to ~30 nm). These preferably oriented and periodically regulated nanograins have resulted in a small remnant polarization and a delayed polarization saturation in the film’s P–E behavior, leading to a high recyclable energy density. Meanwhile, an improved polarizability/dielectric constant of the BaTiO₃ film has produced a much larger maximum polarization than those deposited at lower temperatures at the same electric field, leading to a record-breaking responsivity for this simple perovskite.

The excellent energy storage performances of dielectric materials, a high energy density and efficiency, the stability in a wide range of temperature, frequency and cycling time, are surely desirable for the energy storage devices. A trade-off relationship between polarization and breakdown strength, however, limits the enhancement of energy storage properties of dielectric materials. To effectively boost the energy density and efficiency of dielectric capacitors, by inserting a BiFeO₃ layer into the BaTiO₃ film in present case, the symmetric BaTiO₃/BiFeO₃/BaTiO₃ tri-layer film heterostructure with antiferroelectric-like characteristics was constructed based on the dual-interlayer coupling effect, what's more, its antiferroelectric-like characteristics will evolve with electric field. Such the tunable polarization behavior endows it with an enhanced maximum polarization but a reduced remnant one, a delayed saturation of polarization and a high breakdown strength, which are synergistically accountable for a large energy density (W_rec~109 J/cm³) and a high efficiency (η~82.6%), together with the good thermal (T_R~200 ℃, ΔW_rec<3% & Δη<10%) and frequency (50 Hz–10 kHz, ΔW_rec<7% & Δη<13%) stabilities, particularly an outstanding cycling reliability (10⁹ cycles, both ΔW_rec and Δη<1%). Hence these findings can provide some innovative ideas for enriching the performance tuning of ferroelectrics, especially in enhancing their energy storage characteristics.

Achieving an excellent energy storage performance, together with high cycling reliability, is desirable for expanding technological applications of ferroelectric dielectrics. However, in well-crystallized ferroelectric materials, the concomitant high polarizability and low polarization-saturation field have led to a square-shaped polarization–electric field loop, fatally impairing both recoverable energy density (W_rec) and efficiency (η). Nanocrystalline ferroelectric films with a macroscopically amorphous structure have shown an improved W_rec and η, but their much lower polarizability demands an extremely high electric field to achieve such performances, which is undesirable from an economic viewpoint. Here, we propose a strategy to boost the energy storage performances and stability of ferroelectric capacitors simultaneously by constructing a tri-layer film in which a well-crystallized ferroelectric layer was sandwiched by two pseudo-linear dielectric layers with a dominant amorphous structure. In sol–gel-derived BaTiO₃/(Pb,La,Ca)TiO₃/BaTiO₃ (BTO/PLCT/BTO) tri-layer films, we show that the above design is realized via rapid thermal annealing which fully crystallized the middle PLCT layer while left the top/bottom BTO cap layers in a poor crystallization status. This sandwiched structure is endowed with an enhanced maximum polarization while a small remnant one and a much-delayed polarization saturation, which corresponds to large W_rec ≈ 80 J/cm³ and high η ≈ 86%. Furthermore, the film showed an outstanding cycling stability: its W_rec and η remain essentially unchanged after 10⁹ electric cycles (ΔW/W < 4%, Δη/η < 2%). These good energy storage characteristics have proved the effectiveness of our proposed strategy, paving a way for the utilization of sandwiched films in applications of electric power systems and advanced pulsed-discharge devices.

Due to its lead-free composition and a unique double polarization hysteresis loop with a large maximum polarization (P_max) and a small remnant polarization (P_r), AgNbO₃-based antiferroelectrics (AFEs) have attracted extensive research interest for electric energy storage applications. However, a low dielectric breakdown field (E_b) limits an energy density and its further development. In this work, a highly efficient method was proposed to fabricate high-energy-density Ag(Nb,Ta)O₃ capacitor films on Si substrates, using a two-step process combining radio frequency (RF)-magnetron sputtering at 450 ℃ and post-deposition rapid thermal annealing (RTA). The RTA process at 700 ℃ led to sufficient crystallization of nanograins in the film, hindering their lateral growth by employing short annealing time of 5 min. The obtained Ag(Nb,Ta)O₃ films showed an average grain size (D) of ~14 nm (obtained by Debye–Scherrer formula) and a slender room temperature (RT) polarization–electric field (P–E) loop (P_r ≈ 3.8 μC·cm⁻² and P_max ≈ 38 μC·cm⁻² under an electric field of ~3.3 MV·cm⁻¹), the P–E loop corresponding to a high recoverable energy density (W_rec) of ~46.4 J·cm⁻³ and an energy efficiency (η) of ~80.3%. Additionally, by analyzing temperature-dependent dielectric property of the film, a significant downshift of the diffused phase transition temperature (T_M2–M3) was revealed, which indicated the existence of a stable relaxor-like AFE phase near the RT. The downshift of the T_M2–M3 could be attributed to a nanograin size and residual tensile strain of the film, and it led to excellent temperature stability (20–240 ℃) of the energy storage performance of the film. Our results indicate that the Ag(Nb,Ta)O₃ film is a promising candidate for electrical energy storage applications.