Large Remanent Polarization Research Articles

Ferroelectric Nanomaterials for Energy Harvesting and Self‐Powered Sensing Applications

AbstractThe rapid development of the Internet of Things has introduced new challenges for miniaturized, highly integrated energy harvesters and sensors, promoting the exploration of various novel nanomaterials. Ferroelectric nanomaterials, characterized by large remanent polarization, exceptional dielectric properties, outstanding chemical stability, and diverse electricity generation capabilities, are emerging as promising candidates in a variety of fields. Possessing various mechanisms for electricity generation, including piezoelectric, pyroelectric, photovoltaic, and triboelectric effects, ferroelectric nanomaterials demonstrate their capability for harvesting and sensing multiple energies simultaneously, including light, thermal, and mechanical energies. This capability contributes to the miniaturization and high integration of electronic devices. This article reviews recent achievements in ferroelectric nanomaterials and their applications in energy harvesting and self‐powered sensing. Different categories of ferroelectric nanomaterials, their ferroelectric properties, and fabrication methods are introduced. The working mechanisms and performance of ferroelectric energy harvesters and self‐powered sensors are described. Additionally, future prospects are discussed.

Phase transition kinetics and sublayer optimization of HfO2/ZrO2 superlattice ferroelectric thin films

The sublayer thickness of superlattices, as a key factor affecting lattice integrity, interface defects, and strain, deserves in-depth studies about its impact on improving ferroelectric properties. This study described and analyzed the performance of HfO2/ZrO2 superlattices with various sublayer thicknesses. It can be concluded that the structure of the thicker layers will guide the trend of the phase composition of the entire device: when ZrO2 layers are thicker, the superlattices will exhibit antiferroelectricity due to the higher content of the tetragonal phase (t-phase); when HfO2 layers become thicker, the fraction of the monoclinic phase (m-phase) will increase, leading to a decrease in ferroelectricity and an increase in leakage current. In this way, the device with a 1:1 HfO2/ZrO2 thickness ratio was optimized to have the largest remanent polarization and the lowest leakage current. Maintaining the same thickness ratio of the HfO2/ZrO2 superlattices, it was found that HfO2/ZrO2 superlattices with thinner sublayers exhibited a larger remanent polarization (Pr) value due to increased interlayer distortion. On the contrary, the thicker sublayers reduced leakage current, which was beneficial for improving the device lifespan.

Physical Properties of CaTiO3-Modified NaNbO3 Thin Films.

NaNbO3(NN)-based lead-free materials are attracting widespread attention due to their environment-friendly and complex phase transitions, which can satisfy the miniaturization and integration for future electronic components. However, NN materials usually have large remanent polarization and obvious hysteresis, which are not conducive to energy storage. In this work, we investigated the effect of introducing CaTiO3((1-x)NaNbO3-xCaTiO3) on the physical properties of NN. The results indicated that as x increased, the surface topography, oxygen vacancy and dielectric loss of the thin films were significantly improved when optimal value was achieved at x = 0.1. Moreover, the 0.9NN-0.1CT thin film shows reversible polarization domain structures and well-established piezoresponse hysteresis loops. These results indicate that our thin films have potential application in future advanced pulsed power electronics.

Improving the ferroelectric properties of Lu doped Hf0.5Zr0.5O2 thin films by capping a CeO x layer

Lu doped Hf0.5Zr0.5O2 (HZO) ferroelectric films were prepared on Pt/TiN/SiO2/Si substrate by chemical solution deposition method, and an interfacial engineering strategy for improving the ferroelectric property was explored by capping the Lu doped HZO films with a cerium oxide layer. Compared with the Lu doped HZO film without the CeO x coating layer, the Lu doped HZO film with the CeO x coating layer has a larger remanent polarization (2P r = 34.72 µC cm−2) and presents weaker wake-up behavior, which result from the higher orthogonal phase ratio and the lower oxygen vacancy of the CeO x coated Lu doped HZO film. In addition, the CeO x coating can remarkably improve the fatigue resistance and retention performance of the Lu doped HZO films. It is hoped that the results can provide an effective approach for the realization of high-performance and highly reliable hafnium oxide based ferroelectric thin films.

Enhanced ferroelectric and piezoelectric properties of Sb/Nb co‐doped BiScO3-PbTiO3-based ceramics via relaxation regulation near morphotropic phase boundary

Here, novel ferroelectric ceramics of Sb/Nb co-doped 0.36BiScO3-0.64PbTiO3 (BS-PT-Sb2O5-xNb2O5) of complex perovskite structure are reported with compositions near the morphotropic phase boundary (MPB) where the dominant monoclinic and tetragonal phases coexist. Excitingly, the incorporation of Sb/Nb dopants leads to a remarkable enhancement in both ferroelectric and piezoelectric properties. Notably, the sample with x = 1.0 demonstrates the optimal properties, boasting a giant piezoelectric coefficient (d33) of 498 pC/N (10.7 % higher than pure BS-PT ceramics), a high Curie temperature (Tc) of 392 °C, a high depolarization temperature of 362 °C and a large remanent polarization of 52.7 μC/cm2 (55.9 % higher than pure BS-PT ceramics). The enhanced ferroelectric and piezoelectric properties are ascribed to a relaxation behavior originating from the formation of polar nanoregions (PNRs) and reduced concentration of oxygen vacancy. The result provides an opportunity to elucidate the relationship between relaxation behavior and electrical properties, offering prospects for substantially improving the performance of piezoelectric ceramics.

Scalable Synthesis of High-Quality Ultrathin Ferroelectric Magnesium Molybdenum Oxide.

The development of ultrathin, stable ferroelectric materials is crucial for advancing high-density, low-power electronic devices. Nonetheless, ultrathin ferroelectric materials are rare due to the critical size effect. Here, a novel ferroelectric material, magnesium molybdenum oxide (Mg2Mo3O8) is presented. High-quality ultrathin Mg2Mo3O8 crystals are synthesized using chemical vapor deposition (CVD). Ultrathin Mg2Mo3O8 has a wide bandgap (≈4.4eV) and nonlinear optical response. Mg2Mo3O8 crystals of varying thicknesses exhibit out-of-plane ferroelectric properties at room temperature, with ferroelectricity retained even at a 2nm thickness. The Mg2Mo3O8 exhibits a relatively large remanent polarization ranging from 33 to 52µCcm- 2, which is tunable by changing its thickness. Notably, Mg2Mo3O8 possesses a high Curie temperature (>980°C) across various thicknesses. Moreover, the as-grown Mg2Mo3O8 crystals display remarkable stability under harsh environments. This work introduces nolanites-type crystal into ultrathin ferroelectrics. The scalable synthesis of stable ultrathin ferroelectric Mg2Mo3O8 expands the scope of ferroelectric materials and may prosper applications of ferroelectrics.

Frequency dependence of wake-up and fatigue characteristics in ferroelectric Al0.93B0.07N thin films

B-doped AlN ferroelectric thin films are promising candidates for non-volatile memory device applications, due to their relatively low deposition temperatures, large remanent polarizations (up to 120 μC/cm2), and outstanding polarization retention properties. In this research, the polarization switching fatigue characteristics in epitaxial B-doped AlN thin films were investigated. Films were deposited via reactive RF magnetron sputtering onto W-coated c-plane sapphire substrates. Polarization switching fatigue measurements were conducted with triangular waveforms for frequencies between 50 and 1000 Hz from room temperature to 200°C. On repeated cycling, the films undergo wake-up; the wake-up process was characterized by an activation energy of 0.15 ± 0.05 eV. After a period of normal switching, the film leakage current increased with additional cycling, and finally the films underwent dielectric breakdown. Unpatterned Al0.93B0.07N films with 100 nm thick Pt top electrodes survived ∼104 bipolar cycles, whereas films with 1000 nm W top electrodes survived ∼105 cycles before dielectric breakdown. Modeling was used to design a field plate, which improved the performance to ∼106 fatigue cycles. It was found that dielectric failure during fatigue was not due to surface flashover but was associated with hard breakdown events in the dielectric. Failure for 100 Hz cycling was not associated with device self-heating but was exacerbated by field concentrations; at higher frequencies, the instantaneous temperature rise during the switching becomes prominent. Based on the fatigue process under 100 Hz under various ambient temperatures, the activation energy of the polarization switching fatigue of B-doped AlN thin films was found to be Ea = 0.10 ± 0.03 eV.

Reliable Accessibility of Intermediate Polarization States in Textured Ferroelectric Al0.66Sc0.34N Thin Film

AbstractFerroelectric materials are promising candidates for neuromorphic computing synaptic devices due to the nonvolatile multiplicity of spontaneous polarization. To ensure a sufficient memory window, ferroelectric materials with a large coercivity are urgently required for practical applications in highly scaled multi‐bit memory devices. Herein, a remarkable reliability of intermediate ferroelectric polarization states is demonstrated in a textured Al0.66Sc0.34N thin film with a coercive field of 2.4 MV cm−1. Al0.66Sc0.34N thin films are prepared at 300 °C on Pt (111)/Ti/SiO2/Si substrates using a radio frequency reactive sputtering method. Al0.66Sc0.34N thin films exhibit viable ferroelectricity with a large remanent polarization value of >100 µC cm−2. Through the conventional current–voltage characteristics, polarization switching kinetics, and temperature dependence of coercivity, the reproducibility of multiple polarization states with apparent accuracy is attributed to a small critical volume (3.7 × 10−28 m3) and a large activation energy (3.3 × 1027 eV m−3) for nucleation of the ferroelectric domain. This study demonstrates the potential of ferroelectric Al1‐xScxN for synaptic weight elements in neural network hardware.

Mitigation of field-driven dynamic phase evolution in ferroelectric Hf0.5Zr0.5O2 films by adopting oxygen-supplying electrode

Ferroelectricity in (Hf,Zr)O2 films under 10 nm has been correlated with various factors, including dopant, oxygen deficiency, strain, and surface energy. Owing to their ultrathin thickness, the interfaces between the (Hf,Zr)O2 and the electrode can also influence the resultant ferroelectricity. Recently, ferroelectric (Hf,Zr)O2 fabricated on electrodes, such as Mo, showed large remanent polarization and wake-up-free behavior. However, the accurate physical/chemical mechanism behind the ferroelectric (Hf,Zr)O2 on Mo remains under debate. This study reports the structural, chemical, and electrical properties of Hf0.5Zr0.5O2 with Mo and TiN as top and bottom electrodes. The remanent polarization and coercive field of Hf0.5Zr0.5O2 with a Mo electrode (23.7 μC/cm2, 1.1 MV/cm) were higher than those with a TiN electrode (9.1 μC/cm2, 0.9 MV/cm). The X-ray diffraction analysis shows that Hf0.5Zr0.5O2 with Mo electrode exhibits a higher ferroelectric orthorhombic phase fraction than the TiN electrode, explaining its higher remanent polarization. The X-ray photoelectron and electron energy loss spectroscopies revealed that the concentration of oxygen vacancies in Hf0.5Zr0.5O2 with Mo electrode was significantly lower than that in Hf0.5Zr0.5O2 with TiN electrode, explaining the wake-up free behavior.

Room-Temperature, Nanoscale Multiferroic Pb(Fe0.5Ta0.5)1−x(Zr0.53Ti0.47)xO3 (x = 0.2, 0.3) Thin Films Grown via the Pulsed Laser Deposition Technique

Multiferroic materials capable of robust magnetoelectric coupling at room temperature are currently being explored for their possible multifunctional device applications. Highly (100)-oriented Pb(Fe0.5Ta0.5)x(Zr0.53Ti0.47)1−x (PZTFTx) thin films (x = 0.2 and 0.3) with a thickness of about 300 nm were grown on La0.67Sr0.33CoO3 (LSCO)-buffered MgO 100-oriented substrates via the pulsed laser deposition method. An analysis of their X-ray diffraction patterns suggests the stabilization of the orthorhombic phase in the thin films at room temperature. Dielectric spectroscopic measurements of the metal–insulator–metal (Pt/PZTFTx/LSCO) thin-film capacitors as a function of temperature revealed a diffuse ferroelectric-to-paraelectric phase transition around Tm ~520 and 560 K for the x = 0.2 and 0.3 thin films, respectively. Well-saturated electrical hysteresis loops with large remanent (Pr) and saturation (Ps) polarizations were observed in these capacitors, which indicates the establishment of intrinsic ferroelectric ordering in the thin films at room temperature. These thin films retained ferromagnetic/ferrimagnetic ordering up to 300 K and showed saturation magnetization values of 8.3 (x = 0.2) and 6.1 (x = 0.3) emu/cm3 at room temperature. The magnetoelectric coupling constants of 2040 mV/cmOe (x = 0.2) and 850 mV/cmOe (x = 0.3), respectively, were obtained at an in-plane bias field at room temperature. The present study demonstrates that PZTFTx thin films are multiferroic at room temperature with large magnetoelectric couplings, and these materials may be suitable for use in magnetic sensors and spintronic device applications.

AgNbO3-Based Multilayer Capacitors: Heterovalent-Ion-Substitution Engineering Achieves High Energy Storage Performances.

The demand for miniaturization and integration in next-generation advanced high-/pulsed-power devices has resulted in a strong desire for dielectric capacitors with high energy storage capabilities. However, practical applications of dielectric capacitors have been hindered by the challenge of poor energy-storage density (Urec) and efficiency (η) caused by large remanent polarization (Pr) and low breakdown strength (BDS). Herein, we take a method of heterovalent ion substitution engineering in combination with the multilayer capacitor (MLCC) technology and thus achieve a large maximum polarization (Pmax), zero Pr, and high BDS in the AgNbO3 (AN) system simultaneously and obtain excellent Urec and η. The substitution of Sm3+ for Ag+ in SmxAN+Mn MLCCs at x ≥ 0.01 decreases the M1-M2 phase transition temperature, and the antiferroelectric (AFE) M2 phase appears at room temperature, which is beneficial to achieving a low Pr value. Due to the low Pr value and high BDS ∼ 1300 kV·cm-1, an excellent Urec ∼9.8 J·cm-3 and PD,max ∼ 34.8 MW·cm-3 were achieved in SmxAN+Mn MLCCs at x = 0.03. The work suggests a paradigm that can enhance the energy storage capabilities of AFE MLCCs to meet the demanding requirements of advanced energy storage applications.

Role of Defects in the Breakdown Phenomenon of Al1-xScxN: From Ferroelectric to Filamentary Resistive Switching.

Aluminum scandium nitride (Al1-xScxN), with its large remanent polarization, is an attractive material for high-density ferroelectric random-access memories. However, the cycling endurance of Al1-xScxN ferroelectric capacitors is far below what can be achieved in other ferroelectric materials. Understanding the nature and dynamics of the breakdown mechanism is of the utmost importance for improving memory reliability. The breakdown phenomenon in ferroelectric Al1-xScxN is proposed to be an impulse thermal filamentary-driven process along preferential defective pathways. For the first time, stable and robust bipolar filamentary resistive switching in ferroelectric Al1-xScxN is reported. A hot atom damage defect generation model illustrates how filament formation and ferroelectric switching are connected. The model reveals the tendency of the ferroelectric wurtzite-type Al1-xScxN system to reach internal symmetry with bipolar electric field cycling. Defects generated from bipolar electric field cycling influence both the energy barrier between the polarization states and that required for the filament formation.

Enhanced Ferroelectric Polarization in Epitaxial BiFeO3−BiMg2/3Nb1/3O3 Films

Remanent polarization is a key parameter of a ferroelectric material. A large remanent polarization is favorable for various ferroelectric applications. Here, we report a large remanent polarization of 116.8 \ensuremath{\mu}C ${\mathrm{cm}}^{\ensuremath{-}2}$ observed in ${\mathrm{Bi}\mathrm{Mg}}_{2/3}{\mathrm{Nb}}_{1/3}{\mathrm{O}}_{3}$ (BMN)-modified ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_{3}$ (BFO) thin films, epitaxially deposited on (001)-oriented ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ substrates buffered with ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{Mn}\mathrm{O}}_{3}$ electrodes. Combined experimental analyses and first-principles calculations reveal that the incorporation of 10 mol % BMN into BFO increases both the intrinsic and extrinsic contributions, resulting in doubled polarization. The intrinsic polarization calculated for BFO-BMN is larger than that of BFO due to enhanced ionic displacements. Compared with epitaxial BFO films in a rhombohedral structure with an interaxial angle of \ensuremath{\alpha} = 89.72\ifmmode^\circ\else\textdegree\fi{} and quasitetragonal c/a = 1.018, the epitaxial BFO-BMN films become more tetragonal-like, with \ensuremath{\alpha} = 89.91\ifmmode^\circ\else\textdegree\fi{} and c/a = 1.037. Moreover, a unique rotated twinning-domain structure is developed in the BFO-BMN thin films, further increasing the out-of-plane polarization by rotating the [111] direction toward the film's normal. These results provide additional opportunities to increase ferroelectric polarization through composition engineering.

Structure, dielectric and energy storage properties of Bi(Mg2/3Nb1/3)O3 modified Na0.5Bi0.5TiO3-NaNbO3 ceramics

Na0.5Bi0.5TiO3-based ceramics are considered to be a prospective material for energy storage applications due to their unique phase transition and crystal structure. However, the large remanent polarization (Pr) and coercive field (Ec) limit their application in energy storage devices. In this work, the composition-dependent structure, dielectric properties and energy storage properties of Bi(Mg2/3Nb1/3)O3 (BMN) modified Na0.5Bi0.5TiO3-NaNbO3 (NBTNN) ceramics were systematically investigated. The introduction of BMN promotes the formation of local random field, which effectively suppresses the hysteresis loss along with Pr, and form “slender” P-E hysteresis loops. The maximum recoverable energy density of 5.26 J/cm3 is obtained in the optimum composition 0.8NBTNN-0.2BMN, which is about three times more than that of the undoped one, while yielding a high energy efficiency of 78%. In addition, the introduction of BMN into NBTNN ceramic results in excellent temperature stability and significant enhancement in band gap of the material. The improved energy storage performance of BMN modified NBTNN ceramic indicates their potential applicability in dielectric capacitors.

Ferroelectricity in layered bismuth oxide down to 1 nanometer.

Atomic-scale ferroelectrics are of great interest for high-density electronics, particularly field-effect transistors, low-power logic, and nonvolatile memories. We devised a film with a layered structure of bismuth oxide that can stabilize the ferroelectric state down to 1 nanometer through samarium bondage. This film can be grown on a variety of substrates with a cost-effective chemical solution deposition. We observed a standard ferroelectric hysteresis loop down to a thickness of ~1 nanometer. The thin films with thicknesses that range from 1 to 4.56 nanometers possess a relatively large remanent polarization from 17 to 50 microcoulombs per square centimeter. We verified the structure with first-principles calculations, which also pointed to the material being a lone pair-driven ferroelectric material. The structure design of the ultrathin ferroelectric films has great potential for the manufacturing of atomic-scale electronic devices.

Realizing high energy density in BiFeO3-based ceramics capacitors via bandgap engineering and polarization optimization

BiFeO3-based lead-free ferroelectrics possess high spontaneous polarization. Nevertheless, achieving ultrahigh recoverable energy density in BiFeO3-based ceramics is limited by large remanent polarization and insufficient breakdown strength. Herein, (0.7-x)BiFeO3-0.3Ba(Hf0.05Ti0.95)O3-xNaTaO3 ceramics were designed and prepared. It exhibited a high recoverable energy density of 6.3 J/cm3 and high efficiency of 86.6 % due to the contribution of large breakdown strength and optimized polarization (high maximum polarization and small remanent polarization). These contributions stem from NaTaO3 being introduced into the 0.7BiFeO3-0.3Ba(Hf0.05Ti0.95)O3 matrix, which successfully brought in widened bandgap Eg, increased resistivity, and enhanced relaxor behavior. Large breakdown strength is also promoted by dense microstructure and decreased average grain size. This work proves that the strategies of bandgap engineering and polarization optimization for improving energy storage performance in BiFeO3 are practicable.

High-Speed and High-Power Ferroelectric Switching Current Measurement Instrument for Materials with Large Coercive Voltage and Remanent Polarization.

A high-speed and high-power current measurement instrument is described for measuring rapid switching of ferroelectric samples with large spontaneous polarization and coercive field. Instrument capabilities (±200 V, 200 mA, and 200 ns order response) are validated with a LiTaO3 single crystal whose switching kinetics are well known. The new instrument described here enables measurements that are not possible using existing commercial measurement systems, including the observation of ferroelectric switching in large coercive field and large spontaneous polarization Al0.7Sc0.3N thin films.

Ultrahigh Polarization Response along Large Energy Storage Properties in BiFeO3-BaTiO3-Based Relaxor Ferroelectric Ceramics under Low Electric Field.

BiFeO3-BaTiO3 (BF-BT) dielectric ceramics are receiving more and more concern for advanced energy storage devices owing to their excellent ferroelectric properties and environmental sustainability. However, the energy density and efficiency are limited in spite of the large remanent polarization. Herein, we proposed a multiscale optimization strategy via a local compositional disorder with a Birich content and nanodomain engineering by introducing the Sr0.7Bi0.2Ca0.1TiO3 (SBCT) into BF-BT ceramics. Interestingly, an extraordinary energy storage property (ESP) with a high reversible energy storage density (Wrec) of ∼3.79 J/cm3 and an ultrahigh polarization discrepancy (ΔP) of ∼58.5 μC/cm2 were obtained in the SBCT-modified BF-BT ceramics under 160 kV/cm. The boosted ESP should be attributed to the fact that the replacement of A/B-sites cations could transform the long-range ferroelectric order of the BF-BT system into polar nanoregions (PNRs) along with the refined grain size, decreased leakage current, and broadened energy band gap. Moreover, good frequency (1-103 Hz) and temperature (25-125 °C) stabilities, high fatigue resistance (× 105), large power density (∼31.1 MW/cm3), and fast discharge time (∼97 ns) were also observed for the optimized ceramics. These results illustrate a potentially effective method for creating high ESP lead-free ceramics at a low electric field.

Quasipolaron Surface Polarization in Bismuth Ferrite

Multiferroic BiFeO${}_{3}$ has been surprising researchers with its properties for decades now, and questions related to the origin of its polarization and the various mechanisms behind its ferroelectric properties are still important. This study shows that in BiFeO${}_{3}$, quasipolaron surface polarization is the origin of a nested domain structure. As more quasipolarons are pinned at the surface per unit area, the surface density of antiferromagnetically ordered spins increases. The large remanent polarization and superior antiferromagnetic properties show that the surface of bismuth ferrite is important for spintronics.

Robust ferroelectricity enhancement of PZT thin films by a homogeneous seed layer

With its excellent ferroelectric properties such as large dielectric constant and large remanent polarization, PZT thin films are extensively used in micro-sensors and other devices. In this study, the sol–gel process was used to fabricate Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1–x)O3 seed islands. The experimental consequences demonstrate that all the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1−x)O3 seeds show pure perovskite phase with no other impurity phases, and the electrical properties of Pb(Zr0.52Ti0.48)O3 thin films modified by Pb(ZrxTi1–x)O3 seed islands with different Zr/Ti ratios are improved, such as remanent polarization increased, dielectric properties increased, coercive electric field decreased, leakage current density decreased. In particular, the electrical properties of the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1–x)O3 seed islands are the most optimal when the x is 0.52. This paper provides a new technique for optimizing the electrical properties of PZT thin films, which is of great significance for breaking through the bottleneck of the development of ferroelectric memory.