Polarization Switching Process Research Articles

Intrinsic Ferroelectric Switching in Two-Dimensional α-In2Se3.

Two-dimensional (2D) ferroelectric semiconductors present opportunities for integrating ferroelectrics into high-density ultrathin nanoelectronics. Among the few synthesized 2D ferroelectrics, α-In2Se3, known for its electrically addressable vertical polarization, has attracted significant interest. However, the understanding of many fundamental characteristics of this material, such as the existence of spontaneous in-plane polarization and switching mechanisms, remains controversial, marked by conflicting experimental and theoretical results. Here, our combined experimental characterizations with piezoresponse force microscope and symmetry analysis conclusively dismiss previous claims of in-plane ferroelectricity in single-domain α-In2Se3. The processes of vertical polarization switching in monolayer α-In2Se3 are explored with deep-learning-assisted large-scale molecular dynamics simulations, revealing atomistic mechanisms fundamentally different from those of bulk ferroelectrics. Despite lacking in-plane effective polarization, 1D domain walls can be moved by both out-of-plane and in-plane fields, exhibiting avalanche dynamics characterized by abrupt, intermittent moving patterns. The propagating velocity at various temperatures, field orientations, and strengths can be statistically described with a universal creep equation, featuring a dynamical exponent of 2 that is distinct from all known values for elastic interfaces moving in disordered media. This work rectifies a long-held misunderstanding regarding the in-plane ferroelectricity of α-In2Se3, and the quantitative characterizations of domain wall velocity will hold broad implications for both the fundamental understanding and technological applications of 2D ferroelectrics.

Electrical de-poling and re-poling of relaxor-PbTiO3 piezoelectric single crystals without heat treatment

Re-poling of unexpected partially depoled piezoelectric materials conventionally needs to be first fully depoled through annealing above their Curie temperature to revive piezoelectric performances. Here, we investigated de-poling and re-poling of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals under electric fields at room temperature. We found that alternating current electric fields with amplitudes near the coercive field at low frequencies (<10 Hz) can be employed to successfully depolarize poled crystals at room temperature. We also demonstrated a reversible polarization switching process with a relaxor-PbTiO3 single crystal ultrasound transducer without device performance degradations. This experimental observation is supported by phase-field simulation, showing that alternating current electric fields can readily induce de-poling at room temperature, while direct current electric fields induce a transient depoled state only within an uncontrollable short period of time. The findings suggest new strategies for unprecedented in-device tailoring of the polarization states of ferroelectric materials.

Effects of thickness and defect on ultrahigh electro strain of piezoelectric ceramics

Piezoelectric ceramics with high electro strain are widely used in actuators, sensors, and so on. Here, this work investigates the electro strain of representative piezoelectric materials, and analyzes the influence of electrode type on the electro strain of PZT41 with significant strain anomalies. It is found that the occurrence of abnormally significant strain is not affected by the electrode but caused by intrinsic defects. Therefore, the ceramic samples with and without defects tailoring in Na0.5Bi0.5TiO3 (NBT) based and PbZr0.52Ti0.48O3 (PZT) based systems were prepared by the solid-state reaction method, and the results show that abnormally ultrahigh electro strains occur in excessively thin samples with diploes due to defects tailoring. Through displacement configuration experiments and comprehensive analysis of polarization switching processes, it is found that the occurrence of asymmetric abnormal significant strains is caused by reversible bending deformation in excessively thin samples. This work provides new support for the emergence of ultrahigh electro strain in piezoelectric ceramics.

Vapor Deposition of Bilayer 3R MoS2 with Room-Temperature Ferroelectricity.

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS2 synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS2/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS2 will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

The Effect of Electric Aging on Vinylidene Fluoride Copolymers for Ferroelectric Memory.

Copolymers based on vinylidene fluoride are potential materials for ferroelectric memory elements. The trend in studies showing that a decrease in the degree of crystallinity can lead to an unexpected increase in the electric breakdown field is noted. An analysis of the literature data reveals that in fluorine-containing ferroelectric polymers, when using a bipolar triangular field, the hysteresis loop has an unclosed shape, with each subsequent loop being accompanied by a decrease in the dielectric response. In this work, the effect of the structure of self-polarized films of copolymers of vinylidene fluoride with tetrafluoroethylene and hexafluoropropylene on breakdown processes was studied. The structure of the polymer films was monitored using infrared spectroscopy (IR) and X-ray diffraction. Kelvin probe force microscopy (KPFM) was applied to characterize the local electrical properties of the polymers. For the films of the first copolymer, which crystallize in the polar β-phase, asymmetry in the dielectric response was observed at fields greater than the coercive field. For the films of the copolymers of vinylidene fluoride with hexafluoropropylene, which crystallize predominantly in the nonpolar α-phase, polarization switching processes have also been observed, but at lower electric fields. The noted phenomena will help to identify the influence of the structure of ferroelectric polymers on their electrical properties.

Frequency-Dependent Multistep Ferroelectric Polarization Switching Mechanism in P(VDF-TrFE)-Based Capacitors Induced by Polystyrene Electret-like Modulation.

In this work, we investigate multistep ferroelectric polarization switching dynamics of a series of poly(vinylidene fluoride-trifluoroethylene)/polystyrene, P(VDF-TrFE)/PS, as active layers in ferroelectric capacitors with variable P(VDF-TrFE)/PS thickness ratios and a wide range of driving voltage frequencies (1-1000 Hz). The PS electret-like modulation effects on the depolarized field fluctuation are proven to be responsible for this multistep ferroelectric polarization switching process. To be specific, the switching current density peak splits into two peaks in both positive and negative voltage ranges according to the stimulus-response (S-R) data from the metal-ferroelectric-electret-metal capacitor driven by a periodic triangular voltage wave. The double-peak current trough appears when the transitorily suppressed ferroelectric polarization switching occurs while the discharge and recharge of the PS electret by external voltage brings a specific dynamic change in the electric field across ferroelectric (EFE). We also propose a theoretical model to simulate the ferroelectric polarization switching process at a current trough zone. This phenomenon provides new concepts on the electret-modulated multistep ferroelectric switching dynamics, and such switching mechanisms are critical for realizing reliable nonvolatile memory applications in flexible electronics.

Polarization Switching and Correlated Phase Transitions in Fluorite-Structure ZrO2 Nanocrystals.

Unconventional ferroelectricity in fluorite-structure oxides enables tremendous opportunities in nanoelectronics owing to their superior scalability and silicon compatibility. However, their polarization order and switching process remain elusive due to the challenges of visualizing oxygen ions in nanocrystalline films. In this work, the oxygen shifting during polarization switching and correlated polar-nonpolar phase transitions are directly captured among multiple metastable phases in freestanding ZrO2 thin films by low-dose integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM). Bidirectional transitions between antiferroelectric and ferroelectric orders and interfacial polarization relaxation are clarified at unit-cell scale. Meanwhile, polarization switching is strongly correlated with Zr-O displacement in reversible martensitic transformation between monoclinic and orthorhombic phases and two-step tetrahedral-to-orthorhombic phase transition. These findings provide atomic insights into the transition pathways between metastable polymorphs and unravel the evolution of polarization orders in (anti)ferroelectric fluorite oxides.

Electrically Switchable Polarization in Bi2 O2 Se Ferroelectric Semiconductors.

Atomically 2D layered ferroelectric semiconductors, in which the polarization switching process occurs within the channel material itself, offer a new material platform that can drive electronic components toward structural simplification and high-density integration. Here, a room-temperature 2D layered ferroelectric semiconductor, bismuth oxychalcogenides (Bi2 O2 Se), is investigated with a thickness down to 7.3nm (≈12 layers) and piezoelectric coefficient (d33 ) of 4.4 ± 0.1 pm V-1 . The random orientations and electrically dependent polarization of the dipoles in Bi2 O2 Se are separately uncovered owing to the structural symmetry-breaking at room temperature. Specifically, the interplay between ferroelectricity and semiconducting characteristics of Bi2 O2 Se is explored on device-level operation, revealing the hysteresis behavior and memory window (MW) formation. Leveraging the ferroelectric polarization originating from Bi2 O2 Se, the fabricated device exhibits "smart" photoresponse tunability and excellent electronic characteristics, e.g., a high on/off current ratio >104 and a large MW to the sweeping range of 47% at VGS =±5V. These results demonstrate the synergistic combination of ferroelectricity with semiconducting characteristics in Bi2 O2 Se, laying the foundation for integrating sensing, logic, and memory functions into a single material system that can overcome the bottlenecks in von Neumann architecture.

Abnormal Domain Growth during Polarization Reversal in Lithium Niobate Crystal Modified by Proton Exchange

The results of an experimental study of the abnormal domain structure kinetics in lithium niobate single crystals with a surface layer modified by soft proton exchange are presented. Domain switching in a wide field range allowed two qualitatively different types of domain structure evolution to be revealed: (1) the traditional growth of hexagonal domains in fields higher than 21.5 kV/mm and (2) the abnormal growth of stripe domains oriented along the Y crystallographic directions in the field range from 3.8 to 21.5 kV/mm. The stripe domains had a width up to 4 µm and depth up to 30 µm. It was shown that the time dependence of the total length of stripe domains could be analyzed in terms of the modified Kolmogorov–Avrami approach, taking into account the transition from the one-dimensional β-model to the one-dimensional α-model. The possibility of the controllable creation of a quasi-periodic structure of stripe domains with an average period of 5 µm by a two-stage polarization switching process was demonstrated. The formation and growth of stripe domains were considered in terms of the kinetic approach to the evolution of the domain structure as a result of the domain walls’ motion under inefficient screening conditions caused by the presence of a modified surface layer. The abnormally low threshold fields were attributed to a presence of a “built-in” field facilitating switching, created by a composition gradient induced by soft proton exchange.

An effective Fourier spectral phase-field approach for ferroelectric materials

In this article, we propose an efficient iterative numerical algorithm applying a Fourier spectral method to accurately compute the coupled elasto–electrostatic equilibrium equations in ferroelectric microstructures. The method is implemented for a phase-field model for domain pattern formation and domain kinetics evolution in tetragonal perovskite ferroelectrics, that uses spontaneous polarization as order parameter and includes the piezoelectric effect in the constitutive equation. The scheme is based on a two-scale homogenization approach, introducing local perturbations for stress and dielectric displacement. Green’s functions are derived in Fourier space for the coupled constitutive equations. Furthermore, using model parameters for BaTiO3 we investigate domain formation in 2D and 3D, and the polarization switching process starting from a critical nucleus subjected to an applied electric field and compressive stress. The simulation results based on the proposed method reveal that for polarization switching under external fields, intermediately appearing domains with 90° rotated polarization are formed, that reduce electrostatic or elastic strain energy.

Giant electromechanical response in layered ferroelectrics enabled by asymmetric ferroelastic switching

Bismuth layer-structured ferroelectrics (BLSFs) are highly desired for high-temperature piezoelectric systems owing to their high Curie temperature and large spontaneous polarization. However, the inferior electromechanical response, which results from their unique layered crystal structure and inherently low piezoelectric activities, has restricted the practical application. Here, we report discovery of a large electric-field-induced strain in a simplest BLSF Bi2WO6, which is comparable to perovskite ferroelectrics and exhibits an intriguing asymmetric loop-like feature. Through comprehensive analysis on the displacement test configuration and polarization switching process, it is demonstrated that the exotic loop-like strain curve originates from the reversible bending deformation induced by asymmetric 90° ferroelastic switching in the upper and lower surfaces. Specifically, the textured Bi2WO6 ceramic exhibits a giant nominal strain value of ∼0.75% with excellent cycling stability. This discovery raises the possibility of utilizing the bending effect and ferroelastic switching to design piezoelectric actuators and may accelerate the development of high-performance lead-free ceramics.

Ferroelectric polarization-enhanced photodegradation performance for dyes in BaTiO3/AgAlO2 heterostructure

Introducing an internal electric field into photocatalysts is an efficient strategy to inhibiting the charge carrier recombination which is the obstacle to the photocatalytic technology. Herein, we propose to construct BaTiO3/AgAlO2 heterostructure with improved photocatalytic performance through a polarization effect. The ferroelectric BaTiO3 particles with nanocube morphology are distributed over ellipsoid-like AgAlO2. The UV–vis absorption edges extend from 415 to 545 nm owing to the energy band overlap. The ultrasound-assisted photocatalysis efficiency of methyl blue under BaTiO3/AgAlO2 is increased by 22 %. Photoelectrochemical results indicate that the separation efficiency of photoinduced carriers is enhanced by the polarization effect. Moreover, the photocurrent of the BaTiO3/AgAlO2 after polarization is increased by 1.6 times of that before polarization. Piezoelectric force microscope shows that the phase angle has a great change, confirming the polarization switching process. The butterfly-shaped displacement-voltage hysteresis loop confirms the local piezoelectric response originated from BaTiO3.

Effect of high pressure anneal on switching dynamics of ferroelectric hafnium zirconium oxide capacitors

Investigation of the polarization switching mechanism in ferroelectric hafnium zirconium oxide (HZO) film is of great importance for developing high-quality ferroelectric memory devices. Recently, several works have been reported to describe the switching process of polycrystalline HZO film using the inhomogeneous field mechanism (IFM) model. However, no report has recorded the effect of high pressure annealing (HPA) on the polarization switching process. In this paper, we have carried out a careful investigation on the switching properties of HZO capacitors annealed at 600 °C with various high pressure conditions (1, 50, and 200 atm) using the IFM model. As pressure increases to 200 atm, the ferroelectric properties were enhanced in the HZO films, and, as a result, highest remanent polarization (Pr of 24.5 μC/cm2) was observed when compared with 1 and 50 atm. Similarly, as HPA increases, the HZO capacitors showed a decrement of the coercive field, which significantly improved the switching properties. The time consumed for reversing 80% polarization was 113.1, 105.7, and 66.5 ns for the sample annealed at 1, 50, and 200 atm, respectively. From the IFM model, the smallest active field (2.997 MV/cm) and a uniform distribution of the local electric field (0.304) were observed at 200 atm. Furthermore, the characteristic time constant (τ0) showed a decreasing trend (34.7, 18.1, and 11.7 ps) with increasing HPA. The improved switching properties and detailed findings recorded in this study may be helpful for developing the ferroelectric hafnia based non-volatile memory applications.

Numerical Discretization of Variational Phase Field Model for Phase Transitions in Ferroelectric Thin Films

Phase field methods have been widely used to study phase transitions and polarization switching in ferroelectric thin films. In this paper, we develop an efficient numerical scheme for the variational phase field model based on variational forms of the electrostatic energy and the relaxation dynamics of the polarization vector. The spatial discretization combines the Fourier spectral method with the finite difference method to handle three-dimensional mixed boundary conditions. It allows for an efficient semi-implicit discretization for the time integration of the relaxation dynamics. This method avoids explicitly solving the electrostatic equilibrium equation (a Poisson equation) and eliminates the use of associated Lagrange multipliers. We present several numerical examples including phase transitions and polarization switching processes to demonstrate the effectiveness of the proposed method.

Humidity Effects on Domain Structure and Polarization Switching of Pb(Zn1/3Nb2/3)O3-x%PbTiO3 (PZN-x%PT) Single Crystals.

The effect of relative humidity on the domain structure imaging and polarization switching process of Pb(Zn1/3Nb2/3)O3-x%PbTiO3 (PZN-x%PT) ferroelectric single crystals has been investigated by means of the piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS) techniques. It was found that the PFM amplitude increases with the relative humidity, and that the ferroelectric hysteresis loops at different relative humidity levels show that the coercive bias decreases with the increase in relative humidity. These observed phenomena are attributed to the existence of the water layer between the probe tip and the sample surface in a humid atmosphere, which could affect the effect of the electric field distribution and screening properties at the ferroelectric sample surface. These results provide a better understanding of the water adsorption phenomena at the nanoscale in regard to the fundamental understanding of ferroelectrics’ properties.

Revealing the mechanism of electric-field-induced phase transition in antiferroelectric NaNbO3 by in situ high-energy x-ray diffraction

Antiferroelectric materials exhibit electric field-induced phase transitions between antiferroelectric and ferroelectric states, which enable their use in energy storage and other applications. However, the mechanisms of these transitions are insufficiently understood. Here, we considered the electric field-induced phase transition in the lead-free antiferroelectric NaNbO3. Macroscopic measurements of polarization and longitudinal, transverse, and volumetric strain were complemented with simultaneous structural investigations using high-energy x-ray radiation, yielding crystallographic strain and unit cell volume changes. The field-induced behavior can be divided into the structural antiferroelectric–ferroelectric phase transition at about 8 kV/mm and the clearly decoupled polarization switching process at about 12 kV/mm, which is associated with a large increase in polarization and strain. Decoupling of the field-induced phase transition and polarization switching is related to the randomly oriented grains and mechanical stress present at the phase boundary.

Effect of the degree of a diffuse phase transitions on the behavior of polarization switching processes and elastic properties in multicomponent ceramics based on PZT

It is known that in multicomponent piezoceramics with a certain ratio of the components of the solid solution and modifying impurities, the phase transition from the ferroelectric phase to the paraelectric one occurs in several stages. The temperature interval of such structural and polarization changes, can reach hundreds of degrees. The aim of the present article is to study an effect of a degree of diffuse phase transitions on a character of repolarization and elastic properties in the multicomponents piezoceramics based on PZT.

Competitive mechanism of temperature-dependent electrical properties in BiFeO3-BaTiO3 ferroelectrics controlled by domain evolution

To understand the structure nature and physical mechanisms of property evolution under electric and temperature fields, temperature-dependent electrical properties of bismuth ferrite-barium titanate (BiFeO3-BaTiO3, BF-BT) ceramics were analyzed from both static structure and domain dynamics. Intriguingly, temperature coefficient of remanent polarization (Pr) and negative strain (Sneg) changes from positive to negative as BT component increases, leading to a nearly zero temperature coefficient of Pr at critical component (BT=0.35). Combined with multi-scale structure characterization techniques and polarization switching/backswitching described by the temperature-dependent Ps and Ps-Pr, such behavior is explained by the evolution of domain size for the first time and a competitive mechanism is also proposed. That is, the evolution of macro stripe domain (0.25≤BT≤0.30) to nanodomain (0.35≤BT≤0.45) determines the transition from the dominated polarization switching process to the dominated backswitching process, further greatly determining the temperature-stability of strain properties: variation of unipolar strain changes from 136% (BT=0.25) to 25% (BT=0.45) with increasing BT content. We believe that these results significantly guide the performance improvement (especially temperature-dependent properties) of BF-BT solid solutions.

Transient Polarization Reversal using an Intense THz Pulse in Silicon‐Doped Lead Germanate

The processes of dynamic polarization switching induced by the strong electric field of a nearly single‐cycle terahertz (THz) pulse are studied in a classical ferroelectric silicon‐doped lead germanate single crystal, Pb5(Ge0.74Si0.26)3O11, in THz pump–second‐harmonic‐generation (SHG) probe geometry depending on the pulse electric field strength (0.05–15.6 MV cm−1). Since the pump parameters correspond to the region of phonon excitations for a given material, THz‐induced polarization is explained by a dynamic change in the ferroelectric order parameter. Numerical solutions of the Landau–Khalatnikov equation using the thermodynamic potential constants for this material show that the effect of powerful THz pulses leads to polarization modulations and a strong nonlinear response, and is in good agreement with the experimental results. Oscillations of the nonlinear optical response after the end of the THz pulse are found, which can be associated with the excitation of the phonon mode in the crystal.

Combined Real-Time Study of Dielectric Response and Piezoresponse of Pb(Mg1/3Nb2/3)O3 Relaxor in an Electric Field

A method of synchronous measurement of the piezoresponse signal and electrical impedance when studying the processes of polarization switching in an external electric field has been described. Using the developed technique, the processes of switching ferroelectric (FE) phase of lead magnoniobate induced by an electric field are studied. It is shown that a polydomain structure with a weak piezoresponse is formed during the initial appearance of the FE phase. A change in the direction of the external field leads to polarization switching, and the switching process passes through an intermediate glass-like phase. Training the sample by multiple switching leads to a sharp increase in piezoresponse, which can be associated with a single-domain state formation.