Polarization Switching Dynamics Research Articles

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.

Phase-field simulations of polarization variations in polycrystalline Hf0.5Zr0.5O2 based MFIM: Voltage dependence and dynamics

In this work, we investigate the device-to-device variations in the remanent polarization of metal–ferroelectric–insulator–metal stacks based on ferroelectric hafnium–zirconium–oxide (HZO). Our study employs a 3D dynamic multi-grain phase-field model to consider the effects of the polycrystalline nature of HZO in conjunction with the multi-domain polarization switching. We explore the dependence of variations on various design factors, such as the ferroelectric thickness and voltage stimuli (set voltage, pulse amplitude, and width), and correlate the trends to the underlying polarization switching mechanisms. Our analysis reveals a non-monotonic dependence of variations on the set voltage due to the coupled effect of the underlying polycrystalline structure variations and the voltage dependence of polarization switching mechanisms. We further report that collapsing of oppositely polarized domains at higher set voltages can lead to an increase in variations, while ferroelectric thickness scaling lowers the overall device-to-device variations. Considering the dynamics of polarization switching, we highlight the key role of voltage and temporal dependence of domain nucleation in dictating the trends in variations. Finally, we show that using a lower amplitude pulse for longer duration to reach a target mean polarization state results in lower variations compared to using a higher amplitude pulse for shorter duration.

Rough FTO electrode interface effect upon ferroelectric switching dynamics in Ag/P(VDF‐HFP)/FTO capacitor structures

AbstractRough FTO electrode interface effect upon the characteristic ferroelectric polarization parameters and switching dynamics in Ag/P(VDF‐HFP)/FTO junctions is systematically evaluated by varying electric field amplitude and frequency. Structural and microstructural studies confirm the ferroelectric phase formation of P(VDF‐HFP) on FTO surface. Tilted rectangular P‐E loops have been recorded in the applied fields ranging over 156.25–468.75 MV/m measured between 250 mHz and 5 Hz frequency ranges. Giant saturation of 21.7 μC/cm2 and remnant polarization of 34.9 μC/cm2 has been noted at low frequencies due to the influence of high electric field induced leakage currents. The coercive field, saturation and remnant polarization response with increase in electric field amplitude and frequency of the applied signal are well fitted with classic domain growth model proposed by Kolmogorov‐Avrami‐Ishibashi. A switching time scales of 32.2–693 ms has been observed for forward switching while for backward switching it is noted as 22.1–618.8 ms, when measured at frequencies from 5 Hz to 250 mHz. Furthermore, the tilted P‐E loops are attributed to the excessive leakage currents due to the high surface roughness of FTO bottom electrode.

The frequency-dependent polarization switching in nanograined BaTiO3 films under high-strength electric field

ABSTRACT The polarization reorientation in ferroelectric nanomaterials under high-strength AC electric fields is intrinsically a frequency-dependent process. However, the related study is not widely seen. We report a phase-field investigation regarding the dynamics of polarization switching and the electromechanical characteristics of a polycrystalline BaTiO3 nanofilm under applied frequency from 0.1 to 80 kHz. The grain boundaries and the in-plane strains are considered in the model. The obtained hysteresis and butterfly loops exhibit a remarkable variety of shapes with the changing frequency. The underlying mechanism for the observed frequency-dependent physical properties was discussed via domain structure-based analysis. In addition, we examined the influence of the kinetic coefficient in the Ginzburg-Landau equation as well as the influence of the electric-field amplitude to the frequency dependency. It was found that a higher value of kinetic coefficient or field amplitude tends to enhance the mobility of polarization switching and to transform high-frequency characteristics to low-frequency ones.

Polarization switching dynamics simulation by using the practical distribution of ferroelectric properties

We investigated the internal bias field and coercive field in a typical ferroelectric thin-film capacitor and simulated polarization switching dynamics using Euler's method. The simulation results agreed well with the experimental results and reflected the well-known polarization domain switching model in which the polarization switching occurs on the order of nucleation, growth, and coalescence. The fit parameters (damping parameters affecting the polarization change rate) also followed a certain distribution. When the expected value was used instead of full distribution, the simulation results did not agree well with corresponding experimental results. The simulation results suggested no domain structure in the polarization switching dynamics, indicating that the polarization domain structure was affected by the distribution of the fit parameters. Our results demonstrate the possibility of simulation using realistic distribution of ferroelectric properties.

Ferroelectric-superconductor structures properties

At the present time the urgent direction is to investigate the possibilities of new materials as a base for further improvement of very large-scale integration in order to create elements of submicron digital circuits with improved characteristics. The paper presents and discusses the results of measurements of volt-farad characteristics of structures with PbTiO3/YBa2Cu3O7-x ferroelectric films on strontium titanate substrate. New data on the characteristics of heterostructures with thin ferroelectric films are obtained, in particular it is shown that in the volt-farad characteristics of the studied structures local violations of the smooth forward sweep in the form of characteristic dips at the values of the applied field strengths close to the coercive ones determined by the ferroelectric hysteresis loops are observed. This effect can be explained by the emergence of built-in charges at the interface between a segmented dielectric and a high-temperature superconductor as a result of the polarization switching dynamics of the segmented dielectric. Quantitative estimates of the corresponding values of the embedded charges are made. Another explanation can be the assumption that the nature of these minima is related to the negative capacitance effect in segmentelectrics during polarization switching. The obtained results of the research serve as a basis for consideration of the observed effects in the design of a number of devices related to this subject area, in particular, non-volatile memory and gates based on MOS transistors, in the design of which heterostructures with ferroelectrics are used.

An efficient deep neural network accelerator using controlled ferroelectric domain dynamics

The current work reports an efficient deep neural network (DNN) accelerator, where analog synaptic weight elements are controlled by ferroelectric (FE) domain dynamics. An integrated device-to-algorithm framework for benchmarking novel synaptic devices is used. In poly(vinylidene fluoride-trifluoroethylene)-based ferroelectric tunnel junctions (FTJs), analog conductance states are measured using a custom pulsing protocol, and associated control circuits and array architectures for DNN training are simulated. Our results show that precise control of polarization switching dynamics in multi-domain polycrystalline FE thin films can produce considerable weight-update linearity in metal–ferroelectric–semiconductor (MFS) tunnel junctions. Ultrafast switching and low junction currents in these devices offer extremely energy-efficient operation. Via an integrated platform of hardware development, characterization and modeling, we predict the available conductance range, where linearity is expected under identical potentiating and depressing pulses for efficient DNN training and inference tasks. As an example, an analog crossbar-based DNN accelerator with MFS junctions as synaptic weight elements showed >93% training accuracy on a large MNIST handwritten digit dataset while, for cropped images, >95% accuracy is achieved. One observed challenge is the rather limited dynamic conductance range while operating under identical potentiating and depressing pulses below 1 V. Investigation is underway to improve the FTJ dynamic conductance range, maintaining the weight-update linearity under an identical pulse scheme.

Time-fractional Landau–Khalatnikov model applied to numerical simulation of polarization switching in ferroelectrics

Ferroelectrics are complex structured media that exhibit fractal properties and memory effects in terms of a number of dynamic characteristics. The most relevant applications of ferroelectrics in science and technology are associated with the primary mechanisms of polarization switching and domain structure dynamics induced by external exposure. In this study, we propose a time-fractional modification of the Landau–Ginzburg–Devonshire–Khalatnikov model to describe the dynamics of ferroelectric polarization switching. To solve the time-fractional cubic-quintic partial differential equation numerically, a computational scheme is derived. The technique combines an iterative procedure and an implicit finite-difference scheme based on an approximation of the Caputo derivative. A series of computational experiments are presented to visualize polarization switching characteristics on the example of thin films of barium titanate. A variation of the order of fractional derivative as a numerical characteristic of the memory effect allows one to “adjust” suitable regimes of simulated dynamical system. The obtained findings indicate that the generalized time-fractional model can provide better reproduction of experimental data in comparison with the classical analogue.

SPICE compatible semi-empirical compact model for ferroelectric hysteresis

This paper reports a semi-empirical, SPICE compatible and computationally efficient compact model for ferroelectric capacitors (Fe-CAP) description. This compact model is inspired by the Jiles–Atherton model of ferromagnets, which features significantly smaller computational effort than other state-of-the-art models. It successfully computes the evolution of the memory window and hysteresis of ferroelectric capacitors for any arbitrary signal. We have successfully calibrated this model with the experimentally characterized polarization switching dynamics of fabricated 10 nm silicon-doped hafnium oxide based Fe-CAP.

Systematic Interpretation of Time Effect on Negative Capacitance of Ferroelectrics Based on Electrostatics and Charge Dynamics

Negative capacitance (NC) effects in ferroelectric (FE) films have attracted intensive attention recently in terms of both polarization kinetics modeling and low-power CMOS applications. Its physical origin, however, is controversial because the experimentally observed NC effects are different from the initial concept, particularly on the frequency/voltage dependence and hysteresis. Recently, we have proposed an electrostatic model, with which the NC effect with hysteresis and voltage dependence in FE/paraelectric (PE) stacks in dc mode is explained from a viewpoint of polarization switching. Based on this, in this work, the NC effect in the dimension of time in both FE/PE and FE capacitor–resistor (FE-R) circuit is analytically formulated and experimentally studied by comprehensively considering the dynamics of polarization switching, charge communication with power supply, and charge leaking through the capacitor. Our results suggest that such NC effects only occur in a time window in both systems and a competition between FE switching and charge compensation is the key to determine the time window. Thus, a systematic understanding is accomplished to explain most of the phenomenon associated with NC effects in FEs.

Soliton Disentangling and Ferroelectric Hysteresis in Bilayer MoS2 Nanostructures with Reconstructed Moiré Superlattices

Emergent interfacial ferroelectricity has been surprisingly found in spatial inversion symmetry broken moiré systems, such as rhombohedral-stacked bilayer transition metal dichalcogenides (TMDs). However, the lattice dynamics of nanoscale polarization switching and ferroelectric hysteresis remain unclear. Here we report on the observation of phonon splitting of soliton networks in bilayer MoS2 nanostructures with reconstructed moiré superlattices. By tuning a perpendicular displacement field, the interval of phonon splitting exhibits a distinct hysteresis behavior, which arises from solitons disentangling and lattice viscosity. Moreover, we demonstrate a proof-of-principle moiré ferroelectric tunneling junction. Our work presents a potential pathway for future moiré-based nanodevices.

New Insights Into Memory Window of Ferroelectric FET Impacted by Read Operations With Awareness of Polarization Switching Dynamics

In this article, the impacts of read operation under transient gate voltage sweep on the memory window (MW) of ferroelectric field-effect transistor (FeFET) are systematically investigated. By taking into consideration the polarization switching dynamics, it reveals a significant dependence on the sweep range and rate of gate voltage. With increasing sweep range, the increase of polarization switching leads to the monotonically increasing MW. However, different from sweep range, the MW of FeFET nonmonotonically varies with the increasing sweep rate, which is caused by the competition between decreased polarization switching and increased voltage drop on ferroelectric (FE) layer during forward sweep. Besides, compared with the MW obtained by pulsed gate voltage, it is found that the MW obtained by swept gate voltage can be the smaller one even without the consideration of charge trapping contribution. Furthermore, the impacts of FE switching time on MW are also discussed, indicating that the MW may not always be a constant even under quasi-static sweeping with low sweep rate, and the relationship between sweep rate of gate voltage and switching speed of FE plays a critical role in MW evaluation.

The negative dielectric permittivity of polycrystalline barium titanate nanofilms under high-strength kHz-AC fields

Unlike the conventional high-frequency dielectric behavior for which the amplitude of the applied field is too small to cause polarization switching, the dynamically-driven polarization reorientation brings remarkable frequency dependence to the nonlinear electromechanical behaviors of ferroelectric nanofilms. In this study, we developed a thermodynamically consistent phase-field model with an introduced second-order kinetic equation to investigate the dynamics of polarization switching in polycrystalline barium titanate nanofilms under high-strength AC fields. The newly introduced kinetic equation consists of an additional dynamic inertial term that brings about collective resonance of polarization within a certain frequency range. Negative dielectric permittivity is observed in a certain frequency band as the applied frequency varies from 30 to 80 kHz, and the maximum absolute value is reached around 40 kHz. It was demonstrated through microstructure-based analysis that such negative dielectric behavior is inherited from the frequency-dependent polarization switching in the nanograins. It also serves as the mechanism for other observed frequency-dependent physical properties, e.g., the remnant polarization, the piezoelectric coefficient at zero electric field and the coercive field. In addition, we further explored the influence of the damping coefficient in the kinetic equation and the influence of the in-plane strain to such frequency-dependent behaviors. It was found that a higher damping coefficient or applied in-plane strain tends to weaken the occurrence of negative permittivity.

Progress on Emerging Ferroelectric Materials for Energy Harvesting, Storage and Conversion

AbstractSince the discovery of Rochelle salt a century ago, ferroelectric materials have been investigated extensively due to their robust responses to electric, mechanical, thermal, magnetic, and optical fields. These features give rise to a series of ferroelectric‐based modern device applications such as piezoelectric transducers, memories, infrared detectors, nonlinear optical devices, etc. On the way to broaden the material systems, for example, from three to two dimensions, new phenomena of topological polarity, improper ferroelectricity, magnetoelectric effects, and domain wall nanoelectronics bear the hope for next‐generation electronic devices. In the meantime, ferroelectric research has been aggressively extended to more diverse applications such as solar cells, water splitting, and CO2 reduction. In this review, the most recent research progress on newly emerging ferroelectric states and phenomena in insulators, ionic conductors, and metals are summarized, which have been used for energy storage, energy harvesting, and electrochemical energy conversion. Along with the intricate coupling between polarization, coordination, defect, and spin state, the exploration of transient ferroelectric behavior, ionic migration, polarization switching dynamics, and topological ferroelectricity, sets up the physical foundation ferroelectric energy research. Accordingly, the progress in understanding of ferroelectric physics is expected to provide insightful guidance on the design of advanced energy materials.

Analog Synapses Based on Nonvolatile FETs With Amorphous ZrO2 Dielectric for Spiking Neural Network Applications

In this article, an analog synapse based on a nonvolatile field-effect transistor with amorphous ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dielectrics has been fabricated and demonstrated. The conductance modulation properties of the devices have been systematically evaluated. Due to the polarization switching dynamics of the ferroelectric-like amorphous ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dielectric, which is attributable to the voltage-driven oxygen vacancies and negative charges dipoles, the proposed ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based devices exhibit superior synaptic characteristics, including good symmetry and linearity for both potentiation and depression, with small cycle-to-cycle variations. The ratio of maximum conductance and minimum conductance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${G}_{max}/{G}_{min})$ </tex-math></inline-formula> of devices reaches 130, with conductance states over 30. Also, spike-timing-dependent plasticity (STDP) has been mimicked in the devices successfully. Furthermore, based on the experimental STDP characteristics and conductance modulation properties of potentiation and depression, a spiking neural network architecture constructed by the proposed ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based synapses has been simulated. High offline and online learning accuracy of 94% and 87%, respectively, on the handwritten digits dataset, has been achieved.

Nucleation-Limited Switching Dynamics Model for Efficient Ferroelectrics Circuit Simulation

The nucleation-limited switching (NLS) model has been shown in the literature to explain successfully the dynamics of polarization switching of polycrystalline ferroelectric films (FE) in various experiments. However, because of its mathematical complexity, the NLS model has yet to be implemented correctly and efficiently in the simulation of actual, experimental circuits with complex driving waveforms. Here, we reformulate NLS as a generalized polarization rate equation that captures correctly the incubation time of nucleation and the intra- and inter-grain switching statistics of kinetics without any restrictions on waveform and switching time scales. This formulation forms the basis for efficient numerical finite-difference computation, which is verified in realistic circuit simulations of a number of experiments.

Four-Terminal Ferroelectric Schottky Barrier Field Effect Transistors as Artificial Synapses for Neuromorphic Applications

In this paper, artificial synapses based on four terminal ferroelectric Schottky barrier field effect transistors (FE-SBFETs) are experimentally demonstrated. The ferroelectric polarization switching dynamics gradually modulate the Schottky barriers, thus programming the device conductance by applying negative or postive pulses to imitate the excitation and inhibition behaviors of the biological synapse. The excitatory post-synaptic current can be modulated by the back-gate bias, enabling the reconfiguration of the weight profile with high speed of 20 ns and low energy (< 1 fJ/spike) consumption. Besides, the tunable long term potentiation and depression show high endurance and very small cycle-to-cycle variations. Based on the good linearity, high symmetricity and large dynamic range of the synaptic weight updates, a high recognition accuracy (92.6%) is achieved for handwritten digits by multilayer perceptron artificial neural networks. These findings demonstrate FE-SBFET has high potential as an ideal synaptic component for the future intelligent neuromorphic network.

Dynamics of ferroelectric polarization and internal electric field in PbTiO3/SrTiO3 superlattices

We study the dynamics of polarization and internal electric field in the PbTiO3/SrTiO3 (PT/ST) superlattices using the Landau–Khalatnikov equation. Apart from the polarization hysteresis loop, the hysteresis loop of the internal electric field is also observed. The effects of thickness ratio and temperature on the switching dynamics of superlattices are investigated. We found that the dynamics of polarization switching is directly correlated with the internal electric field. The change of sign in the internal electric field implies the occurrence of polarization switching in the ferroelectric superlattices.

Positive-to-negative subthreshold swing of a MOSFET tuned by the ferroelectric switching dynamics of BiFeO3

Ferroelectricity can reduce the subthreshold swing (SS) of metal-oxide-semiconductor field-effect transistors (MOSFETs) to below the room-temperature Boltzmann limit of ~60 mV/dec and provides an important strategy to achieve a steeper SS. Surprisingly, by carefully tuning the polarization switching dynamics of BiFeO3 ferroelectric capacitors the SS of a commercial power MOSFET can even be tuned to zero or a negative value, i.e., the drain current increases with a constant or decreasing gate voltage. In particular, in addition to the positive SS of lower than 60 mV/dec, the zero and negative SS can be established with a drain current spanning for over seven orders of magnitude. These intriguing phenomena are explained by the ferroelectric polarization switching dynamics, which change the charge redistributions and accordingly affect the voltage drops across the ferroelectric capacitor and MOSFET. This study provides deep insights into understanding the steep SS in ferroelectric MOSFETs, which could be promising for designing advanced MOSFETs with an ultralow and tunable SS.

Accurate force field of two-dimensional ferroelectrics from deep learning

The discovery of two-dimensional (2D) ferroelectrics with switchable out-of-plane polarization such as monolayer $\alpha$-In$_2$Se$_3$ offers a new avenue for ultrathin high-density ferroelectric-based nanoelectronics such as ferroelectric field effect transistors and memristors. The functionality of ferroelectrics depends critically on the dynamics of polarization switching in response to an external electric/stress field. Unlike the switching dynamics in bulk ferroelectrics that have been extensively studied, the mechanisms and dynamics of polarization switching in 2D remain largely unexplored. Molecular dynamics (MD) using classical force fields is a reliable and efficient method for large-scale simulations of dynamical processes with atomic resolution. Here we developed a deep neural network-based force field of monolayer In$_2$Se$_3$ using a concurrent learning procedure that efficiently updates the first-principles-based training database. The model potential has accuracy comparable with density functional theory (DFT), capable of predicting a range of thermodynamic properties of In$_2$Se$_3$ polymorphs and lattice dynamics of ferroelectric In$_2$Se$_3$. Pertinent to the switching dynamics, the model potential also reproduces the DFT kinetic pathways of polarization reversal and 180$^\circ$ domain wall motions. Moreover, isobaric-isothermal ensemble MD simulations predict a temperature-driven $\alpha \rightarrow \beta$ phase transition at the single-layer limit, as revealed by both local atomic displacement and Steinhardt's bond orientational order parameter $Q_4$. Our work paves the way for further research on the dynamics of ferroelectric $\alpha$-In$_2$Se$_3$ and related systems.