Ferroelectric 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.

Epitaxial Bi5Ti3FeO15 thin films on Nb‐doped SrTiO3 substrates

AbstractWe investigated the ferroelectric switching dynamics as well as the multiferroic and piezoelectric properties of highly a‐oriented epitaxial Bi5Ti3FeO15 (BTFO) thin films on Nb‐doped SrTiO3 single crystal substrates. The BTFO thin films favored highly a‐oriented crystallinity because c‐oriented crystallinity decreased under deposition conditions in which substrate temperature and thin film deposition rate were simultaneously lowered. The highly a‐oriented epitaxial BTFO thin films showed the best ferroelectric properties, whereas the highly c‐oriented epitaxial BTFO thin films showed the best ferromagnetic properties. In particular, the BTFO thin films in which a‐ and c‐oriented crystallinity were properly mixed showed the best piezoelectric properties.

Controllable tuning of ferroelectric switching via the lattice in crystallographically engineered molecular ferroelectrics

Reducing the switching energy and improving the switching speed of ferroelectrics remain an important goal in the pursuit of electronic devices with ultralow energy consumption and ultrafast response. Molecular ferroelectrics with concise dipole switching mechanism and facile structural tunability are a good platform for manipulating the ferroelectric domains. A methodology is demonstrated to manipulation of ferroelectric domain switching by tailor-made lattice parameters of molecular ferroelectrics, by following which, we succeeded in lowering the threshold electric field and improving the dynamics of ferroelectric switching. Our findings advance the fundamental understanding of microscopic mechanism and provide important insights in controllable tuning of ferroelectric domain switching.

Asymmetric Electrode Influence upon Ferroelectric Switching Dynamics in Ag/PVDF‐HFP/ITO Capacitor Structures

Asymmetric electrode (Ag and indium tin oxide (ITO)) influence upon ferroelectric switching dynamics in polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) thin films across Ag/PVDF-HFP/ITO capacitor structures is systematically investigated. Grazing incidence X-Ray diffraction (GI–XRD), atomic force microscopy (AFM), and UV–visible spectroscopy are utilized to comprehend the structural and microstructural characteristics of PVDF–HFP thin films upon ITO interface. Low voltage rectified and asymmetric current–voltage (I–V) curves reveal the varied barrier potential due to the asymmetric electrodes. Typical squared hysteresis loops along with uniform domain growth model are verified from quasi-static and dynamic ferroelectric loop measurements. Further, from low-frequency (10 and 15 mHz) quasi-static and dynamic (100–1000 Hz) triangular pulses, the switching time, domain dimension with respect to frequency, and amplitude of the electric field are separately evaluated and noted that smaller switching times when electron traverses from Ag to ITO electrode.

Acoustic emission study on avalanche dynamics of ferroelectric switching in lead zirconate titanate ceramics

The avalanche dynamics of ferroelectric switching in lead zirconate titanate ceramics was investigated using acoustic emission. Two distinct power-law regimes for ferroelectric switching events were identified by an anomaly in the histogram of probability density, in contrast to the single power-law behavior observed in BaTiO3 single crystals. Such an anomaly is ascribed to the different minimum cut-offs of two power-law distributions. The critical energy exponents were determined to be 1.25 ± 0.10 for energies <103 aJ and 1.51 ± 0.14 for energies >103 aJ. The events in both regimes can be attributed to the depinning of domain walls from two distinct types of defects. The events in the lower energy regime are associated with domain wall junctions due to long-range electric and elastic interactions, whereas the latter is related to extrinsic defects, such as vacancies and dislocations. Moreover, for both regions, the rate of aftershocks follows the Omori power-law, indicating the same critical temporal correlations between the avalanches.

Symmetry of ferroelectric switching and domain walls in hafnium dioxide

Hafnium dioxide $(\mathrm{Hf}{\mathrm{O}}_{2})$ is a promising ferroelectric (FE) material for achieving high-density nonvolatile memory and neuromorphic computing due to its compatibility with the mainstream integrated circuit technology and the surprisingly enhanced ferroelectricity by reduced thickness. The FE switching dynamics is essential to the device performance, but the complexity of $\mathrm{Hf}{\mathrm{O}}_{2}$ atomic structure causes unknown of various FE switching paths and domain-wall configurations. Here, we demonstrate that its low-barrier paths and domain walls can be comprehensively found and understood from a perspective of topological symmetry. By discussing pseudochirality and equivalent transformation relations in a crystal with first principles and lattice modes, we classify and analyze 4 low-barrier FE switching paths and 93 irreducible topology domain-wall configurations in $\mathrm{Hf}{\mathrm{O}}_{2}$. The anisotropic switching mechanism is inferred based on the investigation for 12 types of ${180}^{\ensuremath{\circ}}\text{-side}$ domain walls. This methodology is expected to be generally applicable to displacive ferroelectrics with low unit-cell point-group symmetries and lay a foundation for mechanism study of the switching dynamics.

Continuous and fast magneto-ionic control of magnetism in Ta/Co/BiFeO3/SrRuO3 multiferroic heterostructure

Room temperature electric field controlled magnetism is extremely promising for the next-generation high-performance spintronic devices. Here, based on the ferroelectric switching driven oxygen ion migration in the Ta/Co/BiFeO3/SrRuO3 heterostructures, the magnetic moment, magnetic coercive field, exchange bias field, and junction resistance are reversibly manipulated by tuning the ferroelectric polarization of the BiFeO3 layer. All these phenomena are consistently explained by the oxygen ion migration induced CoOx/Co redox effect, which is evidenced by the synchrotron X-ray absorption spectroscopy measurements. Interestingly, owing to the controllable ferroelectric switching dynamics of the BiFeO3 thin film, the magnetic coercive field of the Co thin film can be continuously and precisely tuned by controlling the ferroelectric polarization of the BiFeO3 thin film, and the manipulating speed of the voltage control of magnetism can be fast to 100 ns. This nonvolatile, stable, reversible, fast, and reproducible voltage control of magnetism shows great potential for designing low-power and high-speed spintronics.

Conditional radiation tolerance of ferroelectric Hf0.5Zr0.5O2 thin film under 60Co gamma-ray irradiation

We report the effect of 60Co gamma-ray irradiation on the ferroelectric properties of metal–ferroelectric Hf0.5Zr0.5O2–metal thin film structures. The pristine Hf0.5Zr0.5O2 films showed strong radiation tolerance against gamma-rays with stable remanant polarization values. When Hf0.5Zr0.5O2 films were exposed to electric field cycling, or “wake-up” process, prior to irradiation, however, their ferroelectricity demonstrated a clear degradation of remanant polarization and coercive voltage shift of the hysteresis curves. The analysis of ferroelectric switching dynamics revealed faster polarization switching with broadening of Lorentzian distribution of characteristic switching time for higher radiation doses, which is contrary to the wake-up behavior. The relationship between the wake-up process and gamma-ray irradiation on the stability of ferroelectric Hf0.5Zr0.5O2 films was discussed in light of domain alignment and defect mechanisms, considering both the redistribution and trapping of defect charges.

The role of lattice dynamics in ferroelectric switching

Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO3) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO3 films, to large (10’s of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.

High-precision and linear weight updates by subnanosecond pulses in ferroelectric tunnel junction for neuro-inspired computing

The rapid development of neuro-inspired computing demands synaptic devices with ultrafast speed, low power consumption, and multiple non-volatile states, among other features. Here, a high-performance synaptic device is designed and established based on a Ag/PbZr0.52Ti0.48O3 (PZT, (111)-oriented)/Nb:SrTiO3 ferroelectric tunnel junction (FTJ). The advantages of (111)-oriented PZT (~1.2 nm) include its multiple ferroelectric switching dynamics, ultrafine ferroelectric domains, and small coercive voltage. The FTJ shows high-precision (256 states, 8 bits), reproducible (cycle-to-cycle variation, ~2.06%), linear (nonlinearity <1) and symmetric weight updates, with a good endurance of >109 cycles and an ultralow write energy consumption. In particular, manipulations among 150 states are realized under subnanosecond (~630 ps) pulse voltages ≤5 V, and the fastest resistance switching at 300 ps for the FTJs is achieved by voltages <13 V. Based on the experimental performance, the convolutional neural network simulation achieves a high online learning accuracy of ~94.7% for recognizing fashion product images, close to the calculated result of ~95.6% by floating-point-based convolutional neural network software. Interestingly, the FTJ-based neural network is very robust to input image noise, showing potential for practical applications. This work represents an important improvement in FTJs towards building neuro-inspired computing systems.

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.

Local field inhomogeneity and ferroelectric switching dynamics in Hf1−xZrxO2 thin films

We report on the inhomogeneity of the internal bias according to the Zr alloy concentration confirmed by the ferroelectric switching dynamics of ${\mathrm{Hf}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{2}$ thin films. The analytic model for the internal bias was considered in terms of the dipole-dipole interaction accompanied by Zr concentration variation. The ferroelectricity and switching dynamics of ${\mathrm{Hf}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{2}$ thin films were investigated using conventional electrical measurements and local piezoresponse microscopy. Analysis of static and dynamic polarization reversal revealed a correlation between the activation field for polarization switching and Zr concentrations through the variation in local ferroelectric coercivity and the characteristic ferroelectric switching time. The spatially inhomogeneous local built-in electric field was attributed to the defect interaction in the ${\mathrm{Hf}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{2}$ thin films.

Monitoring Electrical Biasing of Pb(Zr0.2Ti0.8)O3 Ferroelectric Thin Films In Situ by DPC-STEM Imaging.

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr0.2Ti0.8)O3 thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions.

Electric field cycling-mediated variations in defect distributions associated with wake-up and split-up behaviors of a ferroelectric Si-doped HfO2 thin film

We report variations in the characteristic ferroelectric switching time and interface free carrier concentration associated with wake-up and split-up behaviors of a 4.2 mol. % Si-doped HfO2 thin film. Prior to the development of the split-up behavior, the thin films exhibited the wake-up behavior; the remanent polarization increased on repeated electric field cycling. After the wake-up behavior, Si-doped HfO2 films exhibited degradation of the remanent polarization values and splitting of the ferroelectric switching current peaks when a lower electric field was swept; this is referred to as the split-up behavior. An investigation of the ferroelectric switching dynamics revealed retardation, followed by recovery, of the characteristic switching time for ferroelectric nucleation, coincident with the wake-up and split-up behaviors, respectively. We analyzed the interface free carrier concentration from capacitance–voltage characteristics and revealed that the wake-up and split-up behaviors were associated with changes of numbers of defects as electric field cycling continued.

Barkhausen noise analysis of thin film ferroelectrics

The first direct Barkhausen noise measurement in a ferroelectric thin film is presented. The Barkhausen noise energy loop is reconstructed from the measured Barkhausen noise and is closely related to the classic ferroelectric P vs E hysteresis loop. Grain boundaries act as a dominant ferroelectric domain wall pinning site in a polycrystalline thin film based on the calculated domain wall jump distance using the Barkhausen noise frequency. The technique is promising for the measurement of ferroelectric switching dynamics, and provides a physical insight for improving application performance.

Ferroelectric domain dynamics and stability in graphene oxide-P(VDF-TrFE) multilayer films for ultra-high-density memory application

The rapid growth of miniaturized electronic devices has raised the demand for compact, flexible, wearable, and non-volatile memory units. However, integration into nanoelectronic devices requires a scaled-down data-storage component, but this often results in the deterioration of the ferroelectric switching performance. Herein, we demonstrate a simple and scalable fabrication of poly (vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] film with graphene oxide (GO) nanosheets. Using piezoresponse force microscopy (PFM), the storage features of this multilayer film were investigated, including establishment of two stable memory states, ferroelectric switching dynamics in the point-polarization and linear-polarization modes, and time and thermal stability of information storage. Remarkably, the GO-P(VDF-TrFE) film favored formation of low-temperature (LT) ferroelectric phase with much more ordered sequences of trans conformations relative to pristine P(VDF-TrFE) due to the presence of electrostatic interaction between GO nanosheets and CF dipoles of P(VDF-TrFE), thus affording improved ferroelectric properties. The GO-P(VDF-TrFE) film showed several excellent storage features, such as ultra-high density of more than 300 Gbits in−2, good writing & erasing repeatability, long data retention time, and elevated device operation temperature. In-depth understanding and utilization of the excellent non-volatile memory performance of this new GO-ferroelectric system will open new avenues for the next generation of nanoelectronic devices.

The Influence of Carbosilane Nanosegregation on the Dynamics in 'de Vries-type' Liquid Crystals.

The mesogens QL32-6, QL33-6 and QL-34-6 contain 5-phenylpyrimidine cores and terminal nanosegregating carbosilane end groups of different lengths and are known to exhibit 'de Vries-type' properties of varying strength. We report a systematic study of the influence of the nanosegregating sublayer on the dynamics and rotational viscosities of the collective modes in the smectic A* (SmA*) and smectic C* (SmC*) phase using dielectric spectroscopy. It was found that the dynamics of the Goldstone mode corresponding to phase angle fluctuations are almost not affected while the relaxation time and rotational viscosity of the soft mode are influenced by the degree of nanosegregation. In other words, the nanosegregating sublayer does not influence the dynamics of ferroelectric switching in the SmC* phase, but is critical in inducing 'de Vries-type' properties.

Compact modelling of ferroelectric tunnel memristor and its use for neuromorphic simulation

Ferroelectric tunnel memristor (FTM) was recently discovered. Its resistance can be continuously tuned by controlling the growth of domain in ferroelectric tunnel barrier. Experiments show its large OFF/ON resistance ratio (&amp;gt;102) and high operation speed (∼10 ns), providing great potential to build up multi-level storage and neuromorphic circuits. However, the corresponding electrical model for circuit simulation is still lacking. In this Letter, we developed a compact model for a voltage-controlled ferroelectric tunnel memristor based on experimental results and ferroelectric switching dynamics. The memristive behavior of this model was demonstrated by resistance measurements as a function of the amplitude and duration of programming voltage pulses, and the accuracy was validated by the relative good agreement between simulation results and experimental measurement. Our model was used to simulate a FTM-based spike-timing dependent plasticity circuit to show its use in neuromorphic circuits.

Ferroelectric Switching Dynamics of Topological Vortex Domains in a Hexagonal Manganite

Field-induced switching of ferroelectric domains with a topological vortex configuration is studied by atomic imaging and electrical biasing in an electron microscope, revealing the role of topological defects on the topologically-guided change of domain-wall pairs in a hexagonal manganite.

Accessing intermediate ferroelectric switching regimes with time-resolved transmission electron microscopy

BiFeO3 (BFO) is one of the most widely studied magneto-electric multiferroics. The magneto-electric coupling in BiFeO3, which allows for the control of the ferroelectric and magnetic domain structures via applied electric fields, can be used to incorporate BiFeO3 into novel spintronics devices and sensors. Before BiFeO3 can be integrated into such devices, however, a better understanding of the dynamics of ferroelectric switching, particularly in the vicinity of extended defects, is needed. We use in situ transmission electron microscopy (TEM) to investigate the response of ferroelectric domains within BiFeO3 thin films to applied electric fields at high temporal and spatial resolution. This technique is well suited to imaging the observed intermediate ferroelectric switching regimes, which occur on a time- and length-scale that are too fine to study via conventional scanning-probe techniques. Additionally, the spatial resolution of transmission electron microscopy allows for the direct study of the dynamics of domain nucleation and propagation in the presence of structural defects. In this article, we show how this high resolution technique captures transient ferroelectric structures forming during biasing, and how defects can both pin domains and act as a nucleation source. The observation of continuing domain coalescence over a range of times qualitatively agrees with the nucleation-limited-switching model proposed by Tagantsev et al. We demonstrate that our in situ transmission electron microscopy technique is well-suited to studying the dynamics of ferroelectric domains in BiFeO3 and other ferroelectric materials. These biasing experiments provide a real-time view of the complex dynamics of domain switching and complement scanning-probe techniques.