Ferroelectric Switching Research Articles

Changes over time in ferroelectricity of nanocomposites containing cellulose combined with [NH4][Zn(HCOO)3

ABSTRACT For the first time, a composite based on a ferroelectric metal-organic framework of [NH4][Zn(HCOO)3] combined with nanocellulose at different cellulose concentrations from 0 to 75 wt% was synthesised to investigate the changes over time in phase transition and ferroelectric switching. A comparative study for fresh samples and the samples after 288 h was conducted. Electrophysical parameters were measured in a temperature range of 140 to 245 K under a weak electric field with an amplitude of 2 V.cm−1 at a frequency of 1 kHz. The results indicated that the increase in cellulose content led to increasing the phase transition temperature from 190.1 to 238.5 K. Besides, the coercive field was also found to increase from 3.78 to 14.12 kV.cm−1 along with the reduction of dielectric permittivity and maximum polarisation. After 288 h, this phase transition decreased significantly over time even to 231.6 K in the case of highest cellulose content (75 wt%). Meanwhile, the coercive field was found to decrease from 14.12 to 10.57 kV.cm−1. Remarkable changes were detected for samples with cellulose content of higher 25 wt%. The weaker interaction between ferroelectric part and cellulose over time due to the softness of cellulose was responsible for the obtained phenomena.

The Effect of Chemical Environment and Temperature on the Domain Structure of Free-Standing BaTiO3 via In Situ STEM.

Ferroelectrics, due to their polar nature and reversible switching, can be used to dynamically control surface chemistry for catalysis, chemical switching, and other applications such as water splitting. However, this is a complex phenomenon where ferroelectric domain orientation and switching are intimately linked to surface charges. In this work, the temperature-induced domain behavior of ferroelectric-ferroelastic domains in free-standing BaTiO3 films under different gas environments, including vacuum and oxygen-rich, is studied by in situ scanning transmission electron microscopy (STEM). An automated pathway to statistically disentangle and detect domain structure transformations using deep autoencoders, providing a pathway towards real-time analysis is also established. These results show a clear difference in the temperature at which phase transition occurs and the domain behavior between various environments, with a peculiar domain reconfiguration at low temperatures, from a-c to a-a at ≈60°C. The vacuum environment exhibits a rich domain structure, while under the oxidizing environment, the domain structure is largely suppressed. The direct visualization provided by in situ gas and heating STEM allows to investigate the influence of external variables such as gas, pressure, and temperature, on oxide surfaces in a dynamic manner, providing invaluable insights into the intricate surface-screening mechanisms in ferroelectrics.

Effect of growth temperature on self-rectifying BaTiO3/ZnO heterojunction for high-density crossbar arrays and neuromorphic computing

In the quest for high-density integration and massive scalability, ferroelectric-based devices provide an achievable approach for nonvolatile crossbar array (CBA) architecture and neuromorphic computing. In this report, ferroelectric-semiconductor (Pt/BaTiO3/ZnO/Au) heterojunction-based devices are demonstrated to exhibit nonvolatile and synaptic characteristics. In this study, the ferroelectric (BaTiO3) layer was modulated at various growth temperatures of 350 °C, 450 °C, 550 °C and 650 °C. Growing temperature in the ferroelectric layer has a significant impact on resistive switching. The ferroelectricity of the BaTiO3 thin film enhanced by increasing temperature causes a substantial shift in the interface state density at heterojunction interface, which is crucial for self-rectification. Furthermore, this self-rectifying property advances to reduce the crosstalk problem without any selector device. Enhanced resistive switching and neuromorphic applications have been demonstrated using BaTiO3 heterostructure devices at 550 °C. The dynamic ferroelectric polarization switching in this heterojunction demonstrated linear conductance change in artificial synapses with 91 % recognition accuracy. Ferroelectric polarization reversal with a depletion region at the heterojunction interface is the responsible mechanism for the switching in these devices. Thus, these findings pave the way for designing low power high-density crossbar arrays and neuromorphic application based on ferroelectric-semiconductor heterostructures.

Switching of Hybrid Improper Ferroelectricity in Oxide Double Perovskites

Switching of Hybrid Improper Ferroelectricity in Oxide Double Perovskites

Ultralow Power and Shifting-Discretized Magnetic Racetrack Memory Device Driven by Chirality Switching and Spin Current.

Magnetic racetrack memory has significantly evolved and developed since its first experimental verification and is considered one of the most promising candidates for future high-density on-chip solid-state memory. However, both the lack of a fast and precise magnetic domain wall (DW) shifting mechanism and the required extremely high DW motion (DWM) driving current make the racetrack difficult to commercialize. Here, we propose a method for coherent DWM that is free from the above issues, which is driven by chirality switching (CS) and an ultralow spin-orbit-torque (SOT) current. The CS, as the driving force of DWM, is achieved by the sign change of the Dzyaloshinskii-Moriya interaction, which is further induced by a ferroelectric switching voltage. The SOT is used to break the symmetry when the magnetic moment is rotated in the Bloch direction. We numerically investigate the underlying principle and the effect of key parameters on the DWM by micromagnetic simulations. Under the CS mechanism, a fast (∼102 m/s), ultralow energy (∼5 attoJoule), and precisely discretized DWM can be achieved. Considering that skyrmions with topological protection and smaller size are also promising for future racetracks, we similarly evaluate the feasibility of applying such a CS mechanism to a skyrmion. However, we find that the CS causes it to "breathe" instead of moving. Our results demonstrate that the CS strategy is suitable for future DW racetrack memory with ultralow power consumption and discretized DWM.

Ferroelectric Al1−xBxN thin films integrated on Si

Ferroelectric Al0.93B0.07N thin films are prepared (100) Si substrates. The necessary c-axis out-of-plane orientation to observe macroscopic ferroelectric switching was achieved by implementing an initial Ar/N2 plasma treatment, followed by a thin layer of AlN to initiate the desired texture and a 150 nm W layer. The plasma treatment facilitates crystallinity enhancement of the AlN template layer, allowing for subsequent growth of highly oriented W and Al0.93B0.07N layers. The W layer exhibits random in-plane orientation and exclusive (110) out-of-plane orientation with a rocking curve width of 1.4°. When grown on these W surfaces, 175 nm thick Al0.93B0.07N films exhibit random in-plane orientation and exclusive (001) texture with rocking curve full-width-half-max values of 1.6° and RMS roughness values less than 1 nm. Polarization hysteresis measurements show robust hysteresis with coercive field values of 5.4 MV/cm and remanent polarization values of 136 μC/cm2. XPS depth profile analysis suggests that the plasma treatment converts the existing native oxide to a nitrogen rich oxynitride with approximate composition Si3O0.5N3.67. Cross-sectional TEM reveals that the oxynitride interlayer is amorphous and ∼3.4 nm thick, more than double the native oxide thickness measured by multiwavelength ellipsometry, implying that (oxy)nitride growth continues after conversion of the native oxide. This new family of ferroelectric wurtzites is interesting from an integration perspective given their chemical compatibility with mainstream semiconductors. Developing synthesis routes that promote needed texture while preserving compatible processing windows is an important step toward practical integration.

Induction and Ferroelectric Switching of Flux Closure Domains in Strained PbTiO3 with Neural Network Quantum Molecular Dynamics.

We have developed an extension of the Neural Network Quantum Molecular Dynamics (NNQMD) simulation method to incorporate electric-field dynamics based on Born effective charge (BEC), called NNQMD-BEC. We first validate NNQMD-BEC for the switching mechanisms of archetypal ferroelectric PbTiO3 bulk crystal and 180° domain walls (DWs). NNQMD-BEC simulations correctly describe the nucleation-and-growth mechanism during DW switching. In triaxially strained PbTiO3 with strain conditions commonly seen in many superlattice configurations, we find that flux-closure texture can be induced with application of an electric field perpendicular to the original polarization direction. Upon field reversal, the flux-closure texture switches via a pair of transient vortices as the intermediate state, indicating an energy-efficient switching pathway. Our NNQMD-BEC method provides a theoretical guidance to study electro-mechano effects with existing machine learning force fields using a simple BEC extension, which will be relevant for engineering applications such as field-controlled switching in mechanically strained ferroelectric devices.

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.

Dielectric properties and ferroelectric resistive switching mechanism in the epitaxial (111) BiFeO3 films

Resistive random access memory is promising for next-generation nonvolatile memory, which has high memory density, low power consumption, and fast read/write speed. However, the resistive switching (RS) mechanism in oxide ferroelectric thin film devices is still unclear. In this paper, Pt/BiFeO3/La0.5Sr0.5CoO3(Pt/BFO/LSCO) heterostructures were deposited on (111)SrTiO3(STO) substrates by magnetron sputtering. X-ray diffraction confirmed the epitaxial relationship between (111)BFO//(111)LSCO//(111)STO. Atomic force microscopy (AFM) showed that (111)BFO had an island structure with uniform grain distribution. The Pt/(111)BFO/(111)LSCO heterostructures exhibit obvious ferroelectricity and RS effects. The results of C-V analysis and I-V fitting indicate that the RS is caused by the space-charge-limited conduction mechanism.

Ferroelectric Switching at Symmetry-Broken Interfaces by Local Control of Dislocations Networks.

Semiconducting ferroelectric materials with low energy polarization switching offer a platform for next-generation electronics such as ferroelectric field-effect transistors. Recently discovered interfacial ferroelectricity in bilayers of transition metal dichalcogenide films provides an opportunity to combine the potential of semiconducting ferroelectrics with the design flexibility of 2D material devices. Here, local control of ferroelectric domains in a marginally twisted WS2 bilayer is demonstrated with a scanning tunneling microscope at room temperature, and their observed reversible evolution is understood using a string-like model of the domain wall network (DWN). Two characteristic regimes of DWN evolution are identified: (i) elastic bending of partial screw dislocations separating smaller domains with twin stackings due to mutual sliding of monolayers at domain boundaries and (ii) merging of primary domain walls into perfect screw dislocations, which become the seeds for the recovery of the initial domain structure upon reversing electric field. These results open the possibility to achieve full control over atomically thin semiconducting ferroelectric domains using local electric fields, which is a critical step towards their technological use.

Multiple modulation of magnetism in two-dimensional FeCl2/In2Se3 van der Waals heterostructure

The effective modulation of magnetism for two-dimensional (2D) materials not only has potential applications in nanoscale spintronic devices but also has urgent demands in modern industries. In this work, we report the discovery of ferroelectrically (FE) tunable orbital reconstruction in FeCl2/In2Se3 2D van der Waals (vdWs) heterostructures (HSs), which leads to a transition from ferromagnetic (FM) order to ferrimagnetic order (intra-layer magnetic coupling is AFM3 arrangement). The reversible FE polarization not only enables the easy magnetization axis to be tuned from the out-of-plane to in-plane direction but also reduces the net magnetization strength from 31.87 to −0.18 μB/f.u. Based on the charge density differences and the density of states analysis, the preference of FM and AFM3 arrangement can be reasonably explained by the Goodenough–Kanamori–Anderson rule. The ferroelectric switching enables nonvolatile electric control of magnetic order and anisotropy, offering significant potential for high-efficiency nanodevices and nonvolatile information storage. Moreover, modulation of magnetism (magnetic moment from −0.18 to 0.18 μB/f.u.) can also be achieved through interlayer sliding in the −P case, providing a way to control magnetism in 2D vdWs HSs.

Ferroelectric switching studies in symmetric Ag/Poly(vinylidene fluoride-co-hexafluoropropylene)/Ag capacitor structures

In this article, ferroelectric switching properties of Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) thin films across symmetric Ag/PVDF-HFP/Ag capacitor structures have been investigated. The structural, microstructural and optical properties of the films are evaluated with grazing incidence X-ray diffraction, Atomic Force Microscopy, and UV visible spectrophotometer. Symmetric quasi-static current-voltage loops exhibit ferroelectric switching, and the corresponding electric displacement- electric field loops show an increase in the coercive field, with the increase in the amplitude of the applied electric field. Higher resistivity of (1.66 – 5.16) × 108 Ω-cm along with optical band gap of 4 eV indicated the good quality of the junctions. Dynamic Polarization-Electric field loops reveal a remnant polarization of 3.22 μC/cm2 and 3.16 μC/cm2 for 80 nm and 160 nm PVDF-HFP films. The increase in coercive field, remnant and saturation polarization parameters with rise in the amplitude of the applied electric fields at 1 Hz for 160 nm PVDF-HFP film are linked to the uniform domain growth phenomena.

All‐In‐One Optoelectronic Neuristor Based on Full‐vdW Two‐Terminal Ferroelectric p–n Heterojunction

AbstractPhotoelectric synaptic devices with optical sensing capability are of great importance in simulating human vision systems. Especially realizing all‐in‐one vision neuristor on silicon based new materials and principles is being pursued. 2D van der Waals (vdW) materials have the unique advantage of arbitrary stacking on demand. Herein, a full‐vdW two‐terminal 2D ferroelectric α‐In2Se3/SnSe p–n heterojunction is proposed to construct an optoelectronic neuristor to simulate visual synaptic functions. Implementation of these simulations is attributed to the co‐modulation of the electrical polarization reconfigurable built‐in electric field caused by p–n junction and photo‐inducing ferroelectric polarization switching. These functions include ultra‐high paired‐pulse facilitation index (457%), short synaptic plasticity, long synaptic plasticity, and retina‐like optical adaptations. The high PPF is crucial for high‐precision decoding and processing of visual information. Meanwhile, the classical Pavlovian conditioned reflexes associated with associative learning are also emulated showing the ability of the device to handle complex electrical and optical inputs. This study demonstrates that two‐terminal ferroelectric p–n heterojunctions have great potential in high‐precision multifunctional optoelectronic visual synaptic devices.

Polar response and Fréedericksz transition in cybotactic nematic fluids of unsymmetrical bent-core molecules

The appearance of polar order in nematic fluids is of broad interest rejuvenating affluent concepts of physics and chemistry. In bent-core nematics (BCNs), ferroelectric switching is a challenging quest of considerable interest after its primary observation in 1,2,4-oxadiazole-based BCNs. Herein, we observed induced polar switching in the cybotactic nematic phase (Ncyb) with a polarization value of ∼185 nC/cm2 in an unsymmetrical bent-core compound 8-F-OH. Temperature-dependent SAXS/WAXS analysis and dielectric measurements revealed nano-sized cybotactic clustering within the nematic phase. Electric-field-induced Fréedericksz transitions (FT) in the Ncyb phase at different frequencies and temperatures were investigated. Compound 8-F-OH exhibited two domains that formed and competed when the system was driven by an ac field. Thus, the build-up process of FT involved the reorientation of molecules and the transport of domains, where each had a distinctive characteristic time. We envisage that these detailed investigations can pave a bright way toward the development of advanced-level optical and electro-optical applications of BCN systems.

Ultralow Tip-Force Driven Sizable-Area Domain Manipulation through Transverse Flexoelectricity.

Deterministic control of ferroelectric domain is critical in the ferroelectric functional electronics. Ferroelectric polarization can be manipulated mechanically with a nano-tip through flexoelectricity. However, it usually occurs in a very localized area in ultrathin films, with possible permanent surface damage caused by a large tip-force. Here it is demonstrated that the deliberate engineering of transverse flexoelectricity offers a powerful tool for improving the mechanical domain switching. Sizable-area domain switching under an ultralow tip-force can be realized in suspended van der Waals ferroelectrics with the surface intact, due to the enhanced transverse flexoelectric field. The film thickness range for domain switching in suspended ferroelectrics is significantly improved by an order of magnitude to hundreds of nanometers, being far beyond the limited range of the substrate-supported ones. The experimental results and phase-field simulations further reveal the crucial role of the transverse flexoelectricity in the domain manipulation. This large-scale mechanical manipulation of ferroelectric domain provides opportunities for the flexoelectricity-based domain controls in emerging low-dimensional ferroelectrics and related devices.

Ferroelectric YAlN grown by molecular beam epitaxy

We report the demonstration of ferroelectric switching in yttrium (Y)-doped nitride semiconductors. In this study, single-crystalline, wurtzite Y0.07Al0.93N films were epitaxially grown on GaN/sapphire templates by plasma-assisted molecular beam epitaxy. The ferroelectric switching process has been investigated by current density–electric field (J-E) and polarization–electric field (P-E) loops as well as positive-up-negative-down measurements, showing a coercive field of ∼6 MV/cm and a switchable polarization of ∼130 μC/cm2. Ferroelectric switching was further confirmed via butterfly shape capacitance–voltage (C-V) loops and polarity-sensitive wet etching. The realization of ferroelectric, Y-doped AlN films further extends the family of nitride ferroelectrics and unravels a wealth of intriguing opportunities in III-nitride based electronic, piezo-electronic, and optoelectronic devices.

Hydrogen Bonding-Assisted Complete Ferroelectric β-Phase Conversion in Poly(vinylidene fluoride) Thin Films: Exhibiting an Excellent Memory Window and Long Retention.

Organic nonvolatile memory with low power consumption is a critical research demand for next-generation memory applications. Ferroelectric switching characteristics of poly(vinylidene fluoride) (PVDF) thin films modified with a trace amount of hydrated Cu salt (CuCl2·2H2O) are explored in the present study. Herein, a Cu salt-mediated PVDF (Cu/PVDF) thin film with preferential edge-on β-crystallites is fabricated through the orientation-controlled spin coating (OCSC) technique. This work proposes a convenient and effective approach to produce edge-on-oriented electroactive PVDF thin films with a high degree of polar β-phase, so as to realize the favorable switching under low operating voltages. Herein, chemically modified PVDF is anticipated to form a complex intermediate, which attains its stability by undergoing favorable hydrogen bonding that reorients the C-C structure of PVDF to obtain the β-conformation. Such information is verified by X-ray photoelectron spectroscopy (XPS). Grazing incidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that the Cu salt incorporated into the PVDF matrix favored the formation of the electroactive β-phase with edge-on crystallite lamellae. Consequently, the Cu/PVDF thin film demonstrates a good contrast between electric field-assisted written and erased data bits in the piezoresponse force microscopy (PFM) phase image. Furthermore, to obtain the ferroelectric memory window, a metal-ferroelectric-insulator-semiconductor (MFIS) diode with Cu/PVDF as a ferroelectric layer has been fabricated. The capacitance-voltage (C-V) characteristic of the MFIS diode exhibits a memory window of 12 V with a long-term retention behavior (∼longer than 7 days). In a nutshell, we tried to represent a clear understanding of the interfacial interactions of the Cu salt with PVDF, which favor the edge-on formation that results in the promising low-voltage ferroelectric switching and excellent retention response, where any additional electrical poling and/or external stretching is completely possible to be ruled out, thus offering a new prospect for the evolution of devices with long-lasting nonvolatile memories.

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.

Evolution between ferroelectric photovoltaic effect and resistance switching behavior engineered by the polarization field and barrier characteristics.

The bandgap and polarization field play a key role in the ferroelectric photovoltaic effect. However, narrow bandgap induced electrical conductivity always brings out a depression of the photovoltaic performances. Based on the mechanisms of the photovoltaic effect and resistance switching behaviors in ferroelectric materials, this work realizes an evolution between the two effects by engineering the polarization field and barrier characteristics, which addresses the trade-off issues between the bandgap and polarization for ferroelectric photovoltaic effect. SrCoOx (SC, 2.5≤x≤3) with multivalent transition is introduced into Na0.5Bi0.5TiO3 (NBT) matrix material to engineered the polarization field and barrier characteristics. (1-x)NBT-xSC (x=0.03, 0.05, 0.07) solid solution films present an evolution of ferroelectric photovoltaic effect to grow out of nothing again to the disappearance of the photovoltaic effect and the appearance of resistance switching behavior. The 0.95NBT-0.05SC film achieve the open-circuit voltage of 0.81 V and the short-circuit current of 23.52 µA/cm2, and the 0.93NBT-0.07SC film obtains the resistive switching behavior with switch ratio of 100. This work provides a practicable strategy to achieve the fascinating evolution between photovoltaic effect and resistive switching.

Three-terminal vertical ferroelectric synaptic barristor enabled by HZO/graphene heterostructure with rebound depolarization

Ferroelectric switching devices employing doped hafnium oxide (HfO2)-based thin films have expanded their range of electronic applications from densely-packed memory to brain-inspired artificial synapses. Here, we introduce a novel and distinctive three-terminal vertical device using a heterogeneous stack of hafnium-zirconium-oxide (HZO) thin film and graphene, called ferroelectric synaptic barristor (FSB), which can function as a scalable artificial synapse for energy-efficient neuromorphic computing. Electrostatic gating in the FSB can simultaneously reorient the polarization degree and the direction of HZO as well as the Schottky barrier and the trapping degree at the graphene interface. With these peculiar controlling factors, this process mimics crucial synaptic characteristics with rebound depolarization (RD), which converts an incoming inhibitory pre-spike into cell excitation. Using the RD function mediated by both ferroelectric switching and trap-mediated charge transport, the FSB exhibited a stable long-term potentiation/depression for 5000 identical pulse schemes with excellent linearity, high yield (47/48 cells), and a good state-retaining ability for 5000 s. The learning ability and energy consumption based on a single neural network were evaluated using MNIST handwritten digits and fashion patterns. The FSB with RD exhibited better accuracy (90.03% and 74.65% for handwritten digits and fashion patterns, respectively), fast and low-power learning ability than that without RD. Our FSB device can bring the advantages of conventional ferroelectric synaptic devices while mitigating their ingrained technical and operational issues.