Ferroelectric Switching Research Articles

Time-Resolved Nanobeam X-ray Diffraction of a Relaxor Ferroelectric Single Crystal under an Alternating Electric Field

Lead-containing relaxor ferroelectrics show enormous piezoelectric capabilities relating to their heterogeneous structures. Time-resolved nanobeam X-ray diffraction reveals the time and position dependences of the local lattice strain on a relaxor ferroelectric single crystal mechanically vibrating and alternately switching, as well as its polarization under an alternating electric field. The complicated time and position dependences of the Bragg intensity distributions under an alternating electric field demonstrate that nanodomains with the various lattice constants and orientations exhibiting different electric field responses exist in the measured local area, as the translation symmetry breaks to the microscale. The dynamic motion of nanodomains in the heterogeneous structure, with widely distributed local lattice strain, enables enormous piezoelectric lattice strain and fatigue-free ferroelectric polarization switching.

Atomic Visualization and Switching of Ferroelectric Order in β-In2 Se3 Films at the Single Layer Limit.

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In2 Se3 films: β'- and β*-In2 Se3 are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in β'-In2 Se3 and ferroelectric order of β*-In2 Se3 are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.

Enhanced Tunneling Magnetoresistance Effect via Ferroelectric Control of Interface Electronic/Magnetic Reconstructions.

Magnetic tunnel junctions (MTJs) with tunable tunneling magnetoresistances (TMR) have already been proven to have great potential for spintronics. Especially, when ferroelectric materials are used as insulating barriers, more novel functions of MTJs can be realized due to interface magnetoelectric coupling. Here, we demonstrate a very large ferroelectric modulation of TMR (as high as 570% in low-resistance state) in the ferroelectric/magnetic La0.5Sr0.5MnO3/BaTiO3 (LSMO/BTO) junctions and find robust interfacial electronic and magnetic reconstructions via ferroelectric polarization switching. Through electrical, magnetic, and optical measurements combined with X-ray absorption and magnetic circular dichroism, we reveal that the interfacial electronic and magnetic (ferromagnetic/antiferromagnetic phase transition) reconstructions originate from strong electromagnetic coupling between BTO and LSMO at the interface and are driven by the modulation of hole/electron doping at the interface of LSMO/BTO through ferroelectric polarization switching. As a result, the ferroelectrically controlled interface barrier height and width and spin filter effect enable a giant electrical modulation of TMR. Our results shed new light on the intrinsic mechanisms governing magnetoelectric coupling and offering a new route to enhance magnetoelectric coupling for spin control in spintronic devices.

Electrostatic force evolution during the tip-induced ferroelectric domain switching

Different from the signals of amplitude and phase lag in piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS), the signals of the contact resonance frequency (f0) have not been clearly interpreted. Due to the complexity of the tip–sample system at the nanoscale, the relevant models and mechanisms were continuously improved in the past decades. The variation of f0 has been reported in a few previous studies, but the physical meaning of the unique variation in ferroelectric materials still needs to be investigated. Due to the imperfect tip–sample contact, the existence of a dielectric gap and screening charges causes the discontinuity in the conduction band. The evolution of the electrostatic force during the PFS measurements is significantly affected by the tip-induced ferroelectric domain evolution, resulting in the variation of the effective contact stiffness (k*) and f0. By involving the screening effect and tunneling effect in this model, the anomalies of the on-field f0 and off-field f0 can be well explained, respectively.

Synthesis and mesomorphic properties of new organosiloxane chiral calamitic liquid crystals

A new chiral calamitic compound consisting of a phenyl core that is connected through ester linkers to a benzoate carrying a chiral (S)-2-methylbutoxy group at one side and a biphenyl carboxylate with an undecenyloxy chain at other side has been synthesized. Additionally, the siloxane substituted derivatives and a dimer with these two identical chiral calamitics linked by a disiloxane spacer have been obtained via hydrosilylation reaction to study the effect of combining the chiral moiety and siloxane segments within the same structure on liquid crystalline properties. The chiral calamitic compound and derivatives with a siloxane end-group or a siloxane spacer have been characterized using classical spectroscopic methods (1H NMR, 13C NMR, 29Si NMR and MS) and elemental analysis (EA). The liquid crystalline properties of all new compounds were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray scattering and electro-optic methods. With the introduction of a siloxane end-group to the vinyl-terminated calamitic compound exhibiting enantiotropic chiral nematic phase (N*) and chiral tilted smectic phase (SmC*), the occurrence of stable smectic phase was strongly promoted. In a similar way, the dimer with two identical calamitic mesogens connected by a disiloxane spacer exhibited a wider smectic mesophase interval in addition to the presence of N* mesophase. The new mesogens exhibit a ferroelectric switching with PS of around 200 nC cm−2 in their chiral tilted smectic (SmC*) mesophase range. The transition temperatures of organosiloxanes are considerably lower than that of olefinic precursor.

Crystal Orientations Dependent Polarization Reversal in Ferroelectric PbZr0.2Ti0.8O3 Thin Films for Multilevel Data Storage Applications

AbstractDeterministic creation of multiple ferroelectric states with variant values of polarization in ferroelectric thin films is promising for multilevel data storage applications. However, the intrinsic bi‐stability of ferroelectric switching makes it challenging to achieve. The deterministic ferroelectric states can be achieved by multistep switching through various means such as controlling domain structure and switching by inducing local disorder, and also by crystal orientations. However, to determine the effect of crystal directions on the stability of multistep polarization reversal requires investigation on different crystal directions. A high‐throughput approach namely combinatorial substrate epitaxy (CSE) is utilized to fabricate La0.7Sr0.3MnO3 (LSMO) polycrystalline substrate to grow PbZr0.2Ti0.8O3 (PZT) films epitaxially with different crystal directions in a single sample. The polarization reversal area is determined as a function of applied voltage along different <hkl>. From the analysis, it is found that all <hkl> exhibit multilevel polarization states due to multistep switching events. Further the degree of multilevel polarization states shows a sawtooth‐like oscillatory behavior with increase in [111] miscut angle, which is attributed to influence of local disorder on the domain structure. Therefore, the realization of multilevel data storage device based on deterministic polarization reversal can be achieved with appropriate crystal directed thin films.

High-Quality Sputtered BiFeO3 for Ultrathin Epitaxial Films

BiFeO3 films were grown by RF magnetron sputtering with various O2 gas flow ratios and substrate temperatures. The optimal sputtering conditions for a slightly excess Bi content produced high-quality parameters: an atomically flat surface (Ra < 0.4 nm), low leakage current (Jc < 10–6 A/cm2), high ferroelectric polarization (72 μC/cm2//[001]pc), and large exchange bias (∼140 Oe). In addition to these typical characterizations, the following two advanced analyses were performed: (i) The lattice constant was identified by Bragg’s diffraction specific to a space group of R3c using X-ray diffraction; it was precisely determined as an expanded a-axis (abulk = 0.568 → aepi = 0.572 nm) and a shrunk c-axis (cbulk = 1.398 → cepi = 1.373 nm). (ii) The ferroelectricity was analyzed by first-order reversal curve (FORC) diagrams, which revealed that ferroelectric switching was packed in a narrow electric field area; an internal electric field in the film body was not observed despite the fact that the BiFeO3 films were as-grown samples. A 3 nm thick BiFeO3 film with a continuous and flat surface/interface was confirmed over a wide area. The crystal symmetry might be identified as a space group of R3c in the case of the 3 nm thick film by comparing the nanobeam selected area electron diffraction patterns with the patterns based on structural calculations. The ferroelectricity might be confirmed by the piezoresponse force microscopy of a 2 nm thick BiFeO3 epitaxial film, owing to the optimal condition of low Jc and uniform ferroelectric switching properties. Furthermore, a 0.4 nm thick ultrathin BiFeO3 film was confirmed to be a continuous one-unit cell perovskite (∼0.4 nm) layer, owing to the optimal condition of low Ra. This study provides a method for investigating the crystal symmetry that affects the multiferroic properties of ultrathin films, which can be used as barrier layers in multiferroic tunnel junctions for highly functional sensors.

Model-based analysis of the ferroelectric response of multiferroic (1-x)BiFeO3-xPbTiO3 solid solution thin films around morphotropic phase boundary

It is a challenging task to deposit single phase multiferroic (1-x)BiFeO3-xPbTiO3 (BF-xPT) solid solution thin films on Fluorine-doped tin oxide (FTO) coated glass substrate due to its high crystallization temperature. This study presents the preparation of BF-xPT solid solution thin films covering the morphotropic phase boundary (MPB) region. Films were prepared in ambient conditions via solution processing method on FTO coated glass substrate. Optical absorption in the visible region has been observed for intermediate compositions, promising for optoelectronic applications. The presence of multiple phases, strain, and finite-size effects results in structural complexities that have been discussed in detail. In addition to that, due to the presence of defects such as grain boundaries and porosity, considerable leakage in hysteresis loops has been observed. Linear capacitive and ferroelectric responses have also been observed, indicating that the films are composed of induced dipoles and ferroelectric domains, respectively. The origin of each mechanism has been discussed in detail. Model-based analysis was carried out to separate intrinsic ferroelectric switching by identifying MPB compositions exhibiting optimum results. Based on the intrinsic response MPB compositions are found to lie in the 0.35 < x < 0.45.

Band alignment engineering of a Ruddlesden–Popper perovskite-based heterostructure constructed using Cs2SnI2Cl2 and α-In2Se3: The effects of ferroelectric polarization switching and electric fields

Ferroelectricity is an important source of the fascinating optoelectronic properties of heterostructures. Interfaces formed between ferroelectrics and oxides or transition metal dichalcogenides have undergone rapid development. However, the integration of ferroelectrics and two-dimensional Ruddlesden–Popper (RP) perovskites has rarely been studied so far. Herein, we use first-principle calculations to investigate ferroelectric polarization and electric field control of band alignments within the RP perovskite-based heterostructure α-In2Se3/Cs2SnI2Cl2. Our calculations demonstrate that the band alignment of heterostructure can be changed from type-II to type-III by switching the ferroelectric polarization direction in the α-In2Se3 layer. Furthermore, application of an external electric field can modulate the band structure of the α-In2Se3/Cs2SnI2Cl2 heterostructure and induce the band alignment transition. These findings highlight the importance of ferroelectric polarization switching in band alignment engineering and suggest the possibility of electric field-tunable multi-band alignment in Ruddlesden–Popper perovskite-based heterostructures.

A robust neuromorphic vision sensor with optical control of ferroelectric switching

The rapid development of the artificial intelligence field has increased the demand for retina-inspired neuromorphic vision sensors with integrated sensing, memory, and processing functions. Here, we present a neuromorphic vision sensor with an optoelectronic transistor structure consisting of monolayer molybdenum disulfide and barium titanate ferroelectric film. Beyond conventional electrical tuning of ferroelectric polarization, the optoelectronic transistor can exhibit a light-dosage tunable synaptic behavior with a high switching ratio and good non-volatility, enabled by photo-induced ferroelectric polarization reversal. The wavelength-dependent optical sensing and multi-level optical memory properties are utilized to achieve the in-sensor neuromorphic visual pre-processing. A simulated artificial neural network built from the proposed vision sensors with neuromorphic pre-processing function demonstrated that the image recognition rate for the Modified National Institute of Standards and Technology (MNIST) handwritten dataset could be significantly improved by reducing redundant data. The obtained results suggest that 2D semiconductor/ferroelectric optoelectronic transistors can provide a promising hardware implementation towards constructing high-performance neuromorphic visual systems

Foreword Special Issue on “New Simulation Methodologies for Next-Generation TCAD Tools”

Technology computer-aided design (TCAD) is an integral part of the development process of semiconductor technologies and devices, a field which has become increasingly complex and heterogeneous. Processing of integrated circuits requires nowadays over 400 process steps, and the resulting devices often have an intricate 3-D structure and contain various specifically designed materials. The full device behavior can only be understood by considering effects on all length scales from atomistic (material properties, interfaces, defects, and so on), to nanometric (quantum confinement, non-bulk properties, tunneling, ballistic transport, and so on), to full-chip dimensions (strain, heat transport, and so on), and time scales from femtoseconds (scattering, ferroelectric switching time, and so on) to seconds (trapping times, degradation, and so on). Voltages, currents, and charges have been scaled to such low levels that statistical effects and process variations have a strong impact. Devices based on new materials (e.g., 2-D crystals) and physical principles (ferroelectrics, magnetic materials, qubits, and so on) challenge standard TCAD approaches. While the simulation methods developed by the physics community can describe the basic device behavior, they often lack important simulation capabilities like, for example, transient simulations or integration with other TCAD tools, and are often too slow for daily use. Due to the complexity of semiconductor technology, it becomes more and more difficult to assess the impact of a change in processing or device structure on circuit performance by looking at a single aspect of an isolated device under idealized conditions. Instead, a TCAD tool chain is required which can handle realistic device structures embedded in a chip environment. New methodologies are required for all aspects of TCAD to ensure an efficient tool chain covering from atomistic effects to circuit behavior based on flexible simulation models that can handle new materials, device principles, and the ensuing large-scale simulations and that make use of artificial intelligence for well-chosen (sub)routines to decrease the overall simulation time. This Special Issue features six invited and 18 regular papers that address these problems.

Optically Induced Ferroelectric Polarization Switching in a Molecular Ferroelectric with Reversible Photoisomerization.

Ferroelectrics usually exhibit temperature‐triggered structural changes, which play crucial roles in controlling their physical properties. However, although light is very striking as a non‐contact, non‐destructive, and remotely controlled external stimuli, ferroelectric crystals with light‐triggered structural changes are very rare, which holds promise for optical control of ferroelectric properties. Here, an organic molecular ferroelectric, N‐salicylidene‐2,3,4,5,6‐pentafluoroaniline (SA‐PFA), which shows light‐triggered structural change of reversible photoisomerization between cis‐enol and trans‐keto configuration is reported. SA‐PFA presents clear ferroelectricity with the saturate polarization of 0.84 μC cm−2, larger than those of some typical organic ferroelectrics with thermodynamically structural changes. Benefit from the reversible photoisomerization, the dielectric real part of SA‐PFA can be reversibly switched by light. More strikingly, the photoisomerization enables SA‐PFA to show reversible optically induced ferroelectric polarization switching. Such intriguing behaviors make SPFA a potential candidate for application in next‐generation photo‐controlled ferroelectric devices. This work sheds light on further exploration of more excellent molecular ferroelectrics with light‐triggered structural changes for optical control of ferroelectric properties.

Effect of residual impurities on polarization switching kinetics in atomic-layer-deposited ferroelectric Hf0.5Zr0.5O2 thin films

The ferroelectricity in fluorite-structured ferroelectrics such as HfO2 and ZrO2 has attracted increasing interest in both academia and industry. Although the polarization switching kinetics is one of the research topics which critically requires further study for the novel ferroelectric material, various factors affecting the polarization switching kinetics have not been rarely examined to date. Especially in atomic-layer-deposited films, the defects including residual impurities are expected to strongly influence the polarization switching kinetics. In this study, therefore, the effect of deposition temperature (Tdep) and impurity concentration on the physical/electrical properties and polarization switching kinetics of atomic-layer-deposited Hf0.5Zr0.5O2 thin films is reported based on physical/chemical/electrical analyses. With decreasing Tdep, the concentration of residual impurities, including carbon increased, resulting in decreased grain size. The increased defect concentration decreased the median switching time and increased the deviation of the switching time distribution. This can be attributed to the lowered energy barrier for the nucleation of oppositely polarized domains and increased spatial inhomogeneities, respectively. This result suggests that Tdep is a key process parameter for optimizing the polarization reversal behavior of fluorite-structured ferroelectrics as it has a considerable influence on the ferroelectric properties and polarization switching kinetics, which are strongly correlated to the defect concentrations and microstructure.

Understanding and modulation of resistive switching behaviors in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 multilayer junctions

The insertion layer in the ferroelectric multilayer junctions plays a key role in regulating the energy band structure of interface and their resistive switching behavior. Here, PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 (PZT/LSMO/NSTO) heterostructures with different LSMO thicknesses were prepared by pulsed laser deposition and their ferroelectric properties and resistive switching behaviors were investigated. It is found that the ferroelectric properties of the heterostructures almost do not change with the increasing in the thickness of LSMO, while the resistive switching behaviors are closely related to the LSMO thickness, and the maximum switching ratio of about 103 can be achieved in the PZT/LSMO/NSTO heterostructure with the LSMO thickness of 18 nm. By analyzing the I-V curves and energy band structures, it can be concluded that the resistive switching behaviors depend on the competition between the tunability of depletion layer width and the ability of ferroelectric filed effect. These results provide insights to understand ferroelectric resistive switching in ferroelectric heterostructures, and demonstrate an effective way to improve resistive switching performance by adjusting the band structure through the appropriate thickness of the insertion layer.

Electrical coupling analysis of 2D time-multiplexing memory actuators exhibiting asymmetric butterfly hysteresis

We present the modeling and analysis of electrical coupling in a hysteretic deformable mirror with 2D memory piezoactuators, which are made of a purposely designed piezomaterial sandwiched between electrodes arranged crosswise and actuated by a multiplexing approach. Using a modified Miller model to describe the memory effect which is based on the ferroelectric domain switching processes, the proposed framework is used to simulate the electric-field dependence of the strain in the piezoelectric material that exhibits asymmetric butterfly loops with remnant deformation through the finite element method. The desired butterfly memory effect in the material is obtained by modifying the saturated dipole polarization curve in the Miller model. The proposed method allows us to numerically investigate the electrical coupling between actuators in more detail and correspondingly understand their influence to the mirror facesheet.

Controllable Schottky barriers by ferroelectric switching in graphene/In2Te3 heterostructures

Van der Waals (vdWs) heterostructures have attracted significant research attention due to their tunable electronic structures and potential for device applications. In this study, the interfacial properties of graphene/In2Te3 heterostructures are systematically explored by employing first principles calculations. It is revealed that the electronic structures of graphene and In2Te3 monolayer are well preserved upon contact. The transition between the n-type Ohmic and p-type Schottky contacts can be realized for the graphene/In2Te3 heterostructures by switching the polarity of In2Te3. The findings reported in this study are not only vital for basic theoretical research, but also provide a feasible strategy to control the Schottky barriers for vdWs heterostructures.

Observation of band bending in WTe2 after surface oxidation

Recently, the observations of spontaneous out-of-plane electric polarization and ferroelectric switching in WTe2 related devices with semimetal property a have generated considerable attentions. The polar structure of WTe2 was proposed to be vital, however there is still no consensus about the physical origin, which deserves detailed surface analysis. Here, by using angle-dependent X-ray photoelectron spectroscopy (ADXPS), we observed an obvious downward band bending in WTe2 after slightly surface oxidation, which can't be observed on the freshly cleaved WTe2. And it will disappear after fully oxidation, demonstrating strong correlation between band bending and surface oxidation. Our study suggests surface oxidation and resulted band bending in WTe2 should be taken into account in the WTe2-related devices, such as ferroelectric.

Room-temperature ferroelectric switching of spin-to-charge conversion in germanium telluride

Since its birth in the 1990s, semiconductor spintronics has suffered from poor compatibility with ferromagnets as sources of spin. While the broken inversion symmetry of some semiconductors may alternatively allow for spin-charge interconversion, its control by electric fields is volatile. Ferroelectric Rashba semiconductors stand as appealing materials unifying semiconductivity, large spin-orbit coupling, and non-volatility endowed by ferroelectricity. However, their potential for spintronics has been little explored. Here, we demonstrate the non-volatile, ferroelectric control of spin-to-charge conversion at room temperature in epitaxial GeTe films. We show that ferroelectric switching by electrical gating is possible in GeTe despite its high carrier density. We reveal a spin-to-charge conversion as effective as in Pt, but whose sign is controlled by the orientation of the ferroelectric polarization. The comparison between theoretical and experimental data suggests that spin Hall effect plays a major role for switchable conversion. These results open a route towards devices combining spin-based logic and memory integrated into a silicon-compatible material.

Ferroelectric switching behavior of nanoscale Hf0.5Zr0.5O2 grains

Grain size has shown significant effect on the ferroelectricity of HfO2-based thin films. However, its microscopic mechanism remains unclear. For this purpose, Hf0.5Zr0.5O2 film with different sized grains was prepared by radio frequency magnetron sputtering. The piezoresponse force microscope characterization of the domain switching properties of nanograins shows that domain switching ability of the grain becomes better as its size decreases within the size range we studied. Then, density functional theory (DFT) calculation of grains’ energy proves that the lower surface energy of ferroelectric phase is the key factor to make the ferroelectric phase more stable and larger proportion in smaller sized grain. Finally, a multi-phase coexistence phase field model is developed based on the DFT calculation. It is found out that if the contribution of surface energy to the total energy of nano-grain increases, the volume fraction of ferroelectric phase will increase. And (001) ferroelectric phase with the lowest surface energy density will be the most stable and account for the largest volume fraction. Our finding provides a microscopic insight into the “grain size effect” and also some guidelines for high-performance ferroelectric thin films preparation through “grain size engineering”.

Imprint phenomenon of ferroelectric switching characteristics in BaTiO3/PbTiO3 multilayer thin films

Materialsin which ferroelectric polarization can be locally controlled have potential for high-density information storage applications. Herein, we investigated the ferroelectric polarization switching characteristics of epitaxial BaTiO3/PbTiO3 (BTO/PTO) multilayer thin films by using piezoresponse force microscopy. Pulsed laser deposition was used to form an epitaxial BTO layer on an epitaxial ferroelectric PTO single thin film. This epitaxial BTO/PTO multilayer thin film exhibited an imprint phenomenon, in which the internal electric field was asymmetrically induced and the ferroelectric hysteresis loop was shifted. Owing to this imprint phenomenon, a large difference occurred between the two coercive electric fields of the ferroelectric hysteresis loop. Because the coercive electric field was reduced in one direction, a relatively small switching voltage was required to form ferroelectric polarization nanobits in a local region and the size of the switched nanobits was minimized. This behavior can be utilized as the basis for developing high-density ferroelectric-based storage media.