Ferroelectric Switching Research Articles

Deterministic Manipulation of Multi‐State Polarization Switching in Multiferroic Thin Films

AbstractDeterministically controllable multi‐state polarizations in ferroelectric materials are promising for the application of next‐generation non‐volatile multi‐state memory devices. However, the achievement of multi‐state polarizations has been inhibited by the challenge of selective control of switching pathways. Herein, an approach to selectively control 71° ferroelastic and 180° ferroelectric switching paths by combining the out‐of‐plane electric field and in‐plane trailing field in multiferroic BiFeO3 thin films with periodically ordered 71° domain wall is reported. Four‐state polarization states can be deterministically achieved and reversibly controlled through precisely selecting different switching paths. These studies reveal the ability to obtain multiple polarization states for the realization of multi‐state memories and magnetoelectric coupling‐based devices.

Two-Dimensional Multiferroics with Intrinsic Magnetoelectric Coupling in A-Site Ordered Perovskite Monolayers

The magnetoelectric coupling effect in multiferroics provides a route to realize the control of magnetism by electric field. Here, we demonstrate the coexistence and coupling of ferroelectricity and ferromagnetism in designed A-site ordered perovskite oxide monolayers by combining symmetry analysis and first-principles calculation. These monolayers all exhibit a layered ordering and tilt distortion, and some of them exhibit rotation or Jahn-Teller distortion simultaneously, leading to the emergence of in-plane ferroelectricity. The Mn-based monolayers exhibit robust ferromagnetism, while some monolayers tend to form E-type spin order due to the splitting of the nearest-neighbor exchange interactions. Whether polarization reversal can lead to magnetization reversal depends on the mode of ferroelectric switching, that is, only the ferroelectric switching that reversing the tilt distortion can lead to magnetization reversal. This work demonstrates the feasibility of controlling the direction of magnetization by electric field in the monolayer limit of perovskites.

Sliding induced multiple polarization states in two-dimensional ferroelectrics

When the atomic layers in a non-centrosymmetric van der Waals structure slide against each other, the interfacial charge transfer results in a reversal of the structure’s spontaneous polarization. This phenomenon is known as sliding ferroelectricity and it is markedly different from conventional ferroelectric switching mechanisms relying on ion displacement. Here, we present layer dependence as a new dimension to control sliding ferroelectricity. By fabricating 3 R MoS2 of various thicknesses into dual-gate field-effect transistors, we obtain anomalous intermediate polarization states in multilayer (more than bilayer) 3 R MoS2. Using results from ab initio density functional theory calculations, we propose a generalized model to describe the ferroelectric switching process in multilayer 3 R MoS2 and to explain the formation of these intermediate polarization states. This work reveals the critical roles layer number and interlayer dipole coupling play in sliding ferroelectricity and presents a new strategy for the design of novel sliding ferroelectric devices.

High-Speed and High-Power Ferroelectric Switching Current Measurement Instrument for Materials with Large Coercive Voltage and Remanent Polarization.

A high-speed and high-power current measurement instrument is described for measuring rapid switching of ferroelectric samples with large spontaneous polarization and coercive field. Instrument capabilities (±200 V, 200 mA, and 200 ns order response) are validated with a LiTaO3 single crystal whose switching kinetics are well known. The new instrument described here enables measurements that are not possible using existing commercial measurement systems, including the observation of ferroelectric switching in large coercive field and large spontaneous polarization Al0.7Sc0.3N thin films.

Optimization of Processing Steps for Superior Functional Properties of (Ba, Ca)(Zr, Ti)O3 Ceramics.

Lead-free piezoelectric ceramics with nominal composition at morphotropic phase boundary Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZ) prepared by different processing routes and sintered either by conventional solid-state reaction or by spark plasma sintering (SPS) techniques were comparatively investigated to observe the role of structural modifications and of microstructures on the dielectric, ferroelectric, piezoelectric and electrocaloric responses. The ceramics presented relative densities from 75% to 97% and showed variations in their phase composition as a result of variable mixing and different synthesis and sintering parameters providing local compositional heterogeneity. As result, all of the ceramics showed diffuse phase transition and ferroelectric switching responses, with parameters affected mostly by density (Pr between 3.6 to 10.1 μC/cm2). High values for the electrocaloric response in the Curie range were found for the ceramics with predominantly orthorhombic character. Field-induced structural modifications were probed by tunability anomalies and by XRD experiments in remanence conditions. Piezoelectric effects with notably high figure of merit values were assigned to the better densification and poling efficiency of BCTZ ceramics.

Polarization retention dependence of imprint time within LiNbO3 single-crystal domain wall devices

Ferroelectric LiNbO3 single crystals have wide applications in surface acoustic wave filters, pyroelectric sensors, and electro-optic modulators. Large-area LiNbO3 single-crystal thin films integrated on silicon are promising for high density integration of ferroelectric domain-wall resistance switching memories and transistors. However, the short-time operation of the memory often suffers from poor polarization retention due to the built-in imprint voltage. Here, we observed the strong polarization orientation-dependent imprint effect within either out-of-plane or in-plane LiNbO3 thin-film capacitors. The imprint effect can shift domain switching hysteresis loops toward positive/negative voltages seriously with written negative/positive polarizations that occur within a characteristic imprint time of 5.1 ms–360 s. Once the write time of the memory is shorter than the imprint time, the inverted domain is unstable and switches back into its previous orientation automatically after the termination of a write operation. However, the write failure can be avoided if the write time is longer than the imprint time, and the written domain can be deeply protected by the imprint field. A model of polarization-dependent charge injection at the interface is developed to explain the time-dependent imprint effect. For a mesa-like LiNbO3 memory cell in contact with two side electrodes fabricated at the film surface, the imprint time can be greatly shortened below 30 ns with the extension of one side electrode over the cell surface to screen the tail of the switched domain, enabling ferroelectric domain-wall resistance switching devices in excellent retention and high operation speeds.

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.

Disentangling electronic and thermal contributions to light-induced resistance switching in BaTiO3 ferroelectric tunnel junction

In the presence of asymmetric potential barriers, such as those created by imprint fields, ferroelectric polarization can be reversed by light due to the photoinduced suppression of polarization. Both thermal effects and photocarrier-induced polarization screening may agree with this experimental observation, challenging its understanding. Here, we explore light-induced ferroelectric polarization switching in BaTiO3 thin films. Time-dependent photocurrent and photoresistance experiments at different wavelengths indicate that the optical switch of polarization is mainly driven by photocarriers rather than thermal effects. The effect of light on sample polarization is found to be relatively slow and that an illumination period as long as ≈100 s is required to achieve complete switching when using a 405 nm light wavelength and 1.4 W/cm2 power density. It is shown that this response is governed by the concentration of photo-generated charges, which is low due to the reduced light absorption of BaTiO3 films at the explored wavelengths. Our conclusions can help us to better design optically switching devices based on ferroelectric materials.

Ferroelectric van der Waals heterostructures of CuInP2S6 for non-volatile memory device applications

Two-dimensional (2D) ferroelectric materials are providing promising platforms for creating future nano- and opto-electronics. Here we propose new hybrid van der Waals heterostructures, in which the 2D ferroelectric material CuInP2S6 (CIPS) is layered on a 2D semiconductor for near-infrared (NIR) memory device applications. Using density functional theory, we show that the band gap of the hybrid bilayers formed with CIPS can be tuned and that the optical and electronic properties can be successfully modulated via ferroelectric switching. Of the 3712 heterostructures considered, we identified 19 structures that have a type II band alignment and commensurate lattice matches. Of this set, both the CuInP2S6/PbSe and CuInP2S6/Ge2H2 heterostructures possess absorption peaks in the NIR region that change position and intensity with switching polarisation, making them suitable for NIR memory devices. The CuInP2S6/ISSb, CuInP2S6/ISbSe, CuInP2S6/ClSbSe and CuInP2S6/ZnI2 heterostructures had band gaps which can be switched from direct to indirect with changing the polarisation of CIPS making them suitable for optoelectronics and sensors. The heterostructures formed with CIPS are exciting candidates for stable ferroelectric devices, opening a pathway for tuning the band alignment of van der Waal heterostructures and the creation of modern memory applications that use less energy.

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.

Nanocrystallite Seeding of Metastable Ferroelectric Phase Formation in Atomic Layer-Deposited Hafnia-Zirconia Alloys.

Hafnia-based ferroelectric thin films are promising for semiconductor memory and neuromorphic computing applications. Amorphous, as-deposited, thin-film binary alloys of HfO2 and ZrO2 transform to the metastable, orthorhombic ferroelectric phase during post-deposition annealing and cooling. This transformation is generally thought to involve formation of a tetragonal precursor phase that distorts into the orthorhombic phase during cooling. In this work, we systematically study the effects of atomic layer deposition (ALD) temperature on the ferroelectricity of post-deposition-annealed Hf0.5Zr0.5O2 (HZO) thin films. Seed crystallites having interplanar spacings consistent with the polar orthorhombic phase are observed by a plan-view transmission electron microscope in HZO thin films deposited at an elevated ALD temperature. After ALD under conditions that promote formation of these nanocrystallites, high-polarization (Pr > 18 μC/cm2) ferroelectric switching is observed after rapid thermal annealing (RTA) at low temperature (350 °C). These results indicate the presence of minimal non-ferroelectric phases retained in the films after RTA when the ALD process forms nanocrystalline particles that seed subsequent formation of the polar orthorhombic phase.

Synergistic Enhancement of Luminescent and Ferroelectric Properties through Multi‐Clipping of Tetraphenylethenes

AbstractSynergistically enhancing luminescent and ferroelectric (SELF) properties are observed from a tetraphenylethene (TP) substituted with clipping groups (C), where the C is consisting of a 4‐[3,5‐bis‐(3‐decyloxy‐styryl)‐styryl]‐phenyl (DOS) unit. The DOS units of TPCn are self‐assembled via intermolecular interaction to clip themselves and induce TP aggregation, as evidenced by clip‐induced quenching of emission at DOS units (Eclip) accompanied by aggregation‐induced emission enhancement of TPs (EAIE). TPC4 demonstrates strong photoluminescence in a dilute chloroform solution and large EAIE in aqueous (>50%) THF solution. TPCn demonstrates SELF properties in film state, with high quantum yields of photoluminescence (>80%) and ferroelectric switching. Due to the introduction of four clips, TPC4 has a higher remnant polarization (Pr = 2.27 µC cm−2) at room temperature than TPC1. TPC4 is successfully employed in a light‐emitting electrochemical cell to achieve over 1290 cd m−2 under pulsed current conditions. The TPC4 film on a flexible substrate produced a piezoelectric output voltage of up to 0.13 V and a current density of 1.14 nA cm−2 upon bending. These results indicate that the side chain clipping and TP aggregation resulted in unprecedented flexible SELF properties in a single compound, offering simultaneous enhancement of electroluminescence, mechanical sensitivity, and energy harvesting capacity.

Reversible canted persistent spin textures in two-dimensional ferroelectric bilayer WTe2

The recent discovery of materials hosting persistent spin texture (PST) opens an avenue for the realization of energy-saving spintronics since they support an extraordinarily long spin lifetime. However, the stability of the PST is sensitively affected by symmetry breaking of the crystal induced by external perturbation such as the electric field. In this paper, through first-principles calculations supplemented by symmetry analysis, we report the emergence of the robust and stable PST with large spin splitting in the two-dimensional (2D) ferroelectric bilayer WTe2. Due to the low symmetry of the crystal (Cs point group), we observe a canted PST in the spin-split bands around the Fermi level displaying a unidirectional spin configuration tilted along the yz plane in the first Brillouin zone. Such a typical PST can be effectively reversed by out-of-plane ferroelectric switching induced by interlayer sliding along the in-plane direction. We further demonstrated that the reversible PST is realized by the application of an out-of-plane external electric field. Thus, our findings uncover the possibility of an electrically tunable PST in 2D materials, offering a promising platform for highly efficient and non-volatile spintronic devices.

Sunlight-assisted ferroelectric domain switching and ionic migration in Sn-based ferroelectric

Laser-assisted ferroelectric polarization switching recently has been proved to be an effective mean to manipulate the ferroelectric domain structure, but with the possibility to damage the specimen surface due to high energy input and large thermal expansion. Compared to laser, sunlight with moderate energy is expected to be more accessible. Here, we employed a simulated sunlight illumination instead of high-energy lasers to realize the sunlight-assisted ferroelectric domain switching in Sn2P2S6 single crystals. The origin is the enhancement of localized carrier concentration due to the disproportionation reaction of Sn ions, which induces an additional internal field and assists the domain switching. The migration and accumulation of the Sn ions are also verified with scanning probe technique, which can be utilized as a resistive memory prototype. It is noteworthy that this memory effect can be significantly enhanced by sunlight illumination and, thus, make it suitable for the sunlight control of ferroelectric domain switching and ionic memory devices.

HZO-based FerroNEMS MAC for in-memory computing

This paper demonstrates a hafnium zirconium oxide (HZO)-based ferroelectric NEMS unimorph as the fundamental building block for very low-energy capacitive readout in-memory computing. The reported device consists of a 250×30 μm2 unimorph cantilever with 20-nm-thick ferroelectric HZO on 1 μm SiO2. Partial ferroelectric switching in HZO achieves analog programmable control of the piezoelectric coefficient (d31), which serves as the computational weight for multiply accumulate (MAC) operations. The displacement of the piezoelectric unimorph was recorded by actuating the device with different input voltages Vin. The resulting displacement was measured as a function of the ferroelectric programming/poling voltage VP. The slopes of central beam displacement (δmax) vs Vin were measured to be between 182.9 nm/V (for −8 Vp) and −90.5 nm/V (for 8 Vp), which corresponds to displacement proportionality constant β of 68 nm/V2 for +ve Vp and 47 nm/V2 for −ve Vp, demonstrating linear behavior of the multiplier unit. The resultant δmax from AC actuation is in the range of −18 to 36 nm and is a scaled product of Vin and programmed d31 (governed by the Vp). The multiplication function serves as the fundamental unit for MAC operations with the ferroelectric NEMS unimorph. The displacement from many such beams can be added by summing the capacitance changes, providing a pathway to implement a multi-input and multi-weight neuron. A scaling and fabrication analysis suggests that this device can be CMOS compatible, achieving high in-memory computational throughput.

Anomalous Phase Transition, Polarization Switching, and Relaxation in a Novel Composite Consisting of Dimethylammonium Aluminum Sulfate Hexahydrate and Oxidized Multiwalled Carbon Nanotubes

A novel ferroelectric composite consisting of dimethylammonium aluminum sulfate hexahydrate (CH3)2NH2Al(SO4)2·6H2O and oxidized multiwalled carbon nanotubes was developed to investigate its dielectric relaxation, phase transition, and ferroelectric switching behaviors. Experimental methods combined with theoretical analyses were employed. The results showed that the filler of oxidized multiwalled carbon nanotubes caused the freezing of domain-wall motion in (CH3)2NH2Al(SO4)2·6H2O at 98.2 K, leading to a decrease in dielectric constant, dielectric loss, and the polarization. In addition, an increase in dielectric permittivity and dielectric loss was found at higher temperatures. The role of hydroxyl groups incorporated in the carbon nanotubes after its oxidation was responsible for the obtained anomalies.

Oxygen‐Scavenging Effects of Added Ti Layer in the TiN Gate of Metal‐Ferroelectric‐Insulator‐Semiconductor Capacitor with Al‐Doped HfO2 Ferroelectric Film (Adv. Electron. Mater. 11/2022)

Metal–Ferroelectric–Insulator–Semiconductors The Ti-inserted top electrode structure is efficiently utilized to induce the oxygen scavenging effect at the interfacial layer of a semiconductor-based ferroelectric device. The Al-doped HfO2 ferroelectric film with the high crystallization annealing temperature triggers a more active decomposition process at the interfacial layer to enhance the ferroelectric switching and also reduce the interface trap density, leading to a more reliable device operation. More details can be found in article number 2200310 by Cheol Seong Hwang and co-workers.

Cover Image

AbstractFerro‐Floating Memory (FFM) which operates in dual‐mode mechanisms (ferroelectric switching of FeFET and charge injection of Floating gate memory) was implemented by employing a‐In2Se3 as a floating gate. The dual‐mode operation allowed the multiple stages of conductance and improved the linearity and dynamic range of conductance. On the cover, the authors illustrate the dual‐mode operation of the FFM by combining FeFET and Flash memory. (DOI : 10.1002/inf2.12367) image

Switching Behavior of Dip‐Coated Organic Ferroelectric Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

AbstractSupramolecular organic ferroelectrics have unique properties related to plasticity and freedom of design but the extensive post‐deposition treatments that are typically required to achieve saturation polarization limit their applicability. A possible solution is to pre‐align the molecules. Here, the effect of dip‐coating on the pre‐alignment of the prototypical supramolecular ferroelectric trialkylbenzene‐1,3,5‐tricarboxamide (BTA) and the resulting changes in the ferroelectric switching behavior of the resulting thin films are analyzed. Dip‐coated films are characterized by different BTA concentrations and dip‐coating velocities. Landau–Levich and evaporation regimes are identified. The corresponding ferroelectric switching behavior is investigated and reveals that dip‐coating in general enhances the ferroelectric switching of pristine films. Dip‐coating perpendicular to the electrodes (parallel to the electric field direction) has the most significant effect, and without further alignment steps, leads to equal switching behavior as films that are aligned by post‐deposition treatment, that is, field annealing. The results are expected to be transferable to other small molecular ferroelectrics.

Domain Switching in BaTiO3 Films Induced by an Ultralow Mechanical Force.

Low-energy switching of ferroelectrics has been intensively studied for energy-efficient nanoelectronics. Mechanical force is considered as a low-energy consumption technique for switching the polarization of ferroelectric films due to the flexoelectric effect. Reduced threshold force is always desirable for the considerations of energy saving, easy domain manipulation, and sample surface protection. In this work, the mechanical switching behaviors of BaTiO3/SrRuO3 epitaxial heterostructure grown on Nb:SrTiO3 (001) substrate are reported. Domain switching is found to be induced by an extremely low tip force of 320 nN (estimated pressure ∼0.09 GPa), which is the lowest value ever reported. This low mechanical threshold is attributed to the small compressive strain, the low oxygen vacancy concentration in BaTiO3 film, and the high conductivity of the SrRuO3 electrode. The flexoelectricity under both perpendicular mechanical load (point measurement) and sliding load (scanning measurement) are investigated. The sliding mode shows a much stronger flexoelectric field for its strong trailing field. The mechanical written domains show several advantages in comparison with the electrically written ones: low charge injection, low energy consumption, high density, and improved stability. The ultralow-pressure switching in this work presents opportunities for next-generation low-energy and high-density memory electronics.