Ferroelectric Switching Research Articles

Unraveling the ferroelectric switching mechanisms in ferroelectric pure and La doped HfO2 epitaxial thin films

Epitaxial orthorhombic phase La doped HfO2 films are promising for achieving robust ferroelectric polarization without wake-up effect. However, lowering the coercive field is crucial for achieving low-power memory devices.In this work, we have investigated the influence of the La content effect on the structural and ferroelectric properties of epitaxial HfO2 thin films. We show that while the remanent polarization is optimum for 2–5 at. % La-doped HfO2 films, the coercive field is decreased with La doping. The experimental work is supported by density functional theory (DFT) calculations which show that the polarization switching in epitaxial La:HfO2 films can be understood based on the synergetic contribution of the presence of a non-ferroelectric monoclinic phase and the La doping itself that causes a reduction of the nucleation and DW motion energy barriers for the crossing path, which makes it more probable than the non-crossing one.

Nonvolatile Electrical Control and Reversible Gas Capture by Ferroelectric Polarization Switching in 2D FeI2/In2S3 van der Waals Heterostructures

Nonvolatile electrical control is the core of future magnetoelectric nanodevices. In this work, we systematically explore both the electronic structures and transport properties of multiferroic van der Waals (vdW) heterostructures consisting of a ferromagnetic FeI2 monolayer and a ferroelectric In2S3 monolayer using density functional theory and the nonequilibrium Green's function method. The results reveal that the FeI2 monolayer can be reversibly switched between semiconducting and half-metallic properties by nonvolatile control of the In2S3 ferroelectric polarization states. Correspondingly, the proof-of-concept two-probe nanodevice based on the FeI2/In2S3 vdW heterostructure exhibits a significant valving effect by modulating the ferroelectric switching. Moreover, it is also found that the preference of nitrogen-containing gases such as NH3, NO, and NO2 for adsorption on the surface of FeI2/In2S3 vdW heterostructures strongly depends on the polarization direction of the ferroelectric layer. In particular, the FeI2/In2S3 heterostructure shows reversible capture behavior for NH3. As a result, the FeI2/In2S3 vdW heterostructure-based gas sensor demonstrates high selectivity and sensitivity. These findings may open up a new route for the application of multiferroic heterostructures to spintronics, nonvolatile memories, and gas sensors.

Doping effects on the ferroelectric properties of wurtzite nitrides

Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a β-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.

Theoretical studies of sliding ferroelectricity, magnetoelectric couplings, and piezo-multiferroicity in two-dimensional magnetic materials

It has been predicted that for a series of two-dimensional (2D) materials, their van der Waals bilayers may possess sliding ferroelectricity with vertical polarizations switchable via interlayer sliding. Such intriguing ferroelectricity has been recently experimentally confirmed in a series of nonmagnetic bilayer systems like BN, MoS2, etc., while magnetic bilayers remain unexplored in those reports. In this paper we show first-principles evidence that three most prevalent 2D ferromagnetic materials, CrI3, Cr2Ge2Te6 and Fe3GeTe2, can all be entailed with sliding ferroelectricity (FE) with vertical polarizations over 0.1 pC/m in their bilayers and are thus multiferroic. In particular, Fe3GeTe2 bilayer may exhibit magnetoelectric coupling effect with a net magnetization moment of 0.01 μB per unit cell switchable upon ferroelectric switching. Moreover, it is shown to be both piezoelectric and piezomagnetic as both its vertical electric polarization and the net magnetic moment can be approximately doubled when the interlayer distance is compressed by 10%, giving rise to unprecedented “piezo-multiferroic” effect where the magnetoelectric coupling effect is intensified with reduced interlayer distance.

Random networks of disconnected nanoparticles in dielectric layers as a source of electric responsivity

Many efforts are currently focused on materials with responsive features for neural-inspired devices. Different approaches are followed, based on various mechanisms – from ferroelectric switching to structural phase changes, from magnetic tunnel junctions to metal filament formation. Here we analyze an alternative strategy based on an unconventional electrical response arising from percolative charge transport and charge trapping in discrete random networks of oxide nanostructures in a dielectric matrix. After an analysis of the mechanisms which can potentially be a source of a plastic response in this class of systems, we report evidence of this behavior in a system comprising an alkali-germanosilicate amorphous matrix with incorporated Ga-oxide nanostructures. The active material – consisting in a film 70 nm thick interfaced to p-type Si and Au electrodes – gives a responsive behavior to pulsed bias which is accompanied by bias dependent electric conduction, with resistivity changes of an order of magnitude by applying 2 V, as well as a dielectric response with hysteretic features, as expected by the model. The results represent a first proof of concept of an unexplored strategy for the design of responsive systems.

Competition between Stepwise Polarization Switching and Chirality Coupling in Ferroelectric GeS Nanotubes

Ferroelectricity of group-IV chalcogenides MX (M = Ge, Sn; X = Se, S) monolayers has been extensively investigated. However, how the ferroelectricity evolves in their one-dimensional nanotubes remains largely unclear. Employing an accurate deep-learning interatomic potential of first-principles precision, we uncover a general stepwise mechanism for polarization switching in zigzag and chiral GeS nanotubes, which has an energy barrier that is substantially lower than the one associated with the conventional one-step switching mechanism. The switching barrier (per atom) gradually decreases with increasing the number of intermediate steps and converges to a value that is almost independent of the tube diameter. In the chiral GeS nanotubes, the switching path of polarization with chirality coupling is preferred at less intermediate steps. This study unveils novel ferroelectric switching behaviors in one-dimensional nanotubes, which is critical to coupling ferroelectricity and chirality.

Intersecting multiaxial domain walls in plastic ferroelectric crystal films

Plastic ferroelectrics comprise a class of solution-processable molecular ferroelectrics, in which spherical polar molecules can rotate and/or freeze within crystal lattices. Some compounds exhibit superior multiaxial ferroelectricity and very sharp ferroelectric switching characteristics, although the domain wall (DW) characteristics have not yet been explored. Here, we report distinctive DW characteristics in drop-casted monocrystalline thin films of a typical plastic/ferroelectric $[\mathrm{AH}][\mathrm{Re}{\mathrm{O}}_{4}]$. We demonstrate that the films form ${71}^{\ensuremath{\circ}}, {109}^{\ensuremath{\circ}}$, and ${180}^{\ensuremath{\circ}}$ DWs, unique to the pseudocubic rhombohedral lattice, that emerge and compete with each other, depending on the crystallographic orientation of the films. Particularly, it is observed that all three types of DWs intersect at several single points, whose unique nature should be ascribed to the small elastic deformation. The observed domains and ${180}^{\ensuremath{\circ}}$ DWs also comprise much larger periodicity and scaling than those of conventional inorganic ferroelectric films, which should be associated with the unique polarization reversal mechanism in the compound films. These observed features reveal the exotic multiaxial ferroelectricity of soft plastic/ferroelectrics, paving the way for developing printable ferroelectronics.

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.

Inversion of Molecular Chirality Associated with Ferroelectric Switching in a High-Temperature Two-Dimensional Perovskite Ferroelectric.

Controlling molecular chirality by external stimuli is of great significance in both fundamental research and technological applications. Herein, we report a high-temperature (384 K) molecular ferroelectric of a Cu(II) complex whose spontaneous polarization can be switched associated with flipping of molecular chirality. In this two-dimensional perovskite structure, the inorganic layer is separated by (NH3(CH2)2SS(CH2)2NH3)2+ organic cations skewed in a chiral conformation (P- or M-helicity in an individual crystal). As the stereodynamic disulfide bridge determines the molecular dipole moment along the polar axis, the chiral organic cation can be converted to its enantiomer as a consequence of an electric field-induced shift of the S-S moiety relative to its screw axis during the ferroelectric switching. The variation of the molecular chirality is examined with single-crystal X-ray diffraction and circular dichroism spectra. The simultaneous switching of molecular chirality and spontaneous polarization in this perovskite ferroelectric may lead to novel chiral electronic phenomena.

Achieving High Permittivity Paraelectric Behavior in Mesogen-Free Sulfonylated Chiral Polyethers with Smectic C Liquid Crystalline Self-Assembly

Ferroelectric liquid crystalline polymers (LCPs) with a high spontaneous polarization (Ps) are of great interest for the fabrication of advanced electronic devices. However, conventional ferroelectric LCPs typically exhibit low spontaneous polarization, only around 1 mC/m2. To increase the orientational polarization for ferroelectric LCPs, in this work, we designed mesogen-free comb-shaped LCPs based on isotactic polyepichlorohydrin, which contained highly dipolar sulfonyl groups (dipole moment of 4.5 Debye) in the side chains. The goal was to increase dipole density and induce ferroelectric switching. In this new series of mesogen-free LCPs, the sulfonyl groups were first moved away from the main chain to mitigate strong dipole–dipole interactions. Second, methyl-branched alkyl side chains were implemented to decrease the crystallizability and induce the Smectic C (SmC) self-assembly. Intriguingly, these SmC samples, for the first time, exhibited typical paraelectric behavior (i.e., slim S-shaped single hysteresis loops) with a high dielectric constant in the range of 20–30. Learning from the knowledge obtained from this study, we will be able to design new mesogen-free LCPs with tailor-made dipole–dipole interactions and realize ferroelectricity.

Ferroelectrics‐Electret Synergetic Organic Artificial Synapses with Single‐Polarity Driven Dynamic Reconfigurable Modulation

AbstractMost current studies of artificial synapses only mimic the static plasticity, which is far from achieving the complex behaviors of the human brain. The few reported dynamic reconfigurable synapses based on ambipolar transistors switch the operating states by voltages with opposite polarity, which impedes the development of highly efficient synaptic readout circuits. To improve the efficiency, flexibility, and biocompatibility of dynamic reconfigurable synapses, here a ferroelectrics‐electret synergetic organic synaptic p‐type transistor (FESOST) is devised. Owing to the synergetic action of ferroelectric polarization switching and charge capture, FESOST exhibits single‐polarity driven dynamic reconfigurable operating states with different synaptic behaviors (potentiation and depression) in response to the same gate pulse in different modes (excitatory and inhibitory). In addition, various single‐polarity driven synaptic behaviors including short‐term/long‐term plasticity, paired‐pulse facilitation/depression, spike‐rate‐dependent plasticity, and spike‐number‐dependent plasticity are also simulated. Finally, the reconfigurable artificial temperature perception system is simulated for the complex emotions of humans in response to different weather stimuli for people of different constitutions. The novel device architecture represents a major step forward in the development of dynamic, reconfigurable, high‐efficiency, organic synapses.

Synergistic effect of strain and oxygen vacancy on the ferroelectric properties of hafnium oxide-based ferroelectric films

In this paper, a multiphase coexistence phase field model considering mobile oxygen vacancy is established to investigate the synergistic effect of strain and oxygen vacancy on the ferroelectric properties of hafnium oxide-based ferroelectric films. Uniform strain introduced by flat substrate and heterogeneous strain caused by miscut substrate are considered. Our study demonstrates that whether the HfO2 film is under flat substrate-induced strain or miscut substrate-induced strain, the regulation of oxygen vacancy concentration on the ferroelectric phase transition and domain switching of the film is double-sided. When the oxygen vacancy concentration is 1 × 1020 cm−3, the film under 1.2 % uniform strain reaches a maximum remanent polarization of 0.36 C/m2. The optimal concentration interval of oxygen vacancy can be widened by −1.2 % flat substrate-induced uniform strain or 6° and 8° miscut substrate-induced heterogeneous strain. These results embodied the synergistic relationship between oxygen vacancy and strain in regulating the ferroelectric properties of hafnium-oxide based films, which offers guidance for us to understand the phase transition behavior of hafnium-based ferroelectric films and to optimize the electric properties of thin films in device applications.

Tuneable physical properties of ReI3 through ferroelectric polarization switching in 2D van der Waals multiferroic heterostructures

Ferroelectric switching of atom-thick van der Waals (vdW) magnets is desirable for the future advancement of multiferroic nanodevices. In this work, we systematically investigated the physical properties of a ReI3 layer in contact with an In2Se3 ferroelectric substrate using first-principles calculations. We demonstrated that a phase transition from ferromagnetic to antiferromagnetic occurs in the ReI3 layer under the polarization electric field of the In2Se3 layer, and the magnetocrystalline anisotropy energy is sharply reduced by 54% and 63% for upward and downward polarization directions, respectively. A semiconductor-to-metal transition occurs in the ReI3 layer as the electric polarization of the ferroelectric layer is switched. We further demonstrate that the electric polarization of the In2Se3 substrate breaks the inversion symmetry of the ReI3 layer and generates appreciable Dzyaloshinskii-Moriya (DM) interactions between the magnetic moments of Re3+ ions. Furthermore, the DM interactions can be well controlled by the ferroelectric polarization of the In2Se3 layer, which enables nonvolatile electric control of AFM skyrmions in the ReI3/In2Se3 heterostructure. These results suggest a new strategy to tune the physical properties of ReI3 and may have potential application in future multiferroic devices.

Fatigue and Aging in a Ferroelectric Composite Based on Triglycine Sulfate Combined with Nanocellulose

For the first time, the influence of fatigue and aging effects on phase transition and ferroelectric switching is studied for the composite based on ferroelectric polycrystals of triglycine sulfate and nanocellulose with different concentrations. The fatigue in samples was created by the exposure to a sinusoidal electric field, while the aging was investigated for samples after 240 h under dry conditions without the pre-exposure. It was figured out that the exposure led to the increase in phase transition temperature and the inhibition of polarization switching. On the contrary, the aging resulted in improving polarization switching and reducing the phase transition temperature.

Shape-memory effect in twisted ferroic nanocomposites

The shape recovery ability of shape-memory alloys vanishes below a critical size (~50 nm), which prevents their practical applications at the nanoscale. In contrast, ferroic materials, even when scaled down to dimensions of a few nanometers, exhibit actuation strain through domain switching, though the generated strain is modest (~1%). Here, we develop freestanding twisted architectures of nanoscale ferroic oxides showing shape-memory effect with a giant recoverable strain (>8%). The twisted geometrical design amplifies the strain generated during ferroelectric domain switching, which cannot be achieved in bulk ceramics or substrate-bonded thin films. The twisted ferroic nanocomposites allow us to overcome the size limitations in traditional shape-memory alloys and open new avenues in engineering large-stroke shape-memory materials for small-scale actuating devices such as nanorobots and artificial muscle fibrils.

Effects of Charge Trapping on Memory Characteristics for HfO2-Based Ferroelectric Field Effect Transistors.

A full understanding of the impact of charge trapping on the memory window (MW) of HfO2-based ferroelectric field effect transistors (FeFETs) will permit the design of program and erase protocols, which will guide the application of these devices and maximize their useful life. The effects of charge trapping have been studied by changing the parameters of the applied program and erase pulses in a test sequence. With increasing the pulse amplitude and pulse width, the MW increases first and then decreases, a result attributed to the competition between charge trapping (CT) and ferroelectric switching (FS). This interaction between CT and FS is analyzed in detail using a single-pulse technique. In addition, the experimental data show that the conductance modulation characteristics are affected by the CT in the analog synaptic behavior of the FeFET. Finally, a theoretical investigation is performed in Sentaurus TCAD, providing a plausible explanation of the CT effect on the memory characteristics of the FeFET. This work is helpful to the study of the endurance fatigue process caused by the CT effect and to optimizing the analog synaptic behavior of the FeFET.

Sensing single domains and individual defects in scaled ferroelectrics.

Ultra-scaled ferroelectrics are desirable for high-density nonvolatile memories and neuromorphic computing; however, for advanced applications, single domain dynamics and defect behavior need to be understood at scaled geometries. Here, we demonstrate the integration of a ferroelectric gate stack on a heterostructure tunnel field-effect transistor (TFET) with subthermionic operation. On the basis of the ultrashort effective channel created by the band-to-band tunneling process, the localized potential variations induced by single domains and individual defects are sensed without physical gate-length scaling required for conventional transistors. We electrically measure abrupt threshold voltage shifts and quantify the appearance of new individual defects activated by the ferroelectric switching. Our results show that ferroelectric films can be integrated on heterostructure devices and indicate that the intrinsic electrostatic control within ferroelectric TFETs provides the opportunity for ultrasensitive scale-free detection of single domains and defects in ultra-scaled ferroelectrics. Our approach opens a previously unidentified path for investigating the ultimate scaling limits of ferroelectronics.

Unveiling Resistance Switching Mechanisms in Undoped HfOx Ferroelectric Tunnel Junction Using Low-Frequency Noise Spectroscopy

We demonstrate that the resistance switching (RS) of an undoped hafnium oxide (HfOx)-based ferroelectric tunnel junction (FTJ) is affected not only by ferroelectric domain switching of HfOx but also by the redistribution of oxygen vacancies inside HfOx, known as the working principle of resistive random-access memory. It is revealed that the RS mechanism varies depending on the program bias applied to FTJ. Through low-frequency noise spectroscopy, a precise method for distinguishing two distinct RS processes intrinsic to FTJ is presented.

Design Strategy to Improve Memory Window in Ferroelectric Transistors With Oxide Semiconductor Channel

Oxide semiconductors are promising channel materials for hafnia-based ferroelectric transistor memories because they can constrain the formation of an unwanted interfacial layer that can deteriorate the stability of the device. A major obstacle is the limited memory window, originating from insufficient polarization switching because <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}$ </tex-math></inline-formula> -type oxide semiconductors cannot provide sufficient hole carriers to realize ferroelectric polarization switching. To solve this issue, a novel design strategy is proposed to achieve increased polarization switching while maintaining the stability of oxide semiconductor-based ferroelectric thin-film transistors (FeTFTs). By inserting an additional <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula> -type CuOx layer between the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}$ </tex-math></inline-formula> -type oxide semiconductor InZnOx and ferroelectric HfZrOx, increased polarization switching is achieved owing to the high electron and hole densities in the InZnOx and CuOx layers, respectively. Thus, a memory window of 4 V is achieved, which cannot be obtained using a single oxide-semiconductor channel. We also demonstrate that the proposed method is viable for three-dimensional ferroelectric NAND (3D FeNAND) devices. In 3D FeNAND, replacing the dielectric filler with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula> -type CuOx maximizes polarization switching and enlarges the memory window. The results demonstrate a novel structure and fabrication method for high-performance FeTFTs for advanced 3D non-volatile memory applications.

Deterministic Manipulation of Multi‐State Polarization Switching in Multiferroic Thin Films (Adv. Funct. Mater. 8/2023)

Four-State Nonvolatile Memories In article number 2208244, Deyang Chen and co-workers report that four-state polarization states in multiferroic BiFeO3 thin films are deterministically achieved and reversibly controlled through precisely selecting 71° ferroelastic and 180° ferroelectric switching paths. This study provides the platform for the realization of multi-state memories and magnetoelectric coupling based devices.