Ferroelectric Switching Research Articles

Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Nanoscale Investigation of the Effect of Annealing Temperature on the Polarization Switching Dynamics of Hf0.5Zr0.5O2 Thin Films

AbstractRecently, HfO2‐based ferroelectric thin films have attracted widespread interest in developing next‐generation nonvolatile memories. To form a metastable ferroelectric orthorhombic phase in HfO2, a post‐annealing process is typically necessary. However, the microscopic mechanism underlying the effect of annealing temperature on ferroelectric domain nucleation and growth is still obscure, despite its importance in optimizing the operation speed of HfO2‐based devices. In this study, the ferroelectric properties and polarization switching of Hf0.5Zr0.5O2 thin films annealed at different temperatures (550–700 °C) are systematically investigated. Evidently, the crystal structure, remnant polarization, and dielectric constant monotonically change with annealing temperature. However, microscopic piezoresponse force microscopy images as well as macroscopic switching current measurements reveal non‐monotonic changes in the polarization switching speed with annealing temperature. This intriguing behavior is ascribed to the difference in the ferroelectric‐domain nucleation process induced by the amount of oxygen vacancies in the Hf0.5Zr0.5O2 thin films annealed at different temperatures. This work demonstrates that controlling the defect concentration of ferroelectric HfO2 by tuning the post‐annealing process is critical for optimizing device performance, particularly polarization switching speed.

Competing Charge Transfer and Screening Effects in Two-Dimensional Ferroelectric Capacitors.

Two-dimensional (2D) ferroelectrics promise ultrathin flexible nanoelectronics, typically utilizing a metal-ferroelectric-metal sandwich structure. Electrodes can either contribute free carriers to screen the depolarization field, enhancing nanoscale ferroelectricity, or induce charge doping, disrupting the long-range crystalline order. We explore electrodes' dual roles in 2D ferroelectric capacitors, supported by first-principles calculations covering a range of electrode work functions. Our results reveal volcano-type relationships between ferroelectric-electrode binding affinity and work function, which are further unified by a quadratic scaling between the binding energy and the transferred interfacial charge. At the monolayer limit, charge transfer dictates the ferroelectric stability and switching properties. This general characteristic is confirmed in various 2D ferroelectrics including α-In2Se3, CuInP2S6, and SnTe. As the ferroelectric layer's thickness increases, the capacitor stability evolves from a charge-transfer-dominated state to a screening-dominated state. The delicate interplay between these two effects has important implications for 2D ferroelectric capacitor applications.

Nonvolatile ferroelectric resistive switching in α-In2Se3(2H) ferroelectric semiconductor junctions

Emerging two-dimensional van der Waals (vdW) based ferroelectric semiconductor junctions (FSJs) are promising for low-power and high-density applications. In this work, we report the vdW α-In2Se3(2H) based FSJs of different metal-In2Se3 interfaces. All vertical devices emerge with the nonvolatile ferroelectric resistive switching behavior. The Cr/α-In2Se3(2H)/Au FSJs show a typical ferroelectric resistive switching behavior under ∼1.0 V applied voltage. The switching voltage of the Ag/α-In2Se3(2H)/Au FSJs could be further decreased from 1.0 V to 0.8 V because of Ag migrations and ferroelectric reversal. Further, by optimizing the metal-In2Se3 interfaces, the Ag/α-In2Se3(2H)/Pt FSJs exhibit the ultralow switching voltage (∼0.3 V), excellent endurance (>225 cycles) and retention (>1 × 104 s). The results suggest that vdW α-In2Se3(2H) ferroelectric has great potential for low-power memristive devices.

Molecular beam epitaxy and characterization of ferroelectric quaternary alloy Sc0.2Al0.45Ga0.35N

In this study, we demonstrate ferroelectricity in high-quality monocrystalline quaternary alloy ScAlGaN. Sc0.2Al0.45Ga0.35N films are grown by plasma-assisted molecular beam epitaxy and exhibit a surface roughness of 0.5 nm, limited by the roughness of the underlying molybdenum template. Polarization-electric field and positive-up-negative-down measurements reveal unambiguous ferroelectric switching with a coercive field of ∼5.5 MV cm−1 at 10 kHz and high remanent polarization of ∼150 μC cm−2. Time-dependent measurements suggest that the polarization reversal behavior adheres to the Kolmogorov–Avrami–Ishibashi model and follows a scheme of domain nucleation and growth. Detailed piezoresponse force microscopy studies further elucidate the evolution of polarity reversal domains in wurtzite nitride ferroelectrics and support the notion that the growth of inversion domains occurs via an in-plane motion of the domain walls. The realization of functional ferroelectric quaternary alloys in the wurtzite nitride family extends beyond being a technical demonstration. The additional degree of bandgap, band alignment, lattice parameter, and piezoelectric constant tunability achievable through quaternary alloys unveils a vast dimension through which wurtzite nitride ferroelectrics can be optimally engineered for a broad variety of high-performance electronic, optoelectronic, and acoustic devices and systems.

Domain Wall Migration-Mediated Ferroelectric Switching and Rashba Effect Tuning in GeTe Thin Films

Domain Wall Migration-Mediated Ferroelectric Switching and Rashba Effect Tuning in GeTe Thin Films

Modeling of polarization reversal-induced interface sheet charge in wurtzite-type AlScN/GaN heterostructures

In this work, the value and the polarity of the spontaneous and piezoelectric polarization have been investigated, as the use of two different reference structures for wurtzite-type group-III nitrides, namely, the zinc-blende and the layered-hexagonal crystal lattice, have resulted in different predictions. It was found that although the differences in value and polarity of the polarization for heterostructures such as wurtzite Al1−xScxN/GaN lead to similar interface sheet charges, a significant mismatch is observed when polarization reversal is considered. The interface sheet charge predicted before and after the polarization reversal in the wurtzite Al1−xScxN layer on GaN using the zinc-blende lattice as a reference predominantly shows a change in sign. When using the layered-hexagonal lattice as a reference, not only is the same polarity of the interface sheet charge maintained after polarization reversal, but it is even 30 times larger. In this case, the giant and positive spontaneous polarization values for metal-polar Al1−xScxN extracted from the ferroelectric switching, as well as the alignment of the piezoelectric polarization to it, were observed to be consistent with the predictions referenced to the layered-hexagonal lattice. Thus, it is concluded that the layered-hexagonal reference is not only more suitable for predicting the ferroelectric properties of wurtzite Al1−xScxN but should also be the correct reference when considering polarization reversal in heterostructures. If the significant increase in the interface sheet charge after polarization reversal is experimentally detected, it will allow the design and fabrication of novel devices for future high-frequency and power electronics applications.

Controlled Molecular Orientation through Intercalation in PVDF Thin Films: Exhibiting Ultralong Retention and Improved Leakage Current.

Ferroelectric switching and retention performance of poly(vinylidene fluoride) (PVDF) thin films improve by the incorporation of unmodified smectite montmorillonite (MMT) clay nanodielectric. In the present study, an intercalated PVDF (clay/PVDF) thin film with edge-on β-crystallite is fabricated via a heat-controlled spin coating (HCSC) technique. This provides an efficient and simple way to fabricate the edge-on oriented crystallite lamellae with an electroactive β-phase, facilitating nanoscale ferroelectric switching at a lower voltage compared to the face-on orientation. Here, we demonstrate the polarization retention for periods longer than 20 days (∼480 h, i.e., 1.8 × 106 s), with no degradation in switched nanoscale domains. In addition, by maintaining the relatively high dielectric constant, the incorporation of nanoclay effectively lowers the leakage current by 102 factors. The obtained memory window in the edge-on orientation is 7 V, approximately twice the memory window obtained in the face-on orientation. In short, our findings provide a simple and promising route to fabricate edge-on oriented PVDF thin films, with ultralong retention, high dielectric constant, and improved leakage current.

Rotational magnetoelectric switching in orthorhombic multiferroics

Controlling the direction of ferromagnetism and antiferromagnetism by an electric field in single-phase multiferroics will open the door to the next generation of devices for spintronics and electronics. The typical magnetoelectric coupling such as the linear magnetoelectric effect is very weak in type-I multiferroics and therefore the magnetoelectric switching is rarely achieved. Here, using first-principles simulations, we propose a magnetoelectric switching mechanism to achieve such highly desired control in orthorhombic multiferroics. One class of two-dimensional proper multiferroics (CrX2Se3 and MnX2Te3, X = Sn, Ge) and perovskite multiferroics (EuTiO3 and BiFeO3/LaFeO3 superlattice) are taken as examples to show the mechanism. In the ferroelectric switching process, the proper polarization rotates its direction by 180° and keeps its magnitude almost unchanged, the ferromagnetic or antiferromagnetic vector is rotationally switched by 180° following the rotation of ferroelectric polarization. This rotational magnetoelectric switching results from in-plane structural anisotropy and magnetic anisotropy, and the process of switching is governed by cosine functions from the phenomenological Landau-type models. This study addresses the challenge of magnetoelectric switching in type-I multiferroics by proposing a general magnetoelectric switching mechanism.

Concomitant Light-Reversible Magnetic Response in Multiferroic Oxide Heterostructures for Multiphysics Applications.

The concept of multiphysics, where materials respond to diverse external stimuli, such as magnetic fields, electric fields, light irradiation, stress, heat, and chemical reactions, plays a fundamental role in the development of innovative devices. Nanomanufacturing, especially in low-dimensional systems, enhances the synergistic interactions taking place on the nanoscale. Light-matter interaction, rather than electric fields, holds great promise for achieving low-power, wireless control over magnetism, solving two major technological problems: the feasibility of electrical contacts at smaller scales and the undesired heating of the devices. Here, we shed light on the remarkable reversible modulation of magnetism using visible light in epitaxial Fe3O4/BaTiO3 heterostructure. This achievement is underpinned by the convergence of two distinct mechanisms. First, the magnetoelastic effect, triggered by ferroelectric domain switching, induces a proportional change in coercivity and remanence upon laser illumination. Second, light-matter interaction induces charged ferroelectric domain walls' electrostatic decompensations, acting intimately on the magnetization of the epitaxial Fe3O4 film by magnetoelectric coupling. Crucially, our experimental results vividly illustrate the capability to manipulate magnetic properties using visible light. This concomitant mechanism provides a promising avenue for low-intensity visible-light manipulation of magnetism, offering potential applications in multiferroic devices.

Downscaling grain size toward the nanometre range – A key-factor for tuning the crystalline structure, phase transitions, dielectric and ferroelectric behaviour in Ba0·8Sr0·2TiO3 ceramics

The present study aims to describe the role of the grain size on the properties of submicron- and nano-structured Ba0·8Sr0·2TiO3 (BST) ceramics. Dense (1 − 2% porosity) ceramics with average grain sizes in the range of (77 − 234) nm were consolidated under different spark plasma sintering conditions starting from nanopowders with a mean particle size of 70 nm, synthesized via the acetate variant of the sol-gel method. The structural analysis based on XRD data revealed a mixture of cubic and tetragonal modifications at room temperature for the precursor powders and for all the investigated ceramics. The structural heterogeneity of the individual ceramic grains with coexistence of cubic and tetragonal polymorphs was confirmed by HR-TEM investigations. Accordingly, a “brick-wall" model with cubic grain boundary regions and tetragonal grain cores is proposed. By increasing the grain size, from 77 to 234 nm, a decrease of the phase transitions diffuseness accompanied by an increase of the permittivity maxima (from 650 to 4500) and dielectric losses (from 5 to 7.5%, at 100 Hz), was detected by broadband dielectric spectroscopy. No variation of the Curie temperature in the investigated Ba0·8Sr0·2TiO3 ceramics was detected, unlike typically reported for BaTiO3 ceramics with similar grain sizes. The Curie-Weiss temperature and the Curie constant decrease when grain size is diminished, indicating an overall reduction of the ferroelectric active volume, as a scaling effect. The ferroelectric switching was demonstrated for all the selected fine-grained BST ceramics, either at nanoscale or macroscopically, with an increased ferroelectric character for the coarser submicron-structured ceramics, with respect to the nanocrystalline one. The observed properties of the fine-grained Ba0·8Sr0·2TiO3 ceramics are explained in the frame of multiphase coexistence and ferroelectricity “dilution” due to the increasing role of non-ferroelectric grain boundaries when reducing grain size and complete the knowledge on the scale-dependent properties of dense fine-grained BaTiO3-based ceramics.

Unique Consecutive RTN Characteristics Coupled With Ferroelectric Nanodomain Switching in Advanced Fe-FinFETs

Unique Consecutive RTN Characteristics Coupled With Ferroelectric Nanodomain Switching in Advanced Fe-FinFETs

Tailoring the optoelectronic properties of PZT through the modulation of the thin film

Ferroelectric materials have great promise for use in photodetectors due to their built-in electric field-assisted carrier separation and switchable polarization properties. Carrier separation efficiency is a decisive factor in evaluating photodetector performance. The photodetector optoelectronic performance can be enhanced further by optimizing the thickness of the ferroelectric film to take full advantage of the switchable polarization properties of the ferroelectric material, and by enhancing the built-in electric field to drive carrier separation. In this work, we optimize the performance of PbZr0.52Ti0.48O3 (PZT) photodetectors by modulating the thickness of the film. It is observed that thicker ferroelectric films have lower coercivity fields, which are more favorable for ferroelectric domain switching. On this basis, the ferroelectric properties of ferroelectric PZT films were optimized by thickness tuning, and the photodetection performance of PZT-based self-powered photodetectors was explored. It is found that the polarization enhances the internal electric field, driving photogenerated carrier separation and improving the self-powered current, while also selectively enhancing photodetectivity for devices of different thicknesses. Additionally, both film thickness and ferroelectric polarization significantly impact the response time.

Chemical modification of lead zirconate titanate piezoceramics through cold-sintering process

This study presents a facile strategy to chemically modify the PZT through the cold-sintering process. The presence of lead oxide phases in the XRD patterns of samples cold-sintered at 290 °C confirms the dissolution of Pb ions from the surface of PZT particles. Post-annealing the samples at 900 °C resulted in a phase pure material with a perovskite structure, wherein the Ba ions had diffused into the lattice of PZT. The post-annealed samples had a multi-faceted grain with equilibrated triple points and a relative density of ∼93%. The electromechanical characterization showed that the d33 of the samples increased with increasing content of Ba in the PZT, reaching a value of 201 ± 80 pm/V. The post-annealed samples displayed ferroelectric switching current behaviors with saturated polarisation loops. The experimental findings reported in this study highlight the potential of the cold-sintering process for chemical modification of lead zirconate titanate electroceramics.

Effect and Regulation Mechanism of Post-deposition Annealing on the Ferroelectric Properties of AlScN Thin Films.

Integrating ferroelectric AlScN with III-N semiconductors to enhance the performance and tunability of nitride devices requires high-quality AlScN films. This work focuses on the effect and regulation mechanism of post-annealing in pure N2 on the crystal quality and ferroelectric properties of AlScN films. It is found that the crystal quality improves with increasing annealing temperatures. Remarkably, the leakage current of AlScN films caused by grain boundaries could be reduced by four orders of magnitude after annealing at 400 °C. The crystal growth dynamics simulations and band structure calculations indicate that the energy supplied by the temperature facilitates the evolution of abnormally oriented grains to have a better c-axis orientation, resulting in the defect states at the Fermi-level disappearing, which is mainly the reason for the leakage current decrease. More interestingly, the reduction of leakage current leads to the previously leaking region exhibiting ferroelectric properties, which is of great significance to improve the ferroelectricity of AlScN and ensure the uniformity of devices. Furthermore, annealing enhances the tensile strain on the film, which flattens the energy landscape of ferroelectric switching and reduces the coercive field. However, the risk of incorporation of oxygen will also be increased if the annealing temperatures are higher than 400 °C, which will not only reduce the relative displacement of metal atoms and nitrogen atoms in AlScN but also enhance the ferroelectric depolarization field, leading to the remnant polarization decreasing dramatically. These discoveries facilitate a deeper understanding of the influencing mechanism on the ferroelectric properties of AlScN films and provide a direction for obtaining high-quality AlScN.

Effect of the number and distribution of Al2O3 atomic layer deposition cycles within HfO2 layer on ferroelectric characteristics

We comprehensively analyze the effects of the number and distribution of Al2O3 atomic layer deposition (ALD) cycles into a 10-nm-thick HfO2 matrix on the ferroelectric switching behavior. An ALD cycle containing one pulse for Hf (or Al) precursor and one pulse of water as reactant is repeated 150 times for the given thickness of 10 nm. Spontaneous remnant polarization (Pr) is enabled through the formation of crystalline Al-doped HfO2 (Al:HfO2) by incorporating at least two Al2O3 ALD cycles evenly into the HfO2 film under annealing at 600 °C for 3 min following W top electrode (TE) deposition. When more than four Al2O3 cycles are used, the Al elements function as leakage sources rather than stressors, resulting in an open hysteresis loop and a weak endurance of 105 cycles. Notably, an improved 2 Pr of ∼9 μC/cm2 is achieved when the Al2O3 layers are concentrated near the lower region of the HfO2. On the other hand, as the Al2O3 layers are intensively located in the upper region of the HfO2, a dielectric response is observed in the polarization–voltage and current–voltage measurements. Our results indicate that the two mechanical stresses induced by the Al dopant with a size smaller than Hf and the difference in the thermal expansion coefficient between TE and Al:HfO2 effectively activate both the lower and upper sites. Therefore, many dipoles are observed to participate in the polarization owing to the stresses that are applied evenly throughout the Al:HfO2 layer to form the orthorhombic phase.

Interface engineering for facile switching of bulk-strong polarization in Si-compatible vertical superlattices

Ferroelectric thin films incorporating different compositional layers have emerged as a promising approach for enhancing properties and performance of electronic devices. In recent years, superlattices utilizing various interactions between their constituent layers have been used to reveal unusual properties, such as improper ferroelectricity, charged domain walls, and negative capacitance in conventional ferroelectrics. Herein, we report a symmetry scheme based on the interface engineering in which the inherent cell-doubling symmetry allowed atomic distortions (phonons) in any vertically aligned superlattice activate novel interface couplings among atomic distortions of different symmetries and fundamentally improve the ferroelectric properties. In a materialized case, the ionic size difference between Hf4+ and Ce4+ in the HfO2/CeO2 (HCO) ferroelectric/paraelectric superlattice leads to these couplings. These couplings mitigate the phase boundary between polar and non-polar phases, and facilitate polarization switching with a remarkably low coercive field (Ec\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${E}_{c}$$\\end{document}) while preserving the original magnitude of the bulk HfO2 polarization and its scale-free ferroelectric characteristics. We show that the cell-doubled distortions present in any vertical superlattice have unique implications for designing low-voltage ferroelectric switching while retaining bulk-strong charge storing capacities in Si-compatible memory candidates.

A Scanning Microwave Impedance Microscopy Study of α‐In2Se3 Ferroelectric Semiconductor

AbstractVan der Waals ferroelectric semiconductors, which encompass both ferroelectricity and semiconductivity, have garnered intensive research interests for developing novel non‐volatile functional devices. Previous studies focus on ferroelectricity characterization and device demonstration, with little attention paid to the fundamental electronic properties of these materials and their functional structures, which are essential for both device design and optimization. In this study, scanning microwave impedance microscopy (sMIM) is utilized to investigate the ferroelectric semiconductor of α‐phase indium selenide (α‐In2Se3) and its synaptic field effect transistors. α‐In2Se3 nanoflakes of varying thicknesses are visualized through capacitive signal detection, whose responses are consistent with finite element simulations manifesting dependence on both flake thickness and its semiconductor property. sMIM spectroscopy performed on α‐In2Se3‐based metal‐oxide‐semiconductor (MOS) structures reveals typical MOS capacitance‐voltage characteristics, with additional hysteresis arising from the ferroelectric switching of α‐In2Se3. The local conductance state changes of synaptic α‐In2Se3 ferroelectric semiconductor transistors (FeSFET) in response to gate voltage stimuli are effectively detected by in situ sMIM, in good agreement with electrical device transport properties. This work deepens the understanding of ferroelectric semiconductor physics toward their practical device application.

Tracking intrinsic ferroelectric switching under electric field via operando second harmonic generation

Polarization hysteresis is the defining characteristic of ferroelectrics, though the measurement of ferroelectric hysteresis is often complicated by artifacts such as leakage current and not all materials with apparent electrical hysteresis are ferroelectric. In this Letter, we have set up an operando second harmonic generation (SHG) system to track intrinsic ferroelectric switches under electric field, which is free from leakage current interference, thus yielding a signature for intrinsic ferroelectricity. Taking representative PbZr0.2Ti0.8O3 (PZT) thin films with different thicknesses as examples, the operando SHG system can capture ferroelectric hysteresis not only for PZT films thicker than 50 nm, for which conventional hysteresis measurement works, but also for PZT film as thin as 26 nm, for which conventional measurement fails due to the presence of large leakage current. Different domain evolution processes in these films are also illustrated.

The Robust Ferroelectric and Electrical Response in 2D Bi2O2Se Semiconductor

AbstractThe amelioration of atomically thin ferroelectric materials is imperative for next‐generation outperformed two‐dimensional (2D) electronics, which is elusive by their bulk counterparts. These remarkable materials’ ferroelectric and piezoelectric features are the fundamental urges in optoelectronics, electronics, and energy harvesting. In this work, 2D ferroelectric Bi2O2Se flakes have been synthesized using a modified chemical vapor deposition technique. The 6 nm thick Bi2O2Se flake provides a robust ferroelectric switching under an applied voltage of ±10 V by piezoresponse force microscopy, further confirmed by first principles. Leveraging the successful growth, the high‐quality Bi2O2Se flakes permit the fabrication of a field‐effect transistor (FET) with state‐of‐the‐art performance. The FET device rewards a high current on–off ratio of 108 and field effect mobility of almost 131 cm2 V−1 s−1, owing to the small carrier effective mass of 0.2 m0. Combined, the electric field‐induced local polarization of ferroelectric switching and unprecedented device performance of Bi2O2Se semiconductors are certified for their utilization in advanced nanoelectronics and miniaturization of multifunctional devices with multifunctionalities.