Ferroelectric Superlattices Research Articles

Mimicking Antiferroelectrics with Ferroelectric Superlattices.

Antiferroelectric oxides are promising materials for applications in high-density energy storage, solid-state cooling, and negative capacitance devices. However, the range of oxide antiferroelectrics available today is rather limited. In this work, it is demonstrated that antiferroelectric properties can be electrostatically engineered in artificially layered ferroelectric superlattices. Using a combination of synchrotron X-ray nanodiffraction, scanning transmission electron microscopy, macroscopic electrical measurements, and lateral and vertical piezoresponse force microscopy in parallel-plate capacitor geometry, a highly reversible field-induced transition is observed from a stable in-plane polarized state to a state with in-plane and out-of-plane polarized nanodomains that mimics, at the domain level, the nonpolar to polar transition of traditional antiferroelectrics, with corresponding polarization-voltage double hysteresis and comparable energy storage capacity. Furthermore, it is found that such superlattices exhibit large out-of-plane dielectric responses without involving flux-closure domain dynamics. These results demonstrate that electrostatic and strain engineering in artificially layered materials offers a promising route for the creation of synthetic antiferroelectrics.

Emergence and transformation of polar skyrmion lattices via flexoelectricity

As analogies to magnetic skyrmions, polar skyrmions in ferroelectric superlattices and multilayers have garnered widespread attention for their non-trivial topology and novel properties like negative capacitance and nonlinear optical effect. So far, they have only been theoretically predicted to be able to assemble ordered hexagonal skyrmion lattices (SkLs) in ferroelectric thin films. Here, based on phase-field simulations, we report the critical roles of flexoelectricity playing in the stabilization and transformation of polar SkLs. Different polar SkL patterns can emerge in the ferroelectric thin films, including tetragonal-SkL, and hexagonal-SkLs with diverse orientations, as summarized by phase diagrams. These emergent SkL states are attributed to the material anisotropy modified by the flexoelectric effect. Interestingly, we further found that the hexagonal-SkLs can be rotated by applying strain gradient or in-plane electric field to the films. Moreover, a nonreciprocal bending response of tetragonal-SkL is also induced by the flexoelectric effect. Our results provide useful guidelines for the implementation of polar skyrmion lattices in experiments.

First-principles predictions of HfO2-based ferroelectric superlattices

The metastable nature of the ferroelectric phase of HfO2 is a significant impediment to its industrial application as a functional ferroelectric material. In fact, no polar phases exist in the bulk phase diagram of HfO2, which shows a dominant non-polar monoclinic ground state. As a consequence, ferroelectric orthorhombic HfO2 is stabilized either kinetically or via epitaxial strain. Here, we propose an alternative approach, demonstrating the feasibility of thermodynamically stabilizing polar HfO2 in superlattices with other simple oxides. Using the composition and stacking direction of the superlattice as design parameters, we obtain heterostructures that can be fully polar, fully antipolar or mixed, with improved thermodynamic stability compared to the orthorhombic polar HfO2 in bulk form. Our results suggest that combining HfO2 with an oxide that does not have a monoclinic ground state generally drives the superlattice away from this non-polar phase, favoring the stability of the ferroelectric structures that minimize the elastic and electrostatic penalties. As such, these diverse and tunable superlattices hold promise for various applications in thin-film ferroelectric devices

Stabilization and control of weakly correlated polar skyrmions in ferroelectric thin films

In recent years, polar topological textures like skyrmions and vortices in ferroelectric superlattices and multilayers have received extensive attention. However, the polar topological textures in these systems suffer strong interactions and are difficult to be manipulated as individual elements, limiting their application prospects in storage and logic devices. Here, via phase field simulations, we demonstrate domain engineering as a feasible strategy to create diverse weakly correlated polar topological texture arrays in single-layer tetragonal ferroelectric thin films, including Néel-type skyrmions with positive/negative topological charges and Bloch-type skyrmions with different chirality. We calculate the stability phase diagram and reveal the evolution characteristics of these polar topological textures under local electric and mechanical stimuli. We realize local erasing and writing of single Néel- and Bloch-type skyrmions, as well as deterministic chirality control of single Bloch-type skyrmion, confirming the weak correlation of the polar skyrmions. Our study therefore demonstrates domain engineering as an effective strategy for stabilizing weakly correlated polar topological textures, which extends the existence system of polar topological textures and paves a new way for the design of storage and logic devices based on polar topological textures.

Creating Ferroelectricity in Monoclinic (HfO_{2})_{1}/(CeO_{2})_{1} Superlattices.

Ferroelectricity in CMOS-compatible hafnia (HfO_{2}) is crucial for the fabrication of high-integration nonvolatile memory devices. However, the capture of ferroelectricity in HfO_{2} requires the stabilization of thermodynamically metastable orthorhombic or rhombohedral phases, which entails the introduction of defects (e.g., dopants and vacancies) and pays the price of crystal imperfections, causing unpleasant wake-up and fatigue effects. Here, we report a theoretical strategy on the realization of robust ferroelectricity in HfO_{2}-based ferroelectrics by designing a series of epitaxial (HfO_{2})_{1}/(CeO_{2})_{1} superlattices. The designed ferroelectric superlattices are defects free, and most importantly, on the base of the thermodynamically stable monoclinic phase of HfO_{2}. Consequently, this allows the creation of superior ferroelectric properties with an electric polarization >25 μC/cm^{2} and an ultralow polarization-switching energy barrier at ∼2.5 meV/atom. Our work may open an avenue toward the fabrication of high-performance HfO_{2}-based ferroelectric devices.

High‐throughput combinatorial approach expedites the synthesis of a lead‐free relaxor ferroelectric system

AbstractDeveloping novel lead‐free ferroelectric materials is crucial for next‐generation microelectronic technologies that are energy efficient and environment friendly. However, materials discovery and property optimization are typically time‐consuming due to the limited throughput of traditional synthesis methods. In this work, we use a high‐throughput combinatorial synthesis approach to fabricate lead‐free ferroelectric superlattices and solid solutions of (Ba0.7Ca0.3)TiO3 (BCT) and Ba(Zr0.2Ti0.8)O3 (BZT) phases with continuous variation of composition and layer thickness. High‐resolution x‐ray diffraction (XRD) and analytical scanning transmission electron microscopy (STEM) demonstrate high film quality and well‐controlled compositional gradients. Ferroelectric and dielectric property measurements identify the “optimal property point” achieved at the composition of 48BZT–52BCT. Displacement vector maps reveal that ferroelectric domain sizes are tunable by varying {BCT–BZT}N superlattice geometry. This high‐throughput synthesis approach can be applied to many other material systems to expedite new materials discovery and properties optimization, allowing for the exploration of a large area of phase space within a single growth.image

Improvement of ferroelectric and dielectric properties in BaTiO3/LaNiO3 superlattices by regulating the ferroelectric layer thickness

Ferroelectric superlattices exhibit rich functional characteristics. Yet, the ferroelectric properties are always unsatisfactory due to the presence of depolarization field resulting from the polarization discontinuities between the component materials, which has become a bottleneck problem hindering the improvement of ferroelectric properties and further industrial applications. Here, the BTO/LNO superlattices composed of ferroelectric (BaTiO3, BTO) and metallic (LaNiO3, LNO) materials were prepared by pulsed laser deposition (PLD). The periodic thickness of the LNO layers was fixed to 2 unit cells, while the periodic thickness of the BTO layers was changed. We demonstrate experimentally that by regulating the BTO periodic thickness, BTO/LNO superlattices exhibit excellent and adjustable ferroelectric and dielectric properties. The enhancement of electrical properties is closely related to the increase of tetragonality (c/a) combined with the effect of the ultra-thin LNO layers and the accumulation of oxygen vacancies at superlattice interfaces on reducing the depolarization field. We hope this work will provide an effective strategy for optimizing the ferroelectric properties of ferroelectric superlattices based on the component material and periodic structure regulation.

Giant electric field-induced second harmonic generation in polar skyrmions

Electric field-induced second harmonic generation allows electrically controlling nonlinear light-matter interactions crucial for emerging integrated photonics applications. Despite its wide presence in materials, the figures-of-merit of electric field-induced second harmonic generation are yet to be elevated to enable novel device functionalities. Here, we show that the polar skyrmions, a topological phase spontaneously formed in PbTiO3/SrTiO3 ferroelectric superlattices, exhibit a high comprehensive electric field-induced second harmonic generation performance. The second-order nonlinear susceptibility and modulation depth, measured under non-resonant 800 nm excitation, reach ~54.2 pm V−1 and ~664% V−1, respectively, and high response bandwidth (higher than 10 MHz), wide operating temperature range (up to ~400 K) and good fatigue resistance (>1010 cycles) are also demonstrated. Through combined in-situ experiments and phase-field simulations, we establish the microscopic links between the exotic polarization configuration and field-induced transition paths of the skyrmions and their electric field-induced second harmonic generation response. Our study not only presents a highly competitive thin-film material ready for constructing on-chip devices, but opens up new avenues of utilizing topological polar structures in the fields of photonics and optoelectronics.

Giant tunnel resistance effect in (SrTiO3)2/(BaTiO3)4/(CaTiO3)2 asymmetric superlattice with enhanced polarization.

In this work, we report the effectively enhanced tunneling electroresistance effect in Au/(SrTiO3)2/(BaTiO3)4/(CaTiO3)2/Nb:SrTiO3 superlattice ferroelectric tunnel junction (FTJ). The stable polarization switching and enhanced ferroelectricity were achieved in the nanoscale thickness high-quality epitaxial superlattice. A high ON/OFF current ratio of more than 105 was obtained at room temperature, which is an order of magnitude larger than the BaTiO3 FTJ with the same structure. Nonvolatile resistance switching controlled by nonvolatile polarization switching was observed in the superlattice FTJ. Driven by increased polarization and intrinsic asymmetric ferroelectricity, a highly asymmetric depolarization field is generated compared with the Au/BaTiO3/Nb:SrTiO3 FTJ, resulting in an enhanced tunneling electroresistance effect. These results provide a potential way to construct FTJ memory devices by constructing asymmetric three-component ferroelectric superlattices.

Impact of Zr substitution on the electronic structure of ferroelectric hafnia

HfO2-based dielectrics are promising for nanoscale ferroelectric applications, and the most favorable material within the family is Zr-substituted hafnia, i.e., Hf1−xZrxO2 (HZO). The extent of Zr substitution can be great, and x is commonly set to 0.5. However, the bandgap of ZrO2 is lower than HfO2, thus it is uncertain how the Zr content should influence the electronic band structure of HZO. A reduced bandgap is detrimental to the cycling endurance as charge injection and dielectric breakdown would become easier. Another issue is regarding the comparison on the bandgaps between HfO2/ZrO2 superlattices and HZO solid-state solutions. In this work, we systematically investigated the electronic structures of HfO2, ZrO2, and HZO using self-energy corrected density functional theory. In particular, the conduction band minimum of Pca21-HfO2 is found to lie at an ordinary k-point on the Brillouin zone border, not related to any interlines between high-symmetry k-points. Moreover, the rule of HZO bandgap variation with respect to x has been extracted. The physical mechanisms for the exponential reduction regime and linear decay regime have been revealed. The bandgaps of HfO2/ZrO2 ferroelectric superlattices are investigated in a systematic manner, and the reason why the superlattice could possess a bandgap lower than that of ZrO2 is revealed through comprehensive analysis.

Domain wall state diagram for SrTiO3/BaTiO3 superlattice structures

The domain wall structure of ferroelectric/ paraelectric superlattices can be much more complex due to the influence of the superlattice stacking structure, the in-plane strain induced by the substrate and environmental temperature. In this study, we employed a phase field model to investigate the domain wall state of the SrTi[Formula: see text]/BaTi[Formula: see text] superlattice structure. The domain wall thickness for the SrTi[Formula: see text]/BaTi[Formula: see text] layer was measured using a hyperbolic function. Based on the simulation results, here, we show a domain wall state diagram to distinguish the hard and soft domain states. The polarization profiles across hard/ soft domain walls were illustrated and analyzed. Our simulation results offer a useful concept for the control of the domain wall state in the ferroelectric superlattice.

Photoinduced control of ferroelectricity in hybrid-improper ferroelectric superlattices

We reveal a photoinduced control of ferroelectricity in hybrid-improper ferroelectric superlattices, using ab initio theory. Along with a notable photostriction effect, thermalized carriers from photoexcitation affect octahedral tiltings along different directions. A trilinear coupling between antipolar and tilting modes drives corresponding cationic displacements, and this results in a decrease in polarization under light. Together with recent experimental evidence, our study therefore demonstrates that light is a viable route to engineer functionalities in materials.

Thermodynamics of Light-Induced Nanoscale Polar Structures in Ferroelectric Superlattices.

We study the thermodynamics of nanoscale polar structures in PbTiO3/SrTiO3 ferroelectric superlattices induced by above-bandgap optical excitation using a phase-field model explicitly considering both structural and electronic processes. We demonstrate that the light-excited carriers provide the charge compensation of polarization bound charges and the lattice thermal energy, both of which are key to the thermodynamic stabilization of a previously observed supercrystal, a three-dimensionally periodic nanostructure, within a window of substrate strains, while different mechanical and electrical boundary conditions can stabilize a number of other nanoscale polar structures by balancing the competing short-range exchange interactions responsible for the domain wall energy and long-range electrostatic and elastic interactions. The insights into the light-induced formation and richness of nanoscale structures from this work offer theoretical guidance for exploring and manipulating the thermodynamic stability of nanoscale polar structures employing a combination of thermal, mechanical, and electrical stimuli as well as light.

Domain-Enhanced Energy Storage, Piezoelectric, and Dielectric Properties in Pb(Zr0.52Ti0.48)O3/SrTiO3 Ferroelectric Superlattices

The domain structure and ferroelectric properties are highly sensitive to interfacial strain and electrostatic interaction in the ferroelectric superlattices. Here, we fabricated a series of [Pb(Zr0.52Ti0.48)O3]m/[SrTiO3]3 (PZTm/STO3) ferroelectric superlattices (m = 2, 3, 6... unit cells) on SrTiO3 (001) substrates by pulsed laser deposition. Compared with pure PZT films, the energy storage density, piezoelectric properties, and dielectric constant of the superlattice are improved. Particularly, the energy storage density, which is increased by about 158%. The modulation of PZT/STO superlattice period thickness effectively improves its dielectric and relaxation properties, which in turn enhances its energy density and efficiency. The mechanism underlying the excellent energy storage properties was revealed by the formation of nanodomains analyzed by a piezoresponse force microscope. Adjusting the electrostatic interactions by changing the periodic thickness of ferroelectric superlattices provides a promising strategy for the design of high-performance sensors and energy-storage devices.

Design of high polarization low switching barrier hybrid improper ferroelectric perovskite oxide superlattices.

Hybrid improper ferroelectricity is a useful tool to design ABO3/A'BO3 polar superlattices from non polar building blocks. In this study, we have designed high polarization-low switching barrier hybrid improper ferroelectric superlattices with efficient polarization, and polarization-magnetization switching properties above room temperature, using density functional theory and ab initio molecular dynamics simulations. Superlattices with a chemical formula of (AAlO3)m/(A'AlO3)n, where m/n = 1/1, 1/3, 3/1, 1/5 and 5/1, A, A' = Lanthanide and Y cations are considered to outline the design principles behind polarization switching and (LaFeO3)3/(CeFeO3)1 is investigated for polarization-magnetization switching. We find that the unconventional switching paths via out-of-phase rotation QR- (a0a0c-) and tilt precession QTP always yield lower switching barrier compared to those via in-phase rotation QR+ (a0a0c+) and tilt QT (a-a-c0) of BO6 octahedra. Results from ab initio molecular dynamics simulations estimate the temperature at which the lowest energy barrier can be overcome. It is possible to tune the polarization switching barrier by tuning the tolerance factor, A,A' cation radius mismatch and super lattice periodicity. For switching via QR-, the switching barrier varies exponentially with rotation angle, indicating how high switching barrier is expected for systems, away from cubic symmetry. We provide a recipe to overcome such a bottleneck by tuning superlattice periodicity. Finally, we have proposed the multiferroic device application concept through a proposed polarization-temperature hysteresis loop and magnetization switching.

Anomalous Rashba Effect Driven by Polar and Nonpolar Modes in the Ferroelectric Superlattice

Anomalous Rashba Effect Driven by Polar and Nonpolar Modes in the Ferroelectric Superlattice

Tunable interface states driven by ferroelectric polarization discontinuity in BiFeO3-based superlattice

Interfacial electronic reconstruction is one of the central topics in condensed matter research as it brings in new physics and novel material properties. Typically, it is induced by dipole, valence, or lattice discontinuities near the interfaces. However, ferroelectric polarization discontinuity (FPD) can also induce electronic reconstruction, which is not well understood, particularly in perovskite oxide interfaces. Here, we demonstrate that FPD plays critical roles in determining the electronic properties of ferroelectric superlattices and creates coexisted two-dimensional hole gas (2DHG) and two-dimensional electron gas (2DEG). We further unravel that FPD competes the traditional polar discontinuity, thus, can lead to various final interface states. The present work opens a special door to achieve 2DEG and 2DHG in the ferroelectric perovskite heterostructure via ferroelectric polarization discontinuity and provides a guidance to achieve emergent interfacial phenomena.

Phase transformations and dielectric response in ferroelectric superlattices ferroelectric-dielectric

The influence of electrical interactions between layers on phase transformations in ferroelectric superlattices with alternating layers of isotropic dielectric-ferroelectric is studied theoretically. The change in the temperature of the phase transition to the ferroelectric phase, spontaneous polarization, and dielectric response in these structures are estimated depending on the ratio between the thicknesses of different layers of the lattice. It is shown that the effect of the depolarizing field on the behavior of ferroactive particles in an external field in the ferroelectric component leads to an increase in the dielectric constant of the material.

Strain manipulation of ferroelectric skyrmion bubbles in a freestanding PbTiO3 film: A phase field simulation

So far nanoscale topological polar structures have been mainly observed in ferroelectric superlattices, nanodots, and complex heterostructures, originating from the intricate interplay of built-in electric field, polarization, strain, and their gradient-related energies. However, solid substrates hinder the continuous strain manipulation of the ferroelectric topological textures. Recently, a breakthrough in fabricating freestanding films has demonstrated the possibility of continuous strain manipulation, but there has been little experimental or theoretical investigation of whether polar topological structures exist in freestanding films. Herein, by performing phase field simulation on ${(\mathrm{PbTi}{\mathrm{O}}_{3})}_{20}/{(\mathrm{SrTi}{\mathrm{O}}_{3})}_{10}$ freestanding bilayers, we observed the stabilized ferroelectric skyrmion bubbles in a ferroelectric layer. A thickness-dependent phase diagram indicates that the skyrmion bubbles exist when the ferroelectric layer is between eight and 30 unit cells. Meanwhile, we demonstrated that the uniaxial strain is an effective way to manipulate the skyrmion bubble in freestanding films, which not only induces consolidation and rearrangement of skyrmion bubbles, but also induces a phase transition from the topological skyrmion bubble state to a standard ${a}_{1}/{a}_{2}$ domain state. These results provide us guidance with a mechanical approach to control topological polar structures in freestanding films.

Inherent Spin-Polarization Coupling in a Magnetoelectric Vortex.

Solid-state materials are currently being explored as a platform for the manipulation of spins for spintronics and quantum information science. More broadly, a wide spectrum of ferroelectric materials, spanning from inorganic oxides to polymeric systems such as PVDF, present a different approach to explore quantum phenomena in which the spins are set and manipulated with electric fields. Using dilute Fe3+-doped ferroelectric PbTiO3-SrTiO3 superlattices as a model system, we demonstrate intrinsic spin-polarization control of spin directionality in complex ferroelectric vortices and skyrmions. Electron paramagnetic resonance (EPR) spectra show that the spins in the Fe3+ ion are strongly coupled to the local polarization and preferentially aligned perpendicular to the ferroelectric polar c axis in this complex vortex structure. The effect of polarization-spin directionality is corroborated by first-principles calculations, demonstrating the variation of the spin directionality with the polar texture and offering the potential for future quantum analogues of macroscopic magnetoelectric devices.