Domain-wall pinning and defect ordering in BiFeO3 probed on the atomic and nanoscale

Ferroelectric Domain Modulation with Tip-Poling Engineering in BiFeO3 Films

BiFeO3 (BFO) films with ferroelectricity are the most promising candidates regarding the next generation of storage devices and sensors. The comprehensive understanding of ferroelectric switchable properties is challenging and critical to robust domain wall nanoelectronics. Herein, the domain dynamic was explored in detail under external bias conditions using scanning probe microscopy, which is meaningful for the understanding of domain dynamics and the foundation of ferroelectric devices. The results show that domain reversal occurred under external electric fields with sufficient energy excitation, combined with the existence of a charged domain wall. These findings extend the domain dynamic and current paths in ferroelectric films and shed light on the potential applications for ferroelectric devices.

Transient Ruddlesden-Popper-Type Defects and Their Influence on Grain Growth and Properties of Lithium Lanthanum Titanate Solid Electrolyte

Lithium lanthanum titanate (LLTO) perovskite is one of the most promising electrolytes for all-solid-state batteries, but its performance is limited by the presence of grain boundaries (GBs). The fraction of GBs can be significantly reduced by the preparation of coarse-grained LLTO ceramics. In this work, we describe an alternative approach to the fabrication of ceramics with large LLTO grains based on self-seeded grain growth. In compositions with the starting stoichiometry for the Li0.20La0.60TiO3 phase and with a high excess addition of Li (Li:La:Ti = 11:15:25), microstructure development starts with the formation of the layered RP-type Li2La2Ti3O10 phase. Grains with many RP-type defects initially develop into large platelets with thicknesses of up to 10 μm and lengths over 100 μm. Microstructure development continues with the crystallization of LLTO perovskite, epitaxially on the platelets and as smaller grains with thinner in-grain RP-lamellae. Theoretical calculations confirmed that the formation of RP-type sequences is energetically favored and precedes the formation of the LLTO perovskite phase. At around 1250 °C, the RP-type sequences become thermally unstable and gradually recrystallize to LLTO via the ionic exchange between the Li-rich RP-layers and the neighboring Ti and La layers as shown by quantitative HAADF-STEM. At higher sintering temperatures, LLTO grains become free of RP-type defects and the small grains recrystallize onto the large platelike seed grains via Ostwald ripening. The final microstructure is coarse-grained LLTO with total ionic conductivity in the range of 1 × 10-4 S/cm.

Large Electromechanical Response in a Polycrystalline Alkali-Deficient (K,Na)NbO3 Thin Film on Silicon

The demand for large electromechanical performance in lead-free polycrystalline piezoelectric thin films is driven by the need for compact, high-performance microelectromechanical systems (MEMS) based devices operating at low voltages. Here we significantly enhance the electromechanical response in a polycrystalline lead-free oxide thin film by utilizing lattice-defect-induced structural inhomogeneities. Unlike prior observations in mismatched epitaxial films with limited low-frequency enhancements, we achieve large electromechanical strain in a polycrystalline (K,Na)NbO3 film integrated on silicon. This is achieved by inducing self-assembled Nb-rich planar faults with a nonstoichiometric composition. The film exhibits an effective piezoelectric coefficient of 565 pm V-1 at 1 kHz, surpassing those of lead-based counterparts. Notably, lattice defect growth is substrate-independent, and the large electromechanical response is extended to even higher frequencies in a polycrystalline film. Improved properties arise from unique lattice defect morphology and frequency-dependent relaxation behavior, offering a new route to remarkable electromechanical response in polycrystalline thin films.

Impact of operation voltage and NH3 annealing on the fatigue characteristics of ferroelectric AlScN thin films grown by sputtering

This work investigates the impact of the magnitude of cycling voltage on the fatigue characteristics of 40 nm-thick AlScN ferroelectric thin film. The fatigue rate and the rejuvenation of remanent polarization vary with the cycling voltage. The primary fatigue mechanism is identified to be the interfacial layer formation and domain wall pinning at high and low cycling voltages, respectively. Additionally, annealing the film under the NH3 atmosphere decreases the fatigue rate and improves endurance by eliminating impurities in the film. The amount of trapped charges at the interface also decreases after NH3 annealing, leading to a reduction in leakage current. Furthermore, the ferroelectric performance of the AlScN film is not degraded after the thermal annealing at 900 °C under the NH3 environment, suggesting its robustness against the severe thermal budget. It is concluded that NH3 annealing is a promising method to address the reliability issue of the AlScN film.

Perspective and Prospects for Ordered Functional Materials

Many functional materials are approaching their performance limits due to inherent trade-offs between essential physical properties. Such trade-offs can be overcome by engineering a material that has an ordered arrangement of structural units, including constituent components/phases, grains, and domains. By rationally manipulating the ordering with abundant structural units at multiple length scales, the structural ordering opens up unprecedented opportunities to create transformative functional materials, as amplified properties or disruptive functionalities can be realized. In this perspective article, a brief overview of recent advances in the emerging ordered functional materials across catalytic, thermoelectric, and magnetic materials regarding the fabrication, structure, and property is presented. Then the possibility of applying this structural ordering strategy to highly efficient neuromorphic computing devices and durable battery materials is discussed. Finally, remaining scientific challenges are highlighted, and the prospects for ordered functional materials are made. This perspective aims to draw the attention of the scientific community to the emerging ordered functional materials and trigger intense studies on this topic.

Thermal Stability of the Ferroelectric Properties in 100 nm-Thick Al0.72Sc0.28N

The discovery of ferroelectricity in aluminum scandium nitride (Al_1-xSc_xN) opens technological perspectives for harsh environments and space-related memory applications, considering the high-temperature stability of piezoelectricity in aluminum nitride. The ferroelectric and material properties of 100 nm-thick Al_0.72Sc_0.28N are studied up to 873 K, combining both electrical and in situ X-ray diffraction measurements as well as transmission electron microscopy and energy-dispersive X-ray spectroscopy. The present work demonstrates that Al_0.72Sc_0.28N can achieve high switching polarization and tunable coercive fields in a 375 K temperature range from room temperature up to 673 K. The degradation of the ferroelectric properties in the capacitors is observed above this temperature. Reduction of the effective top electrode area and consequent oxidation of the Al_0.72Sc_0.28N film are mainly responsible for this degradation. A slight variation of the Sc concentration is quantified across grain boundaries, even though its impact on the ferroelectric properties cannot be isolated from those brought by the top electrode deterioration and Al_0.72Sc_0.28N oxidation. The Curie temperature of Al_0.72Sc_0.28N is confirmed to be above 873 K, thus corroborating the promising thermal stability of this ferroelectric material. The present results further support the future adoption of Al_1-xSc_xN in memory technologies for harsh environments like applications in space missions.

Bulk Photovoltaic Current Mechanisms in All-Inorganic Perovskite Multiferroic Materials

After the discovery of bulk photovoltaic effect more than half a century ago, ferro-electrical and magneto-optical experiments have provided insights into various related topics, revealing above bandgap open voltages and non-central symmetrical current mechanisms. However, the nature of the photon-generated carriers responses and their microscopic mechanisms remain unclear. Here, all-inorganic perovskite Bi0.85Gd0.15Fe1-xMnxO3 thin films were prepared by a sol-gel process and the effects of Gd and Mn co-doped bismuth ferrites on their microtopography, grain boundries, multiferroic, and optical properties were studied. We discovered a simple "proof of principle" type new method that by one-step measuring the leakage current, one can demonstrate the value of photo generated current being the sum of ballistic current and shift current, which are combined to form the so-called bulk photovoltaic current, and can be related to the prototype intrinsic properties such as magneto-optical coupling and ferroelectric polarization. This result has significant potential influence on design principles for engineering multiferroic optoelectronic devices and future photovoltaic industry development.

Diverse Defects in Alkali Niobate Thin Films: Understanding at Atomic Scales and Their Implications on Properties

Defects in ferroelectric materials have many implications on the material properties which, in most cases, are detrimental. However, engineering these defects can also create opportunities for property enhancement as well as for tailoring novel functionalities. To purposely manipulate these defects, a thorough knowledge of their spatial atomic arrangement, as well as elastic and electrostatic interactions with the surrounding lattice, is highly crucial. In this work, analytical scanning transmission electron microscopy (STEM) is used to reveal a diverse range of multidimensional crystalline defects (point, line, planar, and secondary phase) in (K,Na)NbO₃ (KNN) ferroelectric thin films. The atomic-scale analyses of the defect-lattice interactions suggest strong elastic and electrostatic couplings which vary among the individual defects and correspondingly affect the electric polarization. In particular, the observed polarization orientations are correlated with lattice relaxations as well as strain gradients and can strongly impact the properties of the ferroelectric films. The knowledge and understanding obtained in this study open a new avenue for the improvement of properties as well as the discovery of defect-based functionalities in alkali niobate thin films.

Water Quenched and Acceptor-Doped Textured Piezoelectric Ceramics for Off-Resonance and On-Resonance Devices

Piezoelectric materials should simultaneously possess the soft properties (high piezoelectric coefficient, d₃₃ ; high voltage coefficient, g₃₃ ; high electromechanical coupling factor, k) and hard properties (high mechanical quality factor, Q_m ; low dielectric loss, tan δ) along with wide operation temperature (e.g., high rhombohedral-tetragonal phase transition temperature T_r-t ) for covering off-resonance (figure of merit (FOM), d₃₃  × g₃₃ ) and on-resonance (FOM, Q_m  × k² ) applications. However, achieving hard and soft piezoelectric properties simultaneously along with high transition temperature is quite challenging since these properties are inversely related to each other. Here, through a synergistic design strategy of combining composition/phase selection, crystallographic texturing, defect engineering, and water quenching technique, <001> textured 2 mol% MnO₂ doped 0.19PIN-0.445PSN-0.365PT ceramics exhibiting giant FOM values of Q_m  × k 31 2 $k_{31}^2$ (227-261) along with high d₃₃  × g₃₃ (28-35 × 10^-12 m² N^-1 ), low tan δ (0.3-0.39%) and high T_r-t of 140-190 °C, which is far beyond the performance of the state-of-the-art piezoelectric materials, are fabricated. Further, a novel water quenching (WQ) room temperature poling technique, which results in enhanced piezoelectricity of textured MnO₂ doped PIN-PSN-PT ceramics, is reported. Based upon the experiments and phase-field modeling, the enhanced piezoelectricity is explained in terms of the quenching-induced rhombohedral phase formation. These findings will have tremendous impact on development of high performance off-resonance and on-resonance piezoelectric devices with high stability.

Direct investigation of the atomic structure and decreased magnetism of antiphase boundaries in garnet

The ferrimagnetic insulator iron garnets, tailored artificially with specific compositions, have been widely utilized in magneto-optical (MO) devices. The adjustment on synthesis always induces structural variation, which is underestimated due to the limited knowledge of the local structures. Here, by analyzing the structure and magnetic properties, two different antiphase boundaries (APBs) with individual interfacial structure are investigated in substituted iron garnet film. We reveal that magnetic signals decrease in the regions close to APBs, which implies degraded MO performance. In particular, the segregation of oxygen deficiencies across the APBs directly leads to reduced magnetic elements, further decreases the magnetic moment of Fe and results in a higher absorption coefficient close to the APBs. Furthermore, the formation of APBs can be eliminated by optimizing the growth rate, thus contributing to the enhanced MO performance. These analyses at the atomic scale provide important guidance for optimizing MO functional materials.

Iron garnets are widely used in magneto-optical devices, but knowledge of the effects of common defects on performance is limited. Here, using high-resolution microscopy and spectroscopy, the authors find that magnetism is weakened near these defects causing reduced performance, but can be avoided by tuning the growth rate.

Exploring Charged Defects in Ferroelectrics by the Switching Spectroscopy Piezoresponse Force Microscopy

Monitoring the charged defect concentration at the nanoscale is of critical importance for both the fundamental science and applications of ferroelectrics. However, up-to-date, high-resolution study methods for the investigation of structural defects, such as transmission electron microscopy, X-ray tomography, etc., are expensive and demand complicated sample preparation. With an example of the lanthanum-doped bismuth ferrite ceramics, a novel method is proposed based on the switching spectroscopy piezoresponse force microscopy (SSPFM) that allows probing the electric potential from buried subsurface charged defects in the ferroelectric materials with a nanometer-scale spatial resolution. When compared with the composition-sensitive methods, such as neutron diffraction, X-ray photoelectron spectroscopy, and local time-of-flight secondary ion mass spectrometry, the SSPFM sensitivity to the variation of the electric potential from the charged defects is shown to be equivalent to less than 0.3 at% of the defect concentration. Additionally, the possibility to locally evaluate dynamics of the polarization screening caused by the charged defects is demonstrated, which is of significant interest for further understanding defect-mediated processes in ferroelectrics.

Domain Wall Patterning and Giant Response Functions in Ferrimagnetic Spinels

The manipulation of mesoscale domain wall phenomena has emerged as a powerful strategy for designing ferroelectric responses in functional devices, but its full potential is not yet realized in the field of magnetism. This work shows a direct connection between magnetic response functions in mechanically strained samples of Mn3 O4 and MnV2 O4 and stripe-like patternings of the bulk magnetization which appear below known magnetostructural transitions. Building off previous magnetic force microscopy data, a small-angle neutron scattering is used to show that these patterns represent distinctive magnetic phenomena which extend throughout the bulk of two separate materials, and further are controllable via applied magnetic field and mechanical stress. These results are unambiguously connected to the anomalously large magnetoelastic and magnetodielectric response functions reported for these materials, by performing susceptibility measurements on the same crystals and directly correlating local and macroscopic data.

Charged Ferroelectric Domain Walls for Deterministic ac Signal Control at the Nanoscale

The direct current (dc) conductivity and emergent functionalities at ferroelectric domain walls are closely linked to the local polarization charges. Depending on the charge state, the walls can exhibit unusual dc conduction ranging from insulating to metallic-like, which is leveraged in domain-wall-based memory, multilevel data storage, and synaptic devices. In contrast to the functional dc behaviors at charged walls, their response to alternating currents (ac) remains to be resolved. Here, we reveal ac characteristics at positively and negatively charged walls in ErMnO₃, distinctly different from the response of the surrounding domains. By combining voltage-dependent spectroscopic measurements on macroscopic and local scales, we demonstrate a pronounced nonlinear response at the electrode-wall junction, which correlates with the domain-wall charge state. The dependence on the ac drive voltage enables reversible switching between uni- and bipolar output signals, providing conceptually new opportunities for the application of charged walls as functional nanoelements in ac circuitry.

Negatively Charged In-Plane and Out-Of-Plane Domain Walls with Oxygen-Vacancy Agglomerations in a Ca-Doped Bismuth-Ferrite Thin Film

The interaction of oxygen vacancies and ferroelectric domain walls is of great scientific interest because it leads to different domain-structure behaviors. Here, we use high-resolution scanning transmission electron microscopy to study the ferroelectric domain structure and oxygen-vacancy ordering in a compressively strained Bi_0.9Ca_0.1FeO_3-δ thin film. It was found that atomic plates, in which agglomerated oxygen vacancies are ordered, appear without any periodicity between the plates in out-of-plane and in-plane orientation. The oxygen non-stoichiometry with δ ≈ 1 in FeO_2-δ planes is identical in both orientations and shows no preference. Within the plates, the oxygen vacancies form 1D channels in a pseudocubic [010] direction with a high number of vacancies that alternate with oxygen columns with few vacancies. These plates of oxygen vacancies always coincide with charged domain walls in a tail-to-tail configuration. Defects such as ordered oxygen vacancies are thereby known to lead to a pinning effect of the ferroelectric domain walls (causing application-critical aspects, such as fatigue mechanisms and countering of retention failure) and to have a critical influence on the domain-wall conductivity. Thus, intentional oxygen vacancy defect engineering could be useful for the design of multiferroic devices with advanced functionality.

Engineering Individual Oxygen Vacancies: Domain-Wall Conductivity and Controllable Topological Solitons

Nanoscale devices that utilize oxygen vacancies in two-dimensional metal-oxide structures garner much attention due to conductive, magnetic, and even superconductive functionalities they exhibit. Ferroelectric domain walls have been a prominent recent example because they serve as a hub for topological defects and hence are attractive for next-generation data technologies. However, owing to the light weight of oxygen atoms and localized effects of their vacancies, the atomic-scale electrical and mechanical influence of individual oxygen vacancies has remained elusive. Here, stable individual oxygen vacancies were engineered in situ at domain walls of seminal titanate perovskite ferroics. The atomic-scale electric-field, charge, dipole-moment, and strain distribution around these vacancies were characterized by combining advanced transmission electron microscopy and first-principle methodologies. The engineered vacancies were used to form quasi-linear quadrupole topological defects. Significant intraband states were found in the unit cell of the engineered vacancies, proposing a meaningful domain-wall conductivity for miniaturized data-storage applications. Reduction of the Ti ion as well as enhanced charging and electric-field concentration were demonstrated near the vacancy. A 3-5% tensile strain was observed at the immediate surrounding unit cells of the vacancies. Engineering individual oxygen vacancies and topological solitons thus offers a platform for predetermining both atomic-scale and global functional properties of device miniaturization in metal oxides.

Phase Transition Effect on Ferroelectric Domain Surface Charge Dynamics in BaTiO3 Single Crystal

The ferroelectric domain surface charge dynamics after a cubic-to-tetragonal phase transition on the BaTiO₃ single crystal (001) surface was directly measured through scanning probe microscopy. The captured surface potential distribution shows significant changes: the domain structures formed rapidly, but the surface potential on polarized c domain was unstable and reversed its sign after lengthy lapse; the high broad potential barrier burst at the corrugated a-c domain wall and continued to dissipate thereafter. The generation of polarization charges and the migration of surface screening charges in the surrounding environment take the main responsibility in the experiment. Furthermore, the a-c domain wall suffers large topological defects and polarity variation, resulting in domain wall broadening and stress changes. Thus, the a-c domain wall has excess energy and polarization change is inclined to assemble on it. The potential barrier decay with time after exposing to the surrounding environment also gave proof of the surface screening charge migration at surface. Thus, both domain and domain wall characteristics should be taken into account in ferroelectric application.

Defect engineering in perovskite oxide thin films

Perovskite oxide thin films are a category of multifunctional materials that have intriguing electrical, magnetic, and photovoltaic properties that can be harnessed combinatorially in future microelectronic devices. However, the inevitable existence of defects in perovskites, regardless of the materials' processing conditions, plays a significant role in their functional properties, which could be either detrimental or beneficial, depending on the exact chemical nature of these defects. As such, defect engineering is an important research area in perovskite thin films that aims at understanding the chemical nature of the defects, from which the physical properties of materials can be more precisely manipulated. Here, we review the common defects in perovskite oxide thin films, which include point defects, dopants, domains and domain walls. The factors that impact the appearance and existence of defects and the corresponding mechanisms are also discussed. While summarizing our previous work, the state-of-the-art in the field from other groups has also been discussed. Most of the defects exist as defect dipoles that affect the oxidation states of relevant ions and induce anomalous behaviors, such as ferroelectricity in otherwise non-ferroelectric thin films, as well as enhanced electrical conductivity in insulators. Furthermore, the couplings between defect dipoles and other degrees of freedom including epitaxial strains and interfaces also provide new strategies to modulate the functional properties of perovskite thin films. Particularly, the coupling between defects and domain wall motion can be regarded as a universal tool to modulate the electric and magnetic properties of thin films of perovskite oxides. It is our hope that this review could promote defect engineering as a general regulation strategy to embellish the functional properties of perovskite oxide thin films.

Giant conductivity of mobile non-oxide domain walls

Atomically sharp domain walls in ferroelectrics are considered as an ideal platform to realize easy-to-reconfigure nanoelectronic building blocks, created, manipulated and erased by external fields. However, conductive domain walls have been exclusively observed in oxides, where domain wall mobility and conductivity is largely influenced by stoichiometry and defects. Here, we report on giant conductivity of domain walls in the non-oxide ferroelectric GaV₄S₈. We observe conductive domain walls forming in zig-zagging structures, that are composed of head-to-head and tail-to-tail domain wall segments alternating on the nanoscale. Remarkably, both types of segments possess high conductivity, unimaginable in oxide ferroelectrics. These effectively 2D domain walls, dominating the 3D conductance, can be mobilized by magnetic fields, triggering abrupt conductance changes as large as eight orders of magnitude. These unique properties demonstrate that non-oxide ferroelectrics can be the source of novel phenomena beyond the realm of oxide electronics.

Potential High-Temperature Piezoelectric Ceramics with Remarkable Performances Enhanced by the Second-Order Jahn-Teller Effect

Herein, the second-order Jahn-Teller effect was applied to the design of the bismuth ferrite-based ceramics. A large distortion of an electron structure arranged along the z axis and an asymmetric distribution of charge density were calculated in 0.80(0.725BiFeO₃-0.275BaTiO₃)-0.20PT (0.20 PT) based on the density functional theory, indicating good ferro/piezoelectric properties. The top experimental polarization of 36.89 μC/cm², optimal d₃₃ value of 258 pC/N measured at room temperature, and ultrahigh d₃₃ value of 303 pC/N measured at 370 °C were obtained at 0.20 PT, thereby further confirming the calculations. Furthermore, a high Curie point of 488 °C, as well as outstanding temperature stability ranging from room temperature to 430 °C of the 0.20 PT ceramic was observed. The domain of the 0.20 PT exhibited greater order and smaller size, resulting in easy switching when applying voltage. The distorted electron structure, plumb grains, ordered and easily switchable domains, and coexistences of tetragonal (T) and rhombohedral (R) phases contributed to the large piezoelectric constant of the 0.2 PT ceramic. BFBT-xPT ceramics are potentially promising for high-temperature piezoelectric field applications.