Conductive Domain Walls Research Articles

Implementation of habituation on single ferroelectric memristor

As a basic form of behavioral plasticity, habituation enables organisms to adjust their behaviors in response to external stimulation and is a fundamental ability of organisms. The emulation of habituation in hardware becomes critical in highly efficient artificial neuromorphic computing. However, few devices can mimic habituation because it includes two opposite responses to the same repeated stimulus. Herein, we fabricate a device of Au/LiTaO3/Pt, which not only shows synaptic plasticity, such as long-term potentiation/depression and paired-pulse facilitation, but also demonstrates habituation behavior. In addition, the habituation characteristics, which is dependent on the intensity and frequency of stimulus, are realized on the device. The analysis of electrical transport behavior indicates that the variation in resistance states correlates with the injection/extraction of free carriers during the resistance switching and suggests that the habituation behavior originates from the evolution of conductive domain walls in LiTaO3 ferroelectric.

Nonvolatile Ferroelectric LiNbO3 Domain Wall Crossbar Memory

High-density domain wall memory based on crossbar architecture is a strong contender among next-generation high performance versatile memories due to its ultra-fast operation speed and excellent size scalability. Herein, we report 4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 4 domain wall crossbar memory arrays fabricated on a LiNbO3 single crystal. Reversible creation and erasure of conducting domain walls between two antiparallel/parallel domains at bipolar write voltages enable the storage of digital “0” and “1” information. At the crosspoint of each word and bit lines, the memory cell can be accurately accessed, programmed and erased. The diode-like rectification of readout currents exhibited by all memory cells can inhibit read and write crosstalk. The electrical testing results in the crossbar memory array demonstrated the good stability of on/off currents and good uniformity of operating voltages, which can be reflected by the fact that the distribution of the coercive voltages is within 2.9 V and the “on”/ “off” current ratio is around 100 at a reading voltage of 3 V. Good data retention and fatigue resistance are also exhibited, making it possible to integrate domain wall random access memories in high density and reliability.

Effect of domain wall conductivity on domain formation energy

Domain wall (DW) conductivity is nowadays considered as a general ferroelectric phenomenon. As applied to ferroelectric domain reversal, it is capable of providing a strong and instantaneous compensation of the bound polarization charge on DWs by free charges coming from electrodes. We claim that account for this compensation can strongly lower the domain formation energy as compared to that known from the Landauer theory. This facilitates the domain reversal process and resolves the problem of an excessively large depolarization energy. We provide a derivation and detailed analysis of the domain formation energy in the presence of the DW conduction; this is relevant to both the capacitor and atomic force microscopy experimental configurations.

A Ferroelectric Domain-Wall Transistor

On the basis of novel properties of ferroelectric conducting domain walls, the domain wall nanoelectronics emerges and provides a brand-new dimension for the development of high-density, high-speed and energy-efficient nanodevices. For in-memory computing, three-terminal devices with both logic and memory functions such as transistors purely based on ferroelectric domain walls are urgently required. Here, a prototype ferroelectric domain-wall transistor with a well-designed coplanar electrode geometry is demonstrated on epitaxial BiFeO3 thin films. For the logic function, the current switching between on/off states of the transistor depends on the creation or elimination of conducting domain walls between drain and source electrodes. For the data storage, the transistor can maintain nonvolatile on/off states after the write/erase operations, providing an innovative approach for the development of the domain wall nanoelectronics.

Tuning Local Conductance to Enable Demonstrator Ferroelectric Domain Wall Diodes and Logic Gates

AbstractFundamentally, lithium niobate is a good electrical insulator. However, this can change dramatically when 180° domain walls are present, as they are often found to be strongly conducting. Conductivities depend on the inclination angles of walls with respect to the polarization axis and so, if these angles can be altered, then electrical conduction can be tuned, or toggled on and off. In ≈500 nm thick z‐cut ion‐sliced thin films, localized wall angle variations can be controlled by both the sense and magnitude of applied electrical bias. It is shown that this results in diode‐like behaviour, allowing half‐wave rectification at modest frequencies. Importantly, it is experimentally demonstrated that these domain wall diodes can be used to construct “AND” and inclusive “OR” logic gates, where “0” and “1” output states are clearly distinguishable. Extrapolation to more complex arrangements shows that output states can still be distinguished in two‐level cascade logic. Insights show that simple logic circuits can be realized by localized manipulation of domain wall conductivity. Our research complements that by Jie Sun et al. (Adv. Funct. Mater. 2207418 (2022)), where NOT, NOR, and NAND gates are realized by moving conducting domain walls to make or break electrical contacts.

Estimation of interfacial-layer thickness from voltage-dependent domain switching times in LiNbO3 single-crystal domain wall memory

Conducting domain walls created through the local domain 180° reversal within LiNbO3 mesa-like cells fabricated at the surface of a single-crystal LiNbO3 thin film have enabled ferroelectric domain wall memories, transistors, and sensors, where volatile switching in the domains within the interfacial layers near two side electrodes can rectify diode-like wall conduction under an applied voltage higher than an onset voltage. The diodes at interfaces not only can perform NOT, NAND, and NOR gate logic functions for in-memory computing, but can also work as embedded selectors for the fabrication of crossbar arrays of the memories in adjustable onset voltages that are proportional to the thicknesses of interfacial layers. Here, we develop a method to estimate the interfacial-layer thickness through the measurements of voltage-dependent domain switching times across two interfacial layers and an entire memory cell, respectively. Both dependences can be fitted according to the Merz law using an identical activation field from which we derived the equivalent electrical thickness of the interfacial layer. This non-destructive estimation paves the way to tailoring onset voltages of the diodes through the regulation of interfacial-layer thicknesses within various domain wall nanodevices.

Giant switchable non thermally-activated conduction in 180° domain walls in tetragonal Pb(Zr,Ti)O3

Conductive domain walls in ferroelectrics offer a promising concept of nanoelectronic circuits with 2D domain-wall channels playing roles of memristors or synoptic interconnections. However, domain wall conduction remains challenging to control and pA-range currents typically measured on individual walls are too low for single-channel devices. Charged domain walls show higher conductivity, but are generally unstable and difficult to create. Here, we show highly conductive and stable channels on ubiquitous 180° domain walls in the archetypical ferroelectric, tetragonal Pb(Zr,Ti)O3. These electrically erasable/rewritable channels show currents of tens of nanoamperes (200 to 400 nA/μm) at voltages ≤2 V and metallic-like non thermally-activated transport properties down to 4 K, as confirmed by nanoscopic mapping. The domain structure analysis and phase-field simulations reveal complex switching dynamics, in which the extraordinary conductivity in strained Pb(Zr,Ti)O3 films is explained by an interplay between ferroelastic a- and c-domains. This work demonstrates the potential of accessible and stable arrangements of nominally uncharged and electrically switchable domain walls for nanoelectronics.

Tuning domain wall conductivity in bulk lithium niobate by uniaxial stress

Although ferroelectrics are electrical insulators, their domain walls (DWs) can be charged and can possess an exceptional large conductivity of an order of magnitude larger than in the bulk crystal. To shed light on the correlation between bound charges and conductivity of ferroelectric DWs, the authors in this work investigate the effect of piezoelectric charging via external uniaxial stress on the directionality of DW conductivity in single crystals of $z$-cut 5% MgO doped lithium niobate by combing a piezodriven stress cell with a scanning probe microscope.

Nanometer-sized etching of lithium niobate domain wall devices

Ferroelectric LiNbO3 single crystals have piezoelectric, pyroelectric and electro-optic properties and are widely used in nonvolatile memories, sensors, and electro-optic modulators. The material is hard and relatively inert for dying etching using thick Cr/Ni mask layers which are incompatible with the CMOS manufacturing technologies. Moreover, the etching angles are generally below 80° that could result in poor polarization retention and a large imprint effect. Here we developed an etching technique on the basis of the solid-state reaction and wet solution corrosion together that used CMOS-compatible SiO2 masks to fabricate large arrays of LiNbO3 nanodevices in high precision. The LiNbO3 nanopatterns at the regions beyond the SiO2 masks can react with the deposited Al metal below 500 °C in formation of amorphous layers in targeted thicknesses that vary with the annealing temperature and time at a reduced atmosphere. The amorphous layers can be dissolved in mixed solutions of H2O, NH4OH, and H2O2 at 85 °C, and the etching angles of memory cells in lateral sizes of 20–220 nm can approach 90° after the passivation of the LiNbO3 surface layer. The transmission electron microscope study shows the oxygen deficiency at the surface. After the connection of the cell to two side electrodes, domain switching occurs under an applied horizontal electrode field along the polar Z-axis accompanying the formation of conducting domain walls against the peripheral unswitched domain at the substrate. The read current can highly reach the level of 2–20 μA in high reliability. This etching technique promotes the large-scale fabrication of LiNbO3 nanodevices on the Si platform.

Polarization Topology at the Nominally Charged Domain Walls in Uniaxial Ferroelectrics.

Ferroelectric domain walls provide a fertile environment for novel materials physics. If a polarization discontinuity arises, it can drive a redistribution of electronic carriers and changes in band structure, which often result in emergent 2D conductivity. If such a discontinuity is not tolerated, then its amelioration usually involves the formation of complex topological patterns, such as flux-closure domains, dipolar vortices, skyrmions, merons, or Hopfions. The degrees of freedom required for the development of such patterns, in which dipolar rotation is a hallmark, are readily found in multiaxial ferroelectrics. In uniaxial ferroelectrics, where only two opposite polar orientations are possible, it has been assumed that discontinuities are unavoidable when antiparallel components of polarization meet. This perception has been borne out by the appearance of charged conducting domain walls in systems such as hexagonal manganites and lithium niobate. Here, experimental and theoretical investigations on lead germanate (Pb5 Ge3 O11 ) reveal that polar discontinuities can be obviated at head-to-head and tail-to-tail domain walls by mutual domain bifurcation along two different axes, creating a characteristic saddle-point domain wall morphology and associated novel dipolar topology, removing the need for screening charge accumulation and associated conductivity enhancement.

In‐Memory Computing of Multilevel Ferroelectric Domain Wall Diodes at LiNbO3 Interfaces

AbstractDirect data processing in nonvolatile memories can enable area‐ and energy‐efficient computation, unlike independent performance between separate processing and memory units; repetitive data transfer between these units represents a fundamental performance limitation in modern computers. Spatially mobile conducting domain walls in ferroelectrics can be redirected between drain, gate, and source electrodes to function as nonvolatile transistors with superior energy efficiency, ultrafast operating and communication speeds, and high logic/storage densities. Here, in‐memory computing is demonstrated using multilevel domain wall diodes at LiNbO3 interfaces. Ultrathin domains within interfacial layers between each mesa‐like memory cell and the contact electrodes can rectify diode‐like domain wall currents with on/off current ratios exceeding 107 at low operating voltages, surpassing the performance of traditional p‐n junctions using built‐in potentials across depletion layers. NOT, NAND, and NOR gate logic functions are demonstrated, providing insights into high‐density integration of multilevel storage and computational units in all‐ferroelectric domain wall devices.

Reduction of the lattice strain with increasing field amplitude in polycrystalline BiFeO3

As dynamic interfaces in piezoelectric materials, domain walls can greatly impact the functionality under applied external fields. Recently, conductive domain walls have been found to result in Maxwell-Wagner (M-W)-like dispersion of the lattice strain and ferroelastic domain wall motion as a function of field frequency in polycrystalline BiFeO3. Here, we reveal an anomalous field-dependent behavior of the ferroelastic domain wall motion and lattice strain during field cycling. This response is dominated by a common nonlinear increase of ferroelastic domain wall motion and an unusual nonlinear reduction of the lattice strain with increasing field amplitude. Using analytical modeling, this complex field-dependent behavior of polycrystalline BiFeO3 is interpreted by the generation of opposing fields accompanying domain wall motion. Two contributors to the opposing fields are charge accumulation and movement along conductive domain walls and elastic coupling between adjacent grains.

Large magneto-thermoelectric effect on the verge of metal–insulator and topological transitionsin pyrochlore iridates

We investigate the Seebeck and Nernst effects of pyrochlore iridium oxides (Nd1−xPrx)2Ir2O7 across the thermally induced and magnetic field-induced metal–insulator and topological transitions. Nd2Ir2O7 exhibits the salient temperature dependence of the Seebeck coefficient accompanied by the enhancement of the Nernst effect in the vicinity of the thermal magnetic transitions. Moreover, the Seebeck coefficient shows a remarkable magnetic field hysteresis with the differential magnitude reaching as large as 110 μV/K, as the conductive magnetic domain walls are generated/annihilated by the external field. For x = 0.5, the Nernst signal increases rapidly across the field-induced metal–insulator transitions, exceeding the values reported in existing ferromagnetic oxides. These findings indicate that the thermoelectric effects increase significantly near the topological electronic phase transitions in strongly correlated systems, providing a new guideline for thermoelectric material design.

Ferroelectric Domain Reversal: The Role of Domain Wall Conduction

Ferroelectric domain reversal is a vast research area relevant to the fundamental science and applications as well as to macro- and nanoscales. An instantaneous compensation of typically huge depolarizing electric fields is a key element here. On the other hand, the domain wall (DW) conduction is now regarded as a general ferroelectric phenomenon. Being often separated by big surface energy barriers in macroscopic experiments, the DW conduction becomes easily available under the conditions of atomic-force microscopy. We claim that the DW conduction has to be regarded as a crucial and general ingredient of the domain reversal processes. Its importance is in providing an automatic compensation of the depolarizing electric fields. The presence of DW conduction essentially modifies the basics of domain reversal processes, including the relations for domain nucleus energies, the domain grows velocities, and the domain shapes. Concerning atomic-force microscopy applications, domain reversal theories have to include injection models from conductive tip electrodes. Below we provide specific primary results relevant to the basics of DW conduction mediated ferroelectric domain reversal.

Conductive domain walls in x-cut lithium niobate crystals

Conductive ferroelectric domain walls have been the focus of intensive studies in nanoelectronics due to their promising electronic properties. Here, both head-to-head and tail-to-tail domain walls with a large inclination angle (≈90°) were fabricated on the surface of x-cut nominally pure congruent lithium niobate crystals. Our results show that both types of domain walls are conductive, while the conductivity of the head-to-head domain walls is larger than that of the tail-to-tail domain walls by about two orders of magnitude at the same inclination angle of domain walls.

Nonvolatile ferroelectric domain wall memory integrated on silicon

Ferroelectric domain wall memories have been proposed as a promising candidate for nonvolatile memories, given their intriguing advantages including low energy consumption and high-density integration. Perovskite oxides possess superior ferroelectric prosperities but perovskite-based domain wall memory integrated on silicon has rarely been reported due to the technical challenges in the sample preparation. Here, we demonstrate a domain wall memory prototype utilizing freestanding BaTiO3 membranes transferred onto silicon. While as-grown BaTiO3 films on (001) SrTiO3 substrate are purely c-axis polarized, we find they exhibit distinct in-plane multidomain structures after released from the substrate and integrated onto silicon due to the collective effects from depolarizing field and strain relaxation. Based on the strong in-plane ferroelectricity, conductive domain walls with reading currents up to nanoampere are observed and can be both created and erased artificially, highlighting the great potential of the integration of perovskite oxides with silicon for ferroelectric domain wall memories.

Strain Driven Conducting Domain Walls in a Mott Insulator

AbstractRewritable nanoelectronics offer new perspectives and potential to both fundamental research and technological applications. Such interest has driven the research focus into conducting domain walls: pseudo‐2D conducting channels that can be created, positioned, and deleted in situ. However, the study of conductive domain walls is largely limited to wide‐gap ferroelectrics, where the conductivity typically arises from changes in charge carrier density, due to screening charge accumulation at polar discontinuities. This work shows that, in narrow‐gap correlated insulators with strong charge‐lattice coupling, local strain gradients can drive enhanced conductivity at the domain walls—removing polar‐discontinuities as a criteria for conductivity. By combining different scanning probe microscopy techniques, it is demonstrated that the domain wall conductivity in GaV4S8 does not follow the established screening charge model but likely arises from the large surface reconstruction across the Jahn–Teller transition and the associated strain gradients across the domain walls. This mechanism could turn any structural, or even magnetic, domain wall conducting, if the electronic structure of the host is susceptible to local strain gradients—drastically expanding the range of materials and phenomena that may be applicable to domain wall‐based nanoelectronics.

Nanoscale Conductive Sheets in Ferroelectric BaTiO3: Large Hall Electron Mobilities at Head-to-Head Domain Walls

Strongly charged head-to-head (H2H) domain walls (DWs) that are purposely engineered along the [110] crystallographic orientation into ferroelectric BaTiO$_3$ single crystals have been proposed as novel 2-dimensional electron gases (2DEGs) due to their significant domain wall conductivity (DWC). Here, we quantify these 2DEG properties through dedicated Hall-transport measurements in van-der-Pauw 4-point geometry at room temperature, finding the electron mobility to reach around 400~cm$^2$(Vs)$^{-1}$, while the 2-dimensional charge density amounts to ~7$\times$10$^3$cm$^{-2}$. We underline the necessity to take account of thermal and geometrical-misalignment offset voltages by evaluating the Hall resistance under magnetic-field sweeps, since otherwise dramatic errors of several hundred percent in the derived mobility and charge density values can occur. Apart from the specific characterization of the conducting BaTiO$_3$ DW, we propose the method as an easy and fast way to quantitatively characterize ferroic conducting DWs, complementary to previously proposed scanning-probe-based Hall-potential analyses.

Ferroelectric domain-wall logic units

The electronic conductivities of ferroelectric domain walls have been extensively explored over the past decade for potential nanoelectronic applications. However, the realization of logic devices based on ferroelectric domain walls requires reliable and flexible control of the domain-wall configuration and conduction path. Here, we demonstrate electric-field-controlled stable and repeatable on-and-off switching of conductive domain walls within topologically confined vertex domains naturally formed in self-assembled ferroelectric nano-islands. Using a combination of piezoresponse force microscopy, conductive atomic force microscopy, and phase-field simulations, we show that on-off switching is accomplished through reversible transformations between charged and neutral domain walls via electric-field-controlled domain-wall reconfiguration. By analogy to logic processing, we propose programmable logic gates (such as NOT, OR, AND and their derivatives) and logic circuits (such as fan-out) based on reconfigurable conductive domain walls. Our work might provide a potentially viable platform for programmable all-electric logic based on a ferroelectric domain-wall network with low energy consumption.

Mechanical strain modulation of domain wall currents across LiNbO3 nanosensors

Recently, studies on low-dimensional conducting domain walls (DWs) in insulating ferroelectrics have opened up new research areas that allow information to be mechanically written and electrically read on the nanoscale. Large strains in thin films can change the polarization gradient across the DW region and thus increasing the DW current significantly. This phenomenon can enable the development of high sensitivity mechanical vibration sensors. In this study, the effects of variable uniaxial strain on the structures of 180° conducting DWs in LiNbO3 (LNO) single-crystal thin films bonded onto Si/SiO2 substrates were investigated. After the creation of antiparallel domains within each LNO nanosensor integrated at the film surface, strain modulation of DW currents was observed through simple mechanical bending of the film. The DW current increases under application of tensile strain along the axis of polarization, but decreases under application of in-plane compression by a factor of approximately 25. Phase field simulations showed the dramatic change in polarization gradients around the DW regions under the increase in tensile strain, which reduced the band gap. Repetitive band-gap narrowing/broadening with change in local electric field intensity under vibrating mechanical forces can periodically modulate both the carrier density and the DW conduction in the sensors. This finding not only provides the new fundamental physics to enrich the ferroelectric theory, but also paves the way to the near-future development of bending actuators, piezolighters, and micro-/nano-manipulators, etc.