Article

Charged Domain Wall Modulation of Resistive Switching with Large ON/OFF Ratios in High Density BiFeO3 Nano-Islands

Authors:
  • Institute of Metal Research, Chinese Academy of Sciences, shenyang
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Abstract

Ferroelectrics exhibit polarization tunable resistance switching behaviors, which are promising for next-generation non-volatile memory devices. For technological applications, thinner nanoscale arrays are expected, which feature with higher density and larger ON/OFF (RON/OFF) ratios in the metal/ferroelectrics/semiconductor heterojunction. Here, we acquire high density BiFeO3 (BFO) nano-islands around 10 nm in thickness displaying a high RON/OFF ratio of 10³, comparable to the tunnel junctions. Moreover, both the macroscopic and microscopic resistive switching behaviors of the present BFO films reveal an unexpected filamentary-type resistive switching which is modified by the charged domain walls in nano-islands dominated by the center-type domains. Particularly, the charged domain walls spontaneously formed within the BFO nano-islands are proposed as the conductive paths based on the redistribution of carriers under the applied voltages. Potential applications for memories with large RON/OFF ratios of such kind of configurable charged domain walls are demonstrated.

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... The fabrication parameters for BFO memristive devices using PLD are highly variable and are contingent upon the specific research or fabrication process. Typical parameters for fabricating BFO memristive devices by PLD include laser energy density of 1-2 J/cm 2 , laser repetition rate of 2-10 Hz, substrate temperature of 450-800°C, oxygen pressure of 10-120 mTorr, and target-substrate distance of 3-6 cm [5,[17][18][19][20][21]. For example, higher laser repetition rates during deposition can affect the RS behavior of BFO memristive devices by changing the thickness and quality of the BFO film, resulting in thicker films and a higher on/off ratio [21]. ...
... Typical parameters for fabricating BFO memristive devices by PLD include laser energy density of 1-2 J/cm 2 , laser repetition rate of 2-10 Hz, substrate temperature of 450-800°C, oxygen pressure of 10-120 mTorr, and target-substrate distance of 3-6 cm [5,[17][18][19][20][21]. For example, higher laser repetition rates during deposition can affect the RS behavior of BFO memristive devices by changing the thickness and quality of the BFO film, resulting in thicker films and a higher on/off ratio [21]. Lamichhane et al. increased the on/off ratio and reduced the leakage current of a BFO memristive device by increasing the laser energy density [22]. ...
... These findings highlight the potential of multiferroic heterojunctions based on BFO for achieving fast and tunable memristive devices with interfacial magnetoelectric coupling. Similar to the preparation method of the device in the above work, Han et al. [21] fabricated RS devices based on high-density self-assembled BFO nanoislands grown on an NSTO substrate, as shown in Figure 5c. The formation and rupture of conductive filaments at charged domain walls caused the RS properties due to changes in the charge carrier concentration under an applied voltage. ...
Article
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This review provides a comprehensive examination of the state-of-the-art research on resistive switching (RS) in BiFeO3 (BFO)-based memristive devices. By exploring possible fabrication techniques for preparing the functional BFO layers in memristive devices, the constructed lattice systems and corresponding crystal types responsible for RS behaviors in BFO-based memristive devices are analyzed. The physical mechanisms underlying RS in BFO-based memristive devices, i.e., ferroelectricity and valence change memory, are thoroughly reviewed, and the impact of various effects such as the doping effect, especially in the BFO layer, is evaluated. Finally, this review provides the applications of BFO devices and discusses the valid criteria for evaluating the energy consumption in RS and potential optimization techniques for memristive devices.
... This inverse design is distinct from previous investigations on CFO as isolated nanostructures embedded in a surrounding high molar ratio BFO matrix, [1,2,4,5,[7][8][9][15][16][17][18][19][20][21]36] and also different from previous works on (111)-oriented, lower molar ratio BFO pillars in a CFO matrix. [37] Given recent demonstrations of topological polarization states and other unique functionalities in geometrically-confined nanostructures, [38][39][40][41][42] direct evidence for strain-induced interfacial effects on ferroelectricity is important both fundamentally, and for future applications of vertically aligned functional nanocomposites, and other strain engineered devices. ...
... [73] However, in this work, the effect of the large out-of-plane tensile strain in the edge region of BFO nanopillars-which raises the potential barrier of polarization switching and hence the coercive voltage-outweighs the potential-barrier-lowering effect of the interfacial defects. [71] Additionally, although previous studies by Han et al., of geometrically-confined BFO nanoislands have demonstrated stable polar topological states with exotic center-type domains and resistive switching behavior, [41,42] no clear center-type domains, resistive switching, or charged domain walls were observed in our BFO nanopillars according to the PFM tomograms ( Figure 2) and C-AFM results ( Figure S1, Supporting Information). This could be due to the completely different BFO-CFO composite sample structure and epitaxial growth conditions from the previous self-assembled BFO nanoislands grown directly on BFO thin film matrices. ...
... This could be due to the completely different BFO-CFO composite sample structure and epitaxial growth conditions from the previous self-assembled BFO nanoislands grown directly on BFO thin film matrices. [39,41,42] Also, the semiconducting characteristics of the CFO matrix ( Figure S1, Supporting Information), and/or the interfacial strain, could destabilize the polar topological states for our VANs. More generally, given the capability to directly map polarization orientations in 3D as demonstrated in this work, T-AFM can be a powerful tool to visualize polar topological states and their corresponding exotic properties. ...
Article
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Self-assembled BiFeO3-CoFe2O4 (BFO-CFO) vertically aligned nanocomposites are promising for logic, memory, and multiferroic applications, primarily due to the tunability enabled by strain engineering at the prodigious epitaxial vertical interfaces. However, local investigations directly revealing functional properties in the vicinity of such critical interfaces are often hampered by the size, geometry, microstructure, and concomitant experimental artifacts. Ferroelectric switching in the presence of lateral distributions of vertical strain thus remains relatively unexplored, with broader implications for all strain-engineered functional devices. By implementing tomographic atomic force microscopy, 3D domain orientation mapping, and spatially-resolved ferroelectric switching movies, local tensile strain significantly impacts the ferroelectric switching, principally by retarding domain nucleation in the BFO nearest to the vertically epitaxial tensile-strained interfaces. The relaxed centers of the BFO pillars become preferred domain nucleation and growth sites for low biases, with up to an order of magnitude change in the edge:center switching ratio for high biases. The new, multi-dimensional imaging approach—and its corresponding insights especially for directly strained interface effects on local properties—thereby advances the fundamental understanding of polarization switching and provides design principles for optimizing functional response in confined nanoferroic systems.
... This mechanism is similar to other ferroelectrics and is limited by the thickness of the films of a few nm [21,22], where tunneling can be realized [23,24]. For thicker films, the RS is usually explained by the electromigration of the oxygen vacancies creating conductive "filaments" consisting of trap centers for electrons [25][26][27][28] or modulating the height of the potential barrier at the electrode-sample interface [2,10]. The mobility of the oxygen vacancies in bismuth ferrite is a widely accepted explanation for the RS because of belief of high oxygen vacancies mobility. ...
... A conductivity along the ferroelectric domain walls also provides a track for electronic transport [30][31][32]. As an example, creating/erasing the domain walls was postulated to be responsible for RS in BFO nano-dots [27]. Though a pure electronic trapping/detrapping explanation of RS has been proposed for the explanation of the RS in BFO [6] similar to other materials [33], this hypothesis is not popular. ...
Article
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Ferroelectric materials attract much attention for applications in resistive memory devices due to the large current difference between insulating and conductive states and the ability of carefully controlling electronic transport via the polarization set-up. Bismuth ferrite films are of special interest due to the combination of high spontaneous polarization and antiferromagnetism, implying the possibility to provide multiple physical mechanisms for data storage and operations. Macroscopic conductivity measurements are often hampered to unambiguously characterize the electric transport, because of the strong influence of the diverse material microstructure. Here, we studied the electronic transport and resistive switching phenomena in polycrystalline bismuth ferrite using advanced conductive atomic force microscopy (CAFM) at different temperatures and electric fields. The new approach to the CAFM spectroscopy and corresponding data analysis are proposed, which allow deep insight into the material band structure at high lateral resolution. Contrary to many studies via macroscopic methods, postulating electromigration of the oxygen vacancies, we demonstrate resistive switching in bismuth ferrite to be caused by the pure electronic processes of trapping/releasing electrons and injection of the electrons by the scanning probe microscopy tip. The electronic transport was shown to be comprehensively described by the combination of the space charge limited current model, while a Schottky barrier at the interface is less important due to the presence of the built-in subsurface charge.
... 6 Duan and co-workers demonstrated RS polarity reversal due to ferroelectrically induced phase transition in BFO/Ca 0.96 Ce 0.04 MnO 3 bilayers. 7 So far, RS behavior is basically concentrated in oxide thin films with the thickness of decade nanometers to several hundred nanometers, [8][9][10][11][12][13] while ultrathin BFO thin films below 10 nm has rarely been reported yet. Investigating the resistive switching RS behavior of ultrathin ferroelectric films is of significant importance, as it might not only allow for a deeper understanding of the RS mechanisms but also provide new insights for the development of high-performance RRAMs. ...
Article
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Electrically induced resistive switching (RS) effects have been proposed as the basis for future non-volatile memories. In this work, 9 nm-thick BiFeO3 (BFO) epitaxial thin films were deposited on (001)-oriented SrTiO3 substrates by pulsed laser deposition and their resistive switching (RS) behaviors were investigated. A large resistive switching with ON/OFF ratio of ∼10⁶ is observed, surpassing the performance of most resistive random access memories ever reported. The conducting filament is proposed to dominate the RS behavior in the positive voltage region, while the modulation of ferroelectric polarization is suggested to play a significant role in the negative voltage region. Our study significantly deepens the understanding of the physical origin of RS and could provide a reference for designing high-performance memories and memristors based on ultrathin ferroelectric films.
... This indicates that slower mechanisms such as oxygen vacancies can be safely neglected in the presence of other resistive switching (RS) mechanisms. In bismuth ferrite, polarisation reversal can aid the formation and rupture of conductive filaments at the charged domain walls [65,66]. The changes in the charge carrier density with the applied voltage help to sustain the resistive switching properties. ...
Article
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Bismuth ferrite (BFO) is a prime candidate for room-temperature magnetoelectric coupling and multiferroic applications. The rhombohedral R3c phase of BFO is the source of many properties, but the phase purity and oxygen vacancies are still the biggest obstacles to its real-world application. Considering these facts, the present work investigates the effects of oxygen vacancies on the functional properties through manipulation of drying temperatures of spin-cast films, especially at temperatures around 280 °C, where both the secondary phase and oxygen vacancies are prevalent. One of the biggest sources of oxygen vacancy is bismuth volatilisation, and our work deals with the situation head-on, uncovering the effect of bismuth volatilisation on functional properties. The structural properties were studied using x-ray diffraction (XRD), and deeper insights into the surface topography of the samples were obtained using AFM imaging. The electrical and dielectric characteristics help distinguish and analyse the samples in terms of the presence of resistive switching. PUND studies were performed to determine the ferroelectric properties of the samples. A fifty percent reduction in the oxygen vacancies in the presence of secondary phases was observed when compared with the phase-pure sample, as shown by the XPS analysis. Deeper insights were provided into the valence band spectra by first-principles studies. This work shows that phase purity may not be the singular condition for enhancing functional properties, and fine-tuning the presence of secondary phases and oxygen vacancies may be the way forward. The ferroelectric polarisation in one of the samples exhibits a notably higher value when using chemical solution deposition methods, making it a promising candidate for memory devices.
... In comparison to the three-dimensional domains in ferroelectric thin films, the two-dimensional domain walls or boundaries, which separate domains of distinct crystallographic and electronic features, typically have the reduced dimensionality of only a few atomic layers. These domain walls possess broken local symmetry and perturbed chemical environments, rendering them with promising aspects for nanoelectronics and nano-optoelectronics [1][2][3][4][5][6][7][8] . A wealth of physico-chemical properties have been discovered in ferroelectric domain walls, including domain-wall conductivity [9][10][11][12][13][14][15] , unconventional magnetism 16 , optical response 17,18 , giant electromechanical 19 and thermotropic behaviors 20 . ...
Article
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The deterministic creation and modification of domain walls in ferroelectric films have attracted broad interest due to their unprecedented potential as the active element in non-volatile memory, logic computation and energy-harvesting technologies. However, the correlation between charged and antiphase states, and their hybridization into a single domain wall still remain elusive. Here we demonstrate the facile fabrication of antiphase boundaries in BiFeO 3 thin films using a He-ion implantation process. Cross-sectional electron microscopy, spectroscopy and piezoresponse force measurement reveal the creation of a continuous in-plane charged antiphase boundaries around the implanted depth and a variety of atomic bonding configurations at the antiphase interface, showing the atomically sharp 180° polarization reversal across the boundary. Therefore, this work not only inspires a domain-wall fabrication strategy using He-ion implantation, which is compatible with the wafer-scale patterning, but also provides atomic-scale structural insights for its future utilization in domain-wall nanoelectronics.
... In comparison to the three-dimensional domains in ferroelectric thin films, the two-dimensional domain walls or boundaries, which separate domains of distinct crystallographic and electronic features, typically have the reduced dimensionality of only a few atomic layers. These domain walls possess broken local symmetry and perturbed chemical environments, rendering them with promising aspects for nanoelectronics and nano-optoelectronics [1][2][3][4][5][6][7][8] . A wealth of physico-chemical properties have been discovered in ferroelectric domain walls, including domain-wall conductivity [9][10][11][12][13][14][15] , unconventional magnetism 16 , optical response 17,18 , giant electromechanical 19 and thermotropic behaviors 20 . ...
Preprint
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The deterministic creation and modification of domain walls in ferroelectric films have attracted broad interest due to their unprecedented potential as the active element in non-volatile memory, logic computation and energy-harvesting technologies. However, the correlation between charged and antiphase states, and their hybridization into a single domain wall still remain elusive. Here we demonstrate the facile fabrication of antiphase boundaries in BiFeO3 thin films using a He-ion implantation process. Cross-sectional electron microscopy, spectroscopy and piezoresponse force measurement reveal the creation of a continuous in-plane charged antiphase boundaries around the implanted depth and a variety of atomic bonding configurations at the antiphase interface, showing the atomically sharp 180° polarization reversal across the boundary. Therefore, this work not only inspires a domain-wall fabrication strategy using He-ion implantation, which is compatible with the wafer-scale patterning, but also provides atomic-scale structural insights for its future utilization in domain-wall nanoelectronics.
... Furthermore, authors attributed analogue switching to forming an interfacial Al 2 O 3 layer between the Al contact and BiFeO 3 top surface, which acts as the reservoir for oxygen vacancies. 27,28 This article presents the mimicking behavior of a MIM (metal−insulator−metal) device structure, that is, Cu/ BiFeO 3 /FTO, where BiFeO 3 is an active layer synthesized using a sol−gel process. The devices are evaluated for multiple cycles, exhibiting very high retention and endurance. ...
... STEM reveals the center-divergent domain structure, which may be caused by surface charge accumulation and the shape of the nanoislands. A filamentary-type resistive switching with large ON/OFF ratio that is modified by the CDWs in nanoislands is observed [375]. Recently, radial quadrant domain texture with vortices and anti-vortices was also observed in square nanoplate of BFO [104]. ...
Article
Bismuth ferrite (BiFeO3, BFO) as one of the few single-phase room-temperature multiferroics, has aroused ever-increasing enthusiasm in research communities during the past two decades. The robust ferroelectricity, promising magnetoelectric coupling and remarkable optical behaviors of BFO all enrich its physical phenomena and functional properties. The microscopic ferroic domain structures in BFO determine both the static configurations and dynamic behaviors of order parameters, which is the fundamental basis for understanding and controlling of macroscopic properties. Here, we provide a comprehensive and up-to-date review of the intensive research advances of BFO, in the framework of domain engineering. We begin with an introduction to the rich domain structures of BFO and typical domain engineering strategies, such as chemical modification, electrostatic boundary control, strain engineering, substrate engineering, etc. Then, electrical properties (ferroelectricity, piezoelectricity and conduction), magnetoelectric couplings and optical effects (photovoltaic, photocatalytic, mechanical-optical, etc.) modulated by domain engineering in BFO are discussed in sequence. Remarkable electrical, magnetic and optical phenomena at the domain walls of BFO, which have been discovered and intensively explored recently, are also summarized. Finally, remaining challenges and perspectives are proposed for further domain engineering in BFO-based functional materials, devices and applications.
... Furthermore, a biological synapse is the cardinal component that performs learning and memory by controlling the synaptic plasticity [7][8][9][10][11][12][13]. Therefore, the fabrication of an artificial synapse with the ability to emulate biological synaptic characteristics is important to realize a neuromorphic computing system. Recently, various resistive switching random access memory (RRAM) devices have been considered as potential artificial synapses because of their simple structure, low operating power, and good interchangeability with the CMOS process [14][15][16][17]. ...
Article
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An amorphous Pr0.7Ca0.3MnO3 (PCMO) film was grown on a TiN/SiO2/Si (TiN–Si) substrate at 300 °C and at an oxygen pressure (OP) of 100 mTorr. This PCMO memristor showed typical bipolar switching characteristics, which were attributed to the generation and disruption of oxygen vacancy (OV) filaments. Fabrication of the PCMO memristor at a high OP resulted in nonlinear conduction modulation with the application of equivalent pulses. However, the memristor fabricated at a low OP of 100 mTorr exhibited linear conduction modulation. The linearity of this memristor improved because the growth and disruption of the OV filaments were mostly determined by the redox reaction of OV owing to the presence of numerous OVs in this PCMO film. Furthermore, simulation using a convolutional neural network revealed that this PCMO memristor has enhanced classification performance owing to its linear conduction modulation. This memristor also exhibited several biological synaptic characteristics, indicating that an amorphous PCMO thin film fabricated at a low OP would be a suitable candidate for artificial synapses.
... Finally, we should mention that M. J. Han fabricated a RS device based on high-density self-assembled BiFeO 3 nano-islands grown on a Nb doped SrTiO 3 (0.7 wt%) (NSTO) substrate. 195 The RS property was induced by the formation and rupture of the conductive filament at the charged domain wall due to the change of charge carrier concentrations under the applied voltage. Fig. 19f shows a schematic diagram of the charge distribution in a charged domain wall under different applied voltages. ...
Article
The unique electron spin, transfer, polarization and magnetoelectric coupling characteristics of ABO3 multiferroic perovskite materials make them promising candidates for application in multifunctional nanoelectronic devices. Reversible ferroelectric polarization, controllable defect concentration and domain wall movement originated from the ABO3 multiferroic perovskite materials promotes its memristive effect, which further highlights data storage, information processing and neuromorphic computing in diverse artificial intelligence applications. In particular, ion doping, electrode selection, and interface modulation have been demonstrated in ABO3-based memristive devices for ultrahigh data storage, ultrafast information processing, and efficient neuromorphic computing. These approaches presented today including controlling the dopant in the active layer, altering the oxygen vacancy distribution, modulating the diffusion depth of ions, and constructing the interface-dependent band structure were believed to be efficient methods for obtaining unique resistive switching (RS) behavior for various applications. In this review, internal physical dynamics, preparation technologies, and modulation methods are systemically examined as well as the progress, challenges, and possible solutions are proposed for next generation emerging ABO3-based memristive application in artificial intelligence.
... The finding of fluxclosure provides a new similarity between magnet and ferroelectrics, which implies that there is plenty of room to explore the dipole topologies in ferroelectric or polar materials corresponding to that in ferromagnetic materials. 1,2 Since the first observation of flux-closures, 39 several topological polar structures in ferroelectric [51][52][53][54][55][56][57][58] The lateral dimensions are within ∼15 to ∼40 nm for the flux-closures, ∼4 to ∼10 nm for vortices, ∼7 nm for skyrmions, and ∼4 to ∼10 nm for merons. In each of the above explorations, the aberration-corrected STEM imaging together with related quantitative analysis has played critical roles. ...
Article
The continuous rotation of electric dipoles, which is inspired by unusual spin textures in magnetic materials, has been envisioned by theoretical modelings in last two decades. Although in electrically polar systems it was thought to be difficult to introduce continuous rotation of electric dipoles since similar Dzyaloshinskii–Moriya interaction like that of ferromagnets is still under study, external strains and interface depolarization fields have been then identified to be critical for rotating electric dipoles in nano-scale oxide films/superlattices. In this Perspective, we will briefly summarize the experimental finding of the newly identified topological polar structures and corresponding properties, such as polar flux-closure, vortex, skyrmion lattice, and meron. The critical importance of microscopy technologies, especially the advanced aberration-corrected transmission electron microscopy with ultra-high spatial resolutions, will be emphasized. Moreover, physical aspects to be addressed in the future, such as the strain maintenance and relaxation mechanisms of polar systems/superlattices, atomic maps of three-dimensional topological polar structures, and flexoelectricity-related properties, will be highlighted and envisioned.
... Resistive switching characteristics have been extensively studied in different materials, such as semiconductor oxides, [5][6][7][8] perovskites, [9,10] chalcogenides, [11][12][13][14][15] and ferroelectrics. [16,17] Although metal oxides have attracted much attention in volatile memory technology but the nonuniformity and stability issues diverted the attention of researchers to other materials. The electric field induced switchable spontaneous polarization in ferroelectric thin films make them a promising candidates for nonvolatile memories. ...
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Ferroelectric crystals feature asymmetric or polar structures that are switchable under an external electric field, holding promise for information storage. Nanoscale ferroelectrics might exhibit various exotic domain configurations and polar topologies, such as full flux-closure, vortex, skyrmion, and meron. These topological domains were theoretically switchable and may give rise to an unusually high density of memory bits. They would also undergo unusual phase transitions and form hidden, collective polar topological states under external stimulations. Similar domains and spin topologies are well known in ferromagnetic materials, and their topological properties and dynamics are under intensive investigation. However, in ferroelectric materials, the coupling of polarizations to spontaneous strains would be so pronounced that the formations of polar topologies were believed to be impossible. How to stabilize the polar topologies in ferroelectrics, especially in nanoscale ferroelectrics, was known as a big challenge. In this overview, we summarize the recent progress in polar topologies in ferroelectric oxides. We start from a review the discovery of polar topologies, including flux-closure quadrant, vortex, bubble, skyrmion, meron lattice, polar waves, and center-type domains. We also focus on the effects of mechanical and electrical boundary conditions and sample size on the formation of topological structures. In the meanwhile, we emphasize the use of aberration-corrected transmission electron microscope which enables to visualize the ion displacement at a sub-Ångström resolution in real space. And at the end, we envision several aspects to be considered in the future, such as imaging three dimensional (3D) atomic morphology of the topological polar structures, exploring novel polar topologies in other possible systems, and addressing the coupling of polar topologies with flexoelectricity by a combination of quantitative transmission electron microscopy and relevant theoretical approaches.
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Charged domain walls (CDWs) have attracted considerable attention owing to their tunable properties corresponding to high-density information storage and nanoelectronics devices. The excellent time endurance of conductivity is in an...
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Charged domain walls in ferroelectrics exhibit a quasi-two-dimensional conduction path coupled to the surrounding polarization. They have been proposed for use as non-volatile memory with non-destructive operation and ultralow energy consumption. Yet the evolution of domain walls during polarization switching makes it challenging to control their location and conductance precisely, a prerequisite for controlled read-write schemes and for integration in scalable memory devices. Here, we explore and reversibly switch the polarization of square BiFeO3 nanoislands in a self-assembled array. Each island confines cross-shaped, charged domain walls in a centre-type domain. Electrostatic and geometric boundary conditions induce two stable domain configurations: centre-convergent and centre-divergent. We switch the polarization deterministically back and forth between these two states, which alters the domain wall conductance by three orders of magnitude, while the position of the domain wall remains static because of its confinement within the BiFeO3 islands.
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The exotic topological domains in ferroelectrics and multiferroics have attracted extensive interest in recent years due to their novel functionalities and potential applications in nanoelectronic devices. One of the key challenges for these applications is a realization of robust yet reversibly switchable nanoscale topological domain states with high density, wherein spontaneous topological structures can be individually addressed and controlled. This has been accomplished in our work using high-density arrays of epitaxial BiFeO3 (BFO) ferroelectric nanodots with a lateral size as small as ~60 nm. We demonstrate various types of spontaneous topological domain structures, including center-convergent domains, center-divergent domains, and double-center domains, which are stable over sufficiently long time but can be manipulated and reversibly switched by electric field. The formation mechanisms of these topological domain states, assisted by the accumulation of compensating charges on the surface, have als
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Although elastic strains, particularly inhomogeneous strains, are able to tune, enhance or create novel properties of some nanoscale functional materials, potential devices dominated by inhomogeneous strains have not been achieved so far. Here we report a fabrication of inhomogeneous strains with a linear gradient as giant as 10⁶ per metre, featuring an extremely lower elastic energy cost compared with a uniformly strained state. The present strain gradient, resulting from the disclinations in the BiFeO3 nanostructures array grown on LaAlO3 substrates via a high deposition flux, induces a polarization of several microcoulomb per square centimetre. It leads to a large built-in electric field of several megavoltage per metre, and gives rise to a large enhancement of solar absorption. Our results indicate that it is possible to build up large-scale strain-dominated nanostructures with exotic properties, which in turn could be useful in the development of novel devices for electromechanical and photoelectric applications.
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Recently, ferroelectric tunnel junctions have attracted much attention due to their potential applications in non-destructive readout non-volatile memories. Using a semiconductor electrode has been proven effective to enhance the tunnelling electroresistance in ferroelectric tunnel junctions. Here we report a systematic investigation on electroresistance of Pt/BaTiO3/Nb:SrTiO3 metal/ferroelectric/semiconductor tunnel junctions by engineering the Schottky barrier on Nb:SrTiO3 surface via varying BaTiO3 thickness and Nb doping concentration. The optimum ON/OFF ratio as great as 6.0 × 10⁶, comparable to that of commercial Flash memories, is achieved in a device with 0.1 wt% Nb concentration and a 4-unit-cell-thick BaTiO3 barrier. With this thinnest BaTiO3 barrier, which shows a negligible resistance to the tunnelling current but is still ferroelectric, the device is reduced to a polarization-modulated metal/semiconductor Schottky junction that exhibits a more efficient control on the tunnelling resistance to produce the giant electroresistance observed. These results may facilitate the design of high performance non-volatile resistive memories.
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We report resistance switching effects in polycrystalline pure BiFeO3 films prepared by a sol-gel method. By current-voltage and conductive atomic force microscope (c-AFM) measurements, resistance switching effects are observed in BiFeO3 films annealed at and above 650 °C. A fresh sample can be transformed into a low-resistive state by applying a high positive voltage without forming process and then be switched to a high-resistive state by applying a negative voltage. Both c-AFM and retention results suggest that the redistribution of oxygen vacancies in grain boundaries could play a key role on the resistance switching in the polycrystalline pure BiFeO3 films.
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Epitaxial c-axis oriented BiFeO3 (BFO) thin films were deposited on (001) Nb-doped SrTiO3 (Nb-STO) substrates by pulsed laser deposition. Introducing Bi vacancies caused the BFO thin film to evolve to a p-type semiconductor and formed a p-n heterojunction with an n-type semiconductor Nb-STO. The current density versus voltage (J-V) and capacitance versus voltage (C-V) characteristics of the heterojunction were investigated. A typical rectifying J-V effect was observed with a large rectifying ratio of 5×104. Reverse C-V characteristics exhibited a linear 1/C2 versus V plot, from which a built-in potential of 0.6 V was deduced. The results show a potential application of BFO/Nb-STO heterojunction for oxide electronics.
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Current-voltage hysteresis and switchable rectifying characteristics have been observed in epitaxial multiferroic BiFeO3 (BFO) thin films. The forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching. With analyzing the potential barriers and their variation with ferroelectric switching at the interfaces between the metallic electrodes and the semiconducting BFO, the switchable diode effect can be explained qualitatively by the polarization-modulated Schottky-like barriers.
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Typically, polarization and strain in ferroelectric materials are coupled, leading to the generally accepted direct relation between polarization and unit-cell tetragonality. Here, by means of high-resolution transmission electron microscopy we map, on the unit-cell scale, the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultrathin epitaxial PbZr0.2Ti0.8O3 film on a SrRuO3 electrode layer deposited on a SrTiO3 substrate. The lattice is highly tetragonal at the centre of the film, whereas it shows reduced tetragonality close to the interfaces. Most strikingly, we find that the maximum off-centre displacements for the central area of the film do not scale with the tetragonality. This challenges the fundamental belief in a strong polarization–tetragonality coupling in PbTiO3-based ferroelectrics, at such thicknesses. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces, suggesting that interface-induced suppression of the ferroelectric polarization plays a critical role in the size effect of nanoscale ferroelectrics.
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Rapid advances in information technology rely on high-speed and large-capacity nonvolatile memories. A number of alternatives to contemporary Flash memory have been extensively studied to obtain a more powerful and functional nonvolatile memory. We review the current status of one of the alternatives, resistance random access memory (ReRAM), which uses a resistive switching phenomenon found in transition metal oxides. A ReRAM memory cell is a capacitor-like structure composed of insulating or semiconducting transition metal oxides that exhibits reversible resistive switching on applying voltage pulses. Recent advances in the understanding of the driving mechanism are described in light of experimental results involving memory cells composed of perovskite manganites and titanates.
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Ferroelectric materials have emerged in recent years as an alternative to magnetic and dielectric materials for nonvolatile data-storage applications. Lithography is widely used to reduce the size of data-storage elements in ultrahigh-density memory devices. However, ferroelectric materials tend to be oxides with complex structures that are easily damaged by existing lithographic techniques, so an alternative approach is needed to fabricate ultrahigh-density ferroelectric memories. Here we report a high-temperature deposition process that can fabricate arrays of individually addressable metal/ferroelectric/metal nanocapacitors with a density of 176 Gb inch(-2). The use of an ultrathin anodic alumina membrane as a lift-off mask makes it possible to deposit the memory elements at temperatures as high as 650 degrees C, which results in excellent ferroelectric properties.
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We calculate the exact analytical solution to the domain wall properties in a multiferroic system with two order parameters that are coupled bi-quadratically. This is then adapted to the case of a magnetoelectric multiferroic material such as BiFeO3, with a view to examine critically whether the domain walls can account for the enhancement of magnetization reported for thin films fo this material, in view of the correlation between increasing magnetization and increasing volume fraction of domain walls as films become thinner. The present analysis can be generalized to describe a class of magnetoelectric devices based upon domain walls rather than bulk properties.
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Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90degrees domain wall is observed. Nucleation of new reversed 180degrees domains at the 90degrees domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time. (C) 2002 American Institute of Physics.
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Unidirectional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal/semiconductor interfaces with Schottky barriers. We report on a diode effect associated with the direction of bulk electric polarization in BiFeO3: a ferroelectric with a small optical gap edge of ∼2.2 electron volts. We found that bulk electric conduction in ferroelectric monodomain BiFeO3 crystals is highly nonlinear and unidirectional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. A substantial visible-light photovoltaic effect is observed in BiFeO3 diode structures. These results should improve understanding of charge conduction mechanisms in leaky ferroelectrics and advance the design of switchable devices combining ferroelectric, electronic, and optical functionalities.
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Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO(3). The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features.
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Oxide layers grown on the surface provide an effective way of protecting metallic materials against corrosion for sustainable use in a broad range of applications. However, the growth of cavities at the metal/oxide interface weakens the adherence of the protective layer and can promote its spallation under service conditions, as observed for alumina layers formed by selective oxidation of aluminide intermetallic alloys used in high-temperature applications. Here we show that direct atomic-scale observations of the interface between an ultrathin protective oxide layer (alumina) grown on an intermetallic titanium aluminide substrate (TiAl) can be performed with techniques sensitive to the topmost atomic layers at the surface. Nanocavities resulting from the self-assembling of atomic vacancies injected at the interface by the growth mechanism of the protective oxide are observed for the first time, bringing new insight into the understanding of the fate of injected cavities in oxidation processes.
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Topological defects showing exotic properties and diverse functionalities provide us a potential utilization in nanoscale electronic devices. However, the formation mechanism and density manipulation of topological defects such as center-type domains which are crucial for applications remain elusive. Usually, these center-type domains are generated by applying external electric fields in ferroelectrics. In contrast, here we have prepared high density center-divergent domains in BiFeO3 as self-assembled nano-islands deposited on both Nb and Fe doped SrTiO3 substrates. The size and density of these domains can be easily manipulated by varying doping level in substrates. The panorama polar configurations of the center-divergent domains are revealed by piezoresponse force microscopy (PFM) and Cs-corrected scanning transmission electron microscopy. Phase-field simulations prove that both the surface charge accumulation and the shape of the nano-islands take great effect in the formation of center-type domains. The controllable growth of the nano-islands offers us a promising way to acquire high density nanoscale non-volatile memories.
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We demonstrate memristive behaviors in Pt/BaTiO3/Nb:SrTiO3 metal/ferroelectric/semiconductor ferroelectric tunnel junctions, in which the semiconductor electrode can be switched between the accumulated and the depleted states by polarization reversal in the BaTiO3 barrier via the ferroelectric field effect. An extra barrier, against electron tunneling, forms in the depleted region of the Nb:SrTiO3 electrode surface, which together with the ferroelectric barrier itself modulate the tunneling resistance with the change of effective polarization. Continuous resistance modulation over four orders of magnitude is hence achieved by application of programmed voltage pulses with different polarity, amplitude, and repetition numbers, as a result of the development of the extra barrier. (C) 2014 AIP Publishing LLC.
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Here we describe the Wulff shape of strontium titanate nanocuboids prepared by a hydrothermal method and annealed at high temperature. Transmission electron microscopy was used to measure the faceting ratios d(110):d(100) which are compared with surface energy ratios γ(110):γ(100) from first-principles calculations. Internal voids attributed to the Kirkendall effect were also measured and show agreement with the external faceting. Experiment and theory are shown to agree strongly within statistical and density functional theory error.
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This work reports a resistive switching effect observed at rectifying Pt/Bi1–δFeO3 interfaces and the impact of Bi deficiencies on its characteristics. Since Bi deficiencies provide hole carriers in BiFeO3, Bi-deficient Bi1–δFeO3 films act as a p-type semiconductor. As the Bi deficiency increased, a leakage current at Pt/Bi1–δFeO3 interfaces tended to increase, and finally, rectifying and hysteretic current–voltage (I–V) characteristics were observed. In I–V characteristics measured at a voltage-sweep frequency of 1 kHz, positive and negative current peaks originating from ferroelectric displacement current were observed under forward and reverse bias prior to set and reset switching processes, respectively, suggesting that polarization reversal is involved in the resistive switching effect. The resistive switching measurements in a pulse-voltage mode revealed that the switching speed and switching ratio can be improved by controlling the Bi deficiency. The resistive switching devices showed endurance of >105 cycles and data retention of >105 s at room temperature. Moreover, unlike conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at Pt/Bi1–δFeO3 interfaces to nonvolatile memory.
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We report reversible resistance switching behaviors in Pt/BiFeO3/Nb:SrTiO3 memristor. The resistance of the junctions can be tuned up to about five orders of magnitude by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The high performances are promising for employing ferroelectric junctions in nonvolatile memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the formation and annihilation of trap centers in the BFO films, resulting in Poole-Frenkel emission for low resistance state and the thermionic emission for high resistance state, respectively.
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All-polymer, Write-Once-Read-Many times resistive memory devices have been fabricated on flexible substrates using a single polymer, poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS). Spin-cast or inkjet-printed films of solvent-modified PEDOT:PSS are used as electrodes while the unmodified or as-is PEDOT:PSS is used as the semiconducting active layer. The all-polymer devices exhibit an irreversible but stable transition from a low resistance state (ON) to a high resistance state (OFF) at low voltages caused due to an electric field induced morphological rearrangement of PEDOT and PSS at the electrode interface. However, in the metal-PEDOT:PSS-metal devices, we have shown a metal-filament formation switching the device from an initial high resistance state (OFF) to the low resistance state (ON). The all-PEDOT:PSS memory device has low write voltages (< 3V), high ON/OFF ratio (> 103), good retention characteristics (> 10,000 cycles) and stability in ambient storage (> 3 months).
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Nanometer-scale electronic transport in engineered interfaces in ferroelectrics, such as domains and topological defects, has emerged as a topic of broad interest due to potential applications in information storage, sensors and photovoltaic devices. Scanning probe microscopy (SPM) methods led to rapid growth in the field by enabling correlation of the unique functional properties with microstructural features in the aforementioned highly localized phenomena. In addition to conduction localized at interfaces, polarization-mediated control of conduction through domains in nanoscale ferroelectrics suggests significant potential for use in memristor technologies. In parallel with experiment, theory based on thermodynamic Landau-Ginzburg-Devonshire (LGD) framework has seen rapid development, both rationalizing the observations, and hinting at possibilities for local, deterministic control of order parameters. These theories can successfully account for static interface conductivity at charged, nominally uncharged and topologically protected domain walls. Here, recent experimental and theoretical progress in SPM-motivated studies on domain wall conduction in both standard and improper ferroelectrics are reviewed. SPM studies on transport through ferroelectrics reveal that both domains and topological defects in oxides can be exploited as individual elements for use in functional nanoscale devices. Future prospects of the field are discussed.
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Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or Néel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines’ and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.
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We have investigated the effect of the deposition rate, as a direct function of the laser pulse repetition rate, on the surface morphology of CeO2 films deposited by the pulsed laser deposition (PLD) technique on r-cut Al2O3 (11¯02) substrates. The critical thickness is defined as the thickness before the onset of increased growth of large islands and abrupt increase in surface roughening. Two regimes of growth were found within the investigated range of deposition rate. It is found out that in the high deposition rate-regime (within 2-4 nm/min), the critical thickness is ~90 nm, but in the low deposition rate-regime (less than 1 nm/min), the critical thickness is shifted to ~40 nm. Films belonging to these two regimes of crystalline growth were found to have characteristically different formations and surface morphologies. As observed through atomic force microscopy (AFM), the surface morphology is composed of longitudinal islands forming a maze-like pattern in the high deposition rate-regime, while the characteristic morphology was composed of rounded islands in the low deposition rate-regime. Significant reduction in the areal density of large islands and characteristically smoother films was achieved using a low deposition rate.
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Comparison between piezoelectric force microscopy images and current-voltage data consecutively obtained using conductive atomic force microscopy below transition voltages for a highly oriented ferroelectric BiFeO3 nano-island confirms that ferroelectric polarization reversal induces transitions of forward-direction, and thus down- and up-polarization is accompanied by positive- and negative-forward diode-like behavior, respectively.
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This review covers resistive random access memories which utilize redox processes and ionic motion on the nanoscale as their storage principle (ReRAM). Generic aspects are described in order to provide the physics and chemistry background for the explanation of the microscopic switching mechanism and of the high nonlinearity in the switching kinetics. The valence change memory (VCM) effect is elaborated in more detail. As common features, ReRAM typically show very short switching times, low switching energies, and long data retention times. In addition, they offer a scalability potential down to feature sizes in the order of 5 nm and below.
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LiNbO3/Nb-doped SrTiO3 heterojunction was fabricated by pulsed laser deposition. The current-voltage curve of this heterojunction shows good rectifying property and changes with temperature dramatically. For the forward bias, the conduction mechanism changes from Ohmic-like for the low bias voltages to space charge limited current for the high bias voltages. While for the reverse bias, it changes from Schottky emission to avalanche breakdown with increasing bias voltage. The results were explained by considering the band structures of the junction. This work demonstrates that ferroelectric materials, combined with other oxides, can lead to some interesting property which may have potential applications.
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A large electric-pulse-induced reversible resistance change active at room temperature and under zero magnetic field has been discovered in colossal magnetoresistive (CMR) Pr0.7Ca0.3MnO3 thin films. Electric field-direction-dependent resistance changes of more than 1700% were observed under applied pulses of ∼ 100 ns duration and as low as ±5 V magnitude. The resistance changes were cumulative with pulse number, were reversible and nonvolatile. This electrically induced effect, observed in CMR materials at room temperature has both the benefit of a discovery in materials properties and the promise of applications for thin film manganites in the electronics arena including high-density nonvolatile memory. © 2000 American Institute of Physics.
Article
The application of ferroelectric materials for nonvolatile memory and ferroelectric data storage necessitates quantitative studies of local switching characteristics and their relationship to material microstructure and defects. Switching spectroscopy piezoresponse force microscopy (SS-PFM) is developed as a quantitative tool for real-space imaging of imprint, coercive bias, remanent and saturation responses, and domain nucleation voltage on the nanoscale. Examples of SS-PFM implementation, data analysis, and data visualization are presented for epitaxial lead zirconate titanate (PZT) thin films and polycrystalline PZT ceramics. Several common artifacts related to the measurement method, environmental factors, and instrument settings are analyzed.
Article
Bismuth ferrite (BiFeO3, BFO) thin films were spin-coated on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 103 for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole–Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole–Frenkel model after forming.
Article
Conductance quantization phenomena are observed in oxide-based resistive switching memories. These phenomena can be understood by the formation and disruption of atomic-scale conductive filaments in the insulating oxide matrix. The quantum conductance effect can be artificially modulated by controlling the electrical parameters in Set and Reset processes, and can be used for multi-level data storage and help understand and design one-dimensional structures at atomic scales in various materials systems.
Article
One problem with the growth of high quality c‐axis oriented YBa 2 Cu 3 O 7-x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c‐axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c‐axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a‐axis films. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.
Article
The fundamental building blocks of modern silicon-based microelectronics, such as double gate transistors in non-volatile Flash memories, are based on the control of electrical resistance by electrostatic charging. Flash memories could soon reach their miniaturization limits mostly because reliably keeping enough electrons in an always smaller cell size will become increasingly difficult . The control of electrical resistance at the nanometer scale therefore requires new concepts, and the ultimate resistance-change device is believed to exploit a purely electronic phase change such as the Mott insulator to insulator transition [2]. Here we show that application of short electric pulses allows to switch back and forth between an initial high-resistance insulating state ("0" state) and a low-resistance "metallic" state ("1" state) in the whole class of Mott Insulator compounds AM4X8 (A = Ga, Ge; M= V, Nb, Ta; X = S, Se). We found that electric fields as low as 2 kV/cm induce an electronic phase change in these compounds from a Mott insulating state to a metallic-like state. Our results suggest that this transition belongs to a new class of resistive switching and might be explained by recent theoretical works predicting that an insulator to metal transition can be achieved by a simple electric field in a Mott Insulator. This new type of resistive switching has potential to build up a new class of Resistive Random Access Memory (RRAM) with fast writing/erasing times (50 ns to 10 {\mu}s) and resistance ratios \Delta R/R of the order of 25% at room temperature.
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A computationally rapid image analysis method, weighted overdetermined regression, is presented for two-dimensional (2D) Gaussian fitting of particle location with subpixel resolution from a pixelized image of light intensity. Compared to least-squares Gaussian iterative fitting, which is most exact but prohibitively slow for large data sets, the precision of this new method is equivalent when the signal-to-noise ratio is high and approaches it when the signal-to-noise ratio is low, while enjoying a more than 100-fold improvement in computational time. Compared to another widely used approximation method, nine-point regression, we show that precision and speed are both improved. Additionally, weighted regression runs nearly as fast and with greatly improved precision compared to the simplest method, the moment method, which, despite its limited precision, is frequently employed because of its speed. Quantitative comparisons are presented for both circular and elliptical Gaussian intensity distributions. This new image analysis method may be useful when dealing with large data sets such as those frequently met in astronomy or in single-particle and single-molecule tracking using microscopy and may facilitate advances such as real-time quantification of microscopy images.
  • J Seidel
  • L W Martin
  • Q He
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  • Y H Chu
  • A Rother
  • M E Hawkridge
  • P Maksymovych
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  • M Gajek
  • N Balke
  • S V Kalinin
  • S Gemming
  • F Wang
  • G Catalan
  • J F Scott
  • N A Spaldin
  • J Orenstein
  • R Ramesh
J. Seidel, L.W. Martin, Q. He, Q. Zhan, Y.H. Chu, A. Rother, M.E. Hawkridge, P. Maksymovych, P. Yu, M. Gajek, N. Balke, S.V. Kalinin, S. Gemming, F. Wang, G. Catalan, J.F. Scott, N.A. Spaldin, J. Orenstein, R. Ramesh, Conduction at domain walls in oxide multiferroics, Nat. Mater. 8 (2009) 229-234.
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  • S Wu
  • X Lu
  • M Zeng
  • X Gao
  • J Dai
  • J M Liu
L. Zhao, Z. Lu, F. Zhang, G. Tian, X. Song, Z. Li, K. Huang, Z. Zhang, M. Qin, S. Wu, X. Lu, M. Zeng, X. Gao, J. Dai, J.M. Liu, Current rectifying and resistive switching in high density BiFeO 3 nanocapacitor arrays on Nb-SrTiO 3 substrates, Sci. Rep. 5 (2015) 9680.
  • Z Li
  • Y Wang
  • G Tian
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  • L Zhao
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  • H Fan
  • X Song
  • D Chen
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  • M Qin
  • M Zeng
  • Z Zhang
  • X Lu
  • S Hu
  • C Lei
  • Q Zhu
  • J Li
  • X Gao
  • J M Liu
Z. Li, Y. Wang, G. Tian, P. Li, L. Zhao, F. Zhang, J. Yao, H. Fan, X. Song, D. Chen, Z. Fan, M. Qin, M. Zeng, Z. Zhang, X. Lu, S. Hu, C. Lei, Q. Zhu, J. Li, X. Gao, J.M. Liu, Highdensity array of ferroelectric nanodots with robust and reversibly switchable topological domain states, Sci. Adv. 3 (2017) e1700919.