Ferroelectric tunnel junctions with graphene electrodes

Enhancement of tunneling electroresistance in metal/two-dimensional ferroelectric tunnel junctions: route for polarization-modulated interface transport

In fabricating ferroelectric tunnel junction (FTJ) devices, it is essential to employ low-resistance metals as electrodes interfacing with two-dimensional (2D) ferroelectric materials. For FTJs with a top contact configuration, two interfaces for charge transport are present, namely the vertical interface between the metal electrode and the 2D ferroelectric material, and the lateral interface between the electrode and the central scattering region. These interfaces significantly influence the tunneling electroresistance (TER) of FTJs. However, there exists a notable deficiency in comprehension concerning the physics of charge transport at the interface. In this work, we explore the interface transport properties in FTJs featuring a top contact configuration between metal and the typicalα-In2Se3monolayer. By employing the non-equilibrium Green's function method, we observe a TER ratio of1.15×105% for the Pd top contact interfacing with anα-In2Se3monolayer. The significant TER effect is attributed to polarization-controlled interface transport, which is further elucidated through an analysis of the transport mechanisms influenced by the out-of-plane polarization ofα-In2Se3at the vertical interface and the in-plane polarization at the lateral interface. This investigation of the fundamental physical mechanisms of polarization-controlled interface transport demonstrates significant potential for enhancing non-volatile memory devices.

Super-tetragonal Sr4Al2O7 as a sacrificial layer for high-integrity freestanding oxide membranes

Identifying a suitable water-soluble sacrificial layer is crucial to fabricating large-scale freestanding oxide membranes, which offer attractive functionalities and integrations with advanced semiconductor technologies. Here, we introduce a water-soluble sacrificial layer, "super-tetragonal" Sr4Al2O7 (SAOT). The low-symmetric crystal structure enables a superior capability to sustain epitaxial strain, allowing for broad tunability in lattice constants. The resultant structural coherency and defect-free interface in perovskite ABO3/SAOT heterostructures effectively restrain crack formation during the water release of freestanding oxide membranes. For a variety of nonferroelectric oxide membranes, the crack-free areas can span up to a millimeter in scale. This compelling feature, combined with the inherent high water solubility, makes SAOT a versatile and feasible sacrificial layer for producing high-quality freestanding oxide membranes, thereby boosting their potential for innovative device applications.

Realizing tunneling electroresistance effect in the Au/h-BN/In2Se3/Au vertical ferroelectric tunnel junction

Two-dimensional (2D) ferroelectric tunnel junctions (FTJs) have great potential in the design of non-volatile memory devices due to the tunneling electroresistance (TER) effect and the fact that it is not constrained by critical thickness. Incorporation of 2D ferroelectric materials in realistic FTJs inevitably involves the contacts to the traditional three-dimensional (3D) metals. However, how to design the FTJs by combining the 2D ferroelectric materials with the 3D metals still needs to be studied. In this work, we design a vertical 3D FTJ by adopting the 3D metal Au as the left and right electrodes and the 2D ferroelectric material In2Se3 together with h-BN as the central scattering region. By density functional theory combined with the non-equilibrium Green's function (NEGF) method, we demonstrate that the h-BN intercalation with a large bandgap plays the role of good "insulator," which breaks the symmetry of the left and right electrodes. As a result, we obtain the TER ratio of about 170%, and it can be further improved to about 1200% if two layers of In2Se3 (2L-In2Se3) are adopted as the tunneling barrier layer. Our results provide another way for the design and application of ferroelectric memory devices based on 2D ferroelectric materials.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Giant tunneling electroresistance in a 2D bilayer-In2Se3-based out-of-plane ferroelectric tunnel junction

Ferroelectric tunnel junctions (FTJs) have great potential in nonvolatile memory devices and have been extensively studied in recent years. Compared with conventional FTJs based on perovskite-type oxide materials as the barrier layer, two-dimensional (2D) van der Waals ferroelectric materials are advantageous in improving the performance of FTJs and achieving miniaturization of FTJ devices due to the features such as atomic thickness and ideal interfaces. In this work, we present a 2D out-of-plane ferroelectric tunnel junction (FTJ) constructed using graphene and bilayer-In₂Se₃. Using density functional calculations combined with the nonequilibrium Green's function technique, we investigate the electron transport properties in the graphene/bilayer-In₂Se₃ (BIS) vdW FTJ. Our calculations show that the FTJ we constructed can be switched from ferroelectric to antiferroelectric by changing the relative dipole arrangement of the BIS to form multiple nonvolatile resistance states. Since the charge transfer between the layers varies for the four different polarization states, the TER ratios range from 10³% to 10¹⁰%. The giant tunneling electroresistance and multiple resistance states in the 2D BIS-based FTJ suggest that it has great potential for application in nanoscale nonvolatile ferroelectric memory devices.

Two-Dimensional Layered Materials Meet Perovskite Oxides: A Combination for High-Performance Electronic Devices

As the Si-based transistors scale down to atomic dimensions, the basic principle of current electronics, which heavily relies on the tunable charge degree of freedom, faces increasing challenges to meet the future requirements of speed, switching energy, heat dissipation, and packing density as well as functionalities. Heterogeneous integration, where dissimilar layers of materials and functionalities are unrestrictedly stacked at an atomic scale, is appealing for next-generation electronics, such as multifunctional, neuromorphic, spintronic, and ultralow-power devices, because it unlocks technologically useful interfaces of distinct functionalities. Recently, the combination of functional perovskite oxides and two-dimensional layered materials (2DLMs) led to unexpected functionalities and enhanced device performance. In this paper, we review the recent progress of the heterogeneous integration of perovskite oxides and 2DLMs from the perspectives of fabrication and interfacial properties, electronic applications, and challenges as well as outlooks. In particular, we focus on three types of attractive applications, namely field-effect transistors, memory, and neuromorphic electronics. The van der Waals integration approach is extendible to other oxides and 2DLMs, leading to almost unlimited combinations of oxides and 2DLMs and contributing to future high-performance electronic and spintronic devices.

Observation of Electric Hysteresis, Polarization Oscillation, and Pyroelectricity in Nonferroelectric p-n Heterojunctions

The switchable electric polarization is usually achieved in ferroelectric materials with noncentrosymmetric structures, which opens exciting opportunities for information storage and neuromorphic computing. In another polar system of p-n junction, there exists the electric polarization at the interface due to the Fermi level misalignment. However, the resultant built-in electric field is unavailable to manipulate, thus attracting less attention for memory devices. Here, we report the interfacial polarization hysteresis (IPH) in the vertical sidewall van der Waals heterojunctions of black phosphorus and quasi-two-dimensional electron gas on SrTiO_{3}. A nonvolatile switching of electric polarization can be achieved by reconstructing the space charge region (SCR) with long-lifetime nonequilibrium carriers. The resulting electric-field controllable IPH is experimentally verified by electric hysteresis, polarization oscillation, and pyroelectric effect. Further studies confirm the transition temperature of 340 K, beyond which the IPH vanishes. The second transition is revealed with the temperature dropping below 230 K, corresponding to the sharp improvement of IPH and the freezing of SCR reconstruction. This work offers new possibilities for exploring the memory phenomena in nonferroelectric p-n heterojunctions.

Tunable Molecular Electrodes for Bistable Polarization Screening

The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, screening is done by the electrodes, while in bare ferroelectric surfaces it is typically accomplished by atmospheric adsorbates. Although chemisorbed species can have even better screening efficiency than conventional electrodes, they are subject to unpredictable environmental fluctuations and, moreover, dominant charged species favor one polarity over the opposite. This paper proposes a new screening concept, namely surface functionalization with resonance-hybrid molecules, which combines the predictability and bipolarity of conventional electrodes with the screening efficiency of adsorbates. Thin films of barium titanate (BaTiO₃ ) coated with resonant para-aminobenzoic acid (pABA) display increased coercivity for both signs of ferroelectric polarization irrespective of the molecular layer thickness, thanks to the ability of these molecules to swap between different electronic configurations and adapt their surface charge density to the screening needs of the ferroelectric underneath. Because electron delocalization is only in the vertical direction, unlike conventional metals, chemical electrodes allow writing localized domains of different polarity underneath the same electrode. In addition, hybrid capacitors composed of graphene/pABA/ferroelectric have been made with enhanced coercivity compared to pure graphene-electode capacitors.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

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.

2D materials-based homogeneous transistor-memory architecture for neuromorphic hardware

[Figure: see text].

Giant tunneling electroresistance arising from reversible partial barrier metallization in the NaTiO3/BaTiO3/LaTiO3 ferroelectric tunnel junction

Tunneling electroresistance (TER) is the change in tunneling resistance induced by ferroelectric polarization reversal in ferroelectric tunnel junctions (FTJs), and how to achieve a giant TER has always been a central topic in the study of FTJs. In this work, by considering the NaTiO3/BaTiO3/LaTiO3 junction with asymmetric polar interfaces as an example, we propose a novel scheme to realize a giant TER based on the reversible partial metallization of ferroelectric barrier upon the switching of ferroelectric polarization. Density functional theory calculations indicate that high on-state and low off-state conductances are obtained and the TER ratio is as high as 3.20 × 108% due to the reversible partial barrier metallization, which leads to a great difference in the effective tunneling barrier widths. The reversible partial barrier metallization, accompanied by the ferroelectric polarization reversal, is driven by the parallel or anti-parallel alignment of the depolarization electrical field of the ferroelectrical barrier and a strong built-in electrical field cooperatively contributed by the asymmetric polar interfaces and the difference in the work functions of the two leads. The findings suggest a feasible scheme for constructing promising high performance FTJ memory devices by combining both asymmetric polar interfaces and substantially different work functions.

Emerging 2D Memory Devices for In-Memory Computing

It is predicted that the conventional von Neumann computing architecture cannot meet the demands of future data-intensive computing applications due to the bottleneck between the processing and memory units. To try to solve this problem, in-memory computing technology, where calculations are carried out in situ within each nonvolatile memory unit, has been intensively studied. Among various candidate materials, 2D layered materials have recently demonstrated many new features that have been uniquely exploited to build next-generation electronics. Here, the recent progress of 2D memory devices is reviewed for in-memory computing. For each memory configuration, their operation mechanisms and memory characteristics are described, and their pros and cons are weighed. Subsequently, their versatile applications for in-memory computing technology, including logic operations, electronic synapses, and random number generation are presented. Finally, the current challenges and potential strategies for future 2D in-memory computing systems are also discussed at the material, device, circuit, and architecture levels. It is hoped that this manuscript could give a comprehensive review of 2D memory devices and their applications in in-memory computing, and be helpful for this exciting research area.

Emerging Opportunities for 2D Semiconductor/Ferroelectric Transistor-Structure Devices

Semiconductor technology, which is rapidly evolving, is poised to enter a new era for which revolutionary innovations are needed to address fundamental limitations on material and working principle level. 2D semiconductors inherently holding novel properties at the atomic limit show great promise to tackle challenges imposed by traditional bulk semiconductor materials. Synergistic combination of 2D semiconductors with functional ferroelectrics further offers new working principles, and is expected to deliver massively enhanced device performance for existing complementary metal-oxide-semiconductor (CMOS) technologies and add unprecedented applications for next-generation electronics. Herein, recent demonstrations of novel device concepts based on 2D semiconductor/ferroelectric heterostructures are critically reviewed covering their working mechanisms, device construction, applications, and challenges. In particular, emerging opportunities of CMOS-process-compatible 2D semiconductor/ferroelectric transistor structure devices for the development of a rich variety of applications are discussed, including beyond-Boltzmann transistors, nonvolatile memories, neuromorphic devices, and reconfigurable nanodevices such as p-n homojunctions and self-powered photodetectors. It is concluded that 2D semiconductor/ferroelectric heterostructures, as an emergent heterogeneous platform, could drive many more exciting innovations for modern electronics, beyond the capability of ubiquitous silicon systems.

Two-Dimensional Antiferroelectric Tunnel Junction

Ferroelectric tunnel junctions (FTJs), which consist of two metal electrodes separated by a thin ferroelectric barrier, have recently aroused significant interest for technological applications as nanoscale resistive switching devices. So far, most existing FTJs have been based on perovskite-oxide barrier layers. The recent discovery of the two-dimensional (2D) van der Waals ferroelectric materials opens a new route to realize tunnel junctions with new functionalities and nm-scale dimensions. Because of the weak coupling between the atomic layers in these materials, the relative dipole alignment between them can be controlled by applied voltage. This allows transitions between ferroelectric and antiferroelectric orderings, resulting in significant changes of the electronic structure. Here, we propose to realize 2D antiferroelectric tunnel junctions (AFTJs), which exploit this new functionality, based on bilayer In_{2}X_{3} (X=S, Se, Te) barriers and different 2D electrodes. Using first-principles density functional theory calculations, we demonstrate that the In_{2}X_{3} bilayers exhibit stable ferroelectric and antiferroelectric states separated by sizable energy barriers, thus supporting a nonvolatile switching between these states. Using quantum-mechanical modeling of the electronic transport, we explore in-plane and out-of-plane tunneling across the In_{2}S_{3} van der Waals bilayers, and predict giant tunneling electroresistance effects and multiple nonvolatile resistance states driven by ferroelectric-antiferroelectric order transitions. Our proposal opens a new route to realize nanoscale memory devices with ultrahigh storage density using 2D AFTJs.

Low-Voltage Domain-Wall LiNbO3 Memristors

Application of conducting ferroelectric domain walls (DWs) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DW density using supercoercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO₃ capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of subcoercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multilevel memories and neuromorphic computing applications.

The Role of Ferroelectric Polarization in Resistive Memory Properties of Metal/Insulator/Semiconductor Tunnel Junctions: A Comparative Study

Recently, tunnel junction devices adopting semiconducting Nb:SrTiO₃ electrodes have attracted considerable attention for their potential applications in resistive data storage and neuromorphic computing. In this work, we report on a comparative study of Pt/insulator/Nb:SrTiO₃ tunnel junctions between ferroelectric BaTiO₃ and nonferroelectric SrTiO₃ and LaAlO₃ barriers to reveal the role of polarization in resistance switching properties. Although hysteretic behaviors appear in current-voltage measurements of all devices regardless of the barrier character, significantly improved current ratios by more than three orders of magnitude are observed in the Pt/BaTiO₃/Nb:SrTiO₃ tunnel junctions due to the dominance of polarization in modulation of junction barrier profiles between the low and high resistance states. The switchable polarization also gives rise to enhanced resistance retention since the electron diffusion that smears the barrier contrast of the bistable resistance states is suppressed by the polar BaTiO₃/Nb:SrTiO₃ interface associated with the ferroelectric bound charges. These polarization-induced effects are absent in the nonferroelectric Pt/SrTiO₃/Nb:SrTiO₃ and Pt/LaAlO₃/Nb:SrTiO₃ devices in which serious resistance state decay, described by Fick's second law, is observed since there are no switchable interface charges on SrTiO₃/Nb:SrTiO₃ and LaAlO₃/Nb:SrTiO₃ to block the electron diffusion. In addition, the Pt/BaTiO₃/Nb:SrTiO₃ device also exhibits an excellent switching endurance up to ∼4.0 × 10⁶ bipolar cycles. These enhancements indicate the importance of ferroelectric polarization for achieving high-performance resistance switching and suggest that metal/ferroelectric/Nb:SrTiO₃ tunnel junctions are promising candidates for nonvolatile memory applications.

Ferroelectric Tunnel Junctions: Modulations on the Potential Barrier

Recently, ferroelectric tunnel junctions (FTJs) have attracted considerable attention for potential applications in next-generation memories, owing to attractive advantages such as high-density of data storage, nondestructive readout, fast write/read access, and low energy consumption. Herein, recent progress regarding FTJ devices is reviewed with an emphasis on the modulation of the potential barrier. Electronic and ionic approaches that modulate the ferroelectric barriers themselves and/or induce extra barriers in electrodes or at ferroelectric/electrode interfaces are discussed with the enhancement of memory performance. Emerging physics, such as nanoscale ferroelectricity, resonant tunneling, and interfacial metallization, and the applications of FTJs in nonvolatile data storage, neuromorphic synapse emulation, and electromagnetic multistate memory are summarized. Finally, challenges and perspectives of FTJ devices are underlined.

Decade of 2D-materials-based RRAM devices: a review

Two dimensional (2D) materials have offered unique electrical, chemical, mechanical and physical properties over the past decade owing to their ultrathin, flexible, and multilayer structure. These layered materials are being used in numerous electronic devices for various applications, and this review will specifically focus on the resistive random access memories (RRAMs) based on 2D materials and their nanocomposites. This study presents the device structures, conduction mechanisms, resistive switching properties, fabrication technologies, challenges and future aspects of 2D-materials-based RRAMs. Graphene, derivatives of graphene and MoS2 have been the major contributors among 2D materials for the application of RRAMs; however, other members of this family such as hBN, MoSe2, WS2 and WSe2 have also been inspected more recently as the functional materials of nonvolatile RRAM devices. Conduction in these devices is usually dominated by either the penetration of metallic ions or migration of intrinsic species. Most prominent advantages offered by RRAM devices based on 2D materials include fast switching speed (<10 ns), less power losses (10 pJ), lower threshold voltage (<1 V) long retention time (>10 years), high electrical endurance (>108 voltage cycles) and extended mechanical robustness (500 bending cycles). Resistive switching properties of 2D materials have been further enhanced by blending them with metallic nanoparticles, organic polymers and inorganic semiconductors in various forms.

Domain-Wall Tunneling Electroresistance Effect

Ferroelectric tunnel junctions (FTJs) utilizing an in-plane head-to-head ferroelectric domain wall (DW) have recently been realized, showing interesting physics and new functionalities. However, the DW state in these junctions was found to be metastable and not reversible after applying an electric field. In this work, we demonstrate that a stable and reversible head-to-head DW state can be achieved in FTJs by proper engineering of polar interfaces. Using density functional theory (DFT) calculations and phenomenological modeling, we explore the DW stability by varying stoichiometry of the La_{1-x}Sr_{x}O/TiO_{2} interfaces in FTJs with La_{0.5}Sr_{0.5}MnO_{3} electrodes and a ferroelectric BaTiO_{3} tunnel barrier. For, x≤0.4 we find that the DW state becomes a global minimum and the calculated hysteresis loops exhibit three reversible polarization states. For such FTJs, our quantum transport calculations predict the emergence of a DW tunneling electroresistance effect-reversible switching of the tunneling conductance between the highly conductive DW state and two much less conductive uniform polarization states.

Reconfigurable Dipole-Induced Resistive Switching of MoS2 Thin Layers on Nb:SrTiO3

The controllable band gap and charge-trapping capability of MoS₂ render it suitable for use in the fabrication of various electrical devices with high-k dielectric oxides. In this study, we investigated reconfigurable resistance states in a MoS₂/Nb:SrTiO₃ heterostructure by using conductive atomic force microscopy. Low-resistance and high-resistance states were observed in all MoS₂ because of barrier height modification resulting from redistribution of charge and oxygen vacancies in the vicinity of interfaces. In a thin layer of the MoS₂ film, the carrier density was high, and layer-dependent transport properties appeared because of the charge separation in MoS₂. The hysteresis and switching voltage of the MoS₂/Nb:SrTiO₃ heterostructure could be varied by controlling the number of layers of MoS₂.

Ferroelectric Tunnel Junctions Enhanced by a Polar Oxide Barrier Layer

Ferroelectric tunnel junctions (FTJs) have recently aroused significant interest due to the interesting physics controlling their properties and potential application in nonvolatile memory devices. In this work, we propose a new concept to design high-performance FTJs based on ferroelectric/polar-oxide composite barriers. Using density functional theory calculations, we model electronic and transport properties of LaNiO₃/PbTiO₃/LaAlO₃/LaNiO₃ tunnel junctions and demonstrate that an ultrathin polar LaAlO₃(001) layer strongly enhances their performance. We predict a tunneling electroresistance (TER) effect in these FTJs with an OFF/ON resistance ratio exceeding a factor of 10⁴ and ON state resistance as low as about 1 kΩμm². Such an enhanced performance is driven by the ionic charge at the PbTiO₃/LaAlO₃ interface, which significantly increases transmission across the FTJ when the ferroelectric polarization of PbTiO₃ is pointing against the intrinsic electric field produced by this ionic charge. This is due to the formation of a two-dimensional (2D) electron or hole gas, depending on the LaAlO₃ termination being (LaO)⁺ or (AlO₂)^-, respectively, which is formed to screen the polarization charge of the nonuniform polarization state. This 2D electron (hole) gas can be switched ON and OFF by the reversal of ferroelectric polarization, resulting in the giant TER effect. The proposed design suggests a new direction for creating FTJs with a stable and reversible ferroelectric polarization, a sizable TER effect, and a low-resistance-area product, as required for memory applications.

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO₃ , graphene/LaAlO₃ /SrTiO₃ , graphene/ferroelectric oxide, MoS₂ /SrTiO₃ , and FeSe/SrTiO₃ heterostructures include voltage scaling in field-effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next-generation optoelectronic devices.

Nanodomain Engineering for Programmable Ferroelectric Devices

We introduce a concept of programmable ferroelectric devices composed of two-dimensional (2D) and ferroelectric (FE) materials. It enables precise modulation of the in-plane conductivity of a 2D channel material through nanoengineering FE domains with out-of-plane polarization. The functionality of these new devices has been demonstrated using field-effect transistors (FETs) fabricated with monolayer molybdenum disulfide (MoS₂) channels on the Pb(Zr,Ti)O₃ substrates. Using piezoresponse force microscopy (PFM), we show that local switching of FE polarization by a conductive probe can be used to tune the conductivity of the MoS₂ channel. Specifically, patterning of the nanoscale domains with downward polarization creates conductive paths in a resistive MoS₂ channel so that the conductivity of an FET is determined by the number and length of the paths connecting source and drain electrodes. In addition to the device programmability, we demonstrate the device ON/OFF cyclic endurance by successive writing and erasing of conductive paths in a MoS₂ channel. These findings may inspire the development of advanced energy-efficient programmable synaptic devices based on a combination of 2D and FE materials.

Chemical vapor deposition and characterization of two-dimensional molybdenum dioxide (MoO2) nanoplatelets

We report on the chemical vapor deposition synthesis of MoO₂ nanoplatelets by sublimation of MoO₃ and its reduction in a hydrogen atmosphere at 750 °C. When grown on Si/SiO₂ substrates, the platelets primarily assume a rhomboidal shape and have thicknesses ranging from several to tens of nm. The morphology of MoO₂ crystals was found to depend on the chemical nature of substrates. MoO₂ platelets on Si/SiO₂ were characterized by a number of microscopic and spectroscopic techniques, and the electrical measurements revealed the metallic nature of their conductivity averaging at 2400 ± 1000 S cm^-1. Raman spectroscopy of MoO₂ platelets on graphene indicates their strong hole injection property. Small thickness, planar morphology, high chemical stability and metallic conductivity of ultrathin MoO₂ platelets make them potentially interesting for integration different other two-dimensional materials in a variety of electronic structures and devices.

Ferroelectric resistive switching behavior in two-dimensional materials/BiFeO3 hetero-junctions

Integrating two-dimensional (2D) materials with ferroelectric thin films may result in unique characteristics and novel applications due to the coupling between their intrinsic characters. Here, we observed the ferroelectric resistive switching behavior in both graphene/BFO and MoS2/BFO heterojunctions, which stems from the modulation of contact barriers and depletion width at the hetero-interface induced by the ferroelectric polarization. Besides, the ferroelectric resistive switching behavior in both graphene/BFO and MoS2/BFO depends on the thicknesses of the corresponding 2D materials, because the thickness-dependent work function or conductivity of 2D materials could change the contact barrier heights and widths at the interface of 2D materials and ferroelectrics. Our results will widen the memristive applications of 2D/ferroelectrics hetero-junctions and provide a pathway for the novel memory devices based on hetero-structures with 2D/3D materials in the future.

Defect-Assisted Tunneling Electroresistance in Ferroelectric Tunnel Junctions

Recent experimental results have demonstrated ferroelectricity in thin films of SrTiO_{3} induced by antisite Ti_{Sr} defects. This opens a possibility to use SrTiO_{3} as a barrier layer in ferroelectric tunnel junctions (FTJs)-emerging electronic devices promising for applications in nanoelectronics. Here using density functional theory combined with quantum-transport calculations applied to a prototypical Pt/SrTiO_{3}/Pt FTJ, we demonstrate that the localized in-gap energy states produced by the antisite Ti_{Sr} defects are responsible for the enhanced electron tunneling conductance which can be controlled by ferroelectric polarization. Our tight-binding modeling, which takes into account multiple defects, shows that the predicted defect-assisted tunneling electroresistance effect is greatly amplified when the defect energy levels are brought to the Fermi energy by one of the polarization states. Our results have implications for FTJs based on conventional ferroelectric barriers with defects and can be employed for the design of new types of FTJs with enhanced performance.

Water on Graphene-Coated TiO2: Role of Atomic Vacancies

Beyond two-dimensional (2D) materials, interfaces between 2D materials and underlying supports or 2D-coated metal or metal oxide nanoparticles exhibit excellent properties and promising applications. The hybrid interface between graphene and anatase TiO₂ shows great importance in photocatalytic, catalytic, and nanomedical applications due to the excellent and complementary properties of the two materials. Water, as a ubiquitous and essential element in practical conditions and in the human body, plays a significant role in the applications of graphene/TiO₂ composites for both electronic devices and nanomedicine. Carbon vacancies, as common defects in chemically prepared graphene, also need to be considered for the application of graphene-based materials. Therefore, the behavior of water on top and at the interface of defective graphene on anatase TiO₂ surface was systematically investigated by dispersion-corrected hybrid density functional calculations. The presence of the substrate only slightly enhances the on-top adsorption and reduces the on-top dissociation of water on defective graphene. However, at the interface, dissociated water is largely preferred compared with undissociated water on bare TiO₂ surface, showing a prominent cover effect. Reduced TiO₂ may further induce oxygen diffusion into the bulk. Our results are helpful to understand how the presence of water in the surrounding environment affects structural and electronic properties of the graphene/TiO₂ interface and thus its application in photocatalysis, electronic devices, and nanomedicine.

Time-dependent transport characteristics of graphene tuned by ferroelectric polarization and interface charge trapping

Graphene-based field effect transistors (FETs) were fabricated by employing ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) as a gate insulator. The co-existing effects of ferroelectric gating and interface charge trapping on the transport properties of graphene were investigated with respect to the FET structure. The sheet resistance (R_s) of graphene shows a slight decay under a small applied voltage, which is much less than the coercive voltage of the ferroelectric PMN-PT, suggesting non-negligible charge trapping effects. Moreover, when the applied voltage is increased up to a value larger than the coercive voltage, R_s exhibits three states: an initial rapid change, followed by a slow nearly exponential evolution, and finally a saturated state either during the applied voltage is retained or after it is released. In particular, a high-resistance state is finally reached due to the ferroelectric gating, implying that ferroelectric effects dominate this process. The underlying physical mechanism was fully investigated to effectively address the observed evolution of time-dependent R_s. Such a finding provides us an opportunity to understand the co-existing effects of ferroelectric gating and charge trapping and tune the transport properties of graphene through the interface effects.

Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene

Quantum Hall conductance in monolayer graphene on an epitaxial SrTiO₃ (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate-voltage sweep range is gradually increased in the device, systematic generation and annihilation of oxygen vacancies, evidenced from the hysteretic conductance behavior in the graphene, are observed. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and noninvasive probe for understanding the electronic and ionic phenomena in complex transition-metal oxides without impairing the oxide layer underneath.

Effect of Extrinsically Introduced Passive Interface Layer on the Performance of Ferroelectric Tunnel Junctions

We report the effect of the top electrode/functional layer interface on the performance of ferroelectric tunnel junctions. Ex situ and in situ fabrication process were used to fabricate the top Pt electrode. With the ex situ fabrication process, one passive layer at the top interface would be induced. Our experimental results show that the passive interface layer of the ex situ devices increases the coercive voltage of the functional BaTiO₃ layer and decreases the tunneling current magnitude. However, the ex situ tunneling devices possess more than 1000 times larger ON/OFF ratios than that of the in situ devices with the same size of top electrode.

Polarization-Mediated Modulation of Electronic and Transport Properties of Hybrid MoS2-BaTiO3-SrRuO3 Tunnel Junctions

Hybrid structures composed of ferroelectric thin films and functional two-dimensional (2D) materials may exhibit unique characteristics and reveal new phenomena due to the cross-interface coupling between their intrinsic properties. In this report, we demonstrate a symbiotic interplay between spontaneous polarization of the ultrathin BaTiO₃ ferroelectric film and conductivity of the adjacent molybdenum disulfide (MoS₂) layer, a 2D narrow-bandgap semiconductor. Polarization-induced modulation of the electronic properties of MoS₂ results in a giant tunneling electroresistance effect in the hybrid MoS₂-BaTiO₃-SrRuO₃ ferroelectric tunnel junctions (FTJs) with an OFF-to-ON resistance ratio as high as 10⁴, a 50-fold increase in comparison with the same type of FTJs with metal electrodes. The effect stems from the reversible accumulation-depletion of the majority carriers in the MoS₂ electrode in response to ferroelectric switching, which alters the barrier at the MoS₂-BaTiO₃ interface. Continuous tunability of resistive states realized via stable sequential domain structures in BaTiO₃ adds memristive functionality to the hybrid FTJs. The use of narrow band 2D semiconductors in conjunction with ferroelectric films provides a novel pathway for development of the electronic devices with enhanced performance.

Ferroelectric-Driven Performance Enhancement of Graphene Field-Effect Transistors Based on Vertical Tunneling Heterostructures

A vertical graphene heterostructure field-effect transistor (VGHFET) using an ultrathin ferroelectric film as a tunnel barrier is developed. The heterostructure is capable of providing new degrees of tunability and functionality via coupling between the ferroelectricity and the tunnel current of the VGHFET, which results in a high-performance device. The results pave the way for developing novel atomic-scale 2D heterostructures and devices.

Nanodomain Engineering in Ferroelectric Capacitors with Graphene Electrodes

Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.

Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3 Composite Barriers

Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for nonvolatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs) and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities.

Catalysis under Cover: Enhanced Reactivity at the Interface between (Doped) Graphene and Anatase TiO2

The "catalysis under cover" involves chemical processes which take place in the confined zone between a 2D material, such as graphene, h-BN, or MoS2, and the surface of an underlying support, such as a metal or a semiconducting oxide. The hybrid interface between graphene and anatase TiO2 is extremely important for photocatalytic and catalytic applications because of the excellent and complementary properties of the two materials. We investigate and discuss the reactivity of O2 and H2O on top and at the interface of this hybrid system by means of a wide set of dispersion-corrected hybrid density functional calculations. Both pure and boron- or nitrogen-doped graphene are interfaced with the most stable (101) anatase surface of TiO2 in order to improve the chemical activity of the C-layer. Especially in the case of boron, an enhanced reactivity toward O2 dissociation is observed as a result of both the contribution of the dopant and of the confinement effect in the bidimensional area between the two surfaces. Extremely stable dissociation products are observed where the boron atom bridges the two systems by forming very stable B-O covalent bonds. Interestingly, the B defect in graphene could also act as the transfer channel of oxygen atoms from the top side across the C atomic layer into the G/TiO2 interface. On the contrary, the same conditions are not found to favor water dissociation, proving that the "catalysis under cover" is not a general effect, but rather highly depends on the interfacing material properties, on the presence of defects and impurities and on the specific reaction involved.

Encoding, training and retrieval in ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) are quantum nanostructures that have great potential in the hardware basis for future neuromorphic applications. Among recently proposed possibilities, the artificial cognition has high hopes, where encoding, training, memory solidification and retrieval constitute a whole chain that is inseparable. However, it is yet envisioned but experimentally unconfirmed. The poor retention or short-term store of tunneling electroresistance, in particular the intermediate states, is still a key challenge in FTJs. Here we report the encoding, training and retrieval in BaTiO₃ FTJs, emulating the key features of information processing in terms of cognitive neuroscience. This is implemented and exemplified through processing characters. Using training inputs that are validated by the evolution of both barrier profile and domain configuration, accurate recalling of encoded characters in the retrieval stage is demonstrated.

Emerging ferroelectric transistors with nanoscale channel materials: the possibilities, the limitations

Combining the nonvolatile, locally switchable polarization field of a ferroelectric thin film with a nanoscale electronic material in a field effect transistor structure offers the opportunity to examine and control a rich variety of mesoscopic phenomena and interface coupling. It is also possible to introduce new phases and functionalities into these hybrid systems through rational design. This paper reviews two rapidly progressing branches in the field of ferroelectric transistors, which employ two distinct classes of nanoscale electronic materials as the conducting channel, the two-dimensional (2D) electron gas graphene and the strongly correlated transition metal oxide thin films. The topics covered include the basic device physics, novel phenomena emerging in the hybrid systems, critical mechanisms that control the magnitude and stability of the field effect modulation and the mobility of the channel material, potential device applications, and the performance limitations of these devices due to the complex interface interactions and challenges in achieving controlled materials properties. Possible future directions for this field are also outlined, including local ferroelectric gate control via nanoscale domain patterning and incorporating other emergent materials in this device concept, such as the simple binary ferroelectrics, layered 2D transition metal dichalcogenides, and the 4d and 5d heavy metal compounds with strong spin-orbit coupling.

Voltage Scaling of Graphene Device on SrTiO3 Epitaxial Thin Film

Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to promote gate-voltage scaling using a high-k dielectric material. The atomically flat surface of thin STO layer epitaxially grown on Nb-doped STO single-crystal substrate offers good adhesion between the high-k film and graphene, resulting in nonhysteretic conductance as a function of gate voltage at all temperatures down to 2 K. The two-terminal conductance quantization under magnetic fields corresponding to quantum Hall states survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. Our experimental data prove that the operating voltage reduction can be successfully realized due to the underlying high-k STO thin film, without any noticeable degradation of graphene device performance.

Optically controlled electroresistance and electrically controlled photovoltage in ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) have recently attracted considerable interest as a promising candidate for applications in the next-generation non-volatile memory technology. In this work, using an ultrathin (3 nm) ferroelectric Sm_0.1Bi_0.9FeO₃ layer as the tunnelling barrier and a semiconducting Nb-doped SrTiO₃ single crystal as the bottom electrode, we achieve a tunnelling electroresistance as large as 10⁵. Furthermore, the FTJ memory states could be modulated by light illumination, which is accompanied by a hysteretic photovoltaic effect. These complimentary effects are attributed to the bias- and light-induced modulation of the tunnel barrier, both in height and width, at the semiconductor/ferroelectric interface. Overall, the highly tunable tunnelling electroresistance and the correlated photovoltaic functionalities provide a new route for producing and non-destructively sensing multiple non-volatile electronic states in such FTJs.

Tunnelling electroresistance is the variation of resistance of a thin-film junction with the polarization state of its ferroelectric tunnel barrier. Here the authors demonstrate a large light-modulated tunnelling electroresistance and a hysteretic photovoltaic effect in a complex oxide heterostructure.

High-Tc Layered Ferrielectric Crystals by Coherent Spinodal Decomposition

Research in the rapidly developing field of 2D electronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as nonlinear dielectrics are needed to enable realistic device architectures. Candidate materials require tunable dielectric properties and pathways for heterostructure assembly. Here we report on a family of cation-deficient transition metal thiophosphates whose unique chemistry makes them a viable prospect for these applications. In these materials, naturally occurring ferrielectric heterostructures composed of centrosymmetric In4/3P2S6 and ferrielectrically active CuInP2S6 are realized by controllable chemical phase separation in van der Waals bonded single crystals. CuInP2S6 by itself is a layered ferrielectric with a ferrielectric transition temperature (Tc) just over room temperature, which rapidly decreases with homogeneous doping. Surprisingly, in our composite materials, the ferrielectric Tc of the polar CuInP2S6 phase increases. This effect is enabled by unique spinodal decomposition that retains the overall van der Waals layered morphology of the crystal, but chemically separates CuInP2S6 and In4/3P2S6 within each layer. The average spatial periodicity of the distinct chemical phases can be finely controlled by altering the composition and/or synthesis conditions. One intriguing prospect for such layered spinodal alloys is large volume synthesis of 2D in-plane heterostructures with periodically alternating polar and nonpolar phases.

Ferroelectric Single-Crystal Gated Graphene/Hexagonal-BN/Ferroelectric Field-Effect Transistor

The effect of a ferroelectric polarization field on the charge transport in a two-dimensional (2D) material was examined using a graphene monolayer on a hexagonal boron nitride (hBN) field-effect transistor (FET) fabricated using a ferroelectric single-crystal substrate, (1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] (PMN-PT). In this configuration, the intrinsic properties of graphene were preserved with the use of an hBN flake, and the influence of the polarization field from PMN-PT could be distinguished. During a wide-range gate-voltage (VG) sweep, a sharp inversion of the spontaneous polarization affected the graphene channel conductance asymmetrically as well as an antihysteretic behavior. Additionally, a transition from antihysteresis to normal ferroelectric hysteresis occurred, depending on the V(G) sweep range relative to the ferroelectric coercive field. We developed a model to interpret the complex coupling among antihysteresis, current saturation, and sudden conductance variation in relation with the ferroelectric switching and the polarization-assisted charge trapping, which can be generalized to explain the combination of 2D structured materials with ferroelectrics.

Domain control of carrier density at a semiconductor-ferroelectric interface

Control of charge carrier distribution in a gated channel via a dielectric layer is currently the state of the art in the design of integrated circuits such as field effect transistors. Replacing linear dielectrics with ferroelectrics would ultimately lead to more energy efficient devices as well as the added advantage of the memory function of the gate. Here, we report that the channel-off/channel-on states in a metal/ferroelectric/semiconductor stack are actually transitions from a multi domain state to a single domain state of the ferroelectric under bias. In our approach, there is no a priori assumption on the single or multi-domain nature of the ferroelectric layer that is often neglected in works discussing the ferroelectric-gate effect on channel conductivity interfacing a ferroelectric. We also predict that semiconductor/ferroelectric/semiconductor stacks can function at even lower gate voltages than metal/ferroelectric/semiconductor stacks when an n-type semiconductor is placed between the ferroelectric and the gate metal. Our results suggest the ultimate stability of the multidomain state whenever it interfaces a semiconductor electrode and that a switchable single domain state may not be necessary to achieve effective control of conductivity in a p-type channel. Finally, we discuss some experimental results in the literature in light of our findings.

Optoelectrical Molybdenum Disulfide (MoS2)--Ferroelectric Memories

In this study, we fabricated and tested electronic and memory properties of field-effect transistors (FETs) based on monolayer or few-layer molybdenum disulfide (MoS2) on a lead zirconium titanate (Pb(Zr,Ti)O3, PZT) substrate that was used as a gate dielectric. MoS2-PZT FETs exhibit a large hysteresis of electronic transport with high ON/OFF ratios. We demonstrate that the interplay of polarization and interfacial phenomena strongly affects the electronic behavior and memory characteristics of MoS2-PZT FETs. We further demonstrate that MoS2-PZT memories have a number of advantages and unique features compared to their graphene-based counterparts as well as commercial ferroelectric random-access memories (FeRAMs), such as nondestructive data readout, low operation voltage, wide memory window and the possibility to write and erase them both electrically and optically. This dual optoelectrical operation of these memories can simplify the device architecture and offer additional practical functionalities, such as an instant optical erase of large data arrays that is unavailable for many conventional memories.

Effect of epitaxial strain on tunneling electroresistance in ferroelectric tunnel junctions

We report the effect of compressive strain on the tunneling electroresistance (TER) effect in BaTiO3/SrRuO3 (BTO/SRO) heterostructures. We find that epitaxial strain imposed by the mismatch of NdGaO3 and SrTiO3 lattice parameters with the BTO and SRO layers improves ferroelectric polarization of BTO and concurrently promotes the metallicity of the SRO films. While the enhanced polarization is beneficial for the TER magnitude, the reduced asymmetry in the tunneling barrier due to the shortened screening length of SRO is detrimental for the effect. Thus, a combined effect of strain on the polarization of the ferroelectric barrier and the screening properties of the electrodes needs to be taken into account when considering and predicting the TER effect in ferroelectric tunnel junctions.