Tunneling Electroresistance Research Articles

Investigation of ferro-resistive switching mechanisms in TiN/Hf0.5Zr0.5O2/WOx/W ferroelectric tunnel junctions with the interface layer effect

This study presents an in-depth analysis of ferro-resistive switching (FRS) behaviors in a TiN/Hf0.5Zr0.5O2(HZO)/WOx/W ferroelectric tunnel junction (FTJ) device, with a particular focus on the role of the tungsten oxide (WOx) interface layer (IL). Structural examinations confirm the presence of the WOx IL, which significantly influences the FRS properties of the device. Electrical measurements indicate the devices exhibit stable and reproducible FRS characteristics with an ON/OFF ratio of 9.7, predominantly attributed to the tunneling electro-resistance (TER) effect driven by the ferroelectric polarization. Comprehensive numerical simulations, incorporating the nucleation-limited switching model and Simmons tunneling mechanism, provide detailed insights into how the WOx IL and the trapped charges at the HZO/WOx interface affect polarization switching mechanisms and the electronic potential barrier profile. These findings underscore the importance of interface effects in HfO2-based FTJs and advance the understanding of the TER mechanism in multilayer ferroelectric systems.

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.

Ferroelectric tunnel junctions based on a HfO2/dielectric composite barrier

The ferroelectric tunnel junction (FTJ), which possesses a simple structure, low power consumption, high operation speed, and nondestructive reading, has attracted great attention for the application of next-generation nonvolatile memory. The complementary metal–oxide–semiconductor-compatible hafnium oxide (HfO2) ferroelectric thin film found in the recent decade is promising for the scalability and industrialization of FTJs. However, the electric performance, such as the tunneling electroresistance (TER) effect, of the current HfO2-based FTJs is not very satisfactory. In this work, we propose a type of high-performance HfO2-based FTJ by utilizing a ferroelectric/dielectric composite barrier strategy. Using density functional theory calculations, we study the electronic and transport properties of the designed Ni/HfO2/MgS/Ni (001) FTJ and demonstrate that the introduction of an ultra-thin non-polar MgS layer facilitates the ferroelectric control of effective potential barrier thickness and leads to a significant TER effect. The OFF/ON resistance ratio of the designed FTJ is found to exceed 4 × 103 based on the transmission calculation. Such an enhanced performance is driven by the resonant tunneling effect of the ON state, which significantly increases transmission across the FTJ when the ferroelectric polarization of HfO2 is pointing to the non-polar layer due to the aroused electron accumulation at the HfO2/MgS interface. Our results provide significant insight for the understanding and development of the FTJs based on the HfO2 ferroelectric/non-polar composite barrier.

Electronic and Transport Engineering of A-Type Antiferromagnets with Ferroelectric Sandwich Structure: Toward Multistate Nonvolatile Memory Applications.

Achieving higher-order multistates with mutual interstate switching at the nanoscale is essential for high-density storage devices; yet, it remains a significant challenge. Here, we demonstrate that integrating A-type antiferromagnetic semiconductors sandwiched between ferroelectric layers is an effective strategy to achieve high-performance multistate data storage. Taking the Sc2CO2/VSi2P4 bilayer (bi-VSi2P4)/Sc2CO2 van der Waals multiferroic heterostructure as an example, our first-principles calculations show that by switching the polarization direction of the upper and bottom ferroelectric Sc2CO2 layers, antiferromagnetic bi-VSi2P4 can exhibit four distinct states with different band structures. The intriguing band structure engineering stems from the polarization-field-induced band shift and interface charge transfer. Accordingly, the proposed Sc2CO2/bi-VSi2P4/Sc2CO2-based multiferroic device can achieve four different resistance states, accompanied by fully spin-polarized currents and giant tunneling electroresistance ratios. Our results propose a viable strategy for realizing nonvolatile electrical control of antiferromagnets at the nanoscale and provide insights into the development of advanced memories.

Giant Tunneling Electro‐Resistance in Ultrathin Ferroelectric Tunnel Junctions: The Interface Barrier Gain Mechanism

AbstractUltrathin ferroelectric tunnel junctions (FTJs) hold considerable promise for next‐generation, high‐speed, low‐power, and high‐density nonvolatile memory applications. Achieving a substantial tunneling electro‐resistance (TER) remains a challenge as the ferroelectric layer is thinned to nanoscale dimensions, often resulting in a diminished or lost polarization. An innovative interface barrier gain mechanism is introduced, employing interface electronic state modulation to precisely control the size of an additional interface barrier. This strategy lessens the dependency of the tunneling barrier on ferroelectric polarization strength, facilitating a remarkable TER even at ferroelectric thicknesses as minimal as ≈1 nm. The focus is on composite FTJs using In2Se3/MTe2 (M = Mo, W), where the inclusion of an MTe2 monolayer disrupts the asymmetric electrode configuration. The weak ferroelectric polarization reversal of the In2Se3 monolayer effectively modulates the electronic state coupling at the In2Se3/MTe2 interface. This modulation leads to variations in the width and height of the Schottky barrier at the heterojunction‐electrode interface corresponding with the ferroelectric polarization reversal, establishing a beneficial Ohmic contact in the “on” state and resulting in an exponential TER increase up to 5.4 × 106%. This work introduces a universal mechanism to overcome the thickness limitations traditionally associated with enhancing TER, marking a significant advancement in the development of ultrathin ferroelectric nonvolatile devices.

High Tunneling Electroresistance in Ferroelectric Tunnel Junctions Based on 2-D Bilayer Ferroelectric CuInP₂Se₆/Ga₂O₃ van der Waals Heterostructure

High Tunneling Electroresistance in Ferroelectric Tunnel Junctions Based on 2-D Bilayer Ferroelectric CuInP₂Se₆/Ga₂O₃ van der Waals Heterostructure

Theoretical study of metal contacts to the monolayer ferroelectric material CuInP2S6 and its device applications

Two-dimensional (2D) ferroelectric materials exhibit significant potential for applications in nonvolatile memory and device miniaturization. In the device design stage, it is essential to consider the compatibility between 2D ferroelectric materials and three-dimensional (3D) metal. However, the interface between them introduces complex interactions that could impact the device's performance. In this work, based on the first-principles method, we simulate several 3D metal–2D ferroelectric material contact systems by utilizing different 3D metals in contact with the 2D ferroelectric monolayer CuInP2S6 (CIPS). By calculating the electronic structures of the systems, we find that the Cd(001)–CIPS configuration is the most stable structure, followed by the Ag(111)–CIPS and Au(111)–CIPS systems. Both the Cd(001)–CIPS and Ag(111)–CIPS systems undergo a transition from Schottky to Ohmic contact. Finally, we theoretically design a ferroelectric tunnel junction (FTJ) based on the Cd(001)–CIPS contact system, achieving a tunneling electroresistance ratio of 2.394×105% and a remarkably low resistance–area product of 0.78 Ω·μm2, which makes the proposed FTJ superior to the conventional 3D FTJ. This work provides some insights for the design of nonvolatile storage devices.

Volatile memory characteristics of CMOS-compatible HZO ferroelectric layer for reservoir computing

Recently, ferroelectric memory utilizing hafnium oxide has emerged as an attractive option compared to existing memory technologies, primarily due to its scalability and energy-efficient advantages. Among them, hafnium zirconium oxide (HZO) is examined for its short-term memory characteristics to achieve a reservoir computing system known to exhibit remarkable polarization properties, being able to switch between distinct polarization states under the influence of an electric field. These unique properties are of utmost importance in ferroelectric memory applications, where they play a pivotal role in the storage and retrieval of binary data. In this study, we identify and experiment with the electrical characteristics of a ferroelectric tunnel junction (FTJ) device with a metal-ferroelectric-semiconductor (MFS) structure using TiN as the top electrode and HZO as the ferroelectric layer. Moreover, we assess the performance of the device by evaluating its maximum 2Pr (remnant polarization) and tunneling electro resistance (TER) values in different conditions of cell area. Furthermore, we analyze and show short-term memory (STM) characteristics and synaptic properties with 5 cycles of potentiation and depression under conditions of stable dynamic range by coordinating identical and incremental pulses. In the case of incremental pulses (>95 %), the MNIST pattern recognition accuracy is higher than in the case of identical pulses (>94 %). Through a sequence of procedures, the synaptic characteristics of FTJs are confirmed to assess their suitability for use as an artificial synaptic device.

Band Alignment Engineering in 2D Ferroelectric Van der Waals Heterostructures for All‐In‐One Optoelectronic Architecture

Abstract2D van der Waals (vdW) heterostructures consisting of vertically stacking atomically thin semiconductors with different band structures provide a flexible platform to design integrated electronic and optoelectronic devices with multi‐functionalities. However, the realization of device multifunctionality requires the heterostructures with tunable band alignments. Here an efficient strategy is proposed by constructing 2D vdW ferroelectric semiconductor heterostructures composed of atomically thin ferroelectrics and semiconductors to achieve this goal. These calculated results indicate that the local built‐in electric field derived from the ferroelectric polarization can effectively modulate the band alignment of the heterostructures, leading to 36 potential band‐alignment transition pathways. Using SnS/In2Se3 vdW heterostructure as a prototype example, a reversible switching from high‐resistance to low‐resistance state is demonstrated by the band‐alignment transition from type‐II to type‐III driven by ferroelecric polarization switching, consequently leading to giant tunneling electroresistance (TER) ratio as high as 1012%. Moreover, the heterostructure with the momentum‐space matching band structure and in‐plane anisotropy exhibits broadband photoresponse from near‐infrared to ultraviolet regions and excellent polarization sensitivity with the dichroic ratio up to 10.3. The ferroelectric polarization‐dependent conductance state and photoresponse in the heterostructures make them large potential for the realization of all‐in‐one optoelectronic architecture in artificial vision system.

Orientation-dependent tunneling electroresistance in Pt/Pb(Zr,Ti)O3/Nb:SrTiO3 ferroelectric tunnel junctions

Orientation anisotropy is a well-known essential character for polarization characteristics of ferroelectric materials, which has been widely investigated in conventional ferroelectric random access memories. In this work, we study the effects of orientation on the tunneling electroresistance (TER) of ferroelectric tunnel junctions (FTJs). Rhombohedral Pb(Zr[Formula: see text],Ti[Formula: see text])O3 (PZT) that has the polar axis along the [Formula: see text] orientation is adopted as potential barriers and two kinds of FTJs that are composed of (001)- and (111)-oriented PZT barriers and Nb:SrTiO3 (Nb:STO) electrodes, respectively, are fabricated. The (111)-oriented Pt/PZT/Nb:STO FTJ exhibits a giant ON/OFF ratio of [Formula: see text], about 30 times that of the (001)-oriented device, due to the lowered PZT barrier in the ON state and the widen Schottky barrier in the OFF state based on current and capacitance analyses. In addition, compared to the (001)-oriented device, the (111)-oriented FTJ shows a sharper and faster switching between the ON and OFF states according to the nucleation-limited-switching dynamics model, giving rise to good linearity in memristive behaviors for synaptic plasticity and reliable retention and endurance properties for the resistance switching. The improved TER properties are ascribed to larger effective polarizations and 180∘ switching in the (111)-oriented PZT barrier. These results facilitate the design and fabrication of high-performance FTJ devices with the optimization of crystallographic orientation and polarization switching characteristics.

Significantly enhanced tunneling electroresistance in two-dimensional ferroelectric tunnel junctions realized by reversible Schottky-Ohmic contact switching

Significantly enhanced tunneling electroresistance in two-dimensional ferroelectric tunnel junctions realized by reversible Schottky-Ohmic contact switching

Al2O3/SnC heterostructure: Physical properties, regulation effect and device design

Two van der Waals (vdW) heterostructures between Al2O3 and SnC monolayers are constructed by the Generalised Lattice Matching (GLM) method. We calculate the elastic constants of monolayers and heterostructure and find that the heterostructures show larger Young’s modulus (220−234) N/m compared with Al2O3 (SnC), the reasonable Poisson’s ratio is about 0.3 and gravity-induced bending is the order of 104, which is comparable to graphene. The electronic properties, optical absorption coefficient and transmission properties of the heterostructure also have been investigated. Some important discoveries can be obtained: The Al2O3/SnC heterostructure shows an indirect bandgap (Eg) semiconductor with 1.70 eV and type II band arrangement when the ferroelectric polarization of Al2O3 points to SnC. While the polarization direction of Al2O3 is opposite, the Eg is zero. The physical properties are manipulated by some conditions (strain and electric field), leading to transitioning between Type II and I semiconductors, semiconductors and metal can be experienced. Moreover, the heterostructures prefer the higher light adsorption coefficient (105 cm−1) compared to monolayers. The transport properties of the Al2O3/SnC linked by two silicene electrodes show an excellent tunnel electroresistance (TER) effect with 108, moreover, the negative differential resistance (NDR) effect and switching effect also can be observed.

Epitaxial Strain Enhanced Ferroelectric Polarization toward a Giant Tunneling Electroresistance.

A substantial ferroelectric polarization is the key for designing high-performance ferroelectric nonvolatile memories. As a promising candidate system, the BaTiO3/La0.67Sr0.33MnO3 (BTO/LSMO) ferroelectric/ferromagnetic heterostructure has attracted a lot of attention thanks to the merits of high Curie temperature, large spin polarization, and low ferroelectric coercivity. Nevertheless, the BTO/LSMO heterostructure suffers from a moderate FE polarization, primarily due to the quick film-thickness-driven strain relaxation. In response to this challenge, we propose an approach for enhancing the FE properties of BTO films by using a Sr3Al2O6 (SAO) buffering layer to mitigate the interfacial strain relaxation. The continuously tunable strain allows us to illustrate the linear dependence of polarization on epitaxial strain with a large strain-sensitive coefficient of ∼27 μC/cm2 per percent strain. This results in a giant polarization of ∼80 μC/cm2 on the BTO/LSMO interface. Leveraging this large polarization, we achieved a giant tunneling electroresistance (TER) of ∼105 in SAO-buffered Pt/BTO/LSMO ferroelectric tunnel junctions (FTJs). Our research uncovers the fundamental interplay between strain, polarization magnitude, and device performance, such as on/off ratio, thereby advancing the potential of FTJs for next-generation information storage applications.

Nonvolatile memory cells from hafnium zirconium oxide ferroelectric tunnel junctions using Nb and NbN electrodes

Ferroelectric tunnel junctions (FTJs) utilizing hafnium zirconium oxide (HZO) have attracted interest as non-volatile memory for microelectronics due to ease of integration into back-end-of-line (BEOL) complementary metal oxide semiconductor fabrication. This work examines asymmetric electrode NbN/HZO/Nb devices with 7 nm thick HZO as FTJs in a memory structure, with an output resistance that can be controlled by read and write voltages. The individual FTJs are measured to have a tunneling electroresistance of 10 during the read state without significant filament conduction formation and reasonable ferroelectric performance. Endurance and remanent polarizations of up to 105 cycles and 20 μC/cm2, respectively, are measured and are shown to be dependent on the cycling voltage. Electrical measurements demonstrate how magnitude of the write pulse can modulate the high state resistance and the read pulse influences both resistance values as well as separation of resistance states. Then, by using two opposite switching FTJ devices in series, a programmable nonvolatile resistor divider is demonstrated. Measurements of these two FTJ unit memory cells show wide applicability to a BEOL microfabrication process for a re-readable, rewritable, and nonvolatile memory cell.

Performance comparison of planar and cylindrical ferroelectric tunnel junctions

A comprehensive comparison of the electrical characteristics between the planar (P-FTJ) and cylindrical ferroelectric tunnel junction (C-FTJ) is conducted based on physical modeling and simulation. The FTJ architecture consists of metal-ferroelectric–dielectric-metal stacks. Two configurations of C-FTJ are considered depending on whether the position of the ferroelectric (FE) layer is close or away from the inner electrode. The differences between the P-FTJ and C-FTJs in the distributions of the electric field and FE polarization are analyzed. The resultant tunneling electroresistance (TER) is explored as a function of the inner radius, FE thickness, dielectric thickness, and remnant polarization. These simulation results offer physical insights into achieving highly integrated three-dimensional storage structures by improving the TER ratio.

Enhanced tunneling electroresistance through polarization-controlled band alignments in α-In2Se3-based ferroelectric tunnel junctions

The manipulation of tunneling resistance is of paramount importance in the realm of ferroelectric tunnel junction (FTJ) devices. In this work, we propose a novel mechanism that enables the manipulation of tunneling resistance through polarization-controlled band alignments in α-In2Se3-based FTJs. This mechanism exploits the transition between metallic and insulating states, induced by polarization-controlled band alignments, thus resulting in the emergence of a significant tunneling electroresistance (TER) effect. To investigate this intriguing possibility, we have designed two-dimensional (2D) FTJs based on a PtTe2/α-In2Se3 van der Waals heterostructures. By employing first-principles calculations, we demonstrate that the manipulation of band alignments between insulating and metallic states can be achieved via ferroelectric polarization. Leveraging nonequilibrium Green's function methods, we have successfully achieved a remarkably large TER effect with a ratio reaching up to 2.40 × 104. This work unveils substantial potential for the development of nano-field high-density memories and ferroelectric field-effect transistors utilizing 2D layered ferroelectric/semiconductor heterostructures.

High Tunneling Electro-Resistance Due to Resonant-Type Tunneling in a Ferroelectric Tunnel Junction (FTJ) with a Composite Barrier

Recently, ferroelectric tunnel junctions (FTJs) have emerged as potential devices for memory sector wherein the tunneling current is used to sense the direction of polarization with a nondestructive reading operation. The tunneling electro-resistance ratio (TER) is the figure of merit of this device. A composite ferroelectric-dielectric barrier is expected to achieve a high TER with enhanced performance. A theoretical modeling of a FTJ with a composite barrier (CB) consisting of a dielectric (SrTiO3) and ferroelectric (BaTiO3) layers is proposed. The potential energy profile and the transmission characteristics of a typical (20Å) SrRuO3/(20Å) SrTiO3/(20Å) BaTiO3/(12Å) Pt FTJ system were extensively studied, for both OFF and ON states. The tunneling current density and TER were computed with the variation of bias as well as with varying ferroelectric layer width. Under electric bias, an asymmetric triangular well develops at the dielectric-ferroelectric interface in its ON state whose effect in producing a resonant-type tunneling and a high tunneling current density in its ON state is explained.

Tunable multiple nonvolatile resistance states in a MnSe-based van der Waals multiferroic tunnel junction.

Two-dimensional (2D) van der Waals (vdW) multiferroic tunnel junctions (MFTJs) composed of a ferromagnetic metal and a ferroelectric barrier have controllable thickness and clean interface and can realize the coexistence of tunneling magnetoresistance (TMR) and tunneling electroresistance (TER). Therefore, they have enormous potential application in nonvolatile multistate memories. Here, using first principles combined with non-equilibrium Green's function method, we have systematically investigated the spin-dependent transport properties of Fe3GeTe2/MnSe/Fe3GeTe2 vdW MFTJs with various numbers of barrier layers. By controlling the polarization orientation of the ferroelectric barrier MnSe and the magnetization alignment of the ferromagnetic electrodes Fe3GeTe2, the MnSe-based MFTJs exhibit four nonvolatile resistance states, with the TMR (TER) becoming higher and reaching a maximum of 1.4 × 106% (4114%) as the MnSe layers increase from a bilayer to a tetralayer. Using asymmetric Cu and Fe3GeTe2 as the electrodes, the TER can be further improved from 349% to 618%. Moreover, there is a perfect spin filtering effect in these MFTJs. This work demonstrates the potential applications of MnSe-based devices in multistate nonvolatile memories and spin filters, which will stimulate experimental studies on layer-controllable spintronic devices.

Magnetic-ferroelectric synergic control of multilevel conducting states in van der Waals multiferroic tunnel junctions towards in-memory computing.

van der Waals (vdW) multiferroic tunnel junctions (MFTJs) based on two-dimensional materials have gained significant interest due to their potential applications in next-generation data storage and in-memory computing devices. In this study, we construct vdW MFTJs by employing monolayer Mn2Se3 as the spin-filter tunnel barrier, TiTe2 as the electrodes and In2S3 as the tunnel barrier to investigate the spin transport properties based on first-principles quantum transport calculations. It is highlighted that apparent tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects with a maximum TMR ratio of 6237% and TER ratio of 1771% can be realized by using bilayer In2S3 as the tunnel barrier under finite bias. Furthermore, the physical origin of the distinguished TMR and TER effects is unraveled from the k||-resolved transmission spectra and spin-dependent projected local density of states analysis. Interestingly, four distinguishable conductance states reveal the implementation of four-state nonvolatile data storage using one MFTJ unit. More importantly, in-memory logic computing and multilevel data storage can be achieved at the same time by magnetic switching and electrical control, respectively. These results shed light on vdW MFTJs in the applications of in-memory computing as well as multilevel data storage devices.

Nanotube ferroelectric tunnel junctions with an ultrahigh tunneling electroresistance ratio.

Low-dimensional ferroelectric tunnel junctions are appealing for the realization of nanoscale nonvolatile memory devices due to their inherent advantages of device miniaturization. Those based on current mechanisms have limitations, including low tunneling electroresistance (TER) effects and complex heterostructures. Here, we introduce an entirely new TER mechanism to construct a nanotube ferroelectric tunnel junction with ferroelectric nanotubes as the tunneling region. When rolling a ferroelectric monolayer into a nanotube, due to the coexistence of its intrinsic ferroelectric polarization with the flexoelectric polarization induced by bending, a metal-insulator transition occurs depending on the radiative polarization states. For the pristine monolayer, its out-of-plane polarization is tunable by an in-plane electric field, and the conducting states of the ferroelectric nanotube can thus be tuned between metallic and insulating states via axial electric means. Using α-In2Se3 as an example, our first-principles density functional theory calculations and nonequilibrium Green's function formalism confirm the feasibility of the TER mechanism and indicate an ultrahigh TER ratio that exceeds 9.9 × 1010% of the proposed nanotube ferroelectric tunnel junctions. Our findings provide a promising approach based on simple homogeneous structures for high density ferroelectric microelectric devices with excellent ON/OFF performance.