Tunneling Electroresistance Research Articles

Extraordinary tunnel electroresistance in layer-by-layer engineered van der Waals ferroelectric tunnel junctions

<h2>Summary</h2> The ability to engineer potential profiles of multilayered materials is critical for designing high-performance tunneling devices such as ferroelectric tunnel junctions (FTJs). FTJs comprise asymmetric electrodes and a ferroelectric spacer, promising semiconductor-platform-compatible logic and memory devices. However, traditional FTJs consist of metal/oxide/metal multilayered structures with unavoidable defects and interfacial trap states, which often cause compromised tunneling electroresistance (TER). Here, we constructed van der Waals (vdW) FTJs by a layered ferroelectric CuInP₂S₆ (CIPS) and graphene. Owing to the gigantic ferroelectric modulation of the chemical potentials in graphene by as large as ∼1 eV, we demonstrated a giant TER of 10⁹. While inserting just a monolayer MoS₂ between CIPS/graphene, the off state is further suppressed, leading to >10¹⁰ TER. Our discovery opens a new solid-state paradigm where potential profiles can be unprecedentedly engineered in a layer-by-layer fashion, fundamentally strengthening the ability to manipulate electrons' tunneling behaviors and design advanced tunneling devices.

Performance Enhancement and Transient Current Response of Ferroelectric Tunnel Junction: A Theoretical Study

A comprehensive physical model is established to understand the device operation and optimization strategy of the ferroelectric tunnel junction (FTJ). This model is capable of simulating write (switching polarity), read [tunnel electroresistance (TER)], and ac transient operations with a good agreement with experiments. The strategy of optimizing the thickness of the ferroelectric layer and nonpolar interfacial layer is discussed for enlarging TER ratio. We also discussed the possible misinterpretation of the measured TER ratio according to our model.

Gate-tunable giant tunneling electroresistance in van der Waals ferroelectric tunneling junctions

Ferroelectric tunneling junctions (FTJs) have attracted great interest due to their potential applications in non-volatile memories and neurosynaptic computing. In this work, high performance FTJs constructed with graphene and two-dimensional (2D) layered ferroelectric CuInP2S6 (CIPS) with out-of-plane polarization have been demonstrated. These van der Waals (vdW) heterostructure tunneling devices show tunneling electroresistance (TER) up to 107. Furthermore, the FTJs exhibit noticeable gate tunability, for which the on-state tunneling current can increase by 100% by applying a 50 V gate voltage through the conventional 260-nm-thick SiO2 dielectric layer. Our demonstration of gate-tunable, giant tunneling electroresistance highlights its potential in energy-efficient non-volatile memories and computing-in-memory functions.

Multilevel polarization switching in ferroelectric thin films

Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr0.52Ti0.48O3 films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance.

Nonvolatile Ferroelectric Memory with Lateral β/α/β In2Se3 Heterojunctions.

The electric dipole locking effect observed in van der Waals (vdW) ferroelectric α-In2Se3 has resulted in a surge of applied research in electronics with nonvolatile functionality. However, ferroelectric tunnel junctions with advantages of lower power consumption and faster writing/reading operations have not been realized in α-In2Se3. Here, we demonstrate the tunneling electroresistance effect in a lateral β/α/β In2Se3 heterojunction built by local laser irradiation. Switchable in-plane polarizations of the vdW ferroelectric control the tunneling conductance of the heterojunction device by 4000% of magnitude. The electronic logic bit can be represented and stored with different orientations of electric dipoles. This prototype enables a new approach to rewritable nonvolatile memory with in-plane ferroelectricity in vdW 2D materials.

Insights into Electron Transport in a Ferroelectric Tunnel Junction.

The success of a ferroelectric tunnel junction (FTJ) depends on the asymmetry of electron tunneling as given by the tunneling electroresistance (TER) effect. This characteristic is mainly assessed considering three transport mechanisms: direct tunneling, thermionic emission, and Fowler-Nordheim tunneling. Here, by analyzing the effect of temperature on TER, we show that taking into account only these mechanisms may not be enough in order to fully characterize the performance of FTJ devices. We approach the electron tunneling in FTJ with the non-equilibrium Green function (NEGF) method, which is able to overcome the limitations affecting the three mechanisms mentioned above. We bring evidence that the performance of FTJs is also affected by temperature–in a non-trivial way–via resonance (Gamow-Siegert) states, which are present in the electron transmission probability and are usually situated above the barrier. Although the NEGF technique does not provide direct access to the wavefunctions, we show that, for single-band transport, one can find the wavefunction at any given energy and in particular at resonant energies in the system.

Photoenhanced Electroresistance at Dislocation-Mediated Phase Boundary

Ferroelectric tunneling junctions have attracted intensive research interest due to their potential applications in high-density data storage and neural network computing. However, the prerequisite of an ultrathin ferroelectric tunneling barrier makes it a great challenge to simultaneously implement the robust polarization and negligible leakage current in a ferroelectric thin film, both of which are significant for ferroelectric tunneling junctions with reliable operating performance. Here, we observe a large tunneling electroresistance effect of ∼1.0 × 104% across the BiFeO3 nanoisland edge, where the intrinsic ferroelectric polarization of the nanoisland makes a major contribution to tuning the barrier height. This phenomenon is beneficial from the artificially designed tunneling barrier between the nanoscale top electrode and the inclined conducting phase boundary, which is located between the rhombohedral-island and tetragonal-film matrix and arranged with the dislocation array. More significantly, the tunneling electroresistance effect is further improved to ∼1.6 × 104% by the introduction of photoinduced carriers, which are separated by the flexoelectric field arising from the dislocations.

Tunnel Electroresistance in Hf0.5Zr0.5O2-Based Ferroelectric Tunnel Junctions under Hysteresis: Approach of the Point Contact Model and the Linearized Thomas–Fermi Screening

Tunnel Electroresistance in Hf_0.5Zr_0.5O₂-Based Ferroelectric Tunnel Junctions under Hysteresis: Approach of the Point Contact Model and the Linearized Thomas–Fermi Screening

Enhanced tunneling electroresistance effect by designing interfacial ferroelectric polarization in multiferroic tunnel junctions

Due to the long electrical screening length of an insulator, changes in local ferroelectric (FE) polarization can cause significant electrostatic potential drops or rises across the whole insulating region, which can be designed to enhance the tunneling electroresistance (TER) effect. First-principles calculations of ${\mathrm{Co}}_{2}\mathrm{MnSi}/{\mathrm{BaTiO}}_{3}/{\mathrm{SrRuO}}_{3}$ multiferroic tunnel junctions predict a unique local FE polarization at the MnSi-${\mathrm{TiO}}_{2}$ interface, which is giant (tiny) in the right-polarized (left-polarized) state. For the right-polarized state, the large interfacial FE polarization greatly lowers the FE barrier height, making it close to zero. However, for the left-polarized state, because its band edges near the MnSi-${\mathrm{TiO}}_{2}$ interface are usually below the Fermi level, the tiny interfacial FE polarization has a weak effect on the electrostatic potential, maintaining a sizable barrier height. This FE-controlled barrier height produces an optimistic TER ratio of up to $1.1\ifmmode\times\else\texttimes\fi{}{10}^{3}$. Even if the large interface polarization is unexpectedly fixed, the TER ratio remains ultrahigh, increasing the fault tolerance of the interface-enhanced TER effect in applications.

Giant Tunneling Electroresistance Induced by Interfacial Doping in Pt/BaTiO3/Pt Ferroelectric Tunnel Junctions

Ferroelectric tunnel junctions (FTJs) are very promising candidates for nonvolatile memory devices and a large tunneling electroresistance (TER) ratio is essential for their high performance. This work intends to achieve large TER ratio by interfacial doping in FTJs by taking ${\mathrm{Pt}/\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}/\mathrm{Pt}$ tunnel junctions as an example. By introducing Na (or $\mathrm{Li}$) substitutions for $\mathrm{Ti}$ atoms at the right interface, the resultant strong Coulomb repulsion from the negatively charged ${\mathrm{Na}\mathrm{O}}_{2}$ interface pushes the electrons to higher energy in an increasing manner from left to right in the whole ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$ barrier, which leads to rapidly increasing potential energy profile and partial metallization close to the right interface in the left polarization state. However, in the right polarization state, since the right ferroelectric polarization produces a decreasing potential energy profile from left to right, although the ${\mathrm{Na}\mathrm{O}}_{2}$ interface also pushes the electrons to much higher energy and the slope of the potential energy profile changes from negative to positive, the final slope of the potential energy profile is much less steeper and the Fermi level is always inside the band gap, leading to a completely insulating state. The substantially different distributions of the electrostatic potential energy profile in the two polarization states lead to great differences in the transport properties. Based on density-functional-theory calculations, a TER ratio up to ${10}^{5}$% is achieved. The results indicate that a negatively charged interface based on interfacial substitution is a promising method for obtaining a large TER ratio in FTJs, and thus will have implications for the further understanding and design of high-performance FTJs.

Engineering Hf0.5Zr0.5O2 Ferroelectric/Anti- Ferroelectric Phases With Oxygen Vacancy and Interface Energy Achieving High Remanent Polarization and Dielectric Constants

In this study, a decreased oxygen vacancy concentration [V_o] and an increased ZrO₂-HfO₂ interface area were experimentally and theoretically demonstrated to favor the formation of the ferroelectric orthorhombic phase (FE o-phase), whereas an increased [V_o] and a decreased ZrO₂-HfO₂ interface area (by forming alloy) favored the anti-ferroelectric tetragonal phase (AFE t-phase). High remanent polarization (2P<inline-formula> <tex-math notation="LaTeX">$_{r} = 51\,\,\mu \text{C}$ </tex-math></inline-formula>/cm²) was achieved, and therefore, a high tunneling electroresistance (TER) ratio (46) was obtained in metal-ferroelectric-metal (MFM) superlattice structures. Moreover, the optimized AFE phase film exhibited a high dielectric constant 50, as measured by non-hysteretic C-V, by using metal-insulator-metal (MIM) capacitors with the ZrO₂-HfO₂ alloy. The thermal budget used in this study was as low as 450&#x00B0;C.

Integration of resonant band with asymmetry in ferroelectric tunnel junctions

We propose that the asymmetry-induced tunneling electroresistance (TER) effect in a ferroelectric tunnel junction (FTJ) could be improved by integrating a polarization-controlled resonant band. Using first-principles calculations and a quantum-mechanical tunneling model, we studied an asymmetric FTJ SrRuO3/BaTiO3/SrTiO3/SrRuO3. The resonant band is integrated into this FTJ by two atomic layers of BaSnO3 embedded in the barrier. In the elaborated FTJ SrRuO3/BaTiO3/BaSnO3/SrTiO3/SrRuO3, both resonant band and asymmetry work together. For one polarization direction, the BaSnO3 and SrTiO3 dielectric layers work together as barriers to provide considerable efficient barrier height for direct tunneling and lead to large tunneling resistance. For the opposite polarization, the BaSnO3 layer serves as a quantum well to induce resonant tunneling across the barrier and considerably reduces the tunneling resistance of the ON state. The integration of resonant band with asymmetry may provide a more efficient and applicable way to further improve the functionalities of FTJs.

Atomic layer etching of ferroelectric hafnium zirconium oxide thin films enables giant tunneling electroresistance

The ferroelectric properties of hafnium oxide and zirconium oxide based thin films are promising for applications in low power electronics, such as ultra-thin ferroelectric tunneling devices. However, the amount of ferroelectric phase in the film depends on their polycrystalline morphology, which changes with film thickness. Therefore, controlling the film thickness without changing the ferroelectric properties has remained challenging. Here, we propose the use of thermal atomic layer etching to decouple the ferroelectric phase stabilization from the film thickness. First, the ferroelectric phase fraction is maximized by crystallizing the film at an optimized film thickness. Subsequently, the ferroelectric film thickness is reduced to the desired range by atomic layer etching. We demonstrate the feasibility of this approach for a ferroelectric hafnium zirconium oxide film of 10 nm initial thickness, which we integrate into a double-layer ferroelectric tunnel junction. The atomic layer etch rate of ferroelectric hafnium zirconium oxide using HF and dimethylaluminum chloride is found to be ∼0.2 Å/cycle. Although the ferroelectric phase persists after atomic layer etching, the etching increases the surface roughness. For applications in ferroelectric tunnel junctions, we show that atomic layer etching of ferroelectric hafnium zirconium oxide can improve the read current by more than a factor of 200, while at the same time reducing the read voltage by 43%. The resulting tunneling electroresistance of about 2500 is the highest reported so far for polycrystalline hafnium zirconium oxide-based materials.

Si-Doped HfO2-Based Ferroelectric Tunnel Junctions with a Composite Energy Barrier for Non-Volatile Memory Applications.

Ferroelectric tunnel junctions (FTJs) have attracted attention as devices for advanced memory applications owing to their high operating speed, low operating energy, and excellent scalability. In particular, hafnia ferroelectric materials are very promising because of their high remanent polarization (below 10 nm) and high compatibility with complementary metal-oxide-semiconductor (CMOS) processes. In this study, a Si-doped HfO2-based FTJ device with a metal-ferroelectric-insulator-semiconductor (MFIS) structure was proposed to maximize the tunneling electro-resistance (TER) effect. The potential barrier modulation effect under applied varying voltage was analyzed, and the possibility of its application as a non-volatile memory device was presented through stability assessments such as endurance and retention tests.

Giant tunneling magnetoresistance and electroresistance in α−In2Se3 -based van der Waals multiferroic tunnel junctions

In the multiferroic tunnel junction (MFTJ) composed of ferromagnetic and ferroelectric materials, the tunneling electroresistance (TER) coexists with the tunneling magnetoresistance (TMR), making it an ideal platform for designing multifunctional electronic devices. Recently, the rapid development of van der Waals (vdW) materials opened up a new avenue of MFTJ due to their atomic thickness and significance in miniaturizing device sizes. Here by employing the nonequilibrium Green's function combined with density-functional theory, we systemically study the spin-dependent electronic transport properties of ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ (FGT)/bilayer $\ensuremath{\alpha}\ensuremath{-}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ (BIS)/FGT vdW MFTJs. We find that the MFTJ can form multiple nonvolatile resistance states by altering the polarization orientation of the ferroelectric barrier BIS and the magnetization alignment of the two ferromagnetic FGT electrodes, with a maximum TMR (TER) ratio up to $1.1\ifmmode\times\else\texttimes\fi{}{10}^{7}$% (744%). The TER ratio can be further increased to 1868% by using left and right symmetrical copper electrodes. More interestingly, the perfect spin filtering effect can be realized in our MFTJs and the spin current can be controlled by the sign of bias voltages, suggesting a promising route for spin valves that can flexibly manipulate spin currents. Our results demonstrate that giant TMR, large TER, as well as a tunable spin filter can coexist in one system, and that the feasible tunability of such kind of vdW MFTJs is beneficial in designing next-generation logic and memory devices.

Top Electrode Engineering for Freedom in Design and Implementation of Ferroelectric Tunnel Junctions Based on Hf1–xZrxO2

Ferroelectric tunnel junctions (FTJs) based on ultrathin HfO2 have great potential as a fast and energy-efficient memory technology compatible with complementary metal oxide semiconductors. FTJs consist of a ferroelectric film sandwiched between two distinct electrodes, the properties of which are intricately linked to the electrical properties of the FTJs. Here we utilize a W crystallization electrode (CE) to achieve a high and reproducible remanent polarization, combined with a metal replacement process in which the W is carefully removed and replaced by another top electrode (TE). In this way we separate the ferroelectric film properties from the device design and can thereby evaluate the effect of the TE work function (WF) and conduction band electron density (ne) on the tunneling electroresistance (TER) and device reliability. We compare FTJs designed with a TiN bottom electrode and W, Cr, or Ni TE and find that the use of high electron density metals such as Ni or Cr as TE allows for an improved TER, albeit at the cost of reliability due to a large built-in electric field. To bypass this effect, a bilayer Cr/Ni TE is implemented, which allows for a high TER and minimal built-in field, leading to excellent retention and endurance beyond 108 cycles. The results presented here thus highlight a process flow for reliable design and implementation of FTJs.

Enhanced Tunneling Electro-Resistance Ratio for Ferroelectric Tunnel Junctions by Engineering Metal Work Function

The structure of metal electrode/HfZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (HZO)/AlON/n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si was employed as the platform to study how the effective work function (EWF) of the electrode affects the characteristics of the FTJ devices. By introducing TiAl into the conventional TiN electrode, the EWF is reduced which leads to a higher tunnel electroresistance (TER) ratio of 30 due to the improved LRS current arising from the lower barrier height for thermionic emission. In addition, the TiAl-based device also displays high switching speed of 500 ns and robust endurance up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> , showing competitive advantages compared to others. Furthermore, the implementation of long-term plasticity is also demonstrated, implying potential applications for neuromorphic computing. The prominent properties make TiAl-based electrode as a compelling enabler for FTJ devices with even higher performance.

Strain-tunable in-plane ferroelectricity and lateral tunnel junction in monolayer group-IV monochalcogenides

2D ferroelectric materials are promising for designing low-dimensional memory devices. Here, we explore strain-tunable ferroelectric properties of group-IV monochalcogenides MX (M=Ge, Sn; X=S, Se) and their potential application in lateral field tunnel junction devices. We find that these monolayers have in-plane ferroelectricity, with their ferroelectric parameters being on par with other known 2D ferroelectric materials. Among SnSe, SnS, GeSe, and GeS, we find that GeS has the best ferroelectric parameters for device applications, which can be improved further by applying uniaxial tensile strain. We use the calculated ferroelectric properties of these materials to study the tunneling electroresistance (TER) of a 4 nm device based on a lateral ferroelectric tunnel junction. We find a substantial TER ratio of 103–105 in the devices based on these materials, which can be further improved up to a factor of 40 on the application of tensile strain.

Giant magnetoresistance and tunneling electroresistance in multiferroic tunnel junctions with 2D ferroelectrics.

Multiferroic tunneling junctions (MFTJs), composed of two magnetic electrodes separated by an ultrathin ferroelectric (FE) thin film as a barrier, have received great attention in multi-functional devices. Recent theoretical and experimental works have revealed that ferroelectric polarization exists at room temperature in two-dimensional ferroelectric (2D FE) materials within the ultrathin thickness. Here we propose a novel MFTJ Ni/bilayer In2Se3/BN/Ni, in which the resistance of the tunneling spin polarization electrons can be modulated by different magnetization alignments of the electrode and electric polarization direction of the 2D FE In2Se3 layer, leading to multiple tunneling resistance states. The tunneling magnetoresistance (TMR) and electroresistance (TER) of MFTJs are enhanced by the inserted h-BN layer, achieving an ON/OFF TER ratio of 4188% as well as a TMR ratio of 581% with a much lower resistance area. The giant tunneling resistance ratio, multiple resistance states, and ultra-low energy consumption in 2D FE-based MFTJs suggest their great potential in non-destructive non-volatile memories.

Triangular Well Bound State Driven High Tunneling Electro-Resistance in Composite Barrier Ferroelectric Tunnel Junction

Triangular Well Bound State Driven High Tunneling Electro-Resistance in Composite Barrier Ferroelectric Tunnel Junction