Enhanced Tunneling Electroresistance Research Articles

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

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.

Giant tunnel resistance effect in (SrTiO3)2/(BaTiO3)4/(CaTiO3)2 asymmetric superlattice with enhanced polarization.

In this work, we report the effectively enhanced tunneling electroresistance effect in Au/(SrTiO3)2/(BaTiO3)4/(CaTiO3)2/Nb:SrTiO3 superlattice ferroelectric tunnel junction (FTJ). The stable polarization switching and enhanced ferroelectricity were achieved in the nanoscale thickness high-quality epitaxial superlattice. A high ON/OFF current ratio of more than 105 was obtained at room temperature, which is an order of magnitude larger than the BaTiO3 FTJ with the same structure. Nonvolatile resistance switching controlled by nonvolatile polarization switching was observed in the superlattice FTJ. Driven by increased polarization and intrinsic asymmetric ferroelectricity, a highly asymmetric depolarization field is generated compared with the Au/BaTiO3/Nb:SrTiO3 FTJ, resulting in an enhanced tunneling electroresistance effect. These results provide a potential way to construct FTJ memory devices by constructing asymmetric three-component ferroelectric superlattices.

Enhanced tunneling electroresistance through interfacial charge-modulated barrier in α-In2Se3-based ferroelectric tunnel junction

The manipulation of tunneling resistance is critical for ferroelectric tunnel junction (FTJ) devices. In this work, we propose a mechanism to manipulate tunneling resistance through interfacial charge-modulated barrier in two-dimensional (2D) n-type semiconductor/ferroelectric FTJs. Driven by ferroelectric reversal, different effective tunneling barriers are realized by the depletion or accumulation of electrons near the n-type semiconductor surface in such devices. Thus, the tunneling resistance in FTJs undergoes significant changes for different polarization orientations, resulting in a giant tunneling electroresistance (TER) effect. To illustrate this idea, we construct 2D FTJs based on n-InSe/α-In2Se3 van der Waals (vdW) heterostructures. Based on the electronic transport calculations, it is found that TER ratio can reach 4.20 × 103% in the designed FTJs. The physical origin of the giant TER effect is verified through analysis of the effective potential energy of the n-InSe/α-In2Se3 vdW heterostructures and the real-space transmission eigenstates of the designed FTJs. This work contributes to the knowledge of carrier tunneling mechanisms at the interface of semiconductor/In2Se3 vdW heterostructures, and providing a significant insight into the TER effect of this FTJ systems, also presenting an alternative approach for the design of FTJ-based devices.

Greatly enhanced tunneling electroresistance in ferroelectric tunnel junctions with a double barrier design

We propose that the double barrier effect is expected to enhance the tunneling electroresistance (TER) in the ferroelectric tunnel junctions (FTJs). To demonstrate the feasibility of this mechanism, we design a model structure of Pt/BaTiO3/LaAlO3/Pt/BaTiO3/LaAlO3/Pt double barrier ferroelectric tunnel junction (DB-FTJ), which can be considered as two identical Pt/BaTiO3/LaAlO3/Pt single barrier ferroelectric tunnel junctions (SB-FTJs) connected in series. Based on density functional calculation, we obtain the giant TER ratio of 2.210 × 108% in the DB-FTJ, which is at least three orders of magnitude larger than that of the SB-FTJs of Pt/BaTiO3/LaAlO3/Pt, together with an ultra-low resistance area product (0.093 KΩμm2) in the high conductance state of the DB-FTJ. Moreover, it is possible to control the direction of polarization of the two single ferroelectric barriers separately and thus four resistance states can be achieved, making DB-FTJs promising as multi-state memory devices.

Enhanced tunneling electroresistance in multiferroic tunnel junctions through the barrier insulating-metallic transition: A first-principles study

The tunneling electroresistance (TER) ratio is an important device merit of ferroelectric tunnel junction (FTJ) and multiferroic tunnel junction (MFTJ) devices. Here, through first-principles calculations, we propose an efficient way to achieve a sizable TER effect through the interface engineering in both SrRuO3/PbTiO3/FeO/Fe and SrRuO3/PbTiO3/CoO/Co MFTJs. It is found that the interfacial FeO or CoO layer can significantly modify the band alignment between PbTiO3 barrier and electrodes through its large depolarization field, causing the insulating–metallic transition of PbTiO3 barrier upon polarization reversal. As a result, the tunneling resistance changes significantly, leading to a giant TER effect of 105%. Our results suggest a practical way to enhance the TER effect in MFTJs.

Enhanced tunneling electroresistance effect in Pt/BiAlO3/Pt ferroelectric tunnel junctions by a graphene interlayer

Realizing a large tunneling electroresistance (TER) effect with a low ON-state resistance-area product (RAP) is essential for the applications of ferroelectric tunnel junctions (FTJs) in information storage. Herein, taking Pt/BiAlO3/Pt (Pt/BAO/Pt) FTJs as examples, we theoretically demonstrate that a giant TER ratio of about 1458%, which is comparable to most van der Waals tunnel junctions, can be achieved by switching the electric polarization of ferroelectric BAO(0001) barrier due to the reversible metallization of ferroelectric in conjunction with the Schottky barrier. More interestingly, with the effective modulation on barrier height and width via polarization reversal, an enhanced TER effect close to 104% can be obtained by inserting a graphene monolayer into right (or left) Pt/BAO interface of the FTJs and the writing endurance of the FTJ memories is expected to be significantly improved. Furthermore, the calculated ON-state RAPs in all three FTJ devices are not more than 100 Ω μm2. These fascinating findings suggest that the proposed BAO-based FTJs hold great potential for designing next-generation nanoscale high-density and low-power nonvolatile memories with excellent performance.

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.

Ultra-thin Hf0.5Zr0.5O2 thin-film-based ferroelectric tunnel junction via stress induced crystallization

We report on 4.5-nm-thick Hf0.5Zr0.5O2 (HZO) thin-film-based ferroelectric tunnel junctions (FTJs) with a tungsten (W) bottom electrode. The HZO on the W electrode exhibits stable ferroelectricity with a remanent polarization of 14 μC/cm2, an enhanced tunneling electroresistance of 16, and excellent synaptic properties. We found that a large tensile stress was induced on a HZO thin film, owing to a low thermal expansion coefficient of the W bottom electrode. The low thermal expansion coefficient results in the effective formation of an orthorhombic phase, even in an ultra-thin HZO film. This was verified by a comparative study of the electrical characteristics, grazing-angle incidence x-ray diffraction, and residual stress measurement of the HZO film on various bottom electrodes with different thermal expansion coefficients. In addition, this study demonstrates the suitable functions of the FTJ for electronic synapses, such as analog-like resistance transition under various pulse schemes. The fabricated stress-engineered FTJ exhibits an appropriate conductance ratio, linearly modulated long-term potentiation and depression characteristics, and excellent reliability. These characteristics render FTJs ideal electronic devices for neuromorphic computing systems.

A perspective on electrode engineering in ultrathin ferroelectric heterostructures for enhanced tunneling electroresistance

The combination of ferroelectricity and quantum tunneling enables the tantalizing possibility of next-generation nonvolatile memories based on ferroelectric tunnel junctions (FTJs). In the last two decades, significant progress has been achieved in the understanding of FTJs in terms of the role of the critical thickness for ferroelectricity, interface-related factors that yield an enhanced tunneling electroresistance effect, as well exploiting the combination of magnetism and ferroelectricity to realize multiferroic or magnetoelectric tunnel junctions. One key ingredient in the successful design of FTJs is the type and nature of the electrode used—indeed device performance strongly hinges on the ability to precisely tune and modulate the electrostatic boundary conditions. This perspective presents an overview of the experimental state of the art in electrode engineering for FTJs. We discuss related governing factors and methods for various electrode-FTJ combinations, highlighting and comparing the advantages and weaknesses for each system. Finally, we also reveal the challenges and identify the opportunities for the future development of FTJs. In summary, we aim to provide significant insights into electrode engineering of high-quality FTJs with excellent tunneling electroresistance performance.

Tunneling through a dielectric/ferromagnetic/ferroelectric three-step-like composite barrier

Electron transport in ferroelectric tunnel junctions (FTJs) with a three-step-like barrier combining a dielectric, ferromagnetic, and ferroelectric was investigated theoretically. A significant enhanced tunneling electroresistance effect was observed as compared to traditional FTJs with a bilayer composite barrier. The spin polarization also could be enhanced in a certain orientation of ferroelectric polarization. A metal/dielectric/ferromagnetic/ferroelectric/metal FTJ with a large ferromagnetic thickness is preferred for the optimal performance. The ferroelectricity in the ferroelectric layer has a weak modulation effect on the spin polarization. This work provides a way to enhance the performance of FTJs and/or to control the spin-polarized electronic transport by structure engineering.

Unusual Hole and Electron Midgap States and Orbital Reconstructions Induced Huge Ferroelectric Tunneling Electroresistance in BaTiO3/SrTiO3.

Oxide heterostructures have attracted a lot of interest because of their rich exotic phenomena and potential applications. Recently, a greatly enhanced tunneling electroresistance (TER) of ferroelectric tunnel junctions (FTJs) has been realized in such heterostructures. However, our understanding on the electronic structure of resistance response with polarization reversal and the origin of huge TER is still lacking. Here, we report on electronic structures, particularly at the interface and surface, and the control of the spontaneous polarization of BaTiO3 films by changing the termination of a SrTiO3 substrate. Interestingly, unusual electron and hole midgap states are concurrently formed and accompanied by orbital reconstructions, which determine the ferroelectric polarization orientation in the BaTiO3/SrTiO3. Such unusual midgap states, which yield a strong electronic screening effect, reduce the ferroelectric barrier width and height, and pin the ferroelectric polarization, lead to a dramatic enhancement of the TER effect. The midgap states are also observed in BaTiO3 films on electron-doped Nb/SrTiO3 revealing its universality. Our result provides new insight into the origin of the huge TER effect and opens a new route for designing ferroelectric tunnel junction-based devices with huge TER through interface engineering.

Enhanced tunneling electroresistance effect in composite ferroelectric tunnel junctions with asymmetric electrodes

Abstract

Enhanced tunneling electroresistance effects in HfZrO-based ferroelectric tunnel junctions by high-pressure nitrogen annealing

Greatly improved ferroelectricity with an excellent remanent polarization of 20 μC/cm2 and enhanced tunneling electroresistance (TER) were achieved with TiN/HfZrO(HZO)/p-type Ge ferroelectric tunnel junctions (FTJs) annealed at a high pressure of 200 atmosphere (atm.). We found that the enhanced ferroelectric characteristics can be ascribed to the effective formation of an orthorhombic phase at high pressures. This was verified by the combined study of grazing angle incidence X-ray diffraction, transmission electron microscopy, and hysteresis polarization curve analyses. In addition, using pulse switching measurements, we quantitatively evaluated the interfacial paraelectric capacitance (Ci) of HZO FTJs according to the annealing temperature. HZO films annealed at 550 °C and 200 atm. exhibited an excellent TER effect ratio of 20 due to the extra paraelectric layer between the ferroelectric layer and the bottom electrode and a relatively high remanent polarization.

Enhanced tunneling electroresistance in multiferroic tunnel junctions due to the reversible modulation of orbitals overlap

We report a first-principles study of the ferroelectricity and spin-dependent transport through Co/BaTiO3/CoO/Co multiferroic tunnel junctions (MFTJs). We find the coexistence of large tunneling magnetoresistance (TMR) ratio and large tunneling electroresistance (TER) ratio in the MFTJs. The large TMR effect originates from the spin-filter tunneling through the BaTiO3 barrier, while the TER effect is due to the modulation of orbitals overlap by polarization reversal. The microscopic physics of TER are identified and understood through the analysis of metal-oxygen relative displacements, local polarization magnitude, transmission in momentum space and real space scattering states. Our results provide a practical way to achieve the coexistence of large TER and TMR effects in MFTJs.

Enhanced tunneling electroresistance in Pt/PZT/LSMO ferroelectric tunnel junctions in presence of magnetic field

ABSTRACTWe report an enhancement of tunneling electroresistance (TER) in the presence of magnetic field for 7-nm PbZr0.52Ti0.48O3 (PZT) ferroelectric tunnel barriers sandwiched between platinum metal (Pt) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) layers. The significant variation in TER was observed when Pt/PZT/LSMO junction was exposed to magnetic field and its values at zero bias changed from 57 (at 0 G) to 110 under 10 kG of magnetic field. We attributed this enhancement to change in resistance near the PZT/LSMO interface under magnetic field. Ferroelectric polarization reversal and application of magnetic field changed lattice strain, chemical bonding and charge modulation near PZT/LSMO interface which in turn affects the charge carrier density and transmission probability. Our results suggest the possibility to manipulate TER by magnetic field in ferroelectric tunnel junctions. These results should lead to performance improvement in the devices based on nano-scale ferroelectrics in quantum nanostructures, with application to low energy memories and logic devices.

Enhanced Tunneling Electroresistance in Ferroelectric Tunnel Junctions due to the Reversible Metallization of the Barrier.

Realizing a large tunneling electroresistance (TER) effect is crucial for device application of ferroelectric tunnel junctions (FTJs). FTJs are typically composed of a thin ferroelectric layer sandwiched by two metallic electrodes, where TER generally results from the dependence of the effective tunneling barrier height on the ferroelectric polarization. Since the resistance depends exponentially not only on barrier height but also on barrier width, TER is expected to be greatly enhanced when one of the electrodes is a semiconductor where the depletion region near the interface can be controlled via ferroelectric polarization. To explore this possibility, we perform studies of SrRuO_{3}/BaTiO_{3}/n-SrTiO_{3} FTJs, where n-SrTiO_{3} is an electron doped SrTiO_{3} electrode, using first-principles density functional theory. Our studies reveal that, in addition to modulation of the depletion region in n-SrTiO_{3}, the BaTiO_{3} barrier layer becomes conducting near the interface for polarization pointing into n-SrTiO_{3}, leading to dramatic enhancement of TER. The effect is controlled by the band alignment between the semiconductor and the ferroelectric insulator and opens the way for experimental realization of enhanced TER in FTJs through the choice of a semiconducting electrode and interface engineering.

Ultrathin BaTiO3-Based Ferroelectric Tunnel Junctions through Interface Engineering

The ability to change states using voltage in ferroelectric tunnel junctions (FTJs) offers a route for lowering the switching energy of memories. Enhanced tunneling electroresistance in FTJ can be achieved by asymmetric electrodes or introducing metal-insulator transition interlayers. However, a fundamental understanding of the role of each interface in a FTJ is lacking and compatibility with integrated circuits has not been explored adequately. Here, we report an incisive study of FTJ performance with varying asymmetry of the electrode/ferroelectric interfaces. Surprisingly high TER (∼400%) can be achieved at BaTiO3 layer thicknesses down to two unit cells (∼0.8 nm). Further our results prove that band offsets at each interface in the FTJs control the TER ratio. It is found that the off state resistance (R(Off)) increases much more rapidly with the number of interfaces compared to the on state resistance (ROn). These results are promising for future low energy memories.

Temperature-dependent tunneling electroresistance in Pt/BaTiO3/SrRuO3 ferroelectric tunnel junctions

Tunneling electroresistance of Pt/BaTiO3/SrRuO3 ferroelectric tunnel junctions is investigated as a function of temperature. Two distinct resistance states that are dependent on polarization direction in the BaTiO3 barrier layer and bipolar resistance switching are observed at various temperatures from 10 to 290 K. The ON/OFF current ratio of Pt/BaTiO3/SrRuO3 tunnel junctions increases monotonically with decreasing temperature above 50 K and saturates below 50 K. The enhanced tunneling electroresistance at low temperatures can be ascribed to the suppression of thermally assisted indirect tunneling, which is less sensitive to the polarization reversal of BaTiO3 compared to the direct tunneling.

Effects of ferroelectric dead layer on the electron transport in ferroelectric tunneling junctions

Ferroelectric dead layer is intrinsic and inevitable at the metal-ferroelectrics interface. In general, it is detrimental to the application of nanoscale devices; however, in ferroelectric tunneling junctions with ferroelectric dead layer, an enhanced tunneling electroresistance (TER) can be achieved when the nonswitchable interface polarization at both metal-ferroelectrics interfaces points to the ferroelectric center. The larger the interface polarization, the stronger the TER. In addition, low dielectric constant of dead layer will favor such enhancement. Our results provide an alternative route for achieving larger TER in ferroelectric junctions.