Large Electroresistance Research Articles

High electroresistance in all-oxide ferroelectric tunnel junctions enabled by a narrow bandgap Mott insulator electrode

Ferroelectric tunnel junctions (FTJs) based on epitaxial complex oxide heterostructures are promising building blocks for developing low power nanoelectronics and neuromorphic computing. FTJs consisting of correlated oxide electrodes have distinct advantages in size scaling but only yield moderate electroresistance (ER) at room temperature due to the challenge in imposing asymmetric interfacial screening and large modulation of the tunneling potential profile. Here, we achieve large ER in all-oxide FTJs by paring a correlated metal with a narrow bandgap Mott insulator as electrodes. We fabricate epitaxial FTJs composed of 2.8 and 4 nm PbZr0.2Ti0.8O3 tunnel barriers sandwiched between correlated oxides LaNiO3 and Sr3Ir2O7 electrodes. An ER of 6500% has been observed at room temperature, which increases to over 105% at 100 K. The high ER can be attributed to ferroelectric polarization induced metal–insulator transition in interfacial Sr3Ir2O7, which enhances the potential asymmetry for the tunnel barrier. The temperature dependence of tunneling current shows that direct tunneling dominates in the on state, while the off-state conduction transitions from thermally activated behavior at high temperatures to Glazman–Matveev defect-mediated inelastic tunneling at low temperatures. Our study provides a viable material strategy for designing all-oxide FTJs with high ER, facilitating their implementation in nonvolatile memories and energy-efficient computing devices.

Electric current driven insulator–metal transition and large electroresistance in single crystal Sr2Ir1–xRuxO4

A series of single crystal Sr2Ir1–xRuxO4 (0 ≤ x ≤ 0.55) have been synthesized, and their physical properties have been investigated. Ru-substitution at Ir-site can drastically reduce the resistivity, which is accompanied by serious suppression of the long range antiferromagnetic order. With the application of electric current, remarkable electroresistance exceeding 90% is obtained at x ∼ 0.2, which may contain significant contribution from the strong magnetic competitions. Interestingly, insulator–metal transition driven by electric current is observed (e.g., below 100 K), which can be ascribed to the lattice modulation. The findings indicate that the electric current can be an efficient route to tune the physical properties of the Jeff = 1/2 Mott insulator.

Effects of Li substitution on structural, magnetic properties and electroresistance behaviour in La0.8Na0.2-xLixMnO3 monovalent-doped manganite

The effects of Li substitution on structural, magnetic properties, and electroresistance (ER) behaviour in La0.8Na0.2-xLixMnO3 (x = 0 – 0.15) manganites prepared via the conventional solid-state method were reported. All samples exhibited a metal–insulator transition behaviour with a decrease in the metal–insulator transition temperature, TMI from 270 K (x = 0) to 255 K (x = 0.15) and reduction in Curie temperature, TC from 272 K (x = 0.10) to 235 K (x = 0.15). All samples exhibited a reduction in resistivity under higher applied current of 5 mA than that of 1 mA, which led to an ER effect. Large ER value of −59 % is achieved for sample × = 0.15 at 300 K, thereby indicating its potential application for spintronic-based devices. The competition between the occurrence of lattice distortion and the presence of magnetic inhomogeneity induced by A-site disorder (σ2) is suggested responsible for the observed ER effect.

Resistive Memory Based on the Spin-Density-Wave Transition of Antiferromagnetic Chromium

Spin density waves (SDWs) are antiferromagnetic ground states characterized by real-space spin modulation. When an electronic system undergoes a paramagnetic-SDW transition, the translational symmetry is spontaneously broken and energy gaps are developed near the Fermi level, which offers potential for constructing various SDW components. Here we report a prototype resistive memory device based on a prototypical SDW metal, antiferromagnetic chromium. Transport and magnetic measurements show that the paramagnetic-SDW transition, i.e., the SDW antiferromagnetism, can be effectively suppressed by the electric-field-generated piezoelectric strain in epitaxial $\mathrm{Cr}/0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$--$0.3{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ (PMN-PT) heterostructures. This enables a large electroresistance effect for metallic systems as the SDW band gaps can be intentionally controlled to vanish or develop. Combining this electroresistance effect with the different remanent piezoelectric strain of PMN-PT after poling by electric-field pulses of opposite polarity, we obtain two nonvolatile resistance states differing by about 1.8% and stable against a magnetic field of 3 T at room temperature. Our work unveils the electric-field controllability of the SDW transitions in thin films and the consequent wide application prospects of SDW materials.

Effects of Bi substitution on electroresistance behaviours in La0.8-xBixNa0.2MnO3 manganites

The observation of the electroresistance (ER) effect related to the reduction of resistivity under increased applied current indicates the potential of manganite material for next-generation spintronic-based devices. The observed behaviour was attributed to the presence of magnetic inhomogeneity. However, such relation is still not well understood, hence the current-induced effect on the resistivity in monovalent-doped La0.8-xBixNa0.2MnO3 (x = 0–0.20) manganite prepared via solid-state method is reported. All samples exhibited low resistivity in the temperature ranges of 30 K–300 K under a higher applied current of 5 mA compared with 1 mA, leading to the observation of the ER effect. The reduction of resistivity in the metallic region was due to the enhancement of DE itinerant hopping and the decrease in the scattering effect of conduction electrons, whereas in the insulating region, resistivity reduction is suggested to be related to the weakening of the electron–lattice attraction, as indicated by the reduction in activation energy (Ea) of charge carriers. Both x = 0.15 and x = 0.20 samples respectively exhibited large ER effects of 158% and 171% at 300 K amongst the studied samples. The enhancement of ER with Bi substitution is suggested due to the presence of magnetic inhomogeneities induced by MnO6 distortion which favours the formation of filamentary conduction paths under the presence of high applied current. The findings indicate that Bi substitution in La0.8-xBixNa0.2MnO3 (x = 0–0.20) manganites enhanced the sensitivity to the changes in electric field which shows that the investigated samples could be potentially used for non-volatile memory devices.

One Nanometer HfO2‐Based Ferroelectric Tunnel Junctions on Silicon

AbstractIn ferroelectric materials, spontaneous symmetry breaking leads to a switchable electric polarization, which offers significant promise for nonvolatile memories. In particular, ferroelectric tunnel junctions (FTJs) have emerged as a new resistive switching memory which exploits polarization‐dependent tunnel current across a thin ferroelectric barrier. This work integrates FTJs with complementary metal‐oxide‐semiconductor‐compatible Zr‐doped HfO2 (Zr:HfO2) ferroelectric barriers of just 1 nm thickness, grown by atomic layer deposition on silicon. These 1 nm Zr:HfO2 tunnel junctions exhibit large polarization‐driven electroresistance (>20 000%), the largest value reported for HfO2‐based FTJs. In addition, due to just a 1 nm ferroelectric barrier, these junctions provide large tunneling current (>1 A cm−2) at low read voltage, orders of magnitude larger than reported thicker HfO2‐based FTJs. Therefore, this proof‐of‐principle demonstration provides an approach to simultaneously overcome three major drawbacks of prototypical FTJs: a Si‐compatible ultrathin ferroelectric, large electroresistance, and large read current for high‐speed operation.

Stabilization of the Ferroelectric Phase in Epitaxial Hf1–xZrxO2 Enabling Coexistence of Ferroelectric and Enhanced Piezoelectric Properties

Systematic studies on polycrystalline Hf1–xZrxO2 films with varying Zr contents show that HfO2 films are paraelectric (monoclinic). If the Zr content is increased, films become ferroelectric (orthorhombic) and then antiferroelectric (tetragonal). HfO2 shows very good insulating properties and it is used in metal-oxide-semiconductor field-effect devices, while ZrO2 shows good piezoelectric properties, but it is antiferroelectric. In between, Hf0.5Zr0.5O2 shows good ferroelectric properties at the expense of poorer insulating and piezoelectric properties than HfO2 and ZrO2, respectively. Here, we explore the ferroelectric, insulating, and piezoelectric properties of a series of epitaxial films of Hf1–xZrxO2 with different compositions. We show that epitaxial growth permits the stabilization of the ferroelectric phase in a whole range of Zr content (from x = 0 to x = 1). In epitaxial ZrO2 films, ferroelectricity coexists with better piezoelectric and insulating properties than Hf0.5Zr0.5O2, and in HfO2 epitaxial films, ferroelectricity coexists with better insulating properties than Hf0.5Zr0.5O2. For the case of ZrO2 films, large electroresistance is also observed. In both cases, the ferroelectric endurance is poorer than that for Hf0.5Zr0.5O2.

Electroresistance effect in oxygen-deficient La0.8Ba0.2MnO3−δ thin films

Electroresistance (ER) has been intensively studied in low- and intermediate-bandwidth manganites, which possess phase separation characteristics. As for the Sr- and Ba-doped large-bandwidth manganites, however, few results about ER have been reported so far. Here we report ER effect in oxygen-deficient La0.8Ba0.2MnO3−δ thin films, which were obtained by applying a large electric current (33 mA) to the pristine films in vacuum. While the pristine film displays a negligible change in resistivity with respect to the test current, the oxygen-deficient film shows significant ER effect, i.e. ER ratio of −22% at 260 K under a test current of 0.3 mA. By gradually restoring oxygen content in the films, it is found that the ER effect is closely related to the residual resistivity at low temperatures, demonstrating the key role of grain boundaries. Furthermore, the residual resistivity can readily be tuned by heating the oxygen-deficient films in air, suggesting strong oxygen activity in the grain boundaries. The magnetoresistance (MR) data show current dependent feature, also revealing the role of grain boundaries. At 40 K, the MR ratio of the 100 °C restored film under 30 kOe increases from −15% to −25% when decreasing the test current from 1 to 10−3 mA. The large ER effect in the oxygen-deficient films is discussed based upon the conductive filament picture in grain boundaries. Our approach to controlling the ER effect through oxygen deficiency makes oxide films more promising for potential applications in the memristive devices and neuomorphic computing.

Transport properties and electroresistance of manganite based heterostructure

In the present communication, ZnO/La0.7Sr0.3MnO3/Al2O3 (ZnO/LSMO/Al2O3) heterostructure was grown using chemical solution deposition (CSD) technique. X–ray diffraction (XRD) measurement confirms the (100) crystallographic oriented growth of LSMO manganite and polycrystalline hexagonal growth of ZnO layer. Temperature dependent resistivity behavior under different applied magnetic fields, performed for two different geometries [current in plane (CIP) and current perpendicular to plane (CPP)], suggests the modifications in metal to insulator transition temperature TP and alterations in resistivity with applied magnetic field and measurement geometry. This also implies the better charge conduction across the ZnO/LSMO interface (CPP) as compared to LSMO film layer (CIP). Various charge conduction mechanisms have been employed to understand the charge transport for observed low temperature resistivity minimum, metallic behavior, insulating state and magnetoresistance (MR) of LSMO film and ZnO/LSMO interface. Electroresistance (ER) and field effect configuration (FEC) have been investigated across the ZnO/LSMO interface by recording the LSMO channel resistance, at room temperature, that suggest the tuning of signature and value of ER by considering forward and reverse bias modes across the interface. FEC studies show the large ER ∼ +750% and −85% across the LSMO channel with its tunability under different interface biases.

Giant Electroresistance in Ferroionic Tunnel Junctions

SummaryOxide-based resistive switching devices, including ferroelectric tunnel junctions and resistance random access memory, are promising candidates for the next-generation non-volatile memory technology. In this work, we propose a ferroionic tunnel junction to realize a giant electroresistance. It functions as a ferroelectric tunnel junction at low resistance state and as a Schottky junction at high resistance state, due to interface engineering through the field-induced migration of oxygen vacancies. An extremely large electroresistance with ON/OFF ratios of 5.1×107 at room temperature and 2.1×109 at 10 K is achieved, using an ultrathin BaTiO3-δ layer as the ferroelectric barrier and a semiconducting Nb-doped SrTiO3 substrate as the bottom electrode. The results point toward an appealing way for the design of high-performance resistive switching devices based on ultrathin oxide heterostructures by ionic controlled interface engineering.

Polarization-induced resistance switching phenomenon in metal Au/ferroelectric vinylidene fluoride–trifluoroethylene/semiconductor Si junction

The polarization-induced resistance switching effect in Au/vinylidene fluoride–trifluoroethylene (VDF–TrFE)/Si capacitor was investigated. The 20-nm-thick VDF/TrFE copolymer fabricated on n-Si with 1–3 mΩ·cm resistivity exhibited an entire polarization reversal, which is almost comparable to the ferroelectric properties of a metal–ferroelectric–metal structure. The conduction current in the ON state, in which the polarization direction is from Au to Si, was 60 times larger than that in the OFF state. The large electroresistance effect was switchable by the polarization reversal of the VDF/TrFE copolymer film, which is essentially explained by the Thomas–Fermi screening theory.

Comparison of charge transport studies of chemical solution and pulsed laser deposited manganite-based thin film devices

The electrical transport properties of manganites play a significant role in the diverse applications of spintronics. In this communication, we report the variation in the electrical transport properties of Y0.95Ca0.05MnO3 (YCMO) films deposited on (100) single crystalline Si substrates by two different deposition techniques: chemical solution deposition (CSD) and pulsed laser deposition (PLD). The charge conduction mechanisms responsible for the electrical transport properties of these films have been studied across the YCMO/Si junctions using temperature-dependent current–voltage (I–V) and resistance–voltage characteristics and results have been compared for both the devices fabricated by CSD and PLD techniques. The role of structural strain and space charge limited conduction (SCLC) processes has been discussed to understand the results of I–V and electroresistance (ER) behavior across the junctions. The CSD grown films exhibits better electrical transport properties at the junction, i.e., high current across the junction, low junction resistance and large ER, and this has been discussed in detail on the basis of low structural strain present at the interface.

Large electroresistance and tunable photovoltaic properties of ferroelectric nanoscale capacitors based on ultrathin super-tetragonal BiFeO3films

Ferroelectric nanocapacitors with tunable resistance and photovoltaic effect have potential for multifunctional opto-electronic nanodevices.

Electric field controlled magnetization and transport properties of La0.7Ca0.3MnO3 ultrathin film

We have investigated the effect of electric field on the magnetization in La0.7Ca0.3MnO3 (LCMO) ultrathin film (∼10 nm) by using it as the semiconductor channel material of a prototypical field effect device and SiO2 as dielectric gate. The direct magnetization measurements confirm the formation of a large ferromagnetic phase with the increase in the negative applied gate voltage. We have also shown that a large electroresistance (ER) of ∼78% with Vg = −8 V can obtained in LCMO by using a dielectric gate by reducing the thickness of the LCMO channel.

Temperature-dependent I–V and C–V characteristics of chemically-grown Y0.95Ca0.05MnO3/Si thin films

In this communication, we report the results of the studies on temperature-dependent current—voltage (I–V) and capacitance—voltage (C–V) characteristics of chemical solution deposition-grown Y0.95Ca0.05MnO3/Si films and annealing temperature-induced modified interface dependence on device characteristics. X-ray diffraction results reveal the single phasic nature of films having polycrystalline growth on single crystalline (100) Si substrate. The magnetic nature of the films is confirmed from magnetic force microscopy studies. I–V and C–V characteristics show a strong dependence on the temperature and nature of the film—substrate interface, which has been understood on the basis of the annealing effect involved. Various models and theories have been used to understand the mechanism responsible for the transport across the film—substrate interface. A large rectifying ratio of ∼1.2 × 104 has been obtained across the interface annealed at a lower temperature, which becomes almost double in the film annealed at a higher temperature. A large electroresistance of ∼600% has been achieved for the interface annealed at a lower temperature. The temperature dependence of C–V behavior recorded across the interfaces of the films is discussed in detail on the basis of free and trapped charge carrier density and interface modifications.

Space-charge Effect on Electroresistance in Metal-Ferroelectric-Metal capacitors.

Resistive switching through electroresistance (ER) effect in metal-ferroelectric-metal (MFM) capacitors has attracted increasing interest due to its potential applications as memories and logic devices. However, the detailed electronic mechanisms resulting in large ER when polarisation switching occurs in the ferroelectric barrier are still not well understood. Here, ER effect up to 1000% at room temperature is demonstrated in C-MOS compatible MFM nanocapacitors with a 8.8 nm-thick poly(vinylidene fluoride) (PVDF) homopolymer ferroelectric, which is very promising for silicon industry integration. Most remarkably, using theory developed for metal-semiconductor rectifying contacts, we derive an analytical expression for the variation of interfacial barrier heights due to space-charge effect that can interpret the observed ER response. We extend this space-charge model, related to the release of trapped charges by defects, to MFM structures made of ferroelectric oxides. This space-charge model provides a simple and straightforward tool to understand recent unusual reports. Finally, this work suggests that defect-engineering could be an original and efficient route for tuning the space-charge effect and thus the ER performances in future electronic devices.

Magnetic field cycling effect on the non-linear current-voltage characteristics and magnetic field induced negative differential resistance in α-Fe1.64Ga0.36O3 oxide

We have studied current-voltage (I-V) characteristics of α-Fe1.64Ga0.36O3, a typical canted ferromagnetic semiconductor. The sample showed a transformation of the I-V curves from linear to non-linear character with the increase of bias voltage. The I-V curves showed irreversible features with hysteresis loop and bi-stable electronic states for up and down modes of voltage sweep. We report positive magnetoresistance and magnetic field induced negative differential resistance as the first time observed phenomena in metal doped hematite system. The magnitudes of critical voltage at which I-V curve showed peak and corresponding peak current are affected by magnetic field cycling. The shift of the peak voltage with magnetic field showed a step-wise jump between two discrete voltage levels with least gap (ΔVP) 0.345(± 0.001) V. The magnetic spin dependent electronic charge transport in this new class of magnetic semiconductor opens a wide scope for tuning large electroresistance (∼500-700%), magnetoresistance (70-135 %) and charge-spin dependent conductivity under suitable control of electric and magnetic fields. The electric and magnetic field controlled charge-spin transport is interesting for applications of the magnetic materials in spintronics, e.g., magnetic sensor, memory devices and digital switching.

Mapping Chemical Disorder and Ferroelectric Distortions in the Double Perovskite Compound Sr2-xGdxMnTiO6 by Atomic Resolution Electron Microscopy and Spectroscopy

In this work we report a study of the chemical and structural order of the double perovskite compound Sr 2-x Gd x MnTiO6 for compositions x=0, 0.25, 0.5, 0.75, and 1. A noticeable disorder at the B-site in the Mn and Ti sublattice is detected at the atomic scale by electron energy-loss spectroscopy for all x values, resulting in Mn-rich and Ti-rich regions. For x ≥ 0.75, the cubic unit cell doubles and lowers its symmetry because of structural rearrangements associated with a giant ferroelectric displacement of the perovskite B-site cation. We discuss this finding in the light of the large electroresistance observed in Sr 2-x Gd x MnTiO6, x ≥ 0.75.

Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics.

Electric field modulation of magnetism and electric properties in La-Ca-MnO3/Pb(Zr0.52Ti0.48)O3 magnetoelectric laminate

We investigated the effects of electric field on the magnetization and electric transport properties in La0.6Ca0.4MnO3/Pb(Zr0.52Ti0.48)O3 (LCMO/PZT) thin film prepared by sol-gel method and simple spin coating technology. Obvious converse magnetoelectric effect (the relative change in magnetization, ΔM ∼ 15%), as well as large electroresistance effect (the relative change in resistance, ER ∼ 61%), has been obtained in LCMO thin film with applying electric field on the PZT ceramic. The efficient mechanical coupling, which originated from ferroelectric polarization and inverse piezoelectric effect in PZT substrate, leads to large changes in the magnetism and resistance of LCMO thin film. The experimental results can be interpreted by the changes of both spin exchange coupling and electron-lattice coupling strengths in LCMO film, which result from the strain at the ferromagnetic/ferroelectric interface.