Bipolar Resistive Switching Modes Research Articles

Complementary bipolar resistive switching behavior in lithium titanate memory device

In this paper, we report the coexistence of usual bipolar and unique complementary bipolar resistive switching behaviors in an Ag/Li x Ti5O12/Pt memory device. The SET and RESET polarities of the complementary bipolar resistive switching mode were found to be opposite to those of the usual bipolar resistive switching mode. The two bipolar switching modes can be freely converted without altering the compliance current. Based on the conducting filament model, the normal bipolar resistive switching mode is explained by an Ag filament and electrochemical metallization mechanism. Whereas, the complementary bipolar resistive switching behavior is ascribed to Li diffusion and phase transition between Li x Ti5O12 and conducting lithium-rich Li x Ti5O12.

High‐Throughput Screening Thickness‐Dependent Resistive Switching in SrTiO3 Thin Films for Robust Electronic Synapse

AbstractThe functionalities and applications of oxide thin films are highly dependent on their thickness. Most thickness‐dependent studies on oxide thin films require the preparation of independent samples, which is labor‐intensive and time‐consuming and inevitably introduces experimental errors. To address this challenge, a general strategy based on high‐throughput pulsed laser deposition technology is proposed to precisely control the thin‐film thickness in local regions under similar growth conditions. The as‐proposed synthesis strategy is demonstrated using typical complex oxide materials of SrTiO3 (STO). Consequently, high‐throughput STO thin films with nine gradient thicknesses ranging from 10.1 to 30.5 nm are fabricated. Notably, a transition from the unipolar to the bipolar resistive switching mode is observed with increasing STO thickness. Moreover, a physical mechanism based on the heterostructure‐mediated redistribution of oxygen vacancies is employed to interpret the transition between the two memristive patterns. The screening of STO thin films with different resistive switching behaviors revealed that the STO thin film with a thickness of 20.3 nm exhibit excellent conductance modulation properties under the application of electrical pulses as well as significant reliability for the emulation of various synaptic functions, rendering it a promising material for artificial neuromorphic computing applications.

Dual Resistive Switching Performance Derived from Ionic Migration in Halide Perovskite Based Memory

Herein, we report an environmentally stable and friendly halide perovskite based resistive random access memory device with an Ag/PMMA/(PMA)2CuBr4/FTO (PMMA = poly(methyl methacrylate); PMA = C6H5CH2NH3) architecture. The device exhibits the coexistence of two bipolar resistive switching modes, including counterclockwise and clockwise switching characteristics. The devices with both switching modes show stable endurance (>100 cycles) and long retention performance (>104 s). By applying a suitable electrical stimulation, the counterclockwise and clockwise switching behaviors are interconvertible. Furthermore, the Au/PMMA/(PMA)2CuBr4/FTO and Ag/(PMA)2CuBr4/FTO devices were fabricated to verify the origin of dual resistive switching behaviors. The similar dual resistive switching behaviors after electroforming processes of three types of memory devices suggest that the interconvertible dual resistive switching characteristics could be attributed to the ionic migration in the (PMA)2CuBr4 perovskite layer.

Understanding the coexistence of unipolar and bipolar resistive switching in NiFe2O4 resistive memory devices

In this Letter, spinel ferrite NiFe2O4 films prepared through pulsed laser deposition are used to fabricate Pt/NiFe2O4/Pt devices with the coexistence of unipolar resistive switching (URS) and bipolar resistive switching (BRS). The device demonstrated nonvolatile and stable resistive switching (RS) properties under URS and BRS modes. The current–voltage (I–V) fitting analysis and temperature dependence measurements suggested that one Pt/NiFe2O4/Pt RRAM cell follows two significantly different RS and carrier transport mechanisms under URS and BRS modes. Under URS mode, the carrier mechanism alternates between the high resistance state (HRS) with Schottky emission and low resistance state (LRS) with the Ohmic mechanism. In contrast, under BRS mode, carrier transport at HRS and LRS is dominated by space-charge-limited current and nearest-neighboring hopping, respectively. Finally, a physical model, combining oxygen ion (O2−) migration and electric field distribution simulated by COMSOL Multiphysics, is proposed to further clarify the coexistence of two distinct RS and carrier transport mechanisms in the single Pt/NiFe2O4/Pt RRAM device.

Understanding the coexistence of two bipolar resistive switching modes with opposite polarity in CuxO (1 ≤ x ≤ 2)-based two-terminal devices

Understanding the coexistence of two bipolar resistive switching modes with opposite polarity in CuxO (1 ≤ x ≤ 2)-based two-terminal devices

Improved resistive switching characteristics of a multi-stacked HfO2/Al2O3/HfO2 RRAM structure for neuromorphic and synaptic applications: experimental and computational study.

Atomic Layer Deposition (ALD) was used for a tri-layer structure (HfO2/Al2O3/HfO2) at low temperature over an Indium Tin Oxide (ITO) transparent electrode. First, the microstructure of the fabricated TaN/HfO2/Al2O3/HfO2/ITO RRAM device was examined by the cross-sectional High-Resolution Transmission Electron Microscopy (HRTEM). Then, Energy Dispersive X-ray Spectroscopy (EDS) was performed to probe compositional mapping. The bipolar resistive switching mode of the device was confirmed through SET/RESET characteristic plots for 100 cycles as a function of applied biasing voltage. An endurance test was performed for 100 DC switching cycles @0.2 V wherein; data retention was found up to 104 s. Moreover, for better insight into the charge conduction mechanism in tri-layer HfO2/Al2O3/HfO2, based on oxygen vacancies (VOX), total density of states (TDOS), partial density of states (PDOS) and isosurface three-dimensional charge density analysis was performed using WEIN2k and VASP simulation packages under Perdew–Burke–Ernzerhof _Generalized Gradient approximation (PBE-GGA). The experimental and theoretical outcomes can help in finding proper stacking of the active resistive switching (RS) layer for resistive random-access memory (RRAM) applications.

Quantum conductors formation and resistive switching memory effects in zirconia nanotubes

The prospects of the development of non-volatile memory elements that involve memristive metal-dielectric-metal sandwich structures are due to the possibility of reliably implementing sustained functional states with quantized conductance. In the present paper, we have explored the properties of Zr/ZrO2/Au memristors fabricated based on an anodic zirconia layer that consists of an ordered array of vertically oriented non-stoichiometric nanotubes with an outer diameter of 30 nm. The operational stability of the designed memory devices has been analyzed in unipolar and bipolar resistive switching modes. The resistance ratio ≥105 between high-resistance (HRS) and low-resistance (LRS) states has been evaluated. It has been found that the LRS conductivity is quantized over a wide range with a fundamental minimum of 0.5G 0 = 38.74 μS due to the formation of quantum conductors based on oxygen vacancies (VO). For Zr/ZrO2/Au memristors, resistive switching mechanisms to be sensitive to the migration of VO in an applied electric field have been proposed. It has been shown that the ohmic type and space-charge-limited conductivities are realized in the LRS and HRS, respectively. Besides, we have offered a brief review of parameters for functional metal/zirconia/metal nanolayered structures to create effective memristors with multiple resistive states and a high resistance ratio.

Complementary resistive switching in core–shell nanowires

Highly dense, energy-efficient, and fast neuromorphic architectures emulating the computational abilities of the brain use memristors to emulate synapses in the analog or digital systems. Core–shell nanowires provide us with new opportunities for neuromorphic hardware integration. In this work, we have fabricated core–shell nanowires using a combination of bottom-up and top-down techniques. Additionally, we have demonstrated eightwise and counter-eightwise bipolar resistive switching (BRS). Remarkably, for the first time along with BRS, we have also demonstrated complementary resistive switching (CRS) in core–shell nanowires. Here, Pt was used as the conductive core and HfO2 as the memristive shell with Ti as the top electrode. The resistive switching properties were characterized by I–V curves and pulse operation modes. The cycling endurance in the BRS mode was 1000 cycles with an off–on ratio of ∼13 and resistance was retained for 104 s. Additionally, the compliance current used to form the nanowire in the BRS mode influenced the CRS operation by lowering the peak operating current. Additionally, current density–electric field analysis performed to determine charge conduction mechanisms revealed that the wires exhibit a thermionic emission mechanism in the high resistance state and Ohmic conduction mechanism in the low resistance state during the BRS mode of operation and hopping conduction mechanism in state 0 and space-charge-limited conduction mechanism in state 1 during the CRS mode of operation. This observed versatility in the mode of operation makes core–shell nanowires of significant interest for use as synaptic elements in neuromorphic network architectures.

Nonideal resistive and synaptic characteristics in Ag/ZnO/TiN device for neuromorphic system

Ideal resistive switching in resistive random-access memory (RRAM) should be ensured for synaptic devices in neuromorphic systems. We used an Ag/ZnO/TiN RRAM structure to investigate the effects of nonideal resistive switching, such as an unstable high-resistance state (HRS), negative set (N-set), and temporal disconnection, during the set process and the conductance saturation feature for synaptic applications. The device shows an I–V curve based on the positive set in the bipolar resistive switching mode. In 1000 endurance tests, we investigated the changes in the HRS, which displays large fluctuations compared with the stable low-resistance state, and the negative effect on the performance of the device resulting from such an instability. The impact of the N-set, which originates from the negative voltage on the top electrode, was studied through the process of intentional N-set through the repetition of 10 ON/OFF cycles. The Ag/ZnO/TiN device showed saturation characteristics in conductance modulation according to the magnitude of the applied pulse. Therefore, potentiation or depression was performed via consecutive pulses with diverse amplitudes. We also studied the spontaneous conductance decay in the saturation feature required to emulate short-term plasticity.

Synaptic Behavior and Conduction Mechanisms in Core-Shell Nanowires for Neuromorphic Hardware Applications

To tackle the ever-increasing demand for faster computation time and energy efficiency, analog neuromorphic architectures have been proposed in place of current artificial neural networks (ANNs) to emulate the computational abilities of the brain. Non-volatile, two terminal devices like memristors have been proven as viable candidates as an artificial synapse in these architectures. Presently, these devices are fabricated as crossbar, thin-film or nanowires. Nanowire-based architectures can play an important role in enhancing sparsity and randomness of a network, particularly when built using core-shell structures. We have previously shown that, depending on the forming process, a Pt core and HfO2 shell nanowire with a Ti top electrode can display eightwise (8W) or counter eightwise (C8W) bipolar resistive switching (BRS) at low potential ranges (±1 V). We will show that the symmetry of the core-shell nanowires combined with the 8W and C8W switching observed, are indicative of anti-serial memristor arrangement. Moreover, this configuration allows for the individual nanowires to exhibit complementary resistive switching (CRS) at higher potentials (±2 V). We will also show the current conduction mechanisms of this system, where thermionic emission dominates in the low resistance state while ohmic conduction dominates in the high resistance state, when the device is operated in BRS mode; and hopping conduction dominates in state 0 while space-charge-limited conduction dominates in state 1 when, the device is operated in CRS mode. When in BRS mode, the LRS and HRS states are stable for at least 1000 cycles and retain memory for 104 s. Finally, when testing for synaptic plasticity via long-term potentiation and depression (LTP and LTD), non-linear weight update behavior was observed. The effect of voltage pulse schemes on the synaptic response will be discussed in detail.

Any-polar resistive switching behavior in Ti-intercalated Pt/Ti/HfO2/Ti/Pt device* *Project supported by the National Natural Science Foundation of China (Grant Nos. 62004087, 61474081, and 61534005), the Natural Science Foundation of Fujian Province, China (Grant No. 2020J01815), the Natural Science Foundation of Zhangzhou, China (Grant No. ZZ2020J32), and the Natural Science Foundation of Jiangxi Province, China (Grant No. 20192ACBL20048).

The special any-polar resistive switching mode includes the coexistence and stable conversion between the unipolar and the bipolar resistive switching mode under the same compliance current. In the present work, the any-polar resistive switching mode is demonstrated when thin Ti intercalations are introduced into both sides of Pt/HfO2/Pt RRAM device. The role of the Ti intercalations contributes to the fulfillment of the any-polar resistive switching working mechanism, which lies in the filament constructed by the oxygen vacancies and the effective storage of the oxygen ion at both sides of the electrode interface.

Bipolar and Complementary Resistive Switching Characteristics and Neuromorphic System Simulation in a Pt/ZnO/TiN Synaptic Device.

In this work, a ZnO-based resistive switching memory device is characterized by using simplified electrical conduction models. The conventional bipolar resistive switching and complementary resistive switching modes are accomplished by tuning the bias voltage condition. The material and chemical information of the device stack including the interfacial layer of TiON is well confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis. The device exhibits uniform gradual bipolar resistive switching (BRS) with good endurance and self-compliance characteristics. Moreover, complementary resistive switching (CRS) is achieved by applying the compliance current at negative bias and increasing the voltage at positive bias. The synaptic behaviors such as long-term potentiation and long-term depression are emulated by applying consecutive pulse input to the device. The CRS mode has a higher array size in the cross-point array structure than the BRS mode due to more nonlinear I–V characteristics in the CRS mode. However, we reveal that the BRS mode shows a better pattern recognition rate than the CRS mode due to more uniform conductance update.

The Resistive Switching Characteristics of TiN/HfO2/Ag RRAM Devices with Bidirectional Current Compliance

The performance of TiN/HfO2/Ag resistive random-access memory (RRAM) devices combining the oxygen-based RRAM (OxRRAM) and conducting bridge random-access memory (CBRAM) was studied. Current compliance (CC) values could significantly affect the resistive switching process: with unidirectional CC, permanent breakdown for the devices was observed. With bidirectional smaller CC, the stable bipolar resistive switching mode was obtained, and the cation injection played a leading role in the resistive switching process, while the devices with higher CC always remained in low-resistance state (LRS). The resistance–voltage (R–V) curve and conducting mechanism were analyzed for the bidirectional smaller CC devices, indicating ohmic conduction for the LRS, while the space charge limited the current for the high-resistance state. The results of x-ray photoelectron spectroscopy showed that oxygen vacancies participated in the resistive switching process. It could be concluded that conducting paths were formed from conducting filaments of oxygen vacancies and conducting bridges of Ag grains, and the different formation rate of conducting filaments related to the values of CC was the prime reason for the various changes. Moreover, the characteristics of TiN/HfO2/Ta devices proved the universality of bidirectional CC. The method of making the OxRRAM and CBRAM coexist in the RRAM devices may offer a way to fabricate devices with low power consumption.

Understanding the Coexistence of Two Bipolar Resistive Switching Modes with Opposite Polarity in Pt/TiO2/Ti/Pt Nanosized ReRAM Devices

Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO2 cells, is still not completely understood. Here, we report on nanosized (<0.01 μm2) asymmetric memristive cells from 3 to 6 nm thick TiO2 films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO2/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO2 cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO2 Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO2/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO2 interface.

Impact of Schottky junctions in the transformation of switching modes in amorphous Y2O3-based memristive system

In this report, we study factors that dominate the mode transformation of resistive switching (RS) in yttria based memristive devices. It is found that amorphous yttria films are more suitable for RS whereas highly crystalline films are counterproductive for RS. The transformation from unipolar to bipolar resistive switching mode is demonstrated in our devices via moving from a system of single Schottky barrier diode (SBD) to double SBD. The conduction mechanism behind these transformation mechanisms is found to be predominantly interfacial. We also report a forming-free Al/Y2O3/Al based memristor fabricated by the dual ion beam sputtering without any post-processing steps for the first time. It shows stable switching behavior for >29 000 cycles with good retention (105 s) characteristics.

Role of the Electrode Material on the RESET Limitation in Oxide ReRAM Devices

AbstractMetal‐oxide‐based bipolar resistive switching (BRS) redox‐based resistive switching memory (ReRAM) shows many outstanding properties making it of interest as an emerging nonvolatile memory. However, it often suffers from a low ROFF/RON ratio, while a large ratio is desired to compensate for read margin loss due to the intrinsic variability of the ReRAM cells. Understanding of the physical processes responsible for limitations of the ROFF and RON in ReRAM cells is therefore of high importance. In this paper a study on the RESET process in BRS Ta2O5‐based ReRAM cells is presented. The ROFF is found to be limited by a secondary volatile resistive switching mode that shows an opposite polarity compared to the main BRS mode. Based on results of switching kinetics measurements a physical model is proposed. It involves an oxygen exchange reaction at the metal‐oxide/active electrode interface combined with a drift‐diffusion induced migration of the resulting oxygen vacancy defects within the metal‐oxide. Incorporation of a thin oxygen‐blocking layer at the active interface allows for a suppression of the secondary switching mechanism. The improved RESET characteristic results in a strongly increased maximum ROFF. These results provide new insights into the role of the electrode material on the RESET process in BRS ReRAM cells.

Coexistence of unipolar and bipolar modes in Ag/ZnO/Pt resistive switching memory with oxygen-vacancy and metal-Ag filaments**Project supported by the National Natural Science Foundation of China for Excellent Young Scholars (Grant No. 51422201), the National Natural Science Foundation of China (Grant Nos. 51172041, 51372035, 11304035, 61574031, and 61404026), the National Basic Research Program of China (Grant No. 2012CB933703), the “111” Project,

In this study, the unipolar resistive switching (URS) and bipolar resistive switching (BRS) are demonstrated to be coexistent in the Ag/ZnO/Pt memory device, and both modes are observed to strongly depend on the polarity of forming voltage. The mechanisms of the URS and BRS behaviors could be attributed to the electric-field-induced migration of oxygen vacancies (VO) and metal-Ag conducting filaments (CFs) respectively, which are confirmed by investigating the temperature dependences of low resistance states in both modes. Furthermore, we compare the resistive switching (RS) characteristics (e.g., forming and switching voltages, reset current and resistance states) between these two modes based on VO- and Ag-CFs. The BRS mode shows better switching uniformity and lower power than the URS mode. Both of these modes exhibit good RS performances, including good retention, reliable cycling and high-speed switching. The result indicates that the coexistence of URS and BRS behaviors in a single device has great potential applications in future nonvolatile multi-level memory.

Reversible transition between bipolar and unipolar resistive switching in Cu2O/Ga2O3 binary oxide stacked layer

Both unipolar resistive switching (URS) and bipolar resistive switching (BRS) behaviors are observed in Cu2O/Ga2O3 stacked layer. The conversion between BRS and URS is controllable and reversible. The switching operations in BRS mode requires smaller voltage than that in the URS mode. The oxygen vacancies closed to the Cu2O/Ga2O3 interface contributes to the BRS, and the bias-controlling filament formation/rupture in depletion layer is considered to contribute to the URS. The URS happens only in the negative voltage part due to the nature of directionality of the p-n junction. The process reported here can be developed to design memory device.

Resistivity switching properties of Li-doped ZnO films deposited on LaB6 electrode

Current–voltage (I–V) characteristics of Al/p-ZnO:Li/LaB6 device, measured in voltage sweep mode, show unipolar resistive switching and monostable threshold switching (URS and MTS) for different bias voltage polarities. URS could be transformed to MTS by application of reverse bias voltage. With increasing number of cycles, URS is converted to bipolar resistive switching mode which is lost after certain number of cycles, and device turns into an ordinary resistor. Analysis of linear fitting I–V curves suggests that ohmic and space charge limited current laws are responsible for conductivity mechanism of Al/p-ZnO:Li/LaB6 device.

Compliance current induced non-reversible transition from unipolar to bipolar resistive switching in a Cu/TaOx/Pt structure

Unipolar resistive switching (URS) as well as bipolar resistive switching (BRS) behaviors in a Cu/TaOx/Pt structure were investigated. Upon increasing the compliance current (Ic), the current-voltage characteristics of the Cu/TaOx/Pt structure showed a URS behavior at Ic = 0.1 mA then experienced a non-reversible transition from the URS to a BRS mode at Ic = 10 mA. Through a detailed analysis of the electrical properties in each resistance state of URS and BRS, we revealed that the permanent transition from the URS to the BRS mode was induced by the formation of stronger Cu metal conductive filaments within the TaOx thin film. More interestingly, both URS and BRS modes were governed by the formation and rupture of conductive filaments, whereas the rupture of these filamentary paths in BRS was proposed due to both Joule heating and electric field effects.