Reproducible Bipolar Resistive Switching Research Articles

Evaluation of Nanostructured NiS2 Thin Films from a Single-Source Precursor for Flexible Memristive Devices.

Herein, we report the first demonstration of a single-step, in situ growth of NiS2 nanostructures from a single-source precursor onto a flexible substrate as a versatile platform for an effective nonvolatile memristor. The low temperature, solution-processed deposition of NiS2 thin films exhibits a wide band gap range, spherical-flower-like morphology with high surface area and porosity, and negligible surface roughness. Moreover, the fabricated Au/NiS2/ITO/PET memristor device reveals reproducible bipolar resistive switching (RS) at low operational voltages under both flat and bending conditions. The flexible device shows stable RS behavior for multiple cycles with a good memory window (∼102) and data retention of up to 104 s. The switching of a device between a high-resistance state and a low-resistance state is attributed to the filamentary conduction based on sulfur ion migration and sulfur vacancies and plays a key role in the outstanding memristive performance of the device. Consequently, this work provides a simple, scalable, solution-processed route to fabricate a flexible device with potential applications in next-generation neuromorphic computing and wearable electronics.

Evaluation of Cellulose-MXene Composite Hydrogel Based Bio-Resistive Random Access Memory Material as Mimics for Biological Synapses.

We report a memory device based on organic-inorganic hybrid cellulose-Ti3C2TX MXene composite hydrogel (CMCH) as a switching layer sandwiched between Ag top and FTO bottom electrodes. The device (Ag/CMCH/FTO) was fabricated by a simple, solution-processed route and exhibits reliable and reproducible bipolar resistive switching. Multilevel switching behavior was observed at low operating voltages (±0.5 to ±1 V). Furthermore, the capacitive-coupled memristive characteristics of the device were corroborated with electrochemical impedance spectroscopy and this affirmed the filamentary conduction switching mechanism (LRS-HRS). The synaptic functions of the CMCH-based memory device were evaluated, wherein potentiation/depression properties over 8 × 103 electric pulses were observed. The device also exhibited spike time-dependent plasticity-based symmetric Hebbian learning rule of a biological synapse. This hybrid hydrogel is expected to be a potential switching material for low-cost, sustainable, and biocompatible memory storage devices and artificial synaptic applications.

High-performance and humidity robust multilevel lead-free all-inorganic Cs3Cu2Br5 perovskite-based memristors

Halide perovskites have attracted surge of interest in the memristor field due to their superior electrical property and corresponding remarkable device performances. However, the issues of toxicity and unstable properties still severely restrict their potential applications. Here, the lead-free all-inorganic perovskite Cs3Cu2Br5 films are adopted as the switching layer to fabricate memristors with Al/Cs3Cu2Br5/ITO structure. The prepared Al/Cs3Cu2Br5/ITO memristors exhibit typical reproducible bipolar resistive switching (RS) behavior with striking characteristics, including ultralow operating voltages (0.45, −0.39 V), moderate high resistance state/low resistance state ratio (≈102), and remarkable retention time (>104 s). In addition, the multilevel storage capability can be achieved by controlling compliance current. The RS effect, stemming from the formation/rupture of both localized conductive Br vacancy/Al atom filaments, is proposed to illustrate the memristors. More importantly, the RS behavior of Al/Cs3Cu2Br5/ITO memristors maintains robustness in harsh environments with humidity up to 80%, enabling secure hardware applicable in extreme environments. This work demonstrates the opportunity for exploring the next-generation nonvolatile memories based on lead-free all-inorganic halide perovskites in future environmental-friendly and humidity robust electronics.

Flexible Memristor Devices Using Hybrid Polymer/Electrodeposited GeSbTe Nanoscale Thin Films.

We report on the development of hybrid organic-inorganic material-based flexible memristor devices made by a fast and simple electrochemical fabrication method. The devices consist of a bilayer of poly(methyl methacrylate) (PMMA) and Te-rich GeSbTe chalcogenide nanoscale thin films sandwiched between Ag top and TiN bottom electrodes on both Si and flexible polyimide substrates. These hybrid memristors require no electroforming process and exhibit reliable and reproducible bipolar resistive switching at low switching voltages under both flat and bending conditions. Multistate switching behavior can also be achieved by controlling the compliance current (CC). We attribute the switching between the high resistance state (HRS) and low resistance state (LRS) in the devices to the formation and rupture of conductive Ag filaments within the hybrid PMMA/GeSbTe matrix. This work provides a promising route to fabricate flexible memory devices through an electrodeposition process for application in flexible electronics.

Unraveling the Effect of the Water Content in the Electrolyte on the Resistive Switching Properties of Self-Assembled One-Dimensional Anodized TiO2 Nanotubes.

The applied potential, time, and water content are crucial factors in the electrochemical anodization process because the growth of one-dimensional nanotubes can be accelerated by enhancing the corrosive effect. We investigated the effect of the water content on the resistive switching (RS) properties of Ti foils by anodizing the foils and varying the water content in an electrolyte (1-10 vol %). By increasing the water content, we facilitated a slow transition from nanopores to nanotubes and realized an increase in the tube wall diameter and tube length. All of the fabricated memristive devices exhibited a reliable and reproducible bipolar resistive switching effect. The optimized device exhibited bipolar RS properties with good dc endurance (104 cycles) and data retention capability (105 s). Our results suggest that as the water content increases to 5 vol %, the RS process improves; further increases in the water content impair the RS process.

Low-temperature characteristics of resistive switching memory devices based on reduced graphene oxide-phosphor composites toward reliable cryogenic electronic devices

Low-temperature electronic devices are indispensable for quantum computing and space exploration applications. However, developing memory devices that are an integral part of the control circuits of low-temperature electronic devices is highly challenging. The electrical conductivity of semiconducting materials drops at low temperatures, leading to large variations in SET/RESET voltages. A strategy for the efficient operation of nonvolatile memory devices at low temperatures should be developed. This study investigated the low-temperature resistive switching characteristics of reduced graphene oxide-phosphor composites (GPs). At cryogenic temperatures, the fabricated ITO/GP/Si3N4/p++Si device exhibited a reliable and reproducible bipolar resistive switching performance. The SET/RESET voltages of the devices remained almost constant at all temperatures ranging from 77 to 300 K. The performance of the devices was further investigated by illuminating the devices with blue laser light. The SET/RESET voltages were considerably reduced through light illumination, which is correlated to the lowering of the barrier height at the Si3N4/p++Si interface and enhanced photoconductivity in the GP film. Our studies suggest that light illumination can be a promising tool for investigating issues in ReRAM devices and improving device performance in dark conditions.

Nonvolatile resistive switching memory based on monosaccharide fructose film

In this paper, we report resistive random access memory (RRAM) based on a monosaccharide—fructose for nonvolatile memory in biocompatible and “green” electronics. Fructose thin film acts as the resistive switching layer with Al and Ag top electrodes for comparison. Both devices demonstrated highly reproducible nonvolatile bipolar resistive switching behaviors with a large on/off ratio of ∼106 for the Al electrode and ∼105 for the Ag electrode. The forming voltage, set voltage, and memory window are also larger for the Al electrode than the Ag electrode, but the reset voltages are comparable. Dominant conduction mechanisms of fructose films were proposed. At a high resistance state, both electrodes reveal space charge limited conduction, while at a low resistance state, the governing mechanism is Ohm's law, and in addition, the Ag electrode also shows trap-fill limited conduction when approaching the reset voltage. This observation has yet to be reported in RRAM based on natural bio-organic materials.

Fabrication and investigation of quaternary Ag–In–Zn–S quantum dots-based memristors with ultralow power and multiple resistive switching behaviors

Quaternary Ag–In–Zn–S (AIZS) quantum dots (QDs) play critical roles in various applications since they have advantages of combining superior optical and electrical features, such as tunable fluorescence emission and high carrier mobilities. However, the application of semiconductor AIZS QDs in brain-inspired devices (e.g. memristor) has been rarely reported. In this work, the tunable volatile threshold switching (TS) and non-volatile memory switching (MS) behaviors have been obtained in a memristor composed of AIZS QDs by regulating the magnitude of compliance current. Additionally, the innovative Ag/AIZS structure devices without traditional oxide layer exhibit low operation voltage (∼0.25 V) and programming current (100 nA) under the TS mode. Moreover, the devices achieve reproducible bipolar resistive switching (RS) behaviors with large ON/OFF ratio of ∼105, ultralow power consumption of ∼10–10 W, and good device-to-device uniformity under the MS mode. Furthermore, the charge transport mechanisms of the high- and low-resistance states under the positive and negative bias have been analyzed with space-charge-limited-current and filament conduction models, respectively. This work not only validates the potential of AIZS QDs acting as dielectric layer in RS devices but also provides a new guideline for designing ultralow power and multiple RS characteristics devices.

Green thin film for stable electrical switching in a low-cost washable memory device: proof of concept.

Low-cost and washable resistive switching (RS) memory devices with stable retention and low operational voltage are important for higher speed and denser non-volatile memories. In the case of green electronics, pectin has emerged as a suitable alternative to toxic metal oxides for resistive switching applications. Herein, a pectin-based thin film was fabricated on a fluorine-doped tin oxide glass substrate for RS mechanism. The presence of sp3–C groups with low binding energy corresponds to tunable charged defects and the oxygen vacancies confirmed by the O 1s spectra that plays a decisive role in the resistive switching mechanism, as revealed by X-ray photoemission spectroscopy (XPS). The surface morphology of the pectin film shows homogeneous growth and negligible surface roughness (38.98 ± 9.09). The pectin film can dissolve in DI water (10 minutes) owing to its ionization of carboxylic groups, that meet the trends of transient electronics. The developed Ag/pectin/FTO-based memory cell exhibits stable and reproducible bipolar resistive switching behavior along with an excellent ON/OFF ratio (104) and negligible electrical degradation was observed over 30 repeated cycles. Hence, it appears to be a valuable application for green electronics. Indeed, biocompatible storage devices derived from natural pectin are promising for high-density safe applications for information storage systems, flexible electronics, and green electronics.

Point-contact enabled reliable and low-voltage memristive switching and artificial synapse from highly transparent all-oxide-integration

Mimicking brain-like functionality for enabling higher-level artificial intelligence is one of the ultimate goals in neuromorphic computing, which could be achieved by two-terminal memristors. However, conventional memristors are suffering from severe shortcomings such as temporal (cycle-to-cycle) and spatial (device-to-device) reproducibility along with high operative voltage, albeit all these are crucial for accurate and quick information processing. Here, we demonstrate point-contact enabled reproducible and reliable bipolar resistive switching from all-oxide-based highly transparent memristors with low operating voltage (<0.5 V) and long retention times. Analogous to bio-synapse, memristor mimics the functions like short-term potentiation and short-term depression under the action of applied pulses. Conductive atomic force microscopy unambiguously revealed the formation of the localized conducting channels as well as nanoscale dynamics. Further, finite element simulation confirms that the tip-enhanced electric field could generate localized conduction channels, in contrast to the uniform electrode in which edges are the preferred sites for conduction. These results represent an important milestone toward the use of point-contact to design all-oxide-based highly transparent and reproducible resistive switching devices as well as artificial synapse.

Biomaterial-Induced Stable Resistive Switching Mechanism in TiO2 Thin Films: The Role of Active Interstitial Sites/Ions in Minimum Current Leakage and Superior Bioactivity.

Leakage of current in oxide layers is the main issue for higher speed and denser resistive random-access memory. Defect engineering played a substantial role in meeting this challenge by doping or producing controlled interstitial defects or active sites. These controlled active sites enabled memory cells to form a stable and reproducible conduction filament following an electrochemical metallization model. In this study, a defect-abundant lime peel extract (LPE)-mediated anatase TiO2 thin film was fabricated using a simple hydrothermal route. The detailed structural and morphological analysis of the bioactive anatase TiO2-LPE thin film reveals the homogeneous growth of TiO2 flowers and distinct features in terms of controlled defects as compared to simple anatase TiO2. These interstitial defects (Ti+3 and Ti+4) behave as active sites for cation migrations along highly conductive K1+ ions because of the mediation of LPE. The defect-free surface reveals slight surface roughness (4.8 nm) that successfully minimizes leakage of current. The strategy enabled a reliable conductive bridge filament, which can replicate with no more electric degradation. The Ag/TiO2-LPE/FTO-based memory cell demonstrates reproducible bipolar resistive switching along with a high ON/OFF ratio (>105), excellent endurance (1.5 × 103 cycles), and long-term retention (105 s) without any electrical degradation. Furthermore, green-synthesized TiO2-LPE nanoparticles have shown superior antibacterial activity as compared to other green syntheses of different plants or fruits against the toxic microorganisms present in inorganic media.

Bipolar resistive switching behavior and conduction mechanisms of composite nanostructured TiO2/ZrO2 thin film

In this work, TiO2/ZrO2 bilayer thin film was prepared on fluorine doped tin oxide (FTO)/glass substrates by using a simple and low-cost chemical solution deposition method. Reproducible bipolar resistive switching (RS) characteristics in Au/TiO2/ZrO2/FTO/glass devices are reported in this work. TiO2/ZrO2 bilayer thin films prepared in this work shows reversible bipolar resistive switching and unidirectional conduction performances under applying voltage and these special performances of TiO2/ZrO2 bilayer thin films was first reported. Obvious resistive switching performance can be observed after setting a compliance current, the ratio of high/low resistance reached about 100 at a read voltage of +0.1V and −0.1V and the RS properties showed no obvious degradation after 100 successive cycles tests. The resistive switching characteristics of Au/TiO2/ZrO2/FTO/glass device can be explained by electron trapping/detrapping related with the vacancy oxygen defects in TiO2/ZrO2 bilayer thin film layer. According to slope fitting, the main conduction mechanisms of the sample are Ohmic and Space charge limited current mechanism.

White light-modulated bipolar resistive switching characteristics of Cu/MoS2 NRs/Pt MIM structure

The present work explores the white light controlled resistive switching functionality of MoS2 nanorods (NRs) by fabricating a metal-insulator-metal stack configuration. The Cu/MoS2 NRs/Pt/Si device demonstrates the reproducible two-state bipolar resistive switching characteristics under both dark and light environments. In the dark condition, the resistive switching behavior of the NR device could be attributed to the metallic path formation/rupture between top and bottom electrodes. Whereas the applied white light causes the lowering of SET and RESET voltages by inducing conducting path formation/rupture via electron trapping/detrapping in sulfur vacancies across the MoS2 NRs. The formation of a conducting path under dark and light illumination conditions is well explained by proposing a conceptual model and analyzing the resistance vs temperature measurements. It is observed that the white light acts as an external tool to modulate the resistive switching behavior of the fabricated NR device. The correlation between the applied light intensity and the SET voltage is also demonstrated. The NR structure of the MoS2 device provides good endurance of 1500 cycles and a long retention time of 103 s at room temperature under light illumination because of straight conducting path formation through NRs. These results demonstrate that the optically active MoS2 NR based devices have potential for next generation tunable nonvolatile resistive random access memory applications with additional functionality such as photosensors and optoelectronic switches.

Yttria-stabilized zirconia cross-point memristive devices for neuromorphic applications

An array of cross-point memristive devices has been implemented on the basis of yttria-stabilized zirconia thin film for applications in prototypes of spiking neural networks. The resistive switching phenomena and the plasticity nature of such memristive devices are studied. Reproducible bipolar resistive switching and precise tuning of resistive state are demonstrated and used to implement the plasticity rule according to STDP (spike-timing-dependent plasticity) mechanism. STDP learning is found to be dependent on the memristor's initial resistive state value and discussed in terms of the finite conductance change in studied structures. Obtained results provide the foundation for autonomous neuromorphic circuits with unsupervised learning development.

Bipolar resistive switching characteristics of samarium oxide solid electrolyte thin films for non-volatile memory applications

In this paper, we report the bipolar resistive switching behaviors in Ag/Sm2O3/Pt structures where the Sm2O3 thin films act as solid electrolyte layer of electrochemical metallization memory (ECM) devices. The memory devices show reproducible and stable bipolar resistive switching over 1000 cycles with a resistance ratio (high-resistance state to low-resistance state) of over 4 orders of magnitude and stable retention for over 104[Formula: see text]s at room temperature. Moreover, the benefits of high yield and multilevel storage possibility make the device promising in the next generation non-volatile memory application.

Flexible Pyrene/Phenanthro[9,10‐d]imidazole‐Based Memristive Devices for Mimicking Synaptic Plasticity

Emulation of memory and learning functionalities of biological synapses using a two‐terminal electronic device with bidirectional progressive conductance modulation is an indispensable move toward the development of bio‐inspired neuromorphic networks. Herein, a small molecule 1‐phenyl‐2‐(4‐(pyren‐1‐yl)phenyl)‐1H‐phenanthro[9,10‐d]imidazole (pPPI) is synthesized, and an organic synaptic device is investigated with bipolar resistive switching characteristics and bidirectional gradual conductance regulation for the first time. A facile solution‐processing approach can be used to deposit a uniform active layer on flexible substrates. Our pPPI‐based nonvolatile memory presents a superior electrical performance, such as relatively stable and reproducible bipolar resistive switching phenomena and a robust data retention capability. In particular, our device displays a remarkably large switching window of around 7.0 × 107, which is a record ON/OFF ratio compared with other small molecule‐based memories up to now. In addition, comprehensive cognitive functions of chemical synapses, for example, the excitatory postsynaptic current (EPSC), paired‐pulse facilitation/depression (PPF/PPD), spike‐rate‐dependent plasticity (SRDP), and spike‐time‐dependent plasticity (STDP) are successfully achieved. Our achievement of a synaptic device based on small molecules may boost the development of bio‐inspired neuromorphic systems using organic electronics.

All-inorganic perovskite Cs4PbBr6 thin films in optoelectronic resistive switching memory devices with a logic application

Perovskite-based Cs4PbBr6 has potential applications in optoelectronic devices, such as photodetectors, electroluminescence devices and color converters. All-inorganic perovskite Cs4PbBr6 thin films were successfully prepared by a simple low-temperature synthesis method and were first implemented as an insulating layer in Au/Cs4PbBr6/PEDOT:PSS/Pt devices. The memory devices possess reproducible bipolar resistive switching behavior, low operating voltages, good endurance, and long retention times. Furthermore, the novel sandwich architecture enables the application of the Cs4PbBr6 films as memristors and photoresponsive behaviors. Considering the distinct photoresponses of the resistance state as a nonvolatile memory, the devices can be used as a logical "OR" gate by applying bias voltages and light illumination as input signals. The formation and annihilation of Br- ion vacancy filaments induced by the external bias and light illumination can result in pronounced resistive switching performance. It is believed that solution-processed Cs4PbBr6-based devices have great potential for technological deployment at the forefront as a photonic nonvolatile memory as well as integrated modulating and arithmetic functions.

Characteristics of the bipolar resistive switching behavior in memory device with Au/ZnO/ITO structure

In this work, reproducible and stable bipolar resistive switching behavior without the requirement of forming process is observed in the memory device with Au/ZnO/ITO structure. It shows a high Ron/Roff ratio, where Ron and Roff are the resistance at low resistance state (LRS) and high resistance state (HRS), respectively. The dominated transport mechanisms for LRS and HRS are related to space charge limited current and Ohmic behavior, respectively. This bipolar resistive behavior is attributed to the formation and rupture of conducting filaments which are constructed with oxygen vacancies. The Au/ZnO/ITO device discussed in this work shows huge potential applications in the next generation nonvolatile memory field.

Origin of tri-state resistive switching characteristics in SiCN thin films for high-temperature ReRAM applications

In the present work, the resistive switching (RS) properties of Ag/SiCN/Pt and W/SiCN/Pt devices having electrochemically active (Ag) and inactive (W) top electrodes have been systematically investigated. Both devices revealed stable and reproducible bipolar resistive switching characteristics. The W/SiCN/Pt device exhibits two-state resistive switching behavior, i.e., low resistance state (LRS) and high resistance state (HRS), whereas the Ag/SiCN/Pt device shows tri-state RS characteristics [LRS, intermediate resistance state, and HRS)]. The two resistance state RS characteristics of the W/SiCN/Pt device were ascribed to conduction path formation/rupture via electron trapping/de-trapping in nitride-related traps. However, the tri-state RS behavior of the Ag/SiCN/Pt device could be attributed to conduction path formation via electron trapping in nitride-related traps followed by an additional Ag filament growth between the top and bottom electrodes. The origin of tri-state switching in the Ag/SiCN/Pt device and Ag filament formation were well explained by a conceptual model and the temperature and cell area dependence of the resistance measurement. The Ag/SiCN/Pt device exhibits good reliable properties such as endurance of ∼105 cycles and long retention time ∼105 s at a high temperature of 200 °C. This comprehensive study suggests that nonvolatile multi-level (three-level) resistive switching in the SiCN-based device can be achieved by the formation of different types of conducting filaments sequentially and the Ag/SiCN/Pt device could be capable of futuristic multi-bit storage resistive random access memory which can operate at high temperature.

Bipolar resistive switching behavior in MoS2 nanosheets fabricated on ferromagnetic shape memory alloy

The present study explores the systematic investigation of resistive switching response of magnetron sputtered MoS2 thin films sandwiched between a Ni-Mn-In ferromagnetic shape memory alloy (bottom) and copper (top) electrodes. The Cu/MoS2/Ni-Mn-In device exhibits stable and reproducible bipolar resistive switching behavior. The current-voltage (I-V) analysis suggests that the device shows ohmic conduction behavior in the low resistance state (LRS) while space charge limited conduction is the dominating conduction mechanism in the high resistance state (HRS). The compliance current vs reset current measurements were also performed which reveals that the power consumption of the device can be suppressed by decreasing the compliance current. To explain the resistive switching behavior in the Cu/MoS2/Ni-Mn-In device, a conceptual model based on copper ion migration, is proposed and well explained. The resistive switching parameters such as the set voltage, LRS and HRS resistances, are also investigated in a temperature range overlapping with the martensite phase transformation temperatures of the bottom ferromagnetic shape memory alloy (Ni-Mn-In) electrode. It provides temperature as an extra degree of freedom to modulate the resistive switching characteristics of the fabricated device. The Cu/MoS2/Ni-Mn-In device shows a decent endurance of 500 and a long retention of 103. These results demonstrate the feasibility of the MoS2 based devices for futuristic tunable nonvolatile resistive random access memory applications.