Conductive Filament Research Articles

Resistive Mechanisms and Microscopic Electrical Models of Metal Oxide Resistive Memory

Resistive random access memory (RRAM) has emerged as a competitive candidate for nonvolatile memory due to its high speed, low power consumption, simple structure, strong scalability, and CMOS compatibility. Herein, an overview of the electrodes and resistive layers is first discussed according to the material properties and structure of RRAM. Then, recent advances in the resistive mechanisms of RRAM are comprehensively discussed and evaluated based on experimental research results and fundamental physics principles. Then, the electrochemical metallization mechanism and the valence change metallization mechanism are thoroughly studied, as well as the formation, fracture, and resistance state change behavior of conductive filaments in RRAM based on the ion motion type. Then, various electron transport‐dominated resistance mechanisms are reviewed, and electron transport and resistance switching in the space–charge‐limited current mode is thoroughly examined using energy band theory. As a result, a theoretical reference is provided for the development of nanothin‐film devices.

Physical model simulations of Hf oxide resistive random access memory device with a spike electrode structure

Resistive memory has become an attractive new memory type due to its outstanding performance. Oxide-based resistive random access memory is one type of widely used memory whose resistance can be transformed by applying current or voltage. Memristors are widely used in various kinds of memories and neural morphological calculations. Therefore, it is of vital importance to understand the physical change mechanism of an internal memristor under stimulation to improve electrical properties of the memristor. In our studies, a device model based on Hf oxide was proposed, then completely processes of the forming, reset and set were simulated. Meantime, the generation and recombination of oxygen vacancies were considered in all the processes, making the simulation more practical. In addition, a spike electrode structure was applied, a gathering electric field can be generated in the oxide layer so that the improved device has a faster forming voltage, lower forming current and lower instantaneous power consumption in the ON state. Finally, the effects of spike electrode length on the forming process were studied, the research results reveal that a longer probe electrode can engage a lower forming voltage and accelerate the formation of conductive filaments.

Definition of a Localized Conducting Path via Suppressed Charge Injection in Oxide Memristors for Stable Practical Hardware Neural Networks.

Oxide-based memristors have been demonstrated as suitable options for memory components in neuromorphic systems. In such devices, the resistive switching characteristics are caused by the formation of conductive filaments (CFs) comprising oxygen vacancies. Thus, the electrical performance is primarily governed by the CF structure. Despite various approaches for regulating the oxygen vacancy distributions in oxide memristors, controlling the CF structure without modifying the device configuration related to material compatibility is still a challenge. This study demonstrates an effective strategy for localizing CF distributions in memristors by suppressing charge injection during the formation of conducting paths. As the injected charge quantity is reduced in the electroforming process of the oxide memristor, the CF distributions become narrower, leading to more reproducible and stable resistive switching characteristics in the device. Based on these findings, a reliable hardware neural network comprising oxide memristors is constructed to recognize complex images. The developed memristor has been employed as a synaptic memory component in systems without degradation for a long time. This promising concept of oxide memristors acting as stable synaptic components holds great potential for developing practical neuromorphic systems and their expansion into artificial intelligent systems.

Mechanisms of Formation of Current-Conducting Channels in Bipolar Memristors of Various Designs

The paper shows and describes various mechanisms for the formation of a conductive channel in bipolar memristors of the vacancy and ion types: in the first case, due to the nucleation and growth of conductive filaments, and in the second, due to segregation processes with the formation of spatial associations of ions from the active electrode material. The volt-ampere characteristics for memristors of both types are given and a comparison is made of the design features of these memristors

Controllable analog-to-digital bipolar resistive switching behavior and mechanism analysis in δ-MnO2-based memristor

Memristor is one of the most emerging electronic components to realize synaptic functions in the neuromorphic computing. Memristor with controllable analog-to-digital resistive switching behaviors has the potential to considerably accelerate the realization of high-performance neuromorphic computing. In this work, a memristor with Ag/δ-MnO2/Ti structure was fabricated, and an analog-to-digital resistive switching behavior with voltage regulation was demonstrated. Memristive switching with analogue characteristics can generate a gradual increase in conductance at low voltage of less than 1.3 V to aid in the building of artificial synapses. The Ag/δ-MnO2/Ti memristor with excellent digital resistive switching performance can be applied for data storage and in-memory computation at voltage windows of higher than 1.3 V. Furthermore, the resistive switching mechanism was analyzed by employing the first-principle calculation. The main reason for the emergence of digital memristor characteristics is the Ag+ ions can enter to the δ-MnO2 layer driven by a larger voltage and continue to cluster and form conductive filament. This work enriches the application of analog-to-digital bipolar memristor in neuromorphic computing, and facilitates further realization of memristor in low-power and high-density circuit integration.

Solution-Processed Hydrogen-Bonded Organic Framework Nanofilms for High-Performance Resistive Memory Devices.

The integration of hydrogen-bonded organic frameworks (HOFs) into electronic devices holds great promise due to their high crystallinity, intrinsic porosity, and easy regeneration. However, despite their potential, the utilization of HOFs in electronicdevices remains largely unexplored, primarily due to the challenges associated with fabricating high-quality films. Herein, a controlled synthesis of HOFnanofilms with smooth surface, good crystallinity, and high orientation is achieved using a solution-processed approach. The memristors exhibit outstanding bipolar switching performance with alow set voltage of 0.86V, excellent retention of 1.64 × 104 s, and operational endurance of 60 cycles. Additionally, these robust memristors display remarkable thermal stability, maintaining their performance even at elevated temperatures of up to 200 °C. More strikingly, scratched HOFfilms can be readily regenerated through a simple solvent rinsing process, enabling their reuse for the fabrication of new memristors, which is difficult to achieve with traditional resistive switching materials. Additionally, a switching mechanism based on the reversible formation and annihilation of conductive filaments is revealed. This work provides novel and invaluable insights that have a significant impact on advancing the widespread adoption of HOFs as active layers in electronic devices.

Describing the analog resistance change of HfOx-based neuromorphic synapses using a compact series trap-assisted tunneling and Ohmic conduction model

Changes in the resistance of Ti/HfOx synapses are known to be governed by a thin-oxide barrier associated with the oxidation/reduction of a Hf-rich conducting filament (CF). However, experimental characterization of the CF is challenging. Critical physical properties and processes, such as the barrier location, time-dependent thickness during analog pulsing, and the temperature-effect on current, need to be better established. In this work, a compact model based on Trap-Assisted-Tunneling and Ohmic transport is utilized to analyze the analog switching of HfOx synapses. The model agrees well with the experimentally observed current–voltage relation and its temperature dependence. The extracted barrier heights during analog pulsing are consistent with a barrier situated near the reset anode; the electrode is opposite to the Ti oxygen-reservoir layer. A Finite Element Analysis simulation, which incorporates oxygen-vacancy migration, independently supports this conclusion. The model further permits extraction of the barrier thickness in relation to the analog pulses.

Analog programming of CMOS-compatible Al2O3/TiO2−x memristor at 4.2 K after metal-insulator transition suppression by cryogenic reforming

Exploration of memristors' behavior at cryogenic temperatures has become crucial due to the growing interest in quantum computing and cryogenic electronics. In this context, our study focuses on the characterization at cryogenic temperatures (4.2 K) of TiO2−x-based memristors fabricated with a CMOS-compatible etch-back process. We demonstrate a so-called cryogenic reforming (CR) technique performed at 4.2 K to overcome the well-known metal-insulator transition (MIT), which limits the analog behavior of memristors at low temperatures. This cryogenic reforming process was found to be reproducible and led to a durable suppression of the MIT. This process allowed to reduce by ∼20% the voltages required to perform DC resistive switching at 4.2 K. Additionally, conduction mechanism studies of memristors before and after cryogenic reforming from 4.2 to 300 K revealed different behaviors above 100 K, indicating a potential change in the conductive filament stoichiometry. The reformed devices exhibit a conductance level that is 50 times higher than ambient-formed memristor, and the conduction drop between 300 and 4.2 K is 100 times smaller, indicating the effectiveness of the reforming process. More importantly, CR enables analog programming at 4.2 K with typical read voltages allowing to store up to 4 bits of information on a single CR memristor. Suppressing the MIT improved the analog switching dynamics of the memristor leading to ∼250% larger on/off ratios during long-term depression (LTD)/long-term potentiation (LTP) resistance tuning. This enhancement opens up the possibility of using TiO2−x-based memristors to be used as synapses in neuromorphic computing at cryogenic temperatures.

Genetic evidence that multiple cytochrome nanowires are necessary for Fe(III) oxide reduction in Geobacter sulfurreducens

Geobacter sulfurreducens is a dissimilatory metal-reducing microorganism capable of utilizing insoluble acceptors via extracellular electron transfer. While a large number of multiheme c-type cytochromes expressed by G. sulfurreducens are implicated in linking its cytoplasmic respiratory chain to materials beyond its outer membrane, whether these proteins have specific roles in reduction or recognition of particular metals is unknown. Recently, structures of three extracellular conductive c-type cytochrome filaments, often referred to as nanowires, were reported. Comprised of either OmcS, OmcE, or OmcZ, these nanowires are long polymers of protein subunits with a core of closely spaced hemes, with no similarity in sequence, fold, glycosylation, subunit size, or diameter. We utilized a markerless deletion approach to construct single, double, and triple-deletion strains in an isogenic background to investigate possible roles of OmcS, OmcE, and OmcZ. When soluble Fe(III) or the organic acceptor fumarate were electron acceptors, no defects were observed in any mutant. When freshly precipitated Fe(III) oxide was tested as an electron acceptor, mutants lacking omcE were strongly affected, reducing Fe(III) approximately half as fast. No other single mutant (∆omcS or ∆omcZ) showed a defect. Double mutants containing only omcE (∆omcSZ) also showed a defect, suggesting other proteins could be required in addition to OmcE. The double mutant containing only omcZ (∆omcSE) also showed a partial defect, while double mutants containing only omcS (∆omcEZ) were completely unable to reduce Fe(III) oxide. The triple (∆omcESZ) mutant was also unable to reduce Fe(III) oxides. Taken together, this indicates that genes for two separate nanowires are necessary to completely reduce this form of Fe(III) oxide. This is the first evidence that omcZ, which has only been implicated in electron transfer to electrodes, could also be needed for metal reduction. With the recent discovery of two completely unrelated mutltiheme cytochrome nanowires in thermophilic Archaea, different conductive filaments with different substrate specificities may have repeatedly evolved to facilitate extracellular respiration.

Resistive switching characteristics of MnO2-based thin film for transparent non-volatile ReRAM

The present study covers the fabrication of indium tin oxide (ITO)/manganese dioxide (MnO2)/ITO capacitor-based transparent resistive random access memory (ReRAM) device. The ITO/MnO2/ITO device exhibits 80 % transparency in the visible region along with steady bipolar resistive switching characteristics. The conduction mechanism investigation reveals that the current in low resistance state (LRS) is governed by Ohmic conduction. However, the dominance of trap-controlled space charge limited conduction (SCLC) mechanism in high resistance state (HRS) was observed. The resistive switching phenomenon in ITO/MnO2/ITO device was caused due to the formation and rupture of conduction filament via electron trapping and de-trapping in oxide-related traps (oxygen vacancies). In terms of reliability, the MnO2-based device exhibits good endurance up to 104 cycles and long retention for 104 s at room-temperature as well as at 85 °C. Cumulative probability distribution shows resistance ratio of 103 between LRS and HRS, which is sufficient to read memory states. These outcomes suggest that the ITO/MnO2/ITO-based transparent ReRAM could be a futuristic see-through electronic device for artificial intelligence (AI) application.

Estimation of the activation energy in the Ag/SnSe/Ge2Se3/W self-directed channel memristor

In this study, we conducted an investigation into the Ag/SnSe/Ge2Se3/W ionic memristor, focusing on the determination of activation energies associated with its two primary operational processes: the formation of conductive filaments and memristor degradation. To ascertain the electrical conductivity of the memristor in both its basic electronic states, a low resistance state and a high resistance state, we constructed current-voltage characteristics. The estimation of activation energy values was carried out employing the Arrhenius law and the provisions of irreversible thermodynamics, with specific reference to Onsager's second postulate. This fundamental concept posits that the growth rate of irreversible component of entropy can be expressed as the summation of products involving fluxes and thermodynamic forces when a system tends towards its equilibrium state. In the context of this study, the equilibrium state of the memristor is defined as the condition at which the memristor can no longer function as a resistive memory cell. Our experimentation involved the application of a flux of Ag+ ions (electromigration). The calculated activation energy values were found to be 0.24 eV for the initial process and 1.16 eV for the latter. These divergent activation energy values indicate the differentiation between the agglomerative mechanism that governs the formation of conductive channels, prevalent in the Ag/SnSe/Ge2Se3/W memristor, and the "conventional" substance transfer mechanism based on a group of point defects that manifests itself during the memristor's degradation.

Solving the issue of increasing forming voltage during device miniaturization in hafnium oxide-based resistive access memory using high-k sidewall material

An electrothermal coupling model of resistive random access memory (RRAM) was established based on the oxygen vacancy conduction mechanism. By resolving the partial differential equation for the coefficients, the variation process of the device resistance was simulated. In our studies, a device model was proposed which can accurately simulate the whole process of RRAM forming, reset, and set. Based on the established model, a new high dielectric constant (high-k) material (La2O3) is introduced as the sidewall material. The La2O3 sidewall material can concentrate the electric field and helps to speed up the formation of conductive filaments. The La2O3 sidewall can effectively reduce the forming voltage increase during the miniaturization process. Then, the influence of sidewall thermal conductivity on forming voltage is studied, and it is discovered that low thermal conductivity helps to reduce the model’s forming voltage and increase the temperature concentration. These findings serve as a foundation for more studies on the choice of sidewall materials.

Investigation on the new reliability issues of PCB in 5G millimeter wave application

PurposeThe purpose of this paper is to illustrate the new technical demands and reliability challenges to printed circuit board (PCB) designs, materials and processes when the transmission frequency increases from Sub-6 GHz in previous generations to millimeter (mm) wave in fifth-generation (5G) communication technology.Design/methodology/approachThe approach involves theoretical analysis and actual case study by various characterization techniques, such as a stereo microscope, metallographic microscope, scanning electron microscope, energy dispersive spectroscopy, focused ion beam, high-frequency structure simulator, stripline resonator and mechanical test.FindingsTo meet PCB signal integrity demands in mm-wave frequency bands, the improving proposals on copper profile, resin system, reinforcement fabric, filler, electromagnetic interference-reducing design, transmission line as well as via layout, surface treatment, drilling, desmear, laminating and electroplating were discussed. And the failure causes and effects of typical reliability issues, including complex permittivity fluctuation at different frequencies or environments, weakening of peel strength, conductive anodic filament, crack on microvias, the effect of solder joint void on signal transmission performance and soldering anomalies at ball grid array location on high-speed PCBs, were demonstrated.Originality/valueThe PCB reliability problem is the leading factor to cause failures of PCB assemblies concluded from statistical results on the failure cases sent to our laboratory. The PCB reliability level is very essential to guarantee the reliability of the entire equipment. In this paper, the summarized technical demands and reliability issues that are rarely reported in existing articles were discussed systematically with new perspectives, which will be very critical to identify potential reliability risks for PCB in 5G mm-wave applications and implement targeted improvements.

Nonvolatile and volatile resistive switching characteristics in MoS2 thin film for RRAM application

Resistive random access memory (RRAM) is one of strong candidates for future memory technology. The volatile and nonvolatile properties of devices are important foundations for the construction of neuromorphic hardware systems, which still represents a challenge. In this study, we fabricated the Cu/MoS2/ITO RRAM by RF magnetic sputtering with different thicknesses of MoS2 thin films, and explored the possibility of obtaining the nonvolatile and volatile memristive effects of RRAM devices through different operating voltages. By systematically investigating the resistive switching (RS) characteristics, we proposed and discussed the nonvolatile and volatile conceptual models to elaborate on the RS mechanism of the fabricated devices. The nonvolatile and volatile behaviors were explained respectively by the forming and breaking of Cu conductive filaments between Cu and ITO, and the trapping and de-trapping of electrons leading to the change of the Schottky barrier at the ITO/MoS2 interface.

Development of a Test Bench for the Experimentation of the Electrical Performance of 3D Printed Multi-Material Parts

Obtaining multi-material parts by material extrusion processes is becoming more interesting as the available materials permit achieving superior properties in the 3D printed products. Combining conductive filament with other with elastomeric properties makes it possible to materialise electrical circuits for introducing active elements in specific parts, such as sensors, triggers or antennas. In this context, a test bench has been designed, manufactured and set-up, to evaluate the electrical behaviour of multi-material 3D printed test samples composed of two or more materials, being one a conductor of electricity (at least) and the other(s) non-conductive but flexible. The functionalities of the test bench include the possibility to apply tensile, compressive, shear, or flexural loads to the test samples. The electrical performance of the samples can be assessed in terms of resistivity and capacitance, in real time, when the test bench stands still and when it conducts the series of movements that produce the elastic deformation of the samples. To achieve this, three electronic circuits have been designed with their own corresponding control with Arduino: a circuit to measure the variation of the resistance of the test samples, a circuit to measure the variation of the capacitance of the test samples, and a circuit controlling the movements of the mechanical set (motor and terminals) that generates the deformation of the test samples. The test bench is connected to a desktop computer to ease the data export, treatment, and visualisation. As a set-up of the test bench, several preliminary experimentation measurements have been done to assess factors of interest such as sensitivity and a correlation index. The present work also frames the requirements of the parts to be tested in the bench and outlines the work procedure to carry out the series of experiments.

Effect of oxygen vacancy and Si doping on the electrical properties of Ta2O5 in memristor characteristics

The resistive switching behavior in Ta2O5 based memristors is largely controlled by the formation and annihilation of conductive filaments (CFs) that are generated by the migration of oxygen vacancies (OVs). To gain a fundamental insight on the switching characteristics, we have systematically investigated the electrical transport properties of two different Ta2O5 polymorphs (epsilon-Ta2O5 and λ-Ta2O5), using density functional theory calculations, and associated vacancy induced electrical conductivity using Boltzmann transport theory. The projected band structure and DOS in a few types of OVs, (two-fold (O2fV), three-fold (O3fV), interlayer (OILV), and distorted octahedral coordinated vacancies (OεV)) reveal that the presence of OILV would cause Ta2O5 to transition from a semiconductor to a metal, leading to improved electrical conductivity, whereas the other OV types only create localized mid-gap defect states within the bandgap. On studying the combined effect of OVs and Si-doping, a reduction of the formation energy and creation of defect states near the conduction band edge, is observed in Si-doped Ta2O5, and lower energy is found for the OVs near Si atoms, which would be advantageous to the uniformity of CFs produced by OVs. These findings can serve as guidance for further experimental work aimed at enhancing the uniformity and switching properties of resistance switching for Ta2O5-based memristors.

A research on MoTe2-based memristor and switching stability improvement

Electronic synapse based on two-dimensional material are equivalent to synapses because of their excellent properties, which is very pivotal for constructing neuromorphic computing to break through the traditional Von Neumann architecture. In the study, a vertical double-ended memristor is prepared by using the dispersion of molybdenum disulfide nanosheets. Memristors based on MoTe2 structure show stable bipolar nonvolatile resistive behavior. Based on this, we introduce carbon dots layer into the original device structure, and improve the device performance by using carbon atoms to form new conductive filaments driven by electric field. This research provides a reliable solution for the next generation of neuromorphic computing.

Influences of Cu Doping on the Microstructure, Optical and Resistance Switching Properties of Zinc OxideThin Films.

Copper-doped zinc oxide films (Zn1-xCuxO) (x = 0, 2%, 4%, 6%) were fabricated on conductive substrates using the sol-gel process. The crystal structure, optical and resistive switching properties of Zn1-xCuxO films are studied and discussed. RRAM is made using Zn1-xCuxO as the resistive layer. The results show that the (002) peak intensity and grain size of Zn1-xCuxOfilms increase from 0 to 6%. In addition, PL spectroscopy shows that the oxygen vacancy defect density of Zn1-xCuxO films also increases with the increase in Cu. The improved resistive switching performance of the RRAM device can be attributed to the formation of conductive filaments and the destruction of more oxygen vacancies in the Zn1-xCuxO film. Consequently, the RRAM device exhibits a higher low resistance state to high resistance state ratio and an HRS state of higher resistance value.

Mixed Graphite/Carbon Black Recycled PLA Conductive Additive Manufacturing Filament for the Electrochemical Detection of Oxalate.

Mixing of graphite and carbon black (CB) alongside recycled poly(lactic acid) and castor oil to create an electrically conductive additive manufacturing filament without the use of solvents is reported herein. The additively manufactured electrodes (AMEs) were electrochemically benchmarked against a commercial conductive filament and a bespoke filament utilizing only CB. The graphite/CB produced a heterogeneous rate constant, k0, of 1.26 (±0.23) × 10-3 cm s-1 and resistance of only 155 ± 15 Ω, compared to 0.30 (±0.03) × 10-3 cm s-1 and 768 ± 96 Ω for the commercial AME. Including graphite within the filament reduced the cost of printing each AME from £0.09, with the CB-only filament, to £0.05. The additive manufacturing filament was successfully used to create an electroanalytical sensing platform for the detection of oxalate within a linear range of 10-500 μM, achieving a sensitivity of 0.0196 μA/μM, LOD of 5.7 μM and LOQ of 18.8 μM was obtained. Additionally, the cell was tested toward the detection of oxalate within a spiked synthetic urine sample, obtaining recoveries of 104%. This work highlights how, using mixed material composites, excellent electrochemical performance can be obtained at a reduced material cost, while also greatly improving the sustainability of the system.

Wafer‐Scale Memristor Array Based on Aligned Grain Boundaries of 2D Molybdenum Ditelluride for Application to Artificial Synapses

Abstract2D materials have attracted attention in the field of neuromorphic computing applications, demonstrating the potential for their use in low‐power synaptic devices at the atomic scale. However, synthetic 2D materials contain randomly distributed intrinsic defects and exhibit a stochasitc forming process, which results in variability of switching voltages, times, and stat resistances, as well as poor synaptic plasticity. Here, this work reports the wafer‐scale synthesis of highly polycrystalline semiconducting 2H‐phase molybdenum ditelluride (2H‐MoTe2) and its use for fabricating crossbar arrays of memristors. The 2H‐MoTe2 films contain small grains (≈30 nm) separated by vertically aligned grain boundaries (GBs). These aligned GBs provide confined diffusion paths for metal ions filtration (from the electrodes), resulting in reliable resistive switching (RS) due to conductive filament confinement. As a result, the polycrystalline 2H‐MoTe2 memristors shows improvement in the RS uniformity and stable multilevel resistance states, small cycle‐to‐cycle variation (<8.3%), high yield (>83.7%), and long retention times (>104 s). Finally, 2H‐MoTe2 memristors show linear analog synaptic plasticity under more than 2500 repeatable pulses and a simulation‐based learning accuracy of 96.05% for image classification, which is the first analog synapse behavior reported for 2D MoTe2 based memristors.