Filamentary Switching Mechanism Research Articles

Zeolitic-imidazolate framework (ZIF-67) based bipolar memristor for nonvolatile ReRAM application

In the realm of next-generation nonvolatile memories, there is a strong demand for resistive random access memory (ReRAM) devices that are affordable, stable, and simple to produce. Metal–organic frameworks (MOFs) have garnered extensive research attention across diverse fields due to their remarkable chemical adaptability, stability, and exceptional structural variability. In this work, a ReRAM device has been fabricated that incorporates a solution-processed thin film of zeolitic imidazolate framework (ZIF-67), a material of MOFs class, as the resistive switching (RS) layer. This Al/ZIF-67/ITO structured device possesses a consistent RS behavior with a high on/off resistance ratio of ∼104. The retention of low resistance state and high resistance state have been shown up to 104 s. Furthermore, the device also shows a consistent switching for 500 cycles of pulse switching signals of +6 V/−1.8 V (set/reset). Based on the impedance spectroscopy analysis, a filamentary switching mechanism has been established for the RS behavior of the device. The robust and enduring performance, coupled with the substantial on/off resistance ratio and high retention of the states, makes this device a promising candidate for nonvolatile ReRAM applications.

Vacancy-Engineered Nickel Ferrite Forming-Free Low-Voltage Resistive Switches for Neuromorphic Circuits.

Innovations in resistive switching devices constitute a core objective for the development of ultralow-power computing devices. Forming-free resistive switching is a type of resistive switching that eliminates the need for an initial high voltage for the formation of conductive filaments and offers promising opportunities to overcome the limitations of traditional resistive switching devices. Here, we demonstrate mixed charge state oxygen vacancy-engineered electroforming-free resistive switching in NiFe2O4 (NFO) thin films, fabricated as asymmetric Ti/NFO/Pt heterostructures, for the first time. Using pulsed laser deposition in a controlled oxygen atmosphere, we tune the oxygen vacancies together with the cationic valence state in the nickel ferrite phase, with the latter directly affecting the charge state of the oxygen vacancies. The structural integrity and chemical composition of the films are confirmed by X-ray diffraction and hard X-ray photoelectron spectroscopy, respectively. Electrical transport studies reveal that resistive switching characteristics in the films can be significantly altered by tuning the amount and charge state of the oxygen vacancy concentration during the deposition of the films. The resistive switching mechanism is seen to depend upon the migration of both singly and doubly charged oxygen vacancies formed as a result of changes in the nickel valence state and the consequent formation/rupture of conducting filaments in the switching layer. This is supported by the existence of an optimum oxygen vacancy concentration for efficient low-voltage resistive switching, below or above which the switching process is inhibited. Along with the filamentary switching mechanism, the Ti top electrode also enhances the resistive switching performance due to interfacial effects. Time-resolved measurements on the devices display both long- and short-term potentiation in the optimized vacancy-engineered NFO resistive switches, ideal for solid-state synapses achieved in a single system. Our work on correlated oxide forming-free resistive switches holds significant potential for CMOS-compatible low-power, nonvolatile resistive memory and neuromorphic circuits.

SnO2-Based Memory Device with Filamentary Switching Mechanism for Advanced Data Storage and Computing.

In this study, we fabricate a Pt/TiN/SnOx/Pt memory device using reactive sputtering to explore its potential for neuromorphic computing. The TiON interface layer, formed when TiN comes into contact with SnO2, acts as an oxygen vacancy reservoir, aiding the creation of conductive filaments in the switching layer. Our SnOx-based device exhibits remarkable endurance, with over 200 DC cycles, ON/FFO ratio (>20), and 104 s retention. Set and reset voltage variabilities are impressively low, at 9.89% and 3.2%, respectively. Controlled negative reset voltage and compliance current yield reliable multilevel resistance states, mimicking synaptic behaviors. The memory device faithfully emulates key neuromorphic characteristics, encompassing both long-term potentiation (LTP) and long-term depression (LTD). The filamentary switching mechanism in the SnOx-based memory device is explained by an oxygen vacancy concentration gradient, where current transport shifts from Ohmic to Schottky emission dominance across different resistance states. These findings exemplify the potential of SnOx-based devices for high-density data storage memory and revolutionary neuromorphic computing applications.

Hybrid Precision in Resistive Memory-Based Convolutional Kernel for Fault-Resilient Neuromorphic Systems

As simple convolution computation is intensively and iteratively performed to extract features from input images, cross-point arrays with resistive random access memory (RRAM) serving as a kernel weight can accelerate the relevant mathematical operations in hardware. However, considering actual RRAM characteristics, either variability or unexpected permanent failure from the filamentary switching mechanism is observed, degrading recognition performance. This study investigates the impact of fault in a conventional kernel structure, where two adjacent columns in the array represent a single weight, on feature extraction using MATLAB. First, the fault types of HfO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit{x}}$</tex-math> </inline-formula> -based multilevel RRAM is categorized. The results reveal that the unidirectional fault of RRAM primarily worsens the accuracy of image recognition. This is because the subtraction of negative weights from positive ones is crucial for identifying the edges of images through convolution operations. Therefore, we exploit a kernel structure, in which a single column dedicated to negative weights is located next to a matrix of positive weights. In addition, we reduce the weight precision for negative weights, while quantizing positive weights to higher bits. By mitigating the subtraction errors achieved by the kernel structure with hybrid precision, we improved fault tolerance, minimizing accuracy degradation.

Alternative memristor-based interconnect topologies for fast adaptive synchronization of chaotic circuits

Resistive switching devices (memristors) constitute an emerging device technology promising for a variety of applications that are currently being studied. In this context, the use of memristors as coupling elements of the dynamics of chaotic circuits for adaptive synchronization purposes, was recently proposed and the passive crossbar array was evaluated as target interconnect medium. Nonetheless, memristors may suffer from defects and degradation. Therefore, this work evaluates the impact of memristor switching faults in an adaptive chaotic synchronization scheme, exploring at the same time the fault-tolerance of the crossbar architecture. Moreover, inspired from our observations in the stuck-at-OFF fault analysis of the memristive crossbar, some alternative scalable memristive interconnect patterns are suggested, whose performance is found independent of the number of interconnected chaotic circuits, requiring a much smaller number of total memristors than the crossbar array. All simulations are based on an accurate physics-based model of a bipolar memristor with filamentary switching mechanism. Based on our results, using the alternative topologies instead of the crossbar array leads to significant savings in the synchronization time that increase with the number of interconnected chaotic units, at the cost of more limited scaling capability and fault-tolerance.

First-principles prediction of the native filament:dielectric interfaces for the possible filamentary switching mechanism in chalcogenide selector devices

Chalcogenide selector devices are key components of the high-density cross-point memory/memristor arrays. However, their threshold switching mechanism remains elusive. Apart from the commonly cited purely electronic models, the filamentary models have recently been supported by experiment. In the context of filamentary switching, we study the native filament:dielectric interfaces in typical selector materials, namely, GeSe and SiTe. Local crystallization induced filaments and electrochemically induced filaments are considered. The filament:dielectric contacts are found to have low n-type and p-type Schottky barriers for GeSe and SiTe, respectively. The filamentary mechanism and purely electronic mechanism can be synergetic: thermal excitation of carriers from the filament to the host dielectric can induce electronic and structural changes of the host dielectric, which may in turn facilitate the growth of filament. This work promotes the understanding of filamentary-type switching in chalcogenide selector devices.

Resistive switching behaviour of multi-stacked PVA/graphene oxide + PVA composite/PVA insulating layer-based RRAM devices

In this study, we have investigated the resistive switching behavior of multi-stacked PVA/GO + PVA composite/PVA insulating layer-based RRAM (resistive random-access memory) as the annealing temperature of the insulating layer was varied between 100 °C, 150 °C, and 200 °C. The fabricated RRAM device with a multi-stacked insulating layer annealed at 200 °C showed relatively good switching properties with a high on/off ratio (∼104) and low VSET (3.5 ± 0.29 V) and VRESET (−1.81 ± 0.10 V), which were uniformly distributed over 100 DC sweep cycles. In terms of reliability, multi-stacked insulating layer-based RRAM devices exhibited good retention (>2 × 103 s) and DC sweep endurance (>80) due to the enhanced stability of the insulating layer by good dispersion and the thermal treatment. The conduction mechanisms of the device at low resistance state (LRS) and high resistance state (HRS) were analyzed through Ohmic conduction LRS and Poole-Frenkel emission of HRS, respectively. In addition, we demonstrated the filamentary switching mechanism of resistive switching in our proposed devices.

Recent Advances of Quantum Conductance in Memristors

AbstractMemristors with the filamentary switching mechanism have been acknowledged as a leading candidate for next‐generation nonvolatile memory applications, primarily due to their excellent downscaling potential, fast operation speed, low power consumption, and high switching endurance. In particular, room‐temperature quantum conductance effect can emerge as the size of the conducting filaments is reduced down to atomic scale, offering great opportunities for the physical understanding of memristive switching phenomena and the realization of ultrahigh‐density storage, logic‐in‐memory circuits, atomic scale photodetectors, and etc. This review presents a timely and comprehensive summary of the recent advances in quantum conductance in memristors. After a brief description on the evolution dynamics of conducting filaments, the experimental phenomena, theoretical understanding, effective control, and promising applications of quantum conductance in memristors are summarized and discussed in detail. Finally, current challenges and future prospects concerning quantum conductance in memristors are presented.

In Operando Characterization of Pt-TaOx-Ta Bipolar Vacancy Change Memories

Clear and unambiguous experimental observation of the physical processes leading to resistance change during formation and switching of bipolar valence change memories (VCM) remains one of the main remaining challenges in the characterization and understanding of resistive memory devices[1]. Presented here is a Scanning Transmission Electron Microscopy (STEM) Electron Energy Loss Spectroscopy (EELS) study which reveals a valence change filamentary switching mechanism during low-current bipolar resistive switching operation in Pt-TaOx-Ta resistive memory devices. No change in the film stack is observed in High Angle Annular Dark Field (HAADF) STEM during SET or RESET operation, which is consistent with expectations for a low-current operation VCM. A ~50 nm wide filamentary region is observed in EELS maps analyzed with multivariate statistical analysis disappearing on RESET operation and reappearing during SET operation. In this filamentary region, the Ta O2,3 edge is consistent with oxidized Ta but there is a sharp metallic plasmon feature which is blue shifted in accordance with our previous ab initiopredictions for metallic oxides. [1] Yang Y and Lu W D 2016 Progress in the Characterizations and Understanding of Conducting Filaments in Resistive Switching Devices IEEE Transactions on Nanotechnology 15 465-72

Impact of ultra-thin Al2O3–y layers on TiO2–x ReRAM switching characteristics

Transition metal-oxide resistive random access memory devices have demonstrated excellent performance in switching speed, versatility of switching and low-power operation. However, this technology still faces challenges like poor cycling endurance, degradation due to high electroforming (EF) switching voltages and low yields. Approaches such as engineering of the active layer by doping or addition of thin oxide buffer layers have been often adopted to tackle these problems. Here, we have followed a strategy that combines the two; we have used ultra-thin Al2O3–y buffer layers incorporated between TiO2–x thin films taking into account both 3+/4+ oxidation states of Al/Ti cations. Our devices were tested by DC and pulsed voltage sweeping and in both cases demonstrated improved switching voltages. We believe that the Al2O3–y layers act as reservoirs of oxygen vacancies which are injected during EF, facilitate a filamentary switching mechanism and provide enhanced filament stability, as shown by the cycling endurance measurements.

Resistive Switching Performance Improvement via Modulating Nanoscale Conductive Filament, Involving the Application of Two-Dimensional Layered Materials.

Reversible chemical and structural changes induced by ionic motion and reaction in response to electrical stimuli leads to resistive switching effects in metal-insulator-metal structures. Filamentary switching based on the formation and rupture of nanoscale conductive filament has been applied in non-volatile memory and volatile selector devices with low power consumption and fast switching speeds. Before the mass production of resistive switching devices, great efforts are still required to enable stable and reliable switching performances. The conductive filament, a bridge of microscopic metal-insulator-metal structure and macroscopic resistance states, plays an irreplaceable part in resistive switching behavior, as unreliable performance often originates from unstable filament behavior. In this Review, departing from the filamentary switching mechanism and the existing issues, recent advances of the switching performance improvement through the conductive filament modulation are discussed, in the sequence of material modulation, device structure design and switching operation scheme optimization. In particular, two-dimensional (2D) nanomaterials with excellent properties including and beyond graphene, are discussed with emphasis on performance improvement by their active roles as the switching layer, insertion layer, thin electrode, patterned electrode, and edge electrode, etc.

Doping Trends in HfOx Rram

In resistive random access memory devices (RRAM), materials selection plays a critical role in the tuning of properties like switching voltages, retention, and endurance. Aside from choosing between transition metal-oxide matrices like HfOx, TiOx, TaOx, NiOx, and AlOx, doping in the oxide can also be a powerful tool for device optimization. However, only a handful of dopants have been studied in detail [1-3], and it is currently not known which will offer the greatest benefit for device performance. The filamentary switching mechanism in RRAM devices, in which charged oxygen vacancies (VO’s) coalesce into conductive filaments during switching, has been widely documented [4-9]. Dopants have been shown to affect the switching characteristics of RRAM devices by decreasing the formation energy of VO’s and potentially stabilizing conductive filaments [1-3]. In this investigation, first principles modeling is used to characterize 42 possible dopants in HfOx. In addition to quantifying their effects on HfOx devices, trends are extracted which can serve as general guidelines for doping in any transition metal-oxide, and which are applicable to conductive bridge random access memories (CBRAM) as well. This work identifies a critical new metric for predicting dopant behavior in RRAM and CBRAM: the relative favorability of the dopant forming on substitutional or interstitial sites in the oxide lattice. Dopants that form on interstitial sites, such as Ag, Cu, and Ni, also tend to form cation filaments and thus CBRAM devices. Conversely, dopants that strongly prefer to substitute onto the Hf lattice site are unlikely to produce competing cation filaments, leaving the VO-based switching mechanism intact. The relative favorability is also independent of dopant chemical potential, allowing a direct one-to-one comparison of different dopant species in HfOx. In this work, the quantity of relative stability is formulated and quantified over all 42 cation dopants. The results are shown in Fig. 1. The impact of these dopants on VO formation energy is also analyzed, as shown in Fig. 2. The effect of dopants on device properties is strongly dependent upon dopant species, and Fig.’s 2 and 3 clearly show that these effects follow predictable trends. Finally, the trends underlying dopant behavior are explored, and a model is developed to predict dopant behavior in the oxide system. The behavior of the dopant was found to vary predictably as a function of six key quantities: (1) the valence difference between the dopant and Hf, (2) the dopant atomic radius, (3) the dopant oxide enthalpy of formation, (4) the change in dopant coordination from native oxide to HfOx, (5) the magnetization of the doped system, and (6) the perturbation of neighboring ion charge states by the dopant. The derived composite function predicts the relative dopant site favorability with 95% accuracy. Since these quantities can all be calculated for different oxides, it is expected that these trends can be used as guidelines for doping in any transition metal-oxide for both RRAM and CBRAM applications. [1] D. Duncan, B. Magyari-Köpe, and Y. Nishi, Appl. Phys. Lett. 108, 043501, 2016. [2] L. Zhao, S. W. Ryu, A. Hazeghi, D. Duncan, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., T106-T107, 2013. [3] D. Ning, P. Hua, and W. Wei, Chin. Phys. B 23(10), 107306, 2014. [4] D. Duncan, B. Magyari-Köpe, and Y. Nishi, Elec. Dev. Lett. 37 (4), 400-403, 2016. [5] B. Magyari-Köpe, S.G. Park, H.-D. Lee, and Y. Nishi, J. Mater. Sci. 47, pp. 7498-7514, 2012. [6] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B, 82, 115109, 2010. [7] H.-D. Lee, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B 81, 193202, 2010. [8] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., 2011. [9] B. Magyari-Köpe, M. Tendulkar, S.G. Park, H.-D. Lee, and Y. Nishi, Nanotechnology 22, 254029, 2011. Figure 1

Self‐Limited Switching in Ta2O5/TaOx Memristors Exhibiting Uniform Multilevel Changes in Resistance

To facilitate the development of memristive devices, it is essential to resolve the problem of non‐uniformity in switching, which is caused by the random nature of the filamentary switching mechanism in many resistance switching memories based on transition metal oxide. In addition, device parameters such as low‐ and high‐state resistance should be regulated as desired. These issues can be overcome if memristive devices have switching limits for both the low‐ and high‐resistance states and if their resistance values are highly controllable. In this study, a method termed self‐limited switching for uniformly regulating the values of both the low‐ and high‐resistance states is suggested, and the circuit configuration required for the self‐limited switching is established in a Ta2O5/TaOx memristive structure. A method of improving the uniformity of multi‐level resistance states in this memristive system is also proposed.

Neuromorphic crossbar circuit with nanoscale filamentary-switching binary memristors for speech recognition.

In this paper, a neuromorphic crossbar circuit with binary memristors is proposed for speech recognition. The binary memristors which are based on filamentary-switching mechanism can be found more popularly and are easy to be fabricated than analog memristors that are rare in materials and need a more complicated fabrication process. Thus, we develop a neuromorphic crossbar circuit using filamentary-switching binary memristors not using interface-switching analog memristors. The proposed binary memristor crossbar can recognize five vowels with 4-bit 64 input channels. The proposed crossbar is tested by 2,500 speech samples and verified to be able to recognize 89.2% of the tested samples. From the statistical simulation, the recognition rate of the binary memristor crossbar is estimated to be degraded very little from 89.2% to 80%, though the percentage variation in memristance is increased very much from 0% to 15%. In contrast, the analog memristor crossbar loses its recognition rate significantly from 96% to 9% for the same percentage variation in memristance.

Resistive Switching Behavior in Gelatin Thin Films for Nonvolatile Memory Application

This paper presents the characteristics of gelatin, which can cause reproducible resistive switching and bipolar resistive switching in aluminum (Al)/gelatin (35 nm)/ITO devices. The memory devices exhibited a high ON/OFF ratio of over 10(6) and a long retention time of over 10(5) seconds. The resistive switching mechanism was investigated using the high-angle dark field transmission electron microscopy image of Al/gelatin/ITO devices in the pristine high-resistance state (HRS) and then in returning to HRS after the RESET process. The energy-dispersive X-ray spectroscopy analysis revealed the aggregation of N and Al elements and the simultaneous presence of carbon and oxygen elements in the rupture of filament paths. Furthermore, via a current-sensing atomic force microscopy, we found that conduction paths in the ON-state are distributed in a highly localized area, which is associated with a carbon-rich filamentary switching mechanism. These results support that the chelation of N binding with Al ions improves the conductivity of the low-resistance state but not the production of metal filaments.

Organic nonvolatile memory devices with charge trapping multilayer graphene film

We fabricated an array-type organic nonvolatile memory device with multilayer graphene (MLG) film embedded in polyimide (PI) layers. The memory devices showed a high ON/OFF ratio (over 106) and a long retention time (over 104 s). The switching of the Al/PI/MLG/PI/Al memory devices was due to the presence of the MLG film inserted into the PI layers. The double-log current–voltage characteristics could be explained by the space-charge-limited current conduction based on a charge-trap model. A conductive atomic force microscopy found that the conduction paths in the low-resistance ON state were distributed in a highly localized area, which was associated with a carbon-rich filamentary switching mechanism.

Direct Observation of Ag Filamentary Paths in Organic Resistive Memory Devices

AbstractWe demonstrate bipolar switching of organic resistive memory devices consisting of Ag/polymer/heavily‐doped p‐type poly Si junctions in an 8 × 8 cross‐bar array structure. The bistable switching mechanism appears to be related to the formation and rupture of highly conductive paths, as shown by a direct observation of Ag metallic bridges using transmission electron microscopy and energy‐dispersive X‐ray spectroscopy. Current images of high‐ and low‐conducting states acquired by conducting atomic force microscopy also support this filamentary switching mechanism. The filamentary formation can be described by an electrochemical redox reaction model of Ag. Our results may also be applied to other kinds of organic materials presenting similar switching properties, contributing to the optimization of device scaling or memory performance improvement.

Noise-Analysis-Based Model of Filamentary Switching ReRAM With $\hbox{ZrO}_{x}/\hbox{HfO}_{x}$ Stacks

In this letter, we propose a model based on noise analysis for the filamentary switching mechanism in resistance random access memory having ZrOx/HfOx bilayer stacks. From the noise analysis results, we concluded that the current flowing during high-resistance state can be described as a trap-assisted current that flows through traps. It has been hypothesized that these traps are the cause of the instability of device parameters. To validate this, the noise analysis results of large-area devices were compared with those of small-area devices. As a consequence, we improved the uniformity of device parameters.

Effect of Scaling $\hbox{WO}_{x}$-Based RRAMs on Their Resistive Switching Characteristics

We investigated the effect of scaling down the device area of WOx resistive random-access memory (RRAM) devices on their switching characteristics. Device dimensions were successfully scaled down to 50 nm using a via-hole structure with additional Al2O3 sidewall process. As compared to the microscale devices, the nanoscale devices exhibited a distinct switching mechanism and better memory performance, such as improved switching uniformity, larger memory window, and stable endurance characteristics for up to 107 cycles. This improvement can be explained by a uniform interfacial switching mechanism in nanoscale device; this is in contrast with the defect-induced filamentary switching mechanism observed in microscale devices. In this way, the intrinsic switching properties of RRAMs were obtained by scaling down of the device area, indicating that RRAMs hold considerable promise for future applications.

Proton mobility in SiO2 thin films and impact of hydrogen and humidity on the resistive switching effect

ABSTRACTSilicon dioxide based Electrochemical Metallization (ECM) cells were intensively studied as a promising candidate for CMOS compatible non-volatile memory devices. The resistance of ECM cells can be switched between a high resistive (OFF) state and a low resistive (ON) state by applying a sufficient voltage or current pulse. This resistance transition is attributed to the formation and rupture of a few nanometers in diameter metallic filament. However, the metal ion transport which is believed to be responsible for the filamentary switching mechanism is not understood in detail. In case of SiO2 we suppose protons or humidity may enhance the metal ion transport.In this work we report our studies on the proton incorporation in amorphous SiO2 thin films focused on the impact of hydrogen and humidity on the resistive switching effect. The switching behavior was analyzed by current-voltage measurements performed at different ambient conditions. The incorporation of hydrogen has been confirmed by Time-of-Flight Secondary-Ion-Mass-Spectroscopy (ToF-SIMS). The results led to an expansion of the defect model proposed in the literature.