High Resistance State Research Articles

Metaplasticity and Reservoir Computing in Bio‐Realistic Artificial Synapses with Embedded Localized Au‐Nanoparticle‐Based Memristors

AbstractThis research reports on the control of short‐term and long‐term memory in transition metal oxides embedded with localized gold nanoparticles (Au‐NPs). The HfTiOx/TiSiOx switching layer, after the orderly and uniform insertion of Au‐NPs, demonstrates uniform cycle‐to‐cycle DC switching with an ON–OFF ratio >10. Stable low‐resistance states (LRS) and high‐resistance state (HRS) are maintained up to 104 s with multilevel memory characteristics due to the control of oxygen vacancy concentrations. The localized Au‐NPs enhance the local electric field near the HfTiOx/Au‐NP interface, forming controlled conductive filaments, while the high concentration of oxygen vacancies creates a permanent conduction path inside TiSiOx after the electroforming process. The ITO/HfTiOx/Au‐NP/TiSiOx/TaN memristor exhibits stable, controllable gradual bipolar switching and mimics several biological memory functions, including pulse‐width‐dependent plasticity, spike‐timing‐dependent plasticity, pulse‐frequency‐dependent plasticity, and experience‐dependent plasticity. Additionally, a performance of 50k SET/RESET cycles without any significant degradation is achieved and the facilitation of long‐term potentiation/depression are demonstrated. With the help of controlled oxygen vacancy generation on the surface of Au‐NP inside the HfTiOx/TiSiOx switching layer, the memristor can emulate metaplasticity. Evaluation of a reservoir computing system utilizing the volatile switching of the memristor shows efficient processing of temporal data information which is essential for neuromorphic systems.

Aromaticity‐Dependent Memristive Switching

AbstractIn recent years, memristors have drawn attention as non‐volatile memory devices for advanced computing engineering. The features of memristors, such as hysteresis, high resistance state to low resistance state ratio, retention time, etc., are determined by the material, structure of the device, the switching mechanism, and the kinetics resulting from the characteristics of the materials. Here, a resistive switching is proposed based on the aromaticity change of the material. A device has been built based on tetracyclone that undergoes the transition from an antiaromatic to an aromatic state upon applied potential. This phenomenon results in the presence of two distinguishable, and more importantly, stable resistive states of the memristor. On the basis of theoretical and experimental investigation, it is demonstrated that the working principle of the device depends on the redox change of the molecules. This study provides the foundation for a new kind of memristive switching mechanism: the formation of virtual conductivity paths. They can be a key to improving the performance of semiconductor memory devices in crucial factors such as stability, reversibility, and nonvolatility because these paths are related to electron delocalization and do not involve any significant structural changes in the device (e.g., diffusion).

Printed High-Entropy Prussian Blue Analogs for Advanced Non-Volatile Memristive Devices.

Non-volatile memristors dynamically switch between high (HRS) and low resistance states (LRS) in response to electrical stimuli, essential for electronic memories, neuromorphic computing, and artificial intelligence. High-entropy Prussian blue analogs (HE-PBAs) are promising insertion-type battery materials due to their diverse composition, high structural integrity, and favorable ionic conductivity. This work proposes a non-volatile, bipolar memristor based on HE-PBA. The device, featuring an active layer of HE-PBA sandwiched between Ag and ITO electrodes, is fabricated by inkjet printing and microplotting. The conduction mechanism of the Ag/HE-PBA/ITO device is systematically investigated. The results indicate that the transition between HRS and LRS is driven by an insulating-metallic transition, triggered by extraction/insertion of highly mobile Na+ ions upon application of an electric field. The memristor operates through a low-energy process akin to Na+ shuttling in Na-ion batteries rather than depending on formation/rupture of Ag filaments. Notably, it showcases promising characteristics, including non-volatility, self-compliance, and forming-free behavior, and further exhibits low operation voltage (VSET = -0.26 V, VRESET = 0.36 V), low power consumption (PSET = 26 µW, PRESET = 8.0 µW), and a high ROFF/RON ratio of 104. This underscores the potential of high-entropy insertion materials for developing printed memristors with distinct operation mechanisms.

Multilevel Optical Storage, Dynamic Light Modulation, and Polarization Control in Filamented Memristor System.

The electrochemical metallization (ECM) mechanism is emerging as a promising approach for the development of optical memristors-nonvolatile memory systems proposed for use as artificial synapses in neuromorphic computing applications. ECM memristors offer exceptional operating dynamics and power efficiency compared to other systems, but challenges with reproducible cycle-to-cycle state switching and the absence of advanced optical functionalities hinder their integration into photonic systems. In this work, an ECM free-standing memristor structure is proposed, which simultaneously offers wavelength-dependent multilevel nonvolatile optical storage, volatile light modulation, and dynamic polarization control. It is demonstrated that in the presence of a resonance, the optical readout provides noise-free, robust, and significantly more accurate information about the memristor's state than electrical measurement. The use of light allows to gain insight into the intermediate electrical levels of the device as it transitions between high and low resistance states and to recover the complete record of applied voltages even when stochastic filament ruptures occur. Finally, the investigations show that spectroscopic ellipsometry provides real-time information on the dynamics of cation movement and the corresponding permittivity changes at the interfaces between the switching layer and the electrodes, thus becoming a complementary characterization method for ECM memristors alongside state-of-the-art techniques.

Graphene Oxide-Doped Indium Oxide Buffer Film Sandwiched between Titanium Oxide Layers for the Development of Photosensitive Resistive Memory Devices.

Over the past decade, metal oxide semiconductors have attracted considerable attention because of their transparency, high intrinsic charge carrier mobility, and charge carrier density. Metal oxide semiconductors also provide a promising route to develop resistive memory devices because of the tunability of their conductivity via the removal of oxygen ions, forming oxygen vacancies that can act as electron donors. Here, this paper reports the fabrication of a resistive random-access memory (ReRAM) device with TiO2 and TiO2-x layers and introduces a solution-processed In2O3-graphene oxide (GO) buffer layer from a water-based precursor solution to tune the switching characteristics. The devices were compared with a ReRAM device with no buffer layer, and the device composition was optimized by tuning the GO concentration in the layers. The devices showed hysteresis under cyclically scanned bias between positive and negative voltages with clear switching between the low resistance state (LRS) and the high resistance state (HRS). The optimized device also showed a more than 3 orders of magnitude difference in resistance between the LRS and HRS and a stable current retention. The devices also showed photoresponsive behaviors. In the LRS, the current increased significantly under illumination, while in the HRS, the current deteriorated slowly when constantly illuminated. These results highlight the dual role of the GO flakes in the buffer layer by increasing the resistance in the HRS and providing a photoresponsive switching capability. The ReRAM device was connected to a TFT device, and its feasibility as a memory component in a digital circuit was successfully demonstrated. These results provide a new avenue for developing metal-oxide-based ReRAM devices with GO-containing active layers.

Enhancing Uniformity, Read Voltage Margin, and Retention in Three-Dimensional and Self-Rectifying Vertical Pt/Ta2O5/Al2O3/TiN Memristors.

This study introduces a Ta2O5-based self-rectifying memristor (SRM) with an Al2O3 interfacial layer adopted to improve switching uniformity, read voltage margin, and long-term retention. The Pt/Ta2O5/Al2O3/TiN (PTAT) device exhibits a 105 rectification ratio, 104 on/off ratio, 2 × 106 endurance, and retention of 104 s at 150 °C. A 3-layer 4 × 4 vertical resistive random access memory structure exhibits uniform switching parameters. The coefficient of variation (CV) for device-to-device measurements is 0.23 for the low resistance state (LRS) and 0.22 for the high resistance state (HRS), while for cycle-to-cycle measurements, the CV is 0.38 for the LRS and 0.11 for the HRS. Finally, the present study demonstrates the superior performance of the PTAT devices in the context of hardware-aware training for a fully connected neural network implementation. These advancements position the PTAT device as a promising candidate for high-density three-dimensional storage class memory and low-power neural networks, offering the consistent performance and reliability necessary for future high-density storage applications.

Impact of Light on Coexistence of Memresistance, Meminductance, and Memcapacitance (MEMRIC) in Nanostructured Copper Oxide (CuO) Based Random Access Memory Devices.

We demonstrated the coexistence of memresistance, memcapacitance, and meminductance non-volatile bipolar analog resistive switching in Cu (top electrode)/CuO (active layer)/SS (bottom electrode) memory devices with and without presence of the light. The onset of memcapacitance and meminductance is noticed from the pinched-shaped characteristics in the first and third quadrants. The variation in the scan rate also impacts the coexistence characteristics by shifting the intercept point for low resistance (LRS) and high resistance (HRS) states in positive and negative biasing voltages. The durability and retention are measured over a period of 150 cycles and 1500 s with and without light illumination. The slopes for the Weibull cumulative distribution plot for set/reset state with and without light are 106.80/114.23 and 70.21/102.25, respectively, suggesting that the stability of the device increases with light illumination. Interestingly, the memcapacitance disappears after 600 cycles and after 60 . The double logarithmic I-V characteristics suggest the trap-assisted conduction (higher slope >2) in the higher external electric field, and Ohmic behavior in the lower applied field region. Thus, the present study provides a way for low-power electronics and photoresistors together with memresistance, memcapacitance, and meminductance, simultaneously, i. e., MEMRIC in a single device.

Machine Learning Enabled High‐Throughput Screening of 2D Ultrawide Bandgap Semiconductors for Flexible Resistive Materials

AbstractThe 2D ultrawide bandgap (UWBG) semiconductors have attracted great attentions for the next generation of electronics and optoelectronics, owing to their superiority on material flexibility, device stability, and power consumption. However, few 2D UWBG semiconductors have been discovered, impeding their prosperous developments and widespread applications. Here, a high‐throughput workflow is constructed to screen 2D UWBG semiconductors assisted by machine learning, and 507 potential candidates are obtained. Moreover, by learning, predicting, and screening Young's modulus and Poisson's ratio, 31 flexible 2D UWBG semiconductors are identified. Then the generation and the diffusion of anion vacancies, as well as the corresponding electronic properties are investigated by using the first‐principles calculations, and 3 of them are demonstrated as the most promising candidates for the flexible resistive materials. The facile interface tunneling and the increased material conductance caused by the anion vacancies will contribute to the transition from high resistive state to low resistive state. This work provides an efficient high‐throughput screening protocol to enrich the family of 2D UWBG semiconductors and is expected to foster their practical applications.

Feasibility study of analogue filters based on memristor

AbstractThis paper reports the critical parameters in fitting the Al:HfO2 memristor and exploits the memristor symbol derived from this fitting to design filter circuits. Memristor‐based filters can obtain at least two different filter ranges with the same filtering effect as conventional filters. Multistate resistance can be achieved by controlling the filamentary gap; adjusting the initial gap in the high resistance state from 1 to 0.1 nm could vary the cut‐off frequency of the filters from 230 MHz to 1.54 GHz. Memristor‐based band‐pass filter is also simulated by cascading a low‐pass filter and a high‐pass filter; the results show that a cut‐off frequency of 5.46 to 22.22 GHz can be tuned by adjusting the initial gap of a low resistance state from 0.4 nm down to 0.2 nm. Replacing the resistor in a conventional filter with a memristor allows the filter to have a variable range; this would solve the limitation where the conventional analogue filters have only a single filter range due to the fixed values of their constituent elements.

Demonstration of bipolar resistive memory fabricated using an ultra-thin BaTiOx resistive switching layer with a thickness of ∼5 nm

In this study, a high-performance non-volatile bipolar resistive random-access memory (RRAM), which was fabricated using an ultra-thin barium titanate (BaTiOx, BTO) film as a resistive switching (RS) layer, was demonstrated. The BTO RS layers, whose thicknesses were only about 5 nm, were prepared using radio-frequency sputtering under different oxygen flow rates. Introduction of oxygen was used to modify the chemical compositions of the BTO films. Copper was used as the top electrode material to realize a Cu/BTO/n+-Si electrochemical metallization memory, where the resistance switching was triggered by electrochemical reaction of Cu electrode. The RRAM could repeatedly and consistently switch between a high-resistance state and a low-resistance state over 300 cycles. A large memory window of 105 and a long data retention time of >1.5 × 104 s both at room temperature and 85 °C were observed. Moreover, the Cu/BTO/n+-Si RRAM showed high switching speeds of 60–70 ns.

Preparation of Zn1-xCexO resistive switching film on SS304 and its corrosion resistance mechanism in NaCl solution

The corrosion-resistant behavior of the films was investigated in conjunction with the principle of resistive switching, and the films were subjected to continuous repair by modulating the oxygen vacancies. Zn1-xCexO resistive switching film was prepared on SS304 surface by one-step hydrothermal method combined with the following heat treatment. The surface morphology, crystal structure, composition, oxygen vacancy concentration and semiconductor type of Zn1-xCexO film were determined. The corrosion resistance and resistive switching properties of Zn1-xCexO films were measured by electrochemical tests, immersion experiment and resistive switching experiment. The oxygen vacancy formation energy, surface adsorption energy and diffusion energy barrier energy of Zn1-xCexO film were calculated by density functional theory (DFT). The experimental results show that with the increasing of Ce doping concentration, the nano-flowers on the surface of Zn1-xCexO film become sparse, and the lattice constants a and c increase, the oxygen vacancy concentration of the film gradually increases, while the corrosion resistance of Zn1-xCexO film decreases. The corrosion resistance of Zn0.98Ce0.02O film is the best (99.36%). During resistive switching process, the polarization treatment will promote the conversion of Ce3+ to Ce4+ in film, reduce the formation energy and diffusion barrier of oxygen vacancies, and thus promote the formation and migration of oxygen vacancies in film and then convert the film into a low resistance state. Zn1-xCexO films can stably switching between high and low resistance states, maintaining excellent corrosion resistance, which is of great significance for expanding the application of resistance switching principles in the field of corrosion protection.

A diffusive memristor with two dimensional ZrCl2

Layered transition metal halides have been identified as potential materials for memristors due to their distinctive structure and exceptional electrical properties. However, systematic studies on their preparation processes and properties have been scarce in recent years. This study marks a significant milestone as it successfully synthesizes two-dimensional layered ZrCl2 nanomaterials using chemical vapor deposition techniques. Subsequently, an Ag/ZrCl2/Au diffusion memristor was fabricated utilizing the synthesized ZrCl2 material, followed by comprehensive performance evaluations. The results demonstrate that under DC voltage conditions, the memristor exhibits stable abrupt resistive switching behavior, with a high resistance state to low resistance state ratio reaching up to 103; this stability persists even after 9×104 cycles. Furthermore, after approximately 1000 operating cycles, the memristor displays gradual resistance switching behavior characterized by continuous changes in its resistance value corresponding to variations in voltage levels. Notably, this study successfully emulates key features of biological nociceptors such as "threshold," "no adaptation," and "plasticity." A proposed mechanism based on silver ion migration elucidates the observed resistance switching phenomenon. These research findings underscore broad application prospects for 2D transition metal halides within the realm of memristors, while also offering a novel approach towards developing high-performance and long-life neuromorphic computing devices.

Threshold Switching and Resistive Switching in SnO2-HfO2 Laminated Ultrathin Films

Polycrystalline SnO2-HfO2 nanolaminated thin films were grown by atomic layer deposition (ALD) on SiO2/Si(100) and TiN substrates at 300 °C. The samples, when evaluated electrically, exhibited bipolar resistive switching. The sample object with a stacked oxide layer structure of SnO2 | HfO2 | SnO2 | HfO2 additionally exhibited bidirectional threshold resistive switching properties. The sample with an oxide layer structure of HfO2 | SnO2 | HfO2 displayed bipolar resistive switching with a ratio of high and low resistance states of three orders of magnitude. Endurance tests revealed distinguishable differences between low and high resistance states after 2500 switching cycles.

Monolayer MoS[formula omitted] CVD-growth on SiC substrates assisted with KCl

This study examines the growth of molybdenum disulfide (MoS2) on semi-insulating silicon carbide (SI-6H-SiC) substrates using chemical vapor deposition (CVD). Raman and photoluminescence measurements were utilized to characterize pristine and grown with a potassium chloride (KCl) nucleation promoter MoS2. The Raman spectra showed a frequency difference (Δω) of 18.7 cm−1 between the A1g and E2g modes for pristine MoS2, confirming its monolayer nature, while KCl-modified MoS2 exhibited a Δω of 20.4 cm−1. Pristine MoS2 demonstrated compressive strain due to lattice mismatch, with values up to −1.4%, and a low dependency of charge doping shift on Δω. Conversely, KCl-modified MoS2 flakes showed smaller strain, up to −0.4%, and a significant dependency of charge doping shift on Δω. Electrical characterization of 2-Terminal devices indicated a ratio of the high-resistance state to low-resistance state close to 1 for pristine MoS2 and 1.75 for KCl-modified MoS2. Low-temperature photoluminescence measurements suggest that the observed behavior is due to the interaction of MoS2 with KCl.

Interface characteristic and performance optimization mechanism for HfOx-based RRAM devices

The HfOx-based resistive random access memory (RRAM) is extensively researched for next-generation nonvolatile memory because of its high integration and excellent compatibility with CMOS technology. The interfacial characteristics, including interface state and series resistance, which impact the HfOx-based RRAM performance. However, a quantitative analysis about interface state and series resistance at metal/hafnium oxide (HfOx) interface is still scarce. The interface state and series resistance are related to the interfacial layer and oxygen vacancy concentration. In this work, we use the Ti/HfOx/Pt structural RRAM device with different oxygen contents to control the interfacial layer and oxygen vacancy concentration at metal/HfOx interface. The series resistance at the HfOx/Pt interface in the low resistance state (LRS) is obtained using the Schottky model, while the interface state density and time constant at the Ti/HfOx interface in the high resistance state (HRS) are extracted by frequency dependence of conductance. The results indicate that the device uniformity is improved by reducing the LRS series resistance and HRS interface state density. Furthermore, the device endurance in the HRS deteriorates with the decrease of the interface state time constant. Overall, the RRAM device performance is optimized by modulating the series resistance, interface state density, and time constant.

Superior probabilistic computing using operationally stable probabilistic-bit constructed by manganite nanowire

Abstract Probabilistic computing has emerged as a viable approach to treat optimization problems. To achieve superior computing performance, the key aspect during computation is massive sampling and tuning on the probability states of each probabilistic bit (p-bit), demanding its high stability under extensive operations. Here, we demonstrate a p-bit constructed by manganite nanowire that shows exceptionally high stability. The p-bit contains an electronic domain that fluctuates between metallic (low resistance) and insulating (high resistance) states near its transition temperature. The probability for the two states can be directly controlled by nano-ampere electrical current. Under extensive operations, the standard error of its probability values is less than 1.3%. Simulations show that our operationally stable p-bit plays the key role to achieve correct inference in Bayesian network by strongly suppressing the relative error, displaying the potential for superior computing performance. Our p-bit also serves as high quality random number generator without extra data-processing, beneficial for cryptographic applications.

Resistive random access memory based on organic-metallic hybrid polymer

Abstract Resistive random access memories (ReRAMs) based on an organic-metallic hybrid polymer, Poly(Fe-btpyb) Purple, were fabricated. This material was synthesized by complexation of metal ion and organic ligand. When applying forward bias, an abrupt resistance change from a low resistance state (LRS) to a high resistance state (HRS), which is known as reset process, was observed. In contrast, a reverse bias switched the resistance from HRS to LRS (set process). The resistive switching phenomenon is probably caused by the electrochemical oxidation-reduction reaction of the metal ion (Fe(II)/Fe(III)). The nonvolatile memory characteristics were measured with data-retention tests, showing no significant degradation over 105 s. The endurance characteristics exhibited sufficient long-term durability, due to no conformational change of the organic ligand. It is proposed that the difference in charge-transfer efficiency between the reduced state (Fe(II)) and oxidized state (Fe(III)) might be the physical mechanism of the resistive switching.

Memristive behaviour of Al/rGO-CdS/FTO device at different temperatures: A MATLAB-integrated study

In the present work, a comprehensive study is carried out to investigate the memristive behaviour of reduced graphene oxide (rGO) conjugated cadmium sulphide quantum dot (CdS QD) nanocomposites (rGO-CdS), offering insights into their dynamic response under varying thermal conditions. The study integrates experimental analysis with MATLAB simulation to provide a detailed understanding of the complex interplay between different operating temperature (300K, 350K, 400K, and 450K) and memristive behavior in rGO-CdS nanocomposites. Different structural and chemical characterizations were carried out which confirms the formation of rGO-CdS nanocomposites. A sandwich structured device was fabricated with the synthesized rGO-CdS nanocomposites using Aluminum (Al) as top and Fluorine doped tin oxide (FTO) as bottom electrode. The influence of operating temperature on hysteresis behaviour of the fabricated Al/rGO-CdS/FTO device was investigated using Keithley 2450 source meter by sweeping a direct current (dc) voltage (−5 V → 5 V → −5 V). Notably, we observe a positive temperature coefficient in the device current, with maximum and minimum recorded current of |1.29mA| and |0.66mA| at 450K and 300K respectively. The current-voltage (I-V) behavior observed in the device reveals that in the low resistance state (LRS), conduction is dominated by bulk-limited mechanisms. However, in the high resistance state (HRS), conduction involves contributions from both Schottky barriers and the Pool-Frenkel effect. A MATLAB based linear drift model was used to simulate the device responses at different temperatures using the experimental data. The study provides first comprehensive analysis of temperature dependent hysteresis behaviour of Al/rGO-CdS/FTO device, integrating MATLAB simulation to glean valuable insights into its operation and possible applications as memristive material across different temperature regimes.

Flexible Artificial Ag NPs:a-SiC0.11:H Synapse on Al Foil with High Uniformity and On/Off Ratio for Neuromorphic Computing.

A neuromorphic computing network based on SiCx memristor paves the way for a next-generation brain-like chip in the AI era. Up to date, the SiCx-based memristor devices are faced with the challenge of obtaining flexibility and uniformity, which can push forward the application of memristors in flexible electronics. For the first time, we report that a flexible artificial synaptic device based on a Ag NPs:a-SiC0.11:H memristor can be constructed by utilizing aluminum foil as the substrate. The device exhibits stable bipolar resistive switching characteristic even after bending 1000 times, displaying excellent flexibility and uniformity. Furthermore, an on/off ratio of approximately 107 can be obtained. It is found that the incorporation of silver nanoparticles significantly enhances the device's set and reset voltage uniformity by 76.2% and 69.7%, respectively, which is attributed to the contribution of the Ag nanoparticles. The local electric field of Ag nanoparticles can direct the formation and rupture of conductive filaments. The fitting results of I-V curves show that the carrier transport mechanism agrees with Poole-Frenkel (P-F) model in the high-resistance state, while the carrier transport follows Ohm's law in the low-resistance state. Based on the multilevel storage characteristics of the Al/Ag NPs:a-SiC0.11:H/Al foil resistive switching device, we successfully observed the biological synaptic characteristics, including the long-term potentiation (LTP), long-term depression (LTD), and spike-timing-dependent plasticity (STDP). The flexible artificial Ag NPs:a-SiC0.11:H/Al foil synapse possesses excellent conductance modulation capabilities and visual learning function, demonstrating the promise of application in flexible electronics technology for high-efficiency neuromorphic computing in the AI period.

Octafluorobiphenyl-4,4’-diyl 9-oxothioxanthene-1,4-diyl polyether – a promising material for organic film based memristors: synthesis, memristive effect and charge transport mechanism

Polycondensation of perfluorobiphenyl with 1,4-dihydroxy- thioxanthen-9-one resulted in the title highly thermally stable polyether containing the thioxanthenone-type electron acceptor in-chain blocks in its structure. Model resistive memory devices based on it as the active medium exhibited a bipolar switching memristive effect with a 4-order difference in resistance between the low and high resistance states. The charge transport in this polyether is quantitatively described by the Nasyrov–Gritsenko model of phonon-assisted electron tunneling between traps.