Resistive Memory Characteristics Research Articles

Demonstration of 4-quadrant analog in-memory matrix multiplication in a single modulation

Analog in-memory computing (AIMC) leverages the inherent physical characteristics of resistive memory devices to execute computational operations, notably matrix-vector multiplications (MVMs). However, executing MVMs using a single-phase reading scheme to reduce latency necessitates the simultaneous application of both positive and negative voltages across resistive memory devices. This degrades the accuracy of the computation due to the dependence of the device conductance on the voltage polarity. Here, we demonstrate the realization of a 4-quadrant MVM in a single modulation by developing analog and digital calibration procedures to mitigate the conductance polarity dependence, fully implemented on a multi-core AIMC chip based on phase-change memory. With this approach, we experimentally demonstrate accurate neural network inference and similarity search tasks using one or multiple cores of the chip, at 4 times higher MVM throughput and energy efficiency than the conventional four-phase reading scheme.

Potential for multi-application advancements from doping zirconium (Zr) for improved optical, electrical, and resistive memory properties of zinc oxide (ZnO) thin films

Transition metal-doped Zinc oxide (ZnO) thin films with an optimal wide band gap and semiconducting nature find numerous applications in optoelectronic devices, gas sensors, spintronic devices, and electronics. In this study, Zirconium (Zr) doped ZnO thin films were deposited on ITO (Indium Tin oxide) coated glass substrate using RF-magnetron sputtering. Optical and electrical properties were examined for their potential use in resistive random-access memory (RRAM) applications. X-ray Diffraction (XRD), UV–vis spectroscopy, x-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM) and Scanning electron microscopy (SEM) were used to investigate structural, optical, and compositional properties and roughness respectively. The results demonstrate that the films possess crystalline properties. Additionally, an augmentation in Zr concentration correlates with an elevation in the optical band gap, ascending from 3.226 eV to 3.26 eV, accompanied by an increase in Urbach energy from 0.0826 eV to 0.1234 eV. The film with the highest Zr content among all the films demonstrated the best electrical performance for resistive memory applications. Incorporating Zr as a dopant shows enhancement in the electrical performance and such ZnO films with optimum concertation of Zr can potentially be used in RRAM. ZnO being a versatile host material, its doping with Zr may extend its applications in catalysis, gas sensing, energy storage, and biomedical engineering. ZnO thin films employ zirconium (Zr) as a dopant, which is a novel way to improve the material’s characteristics. Although ZnO has been thoroughly researched, adding Zr presents a novel technique to enhance optical, electrical, and resistive memory characteristics all at once that has not been fully investigated.

Onset and microscopic origin of resistive random-access memory characteristics on low-cost thermally treated mixed phase CuxO (x = 1,2) on Cu sheet

We demonstrated the resistive random-access memory (RRAM) characteristics in cost-efficient, single-step, and in-situ grown nanostructured mixed-phase CuxO (x = 1, 2) thin films, on a commercially available Cu sheet, fabricated using thermal treatment under ambient conditions. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements explain the surface morphology of grown mixed-phase CuxO thin film with substrate imprints. Fourier-transform Infrared spectroscopy confirmed the mixed phase of Cu2O and CuO of synthesized thin films. The fabricated Al/CuxO/Cu RRAM device showed bipolar digital resistive switching followed by analog resistive switching. The device showed a fast-switching speed of 250 ms, high endurance durability for 2500 cycles, and better retention of 103 s, respectively. The interconnected CuxO with Cu sheet provides efficient charge carrier migration from the bottom electrode to the active layer. The formed defect and trapped site are responsible for the resistive switching behavior. Thus, the present study provides a way to harness the potential of RRAM on either thermally treated Cu sheets or realizing a Cu thin film followed by low-temperature annealing, which can easily be integrated into existing electronic devices for data storage applications.

Multi-level Storage Characteristics of MoSe2 Resistive Random Access Memory

Resistive Random Access Memory (RRAM) is a type of non-volatile memory (NVM) device that stores information by switching between high and low resistance values. It has attracted widespread attention due to its promising potential for miniaturization. In this study, molybdenum diselenide (MoSe2) was successfully synthesized via the hydrothermal method, and the RRAM was fabricated with MoSe2 as the resistance change layer. Furthermore, the MoSe2 samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the prepared MoSe2 forms flower-like nanospheres assembled by nanosheets, with a particle size is about 200 nm. In addition, RRAM has a low operating voltage (< 1V), a high OFF/ON-state resistance ratio (> 102), a good endurance (103 cycles), and its resistance switching mechanism is dominated by the trap-controlled space charge limited current (SCLC) mechanism in the high resistance state (HRS) and by the ohmic mechanism in the low resistance state (LRS). Furthermore, multi-level storage is achieved by adjusting the compliance currents (Icc) and the stop voltage (Vstop).

Effect of heat treatment on resistive switching memory characteristics of NiO nanodots of tens of nanometers shattered by AFM tips

The characteristics of resistive random access memory (RRAM) at the nanometer scale are increasingly important for RRAM cells with further miniaturization. We report the RRAM characteristics of NiO nanodots in the order of tens of nanometers. NiO nanodots of approximately 70 nm were created on a single-crystal Nb-doped SrTiO3 (Nb:STO) substrate by dip-pen nanolithography. For comparison, a NiO thin film was also deposited on the Nb:STO substrate by spin coating. To reduce the size of NiO nanodots, they were shattered by an atomic force microscope tip, obtaining nanodots of approximately 10 nm. Using a conductive atomic force microscope, we characterized the current according to the applied electric field of the NiO nanodots on the Nb:STO substrate with a capacitor configured of Pt/NiO/Nb:STO substrate. When compared with the NiO thin film, the forming voltage of the NiO nanodots decreased. In addition, the shattered NiO nanodots exhibited unipolar switching characteristics, and the forming and set/reset voltages decreased with the reduction in nanodot size. Furthermore, the NiO nanodots exhibited a substantially reduced forming and set/reset voltages after heat treatment in vacuum. We suggest that the oxygen-depleted layer formed on the surface of NiO nanodots by heat treatment plays a crucial role in the reduction of the forming and operation voltages.

Spike Optimization to Improve Properties of Ferroelectric Tunnel Junction Synaptic Devices for Neuromorphic Computing System Applications.

The continuous advancement of Artificial Intelligence (AI) technology depends on the efficient processing of unstructured data, encompassing text, speech, and video. Traditional serial computing systems based on the von Neumann architecture, employed in information and communication technology development for decades, are not suitable for the concurrent processing of massive unstructured data tasks with relatively low-level operations. As a result, there arises a pressing need to develop novel parallel computing systems. Recently, there has been a burgeoning interest among developers in emulating the intricate operations of the human brain, which efficiently processes vast datasets with remarkable energy efficiency. This has led to the proposal of neuromorphic computing systems. Of these, Spiking Neural Networks (SNNs), designed to closely resemble the information processing mechanisms of biological neural networks, are subjects of intense research activity. Nevertheless, a comprehensive investigation into the relationship between spike shapes and Spike-Timing-Dependent Plasticity (STDP) to ensure efficient synaptic behavior remains insufficiently explored. In this study, we systematically explore various input spike types to optimize the resistive memory characteristics of Hafnium-based Ferroelectric Tunnel Junction (FTJ) devices. Among the various spike shapes investigated, the square-triangle (RT) spike exhibited good linearity and symmetry, and a wide range of weight values could be realized depending on the offset of the RT spike. These results indicate that the spike shape serves as a crucial indicator in the alteration of synaptic connections, representing the strength of the signals.

Highly uniform resistive switching characteristics of KGM resistive memory through air plasma technology

In this study, a highly stable resistive switching behavior was obtained by utilizing konjac glucomannan, a biomaterial, as a dielectric layer through a solution-based process, and the ITO bottom electrode was modified by using an air plasma technique. Electrical results showed that the pretreatment device did not observe significant switching characteristics (∼101). The device, which underwent additional plasma treatment, was recognized for its significant improvements in this area, demonstrating a reduced set voltage (Vset = 0.37 V) with a 6% coefficient of variation, more than 200 consecutive cycles, and up to 90% electrical yield. Moreover, the treated device exhibited a dramatically higher memory window (&amp;gt;104) due to the relatively low off-state current. According to the XPS and UPS analysis, the work function increased from an untreated 4.3 to 5.79 eV as the treatment time increased to 180 s. The conductive filaments, which stemmed from oxygen vacancies, were introduced through plasma treatment to enhance the proportion of oxygen vacancies in the ITO films, thereby creating a stable and consistent filament path. Consequently, the systematic and reproducible resistive switching phenomenon was intensified. The reported results confirmed that the reliability and uniformity in bioelectronic devices can be accomplished through a simple and effective plasma technique. This approach paved the way for alternative applications of these devices.

Comparative analysis of low-frequency noise based resistive switching phenomenon for filamentary and interfacial RRAM devices

We investigate the low-frequency noise (LFN) characteristics of resistive switching random access memory (RRAM) devices with metal–insulator–metal structures of TiN/Ti/TiO2/TiN (filamentary) and TiN/Ti/TiO2/HfO2/TiN (interfacial). By comparing filamentary and interfacial RRAM devices, we stipulate guidelines for noise sources according to the resistance states or resistive switching mechanisms of RRAM device. In filamentary RRAM devices, the dominant noise source in the low resistance state is the localized oxygen vacancies, whereas the noise characteristics in the high resistance state represent a bulk effect. In interfacial RRAM devices, the noise characteristics in both resistance states represent the bulk effect. The physical mechanism of the multilevel cell operation is inferred by the I–V characteristics in both Icc and Vreset control modes. Specifically, the changes in the structure of the conductive filament inside the switching layer can be confirmed through LFN measurement.

GeS conducting-bridge resistive memory device with IGZO buffer layer for highly uniform and repeatable switching

A double stacked monochalcogenide GeS-based conducting-bridge random access memory (CBRAM) device with a IGZO buffer layer is investigated for highly improved resistive memory characteristics. The IGZO/GeS double layer is found to provide the CBRAM with a markedly improved sub-1V DC set/reset-voltage distributions (&amp;lt;±0.1 V variation). High endurance (&amp;gt;107 cycles) and retention (&amp;gt;105 s at 85 °C) performance are also achieved. The metal ion diffusion and migration rates in the solid electrolytes along with the redox reaction rates at the electrodes determine the respective resistive switching (RS) mechanism in the CBRAM device. Considering this fact, it is proposed that Ag diffusion into IGZO creates a virtual electrode, when coupled with strong ionic transport in GeS, consistently mediate the formation/dissolution of Ag filament there, thus reducing switching variation. Understanding the RS mechanism based on the materials' physical and chemical properties and tailoring the device structure allow an optimal control over cycle to cycle and device to device variability. The findings show that this material combination or similar oxide/chalcogenide stacks may offer a facile means for mitigating CBRAM variability.

Programming Strategy Optimization of RRAM Fabricated by 28 nm Standard CMOS Process

The switching characteristics of HfOx-based resistive switching random access memory (RRAM) fabricated on front-end of 28 nm standard CMOS process are investigated. The integration of RRAM in the front-end of the CMOS process would minimize the influence of parasitic parameters caused by interconnection line and interlayer oxide on the one transistor one RRAM (1T1R) cells. By adopting pulse program-verify measurement for cycling test, we observe two nonideal phenomena in the commercialized RRAM devices for the first time: 1) low resistance state (LRS) current periodically decays exponentially accompanied by incremental SET pulse number and then increases abruptly when the cycle reaches a certain level and 2) high resistance state (HRS) current fluctuated dramatically during RESET operation. The current fluctuation is mainly due to the generation of new oxygen vacancies (Vos) near the bottom electrode and gap region by electrons trapping/detrapping. By adopting efficient pulse amplitude modulation, the optimized programming strategy has greatly reduced the switching instability and improved the endurance of the devices. Mechanisms of the instability and the optimization strategy are explained by a modified trap-assisted tunneling (TAT) model based on the generation and redistribution of Vos during cycling operations.

Effect of stoichiometry on the resistive switching characteristics of STO resistive memory

Herein, we modify the stoichiometry of an SrTiOx resistive switching layer to fabricate a memristor with a high ON/OFF current ratio and high data retention capability.

Resistive switching memory based on polyvinyl alcohol-graphene oxide hybrid material for the visual perception nervous system

Resistive random-access memory (RRAM) is a new memory technology that can not only realize high-density storage, but also can simulate the neural synapse for use in artificial intelligence applications. In this study, we propose an RRAM device that shows competitive resistive memory characteristics and can similarly be used as a synapse in simulation of the human visual perception nervous system. First, we demonstrate that the polyvinyl alcohol-graphene oxide (PVA@GO) hybrid material-based RRAM device offers competitive resistive memory characteristics, including long retention capability, high durability, repeatability, and mechanical flexibility. Second, we integrate the RRAM (as the artificial synapse) with a light-sensitive electronic component (a photoreceptor cell) to construct an artificial visual perception system, and realize effective emulation of light perception and conversion of light signals into synaptic signals. Under light irradiation at 532 nm, a range of versatile synaptic functions, including short-term plasticity (STP), long-term plasticity (LTP), and paired pulse facilitation (PPF), was imitated. This work provides valuable insight into the development path for next-generation high-density data storage technology, and also offers a new way to imitate the human visual neural network for multi-functional humanoid robots.

The Spectral Response of the Dual Microdisk Resonator Based on BaTiO3 Resistive Random Access Memory.

With the resistive random access memory (ReRAM) devices based on the Al/BaTiO3 (BTO)/ITO structure fabricated at hand, by cross-analyzing the resistive memory characteristics in terms of various barium titanate (BTO) film thicknesses, it is found that the device with 60 nm thick BTO can be switched more than 425 times, while the corresponding SET/RESET voltage, the on-off ratio, and the retention time are −0.69 V/0.475 V, 102, and more than 104 seconds, respectively. Furthermore, the aforementioned ReRAM with a low switching voltage and low power consumption is further integrated with a waveguide resonator in the form of a dual microdisk aligned in a parallel fashion. As the separation gap between the two microdisks is fixed at 15 μm, the ReRAM-mediated dual disk resonator would render a 180° phase reversal between the spectral outputs of the through-port and drop-port. If the gap is shortened to 10 and 5 μm, the expected phase reversal could also be retrieved due to the selective combinations of different memory states associated with each of the two ReRAM microdisks as witnessed by a series of characterization measurements.

Use of a supercritical fluid treatment to improve switching region in resistive random access memory

This work investigates the influence of a supercritical fluid (SCF) treatment on the characteristics of resistive random access memory. A comparison between the experimental results for the device at initial, after the overset process, and after the SCF treatment, shows that the treatment dopes oxygen ions and generates defects in the switching region (SR). Moreover, the changes in the ratio of the components of the SR after the SCF treatment improve memory characteristics, including a lower set/reset voltage (V SET/V RESET), and higher resistances at low resistance state and high resistance state.

Enhanced resistive switching characteristics of FeMnO3 resistive random access memory devices with embedding Au nanoparticles

Au nanoparticles (Au NPs) embedded FMO (Au-FMO) thin films were fabricated on Pt/Ti/SiO2/Si substrates by using a chemical solution deposition method. The sandwich structured Pt/Au-FMO/Pt devices exhibit bipolar resistive switching behavior with uniform Set/Reset voltage distribution, stable high and low resistance values, and excellent retention characteristics. The introduction of Au NPs can enhance the local electric field, which greatly reduces the randomness of the generation and fracture of oxygen vacancy conductive filaments. The conduction mechanisms of Pt/Au-FMO/Pt devices in low resistance state and high resistance state can be described by Ohmic conduction and space charge limited current model, respectively. The results show that the Au-FMO films have great potential in the field of resistive random access memory devices.

Light-assisted defects migration in cuprous iodide (CuI)

We report light-assisted iodine ion migration in CuI – a popular wide bandgap p-type semiconductor, which was synthesized via iodinating Cu film in iodine solution. In-depth crystallographic analysis was performed with the combination methods of X-ray diffraction (XRD), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), crystal modeling and diffraction simulation. The samples show strong diffraction peaks of γ-CuI, yet still exists β phase diffraction. Loop current-voltage (IV) test shows characteristics of resistive random access memory (RRAM), suggesting the existence of large amount of movable native defects, which forms the conductive filaments. UV irradiation was found to be effective to convert the RRAM device from low resistance state (LRS) to high resistance state (HRS), indicating potential application of novel memory device with electric read-in and optical erasure function. To study the properties of native defects, time-lapsed PL were employed, in which the near band edge defects luminescence, related to VCu, increased, whereas mid-band broad luminescence, related to VI, decreased, with the increase of irradiation time. UV irradiation induced defects evolution was also observed in X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). Finally, we propose the microscopic physical mechanism of defect migration in CuI with the assistance of UV irradiation. This work reveals the nature of point defects evolution in CuI under light irradiation and is expected to arise more discussions on defects formation, migration and light-defects interaction of CuI material.

Improved uniformity in resistive switching behaviors based on PMMA films with embedded carbon quantum dots

The growth and rupture of conductive filaments act a crucial part in the reliability of resistive switching behaviors. The random growth and rupture of conductive filaments are the primary reason for the instability of set/reset reproducibility. Hence, we propose a method that embedded carbon quantum dots (CQDs) in polymethylmethacrylate (PMMA) to fabricate the Ag/PMMA&amp;CQDs/FTO resistive switching device. Five different concentrations of CQDs are embedded in PMMA to regulate the resistive switching properties, and the resistive memory characteristics of the optimal group are systematically studied. The optimal group exhibits excellent switching repeatability, low set/reset voltages, and stable forming voltage, which is much better than PMMA without CQDs. Furthermore, we employ the COMSOL software to build a simulation model for exploring the influence of CQDs on the internal electric field of PMMA, which proved that the introduction of CQDs might have a favorable effect on the orderly growth of conductive filaments.

Effect of nitrogen capture ability of quantum dots on resistive switching characteristics of AlN-based RRAM

This Letter studies the effect of the nitrogen capture ability of quantum dots on resistive switching characteristics of AlN-based resistive random access memory. We prepared a single layer AlN device and four types of AlN/PbS quantum dot stacked structure devices with different concentrations. Compared with the single layer AlN device, the AlN/PbS quantum dot stacked structure devices exhibit excellent resistive switching characteristics, such as forming-free, low power consumption, and excellent stability. We propose that the resistive switching process is determined by the migration of nitrogen ions and the lead sulfide (PbS) quantum dot layer as a natural nitrogen ion reservoir, which can improve the resistive switching characteristics. Moreover, the size of the natural nitrogen ion reservoir can be modulated by changing the concentration of quantum dots.

Li-Doping Effect on Characteristics of ZnO Thin Films Resistive Random Access Memory.

In this study, a Pt/Ag/LZO/Pt resistive random access memory (RRAM), doped by different Li-doping concentrations was designed and fabricated by using a magnetron sputtering method. To determine how the Li-doping concentration affects the crystal lattice structure in the composite ZnO thin films, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) tests were carried out. The resistive switching behaviors of the resulting Pt/Ag/LZO/Pt devices, with different Li-doping contents, were studied under direct current (DC) and pulse voltages. The experimental results showed that compared with the devices doped with Li-8% and -10%, the ZnO based RRAM device doped by 5% Li-doping presented stable bipolar resistive switching behaviors with DC voltage, including a low switching voltage (<1.0 V), a high endurance (>103 cycles), long retention time (>104 s), and a large resistive switching window. In addition, quick switching between a high-resistance state (HRS) and a low-resistance state (LRS) was achieved at a pulse voltage. To investigate the resistive switching mechanism of the device, a conduction model was installed based on Ag conducting filament transmission. The study of the resulting Pt/Ag/LZO/Pt devices makes it possible to further improve the performance of RRAM devices.

Chemically Controlled Volatile and Nonvolatile Resistive Memory Characteristics of Novel Oxygen-Based Polymers.

Recent advancements in modern microelectronics continuously increase the data storage capacity of modern devices, but they require delicate and costly fabrication processes. As alternatives to conventional inorganic based semiconductors, semiconducting polymers are of academic and industrial interest for their cost-efficiency, power efficiency, and flexible processability. Here, we have synthesized a series of novel oxygen-based polymers through the postmodification reactions of poly(ethylene-alt-maleate) with various oxybenzyl alcohol derivatives. The oxygen-based polymers are thermally stable up to 180 °C, and their nanoscale film devices exhibit reliable, power efficient p-type unipolar volatile and nonvolatile resistive memory characteristics with high ON/OFF current ratios. Additionally, when given a higher number of oxygen atoms in oxyphenyl side groups, the thin film polymer devices demonstrate a wide operational film thickness range. The memory characteristics depend on the oxyphenyl moieties functioning as charge trap sites, where a combination of Schottky emission and trap-limited space charge limited conductions in OFF-state and hopping conduction in ON-state are observed. This study demonstrates the chemical incorporation of oxyphenyl derivatives into polymer dielectrics as a powerful development tool for p-type resistive memory materials.