Forming-free Resistive Switching Research Articles

Sol-Gel Derived ZnO Thin Films with Nonvolatile Resistive Switching Behavior for Future Memory Applications

Herein we report on a facile sol-gel spin-coating technique to fabricate ZnO thin films that grow preferentially along the (002) plane on FTO substrates. By employing the magnetron sputtering technique to deposit a tungsten (W) top metal electrode onto these ZnO thin films, we successfully realize a W/ZnO/FTO memory device that exhibits self-rectifying and forming-free resistive switching characteristics. Notably, the as-prepared device demonstrates impressive nonvolatile and bipolar resistive switching behavior, with a high resistance ratio (RHRS/RLRS) exceeding two orders of magnitude at a reading voltage of 0.1 V. Moreover, it exhibits ultralow set and reset voltages of approximately +0.5 V and −1 V, respectively, along with exceptional durability. In terms of carrier transport properties, the low resistance state of the device is dominated by ohmic conduction, whereas the high resistance state is characterized by trap-controlled space-charge-limited current conduction. This work highlights the potential of the ZnO-based W/ZnO/FTO memory device as a promising candidate for future high-density nonvolatile memory applications.

Forming-Free, Low-Voltage, and High-Speed Resistive Switching in Ag/Oxygen-Deficient Vanadium Oxide(VOx)/Pt Device through Two-Step Resistance Change by Ag Filament Formation.

Forming-free, low-voltage, and high-speed resistive switching is demonstrated in an Ag/oxygen-deficient vanadium oxide (VOx)/Pt device via the facilitated formation and rupture of Ag filaments. Direct current (DC) voltage sweep measurements exhibit forming-free switching from a high-resistance state (HRS) to a low-resistance state (LRS), called SET, at an average VSET of +0.23 V. The reverse RESET transition occurs at an average VRESET of -0.07 V with a low RESET current of <1 mA. Reversible switching operations are stable with an HRS/LRS resistance ratio >103 during repeated measurements for thousands of cycles. In pulse measurements, switching occurs within 100 ns at an amplitude of +1.5 V. Notably, a two-step resistance change is observed in the SET operation, where the resistance first partially decreases due to Ag+ ion accumulation in VOx and then further decreases to the LRS after hundreds of nanoseconds upon complete filament formation. The VOx layer deposited to be mostly amorphous with oxygen deficiency from V2O5 has abundant vacancies and expedites Ag+ ion migration, thus realizing forming-free, high-speed, and low-voltage switching. These characteristics of the facilitated Ag filament formation using the substoichiometric VOx layer are highly beneficial for use as stand-alone nonvolatile memory and in-memory computing elements.

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.

Study on the Sodium-Doped Titania Interface-Type Memristor.

Memristors integrated into a crossbar-array architecture (CAA) are promising candidates for analog in-memory computing accelerators. However, the relatively low reliability of the memristor device and sneak current issues in CAA remain the main obstacles. Alkali ion-based interface-type memristors are promising solutions for implementing highly reliable memristor devices and neuromorphic hardware. This interface-type device benefits from self-rectifying and forming-free resistive switching (RS), and exhibits relatively low variation from device to device and cycle to cycle. In a previous report, we introduced an in situ grown Na/TiO2 memristor using atomic layer deposition (ALD) and proposed a RS mechanism from experimentally measured Schottky barrier modulation. Self-rectifying RS characteristics were observed by the asymmetric distribution of Na dopants and oxygen vacancies as the Ti metal used as the adhesion layer for the bottom electrode diffuses over the Pt electrode at 250 °C during the ALD process and is doped into the TiO2 layer. Here, we theoretically verify the modulation of the Schottky barrier at the TiO2/Pt electrode interface by Na ions. This study fabricated a Pt/Na/TiO2/Pt memristor device and confirmed its self-rectifying RS characteristics and stable retention characteristics for 24 h at 85 °C. Additionally, this device exhibited relative standard deviations of 27 and 7% in the high and low resistance states, respectively, in terms of cycle-to-cycle variation. To verify the RS mechanism, we conducted density functional theory simulations to analyze the impact of Na cations at interstitial sites on the Schottky barrier. Our findings can contribute to both fundamental understanding and the design of high-performance memristor devices for neuromorphic computing.

Tunable Resistive Switching Behaviors and Mechanism of the W/ZnO/ITO Memory Cell.

A facile sol-gel spin coating method has been proposed for the synthesis of spin-coated ZnO nanofilms on ITO substrates. The as-prepared ZnO-nanofilm-based W/ZnO/ITO memory cell showed forming-free and tunable nonvolatile multilevel resistive switching behaviors with a high resistance ratio of about two orders of magnitude, which can be maintained for over 103 s and without evident deterioration. The tunable nonvolatile multilevel resistive switching phenomena were achieved by modulating the different set voltages of the W/ZnO/ITO memory cell. In addition, the tunable nonvolatile resistive switching behaviors of the ZnO-nanofilm-based W/ZnO/ITO memory cell can be interpreted by the partial formation and rupture of conductive nanofilaments modified by the oxygen vacancies. This work demonstrates that the ZnO-nanofilm-based W/ZnO/ITO memory cell may be a potential candidate for future high-density, nonvolatile, memory applications.

Capacitive and RRAM Forming-Free Memory Behavior of Electron-Beam Deposited Ta2O5 Thin Film for Nonvolatile Memory Application

The present study reports the presence of capacitive memory and forming-free resistive random access memory (RRAM) in a tantalum pentoxide (Ta2O5) thin film (TF) device. The capacitance and voltage analysis of the electron-beam evaporated Ta2O5 TF device exhibits three distinct regimes: inversion, depletion, and accumulation, which decrease as the frequency of operation increases. This behavior is attributed to the interface state density (Dit) and series resistance (Rs). Moreover, the memory window of the Ta2O5 device was found to increase from 1.2 (± 1 V) to 7.9 (± 10 V). Furthermore, the resistive switching mechanism was investigated through the current (I) vs voltage (V) measurement for 1000 cycles. The device exhibits abnormal forming-free bipolar resistive switching. In addition, a desirable and stable switching behavior with resistance ratio of 102 and a retention up to 103 s at +2.5 V was observed. The overall findings of the Au/Ta2O5 TF/Si device could provide a pave way for nonvolatile memory (NVM) application.

Forming-Free, Self-Compliance WTe2-Based Conductive Bridge RAM With Highly Uniform Multilevel Switching for High-Density Memory

In this work, forming-free and self-limited resistive switching characteristics are demonstrated in the transition-metal-chalcogenide-based conductive bridge RAM devices. Owing to the choice of a suitable solid electrolyte, the proposed WTe2-based devices present excellent switching characteristics offering highly desirable attributes such as high pulse endurance (> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 10$ </tex-math></inline-formula> 7 cycles) and stable data retention (10 years at 72°C). The devices also present excellent device-to-device and cycle-to-cycle uniformity which is highly desirable for the practical implementation of resistive random access memory (RRAM) devices in large crossbar arrays. The demonstration of robust and highly repeatable multilevel switching further provides multibit data storage capability for high-density memory.

Effect of annealing on forming-free bipolar resistive switching of Gd2O3 thin films

Oxide-based resistive switching devices are a promising candidate for non-volatile memory applications. Here, we present a Gd2O3-based non-volatile memory device deposited using e-beam evaporation. In order to know the effect of annealing on the device performance, the as-grown sample was annealed at 500 °C. The capacitance (C) – voltage (V) and conductance (G) – voltage (V) measurements were investigated in the range of 200 kHz to 1 MHz. It was observed that the C-V and G-V changes with frequency due to the presence of interface states. The effect of series resistance (Rs) and interface state density (Dit) was also investigated for both devices. Furthermore, the resistive switching was recorded at± 5 V. Both devices exhibit bipolar resistive switching with charge conduction mechanism ascribed to ohmic, space charge limited conduction, and trap-filled space charge limited conduction. Consequently, the annealed device displayed an improved resistance ratio of 102 and a stable endurance of up to 1000 cycles with better data retention of over 103 s.

CMOS-compatible self-aligned 3D memristive elements for reservoir computing systems

Neuromorphic capabilities of a self-aligned complementary metal-oxide-semiconductor compatible W/WOx/HfO2/Ru cell in a 3D vertical memristive structure were investigated. We show that the device exhibits nonfilamentary forming-free multilevel resistive switching with gradual resistance change. In addition, the poor retention of a low resistance state allows integration of these structures in architectures that require short-term memory characteristics such as reservoir computing systems. The ability of the device to rely on the temporal sequence of the stream was tested with the digit recognition task. Since a WOx layer was obtained by thermal oxidization and HfO2 and Ru layers were grown by atomic layer deposition methods, the device is suitable for high-density systems with high connectivity within a neural network.

Anti-bacterial and transparent allantoin biomaterial-based biocomposite for non-volatile memory and brain-inspired computing applications

The allantoin biomaterial-based biocomposite was utilized to fabricate resistive switching (RS) devices for non-volatile memory and brain-inspired computing applications. The polymeric phase, surface morphology, and purity of biomaterial were characterized using different analytical tools. The allantoin biofilm shows anti-bacterial and transparent properties, therefore, it can be useful for wearable electronics. Novel allantoin biomaterial-based composite device exhibits forming-free bipolar resistive switching behavior with low SET (+0.50 V) and RESET (−1.34 V) voltages, good endurance (3 × 103 cycles), and memory retention properties (103 s). Additionally, the device mimics various bio-synaptic properties such as excitatory postsynaptic current, paired-pulse facilitation, and paired-pulse depression.

Plasma fireball-mediated ion implantation for nonvolatile memory application

Extraordinary performance of nonvolatile memories based on resistive switching is expected to fulfill the requirements of next-generation data-intensive technologies. However, due to the separate growth of functional layer and electrodes in the device, resistive random access memories still suffer from their implementation at a commercial scale. Herein, depending upon the polarity, a single device is used to grow Ti thin film followed by plasma fireball-mediated oxygen ion implantation to transform the top surface of Ti film into a functional TiOx layer to fabricate the device. The formation of TiOx layer at the near-surface region and oxygen-concentration gradient with depth in the implanted film is demonstrated. The current–voltage characteristics of the device analyzed at the nanoscale show forming-free bipolar resistive switching, which is further confirmed by the twofold erase-write process. At 5 × 1016 ions cm−2 fluence, resistive switching occurs at higher voltages, while at 5 × 1017 ions cm−2 fluence, it occurs at lower voltages. As an application, the rapid fabrication of periodic arrays of squared memory cells of different sizes is demonstrated. We propose a simple and cost-effective one-step technique to fabricate resistive switching-based nanoscale nonvolatile memory devices, which can be utilized for the production of high-density commercial devices at a large scale.

A flexible resistive switching device for logical operation applications in wearable systems

As an emerging micro/nano device, memristor integrates functions, such as memory, sensing, and computing, and has similar internal dynamics to synapses, showing great potential in neuromorphic computing and logical operations. In this work, a flexible memristor with Al/TiOx/Al/polyethylene terephthalate structure was developed to investigate the synergistic effect from the oxygen vacancy, H2O molecule, and NaCl. A forming-free resistive switching (RS) behavior observed in the flexible memristor can be ascribed to the migration and accumulation of the oxygen vacancy and Schottky emission of the interfaces. This forming-free RS is sensitive to the NaCl and H2O molecule because the Na+ migration disturbs the oxygen vacancy filament and interplay between oxygen vacancy and H2O molecules. The variation conductance modulated by the concentration of NaCl and H2O molecules can represent the weight update incrementally that the online learning required. Therefore, the memristor has the potential to be used as a biosensor in human tissues or to realize the brain-like chip, which points out the research direction for the extension function of the memristor in wearable systems.

Resistive switching properties of monolayer h-BN atomristors with different electrodes

Resistive switching properties based on molecular beam epitaxy-grown monolayer hexagonal boron nitride (h-BN) atomristors are studied by using metal insulator metal configurations with different electrode materials. Au/monolayer h-BN/Ni devices demonstrate a forming-free bipolar resistive switching (BRS) behavior, a good endurance with up to 97 cycles at a high compliance current of 100 mA, an average on/off ratio of 103, and a low set/reset voltage variability. Metal/monolayer h-BN/graphite/Co devices exhibit self-compliant current BRS characteristics. Both metal/h-BN/Ni and metal/h-BN/graphite/Co devices show the coexistence of BRS, unipolar resistive switching (URS), and nonvolatile threshold switching (TH) modes. The formation of conductive filaments is attributed to the diffusion and trapping of metal ions on the defect sites driven by the electric field, while the rupture is driven by the electric field in BRS and by Joule heating in URS and TH modes.

Forming-free bipolar resistive switching characteristics in Al/Mn3O4/FTO RRAM device

The bipolar resistive switching (BRS) phenomenon was demonstrated in Mn3O4 using an aluminium/Mn3O4/fluorine-doped tin oxide resistive random access memory (RRAM) device. The fabricated RRAM device showed good retention, non-volatile behaviour, and forming-free BRS. The current–voltage characteristics and the temperature dependence of the resistance measurements were used to explore conduction mechanisms, thermal activation energy, and temperature coefficient of resistance. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) was ∼102. The device showed different conduction mechanisms in LRS and HRS modes, such as ohmic conduction and space charge limited conduction. The formation and rupture of conducting filaments of oxygen vacancies took place by changing the polarity of external voltage, which may be responsible for resistive switching characteristics in the device. This device is suitable for application in future high-density non-volatile memory RRAM devices.

Memristive synaptic device based on a natural organic material—honey for spiking neural network in biodegradable neuromorphic systems

Spiking neural network (SNN) in future neuromorphic architectures requires hardware devices to be not only capable of emulating fundamental functionalities of biological synapse such as spike-timing dependent plasticity (STDP) and spike-rate dependent plasticity (SRDP), but also biodegradable to address current ecological challenges of electronic waste. Among different device technologies and materials, memristive synaptic devices based on natural organic materials have emerged as the favourable candidate to meet these demands. The metal–insulator-metal structure is analogous to biological synapse with low power consumption, fast switching speed and simulation of synaptic plasticity, while natural organic materials are water soluble, renewable and environmental friendly. In this study, the potential of a natural organic material—honey-based memristor for SNNs was demonstrated. The device exhibited forming-free bipolar resistive switching, a high switching speed of 100 ns set time and 500 ns reset time, STDP and SRDP learning behaviours, and dissolving in water. The intuitive conduction models for STDP and SRDP were proposed. These results testified that honey-based memristive synaptic devices are promising for SNN implementation in green electronics and biodegradable neuromorphic systems.

Highly transparent flexible artificial nociceptor based on forming-free ITO memristor

Designing a flexible transparent electronic device with biological functions is of great interest for the future wearable integrated artificial intelligence equipment. Nociceptor is a vitally important receptor of sensory neuron, which is responsible for providing a warning signal by recognizing noxious stimuli to reduce potential physical injury. Here, a flexible transparent artificial nociceptor device is demonstrated to simulate the biological nociceptor functions based on the indium tin oxide (ITO) memristor, which exhibits forming-free and reproducible threshold resistive switching behaviors. This structurally simple memristor can imitate the key features of biological nociceptor, including “threshold,” “relaxation,” and “no adaptation” behaviors and sensitization phenomena of hyperalgesia and allodynia upon external stimuli. Finally, an alarm system is built to demonstrate the simplicity and feasibility of this artificial nociceptor for future neuromorphic systems. These results indicate a potential application of the ITO memristor in the future flexible invisible neuromorphic cognitive platform.

Self-rectifying and forming-free resistive switching behaviors in Pt/La2Ti2O7/Pt structure

In this work, we report the resistive switching behavior of an amorphous La2Ti2O7 (LTO) film as sandwiched between two Pt electrodes. The resistive switching is forming-free and highly uniform. Furthermore, it exhibits self-rectifying resistive switching behaviors owing to the Schottky contact and quasi-ohmic contact formed at the top and bottom interfaces, respectively. The mechanism of switching behavior in the device is attributed to the trapping/detrapping-mediated electronic bipolar resistance switching. By fitting the current-voltage characteristics, it indicates the coexistent conduction mechanisms of Schottky emission and space-charge-limited-conduction (SCLC), while the Schottky barrier modified by electron trapping/detrapping plays a dominating role in the resistive switching process.

Nanoscale-Resistive Switching in Forming-Free Zinc Oxide Memristive Structures.

This article presents the results of experimental studies of the impact of electrode material and the effect of nanoscale film thickness on the resistive switching in forming-free nanocrystalline ZnO films grown by pulsed laser deposition. It was demonstrated that the nanocrystalline ZnO film with TiN, Pt, ZnO:In, and ZnO:Pd bottom electrodes exhibits a nonlinear bipolar effect of forming-free resistive switching. The sample with Pt showed the highest resistance values RHRS and RLRS and the highest value of Uset = 2.7 ± 0.4 V. The samples with the ZnO:In and ZnO:Pd bottom electrode showed the lowest Uset and Ures values. An increase in the number of laser pulses from 1000 to 5000 was shown to lead to an increase in the thickness of the nanocrystalline ZnO film from 7.2 ± 2.5 nm to 53.6 ± 18.3 nm. The dependence of electrophysical parameters (electron concentration, electron mobility, and resistivity) on the thickness of the forming-free nanocrystalline ZnO film for the TiN/ZnO/W structure was investigated. The endurance test and homogeneity test for TiN/ZnO/W structures were performed. The structure Al2O3/TiN/ZnO/W with a nanocrystalline ZnO thickness 41.2 ± 9.7 nm was shown to be preferable for the manufacture of ReRAM and memristive neuromorphic systems due to the highest value of RHRS/RLRS = 2307.8 ± 166.4 and low values of Uset = 1.9 ± 0.2 V and Ures = −1.3 ± 0.5 V. It was demonstrated that the use of the TiN top electrode in the Al2O3/TiN/ZnO memristor structure allowed for the reduction in Uset and Ures and the increase in the RHRS/RLRS ratio. The results obtained can be used in the manufacturing of resistive-switching nanoscale devices for neuromorphic computing based on the forming-free nanocrystalline ZnO oxide films.

Bipolar resistive switching with improved memory window in W/ZnFe2O4/Pt devices

We have demonstrated stable bipolar resistive switching characteristics with low forming voltage in the Rapid Thermal Annealed (RTA) ZnFe2O4 (ZDR6) devices. The structural, surface morphology, surface roughness, and electrical properties of a ZDR6 film was investigated by employing XRD, SEM, AFM and current-voltage (I-V) measurements. The I–V curves show bipolar resistive switching operations. In addition to this, the devices exhibit uniform SET (<1.6 V) and RESET (< -1.1 V) operation with a memory window (MW) larger than two orders of magnitude and show robust retention (>104 s) characteristics. From the physical and electrical characterization, it is inferred that the oxygen vacancies pre-existing in ZDR6 films induced by RTA play the primary role in achieving low forming voltage and later stable SET/RESET operations in these devices. The dominant conduction mechanism in these devices for the high resistance state (HRS) consists of ohmic at a lower bias (< 0.6 V) and space charge limited current at higher bias (> 0.6 V), while for the low resistance state (LRS) they follow the ohmic conduction for the entire range of applied bias. This present work reveals a suitable process to fabricate forming-free resistive switching memory devices.

Sol-Gel Processed ZrO2 Based Forming-Free Resistive Switching Memory Devices

In this work, we fabricated ZrO2 based resistive random access memory by sol-gel spin coating technique and investigated its structural, optical and resistive switching properties. The X-ray diffraction pattern revealed that 400 °C annealed ZrO2 thin film has tetragonal structure. The optical band gap value of ZrO2 thin film obtained was 5.51 eV. The resistive switching behaviour of W/ZrO2/ITO capacitor like structure was studied. It was found that no initial electroforming process required for the device. The fabricated devices show a self-compliance bipolar resistive switching behaviour and have high on off ratio (&gt;102). Our result suggests that solution processed ZrO2 has great potential to develop transparent and flexible resistive random access memory devices.