Electroforming-free Resistive Switching Research Articles

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.

Effects of thermal annealing on analog resistive switching behavior in bilayer HfO2/ZnO synaptic devices: the role of ZnO grain boundaries.

The effects of thermal annealing on analog resistive switching behavior in bilayer HfO2/ZnO synaptic devices were investigated. The annealed active ZnO layer between the top Pd electrode and the HfO2 layer exhibited electroforming-free resistive switching. In particular, the switching uniformity, stability, and reliability of the synaptic devices were dramatically improved via thermal annealing at 600 °C atomic force microscopy and X-ray diffraction analyses revealed that active ZnO films demonstrated increased grain size upon annealing from 400 °C to 700 °C, whereas the ZnO film thickness and the annealing of the HfO2 layer in bilayer HfO2/ZnO synaptic devices did not profoundly affect the analog switching behavior. The optimized thermal annealing at 600 °C in bilayer HfO2/ZnO synaptic devices dramatically improved the nonlinearity of long-term potentiation/depression properties, the relative coefficient of variation of the asymmetry distribution σ/μ, and the asymmetry ratio, which approached 1. The results offer valuable insights into the implementation of highly robust synaptic devices in neural networks.

Electroforming-Free Bipolar Resistive Switching Memory Based on Magnesium Fluoride.

Electroforming-free resistive switching random access memory (RRAM) devices employing magnesium fluoride (MgFx) as the resistive switching layer are reported. The electroforming-free MgFx based RRAM devices exhibit bipolar SET/RESET operational characteristics with an on/off ratio higher than 102 and good data retention of >104 s. The resistive switching mechanism in the Ti/MgFx/Pt devices combines two processes as well as trap-controlled space charge limited conduction (SCLC), which is governed by pre-existing defects of fluoride vacancies in the bulk MgFx layer. In addition, filamentary switching mode at the interface between the MgFx and Ti layers is assisted by O–H group-related defects on the surface of the active layer.

2D oriented covalent organic frameworks for alcohol-sensory synapses.

Resistive random access memories (RRAMs) based on the electrochemical metallization mechanism (ECM) have potential applications in high-density data storage and efficient neuromorphic computing. However, the high variability of ECM devices still hinders their application in artificial intelligence owing to the random formation of conductive filaments (CFs). Here, we demonstrate 2D covalent organic framework (COF) RRAM with electroforming-free resistive switching behavior, low spatial/temporal variations, and excellent retention capability up to 105 s. The one-dimensional channels of the oriented COF-5 film can not only confine the shape of filaments but also modulate the transition direction of Ag ions. Moreover, alcohol vapors could activate the device to achieve gas-mediated multilevel resistive switching since COF materials can absorb small molecules through host guest interactions to vary the conductivity. An alcohol gas recognition system constructed by integrating the COF RRAM as a sensor and filter part with the k-nearest neighbors (KNN) algorithm as a classifier was demonstrated with a recognition accuracy of 87.2%. Furthermore, the effect of alcohol inhibition stimulation in the human nervous system is successfully emulated by the COF RRAM.

Multiple nano-filaments based efficient resistive switching in TiO2 nanotubes array influenced by thermally induced self-doping and anatase to rutile phase transformation

In this paper, the impact of thermally induced self-doping and phase transformation in TiO2 based resistive random-access memory (ReRAM) is discussed. Instead of a thin film, a vertically aligned one-dimensional TiO2 nanotube array (TNTA) was used as a switching element. Anodic oxidation method was employed to synthesize TNTA, which was thermally treated in the air at 350 °C followed by further annealing from 350 °C to 650 °C in argon. Au/TiO2 nanotube/Ti resistive switching devices were fabricated with porous gold (Au) top electrode. The x-ray diffraction results along with Raman spectra evidently demonstrate a change in phase of crystallinity from anatase to rutile, whereas photoluminescence spectra revealed the self-doping level in terms of oxygen vacancies (OV) and Ti interstitials (Tii) as the temperature of thermal treatment gets increased. The electrical characterizations establish the bipolar and electroforming free resistive switching in all the samples. Among those, the ReRAM sample S3 thermally treated at 550 °C displayed the most effective resistive switching properties with R OFF/R ON of 102 at a read voltage of −0.6 V and a SET voltage of −2.0 V. Moreover, the S3 sample showed excellent retention performance for over 106 s, where stable R OFF/R ON ≈ 107 was maintained throughout the experiment.

Robust resistive switching performance of pulsed laser deposited SiC/Ag/SiC tri-layer thin films deposited on a glass substrate

In this work, the authors developed SiC(10 nm)/Ag/SiC(10 nm) thin films showing an electroforming-free resistive switching (RS) effect with a switching ratio of 102. The observed RS effect is attributed to charging and discharging of Ag nanoparticles in the film layer. Further, SiC/Ag/ SiC film shows an excellent endurance and retention as well as a good thermal stability of RS characteristics. It is also identified that the switching ratio is invariant but the switching voltage of the device greatly depends on the Ag nanoparticles concentration and the operation temperature of the device. Therefore, SiC/Ag/SiC thin films are attractive for next-generation memory devices with enhanced durability.

Synthesis and Memristor Effect of a Forming-Free ZnO Nanocrystalline Films.

We experimentally investigated the effect of post-growth annealing on the morphological, structural, and electrophysical parameters of nanocrystalline ZnO films fabricated by pulsed laser deposition. The influence of post-growth annealing modes on the electroforming voltage and the resistive switching effect in ZnO nanocrystalline films is investigated. We demonstrated that nanocrystalline zinc oxide films, fabricated at certain regimes, show the electroforming-free resistive switching. It was shown, that the forming-free nanocrystalline ZnO film demonstrated a resistive switching effect and switched at a voltage 1.9 ± 0.2 V from 62.42 ± 6.47 (RHRS) to 0.83 ± 0.06 kΩ (RLRS). The influence of ZnO surface morphology on the resistive switching effect is experimentally investigated. It was shown, that the ZnO nanocrystalline film exhibits a stable resistive switching effect, which is weakly dependent on its nanoscale structure. The influence of technological parameters on the resistive switching effect in a forming-free ZnO nanocrystalline film is investigated. The results can be used for fabrication of new-generation micro- and nanoelectronics elements, including random resistive memory (ReRAM) elements for neuromorphic structures based on forming-free ZnO nanocrystalline films.

Electroforming-free resistive switching in yttrium manganite thin films by cationic substitution

We report unipolar resistive switching in polycrystalline, hexagonal yttrium manganite thin films grown on unpatterned Pt metal coated SiO2/Si substrates with circular Al top electrodes. Electroforming-free or electroforming-based resistive switching is observed, depending on the chemical composition (Y1Mn1O3, Y0.95Mn1.05O3, Y1Mn0.99Ti0.01O3, and Y0.94Mn1.05Ti0.01O3). The number of loading cycles measured at room temperature for samples with Y1Mn1O3 and Y0.95Mn1.05O3 composition is larger than 103. The dominant conduction mechanism of the metal–insulator–metal structures between 295 K and 373 K in the high resistance state is space charge limited conduction and in the low resistance state is ohmic conduction. Activation energies in Ohm's law region in the high resistance state are calculated from the Arrhenius equation and are evaluated to be 0.39 ± 0.01 eV (Y1Mn1O3), 0.43 ± 0.01 eV (Y0.95Mn1.05O3), 0.34 ± 0.01 eV (Y1Mn0.99Ti0.01O3), and 0.38 ± 0.02 eV (Y0.94Mn1.05Ti0.01O3).

Ultralow Power Consumption Flexible Biomemristors.

Low power consumption is the important requirement in memory devices for saving energy. In particular, improved energy efficiency is essential in implantable electronic devices for operation under a limited power supply. Here, we demonstrate the use of κ-carrageenan (κ-car) as the resistive switching layer to achieve memory that has low power consumption. A carboxymethyl (CM) group is introduced to the κ-car to increase its ionic conductivity. Ag was doped in CM:κ-car to improve the resistive switching properties of the devices. Memory devices based on Ag-doped CM:κ-car showed electroforming-free resistive switching. This device exhibited low reset voltage (∼0.05 V), fast switching speed (50 ns), and high on/off ratio (>103) under low compliance current (10-5 A). Its power consumption (∼0.35 μW) is much lower than those of the previously reported biomemristors. The resistive switching may be a result of an electrochemical redox process and Ag filament formation in the CM:κ-car under an electric field. This biopolymer memory can also be fabricated on flexible substrate. This study verifies the feasibility of using biopolymers for applications to future implantable and biocompatible nanoelectronics.

Highly flexible and electroforming free resistive switching behavior of tungsten disulfide flakes fabricated through advanced printing technology

Tungsten disulfide (WS2) is a transition metal dichalcogenide that differs from other 2D materials such as graphene owing to its distinctive semiconducting nature and tunable band gap. In this study, we have reported the structural, electrical, physical, and mechanical properties of exfoliated WS2 flakes and used them as the functional layer of a rewritable bipolar memory device. We demonstrate this concept by sandwiching few-layered WS2 flakes between two silver (Ag) electrodes on a flexible and transparent PET substrate. The entire device fabrication was carried out through all-printing technology such as reverse offset printing for patterning bottom electrodes, electrohydrodynamic (EHD) atomization for depositing functional thin film and EHD patterning for depositing the top electrode respectively. The memory device was further encapsulated with an atomically thin layer of aluminum oxide (Al2O3), deposited through a spatial atmospheric atomic layer deposition system to protect it against a humid environment. Remarkable resistive switching results were obtained, such as nonvolatile bipolar behavior, a high switching ratio (∼103), a long retention time (∼105 s), high endurance (1500 voltage sweeps), a low operating voltage (∼2 V), low current compliance (50 μA), mechanical robustness (1500 cycles) and unique repeatability at ambient conditions. Ag/WS2/Ag-based memory devices offer a new possibility for integration in flexible electronic devices.

Light controlled resistive switching and photovoltaic effects in ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 thin films

In this work, the structural and ferroelectric properties of 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (0.5BZT-0.5BCT) thin films deposited at different pulse repetition rates were studied. The films deposited at pulse repetition rate of 1Hz display the optimum values of ferroelectric polarization and dielectric permittivity and are chosen for the investigation of resistive switching and photovoltaic studies. The Pt/0.5BZT-0.5BCT/ITO capacitors show the electroforming free resistive switching (RS) and is explained based on the polarization modulation of the Schottky barrier at the 0.5BZT-0.5BCT/ITO interface. Furthermore, it is shown that the RS ratio and switching voltage can be tuned with white light illumination. The capacitors display photovoltaic effect with the open circuit voltage ≈0.8V and the short circuit current density ≈72.6μAcm−2. The photovoltaic efficiency is found to be ≈0.010% and is greater than that of other perovskite ferroelectric thin films. The underlying mechanism for enhanced RS and photovoltaic effects is highlighted.

Resistive switching in ferroelectric lead-free 0.5Ba (Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 thin films

In this work, resistive switching in pulsed laser deposited ferroelectric lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (0.5BZT–0.5BCT) thin films was investigated. This study reveals that films grown at 5.5 J cm−2 have shown optimal ferroelectric and resistive switching response, which are attributed to high tetragonality, large grain size and less defect concentration. Au/0.5BZT–0.5BCT/Pt capacitors show the electroforming free resistive switching that is explained based on the polarization modulation of the Schottky-like barrier at the 0.5BZT–0.5BCT/Au interface. The polarization induced resistive switching is evidenced by its disappearance as the temperature increases to the Curie temperature. The capacitor based on film grown at 5.5 J cm−2 shows resistive switching characterized by high switching ratio of 106 at a low set/reset voltage ≈1 V, and by a stable memory window, which are highly required for memory applications.

Organolead Halide Perovskites for Low Operating Voltage Multilevel Resistive Switching.

Organolead halide perovskites are used for low-operating-voltage multilevel resistive switching. Ag/CH3 NH3 PbI3 /Pt cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10(3) V cm(-1) for four distinguishable ON-state resistance levels. The migration of iodine interstitials and vacancies with low activation energies is responsible for the low-electric-field resistive switching via filament formation and annihilation.

Effect of annealing temperature on photoluminescence and resistive switching characteristics of ZnO/Al2O3 multilayer nanostructures

This work demonstrates the role of defects generated during rapid thermal annealing of pulsed laser deposited ZnO/Al2O3 multilayer nanostructures in presence of vacuum at different temperatures (Ta) (500–900°C) on their electrical conductance and optical characteristics. Photoluminescence (PL) emissions show the stronger green emission at Ta≈600°C and violet–blue emission at Ta⩾800°C, and are attributed to oxygen vacancies and zinc related defects (zinc vacancies and interstitials) respectively. Current–voltage (I–V) characteristics of nanostructures with rich oxygen vacancies and zinc related defects display the electroforming free resistive switching (RS) characteristics. Nanostructures with rich oxygen vacancies exhibit conventional and stable RS behavior with high and low resistance states (HRS/LRS) ratio ≈104 during the retention test. Besides, the dominant conduction mechanism of HRS and LRS is explained by trap-controlled-space-charge limited conduction mechanism, where the oxygen vacancies act as traps. On the other hand, nanostructures with rich zinc related defects show a diode-like RS behavior. The rectifying ratio is found to be sensitive on the zinc interstitials concentration. It is assumed that the rectifying behavior is due to the electrically formed interface layer ZnAl2O4 at the Zn defects rich ZnO crystals – Al2O3−x interface and the switching behavior is attributed to the electron trapping/de-trapping process at zinc vacancies.

Structural properties and electroforming-free resistive switching characteristics of GdOx, TbOx, and HoOx memory devices

Resistive random access memory (RRAM) using the Ru/REOx/TaN (rare-earth, RE, RE = Gd, Tb and Ho) structures is fabricated with full room temperature process and is not required the electroforming process. X-ray diffraction and X-ray photoelectron spectroscopy were used to study the structural and chemical features of REOx films. The conduction mechanism of REOx-based RRAM memory devices in the low-resistance state is Ohmic emission, whereas the high-resistance state is space-charge-limited conduction. The GdOx-based RRAM devices show high-resistance ratio of about 104, reliable data retention of 105 s, and stable cycling behaviors for up to 190 cycles. This result suggests the high concentration of the metallic Gd0 and oxygen vacancies in GdOx film. The Ru/GdOx/TaN structure memory is a possible candidate for next-generation nonvolatile memory applications.

Resistive Switching Characteristics of Tm$_{2}$O $_{3}$, Yb$_{2}$O $_{3}$, and Lu$_{2}$O$_{3}$-Based Metal–Insulator–Metal Memory Devices

In this paper, we investigated the electroforming-free resistive switching (RS) behavior in the Ru/RE2O3/TaN (rare-earth, RE, RE = Tm, Yb, and Lu) memory device fabricated with full room temperature process. The conduction mechanism of RE2O3-based memory devices in the low-resistance state is ohmic emission, whereas Tm2O3, Yb2O3, and Lu2O3 memory devices in the high-resistance state are space charge limited conduction (SCLC), ohmic behavior, and SCLC, respectively. The Ru/Lu2O3/TaN device showed a high-resistance ratio of ~ 104, a high device yield of ~70%, a good data retention as long as 105s measured at 85°C, and a reliable endurance for up to 100 cycles, suggesting the optimal chemical defects (metallic Lu and nonlattice oxygen ion) in Lu2O3 film. All of these results suggest that Ru/Lu2O3/TaN structure memory is a good candidate for future nonvolatile RS memory applications.