Forming-free Bipolar Resistive Switching Research Articles

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.

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.

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.

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.

Tunable multiferroic and forming-free bipolar resistive switching properties in multifunctional BiFeO3 film by doping engineering

The advent of multiferroic-based materials has opened the plethora for high tunable multifunctional materials and ultra-fast operation for future non-volatile memory technology. Multifunctional rhombohedral Bi0.94Y0.06Fe0.95Mn0.05O3 (BYFMO) film is grown on fluorine-doped tin oxide to investigate the electromechanical and resistive switching properties in Ag/BYFMO/FTO RRAM configuration. UV–visible absorbance spectra reveal the semiconducting behavior of BYFMO, and the band-gap is found to be 2.37 eV. The magnetic hysteresis curve manifests the soft ferromagnetic nature by suppressing the spiral spin modulated structure, supported by MFM imaging. The Y-Mn co-doped BFO possesses highly tunable piezoelectric and ferroelectric features with maximum domains preferred along 710 and 1090. Lateral domain growth is observed with the increase in tip bias voltage. The Ag/BYFMO/FTO RRAM shows distinct bipolar resistive switching behavior at the SET (ON), and RESET (OFF) processes are obtained at voltage VSET = +1.7V and VRESET = −2.8V, respectively. The memory window (ON/OFF) between high resistance state and low resistance state is about ~ 100, which can be sustained up to 100 testing cycles and 103s without any degradation, indicating that the BYFMO based device exhibits better endurance and retention properties. Moreover, the resistive switching mechanism of the device can be well explained by space charge limited current conduction, which is well supported by conducting a filamentary model. With excellent piezoelectric and resistive switching performance, the multifunctional BYFMO has enough potential for future non-volatile memory technology.

Annealing effect on forming-free bipolar resistive switching characteristics of sol-gel WOx resistive memories with Al conductive bridges

This paper studies the annealing-temperature dependent resistive switching (RS) behavior of forming-free Al/WOx/ITO memories, where the WOx active layers were prepared using a sol-gel process. The as-fabricated WOx resistive random-access memory (RRAM) is highly conductive and shows no RS characteristic. Bipolar RS behavior is obtained when the 200 °C-annealed WOx film was employed as the RS layer. The ratio of the resistance in the high-resistance state (HRS) to that in the low-resistance state (LRS) is 10. When the annealing temperature is increased to 300–400 °C, a significant increase in the resistance ratio to ~103 is obtained. A clear resistance window can be observed in the endurance test of the 300 °C-annealed RRAM, while a failure in set operation, switching from the HRS to LRS, is detected in that of the 400 °C-annealed RRAM. The performance change caused by the annealing process is attributed to the differences in the crystallinities, content of oxygen-vacancy defects and surface morphologies between the RRAMs. Aluminum conductive bridges are able to explain the RS mechanisms of the WOx RRAMs.

Fabrication of solution-processed SnO2–Based flexible ReRAM using laser-induced graphene transferred onto PDMS

In this paper, we are reporting the fabrication of a solution-processed SnO2-based flexible ReRAM using laser-induced graphene (LIG) transferred onto polydimethylsiloxane (PDMS). The fabricated ReRAM showed forming-free and self-compliance bipolar resistive switching characteristics when the applied voltage was swept from 0 V to 4.5 V for SET and from 0 V to - 4.5 V for RESET. The device operates as a filamentary type ReRAM and its conduction mechanism analysis indicates that the space charge limited conduction (SCLC) is dominant mechanism in the analog resistive switching of the fabricated device. For the reliability analysis, 100 cycles of endurance test and 1.8 × 103 s of retention test were performed. The flexibility of the fabricated ReRAM device was demonstrated by showing that the resistive switching characteristics were still obtained after bending 200 times repeatedly down to 1 mm radius. Our study suggests the new fabrication process of a solution-processed flexible ReRAM and proves its potential applications to flexible electronics.

Barium Titanate Nickelate Nanostructured Materials Prepared by Solution Process for Resistive Random Access Memory Application

Barium titanate nickelate (BTN) nanorod (NR)-based resistive random access memory (RRAM) has been demonstrated by using the hydrothermal method. The prepared BTN NR materials have the advantages of ease of fabrication, low temperature application, ability to grow various materials, and relative cost effectiveness. In addition, the BTN NR-based RRAM displayed a highly repeatable and forming-free bipolar resistive switching behavior with an ON/OFF ratio of over 105. The resistive switching behavior may be related to the oxygen vacancies on the surface of the BTN NRs, giving rise to the formation of straight and extensible conducting filaments along each vertically aligned BTN NR. Compared to its counterpart of BTN thin film, superior reproducibility was also observed, demonstrating that the nanostructured material provides an effective way to improve the memory properties.

Effects of top electrode material in hafnium-oxide-based memristive systems on highly-doped Si

This work provides useful insights into the development of HfO2-based memristive systems with a p-type silicon bottom electrode that are compatible with the complementary metal–oxide–semiconductor technology. The results obtained reveal the importance of the top electrode selection to achieve unique device characteristics. The Ag/HfO2/Si devices have exhibited a larger memory window and self-compliance characteristics. On the other hand, the Au/HfO2/Si devices have displayed substantial cycle-to-cycle variation in the ON-state conductance. These device characteristics can be used as an indicator for the design of resistive-switching devices in various scenes such as, memory, security, and sensing. The current–voltage (I–V) characteristics of Ag/HfO2/Si and Au/HfO2/Si devices under positive and negative bias conditions have provided valuable information on the ON and OFF states of the devices and the underlying resistive switching mechanisms. Repeatable, low-power, and forming-free bipolar resistive switching is obtained with both device structures, with the Au/HfO2/Si devices displaying a poorer device-to-device reproducibility. Furthermore, the Au/HfO2/Si devices have exhibited N-type negative differential resistance (NDR), suggesting Joule-heating activated migration of oxygen vacancies to be responsible for the SET process in the unstable unipolar mode.

Study on Multilevel Resistive Switching Behavior With Tunable ON/OFF Ratio Capability in Forming-Free ZnO QDs-Based RRAM

Resistive random access memories (RRAMs) have attracted tremendous attention due to their miniaturized size, low power consumption, high response speed, and simple fabrication requirements, which are regarded as a promising candidate for future nonvolatile memories. However, the traditional binary storage RRAMs fail to meet the high-density requirement in the area of big data. One of the ways to overcome this obstacle is to realize multilevel state transitions in a unit RRAM. Herein, we propose a facile spin-coating method to develop a multilevel RRAM based on Al/poly(methyl methacrylate) (PMMA)/ZnO quantum dots (QDs)/PMMA/ZnO QDs/PMMA/fluorine-doped tin oxide (FTO) structure, with the significant merits of typical forming-free bipolar resistive switching behavior, low-power operating voltage, stable endurance, and excellent retention. In specific, the ON/OFF ratio of the proposed ZnO QDs-based device can be adjusted by tuning the concentration of the ZnO QDs. Moreover, a multilevel RRAM performance with distinct four-level resistance states is obtained by varying the compliance current, which demonstrates that the proposed device has a specific application in multilevel data storage. The conductive filament model, combining the enhanced internal field and robust formation of the conductive filament, is introduced to explain the mechanism of the resistive switching behavior and multilevel performance. This work paves a new way for its prospect in constructing high-density data storage RRAMs.

Forming-free bipolar and unipolar resistive switching behaviors with low operating voltage in Ag/Ti/CeO2/Pt devices

Resistive switching devices are promising candidates to replace today’s nonvolatile memory device, and find applications in neuromorphic computing. In this study, bipolar resistive switching (BRS) and unipolar resistive switching (URS) behaviors at room temperature were shown for Ag/Ti/CeO2/Pt devices. Without forming step, the device exhibited low set voltage (about 0.3 V) and reset voltage (about −0.3 V to −0.6 V) in BRS behavior, which is beneficial for nonvolatile memory application. The I-V characteristics were analyzed, and it was concluded that the switching mechanism in BRS is dominated by the electrochemical metallization mechanism (ECM), while in URS, conduction is dominated by thermochemical mechanism (TCM).

Flexible resistive switching device based on the TiO2 nanorod arrays for non-volatile memory application

In this article, a flexible resistive switching memory is developed based on TiO2 nanorod arrays with interstice between the nanorods to release the bending stress/strain. The device shows a stable forming-free bipolar resistive switching behavior with low operating voltage of below 3 V, high ON/OFF ratio of above 102, good retention for more than 105 s, good endurance of 107 cycles, and good thermal stability of the high resistance and low resistance state at 85 °C during the retention and fatigue tests. Moreover, under the different bending radius from 12 mm to 5 mm, the electrical performances referring to operating voltage and ON/OFF ratio of the device show few changes, and under the bending cycles of 400 times with the bending radius of 12 mm, all of the devices show no interfacial crack or delamination as well as keep the bipolar resistive switching behavior with the low operating voltage and high ON/OFF ratio. The good performances of the devices suggest that the TiO2 nanorod arrays resistive switching device may open up new possibilities in realizing high-performance flexible memory.

Forming-free bipolar resistive switching and quantum conductance in NiO/FTO structures

Forming-free bipolar resistive switching and quantum conductance effects are reported for Au/NiO/FTO structures. The devices show multistep conductance changes during the RESET process for temperatures in the range from 295 to 373 K. Statistical histograms based on 650 switching cycles indicate that the conductivity changes are well represented by integer or half integer multiples of the quantum conductance G0. Temperature dependent I–V measurements below the switching voltages can be described using a Poole–Frenkel model. The mechanism of forming-free bipolar switching and the quantum conductance effect in the Au/NiO/FTO structure is interpreted by multiple quasi-filaments existing in as-grown NiO films and the numbers for the multiple conducting filaments decrease gradually in RESET processes when they rupture gradually.

Negative differential resistance and multilevel resistive switching in BaSrTiO3 films

Epitaxial BaxSr1−xTiO3 (BST) films are grown on SrTiO3:Nb (NSTO) substrates by hydrothermal means. A Pt/BST/NSTO device exhibits typical rectification characteristics under relatively low voltage and forming-free bipolar resistive switching under relatively high voltage, with the resistance ratio between the high and low resistance states exceeding four orders of magnitude. Furthermore, negative differential resistance arises during the reset process, and multilevel resistive switching is achieved under various reset voltages. These behaviors in the Pt/BST/NSTO device are likely due to trapping/detrapping of electrons by oxygen vacancies at the Pt/BST interface.

Photoresponse of nonvolatile resistive memory device based on all-inorganic perovskite CsPbBr3 nanocrystals

In this work, the resistive memory devices were fabricated based on all-inorganic CsPbBr3 nanocrystals (NCs) embedded into the insulating polymethylmethacrylate (PMMA) with a configuration of Au/PMMA/PMMA: CsPbBr3 NCs/PMMA/indium tin oxide (ITO). The as-fabricated devices exhibit forming-free bipolar resistive switching with resistance ratio of HRS to LRS (RH/L) evolving from metastable 106 to stable 10. In HRS with high RH/L, the devices witness the prompt photoresponse under light illumination of 365 nm, 405 nm, 420 nm, and 500 nm. Also, set voltages of resistive switching with low RH/L can be reduced by the illumination. Furthermore, the resistive switching and photoresponse of Au/PMMA/PMMA: CsPbBr3 NCs/PMMA/ITO devices were elucidated by considering the photosensitive property and role of trapping/de-trapping centers of CsPbBr3 NCs. This work suggests the tunable resistive switching of Au/PMMA/PMMA: CsPbBr3 NCs/PMMA/ITO devices through light illumination, and controllable photoresponse by resistive states.

Bipolar Resistive Switching Characteristics of Thermally Evaporated V2O5 Thin Films

Forming-free bipolar resistive switching has been reported in ITO/V2O5/ITO structures. The sandwiched V2O5 film was synthesized by the sol–gel process, and it suffers from the issues of non-uniformity, low thermal stability, and short lifetime due to the presence of intercalated water molecules. In this work, thermal evaporation was employed for the deposition of amorphous V2O5 in ITO/V2O5/ITO structures. Bipolar resistive switching characteristics were preserved and insensitive to annealing temperature below 300 °C. Switching disappears when the annealing temperature exceeds 350 °C due to the re-crystallization of the amorphous film. Ab initio molecular dynamics method was utilized to simulate the melt-quench process and generate the amorphous V2O5 structure. Significantly, lowered formation energies were found at some oxygen vacancy sites in amorphous structure. The simulation was in agreement with the proposed switching mechanism that the generation of mobile oxygen ions in V2O5 film was essential for the occurrence of resistive switching. The lifetime and the compatibility with IC fabrication of the devices have also been greatly improved.

Forming-Free One-Selector/One-Resistor Characteristics of Oxygen-Rich ITO Based Transparent Resistive Switching Memory via Defect Engineering Using the Reactive Sputtering Process.

In recent research of resistive random access memory (RRAM), solving the degradation phenomenon induced by both a high forming voltage to form the conducting filaments (CFs) and a high reset current is one of the main issues encountered. In this study, to overcome these problems, we propose forming-free bipolar resistive switching (BRS) behaviors by employing an ITO film with abundant oxygen vacancies, instead of conventional CF based RRAM requiring a forming process, and systematically investigate the feasibility of forming free BRS behaviors and a possible switching mechanism. Compared to conventional CF based RRAM devices, it is possible for the proposed devices to achieve stable BRS properties (i.e., narrow variations of operating current and voltage, and retention) without the forming process, under an operating current of sub-nano ampere. In addition, the proposed cell shows a stable hysteresis of current-voltage curves, which is well matched with the Poole-Frenkel emission, and currents at a low voltage are limited due to a formed barrier height like Schottky diode between the active layer and electrodes.