Resistive Switching Properties Research Articles

Controllable Electrical Properties of ZrO2/BiFeO3 Bilayer Memristor from Synaptic Mimic to TRNG Circuit Application by Modulating Compliance Currents

AbstractApart from simulating biological synapses, memristors can also be used in the secure encryption by exploiting their inherent random resistive switching (RS) properties. In this work, nonvolatile Ta/BiFeO3/ZrO2/Pt memristor is fabricated with 2 nm inserting BiFeO3 layer. At a high compliance current of 10 mA, it presents gradual RS characteristics during the set process, which resulting in 30 conductance states. Synaptic behavior can be successfully mimicked by precise conductance modulating under pulse electrical stimulation. Whereas under a low compliance current of 500 µA, it exhibits abrupt RS behavior, and the set voltage ranged from 0.3–0.8 V, which can be effectively as an entropy source for true random number generator (TRNG). Due to its Shannon entropy of 300 bits is 0.99987, Hamming weights and intra‐Hamming distances tend to be 50%. Furthermore, the software simulation of encryption and decryption of cat image is achieved by combining the key generated by TRNG. The controllable electrical properties of the Ta/BiFeO3/ZrO2/Pt device can be modulated by the compliance current, thus meeting the computing or safety requirements, which provides a solid foundation for the development of integrating computation and security at the hardware level.

Nymphaea Alba for Resistive Switching Devices: Exploring the Non‐Volatile Memory and Neuromorphic Computing Applications of the Plant Leaves

AbstractBiomaterial‐based resistive switching memory is one promising technology that could significantly impact the current direction of non‐volatile memory and neuromorphic computing. Given this, we report the fabrication of a resistive switching (RS) device based on Nymphaea Alba as an active switching layer and investigate its RS properties for memory and neuromorphic computing applications. The Nymphaea Alba switching layer was deposited by the conventional doctor blade method. Further, this is characterized by various analytical techniques. The fabricated device showed bipolar RS behavior and demonstrated good non‐volatile memory properties. Additionally, the device mimics various synaptic properties of the human brain, such as potentiation‐depression, Excitatory post‐synaptic current (EPSC), and Paired‐pulse facilitation (PPF). The charge‐flux relation reveals the non‐ideal memristor‐type behavior of the device. The Ohmic and Child's square law dominates the conduction of the device and the filamentary process responsible for the RS process of the device. These results show that the Nymphaea Alba is a promising biomaterial for non‐volatile memory applications.

Resistive switching properties in polycrystalline LiNbO3 thin films

LiNbO3 (LNO) is currently intensively studied as an important ferroelectric material. In this work, polycrystalline LNO films were prepared through a sputtering technique, and their ferroelectricity-related resistive switching property was investigated using a device structure of PtSi/SiO2/LNO/Pt. The device exhibits a volatile resistance switching property at lower positive sweeping voltages and a stable bipolar nonvolatile switching property at higher sweeping voltages. The resistive switching mechanism of the device is discussed based on the domain wall conductivity characteristics of the polycrystalline LNO thin films. The PtSi/SiO2/LNO/Pt memristor device has potential applications in memory and artificial neural synapses.

Forming-less flexible memristor crossbar array for neuromorphic computing applications produced using low-temperature atomic layer deposition

The rapid development of wearable artificial intelligence devices based on memristor crossbar arrays has increased the demand for flexible electronics. However, fabricating crossbar arrays on flexible substrates faces inherent challenges, notably due to the complex fabrication process at low temperatures. Moreover, ensuring the stability and reliability of memristor device remains a crucial issue. To address these issues, this study introduced N-doped TaOx (N:TaOx) as a flexible memristor crossbar array fabricated via atomic layer deposition directly on a flexible substrate at an exceptionally low temperature (150 °C). The flexible PET/ITO/N:TaOx/TiN memristor device exhibited forming-less bipolar resistive switching properties with analog memory characteristics. This was accomplished by increasing the nitrogen doping concentration and effectively reducing oxygen-related defects in TaOx. The amorphous-phase flexible N:TaOx memristor demonstrated remarkable stability, enduring 500 switching cycles and retaining its state for 24 h during bending tests involving 104 bending cycles at 2.5 mm bending radius. Furthermore, a 6 × 6 flexible memristor crossbar array was fabricated successfully, with all 36 devices exhibiting well-defined memristor behavior and minimal variation attributed to the film's uniformity achieved through ALD process. Additionally, by programming the tunable conductance of each device in the array, characters of desired shapes can be formed and read. Regarding their synaptic behavior via long-term potentiation and depression, the proposed flexible memristor device showed an outstanding accuracy of 96.44 % in image recognition tasks under extreme bending conditions, employing the MNIST and Fashion-MNIST datasets. The N:TaOx memristor deposited at low temperatures thus exhibits significant potential for use in wearable neuromorphic hardware applications due to its high density, high performance, reliability, and precise image recognition.

Atomic Layer Deposition Films for Resistive Random‐Access Memories

AbstractResistive random‐access memory (RRAM) stands out as a promising memory technology due to its ease of operation, high speed, affordability, exceptional stability, and potential to enable smaller memory devices with sizes under 10 nm. This has drawn significant attention, with atomic layer deposition (ALD) emerging as an ideal technology to tackle the challenges of nanoscale fabrication in the micro‐ and nanomanufacturing industry. ALD offers technological advantages such as functional multiple‐layer stacking, doping capabilities, and incorporating oxygen reservoirs or reactive layers. These factors contribute to achieving more intriguing, stable, and reliable nonvolatile resistance switching behaviors in RRAM. Specifically, ALD greatly benefits RRAM, that relies on the valence change mechanism, where high‐k transition metal oxides are commonly used as switching materials, and precise control over oxygen vacancies is achievable. This review provides a comprehensive overview of ALD films used in RRAM, delves into resistive switching properties and microscopic mechanisms in binary and ternary oxides and nitrides, and explores the impact of ALD‐prepared electrodes. Furthermore, the current status and future prospects of ALD‐based RRAM are highlighted, which is poised to catalyze further advancements in the fields of information storage and neural networks.

Versatile Titanium Carbide MXene Thin-Film Memristors with Adaptive Learning Behavior.

With the advent of the modern era, there is a huge demand for memristor-based neuromorphic computing hardware to overcome the von Neumann bottleneck in traditional computers. Here, we have prepared two-dimensional titanium carbide (Ti3C2Tx) MXene following the conventional HF etching technique in solution. After confirmation of Ti3C2Tx properties by Raman scattering and crystallinity measurements, high-quality thin-film deposition is realized using an immiscible liquid-liquid interfacial growth technique. Following this, the memristor is fabricated by sandwiching a Ti3C2Tx layer with a thickness of 70 nm between two electrodes. Subsequently, current-voltage (I-V) characteristics are measured, revealing a nonvolatile resistive switching property characterized by a swift switching speed of 30 ns and an impressive current On/Off ratio of approximately 103. Furthermore, it exhibits endurance through 500 cycles and retains the states for at least 1 × 104 s without observable degradation. Additionally, it maintains a current On/Off ratio of about 102 while consuming only femtojoules (fJ) of electrical energy per reading. Systematic I-V results and conductive AFM-based current mapping image analysis are converged to support the electroforming mediated filamentary conduction mechanism. Furthermore, our Ti3C2Tx memristor was found to be truly versatile as an all-in-one device for demonstrating edge computation, logic gate operation, and classical conditioning of learning by the brain in Psychology.

The effect of substrate-induced defects on structural and resistive switching properties in Gd0.2Ca0.8MnO3 thin films

Gd0.2Ca0.8MnO3 thin films were deposited on various substrate materials and their structural and resistive switching (RS) properties were investigated. The deposition resulted in epitaxial and polycrystalline films, with the latter also exhibiting distorted film surfaces. Both epitaxial and a part of polycrystalline films used as RS devices showed consistent RS performance in which an order of magnitude, or higher, switching ratios were achieved between high and low resistance states. The devices showed strong endurance during repeated switching cycles. However, under retention characterization, the resistance states did not remain distinguishable in devices constructed on polycrystalline films, while other devices maintained separable resistance states. The RS results are discussed in relation to the structural characteristics of the films, and this work helps us understand the RS mechanisms that still remain elusive in manganite-based devices.

Modulating resistive switching in Ni0.5Zn0.5Fe2O4 memristors: The role of manganese doping concentrations

Memristors, recognized as key devices for mitigating the von Neumann bottleneck, show great potential in non-volatile memory technology. Their performance is fundamentally linked to the chemical composition and structural characteristics of their intermediate dielectric layer. Owing to the inherent advantages of metal oxides, such as their simplistic chemical makeup and superior thermal stability, memristors featuring a Metal-Insulator-Metal (MIM) sandwich structure with metal oxides as the intermediate layer have received extensive focus. In this study, a Pt/Ni0.5Zn0.5MnxFe2-xO4(NZMFO)/Pt composite film was prepared, and its resistance switching behavior was modulated by altering the doping concentration of Mnn+ in NZMFO. The experimental results indicate that altering the Mnn+ doping concentration does significantly impact the content of oxygen vacancies. As the concentration of Mnn+ increases from 0.10 to 0.20 at.%, the content of oxygen vacancies correspondingly rises from 46.58 % to 76.42 %. Concurrently, variations in Mnn+ doping concentration significantly alter the resistance switching properties of the Pt/NZMFO/Pt composite films. This includes elevating the set/reset voltage, diminishing the resistance ratio, and enhancing the durability with the augmentation of Mnn+ concentration. By meticulously adjusting the Mnn+ doping concentration, we can achieve a memristor characterized by superior performance across all metrics. Considering all performance aspects, this study determined that the Pt/NZMFO/Pt thin film memristor with x = 0.15 exhibits adequate endurance (100 cycles), high data retention, and stable set/reset voltage. This work underscores the critical role of Mnn+ doping concentration in optimizing the electrochemical properties of NZFO memristors, illustrating a strategic pathway to engineer memristive devices with enhanced performance for future non-volatile memory applications.

Inkjet printed IGZO memristors with volatile and non-volatile switching

Solution-based memristors deposited by inkjet printing technique have a strong technological potential based on their scalability, low cost, environmentally friendlier processing by being an efficient technique with minimal material waste. Indium-gallium-zinc oxide (IGZO), an oxide semiconductor material, shows promising resistive switching properties. In this work, a printed Ag/IGZO/ITO memristor has been fabricated. The IGZO thickness influences both memory window and switching voltage of the devices. The devices show both volatile counter8wise (c8w) and non-volatile 8wise (8w) switching at low operating voltage. The 8w switching has a SET and RESET voltage lower than 2 V and − 5 V, respectively, a retention up to 105 s and a memory window up to 100, whereas the c8w switching shows volatile characteristics with a low threshold voltage (Vth < − 0.65 V) and a characteristic time (τ) of 0.75 ± 0.12 ms when a single pulse of − 0.65 V with width of 0.1 ms is applied. The characteristic time alters depending on the number of pulses. These volatile characteristics allowed them to be tested on different 4-bit pulse sequences, as an initial proof of concept for temporal signal processing applications.

A Flexible Nickel-Oxide-Based RRAM Device Prepared Using the Solution Combustion Method

Binary metal oxide materials, such as nickel oxide, are widely used in flexible resistive variable memory devices due to advantages such as their easily controllable material composition, simple structural composition, and good compatibility between manufacturing processes and complementary metal oxide processes. In this work, a solution combustion method was employed to prepare NiOx thin films for use as a resistive switching layer of resistance random-access memory. The formation temperature of the NiOx layer in the RRAM device was kept as low as 175 °C, making the device compatible with flexible substrates. With polyethylene naphthalenediate as the substrate, the NiOx-based RRAM exhibited obvious bipolar resistance-switching properties, superb bending resistance, and good stability. Through theoretical fitting and structural characterization, the conduction mechanisms were attributed to the combination of the space-charge-limited current and Ohmic mechanisms, while the valence change mechanism was determined to be the resistive switching mechanism. This study demonstrates a low-temperature and scalable approach to constructing NiOx-based RRAM devices on flexible substrates.

Resistive Switching Property of Euforbia Cotinifolia Plant Extract for Potential Use in Eco-Friendly Memory Devices

Resistive switching memory devices based on organic materials are intriguing. These devices are biodegradable and nontoxic to living organisms. In this work, using euphorbia cotinifolia plant extract was investigated for its applicability as an active layer of a resistive switching memory device consisting of silver top electrode and indium-doped tin oxide bottom electrode. This study selected Euphorbia cotinifolia because it is a common, prolific and simple-to-grow plant in many South African homes. When the euphorbia cotinifolia plant is broken, an extract resembling milk is emitted. This extract was collected directly onto the bottom electrode, which was then dried at ambient temperature. For top electrode, silver paste was applied. The entire fabrication process was devoid of heat and electricity. The fabricated device showed impressive properties such as memory hysteresis with [Formula: see text], endurance of over [Formula: see text] write/erase cycles, and an impressive retention of [Formula: see text]. Therefore, this system may be a candidate for a nonvolatile and disposable memory device.

Effects of Dy on structural, morphology, optical, and resistive switching properties of HfO2 thin films

This paper studies the effect of Dy on the structure, morphology, and optical properties of HfO2 thin films deposited on fused silica substrates and the optical transparency and resistive switching (RS) behavior of the thin films as an active insulator layer in Ag/active insulator layer/ITO/glass memories. Thin films were deposited by spin-coating using a precursor solution obtained by the Pechini-type sol-gel method. The structural analysis revealed that the insertion of Dy ions into the HfO2 lattice caused the phase transformation from the monoclinic to the cubic phase. Morphological analysis revealed that the thin films had thicknesses between 98 and 101 nm, and their surfaces were composed of 3D islands formed by aggregates of spherical particles ranging from 8 to 5 nm. The active insulator layers had thicknesses ranging from 235 to 218 nm, with no cavities or cracks between them. The optical transparency of the active insulator layer/ITO/glass memory structures was between 70 and 87 %. Optical transmittance, optical bandgap, and optical constants were modified incorporating Dy ions into HfO2 thin films compared to the undoped HfO2 thin films. Incorporating Dy ions into HfO2 increased oxygen vacancies in the active insulator layers. The memory with 10 w% of Dy had the highest resistance off/on ratio of 9.04 compared with undoped HfO2 memory. Also, the conduction mechanism in the high resistive state (HRS) of the memory with 10 w% of Dy was related to ohmic conduction and space charge limited conduction (SCLC).

Structural phase transition and resistive switching properties of Cu x O films during post-thermal annealing

We studied the phase change and resistive switching characteristics of copper oxide (Cu x O) films through post-thermal annealing. This investigation aimed to assess the material’s potential for a variety of electrical devices, exploring its versatility in electronic applications. The Cu x O films deposited by RF magnetron sputtering were annealed at 300, 500, and 700 °C in ambient air for 4 min by rapid thermal annealing (RTA) method, and then it was confirmed that the structural phase change from Cu2O to CuO occurred with increasing annealing temperature. Resistive random-access memory (ReRAM) devices with Au/Cu x O/p+-Si structures were fabricated, and the ReRAM properties appeared in CuO-based devices, while Cu2O ReRAM devices did not exhibit resistive switching behavior. The CuO ReRAM device annealed at 500 °C showed the best properties, with a on/off ratio of 8 × 102, good switching endurance of ∼100 cycles, data retention for 104 s, and stable uniformity in the cumulative probability distribution. This characteristic change could be explained by the difference in the grain size and density of defects between the Cu2O and CuO films. These results demonstrate that superior and stable resistive switching properties of RF-sputtered Cu x O films can be obtained by low-temperature RTA.

Photovoltaic and resistance switching properties of NiO/Pb0.92La0.08(Zr0.52Ti0.48)O3/LaNiO3

Due to its excellent ferroelectricity and photovoltaic effect (PE), Pb0.92La0.08(Zr0.52Ti0.48)O3 (PLZT) has considerable application potential in the field of multifunctional electronic devices. In this study, an Ag/NiO/PLZT/LaNiO3 structure is prepared on a Si substrate via a combination of sol–gel and pulsed laser deposition methods, and its ferroelectricity, PE and resistance switching (RS) properties are studied in detail. With decrease in temperature, coercive field of PLZT increases, and its residual polarization initially remains constant and then decreases. The results of ultraviolet-irradiation experiments show that the photogenerated open-circuit voltage (Vo) of PLZT first increases slightly and then decreases slightly as the temperature decreases. By changing the direction of PLZT polarization, the polarization field can be oriented in the same direction or opposite to the built-in electric field formed by p–n junction at NiO/PLZT interface. Thus, the total electric field required to drive the photogenerated carriers inside the structure can be tuned, which results in different Vo values. Furthermore, current–voltage characteristics of the structure are tested. It is found that as different polarization charges are generated at the NiO/PLZT interface for different PLZT polarization directions, the holes in NiO can be attracted or repelled by these polarization charges. Thereby, the interface barrier height of the p–n junction can be varied. Consequently, the structure exhibits a pronounced RS effect suitable for multilevel storage applications. These results represent an important experimental basis for the development and application of multifunctional electronic devices for optoelectronics and RS-based memories.

Carbon quantum dots boosted resistive switching and magnetic properties in Co3O4-based memory devices

Carbon quantum dots (CQDs) could create opportunities of modulating resistive switching behaviors due to their unique electrical performance. Here, CQDs are introduced to enhance switching uniformity and magnetic properties in Co3O4 thin film memory devices. Compared with the pure Co3O4-based device, the device with CQDs exhibited better performance, such as a reduced initialization voltage, uniform distribution of Set and Reset voltages, stable endurance behavior, and enhanced magnetization performance. The fitting results of current-voltage curves showed that the low-resistance states and high-resistance states of the devices obey Ohmic conduction and Schottky emission, respectively. The improvement in resistive switching properties is ascribed to the formation and rupture of confined conductive filaments due to that CQDs enhanced the local electric field, which can efficiently guide the filament growth and suppress the randomness of filament growth. Magnetization modulation associated with resistive switching involves in the variation of oxygen vacancies concentration and the conversion between Co2+ and Co3+. The present study exhibits an effective path to enhance resistive switching and magnetic properties, which shows promising application to design multifunctional electro-magnetic integrated devices.

Quantum Conductance in Vertical Hexagonal Boron Nitride Memristors with Graphene-Edge Contacts.

Two-dimensional materials (2DMs) have gained significant interest for resistive-switching memory toward neuromorphic and in-memory computing (IMC). To achieve atomic-level miniaturization, we introduce vertical hexagonal boron nitride (h-BN) memristors with graphene edge contacts. In addition to enabling three-dimensional (3D) integration (i.e., vertical stacking) for ultimate scalability, the proposed structure delivers ultralow power by isolating single conductive nanofilaments (CNFs) in ultrasmall active areas with negligible leakage thanks to atomically thin (∼0.3 nm) graphene edge contacts. Moreover, it facilitates studying fundamental resistive-switching behavior of single CNFs in CVD-grown 2DMs that was previously unattainable with planar devices. This way, we studied their programming characteristics and observed a consistent single quantum step in conductance attributed to unique atomically constrained nanofilament behavior in CVD-grown 2DMs. This resistive-switching property was previously suggested for h-BN memristors and linked to potential improvements in stability (robustness of CNFs), and now we show experimental evidence including superior retention of quantized conductance.

Ultrasensitive Ionic Conductors with Tunable Resistance Switching Temperature Enabled by Phase Transformation of Polymer Cocrystals.

Phase-transformable ionic conductors (PTICs) show significant prospects for functional applications due to their reversible resistance switching property. However, the representative design principle of PTICs is utilizing the melt-crystallization transition of ionic liquids, and the resistance switching temperatures of such PTICs cannot be tuned as desired. Herein, a new strategy is proposed to design PTICs with on-demand resistance switching temperatures by using the melt-crystallization transition of polymer cocrystal phase, whose melting temperature shows a linear relationship with the polymer compositions. Owing to the melt of polymer cocrystal domains and the tunable migration of ions in the resistance switching region, the obtained PTICs display ultrahigh temperature sensitivity with a superior temperature coefficient of resistance of -8.50% °C-1 around human body temperature, as compared to various ionic conductors previously reported. Therefore, the PTICs can detect tiny temperature variation, allowing for the intelligent applications for overheating warning and heat dissipation. It is believed that this work may inspire future researches on the development of advanced soft electrical devices.

Cell Size Effects and Distinct Current Conduction Behaviors for Hafnium‐Oxide‐Based Selectors with Vanadium as Top Electrode

Selector devices have been incorporated in the resistance random access memory to realize one‐selector‐one‐resistor device for the achievement of high‐density storage applications. Among many possible materials for selector devices, the metal–insulator transition mechanism of the vanadium oxide (VOx) is interesting due to its formation under the influence of temperature. Previous studies have also indicated that VOx is formed after high‐temperature annealing. Herein, the V/HfO2/TiN with different deice cell sizes is investigated. After the forming process, devices with small cell sizes can exhibit threshold switching (TS) characteristics, while devices with large cell sizes can show resistive switching property. Electrical measurements of DC and AC operations are also conducted to verify stable TS characteristics for selector applications. The current fitting method and the computer‐aid simulation are also carried out to compare different current conduction behaviors of devices with large and small cell sizes, which indicates that heat plays an important role in the formations of VOx layer of the V/HfO2/TiN device.

Memory device based on a nano-granulars and nano-worms structured MoS2 active layer: The origin of resistive switching characteristics

This study systematically demonstrates resistive switching (RS) functionality of MoS2 nanostructured [nano-granulars (NGs) and nano-worms (NWs)] based non-volatile resistive random access memory (ReRAM) device. Interestingly, the SET and RESET voltage sharply changes from NGs to NWs device. The RS behavior of devices is interpreted by proposing and discussing a model. In the Ag/MoS2(NGs)/ITO device, a high resistance state (HRS) and low resistance state (LRS) could be ascribed due to the formation/disruption of conducting filament via trapping-detrapping of electrons across the NGs active layer. In contrast, the RS behavior of Ag/MoS2(NWs)/ITO device is caused by forming of an Ag metallic filamentary route between the electrodes. The NWs structured device show substantially better endurance and homogeneity than the device with an NGs structure. This nanostructural-dependent device functionality allows extended RS properties and establishes a relationship between the material's nano-level structural and electrical features.

Preparation and resistive switching properties of ZrO2 films with large room-temperature ferromagnetism

In this study, ordered porous ZrO2 films were prepared on porous Al2O3 films by magnetron sputtering. The films showed large room-temperature ferromagnetism, and it was also observed that the porous ZrO2 films have magnetic anisotropy. The magnetism in the direction perpendicular to the film surface was much greater than in the direction parallel to the film surface. A saturation magnetization value which was up to 3.19 × 10−4 emu was obtained for the ZrO2 film with pores of 42.55 nm in diameter when the external field was perpendicular to the film surface. The saturation magnetization strength per unit volume is 509.26 emu/cm3. After a photoluminescence (PL) spectrum and X-ray photoelectron spectroscopy (XPS) test on the film, it was found that oxygen vacancy content has a direct effect on the film’s magnetism. In addition, some Ag/ZrO2/PAA/Al devices were prepared, and the resistive switching properties of the ZrO2 films were tested. The research in this paper provides ideas for the development of spintronic devices.