Reliable and Low-Power Multilevel Resistive Switching in TiO2 Nanorod Arrays Structured with a TiOx Seed Layer

Investigation of Resistance Switching and Synaptic Properties of VO x for Neuromorphic Applications

The taking run on artificial intelligence in the last decades is based on the von Neumann architecture where memory and computation units are separately located from each other. This configuration causes a large amount of energy and time to be dissipated during data transfer between these two units, in contrast to synapses in biological neurons. A new paradigm has been proposed inspired by biological neurons in human brains, known as neuromorphic computing. Due to the unusual current-voltage characteristic of memristor devices such as pinched hysteresis loops, memristors are considered a key element of neuromorphic architecture. In this study, we report the basic current-voltage characteristic of the memristor devices in the form of Si/SiO2/Pt(30 nm)/VO x (3, 13, 25 nm)/Pt (30 nm) sandwich structure. Synaptic functions such as spike-time-dependent plasticity (STDP), paired-pulse facilitation (PPF), long-term potentiation (LTP), and long-term depression (LTD) of memristor devices were examined in detail. The oxide layer VO x has been grown by using the VO2 target in a pulsed laser deposition (PLD) chamber. The composition and oxidation states of the oxide layer were examined using the X-ray photoelectron spectroscopy (XPS) technique. The status of oxygen vacancies, which play an active role in the operation of the devices, was examined with a photoluminescence (PL) technique. The experimental results showed that the thickness of the oxide layer can significantly influence the synaptic and resistive switching properties of the devices.

1-Selector 1-Memristor Configuration with Multifunctional a-IGZO Memristive Devices Fabricated at Room Temperature

Serving as neuromorphic hardware accelerators, memristors play a crucial role in large-scale neuromorphic computing. Herein, two-terminal memristors utilizing amorphous indium-gallium-zinc oxide (a-IGZO) are fabricated through room-temperature sputtering. The electrical characteristics of these memristors are effectively modulated by varying the oxygen flow during the deposition process. The optimized a-IGZO memristor, fabricated under 3 sccm oxygen flow, presents a 5 × 103 ratio between its high- and low-resistance states, which can be maintained over 1 × 104 s with minimal degradation. Meanwhile, desirable properties such as electroforming-free and self-compliance, crucial for low-energy consumption, are also obtained in the a-IGZO memristor. Moreover, analog conductance switching is observed, demonstrating an interface-type behavior, as evidenced by its device-size-dependent performance. The coexistence of negative differential resistance with analog switching is attributed to the migration of oxygen vacancies and the trapping/detrapping of charges. Furthermore, the device demonstrates optical storage capabilities by exploiting the optical properties of a-IGZO, which can stably operate for up to 50 sweep cycles. Various synaptic functions have been demonstrated, including paired-pulse facilitation and spike-timing-dependent plasticity. These functionalities contribute to a simulated recognition accuracy of 90% for handwritten digits. Importantly, a one-selector one-memristor (1S1M) architecture is successfully constructed at room temperature by integrating a-IGZO memristor on a TaOx-based selector. This architecture exhibits a 107 on/off ratio, demonstrating its potential to suppress sneak currents among adjacent units in a memristor crossbar.

Multilevel resistive switching in stable all-inorganic n-i-p double perovskite memristor

Memristors are promising information storage devices for commercial applications because of their long endurance and low power consumption. Particularly, perovskite memristors have revealed excellent resistive switching (RS) properties owing to the fast ion migration and solution fabrication process. Here, an n-i-p type double perovskite memristor with "ITO/SnO₂/Cs₂AgBiBr₆/NiO_x/Ag" architecture was developed and demonstrated to reveal three resistance states because of the p-n junction electric field coupled with ion migration. The devices exhibited reliable filamentary with an on/off ratio exceeding 50. The RS characteristics remained unchanged after 1000 s read and 300 switching cycles. The synaptic functions were examined through long-term depression and potentiation measurements. Significantly, the device still worked after one year to reveal long-term stability because of the all-inorganic layers. This work indicates a novel idea for designing a multistate memristor by utilizing the p-n junction unidirectional conductivity during the forward and reverse scanning.

The Effect of Nitrogen Annealing on the Resistive Switching Characteristics of the W/TiO2/FTO Memory Device

In this paper, the effect of nitrogen annealing on the resistive switching characteristics of the rutile TiO₂ nanowire-based W/TiO₂/FTO memory device is analyzed. The W/TiO₂/FTO memory device exhibits a nonvolatile bipolar resistive switching behavior with a high resistance ratio (R_HRS/R_LRS) of about two orders of magnitude. The conduction behaviors of the W/TiO₂/FTO memory device are attributed to the Ohmic conduction mechanism and the Schottky emission in the low resistance state and the high resistance state, respectively. Furthermore, the R_HRS/R_LRS of the W/TiO₂/FTO memory device is obviously increased from about two orders of magnitude to three orders of magnitude after the rapid nitrogen annealing treatment. In addition, the change in the W/TiO₂ Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO₂ interface is suggested to be responsible for the resistive switching characteristics of the W/TiO₂/FTO memory device. This work demonstrates the potential applications of the rutile TiO₂ nanowire-based W/TiO₂/FTO memory device for high-density data storage in nonvolatile memory devices.

Optoelectronic Memristive Synapse Behavior for the Architecture of Cu2ZnSnS4@BiOBr Embedded in Poly(methyl methacrylate)

The great potential of artificial optoelectronic devices that are capable of mimicking biosynapse functions in brain-like neuromorphic computing applications has aroused extensive interest, and the architecture design is decisive yet challenging. Herein, a new architecture of p-type Cu₂ZnSnS₄@BiOBr nanosheets embedded in poly(methyl methacrylate) (PMMA) films (CZTS@BOB-PMMA) is presented acting as a switching layer, which not only shows the bipolar resistive switching features (SET/RESET voltages, ∼ -0.93/+1.35 V; retention, >10⁴ s) and electrical- and near-infrared light-induced synapse plasticity but also demonstrates electrical-driven excitatory postsynaptic current, spiking-time-dependent plasticity, paired pulse facilitation, long-term plasticity, long- and short-term memory, and "learning-forgetting-learning" behaviors. The approach is a rewarding attempt to broaden the research of optoelectric controllable memristive devices for building neuromorphic architectures mimicking human brain functionalities.

Centimetre-scale single crystal α-MoO3: oxygen assisted self-standing growth and low-energy consumption synaptic devices

High-density storage and neuromorphic devices based on 2D materials are hindered by large-scale growth. Moreover, the lack of a mature mechanism makes it difficult to obtain high-quality single crystals in large-scale 2D materials. In this work, we prepared a centimeter-scale single crystal α-MoO₃via an oxygen assisted substrate-free self-standing growth method and mechanism and constructed high-performance synaptic devices based on the centimeter-scale α-MoO₃. The oxygen assisted growth mechanism of α-MoO₃ was developed from the periodic bond chain theory. The large-scale α-MoO₃ is up to 2 cm and exhibits high homogeneity and single crystalline characteristic. Furthermore, with an optimized oxygen partial pressure (18%), the centimeter-scale α-MoO₃ makes the as-prepared memristor achieve continuous conductance modulation. Moreover, the trap-controlled electron conducting mechanism of the memristor was demonstrated through I-V curve fitting analysis at various temperatures, in which the high resistance state section demonstrates space-charge-limited conduction (SCLC) mode. Moreover, the as-prepared α-MoO₃ memristors exhibit low-energy consumption and well emulate the essential synaptic behaviors including excitatory/inhibitory postsynaptic current, paired-pulse facilitation and long-term plasticity.

Filamentary and Interface-Type Memristors Based on Tantalum Oxide for Energy-Efficient Neuromorphic Hardware

To implement artificial neural networks (ANNs) based on memristor devices, it is essential to secure the linearity and symmetry in weight update characteristics of the memristor, and reliability in the cycle-to-cycle and device-to-device variations. This study experimentally demonstrated and compared the filamentary and interface-type resistive switching (RS) behaviors of tantalum oxide (Ta₂O₅ and TaO₂)-based devices grown by atomic layer deposition (ALD) to propose a suitable RS type in terms of reliability and weight update characteristics. Although Ta₂O₅ is a strong candidate for memristor, the filament-type RS behavior of Ta₂O₅ does not fit well with ANNs demanding analog memory characteristics. Therefore, this study newly designed an interface-type TaO₂ memristor and compared it to a filament type of Ta₂O₅ memristor to secure the weight update characteristics and reliability. The TaO₂-based interface-type memristor exhibited gradual RS characteristics and area dependency in both high- and low-resistance states. In addition, compared to the filamentary memristor, the RS behaviors of the TaO₂-based interface-type device exhibited higher suitability for the neuromorphic, symmetric, and linear long-term potentiation (LTP) and long-term depression (LTD). These findings suggest better types of memristors for implementing ionic memristor-based ANNs among the two types of RS mechanisms.

Interaction of a Phospholipid and a Coagulating Protein: Potential Candidate for Bioelectronic Applications

In the present communication, we have investigated the interaction between a biomembrane component 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and a coagulating protein protamine sulfate (PS) using the Langmuir-Blodgett (LB) technique. The π-A isotherm, π-t characteristics, and analysis of isotherm curves suggested that PS strongly interacted with DOPC, affecting the fluidity of the DOPC layer. Electrical characterization indicates that PS as well as the PS-DOPC film showed resistive switching behavior suitable for Write Once Read Many (WORM) memory application. Trap-controlled space charge-limited conduction (SCLC) was the key mechanism behind such observed switching. The presence of DOPC affected the SCLC process, leading to lowering of threshold voltage (V _Th), which is advantageous in terms of lower power consumption.

Multilevel resistive random access memory achieved by MoO3/Hf/MoO3stack and its application in tunable high-pass filter

In this work, the multilevel resistive random access memories (RRAMs) have been achieved by using the structure of Pt/MoO₃/Hf/MoO₃/Pt with four stable resistance states. The devices show good retention property of each state (>10⁴s) and large memory window (>10⁴). The simulation and experimental study reveal that the resistive switching mechanism is ascribed to combination of the conductive filament in the stack of MoO₃/Hf next to the top electrode and redox reaction at the interface of Hf/MoO₃next to bottom electrode. The fitting results of current-voltage characteristics under low sweep voltage indicate that the conduction of HRSs is dominated by the Poole-Frenkel emission and that of LRS is governed by the Ohmic conduction. Based on the RRAM, the tunable high-pass filter (HPF) with configurable filtering characteristics has been realized. The gain-frequency characteristics of the programmable HPF show that the filter has high resolution and wide programming range, demonstrating the viability of the multilevel RRAMs for future spiking neural network and shrinking the programmable filters with low power consumption.

Multi-level characteristics of TiOx transparent non-volatile resistive switching device by embedding SiO2 nanoparticles

TiOx-based resistive switching devices have recently attracted attention as a promising candidate for next-generation non-volatile memory devices. A number of studies have attempted to increase the structural density of resistive switching devices. The fabrication of a multi-level switching device is a feasible method for increasing the density of the memory cell. Herein, we attempt to obtain a non-volatile multi-level switching memory device that is highly transparent by embedding SiO2 nanoparticles (NPs) into the TiOx matrix (TiOx@SiO2 NPs). The fully transparent resistive switching device is fabricated with an ITO/TiOx@SiO2 NPs/ITO structure on glass substrate, and it shows transmittance over 95% in the visible range. The TiOx@SiO2 NPs device shows outstanding switching characteristics, such as a high on/off ratio, long retention time, good endurance, and distinguishable multi-level switching. To understand multi-level switching characteristics by adjusting the set voltages, we analyze the switching mechanism in each resistive state. This method represents a promising approach for high-performance non-volatile multi-level memory applications.

Excellent Bidirectional Adjustable Multistage Resistive Switching Memory in Bi2FeCrO6 Thin Film

As an important method to further improve the storage density of resistive memory, multistage resistive switching devices have become an important research direction. However, no stable and controllable multistage resistive switching device has been prepared, and the working mechanism is still unclear. Here, a sandwich-structured device is simply prepared by spin coating, with the work layer is the Bi₂FeCrO₆ thin film. The device can realize bidirectional controllable multistage resistive switching behavior, the biggest on/off ratio is 10⁴, and it can maintain stability without attenuation at 100 times slow loop and 10⁴ times pulse cycle. The analyzes showed that the charged ions formed by defects in the device migrated under the action of an external electric field lead to the Schottky barrier height reversible changed. Which is the key to cause multistage resistive switching behavior. This work is the first report about the voltage control of bidirectional adjustable multistage resistive switching behavior in the Bi₂FeCrO₆ thin film. The principle of generation is analyzed, and important ideas and insights are provided for the preparation and treatment of related multistage resistive problems.

Floating metal layer as top electrode over vertically aligned nanorod arrays using angle deposition technique

A floating metal layer (FML) is realized over vertically aligned nanorod arrays (NRAs) using a newly developed angle deposition technique (ADT) that utilizes simultaneous metallization from two identical metal sources. The angle of the sources formed with the tip of the nanorod creates a shadow onto adjacent nanorods in the deposition direction. Computational estimation suggests the length of nanorods embedded in FML depends on the length of NRAs and separation distance between them, and normal height and lateral distance of sources from surface of the substrate. A layer of copper (Cu) is metalized using the proposed ADT on top of hydrothermally grown titanium dioxide NRAs (TiO₂-NRAs) formed over fluorine-doped tin oxide (FTO) coated glass substrate (Cu/TiO₂-NRA/FTO). Current-voltage characteristics through the resulting Cu/TiO₂-NRA/FTO vertical device structure in macroscopically large area recorded by sweeping DC-voltage in cycles of [Formula: see text] exhibits resistive switching with transition from high to low resistance state during [Formula: see text] and regaining of the original high resistance state following negative differential resistance behavior during [Formula: see text].

Filamentary Resistive Switching and Capacitance-Voltage Characteristics of the a-IGZO/TiO2 Memory

In this study, molybdenum tungsten/amorphous InGaZnO (a-IGZO)/TiO₂/n-type Si-based resistive random access memory (ReRAM) is manufactured. After deposition of the a-IGZO, annealing was performed at 200, 300, 400, and 500 °C for approximately 1 h in order to analyze the effect of temperature change on the ReRAM after post annealing in a furnace. As a result of measuring the current-voltage curve, the a-IGZO/TiO₂-based ReRAM annealed at 400 °C reached compliance current in a low-resistance state, and showed the most complete hysteresis curve. In the a-IGZO layer annealed at 400 °C, the O₁/O_total value increased most significantly, to approximately 78.2%, and the O₃/O_total value decreased the most, to approximately 2.6%. As a result, the a-IGZO/TiO₂-based ReRAM annealed at 400 °C reduced conductivity and prevented an increase in leakage current caused by oxygen vacancies with sufficient recovery of the metal-oxygen bond. Scanning electron microscopy analysis revealed that the a-IGZO surface showed hillocks at a high post annealing temperature of 500 °C, which greatly increased the surface roughness and caused the surface area performance to deteriorate. Finally, as a result of measuring the capacitance-voltage curve in the a-IGZO/TiO₂-based ReRAM in the range of −2 V to 4 V, the accumulation capacitance value of the ReRAM annealed at 400 °C increased most in a nonvolatile behavior.

Hydrothermally Grown TiO2 Nanorod Array Memristors with Volatile States

In the present study, the memristive characteristics of hydrothermally grown TiO₂ nanorod arrays, particularly, the difference in the retention time of the resistance state, are investigated in dependence of the array growth temperature. A volatile behavior is observed and related to a redistribution of oxygen vacancies over time. It is shown that the retention time increases for increasing array growth temperatures from several seconds up to 20 min. The relaxation behavior is also seen in the current-voltage characteristics, which do not show the common unipolar, bipolar, or complementary switching behavior. Instead, the temporal evolution depends on the duration of the applied voltage and on the nanowire growth temperature. Therefore, electronic measurements are combined with scanning electron and scanning transmission electron microscopy, so that the amount of oxygen defect-rich grain boundaries in the upper part of the nanowires can be linked to the differences in the current-voltage behavior and retention time.

Resistive switching and synaptic learning performance of a TiO2 thin film based device prepared by sol-gel and spin coating techniques

A resistance random access memory device based on TiO₂ thin films was fabricated using a sol-gel spin and coating techniques. The composition, surface morphology, and microstructure of the TiO₂ films were characterized using x-ray diffraction, Raman spectroscopy, scanning electronic microscopy, and transmission electron microscopy, respectively. The fabricated Al/TiO₂ film/fluorine-doped tin oxide device exhibited electroforming-free bipolar resistive switching characteristics with a stable ON/OFF ratio higher than 300. The performance of the endurance cycling was still good after 100 direct sweeping cycles. A retention time of no less than 10⁴ s was confirmed. A switching mechanism is systematically discussed based on the test results, and space-charge-limited current was found to be responsible for the switching behavior. Multilevel memory performance was realized in the as-fabricated devices. The synaptic performance was investigated by applying consecutive positive (0-2 V) and negative (0 to -1.6 V) voltage sweeps. The fabricated devices were found to exhibit 'learning-experience' behavior.

Synaptic learning behavior of a TiO2 nanowire memristor

TiO₂ nanowire memristors were fabricated by dielectrophoresis. The responding current of the memristor continuously increases and decreases with the consecutive positive and negative sweep voltage, which is similar to the nonlinear transmission characteristics of biological synapses. Spike-rate-dependent plasticity and learning behaviors of TiO₂ memristor were studied by applying programmed pulses. The pulses with higher amplitude, bigger width and smaller interval cause a larger excitatory postsynaptic current. The number of relearning pulses is decreased with the learning experience, and a deepening memory will be consolidated by the repeated learning process. A mechanism based on the oxygen vacancy migration is proposed for the learning behavior. Excess oxygen vacancies are generated during the learning process and the conducting pathways are formed by the vacancy drift under the applied voltage. After removing the voltage at the forgetting process, back diffusion and electron trapping of the oxygen vacancies dominate the relaxation time, and the metastable atoms are formed with the involvement of the oxygen atoms. However, weak chemical bonding among the metastable atoms leads to the migration of the regenerated oxygen vacancies again, contributing to the enhanced current in the relearning process.

A Highly Transparent Artificial Photonic Nociceptor

A nociceptor is an essential element in the human body, alerting us to potential damage from extremes in temperature, pressure, etc. Realizing nociceptive behavior in an electronics device remains a central issue for researchers, designing neuromorphic devices. This study proposes and demonstrates an all-oxide-based highly transparent ultraviolet-triggered artificial nociceptor, which responds in a very similar way to the human eye. The device shows a high transmittance (>65%) and very low absorbance in the visible region. The current-voltage characteristics show loop opening, which is attributed to the charge trapping/detrapping. Further, the ultraviolet-stimuli-induced versatile criteria of a nociceptor such as a threshold, relaxation, allodynia, and hyperalgesia are demonstrated under self-biased condition, providing an energy-efficient approach for the neuromorphic device operation. The reported optically controlled features open a new avenue for the development of transparent optoelectronic nociceptors, artificial eyes, and memory storage applications.

Self-limited single nanowire systems combining all-in-one memristive and neuromorphic functionalities

The ability for artificially reproducing human brain type signals’ processing is one of the main challenges in modern information technology, being one of the milestones for developing global communicating networks and artificial intelligence. Electronic devices termed memristors have been proposed as effective artificial synapses able to emulate the plasticity of biological counterparts. Here we report for the first time a single crystalline nanowire based model system capable of combining all memristive functions – non-volatile bipolar memory, multilevel switching, selector and synaptic operations imitating Ca²⁺ dynamics of biological synapses. Besides underlying common electrochemical fundamentals of biological and artificial redox-based synapses, a detailed analysis of the memristive mechanism revealed the importance of surfaces and interfaces in crystalline materials. Our work demonstrates the realization of self-assembled, self-limited devices feasible for implementation via bottom up approach, as an attractive solution for the ultimate system miniaturization needed for the hardware realization of brain-inspired systems.

Memristors have become an emerging technology capable in emulating human brain information processing, but understanding and controlling the switching mechanism remains elusive. Here, Milano et al. combine memristive and neuromorphic functionalities in a single crystalline nanowire model system.

Reversible Negative Resistive Switching in an Individual Fe@Al2O3 Hybrid Nanotube for Nonvolatile Memory

Hybrid nanostructures can show enormous potential in different areas because of their unique structural configurations. Herein, Fe@Al₂O₃ hybrid nanotubes are constructed via a homogeneous coprecipitation method followed by subsequent annealing in a reducing atmosphere. The introduction of zero band gap Fe nanocrystals in the wall of ultrawide band gap Al₂O₃ insulator nanotubes results in the formation of charge trap centers, and correspondingly a single hybrid nanotube-based two-terminal device can show reversible negative resistive switching (RS) characteristics with symmetrical negative differential resistance (NDR) at relatively high operation bias voltages. At a large bias voltage, holes and electrons can be injected into traps at two ends from electrodes, respectively, and then captured. The bias voltage dependence of asymmetrical filling of charges can lead to a reversible variation of built-in electromotive force, and therefore the symmetrical negative RS with NDR arises from two reversible back-to-back series bipolar RS. At a low readout voltage, the single Fe@Al₂O₃ hybrid nanotube can show an excellent nonvolatile memory feature with a relatively large switching ratio of ∼30. The bias-governed reversible negative RS with superior stability, reversibility, nondestructive readout, and remarkable cycle performance makes it a potential candidate in next-generation erasable nonvolatile resistive random access memories.

A facile approach for reducing the working voltage of Au/TiO2/Au nanostructured memristors by enhancing the local electric field

Memristor devices have attracted tremendous interest due to different applications ranging from nonvolatile data storage to neuromorphic computing units. Exploring the role of surface roughness of the bottom electrode (BE)/active layer interface provides useful guidelines for the optimization of the memristor switching performance. This study focuses on the effect of surface roughness of the BE electrode on the switching characteristics of Au/TiO₂/Au three-layer memristor devices. An optimized wet-etching treatment condition was found to modify the surface roughness of the Au BE where the measurement results indicate that the roughness of the Au BE is affected by both duration time and solution concentrations of the wet-etching process. Then we fabricated arrays of TiO₂-based nanostructured memristors sandwiched between two sets of cross-bar Au electrode lines (junction area 900 μm²). The results revealed a reduction in the working voltages in current-voltage characteristic of the device performance when increasing the surface roughness at the Au(BE)/TiO₂ active layer interface. The set voltage of the device (V_set) significantly decreased from 2.26-1.93 V when we increased the interface roughness from 4.2-13.1 nm. The present work provides information for better understanding the switching mechanism of titanium-dioxide-based devices, and it can be inferred that enhancing the roughness of the Au BE/TiO₂ active layer interface leads to a localized non-uniform electric field distribution that plays a vital role in reducing the energy consumption of the device.

High-Performance Single-Active-Layer Memristor Based on an Ultrananocrystalline Oxygen-Deficient TiOx Film

The theoretical and practical realization of memristive devices has been hailed as the next step for nonvolatile memories, low-power remote sensing, and adaptive intelligent prototypes for neuromorphic and biological systems. However, the active materials of currently available memristors need to undergo an often destructive high-bias electroforming process in order to activate resistive switching. This limits their device performance in switching speed, endurance/retention, and power consumption upon high-density integration, due to excessive Joule heating. By employing a nanocrystalline oxygen-deficient TiO_x switching matrix to localize the electric field at discrete locations, it is possible to resolve the Joule heating problem by reducing the need for electroforming at high bias. With a Pt/TiO_x/Pt stacking architecture, our device follows an electric field driven, vacancy-modulated interface-type switching that is sensitive to the junction size. By scaling down the junction size, the SET voltage and output current can be reduced, and a SET voltage as low as +0.59 V can be obtained for a 5 × 5 μm² junction size. Along with its potentially fast switching (over 10⁵ cycles with a 100 μs voltage pulse) and high retention (over 10⁵ s) performance, memristors based on these disordered oxygen-deficient TiO_x films promise viable building blocks for next-generation nonvolatile memories and other logic circuit systems.