32 × 32 Crossbar Array Resistive Memory Composed of a Stacked Schottky Diode and Unipolar Resistive Memory

Various array types of 1‐diode and 1‐resistor stacked crossbar array (1D1R CA) devices composed of a Schottky diode (SD) (Pt/TiO₂/Ti/Pt) and a resistive switching (RS) memory cell (Pt/TiO₂/Pt) are fabricated and their performances are investigated. The unit cell of the 1D1R CA device shows high RS resistance ratio (≈10³ at 1.5 V) between low and high resistance state (LRS and HRS), and high rectification ratio (≈10⁵) between LRS and reverse‐state SD. It also shows a short RS time of <50 ns for SET (resistance transition from HRS to LRS), and ≈600 ns for RESET (resistance transition from LRS to HRS), as well as stable RS endurance and data retention characteristics. It is experimentally confirmed that the selected unit cell in HRS (logically the “off” state) is stably readable when it is surrounded by unselected LRS (logically the “on” state) cells, in an array of up to 32 × 32 cells. The SD, as a highly non‐linear resistor, appropriately controls the conducting path formation during the switching and protects the memory element from the noise during retention.     

Bipolar resistive switching of chromium oxide for resistive random access memory

This study investigates the resistance switching characteristics of Cr₂O₃-based resistance random access memory (RRAM) with Pt/Cr₂O₃/TiN and Pt/Cr₂O₃/Pt structures. Only devices with Pt/Cr₂O₃/TiN structure exhibit bipolar switching behavior after the forming process because TiN was able to work as an effective oxygen reservoir but Pt was not. Oxygen migration between Cr₂O₃ and TiN was observed clearly before and after resistance switching from Auger electron spectroscopy (AES) analysis. Both low resistance state, ON state, and high resistance state, OFF state, of Pt/Cr₂O₃/TiN structures are stable and reproducible during a successive resistive switching. The resistance ratio of ON and OFF state is over 10², on top of that, the retention properties of both states are very stable after 10⁴ s with a voltage of −0.2 V.     

Effect of sweeping voltage and compliance current on bipolar resistive switching and white-light controlled Schottky behavior in epitaxial BaTiO3 (111) thin films

Bipolar resistive switching (RS) phenomenon without required electroforming has been observed in epitaxial (111)-oriented BaTiO₃ (BTO) thin films deposited by PLD technique on conducting Nb-doped substrate of SrTiO₃ (NSTO). Negative differential resistance (NDR) is observed at about −5 V when the maximum of positive voltage exceeds 7 V and the compliance current is more than 1.5 mA. And bipolar resistive switching has also been observed. In addition, the resistance of LRS decreases with increasing compliance current or the maximum of positive voltage while that of HRS barely changes, and the resistance of HRS increases with increasing the absolute of maximum of negative voltage while that of LRS scarcely changes. A typical rectifying behavior is observed when the maximum of positive voltage is less than 4 V (such as 2 V). In this case, the reverse biased current is strongly enhanced under illumination of white-light, and vice versa. The resistance of LRS and HRS can be controlled by the applied voltage or the compliance current. The rectifying behavior can be controlled by the white-light. The transition from rectifying behavior to bipolar resistive switching can be controlled by the applied voltage. The above results were discussed by considering the oxygen vacancies that can trap or release electrons as a trapping layer at the Pt/BTO interface.     

Conductive filament formation at grain boundary locations in polycrystalline HfO2 -based MIM stacks: Computational and physical insight

Resistive switching in high-κ (HK) dielectric based metal-insulator-metal (MIM) devices occurs locally and is accompanied by dynamic changes in the structural and electrical properties of the HK dielectric. In polycrystalline HfO₂ HK dielectric based MIM devices, grain boundaries (GBs) play a significant role in the formation of a percolation path for the resistive switching as the GB regions contain a large number of defects and favor the formation of conductive/low resistive paths. In this work, we present a multi-physics based combined Kinetic Monte Carlo-Finite element model (KMC-FEM) 3D percolation framework to simulate the resistive switching (high resistive state (HRS) to low resistive state (LRS)) process in TiN/HfO₂ (5 nm)/Pt MIM stacks. The KMC-FEM model describes the effect of GBs on the formation of conductive path during the HRS to LRS resistive switching. In addition, this model is used to find the statistical distribution of conductive filament/path formation in amorphous and polycrystalline HfO₂ dielectrics. Conductive atomic force microscopy and transmission electron microscopy observations on the characteristics of the HfO₂ dielectrics at the nanometer scale complement the simulation results. The results clearly show that the HRS to LRS resistive switching occurs preferably at the GB regions in polycrystalline HfO₂ and at random locations in amorphous HfO₂ -based MIM stacks.     

Light‐Responsive Ion‐Redistribution‐Induced Resistive Switching in Hybrid Perovskite Schottky Junctions

Hybrid Perovskites have emerged as a class of highly versatile functional materials with applications in solar cells, photodetectors, transistors, and lasers. Recently, there have also been reports on perovskite‐based resistive switching (RS) memories, but there remain open questions regarding device stability and switching mechanism. Here, an RS memory based on a high‐quality capacitor structure made of an MAPbBr₃ (CH₃NH₃PbBr₃) perovskite layer sandwiched between Au and indium tin oxide (ITO) electrodes is reported. Such perovskite devices exhibit reliable RS with an ON/OFF ratio greater than 10³, endurance over 10³ cycles, and a retention time of 10⁴ s. The analysis suggests that the RS operation hinges on the migration of charged ions, most likely MA vacancies, which reversibly modifies the perovskite bulk transport and the Schottky barrier at the MAPbBr₃/ITO interface. Such perovskite memory devices can also be fabricated on flexible polyethylene terephthalate substrates with high bendability and reliability. Furthermore, it is found that reference devices made of another hybrid perovskite MAPbI₃ consistently exhibit filament‐type switching behavior. This work elucidates the important role of processing‐dependent defects in the charge transport of hybrid perovskites and provides insights on the ion‐redistribution‐based RS in perovskite memory devices.     

C-AFM study on multi - resistive switching modes observed in metal–organic frameworks thin films

Metal-organic framework (MOF) materials have recently attracted much attention for use in resistive random-access memory due to the advantages of having high insulative properties, well-defined structures, a large specific surface area, and an adjustable pore size. In this study, the memory device based on zirconium (IV)-carboxylate MOF (UiO-66) nanoparticles exhibits the low operation voltage (V < 0.5 V), high ON/OFF ratio (~10⁴), excellent endurance (5 × 10² cycles), and longtime retention (10⁴ s). To clarify the resistive switching mechanism of the Ag/PVA-MOF/FTO device, conductive atomic force microscopy (C-AFM) was used. The results indicate that all of the electrode, Zr₆ clusters of UiO-66, PVA, and UiO-66 conjugation have simultaneous contributions to the resistance switching behavior. The resistive switching can be controlled either by the electron hopping process between Ag⁺ ions and Zr₆ nodes in threshold mode or the formation/rupture of the metal filaments in the bipolar switching mode. Interestingly, inherent characteristics of MOF materials, such as high porosity and large size cages (octahedral, tetrahedral), strongly influence the transport properties and switching mechanism of the device which is also discussed in detail. These resistive switching characteristics and mechanisms of UiO-66 could provide a thorough understanding for future research and application not just for UiO-66 but also for the general MOFs materials.     

Thickness effect on the bipolar switching mechanism for nonvolatile resistive memory devices based on CeO2 thin films

Impact of switching layer thickness on the bipolar resistive memory performance, stability and uniformity has been investigated in Ti/CeO₂/Pt devices. XRD and FTIR analyses demonstrate polycrystalline nature of CeO₂ films and the formation of a TiO interface layer. The bipolar switching characteristics like HRS and LRS dispersion are found to be dependent on the thickness of CeO₂ layer. As it is noted that forming as well as SET voltages gradually increase with increasing CeO₂ layer thickness however RESET voltages are slightly affected. Oxygen gettering ability of Ti causes the formation of TiO layer, which not only extracts oxygen ions from the ceria film but also acts as ion reservoir, hence plays a key role in stable functioning of the memory devices. Current transport behavior is based upon Ohmic and interface modified space charge limited conduction. Based on unique distribution characteristics of oxygen vacancies in CeO₂ films, a possible mechanism of resistive switching in CeO₂ RRAM devices has been discussed.     

One‐Step All‐Solution‐Based Au–GO Core–Shell Nanosphere Active Layers in Nonvolatile ReRAM Devices

Nonvolatile resistive random‐access memory devices based on graphene‐oxide‐wrapped gold nanospheres (AuNS@GO) are fabricated following a one‐step room‐temperature solution‐process approach reported herein for the first time. The effect of the thickness of the GO layer (2, 5, and 7 nm) and the size of the synthesized AuNS (15 and 55 nm) are inspected. Reliable bistable switching is observed in the devices made from a flexible substrate and incorporating 5 and 7 nm thick GO‐wrapped AuNS, sandwiched between two metal electrodes. Current–voltage measurements show bipolar switching behavior with an ON/OFF ratio of 10³ and relatively low operating voltage (?2.5 V). The aforementioned devices unveil remarkable robustness over 100 endurance cycles and a retention of 10³ s. Conversely, a 2 nm thick GO layer is shown to be insufficient to allow current passage from the bottom to the top electrodes. The resistive switching mechanism is demonstrated by space charge trapped limited current due to the AuNS in AuNS@GO matrix. The proposed device and methodology herein applied are expected to be attractive candidates for future generation flexible memory devices.     

Uniform bipolar organic memory using vapor phase polymerized poly (3,4-ethylenedioxythiophene)

Organic memory device has emerged as an excellent candidate for the next generation storage devices due to its high performance and low production cost. In this paper, we report the fabrication and electrical characterization of an organic memory device made of vapor-phase polymerized PEDOT thin films that are highly uniform and free of PSS and free of unreacted reactants. The PEDOT memory device exhibited a typical bipolar resistive switching with a high ON/OFF current ratio of at least 10³, which was maintained for more than 10³ dc sweeping cycles. The device performance was stable for more than 10⁵ s. Moreover, the device containing 64 cells has very high cell to cell uniformity as demonstrated by (1) at least 93% of the cells displaying the ON/OFF current ratio of at least 10³ and (2) the deviation of the set and reset voltages from the average values being less than 0.5 V and 0.4 V, respectively. The maximum current before switching in the reset process was found to increase linearly with increase in the compliance current applied during the set process.     

Resistive switching effects of HfO2 high-k dielectric

Resistive switching behavior of HfO₂ high-k dielectric has been studied as a promising candidate for emerging non-volatile memory technology. The low resistance ON state and high resistance OFF state can be reversibly altered under a low SET/RESET voltage of ±3 V. The memory device shows stable retention behavior with the resistance ratio between both states maintained greater than 103. The bipolar nature of the voltage-induced hysteretic switching properties suggests changes in film conductivity related to the formation and removal of electronically conducting paths due to the presence of oxygen vacancies induced by the applied electric field. The effect of annealing on the switching behavior was related to changes in compositional and structural properties of the film. A transition from bipolar to unipolar switching behavior was observed upon O₂ annealing which could be related to different natures of defect introduced in the film which changes the film switching parameters. The HfO₂ resistive switching device offers a promising potential for high density and low power memory application with the ease of processing integration.     

基于TiO2薄膜电阻式随机存储器电致阻变性质的研究

本论文采用溶胶凝胶法制备了TiO2薄膜,研究Pt/TiO2/Pt器件的电致阻变性质,并结合I-V特性曲线分析器件内部的阻变机制,器件高低阻态的导电机制分别为肖特基发射机制和欧姆导电机制.结果表明,Pt/TiO2/Pt器件在非易失性存储器领域具有潜在的应用.     

All-printed and highly stable organic resistive switching device based on graphene quantum dots and polyvinylpyrrolidone composite

We propose all printed and highly stable organic resistive switching device (ORSD) based on graphene quantum dots (G-QDs) and polyvinylpyrrolidone (PVP) composite for non-volatile memory applications. It is fabricated by sandwiching G-QDs/PVP composite between top and bottom silver (Ag) electrodes on a flexible substrate polyethylene terephthalate (PET) at ambient conditions through a cost effective and eco-friendly electro-hydrodynamic (EHD) technique. Thickness of the active layer is measured around 97 nm. The proposed ORSD is fabricated in a 3 × 3 crossbar array. It operates switching between high resistance state (HRS) and low resistance state (LRS) with OFF/ON ratio ∼14 for more than 500 endurance cycles, and retention time for more than 30 days. The switching voltage for set/reset of the devices is ±1.8 V and the bendability down to 8 mm diameter for 1000 cycles are tested. The elemental composition and surface morphology are characterized by XPS, FE-SEM, and microscope.     

Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching

CsPbX₃ (X = halide, Cl, Br, or I) all‐inorganic halide perovskites (IHPs) are regarded as promising functional materials because of their tunable optoelectronic characteristics and superior stability to organic–inorganic hybrid halide perovskites. Herein, nonvolatile resistive switching (RS) memory devices based on all‐inorganic CsPbI₃ perovskite are reported. An air‐stable CsPbI₃ perovskite film with a thickness of only 200 nm is successfully synthesized on a platinum‐coated silicon substrate using low temperature all‐solution process. The RS memory devices of Ag/polymethylmethacrylate (PMMA)/CsPbI₃/Pt/Ti/SiO₂/Si structure exhibit reproducible and reliable bipolar switching characteristics with an ultralow operating voltage (<+0.2 V), high on/off ratio (>10⁶), reversible RS by pulse voltage operation (pulse duration < 1 ms), and multilevel data storage. The mechanical flexibility of the CsPbI₃ perovskite RS memory device on a flexible substrate is also successfully confirmed. With analyzing the influence of phase transition in CsPbI₃ on RS characteristics, a mechanism involving conducting filaments formed by metal cation migration is proposed to explain the RS behavior of the memory device. This study will contribute to the understanding of the intrinsic characteristics of IHPs for low‐voltage resistive switching and demonstrate the huge potential of them for use in low‐power consumption nonvolatile memory devices on next‐generation computing systems.     

Characterization of PI:PCBM organic nonvolatile resistive memory devices under thermal stress

In this study, we fabricated nonvolatile organic memory devices using a mixture of polyimide (PI) and 6-phenyl-C61 butyric acid methyl ester (PCBM) (denoted as PI:PCBM) as an active memory material with Al/PI:PCBM/Al structure. Upon increasing the temperature from room temperature to 470 K, we demonstrated the good nonvolatile memory properties of our devices in terms of the distribution of ON and OFF state currents, the threshold voltage from OFF state to ON state transition, the retention, and the endurance. Our organic memory devices exhibited an excellent ON/OFF ratio (I_ON/I_OFF > 10³) through more than 200 ON/OFF switching cycles and maintained ON/OFF states for longer than 10⁴ s without showing any serious degradation under measurement temperatures up to 470 K. We also confirmed the structural robustness under thermal stress through transmission electron microscopy cross-sectional images of the active layer after a retention test at 470 K for 10⁴ s. This study demonstrates that the operation of PI:PCBM organic memory devices could be controlled at high temperatures and that the structure of our memory devices was maintained during thermal stress. These results may enable the use of nonvolatile organic memory devices in high temperature environments.     

A Polymer‐Electrolyte‐Based Atomic Switch

Studies on a resistive switching memory based on a silver‐ion‐conductive solid polymer electrolyte (SPE) are reported. Simple Ag/SPE/Pt structures containing polyethylene oxide–silver perchlorate complexes exhibit bipolar resistive switching under bias voltage sweeping. The switching behavior depends strongly on the silver perchlorate concentration. From the results of thermal, transport, and electrochemical measurements, it is concluded that the observed switching originates from formation and dissolution of a silver metal filament inside the SPE film caused by electrochemical reactions. This is the first report of an electrochemical “atomic switch” realized using an organic material. The devices also show ON/OFF resistance ratios greater than 10⁵, programming speeds higher than 1 μs, and retention times longer than 1 week. These results suggest that SPE‐based electrochemical devices might be suitable for flexible switch and memory applications.     

Ultra-low power non-volatile resistive crossbar memory based on pull up resistors

To realize a flexible and ultra-low power non-volatile resistive random access memory (NVRRAM), we propose a 3 × 3 resistive memory array comprised of crossbar memristors and a pull-up resistor connected to each column bar (1R-CM). This architecture forms a voltage divider network, which directly reads a parallel data of a row in the form of voltage instead of current. In the proposed structure, the optimum value of the pull-up resistor was found to be 10 MΩ. Poly(4-vinylphenol) (PVP) material is utilized in the memristor to achieve high OFF/ON resistance ratio of ∼1000 as a high resistance state (HRS) is 10 GΩ and a low resistance state (LRS) is 10 MΩ. The operating voltage for writing and reading are ±2 V and 0.5 V in current bound of 10–100 nA, respectively, and consumes ultra-low power (<8.33 nW) during operation. The proposed memory demonstrates bendability down to 10 mm with bending endurance of 1000 cycles and retention time for more than 180 days. It is fabricated on a plastic substrate by using direct-printing technique electrohydrodynamic (EHD) at ambient conditions that can be used in wearable electronics.     

Dispersion improvement of unipolar resistive switching Ni/CuxO/Cu device by bipolar operation method

Thermally grown Cu_xO thin films were adopted to fabricate a Ni/Cu_xO/Cu structure and investigate its resistive switching properties. The resistance of the device can reversibly switch between the high-resistance-state (HRS) and the low-resistance-state (LRS) by dc voltages. The device with the unipolar switching behavior can be either operated by dc voltages in the same direction (unipolar operation method) or in the opposite directions (bipolar operation method). The switching dispersions when using the bipolar operation method were smaller than those when using the unipolar operation method. This may be attributed to the compensation of defect migration during switching cycles. The switching behaviors of the bipolar operation method and of the unipolar operation method were similar. The conducting filament model with the thermochemical effect was suggested to explain the resistive switching behaviors.     

Low voltage bipolar resistive switching in self-assembled PVDF nanodot network in capacitor like structures on Au/Cr/Si with Hg as a top electrode

In this letter, resistive switching phenomena in self-assembled nanodot network of Polyvinylidene fluoride (PVDF) polymer in a capacitor geometry of Hg/PVDF/Au/Cr/Si is investigated. A stable & bipolar resistive switching with a set voltage ranging from 0.35 V to 0.9 V & reset voltage with a range of −0.08 V to −0.25 V is detected. A practical resistance ratio between HRS and LRS of 10–25 may have great potential in organic memories. Possible mechanism for the bipolar switching is discussed with the filament type conduction mechanism. Furthermore, the low voltage switching is elucidated with the high current density associated filament formation and it is explicated using the parallel resistor model.     

Voltage-Polarity-Independent and High-Speed Resistive Switching Properties of V-Doped SrZrO3 Thin Films

In this paper, nonpolar resistive switching behavior is reported for the first time in a SZO-based memory device. The electrode materials used which have different conductivities affect the resistive switching properties of the device. The Al/V:SZO-LNO/Pt device shows nonpolar switching behavior, whereas the Al/V:SZO/LNO device has bipolar switching property. The resistance ratios of these two devices are quite distinct owing to the difference between the resistance of low resistance states. The Al/V:SZO-LNO/Pt device with lower resistive switching voltages (mnplus7 V turn on and mnplus2 V turn off) and higher resistance ratio is more suitable for practical applications compared to the Al/V:SZO/LNO device. The switching speed of the Al/V:SZO-LNO/Pt device is 10 ns, which is the fastest speed that has ever been reported. The conduction mechanisms, nondestructive readout property, retention time, and endurance of this device are also reported in this paper.     

Unipolar nonvolatile memory devices with composites of poly(9-vinylcarbazole) and titanium dioxide nanoparticles

Organic-based devices with an 8 × 8 array structure using titanium dioxide nanoparticles (TiO₂ NPs) embedded in poly(9-vinylcarbazole) (PVK) film exhibited bistable resistance states and a unipolar nonvolatile memory effect. TiO₂ NPs were a key factor for realizing the bistability and the concentration of TiO₂ NPs influenced ON/OFF ratio. From electrical measurements, switching mechanism of PVK:TiO₂ NPs devices was closely associated with filamentary conduction model and it was found that the OFF state was dominated by thermally activated transport while the ON state followed tunneling transport. PVK:TiO₂ NPs memory devices in 8 × 8 array structure showed a uniform cell-to-cell switching, stable switching endurance, and a high retention time longer than 10⁴ s.