Bipolar Resistive Switching Memory Research Articles

Highly reliable bipolar resistive switching of tantalum oxide-based memory using Al2O3 diffusion barrier layers

We present a novel bipolar resistive switching memory based on TaOx, featuring a Ru/Al2O3/Ta2O5/TaOx/Al2O3/W structure. Thin Al2O3 layers play a crucial role as diffusion barriers, preventing undesirable interfacial reactions at the top and bottom interfaces. They support the stable formation of the Schottky barrier near the Ru top electrode through redox reactions during operation, resulting in highly reliable bipolar resistive switching. The device exhibits excellent memory performance, including a fast operation speed (∼10 ns), good switching endurance (∼106 cycles), and robust data retention (>104 s at 200 °C).

Semiempirical Two-Dimensional Model of the Bipolar Resistive Switching Process in Si-NCs/SiO2 Multilayers.

In this work, the SET and RESET processes of bipolar resistive switching memories with silicon nanocrystals (Si-NCs) embedded in an oxide matrix is simulated by a stochastic model. This model is based on the estimation of two-dimensional oxygen vacancy configurations and their relationship with the resistive state. The simulation data are compared with the experimental current-voltage data of Si-NCs/SiO2 multilayer-based memristor devices. Devices with 1 and 3 Si-NCs/SiO2 bilayers were analyzed. The Si-NCs are assumed as agglomerates of fixed oxygen vacancies, which promote the formation of conductive filaments (CFs) through the multilayer according to the simulations. In fact, an intermediate resistive state was observed in the forming process (experimental and simulated) of the 3-BL device, which is explained by the preferential generation of oxygen vacancies in the sites that form the complete CFs, through Si-NCs.

Nonvolatile resistive switching memory behavior in WOx/BiFeOy heterojunction based memristor

Memristors with a two-terminal structure are considered to be one of the most promising electronic devices capable of overcoming the Von Neumann bottleneck, which is highly anticipated in the post-Moore era and next-generation artificial intelligence applications. In this work, a memristive device was fabricated using a WOx/BiFeOy heterojunction as functional layer on F-doped SnO2 (FTO) substrate by magnetron sputtering. The Ag/WOx/BiFeOy/FTO device exhibits enhanced bipolar nonvolatile resistive switching (RS) memory behavior compared with single-layer BiFeOy-based memristors, which can meet the requirements of high-density information storage. By performing a comprehensive conductivity analysis on the current-voltage (I-V) curve, it was proposed a reasonable physical model to explain the RS memory behavior of the device based on space-charge-limited current (SCLC) mechanism and the Schottky emission. Therefore, this work indicates that the bilayer WOx/BiFeOy heterojunction as functional layer can effectively improve the performance of memristive devices, which will further expand the application of ferroelectric/metal oxide heterojunction in the field of memristors.

Improved Performance of the Al2O3-Protected HfO2-TiO2 Base Layer with a Self-Assembled CH3NH3PbI3 Heterostructure for Extremely Low Operating Voltage and Stable Filament Formation in Nonvolatile Resistive Switching Memory.

Herein, we report intriguing observations of an extremely stable nonvolatile bipolar resistive switching (NVBRS) memory device fabricated using HfO2-TiO2 topologically protected by Al2O3 as a stacked base layer for a CH3NH3PbI3 (MAPI) electrolyte layer sandwiched between Ag and fluorine-doped tin oxide (FTO) electrodes. MAPI has been successfully synthesized by a rapid microwave-solvothermal (MW-ST) method within 10 min at 120 °C without requiring any inert gas atmosphere using low-cost precursors and solvents. Subsequently, MAPI powders are dissolved in aprotic solvents (DMF/DMSO = 8:2), and a spin-coated thin film is allowed to recrystallize upon annealing at 120 °C via a solution-based nanoscale self-assembly process. The fabricated memory device with the Ag/MAPI/Al2O3/TiO2-HfO2/FTO configuration shows an enhanced resistance ratio of 105 for >104 s at an extremely lower operating voltage (SET +0.2 V, RESET -0.2 V) when compared to that of the pristine MAPI device (±1 V, 102, 104 s). We show that the memory device also exhibits a remarkable endurance of ≥3500 cycles due to the Al2O3 robust coating on the HfO2-TiO2 layer, facilitating prompt heterojunction formation. Thus, the adopted innovative strategies to prepare structurally and optically stable (∼1.5 years) MAPI under high-humid conditions could offer enhanced performance of NVBRS memory devices for medical, security, internet of things (IoT), and artificial intelligence (AI) applications.

Improved resistive switching of RGO and SnO2 based resistive memory device for non-volatile memory application

Resistive switching memory, which combines simple architectures with the ability of high density, high switching speed, and low power consumption, has sparked a lot of attention in the field of non-volatile memory devices. In the present work, the resistive switching performance of reduced graphene oxide (RGO) and tin oxide (SnO2) based nanocomposite have been carried out. RGO, SnO2, and RGO-SnO2 nanocomposite were synthesized via hydrothermal technique and the formation of nanocomposite is confirmed by X-ray diffraction and Raman spectroscopy techniques. We herein present a bipolar resistive switching memory based on SnO2 and RGO-SnO2 fabricated by a simple spin-coating process. The two different memory devices have been fabricated by depositing pure SnO2, and RGO-SnO2 films over the bottom ITO electrode and top Al electrodes deposited through a shadow mask. It was observed that devices with RGO-SnO2 showed improved resistive switching in comparison to pure SnO2 film. This improvement of the composite-based film is recognized in terms of switching parameters, such as a reduction in the device operating voltage from 4.11 V for SnO2 to 1 V for RGO-SnO2 composite-based devices. In addition, the resistance ratio in the composite-based films showed significant enhancement. Further, the retention and endurance behavior of the fabricated composite film-based device was tested up to 1 * 103 s and up to 100 cycles respectively which didn’t exhibit any degradation. The operating mechanism of the memory device was explained by fitting the current-voltage characteristics in the low-resistance and high-resistance states. It may be highlighted that the present metal oxide and RGO-based devices have great potential for future non-volatile memory devices.

Negative Photoconductance Effect: An Extension Function of the TiOx‐Based Memristor

AbstractThe negative photoconductance (NPC) effect, defined as an increase in resistance upon exposure to illumination, holds great potential for application in photoelectric devices. A prepared memristor with the structure of Ag|graphene quantum dots (GQDs)|TiOx|F‐doped SnO2 exhibits typical bipolar resistive switching (RS) memory behavior. The NPC effect is impressively observed in the high resistance state branch of the RS memory, enabling the memristor function to be extended to both memory logic display and multistate data storage. The observed NPC effect is attributed to the excitation, migration, and compensation of oxygen vacancy at the GQDs/TiOx interface, at which the electron transportation is efficiently restricted because of the variation in the charge distribution and electrostatic potential under illumination. Experiments, theoretical calculations, and physical models are used to provide engineer the interface with the aim of building the NPC effect in the memristive device. These results unveil a new horizon on extending the functionality of the memristor.

Annealed AlOx film with enhanced performance for bipolar resistive switching memory

Although some efforts have been dedicated to explore aluminum oxide (AlOx) based memory devices in the past few years, the disappointed stability and complicated fabrication process strictly limit its wide applications. In this study, we introduce the annealing treatment as an effective method to substantially improve the non-volatile resistive switching performance of AlOx film based resistive memory devices. Fabricated by radio frequency magnetron sputtering method at room temperature, the AlOx resistive memory devices are subsequently annealed in nitrogen atmosphere with the temperature range from 100 °C to 500 °C. The devices exhibit enhanced bipolar resistive switching properties with forming-free nature, high HRS/LRS ratio (>103), long retention time (>104s) and statistical switching performance (>200 cycles). The conductive filaments constructed by oxygen vacancies has been demonstrated as a dominant factor for the resistive switching behavior in the AlOx film based memory devices, and introducing oxygen-free atmosphere annealing treatment will increase the proportion of oxygen vacancies, thus resulting in a considerable resistive switching improvement. Our work proves the great potential of annealed AlOx film as resistive switching layer in resistive random access memory devices, and also paves the way for optimization of relevant devices based on binary metal oxides.

Type-I Core-Shell ZnSe/ZnS Quantum Dot-Based Resistive Switching for Implementing Algorithm.

Core-shell semiconductor quantum dots (QDs) are one of the biggest nanotechnology successes so far. In particular, type-I QDs with straddling band offset possess the ability to enhance the charge carriers capturing which is useful for memory application. Here, the type-I core-shell QD-based bipolar resistive switching (RS) memory with anomalous multiple SET and RESET processes was demonstrated. The synergy and competition between space charge limited current conduction (arising from charge trapping in potential well of type-I QDs) and electrochemical metallization (ECM, originating from redox reaction of Ag electrode) process were employed for modulating the RS behavior. Through utilizing stochastic RS mechanisms in QD-based devices, four situations of RS behaviors can be classified into three states in Markov chain for implementing the application of a true random number generator. Furthermore, a 6 × 6 cross-bar array was demonstrated to realize the generation of random letters with case distinction.

Bipolar resistive switching, synaptic plasticity and non-volatile memory effects in the solution-processed zinc oxide thin film

The memristive devices are getting a lot of interest in recent years due to its applications in the field of resistive memory and brain-inspired computing. In the present work, we have developed a ZnO memristive device using the reflux method. We have demonstrated non-volatile memory properties and mimicked the basic synaptic properties of using Al/ZnO/FTO thin film device. The structural, morphological and compositional studies of ZnO thin film were carried out by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The co-existence of analog and digital resistive switching in a ZnO memristive device was achieved by properly tuning the external electrical stimulus. The developed ZnO memristive device mimics the basic synaptic properties such as potentiation-depression, symmetric Hebbian and antisymmetric Hebbian learning rules at a lower voltage bias (±1 V to ± 3 V). Whereas, non-volatile memory properties (endurance and retention) were achieved at a higher voltage bias (±4 V to ± 6 V). The Al/ZnO/FTO thin film memory device shows good resistive switching memory properties such as 104 endurance cycles and 104 s retention period with good uniformity during the cycle to cycle operation. The detailed analysis of I–V results suggested that the Schottky conduction model is responsible for the analog mode of operation, whereas, space charge limited current governs the device dynamics during the digital mode of operation. A possible resistive switching model is also presented.

Dynamics of set and reset processes on resistive switching memories

In this work, we have characterized hafnium oxide based bipolar resistive switching memories (RRAM) by measuring the small-signal conductance. The samples under study exhibit a continuum of intermediate states which can be accurately controlled by means of adequate sequence of the applied stimulus. The experimental results are analyzed to obtain information on the dynamics of the set and reset processes. This study reveals that ON-to-OFF (reset) and OFF-to-ON (set) processes are not symmetrical. Set transition is gradual and depends on the voltage stimulus according to an Erlang function which consists of a sum of k independent and identically distributed mechanisms, each having an exponential distribution. In contrast, reset process is more abrupt and can be described by a sigmoidal law. Time dependencies of set and reset process at fixed voltage values are explored as well. Set process is gradual at any positive voltage, whereas reset process is characterized by a time constant which depends on the applied voltage. Experimental results are explained in terms of the formation of interfacial barrier between the top electrode and the conductive filament.

Liquid Phase Exfoliation of MoS2 Nano-sheets and Observation of Resistive Switching Memory in MoS2 Nano-sheets-PVDF-HFP Composite Films

Abstract To realise the true and versatile application potentials of 2-D materials like MoS2, the preparation of mono-layer or few layer nano-sheets holds the key. In the present study, we report the successful exfoliation of bulk MoS2 into few layer nano-sheets by adopting a two-step process of grinding and then two hours of ultrasonication by using a probe sonicator. The yield of exfoliation was 32 mg/10 ml and the exfoliated MoS2 was very stable without any re-stacking for more than one month. Further, the exfoliated MoS2 nano-sheets are added to Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to prepare the nanocomposite thin films. Two-terminal devices are prepared with ITO and aluminium as bottom and top electrodes, respectively on a plastic substrate. Its electrical properties are investigated to observe the electrical bistability. Nanocomposite based devices showed bipolar resistive switching memory. For composite film with 1wt.% of MoS2, resistance switching is observed with SET and RESET voltages at 2.74 V and -0.9 V respectively. Resistance ratio of the order of 104 is achieved between the ON and OFF states.

Annealing effect on the bipolar resistive switching memory of NiZn ferrite films

A detailed understanding of the resistive switching behaviors and relevant physical mechanism is key to controlling nonvolatile memory devices. Pt/Ni0.5Zn0.5Fe2O4/Pt was synthesized by radio frequency magnetron sputtering method at room temperature. The typical bipolar resistive switching effects were detected in the annealed Ni0.5Zn0.5Fe2O4 thin films. Annealing effect on the bipolar resistive switching memory have been investigated. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in Pt/NZFO/Pt memory devices. The present results further enhance the applicability of spinel ferrite oxides in nonvolatile memory devices.

White-light-enhanced resistive switching in C/ZnO/C heterostructures

The C/ZnO/C heterostructure is fabricated on single silicon substrate using magnetron sputtering system. The device shows an obvious bipolar resistive switching (RS) memory behavior, which can be modulated by the white light at room temperature. The RS ratio and Set/Reset voltage can be controlled by the white light. A high RS ratio of 105 under illumination can be obtained, while the RS ratio in the dark is about 3. Not only the favorable RS ratio can be realized, but also the RS behaviors show excellent stability and endurance characteristics in C/ZnO/C memory cells. Based on the double-logarithm I-V fitting, the electrons trapping and de-trapping should result in the resistive switching behavior of C/ZnO/C memory cells.

Electrochemical and Electronic Properties of Transparent Coating from Highly Solution Processable Graphene Using Block Copolymer Supramolecular Assembly: Application toward Metal Ion Sensing and Resistive Switching Memory.

Here, we have discussed the preparation of a highly solution processable graphene from a novel supramolecular assembly consisting of block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and pyrenebutyric acid (PBA)-modified reduced graphene oxide (RGO). The PBA molecules anchored on the graphene surface form supramolecules with PS-b-P4VP through H-bonding between the carboxylic acid group of 1-pyrenebutyric acid and the pyridine ring of P4VP. The formation of a supramolecular assembly results in a highly stable solution of reduced graphene oxide in common organic solvents, such as 1,4-dioxane and chloroform. Highly transparent and mechanically stable thin films can be deposited from these supramolecular assemblies on a relatively smooth surface of different substrates such as silicon wafer, glass, indium tin oxide, and flexible polymer substrates like poly(ethylene terephthalate). The graphene surface modifier (PBA) can be selectively removed from the thin film of the hybrid material by simple dissolution, resulting in a porous structure. Hybrid thin films of around 50 nm thickness exhibit interesting electrochemical properties with an areal capacitance value of 17.73 μF/cm2 at a current density of 2.66 μA/cm2 and good electrochemical stability. The pendent P4VP chains present in the composite thin film were further exploited for electrochemical detection of metal ions. The electrical measurement of the thin film sandwich structure of the composite shows a bipolar resistive switching memory with hysteresis-like current–voltage characteristics and electrical bistability. The OFF state shows ohmic conduction at a lower voltage and trap-free space-charge-limited current (SCLC) conduction at high voltage, whereas the ON state conduction is controlled by ohmic at low bias voltage, trap-free SCLC at moderate voltage, and tarp-assisted SCLC at high voltage.

From natural biomaterials to environment-friendly and sustainable nonvolatile memory device

The memory device based on biomaterials is of great value in future applications because the biomaterials are environment-friendly, renewable and pollution-free. In this work, natural biomaterials, which were firstly extracted and processed from orange peel, were used as an active layer for fabricating an environment-friendly and sustainable nonvolatile memory device with Ag/Orange peel/FTO sandwich structure. An obvious memory behavior with reproducible bipolar resistive switching memory performance and large resistive storage window has been observed. This work reveals that the orange peel is a promising material for fabricating an environment-friendly and sustainable memory device in nonvolatile memory applications.

Bipolar resistive switching and nonvolatile memory effect in poly (3-hexylthiophene) –carbon nanotube composite films

The resistive switching behavior and nonvolatile memory effects have been investigated in poly (3-hexylthiophene) (P3HT) and carbon nanotubes (CNT) composite thin films. The memory devices with FTO/P3HT-CNT/Al structure were fabricated by varying the CNT content in the composite. The memory device with 4% CNT in P3HT exhibited a typical bipolar resistive switching with set voltage of ∼1.8 V and a high ON/OFF ratio of >102. The nonvolatile behavior of the device has been studied by the retention test and showed good retention properties for >103 s. The resistance-temperature dependence of different resistance states has also been investigated. Ohmic conduction was found to be a dominant conduction mechanism in low bias region, whereas for the high voltage regime space charge limited conduction was found to be the ruling current conduction mechanism. Bipolar resistive switching characteristic of the nanocomposite film is explained on the basis of rupture and formation of carbon-rich filaments.

Evidence of Bipolar Resistive Switching Memory in Perovskite Solar Cell

In hybrid inorganic-organic perovskite solar cells a very stable bipolar resistive switching behavior in the dark current-voltage characteristics at low-voltages has been observed. The possible use of the solar cell as an electrical memory with a moderate on-off contrast but very good stability over a prolonged time has been suggested. The reversible behavior and the long dynamics during the write/erase processes indicate that the physical mechanism behind the switching is related to polarization effects. A detailed analysis of the charge carrier trapping/detrapping, transport, and recombination mechanisms has been performed by taking the ion migration and the consequent charge carrier accumulation within the device into account. The charge transport during the write operation can be described by space-charge-limited conduction process. The formation and subsequent interruption of conducting pathways due to ion migration have been identified as the main cause of the resistive switching within the perovskite material. The strong interaction between the ion movement and the electron transport enables the operation of the perovskite solar cell also as a non-volatile memory.

Investigation of microstructural and electrical properties of composition dependent co-sputtered Hf1−xTaxO2 thin films

Tantalum doped HfO2 gate dielectric thin films were deposited on silicon substrates using RF reactive co-sputtering by varying RF power of Ta target from 15 W to 90 W. The morphological, compositional and electrical properties of Hf1−xTaxO2 films were systematically investigated. The Ta content was found to be increased up to 21% for a Ta target power of 90 W. The evolution of monoclinic phase of Hf1−xTaxO2 was seen from XRD study upto RF power of 60 W and afterwards, the amorphous like behaviour is appeared. The featureless smooth surface with the decrease in granular morphology has been observed from FESEM micrographs of the doped films at higher RF powers of Ta. The flatband voltage is found to be shifted towards negative voltage in the capacitance–voltage plot, which was attributed to the enhancement in positive oxide charge density with rise in RF power. The interface charge density has a minimum value of 7.85 × 1011 eV−1 cm−2 for the film deposited at Ta RF power of 75 W. The Hf1−xTaxO2 films deposited at Ta target RF power of 90 W has shown lower leakage current. The high on/off ratio of the current during the set process in Hf1−xTaxO2 based memristors is found suitable for bipolar resistive switching memory device applications.

Ag filament induced nonvolatile resistive switching memory behaviour in hexagonal MoSe2 nanosheets

In this work, hexagonal MoSe2 nanosheets were prepared by hydrothermal process. Next, the resistive switching memory behaviour of single MoSe2 nanosheets was further investigated. We observed that MoSe2 nanosheets based memory device show reproducible and stable bipolar resistive switching memory characteristics. Through the analysis for conductive mechanism, the formation and rupture of nanoscale Ag filament inside the MoSe2 nanosheets is suggested to explain the memory behaviour.

Mechanism for bipolar resistive switching memory behaviors of a self-assembled three-dimensional MoS2 microsphere composed active layer

A self-assembled three-dimensional (3-D) MoS2 microsphere-based memristor with a favorable ON/OFF resistance ratio of ∼104, endurance, and retention time is demonstrated at room temperature. The formation and rupture of a localized Ag metallic filament, establishment and destruction of a boundary-based hopping path, and charge trapping and detrapping from the space charge region co-contribute to the bipolar resistive switching memory behaviours observed in the device of Ag/MoS2/ITO. This work may give insight into the mechanism of the resistive switching memory behaviours of a device with a 3-D micro-scale.