Unipolar Resistive Switching Behavior Research Articles

Resistive switching in FeNi/Al2O3/NiO/Pt structure with various Al2O3 layer thicknesses

Abstract Noble metals are generally used as electrodes in resistive switching devices. In this work, ferromagnetic metallic NiFe is used as top electrode on NiO films, and stable unipolar resistive switching (URS) behavior has been observed after a forming process. By inserting a thin Al2O3 layer between NiFe and NiO, URS persists with increasing the thickness of Al2O3 layer up to 20 nm. The URS behavior can be attributed to the formation and rupture of metallic Ni filaments in NiO layer, which penetrate through the thin Al2O3 layer, and have been confirmed by the observation of anisotropic magnetoresistance. With further increasing the thickness of Al2O3 layer up to 30 nm, forming free bipolar resistive switching (BRS) behavior is observed, which is due to the oxygen vacancy accumulation and dissipation at the interface because the thick Al2O3 layer blocks the penetration of Ni filaments. By inserting a thin Al2O3 layer with thickness below 20 nm, the distribution of the reset voltages in URS is significantly narrowed.

Highly stable switching and long retention property of spin coated ZnO thin film for resistive non-volatile memory application

Highly stable unipolar resistive switching behaviour of Pt/ZnO/Al structure with long retention (in the order of 105 min) and stable endurance (for >200 cycles) is reported in this work. Even at lower SET and RESET voltages excellent stability for >200 SET and RESET cycles was observed. The non-linear fitting of the data indicates that conduction mechanism at low resistance state and high resistance state was dominated by the space charge limited conduction/Ohmic and Poole–Frenkel emission, respectively. The migration of oxygen vacancies under applied field along a filament is responsible for the resistive switching action.

Interpretations for coexistence of unipolar and bipolar resistive switching behaviors in Pt/NiO/ITO structure

In this work, a Pt/NiO/ITO memory device is fabricated and characterized. Unipolar resistive switching (URS) and bipolar resistive switching (BRS) are both observed and reproduced. Results of X-ray photoelectron spectroscopy imply the presence of oxygen vacancy (VO) in the NiO film, and such defect is later proven to be intimated with both URS and BRS effects. Via current fitting to derive the conductive mechanisms for URS and BRS, we deduce that the impact of Schottky emission on the switching behavior could be mainly connected to the migration of oxygen anions (O2−) and recombination between O2− ions and VOs. However, the fitting results of I–V curves suggest the plausible principles dominating the URS and BRS could be quite different although the device is dominated by the same mechanisms in both URS and BRS operations.

Size-Independent Unipolar and Bipolar Resistive Switching Behaviors in ZnO Nanowires

In this work, we report on the mixed bipolar and unipolar resistive switching behavior in ZnO nanowires observed through conductive atomic force microscopy measurements on an individual nanowire. Both bipolar and unipolar resistive switching in ZnO nanowires are size-independent, suggesting that the switching is due to conductive filaments with dimensions much smaller than the cross-section of the nanowire. During bipolar resistive switching, both the low resistance and the high resistance states exhibit ohmic conduction at low voltage, consistent with the filament-based model. During unipolar switching, the low-resistance state is a mix of defect-free ohmic conduction and defect-dominated space-charge-limited conduction, which may not be observed in larger devices. Defects also influence transport in the high resistance state, which exhibits trap-controlled space-charge-limited conduction. Our results clearly demonstrate the key role of defects on the resistive switching behavior of ZnO-nanowires, an important consideration for optimizing the material for nonvolatile memory applications.

A unipolar nonvolatile resistive switching behavior in a layered transition metal oxide.

Two-dimensional layered materials have been considered as promising candidates for resistive random access memory, one of the most promising next-generation nonvolatile memories. However, due to the types of defects, most of the devices still suffer from poor environmental stability, defects inducing complexity, and uncontrollability. Here, we fabricate memory cells based on synthesized high-quality two-dimensional layered transition-metal oxide (α-MoO3) nanosheets which can be thinned to 8.68 nm (∼6 layers) and find a unipolar nonvolatile resistive switching behavior. Driven by the migration of intrinsic oxygen vacancies, the devices show a large memory window (∼105), good memory voltage stability, long-term endurance (for durations of over 3 days and 50 manual DC switching cycles) and multi-bit memory states. Furthermore, we find the devices with an excellent temperature tolerance of lower SET/RESET voltages and a larger memory window (>104 at 380 K) at higher temperatures, suggesting their potential in practical applications. Finally, all 2D memory devices are demonstrated using graphene/α-MoO3/graphene heterostructures.

Coexistence of Bipolar and Unipolar Resistive Switching Behavior in Ag/ZnMn2O4/p+-Si Device

ZnMn2O4 thin films were deposited by a sol-gel technique onto a p+-Si substrate, and a RRAM device with the Ag/ZnMn2O4/p+-Si structure was fabricated. The microstructure of ZnMn2O4 films and the resistive switching behavior of Ag/ZnMn2O4/p+-Si device were investigated. ZnMn2O4 thin films had a spinel structure after annealing at 650 °C for 1 h. The Ag/ZnMn2O4/p+-Si device showed unipolar and/or bipolar resistive switching behavior, exhibiting different ION/IOFF ratio and switching endurance properties. In bipolar resistive switching, high-resistance-state (HRS) conduction was dominated by the space-charge-limited conduction mechanism, whereas the filament conduction mechanism dictated the low resistance state (LRS). For unipolar resistive switching, HRS and LRS were controlled by the filament conduction mechanism. For bipolar resistive switching, the conduction process can be explained by the space-charge region of the p-n junction.

Effects of hydrogen annealing temperature on the resistive switching characteristics of HfOx thin films

We investigated the effects of hydrogen annealing temperature on Pt/HfOx/Pt resistive switching memory devices. Memory devices annealed in a hydrogen environment exhibited unipolar resistive switching behaviors with low set, reset, and forming voltages owing to reduction of the HfOx film by the injected hydrogen. The devices annealed at 400 °C exhibited good endurance, lasting 7 times as long as non-annealed device. This improvement could be attributed to the generation of more oxygen vacancies in HfOx, which resulted in more conducting pathways forming during the resistive switching process. The devices annealed in the hydrogen environment also showed longer retention times than those of non-annealed devices. Therefore, hydrogen annealing improves the performance of this nonvolatile switching memory by enabling low power operation at lower operation voltages with greater reliability.

Heterogeneous CFs induced unipolar and bipolar resistive switching behaviors in InGaZnO thin films

This paper investigates resistive switching behaviors of amorphous InGaZnO (a-IGZO) thin films for non-volatile resistive random access memory applications. Reversible transition process between bipolar and unipolar modes in Ag/InGaZnO/Pt memory cell takes place through changing the polarities of sweeping voltages, whereas the reset process cannot be operated by applying a positive voltage. The dominant conduction mechanisms in both modes are Ohmic conduction for the low resistance state, but different for the high resistance state, that is, Poole-Frenkel emission in the unipolar mode and trap-controlled space charge limited conduction in the bipolar mode. Temperature dependence of low resistance state and transmission electron microscopy analysis further indicate that the dominant conductive filaments (CFs) are composed of Ag atoms for the bipolar mode and oxygen vacancies for the unipolar mode, respectively. These two kinds of conductive filaments play a coordination role in resistive switching transition between bipolar and unipolar modes in Ag/InGaZnO/Pt memory devices. Based on the conductive filament model, the physical mechanisms for resistive switching are discussed.

Critical role of a double-layer configuration in solution-based unipolar resistive switching memories

Lately, resistive switching memories (ReRAM) have been attracting a lot of attention due to their possibilities of fast operation, lower power consumption and simple fabrication process and they can also be scaled to very small dimensions. However, most of these ReRAM are produced by physical methods and nowadays the industry demands more simplicity, typically associated with low cost manufacturing. As such, ReRAMs in this work are developed from a solution-based aluminum oxide (Al2O3) using a simple combustion synthesis process. The device performance is optimized by two-stage deposition of the Al2O3 film. The resistive switching properties of the bilayer devices are reproducible with a yield of 100%. The ReRAM devices show unipolar resistive switching behavior with good endurance and retention time up to 105 s at 85 °C. The devices can be programmed in a multi-level cell operation mode by application of different reset voltages. Temperature analysis of various resistance states reveals a filamentary nature based on the oxygen vacancies. The optimized film was stacked between ITO and indium zinc oxide, targeting a fully transparent device for applications on transparent system-on-panel technology.

Nonvolatile Resistive Switching and Physical Mechanism in LaCrO3 Thin Films**Supported by the Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences’ Large-Scale Scientific Facility under Grant No U1532149, and the National Basic Research Program of China under Grant No 2014CB931704.

Polycrystalline LaCrO3 (LCO) thin films are deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition and used as the switching material to construct resistive random access memory devices. The unipolar resistive switching (RS) behavior in the Au/LCO/Pt devices exhibits a high resistance ratio of ∼ 104 between the high resistance state (HRS) and low resistance state (LRS) and exhibits excellent endurance/retention characteristics. The conduction mechanism of the HRS in the high voltage range is dominated by the Schottky emission, while the Ohmic conduction dictates the LRS and the low voltage range of HRS. The RS behavior in the Au/LCO/Pt devices can be understood by the formation and rupture of conducting filaments consisting of oxygen vacancies, which is validated by the temperature dependence of resistance and x-ray photoelectron spectroscopy results. Further analysis shows that the reset current IR and reset power IR in the reset processes exhibit a scaling law with the resistance in LRS (R0), which indicates that the Joule heating effect plays an essential role in the RS behavior of the Au/LCO/Pt devices.

Control the switching mode of Pt/HfO2/TiN RRAM devices by tuning the crystalline state of TiN electrode

In order to increase the resistive switching performance of memory devices based on TiN electrode, it is desirable to fabricate TiN with good oxygen reservoir ability. In the present paper, we report that the crystallized (200) textured TiN electrode were achieved by epitaxial grown on CrRu intermediate layer, which had better oxygen reservoir ability than the amorphous TiN. It is found that the Pt/HfO2/TiN devices with crystallized TiN electrode exhibited a typical bipolar resistive switching behavior, while the I-V curves of the Pt/HfO2/TiN devices with amorphous TiN electrode showed the unipolar resistive switching behavior regardless of voltage polarity, which is similar to the Pt/HfO2/Pt devices. It is believed that this originates from the different oxygen reservoir ability of TiN with different fabricating process. Moreover, the RRAM device with the new structure of TiN (amorphous)/HfO2/TiN (crystallized) exhibited the bipolar resistive switching behavior. This further proved the amorphous TiN electrode had poorer ability of oxygen reservoir than crystallized TiN, which was similar to a noble electrode like Pt electrode. Tuning the resistive switching behaviors of RRAM devices by controlling the crystalline state of TiN electrode may offer a method for fabrication of transition metal oxides based RRAM device with good performances.

Coexistence of unipolar and bipolar resistive switching behaviors in NiFe2O4 thin film devices by doping Ag nanoparticles

The coexistence of unipolar and bipolar resistive switching (RS) behaviors of Ag-nanoparticles (Ag-NPs) doped NiFe2O4 (NFO) based memory devices was investigated. The switching voltages of required operations in the unipolar mode were smaller than those in the bipolar mode, while ON/OFF resistance levels of both modes were identical. Ag-NPs doped NFO based devices could switch between the unipolar and bipolar modes just by preferring the polarity of RESET voltage. Besides, the necessity of identical compliance current during the SET process of unipolar and bipolar modes provided an additional advantage of simplicity in device operation. Performance characteristics and cycle-to-cycle uniformity (>103 cycles) in unipolar operation were considerably better than those in bipolar mode (>102 cycles) at 25 °C. Moreover, good endurance (>600 cycles) at 200 °C was observed in unipolar mode and excellent nondestructive retention characteristics were obtained on memory cells at 125 °C and 200 °C. On the basis of temperature dependence of resistance at low resistance state, it was believed that physical origin of the RS mechanism involved the formation/rupture of the conducting paths consisting of oxygen vacancies and Ag atoms, considering Joule heating and electrochemical redox reaction effects for the unipolar and bipolar resistive switching behaviors. Our results demonstrate that 0.5% Ag-NPs doped nickel ferrites are promising resistive switching materials for resistive access memory applications.

Self-rectifying resistive switching behavior observed in Al2O3-based resistive switching memory devices with p-AlGaN semiconductor bottom electrode

In this work, self-rectifying unipolar resistive switching (RS) behavior is demonstrated in Al2O3-based resistive random access memory (RRAM) devices by employing wide bandgap compound semiconductors, such as p-GaN and p-AlGaN, as a bottom electrode (BE). Compared to an RS characteristic of the Al2O3 RRAM with p-GaN BE, the memory cell with p-AlGaN BE shows a lager current ratio while a current-limited region is also more clearly obtained in a low voltage region, which can result in one diode type RRAM with self-selecting properties. Consequently, these results infer that the proposed Al2O3-based RRAM cells with a wide bandgap BE warrant the realization of selector-free RRAM cell without any additional peripheral elements to suppress a disturbance in reading operation.

Resistive Switching Characteristics of Al 2 O 3 /ZnO Bilayer Thin Films for Flexible Memory Applications

Unipolar resistive switching behaviors of the ZnO and Al2O3/ZnO films fabricated on flexible substrates by pulse laser deposition were studied in this paper. The films were deposited at room temperature without post-annealing treatment during the process. X-ray diffraction results indicated that ZnO film has a dominant peak at (002). Scanning electron microscopy observation showed a columnar grain structure of the ZnO film to the substrate. The bilayer device of Al2O3/ZnO films had stable resistive switching behaviors with a good endurance performance of more than 200 cycles, high resistive switching ratio of over 103 at a read voltage of 0.1 V, which is better than that of the single oxide layer device of ZnO film. A possible resistive switching filamentary mode was demonstrated in this paper. The conduction mechanisms of high and low resistance states can be explained by space charge limited conduction and Ohmic’s behaviors. The endurance of the bilayer (BL) device was not degraded upon bending cycles, which indicates the potential of the flexible resistive switching random access memory applications.

Improved unipolar resistive switching characteristics of Au-doped nickel ferrite magnetic thin films for nonvolatile memory applications

Au-doped nickel ferrite (NiFe2O4-Au, NFO-Au) magnetic thin films were prepared on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method to explore the effect of Au doping on unipolar resistive switching (RS) behavior. Experimental results reveal that 1% Au doped NFO film demonstrated considerably optimum switching parameters such as lower electroforming voltage, narrow distribution of SET/RESET voltages, good cycle-to-cycle endurance (103 cycles) and long data retention (105 s at 125 °C) as compared to NFO and Au-(0.5% and 2%) doped NFO films. The improved performance is attributed to suppressing of randomness of oxygen vacancies based filament by Au doping. Current transport conduction mechanisms are found to be Ohmic in low resistance state and Schottky emission in high resistance state. Physical model concerns for the rupture and formation of Au atoms assisted conductive filament inside the Au-doped NFO films based on the thermal effect of oxygen vacancies. Magnetic properties correlated with RS were measured and analyzed. The variation of saturation magnetization with different resistance state may be attributed to Au-doping induced different oxygen vacancies concentrations and make the conversion of valence states of Fe3+ and Fe2+ ions in octahedral sites easier. Our results provide a pathway towards the applications in nonvolatile RS memory and magnetoelectronic coupling devices.

Bipolar switching properties and electrical conduction mechanism of manganese oxide RRAM devices

Abstract Electrical conduction mechanism and bipolar resistive switching properties of the manganese oxide thin films resistive random access memory (RRAM) devices for high resistive status and low resistive status (HRS/LRS) were investigated and discussed. The electrical and physical properties of post-treatment manganese oxide thin films for different oxygen concentration parameters were measured by x-ray diffraction (XRD) and operation current versus applied voltage ( I-V ) measurement. For I-V curves in different temperature, the ohmic conduction, space charge limited conduction (SCLC), schottky emission conduction, and hopping conduction mechanism of manganese oxide RRAM devices were discussed and verified in initial metallic filament path forming process. Finally, the excess oxygen ions, vacancy, and defect factor resulting in bipolar and unipolar resistive switching behavior of thin films RRAM devices will be explained and investigated by metallic filament path forming model.

Self-Compliant Bipolar Resistive Switching in SiN-Based Resistive Switching Memory

Here, we present evidence of self-compliant and self-rectifying bipolar resistive switching behavior in Ni/SiNx/n+ Si and Ni/SiNx/n++ Si resistive-switching random access memory devices. The Ni/SiNx/n++ Si device’s Si bottom electrode had a higher dopant concentration (As ion > 1019 cm−3) than the Ni/SiNx/n+ Si device; both unipolar and bipolar resistive switching behaviors were observed for the higher dopant concentration device owing to a large current overshoot. Conversely, for the device with the lower dopant concentration (As ion < 1018 cm−3), self-rectification and self-compliance were achieved owing to the series resistance of the Si bottom electrode.

Unipolar nonvolatile memory devices based on the composites of poly(9-vinylcarbazole) and zinc oxide nanoparticles

The application of poly(9-vinylcarbazole) (PVK) to make a composite with zinc oxide (ZnO) as the active layer has been reported. Unipolar resistive switching behaviors were observed from ITO/PVK + ZnO/Al memory devices. The reset voltages were higher than the set voltages. These devices present a low resistance state (LRS)/high resistance state (HRS) current ratio of 104 when read at −0.5 V, retaining the information for a time of 105 s. The bistable resistive switching behaviors were entirely steady within 104 cycles. The fitted results of I–V curves shown that the dominant conduction mechanisms in LRS and HRS were Ohmic conductive behavior and space-charge-limited current mechanism, respectively.

Enhanced Unipolar Resistive Switching Characteristics of Hf0.5Zr0.5O2 Thin Films with High ON/OFF Ratio.

A metal–insulator–metal structure resistive switching device based on H0.5Z0.5O2 (HZO) thin film deposited by pulse laser deposition (PLD) has been investigated for resistive random access memory (RRAM) applications. The devices demonstrated bistable and reproducible unipolar resistive switching (RS) behaviors with an extremely high OFF/ON ratio over 5400. The retention property had no degradation at 6 × 104 s. The current–voltage characteristics of the HZO samples showed a Schottky emission conduction in the high voltage region (Vreset < V < Vset), while at the low voltage region (V < Vreset), the ohmic contact and space charge limited conduction (SCLC) are suggested to be responsible for the low and high resistance states, respectively. Combined with the conductance mechanism, the RS behaviors are attributed to joule heating and redox reactions in the HZO thin film induced by the external electron injection.

An All-Organic Composite System for Resistive Change Memory via the Self-Assembly of Plastic-Crystalline Molecules.

An all-organic composite system was introduced as an active component for organic resistive memory applications. The active layer was prepared by mixing a highly polar plastic-crystalline organic molecule (succinonitrile, SN) into an insulating polymer (poly(methyl methacrylate), PMMA). As increasing concentrations of SN from 0 to 3.0 wt % were added to solutions of different concentrations of PMMA, we observed distinguishable microscopic surface structures on blended films of SN and PMMA at certain concentrations after the spin-casting process. The structures were organic dormant volcanos composed of micron-scale PMMA craters and disk type SN lava. Atomic force microscopy (AFM), cross-sectional transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometer (EDX) analysis showed that these structures were located in the middle of the film. Self-assembly of the plastic-crystalline molecules resulted in the phase separation of the SN:PMMA mixture during solvent evaporation. The organic craters remained at the surface after the spin-casting process, indicative of the formation of an all-organic composite film. Because one organic crater contains one SN disk, our system has a coplanar monolayer disk composite system, indicative of the simplest composite type of organic memory system. Current-voltage (I-V) characteristics of the composite films with organic craters revealed that our all-organic composite system showed unipolar type resistive switching behavior. From logarithmic I-V characteristics, we found that the current flow was governed by space charge limited current (SCLC). From these results, we believe that a plastic-crystalline molecule-polymer composite system is one of the most reliable ways to develop organic composite systems as potential candidates for the active components of organic resistive memory applications.