Rupture Of Conductive Filaments Research Articles

Improvement of Resistive Random Access Memory Device Performance via Embedding of Low-K Dielectric Layer.

The switching mechanisms of resistive random access memories (ReRAMs) were strongly related to the formation and rupture of conduction filaments (CFs) in the transition metal oxide (TMO) layer. The novel method approached to enhance the electrical characteristics of ReRAMs by introducing of the local insertion of the low-k dielectric layer inside the TMO layer. Simulation results showed that the insertion of the low-k dielectric layer in the TMO layer reduced the switching volume and the generation of CFs. The large variation of resistive switching properties was caused by the stochastic characteristics of the CFs, which was involved in switching by generation and rupture. The electrical characteristics of the novel ReRAMs exhibited a low reset current of below 20 microA, the high uniformity of the resistive switching, and the narrow variation of the resistance for the high resistance state.

Nanoscopic structural rearrangements of the Cu-filament in conductive-bridge memories.

The electrochemical reactions triggering resistive switching in conductive-bridge resistive random access memory (CBRAM) are spatially confined in few tens of nm(3). The formation and dissolution of nanoscopic Cu-filaments rely on the displacement of ions in such confined volume, and it is driven by the electric field induced ion migration and nanoscaled redox reactions. The stochastic nature of these fundamental processes leads to a large variability of the device performance. In this work, a combination of two- and three-dimensional scanning probe microscopy (SPM) techniques are used to study the conductive filament (CF) formation, rupture and its nanoscopic structural rearrangements. The high spatial confinement of our approach enables to locally induce RS in a confined area and image it in 3D. A conical shape of the CF is consistently observed, indicating that the ion migration is the rate limiting step in the filament formation when using high quality dielectrics as switching layers. The sub-10 nm electrical contact size of the AFM tip is used to study the filament's dissolution and detect the hopping conduction of Cu during the CF rupture. We consistently observe a tunnel gap formation associated with the tip-induced filament reset. Finally, aiming to match the fundamental understanding with the integrated device operations, we apply scalpel SPM to failed memory cells and directly observe the appearance of filament multiplicity as a major source of failures and variability in CBRAM.

Nonvolatile bio-memristor fabricated with natural bio-materials from spider silk

The employ of nontoxic biomaterials as the basic building blocks of electronic devices is of growing interest for biocompatible and environmentally friendly electronics. Herein the fabrication and characterization of natural biomaterials-based bio-memristors with Ag/Fibroin/Au/Si structure is demonstrated. We observed a significant bipolar resistive switching behavior in Ag/Fibroin/Au/Si structure at room temperature. The results suggested that the memory behavior originates from the formation and rupture of conductive filaments. This work reveals that fibroin from spider silk is a useful natural biomaterial for nonvolatile memory applications, suggesting that spider silk possesses the potential for sustainable electronics and data storage.

(Invited) Visualization of Conductive Filament of ReRAM during Resistive Switching by in-situ TEM

Performance of in-situ-transmission electron microscopy (TEM) applied to analysis of the resistive random access memory (ReRAM) is reported. Recent progress of ReRAMs makes it indispensable to understand the resistive switching mechanism in order to guarantee the reliability of ReRAMs. Since the TEM provides a high resolution images of the nanostructure, in-situ TEM should be a powerful tool for the analysis if electrical characteristics of ReRAMs can be measured in the TEM. The two sample fabrication methods are applied to two different ReRAM devices. Switching characteristics, which are almost the same as those measured at the outside of TEM by the use of conventional ReRAM cells, are achieved together with clear images of formation and rupture of conductive filaments corresponding to the low and high resistance states. In addition, retention characteristics longer than 106 s, and SET/RESET pulse operation more than 100k times are confirmed during TEM observation. These results indicate that the in-situ TEM will be a powerful tool to analyze the reliability of ReRAMs.

Wearable non-volatile memory devices based on topological insulator Bi2Se3/Pt fibers

Pt fibers (15 μm) were coated with topological insulator Bi2Se3 nanoplates via a single mode microwave-assisted synthesis technique. With the Bi2Se3/Pt fibers, flexible memory devices were facilely assembled, and they were demonstrated to exhibit rewritable nonvolatile resistive switching characteristics of low switching voltage (−1.2 V and +0.7 V), high ON/OFF current ratio (106), and good retention (4500 s), showing the potential application in data storage. The resistive switching mechanism was analyzed on the bases of formation and rupture of conductive filaments.

Compliance current induced non-reversible transition from unipolar to bipolar resistive switching in a Cu/TaOx/Pt structure

Unipolar resistive switching (URS) as well as bipolar resistive switching (BRS) behaviors in a Cu/TaOx/Pt structure were investigated. Upon increasing the compliance current (Ic), the current-voltage characteristics of the Cu/TaOx/Pt structure showed a URS behavior at Ic = 0.1 mA then experienced a non-reversible transition from the URS to a BRS mode at Ic = 10 mA. Through a detailed analysis of the electrical properties in each resistance state of URS and BRS, we revealed that the permanent transition from the URS to the BRS mode was induced by the formation of stronger Cu metal conductive filaments within the TaOx thin film. More interestingly, both URS and BRS modes were governed by the formation and rupture of conductive filaments, whereas the rupture of these filamentary paths in BRS was proposed due to both Joule heating and electric field effects.

Resistance Switching Phenomenon Associated with Anisotropic Magnetoresistance of the ReRAM Device with Ferromagnetic Electrodes

A resistive random access memory (ReRAM) is expected to be a next-generation nonvolatile memory substituting NAND Flash because of good scalability against downsizing, fast switching speed and low power consumption. ReRAM operation relies on the ability of the metal oxide to reversely change its resistance from high resistance to low resistance and vice versa by SET or RESET operation. The device structure is composed of a metal oxide layer sandwiched between two metal electrodes. In general, it is considered that resistive switching is caused by the formation and rupture of conductive filament (CF) such as metallic filaments or an oxygen vacancy filaments formed in the oxide layer. It is widely accepted that the type of filaments formed in the ReRAM devices depends on the combination of materials used as the electrodes and the oxide layer. The metallic filament is considered to be formed by metal ions migrated from the oxidizable electrode when a positive bias is applied to the oxidizable electrode. In the previous study, we found that ferromagnetic filament was formed at the on-state of the Ni/TiO2/Pt device [1]. Actually we observed a weak anisotropic magnetoresistance (MR) at room temperature. This study suggested that the conductive filament was formed with migration of Ni atoms from Ni electrode. In this study, we fabricated ReRAM device having two ferromagnetic electrodes; Ni and Fe-Ni. Both electrodes may be active under the voltage stress. Figure 1 shows the MR characteristics measured for the Ni/HfOx/Fe-Ni device at the room temperature. MR ratio is defined as α = [R(H)-R(H0)/R(H0)]×100. The ferromagnetism of filament was confirmed because the resistance of filament changed against magnetic field. It is most likely that this behavior is categorized as anistropic magnetoresistance. From the results, it is suggested that the conductive filament of Ni/HfOx/Fe-Ni device is formed by ferromagnetic materials. Further discussions and analysis will be presented in the symposium.

Resistive switching characteristics of mixed oxides

The unipolar resistive switching (RS) properties of amorphous lanthanum gadolinium oxide (LaGdO3) and samarium gadolinium oxide (SmGdO3) thin films deposited by pulsed laser deposition on platinised silicon substrate with platinum top electrode have been investigated. Reliable and repeatable non-volatile switching of the resistance of these materials was obtained with sufficiently large resistance ratio and non-overlapping and low switching voltages. In the case of SmGdO3, a multilevel RS with four resistance states was observed by controlling the compliance current that opens the possibility of multi-bit storage. The switching between low- and high-resistance states was attributed to the formation and rupture of conductive filaments, while multilevel switching was attributed to the variation in diameter of conducting filaments with changing compliance current. On the other hand, forming free bipolar resistive switching behaviour was found in graphene oxide (GO) thin films on indium tin oxide (ITO) substrate with platinum as the top electrode. The switching between the low-resistance state and high-resistance state showed a reliable and repeating behaviour with an on/off ratio of 104 at room temperature. The device showed good endurance and retention characteristics. The switching mechanism was found to be governed by the migration of oxygen between the GO layer and bottom ITO electrode.

Bipolar resistive switching characteristics in tantalum nitride-based resistive random access memory devices

This paper reports the bipolar resistive switching characteristics of TaNx-based resistive random access memory (ReRAM). The conduction mechanism is explained by formation and rupture of conductive filaments caused by migration of nitrogen ions and vacancies; this mechanism is in good agreement with either Ohmic conduction or the Poole-Frenkel emission model. The devices exhibit that the reset voltage varies from −0.82 V to −0.62 V, whereas the set voltage ranges from 1.01 V to 1.30 V for 120 DC sweep cycles. In terms of reliability, the devices exhibit good retention (>105 s) and pulse-switching endurance (>106 cycles) properties. These results indicate that TaNx-based ReRAM devices have a potential for future nonvolatile memory devices.

Understanding the Dual Nature of the Filament Dissolution in Conductive Bridging Devices.

The formation and rupture of conductive filaments (CFs) inside an insulating medium is used as hardware encoding of the state of a memory cell ("1" - "0") in filamentary-based conductive bridging memories. Currently accepted models explain the filament erase (reset) as the subtraction of conductive metal atoms from the CF; however, they do not fully account for the rich set of phenomena experimentally observed during the reset. The details of the filament erase are unraveled on the nanometer scale by means of an atomic force microscopy-based tomography technique enabling the 3D observation of erased CFs. "Non-broken" and "broken" CFs are observed, whereby the increase in resistance originates, respectively, from a constriction point in the current path and from an interrupted CF. We demonstrate that their existence and morphology can be related to the specific formation history of the CF, and we identify the physical volume of the CF as being mainly responsible for the type of filament erase.

Nonvolatile bio-memristor fabricated with egg albumen film.

This study demonstrates the fabrication and characterization of chicken egg albumen-based bio-memristors. By introducing egg albumen as an insulator to fabricate memristor devices comprising a metal/insulator/metal sandwich structure, significant bipolar resistive switching behavior can be observed. The 1/f noise characteristics of the albumen devices were measured, and results suggested that their memory behavior results from the formation and rupture of conductive filaments. Oxygen diffusion and electrochemical redox reaction of metal ions under a sufficiently large electric field are the principal physical mechanisms of the formation and rupture of conductive filaments; these mechanisms were observed by analysis of the time-of-flight secondary ion mass spectrometry (TOF-SIMS) and resistance–temperature (R–T) measurement results. The switching property of the devices remarkably improved by heat-denaturation of proteins; reliable switching endurance of over 500 cycles accompanied by an on/off current ratio (Ion/off) of higher than 103 were also observed. Both resistance states could be maintained for a suitably long time (>104 s). Taking the results together, the present study reveals for the first time that chicken egg albumen is a promising material for nonvolatile memory applications.

Reversible switching of ferromagnetism in ZnCuO nanorods by electric field

The reproducible switching of ferromagnetism in ZnCuO nanorods by applying a reversible electric field has been realized. High-resolution transmission electron microscopy images showed a hexagonal wurtzite structure with no detectable trace of secondary phase or precipitation of Cu impurity in the ZnCuO nanorods. The Cu concentrations in the ZnCuO nanorods were tested by energy dispersive spectroscopy and x-ray photoelectron spectroscopy and found to be about 2.7 at. %. The switching mechanism is confirmed in terms of the formation and rupture of conductive filaments, with oxygen vacancies (VO) localized mainly on surface of the ZnCuO nanorods. Subsequently, the variation of VO concentration during the resistive switching process modulates the ferromagnetism of the ZnCuO nanorods. The saturation magnetization at low resistance state is apparently 6.4 times larger than that at high resistance state for an Au/ZnCuO/ITO structure. An indirect double-exchange model has been used to explain the ferromagnetism in ZnCuO nanorods.

Improved Memory Effect of ZnO Nanorods Embedded in an Insulating Polymethylmethacrylate Layer.

Fabrication and characterization of memory devices using ZnO nanorod layer grown by chemical-bath method is reported. The fabricated memory device was found exhibit electrical bistability and nonvolatile memory phenomenon. An additional Polymethylmethacrylate (PMMA) polymer layer coated on ITO substrate prior to nanorod deposition has been found improve the LRS/HRS ratio of the device. The current-voltage characteristics of the memory devices are discussed in terms of formation and rupture of conductive filaments. The devices have shown consistent electrical bistable behavior even for 10(5) resistance-switching cycles. This hybrid ITO/PMMA-ZnO NRs/Al device has potential applications in the field of bistable random access memories.

Enhanced unipolar resistive switching in substituted Bi(Fe0.95Mn0.05)O3 bilayer films

In this work, Bi(Fe0.95Mn0.05)O3 (BFMO) and 15% Pr-substituted BFMO (BPFMO) thin films are prepared by chemical solution deposition. Unipolar resistive switching is achieved in both Pt/BFMO/Pt and Pt/BFMO/BPFMO/Pt thin-film capacitors as a result of the formation and rupture of conductive filaments that consist of positively charged oxygen vacancies. The Pt/BFMO/Pt capacitor exhibits a resistance ratio of ∼40 between the high and the low resistance states, in which the programming voltages are 3.1 and 4.5 V, respectively, for the Reset and the Set processes. The resistive switching properties can be significantly enhanced by inserting a BPFMO thin layer between the BFMO and the bottom Pt electrode. In the Pt/BFMO/BPFMO/Pt capacitor, not only is the resistance ratio increased by more than two orders of magnitude, up to ∼5 × 103, but also the Reset and Set voltages are drastically reduced to ∼0.8 and ∼2.6 V, respectively. The possible mechanisms of the enhancement in overall resistive switching are discussed in terms of the reduced concentration of oxygen vacancies in the BPFMO layer, which results in the controllable formation/rupture of conductive filaments in the Pt/BFMO/BPFMO/Pt capacitor.

Resistive Switching Characteristics in TiO2/LaAlO3Heterostructures Sandwiched in Pt Electrodes

TiO2/LaAlO3(TiO2/LAO) heterostructures have been deposited on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. Resistive switching characteristics of Pt/TiO2/LAO/Pt have been studied and discussed in comparison with those of Pt/TiO2/Pt. It is observed that the switching uniformity and the ON/OFF resistance ratio can be greatly improved by introducing the LAO layer. The observed resistive switching characteristics are discussed as a function of LAO thickness and explained by the preferential formation and rupture of conductive filaments, composed of oxygen vacancies, in the LAO layer.

Resistive switching properties of manganese oxide nanoparticles with hexagonal shape

Uniformly sized hexagonal shaped manganese oxide (MnO) nanoparticles were chemically synthesized. The bipolar resistive switching characteristics were investigated in the Ti/MnO/Pt structure. The nanoparticles were assembled as close-packed monolayer with a thickness of 30 nm by dip-coating and annealing procedures. The bipolar resistive switching behaviors in Ti/MnO/Pt device could be caused by the formation and rupture of conductive filaments in the nanoparticles. The temperature dependence of resistance was discussed. The resistance of HRS presented a negative temperature dependence at high temperature, indicating a typical semiconducting behavior. The resistance of LRS increased with the elevated temperature exhibiting a metallic state. Ohmic conduction, space charge limited conduction (SCLC), and Schottky conduction have been investigated for the conduction and switching mechanism.

Improved Bipolar Resistive Switching Memory Characteristics in Ge<SUB>0.5</SUB>Se<SUB>0.5</SUB> Solid Electrolyte by Using Dispersed Silver Nanocrystals on Bottom Electrode

Resistive switching random-access memory (ReRAM) devices based on chalcogenide solid electrolytes have recently become a promising candidate for future low-power nanoscale nonvolatile memory application. The resistive switching mechanism of ReRAM is based on the formation and rupture of conductive filament (CF) in the chalcogenide solid electrolyte layers. However, the random diffusion of metal ions makes it hard to control the CF formation, which is one of the major obstacles to improving device performance of ReRAM devices. We demonstrate the spin-coated metal nanocrystals (NCs) enhance the bipolar resistive switching (BRS) memory characteristics. Compared to the Ag/Ge0.5Se0.5/Pt structure, excellent resistive switching memory characteristics were obtained from the Ag/Ge0.5Se0.5/Ag NCs/Pt structure. Ag NCs improve the uniformity of resistance values and reduce the reset voltage and current. A stable DC endurance (> 100 cycles) and a high data retention (> 10(4) sec) were achieved by spin coating the Ag NCs on the Pt bottom electrode for ReRAMs.

An overview of the switching parameter variation of RRAM

Resistive random access memory (RRAM) has been considered as one of the most promising candidates for next-generation nonvolatile memory, due to its advantages of simple device structure, excellent scalability, fast operation speed and low power consumption. Deeply understanding the physical mechanism and effectively controlling the statistical variation of switching parameters are the basis of fostering RRAM into commercial application. In this paper, based on the deep understanding on the mechanism of the formation and rupture of conductive filament, we summarize the methods of analyzing and modeling the statistics of switching parameters such as SET/RESET voltage, current, speed or time. Then, we analyze the distributions of switching parameters and the influencing factors. Additionally, we also sum up the analytical model of resistive switching statistics composed of the cell-based percolation model and SET/RESET switching dynamics. The results of the model can successfully explain the experimental distributions of switching parameters of the NiO- and HfO2-based RRAM devices. The model also provides theoretical guide on how to improve the uniformity and reliability such as disturb immunity. Finally, some experimental approaches to improve the uniformity of switching parameters are discussed.

Compact Ga-Doped ZnO Nanorod Thin Film for Making High-Performance Transparent Resistive Switching Memory

Fully transparent and stable resistive switching characteristics in resistive random access memory (RRAM) device consisting of ITO/Ga doped ZnO (GZO)/ZnO/ITO architecture are proposed. The GZO nanorods with well aligned and extremely dense properties are considered as a thin film for RRAM device. The oxygen vacancies can be confined and migrate along grain boundaries in the GZO nanorod film. Therefore, the weakest point for formation and rupture of conductive filament can be limited at the interface between GZO nanorod film and ZnO seeding layer. Compared with ITO/ZnO/ITO device, a significant improvement in the distribution of high resistance state (HRS) and low resistance state (LRS) during resistance switching is demonstrated in the present device. In addition, a high endurance of more than 7000 cycles with the resistance ratios of HRS/LRS about 200 times is achieved in this device. The ITO/GZO/ZnO/ITO device is a good candidate for the transparent RRAM application.

Resistive switching in polycrystalline YMnO3 thin films

We report a unipolar, nonvolatile resistive switching in polycrystalline YMnO3 thin films grown by pulsed laser deposition and sandwiched between Au top and Ti/Pt bottom electrodes. The ratio of the resistance in the OFF and ON state is larger than 103. The observed phenomena can be attributed to the formation and rupture of conductive filaments within the multiferroic YMnO3 film. The generation of conductive paths under applied electric field is discussed in terms of the presence of grain boundaries and charged domain walls inherently formed in hexagonal YMnO3. Our findings suggest that engineering of the ferroelectric domains might be a promising route for designing and fabrication of novel resistive switching devices.