Fabrication of solution-processed SnO2–Based flexible ReRAM using laser-induced graphene transferred onto PDMS

In this paper, we are reporting the fabrication of a solution-processed SnO₂-based flexible ReRAM using laser-induced graphene (LIG) transferred onto polydimethylsiloxane (PDMS). The fabricated ReRAM showed forming-free and self-compliance bipolar resistive switching characteristics when the applied voltage was swept from 0 V to 4.5 V for SET and from 0 V to - 4.5 V for RESET. The device operates as a filamentary type ReRAM and its conduction mechanism analysis indicates that the space charge limited conduction (SCLC) is dominant mechanism in the analog resistive switching of the fabricated device. For the reliability analysis, 100 cycles of endurance test and 1.8 × 10³ s of retention test were performed. The flexibility of the fabricated ReRAM device was demonstrated by showing that the resistive switching characteristics were still obtained after bending 200 times repeatedly down to 1 mm radius. Our study suggests the new fabrication process of a solution-processed flexible ReRAM and proves its potential applications to flexible electronics.     

Effect of fatigue fracture on the resistive switching of TiO2-CuO film/ITO flexible memory device

We fabricated the GaIn/TiO₂-CuO/ITO resistive memory and studied the effect of fatigue fracture on the switching performance. The device shows the stable bipolar resistive switching over 10⁸ s under ambient condition. The ON/OFF ratio decreases seriously with increase of bending cycles. The main fatigue fracture caused by dynamic strain includes micro defect between nanoparticles, vertical crack along the film thickness and interfacial delamination between layers. Finite element analysis indicates that channel crack plays a key role to cause the interfacial delamination between function layer and ITO electrode. The channel crack and interfacial delamination can hinder the formation of tree−like conduction filaments. Moreover, oxygen via the cracks can be easily transformed to ions and reduce the density of oxygen vacancies under the catalytic assistance of CuO. Our studies may provide some useful information for inorganic materials applied in flexible nonvolatile memory.     

Resistive switching characteristics of sol–gel based ZnO nanorods fabricated on flexible substrates

The authors report the resistive switching characteristics of sol–gel based ZnO nanorods (NRs) fabricated on flexible substrates. A resistance ratio of 10, endurance of over 100 cycles, and narrower dispersion in the ON/OFF voltages and resistances compared to ZnO thin‐film devices are demonstrated. Furthermore, the resistive switching characteristics on flexible substrates are maintained under severe substrate bending because of the ductile properties of the nanorods. Devices composed of the Au/sol–gel based NRs/Au structure have the potential for low‐temperature flexible nonvolatile memory applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of AlN layer on the resistive switching properties of TiO2 based ReRAM memory devices

The present study reports the resistive switching behaviour in Titanium Dioxide (TiO₂) material, with possible implementations in non volatile memory device. The Cu/TiO₂/Pt memory device exhibit uniform and stable bipolar resistive switching behaviour. The current-voltage (I-V) analysis shows two discrete resistance states, the High Resistance State (HRS) and the Low Resistance State (LRS). The effect of an additional AlN layer in the resistive memory cell is also investigated. The Cu/TiO₂/AlN/Pt device shows a multilevel (tri-state) resistive switching. Multilevel switching is facilitated by ionic and metallic filament formation, and the nature of the formed filaments is confirmed by performing a resistance vs. temperature measurement. The bilayer device shows improved reliability over the single layer device. The formation of high thermal conductive interfacial oxy-nitride (AlON) layer is the main reasons for the enhancement of resistive switching properties in Cu/TiO₂/AlN/Pt cell. The performance of device was measured in terms of endurance and retention, which exhibits good endurance over 10⁵ cycles and long retention time of 10⁵ s at 125 °C. The above result suggests the feasibility of Cu/TiO₂/AlN/Pt devices for multilevel non volatile ReRAM application.     

Colossal resistive switching behavior and its physical mechanism of Pt/<Emphasis Type="Italic">p</Emphasis>-NiO/<Emphasis Type="Italic">n</Emphasis>-Mg<Subscript>0.6</Subscript>Zn<Subscript>0.4</Subscript>O/Pt thin films

A heterojunction structure of p-NiO/n-Mg_0.6Zn_0.4O with an aim to tuning or improving the resistive switching properties was fabricated on Pt/TiO₂/SiO₂/Si substrates by the sol-gel spin-coating technique. The Pt/NiO/Mg_0.6Zn_0.4O/Pt heterojunction thin-film device shows excellent resistive switching properties, such as a reduced threshold current of 1 μA for device initiation, a small dispersion of reset voltage ranging from 0.54 to 0.62 V, long retention time and a high resistance ratio of high-resistance state to low-resistance state about six orders of magnitude. These results indicate that the resistive switching properties can be greatly improved by constructing the p-NiO/n-Mg_0.6Zn_0.4O heterojunction for nonvolatile memory applications. The physical mechanism responsible for colossal resistive switching properties of the heterojunction was analyzed based on interfacial defect effect and formation and rapture of conductive filaments.     

Low power resistive random access memory using interface-engineered dielectric stack of SiOx/a-Si/TiOy with 1D1R-like structure

In this study, we report a resistive random access memory (RRAM) using trilayer SiO_x/a-Si/TiO_y film structure. The low switching energy of <10 pJ, highly uniform current distribution (<13% variation), fast 50-ns speed and stable cycling endurance for 10⁶ cycles are simultaneously achieved in this RRAM device. Such good performance can be ascribed to the use of interface-engineered dielectric stack with 1D1R-like structure. The SiO_x tunnel barrier in contact with top Ni electrode to form diode-like rectifying element not only lowers self-compliance switching currents, but also improves cycling endurance, which is favorable for the application of high-density 3D memory.     

Flexible and transparent resistive switching devices using Au nanoparticles decorated reduced graphene oxide in polyvinyl alcohol matrix

We have investigated the resistive switching mechanism in solution processed Au-reduced graphene oxide-polyvinyl alcohol (PVA) nanocomposites on flexible substrates. Monodispersed gold nanoparticles (Au NPs) attached to reduced graphene oxide (RGO) in aqueous PVA solution have been synthesized using a novel one pot technique. The fabricated hybrid device showed high On/Off switching ratio more than 10³ with low operating voltages. The performance of hybrid device can be effectively enhanced over control RGO device. The switching mechanism occurs from the electrochemical reduction/oxidation process of partially reduced graphene oxide. The proposed devices reveal superior asymmetric bipolar resistive switching characteristics attractive for solution processable flexible and transparent non-volatile memory applications.     

Enhanced resistance switching stability of transparent ITO/TiO2/ITO sandwiches

We report that fully transparent resistive random access memory(TRRAM) devices based on ITO/TiO2/ITO sandwich structure,which are prepared by the method of RF magnetron sputtering,exhibit excellent switching stability.In the visible region(400-800 nm in wavelength) the TRRAM device has a transmittance of more than 80%.The fabricated TRRAM device shows a bipolar resistance switching behaviour at low voltage,while the retention test and rewrite cycles of more than 300,000 indicate the enhancement of switching capability.The mechanism of resistance switching is further explained by the forming and rupture processes of the filament in the TiO 2 layer with the help of more oxygen vacancies which are provided by the transparent ITO electrodes.     

Multilevel resistive switching performance of TiO2-based flexible memory prepared by low-temperature sol-gel method with UV irradiation

The flexible Ag/TiO₂/ITO/PET resistive switching memory is prepared by low-temperature sol-gel method with UV irradiation, and the simple method that combined the advantages of sol-gel method and low temperature can be applied to fabricate high-quality film. The flexible Ag/TiO₂/ITO/PET memory device displays good resistive behavior, for instance, the narrow distributions of switching voltages, good cycle endurance, and long retention time. Meanwhile, the multilevel resistance states of the device can be realized by controlling the compliance current or reset voltages, showing the potential of applications in neural networks and high-density storge. In addition, flexibility of the Ag/TiO₂/ITO/PET is studied, which exhibit good endurance and retention properties under bending condition. The I–V curves are replotted and fitted for analyzing the conductive mechanism of the device. The fitting results show that SCLC and Ohmic mechanism are main mechanisms of high resistance state and low resistance state respectively. The electrochemical and thermochemical modes are adopted to explain resistive switching behavior. Our results indicate the Ag/TiO₂/ITO/PET memory has potential application in wearable and foldable electronics.     

Low power consumption bipolar resistive switching characteristics of ZnO-based memory devices

Ag/ZnO/Pt structure resistive switching devices are fabricated by radio frequency (RF) magnetron sputtering at room temperature. The memory devices exhibit stable and reversible resistive switching behavior. The ratio of high resistance state to low resistance state can reach as high as 10 2 . The retention measurement indicates that the memory property of these devices can be maintained for a long time (over 10 4 s under 0.1-V durable stress). Moreover, the operation voltages are very low, -0.4 V (OFF state) and 0.8 V (ON state). A high-voltage forming process is not required in the initial state, and multi-step reset process is demonstrated. Resistive switching device with the Ag/ZnO/ITO structure is constructed for comparison with the Ag/ZnO/Pt device.     

Flexible and degradable resistive switching memory fabricated with sodium alginate

Transient electronics has attracted interest as an emerging technology to solve electronic-waste problem,due to its physically vanishing ability in solution.Here in this work,we demonstrate a flexible and degradable transient resistive switching(RS) memory device with simple structure of Cu/sodium alginate(SA)/ITO.The device presents excellent RS characteristics as well as high flexibility,including low operating voltage(1.5 V) and multilevel RS behavior.No performance degradation occurs after bending the device 50 times.Moreover,our device can be absolutely dissolved in deionized water.The proposed SA-based transient memory device has great potential for the development of green and security memory devices.     

氧分压对Ni/HfO_x/TiN阻变存储单元阻变特性的影响

采用射频磁控溅射的方法,基于不同氧分压制备的氧化铪构建了Ni/HfO_x/TiN结构阻变存储单元.研究发现,随着氧分压的增加,薄膜表面粗糙度略有降低;另一方面,阻变单元功耗降低,循环耐受性能可达10~3次,且转变电压分布的一致性得到改善.结合电流-电压曲线线性拟合结果及外加温度测试探究了器件的转变机理,得出在低阻态的传导机理为欧姆传导机理,在高阻态的传导机理为肖特基发射机理,并根据氧空位导电细丝理论,对高低阻态的阻变机理进行了详细的理论分析.     

Highly improved resistive switching performances of the self-doped Pt/HfO2:Cu/Cu devices by atomic layer deposition

Metal-oxide electrochemical metallization (ECM) memory is a promising candidate for the next generation nonvolatile memory. But this memory suffers from large dispersion of resistive switching parameters due to the intrinsic randomness of the conductive filament. In this work, we have proposed a self-doping approach to improve the resistive switching characteristics. The fabricated Pt/HfO₂:Cu/Cu device shows outstanding nonvolatile memory properties, including high uniformity, good endurance, long retention and fast switching speed. The results demonstrate that the self-doping approach is an effective method to improve the metal-oxide ECM memory performances and the self-doped Pt/HfO₂:Cu/Cu device has high potentiality for the nonvolatile memory applications in the future.     

Fabrication of printed memory device having zinc-oxide active nano-layer and investigation of resistive switching

An all printed resistive memory device, a 9-bit memristor, has been presented in this study consisting of 3 × 3 memristor crossbars deposited via electrohydrodynamic inkjet printing process at room conditions. Transparent zinc oxide active nano-layers, directly deposited by electrospray process, are sandwiched between the crossbars to complete the metal–insulator metal structure consisting of copper–zinc oxide–silver, where Cu and Ag are used as bottom and top electrodes respectively. The 9-bit memristor device has been characterized using current–voltage measurements to investigate the resistive switching phenomenon thereby confirming the memristive pinched hysteresis behavior signifying the read–write and memory characteristics. The memristor device showed a current bistability due to the existence of metal–oxide layer which gives rise to oxygen vacancies upon receiving the positive voltage hence breaking down into doped and un-doped regions and a charge transfer takes place. The maximum ON/OFF ratio of the current bi-stability for the fabricated memristor was as large as 1 × 10³, and the endurance of ON/OFF switchings was verified for 500 read–write cycles. The metal–insulator–metal structure has been characterized using X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope techniques.     

Investigation of resistive switching behaviours in WO3-based RRAM devices

In this paper, a WO₃-based resistive random access memory device composed of a thin film of WO₃ sandwiched between a copper top and a platinum bottom electrodes is fabricated by electron beam evaporation at room temperature. The reproducible resistive switching, low power consumption, multilevel storage possibility, and good data retention characteristics demonstrate that the Cu/WO₃/Pt memory device is very promising for future nonvolatile memory applications. The formation and rupture of localised conductive filaments is suggested to be responsible for the observed resistive switching behaviours.     

Resistive switching of silicon-rich-oxide featuring high compatibility with CMOS technology for 3D stackable and embedded applications

This paper discusses the resistive switching devices based on highly compatible silicon-rich-oxide, including silicon monoxide (SiO) and SiO _x N _y material, which can be fabricated by low temperature process, and thus fully compatible with the back-end CMOS technology. The demonstrated SiO based RRAM suitable for 3D stackable applications shows repeatable unipolar resistive switching behavior with excellent on/off resistance ratio and good retention performance, but a little bit high switching voltage. The presented silicon-rich silicon-oxynitride RRAM device can effectively reduce the switching voltages (∼1 V) and shows good retention capability under 180°C baking as well as fast speed, giving great potentials for 3D stackable and embedded applications. The switching mechanisms in the studied devices are discussed. The method of switching voltage reduction through nitrogen doping, as a kind of defect engineering, can provide some guidelines for RRAM design.     

Ni/Au Schottky gate oxidation and BCB passivation for high-breakdown-voltage AlGaN/GaN HEMT

A new AlGaN/GaN high electron mobility transistor (HEMT) employing Ni/Au Schottky gate oxidation and benzocyclobutene (BCB) passivation is fabricated in order to increase a breakdown voltage and forward drain current. The Ni/Au Schottky gate metal with a thickness of 50/300 nm is oxidized under oxygen ambient at 500 C and the highly resistive NiO is formed at the gate edge. The leakage current of AlGaN/GaN HEMTs is decreased from 4.94 μA to 3.34 nA due to the formation of NiO. The BCB, which has a low dielectric constant, successfully passivates AlGaN/GaN HEMTs by suppressing electron injection into surface states. The BCB passivation layer has a low capacitance, so BCB passivation increases the switching speed of AlGaN/GaN HEMTs compared with silicon nitride passivation, which has a high dielectric constant. The forward drain current of a BCB-passivated device is 199 mA /mm, while that of an unpassivated device is 172 mA /mm due to the increase in two-dimensional electron gas (2DEG) charge.     

Flexible resistance memory devices based on Cu/ZnO:Mg/ITO structure

Resistance memory devices based on a Cu/Mg‐doped ZnO/indium‐tin‐oxide structure on a PET (polyethylene terephthalate) flexible substrate were fabricated. The devices showed stable bipolar resistance switching property and good flexibility. The high to low resistance ratio was larger than 30 times, the endurance was more than 10² cycles, and the resistance retention was longer than 10⁴ s. The resistance values of both high and low resistance states were not significantly changed by bending in a radius (≥20 mm) for more than 10³ times. This resistance switching phenomenon of our devices can be explained by creation/rupture of metal conductive channels induced by electrochemical migration of Cu ions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermochemical resistive switching: materials,mechanisms, and scaling projections

In this article, resistive switching based on the thermochemical mechanism (TCM) is reviewed. This mechanism is observed when thermochemical redox processes dominate over electrochemical processes. As the switching is based on thermal effects, it is inherently unipolar, i.e., the transitions between the resistive states can be induced by the same bias voltage polarity. NiO has emerged as a “model material” for resistive switching based on the TCM effect and the discussion of the resistance states and the switching processes are focused on this material with the appropriate electrodes, mainly Pt. Unipolar switching is unambiguously filamentary. Conductive filaments are formed during the electroforming process needed prior to memory switching. The SET operation is interpreted as a sequence of threshold switching and subsequent Joule heating which triggers local redox reactions in which oxygen deficient NiO and, if the amount of released oxygen exceeds a certain amount, also metallic Ni will form. The RESET transition can be described as a thermally activated solid-state process resulting in a local decrease of the metallic Ni species. In terms of operation and reliability, a trade-off between RESET current reduction and retention was experimentally found. This is due to the decreasing long-term stability of the filaments with decreasing size. In addition, the scaling projection of a TCM-based memory technology with NiO is directly related to RESET currents and the availability of appropriate select devices.     

Thermal Stability of Reliable Polycrystalline Zirconium Oxide for Nonvolatile Memory Application

Thermal stability of resistive switching of stoichiometric zirconium oxide thin films is investigated for high yielding nonvolatile memory application. The A1/ZrO2/AI cell fabricated in the conventional device process shows highly reliable switching behaviour between two distinct stable resistance states. The retention capabilities are also tested under various conditions and temperatures. The excellent performance of Ai/ZrO2/AI ceil can be explained by assuming that anode/ZrO2 interface exists and by conducting filament forming/rupture mechanism. The device failure is illustrated in terms of permanent conducting filaments formation.