Observation of two resistance switching modes in TiO2 memristive devices electroformed at low current

We report the observation of two resistance switching modes in certain 50 nm × 50 nm crossbar TiO(2) memristive devices that have been electroformed with a low-current process. The two switching modes showed opposite switching polarities. The intermediate state was shared by both modes (the ON state of the high-resistance mode or the OFF state of the low-resistance mode) and exhibited a relaxation to a more resistive state, including an initial transient decay. The activation energies of such a decay and ON-switching to the intermediate state were determined to be 50-210 meV and 1.1 eV, respectively. Although they are attributed to the coexistence of charge trapping and ionic motion, the ionic motion dominates in both switching modes. Our results indicate that the two switching modes in our system correspond to different switching layers adjacent to the interfaces at the top and bottom electrodes.     

Demonstration of synaptic and resistive switching characteristics in W/TiO2/HfO2/TaN memristor crossbar array for bioinspired neuromorphic computing

In this study,resistive random-access memory (RRAM)-based crossbar arrays with a memristor W/TiO2/HfO2/TaN structure were fabricated through atomic layer deposi...     

Improvement of resistive switching characteristics in ZrO2 film by embedding a thin TiOx layer

The stabilization of the resistive switching characteristics is important to resistive random access memory (RRAM) device development. In this paper, an alternative approach for improving resistive switching characteristics in ZrO(2)-based resistive memory devices has been investigated. Compared with the Cu/ZrO(2)/Pt structure device, by embedding a thin TiO(x) layer between the ZrO(2) and the Cu top electrode, the Cu/TiO(x)-ZrO(2)/Pt structure device exhibits much better resistive switching characteristics. The improvement of the resistive switching characteristics in the Cu/TiO(x)-ZrO(2)/Pt structure device might be attributed to the modulation of the barrier height at the electrode/oxide interfaces.     

Electronic bipolar resistance switching in an anti-serially connected Pt/TiO2/Pt structure for improved reliability

Electronic bipolar resistive switching and its degradation in the Pt/TiO2/Pt structure were studied. The electronic bipolar switching was induced from the asymmetric trap distribution of the structure under its unipolar reset state. The imbalanced migration of oxygen accompanied by electronic switching significantly degrades switching endurance. Instead, the anti-serial connection of Pt/TiO2/Pt cells resulted in substantial improvements in endurance, underscoring the importance of vacancy migration in device reliability. In addition, the independent control of resistance states of the two connected cells provides the freedom to control resistance ratio, switching direction, and reliability.     

Schottky barrier mediated single-polarity resistive switching in Pt layer-included TiO(x) memory device

A distinct unipolar but single-polarity resistive switching behavior is observed in a TiO(x)/Pt/TiO(x) trilayer structure, formed by thermal oxidation of a Ti/Pt/Ti stack. As a comparison, a memory device with a single TiO(x) active layer (without addition of Pt midlayer) is also fabricated but it cannot perform resistive switching. Energy band diagrams are illustrated to realize the modulation of Schottky barrier junctions and current conduction in TiO(x)-based devices under various biasing polarities. Introduction of the Pt midlayer creates two additional Schottky barriers, which mediate the band bending potential at each metal-oxide interface and attains a rectifying current conduction at the high-resistance state. The rectifying conduction behavior is also observed with an AFM-tip as the top electrode, which implies the rectifying property is still valid when miniaturizing the device to nanometer scale. The current rectification consequently leads to a single-polarity, unipolar resistive switching and electrically rewritable performance for the TiO(x)/Pt/TiO(x) device.     

Fabrication of a 3D Nanoscale Crossbar Circuit by Nanotransfer‐Printing Lithography

Nanotransfer‐printing lithography simplifies the fabrication of a 3D nanoscale crossbar circuit. Gold nanowires 100 nm in width and with 100 nm spacing are printed onto a polymer layer of electrically switchable, LiClO₄‐doped poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene] mixed with an epoxy. The transfer process can be repeated to obtain a multilayer nanoscale crossbar structure. This process paves the way toward fabricating 3D circuits with ultrahigh device density and neuromorphic architectures.     

Improved performance of ZnO-based resistive memory by internal diffusion of Ag atoms

An Ag/ZnO/Pt memory device, which has much better resistive switching behaviour than Pt/ZnO/Pt device was demonstrated. The detailed resistive mechanisms for the Pt/ZnO/Pt and the Ag/ZnO/Pt systems are proposed and investigated. Microstructures are observed by transmission electron microscope (TEM), indicating that the formation of conducting path for both systems is different. For the Pt/ZnO/Pt device, the conductive filament path is constructed by the oxygen vacancies from top to bottom electrodes under a larger enough bias at a forming process. For the Ag/ZnO/Pt device, the filament path was grown by oxygen vacancies combined with an internal diffusion of Ag atoms under a large bias and can provide the lowest energy barrier for electrons transported between two electrodes during set and reset processes, which reduces formation of other conducting paths after each switching. Accordingly, the stable switching performance of the Ag/ZnO/Pt device can be achieved over 100 cycles even the thickness of ZnO film <25 nm.     

Ag／La0.67Sr0.33MnO3/Pt异质结构的电阻开关特性

采用脉冲激光沉积技术在Pt／Ti／SiO2／Si（100）衬底上制备出多晶La0.67Sr0.33MnO3（LSMO）薄膜,对其电脉冲致非挥发可逆电阻开关特性进行研究．结果表明,Ag/LSMO／Pt结构具有明显的室温电脉冲诱发电阻开关特性,且在宽电压脉冲作用下表现出较低的开关电压和较快的变阻饱和速度．由此可见,总脉冲能量或电荷（电流作用）为该结构的电阻开关效应提供驱动力．对Ag／LsMO／Pt结构进行了耐久性测试,表明该结构具有良好的疲劳特性与保持特性,可应用于新型不挥发存储器、传感器及可变电阻等电子元器件的研制     

退火温度对TiO2基电阻开关器件性能的影响

采用直流磁控溅射法在n+-Si上制备了TiO2薄膜,采用电子束蒸发镀膜仪在TiO2薄膜上沉积Au电极,获得了Au/TiO2/n+-Si结构的器件.研究了退火温度对薄膜结晶性能及器件电阻开关特性的影响.Au/TiO2/n+-Si结构的器件具有单极性电阻开关特性,置位(set)电压,复位(reset)电压、reset电流及功率的大小随退火温度的不同而不同,并基于灯丝理论对器件的电阻开关效应的工作机理进行了探讨.研究结果表明,500℃退火的器件具有良好的非易失性.器件高低阻态的阻值比大于103,其信息保持特性可达10年之久.在读写次数为100次时,器件仍具有电阻开关效应.     

A novel fabrication method for titania resistive switching device

The titania showing reversible resistive switching are attractive for today's semiconductor technology in nonvolatile random-access memories. A novel fabrication method for titania resistive switching device with vertical structure is proposed. First, the Pt electrode was fabricated the bottom using conventional photolithography and chemical etching technique. Next, the titania thin films with the thickness about 50 nm was deposited on the bottom electrode by electron beam evaporation (EBE). Then, the trench of photoresist for electrode deposit was etched with mild chemical process to preserve the original structure of titania layer. After that, the platinum was deposited in the trench of photoresist using ion sputter. A final lift-off process to define the Pt top electrodes was performed with acetone in an ultrasonic bath to remove the resist. The resistive bistability was observed in this device. The on-threshold voltage is +1.5 V and the off-threshold voltage is -0.6 V. The resistance ratio between the two stable states of the device including Al electrode is approximately 1 x 10(3), the state is nonvolatile and the retention-time test performed over an hour in sweeping mode measurement. The results indicate the forming and rupture of conductive channel relate to the defects and distributing of oxygen vacancy. This method is low-cost, high-yielding, and easy to implement, which is applicable to the fabrication of nonvolatile memories.     

Regular arrays of monodisperse platinum/erbium disilicide core-shell nanowires and nanoparticles on Si(001) via a self-assembled template

We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.     

Rectifying switching characteristics of Pt/ZnO/Pt structure based resistive memory

40 nm thick amorphous ZnO thin films were deposited by radio frequency magnetron sputtering at room temperature and asymmetric electrical switching characteristics are observed in the macroscopic symmetric Pt/ZnO/Pt structure. The crystal structure was examined by X-ray diffraction (XRD). The chemical bonding states of ZnO resistive switching layer was investigated by X-ray photoelectron spectroscopy (XPS). Keithley 4200 semiconductor characterization system was used to measure the current-voltage (I-V) characteristics of the fabricated devices. The results reveal that a reversible resistive switching behavior between the high resistance state and the low resistance state with rectifying effects can be repeated for more than 100 dc cycles. This asymmetric electrical behavior is thought to be related to the naturally self-formed PtOx between ZnO film and the Pt bottom electrode, which introduces an energy barrier when electrons flow from top electrode towards the bottom electrode. The model of Pt/ZnO/Pt memory cell is expected to be able to alleviate the misreading error in cross-point array for high density integrations.     

Fabrication of large area nanoprism arrays and their application for surface enhanced Raman spectroscopy

This work demonstrates the fabrication of metallic nanoprism (triangular nanostructure) arrays using a low-cost and high-throughput process. In the method, the triangular structure is defined by the shadow of a pyramid during angle evaporation of a metal etching mask. The pyramids were created by nanoimprint lithography in polymethylmethacrylate (PMMA) using a mould having an inverse-pyramid-shaped hole array formed by KOH wet etching of silicon. Silver and gold nanoprism arrays with a period of 200?nm and an edge length of 100?nm have been fabricated and used as effective substrates for surface enhanced Raman spectroscopy (SERS) detection of rhodamine 6G (R6G) molecules. Numerical calculations confirmed the great enhancement of electric field near the sharp nanoprism corners, as well as the detrimental effect of the chromium adhesion layer on localized surface plasmon resonance. The current method can also be used to fabricate non-equilateral nanoprism and three-dimensional (3D) nanopyramid arrays, and it can be readily extended to other metals.     

<Emphasis Type="Italic">In situ</Emphasis> grazing-incidence small-angle X-ray scattering observation of block-copolymer templated formation of magnetic nanodot arrays and their magnetic properties

The fabrication of bit-patterned media (BPM) is crucial for new types of hard disk drives.The development of methods for the production of BPM is progressing rapidly.Conventional lithography reaches the limit regarding lateral resolution,and new routes are needed.In this study,we mainly focus on the dependence of the size and shape of magnetic nanodots on the Ar+-ion etching duration,using silica dots as masks.Two-dimensional (2D) arrays of magnetic nanostructures are created using silica-filled diblock-copolymer micelles as templates.After the self-assembly of the micelles into 2D hexagonal arrays,the polymer shell is removed,and the SiO2 cores are utilized to transform the morphology into a (Co/Pt)2-multilayer via ion etching under normal incidence.The number of preparation steps is kept as low as possible to simplify the formation of the nanostructure arrays.High-resolution in situ grazing-incidence small-angle X-ray scattering (GISAXS) investigations are performed during the Ar+-ion etching to monitor and control the fabrication process.The in situ investigation provides information on how the etching conditions can be improved for further ex situ experiments.The GISAXS patterns are compared with simulations.We observe that the dots change in shape from cylindrical to conical during the etching process.The magnetic behavior is studied by utilizing the magneto-optic Kerr effect.The Co/Pt dots exhibit different magnetic behaviors depending on their size,interparticle distance,and etching time.They show ferromagnetism with an easy axis of magnetization perpendicular to the film.A systematic dependence of the coercivity on the dot size is observed.     

Integrated freestanding single-crystal silicon nanowires: conductivity and surface treatment

Integrated freestanding single-crystal silicon nanowires with typical dimension of 100 nm × 100 nm × 5 μm are fabricated by conventional 1:1 optical lithography and wet chemical silicon etching. The fabrication procedure can lead to wafer-scale integration of silicon nanowires in arrays. The measured electrical transport characteristics of the silicon nanowires covered with/without SiO(2) support a model of Fermi level pinning near the conduction band. The I-V curves of the nanowires reveal a current carrier polarity reversal depending on Si-SiO(2) and Si-H bonds on the nanowire surfaces.     

Formation of high aspect ratio GaAs nanostructures with metal-assisted chemical etching

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.     

Memristive tri-stable resistive switching at ruptured conducting filaments of a Pt/TiO₂/Pt cell

A tri-stable memristive switching was demonstrated on a Pt/TiO?/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO?/TiO(2-x)/Pt cell.     

小型微纳米图形电子束曝光制作系统(英文)

为了满足科学实验过程中对制作半导体器件和微纳米结构的需要,同时避免受到昂贵的工业级电子束曝光(electron beam lithography,EBL)机的条件制约,构建了一种基于普通扫描电子显微镜(scanning electron microsco-py,SEM)的桌面级小型电子束曝光系统.建立了以浮点DSP为控制核心的高速图形发生器硬件系统.利用线性计算方法实现了电子束曝光场的增益、旋转和位移的校正算法.在本曝光系统中应用了新型压电陶瓷电机驱动的精密位移台来实现纳米级定位.利用此位移台所具有的纳米定位能力,采用标记追逐法实现了电子束曝光场尺寸和形状的校准.电子束曝光实验结果表明,场拼接及套刻精度误差小于100 nm.为了测试曝光分辨率,在PMMA抗蚀剂上完成了宽度为30 nm的密集线条曝光实验.利用此系统,在负胶SU8和双层PMMA胶表面进行了曝光实验;并通过电子束拼接和套刻工艺实现了氮化物相变存储器微电极的电子束曝光工艺.     

Wafer-scale patterning of sub-40 nm diameter and high aspect ratio (>50:1) silicon pillar arrays by nanoimprint and etching

We demonstrate wide-area fabrication of sub-40?nm diameter, 1.5?μm tall, high aspect ratio silicon pillar arrays with straight sidewalls by combining nanoimprint lithography (NIL) and deep reactive ion etching (DRIE). Imprint molds were used to pre-pattern nanopillar positions precisely on a 200?nm square lattice with long range order. The conventional DRIE etching process was modified and optimized with reduced cycle times and gas flows to achieve vertical sidewalls; with such techniques the pillar sidewall roughness can be reduced below 8?nm (peak-to-peak). In some cases, sub-50?nm diameter pillars, 3?μm tall, were fabricated to achieve aspect ratios greater than 60:1.     

One-dimensional nanorod arrays: independent control of composition, length, and interparticle spacing with nanometer precision

We report the synthesis of solution dispersible, one-dimensional metal nanostructure arrays as small as 35 nm in diameter using on-wire lithography, wherein feature thickness and spacing in the arrays is tailorable down to approximately 6 and 1 nm, respectively. Using this unique level of control, we present solution-averaged extinction spectra of 35 nm diameter Au nanorod dimers with varying gap sizes to illustrate the effect of gap size on plasmon coupling between nanorods. Additionally, we demonstrate control over the composition of the arrays with Au, Ni, and Pt segments, representing important advances in controlling the ordering of sub-100 nm nanostructures that are not available with current synthesis or assembly methods.