Multistate resistive switching in silver nanoparticle films

Abstract

Resistive switching devices have garnered significant consideration for their potential use in nanoelectronics and non-volatile memory applications. Here we investigate the nonlinear current–voltage behavior and resistive switching properties of composite nanoparticle films comprising a large collective of metal–insulator–metal junctions. Silver nanoparticles prepared via the polyol process and coated with an insulating polymer layer of tetraethylene glycol were deposited onto silicon oxide substrates. Activation required a forming step achieved through application of a bias voltage. Once activated, the nanoparticle films exhibited controllable resistive switching between multiple discrete low resistance states that depended on operational parameters including the applied bias voltage, temperature and sweep frequency. The films’ resistance switching behavior is shown here to be the result of nanofilament formation due to formative electromigration effects. Because of their tunable and distinct resistance states, scalability and ease of fabrication, nanoparticle films have a potential place in memory technology as resistive random access memory cells.     

Multistate resistive switching in silver nanoparticle films

Resistive switching devices have garnered significant consideration for their potential use in nanoelectronics and non-volatile memory applications. Here we investigate the nonlinear current–voltage behavior and resistive switching properties of composite nanoparticle films comprising a large collective of metal–insulator–metal junctions. Silver nanoparticles prepared via the polyol process and coated with an insulating polymer layer of tetraethylene glycol were deposited onto silicon oxide substrates. Activation required a forming step achieved through application of a bias voltage. Once activated, the nanoparticle films exhibited controllable resistive switching between multiple discrete low resistance states that depended on operational parameters including the applied bias voltage, temperature and sweep frequency. The films’ resistance switching behavior is shown here to be the result of nanofilament formation due to formative electromigration effects. Because of their tunable and distinct resistance states, scalability and ease of fabrication, nanoparticle films have a potential place in memory technology as resistive random access memory cells.     

Charge trapping phenomena of tetraethylorthosilicate thin film containing Si nanocrystals synthesized by solid-state reaction

In this work, we report on the fabrication of tetraethylorthosilicate (TEOS) thin dielectric film containing silicon nanocrystals (Si nc), synthesized by solid-state reaction, in a capacitor structure. A metal-insulator-semi-conductor (MIS) capacitor, with 28?nm thick Si nc in a TEOS thin film, has been fabricated. For this MIS, both electron and hole trapping in the Si nc are possible, depending on the polarity of the bias voltage. A V(FB) shift greater than 1?V can be experienced by a bias voltage of 16?V applied to the metal electrode for 1?s. Though there is no top control oxide, the discharge time for 10% of charges can be up to 4480?s when it is biased at 16?V for 1?s. It is further demonstrated that charging and discharging mechanisms are due to the Si nc rather than the TEOS oxide defects. This form of Si nc in a TEOS thin film capacitor provides the possibility of memory applications at low cost.     

The influence of external bias voltage on electrical properties of YBa2Cu3O7−x /metal point contact interface

We have investigated a change of electrical properties of YBa₂Cu₃O₇−δ/metal point contact immediately after its preparation depending on time, temperature as well as external bias voltage. The increase of the point contact differential resistance in time was experimentally observed at temperature above 200 K even if no external bias voltage was applied. The low external bias voltage considerably influences the time increase of the differential resistance indicating an important role of oxygen diffusion. It is shown on differential characteristics that for Au, In the parameters of tunneling barrier such as the average height and width are constant in time whereas for Al, Pb an evolution of the tunneling barrier was observed because of oxidation of Al and Pb. Applying of higher bias voltage (up to 1 V) enables the transport of oxygen even below 200 K (down to 4.2 K) and changes the electrical properties of YBCO/metal point contact interface. The differential characteristics change their behaviour from that typical for NIS contact with strong tunneling barrier to NS contact with a high transparency of the interface. All the above changes are reversible upon changing the bias voltage polarity.     

约瑟夫森结Nb电极的实验研究

通过溅射Nb膜张力与氩（Ar）压强的关系，超导转变温度Tc，室温阻扰与液氮温度阻抗比RRT／RLN2，沉积中Ar浓度CAr与负偏压关系的测量和扫描电子显微镜的观察分析，对约瑟夫森结Nb电极作了研究。发现Ar压强在1．1Pa时，Nb膜呈现无应力状态；低负偏压下沉积的Nb膜晶粒结构是由致密膜到圆柱状。在偏压Ub＝－50V时，获得表面致密均匀、晶粒结构合适的Nb膜。对Nb膜用阳极氧化电压谱图（AVS）分析，证实沉积的Nb膜内不存在氧化物、寄生结和分层界面。     

Non-local Quasi-Particles Current in the Lateral SNS Junction with Transparent Interfaces

We have fabricated multi-terminal superconductor/normal metal/superconductor junctions comprising superconductor Nb and normal metal Cu. By means of the local and non-local measurements of the junction, we successfully detected the non-existence of a bias current in the Cu wire and the non-local voltage. These results indicate that the current flowing non-locally over the junction is due to the proximity effects and the conversion of the supercurrent and normal current.     

氮化镓肖特基结紫外探测器的异常特性测量

测量了CaN肖特基结紫外探测器在有、无光照下的I-V异常特性。分别用362nm和368nm光束对有源区进行横向扫描,得到了光照不同部位时探测器在无偏压、2V反向偏压下的电流。紫外光照到肖特基结压焊电极附近及透明电极边沿附近区域时,探测器在反向偏压下有较大增益,空间响应均匀性变差,在禁带内有两个增益响应峰波长——364nm和368nm。探测器在810nm光照射下,反向偏压下的光响应增益、持续光电导存在光淬灭现象。探测器紫外光照完后,俘获中心及表面陷阱所俘获的部分电荷在高反向偏置电压下老化可以通过隧穿或发射效应释放出来,经过高反向偏置电压老化完后的探测器在同一低反向偏置电压下暗电流比老化前的要小。测量结果为GaN器件的研制提供了参考数据。     

Large photocurrents in single layer graphene thin films: effects of diffusion and drift

This paper reports large photocurrents in air-assisted depositions of single layer graphene (derived from reduced single layer graphene oxide) upon illumination with near-infrared (NIR) light. NIR-induced charge carrier generation and subsequent separation at the metal-graphene interface resulted in photocurrent generation. Varying bias voltages were applied to test samples and allowed for evaluating photoresponses in either diffusion- or drift-dominated regions. In the diffusion-dominated region, position-dependent effects of photoconductivity were demonstrated. The photocurrent exhibited increase when the positive electrode was illuminated, decrease when the negative electrode was illuminated, and negligible response when the area between the electrodes was illuminated. At a 100 μV bias voltage, a per cent change in current from ～150% (40 mW NIR) to ～1800% (335 mW NIR) is reported. Such large photocurrent responses result from built-in electric fields and optically generated temperature gradients (maximum NIR-induced temperature rise ～70?°C). The per cent photocurrent change was observed to depend on both annealing temperature and NIR power, but not resistance value. In the drift-dominated realm, a Gaussian photocurrent profile was obtained, signaling drift of charge carriers with increase in localized electric field, akin to the classic Haynes-Shockley experiment. A minority carrier mobility value of μ ～700 cm2 V?1 s?1 is reported. The simple low cost graphene devices presented in this paper were fabricated without lithographic processing and are ideal candidates for assorted infrared imaging applications.     

The size effect in current–voltage characteristic of VO<Subscript>2</Subscript>-based ceramics in the on-state

Current–voltage characteristics of (wt.%) 45VO₂–15VPG–5Cu–35SnO₂ ceramics in the on-state were investigated in the electric current interval between 0.3 A and 5 A. Transition from the region with negative differential resistance to the region with positive differential resistance is observed in current–voltage characteristic when electric current increases. The reason of such behaviour is the limitation of lateral expansion for the filament of VO₂ metal phase by the sample’s size. The change in the sign of differential resistance occurs, when the cross-section area of the filament reaches the sample’s cross-section area. In this case the phase transition of VO₂ into metal state is completed in the whole volume of the sample. Therefore at further increasing of current the current–voltage characteristic has the positive differential resistance.     

Thermoelectric voltage at a nanometer-scale heated tip point contact

We report thermoelectric voltage measurements between the platinum-coated tip of a heated atomic force microscope (AFM) cantilever and a gold-coated substrate. The cantilevers have an integrated heater-thermometer element made from doped single crystal silicon, and a platinum tip. The voltage can be measured at the tip, independent from the cantilever heating. We used the thermocouple junction between the platinum tip and the gold substrate to measure thermoelectric voltage during heating. Experiments used either sample-side or tip-side heating, over the temperature range 25-275?°C. The tip-substrate contact is ～4?nm in diameter and its average measured Seebeck coefficient is 3.4?μV?K(-1). The thermoelectric voltage is used to determine tip-substrate interface temperature when the substrate is either glass or quartz. When the non-dimensional cantilever heater temperature is 1, the tip-substrate interface temperature is 0.593 on glass and 0.125 on quartz. Thermal contact resistance between the tip and the substrate heavily influences the tip-substrate interface temperature. Measurements agree well with modeling when the tip-substrate interface contact resistance is 10(8)?K?W(-1).     

Substrate bias voltage influenced structural, electrical and optical properties of dc magnetron sputtered Ta2O5 films

Tantalum oxide (Ta₂O₅) films were formed on silicon (111) and quartz substrates by dc reactive magnetron sputtering of tantalum target in the presence of oxygen and argon gases mixture. The influence of substrate bias voltage on the chemical binding configuration, structural, electrical and optical properties was investigated. The unbiased films were amorphous in nature. As the substrate bias voltage increased to −50 V the films were transformed into polycrystalline. Further increase of substrate bias voltage to −200 V the crystallinity of the films increased. Electrical characteristics of Al/Ta₂O₅/Si structured films deposited at different substrate bias voltages in the range from 0 to −200 V were studied. The substrate bias voltage reduced the leakage current density and increased the dielectric constant. The optical transmittance of the films increased with the increase of substrate bias voltage. The unbiased films showed an optical band gap of 4.44 eV and the refractive index of 1.89. When the substrate bias voltage increased to −200 V the optical band gap and refractive index increased to 4.50 eV and 2.14, respectively due to the improvement in the crystallinity and packing density of the films. The crystallization due to the applied voltage was attributed to the interaction of the positive ions in plasma with the growing film.     

A Self-consistent Calculation and an Anisotropic Wavelength Cutoff Energy of Spin-wave Spectrum in Magnetic Tunnel Junctions

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature (RT), applied de bias voltage dependence of the TMR ratio and resistances at 4.2 K and RT, tunnel current I and dynamic conductance dI/dV as functions of the de bias voltage at 4.2 K, and inelastic electron tunneling (IET) spectroscopy, d(2)I/dV(2) versus V, at 4.2 K for a tunnel junction of Ta(5 nm)/Ni79Fe21(25 nm)/Ir22Mn78(12 nm)/Co75Fe25(4 nm)/Al(0.8 nm)-oxide/Co75Fe25(4 nm)/Ni79Fe21(25 nm)/Ta(5 nm) were systematically investigated. High TMR ratio of 59.2% at 4.2 K and 41.3% at RT were observed for this junction after annealing at 275℃ for an hour. The temperature dependence of TMR ratio and resistances from 4.2 to 300 K at 1.0 mV bias and the de bias voltage dependence of TMR ratio at 4.2 K from 0 to 80 mV can be evaluated by a comparison of self-consistent calculations with the experimental data based on the magnon-assisted inelastic excitation model and theory. An anisotropic wavelength cutoff energy of spin-wave spectrum in magnetic tunnel junctions (MTJs) was suggested, which is necessary for self-consistent calculations, based on a series of IET spectra observed in the MTJs.     

石墨烯介电电泳组装及电学特性

将石墨烯有效地集成到微纳器件上实现组装是石墨烯得以应用的重要先决条件。采用介电电泳法对二维纳米材料石墨烯进行组装,研究介电电泳组装过程参数包括外加交变电压幅值、石墨烯悬浮液浓度和外加电场作用时间对组装的影响。结果表明:组装到电极之间的石墨烯数量随着上述组装参数值的增大而增加,其中石墨烯悬浮液浓度的影响最为显著。组装后石墨烯的I-V特性曲线呈现良好的直线性,依据组装石墨烯数量的不同,电阻在数kΩ到数百kΩ之间,表明石墨烯与金属电极之间具有较高的接触电阻。采用局部焦耳热法可以有效地降低石墨烯的接触电阻,在电压幅值为3.6V时,降阻效果最优,电阻下降幅度为47.91%。     

Sur quelques facteurs intervenant dans le bombardement de couches pulvérisées

The voltage distributions of the discharges employed in sputtering are reviewed. In bias sputtering the voltage difference between the substrates and the discharge plasma is used to obtain film bombardment by charged particles during growth. This voltage difference, which is the effective bias, is related to the externally applied bias and to the voltage distribution for the discharge in various sputtering systems. It is shown that the effective bias is generally different from the external bias and may even exist without it; this makes the externally applied bias illusory as a process controlling parameter. Both positive and negative biasing is discussed. Due to the inconvenience of using positive values of the effective bias in film deposition, only negative values of the effective bias are dealt with when discussing the measurement of the effective bias and the energy distribution of ions bombarding the films.     

Programmable polymer thin film and non-volatile memory device

Building on the success of organic electronic devices, such as light-emitting diodes and field-effect transistors, procedures for fabricating non-volatile organic memory devices are now being explored. Here, we demonstrate a novel organic memory device fabricated by solution processing. Programmable electrical bistability was observed in a device made from a polystyrene film containing gold nanoparticles and 8-hydroxyquinoline sandwiched between two metal electrodes. The as-prepared device, which is in a low-conductivity state, displays an abrupt transition to a high-conductivity state under an external bias of 2.8 V. These two states differ in conductivity by about four orders of magnitude. Applying a negative bias of 1.8 V causes the device to return to the low-conductivity state. The electronic transition is attributed to the electric-field-induced charge transfer between the gold nanoparticles and 8-hydroxyquinoline. The transition from the low- to the high-conductivity state takes place in nanoseconds, and is non-volatile, indicating that the device may be used for low-cost, high-density memory storage.     

Solid-state memories based on ferroelectric tunnel junctions

Ferroic-order parameters are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1?×?10(6)?A?cm(-2)). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ～1?×?10(4)?A?cm(-2) at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories, and have the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism.     

Anomalies in current-voltage characteristics of YBCO/PBCO/YBCO sandwich junctions

The current-voltage characteristics (IVCs) of YBa₂Cu₃O_7−x/PrBa₂Cu₃O_7−x/YBa₂Cu₃O_7−x sandwich junctions of dimensions 14×20 μm² at the temperatures below 300 K were studied. A few types of pecularities (as sudden junction conductance increase, spreading of resistance, voltage jumps, regions of negative differential resistance, sample voltage polarity change at increasing positive bias current) were observed in IVCs of the sandwich junctions at high bias current (above 15 mA). We found out that all these effects are connected with sample overheating by the bias current and the shape of the temperature dependence of the junction resistance.     

Fundamental theory of piezotronics

Due to polarization of ions in crystals with noncentral symmetry, such as ZnO, GaN, and InN, a piezoelectric potential (piezopotential) is created in the crystal when stress is applied. Electronics fabricated using the inner-crystal piezopotential as a gate voltage to tune or control the charge transport behavior across a metal/semiconductor interface or a p-n junction are called piezotronics. This is different from the basic design of complimentary metal oxide semiconductor (CMOS) field-effect transistors and has applications in force and pressure triggered or controlled electronic devices, sensors, microelectromechanical systems (MEMS), human-computer interfacing, nanorobotics, and touch-pad technologies. Here, the theory of charge transport in piezotronic devices is investigated. In addition to presenting the formal theoretical frame work, analytical solutions are presented for cases including metal-semiconductor contact and p-n junctions under simplified conditions. Numerical calculations are given for predicting the current-voltage characteristics of a general piezotronic transistor: metal-ZnO nanowire-metal device. This study provides important insight into the working principles and characteristics of piezotronic devices, as well as providing guidance for device design.     

Coupled surface-enhanced Raman spectroscopy and electrical conductivity measurements of 1,4-phenylene diisocyanide in molecular electronic junctions

Probing the structure of molecules in a metal-molecule-metal junction under an applied voltage is critical for understanding molecular electron transport properties. We present an approach that allows recording surface-enhanced Raman spectra simultaneously with electrical measurements of a monolayer of molecules in molecular electronic junctions. 1,4-Phenylene diisocyanide in two different types of junctions was used to illustrate the approach. The results show that the molecular integrity was intact in the molecular junctions and under the applied bias. The monolayer sensitivity of the approach provides a new powerful tool for characterizing molecular structure in a molecular electronic junction.     

Morphological, microstructural and electrical examinations on ZnO film on p-Si wafer

This study reports not only main electrical and dielectric characteristics of Ag/ZnO/p-Si heterostructure with the aid of the experimental admittance measurements at room temperature and theoretical approaches but also the microstructure and surface morphology of the heterostructure by means of X-ray diffraction, scanning electron microscopy and atomic force microscopy measurements. The results obtained show that the sample, obtaining Wurtzite structure with the (002) preferred orientation, has a fine crystalline microstructure consisting of micro-sized hexagonal rods growing uniformly in large scale on the film surface. When the diameters of the rods are found to vary from 0.5?μm to 1.5?μm, thickness values are observed to be about 2?μm. Further, series resistance (R_s) and some other electronic parameters of the heterostructure are obtained by the capacitance–voltage (C–V), conductance–voltage (G–V) and C^?2–V measurements. Moreover, voltage (V) and frequency (f) dependence of dielectric parameters such as dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tanδ), real and imaginary parts of electric modulus (Μ′ and Μ″) are determined and discussed. It is found that both electrical and dielectric parameters of the heterostructure prepared in this work depend strongly on the applied bias voltage and frequency.