Memristive switching mechanism for metal/oxide/metal nanodevices

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the 'memristor' (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron-ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance.     

Embossed structure embedded organic memory device

Metal filament based organic memory device has unique advantages of long retention time and thermal stability. However, it has suffered from a large variation in the switching delay time (∼ 100 ms), in spite of the fast real switching time of hundreds nanoseconds. Among many possible reasons for the broad delay time, the effect of structural nonuniformity in active area was mainly considered in this work. To solve this problem, we introduced an embossed structure into previous organic memory device, which significantly narrowed the distribution of the delay time. With this device, we could directly observe that the switching preferentially occurs at the summits of the embossed structure through optical microscope image.     

Mechanically induced deterministic 180° switching in nanomagnets

The mechanically induced magnetization switching in nanomagnets is studied by a constraint-free phase field model, which permits exactly constant magnetization magnitude and explicit magneto-mechanical coupling. Depending on the geometry of the nanomagnets, there exist two distinct switching modes: one is the coherent mode where the magnetization vector remains homogeneous during the switching, and the other is the incoherent mode where heterogeneous magnetization distribution occurs. For the application of nanomagnets-based logic and memory devices, the coherent mode is of great interest. Results show that a deterministic 180° switching can happen if mechanical loading is removed once the magnetization rotates to the largest switching angle. The switching time decreases with the magnitude of the applied strain. In addition, the 180° switching under a combination of magnetic field and mechanical strain is also investigated. Simulations demonstrate that an optimum additional strain to reduce the switching time is around 0.2%. The mechanically induced switching is further shown to be damping dependent. A larger damping coefficient is favorable for a faster switching only when the deterministic 180° switching can be guaranteed. This work provides a foundation for the study of mechanically driven/assisted nanomagnets-based logic and memory devices.     

Spectral Radiation Drift of LEDs Under Step-Mode Operation and Its Effect on the Measurement of the Non-Linearity of Radiation Thermometers

The non-linearity (NL) of radiation thermometers is critically involved when realizing ITS-90 above the silver point. It has to be corrected for, and its uncertainty should be adequately specified. In this article, results are presented of NL measurements based upon the superposition method and involving light emitting diodes (LEDs) with high radiance output, peaked at a wavelength of 645 nm. To this end, the two LEDs in question have been operated in the pulse mode with a fixed phase shift. Their spectral radiances have been measured by the radiation thermometers to be tested, separately or superimposed by means of a beam splitter. Still the drift in the spectral radiance observed after switching on the LEDs has to be taken into account and corrected for, since this drift, interfering during the superposition procedure, could corrupt the sum rule for the fluxes involved, which is the crux of the superposition method. Therefore, experimental research has been carried out to characterize the drift of LEDs operated in the pulse mode with a fixed phase shift. The result indicated that the drift could be roughly specified in terms of characteristic time intervals: non-linear drift in the first tens of seconds followed by a quasi-linear drift in the subsequent time interval. The crossover time from non-linear to quasi-linear drift could be ascertained by experiment. In the course of the superposition process, switching of the LEDs was done in such a way that only the quasi-linear part of the drift was involved which allowed for the correction of the drift in the NL measurements, as will be reported. Two radiation thermometers were involved in the NL measurements at radiance temperatures between the silver point and 3153 K: an LP-4, with a central wavelength of 650 nm and 10 nm bandwidth, manufactured by KE-Technologie and the primary standard pyrometer PSP, with a central wavelength of 660 nm and 10 nm bandwidth, developed by NIM.     

An optical channel modeling of a single mode fiber

The evaluation of the optical channel model that accurately describes the single mode fibre as a coherent transmission medium is reviewed through analytical, numerical and experimental analysis. We used the numerical modelling of the optical transmission medium and experimental measurements to determine the polarization drift as a function of time for a fixed length of fibre. The probability distribution of the birefringence vector was derived, which is associated to the ‘Poole’ equation. The theory and experimental evidence that has been disclosed in the literature in the context of polarization mode dispersion – Stokes & Jones formulations and solutions for key statistics by integration of stochastic differential equations has been investigated. Besides in-depth definition of the single-mode fibre-optic channel, the modelling which concerns an ensemble of fibres each with a different instance of environmental perturbation has been analysed.     

Optical characteristics of the compound PLZT

We present a summary of measured characteristics of lanthanum-doped lead zirconium titanate (PLZT) compound in its mechanical housing. It is expected that the PLZT device will be used as the main component in an ultrafast electro-optic switch. We have performed several experiments to measure and calculate the following characteristics: optical power transmission, thermodynamic effects, switching speed, and dc drift phenomenon.     

Photon Statistics of Fluorescence Radiation

We study the general properties of the photon statistics of fluorescence radiation, emitted by a two-level atom in a strong laser field and a perturber bath. We investigate the deviation of the factorial moments from a Poisson distribution, and we show that for long counting times the lowest-order correction can be expressed entirely by the quantity Q, which represents the deviation of the variance from the average. We introduce and evaluate the quantities gn(t), which serve as a measure for the deviation of the higher-order statistics from Poissonian statistics. Subsequently we obtain explicit expressions for the average waiting time for the appearance of the n th photon, after an arbitrary initialization of the counting process. It turns out that the average time delay is again determined by Q. Hence this parameter can be measured in a photon-counting experiment, which involves only the observation of a single photon, rather than an (in principle) infinite number of photons.     

CMOS compatible nanoscale nonvolatile resistance switching memory

We report studies on a nanoscale resistance switching memory structure based on planar silicon that is fully compatible with CMOS technology in terms of both materials and processing techniques employed. These two-terminal resistance switching devices show excellent scaling potential well beyond 10 Gb/cm2 and exhibit high yield (99%), fast programming speed (5 ns), high on/off ratio (10(3)), long endurance (10(6)), retention time (5 months), and multibit capability. These key performance metrics compare favorably with other emerging nonvolatile memory techniques. Furthermore, both diode-like (rectifying) and resistor-like (nonrectifying) behaviors can be obtained in the device switching characteristics in a controlled fashion. These results suggest that the CMOS compatible, nanoscale Si-based resistance switching devices may be well suited for ultrahigh-density memory applications.     

Memory and logic circuits using semiconductor-barrier Josephson junctions

Theoretical and experimental studies on the use of semiconductor-barrier Josephson junctions in switching circuits are reported. This work includes memory loops as well as latching and nonlatching logic circuits. It has been found previously that the switching time of a single junction (67 ps) is comparable with a similar oxide-barrier junction and that the Q of the junction is considerably lower than the oxide-barrier counterpart, as was predicted theoretically, so cavity resonance effects are nearly absent. The memory loop switching time measured is comparable with those employing oxide-barrier junction. The current experimental work on logic circuits is using semiconductor-barrier junctions as they can be made to give the desired values of the McCumber parameter βcrequired to achieve nonlatching operation. In all cases the junctions used are Pb-Te-Pb sandwich structures.     

Unipolar bistable switching of organic non-volatile memory devices with poly(styrene-co-styrenesulfonic acid Na)

We demonstrated unipolar organic bistable memory devices with 8 x 8 cross-bar array type structure. The active material for the organic non-volatile memory devices is poly(styrene-co-styrenesulfonic acid Na) (PSSANa). From the electrical measurements of the PSSANa organic memory devices, we observed rewritable unipolar switching behaviors with a stable endurance and narrow cumulative probability. Also the PSSANa memory devices exhibited a uniform cell-to-cell switching with a high ON/OFF ratio of approximately 10(5) and good retention time of approximately 10(4) seconds without significant degradation.     

Organic nonvolatile memory devices with charge trapping multilayer graphene film

We fabricated an array-type organic nonvolatile memory device with multilayer graphene (MLG) film embedded in polyimide (PI) layers. The memory devices showed a high ON/OFF ratio (over 10(6)) and a long retention time (over 10(4)?s). The switching of the Al/PI/MLG/PI/Al memory devices was due to the presence of the MLG film inserted into the PI layers. The double-log current-voltage characteristics could be explained by the space-charge-limited current conduction based on a charge-trap model. A conductive atomic force microscopy found that the conduction paths in the low-resistance ON state were distributed in a highly localized area, which was associated with a carbon-rich filamentary switching mechanism.     

Observation of Resistive Switching Memory by Reducing Device Size in a New Cr/CrO x /TiO x /TiN Structure

Nano-Micro Letters - The resistive switching memory characteristics of 100 randomly measured devices were observed by reducing device size in a Cr/CrO x /TiO x /TiN structure for the first time....     

Chalcogenide phase-change memory nanotubes for lower writing current operation

We report the synthesis and characterization of Sb-doped Te-rich nanotubes, and study their memory switching properties under the application of electrical pulses. Te-rich nanotubes display significantly low writing currents due to their small cross-sectional areas, which is desirable for power-efficient memory operation. The nanotube devices show limited resistance ratio and cyclic switching capability owing to the intrinsic properties of Te. The observed memory switching properties of this new class of nanostructured memory elements are discussed in terms of fundamental materials properties and extrinsic geometrical effects.     

Bipolar resistance switching characteristics in TiN/ZnO:Mn/Pt junctions developed for nonvolatile resistive memory application

As conventional flash memory is approaching its fundamental scaling limit, there is an urgent demand for an alternative nonvolatile memory technology at present. Resistance-switching random access memory has attracted extensive interests due to its nonvolatile nature, good scalability, and simple structure. In this work, TiN/ZnO:Mn/Pt junctions, which employ a conductive compound TiN as the top electrode to replace regular metal electrodes, were fabricated and investigated for nonvolatile resistive memory applications. These junctions exhibit bistable resistance state at room temperature, and the devices can be reproducibly switched between the two resistance states by applying bidirectional voltage biases. Moreover, both resistance states are demonstrated to retain for more than 10(4) s without electrical power, demonstrating a nonvolatile nature of the memory device. The mechanism of resistance switching effects in TiN/ZnO:Mn/Pt junctions is interpreted in terms of the drift of oxygen vacancies and the resultant formation/annihilation of local conductive channels through ZnO:Mn/Pt Schottky barrier.     

Calculation of the switching constant of magnetic recording media

We studied the dynamic switching time in two classes of media by considering two different particle orientation distribution functions. We calculated the switching constant directly from the Landau-Lifshitz-Gilbert equation of motion, which was chosen to simulate the dynamic properties of the media. A strong linear relation between the reciprocal of the switching time and the difference between the applied and anisotropy fields is illustrated. In media for which experimental results are available, the values we obtained here agree within a factor of 2     

Measurement of useful properties of memory devices

It is customary to measure properties of a memory device in terms of a specific mode of memory operation. Estimating device properties relevant to a different operating mode is usually difficult, if not impossible. At the present time, memory configurations are growing in number and in basic function, ranging widely in size, speed, power, and read/write patterns. It is often necessary to select an available device for a new memory scheme. It is occasionally of interest to select a memory scheme to make best use of a device. A set of simply measured characteristic curves is proposed, which would make it possible to evaluate a given memory device in a large number of operating modes. The quantities involved are the flux, switched by pulses limited in time, amplitude, and number; reversible and irreversible flux switching; and dc properties. Examples of the use of the curves are given.     

线性模型中基于加权残差的渐进变点检测

本文研究线性模型中渐变变点的检验问题．首先,提出了基于加权残差部分和Weighted-CUSUM的检验统计量．其次,证明了原假设下检验统计量依分布收敛于标准布朗桥,备择假设下检验统计量依分布收敛于带有漂移项的布朗桥,并给出检验相合性的条件．最后,模拟结果表明所提方法是可行的．     

Capacity based nondestructive readout for complementary resistive switches

Complementary resistive switches (CRS) were recently suggested to solve the sneak path problem of larger passive memory arrays. CRS cells consist of an antiserial setup of two bipolar resistive switching cells. The conventional destructive readout for CRS cells is based on a current measurement which makes a considerable call on the switching endurance. Here, we report a new approach for a nondestructive readout (NDRO) based on a capacity measurement. We suggest a concept of an alternative setup of a CRS cell in which both resistive switching cells have similar switching properties but are distinguishable by different capacities. The new approach has the potential of an energy saving and fast readout procedure without decreasing cycling performance and is not limited by the switching kinetics for integrated passive memory arrays.     

Investigation of timing properties of a thin multiwire proportional chamber with Csl photocathode

A measurement of the time distribution of signals due to single electrons created at the CsI photocathode in a thin multiwire proportional chamber is described. The full width at half maximum (FWHM) has been measured to be 8.3 ns. Calculations, which include different drift lengths, diffusion and electronic noise, agree well with the measured distribution. A timing resolution of about 1 ns FWHM is expected for Cherenkov radiation in 1 cm of NaF crystal, giving rise to approximately 30 photoelectrons from the CsI photocathode.     

EMD与SLS法在爆破振动加速度信号时域积分中的应用

实测差异分析表明,速度振幅峰值分布受外界因素影响更具规律性,加速度信号主频分布比速度信号更分散。提出一种加速度信号时域积分算法,先基于经验模态分解法对信号进行趋势项剔除、去均值化和高频降噪,再对存在漂移现象的各固有模态函数分量时域积分速度分量进行最小二乘法处理。研究表明:各固有模态函数分量在处理过程中应分别对待;为达到较好降噪效果,阈值修正系数k应不断调整;分段最小二乘法对漂移现象的消除能力优于单段最小二乘法和经验模态分解法。最后,通过本文定义的全局参量和局部参量验证算法的优越性。