Clockwise Switching Research Articles

Dual Resistive Switching Performance Derived from Ionic Migration in Halide Perovskite Based Memory

Herein, we report an environmentally stable and friendly halide perovskite based resistive random access memory device with an Ag/PMMA/(PMA)2CuBr4/FTO (PMMA = poly(methyl methacrylate); PMA = C6H5CH2NH3) architecture. The device exhibits the coexistence of two bipolar resistive switching modes, including counterclockwise and clockwise switching characteristics. The devices with both switching modes show stable endurance (>100 cycles) and long retention performance (>104 s). By applying a suitable electrical stimulation, the counterclockwise and clockwise switching behaviors are interconvertible. Furthermore, the Au/PMMA/(PMA)2CuBr4/FTO and Ag/(PMA)2CuBr4/FTO devices were fabricated to verify the origin of dual resistive switching behaviors. The similar dual resistive switching behaviors after electroforming processes of three types of memory devices suggest that the interconvertible dual resistive switching characteristics could be attributed to the ionic migration in the (PMA)2CuBr4 perovskite layer.

Nonlinear ion drift-diffusion memristance description of TiO2 RRAM devices.

The nature and direction of the hysteresis in memristive devices is critical to device operation and performance and the ability to realise their potential in neuromorphic applications. TiO2 is a prototypical memristive device material and is known to show hysteresis loops with both clockwise switching and counter-clockwise switching and in many instances evidence of negative differential resistance (NDR) behaviour. Here we study the electrical response of a device composed of a single nanowire channel Au–Ti/TiO2/Ti–Au both in air and under vacuum and simulate the I–V characteristics in each case using the Schottky barrier and an ohmic-like transport memristive model which capture nonlinear diffusion and migration of ions within the wire. The dynamics of this complex charge conduction phenomenon is obtained by fitting the nonlinear ion-drift equations with the experimental data. Our experimental results support a nonlinear drift of oxygen vacancies acting as shallow donors under vacuum conditions. Simulations show that dopant diffusion under bias creates a depletion region along the channel which results in NDR behaviour, but it is overcome at higher applied bias due to oxygen vacancy generation at the anode. The model allows the motion of the charged dopants to be visualised during device operation in air and under vacuum and predicts the elimination of the NDR under low bias operation, in agreement with experiments.

Resistive Switching Characteristics of Resistive Random Access Memory Based on a BaxSr1-xTiO3 Thin Film Grown by a Hydrothermal Method

Resistive random access memory (RRAM) is one of the most promising candidates for emerging nonvolatile memory technology. Much effort has been devoted to exploring the resistive switching (RS) mechanism of multioxide films in the RRAM, especially perovskite oxide films. In this letter, we adopted a hydrothermal method to prepare a Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> barium strontium titanate (BST) thin film as a perovskite intermediate layer sandwiched between a top Pt electrode and a bottom TiN/Pt electrode in the RRAM cell. Based on current-voltage measurements, the result illustrated a typical bipolar RS characteristic, and with the increasing cycle count, the clockwise switching mode transformed to a counterclockwise switching mode. The double-logarithm current-voltage curves indicated different mechanisms, including ohmic conduction and space-charge-limited current, in both the set and reset states. A conductive filament (CF) model in which the RS behaviors are dominated by the movement of oxygen vacancies (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> ) in the BST was adopted. In addition, the influence of the compliance current and maximum voltage is considered.

Direct observation of Ag filament growth and unconventional SET-RESET operation in GeTe amorphous films

We report on the direct observation of Ag filament growth and a peculiar resistance switching in amorphous GeTe films with a lateral electrode geometry. The Ag filament growth was monitored by in-situ optical microscopy. The resistance switching was studied in three electrode pairs, Ag–Ag, Pt–Ag, and Pt–Ag/Pt (Ag electrode covered with Pt). In all the three electrode pairs, similar dendritic Ag filaments were clearly observed growing along both directions from one electrode to the other, according to the applied bias polarity. However, the SET and RESET processes are quite different. The Ag-Ag pair produces a unipolar clockwise switching. The Pt–Ag pair shows a bipolar counter-clockwise switching, as predicted in the basic electrochemical metallization theory, but the observed switching polarity is exactly opposite to the basic theory prediction. The Pt–Ag/Pt pair produces a unipolar counter-clockwise switching. The peculiar SET/RESET processes are explained on the basis of strong Ag diffusion into GeTe matrix resulting in an asymmetric effective electrode pair. The findings suggest that the SET/RESET processes are controlled by the amount of Ag and the electrode geometry.

Role of metal-oxide interfaces in the multiple resistance switching regimes of Pt/HfO2/TiN devices

The multiple resistive switching of Pt/HfO2/TiN devices is demonstrated as a result of a competition between the switching at opposite metal/oxide interfaces. Three switching operation modes are demonstrated: clockwise (CW) switching (set for negative voltage and reset for positive voltage at Pt electrode), as already reported in literature for similar material stacks; counterclockwise (CCW) switching and complementary switching (CS) that consist in a set and a reset for increasing voltage of the same polarity. The multiple switching operation modes are enabled by a deep-reset operation that brings the cell resistance close to the initial one. As a consequence, the set transition after a deep-reset occurs at the same voltage and currents as those of the forming and leads to a low resistance state whose resistance can be further decreased in a CCW switching or increased back with a CW switching. With a suitable choice of the stop voltage, a CS in obtained, as well. The coexistence of all three CW, CCW, and CS operations demonstrates that both metal-oxide interfaces are active in the formation and the dissolution of conductive filaments responsible for the switching. All these observations are discussed in terms of a competition between ion migration processes occurring at the opposite metal-oxide interfaces.

Two-Wavelength Switching With a 1310-nm Quantum Dot Distributed Feedback Laser

Two-wavelength switching operation and bistability is reported for the first time with a 1310-nm quantum-dot (QD) distributed feedback (DFB) laser. We demonstrate experimentally the switching of the lasing mode of the QD laser under external optical injection into one of the residual Fabry-Perot (FP) modes of the device. Clockwise switching and bistability occurs for both the lasing and the injected subsidiary modes of the QD DFB laser as the injection strength is increased. Moreover, very high ON-OFF contrast ratio is measured for the switching transition of the lasing mode of the device. We have also analyzed experimentally the influence of two important system parameters, namely the initial wavelength detuning and the applied bias current. The variety of switching behaviors obtained with a 1310-nm QD DFB laser added to the theoretically superior properties of nanostructure lasers offers exciting prospects for novel uses of these devices in all-optical logic and all-optical switching/routing applications in present and future optical telecommunication networks.

A study of resistive switching property in Pr0.7Ca0.3MnO3, CaMnO3, and their bi-layer films

Among the next generation nonvolatile memories, a resistive random access memory is promising due to its simple structure and ultra-fast speed. Pr1−xCaxMnO3 is an attractive resistive random access memory material because it offers resistive switching without a ‘forming process’. However, the resistive switching mechanism has not been fully understood yet. For the application to industry, several barriers of resistive switching materials such as forming process of binary compound, current level, control of resistive switching property, and switching direction have to be overcome. Therefore, this study is progressed for control of resistive switching property using Ti top electrode with Pr0.7Ca0.3MnO3 among the candidate resistive switching materials, CaMnO3 and bi-layer film of these materials. Clockwise switching was observed in all the films, but Pr0.7Ca0.3MnO3, CaMnO3, and the bi-layer films showed definitely different resistive switching property in high resistance state switching of positive voltage and low resistance state switching of negative voltage. From an analysis of the current voltage characteristics, this study demonstrated a difference of a generation mechanism of oxygen vacancy which depends on the resistance of a film and interface and it shows the one method of the switching property control.

Resistive switching characteristics and mechanism of thermally grown WOx thin films

Resistive switching characteristics of thermally oxidized tungsten thin films and their switching mechanism were investigated, modifying thickness of the active layer (WOx) by varying oxidation conditions. Two types of switching were observed in Pt/WOx/W memory devices. Thinner film (t ≤ 15 nm) exhibits clockwise switching (CWS) with filamentary characteristics, whereas thicker film (t ≥ 25 nm) exhibits counter-clockwise switching (CCWS) with more homogeneous conduction. Both switching modes are highly reliable and show good cycling endurance. The conduction phenomena in two different switching modes were examined. In the case of CWS, the conduction mechanism changes from Schottky emission to ohmic conduction due to the local bypass of Schottky barrier formed at Pt/WOx interface by oxygen vacancies. Contrary to CWS, CCWS showed a completely different conduction mechanism. The high resistance state is dominated by the Schottky emission at low electric field and by Poole–Frenkel emission at high electric field, whereas the low resistance state exhibits the Schottky emission. Different types of switching behavior might be attributed to the non-homogenous defect distribution across the active layer. A possible conduction sketch for two types switching behaviors is also discussed.

Bipolar resistance switching of NiO/indium tin oxide heterojunction

We prepared poly-crystalline NiO/indium tin oxide (ITO) heterojunction structure using magnetron sputtering at low temperature (<350 °C) for application to resistance switching memory devices. The NiO/ITO cell showed typical bipolar resistance switching properties similar to those observed in metal/oxide junctions. The switching current decreased as the cell area was reduced. Multi-level resistance states were observed as the maximum applied voltage was varied. The cell could be switched repeatedly for more than 100 cycles. The clockwise switching behavior is attributed to the junction property change due to the field-induced drift of oxygen vacancies in ITO layer and/or oxygen ions in NiO layer.

Coexistence of filamentary and homogeneous resistive switching in graded WOx thin films

AbstractReversible clockwise and counter‐clockwise resistive switching in a Pt/graded WOx /W stack is reported. Two stable switching modes with opposite switching polarity and different electrical characteristics are found to coexist in the same memory cell. Clockwise switching shows filamentary characteristics that lead to relatively faster switching with excellent retention at high temperature. Counter‐clockwise switching exhibits homogeneous conduction with slower switching and moderate retention. The field‐induced switching reversal might be due to inhomogeneous expansion of W during thermal oxidation. Our results provide a clue to modulate the switching type in Pt/WOx /W memory cells. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Switching of the Bacterial Flagellar Motor Near Zero Load

Flagellated bacteria, such as Escherichia coli, are able to swim up gradients of chemical attractants by modulating the direction of rotation of their flagellar motors, which spin alternately clockwise (CW) and counterclockwise (CCW). Chemotactic behavior has been studied under a variety of conditions, mostly at high loads (at large motor torques). Here, we examine motor switching at low loads. Nano-gold spheres of various sizes were attached to hooks (the flexible coupling at the base of the flagellar filament) of cells lacking flagellar filaments in media containing different concentrations of the viscous agent Ficoll. The speeds and directions of rotation of the spheres were measured. Contrary to the case at high loads, motor switching rates increased appreciably with load. Both the CW → CCW and CCW → CW switching rates increased linearly with motor torque. Evidently, the switch senses stator–rotor interactions as well as the CheY-P concentration.

Al electrode dependent transition to bipolar resistive switching characteristics in pure TiO2 films

Stable bipolar resistive switching was demonstrated in polycrystalline TiO2 films involving two different top and bottom Al electrodes of two different structures (Al/TiO2/Pt and Pt/TiO2/Al) after a forming process. With an Al electrode, the transition to bipolar resistive switching was clearly observed, together with counterclockwise and clockwise switching directions, which depended on the position of the Al electrode. The transition from unipolar to bipolar resistive switching seems to be attributable to the redox reaction and trap/detrap at the interfaces between the Al electrode and TiO2 layer due to the migration of oxygen ions and electrons. However, current level analysis of devices reveals that the forming process method basically leads to the formation of conducting paths inside the TiO2 layers. The electrical device properties of the two different structures, the effects of compliance currents, and the operation voltages are also analyzed.

Two-Wavelength Switching With a 1550 nm VCSEL Under Single Orthogonal Optical Injection

We report the first experimental observation of three distinct forms of two-wavelength switching (clockwise, anticlockwise, and butterfly) for a 1550 nm vertical cavity surface emitting laser (VCSEL) operated just above threshold and subject to orthogonally polarized optical injection. The spectrum of the device exhibits two modes, a main mode emitting in a "parallel'' polarization and a subsidiary "`orthogonal'' polarized mode. Low input-power switching of the emission wavelength of the VCSEL by an orthogonally polarized external optical signal injected to the long-wavelength side of the subsidiary mode is observed. Clockwise switching and bistability of the main lasing mode with very high on-off contrast ratio has been measured in all cases. Three different switching shapes: anticlockwise, butterfly, and clockwise, have been measured for the output of the optically injected VCSEL. This diversity of switching behavior with a 1550 nm VCSEL subject to optical injection offers promise for the use of these devices in all-optical switching/routing as well as optical interconnect applications.

The steady-state counterclockwise/clockwise ratio of bacterial flagellar motors is regulated by protonmotive force

A new aspect of the regulation of the bacterial flagellar motor is described. Protonmotive force, i.e. proton electrochemical potential across the cell membrane, not only drives motility, but was found to exert a regulatory effect on the switching of the motor between counterclockwise and clockwise senses. When motor speed was reduced by reducing protonmotive force (but not when speed was reduced by increasing external load), the motor spent a higher fraction of time in counter-clockwise rotation, although the speed in both senses remained equal; at approximately 80% of the maximum motor speed under any given external load condition, no detectable clockwise rotation remained in most cells, i.e. the motor no longer reversed and so cells swam without tumbling. The altered state of the motor remained as long as the state of low protonmotive force remained. Therefore the phenomenon was not a transient tactic response, although it did modify the response to chemotactic stimulation. The regulation of counter-clockwise xxx clockwise switching by protonmotive force appears to be rather general, having been demonstrated in Salmonella, Escherichia coli and Bacillus subtilis. A moderate reduction in protonmotive force from its normal level in actively metabolizing cells did not affect motor reversibility. Motor speed was also unaffected under these conditions (Khan & Macnab, following paper). The existence of a saturation regime with regard to both speed and reversibility suggests that the regulation of motor switching may involve the actual protons driving the motor. All general chemotaxis mutants tested, including those known to be defective in the chemotactic methylation system, were susceptible to protonmotive force regulation in a manner consistent with their normal steady-state counter-clockwise/clockwise ratio. Even mutants that appear “counterclockwise only” could be made more so upon lowering protonmotive force, in that their capacity for clockwise tactic responses was impaired. Since responses to attractants and repellents do not involve an alteration in speed, we conclude that protonmotive force and chemotactic regulatory inputs to the motor operate in parallel. the clockwise/counterclockwise ratio showed an increasing exponential dependence on motor speed. This was a result of both an increased counterclockwise→clockwise switching probability and a decreased clockwise→counterclockwise switching probability. These probabilities were found to be inversely related, so that their product was roughly independent of motor speed. An examination of the pattern of clockwise/counterclockwise switching in tactically stimulated cells and in chemotactic mutants suggests that this is a fundamental property of the motor, which may indicate that switching events in both directions are under the same controlling mechanism. We suggest that suppression of tumbling in circumstances of metabolic deprivation might have survival value by permitting a more rapid dispersal of a population from a region of scarcity of food or oxygen, even when no local gradient is present.