Monostable Switching Research Articles

EMI protection elements on cadmium telluride thin films

The amplitude-time characteristics of cadmium telluride thin films switching were investigated at the influence of single impulses duration 1 μs. It has been founded that with an increase of cadmium telluride layer thickness from 3 μm up to 8 μm, an increase of the operating threshold from 70 V to 105 V is es-tablished. The maximum residual sample voltage was change in the range from 12 V to 40 V, the minimum - from 5 V to 20 V. Samples switching time was no more than 2 nanoseconds; the samples interelectrode capacity does not exceed 2 pF. All test samples were operated without failure up to 20 times. Based on the results of cadmium telluride films structural studies by X-ray diffractometry and scanning electron microscopy we proposed a mechanism of cadmium telluride films with columnar structure monostable switching based on the formation of melted high-conductivity channels in cadmium tel-luride grains oriented in the [111] direction.

The effect of bottom LaB6 electrode and La2O3 interlayer on resistance switching in devices based on Li‐doped ZnO films

Resistance switching (RS) characteristics of Al/ZnO:Li/LaB6 and Al/ZnO:Li/La2O3/LaB6 devices in which LaB6 and lithium‐doped ZnO (ZnO:Li) films are regarded as shallow work function metal and p‐type semiconductor, respectively, are studied. The alternation from bistable unipolar memory switching (URS) to monostable threshold switching (MTS) in the Al/ZnO:Li/LaB6 device is observed. These two switching behaviors can be activated separately depending on the polarity of applied dc voltage: with a positive polarity the URS behavior is measured, while the MTS behavior is observed with a negative polarity. With increase in the number of switching cycles, the MTS and URS behaviors irreversibly transform to bipolar resistance switching (BRS) behavior. The Al/ZnO:Li/La2O3/LaB6 device shows only the BRS behavior, but after certain number of cycles device serves as a rectifying diode. On the basis of the I–V and C–V characteristics, it is concluded that RS properties depend on the barrier height and reactivity between LaB6 metal and ZnO:Li oxide.

Chameleonic electrochemical metallization cells: dual-layer solid electrolyte-inducing various switching behaviours.

We present 'unusual' resistive switching behaviours in electrochemical metallization (ECM) cells utilizing a dual-layer (SiOx/GeSex: SiOx on GeSex) solid electrolyte (SE). The observed switching behaviour markedly varies with the thickness of the upper SiOx layer and compliance current: (i) monostable switching, (ii) counter-eightwise bipolar switching, and (iii) combination of monostable and eightwise bipolar switching behaviours. Focusing on cases (i) and (iii), electrical and chemical analyses on these chameleonic cells were performed in an attempt to gain clues to the understanding of the observed complexity. The chemical analysis indicated the upper SiOx layer as a chemical potential well for Cu ions-Cu ions were largely confined in the well. This non-uniform distribution of Cu across the SE perhaps hints at the mechanism for the complex behaviour; it may be a 'zero-sum game' between SiOx and GeSex layers, in which the two layers fight over the limited number of Cu atoms/ions.

Resistivity switching properties of Li-doped ZnO films deposited on LaB6 electrode

Current–voltage (I–V) characteristics of Al/p-ZnO:Li/LaB6 device, measured in voltage sweep mode, show unipolar resistive switching and monostable threshold switching (URS and MTS) for different bias voltage polarities. URS could be transformed to MTS by application of reverse bias voltage. With increasing number of cycles, URS is converted to bipolar resistive switching mode which is lost after certain number of cycles, and device turns into an ordinary resistor. Analysis of linear fitting I–V curves suggests that ohmic and space charge limited current laws are responsible for conductivity mechanism of Al/p-ZnO:Li/LaB6 device.

Unipolar Resistive Switching in ZrO2 Thin Films

Unipolar resistive switching behaviors including bistable memory switching and monostable threshold switching were found in ZrO2 thin films fabricated by a simple sol–gel method with the Ti/ZrO2/Pt structure. The multilevel resistive switching behaviors were also revealed by varying the compliance current from 9 to 38 mA. Physical mechanisms based on a conductive filament model were proposed to explain the resistive switching phenomena and the device breakdown. A figure of merit Z = ρa/ρf was defined as a criterion for evaluating OFF/ON resistance ratio, where ρf and ρa represent the resistivities of the conductive filament and the fracture region of the filament, respectively. The advantages such as unipolar resistive switching, multilevel resistive switching, good scalability, low operation voltage (<5 V), high OFF/ON resistance ratio (>103), nondestructive readout, long retention (>104 s), and simple fabrication method make the ZrO2-based resistive switching device a promising candidate for next-generation nonvolatile memory applications.

Coexistence of bi-stable memory and mono-stable threshold resistance switching phenomena in amorphous NbOx films

Both bi-stable memory and mono-stable threshold switching are observed in amorphous NbOx films. In addition, the transition between memory and threshold switching can be induced by changing external electrical stress. Raman spectroscopy and transmission electron microscope data show that the NbOx film is self-assembled into a layered structure consisting of a top metal-rich region and a bottom oxygen-rich region. The volume ratio of the two regions depends on the film thickness. Our experimental results suggest that different characteristics of conducting filaments in the two regions result in thickness dependence of switching types and the transition between memory and threshold switching.

A systems-based mathematical modelling framework for investigating the effect of drugs on solid tumours

BackgroundElucidating the effects of drugs on solid tumours is a highly challenging multi-level problem, since this involves many complexities associated with transport and cellular response, which in turn is characterized by highly non-linear chemical signal transduction. Appropriate systems frameworks are needed to seriously address the sources of these complexities, especially from the cellular side.ResultsWe develop a skeletal modelling framework incorporating interstitial drug transport, intracellular signal processing and cell population descriptions. The descriptions aim to appropriately capture the nature of information flow. The model is deliberately formulated to start with simple intracellular descriptions so that additional features can be incorporated in a modular fashion. Two kinds of intracellular signalling modules which describe the drug effect were considered, one a monostable switch and the other a bistable switch. Analysis of our model revealed how different drug stimuli can lead to cell killing in the tumour. Interestingly both modules considered exhibited similar trends. The effects of important parameters were also studied.ConclusionsWe have created a predictive systems platform integrating drug transport and cellular response which can be systematically augmented to include additional layers of cellular complexity. Our results indicate that intracellular signalling models which are qualitatively different can give rise to similar behaviour to simple (and typical) stimuli, and that validating intracellular descriptions must be performed with care by considering a variety of drug stimuli.

Resistive switching transition induced by a voltage pulse in a Pt/NiO/Pt structure

We have observed a switching transition between bistable memory switching and monostable threshold switching in Pt/NiO/Pt structure. Bistable memory switching could be changed to monostable threshold switching by applying a positive electrical pulse with height of 2 V and width between 10−2 and 10−4 s. The change is reversible by applying a negative electrical pulse with the same height and width. By considering polarity- and width-dependence of the switching transition and compositional difference on electrical properties in NiOx, we have proposed a model in which the migration of oxygen ions (O2−) is responsible for the switching transition in Pt/NiO/Pt structures.

Bipolar resistance switching in high-performance Cu/ZnO:Mn/Pt nonvolatile memories: active region and influence of Joule heating

Manganese-doped ZnO dielectric films sandwiched between Cu and Pt electrodes were prepared and investigated for nonvolatile resistive memory applications. These structures exhibit promising bipolar resistive switching (RS) behavior with a large ON/OFF ratio (∼103), suitable threshold voltages (1.4 and −0.7 V for SET and RESET, respectively), long retention (>104 s at 85 °C) and low write current (10 μA). A study on the ZnO:Mn thickness dependence of threshold voltages reveals that RS should be an interfacial effect rather than bulk behavior. By elevating current compliance during the SET process, an anomalous transition from bistable memory switching to monostable threshold switching was observed, which is attributed to the instability of conductive filaments induced by Joule heating effects. Apart from this, fast voltage sweep cycles without efficient heat dissipation were also found to accelerate the hard dielectric breakdown of the device, reflecting the impact of accumulative Joule heating. These results reveal the possible influences of Joule heating effects on bipolar resistance switching and thus the necessity of avoiding them in future high-density memory applications. Conceivable solutions are considered to be reducing the operating currents and improving the heat dissipation of memory devices based on our experiments.

Photorefractive Effect of Polymer-Stabilized Ferroelectric Liquid Crystals

The photorefractive effect of polymer-stabilized ferroelectric liquid crystals (FLC) was investigated. Two-types of monomers, a liquid crystalline monomer and a photoconductive monomer, were mixed with an FLC and the mixtures were photopolymerized in the ferroelectric phase. The polymer-network was formed in the FLC medium and a polymer-stabilized FLC was obtained. The polymer-stabilized FLC exhibited monostable switching properties. The photorefractive effect was evaluated by a two-beam coupling experiment. It was found that the photorefractive effect was strongly dominated by the preparation condition of the polymer-stabilized FLCs.

Molecular Alignment Structure and Switching of a Ferroelectric Liquid Crystal Stabilized by a Polymer Network Created in the SmA Phase

ABSTRACT In order to realize monostability in surface-stabilized ferroelectric liquid crystals, polymer stabilization techniques have been proposed, and the polymer-stabilized ferroelectric liquid crystals (PSFLCs), which is fabricated by UV photocure of doped photocurable mesogenic monomers at a temperature where the FLC medium is in the SmA phase, has been reported to show V-shaped electrooptical characteristics. However, the reason why the monostable V-shaped switching can be realized has not been clear yet. In this research, the electrooptical characteristics, the microscopic textures, the tilt angle and the chevron angle of the PSFLCs photocured in the SmA phase were measured, and then the molecular alignment structure was discussed from these results.

A New Insight on the Bistability and Relevant Phenomena in Ferroelectric Smectic C* Liquid Crystals

ABSTRACT The physics of the bistability and relevant phenomena in smectic C* liquid crystals has been discussed. The problem is treated in terms of the director and not of its projection onto the smectic plane (the so-called c-director). As in the case of nematic liquid crystals, two anchoring energies are considered to be important, the zenithal and the azimuthal ones, both are unpolar. This allows the derivation of a new, basic criterion for the bistability of the director switching in the ferroelectric cells without alignment layers or with strongly conductive alignment layers. In the case of weakly conductive alignment layers, an additional criterion has been found that originates from the internal field in a liquid crystal cell opposite to the external field. This field can cause monostable director switching or an inverse bistability. It is also responsible for the hysteresis-free switching and the electrooptical V-shape effect in the smectic C* phase.

Reproducible resistance switching in polycrystalline NiO films

Negative resistance behavior and reproducible resistance switching were found in polycrystalline NiO films deposited by dc magnetron reactive sputtering methods. Oxygen to argon gas ratio during deposition was critical in deciding the detailed switching characteristics of either bi-stable memory switching or mono-stable threshold switching. Both metallic nickel defects and nickel vacancies influenced the negative resistance and the switching characteristics. We obtained a distribution of low resistance values which were dependent on the compliance current of high-to-low resistance switching. At 200°C, the low-resistance state kept its initial resistance value while the high-resistance state reached 85% of its initial resistance value after 5×105s. We suggested that the negative resistance and the switching mechanism could be described by electron conduction related to metallic nickel defect states existing in deep levels and by small-polaron hole hopping conduction.

Local Layer Structures in Ferroelectric Liquid Crystal Cells Filled with a Material Which Exhibits Cholesteric-Chiral Smectic C Phase Transition

We investigated the local layer structures in ferroelectric liquid crystal (FLC) cells which have a monostable switching mode using synchrotron X-ray micro diffraction. The monostable switching mode is realized by the FLC cells filled with a material which has cholesteric–chiral smectic C (Ch–SmC*) phase transition sequences. On cooling these cells from the isotropic phase to the SmC* phase, the texture of domains and stripes was observed. The X-ray micro diffraction study revealed that the smectic layer bending with the layer shrinkage generate the texture.

Newly Developed Polymer-Stabilized Ferroelectric Liquid Crystals: Microsized Bistable Domains and Monostable V-Shaped Switching

We fabricated new polymer-stabilized ferroelectric liquid crystal displays (PS-FLCDs) by UV photocure of doped monoacrylates with a mesogenic side chain at a temperature where the LC medium is in the SmC* phase under the application of a bipolar square-pulsed AC electric field. Then, we investgated the effect of polymer density on the molecular alignment structure of the PS-FLC by observation of the microscopic textures and measurement of the electrooptical characteristics. The PS-FLC with low concentration of polymer (4 wt%) exhibits bistable electrooptical characteristics accompanied with microsized domain switching. On the other hand, the PS-FLC with high concentration of polymer (10 wt%) exhibits monostable V-shaped electrooptical performance without a threshold.

Superconducting Negative Resistance Switches

A new type of superconducting switch using the negative differential resistance of the CVC (current-voltage characteristic) curve of a nontunneling Josephson junction is suggested. With the proper choice of the load resistor monostable or bistable switching circuits may be designed. The power consumption, the switching speed and other relevant characteristic features of the proposed superconducting switches are analyzed.

Equivalent circuit of Gunn devices operating in the monostable switching mode

An equivalent circuit describing the switching properties of highly doped Gunn devices is deduced from detailed partial differential computer simulations. The circuit elements can be derived from device measurements or stationary calculations. Transients of devices with arbitrary external circuits are calculated by ordinary differential equations. The resulting switching time, current amplitude, and relaxation processes are in good agreement with the partial differential computer simulations and with experimental data.

The characteristics of monostable switching in amorphous niobate films

Describes the preparation of monostable devices using amorphous films of 19 at.% Nb, 13 at.% Na, 2 at.% Si, 66 at.% O. Forming occurs in these devices, similar to the behaviour observed in chalcogenide films. The threshold voltages of both virgin and formed devices are proportional to the square of the electrode separation. This observation has been interpreted as suggesting that excess charge is injected into the device prior to switching. It was found that small DC currents flowing during the on-state do not degrade the characteristics of a monostable device; hence potential applications include use as latching devices in matrix-addressed multi-element displays and other volatile memories.

The temperature dependence of the non-ohmic current and switching characteristics of a chalcogenide glass

The current-voltage characteristics of the blocking state (OFF state) of formed monostable devices of amorphous films of Si12Te48As30Ge10 are described at temperatures between 7 and 140K. Between 7 and 40K it is suggested that the characteristics are consistent with a Fowler-Nordheim description of the current. At higher temperatures it has been supposed that the current is controlled by space-charge existing in traps at the Fermi level. The threshold voltage, holding current and holding voltage have also been examined at temperatures between 7 and 240K. Between 7 and 40K, it is concluded that monostable switching could arise from an impact ionization process.

Monostable switching in amorphous chalcogenide semiconductors

Discusses the thickness dependence of the threshold voltage of amorphous chalcogenide films operated as monostable devices. For pristine films the threshold field is independent of both composition and thickness. Since no correlation is observed with the weak-field resistance of the blocking state, it is proposed that monostable switching of virgin devices arises from impact ionization. In formed devices the threshold voltage, holding current and holding voltage appear to be related to transition temperature. Such behaviour appears to be compatible with thermal avalanching.