Switching Threshold Voltage Research Articles

Impact of scaling to the resistive switching effect in organic polymer – based structures

AbstractThe resistive switching effect has been studied in a set of organic polymer - based structures of a different composition and size scale from macro to micro. It is shown that scaling down reduces both the threshold switching voltage Vth and the respective effective electric field Eth. Furthermore, introduction of metal micro particles into a macro scale polymer matrix provides the same effect. Therefore the metal particle incorporation may be regarded as an alternative method of effective scaling, depending on an application. Switching speed of less than 15 ns, threshold voltage Vth ~ (2 – 25) V, 105 cycle endurance, no significant moisture dependence and high retention time 3.5 months for scaled down samples aswell as for metal doped macro samples have been demonstrated. These characteristics are suitable for constructing memory devices. The switching effect mechanisms are discussed.

Electrical Coupling of Monolithic 3-D Inverters

In this brief, we investigate the electrical coupling between stacked field-effect transistors (FETs) in monolithic 3-D inverters (M3INVs). We study the range of interlayer dielectric (ILD) thickness ( $T_{\text {ILD}}$ ) and channel length $L_{g}$ values that lead to a strong coupling between stacked FETs. Our device simulations show that M3INVs with $T_{\text {ILD}} \geq 50$ nm lead to negligible interaction between the stacked FETs. In addition, our 3-D mixed-mode circuit simulations show that the switching threshold voltages, propagation delays, and fall times of M3INVs with $T_{\text {ILD}} nm are significantly affected by other tiers. This means that new circuit simulation techniques that consider the electrical coupling between the stacked devices are required in monolithic 3-D integrated circuits.

Rational Design of Small Molecules to Implement Organic Quaternary Memory Devices

Organic small‐molecule‐based devices with multilevel electroresistive memory behaviors have attracted more and more attentions due to their super‐high data‐storage density. However, up to now, only ternary memory molecules have been reported, and ternary storage devices may not be compatible with the binary computing systems perfectly. In this work, a donor–acceptor structured molecule containing three electron acceptors is rationally designed and the field‐induced charge‐transfer processes can occur from the donors. Organic quaternary memory devices based on this molecule are successfully demonstrated for the first time. The switching threshold voltages of the memory device are –2.04, –2.73, and –3.96 V, and the current ratio of the “0,” “1,” “2,” and “3” states is 1:101.78:103.47:105.36, which indicate a low possibility of read and write errors. The results represent a further step in organic high‐density data‐storage devices and will inspire the further study in this field.

Hybrid anchoring for a color-reflective dual-frequency cholesteric liquid crystal device switched by low voltages

Cholesteric liquid crystal (CLC) materials used in electro-optical (EO) devices are characterized by high operating voltage and slow response speed, which hinders their further development in display applications. Dual-frequency CLCs (DFCLCs) can solve the problem of slow bistable transition, but the operating voltage is still high, especially in color-reflective DFCLC cells. Here we report a simple approach to lowering the switching voltage as well as to shortening the response time. This technique adopts hybrid surface treatment to modulate the structural arrangement of CLC molecules. Both planar- and vertical-alignment layers are employed and coated on one and the other substrates separately to improve the electro-optical properties of DFCLCs. We show that the threshold voltage for switching can be decreased to as low as 5 V and the shortest response time is measured to be 0.8 ms, which renders CLC EO devices including displays more practical for commercial purpose.

Effect of TiO2 nanoparticle doping on the performance of electrically-controlled nematic liquid crystal core waveguide switch

A liquid crystal (LC) core waveguide has been designed and fabricated using nanocolloid of the nematic liquid crystal 5CB, doped with TiO2 nanoparticles, as the core. Nematic liquid crystals doped with small amount of nanoparticles can have significantly altered electro-optic response. The performance of the fabricated LC core waveguide as an optical-switch has been studied, and the experimental result shows that the threshold voltage for switching is reduced from 1V to 0.25V due to TiO2 nanoparticle dopant-concentration of 2.0wt.%. The overall optimal performance of the waveguide switch, in terms of threshold voltage, extinction ratio, and response time, is achieved for a dopant-concentration of 0.5wt.%.

Intrinsic space charge layers and field enhancement in ferroelectric nanojunctions

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed self-consistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr0.2Ti0.8)O3) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of Pb(Zr0.2Ti0.8)O3. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltages for charge injection, electrochemical and photoelectrochemical processes.

Altering the Position of Phenyl Substitution to Adjust Film Morphology and Memory Device Performance.

In this study, two structural isomers α-PBT and β-PBT, which only differ in the phenyl substituent position on the quinoline chromophore, have been designed and successfully synthesized. The influences of substituent position on the film morphology and the storage performance of the devices were investigated. Both molecules employed in the memory devices exhibited same nonvolatile binary (write-once-read-many-times; WORM) characteristics, but the switch threshold voltage (Vth) of the β-PBT-based device was clearly lower than that of the α-PBT-based device. Simulation results demonstrate that the variation of the phenyl substituent position led to different intermolecular stacking styles and thus to varied grain sizes for each film morphology. This work illustrates that altering the phenyl substituent position on the molecular backbone could improve the quality of the film morphology and reduce power consumption, which is good for the rational design of future advanced organic memory devices (OMDs).

Cellulose-Derivative-Based Gate Dielectric for High-Performance Organic Complementary Inverters.

been realized using organic thin-fi lm transistors (OTFTs) as the essential element. In the majority of these applications OTFTs are integrated either in the form of arrays or as digital logic. The fundamental building block of digital logic is the inverter, a circuit that inverts an input signal. Much work has been devoted in the last fi ve years to design unipolar inverters using p-typeonly OTFTs. [ 18‐23 ] Unfortunately, unipolar logic based on single gate OTFTs always suffers from high power consumption and low noise margin coupled with high noise margin variability and therefore impedes the fabrication of complex circuits having hundreds or more logic gates. [ 19,22,23 ] In this context, it stands

Nematic liquid crystal anchoring strengths of high density polymer brush surfaces

The azimuthal anchoring coefficient A2 of a nematic liquid crystal (NLC) on high-density polymer brushes was investigated using the threshold voltage for the in-plane switching of LC cells consisting of polymer-grafted substrates. A2 was affected by the bulk glass transition temperature (Tg) of the grafted polymers and decreased with increasing temperature. The NLCs on poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(styrene) (PS) brushes had an A2 of 10−5 J m−2 at 25°C. In contrast, the NLC on a poly(hexyl methacrylate) (PHMA) brush exhibited a small A2 (1.1 × 10−6 J m−2) at 25°C and was oriented along the electrodes owing to the brush surface unevenness. With increasing temperature to 90°C, A2 of the NLCs on the PEMA and PS brushes decreased to the values of 10−7 J m−2, and the NLCs were oriented similar to the PHMA brush cell. Although the Tgs of PMMA and PS are similar, at 90°C the A2 of the NLC on the PMMA brush was 4.5 × 10−6 J m−2 and required a magnetic field for LC orientation. Such a difference in A2 is explained by the fact that PS has better compatibility with the NLC than PMMA.

A Model for Multilevel Phase-Change Memories Incorporating Resistance Drift Effects

Phase change memories are emerging as a most promising technology for future nonvolatile, solid-state, electrical storage. However, to compete effectively in mainstream storage applications, a multilevel cell capability is most desirable. Unfortunately, phase-change memories exhibit a temporal drift in programmed resistance (and in threshold switching voltage) which appears to be a fundamental and universal property of the amorphous or partially amorphous phase. Phase-change device models should therefore include these drift effects in a realistic way so that circuit and systems designers can assess the likely performance of multilevel phase-change memories in a variety of potential applications. In this paper, therefore, we present a comprehensive SPICE-based model for phase-change devices that includes the capability for programming into multiple resistance levels, the prediction of the drift of cell resistance (and threshold voltage) with time, and the capability for modeling the randomness inherent to the resistance drift phenomenon. Simulations of multilevel programming and drift phenomena using the model are presented and compared to experimental results, with which there is very good agreement.

Effect of Deposition Rates on Vacuum Deposited Pentacene Thin Films for Memory Device Applications

In this paper, the effect of deposition rates on the vacuum deposited pentacene (C22H14) thin films on ITO coated glass substrate for use as write once read many (WORM) memory devices are reported. IV characteristics of pentacene devices deposited for different deposition rates of 5Å/s, 10Å/s and 20Å/s were studied. Results revealed that the ON/OFF ratio was more for the device with active material deposited at a higher deposition rate of 20Å/s. This device showed a stable switching with an ON/OFF current ratio as high as nearly 108 and a retention time of more than 5x104s with a switching threshold voltage of 1.1V. The irreversible switching of all the devices makes it suitable for a write once read many memory device applications. The structural and morphological studies of these pentacene thin films on glass substrates were also done and the dependence of the grain size of the films with deposition rate is also reported.

The effect of doping Sb on the electronic structure and the device characteristics of Ovonic Threshold Switches based on Ge-Se.

The Ovonic Threshold Switch (OTS) based on an amorphous chalcogenide material has attracted much interest as a promising candidate for a high-performance thin-film switching device enabling 3D-stacking of memory devices. In this work, we studied on the electronic structure of amorphous Sb-doped Ge0.6Se0.4 (in atomic mole fraction) film and its characteristics as to OTS devices. From the optical absorption spectroscopy measurement, the band gap (Eg) was found to decrease with increasing Sb content. In addition, as Sb content increased, the activation energy (Ea) for electrical conduction was found to decrease down to about one third of Eg from a half. As to the device characteristics, we found that the threshold switching voltage (Vth) drastically decreased with the Sb content. These results, being accountable in terms of the changes in the bonding configuration of constituent atoms as well as in the electronic structure such as the energy gap and trap states, advance an effective method of compositional adjustment to modulate Vth of an OTS device for various applications.

Decreasing the energy consumption of memory devices by enhancing the conjugation extent of the terminal electron-donating moieties within molecules.

Three small organic molecules that contained a phenothiazine backbone and triphenylamine (TPA), carbazole (CZ), or anthracene (AN) as a terminal electron donor were synthesized and fabricated in ITO/organic film/Al sandwiched memory devices. The influence of the extent of conjugation in the three molecules on the performance of their corresponding devices was investigated and the results showed that all of the fabricated devices exhibited nonvolatile ternary WORM character, whilst the switch threshold voltages decreased on moving from TPA to CZ and AN, which is promising for low-power-consumption data storage. These results revealed that tailoring the extent of conjugation in the terminal electron donor in the D-A molecules could effectively optimize the device performance, in particular the switch-threshold voltage, which could be instructive for the design of low-energy-consumption memory materials.

Electro-optical and dielectric properties of CdSe quantum dots and 6CHBT liquid crystals composites

We have prepared the composites of a room temperature nematic liquid crystal namely 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and Cadmium Selenide Quantum Dots (CdSe-QDs) and investigated their electro-optical and dielectric properties. Effect of dispersion of CdSe-QDs on various electro-optical and display parameters of host liquid crystalline material have been studied. Physical parameters, such as switching threshold voltage and splay elastic constant have been altered drastically for composites. Dispersion of QDs in a liquid crystals medium destabilizes nematic ordering of the host and decreases the nematic-to-isotropic transition temperature.

Synthesis of Methyl Methacrylate/Acrylic Acid/N, N’-4, 4’-Diphenylmethane-bismaleimide Copolymer and its Electricity Storage Characteristics

Methyl Methacrylate (MMA)/acrylic acid (AA)/N,N’-4,4’-Diphenylmethane-bismaleimide (BMI) copolymer was synthesized by free radical polymerization. The copolymer was characterized by the Fourier Transform Infrared (FTIR), and the memory characteristic of the device was investigated by current-voltage (I-V) test. The structure of the memory device which is based on MMA/AA/BMI copolymer was sandwich, the bottom electrode was indiumtin oxide (ITO) and the top electrode was Al. The ON/OFF current ratio of the ITO/(MMA/AA/BMI copolymer)/Al memory device is ≥1×102 and the switching threshold voltage is about 1.8v.

Facile anionic synthesis of well-defined block copolymers with pendent triphenylamine and ethynylpyridine for nonvolatile memory device applications with high performances

The sequential block copolymerization of 4,4′-vinylphenyl-N,N-bis(4-tert-butylphenyl)benzenamine (A) with 2-(2-(4-vinylphenyl)ethynyl)pyridine (B) was simply carried out using only potassium naphthalenide (K-Naph) as an initiator without any additives in tetrahydrofuran (THF) at −78 °C. The well-defined functional block copolymers containing A block as an electron donor and B block as a weak electron acceptor had predictable molecular weights (Mn = 8,800–14,500 g/mol) and narrow molecular weight distributions (Mw/Mn = 1.09–1.10). The bicontinuous microphase-separated film morphology of the precisely synthesized poly(B-b-A-b-B) (PBAB) with 0.71 of fpoly(A), formed by thermal annealing at 230 °C for 9 h, was expected to be a potential active layer for nonvolatile memory device applications. Indium tin oxide (ITO)/PBAB/aluminum (Al) memory devices with an 8 × 8 cross-bar array structure exhibited nonvolatile resistive switching characteristics. The memory devices showed reliable memory performance in terms of ON/OFF ratios of ∼104, endurance cycles and retention time, and statistical data with regard to cumulative probability of the switching currents and threshold voltage distribution. Filamentary conduction mechanism was proposed to explain the switching of PBAB-based memory devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2625-2632

Effect of memory electrical switching in metal/vanadium oxide/silicon structures with VO2 films obtained by the sol–gel method

Electrical switching and rectifying properties of the metal–VO2–Si structures, on both p-type and n-type silicon, with vanadium dioxide films obtained by an acetylacetonate sol–gel method, are studied. The switching effect is shown to be due to the semiconductor-to-metal phase transition (SMPT) in vanadium dioxide. The shift of the switching threshold voltage, accompanied by the memory effect, in forward bias of the p-Si–VO2 anisotype heterojunction is observed. To explain this effect, a model is proposed which suggests the existence of an additional series resistance associated with a channel at the VO2/Si interface, where a SiOх layer forms during the VO2 deposition process. This resistance is responsible for both threshold switching characteristics, and the memory effect, and the oxygen ion electromigration process is shown to underlie this effect. Potential applications of the observed phenomena, combining the effects of ReRAM and SMPT, in oxide electronics are discussed.

A voltage mode buck DC–DC converter with automatic PWM/PSM mode switching by detecting the transient inductor current

This paper presents a voltage mode buck DC---DC converter that integrates pulse-width modulation (PWM) and pulse-skipping modulation (PSM) to achieve high efficiency under heavy and light load conditions, respectively. Automatic mode-switching is implemented simply by detecting the voltage drop of high-side power switch when it is on, which indicates the transient current flowing through the inductor. Unlike other methods based on average current sensing, the proposed auto-mode switching scheme is implemented based on voltage comparison and simple control logic circuit. In order to avoid unstable mode switching near the load condition boundary, the mode switching threshold voltage is set differently in PWM and PSM mode. Besides, a 16-cycle counter is also used to ensure correct detection of the change in the load condition and fast response of the converter. In addition, a dual-path error amplifier with clamp circuit is also adopted to realize loop compensation and ensure 100 % duty cycle operation. Fabricated in a 0.18-μm standard CMOS technology, the DC---DC converter is able to operate under supply voltage from 2.8 to 5.5 V with 3-MHz clock frequency. Measurement results show that the converter achieves a peak efficiency of 93 %, and an output voltage ripple of less than 40 mV, while the chip area is 1.02 mm2.

Investigations of stochastic resonance in two-terminal device with vanadium dioxide film

The results of stochastic resonance investigation in a nonlinear system, consisting of a microstructure with a polycrystalline vanadium dioxide (VO2) film grown on sapphire and resistor in series are reported. Nonlinearity of the system was provided due to insulator-metal phase transition in VO2. In the stochastic resonance regime at 100 Hz signal frequency, the transition coefficient of signal-to-noise ratio reached 87 in contrast to 250 for microstructures with VO2 films grown on silica in our previous investigations. The measured characteristics of microstructures with VO2 films grown on silica and sapphire substrates were found to be qualitatively similar. For both substrates, a stochastic resonance was observed at threshold switching voltage from insulating to metallic state of VO2. For sapphire substrate the output signal-to-noise ratio rose at higher signal frequencies. The stochastic resonance phenomenon in VO2 films is explained in terms of the monostable damped oscillator model.

Investigation of resistive switching in graphite-like carbon thin film for non-volatile memory applications

Graphite-like carbon films were deposited on Al/SiO2/Si substrates by using Direct Current magnetron sputtering method. Stable and reliable bipolar resistive switching (RS) characteristics were observed in Cu/a-C/Al/SiO2/Si multi-layer structures. An ON/OFF ratio of about 3, a retention time of more than 105 s, and switching threshold voltages of less than 3 V were achieved. I–V properties in both low resistance state and high resistance state can be well explained by the space charge limited current model. The observed RS behaviors are attributed to the electron trapping and detrapping at deep level defects in a-C films.