High Voltage Regime Research Articles

Investigation on temperature dependent DC characteristics of gallium oxide metal-oxide-semiconductor field-effect transistors from 25 °C to 300 °C

High temperature performance of Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) was investigated at temperatures of 25 °C–300 °C. A 30.7% on-state current reduction was observed when the temperature increased from 25 °C to 300 °C. On-state current and peak intrinsic transconductance decreased when temperature increases and followed a power law, indicating that high temperature electron mobility is limited by phonon scattering. Threshold voltage also shift positively with an increase rate of ∼2.3 mV °C−1 when the temperature increased. In terms of off-state currents, traps with an activation energy of 0.42 eV were responsible for increased off-state current at high temperature and high drain voltage regime.

Atomic Force Microscopy Local Oxidation of GeO Thin Films

Metastable germanium monoxide (GeO) thin-insulating films have been investigated as a new promising material for atomic force microscope (AFM) oxidation lithography. The kinetics of GeO layer oxidation performed in the tapping mode of AFM was found to have a logarithmic relationship to oxide height versus pulse duration. Effect of humidity on oxidation of germanium monoxide thin films was studied at relative humidity 40, 60, 80%. When local anodic oxidation of GeO layer was carried out by AFM operating in the contact mode in high voltage (≥9 V) regime and at high relative humidity (RH = 80%), the size and shape of fabricated oxide was changed drastically.

Heterophase materials for fused filament fabrication of structural electronics

In this work, new electrically conductive composite filaments are developed for the fabrication of conductive paths, 3D printed with FDM technology. These composite materials consist of electrically conductive copper powder and a polymer matrix. The influence of three different polymers (ABS, PLA, PS) on the electrical properties of the composites was examined. Electrical measurements of the composite filaments with the increasing copper powder concentrations, allow identifying the percolation threshold for elaborated composites. Results show that the lowest resistivity (0.156 × 10−5 Ωm) was achieved for the ABS/Cu composite at the 84.6 wt% Cu concentration. The obtained resistivity values are much lower than for other conductive composites and nanocomposites filaments reported in the literature. Voltage-current characteristics determined for each composite material showed that composites have Ohmic characteristics in low voltage regime. At high voltage regime, the electrical power dissipated in the composites caused a rapid increase in temperature. It was discovered that a polymer matrix influences the maximum value of the electrical power that can be dissipated in the filament before losing electrical conductivity. Examples of conductive 3D printed structures made from elaborated composites are also presented.

Features of transition of the open pulse–periodical high-voltage discharge in D2 at low pressure into the low-voltage mode

The experimental results on properties of a strongly overvoltage open pulsing discharge being formed in a short gap (3 mm) filled with deuterium at low pressure (P=0.5-2 Torr) are presented. This discharge generates a high-current pulsed-periodical e-beam of 35 mm in diameter with a frequency up to 1.5 kHz and energy up to 25 keV. The repetition frequency influence on the reproducibility of e-beam generation was studied. It was found out that the high-current cathode spots being formed after the transition of the high-voltage regime into low-voltage regime restrict the maximum repetition frequency of a strongly overvoltage open pulsing discharge.

Half-Cell Ion Concentration Polarization on Nafion-Coated Electrode.

On ion-selective membranes, cation/anion-selective transport under electric field initiates ion concentration polarization (ICP); ion concentration increases at one side of the membrane (ion enrichment), whereas it decreases at the other side (ion depletion). This polarization always occurs as the pair of ion enrichment and ion depletion. Departing from such pair generation, we demonstrate that only half of ICP (either ion enrichment or ion depletion) can be solitary on a Nafion-coated electrode. Current-voltage-time responses and conductance measurement capture this half-cell ICP with qualitative in situ pH/ion concentration visualization. In this half-cell, ion depletion hinders an ion flux, whereas ion enrichment facilitates the flux, so a diode-like current rectification is observed even in high-voltage regime (<±200 V) with a rectification factor up to 500. The results in this work give us deeper understanding about ICP on the electrodes and also open the possibility to use half-cell ICP as a high-voltage ionic diode and related sensing/energy applications.

Scaling Effect on Silicon Nitride Memristor with Highly Doped Si Substrate.

A feasible approach is reported to reduce the switching current and increase the nonlinearity in a complementary metal-oxide-semiconductor (CMOS)-compatible Ti/SiNx /p+ -Si memristor by simply reducing the cell size down to sub-100 nm. Even though the switching voltages gradually increase with decreasing device size, the reset current is reduced because of the reduced current overshoot effect. The scaled devices (sub-100 nm) exhibit gradual reset switching driven by the electric field, whereas that of the large devices (≥1 µm) is driven by Joule heating. For the scaled cell (60 nm), the current levels are tunable by adjusting the reset stop voltage for multilevel cells. It is revealed that the nonlinearity in the low-resistance state is attributed to Fowler-Nordheim tunneling dominating in the high-voltage regime (≥1 V) for the scaled cells. The experimental findings demonstrate that the scaled metal-nitride-silicon memristor device paves the way to realize CMOS-compatible high-density crosspoint array applications.

Electronic signature of single-molecular device based on polyacetylene derivative

We reported for polyacetylene chains containing N = 6, 10, 20 carbon atoms making bridges with gold electrodes which exhibit linear and nonlinear current/voltage signatures. The non-equilibrium quantum transport calculations are obtained from (1) low-voltage regime of 0–0.1 V and (2) high-voltage regime of 0–1.0 V. The nonlinear current/voltage behavior for high-voltage regime 0–1.0 V suggests that the system can operate as follows: (1) at 0.27 V there is a resonance captured by the I–V plot; (2) from 0.42 to 0.65 V a plateau is reached presenting a field effect transistor behavior; (3) there is a decrease in conductance from 0.66 V, and a negative differential resonance emerges with minimum value of 0.72 V. The linear I–V behavior will be reviewed, and we discuss the destructive quantum interference status. The effect occurs when we analyze the (1) low-voltage regime, and our results show that the conductance in oscillations maximizes the impact of quantum interferences (QI) on the I–V curve. In the present work, we demonstrate that QI in nanowire molecules is intimately related to the topology of the molecule’s $$\uppi $$ system and establish the existence of QI-induced transmission antiresonances when different voltage regimes are triggered.

Physical Insights on Negative Capacitance Transistors in Nonhysteresis and Hysteresis Regimes: MFMIS Versus MFIS Structures

We present a comprehensive comparison of the two different types of ferroelectric negative capacitance FET (NCFET) structures: metal-ferroelectric-metal-insulator-semiconductor (MFMIS) and metal-ferroelectric-insulator-semiconductor (MFIS). A new segmentation approach is proposed to simulate MFIS NCFET, which correctly takes care of the nonuniformity in potential and horizontal electric field at the ferroelectric–oxide interface. We show that MFMIS NCFET provides a higher ON-current than MFIS NCFET except for the ferroelectrics with very low remnant polarization ( ${P}_{r}$ ) in the high operating voltage regime. We find that this behavior is caused by a reduction or enhancement of the longitudinal electric field in the channel of MFIS structure depending upon ${P}_{r}$ of the ferroelectric and the operating voltage. Moreover, there exists an optimum ${P}_{r}$ which provides maximum ON-current for both the devices. We also find that MFIS NCFET is more prone to hysteresis and starts showing a hysteretic behavior at a lower ferroelectric thickness compared with MFMIS NCFET.

Bipolar resistive switching and nonvolatile memory effect in poly (3-hexylthiophene) –carbon nanotube composite films

The resistive switching behavior and nonvolatile memory effects have been investigated in poly (3-hexylthiophene) (P3HT) and carbon nanotubes (CNT) composite thin films. The memory devices with FTO/P3HT-CNT/Al structure were fabricated by varying the CNT content in the composite. The memory device with 4% CNT in P3HT exhibited a typical bipolar resistive switching with set voltage of ∼1.8 V and a high ON/OFF ratio of >102. The nonvolatile behavior of the device has been studied by the retention test and showed good retention properties for >103 s. The resistance-temperature dependence of different resistance states has also been investigated. Ohmic conduction was found to be a dominant conduction mechanism in low bias region, whereas for the high voltage regime space charge limited conduction was found to be the ruling current conduction mechanism. Bipolar resistive switching characteristic of the nanocomposite film is explained on the basis of rupture and formation of carbon-rich filaments.

Selective production of hydrogen ion species at atomically designed nanotips

Hydrogen scanning ion microscopy systems rely on nanotip gas field ion sources to generate the hydrogen ion beam. The exact structure of the nanotip and the applied electric field are shown to be important. It is demonstrated that hydrogen ion beams are found to occur as mixtures of H+, H2+ and H3+ depending on the electric field strength and the nanotip structure. Various nanotips were prepared, including single atom tips (SATs), trimers and other nano-structured tips to compare the contents of hydrogen ion beams. It was found that single atom tips produce primarily H2+ at low operating voltages, but as the voltage is increased, H3+ dominates. For the trimer case, H2+ becomes a significant species and equals the H3+ current but H3+ can be isolated at higher voltages. For the hexamer tip structure, H2+ almost completely dominates with little H3+ being produced. H+ is only observed in small quantities for all tip structures until a high voltage regime, where apex atom resolution is not observed. Comparisons W SATs and Ir SATs showed similar H3+/H2+ product ratios indicating the nanotip structure plays a key role in the catalytic formation of H3+. Temperature affects are also discussed and operating parameters for single species ion beams are discussed.

Field emission from carbon nanotube fibers in varying anode-cathode gap with the consideration of contact resistance

Abstract

Electrohydrodynamics-Induced Abnormal Electro-Optic Characteristics in a Polymer-Dispersed Liquid Crystal Film

This study demonstrates for the first time abnormal electro-optic (EO) characteristics induced by electrohydrodynamics (EHD) in a polymer-dispersed liquid crystal (PDLC) film in the presence of a low-frequency (1 kHz) AC voltage. Large LC droplets (20−40 µm) buried in the film can be obtained after the illumination of one UV light with a weak intensity (~0.96 mW/cm2) for 12 h. This film exhibits abnormal EO features, including the transmittance’s decay at a high voltage regime at normal incidence and the conversion between polarization independence and polarization dependence for the transmittance-voltage curve at normal and oblique incidences, respectively, of which properties are different from those shown in traditional PDLC films with small droplets. The abnormal EO characteristics of the large-droplet PDLC at the high voltage regime are attributed to a strong scattering effect associated with the formation of the foggy LC droplets in the cell. This effect is induced by a vortex-like LC director field with a rotational axis normal to the cell substrates in each dome-like droplet of the cell at the high voltage regime. The vortex-like director field is induced by a vortex-like turbulence of charged impurity generated by the EHD effect under the action of the AC electric field along the cell normal and the confinement of the dome-like boundary of the droplet on the charged impurities in each droplet. The scattering is decided by the degrees of mismatch between the refractive indices of the LC droplet and polymer, and the local fluctuation of the vortex-like director field in the droplet, resulting in the abnormal EO behaviors of the large-droplet PDLC. This investigation provides novel insight into the EHD effect in three dimensional (3D) microdroplets with anisotropic fluid. Such a large-droplet PDLC has potential in photonic applications, such as electrically controlled polarization-based optical components or optical converters between polarization independence and polarization dependence.

Physics-based analytical model for ferromagnetic single electron transistor

A physically based compact analytical model is proposed for a ferromagnetic single electron transistor (FSET). This model is based on the orthodox theory and solves the master equation, spin conservation equation, and charge neutrality equation simultaneously. The model can be applied to both symmetric and asymmetric devices and does not introduce any limitation on the applied bias voltages. This feature makes the model suitable for both analog and digital applications. To verify the accuracy of the model, its results regarding a typical FSET in both low and high voltage regimes are compared with the existing numerical results. Moreover, the model's results of a parallel configuration FSET, where no spin accumulation exists in the island, are compared with the results obtained from a Monte Carlo simulation using SIMON. These two comparisons show that our model is valid and accurate. As another comparison, the model is compared analytically with an existing model for a double barrier ferromagnetic junction (having no gate). This also verifies the accuracy of the model.

Dependence of Ion Transport on the Electronegativity of the Constituting Atoms in Ionic Crystals.

Ion transport in electrode and electrolyte materials is a key process in Li-based batteries. In this work, we study the mechanism and activation energy of ion transport (Ea ) in rock-salt Li-based LiX (X=Cl, Br, and I) materials. It is found that Ea at low external voltages, where Li-X Schottky pairs are the most favorable defect types, is about 0.42 times the Gibbs energy of formation of LiX compound (ΔGf ). The value of 0.42 is the slope of the electronegativity of anions of binary Li-based materials as a function of ΔGf . At high voltages, where the Fermi level is located very close to the valence band maximum (VBM), electrons can be excited from the VB to Li vacancy-induced states close to the Fermi level. Under this condition, the formation of Li vacancies that are compensated by holes is energetically more favorable than that of Li-X Schottky pairs, and therefore, the activation energies are lower in the former case. The wide range of reported experimental values of activation energies lies between calculated values at low and high voltage regimes. This work motivates further studies on the relation between the activation energy for ionic conductivity in solid materials and the intrinsic ground-state properties of their free atoms.

Local anodic oxidation of solid GeO films: The nanopatterning possibilities

Metastable germanium monoxide (GeO) thin-insulating films have been investigated as a new promising material for oxidation scanning probe lithography. Amorphous GeO films were deposited onto cool Si substrates by thermal evaporation of GeO2<Ge-NCs> heterostructure in vacuum. Properties of GeO films were studied by means of IR spectroscopy, Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy. The nanopatterning of GeO films included three stages. First, AFM-induced local anodic oxidation of GeO layer was used to obtain GeO2 nanowires on Si substrate. After local anodic oxidation in high voltage (≥9 V) regime at 80% relative humidity, the cross-section profile of fabricated GeO2 protrusions contained anomalously high double peaks on a broad base (“two-story shape”). Then, thermal annealing was employed to decompose the GeO film into a GeO2 matrix and Ge nanoclusters. Third, after selective etching of GeO2 from the decomposed GeO film, trenches remained in the porous Ge layer instead of GeO2 nanowires. This may be a potentially useful lithographic approach for fabricating nanoscale structures.

Improved multi-level capability in Si3N4-based resistive switching memory using continuous gradual reset switching

In this letter, we compare three different types of reset switching behavior in a bipolar resistive random-access memory (RRAM) system that is housed in a Ni/Si3N4/Si structure. The abrupt, step-like gradual and continuous gradual reset transitions are largely determined by the low-resistance state (LRS). For abrupt reset switching, the large conducting path shows ohmic behavior or has a weak nonlinear current–voltage (I–V) characteristics in the LRS. For gradual switching, including both the step-like and continuous reset types, trap-assisted direct tunneling is dominant in the low-voltage regime, while trap-assisted Fowler–Nordheim tunneling is dominant in the high-voltage regime, thus causing nonlinear I–V characteristics. More importantly, we evaluate the multi-level capabilities of the two different gradual switching types, including both step-like and continuous reset behavior, using identical and incremental voltage conditions. Finer control of the conductance level with good uniformity is achieved in continuous gradual reset switching when compared to that in step-like gradual reset switching. For continuous reset switching, a single conducting path, which initially has a tunneling gap, gradually responds to pulses with even and identical amplitudes, while for step-like reset switching, the multiple conducting paths only respond to incremental pulses to obtain effective multi-level states.

Cathode solid electrolyte interface’s function originated from salt type additives in lithium ion batteries

This is the study about the cathode’s solid electrolyte interface (SEI) formation mechanism of salt type additives (STAs) and its function. To address this issue, we performed several types of chemical analysis and computer simulation techniques. In order to reveal the redox nature and oxidative decomposition dynamics, the electrolyte (EL) solution dynamics by Quantum mechanics and Molecular mechanics (QM/MM) method was applied. The estimation of SEI chemical components agrees with our chemical analyses data and other group’s reports. The molecular dynamics simulation of sub micro second sampling indicates that the SEI phase induced from STAs functions as a lithium ion selective translocation media and protective coating layer against the degradation of the solvent molecules. The results give us an insight how to design additive’s chemical structure to improve longevity of the cell in the high voltage regime.

Uniform second Li ion intercalation in solid state ϵ-LiVOPO4

Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x–ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4 and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is confirmed by direct comparison between O K–edge absorption spectroscopy and density functional theory.

Viscosity of Liquid Crystal Mixtures in the Presence of Electroconvection

We have experimentally investigated the viscosity of nematic liquid crystal mixtures of p-methoxybenzylidene-p′-n-butylaniline (MBBA) and p-ethoxybenzylidene-p′-cyanoaniline (EBCA) in the presence of electroconvection under an ac electric field with 60 Hz. Although the viscosity of the mixtures with negative dielectric anisotropy shows a characteristic decrease in the high-voltage regime, that with positive dielectric anisotropy shows a monotonic increase as the applied voltage is increased. The experimental results suggest that the decrease in viscosity observed only for the mixtures with negative dielectric anisotropy is attributed to the negative contribution of electric stress caused by the anisotropic director distribution of the turbulent state.

Interface Controls of Lithium-Ion Batteries Electrodes through a Polymerized C60 Surface Coating

The increasing demand on high-energy and high-power lithium-ion batteries (LIBs) especially for electric vehicles application has driven the search on the high-capacity anode and cathode materials. For this purpose, SnO2-based anode materials (theoretical capacity of 900 mAhg-1) and LiCoO2 (theoretical capacity of 274 mAhg-1) have been intensively studied. However, the commercialization of SnO2 anode is still hindered due to the alloying mechanism which leads to severe volume expansion and eventually contributes to disintegrated material. While LiCoO2 has been used as commercial cathode materials, only half of its theoretical capacity (~140 mAhg-1) can be extracted at a cut-off voltage of 3.0-4.2 V. Although increasing cut-off voltage to 4.5 V would significantly boost the specific capacity, however severe side reactions between electrode and electrolyte and a structural phase transition from hexagonal to monoclinic causing structural damages of the electrode are the common issues occurred during the electrochemical cycle. Moreover, unstable solid-electrolyte interphase (SEI) layer on the surface of anode and cathode materials may worsen the situation leading to rapid capacity fading. Surface coating is considered as an appropriate approach in order to overcome the above-mentioned problems owing to its function to enhance interfacial kinetics and protect the host material underneath. Some metal oxides- and carbonaceous-based materials has been used as the coating materials for both anode and cathode materials. In this study, the plasma-polymerized C60 coatings both on the anode and cathode thin film electrodes are presented. The C60 coatings prepared by a radio-frequency plasma-assisted thermal evaporation are carried out on the surface of fluorine-doped SnO2 (C60@SnO2:F) anode and three-dimensional LiCoO2 (C60@3D-LiCoO2) cathode. The samples are characterized by SEM, XRD, TEM, EDX, XPS, ToF-SIMS and assembled into pouch-type and coin cells for anodes and cathodes, respectively. The results demonstrate the improved electrochemical performance of both C60-coated anode and cathode materials. For example, the C60@3D-LiCoO2 electrode delivers an initial discharge capacity of 175 mAhg-1 and maintains the capacity retention of 80% after 50 cycles at 0.1 C even with a high-voltage regime (~ 4.5 V). The C60@SnO2:F exhibits a high initial discharge capacity of 1255 mAhg-1 at 0.15 mAcm-2 and increases Coulombic efficiency of 98%. The improved electrochemical performance for C60@3D-LiCoO2 and C60@SnO2:F is attributed to the facts that the unique plasma-polymerized C60 thin film could provide (1) a mechanical strength to the host materials during repeated cycles directing to the material integrity, (2) a stabilized solid-electrolyte interphase (SEI) layer through a minimized oxygen functional groups in the electrode surface and (3) an enhanced interfacial conductivity leading to a facile Li ion diffusivity. The proposed strategy may open a broad range of application, not only for the two-mentioned electrodes, but also for other anode and cathodes materials of LIBs having similar concerns.