Nonlinear Current-voltage Characteristics Research Articles

Nanoscale Fabrication of a Three-Dimensional Stack of Graphene Layers Using a Focused Ion Beam

A top-down nanoscale stack fabrication approach involving the high-precision carving of a stack of graphene layers from a thin graphite flake via focused-ion-beam (FIB) three-dimensional (3D) etching is presented. Using this technology, etching was performed in the c-axis with the height of a few tens of nanometres. By reducing the stack height to 50 nm, a 3D stack of graphene layers coupled in a stacked manner was fabricated. The transport characteristics of the stack in the c-axis were studied and compared with the transport characteristics of the stack with a larger in-plane area and stack height value. The observed conductivity of the nanostack with a 50 nm height was one order smaller than the conductivity of the stack with a larger in-plane area and stack height value. Nonlinear current-voltage (I-V) characteristics were observed in all the studied temperatures, but the stack with a larger in-plane area and stack height value showed nonlinear characteristics only at lower temperatures. The observation of these temperature-dependent I-V characteristics was discussed in detail, and the tunneling characteristics were investigated using the Fowler-Nordheim tunneling model, which showed the best fit to the experiment data in this study.

Self-heating and nonlinear current-voltage characteristics in bilayer graphene

We demonstrate by experiments and numerical simulations that the low-temperature current-voltage characteristics in diffusive bilayer graphene (BLG) exhibit a strong superlinearity at finite bias voltages. The superlinearity is weakly dependent on doping and on the length of the graphene sample. This effect can be understood as a result of Joule heating. It is stronger in BLG than in monolayer graphene (MLG), since the conductivity of BLG is more sensitive to temperature due to the higher density of electronic states at the Dirac point.

Enhanced Conductivity and Nonlinear Voltage–Current Characteristics of Nonstoichiometric BaTiO3 Ceramics

The electrical conductivity of both BaO‐deficient and TiO2‐deficient BaTiO3 ceramics shows nonohmic, low‐field characteristics at temperatures >∼200°C in contrast to stoichiometric BaTiO3 for which the electrical conductivity is independent of applied voltage. The nonlinearity is observed in both bulk and grain‐boundary resistances of ceramics that are both porous (∼82%) and nonporous (∼98%) and is not associated with interfacial phenomena such as Schottky barriers and memristors or with charge injection from the electrodes. Results, shown as a function of time over the temperature range 200°–750°C with field gradients in the range ∼0.5–20 V/mm, indicate that an excited state is reached that is time, temperature, and field dependent. This effect appears to be caused by departures from local electroneutrality in the defect structure of nonstoichiometric BaTiO3 which are reduced by electron transfer on application of a dc bias, leading to a more conducting, low‐level excited state in which holes associated with underbonded oxygens, presumably as O− ions, are the principal charge carriers. Ceramics gradually return to their ground state in two stages on removal of the dc bias and the conductivity decreases overall by two to three orders of magnitude.

Electronic Non-Resonant Tunneling through Diaminoacenes: A First-Principles Investigation

The electron transport through diaminoacenes sandwiched between two Au electrodes is simulated by using a first-principles analysis. The nonlinear current-voltage characteristic is observed. Effects of the ring number and positions of amine groups on equilibrium transport properties are found. For 1,4 series, the greater the number of the rings, the stronger the transmission spectrum near the Fermi energy. For 2,6 series, the larger the number of the rings, the weaker the transmission spectrum near the Fermi energy. This is helpful for understanding the recently reported results on conductance measurements using amines.

Fabrication and characterization of well-aligned, high density ZnO nanowire arrays and their realizations in Schottky device applications using a two-step approach

We report a two-step general and viable approach for preparing a large area of high density and horizontally well-aligned arrays of zinc oxide nanowires (ZnO NWs) for the realization of Schottky device applications on inexpensive, flexible polymer substrate. A modified chemical vapor deposition (CVD) process is initially used for synthesizing the highly efficient and rapid growth of vertical ZnO NW arrays along their [0001] direction, which is perpendicular to the donor substrate surface without using any metal catalyze. This is followed by transferring the NWs to a receiver substrate by a dry contact printing method. Utilizing the ZnO NWs synthesized by our method, a fully controllable and relatively large separation between the adjacent rows of silver (Ag) electrodes for the electrical contact with the NWs can be obtained using a photolithographic process. The printed ZnO NWs are well aligned along their c-axis, resulting in a spontaneous polarization which leads to a potential gradient along the length of the individual NW. This coupled with the effect of the surface states in the ZnO NWs result in the formation of a Schottky contact at the Ag/ZnO NW interface. Hence, virtually all of the ZnO NW arrays are functional as Schottky diodes which display non-linear current–voltage characteristics with good rectifying diode-like behaviour.

Sb-doped CuAlO2: widening of band gap and nonlinear J–E characteristics

The effect of antimony doping on the optical and electrical properties of copper aluminum oxide synthesized by solid state reaction technique was investigated. Formation of single phase CuAlO2 was confirmed by x-ray diffraction studies. Variation of strain, particle size, and preferred orientation of the crystallites with Sb doping concentrations were analyzed by x-ray diffraction. Using the x-ray photoelectron spectroscopy trivalent state of Sb ions was confirmed. A blue shift in the energy band gap with Sb doping was observed, and it was supported by ab initio calculation based on density functional theory. A nonlinear current–voltage characteristic was observed for the Sb-doped CuAlO2 samples whereas the same was linear for the undoped samples. The nonlinearity of the current–voltage characteristics changed significantly at higher temperature, and this behavior was explained.

Sequential tunneling transport characteristics of GaN/AlGaN coupled-quantum-well structures

Vertical electronic transport in periodic GaN/AlGaN multiple-quantum-well structures grown on free-standing GaN substrates is investigated. Highly nonlinear current-voltage characteristics are measured, displaying a clear transition from a high-resistance state near zero applied bias to a low-resistance state as the voltage is increased. The measurement results, including their temperature dependence and the variations in turn-on voltage with subband structure and bias polarity are in full agreement with a picture of sequential tunneling through the ground-state subbands of adjacent coupled quantum wells. Scattering-assisted tunneling due to interface roughness or structural defects appears to be the dominant transport mechanism. The potential role of photon-assisted tunneling is also investigated.

Non-linear current–voltage characteristics of (La 0.5Eu 0.5) 0.7Pb 0.3MnO 3 single crystals: Possible manifestation of the internal heating of chargecarriers

Temperature evolution of the current–voltage characteristics (CVCs) of a single-crystal lanthanum manganite (La 0.5Eu 0.5) 0.7Pb 0.3MnO 3 is investigated in a wide (up to 1 А) range of instrumental currents. Effects of a transport current and an applied electric field on the resistance of the material are studied in view of possible implementation of the charge ordering break in dielectric regions occurring due to phase separation in manganites. A negative differential resistance portion observed in the CVCs suggests the presence of a current switching effect. Below the temperature of the metal–dielectric transition in (La 0.5Eu 0.5) 0.7Pb 0.3MnO 3, the hysteresis is observed in the CVC. The detailed analysis of the internal sample heating on the basis of experimental data on thermal conductivity showed, however, that these CVC features can be explained within the concept of non-equilibrium heating of electron gas.

NEW POSSIBILITIES FOR OBTAINING STEEPLY NONLINEAR CURRENT–VOLTAGE CHARACTERISTICS IN SOME SEMICONDUCTOR STRUCTURES

Electronic processes in a semiconductor system consisting of some resonant tunneling structures, built in the depletion region of a Schottky barrier, are investigated. It is shown that the Schottky barrier can block or unblock the resonant tunneling current effectively. Tunneling processes do reveal the coherent character. Sharply nonlinear current–voltage characteristics are observed for both the forward and the reverse branches.

Effect of vanadium doping on the dielectric and nonlinear current–voltage characteristics of CaCu 3Ti 4O 12 ceramic

Here we report the effect of vanadium doping on the dielectric and electrical properties of giant dielectric material CaCu 3Ti 4O 12 (CCTO), synthesized through conventional solid-state reaction process. Proper crystalline phase formation together with dopant induced lattice constant shrinkage was confirmed through X-ray diffraction studies. The X-ray photoelectron spectroscopic studies confirmed vanadium doping with V 4+ replacing Ti 4+ at its lattice site. The grain boundary resistivity was found to decrease monotonically with the increase of V doping percentages as revealed by impedance spectroscopic measurement and furthermore the grain size was found to follow the similar trend. The reduced grain boundary resistivity was found to be responsible for the overall variation of current density–electric field ( J– E) characteristics.

Improvement of Surface Flashover Performance in Vacuum of A-B-A Insulator by Adopting ZnO Varistor Ceramics as Layer A

In this paper, a new insulation system of ZnO varistor ceramic-Al2O3 ceramic-ZnO varistor ceramic insulators (defined as A-B-A insulators) is proposed to improve surface flashover performance in vacuum so that it is possible to reduce the electric field at the vacuum-insulator-cathode triple junction (CTJ) by changing the conductivity and permittivity of the ZnO varistor ceramic layer of the A-B-A insulators. Then, the influences of the conductivity and permittivity of ZnO varistor ceramics on the dc and impulse surface flashover voltage in vacuum were experimentally investigated, respectively. It was found that the dc surface flashover voltage of the samples increases greatly with a decrease of the dc conductivity of the ZnO varistor ceramic layer and is improved by 81% compared with the Al2O3 ceramic sample. It is attributed to the decrease of the electric field at the vacuum-layer A-CTJ caused by the conductivity of layer A larger than that of layer B and, furthermore, by the nonlinear current-voltage characteristics of layer A, i.e., ZnO varistor ceramics. Additionally, the impulse surface flashover voltage first increases and then decreases with a decrease of the permittivity of the ZnO varistor ceramic layer and is increased by 136%. It is explained by the decrease of the electric field at the triple junction by the permittivity of layer A larger than that of layer B and, furthermore, by the nonlinear current-voltage characteristics of layer A. These results demonstrate the insulation system of the ZnO varistor ceramic-Al2O3 ceramic-ZnO varistor ceramic insulators as an effective way to improve surface flashover performance in vacuum.

Changing nanoslot ion flux with a dynamic nanocolloid ion-selective filter: Secondary overlimiting currents due to nanocolloid-nanoslot interaction

Nanocolloids trapped at the depleted side (anodic) of a fluidic nanoslot entrance are shown to sensitively regulate dc ion transport through the nanoslot, such that a second limiting-overlimiting transition occurs in its nonlinear current-voltage characteristics. The nanocolloids, brought to the entrance by electro-osmosis, are not stationary but are confined to closed circular and toroidal streamlines, driven by a back-pressure corner vortex and an orthogonal electroconvection vortex instability. The transition from the corner vortex to a complex torus with both vortical motions coincides with the first overlimiting transition, while electrostatic interaction of nanocolloids in these vortices with the nanoslot entrance drives the second limiting transition.

Limiting and overlimiting conductance in field-effect gated nanopores

Numerical modeling of the coupled ionic and fluidic transport in field-effect gated nanopores reveals highly nonlinear current-voltage characteristics, including cross-over, rectification, and particularly limiting and overlimiting conductance. The limiting and overlimiting characteristics are shown to be greatly enhanced by the inherently coupled fluid flow and correlate with electrokinetic phenomena such as concentration polarization and vortex formation. The underlying reason for the observed nonlinear characteristics is explained by considering the symmetry properties of the electrical biasing.

Universal resistance capacitance crossover in current-voltage characteristics for unshunted array of overdamped Nb–AlOx–Nb Josephson junctions

We report on some unusual behavior of the measured current-voltage characteristics (CVC) in artificially prepared two-dimensional unshunted array of overdamped Nb–AlOx–Nb Josephson junctions. The obtained nonlinear CVC are found to exhibit a pronounced (and practically temperature independent) crossover at some current Icr=(1/2βC−1)IC from a resistance R dominated state with VR=RI2−IC2 below Icr to a capacitance C dominated state with VC=ℏ/4eCI−IC above Icr. The origin of the observed behavior is discussed within a single-plaquette approximation assuming the conventional resistively shunted junction model with a finite capacitance and the Ambegaokar–Baratoff relation for the critical current of the single junction.

Crystalline Structures and Electrical Properties of High-Nitrogen-Content Hf–Si–N Films

The crystalline structures and electrical properties of high-nitrogen (N)-content Hf–Si–N films were investigated. When the N2 concentration of a sputtering ambient increases from 9.0 to 13.0%, Hf–Si–N resistivity increases by a factor of more than 105 despite the almost constant N content of the film. According to X-ray diffraction profiles and X-ray photoelectron spectra, such high-N-content Hf–Si–N films consist of Hf3N4 and Si3N4, and have energy band gaps. These results indicate that Hf3N4 and high-N-content Hf–Si–N including Hf3N4 have semiconducting features. In Pt/Hf–Si–N/Pt structures with high-N-content Hf–Si–N, nonlinear current–voltage characteristics and hysteresis behaviors are also observed, which markedly change depending on N2 concentration and postdeposition annealing temperature. Although the elemental composition and crystalline structure hardly change, the phase separation and segregation of nanocrystallites clearly develop. Therefore, these unique electrical characteristics are attributed to current conduction at grain boundaries or Pt/Hf–Si–N interfaces.

The sharply nonlinear current-voltage characteristic of a structure with a quantum well built in the depletion region of a Schottky Barrier

The heterostructure in which a quantum well is formed in the depletion region of the Schottky barrier is considered. The forward current-voltage characteristic of this structure is calculated and analyzed. It is shown that an N-shaped sharply nonlinear current-voltage characteristic is observed in a wide range of typical values of parameters of the structure under consideration. The characteristic has a clearly pronounced region with a negative differential resistance and a high peak-to-valley ratio.

Admittance Spectroscopy of Y<sub>2</sub>O<sub>3</sub>-Doped ZnO Varistors Sintered at Different Temperature

ZnO varistors are multicomponent polycrystalline ceramics with highly nonlinear current-voltage characteristics and surge energy absorption capabilities. The voltage gradient of the ZnO varistor is inversely proportional to its average grain size. Recently, the rare-earth oxides have been reported as growth inhibitor of ZnO grains to obtain high voltage gradient. Although the dopant of rare-earth oxides can remarkably enhance the voltage gradients of varistor samples, their leakage currents and nonlinear coefficients deteriorate at the same time. In this paper, the sintering effects on electrical characteristics and admittance spectroscopies of ZnO varistor samples doped with yttrium oxides were investigated. Samples were fabricated under different sintering temperatures, including 1000 °C, 1100 °C, 1200 °C and 1300 °C. Then, the electrical characteristics and admittance spectroscopies of these varistor samples were measured. The measured current-voltage results behave special U-type curves related to sintering temperature. Furthermore, the admittance spectroscopy of these samples revealed the sintering effects on the intrinsic defects of ZnO varistors.

Non-linear current–voltage characteristics in α-(BEDT-TTF) 2I 3

Current–voltage ( I– V) characteristics have been measured to clarify the charge transport mechanism in the charge order state of a two dimensional organic conductor α-(BEDT-TTF) 2I 3. At low temperatures, we find strong non-linear I– V characteristics that follow the power law approximately. The non-linear I– V characteristics are attributed to the electric-filed-assisted unbinding of excited pairs of an electron and a hole that attract each other due to two-dimensional long-range Coulomb interaction. We also report a larger anisotropy in the dielectric constant, which shows the highly two-dimensional nature of the Coulomb interaction in the BEDT-TTF layers.

A Neural-Network-Based Method of Modeling Electric Arc Furnace Load for Power Engineering Study

It is known that artificial neural network is a powerful scheme for function learning and modeling nonlinear loads. However, a direct application of artificial neural network for modeling time-varying loads may lead to inaccuracies. This paper presents an accurate neural-network-based method for modeling the highly nonlinear voltage-current characteristic of an ac electric arc furnace (EAF). The neural-network-based model can be effectively used to assess waveform distortions, voltage fluctuations, and performances of reactive power compensation devices associated with the EAF in a power system. Simulation results obtained by using the proposed model are compared with the actual measured data and two other traditional neural network models. It is shown that the proposed method yields favorable performance and can be applied for modeling similar types of nonlinear loads for power engineering studies.

Rapid Sequencing of Individual DNA Molecules in Graphene Nanogaps

I propose a technique for reading the base sequence of a single DNA molecule using a graphene nanogap to read the DNA's transverse conductance. Because graphene is a single atom thick, single-base resolution of the conductance is readily obtained. The nonlinear current-voltage characteristic is used to determine the base type independent of nanogap-width variations that cause the current to change by 5 orders of magnitude. The expected sequencing error rate is 0% up to a nanogap width of 1.6 nm.