Conductance Characteristics Research Articles

Analog memory and spike-timing-dependent plasticity characteristics of a nanoscaletitanium oxide bilayer resistive switching device

We demonstrated analog memory, synaptic plasticity, and a spike-timing-dependentplasticity (STDP) function with a nanoscale titanium oxide bilayer resistive switchingdevice with a simple fabrication process and good yield uniformity. We confirmed themultilevel conductance and analog memory characteristics as well as the uniformity andseparated states for the accuracy of conductance change. Finally, STDP and a biologicaltriple model were analyzed to demonstrate the potential of titanium oxide bilayer resistiveswitching device as synapses in neuromorphic devices. By developing a simple resistiveswitching device that can emulate a synaptic function, the unique characteristics ofsynapses in the brain, e.g. combined memory and computing in one synapse and adaptationto the outside environment, were successfully demonstrated in a solid state device.

Fabrication of Silicon Nanowire for Detecting <I>β</I>-Amyloid (1-42) by Nanoimprint Lithography

Ultraviolet nanoimprint lithography (UV-NIL) is a high volume and cost-effective patterning technique with sub-10 nm resolution. It has great potential as a candidate for next generation lithography. Using UV-NIL, nanowire patterns were successfully fabricated on a four-inch silicon-on-insulator (SOI) wafer under moderate conditions. The fabricated nanowire patterns were characterized by FE-SEM. Its electrical properties were confirmed by semiconductor parameter analysis. Monoclonal antibodies against beta-amyloid (1-42) were immobilized on the silicon nanowire using a chemical linker. Using this fabricated silicon nanowire device, beta-amyloid (1-42) levels of 1 pM to 100 nM were successfully determined from conductance versus time characteristics. Consequently, the nanopatterned SOI nanowire device can be applied to bioplatforms for the detection of proteins.

Fabrication of high-Ge-fraction strained Si 1−xGe x/Si hole resonant tunneling diode using low-temperature Si 2H 6 reaction for nanometer-order ultrathin Si barriers

Low-temperature Si barrier growth with atomically flat heterointerfaces was investigated in order to improve negative differential conductance (NDC) characteristics of high-Ge-fraction strained Si 1− x Ge x /Si hole resonant tunneling diode with nanometer-order thick strained Si 1− x Ge x and unstrained Si layers. Especially to suppress the roughness generation at heterointerfaces for higher Ge fraction, Si barriers were deposited using Si 2H 6 reaction at a lower temperature of 400 °C instead of SiH 4 reaction at 500 °C after the Si 0.42Ge 0.58 growth. NDC characteristics show that difference between peak and valley currents is effectively enhanced at 11–295 K by using Si 2H 6 at 400 °C, compared with that using SiH 4 at 500 °C. Non-thermal leakage current at lower temperatures below 100 K tends to increase with decrease of Si barrier thickness. Additionally, thermionic-emission dominant characteristics at higher temperatures above 100 K suggests a possibility that introduction of larger barrier height (i.e. larger band discontinuity) enhances the NDC at room temperature by suppression of thermionic-emission current.

Fast DNA sequencing with a graphene-based nanochannel device

Devices in which a single strand of DNA is threaded through a nanopore could be used to efficiently sequence DNA. However, various issues will have to be resolved to make this approach practical, including controlling the DNA translocation rate, suppressing stochastic nucleobase motions, and resolving the signal overlap between different nucleobases. Here, we demonstrate theoretically the feasibility of DNA sequencing using a fluidic nanochannel functionalized with a graphene nanoribbon. This approach involves deciphering the changes that occur in the conductance of the nanoribbon as a result of its interactions with the nucleobases via π-π stacking. We show that as a DNA strand passes through the nanochannel, the distinct conductance characteristics of the nanoribbon (calculated using a method based on density functional theory coupled to non-equilibrium Green function theory) allow the different nucleobases to be distinguished using a data-mining technique and a two-dimensional transient autocorrelation analysis. This fast and reliable DNA sequencing device should be experimentally feasible in the near future.

Topological Effect on Spin Transport in a Magnetic Quantum Wire: Green's Function Approach

We explore spin dependent transport through a magnetic quantum wire which is attached to two non-magnetic metallic electrodes. We adopt a simple tight-binding Hamiltonian to describe the model where the quantum wire is attached to two semi-infinite one-dimensional non-magnetic electrodes. Based on single particle Green's function formalism all the calculations are performed numerically which describe two-terminal conductance and current-voltage characteristics through the wire. Quite interestingly we see that, beyond a critical system size probability of spin flipping enhances significantly that can be used to design a spin flip device. Our numerical study may be helpful in fabricating mesoscopic or nano-scale spin devices.

Theoretical study on switching characteristic of photoisomers

The electron transport properties of the single molecular junctions composed of isomers of 4,4-(ethene-1,2-diyl) dibenzoic acid and two gold electrodes are investigated systematically using elastic scattering Green's function method. The conductance characteristics are quite different for the two isomers which can be used to construct molecular switch. Our study shows that the molecules are chemically adsorbed on the gold surface by the terminal carboxyl group. Each molecular junction has three different stable conductances, which correspond to the different contact structures between the molecules and electrodes. The different conductances result mainly from the variation of the electronic structures, caused by the interaction between the molecules and the electrodes. Our theoretical simulation is consistent with the experimental measurement.

Coulomb oscillations effect in dual gate controlled silicon nanowire

The tunable single electron effect and Coulomb oscillations were observed in Si nanowire transistors. By measuring the channel current as function of applied back-gate and side-gate voltage, the tunable single electron effect and Coulomb oscillations are investigated. From the differential conductance characteristics, the Coulomb diamonds are clearly observed due to the gate voltage-induced quantum dots formation in the Si nanowire.

Tunneling characteristics of graphene

Negative differential conductance and tunneling characteristics of two-terminal graphene devices are observed before and after electric breakdown, respectively. The former is caused by the strong scattering under a high E-field, and the latter is due to the appearance of a tunneling barrier in graphene channel induced by a structural transformation from crystalline graphene to disordered graphene because of the breakdown. Using Raman spectroscopy and imaging, the presence of nonuniform disordered graphene is confirmed. A memory switching effect of 100 000% ON/OFF ratio is demonstrated in the tunneling regime, which can be employed in various applications.

Dynamical mean-field theory for molecular electronics: Electronic structure and transport properties

We present an approach for calculating the electronic structure and transport properties of nanoscopic conductors that takes into account the dynamical correlations of strongly interacting d- or f-electrons by combining density functional theory calculations with the dynamical mean-field theory. While the density functional calculation yields a static mean-field description of the weakly interacting electrons, the dynamical mean-field theory explicitly takes into account the dynamical correlations of the strongly interacting d- or f-electrons of transition metal atoms. As an example we calculate the electronic structure and conductance of Ni nanocontacts between Cu electrodes. We find that the dynamical correlations of the Ni 3d-electrons give rise to quasi-particle resonances at the Fermi-level in the spectral density. The quasi-particle resonances in turn lead to Fano lineshapes in the conductance characteristics of the nanocontacts similar to those measured in recent experiments of magnetic nanocontacts.

Direct transformation of a resist pattern into a graphene field effect transistor through interfacial graphitization of liquid gallium

The authors found that an extremely thin resist pattern on a silicon dioxide can be directly transformed into a graphene channel through interfacial graphitization of liquid gallium. These patterned graphene field effect transistors show p-type field effect conductance characteristics and a maximum conductance modulation of 100% against an applied gate voltage range from −50 to +50 V at room temperature, which is almost identical to the on/off ratio of 2. These conductance modulation ratios improved with decreasing the initial resist thickness below 2 nm; however, the absolute value of the channel conductance also deteriorated with decreasing the resist thickness, suggesting that electron scattering at the domain boundary dominates the channel conductance.

Room-Temperature Resonant Tunneling Diode with High-Ge-Fraction Strained Si1-xGex and Nanometer-Order Ultrathin Si

For the purpose of heterointegration of Si-based group IV semiconductor quantum effect devices into Si large-scale integrated circuit, formation of atomically flat heterointerfaces in quantum heterostructure by lowering Si barrier growth temperature was investigated in order to improve negative differential conductance (NDC) characteristics of high-Ge-fraction strained Si1-xGex/Si hole resonant tunneling diode. It was found that roughness generation at heterointerfaces is drastically suppressed by utilizing, Si barriers with nanometer order thickness deposited using Si2H6 reaction at a lower temperature of 400oC instead of SiH4 reaction at 500oC after the Si0.42Ge0.58 growth. NDC characteristics show that difference between peak and valley currents is effectively enhanced at 11-295K by using Si2H6 at 400oC, compared with that using SiH4 at 500oC. Thermionic-emission dominant characteristics at higher temperatures above 100 K indicates a possibility that introduction of larger barrier height (i.e., larger band discontinuity) enhances the NDC at room temperature by suppression of thermionic-emission current.

Reduction of Graphene Oxide via Bacterial Respiration

Here we present that graphene oxide (GO) can act as a terminal electron acceptor for heterotrophic, metal-reducing, and environmental bacteria. The conductance and physical characteristics of bacterially converted graphene (BCG) are comparable to other forms of chemically converted graphene (CCG). Electron transfer to GO is mediated by cytochromes MtrA, MtrB, and MtrC/OmcA, while mutants lacking CymA, another cytochrome associated with extracellular electron transfer, retain the ability to reduce GO. Our results demonstrate that biodegradation of GO can occur under ambient conditions and at rapid time scales. The capacity of microbes to degrade GO, restoring it to the naturally occurring ubiquitous graphite mineral form, presents a positive prospect for its bioremediation. This capability also provides an opportunity for further investigation into the application of environmental bacteria in the area of green nanochemistries.

Composite solid electrolytes

Studies of composite ionic conductors are overviewed. Mechanisms of defect formation at ionic crystal surfaces and at interphase boundaries in the composites are discussed; the Stern model that allows calculating surface potential has been involved. Methods for the calculating of the composite’s electrical conductance and other physicochemical characteristics are suggested. Thermodynamic stability of nanocomposites and the genesis of their morphology during sintering are analyzed. General regularities of changes in ionic-salt properties over wide range of the “ionic salt-oxide” systems, as well as size effects are discussed.

Analysis of trapping effects in AlGaN/GaN HEMTs based on near zero bias output conductance

AbstractA simple model is developed for characterization of current collapse caused by electron trapping in AlGaN/GaN HEMTs. Threshold voltages are extracted from near‐zero drain bias output conductance characteristics with respect to gate bias based on 0.2 μs pulsed I –V characteristics at different quiescent bias points. Other device parameters such as effective gate length and electron mobility are also estimated by fitting to a model equation. The results show that a higher negative electric field between gate and drain contacts cause a higher threshold voltage shift in the positive direction and a longer effective gate length, resulting in a larger current dispersion due to trapping of more electrons. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved thermal stability of nitrogen annealed sputtered hafnium oxide thin films for VLSI technology

PurposeHigh‐dielectric thin films are considered as future dielectric for Si based advanced integrated circuit technology and also for the development of organic thin film transistors and micro sensors. The conventional dielectric SiO2 is grown by thermal oxidation of silicon whereas the HfO2 films are grown by both physical and chemical methods. Depending on film deposition technique, the film and interface characteristics are affected. The purpose of this paper is to investigate the effect of thermal annealing in oxygen and nitrogen ambient on the electrical properties of HfO2‐based metal oxide semiconductor (MOS) capacitor and evaluate thermal stability of the characteristics.Design/methodology/approachHfO2 films are deposited by rf sputtering and Al‐HfO2‐Si MOS capacitor fabricated. The electrical I‐V and C‐V characteristics are measured and the effect of temperature in the range of 25‐200°C is evaluated for films annealed in oxygen and nitrogen.FindingsIt is found that thermal annealing in nitrogen reduces oxide trap density and improves the temperature stability compared to the film annealed in oxygen for MOS devices. From the conductance characteristics Dit oxide trap density in the film is estimated to be 2.1×1011/cm2 for nitrogen and 3.23×1011/cm2 for oxygen, which indicates the role of nitrogen in reducing oxide traps. The thermal activation energy of electron traps is found to be about 0.19 eV for nitrogen and 0.58 eV for oxygen annealed films in the temperature range of 25‐150°C.Originality/valueThe paper examines and compares the role and effect of thermal annealing in nitrogen ambient and oxygen ambient on the electrical properties of sputtered HfO2 thin film for improved MOS device reliability.

Electron density and transport in top-gated graphene nanoribbon devices: First-principles Green function algorithms for systems containing a large number of atoms

The recent fabrication of graphene nanoribbon (GNR) field-effect transistors poses a challenge for first-principles modeling of carbon nanoelectronics due to many thousand atoms present in the device. The state of the art quantum transport algorithms, based on the nonequilibrium Green function formalism combined with the density functional theory (NEGF-DFT), were originally developed to calculate self-consistent electron density in equilibrium and at finite bias voltage (as a prerequisite to obtain conductance or current-voltage characteristics, respectively) for small molecules attached to metallic electrodes where only a few hundred atoms are typically simulated. Here we introduce combination of two numerically efficient algorithms which make it possible to extend the NEGF-DFT framework to device simulations involving large number of atoms. We illustrate fusion of these two algorithms into the NEGF-DFT-type code by computing charge transfer, charge redistribution and conductance in zigzag-GNR/variable-width-armchair-GNR/zigzag-GNR two-terminal device covered with a gate electrode made of graphene layer as well. The total number of carbon and edge-passivating hydrogen atoms within the simulated central region of this device is ~7000. Our self-consistent modeling of the gate voltage effect suggests that rather large gate voltage might be required to shift the band gap of the proposed AGNR interconnect and switch the transport from insulating into the regime of a single open conducting channel.

Technology of MISFET with SiO2/BaTiO3 System as a Gate Insulator

The properties of barium titanate (BaTiO3, BT), such as high dielectric constant and resistivity, allow it to find numerous applications in the field of microelectronics. In this work silicon metal-insulator-semiconductor field effect transistor (MISFET) structures with BaTiO3 thin films (containing La2O3 admixture) acting as gate insulator were investigated. The films were produced by means of radio frequency plasma sputtering (RF PS) of sintered BaTiO3 + La2O3 (2% wt.) target. In the paper transfer and output I−V, transconductance and output conductance characteristics of the obtained transistors are presented and discussed. Basic parameters of these devices, such as threshold voltage (VTH) are determined and discussed.

An Analytical Model for the Distribution of CO2 Sources and Sinks, Fluxes, and Mean Concentration Within the Roughness Sub-Layer

A one-dimensional analytical model that predicts foliage CO2 uptake rates, turbulent fluxes, and mean concentration throughout the roughness sub-layer (RSL), a layer that extends from the ground surface up to 5h, where h is canopy height, is proposed. The model combines the mean continuity equation for CO2 with first-order closure principles for turbulent fluxes and simplified physiological and radiative transfer schemes for foliage uptake. This combination results in a second-order ordinary differential equation in which soil respiration (R) and CO2 concentration well above the RSL are imposed as lower and upper boundary conditions, respectively. An inverse version of the model was tested against datasets from two contrasting ecosystems: a tropical forest (h = 40m) and a managed irrigated rice canopy (h = 0.7m), with good agreement noted between modelled and measured mean CO2 concentration profiles within the entire RSL. Sensitivity analysis on the model parameters revealed a plausible scaling regime between them and a dimensionless parameter defined by the ratio between external (R) and internal (stomatal conductance) characteristics controlling the CO2 exchange process. The model can be used to infer the thickness of the RSL for CO2 exchange, the inequality in zero-plane displacement between CO2 and momentum, and its consequences on modelled CO2 fluxes. A simplified version of the solution is well suited for being incorporated into large-scale climate models. Furthermore, the model framework here can be used to a priori estimate relative contributions from the soil surface and the atmosphere to canopy-air CO2 concentration, thereby making it synergetic to stable isotopes studies.

Electron transport of folded graphene nanoribbons

Recently, the AA-stack bilayer graphene nanoribbon (BGN) with a closed edge is observed in experiment. This new type of GN, we called folded GN (FGN), can be formed by folding a monolayer GN (MGN). Electron transport of the folded structures with different edges is studied. The FGNs show unique transport properties different from those of MGNs and BGNs. A metallic MGN with armchair edge (MAGN) is still metallic after folding. However, a semiconducting MAGN can be either semiconducting or metallic after folding, which depends on the width of MAGN and strength of interlayer coupling in the folded structure. The energy gap decreases with the increase of the coupling strength or width. As to the MGNs with zigzag edge (MZGNs), after folding they exhibit interesting conductance characteristics. The conductance steps around the Dirac point are even multiple of G0=2e2/h, while other conductance steps are odd multiple of G0. It indicates that the electron transport around the Dirac point in zigzag-edged FGNs (FZGNs) is similar to that in zigzag-edged BGNs (BZGNs), while electron transport far from the Dirac point is similar to that in zigzag-edged MGNs (MZGNs).

Double-row transport in quantum wires of shallow confinement

The bifurcation of the electron system in a quantum wire has been observed in the form of the suppression and disappearance of the quantised conductance plateau at 2 e 2 / h . We now present low-temperature transport measurements of a top-gated split-gate quantum wire that serve to further characterise this double-row régime of transport. A small distortion of the confinement caused by asymmetrical biasing of the split gates gives rise to a radical change in the conductance characteristics of the wire, beginning with the introduction of a plateau at G = e 2 / h which rises to around 0.7 × 2 e 2 / h with greater differential bias between the split gates. DC source–drain bias measurements in this régime show a split zero-bias peak at low conductances merging into a single peak around G = 0.7 × e 2 / h , which then persists up to the plateau at 4 e 2 / h .