Current Transport Processes Research Articles

A Novel Way for Improving the Overall Photoelectric Conversion Efficiency of Dye Sensitized TiO2 Solar Cells

The past several years has witnessed a sharp increase in the number of scientists and researchers who have attached great importance to studying Dye sensitized solar cells (DSSCs) because of its low cost and potential high photoelectric conversion efficiency. There are two main factors that influence the photoelectric conversion efficiency of dye sensitized solar cells. One is the low photon absorption rate of solar cells, and other one is the low transmission rate of the photo-generated electrons through the thick nanocrystalline film. So in order to further research and improve efficiency, an accurate model using COMSOL Multiphysics is investigated to reveal the photoresponse and current transport processes of DSSC. Specifically, an optical model is studied to determine how much the solar energy each cell takes in, and an electrical model is investigated to determine how much the overall efficiency. Besides, three adjusted micro/nanostructures of DSSCs have been built for improving the efficiency. The study shows that a more suitable structure can collect more solar energy and improve the electronic diffusion efficiency so as to improve the overall photoelectric conversion efficiency of DSSC.

Capabilities of CdTe-Based Detectors With ${\mathrm {MoO}}_{x}$ Contacts for Detection of X- and $\gamma$ -Radiation

The charge transport mechanism and spectrometric properties of the X-ray and $\gamma $ -ray detectors, fabricated by the deposition of molybdenum oxide thin films onto semi-insulating p-CdTe crystals were studied. The current transport processes in the ${\mathrm {Mo}}{-}{\mathrm {MoO}}_{x}/{\mathrm {p}}{-}{\mathrm {CdTe/MoO}}_{x}{-}{\mathrm {Mo}}$ structure are well described in the scope of the carrier’s generation in the space-charge region and the space-charge-limited current models. The lifetime of charge carriers, the energy of hole traps, and the density of discrete trapping centers were determined from the comparison of the experimental data and calculations. Spectrometric properties of ${\mathrm {Mo}}{-}{\mathrm {MoO}}_{x}/{\mathrm {p}}{-}{\mathrm {CdTe/MoO}}_{x}{-}{\mathrm {Mo}}$ structures were also investigated. It is shown that the investigated heterojunctions have demonstrated promising characteristics for practical application in X-ray and $\gamma $ -ray detector fabrication.

A modeling study of the impact of major storms on seabed shear stress and sediment transport on the Grand Banks of Newfoundland

AbstractWaves, current, and sediment transport processes in major storms on the Grand Banks of Newfoundland were simulated using integrated wave, three‐dimensional tide and circulation, and combined‐flow sediment transport models. While the tidal and nontidal currents are generally low and cause little sediment transport, storm‐induced waves and currents enhance bed shear velocity by more than 5 times and cause significant sediment transport over the entire Grand Banks. The impact of storms on shear stress and transport strongly depends on water depths and the greatest impact occurs over the bathymetric highs on southeastern Grand Bank where the maximum shear velocity reaches 15 cm s−1 and the maximum transport rates are >5 kg m−1 s−1. The direction of sediment transport rotates clockwise progressively through nearly 360° during the passage of a storm. Although peak transport typically occurs on central and southeastern Grand Bank with a southeastward direction, the magnitude, direction, and timing of peak transport show strong spatial and temporal variability. The variability of the peak transport largely depends on the timing and relative intensity of the waves and the total bottom currents which in turn depends on the addition of the storm‐induced and tidal currents. The calculation of the maximum transport potential suggests that sediments as coarse as small pebbles are mobile in water depths <80 m under 1:1 year storms and that medium sand is transported in water depths as deep as 200 m during major storms. Results of the sediment transport models corroborate the observed sediment erosion and accretion patterns.

Impact of Non-Linear Piezoelectricity on the Piezotronic Effect of ZnO Nanowires

ZnO is receiving a considerable attention for the development of novel cost-effective nanostructures with outstanding functional properties for applications in electronics and energy. In this paper, we investigate the effects of the nonlinear piezoelectricity, that has been recently observed in ZnO nanostructures, on the piezotronic effect of ZnO nanowires (NWs). We insert a physically-based model of the nonlinear direct piezoelectric effect into a fully-coupled thermo-mechanical-electric scheme to study the current–voltage characteristic of ZnO NWs under a purely vertical compressive/tensile strain. Our results show for the first time that the nonlinear piezoelectricity deeply affects the current transport processes inside the NW and the behavior of devices for piezoelectric-piezotronic applications and provides remarkable insights into the underlying physics.

Barrier inhomogeneities and electronic transport of Pt contacts to relatively highly doped n-type 4H-SiC

The barrier characteristics of Pt contacts to relatively highly doped (∼1 × 1018 cm−3) 4H-SiC were investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements in the temperature range of 160–573 K. The barrier height and ideally factor estimated from the I-V characteristics based on the thermionic emission model are abnormally temperature-dependent, which can be explained by assuming the presence of a double Gaussian distribution (GD) of inhomogeneous barrier heights. However, in the low temperature region (160–323 K), the obtained mean barrier height according to GD is lower than the actual mean value from C-V measurement. The values of barrier height determined from the thermionic field emission model are well consistent with those from the C-V measurements, which suggest that the current transport process could be modified by electron tunneling at low temperatures.

Electrical characteristics and deep level traps study of Au/ZnO:H Schottky diodes

The electrical characteristics and the deep level traps of Au Schottky contacts on ZnO:H films have been investigated by temperature-dependent current–voltage (I–V) and deep level transient spectroscopy (DLTS) measurements. The films were deposited with the DC-sputtering, varying the H flow rate in the Ar/H sputtering gas, so that the hydrogen per volume concentration [H2] was 0%, 20%, 33.3%, 50% and 66.6%. We found that for the Schottky diodes with [H2]=33.3% thermionic emission is the dominant current transport process at low forward voltage, taking into consideration the lateral fluctuations of the barrier height described by a Gaussian distribution model. The domination of trap-assisted current transport mechanisms is possible in the diodes with [H2]=0%, 20%, 50% and 66.6%. DLTS spectra revealed two electron traps, with activation energies of 0.29 and 0.21eV, often observed in ZnO and related to intrinsic defects. The trap concentrations are significantly lower in the 33.3% H2 diodes compared to the 0% H2 ones, indicating an interaction of these traps with hydrogen.

Clay mineral distribution in last Glacial-Holocene sediment cores from the eastern Marmara Sea (Çınarcık Basin–İzmit Gulf Transition), NW-Turkey: Multisources and transport paths in a two-way flow system

The regional variations in grain size, XRD-bulk and clay mineral distribution in 97 sediment samples from 6 gravity cores obtained from 20 to 300 m water depths have been investigated to reveal source and transportation patterns as well as the effects of climatic and sea-level changes during the last 25,000 14C BP in the eastern Marmara Sea (EMS), NW Turkey. The major proportions of sediments in the EMS consist of silt (20–83%) and clay (17–90%), whereas sand and gravel abundances rarely exceed 10%. Of the bulk minerals, 10-Å micas (23–55%) and feldspars (17–55%) dominate over quartz (10–21%) and calcite (10–26%). Smectites (23–62%), illite (18–36%), chlorites (6–21%), kaolinite (2–20%) and 14S–14C mixed layer minerals (0–27%) constitute the principal clay mineral assemblages. Both bulk and clay mineral assemblages in the EMS are similar to those found in the İzmit Gulf, on the southern Marmara shelf, at the Marmara–Aegean approaches and the southern Black Sea. This would suggest that there is no distinct source for mineral assemblages of the study area outside the Marmara Basin. The concentrations of clay fractions and clay mineral assemblages tend to increase or decrease with depth in the cores, but this is limited to some intervals and not to entire sections in the cores. This distribution pattern therefore represents rather local sources of provenance and surrounding fluvial drainage basins together with dilution and distribution effects by current transport processes, rather than the expected paleoclimatic features and sedimentary input by inflowing Black Sea and Aegean Sea water masses throughout the last Glacial to Holocene.

Geometric blockade in a quantum dot coupled to two-dimensional and three-dimensional electron gases

We fabricated a quantum dot coupled laterally to a two-dimensional electron gas and vertically to a three-dimensional electron gas in order to investigate the eigenstate dependence of tunneling rate to these gases. We observed a bias-dependent ``geometric'' current blockade. By tunneling via the asymmetric couplings, population inversion is induced and a dark metastable triplet state is revealed. The metastable state stops the current transport process, suppresses the current, and asymmetrically widens the Coulomb diamond. By analyzing the current as a function of source-drain and gate voltage and the magnetic field, we concluded that this effect is due to the geometric shape of the electronic states in the dot and the current is limited by the tunneling rate due to the eigenstates, that is, artificial $\ensuremath{\sigma}$ coupling and $\ensuremath{\pi}$ coupling.

Simulation of complicated and diverse water system accompanied by human intervention in the North China Plain

AbstractThe author simulated water/heat dynamics in the North China plain (NCP) by coupling the NIES Integrated Catchment‐based Eco‐hydrology (NICE) model series with dam/canal systems in the agricultural and urban areas (NICE‐CNL). This combined model was applied to the Hai River catchment and the lower reach of the Yellow River (530 km wide × 840 km long) at a resolution of 5 km. It satisfactorily reproduced river discharge, soil moisture, evapotranspiration, groundwater level, and crop production of summer maize and winter wheat, correctly estimating—without any detailed statistical data—the amount of crop water use. The simulation showed that irrigation and industrial use of groundwater has a marked effect on the water dynamics in the NCP because the river water volume is smaller than that necessary for irrigation, industrial and, domestic water use. The model reproduced, reasonably well, the trend of groundwater degradation over the previous half century, including cone depressions occurring around bigger cities, which the author was unable to reproduce in previous research. The simulation also clarified that this water shortage is related to the low flow discharge of the Hai River into the ocean. Furthermore, the simulated result for the density current and solute transport processes clarified that these water cycle changes have caused serious seawater intrusion and a decrease of crop productivity in the coastal area, as shown by the gradient of NDVI (Normalized Difference Vegetation Index) estimated from satellite images. This study has important implications for the evaluation of ecosystems and environments located on the moving edge of freshwater and marine areas. Copyright © 2011 John Wiley & Sons, Ltd.

Clay mineral distribution in surface sediments of the northeastern South China Sea and surrounding fluvial drainage basins: Source and transport

Clay mineralogy of surface samples throughout the northeastern South China Sea (SCS) and surrounding fluvial drainage basins has been investigated to reveal source and transport of detrital fine-grained sediments. Three characteristic end-members in clay mineral compositions were observed from three major provenances, respectively: major kaolinite and moderate illite and chlorite from the Pearl River, predominant illite and chlorite from Taiwan, and principal smectite from the Luzon arc system. The transport of four clay mineral species in the northeastern SCS is influenced by various mechanisms. Smectite derived from the Luzon arc system is transported mainly by surface current with significant influence of the Kuroshio intrusion, illite and chlorite from Taiwan are mainly carried by deep water current, while kaolinite from the Pearl River is affected obviously by the westward Guangdong Coastal Current and longshore current. A westerly tongue-shaped distribution of high contents of illite and chlorite mainly from Taiwan on the eastern shelf may result from the westward Guangdong Coastal Current during the winter. The results suggest that clay mineral assemblages in the northeastern SCS are mainly controlled by provenance supply and current transport and their downcore records do not bear contemporaneous paleoclimatic features. The implication of clay minerals for proxies of the late Quaternary East Asian monsoon evolution is realized through both the provenance supply and current transport processes.

Adaptive demand peak management in online transport process planning

We investigate the common and integrated dynamic decision making of the coordinator of a supply chain consortium together with a subordinate fleet managing agent offering transport services for the consortium. While the fleet manager aims at minimizing the costs of the generated transport processes, the goal of the coordinator is to keep the reliability and stability of the processes on a reasonable level. It aims to synchronize the transport processes with upstream and downstream parts of the supply chain. The major innovation presented in this article is a framework that controls and adjusts the decision competence distribution between the two planning agents with respect to the current transport process performance. If the transport process timeliness is endangered to fall below a given threshold and thereby the overall supply chain reliability tends to sink, the coordinator is temporarily granted the right to intervene into the planning of the fleet managing agent. Within simulation experiments, we demonstrate that the proposed system is able to increase the reliability of the generated transport processes. We show that the intervention of the superior coordinator agent during workload peaks ensures higher process timeliness than the transport service providing agent is able to achieve without any coordinator interventions.

Cross-Scale Patterns in Shrub Thicket Dynamics in the Virginia Barrier Complex

To interpret broad-scale erosion and accretion patterns and the expansion and contraction of shrub thickets in response to sea level rise for a coastal barrier system, we examined the fine-scale processes of shrub recruitment and mortality within the context of the influence of ocean current and sediment transport processes on variations in island size and location. We focused on Myrica cerifera shrub thickets, the dominant woody community on most barrier islands along the coastline of the southeastern USA. Observations suggest that M. cerifera, a salt-intolerant species, is increasing in cover throughout the Virginia barrier islands, yet rising sea level in response to climate change is increasing erosion and reducing island area. Our objective was to explain this apparent paradox using pattern–process relationships across a range of scales with a focus on ocean currents and sediment transport interacting with island characteristics at intermediate scales. Multi-decadal comparisons across scales showed a complex pattern. At the scale of the entire Virginia barrier complex, modest decreases in upland area were accompanied by large increases in shrub area. Responses were more variable for individual islands, reflecting inter-island variations in erosion and accretion due to differences in sediment transport via ocean currents. Several islands underwent dramatic shrub expansion. Only for within-island responses were there similarities in the pattern of change, with a lag-phase after initial shrub colonization followed by development of linear, closed canopy thickets. Understanding the fine-scale processes of shrub seedling establishment and thicket development, in conjunction with the influence of ocean currents and sediment transport, provides a framework for interpreting island accretion and erosion patterns and subsequent effects on shrub thicket expansion or contraction across scales of time and space.

Holocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods

Radiocarbon and 230Thexcess measurements were undertaken on the sediments from fifteen box cores recovered between 48–58°N and 12–22°W at water depths of 1100–4500 m in the northeast Atlantic. Eight of the cores were from Feni Drift in the Rockall Trough at depths of 1700–2500 m and the remaining seven formed an approximate north/south transect at various water depths from the abyssal plain north on to Rockall Plateau. Mean Holocene sediment accumulation fluxes for all cores were established from profiles of bulk sediment radiocarbon age against depth to be in the range 3–7 cm ky− 1 off the Drift and 4–23 cm ky− 1 on the Drift. When compared with these radiocarbon-based sediment accumulation rates, precision measurements of 230Th reveal that the 230Thexcess levels present in the sediments match or slightly exceed the potential supply from the vertically overlying water column in a majority of the cores. One core from the open abyssal plain that had the lowest radiocarbon-based accumulation rate in the entire set also had a close balance between predicted and measured 230Thexcess values. With the assumption that only 230Thexcess produced in the overlying water column is supplied to the sediments, the 230Thexcess values in this core and the other six containing carbonate ooze sediments all imply a rather constant regionally averaged sedimentation flux of 2.0 ± 0.2 (1σ, n = 7, min. 1.8, max. 2.3) g cm− 2 ky− 1 rather than matching the 230Thexcess fluxes implied by the radiocarbon data. This singular value is similar to the mean Holocene flux reported for the northeast Atlantic by previous work that utilized the 230Thexcess method, although it is somewhat lower than estimates for the Holocene based on oxygen isotope stratigraphy or radiocarbon methods. The current transport processes that result in the high sediment accumulation rates on Feni Drift have also resulted in a compositional fractionation of the sediments. All sediments from Feni Drift have lower CaCO3 contents than the remainder of the sediments studied, and these are accompanied by lower 230Thexcess specific activities than are found at comparable water column depths elsewhere on the transect. Consistently higher and more variable regional sediment accumulation fluxes are therefore calculated from the Feni Drift 230Thexcess data with the constant flux assumption (average 2.8 ± 0.4 g cm− 2 ky− 1; n = 8, min. 2.3, max. 3.3), some 40% higher than the constant value measured in the other cores. It seems likely that the mean sediment accumulation flux at Feni Drift inside Rockall Trough is consistently higher than on the open ocean margin of the basin, so that the regionally averaged sedimentation fluxes indicated for the Drift by the measured 230Thexcess data are also consistently higher than the singular value measured elsewhere in the northeast Atlantic.

NUMERICAL ANALYSIS OF TIDAL CURRENT AND SEDEMENT TRANPORT IN AN ESTUARY

We analyzed characteristics of tidal current and sediment transport processes in the Ohta flood-way using a 3-D baroclinic numerical model in which a sediment transport model was integrated. The results were compared with the field data obverved at 2.8 km upstream from the river mouth (Sta. A). The numerical model reproduces the main characteristics of the field data at Sta. A. In the Ohta flood-way, strong resuspention occurs at the beginning of flood current. From the horizontal distribution of deposition thickness, it is found that there is characteristic erosion near the river mouth and there is the significant deposition at 3-5 km upstream from the river mouth.

Prediction of barrier inhomogeneities and carrier transport in Ni-silicided Schottky diode

Abstract Based on Quantum Mechanical (QM) carrier transport and the effects of interface states, a theoretical model has been developed to predict the anomalous current–voltage ( I – V ) characteristics of a non-ideal Ni-silicided Schottky diode at low temperatures. Physical parameters such as barrier height, ideality factor, series resistance and effective Richardson constant of a silicided Schottky diode were extracted from forward I – V characteristics and are subsequently used for the simulation of both forward and reverse I – V characteristics using a QM transport model in which the effects of interface state and bias dependent barrier reduction are incorporated. The present analysis indicates that the effects of barrier inhomogeneity caused by incomplete silicide formation at the junction and the interface states may change the conventional current transport process, leading to anomalous forward and reverse I – V characteristics for the Ni-silicided Schottky diode.

Localised feedback in silicon-based avalanche photodiodes

Avalanche photodiode structures with wide-bandgap (WBG) barrier on epitaxial silicon layers of 4–20 μm thickness were studied and fabricated for short-wavelength photons (∼0.4 μm) detector. To optimise avalanche multiplication and current transport processes, a physical model has been developed taking into account charge transport localisation mechanisms in silicon and WBG layer. To reduce lateral spreading of current filaments, the nanostructuring of WBG layer has been proposed. The nanostructured WBG layer includes semiconductor nanoclusters between which electrons may tunnel to reach the field electrode. Thus, additional localisation of every separate avalanche region (filament) is provided. The current outflow mechanism through this structure is characterised by a delay caused by multiple tunnelling of electrons injected from semiconductor local region, where avalanche multiplication takes place, to WBG layer. For this mechanism the noise factor should be lower than for avalanche structures with resistive layer caused by the WBG layer nanoclusters configuration.

Current transport mechanism and I–V characteristics of titanium and indium contacts to p-type GaN

Several Mg-doped p-type GaN samples were grown on sapphire by horizontal MOCVD technique. Concentrations of Mg are in the 10 19 cm −3 range. Free carrier concentrations are p=2–3×10 17cm −3 and Hall mobility μ H=11–13 cm 2 (Vs) −1. Contacts for Hall probes are standard nickel–gold contacts. On such samples the titanium contacts were evaporated and the indium contacts were soldered. We study current transport for various combinations of metal contacts. Measured current voltage curves ( I–V) show similar characteristics for various combinations of metals being dependent more on sample then on the choice of metal. The I–V characteristics showed symmetrical ‘s’-shape, identical in forward and reverse direction. Several expressions are considered in attempt to explain such behavior. It is concluded that the tunneling is dominant current transport process. It is expected that the surface layer properties are critical for the formation of ohmic contacts and the choice of metal is less important.

Propagation of a disturbance created by a probe in electron collection

Experiments in the edge plasma of TEXT-U (the Texas Experimental Tokamak-Upgrade) [K. W. Gentle, Nucl. Technol. Fusion 1, 479 (1981)] have used a positively biased Langmuir probe in conjunction with two triple probes to examine the “wake” or disturbance in the plasma caused by the electron current collected by the driven probe. Spatial measurements of the disturbed volume have been made in the radial and poloidal directions at two points along the disturbed flux tube, at 12 meters and 28 meters away from the driven probe. Both non-time dependent and time-dependent experiments are used to determine the dispersion of the driven wave. A classical collisional two-fluid model of electron collection by a biased surface, which accounts for electric potential perturbations and the associated currents only, successfully predicts many of the qualitative features of the disturbance. The magnitudes of the collision rates, however, have to be increased beyond the best estimates in order to achieve quantitative agreement, indicating the possible involvement of anomalous cross-field current transport processes.

MODELING OF CURRENTS AND TRANSPORT PROCESSES OFCOASTAL OCEAN REGIONS WITH APPLICATION TO THECOAST OF CABO FRIO (RIO DE JANEIRO-BRAZIL)

In this paper a numerical model, which describes the current system and transport processes of coastal ocean regions under wind field forcing, is presented. The model has an active layer overlaying a deep inert layer where the pressure gradient is set to zero. The model includes the turbulent version of the momentum and continuity equations and transport equations to describe the thermodynamic changes of the sea surface temperature(SST) and the changes of pollutants. Solutions are found numerically on a uniform grid and the fundamental equations are approximated by the finite difference method taking the form of a LaxWendroff scheme. Solutions forced by uniform and non-uniform wind fields are evaluated for a schematic straight coastline and for the coastal area of Cabo Frio (Rio de Janeiro, Brazil). The solutions for the coastal area of Cabo Frio differ considerably depending on the orientation of the winds. All the solutions show a very sensitive response of the currents and SST. Solutions forced by an idealized representation of a non-uniform wind field develop a SST pattern similar as the observations, particularly the extension of the cool water plume in south-west direction. The spatial evolution of pollutants concentrations for hypothetical situations shows patterns influenced directly by the current system.

A modified model in positive temperature coefficient of resistance BaTiO3 ceramics

Current–voltage and resistivity–voltage measurements have been made on donor-doped BaTiO3 ceramics in order to investigate the nature of the current transport processes. It is found that the characteristics of the current–voltage do not follow the Heywang model [Solid-State Electron. 3, 51 (1961)]. Hence, the classical Heywang model is modified and the thermionic emission model with smoothly changed barrier resulted from image force is suggested. The modified theory coincides better with the experimental observation. The dependence between current density and voltage is J=J0 exp[−(φ0−βV1/2)/kT]. The relationship between resistivity and voltage (voltage effect) is ρ=(2kT/dJ0β)V1/2 exp[(φ0−βV1/2)/kT . From the modified model, the maximum barrier height of BaTiO3 ceramics can be deduced experimentally as well.