Position-dependent Electric Field Research Articles

Design, optimization, and characterization of deep sub-wavelength evanescent orders in terahertz metagratings

Resonant evanescent orders, being an exclusive deep sub-wavelength phenomenon, are well-known for confining strong EM energy at the air-grating interface when excited utilizing 1-dimensional gratings. However, stimulating prominent evanescent orders demands thoughtful design variations in grating geometry. In this pretext, we have successfully designed and optimized THz gratings that can sustain strong evanescent orders while operating in the subwavelength frequency domain. We have performed a fast Fourier transform (FFT) on the position-dependent electric field distribution of the grating to study the evanescent orders for both of the incidence polarizations (TE and TM). In order to optimize the grating performance, we have systematically increased the grating ridge height at a fixed fill factor (FF = 0.5). In such a way, excited evanescent orders are turned out to be anisotropic in nature at relatively larger grating height. We attribute such anisotropic behaviour to the effective refractive index experienced by the orthogonal THz probe.

Modeling multi-site emission in porous electrosprays resulting from variable electric field and meniscus size

A model predicting the number of emission sites and total current from a porous conical electrospray emitter as functions of voltage is derived. A pressure balance between capillary and electric forces is used to determine an onset criterion for individual menisci, and an ionic emission scaling law is invoked to predict the current each meniscus emits. These submodels are integrated over a phenomenological meniscus size distribution and the area of the emitter to yield a model for emitter performance as a function of five free parameters, two for the ionic emission submodel and three for the meniscus size distribution. Bayesian inference is applied to determine these model parameters from an existing dataset [Dressler et al., J. Propul. Power 38, 809 (2022)]. The model predictions after training are compared to the experimental data, and it is found that the majority of the data are within a 90% credible interval. The ability of the model to capture key trends in the experimental data is attributed to the interplay of two effects: the distribution over meniscus size on the emitter and the position-dependent electric field. The calibrated model results also suggest that the emitter surface is wetted by a series of large but sparsely distributed pools of propellant. The performance and extensibility of the model are examined within the context of model-based design for porous electrospray array thrusters.

Monte Carlo study of electron–electron scattering effects in FET channels

In this work, we study the effect of electron–electron scattering on the high-energy tail of the electron distribution function in FET channels. A new Monte Carlo algorithm for solving a two-particle kinetic equation in the presence of a position-dependent electric field is proposed. In stationary bulk simulations, no visible effect of electron–electron scattering on the distribution function and consequently on its moments is observed. In stationary device simulations, however, a clear effect on the high-energy tail can be seen. The enhancement of the tail relative to the thermal tail is found to scale with the electron concentration.

Investigation of the Boron removal effect induced by 5.5 MeV electrons on highly doped EPI- and Cz-silicon

This study focuses on the properties of the BiOi (interstitial Boron–interstitial Oxygen) and CiOi (interstitial Carbon–interstitial Oxygen) defect complexes by 5.5MeV electrons in low resistivity silicon. Two different types of diodes manufactured on p-type epitaxial and Czochralski silicon with a resistivity of about 10 Ω⋅cm were irradiated with fluence values between 1 × 1015cm-2 and 6 × 1015cm-2. Such diodes cannot be fully depleted and thus the accurate evaluation of defect concentrations and properties (activation energy, capture cross-section, concentration) from Thermally Stimulated Currents (TSC) experiments alone is not possible. In this study we demonstrate that by performing Thermally Stimulated Capacitance (TS-Cap) experiments in similar conditions to TSC measurements and developing theoretical models for simulating both types of BiOi signals generated in TSC and TS-Cap measurements, accurate evaluations can be performed. The changes of the position-dependent electric field, the effective space charge density Neff profile as well as the occupation of the BiOi defect during the electric field dependent electron emission, are simulated as a function of temperature. The macroscopic properties (leakage current and Neff) extracted from current–voltage and capacitance–voltage measurements at 20°C are also presented and discussed.

Position reconstruction for segmented detectors

The topic of the paper is the position reconstruction from signals of segmented detectors. With the help of a simple simulation, it is shown that the position reconstruction using the centre-of-gravity method is strongly biased, if the width of the charge (or e.g. light) distribution at the electrodes (or photo detectors) is less than the read-out pitch. A method is proposed which removes this bias for events with signals in two or more read-out channels and thereby improves the position resolution. The method also provides an estimate of the position–response function for every event. Examples are given for which its width as a function of the reconstructed position varies by as much as an order of magnitude.A fast Monte Carlo program is described which simulates the signals from a silicon pixel detector traversed by charged particles under different angles, and the results obtained with the proposed reconstruction method and with the centre-of-gravity method are compared. The simulation includes the local energy-loss fluctuations, the position-dependent electric field, the diffusion of the charge carriers, the electronics noise and charge thresholds for clustering, A comparison to test-beam-data is used to validate the simulation.

Ion collection efficiency of ionization chambers in ultra-high dose-per-pulse electron beams.

The ion collection efficiency of vented ionization chambers has been investigated in an ultra-high dose-per-pulse (DPP) electron beam. The role of the chamber design and the electric field strength in the sensitive air volume have been evaluated. An advanced Markus chamber and three specially designed parallel plate air-filled ionization chambers (EWC: End Window Chamber) with varying electrode distance of 0.5, 1, and 2mm have been investigated. Their ion collection efficiencies were determined experimentally using two methods: extrapolation of Jaffé plots and comparison against a DPP-independent reference detector. The latter was achieved by calibrating a current transformer against alanine dosimeters. All measurements were performed in a 24 MeV electron beam with DPP values between 0.01 and 3Gy. Additionally, the numerical approach introduced by Gotz et al. was implemented taking into account space charge effects at these ultra-high DPPs. The method has been extended to obtain time-resolved and position-dependent electric field distortions within the air cavity. The ion collection efficiency of the investigated ionization chambers drops significantly in the ultra-high DPP range. The extent of this drop is dependent on the electrode distance, the applied chamber voltage and thus the field strength in the sensitive air volume. For the Advanced Markus chamber, a good agreement between the experimental, numerical and the results of Petersson et al. could be shown. Using the three EWCs with different electrode spacing, an improvement of the ion collection efficiency and a reduction of the polarity effect with decreasing electrode distance could be demonstrated. Furthermore, the results revealed that the determination of the ion collection efficiency from the Jaffé plots and therefore also from two-voltage method typically underestimate the ion collection efficiency in the region of high dose-per-pulse (3 to 130mGy) and overestimate the ion collection efficiency at ultra-high dose-per-pulse (>1Gy per pulse). In this work, the ion collection efficiency determined with different methods and ionization chambers have been compared and discussed. As expected, an increase of the electric field in the ionization chamber, either by applying a higher bias voltage or a reduction of the electrode distance, improves the ion collection efficiency and also reduces the polarity effect. For the Advanced Markus chamber, the experimental results obtained by comparison against a reference agree well with the numerical solution. Based on these results, it seems possible to keep the recombination loss less than or equal to 5% up to a dose-per-pulse of 3Gy with an appropriately designed ionization chamber, which corresponds to the level accepted in conventional radiotherapy dosimetry protocols.

Stark shift of binding energy for on and off-center donor impurities in quantum rings under the influence of charged rods electric fields

In the present work, we study the stark shift of binding energy for a Hydrogenic donor impurity confined within a circular quantum ring composed from three different semiconducting heterostructures AlxGa1-xN/GaN, InxGa1-xN/GaN, Ga1−xAlxAs/GaAs under the electric fields along different directions. We supplied a position-dependent electric field by using a charged rod of radius ‘a' and charge density λ. By placing the charged rod and donor impurity in different positions, we have studied the changes in the on and off-center donor binding energies, system energy levels, probability densities, average position of the electron, and the probability finding electron inside the quantum ring. We describe the redshift and blueshift of the donor impurity binding energy when different parameters change.

Determination of the electric field in highly-irradiated silicon sensors using edge-TCT measurements

A method is presented which allows to obtain the position-dependent electric field and charge density by fits to velocity profiles from edge-TCT data from silicon strip detectors. The validity and the limitations of the method are investigated by simulations of non-irradiated n+p pad sensors and by the analysis of edge-TCT data from non-irradiated n+p strip detectors. The method is then used to determine the position dependent electric field and charge density in n+p strip detectors irradiated by reactor neutrons to fluences between 1 and 10×1015 cm−2 for forward-bias voltages between 25 V and up to 550 V and for reverse-bias voltages between 50 V and 800 V. In all cases the velocity profiles are well described. The electric fields and charge densities determined provide quantitative insights into the effects of radiation damage for silicon sensors by reactor neutrons.

A model study of the role of workfunction variations in cold field emission from microstructures with inclusion of field enhancements

An analytical study of field emission from microstructures is presented that includes position-dependent electric field enhancements, quantum corrections due to electron confinement and fluctuations of the workfunction. Our calculations, applied to a ridge microstructure, predict strong field enhancements. Though quantization lowers current densities as compared to the traditional Fowler–Nordheim process, strong field emission currents can nonetheless be expected for large emitter aspect ratios. Workfunction variations arising from changes in electric field penetration at the surface, or due to interface defects or localized screening, are shown to be important in enhancing the emission currents.

Imaging of Nonuniform Motion of Single DNA Molecules Reveals the Kinetics of Varying-Field Isotachophoresis

The nonuniform motion of charged species in a varying electric field may provide unique separation and focusing power for chemical, biochemical, and nanoscale studies. We imaged in real time the nonuniform motion of single DNA molecules under varying-field isotachophoresis (ITP) conditions. From the trajectories of single molecules, we obtained the time- and position-dependent electric field strength (E) and revealed the behavior of adaption barriers within electro-osmotic flow (EOF)-driven and EOF-independent ITP. We found that the initial terminating electrolyte zone of constant E is split into two zones: a highly adapted high-E zone and a low-E zone of gradually adapting electric field. The formation of the two unique zones is associated with the rate-limiting mass transfer barrier in EOF-driven ITP. As a result of the unique E distribution, DNA molecules first slow to a stop and then rapidly move backward to the leading electrolyte/terminating electrolyte boundary. This provides a novel mechanism for selective focusing of target molecules in dilute solutions of large volume. We show that the ITP focusing can improve the detection of single DNA molecules (limit of detection = 4 × 10(-17) mol/L), which are stochastically distributed at extremely low concentrations. The ITP strategy focuses individual molecules into a small volume that is matched with the focal point of single-molecule imaging.

Skyrmion dynamics in multiferroic insulators

Recent discovery of Skyrmion crystal phase in insulating multiferroic compound Cu$_2$OSeO$_3$ calls for new ways and ideas to manipulate the Skyrmions in the absence of spin transfer torque from the conduction electrons. It is shown here that the position-dependent electric field, pointed along the direction of the average induced dipole moment of the Skyrmion, can induce the Hall motion of Skyrmion with its velocity orthogonal to the field gradient. Finite Gilbert damping produces longitudinal motion. We find a rich variety of resonance modes excited by a.c. electric field.

Time-of-flight mobility of charge carriers in position-dependent electric field between coplanar electrodes

Time-of-flight measurements of the photocurrent in thin organic semiconductor layers represent an effective way to extract charge carrier mobility. A common method to interpret the time-dependence of the photocurrent in these material systems assumes a position-independent electric field between two coplanar electrodes. In this letter, we compare time-dependence of the photocurrent, measured in the samples comprising thin layers of poly-3-hexylthiophene, with the Monte Carlo simulations. In the simulations, we have used both, a position-independent and a position-dependent electric field. We obtained a favorable agreement between the simulations and the measurements only in the case of position-dependent electric field. We demonstrate that the charge carrier mobility may be underestimated by more than one order of magnitude, if a position-independent electric field is used in the calculations of the mobility.

Electrode structure for color shift reduction in fringe-field switching mode

We propose a new electrode structure for the fringe-field switching mode, which not only has a smaller color shift but also transmits more light than the chevron-type structure. While the chevron-type electrode structure mainly uses the different directions of the electric field, the proposed structure makes use of position-dependent strengths as well as different directions of the electric field. Position-dependent electric field strengths bring about rotation of LCs different from position to position in a pixel, which further reduce the color shift by using the multi-domain effect.

Compensation Voltage (CV) Peak Shapes Using a Domed FAIMS with the Inner Electrode Translated to Various Longitudinal Positions

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates ions at atmospheric pressure based on the difference in the mobility of an ion in a strong electric field and in a weak electric field. This field-dependent mobility of an ion is reflected in the compensation voltage (CV) at which the ion is transmitted through FAIMS at an applied asymmetric waveform dispersion voltage (DV). In this report, we show that experimental CV peak shapes using dome tipped inner electrode FAIMS prototypes with inner/outer electrode radii of: (1) 0.2/0.4 cm and (2) 0.4/0.6 cm are a function of the longitudinal position of the inner electrode. Varying the longitudinal position of the inner electrode modifies the electric fields between the surfaces of the hemispherical shaped inner electrode and the outer electrode in the vicinity of the ion outlet. In this region the position-dependent electric field strength (E/N) effectively forms a second tandem FAIMS analyzer region having differing ion separation properties. The final tandem FAIMS separation is the intersection of the CV windows of these two differing FAIMS separations and, therefore, the peak width in the CV scan is dependent on the longitudinal tip displacement (LTD) of the inner electrode. CV scans are shown for a LTD range of 0.14 to 0.4 cm. These scans illustrate that it is possible to control the FAIMS resolution (CV/peak width) from about 1 for the 0.2/0.4 cm electrode set at intermediate longitudinal position to over 10 at the narrowest distance between the inner electrode and the ion outlet.

Avalanche multiplication in forward- and reverse-active mode bipolar junction transistors

Impact ionization in the reverse-biased base-collector space-charge layer of the bipolar junction transistor (BJT) can cause an avalanche collector current as well as a reverse (or negative) base current. We develop analytical models to predict and compare avalanche phenomena in the BJT biased under forward-active and under reverse-active operations. The models consider a position-dependent electric field in the space-charge layer, the effect of the non-uniform doping profile, and the excess free carriers associated with the current passing through the space-charge layer. For the specified device make-up and parameters used, our results suggest that avalanche is more prominent in the reverse-active BJT than the forward-active BJT. The physics underlying this occurrence is given, and experimental data obtained from an advanced BJT is included in support of the models.

Electrostriction: A density functional theory

A study of electrostriction in a dipolar liquid is carried out by using the density functional theory for inhomogeneous systems. A general expression for electrostriction due to a position-dependent electric field is derived. The expression for the electrostriction due to a constant macroscopic electric field is derived in terms of both orientational structure factors and dielectric function. This expression is found to reduce exactly to the expression derived earlier by Rasaiah, Isbister, and Stell when the proper identifications are made. A study of the change in density profile around a point ion in the dipolar liquid is carried out. It is found that the density profile is oscillatory, which is expected. However, our expression which considers only the quadratic nonlinearity in the electric field, breaks down at a very short distance from the ion. The microscopic structure of the liquid is shown to play an important role at the molecular length scales.

Modeling the tunnelling current in reverse-biased p/ n junctions

In this paper we treat the tunnelling current in a more rigorous manner by considering the position-dependent electric field in the space-charge region, rather than treating the electric field as a constant as that in the conventional approach [eqn (1)]

Electric Fields in Thin-Film Dielectrics on Metal Surfaces

Electrostatic fields in oxides and similar compounds formed on metals by thermal reaction with ambient gases are deduced for the case in which rate is determined by charged-particle diffusion through the compound. Specifically, the limiting case where the macroscopic electrostatic potential leads to a condition approximating local charge neutrality is examined. The position-dependent electric field in the compound leads to an electrostatic potential across the layer which is independent of thickness.