Capture Coefficient Research Articles

Deposition of aerosol nanoparticles in filters composed of fibers with porous shells

The diffusion deposition of submicron aerosol particles in model filters consisting of fibers covered with permeable porous shells is studied. An ordered system of parallel cylinders arranged perpendicular to the flow is used as a model filter. The results of calculations are given for the dependences of the capture coefficient on the shell radius, the shell permeability, the packing density of the filters, the particle radius, and the flow velocity. Calculations are performed within a wide range of Peclet numbers. It is shown that the capture coefficient and the quality criterion γ of a filter increase with the diffusion mobility of particles and shell permeability, as well as that the dependence of the quality criterion on the radius of permeable shells has a maximum. It is also shown that the capture coefficients for fibers with porous shells, calculated using the cell model and the isolated row of fibers, almost coincide with one another.

Deposition of nanoparticles in filters composed of permeable porous fibers

The diffusion deposition of nanoparticles is studied from a flow at low Reynolds numbers in model filters composed of permeable circular porous fibers. The field of particle concentration is calculated and the capture coefficient is determined for a cell, as well as the isolated row of parallel fibers within a wide range of Peclet numbers (Pe) depending on the fiber permeability. It is shown that at Pe > 1, the diffusion capture coefficient η increases with permeability, while at Pe → ∞, it tends toward the limiting value, which is equal to the gas flow rate through the porous fiber. The capture coefficients calculated from a cell model and for a row of fibers are almost equal to each other. The diffusion deposition of aerosol particles in the highest penetration range is calculated with an allowance for their finite sizes and it is shown that the radii of most penetrable particles decrease with an increase in fiber permeability.

Proof of Interstitial Cobalt Defects in Silicon Float Zone Crystals Doped during Crystal Growth

In Si crystals, which were doped with cobalt during floating- zone growth, mainly substitutional Co defects with a concentration of about 7 x 1012 cm-3 can be detected by deep- level transient spectroscopy. But a second fundamental defect with a lower concentration around 7 x 1011 cm-3 shows features, which are typical for an interstitial defect with a remarkable mobility even at room temperature. It has a donor level at EV + 0.31 eV as well as an acceptor level at EC - 0.4 eV and is attributed to interstitial Co. In p-type samples pairing of interstitial Co with boron acceptors is observed. The capture coefficient of this pairing reaction is thermally activated with a characteristic energy of 0.9 eV. A binding energy of the Co-B pairs of about 0.6 eV is obtained. In the upper half of the gap an acceptor level was observed at EC - 0.08 eV.

Auger recombination and biexcitons inCu2O: A case for dark excitonic matter

The lifetime of excitons in ${\mathrm{Cu}}_{2}\mathrm{O}$ decreases significantly at high gas densities. This effect has been attributed to an Auger recombination process between two excitons, resulting in a loss rate given by ${\ensuremath{\tau}}^{\ensuremath{-}1}=An$, where $A$ is the Auger constant and $n$ is the exciton gas density. Previous time- and space-resolved photoluminescence measurements, however, yielded an Auger constant that is orders of magnitude larger than theoretical calculations. In addition, the experimental Auger constant varied inversely with temperature, in contrast to the linear-$T$ dependence predicted for direct Auger recombination of two free excitons. To resolve these discrepancies, we propose that excitons form biexcitons that rapidly decay by the Auger process. In this case, the lifetime of excitons is limited by exciton-exciton ``capture'' which occurs more frequently at low temperature. The instantaneous decay rate of excitons due to the capture process is given by $2\overline{C}n$ where $\overline{C}$ is a capture coefficient averaged over orthoexcitons and paraexcitons. Analysis of our photoluminescence data between 2 and $212\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is consistent with a paraexciton capture coefficient that depends inversely with gas temperature. Detailed analysis of the exciton transients following short-pulse excitation yields a biexciton binding energy $\ensuremath{\phi}$ roughly in the range $8--15\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and a biexciton Auger rate ${\ensuremath{\tau}}_{A}^{\ensuremath{-}1}=0.5T\phantom{\rule{0.3em}{0ex}}{\mathrm{ns}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, where $T$ is the gas temperature. Due to their short lifetimes, the density of biexcitons is predicted to be considerably lower than the density of excitons, and it is likely that the ground state of a biexciton is optically inactive. Consequently, it is not surprising that photoluminescence of biexcitons has not been observed in this crystal. The existence of such ``dark matter'' would explain the long-standing difficulties in achieving Bose-Einstein condensation of excitons in this system.

ITON Schottky contacts for GaN based UV photodetectors

Lateral Schottky ultraviolet detectors were fabricated in GaN using indium-tin-oxynitride (ITON) as a contact metal. The GaN semiconductor material was grown on 2 in. sapphire substrate by metal–organic chemical vapor deposition (MOCVD). The Schottky contact has been realized using ITON that has been deposited using sputter techniques. I– V characteristics have been measured with and without UV illumination. The device shows photo-to-dark current ratios of 10 3 at −1 V bias. The spectral responsivity of the UV detectors has been determined. The high spectral responsivity of more than 30 A/W at 240 nm is explained by a high internal gain caused by generation–recombination centers at the ITON/GaN interface. Persistent photocurrent effect has been observed in UV light (on–off) switching operation, time constant and electron capture coefficient of the transition has been determined.

Tunneling recombination in spatially inhomogeneous structures

Methods for finding the parameters (energies and capture coefficients for electrons and holes) of the levels involved in the formation of the recombination flux are suggested. The temperature variation of these parameters for AlGaN/InGaN/GaN and InGaN/SiC structures is discussed. The parameters of the levels responsible for tunneling recombination are determined.

Density-of-states in microcrystalline silicon from thermally-stimulated conductivity

The technique of thermally-stimulated currents has been applied to extract the density-of-states profile in microcrystalline silicon. Exploiting the experimental parameter space a consistent density-of-states profile emerges with an exponential conduction band tail and a broader deeper distribution. Calibrating the absolute density-of-states profile from other techniques like modulated photoconductivity, steady-state photocarrier grating technique and intensity-dependent photoconductivity allows a determination of the capture coefficient of the probed localized states.

The nature of the defect states above midgap and their role in the light-induced metastability of a-Si:H

Defect states of a-Si:H above midgap, with which the electrons interact through trapping and thermal release and make an important contribution to the imaginary (Y) term of the modulated photocurrent, are determined. Analysis of the ‘transition’ region between the low- and high-frequency regimes enables the densities and capture coefficients of defect states to be probed. A reasonable fit to the data is obtained by assuming three defect states, and it is suggested that the defects with the higher capture probability are hydrogen-related centers that are removed during the initial stage of light degradation.

Determination of the density of states of semiconductors from steady-state photoconductivity measurements

We discuss a method to obtain the density of states of photoconductive semiconductors from the light-intensity-dependence of the steady-state photoconductivity. Considering a material having different species of gap states – i.e., with different capture coefficients – we deduce a simple expression relating the defect density to measurable quantities. We show that the relevant capture coefficient appearing into the formula is that of the states that control the recombination. We check the validity of the approximations and the applicability of the final expression from numerical calculations. We demonstrate the usefulness of the method by performing measurements on a standard hydrogenated amorphous silicon sample.

R&D and design for the cryogenic and mechanical vacuum pumping systems of ITER

The next generation thermonuclear fusion machine ITER comprises large cryogenic pumping systems to process very high gas throughputs. The paper starts from a summary of the requirements for the torus exhaust pumping system and the neutral beam injection system for plasma heating and discusses the design solutions derived to match them. All ITER force-cooled cryopumps are based on a modular set-up and incorporate similar design of charcoal-coated cryopanels cooled to 4.5 K with supercritical helium. A systematic approach for how to design a tailor-made cryopump is presented. It is based on Monte Carlo simulation to calculate the overall capture coefficient being the primary design parameter. Experimental results measured with a model pump of representative scale are used to validate the design procedure. In the last part, the paper addresses the design issues to be solved for tritium-compatible forepumps with high pumping speeds.

Relationship between Phase Shift, Square-Wave Response and Density of States in Modulated Photocurrent Spectroscopy

AbstractLow- and high-frequency modulated photoconductivity measurements (LF and HF MPC) have been made on amorphous silicon films prepared by the expanding thermal plasma (ETP) and RF PECVD techniques. Time constants have been measured by decay of square wave excitation and behavior of complex frequency response. The influence of quasi-Fermi level position has been examined. Band tail slopes of 32 meV (ETP) and 37 meV (PECVD), and defect densities of order 1018 and 1017 cm-3 eV-1 respectively are found. Tail state capture coefficients of order 3×10-8 cm3 s-1 are calculated from overlapping LF and HF regimes. Defect state values for ETP (< 10-8 cm3 s-1) are smaller than for PECVD silicon films (> 10-7 cm-3 s-1).

Two-level model of longitudinal magnetic field-induced current instability and chaos inn-GaAs

We extend our previous application of the two-impurity-level model to interpret the cross-over current instability for a $n$-GaAs semiconductor with planar ohmic contacts under an external magnetic field $B$ longitudinal to the applied dc bias. The previous assumption of spatial homogeneity in current flow direction as well as previous considerations of the Landau level shifts for the conduction band electrons and the electron mobility with the field and the carrier electron temperature dependence are all retained. In addition, we further add in a phenomenological effective magnetoresistance $h$ factor from the generation-annihilation dynamic of the filamentary current and the carrier temperature dependence of the impact ionization coefficient from the ground level ${X}_{1}$ and that of the capture coefficient ${T}_{1}^{s}$ in the generation-recombination processes. Increasing the magnetic field above a critical magnitude in the post-breakdown region of the S-shaped current density-field characteristic, we are able to find the system bifurcating to chaos via several period-doubling routes at various bias voltages. Our numerical results are consistent with those observed experimentally by Aoki, Kawase, Yamamoto, and Mugibayahi [J. Phys. Soc. Jpn. 59, 20 (1990)].

Capacitance transient study of the metastableMcenter inn-type4H−SiC

The metastable $M$ center in $n$-type $4H$ silicon carbide is studied in detail after it has been introduced by $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ proton irradiation with a fluence of $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The experimental procedures included deep-level transient spectroscopy, carrier capture coefficient and capacitance versus temperature measurements, and pulse-train measurements. The pulse-train measurements are reproduced by simulations. Three band-gap levels have previously been assigned to the $M$ center: ${M}_{1}$ at ${E}_{C}\ensuremath{-}0.42\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, ${M}_{2}$ at ${E}_{C}\ensuremath{-}0.63\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and ${M}_{3}$ at ${E}_{C}\ensuremath{-}0.83\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, where ${E}_{C}$ is the conduction-band edge. Direct measurements of the majority-carrier capture cross sections show that the cross section values extracted from Arrhenius plots are about two orders of magnitude too large, indicating a large entropy factor. A detailed configuration diagram of the $M$ center is presented, including charge state levels and reconfiguration barriers. Evidence in support of a fourth $M$ center level, not explicitly observed, is presented. Isochronal and isothermal annealing experiments show that the $M$ center anneal out between 310 and $370\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in a process displaying first-order kinetics. The annealing process, which is shown to have an activation energy of $2.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, is identified as dissociation.

Diffusional Deposition of Heavy Submicron Aerosol Particles on Fibrous Filters

The deposition of Brownian submicron aerosol particles of high density in fibrous filters with allowance for interception effect, influence of gravitational, van der Waals forces, and the gas slip on the surface of ultrafine fibers was considered. Based on the numerical solution of the equation of convective diffusion in the field of external forces, the capture coefficient was calculated as a function of particle size and density, the angle between the vectors of gravity force, and the face flow velocity. It was shown that, for descending flow, the radius of most penetrating dense particles appeared to be noticeably smaller than for the ascending flow.

Potential transients on a renewed electrode as a source of information on the surface concentration of catalytically active adatoms

Potential transients on renewable gold and silver electrodes in cyanide solutions containing catalytically active ions of lead and thallium are measured. The transients are recorded both on freshly renewed electrodes and on renewed electrodes kept in solution for specified time periods Δt. The behavior of transients following a change in the concentration of lead and thallium ions, current density, and Δt qualitatively conforms to regularities that could be expected if the catalytically active adatoms were inserted into the deposit similarly to the admixtures. Methods for determining coefficients of capture of such adatoms by the growing metal deposit are suggested. Factors that hamper determination of these coefficients and make it worthwhile to select a method for their determination that would use potential transients are considered. The capture coefficient for the incorporation of lead adatoms in the growing silver deposit is nearly 20 times that for gold.

Inertial deposition of ?heavy? aerosol particles in fibrous filters

The inertial deposition of aerosol particles of a high-density material on filter fibers is studied. By the boundary trajectory method, the capture coefficients are calculated as functions of the size and density of particles under various gas filtration conditions at different filter parameters with simultaneous allowance for the particle inertia, interception, gravity, van der Waals forces, and gas slip on fine fibers. The case is considered where an undisturbed flow upstream of the filter is directed at an angle to the sedimentation velocity vector. The conditions under which particles are not deposited on fibers are found.

Light-induced changes in the gap states above midgap of hydrogenated amorphous silicon

The energy dependence of the capture coefficients and the density of states (DOS) above midgap of hydrogenated amorphous silicon (a-Si:H) in annealed and light exposed states are examined by modulated photocurrent experiments. In the annealed and light exposed states, the electrons are found to interact with two different kinds of gap states through trapping and thermal release. The densities of both gap state distributions at trap depths shallower than 0.67eV below the conduction band decrease rapidly during the initial stage of light degradation. The DOS of the annealed and light exposed states are found to cross at about 0.67eV. In addition, a parallel increase in the capture coefficients of the gap states at trap depths lower than 0.67eV is observed indicating a light-induced disorder. The above behavior is explained with the conversion of strained Si–Si and Si–H–Si bonds, which become highly strained during illumination, into dangling bonds near midgap.

Electric-field-enhanced electron capture coefficient of EL2 level in semi-insulating GaAs

The electric-field dependence of electron capture coefficient to EL2 donor level in semi-insulating GaAs has been quantitatively determined from the analysis of the sublinear J–E characteristics up to 1150V∕cm before the onset of the low-frequency current oscillation. The three-probe guard-ring method was employed to obtain precise J–E characteristics. By considering the rate equation of the capture and emission process in the steady state, a simplified formula was derived to determine the electric-field dependence of capture coefficient. Our results agree well with those calculated from the multiphonon emission capture model.

The Effect of van der Waals Forces on the Deposition of Highly Dispersed Aerosol Particles on Ultrafine Fibers

The effect of van der Waals forces on the collection of highly dispersed aerosol particles with ultrafine fiber filters was studied theoretically. The capture coefficient was found from the numerical solution of the equation of convective diffusion with the account of the particle size, the effect of van der Waals forces acting between a particle and a fiber, and the gas slip effect at the surface of ultrafine fibers. It was shown that allowance being made for van der Waals forces markedly affects the capture coefficient within the maximal particle penetration range and that the radius of the most penetrating particles decreases with the rising effect of these forces.

Investigation of bandgap states using the modulated photocurrent technique in both high and low frequency regimes

The modulated photocurrent (MPC) technique is reviewed, emphasizing both the widely used high frequency regime and the newly developed low frequency regime in order to characterize localized bandgap states. The matching of the density of states reconstructed from both regimes in principle allows a good determination of the ratio of the capture coefficient to the extended states mobility of the type of carriers giving the predominant contribution to the MPC. Experimental discrepancies are discussed in the framework of non-homogeneous generation rates and the existence of states having quite different capture coefficients.