Capture Coefficient Research Articles

Comparison of Experiment and Theory of the Photoconductivity of a-Si:H up to a Generation Rate of 1028cm−3s−1

AbstractWe have studied the dependence of the photoconductivity σp on photocarrier generation rate G in intrinsic a-Si:H at 300K between G=1012cm−3s−1 and 1028cm−3s−1. Below a certain value Go, we find σo =AGγ with γ=0.9±0.05 and the values of A vary considerably with defect concentration Nd which signifies monomolecular recombination through defects. Above Go the recombination is bimolecular, γ=0.5±0.02 and A=(6±3)×10−15 Ω−1cm1/2s1/2 is indpendent of Nd. The transition value Go is about 3×1020cm−3s−1 for high quality annealed a-Si:H and increases with Nd. A simulation of σp(G) assuming conduction in and recombination from extended states fits our experiments within a capture coefficient Ct=(6±2)×10−9cm3s−1 of carriers to their opposite tail states. Our Ct is close to the value (5±2)×10−9cm3s−1 obtained from optical measurements but higher than (0.5±0.1)×10−9cm3s−1 determined from photoelectric studies. Below T=150K our model calculations overestimate σp because the tunneling transitions, becoming important for recombination and conduction, are neglected.

Modulated photoconductivity study of charged and neutral defects in undoped amorphous silicon

A novel spectroscopy technique based on modulated photoconductivity measurements with varying illumination level has been applied to investigate the capture coefficients and the energy distribution of defect states in undoped amorphous silicon. From the experimental data, charged and neutral defect distributions are clearly resolved according to their own capture coefficients. The carrier capture process as well as the defect formation mechanism are both quantitatively discussed.

An analytical method for the thermoluminescence growth curve and its validity

The radiative recombination probability P of a carrier released from an active TL trap is computed for the case when the TL traps are interactive with the deep traps. It is shown that the value of P remains constant throughout the TL readout heating when the capture coefficients of the recombination centre and the deep trap are equal. When these coefficients are not equal, P undergoes change but its variation with temperature is still insignificant under a wide variety of conditions. A net value P applicable to the area of the glow curve is calculated and compared with the initial (when heating is begun) value of P, which is used in the analytical expression for the area of the glow curve.

Impact of field-dependent electronic trapping across Coulomb repulsive potentials on low frequency charge oscillations

We have performed Monte Carlo simulations to obtain the field dependence of electronic trapping across repulsive potentials in GaAs. Such repulsive centers are associated with deep level impurities having multiply charged states. Our results reveal a field-dependent maxima in the electronic capture coefficient, and the overall shape is seen to depend on the background electron density due to the effects of screening. Based on the Monte Carlo calculations, we have examined the stability of compensated semiconductors containing such repulsive centers. Our analysis indicates a potential for low frequency charge oscillations which is in keeping with available experimental data.

Analysis of transients in semiconductor/semi-insulator junctions

A junction between an epitaxial semiconductor and a semi-insulating substrate serves as a tool for analyzing transient behavior in semi-insulators (SI). At equilibrium a narrow region which is fully depleted from electrons exists in the SI. The application of a reverse bias results in an additional partially depleted region, initially depleted just from free carriers, adjacent to the fully depleted one. As the transient progresses electrons are released from the deep traps in the SI. The partially depleted region collapses into a wider fully depleted region. This process is manifested by a substantial current transient through the SI. The charges emitted recombine in the epitaxial layer leading to a decrease in its conductance due to a narrowing conducting path. Thus it is possible to characterize this process by measuring the (large) current through the epilayer rather than the (small) current through the SI. These transients are a function of the emission coefficient. Their analysis provides data also on the energy gap of the traps, their capture coefficient, and the electron lifetime. The long decays can be accelerated by illuminating with photons of energy below the band gap but larger than the energy spacing between the deep traps and the conduction band.

Characterization of deep traps in semi-insulators by current transients

Deep traps in semi-insulators (SI) are characterized using a junction composed of an epitaxial p-type layer grown on SI n-type layer. At a reverse bias electrons are released from the traps resulting in a current transient through the substrate. Simultaneously the depletion region in the epilayer expands until the entire layer is depleted leading to a decaying epitaxial current. The analysis of these transients renders the electron emission and capture coefficients and lifetime, and the energy location of the traps. The long current decay are accelerated by illuminating the sample with photons of energy below the band gap, as long as their energy is larger than the difference between trap energy and the bottom of the conduction band. Thus we determined directly this energy difference.

Contactless local determination of recombination centre parameters in Cd0.3Hg0.7Te by infrared laser interferometry

We describe a method, and the results thereof, for contactless local determination of the excitation energy of recombination centres, their concentration, and the electron and hole capture coefficients in Cd0.3Hg0.7Te. The method suggested is based on analysis of the experimental temperature dependence of electron and hole lifetimes obtained by infrared laser interferometry.

Structural and photoelectronic properties of thallium indium sulphide

A structural investigation by Transmission Electron Microscopy confirmed the presence of planar faults parallel to the (001) tetr-plane of TlInS 2. Their density corresponds to a mean repeat distance of about eight times the lattice parameter c. On the other hand, two energy levels forming two subbands and acting as hole traps at 0.028 and 0.050 eV above the valence band edge of TlInS 2 were determined by an analysis of modulated photocurrents induced by an intensity modulated light beam. Their capture coefficients were of the order of magnitude 10 −13 and 10 −15 cm 3s −1 at 300 K, respectively.

Frequency‐Resolved Admittance Measurements on InAlAs / InGaAs / InAlAs Single‐Quantum Wells to Determine the Conduction Band Offset and the Capture Coefficient

Frequency‐resolved admittance measurements have been used to determine the conduction band offset and the capture time constant of single‐quantum well (SQW) structures. A theoretical analysis of the SQW admittance taking into account nonparabolicity of the conduction band is reported. Analytical expressions for both capacitance and conductance as a function of both bias and frequency have been deduced and used to fit the experimental data. As a result the conduction band offset and the capture time constant have been obtained. Two different well compositions, lattice‐matched and strained, have been studied. In both cases the well width was 5 nm. The obtained conduction band offset at room temperature is for and for . The capture coefficient of the carriers in the first subband in the well is in the range for and in the range for .

Colloidal Properties of Flocculent and Nonflocculent Brewing Yeast Suspensions

AbstractThe orthokinetic flocculation rates of four suspensions of Saccharomyces cerevisiae and Saccharomyces uvarum (calsbergensis) were measured using steady shear rheometry and light microscopy. These results, along with cell ζ potential values previously reported by the authors, allowed us to test the applicability of DLVO (Derjaguin, Landau, Verwey, and Overbeek) mediated orthokinetic flocculation models. The findings also permitted examination of the validity of the elastic floe model and allowed estimation of the force required to separate cell doublets. Cellular suspensions of brewing yeast were subjected to a high shear treatment to reduce any floes to single cells. Periodic samples were then taken while shearing the suspension at 200 s−1, and the extent of flocculation was determined via light microscopy. The logarithm of floe concentration was linearly dependent on shearing time. It was possible to estimate orthokinetic capture coefficients (αo) for the four strains, with values ranging from 0.00025 to 0.0010. By using measured and “typical” colloidal parameters, a theoretical αo value was derived which was much lower than the experimental values (i. e., αot ≪ 0.0001). Thus, it was concluded that the DLVO flocculation model could not be used to explain brewing yeast flocculation. Flow behavior of the yeast suspensions followed the Bingham model above a critical shear rate. Values of this critical shear rate (which predicts the point at which cell doublets separate) were used to calculate the force required to separate doublet cells. A calculated force of 2.2 × 10−11 N was similar to that of lectin‐like bonds that are believed to be involved in yeast flocculation. As well, the elastic floe model could be employed to predict Bingham yield stress values.

Energy dependence of the carrier capture coefficient for neutral shallow impurities in semiconductors

The authors' consider the effect of carrier resonance scattering in slightly doped compensated semiconductors on the carrier capture by neutral shallow impurities, resulting in the formation of H--like centres. The energy dependence of the capture coefficient alpha ( epsilon ) exhibits a weak maximum at Ei, the carrier affinity to the H--like centre. The photoconductivity and photo Hall effect are studied for Si:B samples with a boron concentration NA<1016 cm-3 and a degree of compensation K<10-3 at liquid helium temperature in 'heating' electric fields. The obtained energy dependence of the hole capture coefficient for neutral boron atoms is in agreement with the authors' calculations.

Identification of a nonradiative recombination center in GaAs

We report on the identification of the CuGa acceptor level as a recombination center in GaAs. Using time-resolved photoluminescence (PL) we have studied the recombination of excess charge carriers in metalorganic vapor-phase epitaxy GaAs/AlGaAs double heterostructures. The recombination in one particular set of samples was clearly nonradiative and the trap level derived from our measurements coincides with that of CuGa as seen in the PL spectra. The temperature dependence of the capture coefficients is consistent with a multiphonon process and allows for the determination of the coupling strength for electron and hole capture.

Thermal quenching of bound exciton emission due to phonon-induced non-radiative transitions: experimental data for CdTe and InP

The thermal quenching of (A0X) and (D0X) emission lines due to processes of the type (A0X) to A0+X and (D0X) to D0+X, respectively, is discussed in terms of nonradiative phonon-induced transitions. The onset and the strength of the quenching are determined by the exciton-impurity binding energy as well as the amount of lattice relaxation accompanying these transitions. The latter effect, which has obviously been overlooked in earlier discussions, depends upon the degree of localization of the 'persistent' particle (hole or electron, respectively) and is relatively strong in the case of the 'deep' acceptors (such as CuCd and AgCd in CdTe). The quantitative correlation of the quenching parameters (and also of the capture coefficients related to the inverse processes) with the ionization energies of A0 and D0, which is expected on this basis, is confirmed by experimental results for CdTe and InP.

Decay of Electrostatic Surface Potential on Insulators via Charge Injection, Transport and Trapping

Decay of the surface potential on an initially corona charged insulator via charge injection, transport and trapping is theoretically examined and a general partial differential equation is derived for the space and time dependence of the electric field E(x, t) in terms of the charge carrier mobility, µ, trapping time, τ′, and the trap capture coefficient Ct. The present formulation specifically takes into account that as deep trapping proceeds, the rate of trapping decreases due to trap filling. The numerical solution of this differential equation gives the instantaneous electric field, E(x, t), profiles in the sample which can be integrated to obtain the time evolution of the surface potential, V(t), rate of discharge, dV/dt, final residual potential, Vr, and also the fraction of traps filled as a function of µ, τ′ and Ct. Two experimental conditions are considered. (a) The time dependence of the electric field, E(0, t), at the surface is well defined by the charge injection process, which in the present case, corresponds to a weak step illumination. (b) The spatial dependence of the initial field, E(x, 0), is well defined by an appropriate short light pulse illumination at t=0.

Localized levels parameters in CdInGaS 4 single crystals as determined by photoconductivity measurements

Photoconductivity measurements with the alternative light method were carried out on CdInGaS 4 single crystals in order to determine the parameters of the localized levels present in its energy gap. Nine discrete electron traps were resolved and their levels, concentrations and electron capture coefficients were estimated. Their nature is discussed and their concentrations as obtained by photoconductivity measurements are compared with concentrations deduced from TEM studies.

X-ray induced hole trapping in electroradiographic plates

Changes in the electrical properties of pure a-Se and Cl-doped a-Se:0.35% As electroradiographic layers upon exposure to x-ray radiation has been examined using xerographic techniques based on the first residual potential VR1 and the cycled-up residual potential VR∞ Hole lifetime τ as determined from the Warter expression VR1=L2/2μτ has been found to decrease sharply with the x-ray dose, tending to saturate at higher dose levels in both pure a-Se and Cl-doped a-Se:0.35%As. The hole liftime following x-ray exposure recovers toward its equilibrium value at the anneal temperature. The recovery time is dependent on the dose and is accelerated appreciably by temperature, indicative of a thermally activated process. Cycled-up xerographic residual-potential measurements have indicated that the deep-hole-trap population increases with exposure to x rays. Comparison of cycled-up saturated residual potential and first-cycle residual-potential measurements have shown that as a result of x-ray exposure, the apparent capture coefficient increases, which can only be accounted for by introducing two x-ray-induced effects: (a) generation of deep hole traps and (b) formation of an equal number of trapped electrons and holes in the bulk due to irradiation. Recombination of holes with the trapped electrons in the bulk causes the apparent increase in the capture coefficient. A xerographic spectroscopy technique, based on the analysis of the rate of discharge of the saturated potential after the end of xerographic cycling, was used to map out the energy distribution of the occupied density of states in the mobility gap. It was found that the energy location of the peak of the occupied density of localized states remained unaffected by the exposure to x rays, but the peak value of the density of states increased with the exposure. The x-ray induced trap generation is thus simply an enhancement of the integrated concentration of inherent structural defects in the films located at their characteristic energies: 0.87 eV in pure a-Se and 0.90 eV in Cl-doped a-Se:0.35%As. The present results are consistent with the x-ray-sensitivity predictions of Kalade, Montrimas, and Rakauskas [Phys. Status Solidi A 25, 629 (1974)].

Kinetic analysis of seeded emulsion polymerization of vinyl acetate

AbstractParticle number and size data from a series of seeded, emulsifier‐free, vinyl acetate emulsion polymerization experiments have been analyzed with the aid of polymerization and particle growth models. A secondary population of particles, with a significantly greater number concentration than the seed, was nucleated in all experiments. The two populations (seed and new) had rather narrow size distributions and large diameters. Hence the reactions were in the area normally associated with Smith–Ewart Case III kinetics. Water‐phase termination reactions can be important in this reaction region but radical desorption from such large particles does not significantly influence the kinetics. The results of the analysis were used to evaluate the magnitude of water‐phase termination; to estimate radical capture coefficients; and to evaluate competitive particle growth.

Inhomogeneities in silicon p-n-junctions

Latent macroscopic defects in silicon are detected by electrical and electron microscope measurements. They lead to anomalous temperature dependence of the Fermi level position and growth in the hole capture coefficient. A level with energy of 0.55 eV measured from the conduction zone controls the recombination process. It is proposed that macroscopic defects develop upon association of oxygen-silicon vacancy complexes. Action of an electron beam leads to reversible changes which increase upon multiple scanning, affecting the value of the diffusion length.

Electrostatic deposition of inertially moving charged aerosol particles onto the earthed disk

Deposition of charged aerosol particles onto the earthed disk, placed within the flow of particles is analysed with inertia and electric field of the particles taken into account. Equations of motion for particles and Poisson's equation for the electric field potential are solved numerically for different values of dimensionless parameters determining the aerosol flow. Carrier gas is supposed to remain quiescent. The macroparticle method is used to obtain the one-dimensional steady distributions of particle concentration, velocity and electric field far from the obstacle. Two basic flows, corresponding to large and moderate values of Stokes' number St, are indicated and studied. The results of the calculations are compared with the analytical solution of the problem in the case St → ∞. Numerical solution of the two-dimensional problem for steady motion of disperse phase near the disk shows that deposition of particles onto the obstacle strongly depends on Stokes' number: at St ⪢ 1 there is a particle-free shadow behind the disk, and particle trajectories can intersect. At St ≲ 1 the particle velocity field is single-valued, and deposition onto the back side of the disk is possible. Capture coefficients, which represent the main characteristics of particle deposition, are calculated separately for each side of the disk. Typical distribution diagrams for particle concentration and electric fields are given.

Charge carrier trapping in chalcogenide semiconductors

Interrupted field time-of-flight (IFTOF) transient photoconductivity measurements have been performed in conjunction with first residual and cycled-up xerographic measurements on pure a-Se and chlorinated a-Se:7.4%Te and a-Se:0.2–0.3%As electrophotographic films. The hole lifetime has been measured as a function of Cl doping and location in the sample. Cycled-up xerographic measurements were used in conjunction with IFTOF experiments to estimate the capture coefficient C t and diffusional capture radius r c .