Electron Trapping Mechanism Research Articles

Micro Electron Bunch Acceleration and Trapping by Intense Short Laser Pulse in Vacuum

We propose an electron bunch acceleration and trapping mechanism by an intense short laser pulse in vacuum. By using a short laser pulse obtained by the linear combination of a TEM10 mode and a TEM01 mode with the same amplitude and the relative phase shift of π/2, the electrons are accelerated in the longitudinal direction by a longitudinal ponderomotive force, and the electrons divergence in the transverse direction can be suppressed by a transverse ponderomotive force. In order to analyze the electron motion, the relativistic equation of motion including a relativistic radiation damping effect is solved. In our calculations, the electron is accelerated to 179 [MeV] with an acceleration gradient of 5.12 [GeV/m] at the laser intensity of 1.23×1018 [W/cm2]. The rms emittances of the electron bunch accelerated are (1.06, 1.21, 0.348) [mm mrad] in the x, y and z directions, and the spatial size of this electron bunch accelerated in the radial and longitudinal directions are 856 [μm] and 11.7 [μm], respectively.

Role of As precipitates on ultrafast electron trapping in low-temperature-grown GaAs and AlGaAs alloys

The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons have been studied in a series of low-temperature-grown GaAs and AlGaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross-sectional electron-transmission microscopy was used to determine the density and size of the precipitated arsenic clusters as a function of the growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically investigated by time-resolved pump–probe transient transmission spectroscopy. The electron trapping times follow a τ∝R3/α law which holds for low-temperature-grown GaAs and is applicable for low-temperature-grown AlGaAs as well. This trend generalizes the electron trapping mechanism for low-temperature-grown and annealed GaAs and related semiconductor alloys.

Mechanism of electron trapping in Ge-doped SiO2 glass

We present a possible mechanism of electron trapping in Ge-doped SiO2 glass on the basis of first-principles quantum chemical calculations on clusters of atoms modeling the local structures in the glassy system. The calculations suggest that the so-called “Ge(1) and Ge(2)” centers are distinctly different electron centers rather than GeO4− variants and that an electron trapped at an oxygen vacancy site is a promising candidate for Ge(2).

4Electron self-trapping in intermediate-valentSmB6

SmB6 exhibits intermediate valence in the ground state and unusual behaviour at low temperatures. The resistivity and the Hall effect cannot be explained either by conventional sf-hybridization or by hopping transport in an impurity band. At least three different energy scales determine three temperature regimes of electron transport in this system. We consider the ground state properties, the soft valence fluctuations and the spectrum of band carriers in n-doped SmB6. The behaviour of excess conduction electrons in the presence of soft valence fluctuations and the origin of the three energy scales in the spectrum of elementary excitations is discussed. The carriers which determine the low-temperature transport in this system are self-trapped electron-polaron complexes rather than simply electrons in an impurity band. The mechanism of electron trapping is the interaction with soft valence fluctuations.

Evidence for electron trapping by Sm dimer and trimer centers in Sm doped CaF2

Using coherent anti-Stokes Raman and linear absorption spectroscopy we present experimental evidence for a novel electron trapping mechanism in Sm doped CaF2. The appearance of additional Raman lines after ionization of the Sm2+ ions, their dependence on the Sm3+ concentration and their behavior due to an enhancement by the lowest 4f→5d transition of Sm2+, support strongly that electrons can be trapped by dimers or trimers of Sm3+ ions in a CaF2 lattice. The resulting color change is reversible due to a possible reionization of the traps, rendering this material photochromic.

Electron Localization in Liquid Methanol. Lifetime of the Pre-existing Electron Traps

The structure of liquid methanol, obtained by computer simulation, has been analyzed in order to identify the regions of attractive potential that can serve as pre-existing sites for primary localization of an excess electron. Only about 4% of the regions of attraction produce the binding level and are able to accomodate the electronthese regions have been considered as pre-existing electron traps. The static and dynamical properties of the traps have been investigated and described in terms of the statistical distributions of their geometrical parameters and their lifetimes, respectively. The analysis performed for liquid methanol at room temperature shows that the average lifetime of the electron traps is on the order of a few tens of femtoseconds. However, the concentration of the pre-existing traps that can survive over a period of several hundred femtoseconds is sufficiently high to justify the commonly assumed “trap-seeking” mechanism of electron trapping in polar media.

Electronic processes in chemically-sensitized emulsion grains of silver bromide

Microscopic information on the localized electronic states in chemically sensitized emulsion grains of silver bromide is obtained by luminescence-modulation spectroscopy at liquid helium temperature. Electronic functions of chemical sensitization which is absolutely necessary for the high photographic sensitivity are twofold. One is the formation of hole-trap to suppress electron-hole recombination in the case of reduction- and hydrogen-sensitization. The other is the formation of a deep electron-trap to trigger the nucleation of latent-image in the sulfur- and sulfur-plus-gold-sensitization. The electron-trap essential for sulfur-sensitization is concluded to be due to the dimer of the (substitutional sulfur ~ interstitial silver) complex. A structural model and mechanism of deep electron-trapping are proposed for the sulfur dimer center, and the latent-image nucleation process is discussed for sulfur-sensitized emulsions.

Electron trapping mechanisms in an irradiated single crystal of quartz

Electron trapping states in the near surface region of electron irradiated single crystal quartz have been studied using thermionic and photon simulated electron emission. Electron bombardment at energies between 1 and 4 keV causes oxygen desorption and produces defects and electron trapping states in the material which remain populated for long times. After irradiation the sample is heated and the electron emission is measured as a function of temperature. Five thermionic emission peaks were identified two of which only occur when the sample temperature is below 175 K during irradiation. These peaks occur at temperatures related to the binding energies of the traps. The highest binding energy peaks correlate with oxygen depletion of the surface and we associated these with vacancies both at bulk and surface sites. By measuring the photoelectron emission intensity as a function of incident photon energy, five corresponding photoemission thresholds were identified which are related to electron traps with binding energies ranging from 2.3 to 4.6 eV. Overall, the thermionic emission peaks and corresponding photoemission thresholds provide a direct measure of the trap energies for both intrinsic and radiation induced defects in SiO2.

Experimental evidence for trap-induced photoconductive kink in AlGaAs/GaAs HEMTs

The microwave small-signal and high frequency noise performance of AlGaAs/GaAs HEMTs have been assessed, both with and without illumination at room temperature. The measurements were carried out over the bias range where the photo-induced kink in the DC output characteristics is observed. The results obtained have been compared with our previously reported kink phenomenon in the output characteristics of these devices. The postulated electron trapping mechanism is confirmed as the cause of the kink

Recent measurements of electron trapping and phase area displacement of nonrelativistic electrons employing an electromagnetic wiggler

We report the distinct observation of electron trapping and phase area displacement energy transfer mechanisms in a stimulated Compton scattering free electron laser scheme. The synchronous energy exchange occurred between nonrelativistic electrons and the ponderomotive (beat) force of two counter-propagating intense pulsed CO 2 laser beams. The direction of the energy exchange was dependent on the position where synchronism occurred in the interaction region, as predicted by theory for electron trapping and phase area displacement.

Characterization of thermally oxidized n+polycrystalline silicon

The properties of thermally grown silicon dioxide films on n+polysilicon are studied using cross-sectional TEM, and electrical measurements to evaluate conduction, electron trapping, destructive breakdown and wearout mechanisms. All of the above electrical parameters are found to be degraded by any increase in the degree of surface roughness at the oxide-polysilicon interface. Our results suggest that a significant improvement in the insulating properties of the SiO 2 films can be achieved if the polysilicon is initially deposited in the amorphous phase at 560°C rather than the polycrystalline phase at 620°C. For example, for dry-oxidized diffusion-doped films, there is an increase in oxide breakdown field from 3.0 MV . cm-1to 6.2 MV . cm-1, and a reduction in leakage (Fowler-Nordheim) current of two orders of magnitude. Furthermore, it is shown that the long-term reliability of n+polysilicon/SiO 2 /n+polysilicon structures is directly related to the degree of interface texture; i.e., a smoother interface will result in a significant reduction in electrical wearout and an increase in time to failure.

Trapping and luminescence mechanisms in manganese-doped zinc silicate phosphorsa tunneling model

We have investigated electron trapping and luminescence (phosphorescence, thermoluminescence and photostimulated luminescence) mechanisms in Mn and Mn+As doped Zn2SiO4 phosphors. The intensities of both the phosphorescence and photostimulated luminescence are found to decay with time approximately as (time)-1. The generally accepted model of such a decay of phosphorescence, based on thermal release of electrons from a broad distribution of trap energy levels, cannot explain the observed decay of the photostimulated luminescence. In order to explain both results we have developed a new model based on radiative recombination by tunneling between holes trapped on Mn ions and electrons in shallow traps or in excited states of the deeper traps. We derive a simple expression which describes for both experiments at all times the decay of the light intensity and its dependence on experimental conditions. The agreement between theory and experiment is found to be excellent.

Self-consistent LCAO calculation of the electronic properties of graphite. II. Point vacancy in the two-dimensional crystal

The electronic properties of a point vacancy in the two-dimensional graphite crystal are investigated within the small-periodic-cluster approach using a self-consistent all-valence-electron LCAO (linear combination of atomic orbitals) scheme previously employed for the calculations of the band structure and optical spectra of the regular lattice (part I). Eight crystal bands, 54-96 $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$ points in the Brillouin zone, selected according to the mean value theorem and ${2}^{2}$-${5}^{2}$ primitive unit cells around the site are allowed to interact. A doubly degenerate singly occupied $\ensuremath{\sigma}$ level is shown to appear in the $\ensuremath{\sigma}\ensuremath{-}{\ensuremath{\sigma}}^{*}$ band gap, 3.5 eV above the $\ensuremath{\sigma}$ band edge, with a wave function that is about 80% localized on the three nearest-neighbor atoms. The density of electronic states, charge distribution and Poisson electrostatic potential of the structure are computed and used to discuss the characteristic feature of the in connection with Coulson's defect molecule model and with current models of electron trapping mechanisms used to interpret the experimental data on Hall coefficient, resistivity and diamagnetic susceptibility of damaged graphite. Both symmetric and Jahn-Teller lattice distortions are introduced around the site, the results being used to interpret the experimentally observed decrease in lattice constant, the observed optical absorption and the vibronic parameters of the Jahn-Teller effect. Symmetric lattice relaxations are shown to have a moderate effect on the lattice energy and on the position of the level, these changes being mainly due to the response of the $\ensuremath{\pi}$ subsystem to accumulation of excess $\ensuremath{\pi}$ charge on the surrounding bonds, while Jahn-Teller distortions are shown to have a small effect on the system due to the relative rigidity of the $\ensuremath{\sigma}$ skeleton. The energy of vacancy formation as well as the energy of atom displacement and vacancy migration are directly computed from the change in total lattice energy, the results being in good agreement with experiment. The importance of introducing charge self-consistency in treating the charge redistribution in the system as well as the significance of allowing more distant atoms to interact with the vacancy electrons, is emphasized.

Electron stabilization of drift-cone modes

The mechanism of electron trapping in E×B guiding center motion is proposed as the mechanism for absolute stabilization of drift-cone modes in mirror geometry. The calculated amplitudes are shown to be in reasonable agreement with experimental results in present mirror machines.

Some properties of trapped electrons, radicals and H atoms in glassy hydrocarbons

This autoreview summarizes studies made in the author's laboratory of the properties and reactions of electrons, radicals and hydrogen atoms produced in organic glasses at 77 K and below by γ-radiolysis and photolysis. The goal of the work has been to advance understanding of the mechanisms of formation, trapping and reaction of these intermediates. Optical and E.S.R. spectrometry, electrical conductivity and differential thermal analysis have been the techniques of analysis. The work has included selective wavelength bleaching of the I.R. spectra of trapped electrons in organic glasses: investigation of electron capture by free radicals and subsequent photodetachment from the carbanions; and determination of the heats of reaction of trapped electrons, ions and radicals when the glass is photobleached or warmed to permit diffusion. The effects of γ dose on the yield of trapped electrons, and of light intensity on the photobleaching of the e t − are considered. The data on electrons are discussed with respect to the evidence which they give on the electron trapping mechanism, the variety of trap depths, electron tunneling and multiple retrapping without neutralization. Evidence is given on the fraction the radicals produced by γ-irradiation of a hydrocarbon glass which decay by random diffusive encounters as compared to intraspur reaction. CH 3 radicals produced from methyl halides in 3-methylpentane glass by dissociative electron capture are shown to decay by abstraction of H from the C 6H 14 molecules, whereas in C 6D 14 the decay is attributable to geminate recombination (CH 3+I −→ CH 3I −). An example is given of the use of radical decay kinetics in hydrocarbon glasses to determine diffusion coefficients of small molecules which can react with the radicals. Hydrogen atoms are shown to be absent in the radiolysis of glassy hydrocarbons. These and related observations are discussed with respect to the information they give on the properties of the intermediates and on the total radiolytic process in the glasses.

Electron trapping and luminescence mechanism in thermoluminescence of irradiated polycarbonate

AbstractThermoluminescence of polycarbonate irradiated by electron beams was observed in order to investigate the trapping of electrons and the luminescence mechanism. The thermoluminescence glow curve is composed of three components. The peaks appear at −150°C (α peak), −120°C (β peak), and −30°C (γ peak). The α peak is attributed to recombination of cations with electrons released from trapping sites formed by carbonate groups. The β peak is attributed to small amounts of impurities. The γ peak is perhaps due to untrapping of electrons from the phenoxy anion. The luminescence intensity of the α peak is linearly dependent on the absorbed dose, and that of the β peak is approximately proportional to the cube of the dose. The appearance of these peaks is presumably closely related to local molecular motions.

Decay of Injected Electron-Hole Plasmas in p-Type InSb

The decay of high-density (4×1014−1012 cm−3) injected electron-hole plasmas in pure p-InSb (p0≈1014 cm−3) at 77 K is investigated in detail by means of a double-pulse technique that determines the decay of the plasma conductivity. The decay curves show two distinct modes: a high-density mode (>1013 cm−3) characterized by rapid decay and a low-density mode (<1013 cm−3) of relatively slow decay. The high-density mode is dominated by electrons and lasts for a period of time ranging from several hundred nanoseconds to only a few tens of nanoseconds depending on sample size. The effective time constants in the high-density mode decrease as the plasma density is reduced. This is true whether the reduction is accomplished by letting decay proceed in time or by starting decay at a lower initial density. This trend is observed independent of sample cross section and surface conditions, factors that do affect, however, the magnitude of the time constants. An electron trapping mechanism in the sample bulk accounts for this density-dependent effective plasma lifetime. The maximum high-density lifetime allowed by the trapping mechanism is T∞=0.9 μsec (accurate to within a factor of 2), but the observed time constants are generally smaller because of surface recombination. Since the measurements show no evidence of hyperbolic decay, a characteristic of radiative recombination, and since the theoretical radiative lifetime is much larger than the measured lifetimes, it is concluded that radiative recombination is not important in this decay process. When the low-density mode controls, the shape of the decay curves becomes independent of the initial plasma density. All curves in a family obtained from a sample of given size and surface condition have the same time constant. In the low-density mode the plasma is a one-component plasma composed of holes, the electrons having been bound in traps or returned to the valence band. In large cross-section samples (≥1×1 mm2), the holes decay with the well-known hole lifetime, 1–2 μsec. In both decay modes, surface recombination must be considered. This mechanism causes the measured time constants to be less than would be predicted on the basis of bulk recombination alone, and, in addition, it causes the measured lifetimes to decrease as the sample size is reduced or when the surface recombination velocity of small samples (<0.4×0.4 mm2) is increased. In the high-density mode the observed dependence of the initial time constant on the sample size is in agreement with an approximate theoretical treatment of surface recombination. This work, both theory and experiment, yields a value of the ambipolar diffusion length of 0.14 mm.

Pulse radiolysis study of the mechanism of electron trapping in alcohol glasses at 77°K

Trapped electron spectra in pulse-irradiated 2-propanol glass at 77°K shift toward the blue on a μsec timescale. Solute effects indicate that molecular reorientation around the electron is responsible for the transient spectral shift rather than a redistribution of electrons among traps.

Self-trapping of Radiation Produced Electrons

THE problem treated here is the trapping of free electrons produced by ionizing radiation in dielectric systems. Detailed theoretical descriptions of the equilibrium properties of the solvated electron are available1–3, but its formation is still under discussion. Electrons are formed and thermalized in the coulomb field of the positive ions, but not fully recaptured by the ions. Magee4 considered the auto-ionization of the H3O radicals as a possible secondary source of electrons; Freeman5–7 assumed the electrons to be trapped in the defect sites of the medium at the instant of thermalization, while we thought8,9 that in polar dielectrics the escape of the electrons from the ions can be a free-energy decreasing, and therefore spontaneous, process. In our approach, the electrons which escape are treated as quasi-free, and the question of the kinetics and mechanism of electron trapping arises. Diffusion kinetic calculations on the radiolysis of water10,11 suggest very high initial hydrated electron densities, which require fast trapping processes.

Mechanism of Electron Trapping in Silver Bromide Photographic Grains

The temperature dependence of the electron lifetime and of the time for decay of an internal applied electric field in the silver bromide microcrystals of a simple photographic emulsion has been measured by the technique employing flash exposure of the emulsion in pulsed electric fields. Activation energies were found to be 0.236±0.012 eV for the electron lifetime and 0.245±0.008 for the field-decay time. The agreement of the temperature dependence of the two processes, together with the observation that the room-temperature values of these two times are, without exception, nearly equal in measurements made on a variety of other emulsions, supports the hypothesis that the mechanism of permanent electron-trapping involves the motion of the silver ion to a temporarily trapped electron to form a stable center. These results are not consistent with the alternative suggestion, namely, that the electron is captured at a preexisting deep trap consisting of a silver ion associated with a crystal imperfection.