Electron Flux Distribution Research Articles

Monte Carlo dynamics below the Au-GaAs interface for ballistic-electron-emission microscopy

Monte Carlo simulations of hot electron transport in the {Gamma}, {ital L}, and {ital X} valleys of GaAs were performed to study the transport dynamics for ballistic-electron-emission microscopy (BEEM) of mesoscopic objects beneath the Au-GaAs interface. For the starting electron flux distribution at the interface, the distribution for the strong scattering limit of Smith-Lee-Narayanamurti theory [D. L. Smith, E. Y. Lee, and V. Narayanamurti, Phys. Rev. Lett. {bold 80}, 2433 (1998)] was used. For the subsequent electron transport within GaAs, phonon and intervalley scatterings were simulated. With increasing object depth beneath the Au-GaAs interface, the simulations predict (1) significant cooling of hot electrons, on the order of {approximately}3 meV/nm at 1.5 V, and (2) significant redistribution of electrons among the conduction valleys. These effects should be taken into account for proper interpretation of BEEM experiments, especially for large biases and deep objects. {copyright} {ital 1999} {ital The American Physical Society}

Modeling of nonlocal slow-electron kinetics in a low-pressure negative-glow plasma

The kinetics of slow electrons is studied in a low-pressure negative-glow plasma (NGP). A method based on the nonlocal approach is proposed, which allows the nonlocal (nonequilibrium) nature of slow electrons to be accounted for in a physically transparent and numerically efficient manner. The slow electrons are divided into trapped (cold, Maxwellian) and free (superthermal, non-Maxwellian). It is shown that the superthermal (free) electrons are particularly important because they carry current and supply energy to the system of cold (trapped) electrons. A nonlocal energy-balance equation for the trapped electrons is derived, in which heating by superthermal electrons and diffusion cooling are found to be among the most important mechanisms. Simple expressions for the diffusion-cooling rate and the wall potential are determined. The proposed method is validated by the numerical solution of the full kinetic equation for a NGP in Ar. The energy and space distributions of electron fluxes are analyzed, and flux reversal (in energy space) is observed and explained. A comparison to experiment is carried out and close agreement is obtained. The proposed method can be useful in building fully kinetic, self-consistent models of various NGP-based discharge devices.

On synchrotron radiation energy losses of trapped magnetospheric electrons

Abstract Using an energy balance equation it is explicitly demonstrated that synchrotron radiation (SR) significantly decelerates several MeV electrons confined on geomagnetic L -shells below 2 during their radial diffusive transport into inner magnetosphere. SR losses from the ambient electrons are found to be comparable in importance with Coulomb friction energy losses, a well-known and much modeled cause of energy degradation in the radiation belts. It may also be true that, under some geophysical conditions and at some electron energies, the effective electron lifetime due to wave-induced pitch-angle scattering can be shorter than both the Coulomb and the SR lifetimes. But even when this holds true, the effect of the competing SR energy loss process can substantially influence the outcome of model computations of the ambient inner zone electron flux, pitch angle, spatial and energy distributions. We find that in the inner radiation zone of the Earth, care must be exercised to include the simultaneous effects of all three competing electron loss mechanisms.

Specific features of electron distributions at altitudes of 400 km

New experimental data obtained on the orbital station ‘MIR’ in 1991 during solar maximum are discussed. Electron fluxes with E e>75 keV were registered for three different directions as well as for electrons with E e>300 and 600 keV. Spatial and time distributions of electron fluxes in the trapping region are presented. In the inner radiation belt an additional maximum is observed at L=1.25–1.35, and the fluxes in the 22-05 h MLT interval are 2–3 orders of magnitude smaller, than during other local times. In this region a flattening of the electron spectrum is observed. The results obtained were compared with the AE-8 model.

Interference in the photodecomposition of negative atomic hydrogen ions in an electric field

The quantum interference of electron waves in the photodecomposition of negative hydrogen ions in an external uniform electric field is examined. The structure of the photodetachment amplitudes is discussed. A simple analytic expression for the electron flux distribution is derived. Finally, it is shown that the structure of the electron flux is affected by the angular dependence of the wave function.

Fully adiabatic changes in storm time relativistic electron fluxes

It has been suggested that much of the drop and subsequent recovery of storm time relativistic electron fluxes at geosynchronous orbit can be explained in terms of a fully adiabatic response (all three adiabatic invariants conserved) to magnetic field changes. To calculate this effect, we assume a prestorm electron flux distribution constructed from CRRES satellite data, we use modular magnetospheric magnetic field models to represent the magnetic field configuration before and during the storm, and we use Liouville's theorem to evolve the prestorm electron flux. In this work we focus on the important special case of equatorially mirroring electrons. During the main phase of a storm with a Dst minimum of −100 nT we find that the fully adiabatic effect can cause a flux decrease of up to 2 orders of magnitude, consistent with observed flux decreases. We also find that the magnitude of the fully adiabatic flux decrease is larger for lower energies, again in agreement with observations. The contribution of prestorm electron fluxes to the recovery phase flux increase at synchronous orbit is expected to be small because of losses to the dawnside magnetopause. A comparison of fully adiabatic fluxes with measured electron fluxes for the November 2–5, 1993, storm indicates that for this event the fully adiabatic effect may be contributing to the observed decrease but that nonadiabatic effects are clearly important. Overall we conclude that the fully adiabatic effect can account for a significant fraction of observed flux decreases and that differences between the observed and the fully adiabatic fluxes help to clarify when and where additional loss and source mechanisms exist.

On the radial distribution and nonambipolarity of charged particle fluxes in a nonmagnetized planar inductively coupled plasma

The radial distribution of electron and ion fluxes to conducting and nonconducting walls in a planar inductively coupled plasma has been studied experimentally and theoretically. Measurements of electron and ion currents have been performed using electrostatic probe arrays. Radially resolved measurements of ion impact energies have been performed using an ion energy analyzer array. For conducting walls it is shown by calculations and measurements that electron and ion currents are not in balance locally but that diffusion is nonambipolar. The ion impact energies measured on a conducting surface show a significant radial variation in accord with our theoretical model. For nonconducting surfaces the ambipolar fluxes of electrons and ions result in the formation of a surface charge potential profile across the surface. Voltages of the order of several volts between the center and the periphery of the surface are measured.

Nanometer-scale lithography on Si(001) using adsorbed H as an atomic layer resist

We describe nanometer-scale feature definition in adsorbed hydrogen layers on Si(001) surfaces by exposure to low energy electrons from a scanning tunneling microscope tip. Feature sizes range from <5 to ≳40 nm as a function of bias voltage (5–30 V) and exposure dose (1–104 μC/cm). We show that the cross section for electron stimulated desorption of hydrogen has a threshold at 6–8 eV and is nearly constant from 10 to 30 eV, so that above threshold the feature profiles are a direct reflection of the electron flux profile at the surface. Radial flux distributions are best fit by a simple exponential function, where the decay length is dependent primarily on the tip–sample separation. Low intensity tails at large radius are also observed for high bias emission. Comparison to field emission simulations shows that our tip has an ‘‘effective radius’’ of approximately 30 nm. Simulations demonstrate that tip geometry and tip–sample separation play the dominant role in defining the electron flux distribution, and that optimum beam diameter at the sample is obtained at small tip–sample separation (low bias) with sharp tips. We show that adsorbed hydrogen is a robust resist that can be used as a mask for selective area deposition of metals by chemical vapor deposition. Fe lines 10 nm wide are deposited by pyrolysis of Fe(CO)5 in areas where H has been desorbed, with minimal nucleation in the H-passivated areas.

Inclusion of tunneling and ballistic transport effects in an analytical approach to modeling of NPN InP-based heterojunction bipolar transistors

We describe an analytical approach to modeling NPN InP-based heterojunction bipolar transistors using a Gummel Poon model. Starting from an initial, physical description of the device's epitaxial structure and geometry, this physics-based model is utilized to simulate the device's operation using a thermionic-field-diffusion model for carrier injection at the emitter and diffusive transport across the base. This approach incorporates recent advances in understanding of device operation and improved models of parasitics. The cutoff frequency and maximum frequency of oscillation are calculated as a function of collector current density and compared with experimental measurements. For the cutoff frequency, the results of this model underestimate the high frequency performance of the device by as much as 50% at the higher collector current levels. To improve the model and to allow simulation of narrower base widths, a more rigorous analysis of tunneling through the emitter spike and ballistic transport across the base and base-collector depletion regions has been included. Quantum mechanical tunneling of electrons through the emitter-base spike has been taken into account for accurate description of the energy distribution of electron injection into the base. The resulting electron flux distribution is used as an initial distribution in a regional Monte Carlo simulator to model electron transport from the emitter edge of the base to the subcollector. The results are then used to calculate an average base transit time and base-collector space charge region delay time which can be incorporated in the analytical model.

Monte Carlo simulation of electron motion in the cathode region of a magnetron glow discharge in argon

The motion of electrons in the cathode region of a magnetron glow discharge in low-pressure argon has been studied by Monte Carlo simulation. The influences of the transverse magnetic field on energy distribution of electron flux and ionization have been investigated for normal and abnormal discharges. It has been shown that the magnetic field increases collisions, and consequently decreases the mean electron energy. In normal discharges, under the magnetic field, fewer electrons enter the negative glow region with high energy, while ionization is remarkably enhanced in the cathode region. In abnormal discharges, it is found that the magnetic field required to influence the discharge has to be stronger than that in normal discharges. Under such a magnetic field, for example 0.3 T, though there are still a great number of electrons entering the negative glow region with energy corresponding to the cathode fall potential, the mean electron energy is decreased, and the electrons entering the negative glow region are less beam-like. Analyses for the simulation results and consideration of the practical use of the magnetic field have been given.

A technical description of the TSS-1 ROPE investigation

The Research on Orbital Plasma Electrodynamics (ROPE) investigation was designed to study the complex of phenomena and processes expected to exist around the highly biased satellite of the TSS-1 system. Specifically, ROPE was to study the characteristics of the high-voltage sheath and study ionization processes in the vicinty of the satellite. To accomplish this, two banks of sensors were mounted on the satellite. One was flush-mounted on the satellite's surface and the other was located at the end of the satellite's fixed boom. Of particular note is the ability to drive the boom probes' surface potential to the local floating potential. In conjunction with satellite spin, the surface-mounted sensors could map the distribution of electron flux to the satellite's surface while the boom-mounted sensors could measure the ion and electron fluxes in and out of the satellite's plasma sheath.

Photodetachment from negative ions in the static electric field: effect of strong electromagnetic field

The Green function is constructed for the electron moving in the superposition of uniform static and linearly polarized monochromatic electric fields. It is employed to obtain general integral expressions for electron flux distributions and partial widths in multiphoton detachment or ionization in presence of uniform static electric field. The photodetachment in weak static field is analysed in detail for arbitrary strength of the laser field. The separation of the uniform field and laser field effects allows one to obtain simple and appealing expressions for photodetachment partial widths via static-field-free amplitudes in case of arbitrary number of absorbed photons and quantum numbers of the initial state. The interference oscillations of the photoelectron current are described uniformly for arbitrary direction of the electric vector of the laser field with respect to the static field axis.

Influence of the energy spectrum of electrons ejected from the cathode on electron behaviour in the cathode sheath region of a DC glow discharge in helium and in argon

A multibeam electron flux model has been used to investigate the influence of a key input parameter, namely the energy spectrum of electrons released from the cathode via secondary emission, on electron behaviour in the cathode sheath region of a DC glow discharge in helium and in argon. By comparing the experimental results of Gill and Webb with those predicted by the model for a helium glow discharge, it is found that the structure of the high-energy region of the energy distribution of electron flux can be explained by assuming the electrons are released from the cathode with a range of energies between 0 and 14 eV, and experience only inelastic collisions during subsequent acceleration through the cathode sheath. The gross features of the electron multiplication in the cathode sheath are found to be relatively insensitive to the choice of the initial energy spectrum of the electrons.

Electron multiplication in the glow-discharge cathode fall

A one-dimensional model is formulated for electron multiplication in the cathode fall of a glow discharge. The associated Boltzmann equation in z and v is converted to an integrodifferential equation in new variables that can be solved numerically using standard methods for ordinary differential equations in a single variable. The resulting distribution function in the (z,v) phase space is also the energy distribution of electron flux, which is used to find the current multiplication factor and the energy transport per unit electron current. Even in low-voltage discharges with large current multiplication, the nonionizing collisions play a small role in determining the electron-energy distribution and swarm behavior. Comparison with highly complex simulation, and with experiment, is very favorable.

Oscillations of electron flux in photodetachment of H- in an electric field.

Outgoing electron waves produced in the photodetachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ are propagated to large distances in a homogeneous electric field, and the electron flux is then computed. It is shown that the interference of waves propagating along two distinctive paths from the region of the bound state of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ to the same point induces oscillations in the electron flux distribution. Simple analytic expressions are presented for the flux distribution.

A simulation of electron motion in the cathode sheath region of a glow discharge in argon

Electron motion in the cathode sheath region of a glow discharge in argon has been simulated for a linearly decreasing electric field using a one-dimensional computer model. Distribution functions for the electron flux crossing the sheath have been calculated for 12 values of the cathode fall between 180 and 1400 V encompassing the normal and abnormal regimes. Macroscopic variables including the Townsend ionisation coefficient, the electron multiplication factor and the mean electron energy have been determined for cathode fall values between 180 and 1000 V. It is shown that the proportion of electron flux crossing the sheath without collisions increases as the cathode fall is raised. When the cathode fall is in the range 300-900 V, the electron flux at the sheath/negative-glow boundary is found to be highly directional with distinct beam-like characteristics. Beyond 900 V, the largest component in the distribution of electron flux to reach the boundary region is the collision-free group. These electrons enter the negative glow with energies corresponding to the cathode fall potential and are nearly mono-energetic. In this case, the entire cathode sheath/negative-glow region can be referred to as an 'electron-beam' glow discharge.

Delocalization corrections for electron channeling analysis

A method is developed for quantitative electron channeling analysis employing inelastic excitations which are significantly delocalized, based on the introduction of C- factors . A C- factor is the ratio of the channeling effect of a delocalized excitation to that of a localized excitation on the same atom. Being a ratio it is relatively insensitive to the magnitude of the channeling effect and can be used to correct experimental measurements. Using a simple model of the inelastic scattering process, and electron flux distributions from multislice calculations, good agreement is obtained between experimental and calculated C- factors as a function of energy transfer and accelerating voltage. A simple method for estimating C- factors in any situation is presented. These results indicate that channeling studies can be extended to other low-energy excitations such as electron energy loss and Auger spectroscopy.

Return flux measurements in response to short-time electron beams aboard Spacelab-1

Measurements of the returning electron flux distribution in response to 8 keV, 10 to 100 mA, 20 to 40 ms electron beams of the PICPAB particle accelerator on board Spacelab-1 were performed by an electron spectrometer in the energy range from 0.1 to 12.5 keV (experiment 1ES019A). The electron flux followed almost immediately the beam within the sampling interval of 1 ms. A major flux increase was found at low energies, but also up to the beam energy. The effects of different beam currents and of payload charging are discussed.

A simulation of electron motion in the cathode sheath region of a glow discharge in helium

Electron motion in the cathode sheath region of a glow discharge in helium has been simulated using a one-dimensional computer model. Distribution functions for the electron flux across the cathode sheath have been calculated for fourteen values of the cathode fall between 150 and 1700 V. Macroscopic variables including the Townsend ionisation coefficient, multiplication coefficient and mean electron energy have been determined for the cathode fall range 150-1000 V. It is shown that the proportion of electron flux crossing the cathode sheath without collisions increases as the cathode fall is raised. For a cathode fall between 400 and 1400 V, the electron flux at the cathode sheath/negative glow boundary possesses beam-like characteristics. Beyond 1400 V, the largest component, in the electron flux distribution, to reach the sheath boundary is the collision-free group. These electrons enter the negative glow with energies corresponding to the cathode fall potential, and are practically mono-energetic. The authors propose that this is a suitable criterion to refer to the entire cathode sheath/negative glow region as an 'electron-beam' glow discharge.

Some auroral properties from far ultraviolet observations

Recent far ultraviolet spectra of four nightside auroras observed with the extreme ultraviolet (e.u.v.) spectrometer abroad the STP 78−1 satellite (Bowyer et al., 1981) are discussed in terms of two secondary electron flux distribution models. One of these is based on calculations published by Rees and Maeda (1973) and the other on electron flux observations. Using the 1084 Å NII line as the norm, the relative intensities of several lines in the range 900–1400 Å could be fitted equally well by either model, indicating that some of these emission ratios are somewhat independent of the energy dependence of the secondary electron flux. The ratio of the NI 1243 Å to 1084 Å emission is seen to yield an effective correlation for the other observed emissions and a reasonably consistent estimate of the penetration depth of the aurora.