Showing papers on "Electric field published in 1989"

Trap creation in silicon dioxide produced by hot electrons

Abstract: Trap creation in both the bulk of silicon dioxide films and at its interfaces with silicon and metallic contacting electrodes is shown to depend on the presence of hot electrons in the oxide. For thick oxides (≥100 A), little trap creation is observed in the near‐thermal transport regime at electric field magnitudes less than 1.5 MV/cm. At these low fields, electrons travel in a streaming fashion close to the bottom of the oxide conduction band at energies less than that of the dominant optical phonon mode at 0.153 eV. At higher electric fields, the rate of bulk trap creation is proportional to the average energy of the hot electrons, which move in a dispersive manner and can reach energies as large as 4 eV. For thin oxides (<100 A) where electrons can travel ballistically (i.e., without scattering), traps are not produced unless injected electrons acquire more than 2 eV of kinetic energy from the applied electric field, regardless of the magnitude of this field. All data on both thin and thick oxides are shown to give a threshold for trap creation of about 2.3 eV by the hot electrons in the oxide conduction band. Also, trap creation is shown to be suppressed by lowering the lattice temperature below ≊150 K. Our results are discussed in terms of a model involving hydrogen‐related‐species release from defect sites near the anode by the hot electrons and the subsequent motion of these molecules to regions near the cathode where they can interact with the lattice and form the trapping sites which are measured.

H -mode behavior induced by cross-field currents in a tokamak

Abstract: A sharp transport barrier, accompanied by a bifurcated poloidal rotation and a radial electric field, is formed at the plasma edge by driving a radial current across the outer magnetic surfaces of a tokamak. A decrease in particle transport is observed for negative radial E fields. When the radial current is turned off, the E field and the rotation damp on a time scale comparable with the ion-ion collision time.

Observation of extremely large quadratic susceptibility at 9.6-10.8 microm in electric-field-biased AlGaAs quantum wells.

Abstract: We have observed an extremely large second-order susceptibility for second-harmonic generation of 96--108 \ensuremath{\mu}m radiation due to intersubband transitions in electric-field-biased GaAs quantum wells For 92-A\r{} GaAs wells with 309-A\r{} ${\mathrm{Al}}_{048}$${\mathrm{Ga}}_{052}$As barriers under a bias of 36 kV/cm, the peak value of the susceptibility was 28 nm/V, 73 times larger than bulk GaAs The magnitude and sign of the susceptibility depend on the bias field, and are in accord with theoretical predictions

Electrical linear-response theory in an arbitrary magnetic field: A new Fermi-surface formation

Abstract: We develop a novel formulation of dc electrical linear-response theory for a phase-coherent conductor with multiple leads valid in arbitrarily strong magnetic field and for a given impurity configuration and measuring geometry. This formulation is convenient for discussion of the quantum Hall effect and mesoscopic transport phenomena. We express the total current response ${I}_{m}$ through lead $m$ completely in terms of the voltages ${V}_{n}$ applied at the leads, independent of the electric field in the material, i.e., ${I}_{m}=\ensuremath{\Sigma}{n}^{}{g}_{\mathrm{mn}}{V}_{n}$. We show that while the current-density response is not in general expressible as a Fermi-surface quantity, the total transport current determined by these conductance coefficients ${g}_{\mathrm{mn}}$ does depend only on the wave functions (or Green functions) at the Fermi surface as $T\ensuremath{\rightarrow}0$. This yields new and useful Green-function expressions for the ${g}_{\mathrm{mn}}$ and the longitudinal and Hall resistances. When transformed by appropriate applications of scattering theory, these expressions are shown to be equivalent to the relation ${g}_{\mathrm{mn}}={T}_{\mathrm{mn}}$, where ${T}_{\mathrm{mn}}$ is the sum of all the transmission coefficients between leads $m$ and $n$, as first obtained by B\"uttiker on the basis of phenomenological arguments. A brief discussion of the relation between this formula and other proposed Landauer formulas is given. It is noted that the occurrence of the quantum Hall effect is very natural in this formulation and simple conditions on the scattering matrix of the conductor which imply the quantum Hall effect are derived.

Coulomb explosion imaging of small molecules.

Abstract: Most of our knowledge of the structures of free molecules comes from indirect observations that focus on other properties (for example, energy or mass spectra) that depend on structure. Recently, a new method has been developed that instead yields geometrical images of individual molecules. This method takes advantage of the large Coulomb repulsion of the nuclei within molecules rapidly stripped of their electrons. The first experiments with this new technique have already revealed structural images of several important species, such as positively charged methane ions.

Conformational dynamics of individual DNA molecules during gel electrophoresis.

Abstract: GEL electrophoresis is widely used in molecular biology to separate DNA molecules according to their sizes. The physical basis of this size separation is, however, poorly understood. Here we report observations of individual, fluorescently stained DNA molecules as they migrate during various kinds of gel electrophoresis. Their movement, under the influence of either a steady electric field or a pulsed-field, is characterized by cycles of elongation and contraction. Initially relaxed coils of DNA lengthen into 'hook-shaped' configurations which temporarily 'hang-up' on obstacles in the gel matrix before sliding off, contracting and entering another cycle. The effects of a new electrophoresis technique, termed 'pulse-oriented electrophoresis", which allows the effective angle of the electric field, and hence the molecular orientation of DNA, to be varied without electrode rearrangement, are also studied. In this case the DNA adopts a 'staircase' configuration showing that the net orientation in a direction is given by the vector sum of the pulses used.

Dynamic simulation of electrorheological suspensions

Abstract: A simulation method is developed to investigate structure formation in electrorheological suspensions. The suspension is treated as polarizable, spherical particles in a nonconducting medium, with the spheres subject to electric polarization forces due to an applied electric field and to hydrodynamic resistance due to their motion through the continuous phase. The fibrous structures obtained from these simulations are independent of electric field strength and continuous phase viscosity in agreement with experimental observation. We have also found that the details of the simulated structures are sensitive to the treatment of the short‐range forces preventing particle overlap. When this force is represented by a form that accurately approximates a hard‐sphere interaction, the simulated structures agree well with those obtained experimentally, both with respect to their appearance and the time scale for structure formation.

A liquid crystal microlens obtained with a non-uniform electric field

Abstract: A homogeneously aligned nematic liquid crystal cell with a hole-patterned electrode and with an indium-tin oxide (ITO-) coated counter-electrode has been prepared. A non-uniform electric field can be produced by the asymmetrical electrode structure. The liquid crystal director can be reoriented by applying a voltage across the electrodes, and this produces an axially symmetrical profile of the refractive index. This liquid crystal cell is expected to have a lens effect and so its optical properties have been investigated. The profile of the output light intensity was measured by using a detecting system with an optical fibre. Some relationships between the lens properties, the diameter of the hole and the thickness of the liquid crystal layer have been examined. The liquid crystal cell becomes a convex (converging) lens with a relatively low voltage. A focal length of several millimetres can be obtained by applying voltages of 3-4 V. As the applied voltage increases, the focal length becomes long...

Combined removal of SO/sub 2/, NO/sub x/, and fly ash from simulated flue gas using pulsed streamer corona

Abstract: A pulse-energized electron reactor utilizing pulsed streamer corona has been developed for the combined removal of SO/sub 2/, NO/sub x/, and particles from effluent gases. In the pulse-energized electron reactor process, fast-rising narrow high voltage pulses are superimposed on a DC bias voltage and applied to a nonuniform electric field geometry to generate pulsed streamer corona. The pulsed streamer corona produces energetic free electrons, which dissociate gas molecules, forming radicals. These radicals cause chemical reactions that convert SO/sub 2/ and NO/sub x/ into acid mists and/or solids, which can be removed from the gas stream by conventional means. In pulse-energized electron reactor performance tests on a humid air stream with an initial SO/sub 2/ concentration of 1000 p.p.m., more than 90% of the SO/sub 2/ was removed with an advantageously small power requirement. >

Microwave ion source

Abstract: A microwave ion source suitable for an apparatus which requires ions of an element of high reactivity such as oxygen, fluorine, etc., the microwave ion source being arranged to transmit microwaves between outer and inner conductors of a coaxial line. An ion extraction electrode is formed at least partly of a low magnetic permeability material while an acceleration electrode is formed of a high magnetic permeability material. The acceleration electrode is formed so as to have a structure in which a low magnetic permeability material of a certain thickness is stacked on the high magnetic permeability material at a plasma chamber side and openings of ion exit holes are formed in the portion of the low magnetic permeability material. A permanent magnet constituting a magnetic field generating means is provided to surround the microwave lead-in coaxial line. The direction of magnetization of the permanent magnet is made to coincide with the axial direction of the coaxial line. The end surface of the permanent magnet at the microwave lead-in side is coupled with the periphery of the high magnetic permeability material of the acceleration electrode through another high magnetic permeability material to form a magnetic path. The plasma chamber is formed of a dielectric insulator which transmits microwaves well. It is possible to realize an ion source in which ions can be extracted with a high electric field, and in which a high current ion beam can be extracted for a long time.

Two-dimensional electromagnetic field effects in induction plasma modelling

Abstract: Based on the electromagnetic vector potential representation, a two-dimensional, axisymmetric model is proposed for the calculation of the electromageetic fields in an inductively coupled, radiofrequency (r.f.) plasma. A comparative analysis made between the flow, temperature, and electromagnetic fields obtained using this model and those given by our earlier one-dimensional electromagnetic fields model show relatively little difference between the temperature fields predicted by the two models. Significant differences are observed, however, between the corresponding flow and electromagnetic fields. The new model offers an effective means of accounting for variations in the coil geometry on the flow and temperature fields in the discharge and for achieving a better representation of the electromagnetic fields under higher frequency conditions (f>10 MHz).

Optical molasses and multilevel atoms: experiment

Abstract: The cooling mechanisms for laser cooling of atoms in optical molasses have been investigated experimentally. A significant simplification over the usual three-dimensional geometry has been obtained by studying the optical molasses in one or two dimensions only. By proper choice of polarizations the behavior of a pure two-level system as well as the more complicated effects of polarization gradients on laser cooling of a multilevel atom were observed.

Franz–Keldysh oscillations originating from a well‐controlled electric field in the GaAs depletion region

Abstract: The Franz–Keldysh oscillations induced by the electric field in the depleted zone below the GaAs surface are studied by photoreflectance spectroscopy. The electric field is precisely controlled by a molecular beam epitaxy grown buried highly doped layer and the pinned position of the Fermi level at the surface. It is shown that the electric field value as derived from theory is in disagreement with the value derived from electrostatic calculations. Consequently a determination of the Fermi level pinning is only possible from a measurement of both n‐ and p‐doped samples.

Radiofrequency sheaths and impurity generation by ICRF antennas

Abstract: In general, Faraday screen elements in an ICRF antenna are not aligned precisely along the combined toroidal and poloidal magnetic fields. When plasma of density n > 20V/eg2 ~ 109cm−3 (V being the voltage across the gap and g the gap spacing) is present in the gap between the elements, the electron response to the parallel electric field shorts out the electric field over most of the gap, leaving a narrow sheath of positive space charge and an intense electric field. This intense electric field accelerates ions up to an appreciable fraction of the gap voltage (~1 kV), sufficient to cause physical sputtering of the screen material. Impurities so generated constitute the principal limitationon power density for ICRF antennas. Principles of ICRF antenna and Faraday screen design which minimize sputtering are discussed.

Submicrosecond intercomparison of radiation fields and currents in triggered lightning return strokes based on the transmission-line model

Abstract: The “transmission-line model” of return-stroke radiation, proposed by Uman and McLain (1970), has been tested on rocket-triggered lightning Simultaneous measurements are reported of electric radiation field, current at the channel base, and stroke-propagation velocity Agreement between model and measurements is generally good during the first few microseconds, although several anomalies are pointed out Truncation of the sharp initial peak in many of the field waveforms, motivated by the hypothesis that the return stroke begins a short distance above the ground, leads to an average apparent velocity (derived from the ratio of peak field to peak current) of (151±017) × 108 m/s for 28 strokes, in excellent agreement with the average, photographically measured two-dimensional velocity of (152±017) × 108 m/s for 18 of these same strokes Although the correlation between apparent and photographic velocities for these 18 strokes is poor, the uncertainty in the photographic velocity determinations could explain most of the scatter The best formula for estimating peak currents (ip) in subsequent return strokes from remotely measured, truncated peak electric fields (E′p) at range D appears to be ip = −39 × 10−2 DE′p − 27 × 103, where all variables are in MKS units and the electric field polarity of return strokes lowering negative charge is considered negative

The electric field dependence of the mobility in molecularly doped polymers

Abstract: One of the most puzzling aspects of the hole mobility in molecularly doped polymers, its electric field dependence, is studied. New data over the widest electric field range yet reported have been obtained and are compared with various functions proposed in the literature. Comprehensive linear and nonlinear statistical analysis reveals that only the function exp(E)1/2 is consistent with the data. The theoretical implications of this result are discussed.

Zeeman bifurcation of quantum-dot spectra.

Abstract: We observe the magnetic-field-induced bifurcation of quantum levels into surface states and bulklike Landau States. The disruption of the electric field quantization by a magnetic field is most dramatic for electrons bound in two dimensions perpendicular to the magnetic field. The interplay between competing spatial and magnetic quantization mechanisms results in a pronounced and complex level splitting. The observed splitting of zero-dimensional energy levels depends critically on the size of the quantum dots, and can be explained with a calculated single-particle energy spectrum.

Covariance, CPT and the Foldy-Wouthuysen transformation for the Dirac oscillator

Abstract: The covariance properties and the Foldy-Wouthuysen and Cini-Touschek transformations for the recently proposed Dirac oscillator are found The charge conjugation, parity and time reversal properties are established A physical picture as an interaction of an anomalous (chromo) magnetic dipole with a specific (chromo) electric field emerges for this system This interaction suggests an alternative confinement potential for heavy quarks in QCD

Low capacitance large volume shaped-field germanium detector

Abstract: A large-volume (150 cm/sup 3/) germanium detector with a full-depletion capacitance of only approximately=1 pf has been fabricated. The effect of impurity space charge was utilized to obtain an appropriate electric field distribution in the detector so that carriers are collected on a small-area electrode. Detectors based on this principle are capable of very-low-noise operation and have immediate applications in experiments for the direct detection of dark matter. Detector pulse shapes and carrier-trapping effects were also examined for possible applications involving higher energy radiations. >

Confinement physics of H-mode discharges in DIII-D

Abstract: The authors' data indicate that the L-mode to H-mode transition in the DIII-D tokamak is associated with the sudden reduction in anomalous, fluctuation-connected transport across the outer midplane of the plasma. In addition to the reduction in edge density and magnetic fluctuations observed at the transition, the edge radial electric field becomes more negative after the transition. They have determined the scaling of the H-mode power threshold with various plasma parameters; the roughly linear increase with plasma density and toroidal field are particularly significant. Control of the ELM frequency and duration by adjusting neutral beam input power has allowed us to produce H-mode plasmas with constant impurity levels and durations up to 5 s. Energy confinement time in ohmic H-mode plasmas and in deuterium H-mode plasmas with deuterium beam injection can exceed saturated ohmic confinement times by at least a factor of two. Energy confinement times above 0.3 s have been achieved in these beam-heated plasmas with plasma currents in the range of 2.0 to 2.5 MA. Local transport studies have shown that electron and ion thermal diffusivities and angular momentum diffusivity are comparable in magnitude and all decrease with increasing plasma current.

Two-dimensional simulation and measurement of high-performance MOSFETs made on a very thin SOI film

Abstract: Thinning effects on the device characteristics of silicon-on-insulator (SOI) MOSFETs are discussed. Two-dimensional/two-carrier device simulation revealed the following advantages. An n-channel MOSFET with 500-AA-SOI thickness exhibited a high-punchthrough resistance as well as an improved subthreshold swing down to a deep submicrometer region, even if the film was nearly intrinsic. A capacitance coupling model has been proposed to explain these subthreshold characteristics. The kink elimination effect, which was attributed to a significantly reduced hole density in the SOI film, was reproduced. The low-field channel mobility exhibited a significant increase, which was ascribed to a decrease in the vertical electric field. Moreover, the current-overshoot phenomenon associated with the switching operation was suppressed. Excess holes recombine with electrons quickly after the gate turn-on, bringing about a stabilized potential in the SOI substrate. Experiments were also carried out to verify the simulation. >

Space-charge-limited current in a film

Abstract: Space-charge-limited current is considered in a thin-film n-i-n diode. It is assumed that impenetrable barriers limit the current flow to a film of thickness D. Because the electric field of injected electrons spreads out of the film, the level of injection is substantially higher than in the bulk case described by the classical Mott-Gurney law. As a consequence, the current density in a thin diode can be an order of magnitude larger than in a bulk diode of the same length. It is shown that, in the limit of small D, the total current is independent of D because the decreasing film thickness is exactly compensated by increasing injection. In this limit analytic expressions of the current-voltage characteristics are obtained for several model electrode configurations. The analytic results are confirmed by a numerical simulation of the diode within a drift-diffusion model assuming a field-independent mobility. The numerical results also describe a transition with decreasing D from the Mott-Gurney law to the new law governing space-charge-limited current in a film. >

Structured ion energy distribution in radio frequency glow‐discharge systems

Abstract: The energy distribution of ions incident on the cathode (smaller electrode) in a capacitively coupled asymmetric rf discharge was measured for different process parameters. It was found that the ion energy distribution exhibits a characteristic series of peaks, which are explained by the high‐frequency modulation of the sheath potential combined with charge exchange in the cathode sheath. A parametric model of the ion transport through the sheath is presented showing that the position of the peaks in the ion energy distribution depends only on a discharge scaling parameter and the spatial variation of the electric field e(x) across the sheath region. Good agreement between theoretical and experimental ion energy distributions in an argon discharge is obtained for an electric field variation e(x)∼xν with ν in the range 0.55–0.7.

Plasma processing method and apparatus

Abstract: A plasma process and an apparatus therefor are described A number of substrates are disposed between a pair of electrodes, to which a high frequency electric power is applied in order to generate glow discharge and induce a plasma The substrates in the plasma are applied with an alternating electric field By virtue of the alternating electric field, the substrates are subjected to sputtering action

Model of the H-mode in tokamaks

Abstract: The paper presents a theoretical model of the H-mode in tokamaks which is based on the bifurcation of the radial electric field at the plasma edge. The electric field is determined by the balance of the non-ambipolar fluxes of ions and electrons at the edge. It is found that bifurcations of the radial electric field, the particle flux and the convective energy loss occur when the edge gradient reaches a critical value. This is attributed to L-H or H-L transition. The critical conditions are examined and the role of neutral particles as well as the effect of impurities are incorporated in the model. Combining the confinement scaling laws for both the core plasma and the scrape-off layer plasma, the threshold power for the transition is derived. The temporal evolution associated with the transition is studied.