Field Screening Effect Research Articles

Carbon nanotubes growing on rapid thermal annealed Ni and their application to a triode-type field emission device

In this paper, we demonstrate a new triode-type field emitter arrays using carbon nanotubes (CNTs) as an electron emitter source. In the proposed structure, the gate electrode is located underneath the cathode electrode and the extractor electrode is surrounded by CNT emitters. CNTs were selectively grown on the patterned Ni catalyst layer by using plasma-enhanced chemical vapor deposition (PECVD). Vertically aligned CNTs were grown with gas mixture of acetylene and ammonia under external DC bias. Compared with a conventional under-gate structure, the proposed structure reduced the turn-on voltage by about 30%. In addition, with a view to controlling the density of CNTs, Ni catalyst thickness was varied and rapid thermal annealing (RTA) treatment was optionally adopted before CNT growth. With controlled Ni thickness and RTA condition, field emission efficiency was greatly improved by reducing the density of CNTs, which is due to the reduction of the electric field screening effect caused by dense CNTs.

Nonrelativistic quantum electrodynamic approach to photoemission theory: I. Basic theoretical framework

A new nonrelativistic many-body theory to analyze X-ray photoelectron spectroscopy (XPS) spectra has been developed on the basis of quantum electrodynamic (QED) Keldysh Green’s function approach. To obtain XPS current density we calculate electron Green’s function g < which partly includes electron–photon interactions. We first separate longitudinal and transverse parts of these Green’s functions in the Coulomb gauge. The transverse electron selfenergy describes the electron–photon interaction, whereas the longitudinal electron selfenergy describes the electron–electron interaction. We derive the QED Hedin’s equation from which we obtain systematic skeleton expansion in the power series of the screened Coulomb interaction W and the photon Green’s function D k l . We show the present theory provides a sound theoretical tool to study complicated many-body processes such as the electron propagation damping, intrinsic, extrinsic losses and their interference, and furthermore, resonant photoemission processes. We have also found the importance of the mixed photon Green’s functions D 0 k and D k 0 which have been supposed to be unimportant for the XPS analyses. They, however, directly describe the radiation field screening. In this work, photon field screening effects are discussed in one-step theory, where the electron–photon interaction operator Δ is proved to be replaced by ε − 1 Δ beyond linear approximation. Beyond free photon Green’s function approximation, photon scatterings from the electron density are incorporated within the present QED theory. These photon field effects can directly describe the microscopic photon field spatial variation specific to near the surface region and nanoparticle systems.

A hybrid drift-diffusion-TLM analysis of traveling-wave photodetectors

An efficient hybrid method for numerical analysis of traveling-wave photodetectors (TWPDs) is presented. The method is a combination of two-dimensional semiconductor and three-dimensional full-wave electromagnetic (EM) simulators. The time-domain drift-diffusion method is used to determine the photogenerated currents at the cross section of the device. Those currents are then defined as sources in the EM analysis to calculate the microwave bandwidth and output current of the device. In the analysis, the effects of the carrier velocity and lifetime, optical power, bias voltage, velocity mismatch, and microwave loss are taken into account. The method is tested in case of GaAs and low-temperature-grown GaAs-based TWPDs. A very good agreement between the theoretical and experimental data is observed. It was also possible to predict the saturation behavior of the TWPD under high optical illumination due to the field screening effect.

Growth and field emission properties of carbon nanotubes on rapid thermal annealed Ni catalyst using PECVD

The effect of rapid thermal annealing (RTA) treatment on the growth characteristics of carbon nanotubes (CNTs) was investigated. We observed that Ni catalyst film was agglomerated by RTA treatment, resulting in the formation of Ni nanoparticles. The well-aligned CNTs were grown from the Ni nanoparticles by plasma-enhanced chemical vapor deposition (PECVD). It is shown that the size and distribution of the nanoparticles depend mainly on the annealing temperature and initial thickness of the catalytic metal layer. Also, it was found that the CNTs grown through RTA treatment had the more improved field emission characteristics than those of as-grown CNTs. This might be due to the increase of field enhancement factor, which resulted from the reduction of field screening effect.

Synthesis of conical Si array on Si(100) for a field electron emitter by plasma-enhanced chemical vapor deposition

We synthesized a highly aligned conical Si array on Si(100) for a field electron emitter by plasma-enhanced chemical vapor deposition (PECVD). The number density of these submicron Si cones was controlled in the range 10 6–10 9 cm −2. The 10 6 cm −2 conical Si array exhibited a low threshold voltage for its field electron emission, while for an array with a high number density, the emission property was degraded due to the electric field screening effect.

Comparison of the field emissions between highly ordered carbon nanotubes with closed and open tips

We have studied the field emission from the closed and open tips of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates by changing the tube height. Due to the field-screening effect provoked by the proximity of the neighboring tubes, the field emission from both kinds of the tips was critically affected by the tube height that protruded from the surface. The field emission optimizes when the tube height is similar to the intertube distance for both kinds of tips. The field emission from the closed tips is much more efficient than that from the open ones.

Effects of internal electrical field on transient absorption inInxGa1−xNthin layers and quantum wells with different thickness by pump and probe spectroscopy

The well width dependence of internal electric field effects was investigated using nondegenerate pump and probe spectroscopy at low and room temperature in four types of InGaN-based semiconductors of active layer thicknesses (a) 30 nm (single layer), (b) 10 nm (3 periods), (c) 5 nm (6 periods), and (d) 3 nm (10 periods). For sample (a) and (b) photoinduced absorption was observed due to screening of the internal electric field at low and room temperature. We observed two competing effects, exciton localization and the internal electric field, in sample (c) at low temperature. For sample (d) only photobleaching was observed due to occupation at the localized states. The results show that the field screening effects are more important for increasing well width after carrier generation. Furthermore, the carrier density to observe the photoinduced absorption due to screening the internal electric field is much less than the carrier density for stimulated emission at room temperature.

Effective screened potentials of strongly coupled semiclassical plasma

The pseudopotentials of particle interaction, taking into account both quantum-mechanical effects of diffraction at short distances, and also screening field effects at large distances are obtained for a strongly coupled semiclassical plasma. The limiting cases of potentials are considered.

On the electrical conductivity of semiclassical two-component plasmas

Starting from a memory-function formalism coupled with the Green– Kubo formula and an approximate expression for the generalized Coulomb logarithm, the electric conductivity of a dense high-temperature hydrogen plasma is studied. A pseudopotential model, taking account of short-range quantum effects and long-range screening-field effects, is employed to include quantum mechanical and polarization effects. An analytical formula for the Coulomb logarithm is proposed when the thermal de Broglie wavelengths are rather smaller than the Debye radius. A minimum in the curve of electrical conductivity is found and some physical evidence for its appearance is produced.

Study of the field-screening effect of highly ordered carbon nanotube arrays

We have studied the field-screening effect provoked by the proximity of neighboring tubes by changing the tube height of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates. The field emission was critically affected by the tube height that protruded from the surface. The field emission was optimal when the tube height was similar to the intertube distance. The intertube distance to the tube height for maximum field emission is about one half the intertube distance predicted by Nilsson et al. [Appl. Phys. Lett. 76, 2071 (2000)].

Study of the surface dielectric function: toward screening field effects in optical nonlinearities

A large number of works have been dedicated to the studies of the optical responses of metallic surfaces in the framework of the Jellium model. However, because this model is one dimensional, the fundamental problem of the polarization and azimuthal anisotropies cannot be handled. In the present description, by using a band model going beyond the nearly free electron model, the anisotropy properties are preserved in the description of the surface screening due to the Coulomb interaction. This enables us to evaluate the frequency and wave vector dependence of the dielectric function required in further studies of nonlinear optical responses of surfaces where anisotropies are experimentally observed. Here, the dielectric function of the Al(1 1 1) surface is explicitly calculated.

Magnetic field effects on cross section of inelastic electron–ion collisions

The magnetic field effects on the cross section of inelastic electron–ion collisions on 1s→2p dipole transition of hydrogenic ions are calculated. An atmospheric pressure plasma in thermodynamic equilibrium imbedded in an external, constant, and homogeneous magnetic field is considered. The magnetic field and plasma screening effects on the bound states of hydrogenic ion are included. The longitudinal dielectric function for magnetoactive plasmas is used to describe the interaction potential. The semiclassical straight-line trajectory approximation is used to the motion of the projectile electron. The partial cross section of electron-impact excitation of hydrogenic ions is derived as a function of the impact parameter, projectile energy, Larmor frequency, and plasma parameters. The scaled partial cross section of the electron-impact excitation including the magnetic field effect is always smaller than one without the magnetic field. When the Larmor frequency increases, the magnetic field effect decreases. For larger projectile velocity the differences between the partial cross section of electron-impact excitation for different magnitude of magnetic field becomes more significant.

Carrier dynamics in YBa 2Cu 3O 7− x studied by pump and probe terahertz excitation

We report a pump and probe terahertz (THz) excitation technique to investigate the ultrafast carrier dynamics in YBa 2Cu 3O 7− x (YBCO). THz pulses generated by laser-induced supercurrent modulation are used to probe the time resolved evaluation of the THz emission. The technique is revealed to allow us to study the carrier dynamics, such as fast THz field screening effect inside the YBCO film and changes in the electrodynamics due to Cooper pair breaking and hot carrier excitation, and so on.

Template‐based carbon nanotubes field emitter

The growth of carbon nanotubes(CNTs) in anodic aluminum oxide(AAO) template and their application to a field emitter are described. AAO templates were fabricated by anodizing bulk aluminum and sputtered thin Al film on Nb‐coated Si wafers. After Co catalyst had been electrochemically deposited into the bottom of the pores in AAO template, CNTs were grown by pyrolyzing C2H2. Depending on the reaction conditions, CNTs grew up to or over the top of the pores in AAO template with different structures. The morphology and structure of CNTs were observed with a scanning electron microscope and a transmission electron microscope. The diameter of CNTs strongly depended on the size of the pores in AAO template and the growing conditions. The electron field emission measurement of the samples resulted in the turn‐on field of 1.9–2.2 V/μm and the field enhancement factor of 2450–5200. The observation of high field enhancement factors is explained in terms of low field screening effect.

Pulsar Polar Cap Heating and Surface Thermal X‐Ray Emission. I. Curvature Radiation Pair Fronts

We investigate the effect of pulsar polar cap (PC) heating produced by positrons returning from the upper pair formation front. Our calculations are based on a self-consistent treatment of the pair dynamics and the effect of electric field screening by the returning positrons. We calculate the resultant X-ray luminosities and discuss the dependence of the PC heating efficiencies on pulsar parameters, such as characteristic spin-down age, spin period, and surface magnetic field strength. In this study we concentrate on the regime where the pairs are produced in a magnetic field by curvature photons emitted by accelerating electrons. Our theoretical results are not in conflict with the available observational X-ray data and suggest that the effect of PC heating should significantly contribute to the thermal X-ray fluxes from middle-aged and old pulsars. The implications for current and future X-ray observations of pulsars are briefly outlined.

Charge storage and screening of the internal field in GaN/AlGaN quantum wells

The optical spectra of a set of high quality quantum wells, with well characterized structural parameters, are presented here. We propose a quantitative calculation for the measured emission energies which takes into account the spontaneous and piezoelectric polarization fields and the field-screening effect due to electron-hole photogenerated pairs; such an effect is especially effective in wide QW samples which in fact behave like mesoscopic capacitors.

Template-based carbon nanotubes and their application to a field emitter

Anodic aluminum oxide (AAO) templates were fabricated by anodizing Al films. After the Co catalyst had been electrochemically deposited into the bottom of the AAO template, carbon nanotubes (CNTs) were grown by the catalytic pyrolysis of C2H2 at 650 °C. Overgrowth of CNTs on the AAO templates was observed. The diameter of the CNTs strongly depends on the size of the pores in the AAO template. The electron field emission measurements on the samples showed a turn-on field of 1.9–2.1 V/μm and a field enhancement factor of 3360–5200. Our observation concerning the low turn-on field and high field enhancement factors is explained in terms of a low field screening effect.

Experimental investigation on carrier dynamics in SCH-MQW waveguide saturable absorber of passively mode-locked monolithic laser diode

Decay process of photo-generated carrier density in a saturable absorber (SA) section made of a separate confinement hetrostructure (SCH) waveguide with multiple quantum wells for passively mode-locked monolithic laser diode (MLLD) has been investigated experimentally by means of the pulse mixing method. The measured sample is a realistic SA device, which was cleaved off from the wafer used for the MLLD fabrication. It was found out that the hole accumulation at a SCH interface plays a major role in the dynamics. While the backward and forward carrier flows from the accumulation layer are critical issues, the electric field screening effect on the carrier dynamics is rather small. Numerical simulation based on a new SA model containing these carrier flows and three time constants was performed and was found out to agree quite well with results of the pulse mixing experiment, indicating the appropriateness of the new model.

Thermodynamics of dense high-temperature plasmas: Semiclassical approach

The pseudopotential model of particle interaction of a semiclassical fully ionized plasma, taking into account both quantum effects at short distances and screening field effects at large distances is developed. Radial distribution functions are investigated and it is shown that a short-range order formation can occur in the system under discussion. Correlation energy of dense high-temperature plasma, existing in astrophysical objects is studied and comparison with other methods is performed.

Applied-B ion diode theory: The screening scale, relaxation of a Lagrangian invariant, and the diode stability

The assumption of relaxation to uniformity for the electron drift Lagrangian invariant ne/B results in a voltage–current characteristic U(ji) of an ion diode. The effect of the electric field screening at the magnetic Debye scale rB=B/4πene causes the existence of a maximum in the U(ji) curve at a certain threshold ion current density ji=jcrit. The diode can be stable only at the rising part of the U(ji) curve (ji&amp;lt;jcrit). The theoretical threshold ion current density jcrit is in very good agreement with the ion current density at which the slow two-fluid mode arises in the three-dimensional, particle-in-cell (3D PIC) simulations. This can be an important key to suppression of this mode, which is the main source of the beam divergence.