Electron Emission Density Research Articles

Numerical calculation study on the generalized electron emission phenomenon

There are two kinds of well‐known electron emissions from metal: field and thermionic emission. For thermionic emission, electrons come out of a metal due to the thermal energy, whereas for field emission, electrons tunnel out of a metal through the strong electric field. In this study, the most general electron emission caused by the temperature and electric field with a free electron gas model was considered. The total current density of electron emission comes from the field emission effect, where the electron energy is lower than vacuum, and from the thermionic‐emission effect, where the electron energy is higher than vacuum. The total current density of electron emission is shown as a function of the temperature for a constant electric field, and as a function of the electric field for a constant temperature.

Selective Formation of Carbon Nanotubes and Its Application to Field-Emitter Arrays

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> At room-temperature ambient, carbon nanotubes (CNTs) were selectively synthesized on each bridge of the microheater array (MHA) formed on a glass substrate. Both multiwalled (MW) and single-walled CNTs could be formed by controlling the MHA temperature, which was confirmed from Raman spectroscopy. By incorporating the selectively grown MW CNTs into the lateral-gated field-emitter arrays, high current density of electron emission was observed with low device leakage. </para>

Study of Complex Cathode Emission Characters

Based on the study of explosive emission, the dielectric enhanced electric-field emission model is brought forward, and the metal–dielectric complex cathode material is manufactured. Based on the investigation of surface-flashover explosive emission, the graphite–carbon fiber complex cathode is manufactured, too. Then the experiments are developed respectively, and the results show the electron emission density of these two complex cathode materials is also more than 17 kA/cm, and their lifetime is exceeding 10 pulses.

Enhanced output current density of an active-matrix high-efficiency electron emission device array with 13.75μmpixels

An active-matrix array of high-efficiency electron emission device (HEED) with a sufficient output density has been developed for the use as imaging probe in a high-gain avalanche rushing amorphous photoconductor target. Previously, it was demonstrated that a prototyped image sensor with 20×20μm2pixels could pick up a high definition image with an ultrahigh sensitivity under low-light-level condition. Based on it, an efficient active-matrix HEED with 13.75×13.75μm2pixels is fabricated for pursuing higher sensitivity and resolution. The improvement in the device isolation method enables to enlarge the relative emitting area, and then the emission current density per pixel reaches 3.6A∕cm2 that is about four times of that obtained from the previous one. The active-matrix HEED array with small pixels is available for application to the compact ultrahigh-sensitivity image sensor without affecting on the image definition.

High density electron emission source based on carbon nanotubes for industrial applications

Vertically aligned multi-walled carbon nanotubes (MWCNTs) have been grown by Catalyzed-Chemical Vapour Deposition (C-CVD) both on bare doped silicon and thin TiN film on silicon, using iron and nickel as catalyst, respectively. The aim is the development of high density and stable cold emission sources for industrial applications. Field emission characteristics of MWCNTs have been evaluated, obtaining current densities up to 150 mA/cm 2 at applied electrical field of 6 V/µm. Emission measurements have been performed which prove a medium-term stability. Structural and morphological characterization of MWCNTs has been carried out by SEM and Raman spectroscopy.

Electron emission characteristics of needle type semiconductor diamond electron emitters by pulsed bias operation and X-ray generation

Electron emission characteristics of needle-type semiconductor diamond electron emitters with pulsed bias operation were evaluated. An X-ray generation experiment was performed. Fowler–Nordheim plotting confirmed that field emission completely governed the electron emission. Maximum emission current of 4.2 mA was achieved using an n-type diamond needle. The needle tip, with area smaller than 1 μm 2, had estimated electron emission density greater than 4.2 × 10 5 A/cm 2. The effective emission area obtained from the Fowler–Nordheim plot was several 10 − 13 cm 2. For adopting and emission area of 1 × 10 − 12 cm 2, the estimated electron emission density was higher than 4.2 × 10 9 A/cm 2. Furthermore, the average emission current was 0.5–0.6 mA. This large electron emission was continued for several seconds and repeatable. A threshold electric field existed for electron emission higher than 50 kV/mm; pulsed electron emissions of less than 30 ms were created by slow triangular waveform shaped bias voltage supplied at frequencies of 5–10 Hz. An improved vacuum level and pulsed bias operation prevented damage to diamond electron emitters and steady electron emission better than with thermoelectronic emission and high bias voltage supply in DC mode; continuous X-ray generation of 1 h was achieved.

TH-E-AUD-03: X-Ray Pinhole Camera Images and Electron Beam Modeling of the Xoft Axxent® Source

Purpose: To understand the propagation of electrons through the acceleration gradient of the Xoft micro‐miniature x‐ray source via electrostaticmodeling of the fields and x‐ray pinhole camera imaging. Materials and Methods: 1) A 3D model of the source was built using OmniTrak3, and electron trajectories were traced from the hot cathode to the x‐ray producing anode. A finite element model of the filament temperature profile was input to a Richardson‐Dushman thermionic emission/extraction model to determine the electron emission density distribution. Assumptions made were that the field was graded linearly along the length, and that electrical connection between conductive and resistive components was ideal. 2) A ShadoCam4 x‐ray sensitive camera was used to acquire images of sources through a 30 μm pinhole mounted 15 mm from the source. The camera was 105 mm from the pinhole, providing a magnification of 7×. The camera was read out through a USB computer interface. Images were typically acquired over ten second integration times. Results: The actual images showed distinct patterns that were identifiable with model predictions. Structures associated with particular emission locations and launch angles on the filament were clearly observed. Conversely, patterns in the images were explainable in terms of parameters such as the absolute location of the filament. Image patterns which were relatively similar to the nominal had no correlation with measured spatial distribution patterns of the sources, but where there were notable variations in the image patterns it was possible to infer correlations with polar and/or azimuthal measurements. Conclusions: Electron trajectories within the accelerating structure were explored through pinhole imaging and computer modeling, and sensitivities to mechanical tolerances were established that were valuable in setting manufacturing tolerances. This study was funded by Xoft, Inc. 3 Field Precision LLC, Albuquerque, NM. 4 Rad‐icon Imaging Corp, Santa Clara, CA.

Emission Characteristics of Graphite Nanofiber Field Emitter for Field Emission Display

Graphite nanofiber (GNF) is a field emission material consisting of intricately tangled nano-sized carbon fibers and has similar field emission characteristics to CNT. An important issue in achieving practical use of FED (field emission display) using carbon nano-materials is how to attain emission uniformity. With the objective of uniformity, field emission characteristics of GNF emitter were examined using a scanning Faraday cup emission profiler. The current density of emitted electrons changed considerably over the emitting area at a low field, as high as two orders in intensity in a measuring emitter area of 1 mm2. However, uniformity was significantly improved due to current saturation at a high field. This paper discusses the current saturation in field emission of GNF from the space-charge effect of emitted electrons.

RF gun with ferroelectric cathode

RF guns generate bunched electron beams with pulse current I p∼10 −1–10 3 A and current pulse duration τ p∼10 −6–10 −12 s. The parameters are defined by the type of a cathode used mainly. We propose to apply a ferroelectric cathode in RF gun for the generation of electron beams with I p more than few amperes, τ p∼10 −8 s and current density of electron emission in a ferroelectric cathode j cfe may be ∼10 3 A/cm 2. We consider results of experimental study of S-band RF gun with ferroelectric plasma cathode. Obtained maximum I p at RF gun output is 9 A with τ p≈40–90 ns and electron energy W e≅500 keV.

Combined Effect of Secondary Electron Emission, Plasma Ion and Electron Number Density Variation due to Dust Charging and Ionization-Recombination Processes on Dust Ion Acoustic Wave Propagation

In a dusty plasma, in addition to influencing the dust charge state the secondary electron emission also leads to an enhancement of the electron number density. For proper accounting for secondary electron emission effect one has therefore, for the sake of consistency, also to take into account the ambient electron and ion loss due to charging collisions. The existence of a steady state for the dusty plasma is possible only through compensation of such particle losses or gain – a likely candidate for which are ionization and recombination processes in dusty plasma. Taking into account the combined effects all these processes and assuming that the frequencies of the various types of interparticle collisions associated with the above-mentioned processes are much less than the dust ion acoustic (DIA) frequency it is shown that a small perturbation of the steady state of the dusty plasma may result in DIA wave growth provided the ionization rate is sufficiently high. Threshold values of the ratio of the ionization rate to recombination rate depending on different plasma parameters are shown graphically. The analysis presented is restricted to the case when the dust charge remains negative. It is seen that instability persists even when there is no secondary emission. The threshold for instability is lower for smaller values of the dimensionless charge parameter z0 = Zd0e2/aTe when secondary emission takes place, compared to the case when it is absent. The situation tends to get reversed as z0 increases. The possibility of the existence of such instability was not noted earlier.

Measurements of secondary electron emission and plasma density enhancement for plasma exposed surfaces using an optically isolated Faraday cup

We present secondary electron yield and plasma enhancement factor data for silicon surfaces exposed to Ar, He, N2, O2, H2, and BF3 plasmas, for incident ion energies from 0.5–10 keV. A fiber-optic isolated Faraday cup was used to directly measure the ion current Jion, allowing a direct measurement of the secondary electron yield. This method automatically accounted for the effect of pulse-induced plasma density enhancement due to the ionization of neutral gas by accelerated secondary electrons, which we observed and measured quantitatively. The values of the secondary electron yields measured by this method were higher than published values measured by the conventional (ultraclean surface and ultrahigh vacuum) methods but lower than published values measured by previous plasma immersion ion implantation methods.

Experimental and theoretical characterization of integrated field emission nanotips

An experimental and theoretical analysis of integrated nanotips and field emission arrays is presented in this article. Atomic force and scanning electron microscopy techniques were used to characterize the morphology of the monotips apex. Different positions of the tip with respect to the extraction lenses and morphologies were observed leading to different values of the tip emission current. Numerical calculations were performed considering the integrated tip as an electrostatic two-dimensional system allowing study of potential distributions, electrostatic fields near the tip apex, current density of emitted electrons, and electron trajectories. The correlation between experimental and theoretical results allows us to better understand the effect that the geometrical properties of the integrated nanotips, such as misalignments or morphology failures, produce on the emission current. Experiments and theory support the idea that the best emission is obtained for centered sharper tips with their apex localized at the level of the extraction lenses. Furthermore, slight modifications of the tip geometry lead to significant changes in the current distribution of emitted electrons.

Self-sustained self-sputtering: a possible mechanism for the superdense glow phase of a pseudospark

Self-sustained self-sputtering occurring during high current pseudospark operation (/spl ap/10/sup 4/ A/cm/sup 2/, I>10/sup 3/ A) is shown to be a possible mechanism for the superdense glow. The mean-free-path for ionization of cathode material sputtered in the low-current hollow-cathode phase can be shorter than the cathode-anode gap distance, and ionized atoms can return to the cathode surface, self-sputtering with a yield greater than one. The self-sputtered cathode atoms become ionized in the beam of electrons accelerated in the cathode sheath. A large fraction of the discharge current at the cathode surface can be carried uniformly over the surface by ions and a very high electron emission density is not required to maintain the high current. >

Wave aspects of electron and ion emission from point sources

Electrons emitted from single-atom tips are analysed in a model with the tip as a round tubelike constriction. The source creates two-dimensional minimum wavepackets whose temporal evolution agrees well with observed beam divergences. With few-atom tips interference effects may arise due to the electron's non localized emission from several atoms. The phase space density of emitted electrons is around 10−2 h−3, and electron-electron correlation experiments become feasible. In ion field emission mode observed beam divergences are dominated by thermal broadening and other effects rather than quantum mechanical principles. Applications include electron interferometers of lower energy and wider beam separation than hitherto in use, as well as proton and possibly other ion interferometers. Highly focused low energy beams could also be produced.

A new deflected-electron-beam ionization gauge

A new deflected-electron-beam ionization gauge has been developed. The filament is exterior to one end of the cylindrical grid and is on the axis. Only the electron collector at the other end of the grid can collect electrons, due to electrode voltages which control and deflect the electrons. The X-rays emitted from the electron collector cannot directly irradiate the ion collector placed behind the electron collector, thereby resulting in the lower limit of this gauge being three orders of magnitude lower than that of a B-A gauge. The electron, gas ion and electronic ion desorption (EID) trajectories have been simulated by computer. The electrons emitted from different areas of the cathode have almost the same or equivalent trajectories, so the gauge sensitivity is independent of the electron emission density patterns. Because the cathode is shielded by the electron focal plate and the electrons only travel in the ionization region, the gauge sensitivity is independent of the shape, potential and size of the outer vacuum vessel wall. The gauge has a good stability and reproducibility of sensitivity. The gas phase ions are focused to the ion collector and the collection efficiency is high, the EID ions emitted from the electron collector are not easily focused to the ion collector.

Computer controlled techniques for high emission density mapping of thermionic cathodes

Some of the techniques commonly used (e.g. SLEEP and thermionic emission microscope) for measuring emission or work function uniformity of thermionic cathode surfaces require the use of very low or near zero current densities, thus the cathode is characterized at current densities and temperatures much lower than that of a normally operating cathode. The system reported on here uses a high voltage pulse technique and is capable of measuring emission densities in the range 1 to 80 A/cm 2 at normal cathode operating temperatures. The cathode surface is scanned with an anode having a 0.025 mm aperture whose position is controlled by computer operated stepping motors. The current through the aperture to a collector electrode is measured using a sample-and-hold amplifier. Pulsing and sampling are computer synchronized with the scanning, and data for each pulse are accumulated and can be processed and displayed in several ways using the computer, including a detailed “three-dimensional” map of either the electron emission density or work function variations. The entire surface of the cathode or any portion of it can be mapped in steps as small as 0.001 mm (1μm), but typically steps of 5–100 μm were used. Measurements are presented illustrating the uniformity or nonuniformity of the electron emission densities and work functions for type-B and type-M cathodes.

Superheated-ionization instability of an externally maintained discharge

The authors discuss the instabilities of an externally maintained discharge since they limit the maximum current and voltage on the discharge gap. Instability can develop only through step by step ionization, and the calculated and measured times of development of instability are close for E/N 3.5 x 10/sup -16/ V x cm/sup 2/. The considerable discrepancies at lower values of the ratio E/N were explained by neglect of the field nonuniformity. Increased energy release can also occur due to the higher discharge current density where the local current density of electron emission from the cathode is greater.

Causes of unstable and nonreproducible sensitivities in Bayard–Alpert ionization gauges

The fundamental ionization gauge equation p=i+/(Si−) can be rigorously derived. However, the values of the two currents i+ and i− can only be inferred from measurements of the ion collector current Ic and electron emission current Ie. Measurements are reported in which some physical parameters upon which i+ and i− are dependent were varied under closely controlled conditions. Experimental evidence is presented to show that the sensitivity S of Bayard–Alpert (BA) gauges varies with changes in the electron emission density pattern along the length of the hot cathode. Because the grid intercepts an appreciable fraction of the electron current on each pass through the grid, small changes in the emission density pattern can cause the fraction intercepted to vary widely. Therefore, the amount of ionizing current i− in the ion collection volume varies causing sizable changes in the rate of ion production and in the gauge sensitivity. It is concluded that these causes of changes in sensitivity in BA gauges are inherent in all ionization gauges in which a change in the emission density pattern can effect the rate of ion production.

New ionization gauge geometries providing stable and reproducible sensitivities

A primary cause of inaccuracies in Bayard–Alpert (BA) gauges has been previously identified as the unstable and nonreproducible distribution of electron emission from hot cathodes. Because of uncontrollable variations and instabilities in electron emission density pattern along the length of a hot cathode, the assumptions required to apply the fundamental ionization gauge equation, p=i+/(Si−), are not well satisfied in BA gauges or other ionization gauges employing grids. A new class of ionization gauge geometries is described which avoids these difficulties by closely satisfying the assumptions. New geometries are defined so that regardless of where on the hot cathode an electron is emitted, the electron has the same probability of causing ionization as any other emitted electron. Computer simulated electron and ion trajectories are presented for an optimum simple geometry. Testing of hardware has shown that our computer model is a good predictor of actual electron trajectories. Extensive testing including life testing is being carried out but the limits of performance of this new design have not yet been ascertained. These data indicate it should be possible to duplicate the pressure range of the BA gauge in a simple ionization gauge that is fully compatible with existing gauge controllers and which provides considerably better stability and reproducibilty of gauge sensitivity than does a BA gauge.

High-speed ion pump with a multipactor cathode—The multipactor ion pump

A new type of high-speed ion pump using a multipactor cathode is proposed and the experimental results, obtained with the first trial pump, are described. The multipactor cathode has excellent characteristics as an electron source in order to obtain increased pumping speed of an ion pump, namely: (1) since it is a cold cathode made of pure metal, such as Al, it does not contaminate the vacuum condition, (2) the electron emission density is high, (3) the supply of electrons can be greatly increased in comparison with that of any other supply method, (4) the electrons can be supplied easily into a very wide area. This proposal is to make a high-speed ion pump by combining such an attractive cathode with a normal ion pump. The experimental results showed that the maximum pump current per unit pump volume reached up to 30 times that of a normal ion pump in vacuum conditions of less than 10 −4 Pa.