Study Of Electron Emission Research Articles

Effect of gases on the field emission properties of ultrananocrystalline diamond-coated silicon field emitter arrays

We performed studies of electron emission from ultrananocrystalline diamond (UNCD)-coated, ungated silicon field emitters as a function of in situ exposure to various gases during current versus voltage and current versus time measurements. The emitter arrays were fabricated by a subtractive tip fabrication process and coated with UNCD films using microwave plasma chemical vapor deposition with a CH4/Ar plasma chemistry. The emission characteristics of the coated tip arrays were studied in the diode configuration; using a 2 mm diameter anode with rounded edges to suppress arcing. Significant enhancement of the electron emission was observed, increasing from 35% to 100%, after the emitting surface was exposed to H2 at pressures in the 10−5 and 10−4 Torr range. Upon termination of the H2 exposure, the current decreased to the initial value of 2 μA. The emission current subsequently remained stable at 2 μA upon continued evacuation down to the base pressure below 10−9 Torr. The emission current variation is repeatable with ensuing hydrogen exposure, indicating that the enhancement is due to the hydrogen exposure. Negligible emission current variations are observed at pressures less than 10−5 Torr. Exposure to either Ar or N2 resulted in a reduction of the emission current for ambients of up to 10−5 Torr. This effect is reversible. The effect of the investigated gases on the emission characteristics of UNCD-coated Si tip arrays is attributed to a modification of the effective work function at the localized sites from where electrons are being emitted.

Plasmon excitation in the interaction of protons and electrons with clean and potassium-covered Al surfaces

We present a study of the low-energy electron emission resulting from the interaction of 5‐100 keV protons colliding with clean and K-covered Al~111! surfaces at grazing incidence. In the whole energy range the spectra show structures that are associated with decay of both bulk and surface plasmons. We discuss the dependence of these structures with K coverage and compare them with those seen in the loss spectrum induced by 500 eV electron bombardment. We observe that the surface mode presents a stronger dependence with K coverage. Finally, we discuss the role of the ion velocity on the thresholds for surface and bulk plasmon production by direct excitation, by secondary electrons, and by electron capture processes.

Highly transverse velocity distribution of convoy electrons emitted by highly charged ions

We present a theoretical study of convoy electron emission resulting from highly charged ion (HCI) transport through carbon foils. Employing a classical transport theory we analyze the angular and energy distribution formed by multiple scattering of electrons in the solid. We find that the convoy electron distribution becomes highly transverse at intermediate foil thicknesses representing an oblate spheroidal distribution due to the stepwise excitation of the HCI. The calculated convoy electron spectra are found to be in good agreement with recent measurements.

Field electron emission and nanostructural correlations for diamond and related materials

Results are presented on a complex study of field electron emission (FEE) and structural correlations for nanocrystalline diamond and nitride films. It was found that all the samples studied showed similar dependences of the Fowler–Nordheim work function and effective emitting area on the threshold emission field. Besides it was generally observed that FEE occurred at nanosized regions on the boundary of high and low conducting areas, and peaks of the emission intensity were associated with a lowered surface electron potential. Based on the experimental data, the following mechanism of low-FEE from the materials studied can be supposed. Electrons are transferred from the conducting channel into a vacuum through the low-dimensional region where the emission probability is high due to the quantum well effect. A physical model of the electron escape from a quantum well was analyzed. As follows from the estimations, the quantum size effect, being combined with a moderate field enhancement (the field enhancement factor β=10–100), allows us to explain the observed variation in the energetic parameters for the samples studied. The function of the insulating grains is mainly to support the conducting channels in the sample body. Also, the grains can fulfill an additional function of the heat sink.

A new structural model of glass-like carbon

The results of an experimental study of X-ray diffraction, diamagnetic susceptibility, thermoelectricity and electron emission are presented for glass-like carbons. They lead to a new model of glass-like carbon structure and its modification due to heat treatment. At the initial stage of pyrolysis the stacks of narrow graphite-like sheets are formed being surrounded by a chain-like matrix. The main factor determining such system behavior under heat treatment is mutual parallelism of layer borders and adjacent chains. The most important feature of the model is the destruction of chain-like fragments with simultaneous formation of interstitials above 2000°C. The proposed model is consistent with many other properties of the material.

Bulk plasmon excitation in the interaction of Ne + and Ar + ions with polycrystalline Al surfaces

We report experimental studies of electron emission from decay of bulk plasmons excited in the interaction of slow single charged Ne and Ar ion with polycrystalline Al surfaces. To answer the question of whether bulk plasmons can be excited by charged particles moving outside the surface, we performed measurements as a function of emission angle. Our experiments show that bulk plasmon excitation occurs inside the solid for incident ion energies above a threshold of ∼1 keV. Bulk plasmon excitation in Al by keV single charged Ne and Ar ions is caused by fast electrons traveling inside the solid, which are energized in collisions initiated by the projectile.

Field electron emission study of Ti and Hf adsorption layers on W

Adsorption, nucleation, and crystal growth of hafnium and titanium deposited from vapor in ultra-high vacuum onto thermally cleaned tungsten emitters have been studied by field electron emission microscopy methods. The Ti adsorption on W surface was typical of metals of low intersolubility. The local work function decreases monotonically under Ti adsorption and saturates. Local work function values at saturation were equal to 3.0 eV on the (0 1 6)W face, 3.5 eV on the (0 1 1)W and (1 1 1)W faces, 3.75 eV on the (0 0 1)W face, and 3.95 eV on the (1 1 2)W face. The Hf interaction with W was much stronger and much more anisotropic than the Ti–W interaction. The formation of an intermetallic compound between Hf and W caused strong decrease of the local work function on the {0 0 1}W faces, resulting in an unusually high electron emission from these areas.

Relativistic Electron Transport Through Carbon Foils

We present a theoretical study of convoy electron emission resulting from transmission of relativistic 390 MeV/amu Ar17+ ions through carbon foils of various thicknesses. Our approach is based on a Langevin equation describing the random walk of the electron initially bound to the argon nucleus and later in the continuum. The calculated spectra of ejected electrons in the forward direction exhibit clear signatures of multiple scattering and are found to be in good agreement with recent experimental data.

Monte Carlo study of secondary electron emission

Based on our previous Monte Carlo simulation model of electron interactions with solids, including cascade secondary electron production, in which an optical dielectric function was used to describe electron energy loss and the associated secondary electron excitation, we have systematically investigated secondary electron generation and emission for 19 metals. The calculated secondary yield curve for primary beam energy ranging from 100 eV to 2 keV was found to correspond with the experimental universal curve. The dependence of the secondary yield on the work function was studied numerically, leading to a remarkable scattered deviation from Baroody’s relationship. This deviation shows that the secondary yield relates to different aspects of behavior by electrons in a metal, such as the cascade production process, the stopping power and specific energy loss mechanism for a sample, and the dependence on the electron density of states. The results provide an explanation for the scattered data on the experimental yield versus the work function. The calculations indicate that the characteristic energy loss of primaries may result in a corresponding feature in the energy distribution of secondaries.

Field emission from submicron-grained tungsten

Tungsten specimens subjected to intense plastic strains up to the true logarithmic deformation of e=7 were studied. Transmission electron microscopy revealed a decrease in the mean size of crystal grains down to 100 nm. The field ion-and field electron-emission studies revealed considerable distinctions between the energy distributions for electrons in submicron-grained tungsten and in a coarse-grained metal.

Study of field electron emission from nanocrystalline diamond thin filmsgrown from a N2/CH4 microwave plasma

Nitrogen containing nanocrystalline diamond films have beensynthesized by microwave plasma enhanced chemical vapour deposition usingN2/CH4 as the reactant gas. The films prepared under differentN2/CH4 flow ratios were studied by scanning electron microscopy,transmission electron microscopy, x-ray diffraction and secondary ion massspectroscopy. Their field emission characteristics, i.e. both the distributionof the emission sites and current-voltage characteristics, were recorded using atransparent anode technique. It was found that the grain size of films preparedunder different conditions varies and has a relationship with their turn-onfields. A possible explanation is given, which proposes that nanocrystallineboundary induced defect states might affect the field emission characteristics.

Characterization of a Rydberg atom-based streak camera operating in synchroscan mode

A streak camera that operates in synchroscan mode has been developed with a spectral response throughout the infrared. A gas-phase sample of Rydberg atoms is used as a photocathode. This compact device possesses 5 ps time resolution and can be used with a total infrared energy of about 1 nJ, or 10 −7 of the total macropulse energy of the FELIX free electron laser. This combination of characteristics makes it not only an attractive device for use in a variety of infrared experiments, but also a powerful tool for the study of photo-induced electron emission in atomic systems. As an example, a Rydberg-atom based electron gun which produces about 20 pulses of electrons at a 70 GHz repetition frequency has been characterized using this synchroscan streak camera.

Comparative study of secondary electron emission from solids under positron and electron impacts

Secondary electron emission from gold caused by the impacts of positrons and electrons at keV or less energy is investigated by means of a Monte Carlo simulation of elastic and inelastic collision processes of the projectiles and cascade electrons inside the solid. The cross-section for the elastic collision of each particle with an atom in the solid is calculated using the partial wave expansion technique, whereas the cross-section for the inelastic collision or for the electron excitation is calculated from the Ashley’s optical-data model. Because of less large-angle elastic scattering of the positron, the calculated backscattering coefficient is smaller for the positron than the electron. At low impact energies (<600 eV), however, the calculated secondary electron yield for the positron is larger than that for the electron, due to the larger inelastic energy loss of the positron and the larger number of excited electrons. The difference between the distributions for the positron and the electron calculated at such low impact energy is due to increasing emission of high-energy electrons excited by the projectile without the cascade process; which is more enhanced for the electrons than the positrons. These calculated results are consistent with the recent findings based on experiments.

Effect of sample thickness on electron emission from the TGS ferroelectric crystal

An experimental study of electron emission from nominally pure triglycine sulfate crystal samples of various thicknesses is reported. It is shown that the threshold field of emission, similar to the coercive field, increases with decreasing sample thickness d in inverse proportion to d.

Angular studies of potential electron emission in the interaction of slow ions with Al surfaces

We report energy distributions of electrons emitted from Al surfaces under impact by 1 keV Ar+ and 1-5 keV Ne+ ions. The variation of the energy distributions with the angle of incidence is different for both ions and provides information on the mechanism responsible for electron emission. For Ar+ electron emission results mainly from Auger neutralization, while for Ne+ an important emission mechanism is the decay of plasmon excitations. We find a transition between surface and bulk plasmon excitations as the energy of the ion is increased.

Electron emission in ferroelectrics for different values of the coercive field

Results are presented of experimental studies of electron emission from single crystals of triglycine sulfate, nominally pure and doped with Cr3+. The relation between the parameters of the processes of switching and electron emission from ferroelectrics by a coercive field and the threshold field for the onset of emission is investigated. It is shown that the temperature and concentration dependences of the threshold field can be explained by the corresponding dependences of the coercive field.

Theoretical studies of charge transfer in molecular ion–metal surface collisions

The collision of a molecular ion with a metal surface involves charge transfer. The effects of charge transfer can be readily seen in the increase of dissociation of ionized molecules as compared to neutral molecule dissociation, and to detectable electron emission. In the present work we present the results of a theoretical study of electron emission during the collision of hydrogen and nitrogen with an Al surface. In both cases the dominant electron emission process for slow ions is the de-excitation of the H 2 * and N 2 * molecules formed by resonance neutralization of the molecular ion in front of an Al surface. The de-excitation process is enhanced by the formation of a negative ion, which corresponds to the free-molecule shape resonances 2Σ u + and 2Π g for H 2 and N 2 , respectively. The energy spectra of the emitted electrons are much broadened, making it difficult to recognize the signature of a transient negative ion in the energy spectra of ejected electrons. Finally, the ubiquitous presence of negative ions in molecule-surface scattering is emphasized.

Scanning tunnelling microscopy: application to field electron emission studies

The principles of scanning tunnelling microscopy (STM) are extended to the study of field electron emission from metal, semiconducting and semi-insulating materials. A specially designed, high-vacuum STM device called a scanning tunnelling field emission microscope (STFEM) is constructed, and new measuring procedures are developed to examine complex physical properties of emission centres. Providing high bias voltages and fast mapping of large squares, the STFEM allows one to obtain reliable statistical data on surface properties, namely topography, emission intensity, surface potential distribution and local electroconductivity. Results from a study of low-field electron emission from CVD diamond films are described to illustrate the functional capabilities of the new STM device. It was found that the diamond films studied are composed of nanograined phases distinguished by their physical properties. It has also been noted that the low-field electron emission from the studied samples is associated with the interfaces of these phases.

耐熱絶縁用セラミックスからの電子、イオン放出について

耐熱絶縁用セラミックスからの電子、イオン放出について

Secondary Electron Emission Studies of Diamond and GaN Materials

ABSTRACTSecondary electron emission spectroscopy is used to examine the transport and emission of low-energy electrons in several wide bandgap materials. In particular, the secondary emission properties of C(100), C(111), and CVD diamond samples are compared in order to examine the effect of crystallographic orientation on the emission characteristics. Very high yields are obtained from hydrogenated and cesiated negative-electron-affinity surfaces of all three samples, indicating that low-energy electrons are transported and emitted very efficiently in the diamond materials. While the energy distribution of the emitted electrons is found to be sharply peaked at low energy for all three samples, the energy distributions measured from the C(111) surfaces are broader and reveal structure in the energy gap. The different emission processes at the C(100) and C(111) surfaces may account for the energy distributions observed from the polycrystalline CVD diamond. Finally, initial secondary emission measurements are taken from GaN and AlGaN films grown by molecular beam epitaxy. The secondary emission is not as strong as from the diamond samples, and the measurements reveal the impact of interface and surface barriers on the emission process.