Induced Electron Emission Research Articles

FENNECS: A novel particle-in-cell code for simulating the formation of magnetized non-neutral plasmas trapped by electrodes of complex geometries

This paper presents the new 2D electrostatic particle-in-cell code FENNECS developed to study the formation of magnetized non-neutral plasmas in geometries with azimuthal symmetry. This code has been developed in the domain of gyrotron electron gun design, but solves general equations and can be applied in other domains of plasma physics. FENNECS is capable of simulating electron-neutral collisions using a Monte Carlo approach and considers both elastic and inelastic (ionization) processes. It is also capable of solving the Poisson equation on domains with arbitrary geometries with either Dirichlet or natural boundary conditions. The Poisson solver is based on a meshless Finite Element Method, called web-splines, based on b-splines of any order, and used for the first time in the domain of plasma physics. In addition, the effect of fast ions colliding with the electrodes and causing ion induced electron emission at the electrode surfaces has been implemented in the code. In this paper, the governing equations solved by FENNECS and the numerical methods used to solve them are presented. A number of verification cases are then reported. Finally, the parallelization scheme used in FENNECS and its parallel scalability are presented.

CMOS compatible 2T pixel for on-wafer in-situ EUV detection

A novel 2-transistor (2T) pixel EUV detector is proposed and demonstrated by advanced CMOS technology. The proposed 2T detector also exhibits high spectral range (< 267 nm) and spatial resolution (67 μm) with high stability and CMOS Compatibility. The compact 2T EUV detector pixels arranged in a test array are capable of on-wafer recording the 2D EUV flux distribution without any external power.The compact 2T EUV detector pixels arranged in a test array are capable of on-wafer recording the 2D EUV flux distribution without any external power. Through proper initialization process, EUV induced discharging mechanism is fully investigated and an EUV induced electron emission efficiency model is established. Finally, a 2D array for in-situ EUV detection is demonstrated to precisely reflect the pattern projected on the chip/wafer surface.

Field induced electron emission from graphene nanostructures

Electric fields play a crucial role in modulating the electronic properties of nanoscale materials. Electron emission, induced by an electric field, is a representative phenomenon. Experimental and theoretical aspects of such electron emission from graphene are briefly reviewed. The emission occurs at the edge of graphene flakes, not at the surface, because the edge highly concentrates the electric field. Emission currents are sensitive to the edge shapes and edge functionalization. This review provides guiding principles for designing high-efficiency field-emission devices by using graphene nanostructures.

Ultrafast optical-field-induced photoelectron emission in a vacuum nanoscale gap: An exact analytical formulation

By exactly solving the one-dimensional time-dependent Schrödinger equation, we construct an analytical solution for nonlinear photoelectron emission in a nanoscale metal–vacuum–metal junction driven by a single-frequency laser field, where the impact of image and space charges is neglected. Based on the analytical formulation, we examine the photoelectron energy spectra and emission current under various laser fields and vacuum gap distances. Our calculation shows the transition from direct tunneling to multiphoton induced electron emission as gap distance increases. In the multiphoton regime, the photoemission current density oscillatorily varies with the gap distance, due to the interference of electron waves inside the gap. Our model reveals the energy redistribution of photoelectrons across the two interfaces between the gap and the metals. Additionally, we find that decreasing the gap distance (before entering the direct tunneling regime) tends to extend the multiphoton regime to higher laser intensity. This work provides clear insights into the underlying photoemission mechanisms and spatiotemporal electron dynamics of ultrafast electron transport in nanogaps and may guide the future design of advanced ultrafast nanodevices, such as photoelectron emitters, photodetectors, and quantum plasmonic nanoantennas.

Origin and nature of the emissive sheath surrounding hot tungsten tokamak surfaces

The accurate description of the emitted current that escapes from hot tungsten surfaces is essential for reliable predictions of the macroscopic deformation due to melt motion induced by fast transient events. A comprehensive analytical electron emission model is developed and its implementation in the particle-in-cell 2D3V code SPICE2 is discussed. The properties of emissive sheaths of present tokamaks, where thermionic emission is strongly suppressed by space-charge effects and by prompt re-deposition, are reviewed for arbitrary magnetic field inclination angles. The unique characteristics of emissive sheaths that emerge during ITER ELMs, where weakly impeded thermionic emission is coupled with field emission and competes with electron-induced emission, are revealed. The first ITER simulations are reported for normal inclinations. • The properties of emissive sheaths relevant for present day tokamaks are reviewed. • A comprehensive electron emission model has been implemented in the SPICE2 PIC code. • Electron-induced electron emission is revealed to be important during ITER ELMs. • Thermionic emission is revealed to be coupled to field emission during ITER ELMs.

Surface characterization determined from the secondary electron emission coefficient upon ion bombardment

The ion induced electron emission yield upon Ar+ ion impact at 420 eV was measured for a stainless-steel sample that was partially covered with a 50 nm thick gold layer. The ion induced electron emission yield for the two target materials differs strongly and enables to reproduce the shape of the gold film with a spatial resolution that corresponds to the width of the ion beam, which is 240 µm in the present case. Details in the two-dimensional map of the ion induced electron emission yield are explained by comparison with optical and scanning electron microscopy as well as with X-ray photoelectron spectroscopy measurements of the sample surface. In contrast to the sputtering efficiency, the ion induced electron emission yield does not depend on the orientation of the crystal structure.

Very thin N-doped nanostructured carbon films on quartz and sapphire substrate: Photoelectron emission properties

Very thin N-doped nanostructured carbon films were deposited on quartz and sapphire substrate by radio-frequency reactive magnetron sputtering using carbon target and gas mixture of Ar and N2 or N2+H2 reactive gasses. Rutherford backscattering spectroscopy and Elastic recoil detection analytical methods determined the concentration of elements in the films. Scanning electron microscopy was used to investigate the surface morphology of nanostructured very thin carbon films. Raman spectroscopy was used for the determination of chemical structural properties of the thin nanostructured carbon films. Pulsed laser induced electron emission method was used for the study of photoelectron emission properties of nanostructured carbon films. Measured bunch charge results of fabricated transmission photocathodes showed better photoelectron emission properties of very thin nanostructured carbon films prepared on sapphire substrates. Effects of substrate and technology of very thin nanostructured carbon films on the properties of photo-induced electron emitters as backside illuminated transmission photocathode are discussed.

Integrated modeling of plasma-dielectric interaction: kinetic boundary effects

Kinetic effects of plasma-dielectric interaction are studied theoretically with respect to the mechanisms of electron extraction from solids in response to ion and electron bombardment, coupled with plasma dynamics of a weakly emissive sheath. Emission coefficients of incident beams are first calculated by quantum mechanical as well as semi-classical approaches involving Auger neutralization, energy-dependent ejection due to primary beam, and reflection of low-energy electrons, which are then incorporated into a 1D1V simulation and plasma kinetic theory. Presheath with sheath structures are derived using fluid and kinetic theory regarding ion-induced emission, respectively. The Bohm criterion considering surface emission is evaluated as well. For electron-induced emission, it is found that sheath potential is no longer collinear with plasma electron temperature in our integrated model. Additionally, reflection of low-energy electrons is justified to have a minor impact on the floating sheath for low temperature half-bounded plasma under low pressure. The combined effects of both incident ions and electrons in bounded plasma are analyzed with symmetrical/asymmetrical emission yields at two boundaries. It is then proved that wall potential is barely affected by bulk plasma influx if emission coefficients are symmetrical, while emission due to transiting beam can drastically modify the sheath solution. Electron reflection also becomes more influential if secondary electrons have low initial energy. Finally, we summarize different roles of ion and electron in sheath structure. It is shown that ion-induced emission mitigates sheath potential but cannot reach critical emission on contrast to that of electron flux.

Ionization by an Oscillating Field: Resonances and Photons

We describe new exact results for a model of ionization of a bound state in a 1d delta function potential, induced by periodic oscillations of the potential of period $$2\pi /\omega $$ . In particular we have obtained exact expressions, in the form of Borel summed transseries for the energy distribution of the emitted particle as a function of time, $$\omega $$ and strength $$\alpha $$ of the oscillation of the potential. These show peaks in the energy distribution, separated by $$\hbar \omega $$ , which look like single or multi-photon absorption. The peaks are very sharp when the time is large and the strength of the oscillating potential is small but are still clearly visible for large fields, and even for time-periods of a few oscillations. These features are similar to those observed in laser induced electron emission from solids or atoms (Phys Rev Lett 105:257601, 2010). For large $$\alpha $$ the model exhibits peak-suppression. The ionization probability is not monotone in the strength of the oscillating potential: there are windows of much slower ionization at special pairs $$(\alpha ,\omega )$$ . This shows that ionization processes by time-periodic fields exhibit universal features whose mathematical origin are resonances which pump energy into the system represented by singularities in the complex energy plane. All these features are proven in our simple model system without the use of any approximations.

Electron reflection effects on particle and heat fluxes to positively charged dust subject to strong electron emission

A new model describing dust charging and heating in unmagnetized plasmas in the presence of large electron emission currents is presented. By accounting for the formation of a potential well due to trapped emitted electrons when the dust is positively charged, this model extends the so-called OML+ approach, thus far limited to thermionic emission, by including electron-induced emission processes, and in particular low-energy quasi-elastic electron reflection. Revised semi-analytical formulas for the current and heat fluxes associated with emitted electrons are successfully validated against particle-in-cell simulations and predict an overall reduction of dust heating by up to a factor of 2. When applied to tungsten dust heating in divertor-like plasmas, the new model predicts that the dust lifetime increases by up to 80%, as compared with standard orbital-motion-limited estimates.

The effect of photoemission on nanosecond helium microdischarges at atmospheric pressure

Atmospheric-pressure microdischarges excited by nanosecond high-voltage pulses are investigated in helium-nitrogen mixtures by first-principles particle-based simulations, which include VUV resonance radiation transport via the tracing of photon trajectories. The VUV photons, of which the frequency redistribution in the emission processes is included in some detail, are found to modify the computed discharge characteristics remarkably, due to their ability to induce electron emission from the cathode surface. Electrons created this way enhance the plasma density, and a significant increase of the transient current pulse amplitude is observed. The simulations allow the computation of the density of helium atoms in the 21P resonant state, as well as the density of photons in the plasma and the line shape of the resonant VUV radiation reaching the electrodes. These indicate the presence of significant radiation trapping in the plasma and photon escape times longer than the duration of the excitation pulses are found.

Helium ion beam induced electron emission from insulating silicon nitride films under charging conditions

Abstract Secondary electron emission from thin silicon nitride films of different thicknesses on silicon excited by helium ions with energies from 15 to 35 keV was investigated in the helium ion microscope. Secondary electron yield measured with Everhart-Thornley detector decreased with the irradiation time because of the charging of insulating films tending to zero or reaching a non-zero value for relatively thick or thin films, respectively. The finiteness of secondary electron yield value, which was found to be proportional to electronic energy losses of the helium ion in silicon substrate, can be explained by the electron emission excited from the substrate by the helium ions. The method of measurement of secondary electron energy distribution from insulators was suggested, and secondary electron energy distribution from silicon nitride was obtained.

Transport of 75–1000 eV electrons in metal–insulator–metal devices

Metal–insulator–metal nanostructures with a 50 nm silver film and a 30 nm aluminum film separated by a few nanometer insulator layer (aluminum oxide) are irradiated with a focused e−-beam (diameter ≤500 μm) with kinetic energies in the range of 75–1000 eV. Impact angle and energy dependence of the e−-beam induced electron emission from oxide covered aluminum and from silver show a good coincidence with previous results. The e−-beam induced internal device current measured between the aluminum and the silver film, on the other hand, is found to be independent of the primary electron energy and impact angle. The results suggest that external electron emission may have to be included in the interpretation of the internal transport currents.

Nanodiamond vacuum field emission microtriode

Vacuum field emission (VFE) microtriodes utilizing nanodiamond emitters, integrated with a self-aligned silicon gate and an anode and fabricated by the mold-transfer patterning technique on a silicon-on-insulator (SOI) substrate, have been developed. The nanodiamond VFE microtriodes were fabricated by an integrated circuit-compatible microfabrication process in conjunction with chemical vapor deposition of nanodiamond into the inverted-pyramidal molds micropatterned on the SOI substrate, which provides precision controlled emitter-gate alignment and spacing. The devices exhibited triode characteristics showing anode field induced electron emission with gate controlled emission current modulation at low operating voltages, agreeing with its electron emission transport model. A high current density of 150 mA/cm2 is achievable from the device with the anode-emitter spacing of 4 μm at low operating voltages of Va = 48.5 V and Vg = 5 V. The ac characteristics of the microtriodes for signal amplification were experimentally evaluated, and the results conformed to the proposed small signal equivalent circuit model. The triode small signal parameters were found to be dependent on the device geometry, which could be tailored to meet various applications by designing the physical structures of the device with the desired parameters. These results demonstrate the potential use of the nanodiamond VFE microtriodes for vacuum microelectronic applications.

Geometrical Shape Investigation For Electrodes in Silent Discharge Chamber

Silent discharge is the most prominent method to carry out plasma reaction because. discharge is easily initiated by injecting alternating current in high voltage to the pair of separated electrodes. The electron emission from surface of dielectric placed on instantaneous cathode is stimulated by ion induced electron emission. In this method, spark is avoided by placing insulation material to either one or both of the electrodes. In practical, it is very difficult to determine the exact limit of the voltage that initiate the discharge by mathematical analysis because it depends on many factors, namely dimensions, type and geometrical shapes of electrode, thickness of insulation, and type of electric field inside the discharge gap. To get lower initial voltage for discharge, it is important to find the best geometrical shape of electrode in relation to skin effect that trigger electron emission. This work investigates the behaviour of charges, current, electric field and voltage surrounding electrodes with various geometrical shape.

Modelling the line shape of very low energy peaks of positron beam induced secondary electrons measured using a time of flight spectrometer

In this paper, we present results of numerical modelling of the University of Texas at Arlington’s time of flight positron annihilation induced Auger electron spectrometer (UTA TOF-PAES) using SIMION® 8.1 Ion and Electron Optics Simulator. The time of flight (TOF) spectrometer measures the energy of electrons emitted from the surface of a sample as a result of the interaction of low energy positrons with the sample surface. We have used SIMION® 8.1 to calculate the times of flight spectra of electrons leaving the sample surface with energies and angles dispersed according to distribution functions chosen to model the positron induced electron emission process and have thus obtained an estimate of the true electron energy distribution. The simulated TOF distribution was convolved with a Gaussian timing resolution function and compared to the experimental distribution. The broadening observed in the simulated TOF spectra was found to be consistent with that observed in the experimental secondary electron spectra of Cu generated as a result of positrons incident with energy 1.5 eV to 901 eV, when a timing resolution of 2.3 ns was assumed.

Positron induced electron emission from graphene

We report here the energy spectra of positron-induced electrons emitted from single- and multilayer graphene films. One of UT Arlington’s two Time of Flight positron beam systems was used to deposit very low energy (KE < 1.5 eV) positrons at the surface of graphene samples consisting of a) 1 layer of graphene on polycrystalline Cu and b) 6-8 layers of graphene deposited on polycrystalline Cu. A time of flight spectrometer was used to measure the energy of positron-induced electrons. A peak in the electron energy spectrum was observed at ~263 eV corresponding to annihilation induced Auger electrons as a result of the KVV transition from the two graphene samples, which were comparable in intensity to that observed from a bulk graphite sample. In addition, we have observed a low energy peak in the annihilation induced electron energy spectra from graphene which extends up to ~15 eV. Our observation that the positron annihilation induced KVV Auger signal from graphene has a significant intensity indicates that the low energy positrons can be efficiently trapped on the surface of graphene layer on Cu down to a single-layer.

Double differential cross sections for proton induced electron emission from molecular analogues of DNA constituents for energies in the Bragg peak region.

For track structure simulations in the Bragg peak region, measured electron emission cross sections of DNA constituents are required as input for developing parameterized model functions representing the scattering probabilities. In the present work, double differential cross sections were measured for the electron emission from vapor-phase pyrimidine, tetrahydrofuran, and trimethyl phosphate that are structural analogues to the base, the sugar, and the phosphate residue of the DNA, respectively. The range of proton energies was from 75 keV to 135 keV, the angles ranged from 15° to 135°, and the electron energies were measured from 10 eV to 200 eV. Single differential and total electron emission cross sections are derived by integration over angle and electron energy and compared to the semi-empirical Hansen-Kocbach-Stolterfoht (HKS) model and a quantum mechanical calculation employing the first Born approximation with corrected boundary conditions (CB1). The CB1 provides the best prediction of double and single differential cross section, while total cross sections can be fitted with semi-empirical models. The cross sections of the three samples are proportional to their total number of valence electrons.

Application of AC plasma display panels to radiation detectors

Radiation detector proposals that use plasma display panels are rare. In this work, we confirmed a radiation detector that uses plasma display panels that are focused on the breakdown voltage shift in the ramp waveform. We adapted the cell structures, gas contents, and waveforms of plasma display panels (AC-PDPs) for radiation detectors. Hard X-rays and gamma rays induce electron emission into the discharge gas, resulting in generating electrons, electron multiplication, and charge accumulation on dielectrics. The radiation dose rate of hard X-rays and gamma rays (Cs137) is measured as a breakdown voltage shift between anodes and cathodes. For gamma rays, the detection sensitivity in this experiment is not as high as in the case of hard X-rays, but the detector can locate gamma rays. These results suggest that adapted AC-PDPs can detect both hard X-rays and gamma rays and can be used in a large two-dimensional radiation detector.

Ion source research and development at University of Jyväskylä: Studies of different plasma processes and towards the higher beam intensities

Several ion source related research and development projects are in progress at the Department of Physics, University of Jyväskylä (JYFL). The work can be divided into investigation of the ion source plasma and development of ion sources, ion beams, and diagnostics. The investigation covers the Electron Cyclotron Resonance Ion Source (ECRIS) plasma instabilities, vacuum ultraviolet (VUV) and visible light emission, photon induced electron emission, and the development of plasma diagnostics. The ion source development covers the work performed for radiofrequency-driven negative ion source, RADIS, beam line upgrade of the JYFL 14 GHz ECRIS, and the development of a new room-temperature-magnet 18 GHz ECRIS, HIISI.