Study Of Electron Emission Research Articles

Focused particle streams for electron emission studies from intense laser-plasma interactions

Focused particle streams for electron emission studies from intense laser-plasma interactions

Direct imaging of electron density with a scanning transmission electron microscope

Recent studies of secondary electron (SE) emission in scanning transmission electron microscopes suggest that material’s properties such as electrical conductivity, connectivity, and work function can be probed with atomic scale resolution using a technique known as secondary electron e-beam-induced current (SEEBIC). Here, we apply the SEEBIC imaging technique to a stacked 2D heterostructure device to reveal the spatially resolved electron density of an encapsulated WSe2 layer. We find that the double Se lattice site shows higher emission than the W site, which is at odds with first-principles modelling of valence ionization of an isolated WSe2 cluster. These results illustrate that atomic level SEEBIC contrast within a single material is possible and that an enhanced understanding of atomic scale SE emission is required to account for the observed contrast. In turn, this suggests that, in the future, subtle information about interlayer bonding and the effect on electron orbitals could be directly revealed with this technique.

Breakdown of plane-wave-based theories of field emission from a subnanometer-radius tip

We present a combined experimental and theoretical study of field electron emission from an ultrasharp carbon tip with a subnanometer radius formed by a field-emission-induced structural modification of a diamond needle-like microcrystallite. The tip dimensions were measured by high-resolution transmission electron microscopy and used to calculate the potential distribution outside the emitter. Substitution of the calculated field into the Fowler-Nordheim (FN) theory and the more refined theory based on the Simmons equation overestimate the field emission current by six and five orders, respectively. We show that such a strong discrepancy is due to the incorrect use of plane wavefunctions to describe electrons inside a subnanometer emitter. We present a theory based on more realistic non-planar wavefunctions, which takes into account the effect of localization and one-dimensional density of states inside the emitter. These modifications of the FN theory allowed us to obtain a good order-of-magnitude agreement with the experimental values of the field emission current. The presented results are of great practical importance, since field emitters with nanometer radius are intensively studied experimentally and are the basis of state-of-the-art vacuum nanoelectronic devices.

Simulation studies of secondary electron yield with electron transport from Cu (110) surfaces containing C2, N2, CO2, or NO2 adsorbates

Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.

Interpretation of field emission current–voltage data: Background theory and detailed simulation testing of a user-friendly webtool

In field electron emission (FE) studies, interpretation of measured current–voltage characteristics and extraction of emitter characterization parameters are usually carried out within the framework of “smooth planar metal-like emitter (SPME) methodology”, using a data-analysis plot. This methodology was originally introduced in the 1920s. Three main data-plot types now exist: Millikan–Lauritsen (ML) plots, Fowler–Nordheim (FN) plots, and Murphy–Good (MG) plots. ML plots were commonly used in early FE studies, but most modern analysis uses FN plots. MG plots are a recent introduction.Theoretically, it is now known that ML and FN plots are predicted to be slightly curved in SPME methodology, but a Murphy–Good plot will be very nearly straight. Hence (because 1956 Murphy–Good emission theory is “better physics” than 1928 Fowler–Nordheim emission theory as corrected in 1929), expectation is that parameter extraction using a MG plot will be more precise than extraction using either ML plots or FN plots.In technological FE studies, current–voltage characteristics are often converted into other forms. Thus, measured voltage may be converted to (apparent) macroscopic field, and/or current values may be converted to macroscopic current densities. Thus, four data-input forms can be found in the context of analysing FE current–voltage results.It is also the case that over-simplified models of measurement-system behaviour are very widely assumed, and the question of whether simple use of a data-analysis plot is a valid data-interpretation procedure for the particular system under investigation has often been neglected. Past published studies on field emitter materials development appear to contain a high incidence of spurious values for the emitter characterization parameter “characteristic field enhancement factor”. A procedure (the so-called “Orthodoxy Test”) was described in 2013 that allows a validity check on measurement-system behaviour, and found that around 40% of a small sample of results tested were spuriously high, but has had limited uptake so far.To assist with FE current–voltage data interpretation and validity checks, a simple user-friendly webtool has been under design by the lead author. The webtool needs as user input some system specification data and some “range-limits” data from any of the three forms of data-analysis plot, using any of the four data-input variations. The webtool then applies the Orthodoxy Test, and—if the Test is passed—calculates values of relevant emitter characterization parameters.The present study reports the following: (1) systematic tests of the webtool functionality, using simulated input data prepared using Extended Murphy–Good field electron emission theory; and (2) systematic comparisons of the three different data-plot types, again using simulated input data, in respect of the accuracy with which extracted characterization parameter values match the simulation input values. The paper is introduced by a thorough summary review of the theory on which modern SPME-based current–voltage data-analysis procedures are based. The need in principle to move on (in due course) to data-analysis procedures based on curved-emitter emission theory is noted.An important result is to confirm (by simulations) that, particularly in respect of the extraction of formal emission areas, the performance of the Murphy–Good plot is noticeably better than the performances of Fowler–Nordheim and Millikan–Lauritsen plots. This result is important for field electron emission science because it is now known that differences as between different theories of field electron emission often affect the formal emission area.

Novel ionisation vacuum gauge suitable as a reference standard: Influence of primary electron trajectories on the operation

A detailed study of the operation of a novel ionisation gauge, recently proposed as a reference standard by the consortium of the European project 16NRM05, is performed with focus on the influence of electron trajectories and the interaction of electrons with electrode surfaces. The strategy behind the gauge design was to provide well defined primary electron trajectories that form an electron beam. This investigation seeks to correlate experimental investigations of improper primary beam focusing, electron beam profiling, electron stimulated desorption and outgassing, with charged particle optics simulations that include space charge effects, and a detailed study of secondary electron emission. The main outcome of the research is that control of the primary electron trajectories, quantified by electron transmission efficiency, is essential for its proper operation mainly as a pressure measurement device but also as a vacuum gauge sensitivity reference standard. Once all primary electrons end their trajectories in a Faraday cup, being one of the gauge electrodes, two additional problems are identified: escape of a very small fraction of electrons from the Faraday cup and electron stimulated desorption of neutrals. Their contribution is of low significance when using this device as a sensitivity reference standard in the high vacuum range, which meets the major goal of the 16NRM05 project. However, these effects might represent an obstacle for the gauge application, mainly as a measurement device, at lower pressures. Approaches to mitigate the mentioned potential problems are discussed.

Experimental study of gas breakdown and electron emission in nanoscale gaps at atmospheric pressure

While experiment, simulation, and theory all show that the gas breakdown voltage decreases linearly with gap distance for microscale gaps at atmospheric pressure due to the contribution of field emitted electrons, the continuing reduction in device size motivates a more fundamental understanding of gas breakdown scaling for nanoscale gaps. In this study, we measure current–voltage curves for electrodes with different emitter widths for 20–800 nm gaps at atmospheric pressure to measure breakdown voltage and assess electron emission behavior. The breakdown voltage Vb depends more strongly on effective gap distance deff than the ratio of the emitter width to the gap distance. For 20 and 800 nm gaps, we measure Vb≈5 V and Vb≈275 V. Independent of emitter width, Vb decreases linearly with decreasing deff for deff≳200 nm; for deff<200 nm, Vb decreases less rapidly with decreasing deff, which may correspond to a change in the field enhancement factor for smaller gaps. While gas breakdown usually proceeds directly from field emission, as for microscale gaps, some cases exhibit space-charge contribution prior to the transition to breakdown, as demonstrated by orthodoxy tests. Applying nexus theory, we determine that the range of deff studied is close to the transitions between field emission and space-charge-limited current in vacuum and with collisions, necessitating a coupled theoretical solution to more precisely model the electron emission behavior. Implications on device design and an overall assessment of the dependence of emission and breakdown on gap distance are also discussed.

Deep reddish-orange emitting Sr3Gd(PO4)3: Sm3+ phosphors via modified citrate-gel combustion method

The photoluminescence properties of different concentrations of Sm3+ activated Sr3Gd(PO4)3 phosphors synthesized by modified citrate gel-combustion method were investigated for white light emitting diode (w-LED) applications. The studied phosphors were characterized through powder x-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), photoluminescence excitation, emission and lifetime studies. The PXRD profiles confirmed the body centred cubic structure of undoped and Sm3+ -doped phosphors. The SEM studies showed the distribution and agglomeration of particles. The characteristic emission transitions such as 4G5/2 → 6H5/2 (∼561 nm), 4G5/2 → 6H7/2 (∼598 nm), 4G5/2 → 6H9/2 (∼645 nm) and 4G5/2 → 6H11/2 (∼705 nm) were noticed at their respective positions when excited at 403 nm wavelength. The highest luminescence intensity was found for 1.0 mol% of Sm3+ doped phosphors at 403 nm excitation showing luminescence quenching and it was ascribed to dipole-dipole interaction type of energy transfer among the excited Sm3+ ions at higher concentrations. The reddish-orange color of emitted luminescence was examined by evaluating the chromaticity coordinates. The experimental results confirm the potentiality of studied phosphors for w-LED applications.

Low-threshold field electron emission from graphene nanostructures

This paper presents the results of a study of field electron emission from two-dimensional graphene-like nanostructures synthesized by the method of self-propagating high-temperature synthesis. It is shown that this technology makes it possible to create efficient large-area field emitters. It is found that such structures exhibit the effect of low-threshold field electron emission. The possibility of obtaining high-current electron beams with currents up to several hundred amperes has been confirmed in pulsed electric fields.

Approximate Atomic Green Functions.

In atomic and many-particle physics, Green functions often occur as propagators to formally represent the (integration over the) complete spectrum of the underlying Hamiltonian. However, while these functions are very crucial to describing many second- and higher-order perturbation processes, they have hardly been considered and classified for complex atoms. Here, we show how relativistic (many-electron) Green functions can be approximated and systematically improved for few- and many-electron atoms and ions. The representation of these functions is based on classes of virtual excitations, or so-called excitation schemes, with regard to given bound-state reference configurations, and by applying a multi-configuration Dirac-Hartree-Fock expansion of all atomic states involved. A first implementation of these approximate Green functions has been realized in the framework of Jac, the Jena Atomic Calculator, and will facilitate the study of various multi-photon and/or multiple electron (emission) processes.

Tangential hard x-ray diagnostic array on the EXL-50 spherical tokamak.

A tangential hard x-ray (HXR) diagnostic on the newly constructed ENN XuanLong-50 (EXL-50) spherical tokamak for fast electron emission studies is presented. The HXR detection system consists of a symmetrical CdZnTe semiconductor detector array with a spectral sensitivity range of 20-300 keV. 25 channels have been designed on the 270° horizontal vacuum port with 12 sight lines to observe the forward emission, 12 sight lines to observe the backward emission of fast electrons, and 1 for viewing the central. Currently, ten channels have been in operation in the EXL-50 experiments. The systems are designed to measure the x-ray spectra for the estimation of fast electron temperature and electron velocity distribution in the EXL-50 experiment, which will be useful for understanding the dynamics of fast electrons generated by electron cyclotron resonance heating, for plasma instability and transport studies and for the analysis of plasma heating efficiency.

Thermal effects in field electron emission from idealized arrangements of independent and interacting micro-protrusions

Modelling studies of thermo-field electron emission (TFE) from protrusions at a cathode surface usually use simulations in 2D axial symmetry. Indeed, time-dependent simulations in 3D are very demanding in computation time. Often, 3D simulations have been restricted to stationary pure field electron emission to account for the drastic current decrease caused by electric field screening when the emitters are close. Little interest has therefore been granted to the heat exchanges occurring between nearby emitters. Although the temperature is a second-order parameter in TFE compared to the electric field, thermal effects become non-negligible in high current density regimes, where self-heating is well established. The present study focuses on the thermal effects occurring during the TFE from micro-protrusions. Our model considers a DC voltage but solves in time the temperature evolution coupling the heat equation and the current continuity equation. The protrusions are modelled as hemiellipsoids with 2D axial symmetry. Emission enhancement due to the increase of the temperature in the thermo-field regime compared to the pure field regime is detailed as a test case for isolated protrusions. Then, full 3D simulations are used to investigate the thermal coupling between multiple neighbouring protrusions via their outwards heat fluxes inside the cathode. The results show a higher current increase due to thermal coupling for dome-like protrusions with a low field enhancement factor. The current increases up to 13% of the total current for aspect ratios of 1, but this value is reached for an extreme applied electric field, hardly reachable in experiments. For sharper protrusions with higher field enhancement, the interaction range through the cathode being shorter, the thermal coupling is suppressed by electrostatic screening. Nevertheless, in arrangements of densely distributed field emitter, when the screening is compensated by a higher voltage, our model predicts the possibility of a moderate but noticeable thermal coupling even for sharp protrusions: a parametric study indicates up to 14.5% of the emitted current being caused by a thermal coupling through the cathode bulk, for protrusions with an aspect ratio of 10 under a fixed applied electric field of 0.4 GV m−1 in DC mode.

Applying the Field Emission Orthodoxy Test to Murphy-Good Plots

Abstract: In field electron emission (FE) studies, it is important to check and analyze the quality and validity of results experimentally obtained from samples, using suitably plotted current-voltage [Im(Vm)] measurements. For the traditional plotting method, the Fowler-Nordheim (FN) plot, there exists a so-called "orthodoxy test" that can be applied to the FN plot, in order to check whether or not the FE device/system generating the results is "ideal". If it is not ideal, then emitter characterization parameters deduced from the FN plot are likely to be spurious. A new form of FE Im(Vm) data plot, the so-called "Murphy-Good (MG) plot", has recently been introduced (R.G. Forbes, Roy. Soc. Open Sci. 6 (2019) 190912). This aims to improve the precision with which characterization-parameter values (particularly values of formal emission area) can be extracted from FE Im(Vm) data. The present paper compares this new plotting form with the older FN and Millikan-Lauritsen (ML) forms and makes an independent assessment of the consistency with which slope (and hence scaled-field) estimates can be extracted from an MG plot. It is shown that, by using a revised formula for the extraction of scaled-field values, the existing orthodoxy test can be applied to Murphy-Good plots. The development is reported of a prototype web tool that can apply the orthodoxy test to all three forms of FE data plot (ML, MG and FN). Keywords: Field emission, Field electron emission, Murphy-Good plot, Fowler-Nordheim plot, Millikan-Lauritsen plot, Orthodoxy test.

Implementation of the orthodoxy test as a validity check on experimental field emission data

Abstract In field electron emission (FE) studies, it is important to check and analyse the quality and validity of experimental current-voltage data, which is usually plotted in one of a small number of standard forms. These include the so-called Fowler-Nordheim (FN), Millikan-Lauritsen (ML) and Murphy-Good (MG) plots. The Field emission orthodoxy test is a simple quantitative test that aims to check for the reasonableness of the values of the parameter “scaled field” that can be extracted from these plots. This is done in order to establish whether characterization parameters extracted from the plot will be reliable or, alternatively, likely to be spurious. This paper summarises the theory behind the orthodoxy test, for each of the plot forms, and confirms that it is easy to apply it to the newly developed MG plot. A simple, new, accessible web application has been developed that extracts scaled-field values from any of these three plot forms, and tests for lack of field emission orthodoxy.

Electron emission from CH4 molecule induced by 5.5 MeV/u bare C ions

SynopsisWe present the experimental and theoretical study of electron emission from methane (CH4) in collisions with 5.5 MeV/u bare carbon projectiles. We have measured double differential cross section of electron emission and compared the data with the prior form of the continuum distorted wave – eikonal initial state (CDW-EIS) model calculation. The CDW-EIS calculations are in very good agreement with the data.

Study of secondary electron emission from semiconductors induced by electrons from 20 eV to 800 eV and surface state of semiconductors

Based on processes and characteristics of secondary electron emission (SEE), experimental data and the relations among parameters of secondary electron yield from semiconductors δ, the formula for δ in the range of 20 eV ≤ Epo ≤ 800 eV of semiconductors with Epom ≤ 800 eV as a function of Epo, Epom, atomic number Z and maximum δ (δm) was deduced and proved experimentally, where Epo and Epom are incident energy of primary electron and the Epo of δm, respectively. From the existing formula and the relation among Z, mean escape depth of secondary electrons emitted from semiconductors and primary range R at Epom deduced in this study, the formula for R in the range of 150 eV ≤ Epo ≤ 800 eV in monocrystalline semiconductors with Epom ≤ 800 eV, χ ≥ 0.6 eV and Eg ≥ 0.6 eV was successfully deduced, where χ is original electron affinity, Eg is the width of forbidden band. The method of calculating B(χreal, Eg) of semiconductor with Epom ≤ 800 eV, χreal ≥ 0.5 eV and Eg ≥ 0.6 eV and that of calculating B of semiconductor with Epom ≤ 800 eV were presented, respectively; Where B is the probability that an internal secondary electron escapes into vacuum upon reaching the surface of semiconductor, and the B(χreal, Eg) is the B relative to semiconductor with Eg and real electron affinity χreal. It can be concluded that the two methods are correct. From the analysis of calculated B, experimental Epom and relations among parameters of SEE, it can be concluded that this paper can research the surface state of semiconductor with Epom ≤ 800 eV by SEE.

SECONDARY ELECTRON EMISSION INDUCED BY α-PARTICLES FROM Mg-MgO LAYERS

The paper presents the results of experimental study of forward and backward electron emission induced by α-particles from the deposited film of magnesium. It was shown that during the deposition of magnesium in residual gas atmosphere the deposited film contained a large amount of MgO component, which makes it possible to consider the resulting structure as Mg-MgO. The presence of magnesium oxide on the surface of the target and the collector leads to the fact that the previously obtained dependence of the ratio of forward and backward electron yields on specific energy loss of the ion for various metals is not applicable in the case of deposited magnesium. The differences are explained by the specificity of the emission from magnesium in the presence of a significant amount of MgO. The results obtained can be used to detect MgO on the surface of a magnesium substrate. It was shown the differences in the experimental data for the bulk magnesium collector and the collector with deposited magnesium layer

A study of secondary electron emission from semiconductors and insulators

No rude approximations were made in the course of deducing formula for B(χ, Eg) as a function of parameters of secondary electron emission SEE; where B(χ, Eg) is the probability that an internal secondary electron escapes into vacuum upon reaching the surface of semiconductors and insulators SI with width of forbidden band Eg and original electron affinity χ. It can be concluded that the formula for B(χ, Eg) as a function of parameters of SEE can be used to calculate B(χ, Eg). The B(χ, Eg) calculated with the formula for B(χ, Eg) as a function of parameters of SEE and characteristics of B(χ, Eg) and SEE from SI were studied, the formula for B(χ, Eg) of SI with Eg≥0.6 eV and χ≥0.5 eV as a function of χ and Eg was determined. It concludes that the determined formula can be used to calculate B(χ, Eg) of SI with Eg≥0.6 eV and χ≥0.5 eV. The processes and characteristics of SEE from SI, energy loss of primary electron and parameters of SEE were studied, the formulae for δm(Epom, χreal, Eg) in the general case and 1/α(χreal, Eg) of SI with Eg≥0.6 eV and χ≥0.5 eV were deduced and experimentally proved, respectively; where δm(Epom, χreal, Eg) is the maximum secondary electron yield δm of SI with Eg and χreal, Epom is primary incident energy corresponding to δm, χreal is real electron affinity,1/α(χreal, Eg) is mean escape depth of secondary electrons emitted from SI with Eg and χreal.

Study of electron emission at the cathode in an arc discharge

The work presented in this paper is devoted to the study of the phenomena of electron emission that take place at the cathode in an electric arc discharge. This problem remains a subject of careful research conducted by several specialists. Nevertheless, it seems interesting to make a comparison between all the theories currently available. In an electric arc, electrons are emitted from the surface of the cathode. The cathode spot is the place from which the electrons emerge. The current density of the electrons emitted represents 90% of the total current density. The remaining 10% is provided by the positive ions present in front of the cathode. Thus, the determination of the total current density at the cathode in an arc discharge is reduced to the calculation of that of the emitted electrons. The choice of the method for calculating the current density of the electrons emitted at the cathode is crucial for the evaluation of the total current density at the cathode. In our model, the current density of emitted electrons is evaluated using the Murphy-Good formulation which is in the current state, the most general and the most accurate equation.

Study and mitigation of spurious electron emission from cathodic wires in noble liquid time projection chambers

Noble liquid radiation detectors have long been afflicted by spurious electron emission from their cathodic electrodes. This phenomenon must be understood and mitigated in the next generation of liquid xenon (LXe) experiments searching for WIMP dark matter or neutrinoless double beta decay, and in the large liquid argon (LAr) detectors for the long-baseline neutrino programmes. We present a systematic study of this spurious emission involving a series of slow voltage-ramping tests on fine metal wires immersed in a two-phase xenon time projection chamber with single electron sensitivity. Emission currents as low as 10−18 A can thus be detected by electron counting, a vast improvement over previous dedicated measurements. Emission episodes were recorded at surface fields as low as ∼ 10 kV/cm in some wires and observed to have complex emission patterns, with average rates of 10–200 counts per second (c/s) and outbreaks as high as ∼ 106 c/s. A fainter, less variable type of emission was also present in all untreated samples. There is evidence of a partial conditioning effect, with subsequent tests yielding on average fewer emitters occurring at different fields for the same wire. We find no evidence for an intrinsic threshold particular to the metal-LXe interface which might have limited previous experiments up to fields of at least 160 kV/cm. The general phenomenology is not consistent with enhanced field emission from microscopic filaments, but it appears instead to be related to the quality of the wire surface in terms of corrosion and the nature of its oxide layer. This study concludes that some surface treatments, in particular nitric acid cleaning applied to stainless steel wires, can bring about at least order-of-magnitude improvements in overall electron emission rates, and this should help the next generation of detectors achieve the required electrostatic performance.