Mechanism Of Electron Emission Research Articles

Analysis of application range of simplified models for field to thermo-field to thermionic emission processes from the cathode

Abstract Electrons can escape from the cathode surface by acquiring enough energy greater than the work function or weakening the potential barrier at the cathode surface through tunneling effects in gas discharges, which plays a dominant role in the plasma-cathode interactions and is a key factor in many plasma phenomena and industrial applications. It is necessary to illustrate the various electron emission mechanisms and corresponding applicable description models to evaluate the impacts on discharge properties, especially for numerical simulation studies. However, most current researches usually rely on previous experience to select the appropriate simplified formula to calculate the electron emission current density, and there is little work that can explicitly give the application range of the simplified formulas for describing electron emission. In this work, the detailed expressions of the simplified formulas valid for field emission to thermo-field emission to thermionic emission typically used in the numerical simulation are proposed, and corresponding application ranges are determined in the framework of the Murphy-Good theory, which is commonly regarded as the general model and to be accurate in the full range of conditions of validity of the theory. The dimensionless parametrization is used to evaluate the emission current density of the Murphy-Good formula and a deviation factor is defined to obtain the application ranges for different work functions (2.5~5 eV), different cathode temperatures (300~6000 K), and different emitted electric field (105 ~1010 V‧m-1). The deviation factor is shown to be a non-monotonic function of the three parameters. A comparative study of particle number densities in atmospheric gas discharge with tungsten cathode is performed based on the one-dimensional implicit particle-in-cell with the Monte Carlo Collision (PIC-MCC) method according to the above application ranges. It is found that small differences in emission current density can lead to variation in the distributions of particle number density due to the change of collisional environment. This present work can provide a theoretical basis to select emission models for the subsequent numerical simulation.

Review of electron emission and electrical breakdown in nanogaps

With the continual miniaturization of electronic devices, there is an urgent need to understand the electron emission and the mechanism of electrical breakdown at nanoscale. For a nanogap, the complete process of the electrical breakdown includes the nano-protrusion growth, electron emission and thermal runaway of the nano-protrusion, and plasma formation. This review summarizes recent theories, experiments, and advanced atomistic simulation related to this breakdown process. First, the electron emission mechanisms in nanogaps and their transitions between different mechanisms are emphatically discussed, such as the effects of image potential (of different electrode's configurations), anode screening, electron space-charge potential, and electron exchange-correlation potential. The corresponding experimental results on electron emission and electrical breakdown are discussed for fixed nanogaps on substrate and adjustable nanogaps, including space-charge effects, electrode deformation, and electrical breakdown characteristics. Advanced atomistic simulations about the nano-protrusion growth and the nanoelectrode or nano-protrusion thermal runaway under high electric field are discussed. Finally, we conclude and outline the key challenges for and perspectives on future theoretical, experimental, and atomistic simulation studies of nanoscale electrical breakdown processes.

Collisional space-charge-limited current with monoenergetic velocity: From Child–Langmuir to Mott–Gurney

All theories coupling electron emission theories ultimately approach the space-charge-limited current (SCLC) in vacuum, given by the Child–Langmuir (CL) law, for sufficiently high voltage, or the Mott–Gurney (MG) law for finite electron mobility and high (but not infinite) voltage. These analyses demonstrate the presence of an SCLC regime that cannot be described by either CL or MG. Here, we derive an exact solution for SCLC for general electron mobility and nonzero velocity. We recover the traditional CL with nonzero initial velocity at high voltage. For low mobility (or infinite collision frequency), we derive corrections to the MG law that depend on the ratio of initial velocity to the product of collision frequency and gap distance or initial velocity to drift velocity for low and high voltage, respectively. Increasing collisionality decreases the correction to SCLC for nonzero velocity, indicating that these corrections are less important for low-mobility materials (e.g., solids) than high-mobility materials (e.g., air or vacuum). For a given gap distance (collision frequency), increasing the collision frequency (gap distance) increases the voltage necessary to make the gap appear more like vacuum. These results provide a generalized SCLC for both collisionality and initial velocity when assessing the transitions between electron emission mechanisms.

Experimental study on the full cycle evolution of high-intensity atmospheric dc arc discharge from breakdown to extinguishment.

In this study, the spatiotemporal evolution of full cycle of high-intensity dc argon arc discharge at atmospheric pressure is investigated by using a transferred arc device, which is easy to be directly observed in the experiment. Combining the voltage and current waveforms with high-speed images, the full cycle evolution process of high-intensity atmospheric dc arc can be divided into five different stages: breakdown pulse stage, cathode heating stage, current climbing stage, stable arc discharge stage, and finally arc extinguishing stage. The characteristics of each different stage are analyzed in detail through the electrical properties, high-speed pictures, and spectroscopic measurements. The results show that the strong luminescence region develops from the vicinity of cathode and anode to the middle in the breakdown pulse stage, which is explained from the spatiotemporal evolution of distributions of excited argon atom and ions. The development velocity of emission intensity of argon ions is mainly determined by the dominant stepwise ionization process. Then the cathode heating stage appears with many bright and nonuniformly distributed light spots on the cathode surface, and the electron emission mechanism of cathode gradually changes to the thermionic emission as the surface temperature rises. With the increase of arc current, the discharge channel significantly expands, then becomes stable due to the increment of the Lorentz force. The characteristics of arc extinguishing stage are clarified in terms of the decay of charged particles density.

Plasmonic effects in the neutralization of slow ions at a metallic surface

AbstractSecondary electron emission is an important process that plays a significant role in several plasma‐related applications. As measuring the secondary electron yield experimentally is very challenging, quantitative modelling of this process to obtain reliable yield data is critical as input for higher‐scale simulations. Here, we build upon our previous work combining density functional theory calculations with a model originally developed by Hagstrum to extend its application to metallic surfaces. As plasmonic effects play a much more important role in the secondary electron emission mechanism for metals, we introduce an approach based on Poisson point processes to include both surface and bulk plasmon excitations to the process. The resulting model is able to reproduce the yield spectra of several available experimental results quite well but requires the introduction of global fitting parameters, which describe the strength of the plasmon interactions. Finally, we use an in‐house developed workflow to calculate the electron yield for a list of elemental surfaces spanning the periodic table to produce an extensive data set for the community and compare our results with more simplified approaches from the literature.

Inelastic processes under gas cluster ion bombardment of metals

Total currents of positive and negative particles arising under bombardment of metal surfaces with argon gas cluster ions were measured for the energy range 2–15 keV. Supposing that the negative current was defined by secondary electrons, and the positive current is the flow of the reflected argon ions, the possible mechanisms of electron emission and charge transfer are discussed. The discussion is supported by molecular dynamics simulations of cluster-surface collisions.

Simulation of glow and arc discharges in nitrogen: effects of the cathode emission mechanisms

Experimental evidence in the literature has shown that low-current direct current nitrogen discharges can exist in both glow and arc regimes at atmospheric pressure. However, modelling investigations of the positive column that include the influence of the cathode phenomena are scarce. In this work we developed a 2D axisymmetric model of a plasma discharge in flowing nitrogen gas, studying the influence of the two cathode emission mechanisms—thermionic field emission and secondary electron emission—on the cathode region and the positive column. We show for an inlet gas flow velocity of 1 m s−1 in the current range of 80–160 mA, that the electron emission mechanism from the cathode greatly affects the size and temperature of the cathode region, but does not significantly influence the discharge column at atmospheric pressure. We also demonstrate that in the discharge column the electron density balance is local and the electron production and destruction is dominated by volume processes. With increasing flow velocity, the discharge contraction is enhanced due to the increased convective heat loss. The cross sectional area of the conductive region is strongly dependent on the gas velocity and heat conductivity of the gas.

Transitional behaviors of fractional dimensional models of field-induced and space-charge limited emission from rough surfaces

Electron emission is an important physical phenomenon in both vacuum and solid-state devices. Previously, the effects of surface roughness or interface irregularity have been incorporated in the fractional Fowler–Nordheim (FNα) equation for field emission, the fractional Child–Langmuir (CLα) law for space-charge limited emission (SCLE) in vacuum, and the fractional Mott–Gurney (MGα) law for SCLE in the presence of carrier collisions using a fractional dimensional approach, where the fractional dimensional parameter (0<α≤1) defines the degree of roughness at electrode surface or irregularity of the gap medium. Here, the transitions between FNα, CLα, and MGα regimes have been investigated for electron emission from rough surfaces. It is shown that the fractional second-order or third-order nexuses, where at least two or three of the FNα, CLα, and MGα regimes converge, respectively, have a strong dependence on the surface conditions as captured by the parameter α. This work provides an important insight into the mechanism of electron emission in electron emitters where surface roughness is inevitable due to practical experimental fabrication and/or emitter surface degradation over time. The model shall serve as a versatile analytical tool to characterize the roughness by fitting the model to the experimental measurements.

Charge Dynamics Electron Microscopy: Nanoscale Imaging of Femtosecond Plasma Dynamics.

Understanding and actively controlling the spatiotemporal dynamics of nonequilibrium electron clouds is fundamental for the design of light and electron sources, high-power electronic devices, and plasma-based applications. However, electron clouds evolve in a complex collective fashion on the nanometer and femtosecond scales, producing electromagnetic screening that renders them inaccessible to existing optical probes. Here, we solve the long-standing challenge of characterizing the evolution of electron clouds generated upon irradiation of metallic structures using an ultrafast transmission electron microscope to record the charged plasma dynamics. Our approach to charge dynamics electron microscopy (CDEM) is based on the simultaneous detection of electron-beam acceleration and broadening with nanometer/femtosecond resolution. By combining experimental results with comprehensive microscopic theory, we provide a deep understanding of this highly out-of-equilibrium regime, including previously inaccessible intricate microscopic mechanisms of electron emission, screening by the metal, and collective cloud dynamics. Beyond the present specific demonstration, the here-introduced CDEM technique grants us access to a wide range of nonequilibrium electrodynamic phenomena involving the ultrafast evolution of bound and free charges on the nanoscale.

Gas Desorption and Secondary Electron Emission from Graphene Coated Copper Due to E-Beam Stimulation

The gas desorption and secondary electron multiplication induced by electron bombardment tend to induce severe low-pressure discharge effects in space microwave device cavities. Nevertheless, few studies have focused on both secondary electron emission and electron-stimulated gas desorption (ESD). Although the suppression of secondary electrons by graphene was found to be better in our previous study, it is still unclear whether the surface modification of graphene, which brings about different interfacial states, can also be manifested in terms of ESD. The deep mechanism of gas desorption and secondary electron emission from this extremely thin two-dimensional material under electron bombardment still needs further investigation. Therefore, this paper investigates the mechanism of graphene modification on Cu metal surface on the gas release and secondary electron emission properties under electron bombardment. The surface states of graphene-modified Cu were characterized, and the ESD yield and secondary electron yield of Cu/GoCu were investigated using a self-researched platform and analyzed using molecular dynamics simulations and electron Monte Carlo simulations. The results of the study showed that the most released component on the Cu surface under the bombardment of electrons was H2O molecules, while the most released component on the GoCu surface was H2 molecules. The graphene-modified samples showed a significant suppression effect on the secondary electron yield and ESD only in the low-energy region below 400 eV. This study can provide a valuable reference for suppressing low-pressure discharge and multipactor phenomena in space microwave components.

Influence of the electron field emission on the magnetized direct current high-pressure discharge

The influence of electron field emission on the sub-normal mode of a magnetized direct current high-pressure helium discharge is analyzed using the two-dimensional axisymmetric fluid model. It is observed that in spite of accounting for a more intense electron emission mechanism, discharge still operates in the sub-normal mode. However, the field emission driven discharge is characterized by a smaller discharge voltage and a larger discharge current. For large values of the electric field enhancement factor, the discharge voltage can be as low as ∼40 V, and the discharge current is a few amperes. It is also seen that for large values of the field enhancement factor and small values of the ballast resistor, rather dense plasma (density ∼ 1017 m−3) can be generated on the nanosecond time scale.

Nanoscale Vacuum Channel Hall Sensors

Nanoscale vacuum channel Hall sensors (NVCHSs) were fabricated and simulated. The fabrication flow was compatible with the complementary metal–oxide–semiconductor (CMOS) process, and the active area was etched by using focused ion beam (FIB) technology. The current–voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> ) characteristics of the bias terminals were measured to analyze the electron emission mechanism. Hall measurements were performed at ambient conditions to evaluate the Hall voltage, linearity, and voltage-related sensitivity of the proposed Hall sensor. The linearity error was less than 5%, and the voltage-related sensitivity was 0.28 V/VT. The performance of the sensor was similar to that of the common micrometer-sized Hall sensor, but with the benefits of small size and easy integration. Ultimately, a finite-element method (FEM) simulation was performed to show the potential distribution during normal sensor operation and to demonstrate the possibility of high-frequency operation. This work clearly outlines the potential of nanoscale vacuum channel technology in Hall sensor applications.

Current–voltage characteristics and mechanisms of electron emission from cold cathodes in a helium discharge

The abnormal discharge (AD) in pure helium and in helium containing molecular impurities was investigated. The obtained results were compared with the parameters of wide-aperture AD and different variants of ‘open’ discharge. It was shown that in all of the investigated types of discharges the current–voltage characteristics (CVCs) and, correspondingly, the emission properties of cold cathodes are determined mainly not by their material, but by the doping of cathodes with atoms of working gases and the purity of experimental conditions. With the impurity content less than 10−4% of the helium atom concentration, the CVCs begin to acquire S-shaped form, which is associated with a change in the electron emission mechanism. It was shown that the diversity of the CVCs is caused by uncertainty in the values of the secondary electron emission coefficients γ and the electron multiplication coefficient α in the cathode layer at reduced electric field strength E/N 103 Td. The reproducibility of CVCs and, correspondingly, emission properties of cathodes can be ensured by high purity of the working gas and by maintenance of the cathode doping only by the working gas atoms.

FIB fabrication and electron emission characteristics of overhanging tungsten and platinum nanostructures

The recent advancement in nanofabrication technology has enabled realization of vacuum electron devices with nanogap spacing. These devices have advantages over semiconductor counterparts due to possible operation under hard conditions and some unique functionalities. The challenging task, however, lies with the full understanding of their current–voltage (I–V) characteristics, resulting from various electron emission mechanisms. The reliable extraction of device parameters is, therefore, vital for its potential applications. An attempt has, therefore, been made here to fabricate two three-dimensional overhanging electrodes of tungsten and platinum with a nanoscale gap of 70–100 nm on glass substrates using chemical vapor deposition and focused ion beam milling. Their (I–V) characteristics measured in situ at ∼10−6 mbar are shown to follow the Child–Langmuir law and Fowler–Nordheim field emission at low and high-bias conditions, respectively. The extraction of parameters with a simple procedure suggested earlier yields an effective emission area of ∼3510 Å2, work function of ∼2.5 eV, and field enhancement factor (β) of ∼ 1.8 for tungsten; the values for platinum are 12.5 Å2, 3.0 eV, and 5.0, respectively. The higher β in the case of platinum can be attributed to the formation of a comparatively rough emitter surface with some fine protrusions. The nanostructure gives a current spike at high voltages, which marks its transition to an explosive emission state, breakdown, and dispersion of spherical metal particles over the substrate.

Study of an asymmetry tunnel FET biosensor using junctionless heterostructure and dual material gate

In recent years, label-free sensors have been studied extensively for biomolecule detections. Label-free biosensors based on MOSFETs could achieve high detection sensitivity, the subthreshold swing of such sensors cannot break the limitation of 60 mV Dec−1 due to the physical mechanism of thermal electron emission. However, subthreshold swing less than 60 mV Dec−1 can be achieved in biosensors based on tunnel FETs working in band to band tunneling (BTBT) mode. Usually, label-free biosensors have a nanogap under the gate electrode both in MOSFET and tunnel FET (TFET), which can electrically sense the characteristics of biomolecules by dielectric constant modulation effect. In this article, we propose a novel nanogap embedded and dielectric modulated asymmetry tunnel FET biosensor with junctionless heterostructure and dual material gate, where different biomolecules can be detected effectively by adjusting the workfunctions for gate electrodes. Influences of tunnel gate, auxiliary gate and back gate workfunctions on sensitivities are explored. In addition, device-level gate effects are simulated by considering neutral and charged biomolecules. The influence of different dielectric constant at fixed charge density is also studied. Simulation results show that asymmetry dual material gate junctionless heterostructure tunnel FET (ADMG-HJLTFET) biosensor can provide higher switch ratio and higher sensitivity.

Scandium wetting of tungsten surfaces in “scandate” thermionic cathodes

The surface structure and electron emission mechanism in high-performing “scandate” thermionic cathodes has long been a point of debate, in part due to the challenges inherent in observing emitting surfaces under operational conditions. Critically, the distribution and role of Sc in enabling high emitted current densities at low operating temperatures remains unclear. Here, using density functional theory calculations of the structure and energy of bare and Sc-covered W slabs, the wetting behavior of Sc on W has been explored.Computed surface excess energies reveal that, despite the bulk immiscibility of W and Sc, a single monolayer of Sc wets (001), (110), and (112) W surfaces at very low oxygen chemical potentials. The addition of further layers of Sc was found to be thermodynamically unfavorable. In contrast to previous studies, the present findings suggest the existence of a Sc interlayer directly atop W in scandate cathodes, representing a novel example of surface-energy-driven metal-on-metal wetting leading to self-assembly of atomically-thin layers.

Puzzling time properties of proportional electroluminescence in two-phase argon detectors for dark matter searches

Proportional electroluminescence (EL) in noble gases is a physical process routinely used in two-phase (liquid–gas) detectors for low-energy astroparticle-physics experiments. In this work, the time properties of visible-light EL in two-phase argon detectors have been systematically studied for the first time. In particular, two unusual slow components in the EL signal, with their contributions and time constants increasing with electric field, were observed. This puzzling property is not expected in any of the known mechanisms of photon and electron emission in two-phase media. Time constants of these components is about 4–5 upmu s and 50 upmu s. In addition, a specific threshold behavior of the slow components was revealed: they emerged at a threshold in reduced electric field of 4.8 ± 0.2 Td regardless of the gas phase density, which is about 1 Td above the onset of standard (excimer) EL. There is a conspicuous similarity between this threshold and reduced field threshold of EL in NIR occurring via higher atomic excited states Ar^{*}(3p^{5}4p). An unexpected temperature dependence of slow components was also observed: their contribution decreased with temperature, practically disappearing at room temperature. We show that the puzzling properties of slow components can be explained in the framework of hypothesis that these are produced in the charge signal itself due to trapping of drifting electrons on metastable negative argon ions.

Theoretical Linkage of Thermionic, Field, and Space-Charge Limited Emission for a Vacuum Crossed-Field Gap

Crossed-field devices, such as magnetrons and crossed-field amplifiers, where an external magnetic field is orthogonal to the electric field across the gap, are used in radars, communications systems, and material processing. Improving the efficiency and power density of crossed-field devices requires a robust understanding of electron emission, particularly when multiple emission mechanisms may contribute, such as for thermionic and field emission. Previous studies of nonmagnetic diodes have theoretically identified the transitions, or nexuses, between multiple electron emission mechanisms by examining the relevant equations defining each individual mechanism and then derived exact theoretical solutions that combined these mechanisms and approached the standard emission equations under appropriate limits. This work extends those studies by incorporating the orthogonal magnetic field and assessing the transitions between the Richardson–Laue–Dushman (RLD) formulation for thermionic emission, the Fowler–Nordheim (FN) equation for field emission, the space–charge limit given by both the general (for nonzero electron injection velocity) and (zero electron injection velocity) Child–Langmuir (CL) (generalized CL (GCL) and CL, respectively) laws for space–charge limited current, and the space–charge limited current for a noninsulated (magnetic field below the Hull cutoff) and insulated (magnetic field above the Hull cutoff) diode. We derive exact solutions for the current density for these mechanisms and demonstrate their asymptotic behavior under relevant conditions. Furthermore, we plot the exact solution of current density <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> as a function of applied voltage <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> by incorporating the general thermo-field emission theory for the current source and demonstrate the transition between each emission mechanisms with increasing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> for various temperatures and fixed ratios of magnetic field to the Hull cutoff. We also construct nexus plots to identify transitions between the various regimes for different conditions.

Electron Emission Regimes of Planar Nano Vacuum Emitters

Recent advancements in nanofabrication have enabled the creation of vacuum electronic devices with nanoscale free-space gaps. These nanoelectronic devices promise the benefits of cold-field emission and transport through free space, such as high nonlinearity and relative insensitivity to temperature and ionizing radiation, all while drastically reducing the footprint, increasing the operating bandwidth, and reducing the power consumption of each device. Furthermore, planarized vacuum nanoelectronics could easily be integrated at scale similar to typical microscale and nanoscale semiconductor electronics. However, the interplay between different electron emission mechanisms from these devices is not well understood, and inconsistencies with pure Fowler–Nordheim (FN) emission have been noted by others. In this work, we systematically study the current–voltage characteristics of planar vacuum nanodevices having few-nanometer radii of curvature and free-space gaps between the emitter and the collector. By investigating the current–voltage characteristics of nearly identical devices fabricated from two different materials and under various environmental conditions, such as temperature and atmospheric pressure, we are able to clearly isolate three distinct emission regimes within a single device: Schottky, FN, and saturation. Our work will enable robust and accurate modeling of vacuum nanoelectronics, which will be critical for future applications requiring high-speed and low-power electronics capable of operation in extreme conditions.

Electron emission mechanism of scandium‑tungsten cathodes

Scandium‑tungsten (Sc-W) cathodes have garnered a lot of research attention due to their high emission current density. This study investigates the electronic properties of Sc-W cathodes using the VASP code to explain the operating mechanism of hot electron emission. The results reveal that the work function is decreased by 0.58 eV, whereas the charge density in the crystal structure, the number of high valence electrons and the energy of electrons increase when Sc content is increased from 1% to 2%. Moreover, the bandgap and energy required for electrons to jump from the valence band to the conduction band decreased with the increase of Sc content, enhancing the current emission density and reducing the work function of the cathode.