Emission Current Research Articles

SiO2 etching and surface evolution using combined exposure to CF4/O2 remote plasma and electron beam

Electron-based surface activation of surfaces functionalized by remote plasma appears like a flexible and novel approach to atomic scale etching and deposition. Relative to plasma-based dry etching that uses ion bombardment of a substrate to achieve controlled material removal, electron beam-induced etching (EBIE) is expected to reduce surface damage, including atom displacement, surface roughness, and undesired material removal. One of the issues with EBIE is the limited number of chemical precursors that can be used to functionalize material surfaces. In this work, we demonstrate a new configuration that was designed to leverage flexible surface functionalization using a remote plasma source, and, by combining with electron beam bombardment to remove the chemically reacted surface layer through plasma-assisted electron beam-induced etching, achieve highly controlled etching. This article describes the experimental configuration used for this demonstration that consists of a remote plasma source and an electron flood gun for enabling electron beam-induced etching of SiO2 with Ar/CF4/O2 precursors. We evaluated the parametric dependence of SiO2 etching rate on processing parameters of the flood gun, including electron energy and emission current, and of the remote plasma source, including radiofrequency source power and flow rate of CF4/O2, respectively. Additionally, two prototypical processing cases were demonstrated by temporally combining or separating remote plasma treatment and electron beam irradiation. The results validate the performance of this approach for etching applications, including photomask repair and atomic layer etching of SiO2. Surface characterization results that provide mechanistic insights into these processes are also presented and discussed.

Electromagnetic and Electrostatic Particle-in-Cell Simulations for Multipactor in Parallel-Plate Waveguide

This article introduced a multipactor threshold criterion based on collision and emission currents, and the simulation results were compared using a traditional method. The impact of macroparticle weight on multipactor saturation was studied using electromagnetic particle-in-cell (EM-PIC) simulations of CST Particle Studio. It was found that the simulation results converged when the macroparticle weight was less than 106. A model calculating accumulated charge on the dielectric surface was proposed and validated against theoretical calculations. The dynamic evolution of multipactor in dielectric-loaded parallel-plate waveguide was analyzed, and predictions were validated against results from the existing model. Finally, the effect of surface charge fields and space charge effect on the multipactor mechanism was studied. The results showed that the multipactor could be suppressed when the ratio of the number of accumulated electrons on the dielectric surface to the saturation electron population was greater than 3.96.

Improving the Response Speed of an Active-Current Controlled Field Emitter Arrays by Modifying the Controlled Current

Using an active-current control (ACC) component such as MOSFET in field emitter arrays (FEAs) is an important way to achieve stable emission current. However, the controlled current of ACC may reduce the response speed of FEAs due to the longer charging process, which limits its application. In this work, a general model for the field emission of MOSFET-controlled FEAs has been established to study the influence of controlled saturation current on its response speed. It is found that the response speed which decreases nearly linearly as the saturation current decreases can be reduced down by above 1 order of magnitude comparing to the intrinsic value. The field emission characteristics of a MOSFET-controlled ZnO nanowire FEAs under different saturation current and duty ratio have been investigated, in which the result is in consistency with the model. A solution using a controlled current which is larger than the induced current during the charging process and can rapidly reduce down to the field emission current has been proposed for balancing the stability and response speed in MOSFET-controlled FEAs. All the results provide a method for obtaining fast response ACC-FEAs, which should be useful in the application that requires pulsed driving FEAs.

Improved Sparse Component Analysis for Multi-Point Harmonic Contribution Evaluation Under Incomplete Measurements

The existing methods for multi-point harmonic contribution evaluation usually require some preconditions, such as the node harmonic current is equivalent to the harmonic emission current and the number of measurements is not less than the number of sources. However, these requirements may not be satisfied in practice. In this context, this paper proposes an improved sparse component analysis (SCA) method to solve the underdetermined blind source separation model, which is used to describe the harmonic state under incomplete measurements. The improved SCA adopts a new two-stage technique. First, the mixing matrix containing harmonic transfer impedances is estimated through the synchrosqueezing S-transform. Then, the source signal vector containing harmonic emission currents is reconstructed through the modified subspace projection. By matching the harmonic impedances and currents, the multi-point harmonic contribution evaluation is further achieved. The efficiency of the proposed method is verified by simulations and field case studies. The results demonstrate that the proposed method outperforms the existing methods in terms of accurately estimating the harmonic emission currents and quantifying the harmonic contributions under incomplete measurements.

Biased self-diffusion on Cu surface due to electric field gradients

Under strong electric fields, an arc of strong current flowing through plasma can link two metal surfaces even in ultra high vacuum. Despite decades of research, the chain of events leading to vacuum arc breakdowns is hitherto unknown. Previously we showed that a tall and sharp Cu nanotip exposed to strong electric fields heats up by field emission currents and eventually melts, evaporating neutral atoms that can contribute to plasma buildup. In this work, we investigate by means of molecular dynamics (MD) simulations whether surface diffusion biased by the presence of an electric field gradient can provide sufficient mass transport of atoms toward the top of the nanotip to maintain supply of neutrals for feeding plasma. To reach the necessary timescales and to add electric field in MD, we utilized a novel combination of collective variable-driven hyperdynamics acceleration and coupling to a finite element mesh. In our simulations, we observed biased self-diffusion on Cu surfaces, that can contribute to the continuous replenishment of particle-emitting nanotips. This mechanism implies a need to reduce the rate of surface diffusion in devices that are susceptible to vacuum arcs. Finding suitable alloys or surface treatments that hinder the observed biased diffusion could guide the design of future devices, and greatly improve their efficiency.

RF signals over field emission currents: A theoretical study for MEMS capacitive switches

The existence of (sub)micrometer scale gaps in Micro-Electro-Mechanical-Systems (MEMS) gives rise to field emission currents and this is already considered a reliability issue resulting in device degradation and/or failure. This work aspires to offer another perspective with respect to the field emission related phenomena in Radio Frequency (RF)-MEMS, focusing the attention prior to the failure and emphasizing on a reliability aspect affecting the signal integrity. This stems from the non-linear nature of the field emission currents and instigated during their simultaneous presence with RF signals, particularly of high power. Theoretical calculations reveal that this combination results in the generation of new harmonics in addition to the stimulated one. This effect dependents on the distortion induced in the field emission current by the simultaneous excitation by both the DC and the RF biases. Apart from the applied biases, additional parameters contributing indirectly, such as the operation frequency and the device characteristics are having a major role. These outcomes should therefore be considered when designing (high-power) RF MEMS applications.

Metal-Air Field Emission Devices - Nano Electrode Geometries Comparison of Performance and Stability.

Air-channel devices have a special advantage due to the promise of vacuum-like ballistic transport in air, radiation insensitivity, and nanoscale size. Here, achieving high current at low voltage along with considerable mechanical stability is a primary issue. The comparative analysis of fourplanar and metallic electrode-pair geometries at 10nm channel length is presented. The impact of nano-electrode-pair geometries on overall device performance is investigated. Air-channel devices are operated at the ultra-low voltage of 5mV to demonstrate the device dynamics of air-channel devices at low power. Investigations focus on the direct tunneling (DT) mechanism which is dominant in thelow-voltage regime. Comparative analysis of different electrode-pair geometries revealstwo orders of magnitude increment in the current just by modulating the electrode-pair structure. Theoretical analysis suggests that the emission current is directly related to the active junction area within the metal-air-metal interface at the direct tunneling regime. The geometry-dependent mechanical stability of different electrode pairs is compared by imaging biasing triggered nanoscale structural changes and pulsed biasing stress analysis. The results and claims are confirmed and consolidated with the statistical analysis. Experimental investigations provide strong directions for high-performance and stable devices. In-depth theoretical discussions will enable the accurate modeling of emerging low-power, high-speed, radiation-hardened nanoscale vacuum electronics.

Ion-induced electron emission cathode for a micro-newton HEMP thruster

An ion-induced electron emission cathode is applied for the first time in micro electric propulsion devices. In this investigation, ion-induced electron emission is operated on an annular LaB6 emitter for the neutralization and ion extraction of a microwave discharge HEMP Thruster (High Efficiency Multi-staged Plasma Thruster). A plume plasma diagnosis is performed with a Faraday probe and a retarding potential analyzer to analyze the discharge characteristics and plasma parameters. The results show that the ion-induced electron emission cathode can provides an emission current of over 0.4 mA. Compared with the condition with a thermionic cathode, there is a reduction in the thruster extracted ion current due to the deficiency of the emission current. The emission current shows different trends over anode voltage in different position configurations, which are affected by the incident ion flux and ion energy. Since the ion-induced electron emission cathode is free of power supply and propellant, it is of significant potential for low-power electric propulsion systems.

Multiple connected artificial synapses based on electromigrated Au nanogaps

Building an artificial synaptic device with multiple presynaptic inputs will be a significant step toward realization of sophisticated brain-inspired platforms for neuromorphic computing. However, an artificial synapse that can mimic functions of multiple synapses in a single device has not yet been well developed with existing electronic devices. Here, we experimentally implement the functions of multiple synapses in a single artificial synaptic device consisting of multiple connected nanogap electrodes. The “activation” technique, which is based on electromigration of metal atoms induced by a field emission current, was applied to the device to emulate the synaptic functions. We show that the device, upon application of activation, exhibits conductance changes in response to stimulation voltage, similar to the memory states of biological synapses. Several important synaptic responses—notably, short-term plasticity and long-term plasticity—were successfully demonstrated in multiple connected Au-nanogaps. For further application, a simple network was implemented using multi-input devices based on a two-terminal Au nanogap array, exhibiting the ability to classify the digital input vector pattern. These demonstrations pave the way for brain-inspired computing applications such as associative memory, pattern classification, and image recognition.

Comparative study on atomically heterogeneous surface with conical arrays of field emitters generated using plasma based low-energy ion beams

A comparative study of the field emission properties of conical arrays of atomically heterogeneous, self-organized, micro–submicro–nanodimensional structures, irradiated at normal incidence by high flux of 2 keV argon (flux=6.47×1015cm−2s−1) and krypton ions (flux=4.81×1015cm−2s−1) on copper substrates, without employing any external seeding, is presented. The variation in surface structural growths with ion beam fluence is investigated using scanning electron, atomic force, and transmission electron microscopy. The exposed surfaces are atomically heterogeneous due to the presence of embedded argon and krypton ions in the interstitial layers (≈nm) as observed from the x-ray photoelectron spectroscopy analysis. Kelvin probe force microscopy is employed to analyze the variation in local work function caused by surface deformities and implantation of inert gaseous ions. The conical arrays are naturally selected field emitter sources, and their field enhancement factor is calculated from the Fowler–Nordheim equations. The argon ion treated substrate at a fluence of 4.85×1018cm−2 gives rise to uniformly distributed structures and has a low turn-on voltage of 2.76 kV with an electron emission current of 0.58 nA. Among the krypton ion irradiated substrates, the sample irradiated at the highest fluence of 5.12×1018cm−2 produces self-organized conical arrays having uniform dimension, orientation, distribution, and even a higher electron emission current of 0.81 nA with a lower turn-on voltage of 2.12 kV. Thus, it may be concluded that krypton ion irradiation provides better generation of naturally selected arrays of field emitters.

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.

Optimization of hybridized InAsSb/InGaSb semiconductor topological materials

Generating large topologically protected surface currents using conventional III–V infrared materials such as InAsSb/InGaSbAs quantum wells (QWs) and superlattices (SLs) has been important. In such materials, topological states can be formed at the edge by hybridizing ordinary electronic band structures. However, achieving large surface currents out of these materials is still difficult due to low emission currents and high carrier defects. In this work, we present two hybridized topological structures: one for the 6.22 Å metamorphic QWs and the other for the 6.10 Å pseudomorphic SLs. Both structures are tailored for the same hybridization gap (Δ) of ∼60 meV and optimized for the minimum crystal defects. While the QW grown on metamorphic buffers generates a significant amount of mismatch-related crystal defects, the SL grown on lattice-matched buffers produces an excellent crystalline-quality. Quasiparticle interference mapping and calculations on a SL sample show good agreement of the band structure.

Integrated sub-micron vacuum gaps in semiconductor devices

We present characterization results of integrated vacuum gaps in semiconductors and report the highest breakdown field of dielectric layers ever recorded within microfabricated semiconductor devices. Difficulties associated with the characterization of vacuum gaps in the presence of high electric fields could be overcome by using cylindrical capacitors with silicon electrodes that were manufactured with standard semiconductor technology. With this approach, breakdown fields of up to 6 × 109 V/m were achieved. The vacuum gaps of 175(5) nm were significantly smaller than the mean free path of electrons within the gap such that a breakdown due to avalanche discharge was avoided. As the voltage was increased, initially a field emission current was observed that followed a Fowler–Nordheim tunneling behavior. The tunneling current started to increase at voltages about four times greater as compared to equivalent dielectric layers of silicon oxide. At higher voltages, a mechanical breakdown occurred, where the pillars that formed the central electrode of the capacitor snapped due to electrostatic forces. We provide characteristics of thin vacuum layers, which could be useful for device design in micro- and nanoelectromechanical systems as well as semiconductor devices.

Evolution of field emitters on copper surface during conditioning

To understand the mechanism of structure conditioning under high-power RF fields, a cathode image beamline based on field emission current was built at Tsinghua University. Cathodes made of oxygen-free copper were conditioned on the beamline. To monitor the modification of the cathode surface during conditioning, a designed solenoid focused the field emission current from the cathode to a YAG screen 1m away. The experiment showed that strong field emitters spontaneously appeared after breakdowns occurred, and the emitters were eliminated with increasing RF power. These results suggest that the conditioning process converts subsurface defects into field emitters and eliminates the emitters to improve the gradient threshold of breakdown.

Systematic Field Electron Emission Investigations of Vapor‐Phase‐Grown Sb2Te3 Nanosheets

Micron‐sized Sb2Te3 nanosheets are grown on p‐type Si substrate by facile vapor‐phase route. Physicochemical properties of the Sb2Te3 nanosheets are revealed using X‐ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM), transmission electron microscope (TEM), Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). Owing to their high aspect ratio, field emission (FE) characteristics of the Sb2Te3 nanosheets grown on Si substrate (termed as a “planar emitter”) are investigated. Furthermore, the FE behavior of a single Sb2Te3 nanosheet attached to a blunt tungsten needle is studied. The single nanosheet emitter exhibited superior emission behavior in contrast to the planar emitter. The Fowler–Nordheim (F–N) plot for the single nanosheet emitter exhibits linear nature, whereas for the planar emitter it shows deviation from linearity. An attempt has been made to reveal the FE properties of a single Sb2Te3 nanosheet in an “in‐plane” configuration, similar to the vacuum field emission transistor (VFET). In this case, a maximum emission current of 1 μA is observed at 290 V, which is comparable to other VFETs. The observed results clearly imply the potential of the single nanosheet emitters due to topological insulators, like Sb2Te3, for the development of new generation nanoscale vacuum electronic devices.

Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

ABSTRACT We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn’s ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn’s ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

A stable LaB6 nanoneedle field-emission electron source for atomic resolution imaging with a transmission electron microscope

Field-induced electron emission offers the best performance point electron source for electron microscopes. Herewith we report a new cold field-emission point electron source utilizing a nanoneedle made of lanthanum hexaboride (LaB6) and its implementation in a spherical aberration-corrected transmission electron microscope (TEM). A sub-ångstrom resolution of 0.96 Å has been obtained with the TEM operated at an acceleration voltage of 60 kV and a smaller energy spread was also observed compared with the contemporary W(310) electron source. In particular, the LaB6 nanoneedle cold field-emission electron source required no thermal flashing for continuous operation of extended hours (>100 hours) while exhibiting and sustaining a high stability in emission current (<1%). We attribute the excellent performance of the LaB6 nanoneedle electron source to (i) a low work function of LaB6; (ii) excellent alignment of the nanoneedle emitter by the electron and ion dual-beam processing with nanoscale precision and (iii) robustness of the nanometric structure that suppressed mechanical vibrations of the LaB6 emitter even in a high electric field. This new LaB6 nanoneedle cold field-emission electron source enables stable high-resolution imaging in TEM and it will also benefit cryogenic electron microscopy, scanning electron microscopy, electron beam lithography, and other electron beam technologies.

Semi-analytical theory of emission and transport in a LAFE-based diode

A large area field emitter (LAFE) typically consists of several thousands of nanoscale emitting tips. These are difficult to simulate using purely numerical methods based on finite/boundary element or finite difference methods. We show here that a semi-analytically obtained electrostatic field allows tracking of field emitted electrons of a LAFE fairly accurately using the knowledge of only the LAFE geometry. Using a single and a nine-emitter configuration, the beam parameters calculated using this method are compared with the results of tracking using fields generated by COMSOL. The net emission current, energy conservation, and the transverse trace-emittance are found to be reproduced with reasonable accuracy.

Phonon assisted electron emission from quasi-freestanding bilayer epitaxial graphene microstructures

Electron emission from quasi-freestanding bilayer epitaxial graphene (QFEG) on a silicon carbide substrate is reported, demonstrating emission currents as high as 8.5 μA, at ∼200 °C, under 0.3 Torr vacuum. Given the significantly low turn-on temperature of these QFEG devices, ∼150°C, the electron emission is explained by phonon-assisted electron emission, where the acoustic and optical phonons of QFEG causes carrier acceleration and emission. Devices of differing dimensions and shapes are fabricated via a simple and scalable fabrication procedure and tested. Variations in device morphology increase the density of dangling bonds, which can act as electron emission sites. Devices exhibit emission enhancement at increased temperatures, attributed to greater phonon densities. Devices exhibit emission under various test conditions, and a superior design and operating methodology are identified.

First principle studies revealing the effect of O2, CO2, and H2 adsorption on field emission behaviour of graphene

The effects of the absorption of the gas molecule on the work function and the field emission current of graphene have been theoretically investigated. The results demonstrate that the absorbed O2, CO, and H2 on graphene with different absorption and absorption types have a large effect on its field emission properties. All absorbed O2, CO, and H2 on graphene with different absorption sites and absorption types except H2 on the bridge site lead to a reduction in its field emission current. It agrees with the experimental conclusion that the adsorption of O2 on the graphene will reduce its field emission current in the literature. Numerical results suggest that the density of adsorbed molecules, the types of adsorbed molecules, the type of adsorption and the location of adsorption could be detected by measuring the variation rule of the field emission current under different applied electric fields. This implies that the characteristics of field emission properties of graphene can be implemented as a working principle of a feasible gas sensor.