Emission Current Increases Research Articles

SiC emitters for nanoscale vacuum electronics: A systematic study of cathode–anode gap by focused ion beam etching

Nanoscale vacuum electronics has been receiving much attention recently with the design and fabrication of vacuum field emission transistors and other devices. The performance and lifetime of these devices depend on the material choice for the emitters. Silicon carbide is a robust material with appealing work function and established fabrication processes. Field emission properties of SiC nanoemitters under different cathode–anode gaps are studied in this work with the aid of focused ion beam etching to control the gap precisely. When the electrode gap is as small as 20 nm, a considerable decrease in voltage and increase in field emission current are seen. When the electrode gap is decreased progressively, the emission current increases exponentially at a fixed collector voltage, and the absence of current saturation is ideal for device scaling. Simulations and Fowler–Nordheim equation are used to analyze the field emission characteristics. The emission is enhanced for both extremely short gaps and sharp emitter tips as expected. The present results for the diode-based system are useful for the design of future gated three terminal devices such as vacuum field emission transistors and field emission displays using SiC.

CALCULATIONS OF ION TRAJECTORIES AT MAGNETOPLASMA SEPARATION AND EXPERIMENTS WITH POLYATOMIC GASES

Calculated trajectories of ions with different masses, indicating the possibility of a mixture separation, are obtained. Comparative experiments for plasma of monatomic and polyatomic gases (Ar, N<sub>2</sub>, CO<sub>2</sub>), upon combination of pulsed discharge with a stationary one with incandescent cathode, are carried out. The oscillograms of discharge current and voltage at low emission currents and a constant energy input show that energy is spent on other processes different from ionization. With an increase of emission current, the nonlinear character of the discharge current and voltage, which may be indicative of the role of dissociation and vibrational levels in energy consumption, is observed. In addition, there is connection between the number of atoms in molecule and the values of maximum discharge current and the pressure of injected gas.

Influence of emission threshold and current increase rate on microwave starting time in relativistic backward wave oscillator

Explosive emission cathodes (EECs) are widely used in high power microwave generators. This paper researches the influence of the emission threshold and the current increase rate of annular EECs on the microwave starting time of a relativistic backward wave oscillator (RBWO) when the current amplitude is not affected. The results show that a moderate delay in explosive emission, as long as it's not too long and the current increase rate keeps fast enough, won't bring about a corresponding delay in the starting time of microwave, but inversely, may suppress the mode competition and thus expedite the starting process slightly. The current increase rate, however, has more prominent influence on the starting time of the RBWO. A slower current increase rate will delay the time when the beam current reaches the starting current and lead to a longer starting time.

Influence of Cathode Size on Field Emission Properties of a Single Vertical Carbon Nanotube Material

In this work, we study the field emission properties of a single vertical carbon nanotube with cathode radius changing. The carbon nanotube is considered as purely perfect conductor, and the anode and cathode are modeled as discs. The radius of anode is kept as 24m, then the radius of cathode is changed from 24 m to 1 m. The emitter of that CNT consists of a hemispherical cap of 4 nm radius (r) on top of a cylinder height of 2 m (h). The distance from anode to cathode is w=h+20m. The overall surface area of the CNT and cathode are all grounded, and the anode plate has a constant potential value of 100 V. The distribution of potential and electric field, field emission current are obtained by simulating with the help of COMSOL Multiphysics 4.3b electrostatics module. We find that with cathode radius decrease, the electric field strength over the surface of carbon nanotube (CNT) strengthening and field emission current increase.

Room temperature fabrication of 1D carbon-copper composite nanostructures directly on Cu substrate and their field emission properties

This paper demonstrates a carbon-copper (C-Cu) composite nanostructure directly fabricated on a copper (Cu) substrate using the Ar+ ion irradiation method at room temperature. The morphology of C-Cu composite was controlled by a simultaneous carbon supply during ion irradiation. Conical protrusions formed on the surface of the Cu substrate with the low carbon supply rate (RC), whereas high RC area prominently produced nanoneedle structures. The field electron emission (FEE) tests demonstrated significant improvement between conical protrusions and nanoneedle structures, where the emission current increase from 5.70 μAcm−2 to 4.37 mAcm-2, while the turn-on field reduced from 5.90 to 2.00 Vμm−1.

Controlled electron emission and vacuum breakdown with nanosecond pulses

Vacuum electron sources exploiting field emission are generally operated in direct current (DC) mode. The development of nanosecond and sub-nanosecond pulsed power supplies facilitates the emission of compact bunches of electrons of high density. The breakdown level is taken as the highest value of the voltage avoiding the thermo-emission instability. The effect of such ultra-fast pulses on the breakdown voltage and the emitted electron current is discussed as a result of the thermo-emission modelling applied to a significant protrusion. It is found that pulsing very rapidly the vacuum breakdown occurs at higher voltage values than for the DC case, because it rises faster than the heat diffusion. In addition, the electron emission current increases significantly regardless of the theoretical approach is used. A comparative study of this theoretical work is discussed for several different forms of the protrusion (elliptic and hyperbolic) and different metals (hence varying the melting point), particularly refractory (tungsten) versus conductor (titanium). Pulsed mode operation can provide an increase on breakdown voltage (up to 18%) and a significant increase (up to 330%) of the electron extracted current due to its high non-linear dependency with the voltage, for the case for the case with a hyperbolic protrusion.

Fabrication of vertically aligned carbon nanotubes–zinc oxide nanocomposites and their field electron emission enhancement

Systematic investigation on the fabrications of vertically aligned carbon nanotubes (VACNTs) and zinc oxide (ZnO) nanocomposites for two different configurations were performed to enhance the field electron emission (FEE) performance of the VACNTs synthesised from waste chicken fat. FEE tests demonstrated improvement with the addition of ZnO nanostructures under the VACNT array, where the emission current increased from 212.85 to 259.80μAcm−2, while the turn-on field reduced from 2.66 to 1.60Vμm−1. Meanwhile, the growth of ZnO nanostructures on top of the VACNTs array resulted in a tremendous increase in emission current of up to 2090.00μAcm−2 and a decrease in the turn-on field to 0.49Vμm−1. The difference in the FEE improvement by different composite configuration reflects the significant role of the composite order. The improvement of FEE is influenced by the geometrical shape of the emitters as well as tunnelling effect of electrons through the structure of nanocomposite.

Effect of substrate heating and microwave attenuation on the catalyst free growth and field emission of carbon nanotubes

Carbon nanotubes (CNTs) have been directly grown on Inconel 600 substrates by microwave plasma enhanced chemical vapor deposition without using any external catalyst. Grown CNTs were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and field emission measurements. Characterization results show that field emission current density increases from 200μA/cm2 at ∼5.5V/μm to 14.5mA/cm2 at ∼1.6V/μA when substrate is heat-treated and incident microwave is attenuated before reaching it. Detailed characterization reveals that heat-treatment results in migration of Cr and Fe oxides towards the top surface which completely changes substrate morphology also. Microwave attenuation reduces reflection of microwaves from the substrate and increases residence time of the precursor over the substrate promoting high density growth of CNTs. The combination of these two process parameters resulted in growth of long, dense CNTs with bamboo-like defects that contributes to enhanced current density at lower applied field.

Structural, Field Emission and Ammonia Gas Sensing Properties of Multiwalled Carbon Nanotube-Graphene Like Hybrid Films Deposited by Microwave Plasma Enhanced Chemical Vapor Deposition Technique

This paper reports the direct deposition of multiwalled carbon nanotube (MWCNT)-graphene like hybrid films on nickel substrate using a 2.45 GHz microwave plasma enhanced chemical vapor deposition (MW PECVD) system in the temperature range of 500-700 degrees C at 20 Torr pressure. The films have been characterized by Raman spectra, high resolution transmission electron microscope (HRTEM), scanning electron microscope, high resolution X-ray diffraction and contact angle measurement. Raman spectroscopy and HRTEM reveal the formation of MWCNT and graphene like hybrid carbon sheet structures. The effect of processing temperature on the field emission properties of MWCNT-graphene like hybrid films has been investigated. Field emission measurement reveals that the turn-on field decrease and the emission current density increase with the increase of deposition temperature. The rambutan structure of MWCNT formed at 700 degrees C is responsible for the improvement in the field emission properties. The film deposited at 700 degrees C shows fast response and recovery time of 40 and 96 s, respectively, for ammonia gas sensing due to the high surface area of the film. It has also been found that the hydrophobic surface of the film helps to perform the gas sensing in the humid environment.

钛基底的纳米金刚石掺混纳米碳管的场发射特性

On the basis of field emission characteristics of nano-diamond,the field emission characteristics of diamond doped CNTs were studied. The composite coating of diamond and CNTs was formed using electrophoretic deposition,and the cathode samples were fabricated after heat treatment. The microstructure,field emission and luminescence performance of the samples were characterized. Compared with the undoped diamond cathode,the turn-on field of the composite coating cathode decreases obviously,the field emission current increases. The fluorescent powder on the anode surface can luminescent under lower electric field,but the luminescence is nonuniform and the phenomena of ‘edge luminescence'occurs. The improvement of the field emission characteristics is due the enhancement of the electron transportation capacity of the composite coating and the increase of the effective emitter number. Finally,the reasons of ‘edge luminescence'were explained.

Effect of cold electron emission on diffusion plasma parameters and the sheath structure in a double plasma device

It is observed experimentally that by injecting cold electrons in the discharge region of a double plasma device, the plasma parameters and sheath structure can be controlled in the other region, which is devoid of any electrical discharge. The main discharge region is separated from the region under investigation by a grounded mesh grid. Both cold and hot ionizing electrons are emitted from separate sets of filaments in the discharge region. With an increase in the cold electron emission current, the plasma parameters in the discharge region get changed, which in turn alter the plasma parameters in the other region. Two important effects caused by cold electrons in the diffusion region are the increase in the plasma density and decrease in the plasma potential. The increase in the plasma density and decrease in the sheath potential drop therefore cause the contraction of the sheath.

The Study of Aluminum Pattern on Phosphors Anode in Field Emission Device

The major purpose of this study is to investigate the effect of aluminum thin film overlaid on the phosphor layer. The aluminum film patterns consisted of intersecting lines or dotted structure. The results show that the aluminum thin film with patterns on the phosphor anode surface enable us to improve the life time of the phosphor and eliminates the charging-up effect on the phosphor layer of the field emission device. The emission current increases significantly, and after 5 minutes continuous emission, the phosphor brightness decrement can be restrained effectively in the demonstration.

Dependence of high density polyethylene XPS spectrum on electron flood gun parameters

As specimen charging appears the most significant obstacle in x-ray photoelectron spectroscopy (XPS) analysis of insulators, the application of low energy flood gun was investigated. Dependence of silicon 2s peak properties (position and full width at half maximum), on electron flood gun parameters (emission current and accelerating voltage), was observed. In the first stage of the experiment, flood gun accelerating voltage was kept constant at 2 V, and emission current varied from 2 μA to 50 μA. Peak's position shifted towards lower energy values with increase of emission current, reaching referent value for emission current of 50 μA. Full width at half maximum (FWHM) of Si2s peak increased with increase of emission current. In the second stage of the experiment, emission current was kept constant at 2 μA, and accelerating voltage varied from 1.5 V to 3 V. Peak's position shifted towards lower energy values with increase of accelerating voltage, reaching referent value for accelerating voltage of 3 V. FWHM of Si2s peak increased with increase of accelerating voltage. By changing flood gun emission current and accelerating voltage, it is possible to achieve predictable shifts of peaks' positions, and obtain accurate spectrum.

石墨烯折角场发射特性

Vertical graphene edge is easy to adsorb gas molecules which affects the stability of field emission.The graphene was transferred to insulation wall structure to get the graphene-angle.The insulation wall structure is made on silicon substrate by inductively coupled plasma etching and the method of wet etching,field emission test was made with graphene-angle as field emission tips,and the field emission characteristics of the graphene-angle and the influence of conductive flow on the field emission were studied.The results show that field emission open field strength of graphene-angle is 9.6V/μm.Under the anode voltage 2 000 V,when the bias voltage applied on the both sides of the graphene-angle increases from 0Vto 10 V,the field emission current increases from 4.5μA to 15μA.

Field emission property of nanostructured tungsten formed by helium plasma irradiation

Field emission property of nanostructured tungsten formed by the exposure to a helium plasma is measured to investigate the initiation process of arcing in nuclear fusion devices. Field emission current from the nanostructured tungsten and tungsten with helium bubbles was significantly higher than that from the polished tungsten specimen. However, field enhancement factor was on the order of 100, which was almost comparable to that of bulk tungsten; the increase in the field emission current is attributed to primarily an increase in the effective emission surface area. From the calculation of thermo-field emission current from a heated tungsten, it is thought that a significant current emission can be still initiated even when the field enhancement factor is not so high when tungsten is exposed to high density plasmas such as the one accompanied with ELMs.

Impact of arcing on carbon and tungsten: from the observations in JT-60U, LHD and NAGDIS-II

Arcing is a long-standing plasma–surface interaction issue, and the issue is currently being revived. This paper assesses the impact of arcing in fusion devices based on the observations in JT-60U, the Large Helical Device (LHD) and the linear divertor simulator NAGDIS-II. To investigate the first initiation process of arcing, field emission currents from several tungsten samples are measured. It is shown that the field emission current increases significantly after tungsten is exposed to helium plasmas. A postmortem analysis of JT-60U tiles reveals that arcing phenomena occurred on carbon baffle plates inside the vacuum vessel in JT-60U. From the observation of the arc trails recorded on the baffle plate, the amount of eroded materials is discussed. The arcing seems to occur frequently on the inner baffles rather than the outer baffles. From LHD, it is shown that the arcing can be initiated on nanostructured tungsten even without transient events. The erosion of tungsten by arcing will become an important issue in a fusion reactor, where helium fluence is significantly increased. From the experiments in NAGDIS-II, it is shown that arcing can be initiated even without transient heat load when the target voltage is low enough, e.g. −500 V. Frequent initiation of arcing annihilates the nanostructure growth due to helium plasma irradiation on the surface.

Degradation mechanism of leakage current in AlGaN/GaN high electron mobility transistors

In order to study the degradation mechanism of leakage current in AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated AlGaN/GaN heterojunction Schottky diodes having equivalent structure and characteristics to AlGaN/GaN HEMTs. Step stress tests were then performed to compare the leakage current changes at different gate voltages. The transport mechanism of leakage current before and after degradation was validated based on the current-voltage and capacitance-voltage measurements. The light emission from the device surface was examined by emission microscopy (EMMI) to investigate the time-dependent degradation of leakage current. Experimental results show that the leakage current increases with increasing time and is accompanied by a large noise when the applied gate voltage exceeds a critical value. After introducing the polarization field into the current-field dependence, log(IFT/E) exhibits a good linear relationship with E both before and after degradation, indicating that the leakage current is dominated by the Frenkel-Poole (FP) emission. The slope of log(IFT/E)-E curve decreases after degradation, and the hot spots corresponding to defects are directly observed by EMMI at the gate edge of the degraded device, suggesting that the degradation mechanism is: New defects are induced by high electric field in the AlGaN layer, and the increase of defect density leads to the increase of FP emission current.

Bulb-Shaped Field Emission Lamps Using Carbon Nano-Coil Cathodes

Bulb-shaped field emission lamps (FELs) using carbon nano-coil (CNC) cathodes were fabricated and their performances were characterized. A straight steel wire grown with CNCs was placed on the symmetry axis in the bulb as the cathode. The anode was defined by a Ni film deposited on the bottom hemisphere of a 60-mm-diameter glass bulb. And a phosphor layer was coated on the Ni film for light generation via cathodoluminescence. A numerical simulation model for this geometry also was constructed. The simulated current-voltage curves agree well with the experimental results and reproduce the trends observed in the experiment. The effects of varying the length and diameter of the cathode were studied. Both experiment and simulation showed that the total field emission current increases with the cathode length and decreases with the cathode diameter. Although the design is simple and inherently low-cost, good uniformity of light emission over the entire anode was demonstrated.

Potential barrier decrease at cathode boundary under high-voltage gas-discharge conditions

The influence of atoms, molecules, or layers adsorbed on the cathode surface on the initiation of high-voltage gas discharge is considered. It is shown that the presence of island films of an adsorbate leads to an increase in the field-induced electron emission current that initiates breakdown of the gas-filled interelectrode gap. The formation of a continuous adsorbed film on the cathode accounts for the subsequent weak dependence of the emission characteristics in high-voltage (in particular, picosecond pulsed) discharge on the gas phase composition.

In situ (LaxGd1−x)B6 cathode materials prepared by the spark plasma sintering technique

Polycrystalline hexaborides (LaxGd1−x)B6 (x=0, 0.6, 0.8, 1) were synthesized by the reactive spark plasma sintering (SPS) method using a mixed powder of LaH2, GdH2 and B. The effects of La doping contents on the crystal structure, grain orientation, electrical resistivity and thermionic emission properties were investigated by x-ray diffraction, electron backscattered diffraction and emission current measurements, respectively. The results indicate that the high-purity (LaxGd1−x)B6 cathode sintered by SPS exhibits high densification (>97%) and a high value of the Vickers hardness (2375 kg mm−2). When the La doping contents are increased, the thermionic emission current density increases from 11.00 to 36.20 A cm−2. Thus, the SPS technique is a suitable method for synthesizing dense rare-earth hexaborides with excellent properties.