Structure and composition of tungsten silicide thermal-field microprotrusions

The thermal-field microprotrusions that grow on the surface of a tungsten tip coated with silicon when the tip is heated in an electric field are investigated by a suite of field emission methods, including electron field emission, ion desorption microscopy, and the atomic-probe method. For Si coatings more than a few monolayers thick, microprotrusions are observed to grow in the field desorption regime when the tip is heated to temperatures T=1100–1200 K in an electric field with initial intensity F=5.7–8.6×10⁷ V/cm. The field at which they evaporate is 1.2–1.8×10⁸ V/cm. The set of moving spots (i.e., microprotrusions) forms rings whose collapse signals the dissolution of the thermal-field growths on the developed faces. The most interesting structures are the sharp microprotrusions that grow on the central facet of a {110} tungsten tip under certain conditions. Atomic-probe analysis of their composition reveals that they consist of tungsten trisilicide WSi₃ with a monolayer surface skin whose composition is close to WSi₂. The intense growth of these formations on an initially smooth closepacked {110} face of tungsten is evidence that reconstruction of the latter takes place under the influence of the strong field and the interaction with silicon. Zh. Tekh. Fiz. 67, 102–109 (September 1997)     

Field desorption microscopy of the 〈111〉 trihedral angle of a reconstructed tungsten tip

The surface structure near the 〈111〉 trihedral angle, which forms in an electric-field-heated tungsten tip, is studied by field electron microscopy, continuous-mode field desorption microscopy, and high-temperature field evaporation microscopy. The shape and structure of the surface depend on the temperature, field, and time. The angle is formed by three {011} planes, with the (111) plane at its vertex being retained in the form of a triangle or a hexagon with randomly arranged atomic clusters. The edges between {011} faces represent long and narrow {112} planes having longitudinal or transverse steps. In the absence of field evaporation, the edges and angle sharpen, becoming monoatomic. Field evaporation from the angle or microprotrusions on the edges extends these edges and causes transverse steps to appear on them. The explanation of the changes in the shape and structure of the surface is based on considering the competition of surface diffusion, crystal growth in an electric field, and field evaporation.     

Field-desorption microscopy study of the deformation of a tungsten tip subjected to thermal treatment in an electric field

Deformation of a tungsten tip 500 to 1000 nm in radius subjected to heating in an electric field is studied using field-emission microscopy and continuous-mode field-desorption microscopy. Measurements are performed immediately after the thermal field treatment without any smoothing of the tip by either heating or field evaporation. The edges of the tip (which is shaped into a polyhedron) are found to consist of monatomic steps about 1 nm wide and about 100 nm long. Microscopic protrusions about 10 nm in size are shaped like pyramids or wedges with single-atom apexes or monatomic edges and facets that are a continuation of the facets of the reconstructed tip or an outgrowth on which the protrusions are situated. The outgrowths are shaped like stepped truncated pyramids with monatomic edges. The observed phenomena are explained in terms of competing processes of surface diffusion, crystal growth in an electric field, and field evaporation.     

Preequilibrium thermofield microprotrusions as effective field point sources of electrons and ions

Field emission methods are employed for studying the conditions of formation, crystallographic localization, and emissive properties of preequilibrium thermofield microprotrusions for a number of refractory metals. Individual preequilibrium microprotrusions can be easily obtained using a W emitter of the ordinary 〈110〉 orientation; however, the number of such protrusions on the surface changes with time in the course of ionic emission, as well as their emission parameters (the parameters and the number of microprotrusions do not change in the case of electron emission). Trihedral angles of the rearranged tip, which are formed in the {111} regions, exhibit higher stability to ionic emission. A single trihedral angle stably emitting ions and located on the geometrical axis of the emitter can easily be obtained with the help of a W emitter with the 〈111〉 orientation. Two stable preequilibrium microprotrusions arranged symmetrically about the axis of the emitter in the {111} regions can be obtained using a Ta emitter of the conventional 〈110〉 orientation. Such microprotrusions virtually do not change the emission parameters during long-term extraction of ionic current.     

High-temperature field evaporation of rhenium

High-temperature field emission of Re, Pt, Ta, and W is studied by field-emission methods. Metal ions are found to evaporate mainly from the tops of thermal-field microprotrusions produced by high electric fields and temperatures on the emitter surface. For fi eld intensities of up to F=1–2 V/Å and temperatures of 1500–2000 K, the ion currents i are recorded from the entire emitter surface. They range from several tenths of nanoamperes to several nanoamperes. The activation energies of field evaporation determined from the Arrhenius plots logi=f(1/T) are found to be appreciably lower than those calculated within the charge exchange model for known parameters of the process and the metals evaporated. Reasons for such a difference in the activation energies and mechanisms of ion evaporation at high F and T are discussed.     

High-temperature field evaporation and its connection with surface ionization

High-temperature field evaporation of metals and alloys and its connection with surface ionization are considered. The main parameters of the evaporation process (dependence of the evaporation rate on the emitter temperature and on the electric field at the emitter surface, the charge of the ions being evaporated and its temperature dependence, kinetic parameters of the evaporation process, as well as the state of the emitter surface under simultaneous action of high fields and temperatures) are analyzed. The similarity and the difference between field evaporation at high temperatures and surface ionization in a strong electric field are determined.     

Field evaporation of a Hf-Mo alloy

The field evaporation of a Hf-Mo alloy (15 wt. % Hf) is investigated using a time-of-flight atom probe. A moderately heated tip detects an impurity of Hf and Mo oxides on the surface. Thermofield microprotrusions grown at T=1440–1850 K in an electric field (retarding to electrons) of intensity E=(3.2–5)×10⁷ V/cm are analyzed at room temperature and above. Zh. Tekh. Fiz. 68, 69–73 (March 1998)     

Ionization mechanism in a liquid-metal ion source. Source for high-melting metals

Investigations have been made of the ionization mechanism in a liquid-metal ion source. At the instant of ion current generation the surface of a liquid metal emitter frozen in a highvoltage field exhibits microprotrusions pulled from the liquid metal by the electric field. A double-focusing mass spectrometer identified two components in the ion beam extracted from the aperture in the extractor. One ion component forms at the tip of the emitter and has an energy spread not exceeding a few tens of electronvolts. The other ion component forms in the cathode plasma at the extractor. On the basis of these investigations, a mechanism is proposed for ion formation in a liquid metal ion source and this mechanism is used to produce a modified source design. The source can produce ion beams from high-melting metals and nonmetals, and beams of Ta, W, Mo, C, and Fe ions were obtained. Zh. Tekh. Fiz. 67, 82–87 (November 1997)     

From Field Emission to Vacuum Arc Ignition: A New Tool for Simulating Copper Vacuum Arcs

Understanding plasma initiation in vacuum arc discharges can help to bridge the gap between nano‐scale triggering phenomena and the macroscopic surface damage caused by vacuum arcs. We present a new twodimensional particle‐in‐cell tool to simulate plasma initiation in direct‐current (DC) copper vacuum arc discharges starting from a single, strong field emitter at the cathode. Our simulations describe in detail how a sub‐micron field emission site can evolve to a macroscopic vacuum arc discharge, and provide a possible explanation for why and how cathode spots can spread on the cathode surface. Furthermore, the model provides us with a prediction for the current and voltage characteristics, as well as for properties of the plasma like densities, fluxes and electric potentials in a simple DC discharge case, which are in agreement with the known experimental values. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.     

Field-induced stresses in field emitters

A numerical analysis is performed in order to determine the magnitude and disposition of field-induced stresses in an axisymmetric field emitter whose profile is taken as a calculated profile of a real tungsten emitter. The results are compared to previous analyses and new observations are made. Conclusions are drawn which reflect the effect of these stresses on crystalline defects in the emitter during the imaging process and during field evaporation.     

A One‐Dimensional Particle‐in‐Cell Model of Plasma Build‐Up in Vacuum Arcs

Understanding the mechanism of plasma build‐up in vacuum arcs is essential in many fields of physics. A one‐dimensional particle‐in‐cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point, only an external electric field and an initial enhancement factor of the tip are assumed. General requirements for plasma formation have been identified and formulated in terms of the initial local field and a critical neutral density. The dependence of plasma build‐up on tip melting current, the evaporation rate of neutrals and external circuit time constant has been investigated for copper and simulations imply that arcing involves melting currents around 0.5–1 A/μm², evaporation of neutrals to electron field emission ratios in the regime 0.01 – 0.05, plasma build‐up timescales in the order of ～ 1 – 10 ns and two different regimes depending on initial conditions, one producing an arc plasma, the other one not. Also the influence of the initial field enhancement factor and the external electric field required for ignition has been explored, and results are consistent with the experimentally measured local field value of ～ 10 GV/m for copper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Erosion of the field emitter surface exposed to low-energy ions

The radiation-induced erosion of the tungsten field emitter surface exposed to low-energy ions is studied by field ion microscopy and electron microscopy. During the bombardment, surface atoms are displaced to sites with lower coordination numbers and nanoasperities, generating a locally enhanced electric field, arise on the surface in a jump-like manner, which modifies the characteristics of the emitters. The field evaporation of the asperities produces cavities; hence, the erosion can be described in terms of blistering. Quasi-static surface erosion mechanisms are considered. It is shown that nanoblistering can be related to helium absorption in metal surface layers.     

The work function reduction of the Pt field emitter

We have carried out the field emission experiments to measure the temperature dependence of the work function of Pt field emitter and found that the work function steeply decreases more than 2 eV by annealing at relatively low temperature above about 500 K in ultra high vacuum. The maximum reduction of the work function was 2.59 eV. The reduced work function was restored the original value of Pt clean surface by applying high voltage of only 20% of Pt evaporation field. The experimental results are tentatively interpreted in terms of the formation of complex cyanides on the emitter shank during the electrochemical etching in KCN solution and the surface diffusion of potassium atoms formed by the thermal decomposition of the complex cyanides to the emitter cap.     

阴极金属微凸起电场增强因子数值模拟

 利用有限元方法对无穷大平板二极管中阴极表面金属微凸起的微观电场增强因子进行了计算研究,给出了微凸起微观电场增强因子随凸起参数的变化规律,拟合得到了微观电场增强因子的简单实用的经验表达式,并与文献中的近似公式进行了比较。模拟的结果表明,对锥状球头微凸起的电场增强因子,文献给出的近似公式误差较大,应避免使用。     

碳纳米管薄膜场蒸发效应

在超高真空系统中对基于丝网印刷方法制备的碳纳米管薄膜的场蒸发效应进行实验研究. 实验发现, 碳纳米管薄膜样品存在场蒸发现象, 蒸发阈值场在10.0-12.6 V/nm之间, 蒸发离子流可以达到百皮安量级; 扫描电子显微镜分析和场致电子发射测量结果表明, 场蒸发会使碳纳米管分布变得更加不均匀, 会导致薄膜的场致电子发射开启电压上升(240→300V)、场增强因子下降(8300→4200)、蒸发阈值场上升(10→12.6V/nm), 同时使得薄膜场致电子发射的可重复性明显变好. 场蒸发也是薄膜自身电场一致性修复的表现, 这种修复并非表现在形貌上, 而是不同区域场增强因子之间的差距会越来越小, 这样薄膜场致电子发射的可重复性和稳定性自然会得到改善.     

Hydrogen promoted corrosion of tungsten by oxygen in an electric field: A field ion microscope study

The extensive changes in surface topography observed to occur on tungsten field ion emitters as a result of exposure to oxygen in presence of hydrogen at 78°K and at fields of 2 V/Å have been studied in detail. Field promoted diffusion of gas from the low field region of the emitter shank over the imaged area of the surface removes kink site metal atoms and subsequently deposits them on either side of the well defined diffusion paths along close packed zones; field evaporation of such atoms may also occur when the change in local surface contour causes sufficient field enhancement. Although oxygen is primarily responsible for the corrosion effects, its diffusion at such temperatures is promoted by the presence of hydrogen, and the rate of reaction is dependent, inter alia, on hydrogen pressure.     

Radiation-assisted formation of microprobes for scanning tunnel microscopes

The formation of tipped tungsten microprobes by ion bombardment in high electric fields with subsequent low-temperature field evaporation is reported. During irradiation, the current density was shown to rise nonmonotonically because of heavy particles present in the bombarding beam due to emitter erosion. It was found that the initially hemispheric working part of the probes turns into the axially symmetric complex-shaped surface. A correlation of these effects with inert gas ionization and tungsten sputtering under the action of super-high-density electron beams is discussed. The atomically smooth microprobes obtained by ion bombardment and field evaporation offer high stability and an atomic-level resolution.     

The properties of the nonequilibrium LaB6 surface resulting from field evaporation

The nonequilibrium surface of single-crystal lanthanum hexaboride needles and its modifications are studied with a time-of-flight atomic probe. The surface is obtained by room-temperature field evaporation. The mass spectra of field evaporation shed light on the surface composition at the needle tip immediately after tip etching, corrosion in residual gases, intense cleaning by field evaporation, and the relaxation of the nonequilibrium surface by heating to 1250 K. Conditions for the breakdown of an oxide film on the tip surface and for obtaining the mass spectra of field evaporation for stoichiometric or lanthanum-enriched pure LaB₆ single crystals are discussed.     

Surface reconstruction of a field electron emitter

The surface and emission images of a metal field’s electron cathode in the form of a tip are simulated. The surface structure is calculated in the thin-shell and broken-bond (local-environment) models for the perfect crystal lattice. The cathode shape and macroscopic electric field are represented by the sphere-on-cone model. The amplification of a local electric field is the adjustable parameter of the model. The method of determination of the emitter tip’s crystal faces based on the analysis of the surface atoms’ environment geometry is proposed. It is shown that it is enough to restrict the consideration of geometric environment by the fifth order of the nearest neighbors for the emitter radius of 100–1000 lattice parameters (31.6–316 nm for the tungsten). The crystallographic model of work function anisotropy in the broken-bond approach is used: the local work function’s value is set in accordance with Miller indices of the face containing this area. The model adequacy is corroborated by the comparison of current-voltage characteristics and emission images with the data of the natural experiment.