High-temperature Field Evaporation Research Articles

Stabilization of cold-field-emission current from a CeB6 single-crystal emitter by using a faceted (100) plane

A cerium hexaboride (CeB6) single crystal grown by the floating-zone method has a low work function of about 2.6 eV, and along with lanthanum hexaboride (LaB6), it is one of the most popular cathode materials. It has been widely used as the thermionic emitter of electron microscopes, such as SEMs and TEMs. However, cold-field emitters (CFEs) based on CeB6 and LaB6 have not been put to practical use due to their insufficient emission stability compared to that of conventional tungsten (W)-CFEs. In consideration of that background, in the present study, the stability of the emission current from a CeB6 single-crystal CFE was improved by using the (100) plane at the faceted tip of the single crystal. The CeB6⟨100⟩ single crystal was processed by electrochemical etching and successive high-temperature field evaporation and faceting under an appropriate electric field to make a (100) plane at the apex of the crystal. The improved CeB6(100)-CFE emitted a monochromatic electron beam, which has about three-quarters of the energy width of that of W(310)-CFEs. Emission current from the (100) plane maintained low emission noise, and emission decay in the electron-gun chamber of the SEM was suppressed. The resulting current noise is low enough to produce SEM images without image deterioration, and the relatively small decay makes it possible to use the CeB6(100) emitter for one flashing process per day.

Determination of the critical ionization distance and ionization zone during high-temperature field evaporation of molybdenum

Steady-state field evaporation of molybdenum at high emitter temperatures (T ∼ 2000 K) has been studied using a magnetic mass spectrometer equipped with a field ion source. Only low-charge ions (Mo+2 and Mo+) have been observed in the course of evaporation. The measured ion energies and evaporating field strengths (Fev) were used to determine the critical ionization distances (xcr) and ionization zones (Δ) for singly and doubly charged ions. The obtained xcr and Δ values show that the formation of ions takes place at a certain distance from the emitter surface.

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.

Singly charged tungsten and tantalum ions observed during high-temperature field evaporation

High-temperature field evaporation of tungsten and tantalum emitters in the temperature range from room temperature to 2500 K is studied using a static magnetic mass spectrometer equipped with a field source of ions. At room temperature, triply charged W3+ and Ta3+ ions alone are observed in the mass spectra. However, as the emitter temperature grows, the charge of the ions decreases. At T ≈ 1000 K, doubly charged W2+ and Ta2+ ions dominate in the spectra, and singly charged W+ and Ta+ ions appear in the temperature range 1900 < T < 2500 K. The evaporation rate of the singly charged ions is one to two orders of magnitude lower than the evaporation rate of the doubly charged particles. The energy parameters of field evaporation for differently charged tungsten ions are found.

Atomically sharp 〈111〉 trihedral angle of a tungsten tip

The processes occuring in the course of heating of a tungsten tip in an electric field and resulting in the formation of the 〈111〉 trihedral angle at the intersection of three {011} closest packed planes in the crystal lattice of tungsten are investigated using field-emission microscopy, continuous-mode field-desorption microscopy, and high-temperature field evaporation microscopy. It is demonstrated that atomically sharp angles can be formed at temperatures above 2200 K in the absence of field evaporation. An atom forming the apex of the trihedral angle lies in the triangle of atoms arranged in the (111) plane. In the triangle, each atom is located at the intersection of the 〈111〉 close-packed atomic rows, which are the boundaries of the {011} planes forming the trihedral angle and the {112} planes forming the angle edges two rows in width.

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.

Temperature-and field-induced changes in the form of the Mo-Hf alloy

Simultaneous action of strong electric fields and high temperatures on point field emitters made of the Mo-15%Hf alloy is studied by field emission methods. Such alloys enriched with an emission-active component and containing the Mo2Hf intermetallide exhibit basically the same stages of temperature-and field-induced changes in their form as for pure metals, although a number of peculiarities associated with the surface segregation of Hf also exist. Thermal-field processing of emitters was accompanied with high-temperature field evaporation and emission of predominantly Hf (both atomic and cluster-type) ions. Thermal-field processing also enhances emission localization, but to a smaller extent as compared to that in alloys with a low Hf concentration.

Theory of surface dynamic variation during the high-temperature field evaporation

We consider the heated surface of a metallic tip to which a strong electric field is applied. At temperatures activating surface self-diffusion, crystalline outgrowths and microprotrusions arise on the surface. The latter generate ion fluxes, i.e., act as sources of high-temperature field evaporation, when a positive potential is applied to the emitter. The existence conditions for the microprotrusions on the emitter surface are discussed. It is shown that their stability is provided by the balance between three atomic fluxes: diffusion from the top of the tip, diffusion toward the top, and field evaporation from the top. Different ways of providing such a balance are discussed. In a desorption-type field ion microscope, the microprotrusions and evaporating ions are visualized as bright spots. These spots execute random motion and, at the same time, exhibit ordered cyclic displacements: the microprotrusions first form dotted rings along the developed faces of the crystalline emitter, and then these rings quickly collapse toward the center of the face. A quantitative theory of these cyclic processes is developed for the first time. It explains why the rings “calm down” before collapse and why subsequent collapse develops in an avalanchelike manner. The electric field distribution over the surface in the presence of an outgrowth is calculated, and diffusion fluxes at different stages of its growth and dissolution are analyzed. The calculation shows that the outgrowth heights are relatively small and their slopes are rather smooth.

Field evaporation of carbidized tungsten

The field evaporation of carbidized tungsten (ribbed crystals) was studied by the atom probe method at various temperatures of the sample in the regions of cryogenic, room, and elevated (1300–1600 K) temperatures. For a high pulsed (not less than 40%) component in the evaporating field, the room-temperature field evaporation mass spectra correctly reflect the surface composition of the evaporated sample. The mass spectra reveal a large number of molecular and polymerized (cluster) ions. Data from field ion desorption microscopy indicate that high-temperature field evaporation of a ribbed crystal proceeds via ion emission from walking and merging thermal-field microroughnesses.

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/A 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 of the Metals and its Correlation with Surface Ionization

High temperature field evaporation of It and Pt was investigated at the T = 1400 - 1700 K and electric fields F - I V/A by field electron, ion and evaporation microscopy. The Arrenius plot was obtained for the process of field evaporation of Pt in these conditions. This plot gives the activation energy of the evaporation Qn = 3.05 ± 0.1 eV and preexponential factor vo = 3x10 15 I/s. The value of the heat of evaporation at the presence of high electric field λF for the Pt-ions was obtained by means of image forces model and value of Qn. The value of λF = 3.15 eV is less in comparison with sublimation energy of Pt - λο = 5.85 eV. The reasons of the difference between λF and λο and the mechanism of the evaporation process at these conditions are discussed.

Effects of an electric field in atomic manipulations.

Some recent scanning-tunneling-microscopy (STM) experiments on atomic-scale manipulation of material surfaces are explained in terms of high-temperature field evaporation, directional walks in an applied field gradient, and the formation of liquid-metal cone, etc. The evaporation fields for metals and the charge states of ions formed by an electric field in the STM configuration are shown to depend on the tip-to-sample distance. The possibility of field evaporating as negative ions is also considered. We find this possibility exists especially for alkali metals and adsorbed atoms of large electron affinity and noble gases. Favorable physical conditions for atomic manipulations are also discussed. In addition, a method for creating a thermally stable top surface layer with an atomic pattern of embedded foreign atoms of one's design, using electric-field manipulations and atomic replacement, is described.