Showing papers on "Field electron emission published in 1973"

Recent Advances in Field Electron Microscopy of Metals

Abstract: Publisher Summary The chapter explains how field electron microscopy has established itself as a powerful tool for elucidating a variety of phenomena occurring at metal and semiconductor surfaces. The quasielectron momentum components perpendicular to the tunneling direction are conserved during electron tunneling through a finite potential barrier. The chapter talks about the many-body approach that involves the thermodynamic Green's function method to treat field emission from superconductors, and nonideal metals in which electrons collide with phonons, impurities, and lattice imperfections. The method was also applied to the effect of a finite analyzer resolution. The atomic potential was represented both by a square well with an attractive core parameterized by its depth and width, and by a repulsive delta function potential that was equivalent to orthogonalization of the tunneling electron wavefunction to the occupied, tightly bound adsorbate electron orbitals. Plausible forms for the pseudopotential for both metallic and neutral adsorbates were suggested. The chapter also states that a Stark splitting of an electron–phonon transition can be envisioned in the case of degenerate vibrational, rotational, or bending modes.

Field Emission of Hot Electrons from Tungsten

Abstract: Field emission of hot electrons is observed from a laser-illuminated tungsten tip. The field dependence of the total current is accurately described by a simple theory of barrier penetration.

Color display device

Abstract: A color display device includes a substrate which forms a partition wall for defining a vacuum space, a plurality of cathode protrusions effecting field emission are formed on the substrate, and a transparent insulator which forms the vacuum space along with the substrate. A plurality of phosphor dots are formed on the surface of the transparent insulator on the side of the vacuum space, are arranged in opposition to the electron sources and have mutually different colors of light emission. Anodes are arranged between the electron sources and the phosphor dots which cause electrons to be generated from the electron sources by field emission, and a transparent electrode is disposed between the phosphor dots and the transparent insulator. Apertures are provided in the anodes through which the electrons pass, whereby the electrons are released from the cathode projections by selectively applying voltages to the anodes, so as to impinge on the phosphor dots of desired color.

Point‐cathode electron sources‐electron optics of the initial diode region

Abstract: A realistic model of a point‐cathode electron source is analyzed in this paper, and its electron optical properties are discussed. Both cathode and anode are modeled as equipotential surfaces of a sphere on orthogonal cone (SOC). The model predicts that the radius of the apparent electron source produced by this diode may be as small as 2–3 nm for field emitters and as small as 4–6 nm for strong‐field Schottky emitters. This source is found to be a virtual image of the emitting area of the cathode. The image position and magnification are determined for different representative cathode shapes, and quantitative results are also presented on geometric aberrations, effects arising from the initial energies of emission, and on source current and current density. An optimum cathode apex radius for smallest source sizes is predicted to be about 200 nm for field emission and about 700 nm for Schottky emission. These results are compared with those from the often used, but less accurate, sphere model.

Field emission from carbon fibres

Abstract: The development is described of a carbon-fibre field-emission source with a brightness of 1012 A sr−1 m−2 and currents in excess of 200 μA sr−1 at voltages of 2-3 kV. The source can be run continuously in ambient gas pressures two decades higher than tungsten field emitters at the same current fluctuation level of 2-3%. The source makes possible the operation of field-emission electron guns in conventional unbaked vacuum systems.

Field emission tip and process for making same

Abstract: A field emission tip on which a metal adsorbate has been selectively deposited, and a method by which it may be manufactured. In a vacuum, a clean field emission tip is subjected to heating pulses in the presence of an electrostatic field to create thermal field buildup of a selected plane. Emission patterns from the selected plane are observed, and the process of heating the tip within the electrostatic field is repeated until emission is observed from the desired plane. The adsorbate is then evaporated onto the tip. The tip constructed by this process is selectively faceted, with the emitting planar surface having a reduced work function and the non-emitting planar surfaces having an increased work function. A metal adsorbate deposited on the tip so prepared results in a field emitter tip having substantially improved emission characteristics.

Holographic measurements of the plasmas in a high-current field emission diode

Abstract: Two-exposure holographic interferometry has been employed to obtain the first quantitative measurements of the plasma densities in an intense electron beam diode. Several diode configurations which produce an intense pinch of the electron beam at the anode plane have been investigated. Holograms indicating the temporal and spatial evolution of the plasmas in these diodes are presented. Two types of plasmas are observed in these diodes: a lower density plasma ( 1018/cm3) with a velocity of about 2 cm/μsec. Preliminary analysis indicates that the lower density plasma plays a dominant role in determining the primary beam dynamics.

Initiation of electrical breakdown in vacuum

Abstract: Measurements of prebreakdown current as a function of applied electric field have been made for molybdenum electrodes of unlike surface finish in ultra-high vacuum (10−9 Torr), for a fixed gap separation d = 005 cm. Two sets of electrodes were used: unpolished anode, polished cathode; and polished anode, unpolished cathode. The prebreakdown currents are found to be consistent with the Fowler-Nordheim field emission theory. The breakdown voltage, the field enhancement factor, the current, and the current density and emitting area at breakdown are given as a function of the number of breakdowns. Also, the critical field enhancement factor, the temperature of the anode hot spot and the cathode microgeometry are calculated. Results indicate that the state of the anode surface governs the breakdown voltage and the microgeometry of the cathode surface. Also, in a conditioned gap both anode and cathode are involved in the discharge initiation. Anode initiation is only obtained with a certain set of electrodes.

Resolution limits in the surface scanning electron microscope

Abstract: SUMMARY An approximate theory is developed for estimating the limiting topographical resolution of the scanning electron microscope operating under certain idealized conditions. Limitations imposed by electron beam shot noise and electron diffusion effects within the specimen are considered for the case of an instrument incorporating a field emission source and in which there is ideal collection of the secondary electron signal. The specimen is assumed to be an homogeneous, isotropic solid with the beam incident normally to its surface. It is estimated that, under these ideal conditions, the limiting resolution for a metallic specimen lies in the region of 1 nm. The possibilities of realizing a resolution of this order in a practical instrument are discussed.

Field ion microscope study on the interaction of gallium with metals. I. Pseudomorphic structure and superstructures on tungsten

Abstract: Gallium was deposited on a tip specimen of a field ion microscope by making contact in vacuum. Gallium deposited on a tip area and diffused from the tip shank at 350 to 500 °K was crystallized in the pseudomorphic structure on tungsten. The number of atomic layers of the pseudomorphic gallium was 2 to 3. The sharp helium image of the pseudomorphic gallium appeared at a voltage considerably lower than the helium best‐image voltage for tungsten. The observed high evaporation field of the pseudomorphic gallium, 3.0–4.5 V/A, may indicate that the gallium atoms in the pseudomorphic state are anomalously strongly bound to the substrate tungsten or are field evaporated as the doubly‐ionized ions. Various superstructures were observed on the {112} areas after heating above 900 °K, but their detailed structures did not depend upon temperature or heating time. An unidentified underlayer may play a dominant role in determining the structure of the superstructures. No clear evidence of alloy formation was noticed in the temperature range 21 to 1500 °K.

The Electron Injection Mechanism of the Electroluminescent ZnS: Tb3+ Films

Abstract: The role of interfacial states in the electroluminescence process for Ta-Ta2O5-ZnS: Tb3+-Au and Ta-Ta2O5-ZnS: Tb3+-SiO2-Au structures has been studied. A similar electroluminescence has been observed for both types of cells. This indicates that electron injection via tunneling from the Au contact is not necessarily required to produce electroluminescence. The relaxation times of electrons related to the injection process from interfacial states are measured to be 10–100 ms. These values are the same for three interfacial states of interest (ZnS-Au, ZnS-SiO2 and ZnS-Ta2O5). Their temperature and electric field dependences are also presented. It can be concluded that field emission from the interfacial states is essential to the electron injection process for these types of cells.

V Production of Electron Probes Using a Field Emission Source

Abstract: Publisher Summary This chapter discusses the production of electron probes using a field emission source. Electron probes have a large variety of uses ranging from electron beam evaporators and welders to oscilloscopes, microprobes, and scanning microscopes. These various uses place a variety of different demands upon the electron optical system and upon the electron source itself. By far the majority of these instruments use a hot tungsten filament as the source of electrons because it is simple and easy to use and because it is a well developed technique. It does, however, have its limitations, and therefore the chapter discusses the useful range of the applicability of another source of electrons—field emission—which is superior to the hot filament in brightness but inferior in the total current that can be extracted. It also elaborates the practical features of the process as a source of electrons. A field emission source can be used for the production of electron probes ranging in size from a few A to a few microns and in energy from a few kV to a few hundred kV.

Ionic mobility in liquid3He above 0.25 K

Abstract: A novel technique has been employed to measure the temperature dependence of charge mobilities in liquid 3He under low pressure for 0.25 K < T < 2.0 K. The measurements were made by investigating space-charge-limited currents injected into the liquid from a discharge initiated at a fine metal point by field emission/ionization. The experimental data show less scatter than those obtained previously using conventional methods. The advantages and disadvantages of the space-charge technique, and the feasibility of its use in the millikelvin temperature range, are discussed.

Field emission and field ionization in liquid3He above 0.25 K

Abstract: We have investigated the field emission and field ionization in liquid3He under low pressure between 0.25 and 2.0 K. Experimental data in the high-current regime are strongly affected by the buildup of temperature gradients, but this effect can be avoided by pulsing the emitter. Under pulsed conditions in the space-charge region the data is well described by a simple theory which does not require detailed knowledge of the generating mechanism. Data at low currents, which are completely different for positive and negative ions, are presented and discussed but are not understood in detail. Comparison is drawn between this work and earlier studies of4He, and the advantages of this type of ion source for use in3He at ultra-low temperatures are discussed.

Energy Distributions of Field-Emitted Electrons from Tungsten in the Presence of Adsorbed CO

Abstract: Energy distributions were measured on the (110), (100), (111), and (210) planes under various conditions of CO adsorption. Resonances were seen for the low temperature layer at −7 to 7.5 eV below vacuum on all planes except (100). A comparison of the widths and maximum amplitudes of the resonances shows good agreement with the theory of field emission from adsorbate covered surfaces. On heating, the resonances disappeared at ≤350 K corresponding to conversion of the low temperature states to beta‐precursor states. Readsorption on beta or beta‐precursor layers led to resonances at −7 eV on (210) and (111), but not on (110) or (100). The resonances are interpreted as arising from interaction of CO–π* orbitals with tungsten d‐t2g orbitals in adsorption via the C atom on single W atoms. Disappearance of the resonances on heating is interpreted as activated rearrangement to new bonding configurations‐for instance, bridge bonding via C‐sp2 orbitals. The intensity of the resonances increases initially with cover...

Electron‐beam‐initiated chemical laser in SF6–H2 mixtures

Abstract: A 3.5‐cm × 35‐cm planar field emission electron beam diode, producing a total beam current of 4.65 kA of 120‐keV electrons in 26‐nsec pulses was used to initiate the reaction of F with H2 in SF6–H2 mixtures. From an optical volume of approximately 1 liter we observed HF laser pulses of 20–30 nsec duration with a maximum energy of 0.2 J at 450 Torr. The application of a secondary potential, very near the self‐breakdown limit of the gas, to the laser medium increased the laser output by only a factor of 2. Numerical analysis of the experiments without secondary field shows that 2.7 × 104 F atoms are produced by each 120‐keV electron fully stopped in the gas. This corresponds to an energy of 4.5 eV required for the production of one F atom from SF6 by high‐energy electrons. The conversion efficiency of electron beam energy input to the medium to laser output energy was 5.2%.

Space charge limited field emission in a plane parallel electrode geometry

Abstract: Poisson's equation in parallel geometry is solved analytically with the inclusion of the electron emission velocity spectrum predicted by quantum mechanics. The mathematical difficulty of classically defining the cathode surface is overcome, to a first approximation, by postulating a virtual cathode situated in front of the real cathode such that the majority of the emitted electrons have penetrated the potential barrier at that position. The treatment is nonrelativistic, assumes that the emission is exclusively by barrier penetration, and is expressed in parameters that enable the results to be applied to most situations of interest. It is shown that the effect of nonzero emission velocity is negligible except for the cases of cathodes of low work function operated at very high current densities. Particular attention is paid to the departure of the experimentally measured current from that predicted by the Fowler-Nordheim theory and the subsequent approach to the Langmuir-Child fully space charge limited current. It is shown that the point of departure can quantitatively be defined in terms of the fully limited current and the bearing of this on the design of field emission electron guns is considered.