M.S. Mousa

Hot electron emission from composite metal-insulator micropoint cathodes

Field electron emission measurements have been made on composite emitters consisting of electrolytically etched tungsten micropoint cathodes overlayed by a 40-200 nm thick layer of epoxy resin. Their emission properties include (a) an initial switch-on effect at threshold fields of approximately 109 V m-1, (b) a subsequent reversible I-V characteristic that gives a linear FN plot at low fields ( or approximately=4*108 V m-1, (c) electron spectra whose FWHM and energy shift is strongly field dependent, (d) single-spot emission images. This unusual pattern of behaviour has been interpreted in terms of a hot electron emission mechanism resulting from field penetration in the dielectric overlayer. Consideration is also given to the technological significance of such composite microemitters.

Improvement in the ductility of molybdenum alloys due to grain boundary segregation

Abstract An improvement in the tensile ductility from the traditional 3–20% in 6.35-mm-thick molybdenum weldments has been achieved through the addition of Zr, Al, C and B at the ppm level. Atom probe tomography has revealed segregation of Zr, B and C to and depletion of O at the grain boundaries in the base metal and the heat affected zone.

Field electron emission studies on zinc oxide coated tungsten microemitters

Abstract Details of the preparation of zinc oxide coated tungsten microemitters, their emission characteristics as well as stability and the influence of thermal processing are presented in this paper. Although it was easy to coat the sharp tungsten microemitters with zinc oxide it was difficult to determine the thickness of the coating due to its opacity. A very high switch-on emission current I c ⩾ 10 μ A was typically obtained by applying an anode voltage V A ⩽ 7 kV. The same value of I c was maintained when reducing the applied voltage down to ∼ 3 kV and sometimes even much less (∼ 1 kV). Spatial distributions thus obtained showed very bright emission spots scattered irregularly on the screen. Thermal processing and other conditioning treatments of these emitters considerably reduced the exhibited random fluctuations, thereby producing a high degree of stability of the emission process, especially at temperatures ∼ 200°C. The effect of cooling the specimen to liquid nitrogen temperature is also studied. The insulating zinc oxide coating, in spite of being a good luminescent material, did not produce any luminescence effect during emission.

Beam-energy distribution measurements of liquid gallium field-ion sources

Measurements are reported on the energy distribution of the ion beam produced by Ga liquid metal ion sources, for a range of emission currents, i approximately 1-50 mu A. The full width at half-maximum (FWHM). Delta E1/2, of the energy distribution curves varies from approximately 4.5-5 eV at approximately 1 mu A to approximately 35 eV at 50 mu A. Three regions can be detected in the Delta E1/2 versus i curve. In the first region, between approximately 0.6 and 2 mu A, Delta E1/2 varies very slowly with current. In the second region, above approximately 2 mu A and extending up to approximately 25 mu A, the curve rises steeply with current, in fairly good agreement with the predicted Delta E1/2 varies as i2/3 relation. For i>or approximately=25 mu A the curve deviates markedly from this relation, and tends to flatten. Although a tentative explanation for the deviation from theory at high currents may be given in terms of gross instabilities of the anode, the deviation at very low currents (where Delta E1/2 is almost constant with i) is not understood.

Improved approach to Fowler–Nordheim plot analysis

This article introduces an improved approach to Fowler–Nordheim (FN) plot analysis, based on a new type of intercept correction factor. This factor is more cleanly defined than the factor previously used. General enabling theory is given that applies to any type of FN plot of data that can be fitted using a FN-type equation. Practical use is limited to emission situations where slope correction factors can be reliably predicted. By making a series of well-defined assumptions and approximations, it is shown how the general formulas reduce to provide an improved theory of orthodox FN-plot data analysis. This applies to situations where the circuit current is fully controlled by the emitter characteristics, and tunneling can be treated as taking place through a Schottky–Nordheim (SN) barrier. For orthodox emission, good working formulas make numerical evaluation of the slope correction factor and the new intercept correction factor quick and straightforward. A numerical illustration, using simulated emission...

A new perspective on the hot-electron emission from metal-insulator microstructures

Abstract A combination of a high resolution electron spectrometer and a field emission microscope has been used to study the field emission of electrons from composite metal-insulator micropoint cathodes. These emitters consist of similarly prepared electrolytically etched metallic tungsten micropoints (M), covered with layers of several types of dielectric materials (I). These dielectrics include alumina, hydrocarbon, plastic, lacomit and resin. Some of the dielectric coatings used gave an opaque form which did not make it possible to measure their thicknesses. Other dielectrics produced a measurable regular layer with thicknesses varying from 0.04 to 0.2 μm. Those produced reversible current-voltage characteristic that gives a linear FN plot at low fields (≲ 10 8 V m ) and a saturated emission at high fields ≳ 4 × 10 V m , emission images and electron spectra of both thin and the optimum thick coatings. The observed effects, e.g. the threshold switch-on phenomena and the field-dependence of the FWHM and energy shift of the electron spectra have been identified in terms of a hot-electron emission mechanism resulting from field penetration into the insulating film where conducting channels are formed. Evaluation of the technological importance of such devices as practical electron source is given. These experimental and theoretical findings will be shown to be consistent with recent published analysis of M-I-V systems.

Modification of Mo-Si alloy microstructure by small additions of Zr

Molybdenum and its alloys are potential materials for high-temperature applications. However, molybdenum is susceptible to embrittlement because of oxygen segregation at the grain boundaries. In order to alleviate the embrittlement small amounts of zirconium were alloyed to a solid solution of Mo-1.5Si alloy. Two Mo-based alloys, namely Mo-1.5Si and Mo-1.5Si-1Zr, were investigated by the complementary high-resolution methods transmission electron microscopy and atom probe tomography. The Mo-1.5Si alloy shows a polycrystalline structure with two silicon-rich intermetallic phases Mo(5)Si(3) and Mo(3)Si located at the grain boundaries and within the grains. In addition, small clusters with up to 10 at% Si were found within the molybdenum solid solution. Addition of a small amount of zirconium to Mo-1.5Si leads to the formation of two intermetallic phases Mo(2)Zr and MoZr(2), which are located at the grain boundaries as well as within the interior of the grain. Transmission electron microscopy shows that small spherical Mo-Zr-rich precipitates (<10nm) decorate the grain boundaries. The stoichiometry of the small precipitates was identified as Mo(2)Zr by atom probe tomography. No Si-enriched small precipitates were detected in the Mo-1.5Si-1Zr alloy. It is concluded that the presence of zirconium hinders their formation.

Electron emission from carbon fibre tips

7 μm diameter carbon fibres were electrolytically etched in a 0.1M sodium hydroxide solution to produce micropoint emitters. The behaviour of these tips was tested in a retarding potential spectrometer under a variety of vacuum conditions. At a medium vacuum of ∼10 -3 mbar, the emission of electrons was very noisy and the lifetime of the source was relatively short. At pressures of < 10 -6 mbar, it was found that the performance of the cathodes, including their lifetime, is largely independent of the vacuum pressure. The emission characteristics of carbon fibre tips were recorded. Electron microscopy techniques employed in this investigation were used to study the tip profile. They also showed that the inherent instability associated with carbon fibre emitters resulted from an explosive phenomenon which was encountered during the initial switch-on of the emission current from freshly prepared cathodes. This process led to the destruction of these cathodes. Pre-conditioning treatment of the tip overcame this problem and produced a large reduction in the noise fluctuation of the total emission current. This enabled the recording of energy distributions of field emitted electrons. The spectra obtained demonstrated narrow halfwidths (FWHM) and an energy shift compared with the Fermi level of tungsten. By increasing the applied electric field on the tip, the spectral shift was found to increase. The emission pattern consisted of a random distribution of sub-emission centres, which switch on and off randomly with time. The Fowler-Nordheim current-voltage plots of these carbon fibre tips showed that the work function was insensitive to the vacuum pressure.

Stabilization of carbon-fiber cold field-emission cathodes with a dielectric coating

A comprehensive investigation has been carried out to determine the source of an inherent temporal instability in the spatial distribution and the electron emission current obtained from field-emitting carbon fiber tips. These instability effects were successfully overcome by coating the tip with a sub-micron layer of dielectric epoxy resin coating. The influence of the coating thickness was studied and an optimum thickness of 0.2-0.3 microm that produced high emission stability was found. A large reduction in the intensity fluctuations of the emission image, at this coating thickness is demonstrated by using chart recorder traces in addition to slow scans of an optically monitored screen signal. The current-voltage (I-V) characteristics were obtained at a threshold field that is a few times lower than that of the uncoated tip. At low emission current levels linear F-N plots were obtained with a slope value lower than that of the uncoated emitter. The spatial distribution consisted of a very bright spot without any internal structure. The total energy distribution of the emitted electrons demonstrated a non-metallic behavior. The spectra obtained consisted of a single peak for low currents and a double peak for higher currents. The electron energy was measured relative to the Fermi level of tungsten and a spectral shift was shown to be a function of the current. Experiments have shown that the coated tips are not affected by the variations of pressure conditions down to 10(-6) mbar. These results suggest that a resin coated fiber tip offers superior performance to tungsten as a cold field emission electron source. Numerous improvements in the performance are underway. This includes a variety of polymeric coatings and more emissive carbon fibers.

Influence of barrier form on Fowler-Nordheim plot analysis

Recent research has described an improved method of Fowler–Nordheim (FN) plot analysis, based on the definition and evaluation of a slope correction factor and a new form of intercept correction factor. In this improved approach, there exists a basic approximation that neglects certain terms in the general theory, and focuses on the influence of the form of the tunneling barrier on the values of basic slope (σB) and intercept (ρB) correction factors. Simple formulae exist that allow these to be evaluated numerically for a barrier of arbitrary well-behaved form. This paper makes an initial exploration of the effects of barrier form on FN plot analysis. For a planar emitter, two models for the correlation-and-exchange (C&E) potential energy (PE) are used. For the Schottky–Nordheim barrier, it is shown that numerical and analytical approaches generate equivalent results. This agreement supports the validity of the numerical methods used. Comparisons with results for the Cutler–Gibbons barrier show that small...