Effective field enhancement factor and the influence of emitted space charge
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Although Fowler and Nordheim developed the basics of field emission nearly one century ago with their introduction of the Fowler-Nordheim equation (FNE), the topic continues to attract research interest particularly with the development of new materials that have been proposed as field emitters. The first order analysis of experiments typically relies upon the FNE for at minimum a basic understand of the physical emission process and its parameters of emission. The three key parameters in the FNE are the work function, emission area, and field enhancement factor, all of which can be difficult to determine under experimental conditions. This paper focuses in particular, on the field enhancement factor β. It is generally understood that β provides an indication of the surface roughness or sharpness of a field emitter cathode. However, in this paper, we experimentally and computationally demonstrate that cathodes with highly similar surface morphologies can manifest quite different field enhancements solely through having different emission regions. This fact can cause one to re-interpret results in which a single sharp emitter is proposed to dominate the emission from a field emitting cathode.Thermionic emission
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Concepts of highly bright electron source for beam systems such as electron microscopes and electron exposure systems are given. Recent progress in developing thermal field electron source is reviewed. It is shown that work function of W(100) surface is selectively reduced by modifying the surface with yttrium and with oxygen. The reduced work function is 2.1 eV, which is about 0.6 eV smaller compared with ZrO/W(100) thermal field emitter. It is also shown that the work function of Molybdenum is also reduced on (100) surface selectively by modifying the surface with zirconium and with oxygen. The reduced work function is 2.0eV. Potential abilities of field emission arrays and carbon nano-tubes for electron sources are critically shown.
Electron gun
Field emission microscopy
Field emission gun
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Microfabricated field emitters have shown the potential for very high current densities (>100A∕cm2) and total emission currents (>1A). However, realizing this potential has been elusive, primarily because these cathodes exhibit insufficient emission uniformity over an emitter array. In this article we report the development of an in situ processing method based on emitter tip self-heating during operation that is shown to improve emission uniformity between emitter tips. Two tips differing in emission current by three orders of magnitude for a given voltage as fabricated are shown to be essentially identical in their emission characteristics after controlled pulsing to very high emission current. When the method was applied to a 50 000 tip array, it produced 300 mA of emission (40A∕cm2). The experimental arrangement prevented advancing to higher emission levels due to space charge limitations. It is expected that 1 A of emission at ∼100A∕cm2 is possible with appropriate modifications to the experimental apparatus.
Field emitter array
Field emission display
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Field emission from ZrC films deposited on Si and Mo single emitters and field emitter arrays (FEAs) has been studied. For single emitters, the results show dramatic improvements in emitter performance by reducing work functions—on the order of 1 eV—and increasing stability. For FEAs, deposition of a ZrC film reduced the operating voltage 30%–50% at an emission current of 1.0 μA/tip and increased the emission stability.
Deposition
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Silicidation of the top surface of Si tips with a Nb gate structure has been carried out to improve the emission behavior of Si field emitter arrays (FEAs). A Pt layer with a thickness of 5–10 nm was deposited through the gate opening and annealed at 850 °C. The electron emission was enhanced by a factor of 10 and the average emission per tip was 3.5 μA for a 10×10 FEA. Fowler–Nordheim plots indicated the decrease in work function after silicidation.
Field emitter array
Field emission display
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This contribution is a result of higher emission current investigations with CNT field-emitter cathodes. The emitter resistor must be considered for emission current above 5 mA. Thin and long CNT field-emitters show emission current saturation. A simple R-limited emission model is presented.
Cold cathode
Saturation current
Saturation (graph theory)
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