Influence of a dielectric coating on field electron emission from micro‐point electron sources

Abstract

Field electron (FE) emitters may be prepared by coating a metallic emitter or a carbon-fibre emitter with a layer of dielectric material. Such 'composite' emitters are of interest for both fundamental and technological reasons, and can also be used to simulate low-macroscopic-field emission from flat thin-film emitters. This paper reviews the long-standing work aimed at characterizing and optimizing emitters of this kind, which are sometimes called 'M-I-V' or 'C-D-V' emitters.The coating materials investigated include several types of epoxylite resins, hydrocarbons, alumina, lacomit, plastics, zinc oxide, magnesium oxide, zirconium carbide and tetracyanoethylene. High-resolution electron microscopy was used to monitor the profile of the tip and to measure the coating thickness. The FE emission properties of these composite emitters were studied using both conventional field electron microscope (FEM) techniques and high-resolution FE spectroscopy techniques based on the use of a retarding potential energy analyser.There are common themes to the behaviour of these composite emitters. Characteristic emission properties include: an initial switch-on effect at threshold barrier fields of order 10 9 V/m; a subsequent I -V characteristic that gives linear Fowler-Nordheim (FN) plots at low fields of <10 8 V/m, but saturates at higher fields and may exhibit hysteresis; and an energy distribution with a width and shift that are field-dependent. The coatings can give rise to single-spot emission images, attributed to the formation of a conducting channel in the dielectric. In general, the emission stability is improved, as compared with an uncoated cathode.In the case of carbon-fibre cathodes, an optimum coating thickness of 0.2-0.3 µm has been found. Current-dependent peak shifts of up to 1 eV have been observed in the energy spectra. Surprisingly, these peak shifts are comparable with those found for uncoated carbon fibres and also for the resincoated tungsten emitters. Their origin is not clear, but it seems that resistive drop cannot provide the whole explanation. These peak shifts may be related to the dynamics of surface-state occupation at the material/vacuum interface.

Epoxylite Influece on Field Electron Emission Properties of Tungsten and Carbon Fiber Tips

“…It is found that the field emission characteristics of these micro-emitters are intrinsically changed by coating the emitters with a thin layer of dielectric material, this is in line with the results from previous investigations (AlQudah et al, 2015;Bajic et al, 1989;Latham & Mousa, 1986;Moran Meza et al, 2015;Mousa, 1987Mousa, , 1988Mousa, , 1992Mousa, , 1994Mousa, , 2007Mousa & Kelly, 2003;Mussa et al, 2012). By comparing the effects of different coating materials, we find the following effects.…”

Enhanced Field Electron Emission from Dielectric Coated Highly Emissive Carbon Fibers

“…Further, the coated tips have I-V characteristics that show smoother trends, and greater stability and repeatability than before coating. In contrast with the effects observed with dielectric-coated metallic emitters (Mousa, 2007), no switch-on phenomena were observed, and the effects of thermal processing and relaxation procedures did not change the stability. In addition, the emission images were more concentrated on the phosphor-coated FEM screen, and correspondingly brighter.…”

Relationship of the Distribution Thickness of Dielectric Layer on the Nano-Tip Apex and Distribution of Emitted Electrons&nbsp;

“…The presented results include SEM (FEI Company, 2010;Hitachi S-4700 SEM, 2007;Mousa, 2007) and FEM images taken by digital camera. Fig.…”