Electron Emission From Diamond Films
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Electron Emission From Diamond Films
Diamond Materials
V.V. Zhirnov and JJ. Hren Introduction
Electron Emission From Solids
Diamond always has been considered an exciting material. In addition to its other outstanding properties, diamond's capacity for cold electron emission has become a "hot" topic of research in recent years. The electron emission from diamond films is important for both fundamental and applied purposes, which may be expressed by two questions as follows: "Why does diamond emit electrons?" and "How can an efficient cold cathode be made?" The "diamond-emission era" started in 1991. That year three papers were published, reporting unexpectedly high electron emissivity from diamond. The first paper by Djubua and Chubun1 was very practical, having been written by device engineers. They tried different materials for the fabrication of pointed fieldemission cathodes and found that emitters made from diamondlike carbon (DLC) demonstrated a lower emission threshold compared to other materials. A second paper by Wang et al.2 reported a low emission threshold for chemical-vapordeposition (CVD) diamond films, whereas a third paper by Geis et al.3 described the fabrication and operation of diamond cold cathodes. Since 1991 hundreds of papers about electron emission from diamond and diamondlike materials have been published. The motivations for this increasing activity are prospective applications in vacuum microelectronics—that is, vacuum emission devices fabricated by modern microelectronic technologies. Experts expect that combining the physical advantages of emission devices and the technological progress in solid-state microelectronics will result in the development of an entirely new generation of high-performance electronic devices— among them, flat-panel displays and miniature microwave tubes.5
The different semiconductor emission processes are illustrated in Figure 1. Electrons inside a solid cannot escape into vacuum under equilibrium conditions because of the potential barrier at the surface. The barrier must always be positive for equilibrium electrons because of the requirement for stability. In order to overcome this barrier, external energy is required to excite the electrons (e.g., thermionic, photo- and secondaryelectron emission, or emission through a p-n junction) to create metastable nonequilibrium states (exoemission) or to apply high electric fields to the surface (field emission). Until now only a few results have been reported about thermionic and exoemission from diamond films,67 and photo- and secondary emission from diamond surfaces are reviewed in Reference 8. This article is devoted exclusively to what we term "cold field emission" from diamond films, which is of the
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The considerable amount of difficulty in developing a general explanation for electron emission from diamond comes from the diversity of "diamond materials," particularly those grown by CVD processes. The same name is often used to describe quite different materials, or— conversely—different names are used t
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