Field Emission and Nanostructure of Carbon Films
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ABSTRACT The results of field emission measurements of various forms of carbon films are reported. It is shown that the films' nanostructure is a crucial factor determining the field emission properties. In particular, smooth, pulsed-laser deposited amorphous carbon films with both high and low sp3 contents are poor field emitters. This is similar to the results obtained for smooth nanocrystalline, sp2 - bonded carbon films. In contrast, carbon films prepared by hot-filament chemical vapor deposition (HF-CVD) exhibit very good field emission properties, including low emission turnon fields, high emission site density, and excellent durability. HF-CVD carbon films were found to be predominantly sp -bonded. However, surface morphology studies show that these films are thoroughly nanostructured, which is believed to be responsible for their promising field emission properties. INTRODUCTION Electron field emission from various carbon-based materials recently has drawn significant attention of the industrial and scientific communities world wide, as potential applications of these materials in cold cathodes for field emission displays and other vacuum microelectronic devices appear feasible. Traditionally, cold cathodes have been made in the form of gated microtips because most materials, including Mo and Si which have been used in prototype devices, have a large (- 5 eV) work function (for metals) or electron affinity (for semiconductors). Consequently, a high electric field is required to extract electrons from these materials. The high electric field is achieved by fabricating microtips with a very sharp radius of
curvature which leads to a high geometric enhancement factor (GEF) of the electric field. However, an alternative approach that eliminates complicated and expensive tip fabrication is to use flat cathodes made of a material with either very low electron affinity or intrinsic nanostructure which would provide sufficiently high GEF. For a practical emitter, used for example in a field emission display, many parameters must be taken into account, including low emission turn-on field, determined by the electron affinity or/and GEF, high emission site density, good stability, and long lifetime. 2 It has been established that diamond can possess a negative electron affinity surface ' 3 which, in principle, should allow for electron emission under very low electric fields. Unfortunately, n-doping of bulk diamond is problematic and the transport of electrons to the conduction band, required for field emission applications, is difficult to achieve. As a result, despite its low electron affinity, single crystal diamond is not a good electron emitter. Recently, a few carbon (C) materials such as "coral-like" C4 , CVD diamond5, nanodiamond 6, and C nanotubes7 , were shown to exhibit very encouraging FE characteristics. The emission mechanism for these materials is not completely understood but seems to involve substantial GEF or perhaps electrical field enhancement created by highly non-uniform electronic properties o
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