Field Emission Site Densities of Nanostructured Carbon
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FIELD EMISSION SITE DENSITIES OF NANOSTRUCTURED CARBON J B Cui, J Robertson, W I Milne, Department of Engineering, Cambridge University, Cambridge CB2 1PZ, UK ABSTRACT The field emission properties of nanostructured carbon films deposited by cathodic vacuum arc in a He atmosphere have been studied by measuring the emission currents and the emission site density. The films have an onset field of ~3 V/µm. The emission site density is viewed on a phosphor anode and it increases rapidly with applied field. It is assumed that the emission occurs from surface regions with a range of field enhancement factors but with a constant work function. The field enhancement factor is found to have an exponential distribution. INTRODUCTION Electron field emission from various carbon films, such as chemical vapour deposited (CVD) diamond, CVD graphite, carbon nanotubes, amorphous carbon and nanostructured carbon is of great interest because it occurs from flat films without the need to form microtips [1-19]. Emission from these materials can occur for applied fields of 10 volts per micrometer or less. The emission is however discontinuous, and occurs as the series of spots [20]. The emission site density is therefore an important parameter, particularly for displays. This paper describes field emission properties of nanostructured carbon films deposited at room temperature by cathodic arc. The emission I-V curve and the emission site density were both measured. The discontinuous nature of the emission indicates that emission occurs preferentially from different parts of the surface. We have found that the work function for these carbon films is quite large and varies from 4.5 to 5.0 eV. We propose that the preferential emission therefore arises from a variation in the local field enhancement factor. The emission can be described by a distribution of field enhancement factors. EXPERIMENT The nanostructured carbon films were deposited at room temperature using a cathodic vacuum arc system. The base pressure of the deposition chamber was about 5x10-7 mbar and the deposition pressure could be varied from 1x10-6 to 1 mbar by introducing helium gas into the chamber. The He thermalises the carbon ions and atoms and allows them to form fullerene-like clusters [21], which are incorporated in the growing film. The thickness of the carbon film is between 10 and 100 nm. The films are deposited onto conductive Si substrates on which there is a 1.5 µm thick resistive ballast layer of photoresist. The function of the ballast layer is to limit the emission current from individual emission spots, and to allow the range of the emission sites to be seen, without saturating the phosphor screen [22]. Ballast resistors are frequently used in Spindt tip field emission devices to even out the current distribution and to prevent excessive currents [23]. The emission is measured in a parallel plate configuration. The anode-tosample spacing was 50 or 100 µm by using PTFE spacers. An ITO (indium-tin-oxide) or phosphor coated glass is used as an anode to collect
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