Field Emission from Carbon Systems
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FIELD EMISSION FROM CARBON SYSTEMS
John Robertson Engineering Dept, Cambridge University, Cambridge CB2 1PZ, UK. jr @eng.cam.ac.uk ABSTRACT Electron field emission from diamond, diamond-like carbon, carbon nanotubes and nanostructured carbon is compared. It is found that in all practical cases that emission occurs from regions of positive electron affinity with a barrier of ~5 eV and with considerable field enhancement. The field enhancement in nanotubes arises from their geometry. In diamond, the field enhancement occurs by depletion of grain boundary states. In diamond-like carbon we propose that it occurs by the presence of sp2-rich channels formed by the soft conditioning process. INTRODUCTION Various forms of carbon such as carbon nanotubes, diamond and diamond-like carbon (DLC) can show electron field emission at low applied fields of order 2-20 V/µm. This opens up a number of opportunities for electronic devices. Each device requires a different set of material parameters and so this favours the use of a different type of carbon. The electronic device of primary interest in vacuum microelectronics is the microwave power amplifier [1,2]. The high frequency limit of semiconductor devices is set by their dimensions and the relatively small limiting velocity of electrons in semiconductors, eg 105 m.s-1 for Si. Vacuum microelectronic devices can in principle operate up to THz, as the limiting electron velocity in space is 3.108 m.s-1. These devices require a large, stable emission current density, but place no limits on the substrate or emission site density. It is likely that carbon nanotubes are best here, as they can carry the largest current. Field emission displays (FEDs) are flat panel displays in which the image is formed from a large array of pixels each addressed by field emission sources [3,4]. The use of thin film emitters rather than Spindt tips allows the use of wider gate holes. This makes lithography easier and lower cost for large display areas. A significant constraint is that glass substrates are used for low cost, so deposition and processing temperatures must be kept under about 500°C or lower. The cathodes are ~2 µm in diameter, so it is necessary that there is one emission site per source, equivalent to an emission site density (ESD) of 106 cm-2. FEDs require emission current densities of about 1 mA.cm-2. Carbon field emitters could be used in vacuum power switches [5,6]. Solid state power switches have considerable difficulty in simultaneously withstanding the large breakdown voltages and having small forward voltage drops, as this places opposite constraints on the doping density of the semiconductor. A vacuum emission device can separate these two requirements – the vacuum withstands the off voltage, while the forward voltage drop can be low for a good field emitter. Carbon nanotubes can make field emission electron guns with extremely high brightness and narrow energy distributions for use in scanning transmission electron microscopes [7]. The sharp energy distribution helps to minimise t
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