Field Emission from Diamond Films

Due to the wide bandgap, the conduction band of diamond approximates the vacuum level. Consequently, electrons excited into the conduction band may leave the diamond s surface as there is no significant potential difference between conduction band and vacuum level. In hydrogenated diamond films, the conduction band is even observed to exceed the vacuum level, resulting in a negative electron affinity of the respective film. This causes the emission of excited electrons from the film to occur all the easier. 

Diamond emitter tips (a) array, (b) with gate, (c) diamond nanotips ( Eisevier 

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Field emission from a heteroepitaxial undoped (lOO)-oriented diamond film of 20-[rm thickness deposited on Ir(lOO) substrate was investigated in Ref [441] using a W needle as an anode. As compared with a polycrystalline diamond film also deposited on Ir(lOO), the heteroepitaxial film emitted electrons at lower voltage, roughly 1/3 of the voltage needed for the polycrystalline film. The emission was uniform over the entire area of 3 mm in diameter. The threshold voltage when the emission current was 10 " A was estimated to be 40 V/pm, which was fairly high, because the film surface was flat and the film was thick. 

Figure 6.32 Field emission from a diamond film. Plotted here is the strength of the emission current vs. the intensity of the field applied.

Wang C, Garcia A, Ingram DC, Lake M, Kordesch ME. Cold field-emission from CVD diamond films observed in emission electron-microscopy. Election Lett... 

The next point to realize is that the best emitter is a metal. Many forms of carbon initially studied are semiconductors or even insulators, including nanodiamond [8-11] and diamond-like carbon (DLC) [12-13,4]. Combine this with local field enhancement means that there is never uniform emission from a flat carbon surface, it emits from local regions of field enhancement, such as grain boundaries [8-11] or conductive tracks burnt across the film in a forming process akin to electrical breakdown [13]. Any conductive track is near-metallic and is able to form an internal tip, which provides the field enhancement within the solid state [4]. Figure 13.2 shows the equipoten-tials around an internal tip due to grain boundaries or tracks inside a less conductive region. 

In order to increase the intensity and focus of electrons, a field emission source may be used. This consists of a single crystal tungsten or LaBeClOO) wire that is sharpened to a tip diameter of ca. 100 nm — 1 j,m. For crystalline tungsten, the axis is suitably aligned with respect to the optical axis of the microscope. For example, a beam with a diameter < 5 nm is possible from alignment of the filament planes perpendicular to (310) and (111). In addition to W and LaBe, a number of other materials are proposed for field emission applications, such as silicon, single-walled nanotubes,and ultrananocrystalline diamond (UNCD) or Cu/Li alloy films deposited onto sharpened tips.t ... 

Figure 13.23. Field emission properties from a heteroepitaxial diamond film grown on Ir(lll). The inset shows the experimental setup [442],...

Xu NS, et al. Study of field electron emission from nanocrystalline diamond thin films grown from a N2/CH4 microwave plasma. J Phys D Appl Phys 2000. 

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