Grain boundary diamond

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. 

When a substrate material with a well-defined crystal orientation, or with a special treatment, is used for diamond CVD, it is possible to synthesize a diamond film in which either (100) or (111) diamond faces are parallel to the film surface, and make them align in the same direction. In other words, diamond faces can be azimuthally (in-plane) oriented in the same direction, as seen in Figures 1.1 (b) and (c). In such cases, it often happens that adjacent diamond faces coalesce with each other to form a larger face. If the coalescence develops over the entire surface, then the grain boundaries vanish and a single crystal diamond film is formed on the... 

Since diamond CVD is undertaken at 800 °C, and the stress at the interface is generally very high when the specimen is taken out of the reactor to the ambient environment because of the difference in the thermal expansion coefficients between the CVD diamond film and the substrate. In addition, the intrinsie stress within the CVD diamond film is high if it is polycrystalline. The presence of grain boundaries can be the cause of the intrinsic stress. Thus, reducing both interfacial and intrinsic stresses is an important issue for practical use of CVD diamond films. So far, no effective method has been found to solve this problem. 

An HOD film growth was carried out on 1-inch Si(l 10) substrates [269] using the three-step process. The pretreatment and growth conditions are listed in Table H.3. In this work, (i) there was no apparent grain boundaries in the central area of a diamond film, (ii) an array of misfit dislocations were seen in the periodicity of four D 111 on every three Si 111, and the spacing between the misfit dislocations was... 

Figure 11.32. TEM image of a diamond grain, showing defects at the grain boundaries [319].

In the latest achievement, a fully coalesced diamond film was deposited on a 10-mm(/> Pt(lll) bulk surface, as shown in Figure 12.8. The surface consisted of a grain boundary-less diamond film with numerous bunched steps. Although the detailed analysis of the film has not yet been done, this demonstrated that (lll)-oriented single crystal diamond films can be deposited on Pt(lll). 

Another type of scratch hardness which is a logical development of the Mohs scale consists of drawing a diamond stylus, under a definite load, across the surface to be examined. The hardness is determined by the width or depth of the resulting scratch the harder the material the smaller the scratch. This method has some value as a means of measuring the variation in hardness across a grain boundary. In general, however, the scratch sclerometer is a difficult instrument to operate. 

Alternative methods for introducing electrical conductivity into diamond have been developed, which include dopants such as nitrogen[72,78,81,89], sp2 carbon inclusions in grain boundaries[75], and metal and metal cluster inclusions, and subsurface hydrogen[75]. Other forms of conductive diamond, such as surface conductive[81] or ultracrystalline diamond[78] have also been quoted in literature, suggesting that several types of chemically vapor-deposited diamond may find electrochemical applications[75],... 

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