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Fabrication of Graphene 19 (a) (b) 0.39 nm 0.39 nm 0.38 (0.39) nm graphene layers buffer layer 0.20 (0.17) nm S i C su b st r a te Figure 1.12: Epitaxial Graphene. (a) Schematic view on epitaxial graphene. On top of the SiC substrate, a graphitic layer. It consists of several one-atom thick caron layers. The first one is a buffer layer, which is tightly bound to the substrate, at a distance of 0.2 (0.17) nm for a Si- (C-)terminated surface. The graphene layers are formed on top of this buffer layer at a distance of 0.38 (0.39) nm, and they are equally spaced by 0.39 nm. (b) AFM image of epitaxial graphene on C-terminated SiC substrate. The steps those of the SiC substrate. The 5-10 graphene layers lie on the substrate similar to a carpet which has folds visible as white lines on the image. however, turns out to be rather low, such that the Si-terminated surface is less chosen for the fabrication of samples used in transport measurements. For a C-terminated (000 ̄1) surface, the graphitisation process is very fast, and a large number of graphene layers are formed (up to 100). In contrast to epitaxial graphene on the Si-terminated surface, the electron mobility is, here, rather high. In contrast to exfoliated graphene, the SiC substrate must be consid- ered as an integral part of the whole system of epitaxial graphene. It is indeed the mother compound, and the first graphitic layer formed during the graphitisation process is tightly bound to the SiC substrate (Fig. 1.12). The distance between this layer and the substrate has been estimated numerically to be 0.20 nm for a Si-terminated surface and 0.17 nm for a C-terminated surface [15]. These distances are much smaller than the distance between graphene sheets in crystalline graphite (2.4a = 0.34 nm). 0.38 (0.39) nm above this buffer layer for Si- (C-)terminated SiC, several graphene layers are formed, which are separated by a distance of 0.39 nm. These character- istic distances are roughly 20% larger than the layer distances in crystalline graphite, and one may therefore expect them to be less tightly bound. The above-mentioned numerical values for the distances have been confirmed by X-ray measurements [15].PDF Image | Physical Properties of Graphene
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