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18 Introduction to Carbon Materials graphene (2D metal) Vg 3 2 0 m 0 n SiO (insulator) doped Si (metal) Figure 1.11: Schematic view of graphene on a SiO2 substrate with a doped Si (metallic) backgate. The system graphene-SiO2-backgate may be viewed as a capacitor the charge density of which is controled by a gate voltage Vg. thus given by n2D = αVg with α ≡ ed ≃ 7.2 × 10 V . (1.8) The gate voltage may vary roughly between −100 and 100 V, such that one may induce maximal carrier densities on the order of 1012 cm−2, on top of the intrinsic carrier density which turns out to be zero in graphene, as will be discussed in the next chapter. At gate voltages above ±100 V, the capacitor breaks down (electrical breakdown). 1.3.2 Epitaxial graphene An alternative method to fabricate graphene has been developed by the At- lanta group, led by Walt de Heer and Claire Berger [14]. It consists of expos- ing an epitaxially grown hexagonal (4H or 6H-) SiC crystal8 to temperatures of about 1300o C in order to evaporate the less tightly bound Si atoms from the surface. The remaining carbon atoms on the surface form a graphitic layer (graphitsation). The physical properties of this graphitic layer depend on the chosen SiC surface. In the case of the Si-terminated (0001) surface, the graphitisation process is slow, and one may thus control the number of formed graphene layers (usually one or two). The resulting electron mobility, 8SiC exists also in several other crystal structures, such as the zinc blende structure (3C-SiC), i.e. a diamond structure with different atom types on the two distinct fcc sublattices. ǫ0ǫ 10 cm−2PDF Image | Physical Properties of Graphene
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