Epitaxial Graphene on SiC
Epitaxial graphene on SiC(0001) is commonly prepared by means of silicon sublimation via annealing of the SiC substrate. An initial carbon layer develops - well ordered in a (6√3x6√3)R30° superstructure on the SiC(0001) substrate, which structurally is composed of the typical carbon honeycomb lattice found in graphene. However, about one third of the carbon atoms are still covalently bound to the topmost silicon atoms in the substrate so that the delocalized π-band system cannot develop. This initial carbon layer is called buffer layer or zero layer graphene (ZLG). The covalent bonding situation is sketched with the covalent bonds between Si and C across the interface and dangling bonds (DB) on the remaining Si atoms indicated (a). We note that this bonding induces a strong buckling in the graphitic layer and leads to a very distinct and intense quasi-(6x6)-SiC(0001) diffraction pattern.
By further annealing of the SiC sample a second carbon layer is formed, which in turn adopts the role of the buffer layer. The initial ZLG transforms into a real graphene layer on top with fully developed π-bands and the system is then called monolayer graphene (MLG) (b). The ZLG carbon layer can be decoupled from the SiC substrate by an intercalated atomic layer, as first shown for hydrogen intercalation (c). The covalent bonds at the ZLG interface are broken and all Si atoms in the topmost substrate layer are saturated by hydrogen atoms. Effectively in this way, a quasi-free standing graphene monolayer is obtained exhibiting the well-known graphene Dirac cones. Besides hydrogen many other chemical elements like Ag, Au, Cu, Gd, Ge, Yb, etc. can be used to intercalate the ZLG or MLG. A plentitude of new electronic and chemical properties evolve by the intercalation of graphene with different elements.
ZLG samples with homogeneous coverage are prepared on 6H-SiC(0001) (on-axis, n-doped, purchased from SiCrystal GmbH) by annealing in our home-built RF-furnace in Ar atmosphere, which results in a superb homogeneity on a waver scale. Atomic force microscopy (AFM), low-energy electron diffraction (LEED) and photoelectron spectroscopy (PES) together with synchrotron techniques are used to investigate the chemical and electronic properties of the graphene layer and the interface.
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