Stacking Domains in Epitaxial Bilayer Graphene
Hiroki Hibino1, Seigi Mizuno2, and Hiroyuki Kageshima3
1Materials Science Laboratory, 2Kyushu University, and 3Physical Science Laboratory
Few-layer graphene (FLG) is attracting intense attention as a future electronics material due to its superior electronic transport properties. Epitaxial FLG grown on SiC substrates by thermal decomposition can be easily scaled up and is promising for device integration. We have so far established a method of determining the number of graphene layers microscopically using low-energy electron microscopy (LEEM)  and have succeeded in growing bilayer graphene a few micrometers in size. In this work, we clarified the domain structures in bilayer graphene .
Figure 1 shows LEEM images of FLG 1-3 layers thick grown on 4H-SiC(0001). Bilayer graphene looks continuous in the bright-field LEEM image [Fig. 1(a)] obtained using the specularly reflected (0,0) beam. However, the dark-field LEEM images [Figs. 1(b) and 1(c)] obtained using the diffracted (1,0) and (0,1) beams clearly show that bilayer graphene contains two types of domains. The contrast of the two domains are reversed between the (1,0) and (0,1) dark-field LEEM images, indicating that these domains have three-fold symmetry. Monolayer graphene has six-fold symmetry, and two carbon atoms are located at A and B sites in a unit cell. On the other hand, as shown in Fig. 2, bilayer graphene can have two types of stacking orders with three-fold symmetry: AB stacking in which a B’ carbon atom is on an A carbon atom and BA (usually called AC) stacking in which an A’ carbon atom is on a B carbon atom. Therefore, we speculate that the two types of domains seen experimentally correspond to the AB and AC stackings. To confirm this, we measured the energy dependence of the intensities of the dark-field LEEM images for the two domains and compared it to the calculated energy dependence of the (1,0) and (0,1) low-energy electron diffraction intensities for bulk graphite. Good agreement between these two energy dependences proves that there are stacking domains in bilayer graphene. The stacking domain structures can affect the electronic transport properties in FLG. The formation mechanism and condition for growing a single stacking domain are the next targets.
This work was supported by KAKENHI.
 H. Hibino et al., Phys. Rev. B 77 (2008) 075413.
 H. Hibino et al., Phys. Rev. B 80 (2009) 085406.
Fig. 1. (a) Bright-field and (b)-(c) dark-field LEEM
images of epitaxial FLG. In (a), 1-3 indicate the
number of graphene layers. Electron beam energies
were (a) about 5 eV and (b)-(c) about 58 eV.
Fig. 2. Schematic illustration of AB and AC
stackings in bilayer graphene.
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