Evaluation of Number of Graphene Layers Grown on SiC


Hiroki Hibino1 and Hiroyuki Kageshima2
1Materials Science Laboratory, 2Physical Science Laboratory

  Recently, graphene has attracted much attention as a material for future electronics [1]. So far, electronic device properties have been investigated for graphene layers produced in two ways: graphene flakes exfoliated from bulk graphite [1] and epitaxial graphene grown on SiC substrates by annealing [2]. Epitaxial graphene grows on a wafer scale and is promising for device integration. To make epitaxial graphene applicable, however, we need to grow wide epitaxial graphene with the intended number of layers. As a base of this growth control, we have established a way of determining the number of graphene layers microscopically [3].
  We evaluated the number-of-layers distribution in epitaxial graphene grown on SiC by low-energy electron microscopy (LEEM) using quantized oscillations of electron reflectivity. Figure 1 shows LEEM images of epitaxial graphene on 4H-SiC(0001) at various electron beam energies, which corresponds to the mappings of the secular electron reflectivity in the normal incidence. These images show that the electron reflectivities in different regions change with the energy in different manners. Figure 2 shows the energy dependence of the electron reflectivities in areas A-H. The reflectivity oscillates with the electron beam energy.
  Bulk graphite has continuous electronic bands normal to the graphene sheet, but these bands split into discrete energy levels in graphene layers due to their finite thickness. When the energy of incident electrons coincides with one of the discrete energy levels, the electrons resonantly transmit through the layers, resulting in dips in the reflectivity. Therefore, the number of graphene layers can be counted directly as the number of dips in the reflectivity. The validity of this scenario was confirmed by the result that the quantized conduction band states calculated using tight-binding and first-principles methods well reproduce the dip positions in the reflectivity. In-situ microscopic determination of the number of graphene layers using LEEM would greatly contribute to the growth control of epitaxial graphene.
  This work was partly supported by KAKENHI (19310085) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

[1] K. S. Novoselov and A. K. Geim, Nature Materials 6 (2007) 184.
[2] C. Berger, et al., Science 312 (2006) 1191.
[3] H. Hibino, et al., Phys. Rev. B 77 (2008) 075413; H. Hibino, et al., e-J Surf. Sci. Nanotechnol. 6 (2008) 107.

Fig. 1. LEEM images of graphene layers grown on 4H-SiC(0001) at the electron beam energies of (a) 3.0 and (b) 4.5 eV.
Fig. 2. Electron reflectivities in areas A-H in Fig. 1 versus the electron beam energy.

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