Homogeneous large-area graphene monolayers were successfully prepared ex situ on $6H$-SiC(0001). The samples have been studied systematically and the results are compared with those from a sample cut from the same wafer and prepared by in situ heating. The formation of smaller graphene flakes was found on the in situ prepared sample, which is in line with earlier observations. Distinctly different results are observed from the ex situ graphene layers of different thicknesses, which are proposed as a guideline for determining graphene growth. Recorded $\text{C}1s$ spectra consisted of three components: bulk SiC, graphene $\left(G\right)$, and interface $\left(I\right)$, the latter being a $6\sqrt{3}$ layer. Extracted intensity ratios of $G/I$ were found to give a good estimate of the thickness of graphene. Differences are also revealed in micro low energy electron diffraction images and electron reflectivity curves. The diffraction patterns were distinctly different from a monolayer thickness up to three layers. At a larger thickness only the graphitelike spot was visible. The electron reflectivity curve showed a nice oscillation behavior with kinetic energy and as a function of the number of graphene layers. The graphene sheets prepared were found to be very inert and the interface between the substrate and the layer(s) was found to be quite abrupt. No free Si could be detected in or on the graphene layers or at the interface.

• Received 13 August 2008

DOI:https://doi.org/10.1103/PhysRevB.78.245403