Graphene on silicon: Effects of the silicon surface orientation on the work function and carrier density of graphene
Yiwei Sun, David Holec, Dominik Gehringer, Lei Li, Oliver Fenwick, D. J. Dunstan, C. J. Humphreys
Abstract
Density functional theory has been employed to study graphene on the (111), (100), and (110) surfaces of bare silicon (Si) substrates, which provide three different densities of surface atoms. There are several interesting findings. First, carbon atoms in graphene can form covalent bonds with Si atoms, when placed close enough on Si (111) and (100) surfaces, but not on the (110) surface. The Si (111) surface shifts the Fermi level of graphene into its conduction band, resulting in an increase of the electron density by three orders of magnitude. The work function of graphene is increased by 0.29 eV on the (111) surface, likely due to the surface dipole from the redistribution of $\ensuremath{\pi}$ orbitals. The change in the number of available states below the Fermi level of graphene due to its interaction with the Si surface, is the main cause for the unconventional doping reported in this paper. The electron density can also be increased by eighty times on a Si (100) substrate without the shift of Fermi level, which is another clear example of the proposed doping mechanism. These striking effects that different orientations of a silicon substrate can have on the properties of graphene are related to the surface atom density of the substrate. These results provide valuable guidance to the growth of graphene on Si for various purposes for electronic devices.