  It has been shown that the first C layer on the SiC(0001)(2{\times}2)C
surface already exhibits graphene-like electronic structure, with linear pi
bands near the Dirac point. Indeed, the (2{\times}2)C reconstruction, with a Si
adatom and C restatom structure, efficiently passivates the SiC(0001) surface
thanks to an adatom/restatom charge transfer mechanism. Here, we study the
effects of interface modifications on the graphene layer using density
functional theory calculations. The modifications we consider are inspired from
native interface defects observed by scanning tunneling microscopy. One H atom
per 4 {\times} 4 SiC cell (5 {\times} 5 graphene cell) is introduced in order
to saturate a restatom dangling bond and hinder the adatom/restatom charge
transfer. As a consequence, the graphene layer is doped with electrons from the
substrate and the interaction with the adatom states slightly increases. Native
interface defects are therefore likely to play an important role in the doping
mechanism on the C terminated SiC substrates. We also conclude that an
efficient passivation of the C face of SiC by H requires a complete removal of
the reconstruction. Otherwise, at variance with the Si terminated SiC
substrates, the presence of H at the interface would increase the
graphene/substrate interaction.
