Contribution of the buffer layer to the Raman spectrum of epitaxial graphene on SiC(0001)

We report a Raman study of the so-called buffer layer with (6 root 3 x 6 root 3) R30 degrees periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a non-vanishing signal in the Raman spectrum at frequencies in the range of the D-and G-band of graphene and discuss its shape and intensity. Ab initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer layer.