  We demonstrate that the carrier concentration of epitaxial graphene devices
grown on the C-face of a SiC substrate is efficiently modulated by a buried
gate. The gate is fabricated via the implantation of nitrogen atoms in the SiC
crystal, 200 nm below the surface, and works well at intermediate temperatures:
40K-80K. The Dirac point is observed at moderate gate voltages (1-20V)
depending upon the surface preparation. For temperatures below 40K, the gate is
inefficient as the buried channel is frozen out. However, the carrier
concentration in graphene remains very close to the value set at T\sim 40K. The
absence of parallel conduction is evidenced by the observation of the
half-integer quantum Hall effect at various concentrations at T\sim 4K. These
observations pave the way to a better understanding of intrinsic properties of
epitaxial graphene and are promising for applications such as quantum
metrology.
