  Using highly controlled coverages of graphene on SiC(0001), we have studied
the structure of the first graphene layer that grows on the SiC interface. This
layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity
and x-ray standing waves analysis we have performed a comparative study of the
buffer layer structure with and without an additional monolayer graphene layer
above it. We show that no more than 26\% of the buffer carbon is covalently
bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si
depleted and is the likely the cause of the incommensuration previously
observed in this system. When a monolayer graphene layer forms above the
buffer, the buffer layer becomes less corrugated with signs of a change in the
bonding geometry with the SiC interface. At the same time, the entire SiC
interface becomes more disordered, presumably due to entropy associated with
the higher growth temperature.
