  We report total-energy calculations based on the density-functional theory
that clarify microscopic mechanisms of initial stage of graphene formation on
the SiC(0001) surface. We explore favorable reactions for desorption of either
Si or C atoms from the stepped surface by determining the desorption and the
subsequent migration pathways and calculating the corresponding energy barriers
for the first time. We find that the energy barrier for the desorption of an Si
atom at the step edge and the subsequent migration toward stable terrace sites
are lower than that of a C atom by 0.75 eV, indicative of the selective
desorption of Si from the SiC surface. We also find that the subsequent Si
desorption is an exothermic reaction. This exothermicity comes from the energy
gain due to the bond formation of C atoms being left near the step edges. This
is certainly a seed of graphene flakes.
