Simulations within the density functional theory framework clarify the microscopic mechanism responsible for the initial stage of graphene formation on the SiC(0001) surface. Favorable reaction pathways for the desorption of either Si or C atoms from the stepped surface have been found by determining the desorption and the subsequent migration pathways, quantified in terms of 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 is 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 likely to be a seed of graphene flakes.
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