Probing the structural transition from buffer layer to quasifreestanding monolayer graphene by Raman spectroscopy

The structural transition of a graphene buffer layer epitaxially grown on $6H$ silicon carbide (SiC) to quasifreestanding monolayer graphene by intercalation of oxygen and water molecules at low concentrations is studied by temperature-dependent Raman spectroscopy. We present a detailed investigation of the defect density and strain and doping evolution in the graphene crystal lattice. The structural transition from the buffer layer to monolayer graphene with high defect densities occurs at temperatures from 400 to 500 \ifmmode^\circ\else\textdegree\fi{}C, revealing the nanocrystalline regime of stage 2 of the amorphization trajectory, followed by the transition into stage 1 as evidenced by a gradual reduction of defects in graphene during subsequent annealing up to 900 \ifmmode^\circ\else\textdegree\fi{}C.