Raman fingerprint of the graphene buffer layer grown on the Si-terminated face of 4H-SiC(0001): Experiment and theory

In this work, we present the results of measurements of the Raman spectrum of the √3x√3R30° reconstruction of graphene grown on 4H-SiC(0001), the so-called buffer layer. The extracted Raman spectrum of the buffer layer shows bands, different from those of graphene, which can be attributed to the interaction of the buffer layer with the SiC substrate. In particular, in the high-wavenumber region, at least three bands are observed in the wavenumber regions 1,350–1,420, 1,470–1,490 and 1,520–1,570 cm⁻¹. The assignment of the buffer layer bands is supported here by tight-binding simulations of the one-phonon density of states for structures with a sufficiently large number of Si-C bilayers for reaching convergence. The converged phonon density of states is found to be in semi-quantitative agreement with the latter two bands, and therefore, the tight-binding predictions of the lattice dynamics of the structure can be used for their assignment to buffer layer vibrations. Namely, the Raman band at about 1,550 cm⁻¹ can be assigned to modified in-plane optical phonon branches of graphene, while the Raman band at about 1,490 cm⁻¹ can be assigned to modified folded parts of these branches inside the Brillouin zone of the buffer layer and can be considered as a Raman fingerprint of the buffer layer.