▎ 摘　　要

Epitaxial graphene grows on different SiC polytypes possessing distinct bulk band gaps. In this work we systematically investigate the influence of polytypes on the graphene electronic spectrum employing localdensity approximation (LDA)/Heyd-Scuseria-Ernzerhof (HSE) ab initio calculations including different buffer layer-graphene layer stackings. We find a variation of the Dirac point position with respect to the valence-band edge as a function of the polytype hexagonality. HSE values are in good agreement with recent experimental results, while LDA corroborates the trends. Since the Dirac point, interface-related states, and the Fermi level follow similar polytype-induced shifts, the doping of the epilayer stays practically the same. For the AB stacked buffer and epilayer on a Si-terminated SiC substrate the graphene spectrum exhibits a polytype-dependent energy gap epsilon(g) which ranges 25-40 meV for different polytypes. On the contrary, for the AA stacking the Dirac cone remains intact. We suggest a symmetry-based analytical model which explains the origin of the gap and its absence for the AA geometry and provides a direct connection between epsilon(g) and the buffer-epilayer interaction potential.