▎ 摘　　要

We present a scanning-tunneling microscopy (STM) study of a gently graphitized 6H-SiC(0001) surface in ultrahigh vacuum. From an analysis of atomic scale images, we identify two different kinds of terraces, which we attribute to mono- and bilayer graphene capping a C-rich interface. At low temperature, both terraces show (root 3x root 3) quantum interferences generated by static impurities. Such interferences are a fingerprint of pi-like states close to the Fermi level. We conclude that the metallic states of the first graphene layer are almost unperturbed by the underlying interface, in agreement with recent photoemission experiments.