Electronic band structure of Bi-intercalate layers in graphene and SiC(0001)

A potential way to tune the electronic band structure of graphene is to intercalate foreign atoms in the interface between graphene and substrate. However, such an intercalate layer covered by graphene is difficult to directly study with microscopic probes, and relatively little is known about its crystal and electronic structures. In this work, we study epitaxial graphene on SiC(0001) intercalated by bismuth atoms by means of angle-resolved photoemission spectroscopy. We reveal the electronic band structure of Bi-intercalate layers, from which we could identify two distinct phases, one is metallic and the other is insulating. The metallic phase composed of closely packed Bi atoms shows nearly free electron bands that are repeated to follow the period of SiC(0001)-(1 × 1). The lower coverage insulating phase shows characteristic flat bands in the period of SiC(0001)-(3×3)R30^∘ with respect to the surface lattice constant of SiC(0001). Even though there exists such two distinct phases in the Bi intercalate layer, the doping level of graphene is found to vary rather continuously with the coverage of Bi. Based on the observed band structures and Bi 5d core-level spectra, we suggest a structural model for the metallic and insulating phases of Bi-terminated SiC(0001).