SiGe-intercalated graphene on SiC(0001): Interfacial structures and graphene doping depending on coverage and composition ratio of the alloy

To find out the relation between the charge transfer doping of quasi-free-standing monolayer graphene (QFMLG) formed on 6H-SiC(0001) and the structures of the intercalated SiGe alloy film, Ge atoms were intercalated additionally/substitutionally under QFMLG above the already-built bilayer Si film with a coverage of 1.2 ML. When 0.4-ML Ge is additionally intercalated, the SiGe film with a 31×23 structure is generated and induces weakly n-doped QFMLG whose Dirac point (E_D) is at 0.04 eV below the Fermi level (E_F). When the additionally intercalated Ge amount is over 0.6 ML, the SiGe film with the 2×1.5n3 and/or 4×33 structure composed of adatoms and a bilayer is formed below QFMLG. It turns out that Ge atoms preferentially occupy the top layer, while Si atoms occupy the bottom layer. Once a well-ordered trilayer film is formed at a total Si and Ge coverage of 2.3 MLs, though the concentration of Ge is gradually increased by the substitution of Ge atoms for Si atoms, the E_D of QFMLG is fixed at 0.20 eV above the E_F. Such a result confirms that the amount of charge transferred from the well-ordered trilayer SiGe film to QFMLG is irrelevant to the composition ratio.