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

Abstract

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 (ED) is at 0.04 eV below the Fermi level (EF). 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 ED of QFMLG is fixed at 0.20 eV above the EF. 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.