Origin of ambipolar graphene doping induced by the ordered Ge film intercalated on SiC(0001)

Transfer doping mechanism of the quasi-free-standing monolayer graphene (QFMLG), depending on the thickness as well as the atomic structures of the intercalated Ge film, has been revealed by combined investigations of scanning tunneling microscopy and synchrotron photoemission spectroscopy. The ordered Ge film has been formed between QFMLG and the n-type 6H-SiC(0001) substrate by depositing Ge on the (63×63)R30° zero layer and postannealing. At postannealing temperatures lower than 900 °C, the interfacial structures inducing p-type QFMLG, like 2×1.5n3 and 4×33, are formed. On the other hand, at those higher than 900 °C, the interfacial structures inducing n-type QFMLG, like 7×43 [expressed as a (5−376) matrix], 8×33 and 21×21 [expressed as a (5115) matrix], are formed as a result of partial deintercalation of Ge atoms. Such a transition from p-type to n-type doping turns out to be associated with an increase in the density of states of an upper Hubbard band originating from strong correlation of the electrons at the Ge atoms on the first and the second Ge interfacial layers.