Intercalation and doping of expitaxial graphene
The growth and investigation of graphene (a single layer of carbon atoms) on SiC possesses several major advantages compared to graphene on metallic surfaces. Due to its band gap of around 3 eV SiC is an ideal candidate to investigate electronic properties of graphene. Therefore, no transfer of graphene onto semiconducting or insulating substrates is necessary, which often deteriorates the properties of graphene and is not scalable for mass production. However, after the growth of one carbon layer on SiC by silicon sublimation at high temperature, some of the carbon atoms are still bond to the silicon atoms of the surface and therefore the carbon layer does not show graphene’s typical Dirac like band dispersion. These bonds between Si and the carbon layer can be broken by the intercalation of foreign atomic species, so that the carbon layer is decoupled and the π band system develops its typical conical dispersion. In addition, the carrier density of the epitaxial graphene can be precisely controlled. A milestone in this type of modification of epitaxial graphene (EG) was the preparation of quasi-free standing monolayer graphene (QFMLG) on SiC(0001) by means of atmospheric hydrogen intercalation (Riedl 2009). Intercalation of other elements allows us to functionalize the π band structure, so that p- and n-doped phases could be retrieved by controlled Ge (Emtsev 2011) or Au intercalation (Gierz 2010). Cu intercalation imposes a superperiodic potential onto the graphene layer leading to the appearance of so-called mini-Dirac cones (Forti 2016). New intercalation approaches by implantation and segregation where invented for Bi (Stoer 2016) and also Sb. Besides the generation of quasi-free standing decoupled graphene strongly doped graphene (Yb, Gd) or even 2D metallic bands in-between graphene and the SiC surface (Au, Ag) can be created.
Indeed, recently we were able to show that the intercalation of Gd or Yb leads to a strong electron doping of the graphene, where the Fermi level is pushed to the vicinity of the van Hove singularity at the M-point in graphene’s Brilloiun zone (Rosenzweig 2019, Link 2019). However, this extreme doping does not lead to a rigid shift of the electronic band structure but new effects and electronic structure renormalizations take place. For example at the van Hove singularity a flat band extending over almost 1 Å-1 evolves due to strong electron correlation effects. Furthermore, electron phonon coupling leads to the development of polaron replica bands as observed for the intercalation with Gd (Link 2019). For the intercalation of graphene with Yb a topological transition at the Fermi level and a continuous upshift of the Dirac point are observed by annealing, indicating a decrease in charge carrier density (Rosenzweig 2019). Another strong advantage of the use of SiC over metal substrates is that also the intercalant might be decoupled from the substrate and its electronic properties can be investigated individually with only little interference of the semiconducting substrate. For example, we could recently show that 2-dimensional gold constrained in-between graphene and SiC is a semiconductor with a valence band maximum residing 50 meV below the Fermi level and exhibiting a spin-orbit splitting of 225 meV along the ΓK direction (Forti 2020). By tuning the amount of gold a semiconductor to metal transition of the 2D gold layer can be introduced. The intercalation of silver leads to a similar semiconducting 2D layer while its maximum valence band resides 590 meV below the Fermi level (Rosenzweig and Starke 2020).
Yet, also without intercalation, the deposition of molecular layers or alkali metals can be applied to tailor the charge carrier density in epitaxial graphene. F4-TCNQ was used to obtain charge neutral graphene (Coletti 2010). Further, It could also be demonstrated that Li could potentially induce a superconducting state by an enhanced electron-phonon coupling that leads to a deviation from the Fermi distribution within the cut-off of the topmost valance bands, i.e., evidence for a superconducting gap was found (Ludbrook 2015).
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