Bi-intercalated epitaxial graphene on SiC(0001)

The intercalation of graphene with suitable atomic species is one of the most frequently applied methods to decouple the graphene layer from the substrate in order to establish the classical electronic properties of graphene. In this context, we studied the bismuth (Bi) intercalation of the (63x63)R30 degrees reconstructed so-called 'zeroth layer graphene' on SiC (0001). As reported earlier by Sohn et al (2021 J. Korean Phys. Soc. 78 157) two phases are formed depending on the amount of intercalated Bi, which in turn is controlled by the annealing temperature: The alpha phase, showing a (1x1) periodicity with respect to the substrate, and, at higher temperatures, the (3x3) reconstructed beta phase. We characterise both phases and the transformation from the alpha to the beta phase by photoelectron spectroscopy, normal incidence x-ray standing waves, electron diffraction and electron microscopy. We clearly see an almost complete intercalation of the graphene layers in both phases, with strong (covalent) interaction between the topmost Si atoms of the substrate and the Bi intercalant, but only weak (van der Waals) interaction between Bi and the graphene layer. The n-doping of the graphene found for the alpha phase decreases continuously during the phase transformation, in agreement with a reduced density of the Bi intercalating layer. Missing core level shifts of the surface species as well as the normal incidence x-ray standing waves results indicate that all surface Si atoms remain saturated during the transition and no dangling bonds are formed. Low energy electron microscopy and diffraction reveal the coexistance of both phases after annealing to intermediate temperatures and allow a quantitative analysis of island sizes and numbers.