  The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied
extensively, yet precisely where the Ca resides remains elusive. Furthermore,
the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been
reported. Here, we use low energy electron diffraction, x-ray photoelectron
spectroscopy, secondary electron cut-off photoemission and scanning tunneling
microscopy to elucidate the physical and electronic structure of both Ca- and
Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca
intercalates underneath the buffer layer and bonds to the Si-terminated SiC
surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the
buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene
(Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial
graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC
surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in
Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG).
Ca-intercalation underneath the buffer layer has not been considered in
previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no
evidence that either Ca or Mg intercalates between graphene layers. However, we
do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of
3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to
ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG
remains remarkably stable.
