  We use angle-resolved photoemission spectroscopy to investigate the
electronic structure of bilayer graphene at high n-doping and extreme
displacement fields, created by intercalating epitaxial monolayer graphene on
silicon carbide with magnesium to form quasi-freestanding bilayer graphene on
magnesium-terminated silicon carbide. Angle-resolved photoemission spectroscopy
reveals that upon magnesium intercalation, the single massless Dirac band of
epitaxial monolayer graphene is transformed into the characteristic massive
double-band Dirac spectrum of quasi-freestanding bilayer graphene. Analysis of
the spectrum using a simple tight binding model indicates that magnesium
intercalation results in an n-type doping of 2.1 $\times$ 10$^{14}$ cm$^{-2}$,
creates an extremely high displacement field of 2.6 V/nm, opening a
considerable gap of 0.36 eV at the Dirac point. This is further confirmed by
density-functional theory calculations for quasi-freestanding bilayer graphene
on magnesium-terminated silicon carbide, which show a similar doping level,
displacement field and bandgap. Finally, magnesium-intercalated samples are
surprisingly robust to ambient conditions; no significant changes in the
electronic structure are observed after 30 minutes exposure in air.
