  Monolayer epitaxial graphene (EG) grown on hexagonal Si-terminated SiC
substrates is intrinsically electron-doped (carrier density is about 10^13
cm^(-2)). We demonstrate a clean device fabrication process using a
precious-metal protective layer, and show that etching with aqua regia results
in p-type (hole) molecular doping of our un-gated, contamination-free EG.
Devices fabricated by this simple process can reach a carrier density in the
range of 10^10 cm^(-2) to 10^11 cm^(-2) with mobility about 8000 cm^2/V/s or
higher. In a moderately doped device with a carrier density n = 2.4 x 10^11
cm^(-2) and mobility = 5200 cm^2/V/s, we observe highly developed quantized
Hall resistance plateaus with filing factor of 2 at magnetic field strengths of
less than 4 T. Doping concentrations can be restored to higher levels by heat
treatment in Ar, while devices with both p-type and n-type majority carriers
tend to drift toward lower carrier concentrations in ambient air.
