  Quantum Hall effect (QHE) devices based on epitaxial graphene films grown on
SiC were fabricated and studied for development of the QHE resistance standard.
The graphene-metal contacting area in the Hall devices has been improved and
fabricated using a double metalization process. The tested devices had an
initial carrier concentration of (0.6 - 10)*10^11 1/cm^2 and showed
half-integer quantum Hall effect at a relatively low (3 T) magnetic field.
Application of the photochemical gating method and annealing of the sample
provides a convenient way for tuning the carrier density to the optimum value.
Precision measurements of the quantum Hall resistance (QHR) in graphene and
GaAs devices at moderate magnetic field strengths (<7 T) showed a relative
agreement within 6*10^-9.
