  It is demonstrated that the electric dipole layer due to the overlapping of
electron wavefunctions at metal/graphene contact results in negative
Fermi-level pinning effect on the region of GaAs surface with low
interface-trap density in metal/graphene/n-GaAs(001) junction. The graphene
interlayer takes a role of diffusion barrier preventing the atomic intermixing
at interface and preserving the low interface-trap density region. The negative
Fermi-level pinning effect is supported by the Schottky barrier decreasing as
metal work-function increasing. Our work shows that the graphene interlayer can
invert the effective work-function of metal between $high$ and $low$, making it
possible to form both Schottky and Ohmic-like contacts with identical
(particularly $high$ work-function) metal electrodes on a semiconductor
substrate possessing low surface-state density.
