Negative Fermi-level Pinning Effect Observed in Metal/GaAs Junction with Graphene Insertion Layer*

We report the direct observation revealing that the electric dipole layer due to the chemical interaction at metal/graphene interface and the doping of graphene can induce the negative Fermi-level pinning effect in metal/graphene/n-GaAs(001) junction made on a GaAs substrate containing regions with low interface-trap density in combinational manners. The graphene insertion layer takes a role of diffusion barrier preventing the atomic intermixing at interface and preserving the low interface-trap density region. The change of electrostatic potential across the metal/graphene interface due to the chemical interaction dipole layer and the doping of graphene is found to cause the negative Fermi-level pinning effect, supported by the Schottky barrier decreasing as metal work-function increasing. The low Schottky barrier patches with very small total areal fraction are considered to serve as preferred paths for electron transport through metal/graphene/n-GaAs(001) junctions. This work provides an experimental method to form Schottky contacts (metal/GaAs) and Ohmic contacts (metal/graphene/GaAs) simultaneously on a GaAs substrate covered partially with graphene by using identical metal electrodes.