  By using four layered graphene/gallium nitride (GaN) Schottky diodes with an
undoped GaN spacer, we demonstrate highly effective gating of graphene at low
bias rendering this type of structure very promising for potential
applications. An observed Raman G band position shift larger than 8.5 cm-1
corresponds to an increase in carrier concentration of about 1.2x10^13 cm-2.
The presence of a distinct G band splitting together with a narrow symmetric 2D
band indicates turbostratic layer stacking and suggests the presence of a high
potential gradient near the Schottky junction even at zero bias. The subbands
characterized by the highest Raman energies correspond to the largest
concentration of electrons. An analysis based on electroreflectance
measurements and a modified Richardson equation confirmed that graphene on
n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse
bias. At least 60% of G subband position shifts occur at forward bias, which is
related to a rapid reduction of electric field near the Schottky junction.
Raman micromapping shows a high uniformity of gating efficiency on the surface.
Therefore, our studies demonstrate the usefulness of few layer turbostratic
graphene deposited on GaN for tracing electron-phonon coupling in graphene.
Multilayer graphene also provides uniform and stable electric contacts.
Moreover, the observed bias sensitive G band splitting can be used as an
indicator of charge transfer in sensor applications in the low bias regime.
