Highly effective gating of graphene on GaN

TL;DR

This paper demonstrates highly effective low-bias gating of graphene on GaN using Schottky diodes, revealing insights into electron transfer, carrier concentration, and potential applications in sensors and electronics.

Contribution

It introduces a novel graphene/GaN Schottky diode structure with enhanced gating efficiency and detailed Raman analysis of charge transfer and stacking effects.

Findings

G Raman G band shift >8.5 cm-1 indicates increased carrier concentration.

Graphene on GaN behaves like a capacitor at reverse bias.

High uniformity of gating efficiency across the surface.

Abstract

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.