Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

TL;DR

This study demonstrates the direct growth of nanocrystalline graphene on AlGaN using catalyst-free CVD at high temperature, revealing its structural, chemical, and electrical properties relevant for electronic applications.

Contribution

It presents a novel method for growing uniform graphene on AlGaN without catalysts, and characterizes its nanoscale properties and interactions with the substrate.

Findings

Graphene was successfully grown with uniform coverage.

The graphene is nanocrystalline with a domain size of 7 nm.

The sheet resistance of graphene was measured at 15.8 kOhm/sq.

Abstract

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition (CVD) at temperature of 1350{\deg}C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy (XPS) revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a 2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS), which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size 7 nm) and compressively strained. A Gr sheet resistance of 15.8 kOhm/sq was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy (C-AFM) indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.