Toward a Mechanistic Understanding of the Formation of 2D-GaN x in Epitaxial Graphene.

Ultrathin 2D-GaN can be formed by Ga intercalation into epitaxial graphene (EG) on SiC followed by nitridation in ammonia. Defects in the graphene provide routes for intercalation, but the nature and role of the defects have remained elusive. Here we examine the influence of graphene layer thickness and chemical functionalization on Ga intercalation and 2D-GaN formation using a combination of experimental and theoretical studies. Thin buffer layer regions of graphene near steps on SiC readily undergo oxygen functionalization when exposed to air or a He/O plasma in contrast to thicker regions which are not chemically modified. Oxygen functionalization is found to inhibit Ga intercalation leading to accumulation of Ga droplets on the surface. In contrast, Ga readily intercalates between EG and SiC in the thicker graphene regions that do not contain oxygen. When NH annealing is carried out immediately after Ga exposure, 2D-GaN formation is observed only in the oxygen-functionalized regions, and Ga intercalated under thicker nonfunctionalized graphene does not convert to GaN. Density functional theory calculations demonstrate that oxygen functionalization of graphene alters the binding energy of Ga and NH species to the graphene surface. The presence of hydroxyl groups on graphene inhibits binding of Ga to the surface; however, it enhances the chemical reactivity of the graphene surface to NH which, in turn, enhances Ga binding and facilitates the formation of 2D-GaN. By modifying the EG process to produce oxygen-functionalized buffer layer graphene, uniformly intercalated 2D-GaN is obtained across the entire substrate surface.