Charge neutrality in epitaxial graphene on $6H$-SiC(0001) via nitrogen intercalation

The electronic properties of epitaxial graphene grown on SiC(0001) are known to be impaired relative to those of freestanding graphene. This is due to the formation of a carbon buffer layer between the graphene layers and the substrate, which causes the graphene layers to become strongly $n$-doped. Charge neutrality can be achieved by completely passivating the dangling bonds of the clean SiC surface using atomic intercalation. So far, only one element, hydrogen, has been identified as a promising candidate. We show, using first-principles density functional calculations, how it can also be accomplished via the growth of a thin layer of silicon nitride on the SiC surface. The subsequently grown graphene layers display the electronic properties associated with charge neutral graphene. We show that the surface energy of this structure is considerably lower than that of others with intercalated atomic nitrogen and determine how its stability depends on the ${\mathrm{N}}_2$ chemical potential.

Figure 1

Electronic band structure of (a) the 0LG/SiC(0001) system, (b) the 1LG/SiC(0001) system, and (c) the 2LG/SiC(0001) system.

Figure 2

(a) Relaxed structure of the 2LG/Si-N/SiC(0001) system showing optimized interlayer distances. The dark blue spheres represent Si, the light gray spheres represent N, while the brown spheres represent carbon. Electronic band structure of the (b) 0LG/Si-N/SiC(0001), (c) 1LG/Si-N/SiC(0001), and (d) 2LG/Si-N/SiC(0001) systems.

Figure 3

Electronic structure of the 0LG/SiC(0001) structure including (left) $1/3$ ML and (right) 1 ML nitrogen atoms at the interface. Note the in-plane unit cell of the 1 ML structure is four times that of the $1/3$ ML structure.

Figure 4

The surface free energies of the intercalated N systems relative to the clean surface which includes only the 0LG, as a function of the ${\mathrm{N}}_2$ chemical potential. The relationship between the chemical potential and the ${\mathrm{N}}_2$ pressure is shown on the top axis for two fixed temperatures, 500 K and 1000 K.