Band Engineering and Magnetic Doping of Epitaxial Graphene on SiC (0001)

Using calculations from first principles we show how specific interface modifications can lead to a fine-tuning of the doping and band alignment in epitaxial graphene on SiC. Upon different choices of dopants, we demonstrate that one can achieve a variation of the valence band offset between the graphene Dirac point and the valence band edge of SiC up to 1.5 eV. Finally, via appropriate magnetic doping one can induce a half-metallic behavior in the first graphene monolayer. These results clearly establish the potential for graphene utilization in innovative electronic and spintronic devices.

Figure 1

Side view of the optimized geometry of $2\times 2$ hexagonal unit cell of the graphene/C buffer layer/SiC (0001) system. Red (dark) color atoms are Si and the yellow (light) color are C atoms. The first graphene layer ($c_g$), buffer layer ($c_b$) and the last two SiC bilayers of the substrate are shown. The two types of Si atoms, ${\mathrm{Si}}_l$ and ${\mathrm{Si}}_b$ are indicated. In the passivated interface geometry, an additional atom is bonded to the ${\mathrm{Si}}_l$ atom in the interstitial region between SiC and the buffer layer. In the case of substitutional doping, ${\mathrm{Si}}_l$ is replaced by another atomic species.

Figure 2

The electronic band structures of the monolayer graphene and buffer layer on top of SiC substrate with (a) ${\mathrm{Si}}_l$ is not bonded to any chemical species (intrinsic), (b) ${\mathrm{Si}}_l$ is passivated with a H atom, and (c) ${\mathrm{Si}}_l$ atom is passivated with Li atom. The shaded area is the surface projected bulk band structure of SiC and all the energies are measured relative to the maximum of the valence band in bulk SiC.

Figure 3

The electronic band structures of the monolayer graphene and buffer layer on top of SiC substrate with ${\mathrm{Si}}_l$ atom is replaced by (a) B, (b) N, (c) Al, and (d) P. The valence band offsets ($\mathrm{B}\to 1.32\mathrm{eV}$, $\mathrm{N}\to 1.30\mathrm{eV}$, $\mathrm{Al}\to 0.06\mathrm{eV}$, $\mathrm{P}\to 1.44\mathrm{eV}$) show a clear variation upon surface chemistry. The surface chemistry also controls the charge transfer between the graphene layer and the substrate, as it is described in the text. The shaded area is the surface projected bulk band structure of SiC and all the energies are measured relative to the maximum of the valence band in bulk SiC.

Figure 4

Spin-polarized electronic band structures of the (a) majority and (b) minority spins of the monolayer graphene and buffer layer on top of SiC substrate with one of the C atoms in the ${\mathrm{C}}_b$ layer which is covalently bonded to ${\mathrm{Si}}_b$ atom, is replaced by a Mn atom. The system with Mn impurities in the buffer layer shows half-metallic behavior with a band gap opening of $\sim 80\mathrm{meV}$ for majority spins at the Dirac point.