Interface structure of epitaxial graphene grown on 4H-SiC(0001)

We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer ($\sim 1.7\text{Å}$ compared to $0.63\text{Å}$). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the $\left(000\overline{1}\right)$ surface, as predicted previously by ab initio calculations.

Figure 1

Schematic model of multilayer graphene grown on the 4H-SiC(0001) substrate. Dashed lines are the bulk SiC lattice planes before interface relaxation ($\Delta$’s). The fifth plane of atoms (adatom) is displaced $d_{\text{ad}}$ from the topmost atom plane in the interface. (●) is carbon atoms and (○) is silicon atoms. The shaded circles in the interface (layer-0) can be either carbon or silicon atoms. The graphene layers above the interface layer are referred to as layer-1, -2, -3, etc.

Figure 2

(Color online) Specular reflectivity versus $q_{\perp }$ (in r.l.u.) for a graphitized 4H-SiC(0001) Si-face surface. Circles are the data. Fits to the two model structures in the figure are given. In the schematic models filled and open circles are C and Si atoms, respectively. Shaded circles are Si adatoms. Dotted blue line is a fit to a bulk-terminated SiC(0001) surface with a single relaxed bilayer. Solid red line is a fit to a model similar to the relaxed bilayer but with the addition of a layer of Si adatoms with ${\rho }_{\text{ad}}=0.21$.

Figure 3

Three graphene/SiC interface ball models for graphene grown on the Si face of SiC determined by surface x-ray reflectivity; (b) Si-up model, (c) Si-down model, and (d) C-adatom model. Open circles are silicon atoms and shaded circles are carbon atoms. The densities $\rho$ are relative to the densities of bulk SiC shown in (a).

Figure 4

(Color online) Specular reflectivity versus $q_{\perp }$ (in r.l.u.) for a graphitized 4H-SiC(0001) Si-face surface. Circles are the data. Solid black line is the best fit to the Si-up model in Fig. 3b with $D_0=2.31\text{Å}$ (the C-adatom and Si-down model fits would be indistinguishable). Dashed red line is the same fit but with a smaller $D_0=2.13\text{Å}$. Dotted blue line is the same fit but with a larger $D_0=2.53\text{Å}$.

Figure 5

(Color online) Specular reflectivity versus $q_{\perp }$ (in r.l.u.) for a graphitized 4H-SiC(0001) Si-face surface. Circles are the data. Solid black line is the best fit to the Si-up model with ${\sigma }_{\text{G}}=0.16\text{Å}$. Dashed red line is the same fit but with ${\sigma }_{\text{G}}=0.30\text{Å}$. Dotted blue line is the same fit but with a larger ${\sigma }_{\text{G}}=0.0\text{Å}$.

Figure 6

(Color online) The normalized probability $p_n$ of a $n$-graphene layer stack from a UHV-grown Si-face film as determined by x-ray reflectivity. The x-ray average is $1.9\pm 1.5$ graphene layers while the AES estimate of the average is approximately one layer thicker (3.2 layers).