Pit formation during graphene synthesis on SiC(0001): In situ electron microscopy

We have studied the formation of graphene on the Si face of SiC(0001)-6H and -4H using in situ electron microscopy. By imaging the nucleation and growth of the $6\sqrt{3}$ “buffer layer” during annealing in vacuum we identify key factors responsible for the appearance of deep pits during graphene formation. Pits form because domains of the buffer layer pin decomposing surface steps. Graphene is observed to nucleate in the pits, where the step density is high.

Figure 1

Nucleation of the $6\sqrt{3}$ buffer layer on $\text{SiC}\left(0001\right)\text{−}6\text{H}-\sqrt{3}\times \sqrt{3}$ at $1060°\text{C}$. (a) AFM image of the surface before nucleation with 0.8 nm high steps. (b) 18 eV bright field LEEM image recorded after nucleation of the buffer layer at $1060°\text{C}$. The buffer layer is bright and the $\sqrt{3}$ phase is dark. (c) AFM image from the same surface showing islands, ribbons at the lower sides of the steps, and etched step edges. The arrows indicate buffer layer domains that have nucleated on the terrace.

Figure 2

(a) 5.3 eV bright field LEEM image at $1140°\text{C}$ of large buffer layer domains (bright). (b) AFM image from the same surface illustrating the flow of $\sqrt{3}$ steps around the buffer layer (arrows). (c) AFM phase image from the right of (b) showing contrast between buffer layer (dark) and $\sqrt{3}$ (bright). (d) Line scan from the dotted line indicated in (b). Tall steps (left) are 1.0 nm and short steps (right) are 0.2 nm. The schematic starting profile is shown in gray.

Figure 3

Model of $6\sqrt{3}$ growth on $\text{SiC}\left(0001\right)-\sqrt{3}$. (a) The buffer layer (gray) nucleates at the lower sides of $\sqrt{3}$ steps. (b) $\sqrt{3}$ steps retract uphill as the buffer layer grows. (c) Eventually a state is reached in which all $\sqrt{3}$ steps have been consumed, and with the buffer layer covering roughly half of the surface. (d) Canyons or pits form when the domains of the buffer layer on each terrace are not continuous and $\sqrt{3}$ steps (thick black lines) advance to the next terrace.

Figure 4

(a) 3 eV bright field LEEM image recorded at $1150°\text{C}$ before the completion of the buffer layer. (b) AFM image after heating to $1250°\text{C}$. The canyons marked $A$, $B$, and $C$ are 6 nm deep (or 8 SiC bilayers). Vacancy islands have nucleated in the bottoms of the canyons, increasing their depth. (d) 19 eV bright field LEEM image recorded after graphene formation at $1160°\text{C}$. Graphene is bright in the image and nucleates preferentially in areas of high step density, e.g., in the canyons $\left(D\right)$.