Spontaneous Formation and Growth of a New Polytype on SiC(0001)

Using in situ electron microscopy, we have measured the structure of $\mathrm{SiC}(0001)\mathrm{\text{−}}4H$ during annealing in vacuum. Above $1000°\mathrm{C}$, an additional SiC bilayer forms on the surface that changes the polytype from hexagonal ($4H$) to cubic ($3C$). The interaction with surface steps prevents the cubic layer from growing thicker: the new phase does not wet the steps of the underlying $4H$ substrate. Instead, the cubic layer expands laterally, accelerating step bunching in the surrounding hexagonal regions. During SiC homoepitaxy, this lack of step edge wetting leads to the growth of $3C$ twins separated by deep grooves.

Figure 1

Side view of the $\mathrm{SiC}(0001)\mathrm{\text{−}}4H$ surface ($[11\overline{2}]$ projection). Si atoms are white and carbon atoms are black. Termination $S2$ has a horizontal twin boundary located two bilayers below the surface. Termination $S1$ has a twin boundary one layer below the surface. The cubic termination that forms at high temperature is indicated by $S3$. The notation follows [3]. TB indicates a vertical twin boundary that could form if $S1$ and $S3$ domains meet.

Figure 2

Sequence of 41 eV bright-field LEEM images of $\mathrm{SiC}(0001)\mathrm{\text{−}}(\sqrt{3}\times \sqrt{3})$ surface at $1150°\mathrm{C}$ in a background disilane pressure of $7\times {10}^{-7}\mathrm{Torr}$. The dark features have the cubic termination ($S3$). The bright areas have the hexagonal termination ($S2$). The images are labeled by the time of acquisition.

Figure 3

LEEM images of the $\mathrm{SiC}(0001)\mathrm{\text{−}}(\sqrt{3}\times \sqrt{3})$ surface at $1050°\mathrm{C}$. (a) 34 eV bright-field image with three cubic domains. (b) 14 eV dark-field image of the same area obtained using the ($\frac{2}{3}$, $\frac{1}{3}$) diffraction spot. (c) Same as (b) but using the ($-\frac{2}{3}$, $-\frac{1}{3}$) diffraction spot. Note the contrast reversal between images (b) and (c). A box indicates the location of one of the cubic domains.

Figure 4

(a) Bright-field LEEM image of $\mathrm{SiC}(0001)\mathrm{\text{−}}(3\times 3)$ at $\sim 900°\mathrm{C}$. The $S3$ regions appear dark and the $S2$ ones bright. (b) Dark-field LEEM image of the same area using a (0, $\frac{1}{3}$) spot. The contrast on neighboring terraces is reversed. (c) $2.5\times 2.5\mu \mathrm{m}$ ex situ AFM image of the same area. $S3$ are indicated by white dashed lines. (d) $1\times 1\mu \mathrm{m}$ AFM image of the region indicated by the dashed rectangle in (c). A trench separates the $S3$ from the uphill step. (e) AFM scan along the line indicated in (c). (f) AFM scan along the line indicated in (d), showing that the cubic layer is one bilayer higher than the terrace on which it grows. (g) AFM image after the growth of more than 25 nm of SiC via CVD showing a deep trench that forms between $3C$ twins. Inset shows the line scan over the trench.