Epitaxial graphene on $6H–\mathrm{SiC}\left(0001\right)$: Defects in SiC investigated by STEM

The continuous improvement of the sublimation process of SiC allows using epitaxial graphene nowadays for quantum metrology. While it is known that the interface between graphene and the SiC surface is crucial for graphene's transport properties, almost no information about the composition of the SiC substrate after the sublimation process is available. In this study we present high resolution ${\mathrm{c}}_{\text{s}}$-corrected scanning transmission electron microscopy (STEM) experiments on $6H$-SiC(0001) samples after growth of graphene. A Si deficiency within the first three SiC bilayers was found by atomically resolved energy dispersive x-ray spectroscopy (EDX). The Si concentration within the first bilayer can be reduced up to 50%. In addition, as probed by electron energy loss spectroscopy, the hybridization state of C within the first five bilayers revealed a ${\mathrm{sp}}^2$ contribution, which we refer to as the precipitation of small carbon clusters. Our analysis clearly shows that the electronic interface of epitaxial graphene on $6H$-SiC(0001) is not atomically sharp.

Figure 1

STEM-HAADF image of an epitaxial bilayer graphene on $6H$-SiC(0001). The distances between the carbon layers and to the substrate are denoted. The layers are labeled with ${\mathrm{G}}_{\mathrm{n}}$ for graphene, $\mathrm{B}$ for buffer layer, and ${\mathrm{S}}_{\mathrm{n}}$ for substrate bilayers counted from the interface. The averaged HAADF signal is shown to the right. The envelope shows an increased HAADF intensity of the five bilayers closest to the interface. The depth scale is given with respect to the topmost SiC bilayer.

Figure 2

(a) DF field image of monolayer graphene on $6H$-SiC(0001). (b) Differential phase contrast (iDPC) image of the same sample revealing a better signal to noise ratio compared to the DF image. (c) Characteristic EDX overview spectrum revealing the elements from the sample and sample holder. (d) EELS spectra, being averaged over small horizontal stripes of same color, as marked in (a). The spectra in (c) and (d) were deduced from the spectrum images taken simultaneously with the DF image in (a).

Figure 3

EDX and EELS analysis of the interface area. (a) Differential phase contrast (iDPC) image of the region of interest. (b) EDX data: net intensities of the silicon and carbon EDX peaks together with a line scan of the DF intensity of (a); (c) EELS data: MLLS coefficients for C-K edge (comprising the ${\mathrm{sp}}^3$ hybridized SiC-type and the ${\mathrm{sp}}^2$ hybridized graphene type) and the integrated Si-L edge intensity. All signals were recorded simultaneously during the scan. Line scans were generated by horizontal averaging of the original 2D spectrum image.

Figure 4

Thickness/IMFP plot across the graphene/SiC interface. For better orientation, the background shows the DPC image of the region of interest.

Figure 5

Structural analysis of the TEM lamella fabricated by FIB. (a) Colored SEM image of the lamella. (b) Thickness versus position across the interface deduced from SEM/EDX measurements [41]. (c) AFM profile taken also across the TEM lamella.

Figure 6

(a) Mass densities as a function of $x$ in ${\mathrm{Si}}_x\mathrm{C}$ for three different microstructure models: remaining vacancies on the vacation of Si-sites (gray), replacement of the vacancies by C atoms, phase separation between SiC and graphite-like inclusions (green). (b) Calculated inelastic mean free path $\lambda$ for 80 keV electrons in ${\mathrm{Si}}_x\mathrm{C}$ as a function of the Si fraction $x$.

Figure 7

The depth dependent Si fraction change derived from the IMFP calculation, Si-EDX net intensity, and integral Si-L edge. The depth scale is given with respect to the topmost SiC bilayer.