Direct growth of quasi-free-standing epitaxial graphene on nonpolar SiC surfaces

During the graphitization of polar SiC(0001) surfaces through thermal decomposition, a strongly bound carbon-rich layer forms at the graphene/SiC interface. This layer is responsible for the system's high electron-doping and contributes to the degradation of the electrical properties of the overlying graphene. In this study, with the aid of photoelectron spectroscopy, low-energy electron microscopy, low-energy electron diffraction, and the density functional theory, we show that if the graphitization process starts from the nonpolar (11$\overline{2}$0) and (1$\overline{1}$00) surfaces instead, no buffer layer is formed. We correlate this direct growth of quasi-free-standing graphene over the substrate with the inhibited formation of tetrahedral bonds between the nonpolar surface and the carbon monolayer.

Figure 1

(a) Scheme of a 4H-SiC crystal, showing the low-index (0001), (11$\overline{2}$0), and (1$\overline{1}$00) crystallographic planes. (b) Graphene lattice, showing the hexagonal and the rectangular unit cell. A rectangular supercell can be defined on the basis of the number of its dimer lines $N_a$ and zigzag chains $N_z$. (c) Hexagonal and rectangularly folded graphene Brillouin zones.

Figure 2

$\mathrm{C}1s$ core level spectra of (a) graphene on 4H-SiC(11$\overline{2}$0), (b) graphene on 4H-SiC(1$\overline{1}$00), (c) quasi-free-standing monolayer graphene on H-terminated SiC(0001), (d) monolayer graphene on SiC(0001), and (e) the buffer layer on SiC(0001). The component denoted SiC is due to the emission of C atoms in the SiC substrate. $G$ marks the line is due to the graphene layer. The components $S1$ and $S2$ are characteristic for the buffer layer. All spectra are normalized to the same height of the SiC component and the curves are offset from each other for clarity. The vertical dashed line gives the $\mathrm{C}1s$ core level position of graphite.

Figure 3

(a) Macro-LEED image of graphene on 4H-SiC(11$\overline{2}$0) taken at 140 eV and (b)–(d) $\mu$-LEED images of graphene on 4H-SiC(1$\overline{1}$00) taken at 46 eV with a 1-$\mu$m aperture at various sample positions.

Figure 4

Band structure of graphene on 4H-SiC(11$\overline{2}$0) in the vicinity of the $K$-point measured in the $\overline{\text{KK}}$ direction with $k_y=1.7$ Å${}^{-1}$.

Figure 5

(a) LEEM bright field image at $E_{\text{LEEM}}=4.4$ eV of graphene on 4H-SiC(11$\overline{2}$0). (b) Typical reflectivity spectra offset from each other for clarity. (c) False color image of same area as in (a) colored corresponding to the spectra shown in (b). The length of the scale bar is 1 $\mu$m.

Figure 6

(a) LEEM bright field image at $E_{\text{LEEM}}=2.6$ eV of graphene on 4H-SiC(1$\overline{1}$00). (b) Typical reflectivity spectra offset from each other for clarity. (c) False color image of same area as in (a) colored corresponding to the spectra shown in (b). The length of the scale bar is 1 $\mu$m.

Figure 7

(a) Side view of the relaxed graphene/SiC(11$\overline{2}$0) interface. (b) Band structure of the graphene/SiC(11$\overline{2}$0) heterosystem, showing the contributions of the graphene (substrate) atoms as dark (bright) lines. (c) Valence charge electronic density (right) at the indicated lattice cut (left). Crosses show the positions of the interface atoms (C:$\times$, Si:$+$).

Figure 8

(a) Side view of the relaxed graphene/SiC(1$\overline{1}$00) interface. (b) Band structure of the graphene/SiC(1$\overline{1}$00) heterosystem, showing the contributions of the graphene (substrate) atoms as dark (bright) lines. (c) Detail of the graphene/SiC(1$\overline{1}$00) interface, indicating the structural parameters shown in Table . The red color corresponds to Si-C surface dimers with an excess of holes whereas the blue color indicates an excess of electrons.