Thermal decomposition of SiC shows a great potential for large-area production of graphene. However, the interaction of the formed monolayer graphene (MLG) with the substrate (via a buffer layer (BL)) reduces its carrier mobility in comparison to free-standing monolayer graphene. By thermally annealing a MLG/BL/SiC(0001) sample in air atmosphere, it has been previously shown that it is possible to decouple the BL from the SiC substrate by O intercalation, which leads to the formation of large area and high quality quasi-free-standing bilayer graphene (Oliveira Jr et al., Carbon 52 (2013) 83). The same result could not be obtained by annealing the samples in a pure O atmosphere. It seems that other species rather than only O play a role in the BL decoupling process.
In order to clarify this issue, we investigated the intercalation process in high-quality MLG grown on SiC(0001), which was prepared in an Ar atmosphere. The intercalation experiments consisted of thermal treatments with different proportions of N and O, at 600°C for 40 minutes. In addition, the effect of HO was probed by interposing a bubbler between the N gas flow and the reaction chamber. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the samples. Raman spectra of samples treated in a N+O ambient show that the BL-related features (at ~1300 cm-1) decrease in intensity but do not vanish for all tested O percentages. This indicates that only a partial decoupling of the BL is taking place. Nevertheless, after the introduction of HO in the system, the BL-features completely vanish for all annealing conditions. The BL decoupling (and bilayer graphene formation) is further confirmed by the changes observed for the double-resonance peak (2D) at 2700 cm-1, which offers a larger full-width half maximum (FWHM), and exhibits a characteristic bilayer graphene-related shape. Results obtained by XPS corroborate these findings.
Following annealings, the binding energy of components assigned to the BL (in Si 2p and C 1s core level spectra) decrease in comparison to that related to pristine MLG. In addition, a component associated to the SiC surface oxidation can be observed. Therefore, the present results points to a synergistic effect of HO and O, since a complete decoupling of the BL takes place only in the presence of both gases (annealings in N+HO ambient also promote partial BL decoupling). Considering that a thin silicon oxide layer is formed at BL/SiC interface already in the beginning of the thermal treatment, HO molecules will help in stabilizing further interactions between O and the oxidized SiC surface (Ryu et al., Nano Lett. 10 (2010) 4944), resulting in an acceleration of the intercalation process. Finally, we will also show results obtained from magneto transport measurements. They were conducted to investigate possible changes in the electrical properties which might be associated to the observed physico-chemical modifications.