▎ 摘　　要

Despite the large body of literature reporting on the growth of graphene (Gr) on 6H-SiC(0001) by chemical vapor deposition (CVD), some important issues have not yet been solved, and full-wafer-scale epitaxy of Gr remains challenging, hampering applications in microelectronics. With this study, we shed light on the generic mechanism which produces the coexistence of two different types of Gr domains: Gr on hydrogen (H-Gr) and Gr on buffer layer ((6 x 6) Gr), whose proportion can be carefully controlled by tuning the H-2 flow rate. We show for the first time that the growth of Gr by CVD under a H-2/Ar flow rate proceeds in two stages. First, the nucleation of free-standing epitaxial Gr on hydrogen (H-Gr) occurs; then, H-atoms eventually desorb from either step edges or defects. This gives rise, for a H-2 flow rate below a critical value, to the formation of (6 x 6) Gr domains. The front of H-desorption progresses proportionally to the reduction of H-2. Using the robust and generic X-ray photoelectron spectroscopy (XPS) analysis, we realistically quantify the proportions of H-Gr and (6 x 6) Gr domains of a Gr film synthesized under any experimental conditions. Scanning tunneling microscopy supports the XPS measurements. From these results, we can deduce that the H-assisted CVD growth of Gr developed here is a unique method to grow fully free-standing H-Gr in contrast to the method consisting of H-intercalation below (6 x 6) Gr epitaxial layer. These results are of crucial importance for future applications of Gr/SiC(0001) in nano- and microelectronics and in particular for field-effect transistors, for which maximization of mobility is mandatory. This work also provides the groundwork for the use of Gr as an optimal template layer for van der Waals homo- and heteroepitaxy for optoelectronic applications.