  Despite the large literature focused on the growth of graphene (Gr) on
6H-SiC(0001) by chemical vapour deposition (CVD), some important issues have
not 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, whose proportion can be carefully controlled by tuning the H2 flow
rate. For the first time, we show that the growth of Gr using CVD under H2/Ar
flow rate proceeds in two stages. Firstly, the nucleation of free-standing
epitaxial graphene on hydrogen (H-Gr) occurs, then H-atoms eventually desorb
from either step edges or defects. This gives rise, for H2 flow rate below a
critical value, to the formation of (6x6)Gr domains on 6H-SiC(0001). The front
of H-desorption progresses proportionally to the reduction of H2. Using a
robust and generic X-ray photoelectron spectroscopy (XPS) analysis, we
realistically quantify the proportions of H-Gr and (6x6)Gr domains of a Gr film
synthetized in any experimental conditions. Scanning tunnelling 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 on the contrary to the method consisting of H-intercalation
below epitaxial Gr on buffer layer. These results are of crucial importance for
future applications of Gr/SiC(0001) in nanoelectronics, providing the
groundwork for the use of Gr as an optimal template layer for Van der Waals
homo- and hetero-epitaxy.
