Graphene growth mechanisms during propane/hydrogen CVD on SiC

Propane/hydrogen CVD growth of graphene on SiC, studied since 2010 [1], consists simply to grow graphene from propane in a hydrogen/argon atmosphere. The presence of hydrogen in the gas phase promotes Si excess on the surface, hence making impossible graphene growth without propane flow [2]. This makes propane/hydrogen CVD very different from silicon sublimation where graphene grows from a carbon excess on SiC. Graphene films are mainly grown in a propane/hydrogen/argon gas mixture at high temperature (1550°C) near atmospheric pressure, conditions allowing to grow uniform n-doped monolayers on 2" SiC wafers. Graphene films prepared in such conditions have been widely used for applications in electrical metrology [3] or as a substrate for van der Waals epitaxy [4]. Though, a complete growth study for these specific growth conditions was still missing. Our contribution will present first elements of this study and discuss the growth and hydrogenation mechanisms occurring both during growth step and cooling down.In order to study the different steps of graphene formation, we have grown samples with different growth time in conditions leading to the formation of a buffer layer interface. Surprisingly, incomplete graphene layers presented hydrogenated interfaces, suggesting hydrogenation of the interface during cooling down. This led us to optimize the cooling down to minimize changes in graphene interface during this last step. The new set of graphene samples with different growth time allows to observe the different steps of graphene formation (figure) : first, the rapid (less than 1 minute) formation of a buffer layer fully covering the SiC and the nucleation of graphene ribbons and islands, followed by their lateral growth leading to a coalescence in about 10 minutes. Surprisingly, the growth of the graphene monolayer is self-limited, so that no increase in graphene coverage is observed after 30 minutes of growth. This self-limitation is attributed to a thermodynamical equilibrium [5]. Finally, these self-limited conditions are used to grow graphene films on 2'' wafers. Their uniformity and reproducibility appear to be mainly limited by the local offcut of the SiC substrates.