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 metrology3 or as a substrate for van der Waals epitaxy of nitrides4 or 2D materials.5 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. It will also underline the interest of XPS to study hydrogenation mechanisms in the case of graphene growth on SiC.All our samples were studied with XPS and AFM. A first set of samples, consisting in graphene films grown with different hydrogen/argon ratio, allows to observe the formation of different graphene structures and interfaces with SiC, from disordered multilayer graphene on a hydrogenated interface to monolayer graphene on a buffer layer (typical spectra are presented in Fig. 1). XPS is particularly usefull to evidence the transition from hydrogenated to buffer layer interface. 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, leading to a better understanding of growth. In addition, the optimized cooling down allows to improve the quality of graphene films grown on 2'' wafers.