Our result suggests that a growth chemistry that is more capable of removing carbon is necessary for epitaxial r-B4C film growth in this process. These experimental results cannot explain why r-B4C grows epitaxially on the C-face and polycrystalline on the Si-face. We speculate that the atomistic understanding of this is likely in the initial chemical bonds formed between the surface atoms and the r-B4C and hope that these results spur interest to model the interface by quantum chemical modelling.