7) to search for signs of carbon surface poisoning on the r-B4C films. This analysis reveals free carbon on the surface of the r-B4C films grown only for the C-face. We suggest that carbon accumulation on the growth surface hampers the growth of epitaxial r-B4C films. The polycrystalline r-B4C grown on the Si-face is continuous. We speculate that the absence of surface carbon on the film grown on the Si-face (Fig. ) is due to the surface morphology of that film where the highly faceted surface allows migration and subsequent desorption of hydrocarbon species. The difference in film thickness can also be attributed to a faster polycrystalline film growth rate, as explained by a competitive growth mode. Competitive growth mode is often encountered in thin film growth and is where the dominant growth of crystallites is determined by the fastest-growing crystal facets. In contrast, epitaxial growth occurs only in one crystal orientation. In addition, Raman spectroscopy shows that carbon accumulates on the surface of the epitaxial r-B4C films. Thus, the resulting difference in film thickness could be a combination of carbon accumulation and the difference in growth modes.