  We have determined the growth mode of graphene on SiC(0001) and SiC(000-1)
using ultra-thin, isotopically-labeled Si13C `marker layers' grown epitaxially
on the Si12C surfaces. Few-layer graphene overlayers were formed via thermal
decomposition at elevated temperature. For both surface terminations (Si-face
and C-face), we find that the 13C is located mainly in the outermost graphene
layers, indicating that, during decomposition, new graphene layers form
underneath existing ones.
