Anisotropic growth of long isolated graphene ribbons on the C face of graphite-capped 6H-SiC

Using a graphite cap to cover the silicon carbide (SiC) sample, it is shown that large isolated graphene anisotropic ribbons can be grown on the C face of on-axis, semi-insulating, 6H-SiC wafers. The role of the cap is to modify the physics of the surface reconstruction process during Si sublimation, making more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of a strongly step-bunched morphology with, in between, long (up to 600 mu m) and large (up to 5 mu m) homogeneous monolayers of graphene ribbons. This is shown by optical and scanning electron microscopy, while a closer view is provided by atomic force microscopy (AFM). From Raman spectroscopy, it is shown that most of the ribbons are homogeneous monolayers or bilayers of graphene. It is also shown that most of the thermal stress between the graphene layer and the 6H-SiC substrate is relaxed by wrinkles. The wrinkles can be easily displaced by an AFM tip, which demonstrates evidence of graphene ironing at the nanoscale. Finally and despite the very low optical absorption of a single graphene layer, one shows that differential optical microtransmission can be combined to the micro-Raman analysis to confirm the monolayer character of the thinnest ribbons.