  We demonstrate an all-epitaxial and scalable growth approach to fabricate
single-crystalline GaN nanowires on graphene by plasma-assisted molecular beam
epitaxy. As substrate, we explore several types of epitaxial graphene layer
structures synthesized on SiC. The different structures differ mainly in their
total number of graphene layers. Because graphene is found to be etched under
active N exposure, the direct growth of GaN nanowires on graphene is only
achieved on multilayer graphene structures. The analysis of the nanowire
ensembles prepared on multilayer graphene by Raman spectroscopy and
transmission electron microscopy reveals the presence of graphene underneath as
well as in between nanowires, as desired for the use of this material as
contact layer in nanowire-based devices. The nanowires nucleate preferentially
at step edges, are vertical, well aligned, epitaxial, and of comparable
structural quality as similar structures fabricated on conventional substrates.
