  By combining scanning probe microscopy with Raman and x-ray photoelectron
spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a
function of the deposition temperature in close proximity to the Ge melting
point, highlighting an abrupt change of the graphene's quality, morphology,
electronic properties and growth mode at 930 degrees. We attribute this
discontinuity to the incomplete surface melting of the Ge substrate and show
how incomplete melting explains a variety of diverse and long-debated peculiar
features of the graphene/Ge(001), including the characteristic nanostructuring
of the Ge substrate induced by graphene overgrowth. We find that the
quasi-liquid Ge layer formed close to 930 degrees is fundamental to obtain
high-quality graphene, while a temperature decrease of 10 degrees already
results in a wrinkled and defective graphene film.
