  In this work, the nucleation and growth mechanism of aluminum oxide (Al2O3)
in the early stages of the direct atomic layer deposition (ALD) on monolayer
epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by
atomic force microscopy (AFM) and Raman spectroscopy. Contrary to what is
typically observed for other types of graphene, a large and uniform density of
nucleation sites was observed in the case of EG and ascribed to the presence of
the buffer layer at EG/SiC interface. The deposition process was characterized
by Al2O3 island growth in the very early stages, followed by the formation of a
continuous Al2O3 film (2.4 nm thick) after only 40 ALD cycles due to the
islands coalescence, and subsequent layer-by-layer growth. Raman spectroscopy
analyses showed low impact of the ALD process on the defects density and doping
of EG. The EG strain was also almost unaffected by the deposition in the regime
of island growth and coalescence, whereas a significant increase was observed
after the formation of a compact Al2O3 film. The obtained results can have
important implications for device applications of epitaxial graphene requiring
the integration of ultra-thin high-k insulators.
