  The deposition of dielectric materials on graphene is one of the bottlenecks
for unlocking the potential of graphene in electronic applications. In this
paper we demonstrate the plasma enhanced atomic layer deposition of 10 nm thin
high quality Al$_2$O$_3$ on graphene using a monolayer of hBN as protection
layer. Raman spectroscopy was performed to analyze possible structural changes
of the graphene lattice caused by the plasma deposition. The results show that
a monolayer of hBN in combination with an optimized deposition process can
effectively protect graphene from damage, while significant damage was observed
without an hBN layer. Electrical characterization of double gated graphene
field effect devices confirms that the graphene did not degrade during the
plasma deposition of Al$_2$O$_3$. The leakage current densities were
consistently below 1 nA/mm for electric fields across the insulators of up to 8
MV/cm, with irreversible breakdown happening above. Such breakdown electric
fields are typical for Al$_2$O$_3$ and can be seen as an indicator for high
quality dielectric films.
