  We report a novel fabrication method of graphene Al2O3 gate insulators based
on sputtering. Electrical performance of dual-gated mono- and bilayer
exfoliated graphene devices is presented. Sputtered Al2O3 layers possess
comparable quality to oxides obtained by atomic layer deposition (ALD) with
respect to a high relative dielectric constant of about 8, as well as
low-hysteresis performance and high breakdown voltage. We observe a moderate
carrier mobility of about 1000 cm2/Vs in graphene and 350 cm2/Vs in its bilayer
due to increased resonant scattering on atomic scale defects. Most likely this
originated from the thin Al precursor layer evaporated prior to sputtering the
Al2O3 gate oxide.
