We report the dc and rf performance of graphene rf field-effect transistors,where the graphene films are grown on copper by using the chemical vapour deposition (CVD) method and transferred to SiO2/Si substrates.Composite materials,benzocyclobutene and atomic layer deposition Al2O3 are used as the gate dielectrics.The observation of n- and p-type transitions verifies the ambipolar characteristics in the graphene layers.While the intrinsic carrier mobility of CVD graphene is extracted to be 1200cm2/V·s,the parasitic series resistances are demonstrated to have a serious impact on device performance.With a gate length of 1 μm and an extrinsic transconductance of 72 mS/mm,a cutoff frequency of 6.6 GHz and a maximum oscillation frequency of 8.8 GHz are measured for the transistors,illustrating the potential of the CVD graphene for rf applications.
The carrier doping effects on the magnetic properties of defective graphene with a hydrogen chemisorbed single-atom vacancy(H-GSV)are investigated by performing extensive spin-polarized first-principles calculations.Theoretical results show that the quasi-localized pz-derived states around the Fermi level are responsible for the weakened magnetic moment(MM)and magnetic stabilized energy(MSE)of the H-GSV under carrier doping.The mechanism of reduced MSE in the carrier doped H-GSV can be well understood by the Heisenberg magnetic coupling model due to the response of these p_(z)-derived states to the carrier doping.Within the examined range of carrier doping concentration,the total MM of H-GSV is always larger than 1.0μ_(B) with μ_(B) representing the Bohr magneton,which is mainly contributed by the localized sp^(2) states of the unsaturated C atom around the vacancy.These findings of H-GSV provide fundamental insight into defective graphene and help to understand the related experimental observations.
