The doping efficiency of lithium deposited at cryogenic temperatures on epitaxial and chemical vapor deposition monolayer graphene has been investigated under ultrahigh-vacuum conditions. Change of charge-carrier density was monitored by gate voltage shift of the Dirac point and by Hall measurements in low and high doping regimes. It was found that preannealing the graphene greatly enhanced the maximum levels of doping that could be achieved: doping saturated at $\mathrm{\Delta }n=2\times {10}^{13}{e}^{-}/{\mathrm{cm}}^2$ without annealing, independent of sample type or previous processing; after a 900 K anneal, the saturated doping rose one order of magnitude to $\mathrm{\Delta }n=2\times {10}^{14}{e}^{-}/{\mathrm{cm}}^2$.

• Received 19 September 2016

DOI:https://doi.org/10.1103/PhysRevB.94.201405