  The doping efficiency of lithium deposited at cryogenic temperatures on
epitaxial and CVD monolayer graphene has been investigated under ultra-high
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 pre-annealing the graphene greatly enhanced
the maximum levels of doping that could be achieved: doping saturated at
$\Delta n = 2\times 10^{13}$ e$^-$/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 $\Delta n = 2\times 10^{14}$ e$^-$/cm$^2$.
