Effect of sample preparation on charged impurities in graphene substrates*

The mobility of graphene as fabricated on SiO$_{2}$ has been found to vary widely depending on sample preparation conditions. Additionally, graphene mobility on SiO$_{2}$ appears to be limited to $\sim$20,000 cm$^{2}$/Vs, likely due to charged impurities in the substrate. Here we present a study of the effect of fabrication procedures on substrate charged impurity density (n$_{imp}$) utilizing ultrahigh-vacuum Kelvin probe force microscopy. We conclude that even minimal SEM exposure, as from e-beam lithography, induces an increased impurity density, while heating reduces the number of charges for sample substrates which already exhibit a higher impurity density. We measure both SiO$_{2}$ and h-BN and find that all n$_{imp}$ values observed for SiO$_{2}$ are higher than those observed for h-BN; this is consistent with the observed improvement in mobility for graphene devices fabricated on h-BN over those fabricated on SiO$_{2}$ substrates.