Scanning capacitance microscopy (SCM) is used to probe on nanoscale the capacitive behavior of graphene deposited on a SiO2/Si n(+) substrate (with SiO2 thickness of 300 or 100 nm). The SCM tip provides the contact on graphene, while the Si n(+) substrate acts as the backgate contact in the graphene/SiO2/Si capacitor. The authors studied the screening by the graphene two-dimensional electron gas on the modulating potential applied between the backgate and the SCM tip. In particular, they determined the effect of the oxide thickness on the lateral distribution (i.e., screening length) and the density of the screening charge in graphene. Experimental results indicate that thinner oxide leads to higher screening charge density in the graphene sheet and to higher effectively biased area on graphene. This correspondingly increases the total capacitance (C-tot) of the graphene/SiO2/Si capacitor. We evaluated the dependence on the dielectric thickness of the classical metal-oxide-semiconductor (MOS) capacitance (C-MOS) and of the quantum capacitance (C-q) contributions to C-tot. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3081890]