  Raman spectroscopy, a fast and nondestructive imaging method, can be used to
monitor the doping level in graphene devices. We fabricated chemical vapor
deposition (CVD) grown graphene on atomically flat hexagonal boron nitride
(hBN) flakes and SiO$_2$ substrates. We compared their Raman response as a
function of charge carrier density using an ion gel as a top gate. The G peak
position, 2D peak position, 2D peak width and the ratio of the 2D peak area to
the G peak area show a dependence on carrier density that differs for hBN
compared to SiO$_2$. Histograms of two-dimensional mapping are used to compare
the fluctuations in the Raman peak properties between the two substrates. The
hBN substrate has been found to produce fewer fluctuations at the same charge
density owing to its atomically flat surface and reduced charged impurities.
