In this Letter, we report a quantitative analysis of the n-type doping occurring at SiO2/4H-SiC interfaces during post-deposition-annealing (PDA) in N2O or POCl3 of a 45 nm thick oxide. In particular, a nanoscale characterization using scanning capacitance microscopy on the cross section of metal-oxide-semiconductor capacitors allowed to determine the electrically active nitrogen and phosphorous concentration under the SiO2 layer after PDA in N2O and POCl3, i.e., 5 × 1017 cm−3 and 4.5 × 1018 cm−3, respectively. The technological implications have been discussed considering the possible impact of a PDA-induced “counter doping” of the p-type body region of a n-channel metal-oxide-semiconductor-field-effect-transistor on the device threshold voltage.