In this paper a comparative study of the impact of N₂O and POCl₃ annealing on the SiO₂/SiC system is presented, combining nanoscale electrical characterization of SiC surface doping by scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) to the conventional capacitance-voltage (C-V) and current-voltage (I-V) measurements on MOS-based devices. A significant reduction of the interface states density (from 1.8×10¹² to 5.7×10¹¹ cm^-2eV^-1) and, correspondingly, an increase in the carrier mobility (from 19 to 108 cm²V^-1s^-1) was found moving from N₂O to POCl₃ annealing. Furthermore, SSRM measurements on bare p⁺-type SiC regions selectively exposed to N₂O and POCl₃ at high temperature provided the direct demonstration of the incorporation of N or P-related donors in the SiC surface, leading to a partial compensation of substrate acceptors during N₂O treatment and to an overcompensation during POCl₃ annealing. Finally, cross-sectional SCM profiles performed on epitaxial n-doped 4H-SiC with 45 nm SiO₂ (subjected to post deposition annealing in the two ambients) allowed to quantify the active donors concentrations associated to P or N incorporation under the gate oxide, showing almost a factor of ten higher doping (4.5×10¹⁸cm^-3 vs 5×10¹⁷cm^-3) in the case of P related donors.