Electric field induced second harmonic measurements applying femtosecond laser pulses (1.59 eV, 80±5 fs, 80 MHz) to substantially boron doped p+-Si/SiO2 interfaces reveal a temporal evolution of the second harmonic (SH) signal, which differs drastically from that of weakly doped samples. A significant initial SH signal is observed in native p+-Si/SiO2 interfaces for boron doping concentrations ≫7.5×1017 cm-3. This SH signal is attributed to a built-in interfacial electric field E0 generated by the doping induced accumulation of charges at the Si/SiO2 interface following the ionization of interface defect states. A sign reversal is observed in the azimuthal SH anisotropy pattern of the initial SH signal relative to that of the saturated SH signal in p+-Si/SiO2 indicating that the doping related and electron induced interfacial field components oppose each other. Furthermore, the intensity dependence of the initial SH signal in p+-Si/SiO2 is found to be nonquadratic and, in particular, shows a nonmonotonic behavior at incident intensities above ∼60 GW/cm2. These observations are attributed to ultrafast screening of E0 caused by linearly as well as nonlinearly photoexcited electron-hole pairs.