The authors investigate the fluence and doping dependence of the surface photovoltage (SPV) shifts at SiO₂/Si(001) interfaces by time-resolved photoelectron spectroscopy. Charge carriers are excited by pumping photon energies of hν_pump = 1.2 and 2.4 eV and probed by high-order harmonics of hν_probe = 22.6 eV at 0.2 and 0.7 MHz repetition rates. The authors observe SPV shifts of the non-bonding O_2p state by 240 meV for SiO₂/p-Si and by -140 meV for SiO₂/n-Si upon pumping with hν_pump = 1.2 eV, and their decay rate is estimated from time-resolved measurements. Moreover, the authors observe a striking pumping fluence dependence of SPV at these interfaces, which indicates charge carrier generation by both linear and nonlinear optical excitations.