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The mechanism of dc-electric-field-induced second-harmonic (EFISH) generation at weakly nonlinear buried Si (001)− SiO 2 interfaces is studied experimentally in planar Si (001)− SiO 2− Cr MOS structures by optical second-harmonic generation spectroscopy with a tunable Ti: sapphire femtosecond laser. The spectral dependence of the EFISH contribution near the direct two-photon E 1 transition of silicon is extracted. A systematic phenomenological model of the EFISH phenomenon, including a detailed description of the space-charge region (SCR) at the semiconductor-dielectric interface in accumulation, depletion, and inversion regimes, has been developed. The influence of surface quantization effects, interface states, charge traps in the oxide layer, doping concentration, and oxide thickness on nonlocal screening of the dc-electric field and on breaking of inversion symmetry in the SCR is considered. The …