Strain, dislocations, and electrically active defects at and near the interface of Si/SiO2 and Si/GaP heterostructures are analyzed by optical second-harmonic spectroscopy. For plasma oxides deposited on Si(001) and Si(111), time-dependent second-harmonic experiments reveal that near-interface oxide defects trap charge by the tunneling of photoexcited electrons from the Si conduction band. The space-charge field-induced second-harmonic transients are resonantly enhanced by two-photon E(1) transitions in silicon. In GaP epilayers grown on Si(001) the bulk dipole-allowed electro-optical effect is suppressed by the formation of antiphase domains. In contrast, in Gap films grown on Si(111) and vicinal Si(001) the density of antiphase domains is considerably reduced yielding an enhancement of the second-order nonlinear optical response by two orders of magnitude.