Effects of MgO deposition on SiSi O2 system and charge carrier trapping and recombination in SiSi O2 MgO structures are studied using second-harmonic generation (SHG). An ultrafast 800 nm laser was used both for multi-photon induced electron injection through the Si O2 into a potential well in the MgO, and for monitoring the time-dependent SHG signal, which is sensitive to the electric field at the SiSi O2 interface. Our results indicate that the MgO deposition introduces new trap states, and electrons trapped in the MgO transport more readily through the Si O2 than those in traps on the surface of Si O2. We attribute this to differences in trap energy levels and/or differences in process damage-induced defect densities in the Si O2. © 2006 American Institute of Physics.