Role of photo-assisted tunneling in time-dependent second-harmonic generation from Si surfaces with ultrathin oxides

Optical second-harmonic generation (SHG) from Si surfaces covered with nanometer-thin SiO2 varies with the laser exposure time because of photo-injection and charge trapping. We use UV lamp excitation to decompose the effects of photo-injection and charge trapping on the time-dependence of SHG of 1.65 eV photons at the Si/SiO2 interface. We find that the time-dependence of SHG in air arises mainly from the cooperative effect of three-photon photo-injection and charge trapping by surface O2. When the oxide is ultrathin (≤1.5 nm), the time-dependence also includes a significant contribution from one and two-photon photo-assisted tunneling.