Optical second harmonic generation as a probe for internal electric fields at the Si / SiO2 interface

The second harmonic signal generated in native Si / SiO2 interfaces using femtosecond laser pulses (80 + 5 fs, 10.5 nJ, 1.59 eV) is time dependent. The temporal evolution of the electric field-induced second harmonic signal (EFISH) shows a steady incline and subsequent saturation for incident laser peak intensities below ~45GW/cm2 due to electron injection and trapping as well as defect generation in the SiO2 layer. We used second harmonic generation imaging to visualize defect sample areas. In contrast, a drastically different behaviour of the time-dependent SH response of Si / SiO2 interfaces was observed for peak intensities above ~45 GW/cm2. The SH signal rose to a maximum and showed a subsequent decline over many minutes. We suggest hole injection into the ultra-thin SiO2 layer as an interpretation of this observation.