Charge-carrier dynamics and trap generation in native Si/SiO2 interfaces probed by optical second-harmonic generation -: art. no. 165314

Employing femtosecond laser pulses [1.59 eV, (80+/-5) fs, 80 MHz], we perform time-dependent second-harmonic (SH) measurements to probe the dynamics of charge-carrier transfer processes across native Si/SiO2 interfaces in a new peak intensity regime up to 100 GW/cm(2). The SH signal increases steadily during irradiation up to 45 GW/cm(2) laser peak intensity, but changes its temporal evolution above 45 GW/cm(2): as a different phenomenon, the SH signal surpasses a maximum, decreases slowly, and appears to approach a steady state level. We assign this observed signal decrease to the light induced injection of holes into the SiO2 thin layers involving a four-photon process, which opposes the electron contributions (three-photon process). Furthermore, we investigate the SH signal development after irradiation interrupts and find that preirradiated samples show a drastically accelerated SH response. Both, the electron and the hole injection processes appear to be about two orders of magnitude faster in preirradiated than in "virgin" samples. We assign this acceleration to the laser induced generation of permanent defects, which affect electron as well as hole trapping in the SiO2 layers. In addition the SH signal amplitude is shown to be a function of the dark period duration, and the typical time constant for the depopulation of hole trap sites under dark conditions has been extracted and amounts to (110+/-15) s. We present an empirical model involving four exponential functions, taking into account the electron and hole contributions to the time-dependent SH response. This model fully reproduces the recorded experimental data. It supports our interpretation of opposing three- and four-photon processes being involved in electron and hole injection, respectively. Finally, we employ SH imaging in comparison with scanning electron microscopy to visualize sample areas with laser defects.