Use of high numerical aperture focusing with negative longitudinal spherical aberration is shown to enable deep (> microm), high aspect ratio, nano-scale-width holes to be machined into the surface of a fused-silica (SiO(2)) substrate with single pulses from a 200 fs, 4 microJ Ti-Sapphire laser source. The depths of the nano-holes are characterized by use of a non-destructive acetate replication technique and are confirmed by imaging of sectioned samples with a dual focused ion beam/scanning electron microscope.
Charge generation, transport, and recombination processes in UNIBOND® silicon-on-insulator wafers are studied via an optical second-harmonic generation (SHG) technique. The electric fields at the interfaces vary with time due to charge trapping. The presence of a thin native oxide layer on the top Si film contributes significantly to the SH intensity due to the strong time-dependent electric field generated by electrons transported to the surface. For the thick buried oxide, the electric field is primarily due to carrier trapping at the interface, and it varies with time weakly. The SHG signals depend strongly on the externally applied electric field, which can differentiate the contribution of each interface to the total SH signal.
Effects of MgO deposition on Si∕SiO2 system and charge carrier trapping and recombination in Si∕SiO2∕MgO structures are studied using second-harmonic generation (SHG). An ultrafast 800nm laser was used both for multi-photon induced electron injection through the SiO2 into a potential well in the MgO, and for monitoring the time-dependent SHG signal, which is sensitive to the electric field at the Si∕SiO2 interface. Our results indicate that the MgO deposition introduces new trap states, and electrons trapped in the MgO transport more readily through the SiO2 than those in traps on the surface of SiO2. We attribute this to differences in trap energy levels and/or differences in process damage-induced defect densities in the SiO2.
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