Browsing by Author "Scheidt, T."

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    Femtosecond laser diagnostics of thin films, surfaces and interfaces
    (Academy of Science for South Africa, 2005) Scheidt, T.; Rohwer, E. G.; Bergmann, H. M. V.; Stafast, H.
    An overview is given of optical second harmonic generation (SHG) using femtosecond laser pulses to analyse technologically important crystalline materials. The principle of SHG is briefly explained and a typical experimental setup for SHG is outlined. The second harmonic (SH) measurements performed in prototype compound semiconductors reveal the crystalline structure and orientation of monocrystalline SiC, polycrystalline ZnO, and the ternary compound, PbxCd1-xTe, showing the segregation of a Pb-and a Cd-rich phase. Furthermore, SHG can selectively probe the Si/SiO2 interface and the build-up of its electric field induced by laser-activated charge carrier separation. The examples presented demonstrate that SHG is a versatile technique to probe the structural and electronic properties of crystalline materials and particularly surfaces and interfaces.
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    Optical second harmonic generation as a probe for internal electric fields at the Si/SiO₂ interface
    (Academy of Science for South Africa, 2005) Scheidt, T.; Rohwer, E. G.; Von Bergmann, H. M.
    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 ∼45 GW/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.