Browsing by Author "Von Bergmann, H. M."

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    High-power excimer and CO2 TEA laser technology
    (Academy of Science for South Africa, 2005) Von Bergmann, H. M.
    This article reviews the physical background and design of high-power excimer and CO2 TEA lasers, including a detailed discussion of electrode structures and pre-ionization, gas circulation and acoustic wave management as well as high-voltage switching and magnetic pulse compression technology. Chemical problems associated with the operation of excimer and CO2 lasers, the life-times of various laser components, and operating costs are also discussed.
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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.
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    Reliability of IGBTs in solid-state pulsed power supplies for CO 2 TEA lasers
    (Academy of Science for South Africa, 2005) Stehmann, T.; Von Bergmann, H. M.
    The IGBT (insulated gate bipolar transistor) is a modern solid-state switch with wide applications in power electronics. Little information is, however, available on the performance and reliability of the switch in pulsed power supplies. Results of reliability tests conducted on a high-power, high-voltage IGBT are presented with special focus on applications in high-voltage laser pulsed power supplies. It was found that the IGBT could be reliably used in pulsed power supplies where the peak switching current exceeded the average current rating up to five times. Furthermore, the IGBT could be reliably connected in series to increase the switch operating voltage.
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    Second harmonic generation as a technique to probe buried interfaces
    (Academy of Science for South Africa, 2009) Neethling, P. H.; Scheldt, T.; Rohwer, E. G.; Von Bergmann, H. M.; Stafast, H.
    Since the advances of femtosecond laser technology during the last decade, optical second harmonic generation (SHG) has proven itself a powerful tool to investigate the electronic and structural properties of semiconductor materials. Its advantage lies in the fact that it is a contact-less, non-intrusive method that can be used in situ. It is sensitive to systems with broken symmetry, in particular interfaces and surfaces. The Si/SiO 2 system is technologically important since it forms a component of most modern electronic equipment. Furthermore, it has been shown that it is possible to induce an electric field across this Interface by means of laser irradiation as a result of defect formation and defect population. This electric field can be measured since it determines the SHG signal. The anisotropy of the SHG signal from the Sl/SiO 2 interface was measured and showed four-fold symmetry, illustrating that the SHG technique was able to characterise the electrical properties of the interface below the 5 nm thick oxide layer.