Browsing by Author "Stafast, H."

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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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    First comparison of electric field induced second harmonic of NIR femtosecond laser pulses in reflection and transmission generated from Si/SiO2 interfaces of a silicon membrane
    (Springer, 2011) Nyamuda, G. P.; Rohwer, E. G.; Steenkamp, C. M.; Stafast, H.
    For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ=800 nm, τ=75±5 fs, rep. rate=80 MHz, Epulse≤10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10-μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO2 interfaces, respectively, with SiO2 being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO2 interfaces caused by charge-carrier injection from Si into the oxide. The z-scan results and time-dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation. © 2011 Springer-Verlag.
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    Free charge carrier absorption in silicon at 800 nm
    (Springer, 2016) Heisel, P.-C.; Ndebeka, W. I.; Neethling, P. H.; Paa, W.; Rohwer, E. G.; Steenkamp, C. M.; Stafast, H.
    The transmission of a Ti:sapphire laser beam (c.w. and fs pulsed operation at 800 nm) through a 10-μm-thin oxidized silicon membrane at 45° angle of incidence at first increases with the incident laser power, then shows a maximum, and finally decreases considerably. This nonlinear transmission behavior is the same for c.w. and pulsed laser operation and mainly attributed to free charge carrier absorption (FCA) in Si. A simple FCA model is developed and tested.
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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.