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O: Fachverband Oberflächenphysik
O 99: Ultrafast Electron Dynamics IV
O 99.8: Vortrag
Freitag, 21. März 2025, 12:15–12:30, H11
Structural dynamics of the silicon (111)-(7×7) surface upon optical excitation studied by ultrafast reflection high-energy electron diffraction — •Jonas Darius Fortmann1, Birk Finke1, Christian Brand1, and Michael Horn-von Hoegen1,2 — 1Faculty of Physics, University of Duisburg-Essen, 47057 Duisburg, Germany — 2Center for Nanointegration (CENIDE), University of Duisburg-Essen, 47057 Duisburg
We present first results on the ultrafast structural dynamics of the Si(111)-(7×7) surface subsequent to an optical excitation. The surface is excited by an 800 nm fs-laser pulse at various fluences. The structural dynamics is followed through ultrafast reflection high-energy electron diffraction (URHEED) at a sample temperature of 80 K by means of the Debye-Waller effect. The surface has been excited for incident fluences > 1.6 mJ/cm2 at a drop of RHEED intensity by only 0.8%. For the highest fluence of 5.4 mJ/cm2 the intensity drop was 2% which is equivalent to a rise of surface temperature by 7 K. The fluence dependence is explained by linear absorption through the metallic surface state of the 7×7 reconstruction. For higher fluences we observe an additional three photon absorption due to bulk excitation in order to overcome the direct bandgap. The recovery to the groundstate occur via several processes at different time constants. The system has not fully recovered the groundstate after 200 µs, which indicates the population of a long-lived electronic state.
Keywords: ultrafast electron diffraction; laser excitation; silicon surface reconstruction