Dresden 2014 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 30: Surface Chemical Reactions and Heterogeneous Catalysis II

O 30.7: Vortrag

Dienstag, 1. April 2014, 12:00–12:15, PHY C 213

Non-adiabatic effects in the oxidation of ultrathin crystalline Mg films: quantum size effects and hot charge carrier distribution — •Ulrich Hagemann and Hermann Nienhaus — Faculty of Physics and Center for Nanointegration (CeNIDE), University of Duisburg-Essen, D-47048 Duisburg, Germany

The low-temperature oxidation of ultrathin Mg films in the thickness range between 5 and 30 monolayers (ML) epitaxially grown on Si(111)-7x7 substrates is studied applying the chemicurrent method. The well-ordered films are characterized by electron diffraction and quantum well photoelectron spectroscopy. The exothermic reaction of O₂ molecules with Mg surfaces creates electron-hole pairs which are detected in the Mg:p-Si Schottky diodes as a chemicurrent. For metal layer thicknesses of approximately 8 and 12 ML the chemically induced current is significantly enhanced by a factor of up to three. We show that this result can be explained by an increased initial reactivity due to the normal confinement of the electrons. The resulting quantum well states lead to larger electron densities of states at the Fermi level for 8 and 12 ML. By fabricating different Mg/Si Schottky diodes with varying barrier heights the energy distribution of the excited hot charge carriers for the Mg oxidation is probed. The distribution exhibits an exponential behaviour which may be described by a Boltzmann-type function with an effective temperature of approximately 1600K.