1. Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, D-47057 Duisburg, Germany
2. Helmholtz-Zentrum Dresden-Rossendorf, Institute for Ion Beam Physics and Materials Research, D-01328 Dresden, Germany
3. Department of Physics, Technical University Dortmund, D-44221 Dortmund, Germany