1. Walter Schottky Institut and Physik Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
2. Institute for Advanced Study, Technische Universität München, Lichtenbergstrasse 2a, D-85748 Garching, Germany
3. IOM-CNR and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 5, I-00185 Roma, Italy
4. European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France
5. Laboratoire des Matériaux Semiconducteurs, Institut des Matériaux, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland