1. Walter Schottky Institut and
Physik-Department, Technische Universität München, Am Coulombwall 4a, 85748 Garching, Germany
2. Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
3. Laboratoire des Materiaux Semiconducteurs, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne,
Switzerland