1. Department of Applied Physics, Royal Institute of Technology, Albanova University Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
2. Walter Schottky Institut and Physik Department, Technische Universität München, 85748 Garching, Germany
3. Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, 4040 Linz, Austria
4. Dipartimento di Fisica, Sapienza Università di Roma, Piazzale A. Moro 1, I-00185 Roma, Italy