1. Department of Applied Physics, KTH Royal Institute of Technology, Albanova University
Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
2. Department of Electrical Engineering, KTH Royal Institute of Technology, Electrum 229, 164 40 Kista, Sweden
3. Research Center for Microtechnology, Vorarlberg University of Applied Sciences, Campus V, Hochschulstrasse 1, 6850 Dornbirn, Austria
4. Institute of Semiconductor and Solid State Physics, Johannes Kepler University, 4040 Linz, Austria
5. Department of Physics, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Rome, Italy