1. Semiconductor Materials Division, Department of Physics, Chemistry and Biology-IFM, Linköping University, Linköping S-58183, Sweden
2. Nanoscale Engineering Division, Department of Physics, Chemistry and Biology-IFM, Linköping University, Linköping S-58183, Sweden
3. Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg S-41296, Sweden
4. Department of Physics and I3N, Institute for Nanostructures, Nanomodelling and Nanofabrication, University of Aveiro, Aveiro 3810-193, Portugal