1. Department of Electronics and Nanoengineering, Micronova, Aalto University 1 , P.O. Box 13500, Aalto FI-00076, Finland
2. Department of Physics and Helsinki Institute of Physics, University of Helsinki 2 , P.O. Box 43, Helsinki FI-00014, Finland
3. VTT Technical Research Centre of Finland Ltd 3 , Espoo FI-00014, Finland