1. Department of Applied Physics, Aalto University, P.O. Box 11100, 00076 Aalto, Finland
2. Department of Electrical Engineering and Automation, Aalto University, P.O. Box 11000, 00076 Aalto, Finland
3. Microelectronics Research Unit, University of Oulu, P.O. Box 8000, 90014 Oulu, Finland