1. University of Oslo, Department of Physics/Center for Materials Science and Nanotechnology, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
2. Consiglio Nazionale delle Ricerche, Istituto di Microelettronica e Microsistemi, Sezione di Bologna (CNR-IMM of Bologna), I-40129 Bologna, Italy
3. Royal Institute of Technology KTH, School of Information and Communication Technology (ICT), SE-164 40 Kista-Stockholm, Sweden