1. Laboratorio MDM, IMM-CNR, via C. Olivetti 2, I-20864, Agrate Brianza
(MB), Italy
2. Semiconductor
Physics Laboratory,
Department of Physics and Astronomy, University of Leuven, B-3001 Leuven, Belgium
3. Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via
R. Cozzi 53, I-20126, Milano (MI), Italy