1. CNR-IMM Institute for the Microelectronics and Microsystems, Z. I. VIII Strada 4, I-95121 Catania, Italy
2. Laboratori Nazionali del Sud, via
S. Sofia 62, 95123 Catania, Italy
3. Department
of Physics and Astronomy, University of Catania, v. S. Sofia
64, I-95123 Catania, Italy