[1] Ecole Normale Super, Phys Mat Condensee Lab, F-75005 Paris, France
[2] CNRS, Microstruct & Microelect Lab, F-92220 Bagneux, France
[3] Thomson CSF, Cent Rech Lab, F-91400 Orsay, France
[4] Ctr Elect & Microelect Montpellier, F-34000 Montpellier, France