1. IBM/Macronix Joint Project, IBM T. J. Watson Research Center, 1101 Kitchawan Road, P.O. Box 218, Yorktown Heights, New York 10598, Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France, and Dipartimento di Elettronica e Informazione and the Italian Universities Nanoelectronics Team (IU.NET), Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 - Milano (MI), Italy