1. Department of Materials Science and Engineering, and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20740
2. Department of Physics, and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20740
3. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
4. IBM Semiconductor Technology Research, 257 Fuller Road, Albany, New York 12203