1. Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K., Fritz Haber Institute, D-14195 Berlin-Dahlem, Germany, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium, Arizona State University, Tempe, Arizona 85287-1704, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K., and Institut fuer Kristallzuechtung, Max Born-Strasse 2, D-12489 Berlin, Germany