1. Max Planck Institute of Microstructure Physics, Weinberg 2, Halle D-06120, Germany
2. Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf e.V., P.O. Box 510119, 01314 Dresden, Germany
3. Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 20050, China