1. Department of Materials
Science
and Engineering, University of California, Berkeley, California 94720, United States
2. Department of Materials Science
and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing
100083, People’s Republic of China
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
94720, United States