1. State key laboratory of superlattices and microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 China
2. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China
3. Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Suzhou 215123 China
4. Materials Science Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA