1. State Key Laboratory for Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 China
2. State Key Laboratory on Integrated Optoelectronics; College of Electronic Science and Engineering; Jilin University; Changchun 130012 P. R. China
3. College of Materials Science and Opto-electronic Technology; University of Chinese Academy of Sciences; Beijing 100029 China