1. State Key Laboratory of Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences and College of Materials Science and Opto-Electronic Technology; University of Chinese Academy of Sciences; Beijing 100083 China
2. Hunan Key Laboratory of Super-Microstructure and Ultrafast Process; School of Physics and Electronics; Central South University; Changsha 410083 China
3. Department of Materials Science and Engineering; University of California; Berkeley CA 94720 USA