1. State Key Laboratory of Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences & College of Materials Science and Opto-Electronic Technology; University of Chinese Academy of Sciences; Beijing 100083 China
2. Department of Applied Physics; School of Physics and Electronics; Hunan University; Changsha 410082 China
3. Department of Physics; Henan Normal University; Xinxiang 453007 China
4. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication; National Center for Nanoscience and Technology; Beijing 100190 China