1. National Laboratory of Solid State Microstructures; School of Electronic Science and Engineering; Collaborative Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 China
2. Department of Physics; Southeast University; Nanjing 211189 China
3. Department of Electrical and Computer Engineering; University of Wisconsin-Madison; Madison WI 53706 USA
4. School of Microelectronics; Xidian University; Xian 710071 China
5. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics; Chinese Academy of Sciences; 500 Yu Tian Road Shanghai 200083 China
6. Department of Electronic Engineering and Materials Science and Technology Research Center; The Chinese University of Hong Kong; Hong Kong SAR China