1. Hunan Key Laboratory of Super Microstructure and Ultrafast Process; School of Physics and Electronics; Central South University; Changsha Hunan 410083 China
2. School of Electronic Science and Engineering and Collaborative Innovation Centre of Advanced Microstructures; Nanjing University; Nanjing 210093 China
3. Key Laboratory of Microelectronic Devices and Integrated Technology; Institute of Microelectronics; Chinese Academy of Sciences; Beijing 100029 China
4. Department of Physics and Astronomy; University of Rochester; Rochester NY 14627 USA