1. Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education and Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha 410082, China
2. Key Laboratory of Microelectronics Device and Integrated Technology, Institute of Microelectronics Chinese Academy of Sciences, Beijing 100029, China
3. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30020, Taiwan