1. Key Lab of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi'an, Shaanxi 710071, China
2. Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China
3. School of Advanced Materials and Nanotechnology, Xidian University, Xi'an, Shaanxi 710071, China