1. School of Advanced Materials and Nanotechnology, Xidian University, 266 Xinglong Section of Xifeng Road, Xi’an, Shaanxi 710126, People’s Republic of China
2. State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, Shaanxi 710071, People’s Republic of China
3. School of Microelectronics, Xidian University, Xi’an, Shaanxi 710071, China