1. State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China
2. Shaanxi Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, Xi’an 710071, China
3. State Key Laboratory of Wide Bandgap Semiconductor Devices and Integrated Technology, School of Microelectronics, Xidian University, Xi’an 710071, China