1. School of Advanced Materials and Nanotechnology, Xidian University, Xi'an 710071, People's Republic of China
2. Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China
3. School of Mechano-electric Engineering, Xidian University, Xi'an 710071, People's Republic of China