1. School of Advanced Materials and Nanotechnology, Xidian University, Xi'an 710071, People's Republic of China
2. State Key Discipline Laboratory of Wide Bandgap Semiconductor Technology, School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China
3. Guangzhou Institute of Technology, Xidian University, Guangzhou 510555, People's Republic of China