1. Key Lab of Wide Bandgap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China
2. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China
3. School of Advanced Materials and Nanotechnology, Xidian University, Xi'an 710071, People's Republic of China