1. State Key Laboratory of Wide-Bandgap Semiconductor Devices and Integrated Technology, Faculty of Integrated Circuits, Xidian University 1 , Xi'an 710071, People's Republic of China
2. Institute of Modern Physics, Chinese Academy of Sciences (CAS) 2 , Lanzhou 730000, China
3. The School of Engineering, Liverpool John Moores University 3 , Liverpool L3 3AF, United Kingdom