1. State Key Discipline Laboratory of Wide Bandgap Semiconductor Technology
2. School of Microelectronics
3. Xidian University
4. Xi'an 710071
5. China
6. State Key Laboratory of Surface Physics
7. Department of Physics
8. and Collaborative Innovation Center of Advanced Microstructures
9. Fudan University
10. Shanghai 200433
11. Materials Engineering and Centre for Microscopy and Microanalysis
12. The University of Queensland
13. QLD 4072
14. Australia
15. Beijing Synchrotron Radiation Facility
16. Institute of High Energy Physics
17. Beijing 100039