1. The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Academy of Advanced Interdisciplinary Research, Xidian University, Xi'an 710071, China
2. School of Advanced Materials and Nanotechnology, Xidian University, Xi'an 710126, China
3. Department of Physics, Shaanxi University of Science and Technology, Xi'an 710021, China
4. INRS Centre for Energy, Materials and Telecommunications, 1650 Boul. Lionel Boulet, Varennes, QC J3X 1P7, Canada
5. School of Physics, Ningxia University, No. 489 Helanshan Rd., Xixia District, Yinchuan 750021, China
6. Department of Applied Physical Sciences, The University of North Carolina at Chapel Hill, Chapel Hill, NC, 27514, USA