1. Department of Applied Physics, Xi'an University of Technology, Xi’an 710048, China
2. Key Laboratory for Physical Electronics and Devices of the Ministry of Education, School of Electronic Science and Engineering, Xi'an Jiaotong University, Xi’an 710049, China
3. Key Laboratory of Pulsed Power, Institute of Fluid Physics, China Academy of Engineering Physics, P.O. Box 919-108, Mianyang 621999, China