1. Anhui Key Laboratory of Detection Technology and Energy Saving Devices, Anhui Polytechnic University; Wuhu 241000 P.R. China
2. College of Electrical Engineering, Anhui Polytechnic University; Wuhu 241000 P.R. China
3. Research Center for Wide Gap Semiconductor, School of Physics, Peking University; Beijing 100871 P.R. China