1. School of Microelectronics and Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, Xi'an 710071, China
2. Department of Electrical and Computer Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, 3445 Engineering Hall, Madison, Wisconsin 53706, USA
3. School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071, China