1. Institute of Wide Bandgap Semiconductors, Xi'an Jiaotong University, Xi'an 710049, China
2. Shaanxi Key Laboratory of Photonics Technology for Information, Xi'an Jiaotong University, Xi'an 710049, China
3. Institute of Technology and Science, The University of Tokushima, Tokushima 770-8506, Japan
4. Nation Key Laboratory of ASIC, HSRI, Shijiazhuang 050051, China