1. Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Tokyo 169-8555, Japan
2. Institute of Wide Band Gap Semiconductors, Xi'an Jiaotong University, Xi'an 710049, China
3. Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Tokyo 169-0051, Japan