1. Wide-Gap Semiconductor Group, National Institute for Materials Science, Namiki, Tsukuba 305-0044, Japan
2. Department of Applied Physics, School of Advanced Engineering, Graduate School of Engineering, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
3. Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, Sengen, Tsukuba 305-0047, Japan