1. Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581
2. Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585
3. National Institute of Advanced Industrial Science and Technology, FED, Advanced Power Device Laboratories, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568