1. Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga-koen 6-1, Kasuga, Fukuoka 816-8580, Japan
2. Diamond Materials Team, Advanced Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
3. Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand