1. School of Engineering; The University of Tokyo; 113-8656 Tokyo Japan
2. Advanced Low Carbon Technology Research and Development Program (ALCA); Japan Science and Technology Agency (JST); 332-0012 Saitama Japan
3. Energy Technology Research Institute; National Institute of Advanced Industrial Science and Technology (AIST); 305-8568 Ibaraki Japan
4. Core Research for Evolutional Science and Technology (CREST); Japan Science and Technology Agency (JST); 332-0012 Saitama Japan
5. Wide Bandgap Materials Group; National Institute of Materials Science (NIMS); 305-0044 Ibaraki Japan
6. Department of Physical Electronics; Tokyo Institute of Technology; 152-8550 Tokyo Japan
7. Pure and Applied Sciences; University of Tsukuba; 305-0006 Ibaraki Japan