1. Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
2. Advanced Power Electronics Research Center, AIST, Tsukuba, Ibaraki 305-8568, Japan
3. Research Center for Photovoltaics, AIST, Tsukuba, Ibaraki 305-8568, Japan