1. Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
2. Electron Microscopy Analysis Station, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan
3. Device Technology Research Institute, AIST, Tsukuba, Ibaraki 305-8568, Japan