1. Nanoelectronics Research Institute, Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan
2. IMEC, 3001 Leuven, Belgium
3. Department of Physics and Astronomy, KU Leuven, 3001 Leuven, Belgium
4. Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
5. Department of Materials Science and Engineering, Kyushu Institute of Technology, Tobata, Kitakyushu, Fukuoka 804-8550, Japan