1. Department of Materials Science, Faculty of Engineering, Tohoku University, Aoba Aramaki, Aoba‐ku, Sendai 980, Japan
2. Semiconductor Research Institute, Kawauchi, Aoba‐ku, Sendai 980, Japan
3. Tohoku University, 2‐1‐1 Katahira, Aoba‐ku, Sendai 980, Japan, and Semiconductor Research Institute, Kawauchi, Aoba‐ku, Sendai 980, Japan