1. Department of Materials Science, Faculty of Science and Technology, Keio University, 3‐14‐1 Hiyoshi, Kohoku‐ku, Yokohama, Kanagawa 223, Japan
2. Department of Instrumentation Engineering, Faculty of Science and Technology, Keio University, 3‐14‐1 Hiyoshi, Kohoku‐ku, Yokohama, Kanagawa 223, Japan
3. Department of Electronics, Faculty of Engineering, Tokyo Engineering University, 1404‐1 Katakura, Hachioji, Tokyo 192, Japan