1. Research Institute of Electrical Communication, Tohoku University, 2‐1‐1‐ Katahira, Sendai 980, Japan and Exploratory Research for Advanced Technology, Nishizawa Perfect Crystal Project, Research Development Corporation of Japan. Semiconductor Research Institute, Kawauchi, Sendai 980, Japan
2. Exploratory Research for Advanced Technology, Nishizawa Perfect Crystal Project, Research Development Corporation of Japan, Semiconductor Research Institute, Kawauchi, Sendai 980, Japan
3. Semiconductor Research Institute, Kawauchi, Sendai 980, Japan