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