1. Department of Physics and Electronics, Osaka Prefecture University, Sakai 599-8531, Japan
2. The Research Institute for Molecular Electronic Devices, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan
3. Osaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Jyoto-ku, Osaka 536-8553, Japan
4. Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan