1. Department of Electronic Information Systems Osaka City University 3‐3‐138 Sugimoto, Sumiyoshi‐Ku Osaka 558‐8585 Japan
2. School of Integrated Circuits and Frontier Science Center for Nano‐optoelectronics Peking University Beijing 100871 China
3. SiC Division Air Water Inc. 2290–1 Takibe, Toyoshina Azumino Nagano 399‐8204 Japan
4. Institute for Materials Research (IMR) Tohoku University 2145‐2 Narita Oarai Ibaraki 311‐1313 Japan
5. Department of Physics and Electronics Osaka Metropolitan University 3‐3‐138 Sugimoto, Sumiyoshi‐Ku Osaka 558‐8585 Japan