1. Research Center for Integrated Quantum ElectronicsHokkaido University Sapporo 060-0813 Japan
2. Power Device Lab.Toyota Central R&D Labs., Inc. 41‐1, Yokomichi Nagakute, Aichi 480-1192 Japan
3. Institute of Materials and Systems for SustainabilityNagoya University Nagoya 464-8601 Japan
4. Department of ElectronicsNagoya University Nagoya 464-8603 Japan