1. Research Institute for Advanced Electronics and Photonics National Institute of Advanced Industrial Science and Technology (AIST) 1‐1‐1 Umezono Tsukuba Ibaraki 305–8568 Japan
2. GaN Advanced Device Open Innovation Laboratory National Institute of Advanced Industrial Science and Technology (AIST‐NU) Furo‐cho, Chikusa‐ku Nagoya 464–8601 Japan
3. Device Technology Research Institute National Institute of Advanced Industrial Science and Technology (AIST) 1‐1‐1 Umezono Tsukuba Ibaraki 305–8568 Japan