1. Wide-Gap Semiconductor Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
2. Department of Applied Physics, School of Advanced Engineering, Kougakuin University, Hachioji, Tokyo 192-0015, Japan
3. SCIOCS Co., Ltd., Hitachi, Ibaraki 319-1418, Japan
4. Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, Hyogo 679-5148, Japan
5. Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan
6. Electronic Ceramics Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
7. Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan