1. Institute of Novel Semiconductors, State Key Lab of Crystal Materials, Shandong University, Jinan, 250100, P. R. China
2. Shandong Crystal GaN Semiconductor Co., Ltd, Jinan, 250100, P. R. China
3. Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, P. R. China