1. School of Materials Science and Engineering Zhengzhou University Zhengzhou 450052 China
2. State Key Laboratory of Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 China
3. Henan Key Laboratory of Diamond Optoelectronic Materials and Devices Key laboratory of Materials Physics Ministry of Education, and School of Physics and Microelectronics Zhengzhou University Zhengzhou 450052 China