1. State Key Laboratory of Material Processing and Die & Mould Technology
2. School of Materials Science and Engineering
3. Huazhong University of Science and Technology (HUST)
4. Wuhan 430074, P. R. China
5. State Key Laboratory for Superlattices and Microstructures
6. Institute of Semiconductors
7. Chinese Academy of Sciences
8. Beijing 100083, P. R. China