1. School of Microelectronics, University of Science and Technology of China, Hefei 230026, P. R. China
2. Computer, Electrical, and Mathematical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
3. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR 999077, P. R. China