1. School of Microelectronics Tianjin University Tianjin 300072 P. R. China
2. Tianjin Key Laboratory of Imaging and Sensing Microelectronics Technology Tianjin University Tianjin 300072 P. R. China
3. Department of Materials Science and Engineering City University of Hong Kong 83 Tat Chee Avenue Kowloon Hong Kong 999077 China
4. Center for Analysis and Tests Tianjin University Tianjin 300072 P. R. China