1. School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
2. Division of Microelectronics Science and Engineering, School of Science, Ningbo University, Ningbo 315211, China
3. Institute of Materials Researc h and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634