1. Vacuum Technology Center, Korea Research Institute of Standards and Science, 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, South Korea
2. Low Energy Electronic Systems IRG (LEES), Singapore-MIT Alliance for Research and Technology, 1 Create Way, Singapore 138602
3. School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
4. Korea Photonics Technology and Institute, Gwangju 550-779, South Korea
5. Department of Physics, Yeungnam University, Gyeongbuk 712-749, South Korea
6. Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 and Low Energy Electronic Systems IRG (LEES), Singapore-MIT Alliance for Research and Technology, 1 Create Way, Singapore 138602