1. Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Korea
2. School of Electronic Electrical Engineering, College of Information and Communication Engineering, Sungkyunkwan University, Suwon, 440-746, Korea
3. Department of Materials Science and Engineering, Korea University, Anam-Dong 5-1, Seongbuk-gu, Seoul, 136-701, Korea