1. School of Electrical Engineering, Kookmin University, Seoul 136-702, South Korea
2. Department of Materials Science and Engineering, KAIST, Daejeon 305-701, South Korea
3. School of Electrical and Electronic Engineering, Yonsei University, Seodaemun-gu, Seoul 120-749, South Korea
4. Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, South Korea