1. School of Materials Science and Engineering, Yeungnam University, Gyeongsan-si 712-749, South Korea
2. School of Electrical and Electronic Engineering, Yonsei University, Seodaemun-gu, Seoul 120-749, South Korea
3. Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, South Korea