1. College of Electrical and Computer Engineering, Chungbuk National University, Cheongju 28644, Korea
2. R&D Center, Samsung Display, Co., Ltd., Gyeonggi-do 449-902, Korea
3. School of Electrical and Electronics Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-Gu, Seoul 156-756, Korea