1. Optoelectronic Materials Center, Korea Institute of Science and Technology, Cheongryang, P. O. Box 131, Seoul 130-650, Korea
2. Department of Information Display Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea
3. Department of Physics, Bangalore University, Bangalore 560 056, India
4. Division of Industrial Metrology, Korea Research Institute of Standards and Science, 209 Gajeong-Ro, Yuseong-Gu, Daejeon, 305-340, Korea