1. Department of Applied Physics, Kyung Hee University, Yongin 446-701, Korea
2. Advanced Development Department, Samsung Electronics Co., Ltd, Yongin 446-711, Korea
3. Department of Physics and IPAP, Yonsei University, Seoul 120-749, Korea
4. Department of Chemistry, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea