1. Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Korea
2. School of Electrical and Electronic Engineering, Chung-Ang University, Seoul 06974, Korea
3. Samsung Advanced Institute of Technology, Samsung Electronics Co., Suwon 443-803, Korea
4. Department of Physics, Sejong University, Seoul 143-747, Korea
5. Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea