1. Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University, Seoul 151-747, South Korea
2. Advanced Development Team, LED Business, Samsung Electronics Co., Ltd., San#24 Nongseo-Dong, Giheung-Gu, Yongin-City, Gyeonggi-Do 446–711, South Korea
3. Department of Materials Science and Engineering, Seoul National University, Seoul 151–744, South Korea
4. Department of Physics and Chemistry, Korea Military Academy, Seoul, South Korea