1. Photonics Lab, Samsung Advanced Institute of Technology, Suwon 440‐600, Korea
2. Compound Semiconductor Epitaxy Laboratory, School of Materials Science and Engineering, Seoul National University, Shinlim Dong, Kwan‐Ak Ku, Seoul 151‐742, Korea
3. Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Gwangju 500‐712, Korea