1. Semiconductor Device Laboratory, Samsung Advanced Institute of Technology, Yongin-si,
Gyeonggi-do 446-712, Korea
2. Institute of Physics and Applied
Physics, Yonsei University, Seoul 120-749,
Korea
3. Department of Display and Electronic Information, Soonchunhyang University, Asan-si, Chungcheongnam-do 336-745, Korea