1. Semiconductor Device Laboratory, Samsung Advanced Institute of Technology, Yongin 446-712, Korea
2. Semiconductor R&D center, Samsung Electronics, Yongin 446-711, Korea
3. School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Korea
4. Division of Engineering, Brown University, Providence, Rhode Island 02912, United States
5. School of Electrical Engineering, Korea University, Seoul 136-713, Korea
6. Max Planck Institute of Microstructure Physics, Halle (Saale) 06120, Germany
7. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
8. Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States