1. School of Advanced Materials Science and Engineering, Yonsei University, Seoul 120‐749, Korea
2. Future Technology Research Division, Korea Institute of Science and Technology, Seoul 136‐791, Korea
3. Department of Physics, Chon‐buk National University, Chon‐ju 561‐756, Korea
4. Department of Semiconductor Science and Technology, Chon‐buk National University, Chon‐ju 561‐756, Korea
5. Materials Science Division, Lawrence Berkeley National Laboratory, Department of Chemistry, University of California, Berkeley, CA 94720, USA