1. Department of Physics, Korea Advanced Institute of Science and Technology , Daejeon , South Korea
2. Korea Basic Science Institute , Seoul , South Korea
3. Department of Materials Science and Engineering, Pohang University of Science and Technology , Pohang , South Korea
4. Physics Department and Institute for Nanoscience and Engineering, University of Arkansas , Fayetteville, Arkansas , USA
5. Graduate School of Semiconductor Technology, Department of Electrical Engineering, Korea Advanced Institute of Science and Technology , Daejeon , South Korea