1. School of Materials Science and Engineering, Seoul National University, Seoul 151‐742, Korea
2. Inter‐University Semiconductor Research Center (ISRC), Seoul National University, Seoul 151‐742, Korea
3. School of Electrical Engineering, Seoul National University, Seoul 151‐742, Korea
4. Department of Materials Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305‐701, Korea