1. Department of Physics, Sookmyung Women's University, Seoul 140‐742, Korea
2. Department of Computer and Electronic Physics, Sangji University, Wonju 220‐702, Korea
3. Materials and Devices Laboratory, Samsung Advanced Institute of Technology, Gyeonggi‐Do 449‐711, Korea
4. Department of Physics, Chungbuk National University, Cheongju 361‐763, Korea
5. Department of Physics Education, Kongju National University, Kongju 314‐701, Korea