1. Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea
2. Department of Materials Science and Engineering, Hallym University, Chuncheon 200-702, South Korea
3. Department of Physics, Sookmyung Women's University, Seoul 140-742, South Korea
4. Electronics and Telecommunication Research Institute, Daejeon 305-350, South Korea