1. Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Korea, Quantum Functional Semiconductor Research Center, Dongguk University, 3-26 Pil-dong, Chung-ku, Seoul 100-715, Korea, and Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Korea