1. Electrical Engineering Department, University of California, Los Angeles, California 90095, United States
2. Quantum-Functional Semiconductor Research Center, Dongguk University-Seoul, Seoul 100-715, Korea
3. Samsung Advanced Institute of Technology, Yongin, 446-711, Korea
4. Microelectronics Technology Department, The Aerospace Corporation, Los Angeles, California 90009, United States
5. Department of Physics, Sejong University, Seoul 143-747, Korea