1. Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
2. Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, USA
3. Semiconductor R&D Center, Samsung Electronics Corporation, 1 Samsungjeonja-ro, Hwasung, Kyounggi 445-330, South Korea
4. Advanced Analysis Center, Korea Institute of Science and Technology, Cheongryang, P.O. Box 131, Seoul 130-650, South Korea