1. Micro and Nanotechnology Laboratory and Department of Electrical and Computer Engineering, University of Illinois at Urbana‐Champaign, 208 N. Wright Street, Urbana, IL 61801, USA
2. Department of Electrical and Electronics Engineering, Bilkent University, 06800 Bilkent, Ankara, Turkey
3. School of Electronics and Information, Kyung Hee University, Suwon, 449‐701, Korea