1. Semiconductor
and Dimensional Metrology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Key Laboratory for the Physics and Chemistry
of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China
3. School
of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China
4. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States