1. National Institute of Standards and Technology, Materials Science and Engineering Division, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
2. Department of Physics, Astronomy and Geosciences, Towson University, Towson, Maryland 21252, United States
3. National Institute of Standards and Technology, Semiconductor and Dimensional Metrology Division, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States