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Imaging of Individual Point Defects Inside Silicon Nanowires

Sang Ho Oh*1,2, Klaus van Benthem1,3, Sergio I. Molina1,4, Albina Y. Borisevich1, Weidong Luo1,6, Peter Werner5, Nikolai D. Zakharov5, Dhananjay Kumar2,1, Sokrates T. Pantelides6,1, Stephen J. Pennycook1,6

Nano Lett. 8, 1016 (2008)

Point defects in materials, such as interstitials or vacancies, control diffusion and phase transformations in materials, and can also be electronically and optically active. So far, they have only been studied by indirect means, or through theory. We have directly imaged individual point defect configurations inside silicon nanowires for the first time using a scanning transmission electron microscope (STEM). Gold atoms were found in a number of configurations, substitutional and interstitial. The ability to image point defect configurations such as these inside technologically important materials opens up a new capability to study not only their configurations but also, in future, their motion. Imaging was performed with an aberration-corrected STEM, which has a depth of field less than the thickness of the nanowire. Images obtained focusing in the center of the nanowire show individual gold atoms in substitutional and three interstitial positions. Density functional theory shows the substitutional site to be lowest in energy, with the three interstitial sites of higher energy. Total number densities of the observed point defects were in accord with their relative formation energies obtained from density functional theory, but indicated an effective temperature of about 1000 ??qC, probably due to interaction with the electron beam. The results show that point defect configurations involving heavy atoms, and their dynamics, may now be studied directly with the aberration-corrected STEM.


Z-contrast images of a Si nanowire in?q110?rzone-axis orientation (left panel) obtained in a VG Microscopes HB603U with Nion aberration corrector. Boxes show the regions used for intensity profiles, with Au atoms in various configurations arrowed; (a) substitutional; (b) tetrahedral; (c) hexagonal; (d) buckled Si-Au-Si chain configurations.
   
 

1Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
2Department of Mechanical and Chemical Engineering, North Carolina A & T State University, Greensboro, North Carolina 27411
3Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
4Department of Materials Science, Metallurgical Engineering and Inorganic Chemistry, University of Cadiz, 11510 Puerto Real, Cadiz, Spain
5Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany
6Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235

   
 

 Oak Ridge National Laboratory