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Atomic Configurations and Energetics of Arsenic Impurities in a Silicon Grain Boundary

M. F. Chisholm1, A. Maiti1, S. J. Pennycook1, and S. T. Pantelides2,1

Phys. Rev. Lett., 81, 132 (1998)

Full Article (PDF 224 KB)

Interfaces such as grain boundaries play critical roles in determining the physical properties of complex materials and structures. Impurity segregation in interfaces and grain boundaries often alters these properties in dramatic ways. In this report, we present atomic-resolution Z-contrast images showing that As impurities segregate to particular atomic columns in a Si grain boundary.Figure 1a is a Z-contrast image of a symmetric tilt boundary doped with arsenic revealing periodic bright features. The extra intensity corresponds, on average, to an extra 5 at.% As in each bright triangular feature. These features are parts of mixed dislocation cores identified as 2 or its mirrored core, 2' in the derived boundary structure presented in Fig. 1b. Thus, the image indicates that arsenic not only substitutes for silicon in a few selected sites in this boundary but that it also selects just one of the two mixed dislocation cores in the boundary (two of the six dislocations in each period of the GB). These consistently bright features are not observed in the undoped bicrystal.

The compositional sensitivity of the image is extremely high. We estimate a thickness of the order of 20 nm in this section of the sample, corresponding to approximately 40 atoms per column. With an As concentration of 5% in the selected column, there are on average only two arsenic atoms per column. This small number of impurity atoms explains the variability in the intensities from individual columns.

 
 

  Figure 1. (a) Z-contrast image of a symmetric 23° <001> tilt boundary in Si that has been doped with arsenic. The extra intensity seen at particular sites in the boundary indicates As segregates to special sites in the host interface without otherwise changing the boundary structure. (b) Schematic of the projected atomic column positions of the boundary core, obtained directly from the image, showing the three dislocations that form the boundary. The highlighted column positions in the schematic correspond to bright features in the image.
   
 
  1. Solid State Division, Oak Ridge National Laboratory
  2. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN

 Oak Ridge National Laboratory