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Research Highlight

Tracking dopant diffusion pathways in bulk semiconductors

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Selected frames from a sequence of scanning transmission electron microscope images showing the diffusion pathway of a Ce dopant (the bright atom highlighted with a white arrow) as it moves inside a bulk AlN crystal. The final frame overlays the Ce pathway on the Z-contrast image obtained by averaging each frame.
A scanning transmission electron microscope (STEM) is used to locally excite and directly image the diffusion of single dopant atoms inside bulk single crystals. Although diffusion is a fundamental process that governs the structure, processing and properties of most materials, direct observations of diffusion processes have been elusive and limited to the surfaces of materials, until this work.

Using sequential STEM imaging, a larger number of jumps is observed for heavy Ce dopants, in agreement with a low activation barrier obtained by density functional theory-based calculations. The calculations attribute this low migration barrier to the size-mismatch of the large Ce atom in the smaller AlN host-lattice. In contrast calculations predicted a higher barrier and, hence, a smaller number of jumps for a smaller-size dopant like Mn. This prediction is confirmed by the STEM experiments, which show the Mn atoms to be nearly stationary in AlN. Overall, the results highlight a new methodology to investigate diffusion mechanisms in bulk materials.

 

R. Ishikawa, R. Mishra, A. R. Lupini, S. D. Findlay, T. Taniguchi, S. T. Pantelides, and S. J. Pennycook, “Direct observation of dopant atom diffusion in a bulk semiconductor crystal enhanced by a large size mismatch,” Phys. Rev. Lett. 113, 155501 (2014). 

 

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