Animations of
Defect Nucleation and Migration in Materials:

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(1) Nucleation of a Misfit Dislocation in Gold Film on Nickel Substrate

This is a cross-section of Au/Ni system. Blue circles represent nickel substrate. Green circles are Gold atoms in the film. The starting configuration is coherent Gold film on Nickel substrate. One to one correspondence of Ni-Au atoms is observed across the interface in the starting configuration.
Top surface is free and coherent strain in the film is relieved by relaxing Au atoms in the compressed coherent film. The vacancies move into the film (or atoms move out of the starting configuration). Vacancies migrate towards the interface. The vacancies did not migrate into the substrate.
The final configuration is a well formed 60 degree dislocation at this (111) interface. The extra half plane is in the substrate. The thin white lines show the planes of atoms.
This calculation was performed with ADESH. It took about ten minutes of CPU time on P5-133 PC for the relaxation. A saved log file showed all inetrmediate configurations with the help of MOTION program. "MOTION" utility comes with a registered copy of ADESH 2.1.
NOTE: You can evaluate other heterostructures of your interest with this method. The extension of this method is to investigate the factors that suppress the formation of misfit dislocations. It also shows the level of coherent strain needed to nucleate a dislocation.

(2) Migration of a 90 degree dislocation in Silicon.

Please watch the moves of the two red atoms inside the core. The core of a 90 degree dislocation in silicon consists of a seven atom ring and a five atoms ring. Ideal configuration of Si, viewed from [110] direction, shows six atom rings. Figure shows a 90 degree edge dialocation edge on where the dislocation line is perpendicular to the screen.
With ADESH, the atoms in the core of the dislocation were rearranged and then relaxed. Small initial jumps of two (red) atoms and consequest relaxation of the core atoms successfully migrated the dislocation to a new position one Burgers Vector away.
The white traces show earlier movements of atoms. It shows how little movements inside the core of just a few atoms precipitated the migration of the dislocation.
The energy of migration was about 2 eV. This energy was very close to migration energy of sixty degree dislocation.

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Anjali Nandedkar, Ph. D.
CASA Engineering
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