Molecular dynamics thesis

4.5 Annealing films
To study the effects of annealing, but also, to gather information that would help to
estimate what would happen if room temperature simulations would be possible at
laboratory time scales, two films have been annealed at 2000 K. One was a (110) film with
columnar structures and four monovacancies (vacancy concentration 1.3*10
-3
). It was
annealed for 10 ns. The other was a (100) film with a large void and a total vacancy
concentration of 3 percent, also annealed for 10 ns. Neither contained any argon or helium.
A third film with (110) orientation, containing three vacancies, two argon ions, two self-
interstitials, and a small dislocation loop (formed by clustering of self-interstitials), was
annealed at 1500 K for 2 ns. Also, four bulk samples of molybdenum, each containing a
number of monovacancies, were annealed at 500, 1000, 1500, and 2000 K. The first three
were annealed for 0.7 ns, the fourth was annealed for 2 ns.
4.5.1 Surface diffusion
The most prominent annealing effect that was found for the surface is a decrease of
the number of low-coordinated atoms on the surface. At the start of the simulation there are
a number of atoms that ‘stick out’ and have five or fewer neighbours. During annealing at
2000 K, these atoms are found to be very mobile, even during the very short time of the
simulation. They move over the surface, until they get trapped at more stable, higher-
coordinated sites. In this way, the number of low-coordinated atoms decreases very rapidly.
During the first two tenth of a ns the number of atoms with five or fewer neighbours
decreases from 45 to 2 for the (110) film annealed at 2000 K. There are also atoms,
however, that break free from high-coordinated sites and start moving over the surface. So
the number of mobile atoms does not asymptotically decrease to zero, but to a certain value
dependent on the annealing temperature. As an example, the (110) film before and after
annealing at 2000 K is displayed in fig. 31. It can be seen from fig. 31 that the
‘macroscopic’ roughness has hardly changed at all.
Figure 32 shows the sum over all atoms of the distances by which atoms are
displaced per annealing interval of 0.355 ns, excluding thermal vibrations, for the (100)
film at 2000 K. It can be seen from figure 32, especially from the number of atoms