Shown here are some animations useful in understanding the basic mechanisms of diffusion.
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|There are several atomic mechanisms that lead to the movement of atoms. By far the most prominent are the vacancy mechanism and the direct interstitial mechanism. How they work can be seen in the animations:|
|Simulation of the vacancy mechanism||Simulation of the direct interstitial mechanism|
|Note a fundamental difference! If you consider the diffusion of a particular atom (any blue one for the vacancy mechanism, a specific red on for the interstitial mechanism), this atom always moved a bit in the second case, but may not have done anything in the first case. The diffusion of a particular lattice atom by a vacancy mechanism is inextricably linked to the movements of vacancies, but is something different!|
|Other mechanisms which are quite rare but nonetheless important in semiconductors are:|
|Indirect interstitial mechanism for self-interstitials|
|The simulation shows the elementary step: A self-interstitial (shown in light blue for easier identification) pushes a lattice atom into the interstitial lattice. The net effect is the migration of an self-interstitial from one interstitial site to an different one. The mechanism is however totally different from the regular interstitial mechanism. If in a thought experiment you mark a specific self-interstitial atom (paint it red), it will move a lot with the direct interstitial mechanism, but hardly at all with the indirect one.|
|The "kick-out" mechanism for impurity atoms"|
|Interstitial impurity atoms move rather fast by a
direct interstitial mechanism, until they eventually displace a lattice atom.
This is shown in the simulation. We now have a self-interstitial (that may or
may not be very mobile) and a rather immobile substitutional impurity atom,
which may now diffuse with one of the other (slow) mechanisms.
The total effect of the diffusion now is caused by two, usually very different mechanisms. Au in Si, and possibly some other impurities, diffuse in this fashion .
|Frank-Turnbull mechanisms (or dissociative mechanism).|
|This is the pendant to the kick-out mechanism.
Except that the diffusing impurity atom does not dislodge a lattice atom, but
gets trapped in a vacancy, whereupon it is almost immobile. The total effect
may be quite similar to the kick-out mechanism.
Which mechanism - Frank-Turnbull or kick-out - is operative, is difficult to find out. We must expect that in materials containing predominantly vacancies, the Frank-Turnbull mechanism will prevail, while the kick-out mechanism may be operative in materials with sizeable concentrations of interstitials.
|Various direct diffusion mechanisms|
|Shown is a direct exchange of places between two
atoms. Variants are exchanges involving more that 2 atoms (a whole
"ring" that "rotates").
Direct mechanisms are every now and then suggested in the literature to account for some new diffusion phenomena, but so far do not seem to occur in crystals.
They may, however, play a role (in analogous form) when considering diffusion in amorphous materials.
This is a possibility not yet discussed or observed. It is mentioned just to show that there might be more atomic mechanisms than have been discovered so far.
|Imagine a extended interstitial moving through a crystal. The 10 or so atoms "inside" the extended interstitial move around a bit while the interstitial passes through and may end up on lattice places different from the ones where they were - they have moved! it is totally unknown if this effect plays a role in Si, but it well might occur at high temperatures.|
|Again, it is important to keep in mind that you must clearly keep apart the movement of the "vehicle" - the vacancy, interstitial, etc. - and the movement of the atom(s) whose diffusion is of interest to you!|
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