Fast particle modification of films

Film morphology can also be modified because fast particle bombardment can damage the substrate surface and create local sites with more dangling bonds. These are relatively reactive with adatoms, having extra bonds available, and can serve as preferential nucleation sites. At the same time, disruption of clusters increases the critical nucleus size making nucleation more difficult. These two effects do not cancel each other out because early and late nucleation are subject to different constraints. The first nuclei to form do so in the absence of sinks for adatoms. Therefore the adatom density can increase until the adatom supersaturation favors nucleation enough to counteract the disruption of clusters by fast particles. At the same time, the creation of favored nucleation sites also contributes to favoring nucleation. On balance, fast particle bombardment may accelerate the formation of the early nuclei. 

Once nucleation has occurred sinks for adatoms are available. Thus disruption of later nuclei allows the associated adatoms to escape to previously formed nuclei rather than allowing secondary nucleation to occur. This is equivalent to increasing the capture radius of a nucleus (see Section 10.6). At the same time the effective increase in critical nucleus size due to disruption of small clusters also works against cluster formation. In the presence of existing stable nuclei the formation of preferential nucleation sites is important because not enough adatoms are present to allow nucleation under conditions of fast particle bombardment. 

The combination of accelerated primary nucleation and suppressed secondary nucleation described here has the general effect of increasing the size of crystals on a surface and hence to increasing grain size in a thin film. 

Finally, fast particle bombardment may alter the defect density in the material. For semiconductors this is generally a bad thing because the most common effect is to introduce point defects into the growing film. For example, interstitial defects may be created by displacement cascades. High energy impacts may lead to formation of 

We surmised that if we could mix some Cu with the Mo back contact that we might be able to get a modest amount of Cu diffusion into the CuInSe2 during deposition that would be sufficient to keep the very back side of the material Cu-rich and permit good adhesion while allowing the surface to be In-rich and capable of forming a 

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The primary applications of sputtering are in the deposition of metals, although it has been applied to semiconductors and semiconductor alloys (see for example, Figure 11.33). We will consider an example of sputter deposition of a semiconductor and of a metal alloy here. The two processes are tied together because they involved a project in my laboratory joining the two materials. In particular the deposition of the metal alloy is described because it shows so many of the aspects of fast-particle modification of film properties. 

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