Dislocation tangles

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

Slides Microstructures showing precipitates electron micrographs of dislocation tangles micrographs of polycrystalline metals. 

Fig. 20.33 (top) Transmission electronmicrograph showing dislocation tangles associated with precipitates in an Al-Cu-Mg-Si alloy (x 24 000, courtesy S. Blain) and (bottom) light micrograph showing slip lines in pure lead (x 100)... 

In a similar fashion, the line and planar defects described above are all, strictly speaking, volume defects. For the sake of convenience it is often easiest to ignore this point of view, but it is of importance in real structures, and dislocation tangles, for instance, which certainly affect the mechanical properties of crystals, should be viewed in terms of volume defects. 

Figure 9.13. BF image (g = lOTl) of the dislocation tangles generated at the high-pressure water clusters in wet synthetic quartz deformed at a constant strain-rate at 475°C. (From McLaren et al. 1989.)...

When dislocations moving on inclined glide planes intersect one another they form nodes and locks. After the intersection is complete they will have created jogs or kinks in the other dislocation. The intersection process can instead result in the two dislocations knitting together to form part of a dislocation tangle the... 

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