Dislocation content

Figure 3.1 A schematic representation of the defects common in epilayer structures, (a) mismatch, (b) misorientation, (c) dislocation content, (d) mosaic spread, (e) curvature, (f) relaxation, (g) inhomogeneity...

It is often useful to describe the dislocation content of coherent and semicoherent interfaces in terms of another framework which employs coherency dislocations and anticoherency dislocations. The basic idea is illustrated in Fig. B.8, which shows the same two boundaries shown previously in Fig. B.76 and c. The coherency dislocations possess a stress field equivalent to the long-range coherency stresses associated with the coherent interface. They are not real dislocations in the... 

The line defects that can exist in crystal/crystal interfaces can be classified as pure dislocations, dislocation/ledges (i.e., line defects with both dislocation and ledge character), and pure ledges. Examples of pure dislocations are shown in Figs. B.4c and B.7c. In these cases, there is no ledge in the boundary at the dislocation. An example of a dislocation/ledge is shown in Fig. B.66, and a pure ledge without any dislocation content is shown in Fig. B.9. 

For a material to be soft magnetically, its domain walls must move easily. The principal obstacles to domain wall movement are inclusions and grain boundaries. Low dislocation contents, residual stresses, and a low interstitial content, are also important. 

The most widely used technique for studying the dislocation content and characteristics of metals and alloys, and of all other solids which... 

As the projected surface area of the emitter in a field-ion microseope is typically 10 io A, it is obvious that this technique can reveal emergent dislocations only when the dislocation density is quite high. Nevertheless, much beautiful work on the dislocation content of metals has been carried out (92, 95). It has, for example, been possible to observe a (3T7)[110] type emergent edge dislocation on a platinum surface (92) and slip bands, and complex patterns of intersecting dislocations have been photographed. While it is not possible to locate... 

Many investigators have noted the correspondence between increased dislocation content of a solid and the increased catalytic activity. A large number of gas-solid, liquid-solid, and solid-state systems are known to exhibit such a correspondence we shall, however, deal with but a few of these, and only in rough outline since very few fundamentally important conclusions can be drawn when the identity of the dislocations or dislocation arrays is almost completely unknown. 

For as long as studies are carried out in which no more than the correspondence between the general level of reactivity and the total dislocation content is examined, little real advance will be accomplished in our understanding of the role of defects in chemical reactivity. An effort has to be made to characterize more fully the nature of the dislocations, and to exclude wherever possible effects which may arise from extraneous factors such as the presence of impurities. Quite clearly, model systems need to be investigated using techniques which readily reveal the presence and influence of the dislocations. In this subsection we shall concentrate, in detail, on three major model systems the oxidation of graphite, the thermal decomposition of calcium carbonate, and the solid-state dimerization of anthracene. Related systems will also be discussed where appropriate. 

Calcium carbonate, upon which we shall concentrate here, is particularly convenient since it is available as well-defined highly pure cleavable rhombs of calcite, and the dislocation content is known in considerable detail 197, 201, 202). 

Silicon is obtained from silica, the main component of the Earth s crust it was quickly used in electronics due to its natural abundance. As a consequence, many efforts were devoted to understanding the nature and properties of defects in this material. In addition, silicon appeared as, and is still nowadays, a model material for plasticity investigations because it is an elemental material with a crystal structure that is rather common among semiconducting materials. This material is also obtained as single crystals with zero dislocation content, which allows one to study dislocation nucleation. 

X-ray topography lacks the simplicity of etch patterns and the high resolution of transmission electron microscopy. However, the latter investigations are confined to thin films, and etch pits are not always associated with dislocations. Topographic techniques provide an excellent method for characterizing dislocation content of high-quality crystals up to a few millimeters thick. 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

In general, the shape and character of etch pits may reveal information about the impurity content of the crystal. "Beaked pits (pits with curved apexes, see 12) can indicate impurity haloes. Some forms of the arcuate etching we observed in quartz (16) may be examples of beaking. Very shallow pits can form at aged dislocations while very deep pits form at new dislocations. "Aging" may be related to impurity diffusion in the crystal lattice. 

If a specimen is heavily dislocated it may be impossible to distinguish peaks in double-crystal rocking curves. Figure 7.5 shows a five-layer Si-Ge specimen grown with 0.5 //m thick layers, with the Ge content in each layer respectively 10, 20, 30, 40 and 50% the aim being to produce a moderate amount of relaxation at each layer so that the top layer is fully relaxed 50% Ge but without excessive threading dislocations. This was achieved—the dislocation density was... 

The comparison of the Re content of a molybdenite with its structural polytype has been proposed as a method for assessing the viability of samples for Re-Os dating. 3R molybdenite has been assumed to grow via a screw dislocation mechanism triggered by the incorporation of Re during crystallisation. Low Re 3R ... 

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