Dislocations edge type

The primary consideration we are missing is that of crystal imperfections. Recall from Section 1.1.4 that virtually all crystals contain some concentration of defects. In particular, the presence of dislocations causes the actual critical shear stress to be much smaller than that predicted by Eq. (5.17). Recall also that there are three primary types of dislocations edge, screw, and mixed. Althongh all three types of dislocations can propagate through a crystal and result in plastic deformation, we concentrate here on the most common and conceptually most simple of the dislocations, the edge dislocation. 

This result is easily generalized for mixed dislocations which are partly screw-type and partly edge-type, and also for cases having subsaturated vacancies. For a mixed dislocation, 6 must be replaced by the edge component of its Burgers vector... 

In contrast, in some particular samples a very narrow Aac was observed as shown in TABLE 1, which means that the tilting component is very small. Similar results have been reported by several groups [13,14], However, in many cases, A a for such samples is very wide, which means that the twisting component is very large. Twisting results from the formation of many edge type dislocations around each small grain. 

Figure 11.19. Schematic diagram of a tilted interface caused by an array of edge-type dislocations parallel to the interface [306].

Figure 5.4a shows a representation of an edge dislocation. This type of defect is very common, particularly in metals, and corresponds to major local deformations of the atomic arrangement. The diffracted amplitude is affected by the displacements of the atoms with respect to their reference positions in the same crystal without defects. 

A regular network of edge-type dislocations on an atomic flat interface, as depicted in Figure 8.3, can in principle relax all the misfit strain, allowing the GaN overgrown layer to be totally free of threading dislocations. In real growths, however, the interface is rarely flat, and the dislocation... 

Figure 8.7 Plan-view transmission electron micrograph of the same lateral growth as in Figure 8.6, with the [0001] direction normal to the image plane, showing the bending of screw-type dislocations into edge-type dislocation arrays at (B) during the lateral growth [10]...

Line imperfections are called dislocations and occur in crystalline materials only. Dislocations can be an edge type, screw type, or mixed type, depending on how they distort the lattice, as shown in Figure 8. It is important to note that dislocations cannot end inside a crystal. They must end at a crystal edge or other dislocation, or they must close back on themselves. 

For lattice mismatched III-V semiconductors on Si, two kinds of misfit dislocations are observed one is the pure-edge Lomer misfit dislocation, whose Burgers vector is parallel to the interface (type-I dislocation), and the other is the misfit dislocation, whose Burgers vector is 60° from the dislocation line (60° dislocation or type-II dislocation) [34]. Schematic illustrations of type-I and type-I I dislocations are shown in Fig. 7(a) and 7(b), respectively. 

Fig. 6.33. Screw dislocation with a edge-type segment. The only way to move the edge dislocation segment in the original slip direction is by incorporating or emitting vacancies...

Extended imperfections can also occur. An extra plane of atoms can be common in a crystal lattice. Extended imperfections of this type are called edge dislocations. Another common type of extended imperfection in crystals is the screw dislocation. This type of dislocation occurs when part of a crystal has slipped one atomic distance relative to its adjacent part. 

One type of dislocation is the edge dislocation, illustrated in Fig. VII-7. We imagine that the upper half of the crystal is pushed relative to the lower half, and the sequence shown is that of successive positions of the dislocation. An extra plane, marked as full circles, moves through the crystal until it emerges at the left. The process is much like moving a rug by pushing a crease in it. 

The other major defects in solids occupy much more volume in the lattice of a crystal and are refeiTed to as line defects. There are two types of line defects, the edge and screw defects which are also known as dislocations. These play an important part, primarily, in the plastic non-Hookeian extension of metals under a tensile stress. This process causes the translation of dislocations in the direction of the plastic extension. Dislocations become mobile in solids at elevated temperamres due to the diffusive place exchange of atoms with vacancies at the core, a process described as dislocation climb. The direction of climb is such that the vacancies move along any stress gradient, such as that around an inclusion of oxide in a metal, or when a metal is placed under compression. 

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