Shockley partial dislocations

Figure 3, Area of clean gold (111) surface showing a surface Shockley partial dislocation (arrowed) - see 2. Atomic columns are black.

In contrast to the imperfect dislocation discussed in the preceding paragraph, the Shockley partial dislocation (Fig. Q. 2b) is a mobile imperfect dislocation. A Shockley partial dislocation can be considered as a displacement of a portion of the atoms in one close-packed plane into a new set of positions. For example, in the close-packed layer sequence... ABCABC..., a portion of the atoms in, say, the B layer, are shifted to C sites. 

Values of SFE from 20 to 60 mjm determine intensive mechanical twinning related to TWIP effect. At SFE values higher than about 60 mJm-2, the partition of dislocations into Shockley partial dislocations is difficult, and therefore the glide of perfect dislocations is the dominant deformation mechanism (Hamada, 2007). In TRIPLEX steels with a structure of austenite, ferrite and K-carbides ((Fe,Mn)3AlC) and for SFE > 100 mJm-2, the SIP (Shear Band Induced... 

It has been observed experimentally that the formation of Shockley partial dislocations separated by stacking faults is much more common in diamond cubic epitaxial films in some crystallographic orientations than it is in other orientations... 

Sieradzki 1989), the resistance due to creation or annihilation of a surface ledge in the wake of the threading dislocation (Matthews 1975), the presence of a deposition flux during dislocation motion, and the possibility of dissociation of the dislocations into Shockley partial dislocations (Alexander 1986). Thus, the critical thickness condition could be restated a G = 71. where 7Z is a resisting force representing any or all of these additional effects. 

When the excitation error, s, is small, the presence of the dislocation is revealed in the image as a dark hne [Figs 7(c) and 7(d)]. These lines do not appear if g = (1 /do)(2 2 0) is used for the diffraction condition. This shows that the displacements of the dislocation strain field, which give rise to the contrast, are indeed perpendicular to the dislocation line, as expected for the Shockley partial dislocation of Fig. 4. The calculated images correspond closely to the experimental one [Fig. 7(e)], as illustrated also by the averaged intensity distributions of Figs 7(i)-7(k). 

Fig. 13 shows confocal images of the dislocations on the particle scale. As expected in a hard-sphere crystal, the defects that nucleate are stacking faults on 111 planes, each bounded by a Shockley partial dislocation. The fluctuations that lead up to the nucleation of the defect can be observed directly the defect in Figs 13(b) and 13(e) disappears after about 5 min, and four more of these fluctuations are observed before the nucleation is successful and grows into a large dislocation loop [Figs 13(c and d) and 13(f and g)]. 

The density of partial dislocations allows us also to estimate that the density of Ii-type BSFs is one order of magnitude larger than the density of l2-type faults. However, occasionally it is possible that two Ii faults with different Burgers vectors meet each other, resulting in the appearance of one Shockley partial dislocation instead of two Frank-Shocldey dislocations. This special case is illustrated in Figure 10.8. 

Growth-related defects such as Ii BSFs, which are surrounded by sessile - Shockley partial dislocations, are probably responsible for accommodation of azimuthal rotation of adjacent GaN grains and compensation of lattice mismatch in the [0001] direction between the substrate and the GaN layer. The appearance of BSFs surrounded by glissile Shockley partial dislocations is probably a result of local stress relaxation on the c plane from lattice mismatch and different thermal expansion coefficients between the GaN and the AlN/4H-SiC. If there is no effective way to neutralize stress, then formation of cracks is expected. Such cracks are visible in cross-section samples imaged by TEM. These cracks appear always at some distance from the AlN buffer layer, usually about 100 nm. Once formed, they continue to propagate to the sample surface. The measured average distance between cracks was about 2 p,m. 

Figure n.n (p. 301) Polar plots of the hydrostatic pressure field of a 1 /6[2-203] Frank-Shockley partial dislocation in a-plane GaN (a) assuming the isotropic case, (b) taking anisotropy into account. 

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