Open core dislocations

A7.4 Defects in GaN and related materials open core dislocations and V-defects... 

L., and Gaskill, D. (1995) Open-core screw dislocations in GaN epilayers observed by scanning force microscopy and high-resolution transmission electron microscopy Appl. Phys. Lett. 61, 2284. Copyright 1995, American Institute of Physics. 

Values of r satisfying Equation 3 (corresponding to the minimum and maximum points in Ag) will yield steady state solutions where a pit radius should remain constant, while the rest of the crystal grows or dissolves depending on the chemical affinity (Equation 2). If the term t b2g /2Tt2Y2 > 1, there are no real solutions to Equation 3 and there is no steady state value of r, which indicates that a small pit nucleated at a dislocation core should spontaneously open up to form a macroscopic etch pit. The critical concentration at which this occurs (setting the above term equal to one) is ... 

The dislocation core is always a more open region of the crystal. The number of bonds will be lower or there will be preexisting broken bonds. Thus, vacancies, interstitials, and other point defects can move more easily in the dislocation core. This leads to the concept of dislocation pipe diffusion. 

The technique was first applied to polyethylene and i-polypropylene revealing itself to be sensitive to crystallinity, lamellar orientation, crystal structure, and crystal defects (55). Thus it penetrates between lamellae attacks lamellar side surfaces more than their fold surfaces, a- more than /3-i-polypropylene and removes dislocation cores preferentially. As with chlorosulfonation, it allows systematic study of samples by cutting them open where desired. But it can also reveal the particular character of external or fracture surfaces, neither of which is usually random. 

Figure 21. Schematic representation of singular lines in a smectic C phase, (a, b) The distribution of the directors in four-branch nuclei, the layers being seen from the top open circles indicate the singularity position. (c, d) Cross-sections of singular lines parallel to layers, (e) Superimposed layers of a smectic C phase can be devoid of defects, whereas an edge dislocation is present for the rotation periodicity (left-handed helix). The tilted molecules show a-Ak rotation on the left-hand side of the diagam and only a -2it one on the right-hand side. This situation involves the presence of a line similar to (d) in the defect core. The double lines indicate the locus of directors parallel to the plane of the page.

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