Dislocation contrast

Figure 2 CL micrographs of Te-doped GaAs dark-dot dislocation contrast (a) in GaAs doped with a Te concentration of 10 cm and dot-and-halo dislocation contrast (b) in GaAs doped with a Te concentration of 10 cm. ...

To determine the character of the dislocations and the displacement vector b under dynamic reaction conditions, the g A criteria are used. The dislocation contrast is mapped in several reflections (g) by tilting the crystal, including the reflection in which the dislocation is invisible, i.e. = 0 when b is normal to the reflecting planes. Careful analysis (figure 3.25(f) and (g) of defects from the same crystal area (c)) shows that the displacement vector lies in the plane of shear (i.e. parallel to the shear plane), consistent with glide shear, with no lattice collapse. These criteria show the displacement vector to be 6 = (a/7, 0, c/4). 

The dislocation image lies to one side of the dislocation, in agreement with the intuitive explanation of the origin of dislocation contrast given in Section 5.1. 

Fourier Transform for the hkt) profile component Average dislocation contrast factor Interplanar distance between hkl) planes Scattering (reciprocal space) vector (<7 = 2 sin 0/2) Scattering vector in Bragg condition for the hkl) planes... 

The key here was the theory. The pioneers familiarity with both the kinematic and the dynamic theory of diffraction and with the real structure of real crystals (the subject-matter of Lai s review cited in Section 4.2.4) enabled them to work out, by degrees, how to get good contrast for dislocations of various kinds and, later, other defects such as stacking-faults. Several other physicists who have since become well known, such as A. Kelly and J. Menter, were also involved Hirsch goes to considerable pains in his 1986 paper to attribute credit to all those who played a major part. 

The edge dislocation on the 011 plane is again widely spread on the glide plane w = 2.9 6) and moves with similar ease. In contrast, the edge dislocation on the 001 plane is more compact w = 1.8 6) and significantly more difficult to move (see table 1). Mixed dislocations on the 011 plane have somewhat higher Peierls stresses than either edge or screw dislocations. 

As a consequence edge and mixed (111) dislocations move with relative ease, whereas the Peierls barrier for screw dislocations is as high as 2 GPa. These results are in contrast to previous calculations [6], which have shown a splitting for the screw dislocations and also a much lower Peierls barrier. However, our results can perfectly explain most of the experimental results concerning (111) dislocations which will be discussed in the following section. 

While the c/a ratio deviates only by about 2% from one, it is not ideal and this has significant consequences for the pseudotwin and 120° rotational fault. It results in a misfit at these interface which is compensated by a network of misfit dislocations (Kad and H2izzledine 1992). In contrast, the non-ideal c/a ratio does not invoke any misfit at ordered twins. However, the misfit dislocations present at interfaces are about fifty lattice spacings apart and thus there are large areas between them where the matching of the lamellae is coherent. The structures and... 

Ionic liquids are also not completely randomly arranged but have a structure similar to that of a crystal. However, in contrast to crystals, the ionic liquid structure contains far more vacancies, interstitial cavities, dislocations, and other perturbations. 

There are established criteria for obtaining b by using diffraction contrast (23). Briefly, the dislocation intensity (contrast) is mapped in several Bragg reflections (denoted by vector, g) by tilting the crystal to different reflections and determining the dot product of the vectors g and b (called the g b product analysis). 

The reflections include a particular g in which the dislocation is invisible (i.e., g b = 0 when b is normal to the reflecting plane). With these criteria in diffraction contrast, one can determine the character of the defect, e.g., screw (where b is parallel to the screw dislocation line or axis), edge (with b normal to the line), or partial (incomplete) dislocations. The dislocations are termed screw or edge, because in the former the displacement vector forms a helix and in the latter the circuit around the dislocation exhibits its most characteristic feature, the half-plane edge. By definition, a partial dislocation has a stacking fault on one side of it, and the fault is terminated by the dislocation (23-25). The nature of dislocations is important in understanding how defects form and grow at a catalyst surface, as well as their critical role in catalysis (3,4). 

---