Extrinsic stacking fault,

The core structure of the 1/2 [112] dislocation is shown in Fig. 4. This core is spread into two adjacent (111) plames amd the superlattice extrinsic stacking fault (SESF) is formed within the core. Such faults have, indeed, been observed earlier by electron microscopy (Hug, et al. 1986) and the recent HREM observation by Inkson amd Humphreys (1995) can be interpreted as the dissociation shown in Fig. 4. This fault represents a microtwin, two atomic layers wide, amd it may serve as a nucleus for twinning. Application of the corresponding external shear stress, indeed, led at high enough stresses to the growth of the twin in the [111] direction. 

High-resolution lattice images (e.g., Fig. 8(c)) reveal that the platelets are associated neither with dislocation loops nor with either intrinsic or extrinsic stacking faults. The platelets appear to be microcracks in which the separation between adjacent planes of Si atoms over a finite area is increased due to the slight displacement of these atoms from their substitutional lattice sites. From computer simulations, the lattice images are... 

ESF extrinsic stacking fault lAET irregular atomic environment type... 

The majority of dislocation loops and stacking faults observed by transmission electron microscopy of Si are judged to be of extrinsic or interstitial character. Although there are four proposed mechanisms by which extrinsic-type dislocations may be formed without any self-interstitials being present (12), most workers believe that self-interstitial precipitation is the dominant mechanism in extrinsic-type dislocations. 

Small extrinsic dislocation loops which are not connected with stacking faults were also found in the crystals (FIGURE 8(a)). Sometimes they appear in high density in the central part of the crystal, often also decorated by Ga precipitates. They also were interstitial type with Burgers vector <2203> (FIGURE 8(b)). 

Defects can be further classified into point defects and extended defects. Unassociated point defects are associated with a single atomic site and are thus zero-dimensional. These include vacancies, interstitials, and impurities, which can be intrinsic or extrinsic in nature. Extended defects are multi-dimensional in space and include dislocations and stacking faults. These tend to be metastable, resulting from materials processing. The mechanical properties of solids are intimately related to the presence and dynamics of extended defects. A discussion of extended defects is deferred until Chapter 10. For now, only point defects are covered. Their importance in influencing the physical and chemical properties of materials cannot be overemphasized. 

B-TEM sanple corresponding to the above specimen. From the B-ThM saitples, it was possible to conduct detailed TEM analysis which gave the follcwing information on the above defects. Dislocation loops at depth level I were dominantly a/3 <111> type and extrinsic (extra layers) in nature. The tips of the hair-pin dislocations were of the same character as at I, however, the arms of the hair-pins lay along all six <110> directions. The defects in layer III were found to be stacking fault bundles and microtwins by atomic resolution transmission electron microscopy (17). 

To compute the stacking fault energy, we must evaluate AE = EfauUediN layers) — EperfectiN layers). For the extrinsic case given above only four layers (i.e. 11—1—1 have contributions that are different from the perfect crystal contribu-... 

Three types of BSFs have been identified in the wurtzite structure the intrinsic fi-type with stacking sequence ABABCBCBC, the intrinsic f2-type with stacking sequence ABABCACA, and the extrinsic E type ABABCABAB. These BSE configurations are depicted in Figure 11.12a-c. Zakharov et al. [46] reported the existence of another type of BSE, namely, a double I sequence stacking fault in a-plane GaN grown on SiC. 

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