Perfect dislocation

The discovery of perfect geodesic dome closed structures of carbon, such as C o has led to numerous studies of so-called Buckminster fullerene. Dislocations are important features of the structures of nested fullerenes also called onion skin, multilayered or Russian doll fullerenes. A recent theoretical study [118] shows that these defects serve to relieve large inherent strains in thick-walled nested fullerenes such that they can show faceted shapes. 

Issues associated with order occupy a large area of study for crystalline matter [1, 7, 8]. For nearly perfect crystals, one can have systems with defects such as point defects and extended defects such as dislocations and grain... 

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

But crystals (like everything in this world) are not perfect they have defects in them. Just as the strength of a chain is determined by the strength of the weakest link, so the strength of a crystal - and thus of our material - is usually limited by the defects that are present in it. The dislocation is a particular type of defect that has the effect of allowing materials to deform plastically (that is, they yield) at stress levels that are much less than [Pg.95]

The early understanding of the geometry and dynamics of dislocations, as well as a detailed discussion of the role of vacancies in diffusion, is to be found in one of the early classics on crystal defects, a hard-to-find book entitled Imperfections in Nearly Perfect Crystals, based on a symposium held in the USA in 1950 (Shockley et al. 1952). Since in 1950, experimental evidence of dislocations was as yet very sparse, more emphasis was placed on a close study of slip lines (W.T. Read, Jr.,... 

So far we have discussed the surface of a perfect crystal. But for an imperfect crystal there is another possibility to provide a step source. This is due to the screw dislocation. Assume that one cuts a crystal half-way from one side into the center, and slides the freshly created two faces against each other in... 

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. 

Crystals are distinguished by the regular, periodic order of their components. In the following we will focus much attention on this order. However, this should not lead to the impression of a perfect order. Real crystals contain numerous faults, their number increasing with temperature. Atoms can be missing or misplaced, and dislocations and other imperfections can occur. These faults can have an enormous influence on the properties of a material. 

D. Kuhlmann-Wilsdorf, Frictional Stress Acting on a Moving Dislocation in an Otherwise Perfect Crystal. Phys. Rev., 120,773 (1960). 

Although several types of lattices have been described for ionic crystals and metals, it should be remembered that no crystal is perfect. The irregularities or defects in crystal structures are of two general types. The first type consists of defects that occur at specific sites in the lattice, and they are known as point defects. The second type of defect is a more general type that affects larger regions of the crystal. These are the extended defects or dislocations. Point defects will be discussed first. 

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

The presence of dislocations is able to account for many features of crystal growth that cannot be explained if the growing crystal is assumed to be perfect. In these cases, the dislocation provides a low-energy site for the deposition of new material. 

Figure 3.3 Determination of the Burgers vector of an edge dislocation (a) a circuit around an edge dislocation and (b) the corresponding circuit in a perfect crystal. The vector linking the finishing atom to the starting atom in (b) is the Burgers vector of the dislocation.

The movement of edge dislocations results in slip at much lower stress levels than that needed in perfect crystals. This is because, in essence, only one line of bonds is broken each time the dislocation is displaced by one atomic spacing, and the stress... 

The Burgers vector of a screw dislocation can be determined in exactly the same way as an edge dislocation, following the FS/RH (perfect crystal) convention. A closed Burgers circuit is completed in a clockwise direction around the dislocation (Fig. 3.7a). An identical circuit in both direction and number of steps is completed in a perfect crystal (Fig. 3.7b). This will not close. The vector needed to close the circuit in the perfect crystal, running from the finish atom to the start atom, is the... 

A unit, or perfect, dislocation is defined by a Burgers vector which regenerates the structure perfectly after passage along the slip plane. The dislocations defined above with respect to a simple cubic structure are perfect dislocations. Clearly, then, a unit dislocation is defined in terms of the crystal structure of the host crystal. Thus, there is no definition of a unit dislocation that applies across all structures, unlike the definitions of point defects, which generally can be given in terms of any structure. 

It is seen, therefore, that after the passage of a perfect dislocation through a crystal, the crystal matrix will be perfect and dislocation free. This will not generally be true for imperfect dislocations, which invariably leave a stacking fault in their wake. 

---