Crystal growth dislocations

Edge dislocations play an important role in the strength of a metal, and screw dislocations are important in crystal growth. Dislocations also interact strongly with other defects in the crystal and can act as sources and sinks of point defects. 

As shown in Fig. 10, the zinc blende structure has crystallographic polarity along (111) axes by nature. That is, there are (lll)B and (lll)N surfaces in cBN crystals, which results in the characteristic crystal habit and surface morphology. The crystallographic polarity of cBN should also affect etching behavior, crystal growth, dislocations, strain and abrading properties, chemical reactivity, impurity concentrations, etc. as observed in other sphalerite compounds (97). 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

Dislocations are involved in various important aspects of materials apart from mechanical behaviour, such as semiconducting behaviour and crystal growth. I turn next to a brief examination of crystal growth. 

Crystal growth. As we saw in the preceding section, before World War II the dislocation pioneers came to the concept through the enormous disparity between calculated and measured elastic limiting stresses that led to plastic deformation. The same kind of disparity again led to another remarkable leap of imagination in postwar materials science. 

Figure 3.22. Screw dislocation and crystal growth, after W.T. Read.

Verma, A.R. (1953) Crystal Growth and Dislocations (Butterworths Seientific Publications, London). 

Charles Frank and his recognition, in 1949, that the observation of ready crystal growth at small supersaturations required the participation of screw dislocations emerging from the crystal surface (Section 3.2.3.3) in this way the severe mismatch with theoretical estimates of the required supersaturation could be resolved. 

Figure 5.5 Development of a crystal growth spiral staring from a screw dislocation...

Frank, F.C., 1949. The influence of dislocations on crystal growth. Discussions of the Faraday Society. 5, 48-54. 

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. 

Screw dislocations play an important part in crystal growth. The theoretical background to this fact was first developed in 1949 by Frank and colleagues. It was apparent that crystal growth was rapid as long as ledges and similar sites existed on the face of a crystal because these form low-energy positions for the addition of new atoms or... 

Figure 3.8 Crystal growth at a screw dislocation (a) addition of new material at a step is energetically favored, and (b) a step is always present at an emerging screw dislocation.

Heterogeneous nucleation of CaC03 on 5-AI2O3. Example for the sequence of nucleation and subsequent crystal growth. The latter is plotted as a 2nd order reaction (as is typical for screw dislocation catalysis). 

The classical crystal growth theory goes back to Burton, Cabrera and Frank (BCF) (1951). The BCF theory presents a physical picture of the interface (Fig. 6.9c) where at kinks on a surface step - at the outcrop of a screw dislocation-adsorbed crystal constituents are sequentially incorporated into the growing lattice. 

Surface reaction rate laws for dislocation-free surfaces. No surface diffusion allowed. Crystal growth for InS > 0, dissolution for InS < 0. Solid line, /kT = 3.5 dashed line, d>/kT = 3.0. 

Dislocations. Screw dislocations are the most important defects when crystal growth is considered, since they produce steps on the crystal surface. These steps are crystal growth sites. Another type of dislocation of interest for metal deposition is the edge dislocation. Screw and edge dislocations are shown in Figure 3.4. 

Eig. 2.22 Continuous crystal growth around a central screw dislocation axis. The small blocks are unit cells of the crystalline material and are usually well organized. The displacement site acts as a site for nucleation. 

The concept of dislocations was theoretically introduced in the 1930s by E. Orowan and G. I. Taylor, and it immediately played an essential role in the understanding of the plastic properties of crystalline materials, but it took a further twenty years to understand fully the importance of dislocations in crystal growth. As will be described in Section 3.9, it was only in 1949 that the spiral growth theory, in which the growth of a smooth interface is assumed to proceed in a spiral step manner, with the step serving as a self-perpetuating step source, was put forward [7]. 

---