Growth mechanisms screw dislocation

D nucleation occurs when nuclei at the crystal surfaces act as sources of steps that allow for the further incorporation of growth units. In general, this mechanism accounts for the crystal growth observed at high supersaturations. The screw dislocation mechanism often accounts for growth at lower supersaturations. When the supersaturation is below the threshold for formation of 2-D nuclei, the presence of screw dislocations provides a source of steps for the addition of growth units in an infinite sequence of equidistant and parallel steps. 

In general, the predicted supersaturation required for 2D growth is much larger than has ever been observed. At lower (real) supersaturations the screw dislocation mechanism is much more feasible and two situations can be distinguished. If the supersaturation a is sufficiently smaller than <7i, tanh(cTi/a) becomes equal to 1 (eq. 11). On the other hand, for a>0 one has taxih 0 lo)KO la. The relationships between the growth rate and supersaturation for each case can be written respectively as ... 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

Spiral growth is a mechanism that is expected only on smooth interfaces. The assistance provided by screw dislocations is not necessary in the growth of rough interfaces, where an adhesive-type growth operates. 

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

Multilayer crystals with a central screw dislocation were commonly seen. A micrograph of a beautiful solution-grown multilayer crystal with regularly rotated terraces was presented by Keller [38]. This mechanism to multiply a single crystal layer into many crystal layers is important for the crystal growth from the melt to form spherulites. 

The screw-dislocation theory (sometimes referred to as the BCF theory because of its development by Burton, Cabrera, and Frank) is based on a mechanism of continuous movement in a spiral or screw of a step or ledge on the crystal surface. The theory shows that the dependence of growth rate on supersaturation can vary from a parabolic relationship at low supersaturations to a linear relationship at high supersaturations. In the BCF theory, growth rate is given by ... 

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