Spiral screw dislocation mechanism

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. 

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 ... 

The investigation of the spiral growth mechanism of faces with a low screw dislocation density represents the first quantitative confirmation of Frank s theory. First, the parabolic dependence of the normal growth rate on the supersaturation was... 

At very low supersaturations, the driving force can be even too low to form 2D nuclei and yet growth is still sometimes observed. This fact led Burton et 01. to consider the role of defects within a crystal and to propose the hypothesis of the spiral growth mechanism. The spiral or screw dislocation is a crystallographic defect such that a step is present that can never be grown out or removed (Figure 5). 

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