Rough interface growth mechanisms

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. 

As explained above, three fundamental models of crystal growth mechanism were established in relation to the roughness of interfaces these are illustrated in Fig. 3.14. At present, there is no other known growth mechanism that is essentially different from these three. Therefore, we shall analyze the morphology of crystals, the main topic of this book, based on these three growth mechanisms. 

Figure 3.15. Areas where rough and smooth interfaces are expected. The growth rate versus the driving force relations expected for the three models of growth are indicated on the growth rate, R (vertical), axis versus the driving force (A/x/kT) diagram. Curve A shows the spiral growth mechanism B represents the two-dimensional nucleation growth mechanism C denotes the adhesive-type mechanism.

Crystal faces with curved or wavy surfaces, not exhibiting either striations or step patterns, are rarely encountered. In most cases, these faces appear by dissolution. Rough interfaces grow by the adhesive-type growth mechanism, their normal... 

Fig. 5.1. Crystal growth velocity v as a function of driving free-energy difference AGJ for various mechanisms of interface advance, (a) Two-dimensional nuclea-tion, (6) dislocation mechanism, (c) continuous advance of a rough interface.

We discuss here the reason for the anisotropy in R, the interface structure, and the growth mechanism among the planes. The anisotropic R can be roughly explained from the difference in the arrangement of lattice sites on the surface among the planes [79]. Each H2O molecule in bulk ice has four nearest neighboring HjO molecules and, hence, makes four HBs. However, for all of the basal, prismatic, and secondary prismatic planes, an H2O molecule attached to a lattice site on the ice plane makes only one HB. Therefore, the attached H2O molecule is much less stable than the H2O molecules in bulk ice and, hence, is difficult to be stably captured into the lattice site. 

Geometrical analysis of this interface gave a roughness exponent of 0.62. Analysis of the interface for samples of various Al concentration which were crystallized at various temperatures is in progress in order to supply quantitative information on the interface and its growth mechanism. 

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