Crystal growth surface smoothness

In their pioneering work on crystal growth Burton, Cabrera and Frank predicted that on an atomic length scale the equilibrium structure of a crystal surface should exhibit a transition from a smooth state at low temperatures to... 

Nucleation of two-dimensional clusters mechanism. In cases where the crystal face is smooth, growth can occur by either a two-dimensional nucleation mechanism or by a spiral-growth mechanism. For two-dimensional nucleation, growth occurs by attachment of molecules to the edge of a nucleus on the surface. Under ideal conditions, the growing step on a crystal surface will advance across the crystal face until that particular layer is complete. Before another layer starts, a center of crystallization has to form via surface nucleation. The growth rate forthis mechanism is exponentially dependent on the driving force ... 

The internal structure or polymorphic state represents the molecular arrangement within a crystal and is manifested in the form of a definite heat of fusion (AHf) value. External structure or crystal habit is the outer description of a crystal and is described by its length, width, thickness, and surface appearance (roughness, smoothness, and porosity). Crystal growth may be impeded by adjacent crystals growing simultaneously or contacting container walls. As a result, the development... 

A perfect crystal face should be completely free of any surface defects. In view of its further application for crystal growth studies, however, a face not intersected by screw dislocations can be considered conditionally as perfect. All other defects have either little or no effect on the growth behavior of the face. To meet this situation, the term quasi-ideal or quasi-perfect" has been introduced for the description of faces free of screw dislocations [5.14]. A quasi-perfect face is characterized by extended atomically smooth terraces separated by monatomic steps and absence of emergence points of screw dislocations. A smooth quasi-perfect face without steps can be described as an intact quasi-perfect face". 

At a deposition temperature of 1300°C, surface structures vary with the total pressure. At a total pressure of 50 Torr, growth cones are observed (two cones in Figure 6.12a) and each cone consists of a number of small crystals which have well-defined comers (around 90°). At a total pressure of 60 Torr Figure 6.12b, no growth cones are found and the whole surface is made up of well-grown crystals with clear prominent profiles. The surface of each crystal is very smooth. It is... 

The applicability of this sort of growth mechanism depends, however, upon the assumption that we are dealing with a perfect, smooth crystal surface. This is not often the case. Real crystal surfaces generally have imperfections of various kinds and, under certain circumstances, may even be rough on a molecular scale. The most important type of imperfection, from the point of view of crystal growth, is the screw dislocation. As was first pointed out by Frank (1949), a screw dislocation emerging from a crystal face provides a step on the surface to which atoms can be added continuously without causing it to disappear. The step is anchored... 

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