Progress in Crystal Growth and Characterization of Materials
ReviewThe role of surface and interface structure in crystal growth
Introduction
The growth process of crystals and thin films depends on a large number of parameters, e.g. mass transport, supersaturation and additives that together determine the crystallographic structure, morphology and perfection of the crystal. The actual growth takes place at the interface between the crystal and the growth medium (vapour, solution, melt or even solid), and thus a full understanding of the crystal growth requires a detailed knowledge of the interface structure. In most cases the actual location where growth takes place is at the steps on the surface and their role cannot be overstated. Steps, however, will be covered elsewhere and this paper will mainly focus on the atomic-scale structure at interfaces and its consequences for crystal growth.
Section snippets
Surfaces and interfaces
At the surface of a crystal the environment is very different from the bulk and therefore at a surface structural rearrangements are expected to occur (see Fig. 1). The larger the difference with respect to the bulk environment, the larger these structural rearrangements will be. For this reason crystal surfaces in vacuum often show the most dramatic effects. As an example, at the surface of a metal crystal the atoms find themselves in an environment with lower charge density than the
Methods
When investigating a specific topic in science, most often no single method is available that provides all the answers and the surface/interface structure of growing crystals is no exception. In fact, this is an area that is still at the front of research as many things are not known yet due to the lack of suitable tools. Ideally, one would like to know the static and dynamic structure of the interface down to an atomic scale, including the step structure and for all possible growth
Crystal surface structure: KDP
The kind of structural details on an interface structure that can be obtained using X-ray diffraction will here be illustrated by the growth of potassium-dihydrogen-phosphate (KH2PO4, KDP) crystals. KDP can be easily grown from an aqueous solution and very large, perfect crystals can be grown that are for example used in frequency-doubling of laser light [15]. The growth morphology of this crystal is determined by two facets: {101} and {100}. The role of these two facets is correctly predicted
Conclusion
In situ techniques have increased our understanding of the effect of the atomic-scale structure of interfaces on crystal growth. In view of the progress that continues to be made in experimental techniques and computational methods, a combined use of these methods on specific systems will undoubtedly lead to further insights. In this chapter the focus was on the average structure of homogeneous interfaces, but the future should give a similar understanding of the local structure at surface
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