The mechanism of polytype formation in vapour-phase grown ZnS crystals

Abstract

Morphological, x-ray crystallographic and x-ray topographic studies on many specimens grown in this laboratory lead to a consistent description of polytypism in vapour-phase grown ZnS. Crystal growth begins with the emerging of hexagonal 2H needles from the polycrystalline substrate. The needles thicken during the growth and often expand into platelets. The growth of the needles takes place around a screw dislocation with a Burgers vector 2nc _D, where C ₀ is the interlayer distance along the c axis. Consequently the (00.1) layers form a set of interleaved helical surfaces. Usually n= 1, but occasionally n> 1 and in this case subsequent formation of a poly-type is possible. This takes place by means of stacking faults which are introduced while the specimens are cooling down in the growth tube. Guided by the helical topology the stacking faults expand onto many equidistant (00.1) layers and thus transform the structure. It is demonstrated that all known features of ZnS polytypes in vapour-phase grown specimens