4.4.2 Sources of Growth Dislocations For topological reasons dislocation lines cannot start orend in the interior of a perfect crystal. They either formclosed loops, or they start from external and internal surfaces (e.g., grain boundaries), or from other defects witha break of the crystal lattice. In crystal growth, such defects may arise from all kinds of inclusions (e.g., foreignparticles, liquid inclusions, bubbles, solute precipitates).When inclusions are overgrown and closed by growthlayers, lattice closure errors may occur. These errors arethe origin of growth dislocations which are connectedto the growth front and propagate with it during furthergrowth. It is a very common observation that inclusions arethe source of growth dislocations. Examples are shownin Figs. 4.3, 4.5, 4.10, etc. The appearance of dislocations behind an inclusion (viewed in the directionof growth) is correlated with its size: small inclusionsemit only a few dislocations or are often dislocationfree. Large inclusions (> 50μm) usually emit bundlesof dislocations. In some cases, however, large inclusions(several millimeters in diameter) of mother solutionswithout dislocation generation have been observed (e.g.,in the capping zone of KDP [4.84, 85]). The generation of growth dislocations by foreignparticle inclusions has been experimentally studied byNeuroth [4.86] in crystals growing in aqueous solution (potassium alum) and in supercooled melt (benzophenone (C6H5)2CO, Tm = 48 ~C; salol C13H1003,Tm=42℃).
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