4. Generation and Propagation of Defects During Crystal Growth

This chapter presents a review of the typical growth defects of crystals fully grown on (planar) habit faces, i. e. of crystals grown in all kinds of solutions, in supercooled melt (mainly lowmelting organics) and in the vapour phase. To a smaller extent also growth on rounded faces from the melt is considered when it seemed to be adequate to bring out analogies or discuss results in a more general context. The origins and the typical configurations of defects developing during growth and after growth are illustrated by a series of selected x-ray diffraction topographs (Lang technique) and, in a few cases, by optical photographs. After the introduction (Sect. 4.1) the review starts with the formation of inclusions (Sect. 4.2) which are the main origin of other growth defects such as dislocations and twins. Three kinds of inclusions are treated: foreign particles, liquid inclusions (of nutrient solution) and solute precipitates. Particular attention is directed to the regeneration of seed crystals into a fully facetted shape (capping), and inclusion formation due to improper hydrodynamics in the solution (especially for KDP). Section 4.3 deals shortly with striations (treated in more detail in another Chapter) and more comprehensive with the different kinds of crystal regions grown on different growth faces: growth sectors, vicinal sectors, facet sectors. These regions are usually differently perfect and possess more or less different physical properties, and the boundaries between them frequently are faulted internal surfaces of the crystal. Two subsections treat the optical anomalies of growth and vicinal sectors and the determination of the relative growth rates of neighboured growth faces from the orientation of their common sector boundary. In Sect. 4.4 distinction is made between dislocations connected to and propagating with the growth interface (growth dislocations), and dislocations generated behind the growth front by plastic glide due to stress relaxation. The main sources of both types of dislocations are inclusions. In crystals grown on planar faces, growth dislocations are usually straight-lined and follow (frequently noncrystallographic) preferred directions depending on the Burgers vector, on the growth direction and the elastic constants of the crystal. These directions are explained by a minimum of the dislocation line energy per growth length or, equivalently, by zero force exerted by the growth surface on the dislocation. Calculations based on anisotropic linear elasticity of a continuum confirm this approach. The influence of the discrete lattice structure and core energy on dislocation directions is discussed. Further subsections deal with the Burgers vector determination by preferred directions, with the post-growth movement of grown-in dislocations, with the generation of post-growth dislocations, and with the growth-promoting effect of edge dislocations. In Sect. 4.5, Twinning, the main characteristics of twins and their boundaries, their generation by nucleation and by inclusions, their propagation with the growth front and their growth promoting effect are presented. The post-growth formation of twins by phase transitions and ferroelastic (mechanical) switching is shortly outlined. The last Sect. 4.6 compares the perfection of crystals (KDP and ADP) slowly and rapidly grown from solutions, that the optical and structural quality of rapidly grown crystals is not inferior to that of slowly grown crystals, if particular precautions and growth conditions are met.