Generation and propagation of dislocations during crystal growth

In thr analysis of growth defects two categories of dislocations are distinguished: (i) 'growth dislocations' which are connected with the growth front; (ii) 'post-growth dislocations' which are generated 'behind' growth front, either still during the growth experiment or after growth, eg during cooling to room temperature. Inclusions are the main sources of dislocations. Two mechanisms of dislocation generation are active: (i) when the growth front closes the crystal lattice behind an inclusion 'lattice closure errors', resulting in dislocations, may occur, in which case these dislocations proceed with the advancing growth front ('growth dislocations'); and (ii) inclusions 'emit' dislocations loops or half loops (stress relaxation by plastic processes). In crystals grown on planar faces and in the brittle state, growth dislocations are straight-lined and adopt more or less sharply defined directions which depend on the Burgers vector of the dislocation and the growth direction of the face on which they end. These directions are characterized by a minimum of the dislocation line energy per growth distance, or, in another approach, by the vanishing of the (image) force exerted by the growth surface upon the dislocations lines. The typical 'as-grown' dislocation geometry is observed in crystals grown in their brittle state at relatively low temperatures, e.g. from an aqueous solution. In crystals grown under thermal stress in their plastic region the 'as-grown' geometry may be drastically changed by post-growth movement of growth dislocations and the generation of post-growth dislocations (dislocation glide). The processes of dislocation generation, dislocation propagation and post-growth movement are demonstrated by selected X-ray diffraction topographs (conventional Lang technique) of various crystals grown from solutions and melts. (C) 2000 Elsevier Science S.A. All rights reserved.