The method of directional solidification (DS) is still the major technology to produce silicon wafers for photovoltaic. There are many factors responsible for the quality of the ingot such as impurity concentrations or grain structure. One of the most important parameters characterizing crystal quality is the dislocation density. The major factor responsible for generation and multiplication of dislocations inside the crystal bulk is the thermal stress. The thermal stress is produced by thermal deformation due to the temperature variation. A high stress level leads to the inelastic creep deformation providing the stress relaxation during the growth stage and resulting in high residual stress on the cooling stage. Thus there is a complex interaction of the stress field variation and generation of dislocations. Numerical simulation of transient growth of multi-Si by the DS technique has been performed to study the effect of growth conditions on formation of dislocations. Computer modeling has been provided by the specialized software CGSim [1]. This software allows one to simulate the DS growth as an unsteady continuous process from seeding stage to the end of cooling. The generation of dislocation has been calculated by Haasen- Alexander-Sumino model [2, 3] implemented into CGSim. This model provides the quantitative description of the relationship between plastic deformation and dislocation density. Different recipes of cooling and annealing have been simulated to find a promissing way to control of dislocation density. It is was shown that the most intensive multiplication of dislocations occurring after start of cooling while during the growth stage the dislocation density is quite stable. Despite of this the generation of dislocations on growth stage cannot be ignored because it strongly affects the stress-level inside Si ingot at the start of cooling.

1. www.str-soft.com
2. H. Alexander, P. Haasen, Solid State Phys. 22, 27 (1968)
3. M. Suezawa, K. Sumino, I. Yonegaga, J. Appl. Phys. 51, 217 (1979)

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