DEFECT GENERATION DURING CRYSTAL GROWTH. J.P. van der Eerden, E.A. Huitema, M. Vlot and J. Huinink. Department of Interfaces & Thermodynamics, Debije Institute, Utrecht University, Padualaan 8, 3584CH Utrecht, The Netherlands

Although perfect crystals hardly exist, our fundamental knowledge of the origin of these defects is very limited. In this contribution we introduce general ideas about mechanisms of defect generation during (and due to) crystal growth. Both molecular scale phenomena and thermodynamic descriptions turn out to be relevant. These ideas will be supported by Monte Carlo and Molecular Dynamics simulation results.

In practice impurities often are the starting point for defect generation. From a thermodynamic point of view their incorporation in the crystal is expected to be given by the equilibrium distribution coefficient. In practice often much higher impurity levels are found. Moreover fluctuations in the impurity density couple to the local crystal growth rate and may lead to the well-known Mullins-Sekerka morphological instability.

The accumulation of impurities near the surface leads to macroscopic inclusions which generate planar and linear defects. Even in the absence of impurities such defects are to be expected at large growth rates.

The final defect structure is the result of a competition between generation processes at the growing surface (where sharp free energy gradients exist) and healing processes in a somewhat wider interface region (where relatively fast relaxation towards thermodynamic equilibrium still is possible). Some of the free energy parameters can be obtained from numerical simulations, others have to be estimated from macroscopic observations and descriptions.

The after-growth generation of defects due to temperature gradients, mechanical stresses and solid state phase transitions which occur during cooling of the crystal is not discussed here.