A COMPARATIVE-STUDY OF CRYSTAL-GROWTH PHENOMENA UNDER REDUCED AND ENHANCED GRAVITY

Gravity can act in different ways on the growth of crystals: (i) the own weight of a crystal can cause its plastic deformation; (ii) hydrostatic pressure limits the stability of floating zones; (iii) sedimentation causes the formation of concentration gradients, or the deleterious agglomeration of nuclei, and finally (iv) buoyancy causes an enhanced convective heat and mass transport in the fluid nutrient. It is clear that only in cases (i) and (ii) can a reduced gravity be beneficial for growing crystals. In special cases of homogeneous nucleation microgravity might also be helpful in avoiding sedimentation (iii). On the other hand a very large enhanced gravity of 106 times earth gravity is necessary to produce pronounced (useful) sedimentation effects which limits its applicability. In case (iv) it is not clear, a priori, in what crystal growth configurations a reduction or an enhancement of gravity has advantages. The discussion concentrates, therefore, mainly on effects of gravity driven convection. It is shown that the avoidance of buoyancy convection under reduced gravity can be used to grow more homogeneous crystals. But it is also demonstrated that convective flows with a steady state can be established under enhanced gravity on a centrifuge. This effect can also be used to avoid the formation of doping striations, and also to increase the growth rate in solution growth. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.