Optical and laser crystals grown by Czochralski technique from a solute-rich melt usually suffer defects of melt inclusion or bubble core, which severely affects optical, thermal and mechanical properties of the material. The main purpose of this paper is to study the inclusion mechanisms and to minimize such defects. Two types of mechanisms possibly responsible for inclusion defects are presented. In the current investigation, Czochralski grown optical single crystals are examined to recognize the effects of crystal rotation and natural convection on the melt flow pattern and solidification interface shape. It is established that increasing the rotation rate of crystal or reducing natural convection in the melt will cause the solid-liquid interface change from the convex shape to concave and high concentration of the species may be pushed away from the solidification interface. Simulations were performed to establish the relationships between Gr/Re2 and growth interface shape change, and between Gr/Re2 and stagnant point location were established. A disk submerged into the melt was used to reduce natural convection by reducing the melt height. The idea was similar to the submerged baffle or submerged heater used in Bridgeman crystal growth. The effect of submerged baffle on enhancement of crystal rotation effect was demonstrated. Simulation results showed that the melt flow near the solidification interface depended strongly on the baffle location, which was not surprised. The idea of submerged heater was also examined in Czochralski growth. Different from a constant temperature close to the melting temperature used in Bridgman growth, the submerged heater temperature should be selected on a higher temperature between the melting temperature and crucible temperature. The value depended strongly on the ratio between crystal and crucible diameters. It was proved that a constant temperature was not the best choice in Czochralski growth. In fact, an optimized temperature profile could be found in numerical simulations for melt flow control and inclusion suppression.

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