dc.description.abstract | The Czochralski method is one of important technologies for the crystal industry. For recently, it had been mainly applied for industrial larger size sapphire crystal growth. The thermal and flow transport play significant roles in CZ crystal growth, and it is hard to directly observed in experiments. Moreover, the grown sapphire single crystal is commonly accompanied by small bubbles which might affect the optical properties. Manual control for the amount and distribution of this kind chemical defects strongly depends on the trial experiences. Therefore, we must use a more efficient way to obtain growth parameters or conditions. This thesis is numerically investigated on both thermal-flow and solute transport phenomenon using the finite element method and quasi-steady approximation.
The results presented in this study show the effect of different positions of support, heat shield devices, different upper furnace chambers, different support materials, and different crucible bottom shapes. Strong buoyant flow distorts the isotherms in the melt, and the strength decreases when the power supply decreases. The deflection height of the melt-crystal interface increases, as the melt level goes down. The power supply and temperature gradient inside solid crystal increase, when the support is lifted up. This is not good for crystal quality. Besides, the crystal convexity decreases, when the crucible bottom shape is round or the heat shield device made by carbon fiber is adopted. Furthermore, it is more likely grow more flat crystal and no solidified crystal touched at the crucible bottom, as the ZrO2 bubble insulator of support is used.
Then, we used the solutions of thermal-flow field to discuss its influence on the solute field in CZ system. The results show that the maximum value solute concentration locates at the crucible sidewall and solute distribution strongly depends on the flow motion of molten melt. Besides, solutes are inclined to gather near the melt-crystal interface, and the local-maximum value located at the center sites. The gas bubbles are easily incorporated into solid crystal, as the melt-interface is not stable. The instability of crystallization front is proportional to the solute constitutional supercooling.
The results show the chance of constitutional supercooling increases, when the solute concentration in molten melt increases or pulling rate of system is larger. In addition, the degree of constitutional supercooling is larger particularly near the center sites. These computational results are consistent with the experimental results done by foreign researchers. The temperature degrees in the furnace also should be controlled carefully and we concluded that the heat shield system is better than the others. Based on these results, the crystal quality of sapphire is expected to be improved. | en_US |