Effect Of Rotation On Thermal Convection During Czochralski Crystal Growth Process

The Czochralski(Cz) method is the most frequently-employed crystal growth technique in the industry. In Cz crystal growth, the quality of a single crystal depends mainly on the flow of the melt. The surface tension gradient, which makes the fluid flow along the free surface, is generated by the temperature difference between the crystal and the crucible. Meanwhile, under the condition of the gravity, buoyancy becomes an important driven force for melt flow due to the density difference. In the process of crystallization, due to the effect of segregation of the solute, the solute/impurity concentration are uneven distribution in the melt, which induce the solute capillary force at the free surface. The coupling effect for a variety of driving forces makes the melt flow more complex. In order to make the melt flow more stable and solute distribution more uniform, it tends to make the crystal and crucible rotate in the industry. Therefore, high quality crystal can be obtained. The coupling effect of thermocapillary force, buoyancy, solute-capillary force, centrifugal and Coriolis forces are taken seriously. Although there have been many studies to reveal the flow characteristics and analyze the mechanism of the complex flow in the process of crystal growth with the Cz method. But the experiment researches for the complex flow is limited. Therefore, in this dissertation, the experiment investigation combined with numerical simulation are conducted to understand the effects of crystal and crucible rotations on the thermal convection in a Czochralski configuration with pure and mixed fluids, respectively. The flow structure at the free surface after the flow transits to instability, the internal flow field and temperature field, are obtained. The physical mechanism of flow instability and the transition of the flow structure are also discussed. The main results are as follows:Firstly, the effects of crystal-crucible rotation on the characteristics of the three-dimensional flow and the flow instability are investigated by experiments. Results indicate that there is a transition from stable axisymmetric flow to a three-dimensional oscillatory flow with the increase of the thermocapillary Reynolds number. With or without the crucible rotation, the critical thermocapillary Reynolds number decreases with the increase of the crystal rotation rate. Additionally, the thermocapillary flow for the crystal-crucible counter-rotation is prone to become unstable, as compared to the crystal-crucible co-rotation. When the crystal rotation rate is small, the thermocapillary force is dominant, and the oscillatory flow behaves as the hydrothermal waves(HTWs). The crystal rotation has only a slight effect on the wave number and propagation angle of the hydrothermal waves. As the crystal rotation rate is gradually increased, the rotating waves and the HTWs will coexist in the fluid pool. Finally, the rotating waves occupy the entire pool, and the circular shear instability is taken as the mechanism of the oscillatory flow as the rotation-driven oscillatory flow is predominant. The crucible rotation can suppress the temperature fluctuation of the HTWs, and dominates the azimuthal propagation direction of the HTWs.Secondly, the fundamental characteristics and the flow transition of the rotation-thermocapillary-buoyancy convection during Czochralski crystal growth process are observed by schlieren technique. With the increase of depth, buoyancy has important effects on the flow. Once the temperature gradient exceeds the critical value, the hydrothermal waves transit to the spoke pattern which looks like a bud. With the further increase of liquid pool depth, as the buoyancy is dominant, the flow structure observed at the free surface transits to the spoke pattern with light and shade straight stripes eventually. Thus, the Rayleigh-Bénard instability is taken as the mechanism of the oscillatory flow. The critical Rayleigh number for the onset of instability of thermal convection increases with the increase of the crystal rotation rate without the crucible rotation. When the crucible rotates, the critical Rayleigh number is much higher than that with standing crucible at small crystal rotation rates. With the increase of the crystal rotation rate, the azimuthal propagating velocity of the spoke pattern increases; furthermore, the spoke pattern dims gradually and gives way to the wave pattern. The crystal rotation has a slight effect on the spoke number until the spoke pattern disappears. Compared with the shallow pool, the crystal rotation makes the flow more likely to be disturbed in the deeper pool. On the contrary, the crucible rotation is more conducive to suppressing the oscillatory flow in the deeper pool.In addition, the flow driven by the coupled surface tension gradient, buoyancy and rotation of crystal and crucible is discussed in detail. When the sidewall of the crucible is heated, the motion of the fluid is dominant by the thermocapillary and buoyancy, and a counter-clockwise flow cell occupies the pool. When the radial temperature gradient exceeds a threshold value, steady axisymmetric flow transits into a three-dimensional oscillation flow in the shallow pool and the travelling HTWs can be found. But, it transits into a three-dimensional flow in the deep pool and the spoke pattern with straight fringes can be observed. The wave number of pattern decreases with the increasing depth. In the shallow pool, the maximum temperature oscillatory locates near the crystal edge. On the contrary, it locates near the crucible sidewall in deep pool. The results from the numerical simulation are agreement with these obtaining at the experiments.Finally, the influence of rotation on the coupled buoyancy-solute-thermocapillary in a deep Cz pool with Ge0.98Si0.02 has been numerically investigated. The results show that, when the crystal rotates in small rotation rate, the critical Rayleigh number for the unstable flow increases with crystal rotation rate. The slow rotation rate of the crystal benefit to the flow stability. However, when the rotation rate of the crystal increases and exceeds a certain value, the critical Rayleigh number will decrease with the increase of the rotation rate of crystal. The steady flow loses its stability easily in high rotation rate of crystal and transits into three-dimensional oscillatory flow. When the crystal and crucible keep motionless, the axial symmetry flow becomes three-dimensional steady flow at a rather small Rayleigh number. The slowly increase of the crucible rotation rate results in the growth of the critical Rayleigh number. While the crucible rotation rate is further increasing, the critical point of destabilization decreases slowly. The rotating crucible stabilizes the melt flow. The flow is unstable when the rotating crucible speeds up, even though the temperature difference between the crystal and crucible is week. It suggests that the high rotational speed does not good for the stable flow. At a fixed Rayleigh number without crucible rotation, the “spoke-liked” pattern on the surface rotate in the azimuthal direction with the increase of crystal rotation rate. The fluctuation of azimuthal velocity is reinforced, but the wave number is independent to the crystal rotation rate. The peak Si fluctuation locates between the peak and the trough of azimuthal velocity fluctuation. When the crystal is motionless, the small rotation rate of the crucible not only weakens the Si concentration fluctuation, but also suppresses the azimuthal velocity fluctuation. Once the rotation rate is over the critical point, the fluctuations of Si concentration and azimuthal velocity increase with the crucible rotation. Under the condition of counter-rotation of the crucible and crystal, both the concentration and velocity fluctuations are intensified by the crucible rotation rate. When the rotation rate of crystal is rather high, the flow becomes chaos and the surface pattern is unidentifiable.