A Novel Method for Melt Flow Control and Inclusion Suppression in Optical Crystal Growth

2007 ◽  
Author(s):  
Haisheng Fang ◽  
Lili Zheng ◽  
Hui Zhang ◽  
Yong Hong ◽  
Qun Deng
Keyword(s):  
Crystal Growth ◽  
Natural Convection ◽  
Flow Control ◽  
Melting Temperature ◽  
Constant Temperature ◽  
Czochralski Growth ◽  
Interface Shape ◽  
Melt Flow ◽  
Solidification Interface ◽  
Crystal Rotation

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.

Control of Flow Pattern and Solidification Interface Shape in an Induction Heated Czochralski Crystal Growth System

2007 ◽  
Author(s):  
Haisheng Fang ◽  
Lili Zheng ◽  
Hui Zhang
Keyword(s):  
Crystal Growth ◽  
Natural Convection ◽  
Rotation Rate ◽  
Melt Inclusion ◽  
Interface Shape ◽  
Growth Interface ◽  
Solidification Interface ◽  
Crystal Rotation ◽  
Growth System ◽  
Stagnant Point

Optical crystals grown by Czochralski technique from a solute-rich melt usually suffer defects of melt inclusion or bubble core defects, which severely affect the optical, thermal and mechanical properties of the material. It is well known that the formation of melt inclusion or bubble core is highly related to species distribution in the growth system especially at the solidification interface and the shape of the growth interface. This paper has examined the flow pattern and solidification interface changes by changing the forced convection, e.g., crystal rotation and by changing the natural convection, e.g., inserting a horizontal disk plate. The relative effect of fluid-flow convection modes in the melt associated with crystal rotation rate is represented by a dimensionless parameter, Gr/Re2. Increasing the rotation rate will cause the solid-liquid interface change from the convex shape to concave. When the crystal rotation rate is relatively low and natural convection is strong, Gr/Re2 is large. In this case, the concentration of species pertinent to melt inclusion moves down along the axis of rotation. When the crystal rotation rate is increased, the value of Gr/Re2 decreases. The precipitated composition spreads over the growing interface may then be swiped away from the growth interface by increased crystal rotation. Melt inclusion-free crystals can thus be obtained. The relationship between Gr/Re2 and growth interface shape change is achieved by numerical simulations. The stagnant point location as a function of crystal rotation is also presented, which shows that the stagnant point moves outward by increasing Reynolds number and/or reducing Grashof number. From such understanding, the interface shape and melt inclusion position can then be controlled through control of Gr/Re2 in the growth system. Many times, it is, however, not practical in the experiments to use a high rotation rate for optical crystal growth since high rotation rate will introduce the striation defects. A new design to reduce natural convection is then proposed to improve the effect of crystal rotation and to control the solidification interface shape. Numerical simulations have been performed to demonstrate the possibility of the new design. Results show that such design is very effective and practical to control the melt inclusion and the solidification interface shape.

A numerical and experimental study of natural convection and interface shape in crystal growth

1997 ◽  
Vol 173 (3-4) ◽  
pp. 492-502 ◽  
Author(s):  
G.H. Yeoh ◽  
G. de Vahl Davis ◽  
E. Leonardi ◽  
H.C. de Groh ◽  
M. Yao
Keyword(s):  
Experimental Study ◽  
Crystal Growth ◽  
Natural Convection ◽  
Interface Shape

Mechanisms of Thermo-Solutal Transport and Segregation in High-Pressure Liquid-Encapsulated Czochralski Crystal Growth

1999 ◽  
Vol 121 (1) ◽  
pp. 148-159 ◽  
Author(s):  
Y. F. Zou ◽  
G.-X. Wang ◽  
H. Zhang ◽  
V. Prasad
Keyword(s):  
Boundary Layer ◽  
High Pressure ◽  
Natural Convection ◽  
Driving Forces ◽  
Axisymmetric Flow ◽  
Melt Flow ◽  
Pure Crystal ◽  
Radial Segregation ◽  
Crystal Rotation ◽  
Crucible Rotation

The mechanism of dopant transport and segregation in high-pressure liquid-encapsulated Czochralski (HPLEC) grown III-V compound crystals (e.g., GaAs, InP) has been numerically studied using an integrated model, MASTRAPP. The model approximates the melt flow in the crucible as a quasi-steady-state, laminar, and axisymmetric flow, but the gas flow is considered as turbulent. Based on the physics of the growth process, a two-time-level scheme has been implemented where the dopant transport and growth are simulated at a smaller time scale while flow and temperature solutions are obtained from quasi-static calculations. Detailed numerical analyses are performed for the conditions of pure crystal rotation, pure thermally driven natural convection, and pure crucible rotation as well as for mixed flow with all of these forces present simultaneously. The dopant transport and segregation in these cases are well correlated to the corresponding melt flow pattern. Very weak radial segregation is predicted for pure crystal rotation because the resulting melt flow leads to a fairly flat solute boundary layer. The natural convection, on the other hand, produces a nonuniform boundary layer along the melt/crystal interface. This leads to a strong radial segregation with a high concentration along the central axis of the crystal. The crucible rotation has a similar effect. The combined effect of all of these flow mechanisms produces a strong radial segregation, whose extent depends on the relative strength of the driving forces. In all of these cases, strong melt flows lead to thin boundary layers that result in decreased longitudinal segregation. The predictions agree well with the experimental observations reported in the literature.

An Integrated Model for Czochralski Melt Growth of Optical Crystals

2003 ◽  
Author(s):  
S. P. Song ◽  
B. Q. Li ◽  
K. G. Lynn
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Optical Thickness ◽  
Flow Profile ◽  
Interface Shape ◽  
Melt Flow ◽  
Melt Growth ◽  
Oxide Melt ◽  
Temperature Distributions ◽  
Optical Crystals

This article presents the phenomena of melt flow, heat transfer, and solidification in Czochralski (CZ) melt growth processes of optical crystals, with emphasis on the effect of internal radiative heat transfer on the temperature distributions in oxide melt and crystal, melt convection, and melt-crystal interface shape. An integrated numerical model has been developed for simulating the physical phenomena in generic CZ furnaces, which includes the models for electromagnetic induction in crucible, surface exchange radiation in furnace, internal radiation in semi-transparent oxide melt and crystal, Marangoni convection in the melt, and solidification. Each developed model compares well with available analytical solutions. Numerical simulations were carried out for the prediction of fluid flow and heat transfer in furnaces. The simulation results show that the variation in optical properties of melt and crystal strongly impact their temperature distributions. It also affects the melt flow profile and intensity. The interface shape becomes more deeply convex toward the melt, as the optical thickness of the melt increases. However, the optical thickness of the crystal exhibits a minor impact on the interface shape. The results also show that the natural convection is dominated in the melt and the Marangoni flow enforces the natural convection.

Crystal growth melt flow control by means of magnetic fields

2002 ◽  
Vol 43 (3) ◽  
pp. 309-316 ◽  
Author(s):  
V. Galindo ◽  
G. Gerbeth ◽  
W. von Ammon ◽  
E. Tomzig ◽  
J. Virbulis
Keyword(s):  
Crystal Growth ◽  
Magnetic Fields ◽  
Flow Control ◽  
Melt Flow
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