- Email: [email protected]
Crystal growth through forced stirring of melt or solution in Czochralski configuration
Journal of Crystal Growth 191 (1998) 774—778
Crystal growth through forced stirring of melt or solution in Czochralski configuration A. Kokh Design & Technological Institute of Monocrystals, 630058 Novosibirsk, Russian Federation Received 10 December 1997; accepted 15 February 1998
Abstract An original and effective method for stirring a melt or solution during the process of crystal growth in the Czochralski configuration is proposed. The efficiency of the method was supported by a model experiment. The method allows easy addition of nutrient for steady-state growth. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Gzochralski configuration; Melt/solution stirring; BBO crystal; Feeding
1. Introduction The Czochralski technique is widely used to grow single crystals from melts of various compositions [1]. The modification of this technique applied to the growth of crystals from a solution was named the top-seeded solution growth (TSSG) method. The main parameters which must be controlled for optimized growth of high-quality crystals are as follows: (1) axial and radial temperature gradients in crystal and melt; (2) pulling rate; (3) velocity and direction of rotation of crystal and crucible; (4) diameter of crystal and crucible; (5) depth of the melt (solution); (6) change of furnace heating element power during crystal growth. The character of melt convective movement is one of the most important factors to obtain high-quality single crystals. Convective movement of the liquid depends on both the values of the
above-mentioned parameters and the thermal properties of the liquid. Study of the crystal growth of some materials (borates, for example) from a high-viscous solution showed that convective movement of the liquid is sluggish. The solution becomes heterogeneous due to inadequate stirring. This results in gravitational and/or another differentiation, therefore the production of crystals becomes problematic or even impossible. Commonly, the growth of crystals from viscous melts and solution can be performed only if the crystal growth rate is very low. To intensify the stirring and to control the process of convection many procedures have been suggested: (1) static control over the hydrodynamic structure of the melt by means of construction changes of a flow area, such as a mobile (buoyant) crucible, double crucible, dies, membranes, and partitions [2]; (2) dynamic technique of accelerated crucible rotation (ACRT) [3] inducing the
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 3 5 0 - 9
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
775
convection; this technique is based on periodically accelerating and decelerating rotation of the growth crucible, possibly by changing direction of rotation. This paper presents an original and effective method for direct active control over the hydrodynamic structure of the melt or solution in the process of crystal growth. The apparatus for singlecrystal growth of barium borate (BaB O ) from 2 4 a solution using Na O as a flux [4] is described 2 here. The type of material is not critical in this work.
2. Experimental apparatus and procedure The construction of the growth apparatus is shown in Fig. 1. A platinum growth crucible of 100 mm diameter and 100 mm height was mounted on the pedestal of the rod of the lower mechanism of rotation and vertical displacement. The crucible was filled to about 3 of its volume with an initial 5 charge of 80$1.5 mol% of BaB O and 2 4 20$1.5 mol% of Na O. 2 The axis of a three-zone cylindrical furnace with a heater was installed to coincide with the axis of the growth crucible. The lower and middle zones were controlled by individual temperature controllers with Pt—Pt/10% Rh thermocouples as transducers. The temperature in the upper zone was maintained by an individual temperature controller supplied with a signal from a differential thermocouple with junctions placed in the middle and upper zones. This arrangement maintained a constant axial temperature gradient within a zone of crystal growth. A platinum shape-forming mixer (SFM) of 80 mm outside diameter was fastened to tension members in the upper part of the furnace. The SFM construction is shown in Figs. 1 and 2. The lower part, the mixer itself, consists of several blades (three in this case) which are placed at a slope and directed inside. The blades are attached rigidly to the cylinder at the top and to the stiffening ring at the bottom. The mechanism of rotation and vertical displacement of a crystal was mounted over the furnace. A single-crystal seed was attached to the rod of this mechanism.
Fig. 1. Apparatus for crystal growth by forced stirring of initial melt (solution) in the Czochralski technique configuration: (a) initial position; (b) position during crystal growth; 1 — lower mechanism of rotation and vertical displacement; 2 — pedestal; 3 — Pt growth crucible; 4 — melt (solution); 5 — shape-forming mixer; 6 — crystal; 7 — seed; 8 — mechanism of rotation and vertical displacement of crystal; 9 — furnace; 10 — top; 11 — thermocouples; 12 — differential thermocouple; 13, 14, 15 — lower, middle and upper heating zones, respectively; 16 — light source.
The initial position of the growth crucible was at the bottom of the furnace (Fig. 1a). After the material in the growth crucible melted, the latter was raised up so that the SFM blades and a part of the cylinder was immersed into the liquid (Fig. 1b). Alternatively, the furnace and SFM are lowered until the blades of the latter immerse into the melt. Rotary motion was imparted to the growth crucible. On the melt-solution surface a stationary picture of convective flows appeared as a three-ray star (according to the number of mixer blades) with
776
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
Fig. 2. Construction of the shape-forming mixer: 1 — cylindershape-forming cylinder; 2 — mixer blades; 3 — stiffening ring.
a fixed position of convergence point in the center of the growth crucible. The temperature in the furnace was maintained at 2—3° above the equilibrium melting-crystallization temperature in the convergence point of the rays which is the coldest point of the solution surface. Then, the rotating seed was brought into contact with the solution surface at this point. After the seed adapted to the solution and slightly melted, crystal growth began by regulated temperature decrease in the furnace combined with crystal pulling. The solution surface and growing crystal were viewed through peepholes on the furnace top which were illuminated by a light source. A crystal boule grown by this method along z-axis is shown in Fig. 3. It should be emphasized that SFM is a very reliable device of high structural strength. The experiments showed that the SFM which is produced of 0.8 mm Pt-sheets does not undergo shape distortion resulting from the effect of the viscous liquid flow during multiple growth cycles; in our
Fig. 3. Crystal boule of BBO grown by new method along z-axis.
experiments the dynamic viscosity of the BaB O — 2 4 Na O system is about 250 cP at ¹+900°C [5]. 2 The velocity of rotation of the crucible can reach 30 rpm.
3. Model experiment The action of the SFM was examined under isothermal conditions for a model liquid of high dynamic viscosity. The model liquid was glycerine with dynamic viscosity g"945 cP at ¹"25°C. SFM blades (Fig. 2) were immersed into glycerine in a cylindrical, transparent glass of the same diameter as the growth crucible. Due to the blades, the rotation of the glass relative to the fixed SFM generated liquid ascending flows at the periphery. Correspondingly, the descending flows of 5—10 mm/s velocity were generated in the central part of the glass rotating with 5 rpm velocity. The movement of air bubbles in the glycerine indicated that the convection processes induced by
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
the forced stirring involved almost the whole liquid volume. Change of the rotation direction resulted in an ascending flow originating at the bottom center of the glycerine. Rotation of the glass without the immersed cylindrical mixer generated no notable convection, i.e. the liquid behaved like a solid material.
4. Crystal growth by continuous feeding The advantage of the method is the presence of a free liquid surface which is separated from the crystal growth zone by the SFM. This surface occurs between the shape-forming cylinder and the
777
crucible wall. By supply nutrient material to the solution, large crystals can be grown under isothermal conditions. This is very important for crystal growth from a viscous solution, because the decrease in temperature causes the non-linear increase of melt-solution dynamic viscosity. Experimental apparatus for crystal growth by nutrient-feeding is shown in Fig. 4. The above apparatus of Fig. 1 is fitted with a tube fixed into the furnace. The feed of the nutrient material into the space between the shape-forming cylinder and the growth crucible is performed through this tube. The temperature in this space is higher than that at the crystal growth zone, therefore, the solid pieces dissolve actively, also due to the rotation of cylindrical mixer and growth crucible relative to each other. In this space the solution enrichment in a crystallized component originates a concentration flow into the solution main body which is uniformly distributed in a circle. The presence of the cylindrical mixer, as a partition, prevents the penetration of non-dissolved solid particles into the crystal growth zone, thus excluding spurious crystallization.
5. Conclusions
Fig. 4. Crystal growth by feeding: 1 — tube; 2 — a portion of grown material; 3 — concentration flow.
This method offers the following main advantage: it provides direct and active control of heat- and mass-transfer processes which define the quality of growing crystals. The method allows the whole crystal growth process to proceed under continuous movement of the liquid in the growth crucible. The active stirring maintains a homogenous melt or solution during the whole process. Intensive liquid exchange within the crystal growth zone prevents the accumulation of impurities at the interface and, hence entering the crystal structure. Obviously, this method is applicable to the growth of many materials, in particular, from rather viscous multi-component solutions. Naturally, each specific use of the method to obtain some material needs the optimization of the above-noted parameters and modification of the cylindrical mixer design, e.g. cylinder size, or the number, shape and slope of blades.
778
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
The proposed method provides many possibilities for investigators to form a specific convection pattern. Many aspects of crystal growth processes and crystal growth physical and numerical modeling can be investigated with the help of this method. At present, the laboratory led by the author is performing an experimental work concerning the optimization of the conditions of barium borate single-crystal growth by this method.
References [1] D.T.J. Hurle, B. Cockayne, in: Handbook of Crystal Growth, vol. 2, North-Holland, Amsterdam, 1994, pp. 99—211. [2] I.H. Jafri, V. Prasad, A.P. Anselmo, K.P. Gupts, J. Crystal Growth 154 (1995) 280. [3] H.J. Scheel, E.O. Schulz-Dubois, J. Crystal Growth 8 (1971) 304. [4] R.S. Feigelson, R.J. Raymakers, R.K. Route, J. Crystal Growth 97 (1987) 352. [5] D.Y. Tang, W.R. Zeng, Q.L. Zhao, J. Crystal Growth 123 (1992) 445.
Crystal growth through forced stirring of melt or solution in Czochralski configuration A. Kokh Design & Technological Institute of Monocrystals, 630058 Novosibirsk, Russian Federation Received 10 December 1997; accepted 15 February 1998
Abstract An original and effective method for stirring a melt or solution during the process of crystal growth in the Czochralski configuration is proposed. The efficiency of the method was supported by a model experiment. The method allows easy addition of nutrient for steady-state growth. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Gzochralski configuration; Melt/solution stirring; BBO crystal; Feeding
1. Introduction The Czochralski technique is widely used to grow single crystals from melts of various compositions [1]. The modification of this technique applied to the growth of crystals from a solution was named the top-seeded solution growth (TSSG) method. The main parameters which must be controlled for optimized growth of high-quality crystals are as follows: (1) axial and radial temperature gradients in crystal and melt; (2) pulling rate; (3) velocity and direction of rotation of crystal and crucible; (4) diameter of crystal and crucible; (5) depth of the melt (solution); (6) change of furnace heating element power during crystal growth. The character of melt convective movement is one of the most important factors to obtain high-quality single crystals. Convective movement of the liquid depends on both the values of the
above-mentioned parameters and the thermal properties of the liquid. Study of the crystal growth of some materials (borates, for example) from a high-viscous solution showed that convective movement of the liquid is sluggish. The solution becomes heterogeneous due to inadequate stirring. This results in gravitational and/or another differentiation, therefore the production of crystals becomes problematic or even impossible. Commonly, the growth of crystals from viscous melts and solution can be performed only if the crystal growth rate is very low. To intensify the stirring and to control the process of convection many procedures have been suggested: (1) static control over the hydrodynamic structure of the melt by means of construction changes of a flow area, such as a mobile (buoyant) crucible, double crucible, dies, membranes, and partitions [2]; (2) dynamic technique of accelerated crucible rotation (ACRT) [3] inducing the
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 3 5 0 - 9
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
775
convection; this technique is based on periodically accelerating and decelerating rotation of the growth crucible, possibly by changing direction of rotation. This paper presents an original and effective method for direct active control over the hydrodynamic structure of the melt or solution in the process of crystal growth. The apparatus for singlecrystal growth of barium borate (BaB O ) from 2 4 a solution using Na O as a flux [4] is described 2 here. The type of material is not critical in this work.
2. Experimental apparatus and procedure The construction of the growth apparatus is shown in Fig. 1. A platinum growth crucible of 100 mm diameter and 100 mm height was mounted on the pedestal of the rod of the lower mechanism of rotation and vertical displacement. The crucible was filled to about 3 of its volume with an initial 5 charge of 80$1.5 mol% of BaB O and 2 4 20$1.5 mol% of Na O. 2 The axis of a three-zone cylindrical furnace with a heater was installed to coincide with the axis of the growth crucible. The lower and middle zones were controlled by individual temperature controllers with Pt—Pt/10% Rh thermocouples as transducers. The temperature in the upper zone was maintained by an individual temperature controller supplied with a signal from a differential thermocouple with junctions placed in the middle and upper zones. This arrangement maintained a constant axial temperature gradient within a zone of crystal growth. A platinum shape-forming mixer (SFM) of 80 mm outside diameter was fastened to tension members in the upper part of the furnace. The SFM construction is shown in Figs. 1 and 2. The lower part, the mixer itself, consists of several blades (three in this case) which are placed at a slope and directed inside. The blades are attached rigidly to the cylinder at the top and to the stiffening ring at the bottom. The mechanism of rotation and vertical displacement of a crystal was mounted over the furnace. A single-crystal seed was attached to the rod of this mechanism.
Fig. 1. Apparatus for crystal growth by forced stirring of initial melt (solution) in the Czochralski technique configuration: (a) initial position; (b) position during crystal growth; 1 — lower mechanism of rotation and vertical displacement; 2 — pedestal; 3 — Pt growth crucible; 4 — melt (solution); 5 — shape-forming mixer; 6 — crystal; 7 — seed; 8 — mechanism of rotation and vertical displacement of crystal; 9 — furnace; 10 — top; 11 — thermocouples; 12 — differential thermocouple; 13, 14, 15 — lower, middle and upper heating zones, respectively; 16 — light source.
The initial position of the growth crucible was at the bottom of the furnace (Fig. 1a). After the material in the growth crucible melted, the latter was raised up so that the SFM blades and a part of the cylinder was immersed into the liquid (Fig. 1b). Alternatively, the furnace and SFM are lowered until the blades of the latter immerse into the melt. Rotary motion was imparted to the growth crucible. On the melt-solution surface a stationary picture of convective flows appeared as a three-ray star (according to the number of mixer blades) with
776
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
Fig. 2. Construction of the shape-forming mixer: 1 — cylindershape-forming cylinder; 2 — mixer blades; 3 — stiffening ring.
a fixed position of convergence point in the center of the growth crucible. The temperature in the furnace was maintained at 2—3° above the equilibrium melting-crystallization temperature in the convergence point of the rays which is the coldest point of the solution surface. Then, the rotating seed was brought into contact with the solution surface at this point. After the seed adapted to the solution and slightly melted, crystal growth began by regulated temperature decrease in the furnace combined with crystal pulling. The solution surface and growing crystal were viewed through peepholes on the furnace top which were illuminated by a light source. A crystal boule grown by this method along z-axis is shown in Fig. 3. It should be emphasized that SFM is a very reliable device of high structural strength. The experiments showed that the SFM which is produced of 0.8 mm Pt-sheets does not undergo shape distortion resulting from the effect of the viscous liquid flow during multiple growth cycles; in our
Fig. 3. Crystal boule of BBO grown by new method along z-axis.
experiments the dynamic viscosity of the BaB O — 2 4 Na O system is about 250 cP at ¹+900°C [5]. 2 The velocity of rotation of the crucible can reach 30 rpm.
3. Model experiment The action of the SFM was examined under isothermal conditions for a model liquid of high dynamic viscosity. The model liquid was glycerine with dynamic viscosity g"945 cP at ¹"25°C. SFM blades (Fig. 2) were immersed into glycerine in a cylindrical, transparent glass of the same diameter as the growth crucible. Due to the blades, the rotation of the glass relative to the fixed SFM generated liquid ascending flows at the periphery. Correspondingly, the descending flows of 5—10 mm/s velocity were generated in the central part of the glass rotating with 5 rpm velocity. The movement of air bubbles in the glycerine indicated that the convection processes induced by
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
the forced stirring involved almost the whole liquid volume. Change of the rotation direction resulted in an ascending flow originating at the bottom center of the glycerine. Rotation of the glass without the immersed cylindrical mixer generated no notable convection, i.e. the liquid behaved like a solid material.
4. Crystal growth by continuous feeding The advantage of the method is the presence of a free liquid surface which is separated from the crystal growth zone by the SFM. This surface occurs between the shape-forming cylinder and the
777
crucible wall. By supply nutrient material to the solution, large crystals can be grown under isothermal conditions. This is very important for crystal growth from a viscous solution, because the decrease in temperature causes the non-linear increase of melt-solution dynamic viscosity. Experimental apparatus for crystal growth by nutrient-feeding is shown in Fig. 4. The above apparatus of Fig. 1 is fitted with a tube fixed into the furnace. The feed of the nutrient material into the space between the shape-forming cylinder and the growth crucible is performed through this tube. The temperature in this space is higher than that at the crystal growth zone, therefore, the solid pieces dissolve actively, also due to the rotation of cylindrical mixer and growth crucible relative to each other. In this space the solution enrichment in a crystallized component originates a concentration flow into the solution main body which is uniformly distributed in a circle. The presence of the cylindrical mixer, as a partition, prevents the penetration of non-dissolved solid particles into the crystal growth zone, thus excluding spurious crystallization.
5. Conclusions
Fig. 4. Crystal growth by feeding: 1 — tube; 2 — a portion of grown material; 3 — concentration flow.
This method offers the following main advantage: it provides direct and active control of heat- and mass-transfer processes which define the quality of growing crystals. The method allows the whole crystal growth process to proceed under continuous movement of the liquid in the growth crucible. The active stirring maintains a homogenous melt or solution during the whole process. Intensive liquid exchange within the crystal growth zone prevents the accumulation of impurities at the interface and, hence entering the crystal structure. Obviously, this method is applicable to the growth of many materials, in particular, from rather viscous multi-component solutions. Naturally, each specific use of the method to obtain some material needs the optimization of the above-noted parameters and modification of the cylindrical mixer design, e.g. cylinder size, or the number, shape and slope of blades.
778
A. Kokh / Journal of Crystal Growth 191 (1998) 774–778
The proposed method provides many possibilities for investigators to form a specific convection pattern. Many aspects of crystal growth processes and crystal growth physical and numerical modeling can be investigated with the help of this method. At present, the laboratory led by the author is performing an experimental work concerning the optimization of the conditions of barium borate single-crystal growth by this method.
References [1] D.T.J. Hurle, B. Cockayne, in: Handbook of Crystal Growth, vol. 2, North-Holland, Amsterdam, 1994, pp. 99—211. [2] I.H. Jafri, V. Prasad, A.P. Anselmo, K.P. Gupts, J. Crystal Growth 154 (1995) 280. [3] H.J. Scheel, E.O. Schulz-Dubois, J. Crystal Growth 8 (1971) 304. [4] R.S. Feigelson, R.J. Raymakers, R.K. Route, J. Crystal Growth 97 (1987) 352. [5] D.Y. Tang, W.R. Zeng, Q.L. Zhao, J. Crystal Growth 123 (1992) 445.