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Journal of Crystal Growth 198/199 (1999) 210 --214

Temperature distribution near the interface in sapphire crystals grown by EFG and GES methods
V.M. Krymov *, V.N. Kurlov , P.I. Antonov , F. Theodore , J. Delepine
A.F. Ioffe Physical--Technical Institute, 26 Politekhnicheskaya, St. Petersburg 194021, Russia Institute of Solid State Physics RAS, Chernogolovka, Moscow District 142432, Russia DTA/CEREM/DEM/SPCM, Commisariat a l+Energie Atomique, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France %

Abstract This paper presents results of experiments on in situ temperature measurements during sapphire shaped crystal growth. The temperature distribution difference between the crystals grown by EFG (edge-defined, film-fed growth) and GES (growth from an element of shape) methods is considered. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.05.!t; 81.10.!h; 81.10.Fq Keywords: Shaped crystal growth; Sapphire; EFG/Stepanov method; GES method

1. Introduction The favorable combination of excellent optical and mechanical properties of sapphire complemented with high chemical durability makes it an attractive material for high-technology applications. By now the growth of sapphire single crystals of various shapes is well developed. But the quality of these crystals still remains a serious problem limiting their application in optics. It is very important to know the thermal history of the growing single crystal, in order to improve

* Corresponding author. Fax: #7 812 247 8924; e-mail: antonov@crystal.ioffe.rssi.ru.

the crystal quality and process yield. The temperature distribution in the growing crystal has a dominating influence on the formation of thermal stresses. The thermal stresses in turn can lead to plastic deformations and initiation of defects of structure (dislocations, slip lines and grain boundaries). By adjusting the temperature in the crystal by means of thermal zone modification involving heater, die or shields, these defects can be controlled more effectively [1--3]. The direct temperature measurement in a sapphire single crystal is very difficult because of its high melting point and semitransparency to thermal radiation. The temperature distribution along a growing sapphire tubular crystal was measured with a special IR pyrometer [2], and with the

0022-0248/99/$ -- see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 1 2 9 - 4


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ingrowth of thermocouples [3]. The maximum curvature of temperature distribution just near to crystallization front was demonstrated. This paper reports the direct temperature measurement and comparison of temperature fields near to the interface in sapphire crystals grown by the EFG and GES methods. Sapphire ribbons were grown as a model using both methods.

2. Experimental procedure The experiments were done in an 8 kHz induction heated graphite susceptor/molybdenum crucible setup held within a growth chamber. The special design of this installation allows translation of the pulling shaft in vertical and horizontal directions simultaneously. Sapphire ribbons were grown from the melt by the EFG and GES methods. The techniques utilize capillary rise from a melt source to the top surface of a wetted die. For the EFG technique the outer edges of the die determine the shape of the meniscus, and thus of the growing crystal [4]. The GES method has been developed on the basis of the Stepanov method [5]. The approach of the GES method consists of pulling a shaped crystal from a melt meniscus which is only a small element of the whole transverse cross section of the growing crystal [6]. The crystal grows layer by layer while traversing in the horizontal and vertical directions.

Fig. 1. The scheme of the temperature measurement using the ingrowth of thermocouples: (a) the EFG technique; (b) the GES technique. 1 -- die; 2 -- meniscus; 3 -- thermocouple; 4 -- crystal.

Fig. 2. Photographs of the growth process made by TV camera. EFG technique (a). The successive stages of GES technique: The beginning growth from seed (b), the crystal ribbon growth (c), the view of the ribbon after the quick lifting from the die (d). Marking is the same as in Fig. 1.


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An initial charge was crushed sapphire Verneuil boules. The atmosphere was high purity argon. The molybdenum dies were 2.5;24 mm in cross section for the EFG method (Fig. 1a), and 2.5;3mm in cross section for the GES method (Fig. 1b). The width of the capillary channels was 0.3 mm for both variants. The temperature distribution was measured using WR 5/20 thermocouples, diameter of the wire was 0.1 mm. The thermocouple seal was located on the lower end of the seed ribbon initially grown parallel to the crystallographic c-axis. The sapphire thin tubes were used for insulating the wires. At the beginning of the process the seed ribbon with thermocouple was put down until it made contact with the die and a column of the melt was formed (Fig. 2). After the re-melting of the crystal--melt region, the seal of the thermocouple was on the crystal--melt phase boundary. Then the crystal with ingrown seal of thermocouple was pulled with a constant rate » "0.1 or 1 mm/min (the EFG variant, Fig. 1a). For the GES variant the crystal was pulled in vertical direction with rate » "0.05 mm/min and reverse translated in hori zontal direction with rate » "6.3 mm/min both together (Fig. 1b). Variation of the thermoelectromotive force in time was registered by the "Sefram-8400" recorder.

3. Results and discussion 3.1. EFG variant The axial temperature distribution in sapphire ribbons grown by the EFG technique is shown in Fig. 3. The curve 2 corresponds to the pulling rate » "1.0 mm/min, the curve 3 corres ponds to the rate » "0.1 mm/min. The first dis tinguishing characteristic of the temperature distribution is a sharp decrease of temperature near the crystallization front. At the distance of 0--5 mm from the crystallization front the temperature falls according to the exponential law (exponent 1.2 mm\). With distance from the crystallization front the smooth fall of temperature is observed. The second distinguishing characteristic is the variation of temperature distribution with the pulling rate » . This cool ing rate increases when decreasing the growth rate. This effect is connected to the efficiency of heat radiation out of the crystal that is better the slower the pulling is. Some experimental observations suggest that this effect can also be explained by the crystal quality: the transparency is increased with decreasing the growth rate.

Fig. 3. The axial temperature distribution in sapphire ribbons grown by: 1 -- GES technique (» "0.05 mm/min, » "6.3 mm/min); 2 -- EFG technique (» "1.0 mm/min); 3 -- EFG technique (» "0.1 mm/min).


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Fig. 4. The temperature--time measurement date for the GES method. Marking is the same as in Fig. 1. A -- passing of the thermocouple seal above the die at a movement of a ribbon in one direction; C -- in the opposite direction; B and D -- reversal points.

Fig. 5. The two-dimentional temperature distribution in sapphire ribbon near to crystallization front for the GES technique. X-coordinate along ribbon width (0--20 is equal to 16 mm), ½-coordinate along ribbon length (0--20 is equal to 4 mm), Z-- temperature coordinate.

3.2. GES variant The sapphire ribbons with 2.5;16 mm in cross section have been grown. The average thickness of each growing layer is 100 m. Fig. 4 shows the temperature measurement data for the GES method. The point A corresponds to

a maximum of temperature at passing of the thermocouple seal above the die at a movement in one direction, point C -- in the opposite direction. The points B and D correspond to a moment of reversal of a direction of horizontal translation of a seed holder. In this moment the thermocouple seal is in the extreme position outside of a zone of the die.


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Fig. 4 shows that the temperature maximum (points A, C) decreases with increasing distance from the crystallization front, and disappears absolutely on distance of 4--5 mm. Also the temperature measurements have shown that in extreme points B and D the temperatures difference is 15°C. This is because the heat zone has the radial temperature gradient, approximately equal to 1°/mm. The highresolution TV camera enables observation of the meniscus shape during the crystal growth. A concave crystal--melt phase boundary was established (Fig. 2). After the measurements, the crystal was quickly lifted from the die to preserve the shapedphase boundary in the moment of crystal-die contact (Fig. 2d). The re-melting of crystal above the die is 0.2--0.6 mm, that is, several earlier crystallized layers. Fig. 3 (curve 1) illustrates the axial temperature distribution in the ribbon grown by the GES method. As in the case of EFG method the temperature falls according to the exponential law. The two-dimensional temperature distribution near the crystallization front is shown for the GES method (Fig. 5). This distribution was obtained on the base of time-dependence of temperature distri-

bution in Fig. 4 for the die under the middle part of the ribbon situation (A and C). The die has a pronounced effect on the temperature distribution. In parallel with the axial nonlinear temperature distribution in a crystal near to interface there also has been a strong distortion of the temperature field in the radial direction. Analysis of the temperature distribution and the second derivative of the temperature suggests the high level of thermoelastic stresses in the grown GES crystals.

References
[1] P.I. Antonov, S.I. Bakholdin, E.A. Tropp, V.S. Yuferev, J. Crystal Growth 50 (1980) 62. [2] A.L. Alishoev, L.M. Zatulovsky, Yu.K. Lingard, D.L. Shur, Bull. Acad. Sci. USSR Phys. Ser. 52 (1988) 99. [3] P.I. Antonov, S.I. Bakholdin, M.G. Vasilev, V.M. Krymov, A.V. Moskalev, V.S. Yuferev, Bull. Rus. Acad. Sci. Phys. 58 (1994) 72. [4] H.E. LaBelle, Mater. Res. Bull. 6 (1971) 581. [5] A.V. Stepanov, The Future of Metalworking, Lenizdat, Leningrad, 1963 (in Russian). [6] P.I. Antonov, Yu.G. Nosov, S.P. Nikanorov, Bull. Acad. Sci. USSR Phys. Ser. 49 (1985) 6.