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Journal of Crystal Growth 187 (1998) 107 -- 110

EFG growth of sapphire tubes upto 85 mm in diameter
V.N. Kurlov*, B.M. Epelbaum
Institute of Solid State Physics, RAS, Chernogolovka, Moscow District 142432, Russian Federation Received 27 May 1997; accepted 15 September 1997

Abstract Conditions for large tubular sapphire crystal growth have been examined. Single crystalline tubes with outer diameters of upto 85 mm were grown successfully by the EFG technique using a simple one-heater growth assembly. The dependence of structural perfection on growth conditions and die-top design have been investigated. The process for seed enlargement upto the full-circular cross-section was found to be of primary importance for high-quality growth, and a suitable algorithm has been suggested. 1998 Elsevier Science B.V. All rights reserved. PACS: 81.05.!t; 81.10.!h; 81.10.Fq Keywords: Melt; Crystal growth; Sapphire; EFG method; Large size tubular crystal

1. Introduction The favorable combination of excellent optical and mechanical properties of sapphire complemented with high chemical durability makes it an attractive structural material for high-technology applications. Of particular interest are tubular sapphire crystals of large diameter for special windows, high-pressure reactors, engines, and CVD systems. This letter reports the simple experimental procedure for the growth of macrodefect-free tubular sapphire crystals upto 85 mm in diameter.

* Corresponding author. Fax: #7 096 5764254; e-mail: kurlov@issp.ac.ru. 0022-0248/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 22-02 48( 9 7 )0 084 6 - 4

Fig. 1. Schematic diagram of the growth heating assembly.


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Fig. 2. Faceting of sapphire crystal: (a) the r- and n-planes at the end surface; (b) side view showing crystallographic designations of faces (left) and view down the three fold crystallographic C-axis (right).

2. Experimental procedure The EFG technique is described in detail in Ref. [1]. Our adaptation of a common EFG heating assembly for the growth of large sapphire tubes is sketched in Fig. 1. The molybdenum die was constructed of two tubes each 50 mm in length and the molybdenum crucible was 120 mm in diameter. Capillary feeding was achieved with the use of a ring channel 0.5 mm in width. The die was fixed at the holder plate, which was attached independently to the upper shield cover. This arrangement ensures

that (i) the crucible and the die could be separated easily after the growth process by lowering the former and (ii) a uniform radial temperature distribution near the die top was achieved. The heater was assembled out of two cylindrical graphite parts electrically connected in parallel. The resistivity of the upper part was 20--40% larger than that of the lower one, depending on the processing tube diameter. An initial charge was crushed Verneuil leucosapphire. A high purity Ar atmosphere under a pressure of 1.1 atm was used as an ambient. The 4;4mm seeds were cut along the C-axis from


V.N. Kurlov, B.M. Epelbaum / Journal of Crystal Growth 187 (1998) 107 -- 110

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boules oriented by X-ray diffraction. The pulling rate was varied in the range of 15--40 mm/h. The solidified fraction was nearly 80--90% in all growth experiments. The crystal quality was examined by optical microscopy.

3. Results and discussion The enlargement of the tube upto the die size was found to be the critical factor for high-quality crystal growth. For tubes of large diameter grown along the C-axis, the most remarkable occurrence in this stage was the appearance of inclined facets at the end surface (Fig. 2a). This surface was defined by the sequential appearance of positive rhombohedral planes r (+10 1 1,, +01 11,, +1 101,) and the dipyramidal planes n (+42 2 3,, +24 23,, +2 2 43,, +4 223,, +2 42 3,, +22 4 3,) (Fig. 2). During the enlargement of the tube, upto the die size, the transition from one plane to another may be of three types: nPr, rPn and nPn. The angle between the perpendicular to r-planes and the Caxis is 57.6° and the angle between the perpendicular to n-planes and the C-axis is 61.2° [2]. The total length of enlargement l depends on tube diameter d"(d #d )/2, where d and d are inner and outer crystal diameters, respectively. This l is related to , where is the averaged declination angle of the end crystal surface with respect to the pulling (C) axis, given by the equation l" d/2tn . Obviously, the value of that needs to be maintained in the course of the enlargement stage is in the range of 28.8° (90°!61.2°) to 32.4° (90°!57.6°). We found experimentally the optimal length to be in the range of l"(2.55!2.70)d. The typical shoulder profile is shown in a photograph in Fig. 3. More emphatically the transition from one plane to another can be seen by the development of the projection profile in the same Fig. 3. The development line appears stepped and has three jogs, near 0.6l, 1.4l and 1.8l irrespective of the tube diameter. The experimental data l( , d) have been used for evaluation of the angular coordinate (z) of the boundary crystal segment for the cylindrical system of coordinates (r, , z). On that basis, the calculation of a proper program of mass change for automated crystal weight control became possible [3].

Fig. 3. Enlargement of the sapphire tube crystal: (a) photograph of the typical shoulder profile of sapphire tube and (b) development of the projection profile of sapphire tube.

Die-top design was recognized to be the next critical point for large diameter tube growth. In spite of crystal transparency, the growing tube appears to act as a radiation shield, making the inner die edge temperature lower. As a result, with a flat top die, partial freezing of the crystal to the die occurs. This effect was most pronounced during the growth of tubes with large wall thickness. A simple improvement, changing of die top geometry from flat to concave, enabled us to overcome this problem. Fig. 4 shows the sapphire tube of 85 mm in


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Fig. 4. Die-top improvement, concave die-top surface makes possible high-quality growth of a sapphire tube of 85 mm diameter.

diameter grown with a die of optimal concave-top geometry.

nearly equal and make possible the stable growth of thick-walled 85;75 mm tube.

4. Conclusions The following conclusions can be drawn from the results of EFG growth experiments with sapphire tubes with diameter upto 85 mm: (a) for the first time, transparent tubular macro-defect-free sapphire single crystals with the outer diameter of 85 mm were successfully grown by the EFG technique using a simple one-heater growth assembly; (b) governing conditions for high quality growth were determined to be a regime where the seed expands upto the full-die cross-section, and die-top design; and (c) concave die-top surfaces enable the temperature of inner and outer tube edges to be

Acknowledgements This work was carried out partially under the financial support of the ISF Foundation. Special thanks goes to Evgeni A. Ryabtsev for technical assistance.

References
[1] H.E. LaBelle, J. Crystal Growth 50 (1980) 8. [2] M.V. Klassen-Neklyudova, Kh.S. Bagdasarov (Eds.), Ruby and Sapphire, Nauka, Moscow, 1974 (in Russian). [3] V.N. Kurlov, S.N. Rossolenko, J. Crystal Growth 173 (1996) 417.