Segregation control in horizontal Bridgman crystal growth

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ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 165 (1996) 195-197

L e t t e r to t h e E d i t o r s

Segregation control in horizontal Bridgman crystal growth Y. T a o , S. K o u * Department of Materials Science and Engineering, and Center of Excellence in Solid,cation Processing Technologies of Engineering Materials, Universi~' of Wisconsin, Madison, Wisconsin 53706, USA

Received 11 October 1995; accepted 24 November 1995

Abstract

An attempt was made to reduce dopant segregation in horizontal Bridgman crystal growth by separating the melt into a growth melt and a replenishing melt, with a long passageway to suppress dopant diffusion between them. NaNO 3 was used as a model material for crystal growth. With proper boat tilting, an effective segregation reduction was obtained.

In a recent report [1], attempts to reduce dopant segregation in vertical Bridgman crystals have been described. As for horizontal Bridgman growth, two different attempts to control dopant segregation have been made recently, besides the earlier attempts involving microgravity and magnetic damping. In one attempt, the melt is divided by many partitions into small regions to suppress convection in the melt [2]. The partitions are removed one by one from the melt as the growth front advances during crystal growth. In the other attempt [3], a replenishing melt is injected from a heated syringe above the boat into the melt in the boat. In the present report we describe a simpler approach to segregation control in horizontal Bridgman growth that, unlike microgravity and magnetic damping, is effective even for a segregation coefficient k<
* Corresponding author.

illustrated in Fig. 1. A single crystal can grow at the desired constant dopant concentration C o if the growth melt is kept at the constant dopant concentration C o / k . To do so, the growth melt must be replenished with a replenishing melt of composition C o and dopant diffusion between the two melts must be suppressed. This, in fact, is equivalent to the so-called " z o n e leveling" for achieving uniform compositions in zone melting [4]. The solid feed of composition C o in zone melting is equivalent to the liquid feed of C O here. Sodium nitrate, NaNO 3, was selected as the model material for crystal growth mainly because of its low melting point (307°C) and known segregation coefficient, i.e. k = 0.06 for LiNO 3 [5]. The boat was prepared from a Pyrex glass tube of 3.7 cm ID, its length being about 18 cm excluding the shoulder and seed-holding portions. An aluminum cylinder, 7.6 cm long and about 3.6 cm diameter, was used as the diffusion baffle. In other words, a passageway as long as 7.6 cm formed by the aluminum cylinder and the boat inner wall sepa-

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196

Y. Tao, S. Kou /Journal of Co'stal Growth 165 (1996) 195-197 Rod

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rated the two melts. In order to avoid simplifying assumptions for estimating back diffusion, e.g. plug flow and no back diffusion during waiting and initiation, the passageway was made much too long for back diffusion to occur at all. The connecting rod allowed the aluminum cylinder to be stopped to initiate melt replenishing, while the boat continued to be pulled during crystal growth. Initially, the aluminum cylinder was positioned in the boat at about 1.5 cm from its shoulder. A stationary, two-zone, transparent furnace was used for observation during crystal growth, the lowtemperature zone being on the left. The furnace and the boat were tilted about 2 ° by raising the low-temperature zone slightly. About 110 g NaNO3-0.045 wt% LiNO 3 was charged into the boat, with an additional 0.210 g LiNO 3 added to the left side of the aluminum cylinder to raise the dopant concentration of the growth melt. After complete melting of the charge in the high-temperature zone of the furnace, the boat (with the aluminum cylinder resting in it) was pulled toward the low-temperature zone to initiate crystal growth. The aluminum cylinder was stopped to initiate replenishing of the growth melt, when the crystal just passed the shoulder of the boat. The connecting rod was released when the aluminum cylinder reached the end wall of the boat, i.e. when the replenishing melt ran out. During crystal growth the boat was pulled at 0.5 c m / h . The resultant crystal, about 9 cm long and uniform in thickness, was analyzed for dopant segrega-

Fig. 2. Dopant distributions in a NaNO 3 crystal grown with a 2 ° tilt angle: (a) axial; (b) lateral; (c) vertical. The axial dopant distribution in a similar crystal grown by conventional horizontal Bridgman growth is included in (a) for comparison.

Y. Tao, S. Kou / Journal of Crystal Growth 165 (1996) 195-197

tion by inductively coupled plasma (ICP) emission spectrometer analysis with a detection limit of 0.02 ppm Li. As shown in Fig. 2a, the dopant concentration is uniform except in the last portion of the crystal where it rises rapidly because the replenishing melt ran out. During the last stage of crystal growth, the dopant concentration of the growth melt increased sharply since it was no longer replenished with a melt of a much lower concentration. From the uniform dopant concentration of C O= 0.045 wt% LiNO 3 and k = 0.06, the corresponding composition of the growth melt Co/k is about 0.7 wt% LiNO 3. The dopant distributions in the lateral and vertical directions of the same crystal, at 6 cm from the shoulder, are shown in Figs. 2b and 2c, respectively. As shown, the dopant concentration is also uniform in these directions. A second crystal was grown also with a 2 ° tilting but with the aluminum cylinder resting at the end of the boat throughout the experiment. It is equivalent to conventional Bridgman growth in that there is no melt replenishing. As also shown in Fig. 2a, the dopant concentration increases continually from the beginning to the end, resulting in severe dopant segregation. A third crystal was grown with melt replenishing but without boat tilting. Axial segregation was still severe. Along the crystal central plane, the crystal thickness was about 1.9 cm near the shoulder but only about 1.5 cm at the end. In other words, the

197

crystal top surface was not horizontal but inclined downward about 2 ° . This is because the density of solid NaNO 3 (Ps = 2.1 g / c m 3) is about 10% greater than that of the NaNO 3 melt (PL = 1.9 g/cm3). Since the depth and hence volumetric feed rate of the replenishing melt decreased during crystal growth, the dopant concentration of the growth melt increased, causing the dopant concentration of the crystal to rise significantly. Similar thickness variations due to the density difference have also been reported in zone melting [6]. In conclusion, with proper boat tilting, dopant segregation in horizontal Bridgman growth can be reduced by separating the melt into a growth melt and a replenishing melt with a long passageway to suppress dopant diffusion between them. This work was supported by the National Science Foundation under Grant DMR-9415482.

References [1] Y. Tao and S. Kou, J. Crystal Growth, in press. [2] Fujitsu Ltd., Jpn. Kokai Tokyo Koho, 1987, p. 4 (in Japanese). [3] Mitsubishi Electric Corp., Jpn. Kokai Tokyo Koho, 1986, p. 463 (in Japanese). [4] W.G. Pfann, Zone Melting, 2nd ed. (Wiley, New York, 1966) p. 199. [5] M.H. Lin and S. Kou, J. Crystal Growth 132 (1993) 461. [6] W.G. Pfann, Zone Melting, 2nd ed. (Wiley, New York, 1966, p. 47.