JPS6354679B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for growing rare earth aluminate single crystals by the Czyochralski method.
Rare earth aluminate crystals such as yttrium aluminate (YAlO ₃ ) and gadolinium aluminate (GdAlO ₃ ) have a pseudoperovskite crystal structure at room temperature, but the crystal structure of these crystals changes during the cooling process from high temperatures. . For example _YAlO3
is a high-temperature stable crystal, as is clear from the Y ₂ O ₃ −Al ₂ O ₃ binary system equilibrium phase diagram. Because rare earth aluminate crystals have these properties, they are prone to twinning and cracking during the single crystal growth process, and even if a crystal is successfully grown, the internal strain is large and the crystal often breaks during processing.
The present inventors investigated the relationship between crystal growth conditions and the occurrence of such twins and cracks based on detailed experiments, and found that the conditions of seeding (seeding operation) performed at the initial stage of crystal growth are We found that there is a significant relationship between crystallization and cracking. In other words, it was found that by using high-quality seeds (seed crystals) and performing appropriate seeding, crystals without the above-mentioned defects can be produced.

The present invention provides a method for performing this appropriate seeding extremely easily and reliably.

Hereinafter, conventionally attempted seeding methods and the method according to the present invention will be specifically explained.

Figure 1 is a schematic diagram of a commonly used high-frequency heating-type Tchochralskii single crystal pulling apparatus, and shows an example of the apparatus used to carry out the seeding experiment and the present invention described below. be. However, in the present invention, there is no need to limit this heating method. In Fig. 1, 1 is a housing;
2 is an atmospheric gas inlet, 3 is an outlet, 4 is a crystal rotation axis, 5 is an alumina refractory, 6, 7 and 8 are zirconia refractories, 9 is a zirconia powder, 10 is a heating high frequency coil, 11 is an iridium Smelting pit,
Reference numeral 12 indicates a seed crystal, 13 indicates a grown crystal, and 14 indicates a raw material melt.

Figures 2 to 7 are diagrams showing the situation of the seed and raw material melt during seeding when rare earth aluminate single crystals are grown using the above-mentioned apparatus using a 2 mm to 3 mm square seed. be.

FIG. 2 shows a case where the temperature conditions in and around the raw material melt are much higher than the temperature suitable for seeding. In other words, if the temperature is extremely high, simply bringing the seed 21 close to the melt 22 will cause the seed 2 to
The tip of No. 1 comes off. Of course, in such a situation, if a seed is inserted into the melt, it will melt rapidly, making seeding impossible.

Next, as shown in FIG. 3, when the seed 31 is inserted at a considerably low temperature, the molten material 32 around the seed solidifies and rapidly spreads to the surrounding area. Cracks were observed in most of the crystals grown under these conditions.

Figure 4 shows the seeding situation that is considered appropriate for normal crystal pulling such as Y ₃ Al ₅ O ₁₂ (yttrium aluminum garnet, hereinafter abbreviated as YAG), which is similar to Figure 2. This is achieved at temperatures between FIG. Under these conditions, the moment the seed 41 comes into contact with the melt, the melt 42 is depressed as shown in FIG. 4A, but eventually it blends in with the melt and the liquid level becomes flat as shown in FIG. When the seed 41 is pulled out from the molten body 42 and observed, the tip is sharp, and the seed 41 is pulled out from the molten body 42 again.
This shape does not change much even if you insert the seed 41 inside and pull it out again after a while. Conventionally, in the case of YAG and the like, it has been customary to start crystal growth in this state or at a slightly higher temperature. However, in the case of rare earth aluminate, which is the object of the present invention, when crystal pulling is started from the state shown in Figure 4B, the diameter gradually increases as shown in Figure 4D, and the crystal grows through this process. Cracks and twinning occur in crystals that are pulled apart, regardless of other conditions such as pulling speed. In state B, if you raise the temperature slightly and start pulling after the seed and melt reach a state of thermal equilibrium, the diameter will not increase as in state D, but on the contrary, the crystal will become thinner in a short time. It gets cut off from the molten body.
Therefore, it is conceivable to start raising the temperature at an appropriate rate at the same time as starting pulling from state B, but this would be extremely difficult to control, and in most cases the result would be as shown in Figure 4E. The crystals often broke off from the melt.

As mentioned above, the conventional seeding method, namely
In the case of YAG crystals, even if growth is started under conditions that are said to be good seeding in general crystal growth, if the single crystal to be grown is rare earth aluminate, there are many cases where twins and cracks cannot be avoided. In many cases, it was too difficult to implement on an industrial scale.

By the way, in growing rare earth aluminate single crystals, the present inventors created a neck-shaped region with an extremely small crystal diameter, for example, a neck-like region with a diameter of 1 mm or less and a length of 1 mm or more, thereby preventing cracks or double crystals from forming. It was found that the occurrence of crystals was extremely reduced. However, as explained above, it is almost impossible to create such a small-diameter region immediately after seeding using the normal seeding method, and therefore, it is almost impossible to create a region with such a small diameter immediately after seeding, and therefore, it is difficult to create a high-quality region that does not cause any cracks or twins directly below the seeding. It was extremely difficult to obtain crystals.

The present invention uses a special seeding method to make it possible to produce large single crystals of extremely high quality very easily and reliably in the above-mentioned micro-diameter region on an industrial scale. The method will be explained in detail below based on a specific example.

In carrying out the method according to the present invention, first, the temperature is set so that the conditions for normal seeding are achieved, that is, the conditions shown in FIG. When deeply immersed in the melt 52 for 5 to 10 seconds and pulled out, the
Even angular seeds such as the one shown in Fig. 5B come out with a tapered point. From this state, the temperature is raised very slightly, and a seed 61 is immersed in the melt 62 by about 5 mm to 7 mm, as shown in FIG. 6A. At this time, in about 5 to 15 minutes, the seed crystal in the part below the liquid surface of the melt 62 almost melts, as shown in FIG. Readjust the temperature so that they are completely separated. To achieve this, all you need to do is raise the temperature slightly and check the temperature by putting in and taking out the seeds from time to time.If you actually do this, you can easily set the temperature to this state. Once the Natsutara Seed 61 is melted again about 7 to 10 mm 62
Insert the seed 61 into the seed 61, and repeat pulling out and inserting the seed 61 two or three times in about 15 seconds to shape the tip of the seed 61 into a thin and pointed shape as shown in FIG. 5B. Next, the shaped seeds are immersed in the melt 62 for about 5 to 7 mm, and
After 3 to 3 minutes, when the seeds are lifted all at once at a pulling speed of 5 mm per second or more, the molten material is pulled up by the seed crystals as shown in Figure 7. At this time, if the pulling speed is low, the shape described above, that is, the diameter 1
Not only is it difficult to form a region smaller than mm, but also twins and lattice defects often occur, making it impossible to achieve the object of the present invention. In order to fully obtain the effects of the present invention, the speed of pulling up at once is 5 mm per second.
It has become clear from many experimental results that the above is necessary. The distance to be pulled up is determined by the lifted molten material (the part marked by the diagonal lines in Figure 7).
It is best to stop just before the gravity falls and separates, usually 0.5 mm to 1 mm. This operation is extremely important and will determine the success or failure of the subsequent steps, but fortunately, no matter how many times you try again, as long as you can achieve this state, there will be no negative impact on the final result. Once the state shown in Figure 7 has been achieved in this way, if you leave it for about 5 minutes without changing the temperature setting, the melt that has been pulled up from the seeds (the area hatched with diagonal lines) will disappear. It crystallizes and appears to glow a little. From this state onward, a normal crystal pulling operation can be started and the crystal can be grown into any desired shape by controlling the temperature.

FIG. 8A is an external view of a crystal grown by the conventional method, and FIGS. 8B and C are external views of crystals grown by the method of the present invention.

Crystals exhibiting the appearance shown in Figure 8A are difficult to grow as desired in the first place, but even on the rare occasions when we were able to grow them, the probability of obtaining a crystal without twins or cracks was less than 30%. . However, in those exhibiting the appearance shown in Fig. 8 B and C, large crystals were definitely grown, and more than 80% of the crystals had no twins or cracks. After seeding according to the method of the present invention, whether the diameter is increased gradually as shown in Figure 8B or relatively rapidly as shown in Figure 8C, Crystals without cracks or twins were obtained in a fully satisfactory state.

Next, as an example, a case will be described in which a YAlO ₃ single crystal is grown by the method according to the present invention.

Using the lifting device shown in Figure 1, 50 mm in diameter and 50 mm in depth.
100 grams of Al ₂ O ₃ in a mm iridium crucible,
A mixed powder consisting of 221 grams of Y ₂ O ₃ is dissolved,
Seeding was performed using a prismatic seed with a thickness of 3 mm in the orientation <010>. After setting the seeding temperature as described above, the seeds were immersed in the melt for about 7 mm, and after 1 minute, the seeds were manually pulled up at a speed of 5 mm per second until they were about to separate from the melt, and then stopped.
After leaving it for a minute, normal growth was started. The crystal was pulled at a speed of 2.5 mm per hour while controlling the temperature so that the shoulders of the crystal spread out at an angle of about 20 degrees, and when the diameter reached a constant diameter of about 20 mm, growth was completed when the total length was about 64 mm. The growth was performed in a nitrogen atmosphere, and the seed rotation speed was 15 revolutions per minute. When the crystals were taken out after the growth, the diameter of the crystals immediately after seeding was about 1.5 mm, and the section that maintained this diameter was about 2.0 mm. No cracks or twins were observed in the crystal, and no cracks occurred when the crystal was processed into the desired shape. The method of the present invention was effective even when the seed orientation, rotation speed, pulling speed, etc. were varied.

The method and effects of the present invention have been explained above using the case of growing a YAlO ₃ single crystal as an example, but the growth method of the present invention can also be applied to similar crystals such as other rare earth aluminate crystals such as gadolinium aluminate (GdAlO ₃ ). Of course, this is also valid in the case of.

As described above, in the method according to the present invention, during seeding, the temperature is set higher than in normal crystal pulling, specifically, the temperature is set so that if the seed is inserted, the inserted part will melt in a few minutes to more than ten minutes. After that, the seeds are reinserted, and after a certain period of time has elapsed, the seeds are pulled up at a speed of 5 mm per second or more and held still. At this time, wait for the melt that has been pulled up with the seeds to solidify before starting the normal growing operation. enter,
By implementing this extremely clear and easy seeding method, it has become possible to grow rare earth aluminate single crystals, which were previously considered extremely difficult to grow, while keeping the incidence of cracks and twins extremely low. .

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an example of a general high-frequency heating type Czyochralski type single crystal pulling apparatus used to carry out the present invention. In the figure, 1 is the housing, 2 is the atmospheric gas inlet, 3 is the same outlet, and 4
is a crystal rotation axis, 5 is an alumina refractory, 6, 7 and 8 are zirconia refractories, 9 is a zirconia powder,
10 is a heating high-frequency coil, 11 is an iridium crucible, 12 is a seed crystal, 13 is a grown crystal, 14
indicate the raw material melt, respectively. Figure 2, Figure 3,
and FIG. 4 are diagrams for explaining a conventional seeding method. FIG. 5, FIG. 6, and FIG. 7 are diagrams for explaining the seeding method according to the present invention. 21, 31, 41, 51, 6 in the diagram
1,71 is the seed crystal, and 22,32,4
2, 52, 62, and 72 indicate melts. 8th
The figures show grown crystals; FIG. 8A is grown by the conventional method, and FIGS. 8B and C are grown by the method of the present invention.

Claims (1)

[Claims] 1. When performing a seeding operation to grow a rare earth aluminate single crystal using the Czyochralski method, first, the temperature of the raw material melt is temporarily adjusted so that the insertion part of the seed (seed crystal) inserted into the melt is The temperature is set so that the seeds will almost melt in a few minutes to more than ten minutes, and if left for a while, the seeds will eventually separate from the melt, and then the seeds are reinserted into the raw material melt for 1 to several minutes. Later, the seeds were pulled up all at once at a speed of more than 5 mm per second,
At this time, the molten material that has been pulled up by fitting into the tip of the seed and combined with the interfacial tension is stopped in a state where it is on the verge of being separated by its weight, and the temperature is left as it is without intentionally changing the temperature. A method for growing rare earth aluminate single crystals using the Czyochralski method, which is characterized by waiting for solidification before proceeding to normal crystal growth operations.