JPH032838B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides oxide single crystals produced by the Czyochralski method, particularly eurytite group Bi ₄ (Ge _x Si _1-x ) ₃ O ₁₂ (where x=0~
The present invention relates to the method for growing a single crystal (1).

Single crystals of the eurytite group, represented by bismuth germanate (Bi ₄ Ge ₃ O ₁₂ ), which are useful as scintillator materials for radiation detection, are grown by the Czyochralski method. The detection sensitivity to radiation, that is, the intensity of fluorescence generated from the scintillator, is strongly influenced by the quality of the crystal. In particular, when voids are present in the crystal, the fluorescence generated in the crystal is scattered and absorbed, significantly reducing radiation detection sensitivity. In order to eliminate these bubbles, it is desirable to increase the temperature of the melt and flatten the shape of the solid-liquid interface. A growth method for flattening the shape of the solid-liquid interface and eliminating bubbles is described in JP-A-112789-1989. On the other hand, in order to create a large scintillator for detecting high-energy radiation or to create a scintillator from a single crystal with a high yield, it is desired to increase the size of the grown crystal. However, for example, bismuth germanate single crystals tend to have a twisted crystal shape during the growth process, and this tendency becomes particularly strong when growing large single crystals with a diameter of 35 mm or more.

As an example, the growth of bismuth germanate single crystal (Bi ₄ Ge ₃ O ₁₂ :BGO) will be described. As shown in Figure 1, the diameter is 70mm, the depth is 60mm, and the plate thickness is 2mm.
A single crystal with a crystal diameter of 35 mm was grown from platinum crucible 1. When a platinum crucible is filled with about 1.3 kg of sintered material, the surface of the melt is located 7 mm below the top of the crucible 1. In the Czyochralski single crystal growth using high-frequency induction heating, the positions of the crucible 1 and the high-frequency coil 3 are important because the temperature distribution of the melt in the crucible changes greatly depending on the position. In this method, the center of the high frequency coil 3 in the length direction was positioned 13 mm below the center of the crucible 1 in the depth direction. Growth was started at this position, and a crystal shoulder 5 was created at a crystal rotation speed of 30 min ^-1 as shown in FIG. The cross section of the outside of the crystal was approximately circular, and the shape of the solid-liquid interface of the crystal at that time was convex toward the melt. However, when the solid-liquid interface shape changes from convex to flat at the crystal shoulder, the cross-sectional shape perpendicular to the crystal growth axis becomes distorted from circular. Third
The figure shows the appearance of a crystal whose outer shape has been deformed at the crystal shoulder and the cross-sectional shape perpendicular to the growth axis. 8 in FIG. 3 is a cross section of a parallel crystal part grown 10 mm from the crystal shoulder. It has already been transformed into a square, with dents in four places. As it continued to grow, it became starfish-shaped as shown in Figure 3 (9).

In this way, once the cross section is deformed, it does not return to its original state and the deformation is accelerated. On the other hand, if the crystal rotation speed is reduced in order to prevent this deformation, that is, to make the crystal outer shape circular, bubbles are likely to occur.
Moreover, as the melt level in the crucible decreases as crystallization progresses, the temperature difference throughout the crucible becomes smaller due to radiant heat from the crucible wall, making it impossible to obtain an optimal temperature distribution and increasing the temperature inside the crystal. A large number of bubbles are generated.

The external shape of a single crystal is strongly influenced by the temperature distribution in the melt, and in addition to the crystal rotation speed mentioned above, one way to prevent this crystal external shape from twisting is to place a heater at the upper end of the crucible to control the temperature around the melt. It is already known that the temperature can be increased (Utility Model Publication No. 54-22329). However, in this method, when the surface of the melt in the crucible decreases with crystallization, the heat from the heater no longer reaches the surface of the melt, and the problem of twisting still remains.

Therefore, under the growth conditions described above where the shape of the growth interface is flat, which is suitable for eliminating bubbles, the outer shape of the crystal tends to be twisted, and it is therefore necessary to simultaneously prevent the generation of bubbles and improve the outer shape of the crystal. was an extremely difficult problem.

The purpose of the present invention is to optimize the temperature distribution in the melt,
This method reduces bubbles in the crystal, prevents distortion of the crystal outline, and grows large, high-quality Bi ₄ (Ge _x Si _1-x ) O ₁₂ single crystals.

As a result of investigating the influence of the temperature distribution in the melt on the generation of bubbles in the crystal, it was found that the number of voids decreases when the vertical temperature gradient in the melt is large. In order to achieve such a temperature distribution, it is preferable to place the center of the crucible, which is the heating element, in the depth direction at a higher position than the center of the high frequency coil in the length direction.

On the other hand, if the crucible is located high relative to the high-frequency coil, the outer shape of the grown crystal will be severely twisted. However, it was found that when the crystal cross section perpendicular to the growth axis deviates from a circular shape from the crystal shoulder to the crystal parallel region, twisting occurs, and once twisted, the crystal twists more violently toward the lower part of the crystal. Therefore, if from the crystal shoulder to the crystal parallel part,
If the crystal cross section is circular, even at the bottom of the crystal,
It is thought that the distortion of the crystal external shape will be reduced.

Therefore, in the present invention, in the early stage of crystal growth, that is, in the growth of the crystal shoulder, the crystal rotation speed is reduced, the shape of the solid-liquid interface is made convex toward the melt, and the shape of the crystal cross section is maintained circular. Once growth stabilized, the crystal rotation speed was gradually increased to flatten the shape of the solid-liquid interface, and then the crucible was gradually raised relative to the high-frequency coil.

As a result, the vertical temperature gradient in the melt was maintained large, the number of bubbles in the crystal was significantly reduced, and at the same time, large distortion of the crystal shape was also prevented.

Hereinafter, the present invention will be explained using examples.

Example 1 Bismuth germanate (Bi ₄ Ge ₃ O ₁₂ ) was used as the first
A single crystal with a diameter of 35 mm was grown from platinum crucible 1 with a diameter of 70 mm and a depth of 60 mm as shown in the figure. The center position of the crucible 1 in the depth direction at the start of growth is 13 mm or less with respect to the length direction of the high frequency coil 3. In creating the crystal shoulder portion 5 after seeding, the crystal rotation speed was set to 20 min ^-1 or less. As a result, until it enters the crystal parallel part 6,
The shape of the solid-liquid interface of the crystal was convex toward the melt, and the cross-sectional shape of the crystal was circular as shown in FIG. 411. The crystal rotation speed of 20 min ^-1 is obtained when the furnace structure shown in Fig. 1 is adopted, and the value is not important. It is important that the shape is convex toward the liquid.

When the crystals entered the parallel part and grew about 5 mm, the crystal rotation speed was increased from 20 min ^-1 to 40 min ^-1 in about 1 hour. After increasing the temperature to 40 min ^-1 , the crucible was raised at a rate of 1.1 mm/h, which is the same as the temperature drop of the melt surface, so that the relative position between the melt surface and the high-frequency coil was always constant. The crystal pulling rate was 4.1 mm/h, which was set to be equal to the pulling rate of 3 mm/h when the crucible was stopped.

As a result, distortion of the crystal outer shape was prevented as shown in the crystal appearance and crystal cross section 12 and 13 in FIG. On the other hand, since the position of the crucible was raised relative to the high-frequency coil, the temperature gradient in the vertical direction in the melt was large, and the generation of bubbles could be reduced.

When the rising speed of the crucible is set to 75% of the falling speed of the melt surface, band-shaped bubbles are noticeably generated at the bottom of the crystal, and it is desirable that the rising speed of the crucible be at least 80% of the melt surface falling speed.

Example 2 Bismuth silicate (Bi ₄ Si ₃ O ₁₂ ) was grown using the same growth apparatus as used for growing bismuth germanate described in Example 1. Crucible 1 at the start of cultivation
The center position in the depth direction was 13 mm below the center of the high frequency coil 3 in the length direction. In growing the crystal shoulder 5 after seeding, the crystal rotation speed was set to 25 min ^-1.
I made it. This is because Bi ₄ Si ₃ O ₁₂ tends to have a convex solid-liquid interface. Immediately after the crystal diameter reaches the specified size, the crystal rotation speed is increased to 25 min ^-1.
It increased from 45 min ^-1 in about 1 hour. Thereafter, the crucible was raised at a rate of 1.1 mm/h, the same as the rate at which the melt surface was lowered. As a result, it was found that crystals without twisting of the crystal outer shape could be grown, and the generation of bubbles was also small.

As explained in the examples above, according to the present invention, the number of bubbles in Bi ₄ Ge ₃ O ₁₂ single crystals and Bi ₄ Si ₃ O ₁₂ single crystals is reduced, and single crystals with excellent crystal outlines can be grown. This has made it possible to manufacture more scintillators and larger scintillators from single crystals.

In the examples, Bi ₄ Ge ₃ O ₁₂ and Bi ₄ Si ₃ O ₁₂ were described, but it is clear that these mixed crystals having the same crystal structure also have the same effect.

[Brief explanation of the drawing]

Figure 1 shows the heat insulation structure near the crucible for crystal growth, Figure 2 shows the shape when growing a single crystal using the Czyochralski method, and Figure 3 shows bismuth germanate grown using the conventional method. Figure 4 shows the appearance of a single crystal or bismuth silicate single crystal and the shape of a cross section perpendicular to the crystal growth axis. FIG. 3 is a diagram showing a cross-sectional shape of a crystal. 1...Platinum crucible, 2...Alumina refractory, 3
...High frequency coil, 4...Position of the center of the high frequency coil in the longitudinal direction with respect to the center in the depth direction of the crucible, 5...Crystal shoulder, 6...Crystal parallel part, 7...
Shape of the solid-liquid interface at the crystal shoulder, 8...Cross-sectional shape at 10 mm from the parallel part, 9...70 mm from the parallel part
10... Cross-sectional shape at 150 mm from the parallel part, 11... Cross-sectional shape at 10 mm from the parallel part, 12... Cross-sectional shape at 70 mm from the parallel part,
13...Cross-sectional shape at 150mm from the parallel part.

Claims (1)

[Claims] 1. In growing a Bi ₄ (Ge _x Si _1-X ) ₃ O ₁₂ (where x = 0 to 1) single crystal by the Czyochralski method using high-frequency induction heating, solid-liquid crystal growth a step of having a small crystal rotation speed that makes the interface shape convex with respect to the melt and keeps the cross-sectional shape of the crystal outer shape approximately circular;
Bi ₄ (Ge _x Si _1-x ) ₃ O ₁₂
How to grow single crystals. 2. The relative rising speed of _the crucible is _{80 to 100} % of the falling speed of the surface of _the melt. Cultivation method.