JPH09169592A - Method for growing single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a crystal having a good shape and good quality at a high yield without the twist of the crystal in the second half of growth and the occurrence of cracking by accumulation of thermal strains by regulating the rising speed of a crucible larger than the descending speed of a solid-liquid boundary. SOLUTION: The single crystal is formed by disposing the crucible into which raw material for producing the single crystal are charged movably in a perpendicular direction relatively to a heater, heating and melting the raw material in the crucible with the heater and bringing a seed crystal into contact with the melt thereof. After the single crystal attains a specified crystal diameter, the crucible is risen at the speed higher than the descending speed of the solid-liquid boundary in the crucible relatively to the heater.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing an oxide single crystal such as a lithium niobate single crystal and a lithium tantalate single crystal.

[0002]

2. Description of the Related Art
When growing an oxide single crystal of lithium tantalate or lithium niobate by a pulling method, a high frequency heating furnace is often used. In this case, in the obtained single crystal, defective shape and generation of crystal defects are greatly influenced by the temperature gradient near the liquid surface of the melt in the crucible. In fact, if the temperature gradient becomes too gentle, twisting will occur in the latter half of the crystal growth, and if the temperature gradient is too tight, crystal defects will be more likely to occur from the bottom of the crystal. Usually, when growing a single crystal, the melt contained in the crucible decreases at the same time as the growth, and the level of the solid-liquid interface with respect to the upper end of the crucible decreases. As a result, the temperature gradient in the vicinity of the crucible changes from the conditions at the initial stage of growth, and the quality such as the shape of the single crystal and crystal defects is greatly affected. Conventionally, in order to solve these problems, as proposed in Japanese Patent Publication No. 58-25078, the set position of the central portion of the heating device in the crucible depth direction melts in accordance with the decrease in the liquid level of the melt. A method was adopted to maintain a relative positional relationship at the set position in the center of the depth direction of the material, and there was no change in the temperature gradient in the crucible or the convection of the melt, and consideration was given to stable crystal growth. There is.

However, in the method of maintaining the relative position between the center of the heating device and the center of the melt as in the prior art, the temperature gradient in the crucible and the convection of the melt change in the first half of the crystal growth. Even if it does not exist, from the middle stage to the latter half of the growth, the grown crystal itself blocks the release of solidification heat from the solid-liquid interface, and as a result, the temperature gradient becomes loose and there is a problem that shape defects such as twist occur in the latter half of the crystal. Will occur. Also,
Even if the convection of the melt does not change in the first half of the crystal growth, the amount of the melt decreases in the second half of the growth,
At least that convection changes. For this reason, the convection near the solid-liquid interface, which is the crystal growth point, changes, causing a problem of crystal defects.

Therefore, an object of the present invention is to provide an oxide single crystal having a good shape without fail, and to provide a method for producing a single crystal which is free from cracks due to twisting or thermal strain and has a good yield. To aim.

[0005]

Means for Solving the Problems and Modes for Carrying Out the Invention As a result of intensive studies for achieving the above-mentioned object, the present inventor has found that the solid-liquid interface descent rate in the crucible relative to the heating device. By increasing the speed at a higher speed, particularly 1.3 to 2 times the solid-liquid interface lowering speed, good lithium niobate single crystal or lithium tantalate single crystal can be obtained without causing twisting or cracking. The inventors have found that the oxide single crystal of 1) can be grown with high yield, and have completed the present invention.

Therefore, according to the present invention, the crucible into which the raw material for producing a single crystal is charged is arranged so as to be vertically movable relative to the heating device, and the raw material in the crucible is heated and melted by the heating device. , When growing a single crystal by bringing a seed crystal into contact with the melt, after the single crystal reaches a predetermined crystal diameter, the crucible is relatively moved relative to the heating device from the solid-liquid interface descent rate in the crucible. Provided is a method for producing a single crystal, which comprises raising the temperature at a high rate.

The present invention will be described in more detail below. The method for producing a single crystal of the present invention is effectively used for growing an oxide single crystal such as a lithium niobate single crystal or a lithium tantalate single crystal. As described above, the crucible into which the raw material for producing a single crystal is placed is arranged so as to be movable in the vertical direction relative to the heating device, and the raw material in the crucible is heated and melted by the heating device, and this melting is performed. When growing a single crystal by bringing a seed crystal into contact with an object, after the single crystal reaches a predetermined crystal diameter, the crucible is relatively moved with respect to the heating device at a speed higher than the solid-liquid interface descent rate in the crucible. To raise.

As described above, according to the present invention, the crucible is raised at a rate higher than the rate at which the level of the solid-liquid interface is lowered, and when the rate of the solid-liquid interface is lowered by the rate of rise of the crucible, melting to the heating device is performed. The liquid level of the product rises, and the temperature gradient near the melt in the first half of the growth shifts into the melt as shown in FIG. For this reason, the temperature gradient immediately above the melt becomes larger than that under the conventional conditions, and good crystals can be obtained without causing shape defects such as twisting. In this case, the ascending speed of the crucible is preferably 1.3 to 2 times the solid-liquid interface lowering speed, and if it exceeds 2 times, the temperature gradient becomes tight and cracks due to thermal strain may occur.

Here, the raising of the crucible rises relative to a heating device such as a high-frequency heater, and the crucible itself may be raised or the heating device may be lowered.

The present invention is particularly suitable for the production of lithium tantalate or lithium niobate single crystals, both of which have been grown under a condition with a comparatively steeper temperature gradient than conventional ones. This is because the crucible ascending speed exceeds the descending speed at the solid-liquid interface and the material has no problem in growth even if the temperature gradient just above the melt becomes slightly tight.

The other single crystal growth conditions and manufacturing apparatus of the present invention are the same as those for normal conditions according to the type of single crystal.
The device can be used.

[0012]

EXAMPLES The present invention will be described below in detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[Examples 1 to 4, Comparative Examples 1 to 3] Diameter 15
A lithium tantalate firing raw material was placed in a crucible having a thickness of 0 mm, a height of 150 mm and a thickness of 2 mm, and the raw material was heated by a high frequency heating device to be melted. Then, seed crystals were put into this melt, and a lithium tantalate single crystal having a diameter of 80 mm was grown and pulled up at a growth rate of 6 mm / hr to grow a lithium tantalate single crystal having a diameter of 80 mm and a straight body length of 150 mm. . In this case, the solid-liquid interface descending speed associated with the pulling was 2.3 mm / hr, and the crucible was raised at the speed shown in Table 1 with respect to this speed. Note that FIG. 2 shows the relationship between the moving distance of the crucible and the liquid level lowering distance of the melt.

Table 1 also shows the shape and yield of the single crystal thus obtained.

[0015]

[Table 1]

Examples 5 to 8 and Comparative Examples 4 to 6 Lithium niobate single crystals were grown under the same conditions as in Examples 1 to 4 and Comparative Examples 1 to 3. Table 2 shows the results. Note that FIG. 2 also shows the relationship between the moving distance of the crucible and the liquid level lowering distance.

[0017]

[Table 2]

From the results shown in Tables 1 and 2, it is recognized that the method for growing a single crystal according to the method of the present invention can produce a single crystal having a good yield and a good shape without defects. On the other hand, when the rising speed of the crucible is small, the single crystal is twisted, when the rising speed of the crucible is small, the single crystal is twisted, and when the rising speed of the crucible is too large, cracks due to strain occur, and eventually In the case of, the yield was low.

[0019]

EFFECTS OF THE INVENTION According to the present invention, by defining the crucible ascending rate larger than the solid-liquid interface descending rate, the crystal is not twisted in the latter half of the growth and cracks due to the accumulation of thermal strain are not generated, and the shape is good. In addition, high quality crystals can be obtained with a high yield, and the effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in a temperature gradient due to a rise in a liquid level of a melt with respect to a heating device.
(B) is a diagram of the latter half of the breeding.

FIG. 2 is a graph showing the relationship between the crucible ascending speed and the liquid level descending distance of the melt.

Claims (3)

[Claims] 1. A crucible into which a raw material for producing a single crystal is charged is arranged so as to be movable in a vertical direction relative to a heating device, and the raw material in the crucible is heated and melted by the heating device, and the melt When a single crystal is grown by bringing the seed crystal into contact with, the single crystal reaches a predetermined crystal diameter, and then the crucible is raised at a speed higher than the solid-liquid interface descent rate in the crucible relative to the heating device. A method for producing a single crystal, which comprises: 2. The method according to claim 1, wherein the rising speed of the crucible relative to the heating device is 1.3 to 2 times the falling speed of the solid-liquid interface in the crucible. 3. The production method according to claim 1, wherein the single crystal is a lithium niobate single crystal or a lithium tantalate single crystal.