JP2012224516A - Method for producing oxide single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an oxide single crystal, with which a long garnet crystal without strain is grown with excellent reproducibility by producing no twist in a shape of the crystal, and suppressing occurrence of a facet on a flat growth interface.SOLUTION: In the method for producing the oxide single crystal, in which a raw material for the single crystal is put in a crucible in a furnace, heated and melted, and subsequently the oxide single crystal is grown by a rotating-pulling method in which a seed crystal is brought into contact with a raw material melt and a growing crystal is pulled up with rotation, pulling up of the growing crystal is started with initial rotation speed (ω) of 20 rpm or less, in succession a crystal diameter of the growing crystal is made to increase, interface inversion of the crystal is confirmed, and subsequently a shoulder part is formed by lowering the rotation speed (ω) so as to be in the range indicated by formula (1), ω×(L/L)>ω>ω×(L/L)(1). In formula, ω is rotation speed of the crystal after interface inversion, ωis initial rotation speed, Lis an initial depth of the melt, and L is a depth of the melt during crystal growing.

Description

  The present invention relates to a method for producing an oxide single crystal, and more specifically, reproduces a long garnet crystal without distortion by suppressing generation of facets at a flat growth interface without causing twisting of the crystal shape. The present invention relates to a method for producing an oxide single crystal that can be grown with good performance.

Oxide single crystals, such as garnet single crystals, are widely used as non-magnetic garnet substrates for liquid phase epitaxial growth in the production of magnetic garnet films for various devices used in optical components. The magnetic garnet film has a Faraday rotation effect, and is used as a Faraday rotator in an optical isolator, an optical circulator, an optical switch or the like using this effect.
Garnet single crystals include yttrium / aluminum / garnet (YAG) crystal, gadolinium / gallium / garnet (GGG) crystal, etc., but YAG crystal is a solid laser material, GGG crystal is a rare earth-iron- for optical isolator. It is used as a substrate for growing a garnet (RIG) film.

These garnet crystals for optical use are generally produced by the Czochralski method (Cz method: rotational pulling method) in which a seed crystal is attached to a raw material melted in a crucible and pulled while rotating.
Two types of convection are generated in the raw material melt grown by the Cz method. One is natural convection due to the temperature difference in the melt. Natural convection rises along the crucible wall from the outer periphery of the crucible bottom, flows toward the melt surface center after reaching the melt surface, and sinks toward the crucible bottom at the center. The other is forced convection caused by the centrifugal force exerted on the melt in contact with the growth interface rotating relative to the melt due to the rotation of the growing crystal. Forced convection flows from the center of the crucible bottom toward the crystal growth interface being grown and from the growth interface toward the outer periphery of the crucible. That is, forced convection is a flow in the opposite direction to natural convection.
The natural convection of the material melt changes depending on the viscosity and amount of the melt, the surrounding heat retaining structure, etc., and the forced convection changes depending on the rotational speed of the crystal. The solid-liquid interface is made flat by gradually increasing the forced convection and balancing with natural convection. However, unless this operation is performed by a skilled worker, it cannot be confirmed that the solid-liquid interface is flat.

Therefore, it is proposed to measure the temperature of the material melt, perform Fourier transform to decompose the temperature change for each frequency component, and find a frequency component spectrum smaller than the crystal rotation number (Patent Document 1). Even if it is not an operator, it is easy to confirm that the solid-liquid interface is flat, and by maintaining the crystal rotation speed at this time, variation in material properties in the direction perpendicular to the crystal pulling direction can be reduced. You can do that.
In the Cz method of garnet crystal, when the convection in the melt is grown under natural convection predominance, the growth interface becomes convex with respect to the melt, and a smooth surface in the atomic order called facet develops on the growth interface. The growth mechanism in the facet plane is different from the growth mechanism in the part where the facet is not formed (off-facet part), so the atomic incorporation is different between the facet growth part and the off-facet growth part. Make a difference. If the crystal composition is different, the lattice constant also varies, and distortion occurs at the boundary between the facet portion and the off-facet portion. The portion where this distortion occurs cannot be used for optical applications.

In the conventional growth method, as described above, in order to suppress facet development, the growth interface is increased by increasing the rotation speed of the crystal after the growth crystal has reached a certain diameter and strengthening forced convection due to crystal rotation. An operation called interface inversion to change the shape from convex to almost flat has been performed to eliminate facets (Patent Document 2).
However, when the growth interface shape is flat, the crystal shape is easily twisted, and it becomes difficult to stably control the crystal shape. As crystal growth progresses and the remaining amount of melt in the crucible decreases, the temperature difference in the melt decreases and natural convection becomes weaker, whereas forced convection due to crystal rotation becomes stronger. The interface changes from flat to concave, and it becomes increasingly difficult to control the crystal shape. Therefore, conventionally, since the growth was completed before the twist of the crystal shape occurred, it was difficult to grow a long crystal because the weight of the grown crystal was limited with respect to the amount of raw material input. It was.

JP 2001-068884 A JP 2005-29400 A

  An object of the present invention is to provide a method for producing an oxide single crystal capable of growing a long garnet crystal without distortion with good reproducibility without generating a twist of the crystal shape and suppressing facet generation at a flat growth interface. Is to provide.

  In order to solve the above-mentioned conventional problems, the present inventor has conducted extensive research, and when growing an oxide single crystal such as a garnet crystal by the Cz method, the inversion of the growth interface is induced by forced convection by crystal rotation. After that, by reducing the rotation speed of the crystal under specific conditions in accordance with the decrease rate of the melt amount accompanying crystal growth, it has been found that it is possible to obtain a long crystal while keeping the growth interface shape constant, The present invention has been completed.

That is, according to the first invention of the present invention, the single crystal raw material is put in the crucible in the furnace and heated and melted, and then the seed crystal is brought into contact with the raw material melt to pull the growth crystal while rotating it. In the method for producing an oxide single crystal by growing the oxide single crystal, the pulling of the grown crystal starts at an initial rotation speed (ω ₀ ) of 20 rpm or less, and subsequently the crystal diameter of the grown crystal is increased. After confirming the interface reversal, a method for producing an oxide single crystal is provided in which the shoulder is formed by reducing the rotational speed (ω) within the range represented by the following formula (1).
ω ₀ × (L / L ₀ )>ω> ω ₀ × (L / L ₀ ) ^1/4 (1)
(Where, ω is the rotational speed of the crystal after interface reversal, ω ₀ is the initial rotational speed, L ₀ is the initial melt depth, and L is the melt depth during growth)

According to the second invention of the present invention, in the first invention, the oxide single crystal is an yttrium aluminum garnet (YAG) crystal, a gadolinium gallium garnet (GGG) crystal, or a neodymium gallium garnet. Provided is a method for producing an oxide single crystal, which is a garnet crystal selected from (NGG) crystal, gadolinium / scandium / gallium / garnet (GSGG) crystal, or terbium / gallium / garnet (TGG) crystal. .
According to the third aspect of the present invention, in the first aspect, the interface inversion of the crystal is induced by increasing the rotational speed as the crystal diameter of the grown crystal is increased. A method for producing a crystal is provided.
According to a fourth aspect of the present invention, there is provided the oxide single crystal according to the first aspect, wherein the crystal diameter of the grown crystal at the time of interface inversion is increased to 60% or more of the straight body diameter. A manufacturing method is provided.
Furthermore, according to the fifth aspect of the present invention, there is provided the method for producing an oxide single crystal according to the first aspect, wherein the crystal diameter of the grown crystal is 50 to 90 mm. The

  According to the present invention, it is possible to stably grow a long and distortion-free garnet crystal, which is difficult to manufacture by the conventional growing method. The high-quality garnet crystal thus obtained can provide a material for a solid laser or a material for an optical isolator at a low cost.

5 is a graph showing the relationship between the melt depth after the inversion of the growth interface due to crystal rotation and the crystal rotation speed in growing garnet crystals.

  Hereafter, the manufacturing method of the oxide single crystal of this invention is demonstrated in detail using drawing.

That is, in the method for producing an oxide single crystal according to the present invention, a single crystal raw material is put in a crucible in a furnace and heated and melted, and then a seed crystal is brought into contact with the raw material melt and the grown crystal is rotated while being rotated. In the method of growing an oxide single crystal by the method, the pulling of the growth crystal started at an initial rotation speed (ω ₀ ) of 20 rpm or less, and subsequently the crystal diameter of the growth crystal was increased to confirm the interface inversion of the crystal. After that, the rotational speed (ω) is lowered within a specific range to form the shoulder.

(1) Oxide single crystal In the present invention, the oxide single crystal is not limited by the type of oxide, but garnet crystals, particularly yttrium aluminum garnet (YAG) crystals used as optical products, gadolinium Examples of the garnet crystal include gallium garnet (GGG) crystal, neodymium gallium garnet (NGG) crystal, gadolinium scandium gallium garnet (GSGG) crystal, and terbium gallium garnet (TGG) crystal. Such a garnet crystal includes not only an undoped crystal to which a dopant is not added but also a crystal to which a dopant such as Ca, Zr, or Mg is added.

(2) Pulling method In the growing method of the garnet single crystal of the present invention, the pulling method to be used is not particularly limited. For example, a known method such as the Czochralski method which is a high-frequency heating method and a growth furnace are used. I can do it.

As a growth furnace, a crucible formed of a noble metal such as iridium, a furnace material formed of alumina or the like as a heat insulating material around the crucible, and a high-frequency coil as a heating device arranged outside the furnace material Is used. The apparatus is desirably provided with a window for observing the melt surface and a CCD camera for monitoring the seed crystal and the grown crystal.
For example, an oxide raw material prepared in a predetermined ratio according to the target crystal composition is put in an iridium crucible, and in a garnet single crystal, heated to 1700 to 1900 ° C. and melted to obtain a melt. The seed crystal is in contact with the melt and pulled up at a predetermined speed while rotating at a predetermined crystal rotation speed.
In the growth of the single crystal, the seed crystal is brought into contact with the melt, the neck portion is formed as described above, the shoulder portion is formed by adjusting the rotation speed and the pulling speed, and the straight body portion is subsequently formed. At this time, it is preferable to measure the temperature of the melt surface in the vicinity of the interface between the single crystal and the raw material melt using a radiation thermometer or the like.

The seed crystal used in the growth method of the present invention is not particularly limited. For example, a crystal having good lattice constant matching with the target garnet single crystal is used, and among these, nonmagnetic for liquid phase epitaxial growth is used. When obtaining a garnet substrate, Gd ₃ Ga ₅ O ₁₂ crystal (lattice constant 1.238 nm) or Nd ₃ Ga ₅ O ₁₂ crystal (lattice constant 1.2509 nm) is preferable.

(3) Relationship between melt depth and crystal rotation speed When the seed crystal is brought into contact with the melt and pulled up, the raw material melt decreases as the crystal grows, and its depth in the crucible becomes shallower. The cause of the melt convection changes according to the change in the depth of the raw material melt. For example, in the initial stage of crystal growth, when the raw material melt is deep, forced convection is mainly caused by crucible or seed rotation, but when the raw material melt is reduced and shallow, the influence of the forced convection becomes weaker, and the heater Natural convection by heating is the main.

The natural convection of the material melt changes depending on the viscosity and amount of the melt, the surrounding heat retaining structure, etc., and the forced convection changes depending on the rotational speed of the crystal. The solid-liquid interface was made flat by gradually increasing the forced convection and balancing with natural convection. However, as described above, unless this operation is performed by a skilled worker, it cannot be confirmed that the solid-liquid interface is flat.
In general, a pulled single crystal is grown with a neck portion that is a starting point of crystal growth formed from a seed crystal, a shoulder portion that reaches the straight body portion while increasing the crystal diameter from the neck portion, and a predetermined crystal diameter. In the shoulder, the crystal diameter gradually increases with the distance of crystal growth from the seed crystal (hereinafter sometimes referred to as growth distance). This is because different internal stresses are generated. As a result, crystal defects are introduced at the shoulder or the boundary between the shoulder and the straight body, cracks called cracks enter the crystal grown due to internal distortion due to the shoulder shape, and sometimes the crystal breaks. There was a problem such as falling out.

In conventional silicon single crystal production, when the single crystal is pulled by the CZ method, the contact area between the melt and the quartz crucible decreases as the height of the silicon melt surface decreases. There is a problem that the amount of oxygen dissolved in the liquid decreases and the oxygen concentration in the crystal tends to decrease, and as a result, the oxygen concentration in the axial direction (pull-up direction) of the silicon single crystal tends to be uneven. In order to cope with this, a method of changing the rotation speed of the crucible is considered. This is intended to increase the concentration of oxygen in the crystal pulled up because the amount of dissolution into the liquid increases as the relative rotational speed of the container (crucible) with respect to the liquid (melt) increases. It is.
In general, such crucible rotation is performed in Cz growth of semiconductor crystals such as Si, GaAS, and GaP. In Cz growth of oxide crystals such as garnet crystals including GGG, growth is performed. As the crucible is deformed along with this, the crucible cannot be rotated.

  The present invention proposes a method capable of growing a garnet crystal free from strain with good reproducibility without generating such twisting of the crystal shape, suppressing the generation of facets at a flat growth interface.

In the present invention, unlike the conventional means, the crystal rotation speed is changed according to the decrease in the melt level.
First, at the start of growth, the growth is performed at a low rotation speed, and the interface inversion is induced by increasing the rotation speed as the crystal diameter increases. The rotation speed and the pulling speed are not particularly limited, but the rotation speed is preferably 20 rpm (20 rotations per minute) or less, and preferably 15 to 20 rpm. It is desirable to induce this interface inversion before forming the straight body portion during shoulder growth.

Inversion of the crystal interface can be induced by increasing the rotation speed as the crystal diameter of the grown crystal increases. The interface inversion depends on the crystal diameter, crucible diameter, temperature gradient in the melt, melt depth, and crystal rotation speed.
At this time, the crystal diameter of the grown crystal is preferably increased to 60% or more of the straight body diameter to be grown. For example, in the case of a garnet crystal, when the crystal diameter is 50 mm to 90 mm and the crucible diameter is 100 mm to 200 mm, the temperature gradient in the melt is 2 ° C./cm or less and the initial melt depth is 85 mm or more. It is preferable.

Next, in the present invention, after confirming interface inversion by decreasing the crystal weight increase rate during shoulder growth, the crystal rotation speed is changed under specific conditions.
FIG. 1 is a graph showing the relationship between melt depth and crystal rotation speed in GGG single crystal growth. This is an input value in the growth program, and the melt depth remains in the crucible by subtracting the measured value of the weight of the grown crystal continuously measured during growth from the weight of the raw material charged in the crucible. The depth of the melt is calculated from the amount of the remaining melt and the cross-sectional area of the shape inside the crucible.
In the present invention, as a result of examining the relationship between the melt depth and the crystal rotation speed under various conditions, the crystal growth interface is substantially flat by changing the crystal rotation speed in the manner plotted by ■ and ◆ in FIG. Therefore, high quality crystals could be stably obtained without facets appearing at the growth interface. After the interface inversion, in order to maintain the shape of the grown crystal, the rotational speed is decreased with the reduction of the melt depth. The relationship between the crystal rotation speed and the melt depth in the present invention is in the range represented by the following formula (1). It has been confirmed that this applies not only to GGG single crystals but also to the growth of all garnet crystals including GSGG single crystals.
ω ₀ × (L / L ₀ )>ω> ω ₀ × (L / L ₀ ) ^1/4 (1)
(Where, ω is the rotational speed of the crystal after interface reversal, ω ₀ is the initial rotational speed, L ₀ is the initial melt depth, and L is the melt depth during growth)

  On the other hand, when the crystal rotation speed becomes faster than the condition of the formula (1) as shown by Δ in FIG. 1, the growth interface becomes concave and the crystal outer shape becomes spiral, and the effective length of the straight body portion Will be limited. When the crystal rotation speed becomes slower than the condition of formula (1) as indicated by x in FIG. 1, the growth interface becomes convex and facets develop at the interface, and distortion occurs in the crystal.

  The crystal shape is adjusted by measuring the crystal weight during growth, deriving the diameter, growth rate, and the like by calculation, and adjusting the rotation speed and pulling speed. Also, the change in crystal weight is fed back to the power applied to the high frequency induction coil to control the melt temperature.

(4) Obtained crystal to be obtained According to the present invention, a crystal to be grown has excellent characteristics such that no twist is observed, no cracks, and no optical distortion is observed in the obtained single crystal. Since the grown oxide single crystal has high crystallinity and uniformity, it can be used as a material for electronic parts and optical parts by slicing and polishing the wafer. From the garnet crystal, a magnetic garnet film having a Faraday rotation effect is obtained, and can be used as a Faraday rotator in an optical isolator, an optical circulator, an optical switch or the like using this effect.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[Example 1]
Using an Ir crucible having a size φ150 mm × 150 mmH, a GGG single crystal was grown with a straight barrel portion φ3 in by the Cz method. As the seed crystal, a GGG single crystal of φ7 mm was used. A high-frequency induction heating type single crystal growth furnace was used as the growth furnace.
Gd ₂ O ₃ and Ga ₂ O ₃ having a purity of 4N were weighed and mixed, and then charged in a crucible to obtain a growth raw material. After charging the raw material, it was heated and heated to melt the raw material. Crystal growth was started by adjusting the temperature of the raw material melt and pulling it up while rotating the seed crystal in the melt. The crystal rotation speed at the start of growth was 15 r. p. m. It was. During the growth, the weight of the crystal is monitored by a weight sensor attached to the pulling shaft to automatically control the crystal shape.
When the crystal diameter reached φ70 mm, the crystal weight decreased and it was confirmed that the growth interface was reversed. Thereafter, the crystal rotation speed is 15 r. p. m. Kept. When the crystal diameter exceeds 75 mm, the crystal rotation speed is changed to the melt depth change 15 × (L / L ₀ ) (the melt depth at the start of the crystal rotation speed change: L ₀ , A single crystal having a straight body diameter of φ80 mm and a straight body length of 100 mm was grown by decreasing the melt depth during growth. After separating the crystal from the melt, it was gradually cooled to room temperature, and the crystal was taken out from the growth furnace.
No twist was observed in the shape of the obtained crystal, and no crack was generated. When the crystal was immersed in methylene iodide and the shape of the growth interface was observed under polarized light, it grew on the convex interface from the start of growth until the occurrence of interface inversion, and after the occurrence of interface inversion, it grew on the flat interface until the crystal was released I found out.
When a sample having a diameter of 3 mm and a thickness of 1 mm was cut out from the straight body of the crystal and both ends of the sample were subjected to polarization observation after mirror polishing, no optical distortion was observed, and the crystal was sufficient as an optical crystal. I was able to confirm. In addition, when a lattice constant measurement was performed at one point at the center and four points at the outer periphery of the sample, the difference between the maximum value and the minimum value was 0.0003 mm with respect to the average value of 12.4 mm. It was found that high crystals were obtained. The results are shown in Table 1.

[Example 2]
Using the same raw material, seed crystal, crucible, and growth apparatus as in Example 1, φ3 inch GGG was grown.
When the crystal diameter reached φ70 mm, the crystal weight decreased and it was confirmed that the growth interface was reversed. Thereafter, the crystal rotation speed is 15 r. p. m. Kept. When the crystal diameter exceeds φ75 mm, as shown in FIG. 1, the crystal rotation speed is decreased according to the change in melt depth 15 × (L / L ₀ ) ^1/4 , and the straight body diameter φ80 mm, A single crystal having a trunk length of 100 mm was grown. After separating the crystal from the melt, it was gradually cooled to room temperature, and the crystal was taken out from the growth furnace.
No twist was observed in the shape of the obtained crystal, and no crack was generated. When the crystal was immersed in methylene iodide and the growth interface shape was observed under polarized light, it grew on the convex interface from the start of growth until the occurrence of interface inversion, and after the interface inversion, the interface was almost flat until the separation of the crystal. It turns out that it is growing.
When a sample having a diameter of 3 mm and a thickness of 1 mm was cut out from the straight body of the crystal and both ends of the sample were subjected to polarization observation after mirror polishing, no optical distortion was observed, and the crystal was sufficient as an optical crystal. I was able to confirm. In addition, when a lattice constant measurement was performed at one central part and four outer peripheral parts of the sample, the difference between the maximum value and the minimum value was 0.0005 mm with respect to the average value of 12.4 mm, which was very uniform. It was found that high crystals were obtained. The results are shown in Table 1.

[Example 3]
Using an Ir crucible of size φ100 mm × 100 mmH, GSGG single crystal growth of the straight barrel portion φ2 in was performed by the Cz method. A GSGG single crystal having a diameter of 7 mm was used as a seed crystal. A high-frequency induction heating type single crystal growth furnace was used as the growth furnace.
Gd ₂ O ₃ , Sc ₂ O ₃ and Ga ₂ O ₃ having a purity of 4N were weighed and mixed, and then charged in a crucible to obtain a growth raw material. After charging the raw material, it was heated and heated to melt the raw material. Crystal growth was started by adjusting the temperature of the raw material melt and pulling it up while rotating the seed crystal in the melt. The crystal rotation speed at the start of growth was 15 r. p. m. It was. During the growth, the weight of the crystal is monitored by a weight sensor attached to the pulling shaft to automatically control the crystal shape.
When the crystal diameter reached φ45 mm, the crystal weight decreased and it was confirmed that the growth interface was reversed. Thereafter, until the crystal diameter is 50 mm, the rotation speed of the crystal is 15 r. p. m. Kept. When the crystal diameter exceeds 50 mm, the crystal rotation speed is changed to melt depth change 15 × (L / L0) (melt depth at the start of crystal rotation speed change: L0, melt depth during growth: L), a single crystal having a straight barrel diameter of 55 mm and a straight barrel length of 80 mm was grown. After separating the crystal from the melt, it was gradually cooled to room temperature, and the crystal was taken out from the growth furnace.
No twist was observed in the shape of the obtained crystal, and no crack was generated. When the crystal was immersed in methylene iodide and the shape of the growth interface was observed under polarized light, it grew on the convex interface from the start of growth until the occurrence of interface inversion, and after the occurrence of interface inversion, it grew on the flat interface until the separation of the crystal. I found out.
When a sample having a diameter of 2 inches and a thickness of 1 mm was cut out from the straight body of the crystal, and polarized light was observed after mirror polishing of both end faces of the sample, no optical distortion was observed, and the crystal was sufficient as an optical crystal. I was able to confirm. In addition, when a lattice constant measurement was performed at one central portion and four outer peripheral portions of the sample, the difference between the maximum value and the minimum value was 0.0002 mm, which was very uniform with respect to the average value of 12.6 mm. It was found that high crystals were obtained. The results are shown in Table 1.

[Example 4]
Using the same raw material, seed crystal, crucible, and growth apparatus as in Example 3, φ2 inch GSGG was grown.
When the crystal diameter reached φ45 mm, the crystal weight decreased and it was confirmed that the growth interface was reversed. Thereafter, until the crystal diameter is 50 mm, the rotation speed of the crystal is 15 r. p. m. Kept. When the crystal diameter exceeds φ50 m, the crystal rotation speed is decreased according to the change in melt depth 15 × (L / L0) 1/4, and a single crystal having a straight barrel diameter of 55 mm and a straight barrel length of 80 mm Nurtured. After separating the crystal from the melt, it was gradually cooled to room temperature, and the crystal was taken out from the growth furnace.
No twist was observed in the shape of the obtained crystal, and no crack was generated. When the crystal was immersed in methylene iodide and the growth interface shape was observed under polarized light, it grew on the convex interface from the start of growth until the occurrence of interface inversion, and after the interface inversion, the interface was almost flat until the separation of the crystal. It turns out that it is growing.
When a sample having a diameter of 2 inches and a thickness of 1 mm was cut out from the straight body of the crystal, and polarized light was observed after mirror polishing of both end faces of the sample, no optical distortion was observed, and the crystal was sufficient as an optical crystal. I was able to confirm. In addition, when a lattice constant measurement was performed at one central part and four outer peripheral parts of the sample, the difference between the maximum value and the minimum value was 0.0003 mm with respect to the average value of 12.6 mm, which was very uniform. It was found that high crystals were obtained. The results are shown in Table 1.

[Comparative Example 1]
Using the same raw material, seed crystal, crucible, and growth apparatus as in Example 1, φ3 inch GGG was grown. At this time, the rotation speed of the crystal was 15 r. p. m. And constant.
When the crystal diameter reached φ70 mm, a decrease in crystal weight was observed, confirming that interface inversion occurred. Thereafter, as shown in FIG. p. m. The crystal was grown with a straight barrel diameter of φ80 mm and a straight barrel length of 100 mm. After separating the crystal from the melt, it was gradually cooled to room temperature, taken out of the growth furnace and evaluated.
Although no crack was observed in the crystal, a spiral staircase twist was observed in the crystal shape from the time when the straight body portion was grown about 20 mm. When the crystal was immersed in methylene iodide and the growth interface shape was observed under polarized light, it was observed that it grew on the convex interface from the start of growth until the occurrence of interface inversion, and became a flat interface after the occurrence of interface inversion. It was done. However, as the crystal growth progressed, the interface shape changed from flat to concave. It was found that the crystal shape was twisted from the vicinity where the growth interface shape was concave.
Since the crystal shape is twisted, the portion where the φ3 in substrate can be obtained from this crystal is about 20 mm above the straight body portion, which is very inefficient and costly. The results are shown in Table 1.

[Comparative Example 2]
Using the same raw material, seed crystal, crucible, and growth apparatus as in Example 1, φ3 inch GGG was grown. At this time, the rotation speed of the crystal was 15 r. p. m. And constant.
When the crystal diameter reached φ70 mm, a decrease in crystal weight was observed, confirming that interface inversion occurred. Thereafter, the crystal rotation speed is 15 r. p. m. Kept. When the crystal diameter exceeds φ75 mm, as shown in FIG. 1, the crystal rotation speed is decreased according to the change in melt depth 15 × (L / L ₀ ) ² , and the straight body diameter φ80 mm, the straight body length A single crystal having a thickness of 100 mm was grown. After separating the crystal from the melt, it was gradually cooled to room temperature, and the crystal was taken out from the growth furnace.
Although no crack was observed in the crystal, the shape of the separation interface was convex about 15 mm toward the melt side. When the crystal was immersed in methylene iodide and the growth interface shape was observed under polarized light, it was observed that it grew on the convex interface from the start of growth until the occurrence of interface inversion, and became a flat interface after the occurrence of interface inversion. It was done. However, it was found that the interface shape changed from flat to convex as crystal growth progressed. In addition, it was observed that distortion occurred in the crystal from the vicinity where the interface shape was 5 mm or more convex with respect to the melt side.
Since the strain in the crystal remains, the portion where the φ3 in substrate can be obtained from this crystal is about 30 mm in the upper part of the straight body part, which is very inefficient and costly. The results are shown in Table 1.

[Comparative Example 3]
Using the same raw material, seed crystal, crucible, and growth apparatus as in Example 3, φ2 inch GSGG was grown. At this time, the rotation speed of the crystal was 15 r. p. m. And constant.
When the crystal diameter reached φ45 mm, the crystal weight decreased, confirming that interface reversal occurred. Thereafter, the crystal rotation speed is changed to 15. p. m. The crystal was grown with a straight barrel diameter of 55 mm and a straight barrel length of 80 mm. After separating the crystal from the melt, it was gradually cooled to room temperature, taken out of the growth furnace and evaluated.
Although no crack was observed in the crystal, a spiral staircase twist was observed in the crystal shape from the time when the straight body portion was grown about 20 mm. When the crystal was immersed in methylene iodide and the growth interface shape was observed under polarized light, it was observed that it grew on the convex interface from the start of growth until the occurrence of interface inversion, and became a flat interface after the occurrence of interface inversion. It was done. However, as the crystal growth progressed, the interface shape changed from flat to concave. It was found that the crystal shape was twisted from the vicinity where the growth interface shape was concave.
Since the crystal shape is twisted, the portion from which the φ2in substrate can be obtained from this crystal is very inefficient and costly growing, about 20 mm above the straight body. The results are shown in Table 1.

[Comparative Example 4]
Using the same raw material, seed crystal, crucible, and growth apparatus as in Example 3, φ2 inch GSGG was grown. At this time, the rotation speed of the crystal was 15 r. p. m. And constant.
When the crystal diameter reached φ45 mm, the crystal weight decreased, confirming that interface reversal occurred. Thereafter, the rotation speed of the crystal is 15 r. p. m. Kept. When the crystal diameter exceeds 50 mm, the crystal rotation speed is decreased according to the change in melt depth 15 × (L / L ₀ ) ² to obtain a single crystal having a straight body diameter of 55 mm and a straight body length of 80 mm. I grew up. After separating the crystal from the melt, it was gradually cooled to room temperature, and the crystal was taken out from the growth furnace.
Although no cracks were observed in the crystal, the shape of the separation interface was convex about 12 mm toward the melt side. When the crystal was immersed in methylene iodide and the growth interface shape was observed under polarized light, it was observed that it grew on the convex interface from the start of growth until the occurrence of interface inversion, and became a flat interface after the occurrence of interface inversion. It was done. However, it was found that the interface shape changed from flat to convex as crystal growth progressed. In addition, it was observed that distortion occurred in the crystal from the vicinity where the interface shape was 4 mm or more convex with respect to the melt side.
Since the strain in the crystal remains, the portion where the φ2 in substrate can be obtained from this crystal is about 25 mm in the upper part of the straight body portion, and the growth is very inefficient and expensive. The results are shown in Table 1.

"Evaluation"
From the results shown in Table 1, in Examples 1 to 4, after confirming the interface reversal of the crystal, the rotational speed (ω) was reduced within the range represented by the formula (1) to form the shoulder, A sufficiently large garnet crystal without distortion can be grown without causing twisting or cracking of the crystal shape.
On the other hand, in Comparative Examples 1 and 3, since the shoulder was formed without changing the rotational speed (ω) even after confirming the crystal interface inversion, the crystal shape was twisted and sufficiently large. The garnet crystal could not be grown, and in Comparative Examples 2 and 4, after confirming the inversion of the interface of the crystal, the rotational speed (ω) was changed outside the range indicated by Formula (1) to form the shoulder. Therefore, crystal distortion occurred and a sufficiently large garnet crystal could not be grown.

  According to the present invention, a long and distortion-free garnet crystal is obtained, and this high-quality garnet crystal can be used as a material for a solid-state laser or an optical isolator.

Claims (5)

A single crystal raw material is put in a crucible in a furnace and heated and melted. Then, a seed crystal is brought into contact with the raw material melt, and the grown single crystal is grown by rotating and pulling up while rotating the grown crystal. In the manufacturing method,
The growth crystal is pulled up at an initial rotation speed (ω ₀ ) of 20 rpm or less. Subsequently, after the crystal diameter of the growth crystal is increased and the interface inversion of the crystal is confirmed, the rotation speed (ω) is expressed by the following formula (1 The method for producing an oxide single crystal is characterized in that the shoulder portion is formed by lowering it within the range indicated by.
ω ₀ × (L / L ₀ )>ω> ω ₀ × (L / L ₀ ) ^1/4 (1)
(Where, ω is the rotational speed of the crystal after interface reversal, ω ₀ is the initial rotational speed, L ₀ is the initial melt depth, and L is the melt depth during growth)   The oxide single crystal is an yttrium aluminum garnet (YAG) crystal, gadolinium gallium garnet (GGG) crystal, neodymium gallium garnet (NGG) crystal, gadolinium scandium gallium garnet (GSGG) crystal, or terbium. The method for producing an oxide single crystal according to claim 1, which is a garnet crystal selected from gallium garnet (TGG) crystals.   2. The method for producing an oxide single crystal according to claim 1, wherein the interface reversal of the crystal is induced by increasing the rotation speed as the crystal diameter of the grown crystal increases.   The method for producing an oxide single crystal according to claim 1, wherein the crystal diameter of the grown crystal at the time of interface reversal is increased to 60% or more of the straight body diameter.   2. The method for producing an oxide single crystal according to claim 1, wherein the crystal diameter of the grown crystal is 50 to 90 mm.

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