JPH0114200B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a gallium garnet single crystal, particularly a gallium garnet single crystal having a large diameter of 75 mm or more and having few crystal defects, which is used as a working material for magnetic bubble memories and magnetic refrigerators. This invention relates to an industrial manufacturing method.

As is already known, a gallium garnet single crystal used as a substrate material for a magnetic bubble memory is required to be of extremely high quality with few crystal defects. In particular, recently, highly integrated devices of 1 Mbit/cm ² or more have been manufactured to increase the storage capacity of magnetic bubble memories, and requirements for substrate quality have become stricter. It is required that crystal defects such as minute scratches and local distortions on the surface due to processing are extremely low. The reason for this is that when a rare earth iron-based magnetic garnet thin film is liquid-phase epitaxially grown to generate and drive magnetic bubbles on a substrate, defects on the substrate surface cause defects in the magnetic thin film, making it difficult to drive the magnetic bubbles. This is due to interference or inducing the outflow of unnecessary magnetic bubbles, which becomes a serious cause of failure of magnetic bubble memory devices. In particular, since higher device densities are achieved with bubbles of smaller diameter, the size and number of defects that can be tolerated in magnetic garnet thin films for such high-density, high-integration devices depends on the bubble diameter. Correspondingly, smaller and smaller quantities are required.

Furthermore, in order to produce such highly integrated devices at low cost, single crystal wafers with a large diameter of 75 mm or more are recently required, and stable commercial production of high quality single crystals with such large diameters is difficult. extremely important.

The usefulness of the rare earth gallium garnet single crystal as a working material for magnetic refrigerators has also been experimentally demonstrated, and the development of practical machines for this new refrigerator is progressing. Materials used as working substances are required to have high thermal conductivity at extremely low temperatures in addition to magnetic properties as basic physical properties. Thermal conductivity at cryogenic temperatures is influenced by crystal lattice defects in the material,
It is known that the more defects there are, the lower the thermal conductivity. Therefore, the rare earth gallium garnet single crystal used for this purpose is required to be close to a perfect crystal with few defects.

Gallium garnet single crystals are industrially pulled using the Czyochralski method, but defects in these single crystals include crystal dislocations, micro-inclusions of iridium caused by the crucible material, and defects that have a different crystal structure from garnet. Microscopic inclusions of oxides, voids, oxygen vacancies, etc. are known. Raw materials rare earth oxides and gallium oxide are heated and melted in an iridium crucible, and when pulling a single crystal, lower oxides are generated due to decomposition and evaporation of the raw material materials due to the high temperature, and lower oxides are generated due to redox of iridium. Iridium fine particles are incorporated into the single crystal and become minute inclusions. These generally range in size from 1 μ to several tens of μ, and not only are they crystal defects themselves, but large inclusions cause crystal dislocations and degrade the quality of single crystals.

For this reason, various methods for suppressing such minute inclusions have been studied. For example, there is a method in which the above-mentioned raw material melt is exposed to an atmosphere containing oxygen at 3.8 to 19.4 mmHg for 1 to 10 hours, and then a single crystal is pulled under the same atmospheric gas (Tokuko Showa).
52-46198), a method in which a gas containing 0.25 to 1.5% oxygen is ozonized and a single crystal is pulled in an atmosphere containing this ozone (see Japanese Patent Publication No. 1278/1978), A method has been proposed in which a single crystal is pulled in a nitrogen gas atmosphere containing 0.5 to 3% oxygen (see Japanese Patent Laid-Open No. 136200/1983).

All of these methods are characterized by melting raw materials in the presence of a trace amount of oxygen and then pulling a single crystal. That is, at high temperatures, gallium oxide decomposes according to the following formula: Ga ₂ O ₃ (liquid) → Ga ₂ O (gas) + O ₂ (gas) ... (1), and the generated oxygen oxidizes the iridium of the crucible material. It becomes iridium oxide, Ir (solid) + O ₂ (gas) → IrO ₂ (gas) ...(2) Also, this IrO ₂ (gas) is converted to the following formula IrO ₂ (gas) + Ga ₂ O (gas) → Ir( Solid) + Ga ₂ O ₃ (liquid) ...(3) Or, IrO ₂ (gas) → Ir (solid) + O ₂ (gas) ...(4) is reduced to iridium by the reaction, and fine particles of iridium are formed. becomes. In addition, since the Ga ₂ O mentioned above is incorporated into the single crystal and becomes microscopic inclusions of different oxides, Ga 2 The purpose is to suppress the decomposition of ₂ O ₃ and, as a result, prevent the production of iridium fine particles and foreign oxides.

The present inventors have also confirmed that these methods are effective to some extent in pulling small-diameter gallium garnet single crystals in the early stages of development.
On the one hand, the presence of oxygen in the atmosphere promotes the generation of IrO ₂ according to equation (2), which causes iridium inclusions, and as mentioned above, the reaction mechanism that produces inclusions is Because it is based on a complex and delicate balance, it may not be effective at all depending on conditions such as slight variations in the raw material composition, single crystal pulling equipment, especially the heat insulation structure surrounding the crucible, and the flow of atmospheric gas. .
Therefore, with the above-mentioned known methods, it is difficult to completely suppress inclusions, and in particular, it is extremely difficult to commercially produce gallium garnet single crystals with a diameter of 75 mm or more and extremely few defects. This has become clear through experiments conducted by the present inventors.

In other words, when pulling large gallium garnet single crystals for highly integrated bubble memories or magnetic refrigerators, for example, gadonium gallium garnet single crystals with a diameter of 75 mm or more and a length of 250 mm or more, the diameter is approximately
Since an iridium crucible of 150 mm or more is used and the amount of raw material used at one time is more than 13 kg, the temperature of the crucible is higher than when it is small, which accelerates the decomposition of gallium oxide and the oxidation of Ir, resulting in a single crystal Numerous inclusions were found from the shoulder to the trunk.
It has become clear that known inclusion suppression methods cannot produce the desired single crystal with almost no inclusions.

The present invention overcomes these disadvantages of conventional methods and relates to a method for producing large diameter, extremely defect-free gallium garnet single crystals. That is, in producing a gallium garnet single crystal with a diameter of 75 mm or more by the Czyochralski method, after heating and melting a raw material consisting of gallium oxide and a rare earth metal oxide in an iridium crucible, when pulling the single crystal, Throughout the entire process, the atmospheric gas containing oxygen and CO ₂ is always maintained at an oxygen partial pressure of 20 to 100 mmHg, and the iridium oxide produced under the same oxygen partial pressure, which causes crystal inclusions, is mixed into the raw material melt and the monomer. This method is characterized by forcibly discharging liquid from the vicinity of crystal growth to the outside of the system.

The inventors found that the large diameter of 75 mm or more and the length of 250 mm
As a result of various experimental studies on how to stably and industrially produce high-quality gallium garnet single crystals with a diameter of 1 mm or more and an extremely small amount of inclusions, we found that:
We have discovered a method for producing high-quality, large-diameter, long-length gallium garnet single crystals with almost no inclusions, with different effects from conventional methods. In other words, the present inventors increased the oxygen partial pressure in the atmospheric gas, which was considered disadvantageous in conventional methods, and thereby more effectively suppressed the decomposition of gallium oxide, thereby converting it into lower oxides. The occurrence of inclusions caused by this process is almost completely prevented. On the other hand, IrO ₂ generated under a higher oxygen partial pressure is forcibly discharged from the raw material melt and the vicinity of single crystal growth to prevent Ir inclusions from being taken into the melt, thereby reducing the overall effect of these. As a result, we succeeded in eliminating almost all inclusions in the single crystal. Numerous experiments over a long period of time have shown that it is most effective when the atmosphere contains 20 mmHg (approximately 2.6%) to 100 mmHg (approximately 13.2%), and more preferably 20 mmHg to 40 mmHg (approximately 5.3%). It became. Several methods are effective for forcibly discharging the liquid, such as using an exhaust fan or suction pump, or introducing a large amount of gas into the pulling furnace, thereby removing harmful substances near the surface of the melt. A method of exhausting the atmospheric gas containing the substance can be used. The reason why CO ₂ is used as a component of the atmospheric gas is that it decomposes at high temperatures to generate oxygen, and the oxygen generated by this decomposition effectively acts as oxygen in the atmospheric gas, which is essential for the present invention. It is from. In addition, we have devised a heat insulation structure for the crucible, creating a gap between the lid at the top of the crucible and its upper heat insulation cylinder to serve as a gas passage, so that the fresh gas introduced into the furnace and the gas near the melt surface are separated. It is also effective to facilitate substitution. As described above, various means are effective, but the present invention is not limited to the examples described here. Harmful components are often removed from the system by physically forcibly replacing the atmospheric gas near the melt surface in the presence of a certain effective concentration of oxygen (in an atmosphere with an oxygen partial pressure of 20 to 100 mmHg). There is a philosophy in discharging

In addition, a high oxygen partial pressure is effective in preventing the decomposition of gallium oxide, so it suppresses compositional fluctuations in the raw material melt during pulling, and this allows the crystal to remain stable over the entire length of the long single crystal. It also has the effect of not causing fluctuations in the lattice constant. Substrates with small variations in crystal lattice constants can improve the quality of bubble memories and have significant technical and economical effects on commercial production.

In the method of the present invention, for example, a raw material composition is melted in an iridium crucible with an inner diameter of 145 mm or more, and then a single crystal is pulled. The raw material composition may be a known one,
Gallium oxide (Ga ₂ O ₃ ) with purity greater than 99.9955%
and a rare earth metal oxide such as gadolinium oxide (Gd ₂ O ₃ ), samarium oxide (Sm ₂ O ₃ ), neodymium oxide (Nd ₂ O ₃ ), etc., which also has a purity of 99.995% or more, and pre-mix with 1450% or higher purity. A gallium garnet polycrystal obtained by firing at ℃ is used.

The entire process of heating and melting the raw materials and then pulling the single crystal is performed using oxygen with an oxygen partial pressure of 20 to 100 mmHg.
It is carried out under an atmospheric gas containing _CO2 , and harmful substances that cause inclusions present in the atmospheric gas are continuously forcibly discharged to the outside. Gases other than oxygen and CO ₂ used include Ne,
Inert gases such as Ar, He, N2 _, etc. are used.

Next, this will be explained based on the attached drawings. FIG. 1 shows a schematic longitudinal cross-sectional view of an apparatus for carrying out the method of the present invention, in which an iridium crucible 1 for melting a raw material composition is surrounded by a refractory wall 2 filled with zirconia powder. The heating coil 3 is heated to a predetermined temperature by a high frequency induced current. A gallium garnet single crystal 5 is pulled from the gallium garnet polycrystalline melt 4 melted in the iridium crucible 1 by the Czochralski method.
An iridium lid 7 is provided at the upper end of the iridium crucible 1, and the opening area of this lid must be at least 80 cm ² , preferably 80 to 130 cm ² .A heat-retaining cylinder 8 made of zirconia is placed above it. be placed. Furthermore, a zirconia ring having an opening 6 is disposed at the upper end of this 8, but the opening area needs to be 40 cm ² or more, and the entire ring is further housed in an outer cylinder 9. For melting the gallium garnet polycrystal, the oxygen partial pressure is maintained at 20 to 100 mmHg throughout the entire process from the gas inlet 10 at the bottom of the outer cylinder 9.
The system starts by introducing an atmospheric gas mainly consisting of an inert gas containing oxygen and CO ₂ to fill the entire system with this atmospheric gas. Forced exhaust device 12
is operated to forcibly exhaust the atmospheric gas in the system from the gas outlet 11, and at the same time, the gas inlet 10 is operated.
Fresh atmospheric gas is introduced from the system, and gas containing substances that cause crystal inclusions is discharged from the system.

In addition, as a method for forcibly discharging this atmospheric gas,
Although Fig. 1 shows a forced exhaust method that uses mechanical suction, it is also possible to keep the atmospheric gas containing fresh oxygen and CO ₂ at a low temperature of 5 to 40°C, and pass it through the gas inlet at the top of the outer cylinder. The gas may be introduced from the iridium crucible 1 and its surroundings, and may be effectively discharged by exchanging it with an atmospheric gas containing harmful substances that cause inclusions, which are lightened by heat transfer and radiation from the iridium crucible 1 and its surroundings.

Fig. 2 shows examples of these gas exhaust methods; a) is an exhaust fan, b) is forced exhaust with a suction pump, and c) is oxygen and CO ₂
d) is a case where the gas containing gas is introduced from the top, d) is a case where the gas is introduced using a gas introduction pipe, and e) is a case where fresh gas is introduced from the bottom. Figure 3 shows a method for more effectively discharging gas by providing a gap 14 between the iridium lid 7 and the heat-retaining cylinder 8 above it; g) exemplifies the case where a gas containing oxygen and CO ₂ is introduced from the top (arrows in FIGS. 2 and 3 indicate the main gas flows in the system).

To summarize the above, the method of the present invention is a method of melting a raw material composition in an iridium crucible and pulling a gallium garnet single crystal from this melt.
By using a gas containing oxygen and CO ₂ with an oxygen partial pressure of 20 to 100 mmHg as the atmospheric gas,
In addition to reducing the generation of lower oxides of gallium according to formula (1), the substances that cause the generated inclusions are forcibly discharged from the system. According to the method of the present invention, large diameter gallium garnet single crystals with an inclusion density of 0.2 particles/cm ³ or less can be easily mass-produced.

Next, examples and comparative examples of the method of the present invention will be given.

Example 1 Gallium oxide powder with a purity of 99.99% or more and gadolinium oxide powder with a purity of 99.99% or more were weighed out in amounts to give Gd ₃ Ga ₅ O ₁₂ , then mixed, molded, and heated at 1450°C. It was sintered for 5 hours.

Next, this 13,000 g was placed in an iridium crucible with an inner diameter of 146 mm and a depth of 148 mm in the single crystal pulling apparatus shown in Figure 1, and 2.5% by volume of oxygen gas and 50% by volume of carbon dioxide gas were mixed with nitrogen gas. After creating a gas atmosphere, the mixture is heated and melted. At the same time as melting begins, fresh gas of the same composition is introduced into the gas inlet 10.
After introducing the gallium garnet at a flow rate of 12.5/min and operating the forced exhaust device 12 to continuously replace the atmospheric gas and completely melt the gallium garnet polycrystal, a gallium garnet with a diameter of 20 mm and a length of 250 mm was introduced. A net single crystal was pulled up. When the inclusion density in this single crystal was examined, it was as shown in curve a in FIG. 4.

Comparative Example 1 13,000 g of the same Gd ₃ Ga ₅ O ₁₂ sintered body as in Example 1 was prepared using the device shown in Figure 1 except for the forced exhaust device.
was placed in an iridium crucible with an inner diameter of 146 mm and a depth of 148 mm, and nitrogen gas was added to it through the gas inlet of this device.
Atmosphere gas added with 12.5% oxygen gas by volume
The sintered body was melted at 1750°C while flowing at a flow rate of /min and exhausting it from the gas exhaust port 11.
A gallium garnet single crystal with a diameter of 80 mm and a length of 250 mm was pulled from this melt, and the density of inclusions in this single crystal was determined as shown in curve b in FIG. 4.

Comparative Example 2 420 g of the sintered body of Gd ₃ Ga ₅ O ₁₂ obtained in Example 1 was placed in an iridium crucible with an inner diameter of 47 mm and a depth of 48.5 mm.
It is melted in an atmosphere of nitrogen gas and 3.5% by volume of oxygen gas added to it, and from this melt a diameter of 25 mm is formed.
Pulling a gallium garnet single crystal with a length of 70 mm,
When we investigated the density of inclusions contained in this single crystal, we found that the inclusion density was 200 inclusions/cm ³ at a position 10 mm from the top of the crystal, and 50 to 100 inclusions/cm 3 at a position of 50 to 60 mm. It was warm at ^cm3 .

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a single crystal pulling apparatus for carrying out the method of the present invention, FIG. 2 is a longitudinal cross-sectional view showing a method of replacing atmospheric gas in the apparatus of FIG. 1, and FIG. FIG. 1 is a vertical cross-sectional view showing a method of replacing atmospheric gas when a gap 14 is provided in the apparatus shown in FIG. Furthermore, FIG. 4 illustrates the relationship between inclusion density and crystal length in a gallium garnet single crystal obtained by an example of the method of the present invention. DESCRIPTION OF SYMBOLS 1... Iridium crucible, 2... Fireproof wall, 3... Heating coil, 4... Melt, 5... Single crystal, 6... Opening,
7...Lid made of iridium, 8...Cylinder made of zirconia, 9
...Outer cylinder, 10, 11, 13...Gas inlet/exhaust port,
12... Forced exhaust device, 14... Gap, 15... Suction pump.

Claims (1)

[Claims] 1. A method for producing a gallium garnet single crystal having a diameter of 75 mm or more by the Czyochralski method, in which a raw material consisting of gallium oxide and a rare earth metal oxide is heated and melted in an iridium crucible, and then the single crystal is melted in an iridium crucible. When pulling the crystal, the atmospheric gas containing oxygen and CO ₂ is always maintained at an oxygen partial pressure of 20 to 100 mmHg throughout this entire process, which causes crystal inclusions. A method for producing a gallium garnet single crystal, characterized in that iridium oxide produced under the same oxygen partial pressure is forcibly discharged from the raw material melt and the vicinity of the single crystal growth. 2. The method for producing a gallium garnet single crystal according to claim 1, wherein the atmospheric gas is forcibly discharged using an exhaust fan or a suction pump. 3. Claim 1 in which a large amount of gas is introduced into the furnace and atmospheric gas near the melt surface is forcibly discharged.
A method for producing a gallium garnet single crystal as described in . 4. Claim 1: A gap is provided between the iridium crucible lid and a heat-retaining cylinder placed above the lid, and atmospheric gas near the surface of the melt is forcibly discharged by gas introduced into the furnace. A method for producing a gallium garnet single crystal as described in .