JPH04238888A - Production of oxide single crystal by floating zone method - Google Patents

Abstract

PURPOSE:To suppress entrapment of bubbles in a solid-liquid interface and produce a high-quality oxide single crystal by reducing rotation of a raw mate rial sintered compact. CONSTITUTION:An oxide single crystal is produced at >=15mm/hr growth rate by a condensing heating type floating zone method. In the process, a raw material sintered compact is arranged in the focus of one parabolic mirror where heat rays from a xenon lamp arranged in the focus of the other parabolic mirror converge and the aforementioned raw material sintered compact is lowered at the same speed as the growth rate of the oxide single crystal without rotating the upper and lower shafts supporting the above-mentioned sintered compact or the relative rotational speed of the upper shaft and/or lower shaft is reduced to <=5rpm. Since force in the circumferential direction is not applied to a melt near the solid-liquid interface (a), bubbles are diffused into the melt 1 and entrapment thereof in a crystal 2 is reduced.

Description

Description: FIELD OF INDUSTRIAL APPLICATION The present invention relates to a method for producing a single crystal free of defects such as bubbles by a floating zone method. [0002] When producing single crystals such as ruby and sapphire by the floating zone method, for example, a sintered body obtained by sintering alumina in an air atmosphere at 1550 to 1650°C is used as a raw material. By continuously changing the local heating position of this sintered body, a single crystal was grown. For example, a light source such as a xenon lamp is placed at one focal point of a parabolic mirror, and a raw material placed inside a quartz tube or the like is placed at the other focal point. Then, the heat rays from the light source are concentrated at the other focal point to heat the raw material to a high temperature of 2000° C. or higher. At this time, the upper and lower shafts supporting the upper and lower ends of the sintered body, respectively, are rotated so that the raw material is heated uniformly. [0003] However, raw materials obtained by conventional sintering contain a large number of pores. These pores remain as air bubbles in the obtained single crystal. If such bubbles remain, the properties of the obtained single crystal will deteriorate. Moreover, when the obtained single crystal is used as a light transmitting material, etc., the spectral intensity,
It has a serious adverse effect on light transmittance, straightness, etc. Furthermore, when used as ornaments such as jewelry, it causes an extreme decrease in commercial value. [0004] Air bubbles remaining in the single crystal can be reduced by using raw materials with reduced pores. For example, in Japanese Patent Application Laid-open No. 60-81085, 1
A sintered body sintered at a high temperature of 700℃ or higher, a sintered body sintered in a high vacuum of 10-3 Torr in a temperature atmosphere of 1600℃ or higher, a sintered body sintered by hot pressing, oxidation A sintered body or the like sintered at a temperature of 1600° C. or higher in an atmosphere or a reducing atmosphere is used as a raw material for producing a single crystal. Problems to be Solved by the Invention When a high-density sintered body with suppressed pores is used as a raw material for producing a single crystal, the proportion of air bubbles contained in the obtained single crystal decreases; It is difficult to completely eliminate bubbles. In particular, as the single crystal growth rate increases, the number of bubbles contained in the resulting single crystal tends to increase. Therefore, in order to suppress the residual air bubbles and grow high-quality single crystals, the growth speed was conventionally reduced to 1 m.
m/hour. [0006] Because the growth rate is extremely slow,
Production of single crystals by the floating zone method had very poor productivity. In addition, poor productivity affects production costs, making the obtained single crystal expensive. The present invention was devised in order to solve these problems, and it is possible to solve this problem by inferring the mechanism by which bubbles remain in the obtained single crystal and by establishing the conditions under which this mechanism does not hold. The purpose of this invention is to obtain a high-quality single crystal without residual bubbles even when the single crystal is produced at a high growth rate. [Means for Solving the Problems] In order to achieve the object, the method for producing a single crystal of the present invention produces an oxide single crystal at a growth rate of 15 mm/hour or more using a condensed heating floating zone method. When doing so, the raw material sintered body is placed at the focal point of the other parabolic mirror where the heat rays from the xenon lamp placed at the focal point of one parabolic mirror are concentrated, and the upper shaft and the upper shaft supporting the raw material sintered body are placed. without rotating the lower shaft.
The method is characterized in that the raw material sintered body is lowered at the same speed as the growth speed of the oxide single crystal. [0009] Furthermore, when the raw material sintered body is rotated at a significantly low relative rotational speed of 3 rpm or less, a high-quality oxide single crystal without residual bubbles can be obtained, as in the case where the rotation is stopped. Note that the shaft rotation in the floating zone method employs a method in which the upper shaft and the lower shaft are rotated in the forward direction and the reverse direction, respectively. Therefore, in the present invention, this rotation is expressed by the relative rotation speed of one shaft with respect to the other shaft. for example,
When the upper and lower shafts are rotated forward and backward at 15 rpm, the relative rotational speed is 30 rpm. [Operation] When producing an oxide single crystal using a conventional condensing heating type floating zone, the raw material sintered body in the heating zone is rotated at a rotation speed of about 30 rpm. This rotation was aimed at adjusting the interface between the solid phase and liquid phase in the raw material sintered body heated by the hot wire, maintaining constant growth conditions, and uniformizing the focused heating on the raw material sintered body. It is something. It is also believed by some that centrifugal force is applied to the bubbles present in the liquid phase, causing them to escape radially outward from the raw material sintered body. However, under these growth conditions, when the growth rate exceeds 15 mm/hour, it is inevitable that air bubbles remain in the obtained oxide single crystal. The inventors of the present invention deduced the cause of this bubble retention as follows. [0012] In the raw material sintered body subjected to condensed heating, a single crystal is grown by solidifying the formed melt. At this time, near the solid-liquid interface, the gas dissolved in the melt is
It radiates toward the melt from the point where it is about to solidify. However, when the growth rate is increased, the amount of gas remaining in the melt near the solid-liquid interface exceeds the amount of gas dissipated. When the residual amount of this gas exceeds the concentration at which bubble nuclei are formed, it is incorporated into the crystal as bubbles. When a single crystal is grown from the melt of the raw material sintered body, the solid-liquid interface is inclined with respect to the axial direction of the raw material sintered body. When rotation is applied to such a solid-liquid interface, the entrapment of air bubbles becomes noticeable, especially when the growth rate is high. To simplify this explanation, assume the model of FIG. The heated raw material sintered body becomes a melt 1, and a single crystal 2 is grown from this melt 1. Assuming that the solid-liquid interface 3 between the melt 1 and the single crystal 2 has a surface as shown in the figure, the melt 1
And since the single crystal 2 is rotating in the e direction around the rotation axis 4, the a plane is the part where solidification is going to occur, and the b
This is where the surface is about to melt. At this time, if the crystal growth rate is high, the bubbles are taken into the single crystal 2 on the a-plane. If the single crystal 2 is not sent in the d direction, the crystal that has taken in the bubbles becomes a melt again in the b plane. However, in reality, since there is feeding in the d direction, the crystal containing the bubbles is located below the c-plane. As a result, it does not become a melt, but instead becomes a crystal that incorporates air bubbles. The actual solid-liquid interface is shown in Figure 1.
Although the shape is not as extreme as shown in Figure 2, the basic behavior regarding bubble uptake is similar. Based on this reasoning, as a means of suppressing the inclusion of air bubbles in the crystal, it is possible to use a raw material sintered body that is less likely to cause air bubbles by increasing its density.
Possible methods include reducing the crystal growth rate and stopping or slowing down the rotation around the rotation axis 4 so that the crystal containing bubbles does not become below the c-plane. However, even in a high-density raw material sintered body, it is inevitable that some pores will be included, and it is impossible to completely eliminate the cause of bubble formation. Furthermore, setting the crystal growth rate to be low results in a decrease in productivity, which is not realistic. Therefore, in the present invention, in order to suppress the bubbles in the melt from being incorporated into the crystal on the a-plane, means for stopping or slowing down the rotation around the rotating shaft 4 is adopted. Specifically, when we grew a single crystal without rotating the upper and lower shafts, we found that it was possible to produce a high-quality oxide single crystal with no residual bubbles, as explained in the examples below. became. The raw material sintered body used in the present invention is A
l2O3, SiO2, CrO3, MgO, Ti
It is made by sintering raw materials such as various oxides such as O2 and ZrO2, mixtures of these oxides, and composite oxides. For example, when manufacturing a white sapphire single crystal, sintered Al2O3 is used as the raw material powder. At this time, using spherical powder with a wide particle size distribution in the range of 50 to 100 μm as a raw material increases the density of the sintered body and obtains an oxide single crystal without residual bubbles. preferable. [0017] The raw material powder is prepared using a powder compacting machine such as a rubber press. 5-5. 0 tons/cm2
The powder is pressed into a bar shape of a predetermined size using a moderate amount of pressure. This pressure is 0. If the density is less than 5 mm/cm2, the density of the green compact will be insufficient, the compact will have a strong tendency to lose its shape during sintering, and a sintered compact with the required density and shape will not be obtained. Also, 5. A pressure exceeding ton/cm2 causes the powder molding machine to be large-sized and is not practical. [0018] The compacted raw material powder is sintered at a temperature of 1600°C or higher. The higher the sintering temperature, the higher the density of the obtained sintered body. The sintering time varies depending on the sintering temperature, but it is about 10 hours when the sintering temperature is 1600°C, and about 5 hours when the sintering temperature is 1700°C. Note that when an oxygen-enriched atmosphere is used as the sintering atmosphere, the sintering reaction is accelerated. In order to obtain an oxide single crystal free of residual bubbles, it is preferable to make the density of the sintered body as high as possible by controlling these sintering conditions. The obtained raw material sintered body is heated and melted in a single crystal manufacturing apparatus according to a condensed heating type floating zone method to form an oxide single crystal. At this time, the growth rate of the oxide single crystal is set to 15 mm/hour or more in order to increase productivity. When the growth rate is 15 mm/hour or less, it goes without saying that it takes a very long time to produce a single crystal of a predetermined length, and the effects of stopping or slow rotation of the raw material sintered body are offset. Further, the upper limit of the growth rate is preferably set to 30 mm/hour so that the bubbles float in the liquid phase without being trapped at the solid-liquid interface. [Example] Hereinafter, an example in which white sapphire was manufactured by a condensed heating floating zone method will be described. -Example 1- [0022] As the raw material, alumina powder with a purity of 99.99% and a particle size in the range of 50 to 100 μm was used. This raw material powder is heated to a diameter of 8 mm at a pressure of 1 ton/cm2.
, and molded into a rod-shaped green compact with a length of 70 mm. Then 17
Sintering was carried out in the air at 00° C. for 5 hours to obtain a raw material sintered body with a density of 98%. A white sapphire single crystal was manufactured from this raw material sintered body using a single crystal manufacturing apparatus. The single crystal manufacturing apparatus used was one whose equipment configuration is schematically shown in FIG. That is, a xenon lamp 12 was placed as a heat source at one focal point of the parabolic mirror 11 of the heating furnace 10, and a raw material sintered body 14 inserted into a quartz tube 13 was placed at the other focal point. The raw material sintered body 14 was supported at its upper end and lower end by an upper shaft 15 and a lower shaft 16, respectively. These shafts 15 and 16 can descend in the direction of arrow A as the single crystal grows. Argon gas was fed into the quartz tube 13 as an inert gas 18 from a gas introduction tube 17 provided below to prevent impurities from being introduced into the growing single crystal from the atmosphere. At this time, the flow rate of argon gas was 5Nl/
It was set to minutes. The blown inert gas 18 was discharged to the outside of the system from an exhaust pipe 19 provided above. Heat rays 2 emitted from the xenon lamp 12
0 is reflected by the parabolic mirror 11 and collected on the raw material sintered body 14. As a result, the raw material sintered body 14 is heated and a molten zone 22 is formed. This heating state was observed through a viewing window 23 provided on the side. In addition, in order to accurately grasp the internal situation of the heating furnace 10, the heating furnace 1 located on the viewing window 23 side is
A magnifying lens 24 was attached to the side wall of 0. The heating temperature of the raw material sintered body 14 can be adjusted by changing the output of the xenon lamp 12. Further, when the temperature of the melting zone 22 was measured using a light thermometer or the like through the viewing window 23, the temperature of the melting zone 22 was about 2100°C. When feeding the raw material sintered body 14 in the conveying direction A,
The melting zone 22 moves upward because the heat rays reflected by the parabolic mirror 21 focus on a position above the raw material sintered body 14 . Then, a temperature drop occurs at the lower part of the melting zone 22, a solid phase precipitates from the melting zone 22, and the oxide single crystal 25
is cultivated. In this example, oxide single crystal 25
The growth rate was set at 30 mm/hour, and the upper shaft 15 and lower shaft 16 were moved in the transport direction A at the same speed as this growth rate. In this manner, an oxide single crystal with a length of 60 mm was grown from the raw material sintered body 14 with a length of 70 mm. After cooling, the obtained oxide single crystal was taken out from the heating furnace 10 and its internal structure was observed. As a result, it was found to be a high quality oxide single crystal with no bubbles observed in a visual field of 1 cm2 at 400x magnification. For comparison, the upper shaft 15 is
and lower shaft 16 at 30 rpm (relative rotational speed 60 r
An oxide single crystal was produced under the same conditions as in Example 1, except that the crystal was rotated at pm). When the internal structure of the obtained oxide single crystal was observed, it was found that 10~
10 to 20 bubbles of 100 μm were observed. As is clear from this comparison, when the oxide single crystal is grown without rotating the upper shaft 15 and the lower shaft 16, the pores contained in the raw material sintered body form air bubbles in the oxide single crystal. There will be no residue left. Therefore, even though the growth rate was extremely high compared to conventional methods, high quality oxide single crystals were obtained. -Example 2- [0032] In this example, the single crystal manufacturing apparatus shown in FIG. The effect on the degree of bubbles remaining in the oxide single crystal was investigated. Note that other conditions were the same as in Example 1. the result,
It was found that the following relationship exists between the rotation speed and the presence or absence of bubbles. Rotational speed of the upper and lower shafts in forward and reverse directions Relative rotational speed Presence of air bubbles
1 rpm
2rpm Not observed
2.5rpm
5rpm 〃
5 rp
m 10rpm
[Effects of the Invention] As explained above, in the present invention, when producing an oxide single crystal by the floating zone method, the upper shaft and the lower shaft are not rotated or are rotated. Even in this case, the solid phase of the oxide single crystal is grown from the liquid phase while relative rotation is performed at a low speed of 5 rpm or less. Therefore, there are virtually no remaining bubbles, and high-quality oxide single crystals can be obtained at a high growth rate of 15 mm/hour or more. As described above, according to the present invention, high-quality oxide single crystals can be manufactured with good productivity;
It becomes possible to provide oxide single crystals at low cost.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the mechanism by which air bubbles are incorporated into a grown oxide single crystal.

FIG. 2 is a diagram for explaining a single crystal manufacturing apparatus used in an example of the present invention.

[Explanation of symbols]

1 Melt (liquid phase) 2 Single crystal (solid phase) 3 Solid-liquid interface 4 Axis of rotation a Surface where air bubbles are taken into the crystal b Surface where the crystal containing air bubbles is about to dissolve into the melt c The entire cross section is solid Phased surfaces 10 Heating furnaces 11, 21 Parabolic mirror 12 Xenon lamp 13 Quartz tube 14 Raw material sintered body 15 Upper shaft 16 Lower shaft 17 Gas introduction pipe 18 Inert gas 19 Exhaust pipe 20 Hot wire 22 Melting zone 23 Viewing window 24 Magnifying lens 25 Oxide single crystal

Claims (2)

[Claims] Claim 1: When producing an oxide single crystal at a growth rate of 15 mm/hour or more by the condensed heating floating zone method, the heat rays from the xenon lamp placed at the focal point of one parabolic mirror are concentrated on the other. The raw material sintered body is placed at the focal point of the parabolic mirror of the raw material sintered body, and the raw material sintered body is sintered at the same speed as the growth rate of the oxide single crystal without rotating the upper and lower shafts that support the raw material sintered body. A method for producing an oxide single crystal, characterized by descending the body. [Claim 2] When producing an oxide single crystal at a growth rate of 15 mm/hour or more by the condensed heating floating zone method, the heat rays from the xenon lamp placed at the focal point of one parabolic mirror are concentrated on the other. The raw material sintered body is placed at the focal point of the parabolic mirror, and the raw material sintered body is lowered at the same speed as the growth rate of the oxide single crystal, and the raw material sintered body is lowered at a relative rotation speed of 5 rpm or less. A method for producing an oxide single crystal, characterized by rotating it around an axis.