JPS62288195A - Method for growing single crystal - Google Patents

Abstract

PURPOSE:To grow single crystal free from macroscopic defects such as foams, nontransparent rings, cracks, etc., at high growth rate, growing single crystal Li2B4O7 while controlling protrusion height on the solid-liquid interface and temperature gradient just under the solid-liquid interface properly. CONSTITUTION:Single crystal 10 of Li2B4O7 is grown from melt 11 of Li2B4O7 in a wall face 20 of a crucible by Czochralski method. In the operation, the rotational speed of the crystal 10 is selected so that the value of a ratio H/D of a diameter D of the crystal and protrusion height H on a solid-liquid interface 10a is in a range of 0.1-0. The position of a heater (figure is abbreviated) of the melt is selected simultaneously with the selection and the temperature gradient just under the solid-liquid interface 10a is regulated in a range of 50-100 deg.C/cm. Consequently an impurity 13 in the vicinity of the solid-liquid interface 10a is pushed away to the side of the wall face 20 of the crucible and is prevented from admixing with the crystal 10 to promote growth of the crystal 10.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention is based on a single crystal of lithium tetraborate (Li*Bn(h)), which is promising as an optical element or a surface acoustic wave element. It relates to a method of growing by the Ralsky method (hereinafter referred to as the CZ method).

[Prior Art] For practical purposes, a lithium tetraborate single crystal needs to have high light transmittance and not contain macroscopic defects that cause light scattering.

Conventionally, as a method for growing a single crystal of a bismuth-euritite group compound by the %CZ method, when the diameter of the growing crystal is D and the protrusion height of the solid-liquid interface of this crystal is H, H/D
A method is known in which the rotational speed of the growing crystal is selected so that the value of
9517), according to this method, the number of bubbles in the crystal can be reduced and the yield can be increased.

However, to improve the quality of this type of single crystal, controlling only one factor of H/D can reduce bubbles, but it cannot completely eliminate them.
The method for growing lithium tetraborate single crystals using the Z method is based on the above-mentioned H/D value, the pulling rate of the seed crystal, which is the growth rate, and the temperature gradient just below the solid-liquid interface of the growing crystal, in order to obtain a high quality single crystal. It is used as a controlling factor.

This improved conventional growth method uses high-purity crystal raw materials to completely eliminate macroscopic defects in the single crystal.
The growth rate was kept extremely low, below 0.6 mm/hr, the rotational speed of the growing crystal was set so that the H/D value was within the range of O,OS ~ 0.1, and the solid-liquid [Problems to be solved by the invention] However, while conventional growth methods can yield excellent single crystals without macroscopic defects, the problem is that the growth is extremely slow. was there. In order to solve this problem, attempts have been made to increase the growth rate. However, if the growth rate is simply increased, impurities mainly composed of solid solution gas discharged into the melt from the solid-liquid interface of the growing crystal will not be sufficiently diffused into the melt, and will become bubbles and be incorporated into the crystal. There was a problem with the

As shown in FIG. 3, when the rotation speed v1 of the crystal 10 is low, Li2B4O is transferred to the melt 11.
Since the solid-liquid interface 10a is made convex by thermal convection 12 caused by a high-frequency heating device (not shown), the impurity 13 is caused by the solid-liquid interface 10a.
are gathered at the center of a. The collected impurities 13 remain as bubbles 14 in the crystal core.

Also, as shown in FIG. 4, L12B4O? Crystal I
When the rotational speed ■2 of O is high, the forced convection 1B due to the centrifugal force of the melt 11 overcomes the thermal convection 12 and the solid-liquid interface 1
0a becomes flat or concave. As a result, the bubble at the center of the crystal disappears, but there is a problem in that the impurity 13 remains as a donut-shaped opaque ring 17 that spreads perpendicularly to the pulling direction of the growing crystal 10, in other words, in the radial direction of the crystal 10.

An object of the present invention is to provide a method for growing a single crystal of lithium tetraborate free from macroscopic defects such as bubbles, opaque rings, and cracks inside the crystal at a growth rate about twice as high as that of conventional methods. .

[Means for Solving the Problems] The present inventors microscopically observed an opaque ring, which is one of the macroscopic defects, and found that there were cells ( They found small crystals called cells, with many tiny cavities inside them. It was found that light was diffusely reflected in this cavity, making the crystal opaque. This cell is characterized by compositional supercooling of impurities at the solid-liquid interface of the growing crystal.
The present invention was completed by solving this compositional supercooling.

In the present invention, when Li2B4O in FIG. 1 is in the shape of a crystal 1o, the diameter of this crystal 10 is D, and the protrusion height of the solid-liquid interface 10a of this crystal 1G is H, the value of H/D is −
The rotation speed of this crystal was selected so that it was within the range of 0.
Cm "100"C/'Cm (7) The vertical position of the heating device is selected so as to fall within the range of (7). Here, when H/D is a negative value, the solid-liquid interface 10a
is concave, and when it is zero, it means that the solid-liquid interface 10a is flat.

[Function] As shown in Fig. 2, if the rotational speed of the Li2B40v crystal 1o is selected so that the H/D value is within the range of 10, 1 to 0, the forced convection 18 due to the rotating centrifugal force will occur as shown in Fig. The Li2 at the solid-liquid interface 10a overcomes the thermal convection 12 caused by the external heating device.
B<Ot Push the melt 11 onto the wall 2o of the crucible. Therefore, the impurities 13 discharged from the solid-liquid interface 10a into the melt 11 are also blown away from the solid-liquid interface 10a by riding on this forced convection 1B. At the same time, by setting the temperature gradient just below the solid-liquid interface of the grown crystal 1o within the range of 50°C/am-100"C/cm, even if the growth rate is increased and a large amount of impurity 13 is generated, Compositional supercooling of this solid-liquid interface 10a is prevented.

If the value of H/D is less than -0.1, the shape of the solid-liquid interface 10a becomes too concave, making it easy for cracks to form in the crystal 1o. Also, when the H/D value exceeds 0, thermal convection is stronger than forced convection, so impurities concentrate at the center of the crystal,
Air bubbles remain in the core.

In addition, the temperature gradient just below the solid-liquid interface of the growing crystal is 50”C/
Below cm, compositional supercooling occurs at the solid-liquid interface, creating an opaque ring. Further 100℃/c
If it exceeds m, cracks will easily form in the crystal.

[Effects of the Invention] As described above, according to the present invention, by directing the flow of the melt at the solid-liquid interface of the growing crystal to the outside of the solid-liquid interface, no bubbles are allowed to reach the crystal core. There's nothing left. In addition, by increasing the temperature gradient just below the solid-liquid interface of the growing crystal, even if the amount of impurities generated from this solid-liquid interface increases, compositional supercooling of this solid-liquid interface is prevented and an opaque ring is not formed. , the growth rate can be increased without creating macroscopic defects.

[Example] Next, the present invention will be explained in detail with reference to Examples and Comparative Examples.

A Li2BnO single crystal with a diameter of 25 mm was grown by the CZ method using a platinum crucible with a diameter of 50 mm.

Using a thermocouple, select the vertical position of the high-frequency heating device so that the vertical temperature gradient dT/dz at the center of the solid-liquid interface becomes the value shown in the table below.Then, attach a seed crystal to the pulling shaft. While this axis was pulled up at the growth rate shown in the table below, it was rotated at the rotation speed shown in the table below, so that the H/D value of the grown crystal became the value shown in the table below.

The table below shows the soil formation conditions and the results of visual inspection of the crystals.

As is clear from the table, only Examples 1 to 3, in which one or both of the temperature gradient and the crystal rotation speed were increased, became single crystals without an opaque ring, bubbles in the crystal core, or cracks.

[Brief explanation of drawings]

FIG. 1 is a front view of the main part showing the external shape of the single crystal of the present invention. FIG. 2 is a diagram showing the growth status of the single crystal of the present invention. FIG. 3 is a diagram showing a situation in which bubbles appear in a single crystal core in a conventional example. FIG. 4 is a diagram showing a situation in which an opaque ring appears inside a conventional single crystal. to: crystal of Li2B40, 10a: solid-liquid interface, 11: Li2B+Ot melt, 20: wall of crucible. Figure 1 Figure 2 Figure 3 Figure 4 Procedural amendment July 22, 1985 Commissioner of the Patent Office Kuro 1) Akio Yu 1, Indication of the case 1986 Patent application No. 13217
No. 3 No. 2, Title of the invention Single crystal a-formation method 3, Relationship with the case of the person making the amendment Patent applicant address 401, Shirakawa Central Heights, 27 Tanakagaku Kogen-cho, Sakyo-ku, Kyoto-shi, Kyoto Name Tadashi Shiozaki Address: 1-5-1 Marunouchi, Chiyoda-ku, Tokyo
Name: Mitsubishi Mining Cement Co., Ltd. 4, Agent

Claims (1)

[Claims] 1) In the method of growing a single crystal of lithium tetraborate by the Czochralski method, the diameter of the growing crystal is D.
, when the protrusion height of the solid-liquid interface of this crystal is H,
The rotation speed of this crystal is selected so that the value of /D is within the range of -0.1 to 0, and the temperature gradient directly below the solid-liquid interface is within the range of 50 ° C / cm to 100 ° C / cm. A method for growing a single crystal, characterized in that the vertical position of the heating device is selected as follows.