JPS58104099A - Growth process for tellurium dioxide single crystal - Google Patents

Abstract

PURPOSE:When a single crystal of tellurium dioxide is made to grow by the drawing-up method, after its start, the melt of the raw material is gradually cooled to allow the seed crystal to become thicker gradually to form a bell shape with the flat bottom, thus producing a good-quality single crystal free from striae formation. CONSTITUTION:Tellurium dioxide is melted in a platinum crucible and the single crystal is made to grow by the drawing-up method. At this time, from its start or when the seed crystal comes to have a certain diameter, the melt of the raw material is gradually cooled to allow its diameter to become larger gradually, thus forming a shape of narrow bell or narrow cone 2 with flat or convexed bottom 3. Since the latent heat of solidification is also dissipated toward the melted raw material which is getting lower in its temperature, satisfactory heat dissipation is effected to cause uniform crystallization at the center and inhibit the striae formation.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for growing tellurium dioxide single crystals by a pulling growth method, and particularly relates to a growth method in which striae do not occur.

Tellurium dioxide is known as a substance with a large optical deflection coefficient, and has been put to practical use as a modulator of laser light in devices such as laser facsimiles. In practical use, what is basically desired for this crystal is that, like other optical crystals, when laser light travels through the crystal, it does not undergo multi-refraction and attenuation due to scattering during absorption. These are caused by the presence of impurities, bubbles, striae, etc. The removal of impurities can be solved by using high-purity raw materials, and the generation of bubbles can be prevented by adjusting the pulling speed of the crystal, the number of revolutions, and the temperature distribution in the furnace to appropriate values.

However, since the mechanism by which striae occurs has not been elucidated, a fundamental method for suppressing its occurrence has not been established, leading to a decrease in the yield of high-quality crystals.

The purpose of this invention is to provide a method for growing high-quality tellurium dioxide single crystals that do not contain striae◇
In other words, the present invention is a method for pulling tellurium dioxide single crystals.
At the start of IIK growth, after the seed crystal reaches a predetermined diameter, the raw material melt is gradually cooled and the seed crystal is gradually thickened to a predetermined diameter, and then the aforementioned seed crystal is grown. The crystal is grown so that the diameter increases little by little at a rate equal to or less than the rate of increase in diameter until a predetermined diameter is obtained, resulting in an 8-bell shape, an elongated truncated cone shape, or a long rope bell shape. The crystal is grown in such a shape that the bottom surface of the crystal is flat or convex.

Hereinafter, this invention will be explained in detail with reference to the drawings, together with a growth experiment and a discussion of the results.

The conditions for the growth experiment were as follows. The crucible is made of platinum wire with an inner diameter and depth of 50-1 and a thickness of 1.5 s + s. The rotation speed of the crucible is 30 times per minute, and the pulling speed is 2 III l 11 per hour. The temperature gradient on the molten raw material liquid is 1 cm per hour. It was set to 1200.

Tellurium dioxide (melting point: 740°C) has a relatively large latent heat of solidity, so in order to efficiently dissipate this heat, the temperature is set higher than 50°C per 11 of tantalum-related rhidium and niobium-plated rhidium. However, in the case of tellurium dioxide, under growth conditions that maintain a constant diameter as in general crystal growth, the shape of the crystal bottom lN11 becomes concave as shown in FIG. This indicates that the dissipation of the solidification latent heat is insufficient at an al gradient of 120 °C per 1 (1! The solidification (solidification 1ji) is ideally carried out from the center of the raw material melt, where the temperature is originally the lowest, that is, the center of the bottom surface of the crystal, toward the outer periphery, and the strain during solidification does not remain in the crystal, but heat dissipation If the temperature is insufficient, the center of the crystal will become high temperature due to the heat accumulated in the crystal, and the solidification of the crystal will start from the outer periphery where heat dissipates from the crystal surface, and the center will become hot after lk. Therefore, strain remains, which is considered to be the cause of striae generation.

The striae grow crystals in the direction of the 110 axis, and also □・
,.

Another plate-shaped person perpendicular to the 110 axis, and this #? perpendicular to the tW axis. When viewed from above, it can be seen as several lines as shown in Figure 2. In order to prevent such striae from occurring, it is necessary to find conditions that improve heat dissipation and make the crystal I& direction flat or convex. To achieve this, methods such as reducing the rotation of seeds or increasing the temperature gradient in the furnace can be considered. However, in the case of tellurium dioxide, if the rotation speed is lower than the above-mentioned number, air bubbles may enter the crystal, and if the temperature gradient is 110°C or higher, the distortion will increase and cracks may occur during processing such as cutting the crystal. found. This rounding must be done using a method other than these.

The method used by the inventor here is to lower the temperature of the mackerel liquid itself. According to this method, the latent heat of solidification also diffuses toward the molten raw material whose temperature is the next lowest, so
Since heat dissipates sufficiently and does not remain inside the crystal, it is presumed that crystallization of the raw material does not occur from the outer periphery but occurs from the center or evenly. Next, in Example 4, the growth conditions, the shape of the crystal bottom surface with respect to the cooling rate of the KJrl liquid, and the relationship between the bottom surface shape and the occurrence of striae will be described. The growth conditions were as described above, and the growth direction was 11.
0 axis, the so-called predetermined diameter obtained by gradually increasing the diameter of the seed crystal is set to about 20 + m, and while securing this predetermined diameter, the temperature decrease rate of the subsequent raw material 5hii is set to 0.4°C, 0.5°C, 0.
．． There were three types at 6°C. Then, two single crystals were grown for each type of II. Although this result is qualitative, it appears that the thickening of the crystal (diameter increase rate) is also large when the cooling rate is large. For those with a relatively large diameter increase rate, as shown in Figure 3, the bottom surface is (a) flat or (bl) convex.The diameter increase rate is determined by DA for the predetermined diameter 2 and DI for the bottom diameter 3.

When the length of both IL diameters is defined as (Da-DA)/A as t, the relationship between the diameter increase rate and the bottom shape is as shown in FIG. The relationship between the two is not absolutely strict, but is a guideline 4.
It seems better to think of it as i degree. This is because even when the conditions are exactly the same, the grown crystal has a certain I! This is because differences in degree can be seen. Therefore, it seems advantageous to increase the rate of increase as much as possible.

As expected, no striae were observed in these crystals with flat and convex bottom shapes. As estimated earlier, a large increase rate in crystal diameter means a large temperature drop between the raw material and the liquid, which contributes to the dissipation of latent heat of solidification and ideal crystallization. As a result, the occurrence of striae is suppressed.
[llll 4 This is probably because it will be done. In addition, to determine the presence or absence of striae, directly attach the grown crystal to the second axis.
After cutting to approximately 014111, mirror polishing was performed so that the two cut surfaces were parallel to each other, and this was observed by incision inspection and polarized light microscopy. In addition to the method of lowering the temperature of the raw material melt at a monotonous rate so that the crystal outer shape is equivalent or truncated, the cooling rate is set at a relatively high rate in the W period, and the cooling rate increases as the middle and late stages progress. Good results were also obtained by gradually reducing the size of the ring to create an elongated bell-like shape. The latter method has many advantages over the previous method, such as the rate of increase in diameter is small, which saves valuable raw materials, and the cut elements are relatively uniform in size, making processing easier. .

As detailed above, if the present invention is used to grow a single crystal of tellurium dioxide, a striae-free single crystal of good 1b quality can be obtained.

[Brief explanation of drawings]

Figure 1 shows the bottom 1i1i shape when a tellurium dioxide single crystal is grown in the 110-axis direction when the diameter is kept constant from a predetermined diameter while growing in the 110 axis direction. FIG. 4 is a diagram showing an example of the crystal outer shape and the bottom south shape when #i crystal is grown, and FIG. 4 is a diagram showing the relationship between the diameter increase rate and the bottom shape of the crystal. In the above figures, l is the bottom 11i@% of the single crystal with a concave curve. 2 is the fallen crystal produced by the method of the example, and 3 is the upper part of the 8-bell trapezoid. A similar single crystal has been grown. Indicates the diameter of the hour. Fig. 3 525 - Fig. 4

Claims (1)

[Claims] To grow tellurium dioxide single crystals by the pulling method, the raw material** is gradually cooled from the start of growth or when the seed crystal reaches a certain diameter, so that the diameter gradually increases starting from the seed crystal. And, what's more, the bottom-
A method for growing a tellurium dioxide single crystal, which is characterized by growing the tellurium dioxide single crystal into a shape that is flat or convex mK.