JPH07144997A - Production of x axis-oriented lithium niobate single crystal - Google Patents

Abstract

PURPOSE:To produce an x axis-oriented lithium niobate single crystal without any macrodefect or microdefect. CONSTITUTION:A seed crystal is brought into contact with a melt in a crucible, and the single crystal is rotated and pulled up to grow a lithium niobate single crystal with the growth orientation as the x axis. In this case, the solid-liq. interface is concaved toward the melt by controlling the revolving speed of the crystal, and a/R expressing the concavity ((a) is the concave depth, and R is the crystal diameter) is limited to conform to 0.01<=a/R<=0.10.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a high-quality X-axis direction niobate Riuchimu used for an optical material such as a waveguide type optical device (LiNbO _3: short LN) a process for producing a single crystal.

[0002]

2. Description of the Related Art Conventionally, when a lithium niobate single crystal is used as a waveguide type optical device, a crystal grown in the Z-axis, that is, in the <001> direction is mainly made perpendicular to the growth direction. Z-cut substrates that have been cut and polished have been used. However, the Z-cut substrate exhibits its characteristics only for light having a specific polarization direction, that is, has polarization dependence, and can be arbitrarily inserted in the middle of the existing single-mode optical fiber transmission line. Cannot be used.

Since the element structure is a directional coupling type, it is easily affected by environmental conditions such as temperature. (Reference: Ishikawa, Furuta Technical Report of IEICE vol.91, No.
511, OCS91 75 85 Page 51-58, 1992) and the like, and it was particularly difficult to use for an external modulator for bidirectional optical transmission.

On the other hand, an X-cut substrate obtained by cutting and polishing a lithium niobate single crystal grown in the X-axis, that is, the <100> direction, is cut and polished perpendicularly to the growth direction to form an optical waveguide in the Z-axis direction and in the Y-axis direction. When an electric field is applied, polarization independence can be easily obtained, and there is an advantage that a waveguide type optical device with less characteristic variation due to environmental conditions such as temperature can be manufactured as compared with the Z-cut substrate. However, the coefficient of thermal expansion of the lithium niobate single crystal has anisotropy (X = Y ≠ Z),
When orientations (Y and Z) having different thermal expansion coefficients are mixed in the growth plane as in the case of X-axis growth, strain is likely to be locally concentrated in the crystal during the growth, and cracks may be generated during the growth or cooling. However, there are problems that the crystal defects such as sub-grains and the number of crystal defects such as sub-grains increase. In order to solve this problem, conventionally, the growth conditions were determined so that the solid-liquid interface shape was convex toward the melt side in consideration of the growth stability, but satisfactory results were not obtained. . Therefore, a large-sized lithium niobate single crystal grown in the Z-axis direction is laterally punched in the X-axis direction and processed to obtain an X-cut substrate. This method is not very economical because it has many working steps and the yield is low.

The present invention is directed to an X-axis oriented lithium niobate single crystal which can be used in a waveguide type optical device, particularly an external modulator for bidirectional optical transmission, and a macroscopic defect such as a crack and a microscopic defect such as a subgrain. It is an object of the present invention to provide a manufacturing method without defects.

[0006]

As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention have controlled the crystal rotation speed in the X-axis oriented lithium niobate single crystal growth by the rotation pulling method. Thus, the solid-liquid interface shape is concave toward the melt side, and a / R (a: depth of concave, R: crystal diameter, see FIG. 1) representing the degree is 0.01 ≦ a / R ≦
It has been found that when the range is 0.10, an X-axis oriented lithium niobate single crystal having no macroscopic defects such as cracks and few microscopic defects such as subgrains can be produced.

As described above, according to the present invention, an X-axis azimuth lithium niobate single crystal which can be used for a waveguide type optical device, particularly an external modulator for bidirectional optical transmission, can be manufactured economically and with high quality. It is related to the method of doing. In the X-axis orientation lithium niobate single crystal obtained by the rotary pulling method, the solid-liquid interface shape is concave toward the melt side, and the a / R value representing the degree is in the range of 0.01 ≦ a / R ≦ 0.10. It is controlled, and preferably a / R is controlled in the range of 0.03 to 0.08. By controlling in this way, macro defects such as cracks are not generated, and micro defects such as subgrains are few and high. It is possible to grow a high quality X-axis oriented lithium niobate single crystal. On the other hand, in the case of outside the above range, for example, when the solid-liquid interface shape is flat (a / R = 0) or convex (a / R <0) with respect to the melt side, the solid-liquid interface is Stress is likely to be concentrated in the vicinity and on the crystal surface, and the crystal is likely to be cracked and defects such as subgrains are easily generated. Further, even if the solid-liquid interface shape is concave toward the melt side, if the a / R value exceeds 0.10, the crystals are likely to separate from the melt, making it difficult to maintain stable growth. . Thus, in the present invention, 0.01 ≦
High quality X-axis oriented lithium niobate with no macroscopic defects such as cracks and microscopic defects such as subgrains only when a / R ≦ 0.10. A single crystal can be manufactured.

In the melt in the crucible, there are natural convection mainly due to the temperature difference in the melt and forced convection due to crystal rotation, and by maintaining the advantage of the latter, the solid-liquid interface shape is concave toward the melt. Can be The crystal rotation conditions vary depending on the furnace configuration, the relative position of the heating source and the crucible, the diameter of the grown crystal and the diameter of the crucible, but the solid-liquid interface shape when the grown crystal is separated from the melt is observed, and the result is calculated. By investigating the relationship between the a / R value and the crystal rotation speed, the solid-liquid interface shape is always concave toward the melt side, and the a / R value is 0.01 ≦.
It can be controlled within the range of a / R ≦ 0.10. It is also possible to arbitrarily change the crystal rotation speed (forced convection) according to the strength of natural convection that changes as the growth proceeds.

In practice, the crystal rotation speed is 20 to 5 at the time of seeding.
It is desirable to set it to 0 rpm and decrease it as the growth progresses thereafter. However, from the start of seeding 22-26r
You may make it hold | maintain constant by the rotation speed of pm.

After the crystal is grown for about 24 hours, the crystal is annealed in the atmosphere, and the crystal is cut according to FIG.
/ R ratio is measured.

That is, the crystal is divided into a conical portion and a cylindrical portion,
The bottom part of the cylindrical part is thinly cut to make this part a sample site. Further, the central portion of the cylindrical portion or the body portion is thinly cut in the vertical direction to form a sample portion. The cylindrical portion other than the sample portion is virtually cut into two semicircular cross sections.
The a / R ratio is calculated by polishing both sides of the sample part and observing striation by an optical method such as crossed Nicols. In addition, a sample site is polished on one side and photographed by a Lang camera to observe cyclo defects such as subgrains.

Next, a typical embodiment of the present invention will be shown.

[0013]

【Example】

<Example 1> All single crystal growth was performed by the rotary pulling method (Czochralski method) by high frequency induction heating. As raw materials, lithium carbonate having a purity of 4N and niobium pentoxide were mixed so that the atomic ratio was 48.6: 51.4, and after molding and firing, about 5200 g thereof had a diameter, height and thickness of 130 mmφ, respectively. Platinum crucibles of 130 mm and 2.5 mm in size were filled. The growing conditions were a pulling rate of 3 mm / hr and a growing direction of <100>, and the growing was performed in the atmosphere for about 24 hours. Crystal rotation speed is 30r at seeding
pm, and then gradually decrease, when separating the single crystal 2
It was set to 0 rpm. After pulling up about 60% of the charged raw material, it was cooled to room temperature over about 20 hours. The obtained crystal has no macroscopic defects such as cracks, and has a diameter of 80 mmφ ×
The size of the crystal was 100 mmh, and the shape of the crystal bottom surface was concave toward the melt.

After annealing this crystal in the atmosphere, as shown in FIG.
It was cut as shown in. When the sample site was polished on both sides and directly observed by Nicole, the maximum recess depth a on the melt side was a = 2.
It was 5 mm to 5.0 mm and the diameter R was 80 mm, so that a / R was 0.03 to 0.06. In addition, when the surface of the sample was polished on one side and photographed by a Lang camera, microscopic defects such as subgrains were hardly observed. <Example 2> The crystal rotation speed was 2 from seeding to the end of growth.
All were grown under the same conditions as in Example 1 except that the speed was kept constant at 2 rpm. The obtained crystal is free of macroscopic defects such as cracks, has a size of 80 mmφ × 100 mmh,
The crystal bottom shape was concave toward the melt. After annealing this crystal in the atmosphere, it was cut as shown in FIG. The sample area was polished on both sides and directly observed by Nicole.
R was 0.03 to 0.05. In addition, when the surface of the sample was polished on one side and photographed by a Lang camera, microscopic defects such as subgrains were hardly observed. <Example 3> The growth was performed in the same manner as in Example 2 except that the crystal rotation speed was kept constant at 24 rpm. The obtained crystal has no macroscopic defects such as cracks, and has a diameter of 80 mmφ ×
The size of the crystal was 100 mmh, and the shape of the crystal bottom surface was concave toward the melt. After annealing this crystal in the atmosphere, it was cut as shown in FIG. When the sample site was polished on both sides and directly observed by Nicole, a / R was 0.05 to 0.07. In addition, when the surface of the sample was polished on one side and photographed by a Lang camera, microscopic defects such as subgrains were hardly observed. <Example 4> The growth was performed in the same manner as in Example 2 except that the crystal rotation speed was kept constant at 26 rpm. The obtained crystal has no macroscopic defects such as cracks, and has a diameter of 80 mmφ ×
The size of the crystal was 100 mmh, and the shape of the crystal bottom surface was concave toward the melt. After annealing this crystal in the atmosphere, it was cut as shown in FIG. When the sample site was polished on both sides and directly observed by Nicole, a / R was 0.06 to 0.08. In addition, when the surface of the sample was polished on one side and photographed by a Lang camera, microscopic defects such as subgrains were hardly observed. <Comparative Example 1> Except that the crystal rotation speed was fixed at 6 rpm,
All were raised in the same manner as in Example 2. The obtained crystals are
The size was 80 mmφ × 100 mm, and innumerable cracks were generated around a diameter of 20 mmφ. The shape of the crystal bottom surface was convex toward the melt and was a / R = -0.10. <Comparative Example 2> The same growth as in Example 2 was performed except that the crystal rotation speed was fixed at 10 rpm. The obtained crystal has a size of 80 mmφ × 100 mm and a diameter of 25 mm.
Countless cracks were generated around φ. The crystal bottom shape was convex toward the melt, and was a / R = -0.06. <Comparative Example 3> The growth was performed in the same manner as in Example 2 except that the crystal rotation speed was kept constant at 30 rpm. However, the crystals separated from the melt during the growth. The obtained crystal has no macroscopic defects such as cracks, and has a diameter of 80 mmφ ×
It had a size of 45 mm. The crystal bottom shape was concave toward the melt, and a / R = 0.15. After this crystal was annealed in the air, the straight body was cut and polished perpendicularly to the growth direction and photographed by a Lang camera. Subgrains were observed.

The following table shows a summary of the examples and comparative examples.

[0016] _{Crystal rotation speed a / R Crack} substituting (rpm) line Comparative Example 1 6 -0.10 Yes-Comparative Example 2 10 -0.06 Yes-Example 1 20-30 0.03 to 0.06 No Not observed Example 2 22 0.03 ~ 0.05 None Not observed Example 3 24 0.05 ~ 0.07 None None observed Example 4 26 0.06 ~ 0.08 None None observed No Comparative Example 3 30 0.15 None Yes

[0017]

As shown in Comparative Examples 1 to 3, the solid / liquid interface shape is concave toward the melt side, and a / R represents the degree thereof.
Is not controlled within the range of 0.01 ≦ a / R ≦ 0.10, local stress is concentrated on the crystal and defects such as cracks and subgrains occur.

However, according to the method of the present invention, Examples 1-4 are used.
As shown in, the solid-liquid interface shape is concave toward the melt side, and a / R indicating the degree is 0.01 ≦ a / R ≦ 0.1.
When the content is controlled in the range of 0, an X-axis oriented lithium niobate single crystal can be produced without macroscopic defects such as cracks and microscopic defects such as subgrains.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a solid-liquid interface shape in the method of the present invention.

FIG. 2 is an explanatory diagram showing a state in which a grown crystal is cut to form a sample portion.

Claims (1)

[Claims] 1. When a seed crystal is brought into contact with a melt contained in a crucible and the seed crystal is pulled up while rotating to grow a lithium niobate single crystal having a growth orientation as an X axis,
By controlling the crystal rotation speed, the solid-liquid interface shape is made concave toward the melt side, and a / R (a; depth of concave, R; crystal diameter) representing the degree is 0.01 ≦ a / A method for producing a lithium niobate single crystal, characterized in that R ≦ 0.10.