JP4746925B2 - Lithium tetraborate single crystal and its growth method - Google Patents

Description

  The present invention relates to a lithium tetraborate single crystal and a method for growing the same, and more particularly, a high-quality and large-diameter type lithium tetraborate single crystal grown by a vertical Bridgman method and including no light scatterer. The present invention relates to a lithium tetraborate single crystal that can be suitably used for use, and an industrially useful growth method that can efficiently grow this single crystal.

  As a substrate material for a surface acoustic wave device, lithium niobate, lithium tantalate, crystal, lithium tetraborate and the like have been put into practical use. Among these single crystals, lithium tetraborate has a relatively large electromechanical coupling coefficient and a temperature coefficient with a very small delay time at room temperature, so it is widely used as a material for filters of mobile communication devices. It has been.

  As a method for growing a lithium tetraborate single crystal, a method of growing a seed crystal from a lithium tetraborate melt in a crucible by a vertical Bridgman method (vertical temperature gradient solidification method) or a chocolate ski method (pulling method) has been conventionally used. Are known.

  Of these, the chocolate skiing method is a method in which crystal growth is performed by pulling up while rotating the crucible in the growth furnace, and although there is an advantage that the growth rate can be increased as compared with the vertical Bridgman method, There is a problem that cracks are likely to occur in the crystal being grown due to thermal distortion in the crystal or temperature fluctuations due to gas convection in the furnace. There is also a problem that a relatively large diameter lithium tetraborate single crystal having a diameter of 3 inches or more cannot be grown.

  On the other hand, the vertical Bridgman method, for example, as disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 10-251099) or the like, moves the crucible containing the melt in a growth furnace having a predetermined temperature gradient, A solution that solidifies lithium tetraborate melt from the tip (bottom end) of the crucible containing the seed crystal. The temperature gradient is relatively gradual and the temperature fluctuation is small. It is widely used as a method for producing a large-diameter lithium tetraborate single crystal.

By the way, in recent years, the characteristics of lithium tetraborate, which has a high transmittance from ultraviolet rays to infrared rays, have attracted attention, and the application of lithium tetraborate single crystals to optical elements has been studied.
However, light scatterers are incorporated in the lithium tetraborate single crystal grown by the conventional vertical Bridgman method. When such a single crystal is used as an optical element, it is scattered when laser light is incident. Light is detected and causes optical characteristics to deteriorate. Therefore, application of lithium tetraborate single crystal grown by the vertical Bridgman method to an optical element has not yet been realized.

JP-A-10-251099

The light scatterers contained in the lithium tetraborate single crystal are roughly classified into inclusions of 1 μm or more and minute scatterers of less than 1 μm.
Inclusions of 1 μm or more are those in which impurities are aggregated and taken into the crystal, and can be confirmed with a microscope as bubbles and inclusions. This inclusion can be suppressed by controlling the purity of the lithium tetraborate raw material.
However, no solution has yet been found for very small scatterers of less than 1 μm.

The present invention has been made in view of such a conventional situation, and its intended treatment is that when growing a lithium tetraborate single crystal by the vertical Bridgman method, it is a minute amount of less than 1 μm incorporated into the single crystal. It is an object of the present invention to provide a novel growth method that can significantly reduce scatterers.
In addition, the present invention is a novel lithium tetraborate single crystal grown by the vertical Bridgman method, which hardly contains a minute scatterer of less than 1 μm and can be suitably used for an optical element. The object is to provide crystals.

  In order to achieve the above object, the inventor of the present application has made extensive studies, and as a result, the atmosphere during crystal growth in the vertical Bridgman method is an oxygen-containing atmosphere, and the oxygen concentration in the atmosphere is 20% or less. Thus, it has been found that the appearance of the minute scatterer having a size of less than 1 μm can be significantly reduced. Furthermore, when the oxygen concentration is 10% or less, even if the crucible moving speed (single crystal growth speed) for growing the lithium tetraborate single crystal is set to a speed of 0.25 mm / hour or more, bubbles are generated. The inventors have found that a good quality lithium tetraborate single crystal free of light and light scatterers can be obtained, and have completed the present invention.

That is, the method for growing a lithium tetraborate single crystal according to the present invention is a method in which a crucible is arranged in the interior space of a growth furnace and a lithium tetraborate single crystal is grown from the raw material in the crucible by the vertical Bridgman method. The atmosphere of the internal space (the atmosphere of the single crystal growth space) is an oxygen concentration of 5 to 10 % by volume .
According to this method, a lithium tetraborate single crystal that is a very high-quality single crystal that does not contain bubbles, light scatterers, 1 μm micro scatterers, and the like that can be applied to an optical element is formed into a vertical Bridgman. Can be obtained by law.

Moreover, according to this method, even if the crucible moving speed (single crystal growth speed) for growing the lithium tetraborate single crystal is set to a relatively high speed of 0.25 mm / hour or more, bubbles, light scatterers, less than 1 μm It is possible to obtain an extremely high quality single crystal that does not contain a minute scatterer or the like. However, if the moving speed is as high as 0.75 mm / hour, bubbles are generated in the single crystal, which is not preferable. Therefore, it is preferable to set the moving speed of the crucible for growing the lithium tetraborate single crystal in the range of 0.25 to 0.50 mm / hour.

The lithium tetraborate single crystal obtained by the growth method of the present invention does not generate cracks like the single crystal grown by the Choral Ski method by adopting the vertical Bridgman method, and the comparison is 3 inches or more in diameter. Needless to say, it can be grown as a single crystal having a large diameter.
Moreover, since a single crystal containing almost no light scatterer can be grown, it is possible to obtain a lithium tetraborate single crystal that can be applied as an optical element, which is impossible with the conventional vertical Bridgman method. .
Furthermore, when the atmosphere in the internal space has an oxygen concentration of 5 to 10% by volume, as described above, a lithium tetraborate single crystal having a very high quality and a relatively large diameter can be grown at a high speed. As a method for growing a lithium tetraborate single crystal for use at low cost, it is extremely useful industrially.

  As an example of the atmosphere of the single crystal growth space described above, an oxygen-nitrogen mixed gas atmosphere having an oxygen gas concentration of 5 to 10% by volume and a nitrogen reducing gas concentration of 95 to 90% by volume can be given. Moreover, argon, helium, neon, krypton, xenon, carbon dioxide, hydrogen, etc. can be mentioned as a gas replacing the nitrogen.

As described above, the growing method according to the present invention has the advantage of growing a single crystal by the vertical Bridgman method, and can grow a very good quality lithium tetraborate single crystal that does not contain a light scatterer. Can be preferably used.
In addition, the lithium tetraborate single crystal according to the present invention is a very high quality single crystal that does not contain bubbles, cracks, light scatterers, etc., and therefore can be suitably used for an optical element, and more than 3 inches. Since it has a relatively large diameter, it has many effects such as being able to be provided at low cost.

  Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings and test examples.

  FIG. 1 shows an example of a lithium tetraborate single crystal growing apparatus for carrying out the method of the present invention. Reference numeral 1 in the figure denotes a growth furnace in which a heater 2 is disposed. In the inner space 1a of the growth furnace 1, a crucible base 4 made of a heat-resistant material supported so as to be movable up and down by a support base 3, and the crucible base A crucible 5 is accommodated which is fitted into a recess 4a formed on the upper surface of 4 and can be taken out.

The crucible 5 is provided with a large-diameter body portion 5a and a small-diameter seed tube 5b provided below the body portion 5a in the same manner as conventionally used in the growth of single crystals by the vertical Bridgman method. It has a shape and is supported so as to stand up on the crucible base 4 with its tip (lower end) part fitted in the recess 4 a, and its upper surface opening is a lid 6 made of the same material as the crucible base 4. It is blocked by.
In the present invention, the crucible 5 made of platinum is used because the atmosphere of the internal space 1a of the growth furnace, that is, the atmosphere of the single crystal growth space is an atmosphere containing a predetermined amount of oxygen.
In the crucible 5, a seed crystal b is accommodated in the seed tube 5b, and a lithium tetraborate raw material (lithium tetraborate melt) a is accommodated in the body 5a.

  The support base 3 is supported by a vertical movement mechanism 7, and the crucible base 4 and the crucible 5 are configured to move up and down integrally with the support base 3 by the operation of the vertical movement mechanism 7. The upper opening of the accommodation space 1 a in which the support base 3, the crucible base 4, and the crucible 5 are accommodated is closed with a lid 8.

A plurality of heaters 2 are installed in the growth furnace 1, and the internal space 1 a of the growth furnace is heated by these heaters 2, and a temperature gradient based on the melting point temperature (about 917 ° C.) of lithium tetraborate, The temperature at an arbitrary position in the internal space 1a is set as the melting point temperature of lithium tetraborate (about 917 ° C.), and the temperature above the position is set to be higher than the melting point temperature and the lower temperature is set to be lower than the melting point temperature. Yes.
Using such a growth furnace 1, the crucible 5 is moved in the internal space 1 a at a predetermined speed, so that the raw material a in the crucible 5 is crystallized, and the lithium tetraborate single-piece adapted to the size of the crucible 5 is used. Crystals can be obtained.

  Hereinafter, the growth test of the lithium tetraborate single crystal by the method of the present invention performed using the above-described growth apparatus will be described.

(Test Examples 1 to 4)
First, a lithium tetraborate single crystal raw material (lithium tetraborate melt) a having a purity of 4N (99.99 wt%) prepared in a predetermined molar ratio in the air or in an inert atmosphere is produced with a diameter of 80 mm and a length of 300 mm. After being put into the body portion 5a of the platinum crucible 5, the lithium tetraborate seed crystal b processed in the orientation <110> having a diameter of 5 mm and a length of 100 mm was inserted into the seed tube 5b.
Next, the platinum crucible 5 is installed in the inner space 1a of the growth furnace in which the temperature gradient is set to 20 ° C. and the internal atmosphere is as described in Table 1, and the descending speed (moving speed) is set to 0.3 mm / As time, the crucible 5 was moved 200 mm in the internal space 1a of the growth furnace to grow a single crystal, and then cooled to room temperature. The platinum crucible 5 after cooling was broken to take out a lithium tetraborate single crystal, and the presence or absence of a light scatterer was confirmed with a microscope. The results are shown in Table 1.

From the above results, the presence or absence of a light scatterer in the lithium tetraborate single crystal depends on the atmosphere of the growth space, and the light scatterer is taken into the single crystal in an oxygen-free atmosphere and light in an oxygen-containing atmosphere. It was confirmed that the generation of scatterers can be suppressed.
The reason for this is that in an oxygen-free atmosphere, the oxygen in the lithium tetraborate melt is in short of the chemical composition, and the lithium tetraborate grown from the oxygen-deficient melt incorporates defects in the crystal. As a result of causing distortion, it can be assumed that the refractive index of the distortion part in the crystal changes and is detected as a light scatterer. On the other hand, in the oxygen-containing atmosphere, it is presumed that the oxygen in the lithium tetraborate melt does not become insufficient with respect to the chemical composition, and the phenomenon as described above does not occur.

(Test Examples 5 to 11)
However, when a lithium tetraborate single crystal is grown in an atmosphere containing oxygen by the vertical Bridgman method, bubbles are more easily taken up in the single crystal than in an atmosphere containing no oxygen. In contrast, in the above-described test examples, the present inventors set the atmosphere in the inner space of the growth furnace as the atmosphere of oxygen-nitrogen mixed gas whose oxygen concentration (oxygen partial pressure) is as described in Tables 2 and 3. A test was conducted and it was found that the generation of light scatterers and bubbles could be suppressed by controlling the oxygen concentration.

  From the above results, in the case of growing a lithium tetraborate single crystal by the vertical Bridgman method, the method according to the present invention was adopted in which the atmosphere in the inner space of the growth furnace had an oxygen concentration of 5 to 20% by volume (Test Example 7). 10), and the obtained lithium tetraborate single crystal was confirmed to be of a good quality that does not take in light scatterers (inclusions of 1 μm or more and fine scatterers of less than 1 μm) and bubbles.

(Test Examples 12 to 18)
By the way, in the growth of lithium tetraborate single crystals by the vertical Bridgman method, when considering industrial productivity such as growth costs, the lower the crucible lowering speed, that is, the higher the single crystal growth speed, the better the productivity. Become. However, as the growth rate is increased, bubbles are taken into the crystal and the yield of the product is significantly reduced. Therefore, the present inventors conducted tests in the above-described test examples with the single crystal growth rate described in Tables 4 and 5 and set the oxygen concentration to 5 to 10% by volume, so that the growth rate was set to 0. It has been found that generation of light scatterers and bubbles can be suppressed even at a relatively high speed of 25 to 0.50 mm / hour.

From the above results, in the growth of lithium tetraborate single crystal by the vertical Bridgman method, when the method according to the present invention was adopted in which the atmosphere in the inner space of the growth furnace was an oxygen concentration of 5 to 10% by volume (Test Example 14) 15) Even when the growth rate is set to a relatively high speed of 0.25 to 0.50 mm / hour, the obtained lithium tetraborate single crystal is a light scatterer (inclusion of 1 μm or more and a minute scatterer of less than 1 μm). ) And good quality without taking in bubbles.
In addition, since the blank part in Table 5 is unsuitable about the test example by which inclusion of the bubble was confirmed, it did not confirm the presence or absence of a light-scattering body.

  In FIG. 2, the external appearance observation photograph showing the presence or absence of the bubble in the crystal | crystallization by the difference in the growth rate of the lithium tetraborate single crystal which concerns on the test example 17 is shown. According to this, in the region where the growth rate is 0.25 to 0.50 mm / hour, it is a substantially clear single crystal in which bubbles are not taken in, whereas in the region where the growth rate exceeds 0.50 mm / hour, there is a bubble. Was taken in and it was confirmed that the single crystal had dust.

  Fig. 3 is a photomicrograph of the test results that confirmed the detection of scattered light affecting the presence or absence of light scatterers (inclusions of 1 μm or more and minute scatterers of less than 1 μm) by applying laser light from the back side of the single crystal. Indicates. According to this, in the lithium tetraborate single crystal (right side) obtained by the method according to Test Examples 14 to 17 grown in an atmosphere containing 5 to 20% by volume of oxygen, scattered light is hardly detected, and light scattering It was confirmed that the product was a high-quality single crystal in which the body was not taken up. On the other hand, in the lithium tetraborate single crystal (left side) obtained by the methods according to Test Examples 12 and 13 (less than 5% by volume of oxygen), scattered light is seen in some places and light scatterers are incorporated. Was confirmed.

  The embodiments of the present invention have been described based on the drawings and test examples. However, the present invention is not limited to these illustrated examples and test examples, and is within the scope of the technical idea described in the claims. It goes without saying that various changes can be made.

The simplified sectional view showing an example of the device which enforces the growth method of the lithium tetraborate single crystal concerning the present invention. The external appearance observation photograph showing the presence or absence of the bubble in a crystal | crystallization in the method based on this invention, and the lithium tetraborate single crystal grown by the conventional method. The microscope picture showing the presence or absence of the light-scattering body in the crystal | crystallization in the method based on this invention, and the lithium tetraborate single crystal grown by the conventional method.

Explanation of symbols

1: Growth furnace 1a: Internal space (single crystal growth space)
5: Crucible a: Raw material of lithium tetraborate single crystal b: Seed crystal

Claims (2)

A method of growing a lithium tetraborate single crystal from a raw material in the crucible by arranging a crucible in the inner space of a growth furnace and using a vertical Bridgman method, wherein the atmosphere in the inner space is 5 to 10% by volume of oxygen A method for growing a lithium tetraborate single crystal for an optical element by a vertical Bridgman method, characterized in that the atmosphere includes: The growth rate of the lithium tetraborate single crystal for optical elements according to claim 1, wherein a moving speed of the crucible for growing the lithium tetraborate single crystal from the raw material is 0.25 to 0.50 mm / hour. Method.