JP2881737B2 - Manufacturing method of optical single crystal - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a high-quality optical single crystal for optical waveguides, particularly for optical waveguides and scintillators having a stoichiometric composition. is there.

(Prior Art) Lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO ₃ ) and the like are known as crystals used for optical waveguides, and these crystals are usually manufactured by the Czochralski method. .

However, in this method, it is very difficult to reduce the temperature gradient in the melt, so it is difficult to obtain a high-quality crystal.In this method, only a crystal composition close to the congruent composition can be obtained. However, there is a drawback that a stoichiometric composition required for use cannot be obtained.

(Problems to be Solved by the Invention) Therefore, recently, a method of obtaining a crystal close to the stoichiometric composition by using a condensing floating zone method or a horizontal Bridgman method has been proposed (Kenji Kitamura, Artificial Minerals) However, these methods have the disadvantage that large crystals cannot be obtained.

In addition, in the Czochralski method and the ordinary Bridgman method, since the melt is in contact with the crucible for a long time, they react and the crucible material is mixed into the crystal, which deteriorates the characteristics of the crystal. In particular, bismuth germanate, which is a scintillator crystal, has a drawback that the scintillator characteristics such as energy resolution and light emission amount are deteriorated by mixing platinum, which is a crucible material.

(Means for Solving the Problems) The present invention relates to a method for producing an optical single crystal which has solved such disadvantages, wherein the upper limit of the temperature at the center of the furnace is equal to or higher than the melting temperature of the raw material, In a furnace having a temperature gradient such that the lower limit of the temperature is equal to or lower than the crystallization temperature, the upper crucible containing the raw materials and the lower crucible for receiving and crystallizing the droplets of the raw material melt produced by the upper crucible are placed. Get in,
In a method of gradually lowering these crucibles from the upper part of the furnace and crystallizing raw materials melted in the upper crucible into the lower crucible to produce an optical single crystal, when the stoichiometric ratio is A, it is approximately 1A. Using oxide raw materials blended at a certain ratio,
The operation is performed under the conditions that the temperature gradient near the crystal crystallization portion is 2 to 20 ° C./cm and the crystal growth rate is 5 mm / hour or less. In this case, the optical single crystal is desirably LiTaO ₃ or Bi ₄ Ge ₃ O ₁₂ .

(Operation) This will be described in more detail below.

The present invention relates to a method for producing a high-quality optical single crystal for an optical waveguide or a scintillator, and more specifically, for producing lithium tantalate (LiTaO ₃ ) and bismuth germanate (Bi ₄ Ge ₃ O ₁₂ ). It is about the method.

The production of these optical single crystals is performed by the so-called vertical Bridgman method, which is performed, for example, by an apparatus as shown in FIG. As shown in FIG. 1, this apparatus contains a raw material 3 and has an upper crucible 1 provided with a droplet dropping device 4 and receives droplets from the upper crucible 1, and crystallizes this melt into a single crystal. And a lower crucible 2. These two crucibles are housed in an electric furnace having a temperature gradient shown in FIG. 1 in which the upper limit of the temperature at the center of the furnace is equal to or higher than the melting temperature of the raw material, and the lower limit of the temperature at the upper and lower ends of the path is equal to or lower than the crystallization temperature. ing. Here, the position A is the melting temperature, and the position B is the crystallization start temperature.

In the state shown in FIG. 1, the raw material stored in the upper crucible 1 is heated by the electric furnace, but is not melted because it is still below the melting temperature. In this furnace, the upper and lower crucibles are gradually moved in the direction of the arrow, and when the lower end of the upper crucible passes through the point A, only the raw material 3 is melted into droplets. It is dropped into 2. At this time, the melt dropped into the lower crucible 2 does not crystallize as a single crystal because the temperature is still higher than the crystallization temperature.

However, when the upper and lower crucibles further descend in the furnace, the raw materials 3 stored in the upper crucible 1 are sequentially melted during this time, and droplets of the melt are collected in the lower crucible 2. When the lower end portion reaches the crystal crystallization start temperature point B, the seed crystal 5 is provided in advance in this portion, so that it grows as a single crystal 6, and when the upper and lower crucibles have passed through this electric furnace, All of the raw materials 3 stored in the upper crucible 1 move into the lower crucible as a melt and crystallize as a single crystal 6.

In this case, if the temperature of the melt obtained from the raw materials is excessively heated, a part of the components may volatilize and the melt composition may deviate from the initial raw material composition.
For example, the reaction with the platinum melt proceeds, and platinum easily mixes into the crystal.
The temperature should be kept below 0 ° C. The lower end of the raw material 3 in the upper crucible may be set to the melting start temperature, and the lower end of the melt collected in the lower crucible may be set to the growth start temperature.

The temperature gradient in the vicinity of the growth is 2 ° C./cm or more. If the temperature gradient is 2 ° C./cm or less, there is a problem that bubbles and dislocations are likely to enter.

If the temperature fluctuation is 2 ° C or more, striations and striae tend to occur, so the temperature fluctuation should be 2 ° C.
It is desirable to make the following.

Crystal growth rate of 5 mm / hour or less, preferably 0.3 ~ 3 mm /
Time. If the crystal growth rate is 5 mm / hour or more, there is a problem that bubbles and dislocations are likely to enter.

In this state, if the crystal is grown so that the zone of the melt is 10 mm or more, the heat generated at the solid-liquid interface can be effectively removed, and the generation of bubbles and inclusions in the crystal can be suppressed. In addition, the occurrence of striae and striae can be suppressed by reducing the temperature fluctuation of the melt.

When the operating conditions are set in this manner, the elemental composition of the obtained optical single crystal can be in the range of 0.95 A to 1.05 A, where the stoichiometric ratio is A in oxide units. It is not possible to make the elemental composition of the crystal in the stoichiometric ratio of 0.95A to 1.05A in oxide units by the Czochralski method. According to the present invention, it is possible to grow a large lithium tantalate single crystal having excellent optical properties, and in the case of bismuth germanate as a scintillator crystal, the contact between the melt and the crucible according to the present invention is performed. Since the width of the melt can be limited to a short one, the inclusions and composition changes in the crystal can be greatly reduced compared to the conventional method in which most of the crucible has been in contact with the melt for a long time. A scintillator having characteristics can be obtained.

(Example) Next, an example of the present invention will be described.

Example 1 The upper crucible 1 in FIG. 1 was set to a diameter of 60 mmφ and a length of 100 m.
m crucible made of platinum, lithium carbonate and tantalum oxide were calcined at 1,000 ° C, and lithium oxide: tantalum oxide = 0.50: 0.50 (molar ratio) was charged. A lower crucible 2 made of a platinum crucible was set.

The upper and lower crucibles were placed in an electric furnace having a temperature gradient as shown in FIG. 1, and the lower and upper crucibles were lowered at a speed of 3 mm / hour.
When the raw material was melted and dropped, this droplet was received by the lower crucible, and the melt width at the lower crucible was 10 mm.
When the temperature of the melt reaches 1,250 ° C, the temperature gradient near the growth at this time is 5 ° C / cm, and the temperature fluctuation at the same position is 1 ° C. When all the raw materials in the crucible 1 were melted in the lower crucible 2, the diameter was 60 mmφ and the length was 70 after 30 hours.
mm single crystal was obtained.

Next, when the single crystal was chemically analyzed, it was found to have a composition of lithium oxide: tantalum oxide = 0.499: 0.501 (molar ratio).
A block of 25 mm size was cut out, and a 9 × 25 mm surface previously cut in the <001> direction was mirror-polished, and this surface was irradiated with a 1,310 nm laser using the apparatus shown in FIG. 2 to determine the extinction ratio. When measured, a result of 40 dB was obtained.

Comparative Example 1 However, when a crystal grown by the Czochralski method was subjected to chemical analysis for comparison, the crystal composition was lithium oxide: tantalum oxide = 0.486: 0.514 (molar ratio). When the extinction ratio was measured by the method, the result was 15 dB.

Here, the stoichiometric ratio A of Example 1 and Comparative Example 1 is “0.499 / 0.501 = 0.996” in Example 1, and “0.
486 / 0.514 = 0.946 ".

Example 2 Bismuth oxide (Bi ₂ O) was added to the platinum crucible used in Example 1.
₃ ) 1,122.2 g and germanium oxide (GeO ₂ ) 77.8 g for 1,00
Calcination at 0 ℃ bismuth oxide: germanium oxide = 0.40:
A platinum crucible having the same dimensions was set under the charge of 0.60 (molar ratio). The upper and lower crucibles were placed in an electric furnace having a temperature gradient as shown in FIG. 1, and the upper and lower crucibles were lowered into the furnace at a speed of 2 mm / hour.
When the lower end of the upper crucible 1 reached 1,080 ° C., the raw material was melted and dropped, and the droplets were received by the lower crucible 2. When the melt width in the lower crucible 2 became 10 mm, the lower part of the melt was melted. Is set to 1,050 ° C, the temperature gradient near the growth at this time is set to 10 ° C / cm, and the temperature fluctuation at the same position is set to 2 ° C. In 40 hours, all the raw materials 3 of the upper crucible 1 are set to the lower crucible. As a result, the crystal having a diameter of 60 mmφ and a length of 70 mm could be grown after 45 hours.

Next, when the crystal was chemically analyzed, it had a composition of bismuth oxide: germanium oxide = 0.399: 0.601 (molar ratio). From this, a block of 6 × 12 × 24 mm was cut out, and the 6 × 24 mm surface was cut out. After mirror polishing,
Using the device shown in FIG. 3, γ in which ¹³⁷ Cs is generated
The relationship shown in FIG. 4 was obtained by irradiating a line, and the energy resolution was obtained from the equation ΔE / E × 100 (%). As a result, a result of 10% was obtained.

Comparative Example 2 However, for comparison, a similar crystal was grown by the Czochralski method, and this was chemically analyzed and its crystal composition was examined. As a result, it was found that bismuth oxide: germanium oxide = 0.
424: 0.564 (molar ratio), and the energy resolution of the obtained crystal was examined in the same manner as in Example 2. The result showed that the result was 15% at the maximum.

Here, the stoichiometric ratio A between Example 2 and Comparative Example 2 is “(0.399 / 0.601) × (0.60 / 0.40) = 0.996” in Example 2.
Comparative Example 2 is “(0.424 / 0.544) × (0.60 / 0.40)
= 1.13 ".

(Effect of the Invention) The present invention relates to a method for producing an optical single crystal, and according to the present invention, an optical single crystal for an optical waveguide such as lithium tantalate can be obtained with a high extinction ratio of 40 dB. In addition, bismuth germanate for a scintillator can be easily obtained with a high quality with an energy resolution of 10%. An industrial advantage is provided in that a single crystal having an elemental composition of these oxides of 0.95 A to 1.05 A when the stoichiometric ratio is A in oxide units can be reliably obtained.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of a method for producing a single crystal according to the present invention,
FIG. 2 is a longitudinal sectional view of an optical insertion loss measuring device, FIG. 3 is an explanatory view showing a scintillator characteristic measuring system of a crystal, and FIG. 4 is a γ-ray spectrum diagram. 1 upper crucible 2 lower crucible 3 raw material 4 droplet dropping device 5 seed crystal 6 single crystal

Claims (2)

(57) [Claims] An upper crucible containing raw materials in a furnace having a temperature gradient such that the upper limit of the temperature at the center of the furnace is equal to or higher than the melting temperature of the raw materials and the lower limit of the temperature at the upper and lower ends of the furnace is equal to or lower than the crystallization temperature. Then, put the lower crucible that receives and crystallizes the droplets of the raw material melt made by the upper crucible, gradually lowers these crucibles from the upper part of the furnace, and puts the raw material melted by the upper crucible into the lower crucible. In the method of producing an optical single crystal by crystallizing, an oxide raw material blended at a ratio of approximately 1 A when the stoichiometric ratio is A is used, and the temperature gradient near the crystallized portion is 2 to 20 ° C. / cm, operating under the condition that the crystal growth rate is 5 mm / hour or less, and the element composition constituting the single crystal is in the range of 0.95 A to 1.05 A when the stoichiometric ratio is A in oxide units. A method for producing an optical single crystal, comprising obtaining a single crystal. 2. The method for producing an optical single crystal according to claim 1, wherein the optical single crystal is LiTaO ₃ or Bi ₄ Ge ₃ O ₁₂ .