JPH0751477B2 - Single crystal growth method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a single crystal growing method for growing a single crystal below the surface of a melt by lowering the temperature of the melt as in a cairoporous method.

(Prior Art) The Czochralski method is well known as a method for growing a bulk single crystal.

The crystals that can be produced by this growth method are
In other words, it is limited to substances that crystallize with the same composition as the melt composition.

FIG. 5 is a phase diagram of the conforming molten material.

Since there is no change in the melt composition, there is no change in the melting point and crystallization is always performed at the same temperature.

The Czochralski method is a method of filling a crucible with such a material, immersing the rotated seed crystal in the crystal raw material melt, and then pulling the seed crystal vertically upward to grow a single crystal.

On the other hand, the cairoporous method is used for crystal growth of a peritectic reaction material that decomposes and melts into a solid phase and a liquid phase of another composition at high temperature, that is, a material in which the melt composition and the grown crystal are obtained with different compositions. To be FIG. 6 shows a phase diagram of the peritectic reactant.

In such a substance, the composition of the melt changes and the melting point also changes as the crystals break out.

The device used in the Cairo porous method is almost the same as the device used in the Czochralski method, and there are many parts in common with the operation.

In the Czochralski method, the crystal is pulled up, but in the cairoporous method, the pulling speed is zero, that is, the crystal is broken out.

In the Czochralski method, the crystallization of the melt is carried out at the interface between the crystal and the melt on the surface of the melt, whereas in the cairoporous method it is carried out at the interface in the melt.

When the crystal length is L and the crystal diameter is D, the Czochralski method produces a crystal with a large L / D ratio L / D, and the cairoporous method has a small L / D value. is there.

FIG. 7 shows the state of crystal growth in the Cairo porous method and the Czochralski method.

In this cairoporous method, the seed crystals are kept immersed in the melt for a long time, so that melting of the seed crystals due to temperature change of the melt and rapid crystallization are likely to occur.

In order to prevent these phenomena, it is necessary to accurately control the temperature of the raw material melt in the crucible with a thermally stable and uniform crystal growing furnace.

When crystals grow in the cairoporous method, crystallization begins to cling around the seed crystal immersed in the melt while rotating, and the diameter gradually increases.

Therefore, the dislocations existing in the seed crystal, especially the crystal surface, and the defects are easily propagated to the growing crystal, which may cause deterioration of the quality or the formation of miscellaneous crystals, resulting in polycrystallization. Also in the Czochralski method (pulling method), the same phenomenon as in the case of producing the above-mentioned single crystal occurs. However, in the Czochralski method,
Necking down operation is performed as a method for obtaining a high quality single crystal with few dislocations and defects.

Necking down operation is a step of soaking seed crystals in the raw material melt, allowing them to blend into the melt for a while, and then pulling the seed crystals vertically upward, at first, gradually diminishing the diameter of the seed crystals. , Make an elongated neck.

This is a method of removing various defects and transitions contained in the seed crystal, and then lowering the temperature and increasing the diameter again to obtain a good-quality crystal of a desired size, and most of the crystals by the Czochralski method. Used in breeding.

FIG. 3 shows, in principle, a state in which defects are removed in a crystal manufactured by introducing a necking down step into the Czochralski method.

This necking down, aimed at improving the quality of crystals,
In the Czochralski method, there is an operation of pulling the seed crystal vertically upward, so that it can be easily performed.

(Problems to be Solved by the Invention) If the defects can be removed by the necking down even in the crystal growth by the cairoporous method, the cairoporous method can produce a crystal not suitable for the Czochralski method. .

However, it has been difficult to perform a necking-down operation in a cairoporous method that does not involve a pulling operation or has a very slow pulling speed, and there is no case in which a necking down is performed in a cairoporous method.

KNbO ₃ single crystal exhibits very good nonlinear optical effect,
Also, there is little optical damage to strong laser light.

Therefore, it is a promising crystal as a material for optical modulation and light deflection, including SHG (second harmonic generation) parametric oscillation.

Although many attempts have been made to increase the crystal size and quality, many reports have been made, but due to the complex properties peculiar to KNbO ₃ crystals, there has been no report that they have succeeded in growing large size and high quality crystals.

The crystal of KNbO ₃ crystallizes into a cubic crystal at a melting point of 1034 ° C., and thereafter, a cubic phase-to-tetragonal structure transition occurs at 434 ° C. and a tetragonal to oblique structural phase transition at 232 ° C. before reaching the room temperature phase.

The cubic-to-tetragonal structural phase transition point at 434 ° C is also the Curie point for changing from paraelectric to ferroelectric.

In the KNbO ₃ crystal that undergoes the structural phase transition as described above, many twins are generated when passing through the transition point. Further, with the generation of many twin crystals, various types of domain structure peculiar to the ferroelectric crystal are mixed.

In addition, Si and Ga widely grown by the Czochralski method are used.
Most of semiconductor crystals such as As and dielectric crystals such as LiNbO ₃ and LiTaO ₃ usually grow in a cylindrical shape, but KNbO ₃ crystals are surrounded by {001} due to the cubic system during growth. It grows into a regular square prism shape.

Therefore, the orthorhombic system at room temperature is named as a pseudo cubic system for convenience.

Due to such a complicated property, the grown crystal develops many symptomatic defects in addition to the twin crystal during cooling. Further, macroscopic defects such as cracks are likely to occur, which deteriorates the yield of growing good crystals.

The main object of the present invention is to broadly use the Czochralski method in crystal growth by a single crystal growth method of growing a single crystal below the surface of the melt by lowering the temperature of the melt as in the cairoporous method. Another object of the present invention is to provide a single crystal growth method capable of producing a high quality single crystal by executing the necking down step.

Still another object of the present invention is to provide a KNbO ₃ single crystal growing method for growing a good-quality KNbO ₃ single crystal with high yield.

(Means for Solving Problems) In order to achieve the above object, KNbO ₃ or K according to the present invention is used.
A Ta _x Nb _1-x O ₃ (0 ≦ x <1) single crystal growing method includes: a melt producing step of mixing and melting single crystal materials and maintaining the temperature slightly higher than the melting point; A seed soaking step of rotating seed crystals by contacting seed crystals, acclimatizing the seed crystals by bringing them into contact with the melt, pulling back the seed crystals upward, cooling the melt, and the minimum diameter of the neck is 3 / of the seed crystal diameter. A necking down step for forming a neck smaller than 4 seed crystals so as to be 4 or less, and a crystal growth step for growing crystals by continuously or stepwise pulling at a low pulling rate while lowering the temperature of the melt at a low cooling rate , A crystal growth step of separating the crystal from the melt and cooling to room temperature.

In the method of growing the KNbO ₃ single crystal, the Nb ₂ O ₅ and K ₂ CO
The mixing ratio of ₃ is preferably Nb ₂ O ₅ : K ₂ CO ₃ = 1: 1 to 1: 1.5 in molar ratio.

In the method of growing the KNbO ₃ single crystal, the Nb ₂ O ₅ and K ₂ CO
The mixed object of ₃ is 20 ℃ from the melting point in oxygen atmosphere or air.
It is preferable that the above melting is performed at a high temperature.

It is preferable that the melt once melted in the melt producing step of the method for growing a KNbO ₃ single crystal is cooled to a temperature several degrees higher than the melting point before seeding.

The seed crystal is preferably a seed crystal in the pseudo cubic <001> direction.

The rotation speed of the seed crystal in the seed picking step is 30-60
It is preferably every minute.

The temperature lowering rate in the crystal growth step is preferably 1.0 ° C./hour or less.

450 ~ 400 ℃ near the structural transition point in the crystal cooling step,
The cooling rate in the 250 to 200 ° C. range is preferably 2 ° C. or less per hour.

(Example) Hereinafter, the crystal production method according to the present invention will be described in more detail with reference to the drawings.

In Figure 4, shows a _{K 2 O (K 2 CO 3} ) -Nb 2 O 3 based phase state diagram.

Considering from this phase diagram, in the raw material melt with the Nb ₂ O ₅ content of 50 to 60 mol%, when the temperature is lowered, the melt has a liquidus line ▲ ▼ while precipitating crystals of K ₄ Nb ₆ O ₁₇ composition. The composition is changed from A to B along the line. Therefore, KNbO ₃ crystals do not precipitate in the raw material melt in this composition range.

On the other hand, in the raw material melt having a Nb ₂ O ₅ content of 35 to 50 mol%, crystals of KNbO ₃ composition are precipitated as the temperature decreases, while the melt has a composition of B to C along the liquidus line ▼. Change.

From the above, in order to grow KNbO ₃ crystal, K ₂ O: Nb ₂ O ₅
It is necessary to grow crystals by the chiroporous method on the K ₂ O rich side from the composition of = 1: 1.

In this method, the seed crystals are soaked in the melt for a long time in order to grow the crystals after the seed crystals are soaked in the melt and the temperature of the melt is lowered without pulling up the seed crystals. Therefore, high-performance temperature control of the crystal growth furnace is important.

In this embodiment, the stability and uniformity of the core portion where the crucible is installed are improved so that only a gentle temperature gradient is generated in both the horizontal and vertical directions.

The core temperature can be controlled with an accuracy of 0.1 ° C / hour.

FIG. 2 is a graph showing the temperature distribution of the crucible used when growing a KNbO ₃ single crystal by the method of the present invention.

FIG. 3A shows a temperature gradient in the vertical direction directly above the crucible of the crystal growth furnace.

The same figure (b) has shown the temperature distribution of the horizontal direction in the crystal growth furnace crucible at a fixed distance similarly. Fig. 2 (b)
The curve P ₁ shows the temperature distribution above +2 mm above the melt surface, the curve P ₂ shows the temperature distribution above +25 mm above the melt surface, and the curve P ₃ also shows the temperature distribution above +49 mm. There is.
The temperature gradient of the core part where the used crystal growing furnace crucible is installed is within 10 ° C / cm in both horizontal and vertical directions, and the temperature control is stable and uniform with the accuracy of 0.1 ° C / hour. .

Starting materials niobium pentoxide and potassium carbonate were added to Nb ₂ O ₅ : K ₂ CO
Mix at a ratio of ₃ = 1: 1 to 1: 1.5.

To avoid mixing of impurities, the platinum (Pt) crucible is directly filled without performing the calcination process.

Melt by heating at a temperature 20 ° C higher than the melting point in an oxygen atmosphere or air.

In order to make the melt in the crucible sufficiently uniform by agitation by convection, hold this state for several hours and then melt it (1034
℃) to about 3-10 ℃ higher temperature.

When the temperature becomes stable, the crystals are spun at a rotation speed of 30 to 60 per minute for seed pickling.

The seed crystal orientation is <001> in the pseudo cubic system.

Although potassium niobate crystals are orthorhombic at room temperature, they grow into a unique square prism shape surrounded by {001} planes due to the cubic system during growth.

For convenience, this state is named as pseudo cubic.

Usually, seed pickling is carried out by a seed picking operation so that crystallization does not start immediately after seed pickling so that the crystallization clings around the seed crystal and the seed crystal does not melt. It

In the present invention, the necking-down operation that is usually performed by the Czochralski method is executed.

FIG. 1 shows two examples of necking down operation.

Fig. 1 (a) shows that after seed pickling, the seed crystal diameter d is slightly melted, and the diameter (neck diameter) d'of the crystal after melting is d '<(3/4) d The example which controls the melt temperature at the time is shown.

FIG. 1 (b) shows an example in which the entire seed crystal is not melted, but only the tip is melted so that the melt is connected to the seed crystal by the surface tension of the melt.

After confirming that either of these conditions is maintained, if the temperature is lowered very slowly (for example, 1.0 ° C./hour), a thin neck will be formed, and if cooling is continued, crystal formation will occur in the lower neck after formation of the neck. The expansion of the diameter begins.

As the crystal diameter spreads on the melt surface, crystal growth also progresses into the melt.

Basically, the pulling speed is 0, but if it is desired to promote the growth in the vertical direction, the pulling speed is 0 to 0.5 mm per hour at a constant rate, but the pulling rate is 0 to 3 mm at regular intervals.

Continue cooling, when the crystal grows to the desired size,
Separate from the melt, complete the crystallization process and proceed to cool to room temperature.

FIG. 3 shows an example of a cooling step of cooling the crystal after the crystallization step to room temperature.

In the figure, Tmp is melting point, Tr is room temperature, Tc-t is cubic tetragonal phase transition point, Tt-o ... Tetragonal orthorhombic phase transition point.

Cooling is near the structural phase transition point of KNbO ₃ crystal, cubic → tetragonal 434 ℃, 450-400 ℃, tetragonal → orthorhombic 232 ℃
In the vicinity, the cooling rate is extremely slow at 2 ° C./hr or less in two temperature ranges of 250 ° C. to 200 ° C., and in other temperature regions, it is cooled to room temperature at 10 ° C./hr or less.

Various defects such as cracks and twins are likely to occur when passing through the structural phase transition point.

Therefore, the slower the cooling rate when passing through the structural phase transition point, the better.

In the above examples, KNbO ₃ was described in detail as the single crystal material.

The necking down operation in the Cairo porous method is also used for materials other than this material.

Applicable to all non-conforming melted substances such as KTa ₁ -xNbxO ₃ (KTN) with Ta added, solid solution added with other atoms as impurities, and peritectic reaction substances that decompose and melt into solid phase and liquid phase of different composition It is what is done.

(Effects of the Invention) As described in detail above, the single crystal growth method according to the present invention is a single crystal growth method of growing a single crystal by lowering the temperature of the melt, in which the seed crystal is immersed in the raw material melt in the crucible. After the seed pickling step, a necking down operation is performed to make a narrow neck from the seed crystal, and then the neck is grown to grow a single crystal.

Therefore, KTa ₁ −xNbxO ₅ (KTN) with KNbO ₃ and Ta added
Introducing a necking-down operation to grow crystals made of non-conforming melt materials, such as solid solutions with other atoms added as impurities, or peritectic reactants that decompose and melt into solid and liquid phases with different compositions. As a result, it became possible to grow a high quality single crystal.

The method for growing a KNbO ₃ single crystal according to the present invention is based on Nb ₂ O ₅ and K ₂
Melt production step of mixing and melting CO ₃ and cooling to a temperature slightly higher than the melting point, seed soaking step of rotating seed crystals in the melt and soaking seeds, necking down without pulling up the seed crystals Necking down step to form a narrower neck than the seed crystal, a crystal growth step of growing crystals by pulling at a small pulling rate continuously or stepwise while lowering the temperature of the melt at a small cooling rate, and the crystal. It consists of a crystal cooling step of separating from the melt and cooling to room temperature.

According to this method, KNbO ₃ according to the conventional cairoporous method is used.
Prevents deterioration of crystal quality caused by cracks, twinning, and domain wall formation due to polycrystallization, which causes miscellaneous crystals and preferential growth in one direction during crystal growth. it can.

According to the method of the present invention, necking down operation, and by controlling the temperature and operating speed of the furnace, it is possible to prevent polycrystallization due to the generation of miscellaneous crystals and the like, and it is possible to produce good quality KNbO ₃ crystals with high yield. became.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a necking down operation of the method according to the present invention, in which FIG. 1 (a) shows a state in which a part of the seed crystal is melted, and FIG. It shows a state in which the neck is formed by the surface tension and the viscosity of the melt while melting. FIG. 2 is a graph showing the temperature distribution of the crucible used in the method of the present invention, where FIG. 2 (a) shows the temperature gradient in the vertical direction directly above the crucible of the crystal growth furnace, and FIG. The directional temperature distribution is shown. FIG. 3 is a graph showing an example of a cooling temperature program after the grown crystal is separated from the melt. FIG. 4 is a phase diagram of the K ₂ O (K ₂ CO ₃ ) -Nb ₂ O ₅ system. FIG. 5 is a phase diagram showing a general example of a binary system conforming melting type phase diagram. FIG. 6 is a phase diagram showing a general example of a binary system decomposition melting (peritectic reaction) type phase diagram. FIG. 7 is a schematic diagram for explaining the principle diagram of crystal growth by the (a) cairoporous method and (b) Czochralski method. FIG. 8 is a schematic diagram for explaining the effect of necking down by the Czochralski method. 1 ... Seed crystal, 2 ... Growing crystal d ... Seed crystal diameter, d '... Crystal neck diameter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 55-158196 (JP, A) JP 47-5506 (JP, B1) JP 60-50760 (JP, B2) Joji Ito 1 Masterpiece "Crystal Growth" (Sho 51-12-15) Corona P. 165-179

Claims (8)

[Claims] 1. A melt producing step of mixing and melting single crystal materials and maintaining the temperature slightly higher than the melting point, a seed soaking step in which seed crystals are brought into rotary contact with the melt to soak the seeds, and the seeds Necking down step to form a neck narrower than the seed crystal so that the minimum diameter of the neck is 3/4 or less of the seed crystal diameter by cooling the melt and making the crystal contact with the melt A crystal growth step of growing crystals by pulling at a low pulling rate continuously or stepwise while lowering the temperature of the melt at a low cooling rate; and a crystal cooling step of separating the crystals from the melt and cooling them to room temperature. KNbO ₃ or KTa _x Nb _1-x O ₃ (0 ≦
x <1) A method for growing a single crystal. _2. The mixture ratio of Nb ₂ O ₅ and K ₂ CO _{3 in} the single crystal material is Nb ₂ O ₅ : K ₂ CO ₃ = 1: 1 to 1: 1.5 in molar ratio. The method for growing a single crystal according to item 1. _3. The method for growing a single crystal according to claim 1, wherein the mixed body of Nb ₂ O ₅ and K ₂ CO ₃ is melted at a temperature higher than the melting point by 20 ° C. or more in an oxygen atmosphere or air. . 4. The method for growing a single crystal according to claim 1, wherein the melt once melted in the melt producing step is cooled to a temperature several degrees higher than the melting point before seeding. 5. The method for growing a single crystal according to claim 1, wherein the seed crystal is a seed crystal having a pseudo cubic <001> direction. 6. The method for growing a single crystal according to claim 1, wherein the rotation speed of the seed crystal in the seed pickling step is 30 to 60 times per minute. 7. The cooling rate in the crystal growth step is 1.0
The method for growing a single crystal according to claim 1, wherein the temperature is not higher than ° C per hour. 8. Near the structural transition point in the crystal cooling step
The method for growing a single crystal according to claim 1, wherein the cooling rate in the range of 450 to 400 ° C. and 250 to 200 ° C. is 2 ° C. or less per hour.