JPH0798717B2 - Liquid surface temperature control type single crystal growth method and apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a so-called Czochralski method for growing a single crystal while rotating a pulling crystal using high frequency, and particularly to a crystal defect. It is suitable for growing an optically homogeneous oxide single crystal that is free of defects.

(Prior Art) A so-called Czochralski method is known as one of typical methods for growing a single crystal. This single crystal growth method is
It is a method of growing a pulled crystal while rotating it on the surface of the melt in the crucible using a high frequency.

Currently, most of oxide single crystals for optical and surface acoustic waves are
Although grown by this pulling method, it is well known that many of the crystal defects generated during the growth are greatly affected by the state of the solid-liquid interface during growth. On the other hand, it has been previously pointed out that a high quality crystal can be obtained if it can be made into a gentle convex shape. It has also recently been particularly reported that the shape of the solid-liquid interface is closely related to the convection mode in the crucible melt.

Considering the convection of the melt in the crucible during the growth of the oxide single crystal by the pulling method, there is generally a flow of the melt from the crucible wall toward the center of the crystal, which is considered to include natural convection and Marangoni convection. ing. On the other hand, as the flow from the central part of the crystal toward the crucible wall, there is forced convection accompanying crystal rotation, and there are many other flows, but it is the above three types of flow that govern the solid-liquid interface shape. It is considered that the solid-liquid interface shape is determined by combining these convections.

(Problems to be Solved by the Invention) Conventionally, by changing the number of rotations of a pulled crystal to control the amount of forced convection to some extent, the interface shape of a single crystal with respect to a melt becomes a gentle convex shape. The method is generally used. In other words, as you can see by changing the rotation speed of the pulled crystal and examining the solid-liquid interface shape, there is a crystal rotation speed that reverses from convex to concave with respect to the melt, but the rotation speed is slightly lower than that. Is the method.

However, in practice, it is extremely difficult to stably obtain the desired shape of the solid-liquid interface only by this method, and there is a problem that a complicated shape having a mixture of irregularities is exhibited.

An object of the present invention is to solve the problems of the prior art and to provide a method and an apparatus capable of producing a high-quality single crystal free from crystal defects with high yield.

(Means for Solving the Problems) In view of the above-mentioned circumstances, the present inventor has proposed that an oxide single crystal is used.
Taking PbMoO ₄ as an example, in growing a single crystal with a constant diameter,
Experiments were conducted to examine the solid-liquid interface shape by changing the crystal rotation speed, and to grow single crystals under various conditions to make the solid-liquid interface shape close to a flat surface. Note that FIG. 1 is a diagram showing an apparatus used for that purpose, 1 is a high frequency coil, 2 is a crucible,
Reference numeral 3 is a heat insulating material, and 4 is a crucible stand. The raw material in the crucible is heated by the high-frequency coil 1 to form a melt 5. 6
Is an after-heater and is heated by the high frequency coil 1. 7 is a seed crystal, 8 is a pulled crystal (grown crystal),
Reference numeral 9 is a pulling rotary shaft, 10 is a thermocouple under the crucible, and 11 is a crystal diameter control device (ADC device). The crystal diameter control device 11 is
The difference between the growing crystal weight signal and the set diameter signal is fed back to the high frequency power to control the pulling crystal diameter.

As a result of the experiment, no condition was found that the shape of the solid-liquid interface was smooth over the entire single crystal, and crystal defects such as bubbles and dislocations could not be eliminated.

Specifically, when the crystal rotation speed was changed within the range of 20 to 40 rpm, the solid-liquid interface shape as shown in FIG. 2 was exhibited. Note that FIG. 2 shows the growth of molybdic acid single crystal (PbMoO ₄ ) which is an oxide single crystal by changing the crystal rotation speed while maintaining the crystal diameter at 70 mm by using the apparatus shown in FIG. ,
The obtained single crystal was cut parallel to the growth direction, both surfaces were mirror-polished, and then etched with a 5% NaOH aqueous solution, and striation, bubbles and dislocations were observed by a stereoscopic microscope. .

As shown in Fig. 2, when the number of rotations of the crystal is 25 rpm, it is a narrow area in the center of the crystal, but there is a part that exhibits a gradual solid-liquid interface 14, and this part is a high quality part with neither bubbles nor subgrains. However, defect aggregation was observed before the solid-liquid interface shape at the boundary between the crystal center and the edge changed intricately.

When the crystal rotation speed is less than 25 rpm, the solid-liquid interface shape becomes considerably convex, and there are many distortions on the growth fringes in the grown crystal, and when light is passed, light is scattered along these growth fringes. It was too many to be put to practical use.

Also, when the crystal rotation speed is increased by 25 rpm or more, the solid-liquid interface shape becomes concave with respect to the melt, and the central part of the crystal seems to be wide at first glance, but lattice defects are dispersed in the central part or subgrain exists. And it was not a good crystal.

The reason for this is that in the conventional single crystal growth method by high-frequency heating, the crucible wall surface is not heated uniformly, but the upper end of the crucible is strongly heated compared to the middle part, and the temperature on the melt interface becomes high. As shown in FIG. 3, a large temperature difference occurs from both wall surfaces of the crucible 2 to the central part of the crystal, and as a result, large natural convection (12 in FIG. 1) toward the central part of the crystal occurs. This is the forced convection in the reverse direction due to crystal rotation (13 in Fig. 1).
It is supposed that the complicated interface shape is exhibited due to the collision of both flows.

In order to suppress natural convection, there is a method of uniformly heating the crucible wall, and for that purpose, it is necessary to reduce the temperature difference between the upper, middle, and lower portions of the crucible. In this method, there is a method of changing the pitch and diameter of the high frequency coil, but even if this enables uniform heating of the crucible wall,
In this pulling method, since the melt surface is lowered during the growth of the single crystal, the high frequency coil 1 has to be moved in accordance with the lowering speed, which causes the temperature fluctuation factor to change in a complicated manner. Good results have not been obtained.

There is also a method in which the heating part of the grown crystal 8 (that is, the after-heater 6) and the heating element for the melt heating part (high-frequency coil 1) are independently controlled and heated by different methods. Control is possible, but in reality, the high-frequency coil 1 overheats the upper portion of the crucible, and the upper portion of the crucible becomes a considerably high temperature, so that good results cannot be obtained.

Based on the above consideration, the present inventor has earnestly studied the pulling method using a high frequency from all directions, and as a result, controls the melt interface temperature in the radial direction of the crucible by controlling the heating and cooling in the vicinity of the solid-liquid interface. By controlling a constant temperature difference from the central part to the central part of the crucible, it was possible to effectively suppress melt convection, and it was found that a good quality large oxide single crystal was obtained, and the present invention was reached here. It is a thing.

That is, the present invention, when growing a single crystal while rotating the pulling crystal using a high frequency, by heating and cooling control near the solid-liquid interface, to control the temperature of the fixed point of the melt surface constant. The gist is a characteristic liquid surface temperature control type single crystal growing method.

In another embodiment of the present invention, in an apparatus for growing a single crystal while rotating a pulling crystal by using high frequency, between a crucible heating heating element and an after-heater for heating the pulling crystal, a cooling unit and a heating unit are provided. The gist of the present invention is a liquid surface temperature control type single crystal growing apparatus characterized in that a heating / cooling substrate composed of parts is provided.

The present invention will be described in more detail below.

(Operation) As described above, since the melt heating and the growth crystal heating are both performed by high frequency in the past, it is not possible to prevent overheating of the crucible wall in the vicinity of the solid-liquid interface. However, various measures were attempted on the premise of allowing this natural convection, so there was a limit naturally.

On the other hand, in the present invention, since natural convection at the solid-liquid interface is actively suppressed, it is possible to control the temperature in the radial direction of the crucible.

As a means for this, heating and cooling is controlled in the vicinity of the solid-liquid interface, and thereby the temperature at a fixed point on the melt surface is controlled to be constant.

That is, specifically, as will be described later, an apparatus having a configuration as shown in FIG. 4 is used, but as shown in FIG.
By providing the heating / cooling substrate 15 on the outer side of the crucible wall portion corresponding to the vicinity of the solid-liquid interface 14, it becomes possible to control the temperature of the fixed point of the melt surface at a constant level.
14 can be made into a gentle convex shape. Furthermore, depending on the case, a water cooling body 16 or a heating body 17 can be further added on the solid-liquid interface as shown in FIG. 5 (b).

According to this method, as shown in FIG. 6, it is possible to form a gently convex solid-liquid interface even in a wide range of crystal rotation speeds.

Note that FIG. 6 shows the growth of molybdic acid single crystal (PbMoO ₄ ) which is an oxide single crystal by changing the crystal rotation speed while maintaining the crystal diameter at 70 mm by using the apparatus shown in FIG. 4 described later. The obtained single crystal was cut parallel to the growth direction, both sides were mirror-polished, and then etched with a 5% NaOH aqueous solution, and the results of observing striations, bubbles and dislocations with a stereoscopic microscope were shown. The high-quality portion in the central portion of the crystal was widely spread, and no bubbles or dislocations were observed. In particular, when the crystal rotation speed was 5 rpm, the high quality reached 75% of the whole.

Specifically, the single crystal growth method according to the present invention can have the following various aspects.

FIG. 4 shows an example of a single crystal growth apparatus. In the figure, 1'is a main heating element (high-frequency coil) for melting the raw material in the crucible 2 and heating the melt 5, 7 is a seed crystal, 9 is a pulling shaft, 10 is a lower crucible thermocouple, and 11 is a crystal. It is a diameter control device. Also,
15 is a heating / cooling substrate, 16 is a cooling body, 17 is a heating body,
6'is an after-heater, 18 is a thermocouple for the after-heater, and 19 is an optical thermometer.

In such an apparatus, for example, a heating / cooling substrate 15 is provided between a main heating element 1'which melts a raw material in a crucible and an after-heater 6'which heats a single crystal after being grown, so that the melt surface near the crucible The temperature at a fixed point is measured by the optical thermometer 19, and the liquid surface temperature control unit 20 heats and cools the substrate 15 so that the deviation between this temperature and the temperature signal whose crystal diameter is controlled during crystal growth by the controller 11 becomes zero. By controlling the temperature of, the temperature of the melt surface can be made constant.

Further, since the melt surface is lowered as the crystal growth progresses, the main heating element 1 ', the heating / cooling substrate 15, and the after-heater 6'are moved at the same speed as the melt surface lowering speed.

Further, in order to stably keep the temperature of the melt surface stable, by placing a cooling body 16 near the crucible on the top of the melt and cooling it, or by arranging the heating element 17 near the grown crystal on the melt surface. As a result, a similar effect can be brought about.
The cooling body and the heating element are lowered at the same rate as the liquid level is lowered.

In this way, temperature control in the radial direction of the crucible, which has been impossible at all in the past, becomes possible. Therefore, strong natural convection toward the center of the crucible due to overheating of the end of the crucible can be controlled. That is, the convection control at the solid-liquid interface is remarkably improved, so that the solid-liquid interface can be formed into a gentle convex shape with respect to the melt.

Furthermore, in order to constantly maintain this solid-liquid interface shape,
Considering that there is a close relationship between the solid-liquid interface shape and the crystal rotation speed, the crystal rotation speed may be set at a low speed, and as shown in FIG. 6, it is preferably about 15 rpm or less, more preferably 5 rpm.
It is less than or equal to m.

The device shown in FIG. 4 can be modified in various ways.

For example, as the after-heater 6 ′, the high-frequency coil 1 (FIG. 1) for the main heating element has been used in the past, but in the present invention, such a configuration is also possible.
In order to improve the temperature control of the after-heater, it is preferable to stack substrates of the same or similar type as the heating / cooling substrate 15 in multiple stages to form the after-heater.

As the heating / cooling substrate 15, the one shown in FIG. 7 is preferable. This heating / cooling substrate 15 has a disc-shaped or disc-shaped substrate 30 having a space portion in the center, a concave heat reflection surface 31 having an arbitrary curvature is formed in the space portion of the substrate, and the heat reflection surface 31 is formed on the back side thereof. A water cooling part 32 for cooling the heat reflecting surface 3 is formed, and a heat generating element 33 such as a spiral wire, a round bar, a square plate or a thin film is arranged on the heat reflecting surface 31. 34 is a cooling water inlet / outlet.
The heat reflecting surface 31 is subjected to an appropriate surface treatment or processing such as metal plating. The heating / cooling substrate 15 having such a configuration is
Stacked in multiple stages or in the axial direction of the furnace, and arranged outside the crucible wall. In the case of arranging in a plurality of stages, the concave portion forming the heat reflecting surface 31 is formed in half on the upper side or the lower side of the cross section of each substrate 30, and one concave portion is formed by a pair of adjacent substrates 30. Alternatively, an intermediate body (eg, water-cooled copper plate, heat insulating material, etc.) having no recess may be formed between the substrates 30.

Since the heating / cooling substrate 15 has the above-described configuration, it is possible to efficiently control the temperature in a narrow range by the heat insulating effect of the water cooling unit 32 and the heat collecting effect of the heat reflecting surface 31. Since the thickness can be made thin, especially when stacked in multiple layers, heating and cooling boards
15 interactions can be extremely low.

As the cooling body 16, the embodiment shown in FIG. 8 is preferable. This cooling body 16 is composed of a water cooling ring 40, and a cooling water inlet / outlet hanging rod 41.
Are connected by welding or the like, and can be suspended on the surface of the solution as shown in FIG.

As the heating body 17, the embodiment shown in FIG. 9 is preferable. In this heating element 17, a heating element 51 is held by a holding disk 50, and an insulating suspension rod 52 is connected by a locking pin 53 so that it can be suspended on the melt surface as shown in FIG. . The heating element 17 may be the same as the heating element 33 used for the heating / cooling substrate 15.

Furthermore, in order to integrate the cooling body 16 and the heating body 17,
The mode shown in the figure is preferred. A heating element 51 is wound around the water cooling ring 40 via an insulating heat insulating layer 60, and an insulating suspension rod 52 and a cooling water inlet / outlet suspension rod 41 are attached to these, and as shown in FIG. It can be suspended on the surface.

Next, examples of the present invention will be described.

(Example) A single crystal was grown under the following growth conditions using the apparatus shown in FIG.

Incidentally, the device shown in FIG.
Is a high-frequency coil which is a main heating element, and is temperature-program controlled by the thermocouple 10 under the crucible. The temperature of the 6'after heater is controlled by a dedicated thermocouple 18. The temperature of the heating / cooling substrate 15 provided between the main heating element 1'and the after-heater 6'is the temperature of the melt surface near the crucible measured by the optical thermometer 19 and the temperature in the crystal diameter control device 11. And are controlled to be equal. Since the liquid level decreases as the crystal growth progresses, the main heating body 1 ', the heating / cooling substrate 15, the after-heater 6', and the optical thermometer 19 are lowered at the same speed. However, platinum crucible 2 (150φ x 1
50h x 2.5t) is fixed on the crucible stand 4 and remains stationary. The melt 5 in the crucible 2 is melted at a rate of about 80%. There is a pulling shaft 9 at the tip of the crystal diameter control device 11, a seed crystal 7 is fixed at the tip thereof, and a single crystal grows from this tip.

Using such an apparatus, a lead molybdate single crystal was grown as an example of an oxide single crystal. It was grown by changing the crystal rotation speed while maintaining the crystal diameter at 70 mm.

<Growth conditions> Growing crystal: MoPbO ₄ Temperature gradient (vertical direction): 30 ° C / cm Growth direction: <100> Growth rate: 4 mm / hr Coil moving speed: 4.1 mm / hr Heating substrate moving speed: 1.4 mm / hr Radius Direction temperature gradient: 5 ° C / cm Innermost temperature: 1083 ° C Crystal rotation speed: 3 rpm, 5 rpm, 10 rpm, 15 rpm, 20 rpm The obtained single crystal was sliced in parallel with the growing direction at a thickness of 2 mm, and both sides were mirror-polished. After etching with a 5% NaOH aqueous solution, striations, bubbles and dislocations were observed by a stereoscopic microscope. As a result, a solid-liquid interface shape having a gentle convex shape with respect to the melt was obtained, and there was no bubble or dislocation, and it was a large-sized high-quality single crystal. The yield at a crystal rotation speed of 5 rpm was 75%.

(Effect of the Invention) As described above in detail, conventionally, in growing a single crystal such as an oxide single crystal by a pulling method using a high frequency, overheating occurs at the upper end of the crucible and a large natural convection occurs. , It was extremely difficult to grow a homogeneous single crystal without crystal defects at a high yield, but according to the present invention, by controlling the temperature difference in the crucible radial direction of the melt surface,
While suppressing natural convection, the shape of the solid-liquid interface is controlled by combining with forced convection, so the solid-liquid interface shape can be made into a smooth convex shape, and there are no crystal defects and it is an optically high quality part. It is possible to obtain a single crystal in which The present invention can be applied to the growth of various single crystals, but is particularly suitable for the growth of oxide single crystals of lead molybdate, bismuth silicate, lithium niobate, lithium tantalate, YAG and the like.

[Brief description of drawings]

FIG. 1 is an explanatory cross-sectional view showing a conventional single crystal growth apparatus by high frequency heating, and FIGS. 2 (a) to (e) show the shape of the solid-liquid interface obtained by the conventional single crystal growth method for different crystal rotation speeds. Fig. 3 is an explanatory view showing a temperature gradient in a crucible radial direction on a melt surface in a conventional single crystal growing method, and Fig. 4 is an explanatory sectional view showing an example of a single crystal growing device according to the present invention. 5A to 5C are explanatory views showing the shape of the solid-liquid interface in the single crystal growth method according to the present invention. FIG. 5A is a case where a heating / cooling substrate is provided, and FIG. 5B is a cooling body. 6A to 6E, the shape of the solid-liquid interface in the single crystal growth method according to the present invention is shown for different crystal rotation speeds. Figures 7 (a) and 7 (b) are views showing a heating / cooling substrate,
8A is a sectional view, FIG. 8B is a plan view, FIGS. 8A and 8B are views showing a cooling body, FIG. 8A is a plan view, and FIG. 8B is a side view. 9 (a) and 9 (b) are views showing the heating body, FIG. 9 (a) is a plan view, FIG. 9 (b) is a side view, and FIGS. 10 (a) and 10 (b) are views showing the heating body. Here, (a) is a plan view and (b) is a sectional view. 1 ... high frequency coil, 1 '... main heating element (high frequency coil), 2 ... crucible, 3 ... heat insulating material, 4 ... crucible stand,
5 ... Melt, 6, 6 '... After-heater, 7 ... Seed crystal, 8 ... Growing crystal, 9 ... Crystal pulling rotation axis, 10 ...
… Thermocouple under crucible, 11 …… Crystal diameter control device, 12 …… Natural convection, 13 …… Forced convection, 14 …… Solid-liquid interface, 15… Heating / cooling substrate, 16… Cooling body, 17… Heating Body, 18 ... After-heater thermocouple, 19 ... Optical thermometer, 20 ... Liquid surface temperature control unit, 30 ... Substrate, 31 ... Heat reflection surface, 32 ... Water cooling unit,
33 ... Heating element, 34 ... Cooling water inlet / outlet, 40 ... Water cooling ring,
41 …… Cooling water inlet / outlet hanging rod, 50 …… Holding disk, 51 …… Heating element, 52 …… Insulating hanging rod, 53 …… Stopping pin, 60 …… Insulating heat insulating layer.

Claims (8)

[Claims] 1. When growing a single crystal while rotating a pulling crystal using high frequency, heating and cooling in the vicinity of a solid-liquid interface are controlled so that the temperature at a fixed point on the melt surface is controlled to be constant. And a liquid surface temperature control type single crystal growth method. 2. The method according to claim 1, wherein the temperature of the melt surface in the radial direction of the crucible is controlled so as to have a constant temperature difference from the crucible wall portion to the crucible center portion. 3. The method according to claim 1 or 2, wherein the heating / cooling means is moved at the same speed as the lowering speed of the melt surface. 4. The method according to claim 1, 2 or 3, wherein the pulling crystal is grown at a low rotation speed. 5. An apparatus for growing a single crystal while rotating a pulling crystal by using high frequency, comprising a cooling unit and a heating unit between a crucible heating heating element and an after-heater for heating the pulling crystal. A liquid crystal surface temperature control type single crystal growing apparatus, which is provided with a heated and cooled substrate. 6. The liquid surface temperature according to claim 5, wherein a cooling body having a water-cooled ring-shaped pipe arranged vertically movable is provided above the melt surface in the crucible. Control type single crystal growth device. 7. The liquid surface temperature according to claim 5 or 6, wherein a heating element having a ring-shaped heating element arranged so as to be vertically movable is provided above the melt surface in the crucible. Control type single crystal growth device. 8. The heating element and the cooling element are integrated with each other.
Or the device according to 7.