JP2012001393A - Single crystal ingot and method for growing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for growing a sapphire single crystal of high quality.SOLUTION: The method is for growing a single crystal ingot using the Czochralski method, which has a seed crystal heating process of heating a seed crystal to a predetermined temperature and a necking process of forming a neck part in pulling this seed crystal after the seed crystal is brought into contact with material melt, and the necking process is characterized by passing a crystal grown on the tip surface of the seed crystal through once or more of at least one change in diameter from a contracted diameter to an expanded diameter and an expanded diameter to a contracted diameter.

Description

  The present invention relates to a method for growing a single crystal ingot and a single crystal ingot, and more particularly to a method for growing a high-quality sapphire single crystal ingot.

  A sapphire single crystal is a single crystal of aluminum oxide having a rhombohedral crystal structure or a crystal structure approximated by a hexagonal system. Sapphire single crystal is widely used as an industrial material due to its excellent mechanical properties, chemical stability, and optical properties, and in particular, a GaN film-forming substrate for manufacturing blue and white light emitting diodes (LEDs). It is used as.

  There are various methods for growing such a sapphire single crystal. In particular, a single crystal growth technique called the Czochralski method (CZ method) is used because of the relatively easy growth of large crystals. In general, a method of growing a sapphire single crystal is used.

  This CZ method generally rotates a seed crystal while controlling a pulling speed and a melt temperature after a seeding process in which a single crystal seed crystal (seed crystal) is brought into contact with a polycrystalline raw material melted in a crucible. This is a technique for producing a high-quality single crystal ingot by gradually pulling it up.

  However, when dislocations exist in the seed crystal, that is, when dislocations are already included in the seed crystal production, for example, dislocations are generated on the crystal growth surface of the seed crystal due to thermal shock received during seeding. In this case, there is a problem that these dislocations are succeeded to a single crystal that grows thereafter, and the quality of the single crystal ingot is deteriorated. Therefore, in order to prevent dislocations from being taken over by a single crystal that grows thereafter, for example, when manufacturing an ingot of a silicon single crystal, a process called necking that once narrows the diameter is performed after seeding and before the diameter is increased. It is common to do it.

  However, the growth rate of the sapphire single crystal is about two orders of magnitude lower than that of the silicon single crystal, and the time required for necking is several tens of hours or more. Therefore, conventionally, as described in Patent Document 1, for example, a method of tilting the growth axis of a sapphire single crystal by tilting a seed crystal, or as described in Patent Document 2, a single crystal is heated to a large temperature. Using a method of growing in a gradient or a method of controlling the solid-liquid interface shape as described in Patent Document 3, the sapphire single crystal has been improved in quality.

  However, an LED using a sapphire bulk crystal as a substrate is expected to require higher luminance performance in the future. In order to increase the brightness of the LED, it is essential to improve the crystallinity of the sapphire single crystal serving as the substrate. In order to improve the crystallinity of such a sapphire single crystal, a technique for reducing dislocations and subgrains in the sapphire single crystal is desired.

JP 2008-56518 A JP 2004-256388 A JP 2007-91540 A

  An object of the present invention is to satisfy the above requirements and to provide a method for growing a high-quality sapphire single crystal ingot and a high-quality sapphire single crystal ingot.

  As a result of diligent research to solve the above problems, the present inventor has found that when growing a single crystal ingot using the Czochralski method, the seed crystal is heated to a predetermined temperature to grow the dislocation density of the single crystal. After reducing the above, it has been found that a high-quality sapphire single crystal ingot can be provided by performing a predetermined necking step, and the present invention has been completed.

The present invention is based on the above findings, and the gist of the present invention is as follows.
(1) A method for growing a single crystal ingot using the Czochralski method,
A seed crystal heating step of heating the seed crystal to a predetermined temperature; and a necking step of bringing the seed crystal into contact with a raw material melt and then forming a neck portion while pulling up the seed crystal, the necking step comprising: When pulling up the seed crystal, the crystal grown on the tip surface of the seed crystal has undergone at least one diameter change from reduced diameter to expanded diameter and expanded diameter to reduced diameter at least once, and the diameter has been reduced. A method for growing a single crystal ingot, characterized in that the minimum value of the crystal diameter is 1/5 and less than 5/6 of the maximum value of the crystal diameter when expanded.

(2) In the necking step, when pulling up the seed crystal, the crystal grown on the tip surface of the seed crystal undergoes a change in diameter of at least one of diameter reduction to diameter expansion and diameter expansion to diameter reduction twice or more. The method for growing a single crystal ingot according to (1), which is repeated.

(3) The method for growing a single crystal ingot according to (1) or (2), wherein the seed crystal is an oxide single crystal or a sapphire single crystal.

(4) In the seed crystal heating step, heating is performed so that the temperature of the tip of the seed crystal is in the range of 1400 to 1800 ° C. at a position above the raw material melt by using heat radiation of the raw material melt itself. The method for growing a single crystal ingot according to the above (1), (2) or (3).

(5) The method for growing a single crystal ingot according to (4), wherein the seed crystal heating step includes holding the seed crystal at a position above the raw material melt for 30 minutes or more.

(6) Any of (1) to (5) above, wherein the minimum value of the crystal diameter when the diameter is reduced is not less than 1/3 and not more than 2/3 of the maximum value of the crystal diameter when the diameter is expanded. A method for growing a single crystal ingot according to claim 1.

(7) The method for growing a single crystal ingot according to any one of (1) to (5), wherein the crystal growth surface is a c-plane.

(8) A single crystal ingot grown by the method described in any one of (1) to (7) above, wherein the dislocation density is 10 ² / cm ² or less. .

  According to the present invention, in the method for growing a single crystal ingot using the Czochralski method, a seed crystal heating step of heating the seed crystal to a predetermined temperature, and contacting the seed crystal with the raw material melt, A necking step of forming a neck portion while pulling, and in the necking step, at the time of pulling up the seed crystal, the crystal grown on the tip surface of the seed crystal is at least from the reduced diameter to the expanded diameter and from the expanded diameter to the reduced diameter. One diameter change has passed one or more times, and the minimum value of the crystal diameter when the diameter is reduced is more than 1/5 and less than 5/6 of the maximum value of the crystal diameter when the diameter is expanded. Thus, a high-quality single crystal ingot can be grown.

FIG. 1 is a schematic cross-sectional view showing a part of the manufacturing process of a single crystal ingot. FIG. 2 is a schematic diagram showing a unit cell of a sapphire single crystal when the rhombohedral system is approximated by a hexagonal system. 3A is a schematic cross-sectional view of a sapphire seed crystal for hexagonal crystal growth, and FIG. 3B is a schematic cross-sectional view of a sapphire seed crystal for triangular crystal growth. FIG. 4 is a diagram for explaining a predetermined crystal plane according to the present invention, where the solid lines indicate the case where each crystal plane is a {1-100} plane, and the broken lines are { When the crystal plane is tilted by + 10 ° from 1-100}, the alternate long and short dash line indicates the crystal plane when tilted by −10 ° from {1-100} around the c-axis.

An embodiment of a method for growing a single crystal ingot of the present invention will be described with reference to the drawings.
The method for growing a single crystal ingot of the present invention is a method for growing a single crystal ingot using the Czochralski method, as shown in FIG. 1, and a seed crystal heating step of heating the seed crystal 3 to a predetermined temperature; A necking process in which the seed crystal 3 is brought into contact with the raw material melt 2 and then a neck portion is formed while the seed crystal 3 is pulled up. The necking process grows on the tip surface of the seed crystal when the seed crystal is pulled up. When the crystal to be expanded undergoes at least one diameter change from reduced diameter to expanded diameter and expanded diameter to reduced diameter at least once, and the minimum value of the crystal diameter when the diameter is reduced is the expanded diameter It is characterized in that it is 1/5 and less than 5/6 of the maximum value of the crystal diameter, and by having such a configuration, a high-quality single crystal ingot can be grown.

  When the single crystal 3 is kept at a high temperature, it is known that point defects are collected by a strain field around the dislocation, and move upward to change the shape of the dislocation. It is also known that dislocations slide due to thermal stress due to temperature gradients. In addition, since sapphire has a larger rigidity and dislocation Burgers vector than silicon and has a larger image force, the dislocation tends to be perpendicular to the surface. These effects shorten the total length of dislocations and reduce the dislocation density in the seed crystal.

  In addition, due to the large mirror image force, a force that makes the dislocation perpendicular to the solid-liquid interface works. When the diameter of the crystal to be grown increases (expands), the solid-liquid interface shape becomes convex downward, and when the diameter decreases (shrinks), and when the neck is normal, the solid-liquid interface shape becomes relatively flat. Dislocations are dissipated from the center of the crystal toward the outer periphery in the process of increasing the diameter of the crystal. By reducing the diameter in this state, dislocations other than the center are terminated toward the outside, resulting in a low dislocation density.

  Due to the heat treatment on the seed crystal and the formation of irregularities in the necking step, the dislocation density is greatly reduced with a short neck length.

  The effect of reducing the dislocation density due to the above is remarkable in a crystal having a large rigidity such as sapphire and a Burgers vector.

  In the necking step, when the seed crystal 3 is pulled up, the crystal grown on the tip surface of the seed crystal 3 repeats at least one of diameter change from reduced diameter to expanded diameter and expanded diameter to reduced diameter twice or more. Is preferred.

If the neck diameter is reduced to 1 / n (n is a positive number), the area of the horizontal cross section becomes 1 / n ² , so the total number of dislocations passing through the cross section is reduced by approximately 1 / n ² ( Dislocations that did not remain in the crystal terminate at the crystal surface). After this, the diameter is increased again. By repeating this process N times (N is a positive integer), the total number of dislocations is reduced to (1 / n ² ) ^N.

Similarly, in the case of subgrains, the total length of the subgrain boundary is reduced to (1 / n ² ) ^N by repeating the above process N times.

  Therefore, the minimum value of the crystal diameter when the diameter is reduced is set to be more than 1/5 and less than 5/6 of the maximum value of the crystal diameter when the diameter is expanded. If it is 1/5 or less, the strength of the diameter-reduced portion is insufficient, and if it exceeds 5/6, the effect of reducing dislocations and subgrains cannot be obtained.

  The seed crystal is preferably composed of an oxide single crystal or a sapphire single crystal. The crystal growth surface is preferably a c-plane.

  The sapphire seed crystal has a crystal structure approximated by a rhombohedral system or a hexagonal system, and a side surface which is a crystal plane parallel to the C axis is a crystal plane within {1-100} plane ± 10 °, and It is preferable that the shape is processed to include a hexagonal prism shape or a triangular prism shape, and the (0001) plane is the crystal growth plane.

  FIG. 2 shows a unit cell of a sapphire single crystal. The sapphire single crystal has a rhombohedral crystal structure, but can be approximated by a hexagonal system as shown in FIG. Usually, when a wafer for GaN film formation is cut out from a sapphire single crystal ingot, for example, the wafer is generally cut out so that the main surface of the wafer is the (0001) plane (c-plane) of the sapphire single crystal.

  In order not to waste the material as much as possible, it is desirable to grow a crystal in the c-axis direction to obtain a substantially columnar ingot, and to cut the ingot perpendicularly to the c-axis direction (ingot axial direction). . Therefore, the crystal growth plane is the (0001) plane.

  3 (a) and 3 (b) show schematic cross-sectional views of the sapphire seed crystal when the shape of the sapphire seed crystal is a hexagonal prism shape or a triangular prism shape, respectively. 3 (a) or 3 (b), the side surface of the sapphire seed crystal is within {1-100} plane ± 10 ° (in FIG. It is preferable that the sapphire seed crystal has a hexagonal prism shape or a triangular prism shape. This processing can be performed by measuring the orientation by an X-ray diffraction method and using, for example, a diamond blade.

  Note that the side surface of the sapphire seed crystal is within {1-100} plane ± 10 ° means that the side surface of the sapphire seed crystal is the {1-100} plane as shown by the broken line and the alternate long and short dash line in FIG. It means that the crystal plane is included when rotated by ± 10 ° from the solid line with the c axis as the center of rotation.

  The sapphire seed crystal preferably has a crystal growth surface size in the range of 5 to 12 mm. Here, the size of the crystal growth surface refers to the diameter of a circle circumscribing the hexagon when the shape of the sapphire seed crystal for crystal growth includes a hexagonal column shape, and when the shape includes a triangular prism shape, The diameter of a circle circumscribing a triangle. If the size of the crystal growth surface is less than 5 mm, the seed crystal may be melted due to slight temperature fluctuations in the melt in the seeding process. On the other hand, if the size of the crystal growth surface exceeds 12 mm, the seed crystal may This is because the number of dislocations that exist vertically increases and the dislocations may not be reduced.

  Moreover, the shape of the sapphire seed crystal for crystal growth according to the present invention may be a hexagonal column having a hexagonal pyramid shape as a tip shape or a triangular prism having a tip shape as a triangular pyramid, in addition to a hexagonal column shape or a triangular prism shape. You can also. In particular, when the tip has a pyramid shape, it is possible to effectively prevent the introduction of dislocation due to a heat shock during liquid landing.

  The seed crystal heating step includes heating at a position above the raw material melt using the heat radiation of the raw material melt itself so that the temperature at the tip of the seed crystal is in the range of 1400 to 1800 ° C. preferable. If the heat treatment is less than 1400 ° C., there is a risk that the dislocation ascending movement does not occur sufficiently and the shape change may be insufficient. If the heat treatment exceeds 1800 ° C., the dislocation density grows due to the dislocation sliding motion. This is because it may increase.

  The seed crystal heating step preferably includes moving the seed crystal up and down above the raw material melt. Thereby, the seed crystal can be heated at an appropriate temperature using heat from the raw material melt.

  Dislocations in sapphire seed crystals are most stable in the <1-100> direction of the (0001) plane. In addition, since a large mirror image force acts on the dislocation near the surface of the seed crystal, a large force that works to be perpendicular to the surface acts on the dislocation. As described above, by forming the side surface of the sapphire seed crystal to be a crystal plane within ± 10 ° with respect to the {1-100} plane with the c-axis as the rotation center, the stable direction of dislocation and the image power The direction of the action can be matched. In addition, it is known that when a single crystal is kept at a high temperature, point defects are collected by a strain field around the dislocation and move upward to change the shape of the dislocation. It is also known that dislocations slide due to thermal stress due to temperature gradients. By performing the heat treatment, the dislocation exposed on the surface becomes perpendicular to the surface due to the mirror image force and the ascending motion or sliding motion. Since a shortening force acts on the dislocation, the vertical component in the vicinity of the surface becomes gradually longer at a temperature at which the dislocation can move. As a result, the density of dislocations parallel to the axis can be lowered.

  The seed crystal heating step preferably includes a two-step heat treatment including the heat treatment and a low temperature heat treatment at a position above the raw material melt where the temperature at the tip of the seed crystal is in the range of 1000 to 1200 ° C. This is because when the low temperature heat treatment is less than 1000 ° C., the diffusion of point defects may be delayed, and when the low temperature heat treatment exceeds 1200 ° C., the degree of supersaturation of point defects may be reduced.

  The seed crystal heating step preferably includes a two-stage heat treatment including a heat treatment and a low temperature heat treatment a plurality of times. This is because by performing the two-stage heat treatment a plurality of times, the effect of increasing the degree of supersaturation of point defects and promoting the movement of dislocations is obtained.

  In the seed crystal heating step, high-temperature heat treatment is preferably performed for 30 minutes or longer in order to lengthen the portion perpendicular to the surface due to dislocation ascending or sliding movement. This treatment time can be shortened as the temperature of the heat treatment increases.

  In the seed crystal heating step, it is preferable to perform low-temperature heat treatment for 10 minutes or more in order to sufficiently lower the temperature of the seed crystal to a predetermined temperature. This treatment time can be shortened as the temperature of the heat treatment increases.

By passing through such a seed crystal heating step, the dislocation density on the crystal growth surface of the sapphire seed crystal can be made 10 ³ / cm ² or less.

  The minimum value of the crystal diameter when the diameter is reduced is preferably 1/3 or more and 2/3 or less of the maximum value of the crystal diameter when the diameter is expanded. If it is less than 1/3, the strength of the diameter-reduced portion may be insufficient, and if it exceeds 2/3, the effect of reducing dislocations and subgrains may be insufficient.

By growing by the method according to the present invention described above, the dislocation density of the single crystal ingot can be made 10 ² / cm ² or less.

  The above description shows an example of an embodiment of the present invention, and the present invention is not limited to this embodiment.

Example 1
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1400 ° C. and held for 30 minutes. The neck diameter is 1 cm, and the diameter of the straight body is 15 cm. The process of increasing the neck diameter to 3 cm and returning it to the original 1 cm was repeated twice, and then the crystal was grown through the shoulder process, the straight body process, and the tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, the dislocation density was about 10 ¹ / cm ² . Subgrains were observed by X-ray topography, but no subgrains were confirmed.

(Example 2)
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1800 ° C. and held for 30 minutes. The neck diameter is 2 cm, and the diameter of the straight body is 15 cm. The process of increasing the neck diameter to 3 cm and returning it to the original 2 cm was repeated twice, and then the crystal was grown through the shoulder process, the straight body process, and the tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, the dislocation density was about 10 ⁰ / cm ² . Subgrains were observed by X-ray topography, but no subgrains were confirmed.

(Example 3)
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1400 ° C. and held for 30 minutes. The neck diameter is 1 cm, and the diameter of the straight body is 15 cm. The process of reducing the neck diameter to 0.5 cm and returning it to the original 1 cm was repeated twice, and then a crystal was grown through a shoulder process, a straight body process, and a tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, the dislocation density was about 10 ¹ / cm ² . Subgrains were observed by X-ray topography, but no subgrains were confirmed.

Example 4
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1400 ° C. and held for 30 minutes. The neck diameter is 1 cm, and the diameter of the straight body is 15 cm. The process of increasing the neck diameter to 3 cm and returning it to the original 1 cm was repeated 5 times, and then the crystal was grown through the shoulder process, the straight body process, and the tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, it was dislocation free. Subgrains were observed by X-ray topography, but no subgrains were confirmed.

(Comparative Example 1)
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. No heat treatment or necking of the seed crystal was performed. The diameter of the straight body part is 15 cm. Crystals were grown through the shoulder process, straight body process, and tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, the dislocation density was about 10 ⁴ / cm ² . Moreover, as a result of observing subgrains by X-ray topography, it was observed that subgrains permeated from the crystal surface toward the inside.

(Comparative Example 2)
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1400 ° C. and held for 30 minutes. The neck diameter is 1 cm, and the diameter of the straight body is 15 cm. After the neck diameter was reduced to 0.5 cm, 5 cm growth was carried out while maintaining the diameter at this value, and necking was performed. Thereafter, crystals were grown through a shoulder process, a straight body process, and a tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, the dislocation density was about 10 ⁴ / cm ² . In addition, subgrains were observed by X-ray topography, but subgrains were observed.

(Comparative Example 3)
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1400 ° C. and held for 30 minutes. The neck diameter is 1 cm, and the diameter of the straight body is 15 cm. The process of increasing the neck diameter to 1.2 cm and returning it to the original 1 cm was repeated twice, and then the crystal was grown through the shoulder process, the straight body process, and the tail process. As a result of processing the crystal taken out after cooling into a wafer and measuring the dislocation density, the dislocation density was about 10 ⁴ / cm ² . Moreover, as a result of observing subgrains by X-ray topography, subgrains were confirmed.

(Comparative Example 4)
A sapphire single crystal was grown in the c-axis direction using a rectangular parallelepiped seed crystal (1 cm × 1 cm × 10 cm) by the Czochralski method. After the temperature increase, the seed crystal was held at a position on the melt at 1400 ° C. and held for 30 minutes. The neck diameter is 1 cm, and the diameter of the straight body is 15 cm. As a result of repeating the process of increasing the diameter at the neck to 5 cm and returning it to the original 1 cm twice, cracks were observed at the reduced diameter after the first increase in diameter.

  From the above, Examples 1 to 4 in which single crystal sapphire was grown according to the present invention had a lower dislocation density and subgrains in single crystal sapphire than Comparative Examples 1 to 4, and high quality sapphire. It can be seen that a single crystal is obtained.

  According to the present invention, in the method for growing a single crystal ingot using the Czochralski method, a seed crystal heating step of heating the seed crystal to a predetermined temperature, and contacting the seed crystal with the raw material melt, A necking step of forming a neck portion while pulling, and in the necking step, at the time of pulling up the seed crystal, the crystal grown on the tip surface of the seed crystal is at least from the reduced diameter to the expanded diameter and from the expanded diameter to the reduced diameter. By passing one diameter change one or more times, a high-quality single crystal ingot can be grown.

1 crucible 2 raw material melt 3 seed crystal 4 single crystal ingot

Claims (8)

A method for growing a single crystal ingot using the Czochralski method,
A seed crystal heating step of heating the seed crystal to a predetermined temperature;
After contacting the seed crystal with the raw material melt, a necking step of forming a neck portion while pulling up the seed crystal,
In the necking step, when pulling up the seed crystal, the crystal grown on the tip surface of the seed crystal undergoes at least one diameter change from reduced diameter to expanded diameter and expanded diameter to reduced diameter at least once, The method for growing a single crystal ingot, wherein the minimum value of the crystal diameter when the diameter is reduced is more than 1/5 and less than 5/6 of the maximum value of the crystal diameter when the diameter is expanded.   In the necking step, when pulling up the seed crystal, the crystal grown on the tip surface of the seed crystal repeats at least one of diameter change from reduced diameter to expanded diameter and expanded diameter to reduced diameter twice or more. The method for growing a single crystal ingot according to claim 1.   The method for growing a single crystal ingot according to claim 1, wherein the seed crystal is an oxide single crystal or a sapphire single crystal.   In the seed crystal heating step, heating at a position above the raw material melt using the heat radiation of the raw material melt itself so that the temperature of the tip of the seed crystal is in the range of 1400 to 1800 ° C. The method for growing a single crystal ingot according to claim 1, 2 or 3.   The method for growing a single crystal ingot according to claim 4, wherein the seed crystal heating step includes holding the seed crystal at a position above the raw material melt for 30 minutes or more.   The minimum value of the crystal diameter when the diameter is reduced is not less than 1/3 and not more than 2/3 of the maximum value of the crystal diameter when the diameter is expanded. A method for growing a single crystal ingot.   The method for growing a single crystal ingot according to claim 1, wherein the crystal growth surface is a c-plane. A single crystal ingot grown by the method according to claim 1, wherein a dislocation density is 10 ² / cm ² or less.

Images