WO2010047429A1

WO2010047429A1 - Double layered crucible for crystal growth

Info

Publication number: WO2010047429A1
Application number: PCT/KR2008/006236
Authority: WO
Inventors: Myungjoo Kwon
Original assignee: Myungjoo Kwon
Priority date: 2008-10-22
Filing date: 2008-10-22
Publication date: 2010-04-29

Abstract

The present invention provides a double-layered crucible for growth of a single oxide crystal, for example a PMN-PT crystal, a PIN-PT crystal, a PZN-PT crystal, a PYN-PT crystal, etc., including lead (Pb). In particular, the double-layered crucible of the present invention is configured such that an inner crucible body (20) is fitted into an outer crucible body (30) having the same shape to that of the inner crucible body.

Description

Description DOUBLE LAYERED CRUCIBLE FOR CRYSTAL GROWTH

Technical Field

[1] The present invention relates, in general, to crucibles for the growth of a single oxide crystal, such as a PMN-PT crystal, a PIN-PT crystal, a PZN-PT crystal, a PYN-PT crystal, etc., including lead (Pb) and, more particularly, to a double-layered crucible for single crystal growth which is configured such that an inner crucible body is fitted into an outer crucible body having the same shape as that of the inner crucible body. Background Art

[2] In the VB (vertical Bridgeman) method which is a representative example of conventional methods for the growth of single crystals, a seed crystal is disposed in a front end of a crucible and, thereafter, a single crystal is grown by bringing a raw material melt into contact with the seed crystal and slowly dropping the temperature. To implement the VB method, cylindrical crucibles have been proposed. As shown in FIG. 1, a cylindrical crucible 10 according to a representative example of the conventional technique includes a front end part 11 into which a seed crystal is inserted, and a cylindrical main body part 13 which has a diameter greater than that of the front end part 11 and provides a space for the growth of a single crystal. The crucible 10 further includes a conical part 12 which integrally connects the front end part 11 to the main body part 13 to form a contiguous surface therebetween. The crucible 10 into which polycrystalline raw material is injected is disposed in a crucible installation part (not shown) which is installed in a single crystal producing apparatus (a crystal growth furnace). Subsequently, the polycrystalline raw material is melted by a peripheral heating unit according to a temperature profile including temperature gradient and then brought into contact with a seed crystal in a melted state. Thereafter, the melt 14 is solidified by slowly dropping the temperature thereof from the seed crystal side, thus growing a single crystal 15. After the growth of the single crystal has completed, the crucible 10 is removed and then a single crystal ingot 15 is obtained. Disclosure of Invention

Technical Problem

[3] However, the conventional platinum crucible for growth of a single piezoelectric crystal is problematic because a fine defect of the crucible may have resulted when manufacturing the crucible or because of impurities of a raw material melt, thus resulting in leakage of the melted raw material. In particular, in the case of using an oxide, such as PMN-PT, PIN-PT, PZN-PT, PYN-PT, etc., including lead (Pb) as the raw material of a single piezoelectric crystal, a crucible made of platinum (Pt) or platinum-rhodium (Pt-Rh) is used. In this case, PbO contained in a raw material melt erodes the crucible, so that the raw material melt may leak during the single crystal growth process. Furthermore, in the conventional crucible, because the temperature of the seed crystal disposed in the front end part (seed holder) cannot be directly read, it is difficult to control the position at which the seed crystal is melted in the front end part. Thus, if the temperature profile of the crucible varies, there is a limitation in obtaining a single crystal. In other words, although thermocouples for reading the temperature are disposed as close to the seed holder as possible, it is very difficult to precisely measure the temperature. Despite having the above disadvantage, the thermocouples cannot be directly attached to the seed holder, because the direct attachment of the thermocouples to the seed holder may vary the material characteristics of the crucible. Thereby, a fine crack of the crucible becomes larger, thus further increasing the problem of leakage of the raw material melt. Meanwhile, the cylindrical shape of the front end part undesirably causes heterogeneous nucleation on the circumferential inner surface of the seed holder, thus reducing yield of the single crystal. To solve the problem of a reduced yield of the single crystal, improvements in the internal structure of the front end part, for example, a selector structure which includes a reinforcing member extending a length of 1 mm to 20 mm from the upper end of the front end part downwards, was proposed such that only a single crystal can grow at a central portion in the crucible rather than at a boundary to prevent heterogeneous nucleation on the circumferential inner surface of the crucible. However, it is not easy to manufacture the crucible having this structure. Technical Solution

[4] Accordingly, the present invention has been made keeping in mind the above problems which occur in the prior art, and an object of the present invention is to provide a double-layered crucible for single crystal growth which is configured such that a raw material melt can be prevented from leaking due to a fine defect of the crucible and thermocouples can be directly attached to the circumferential outer surface of the crucible to precisely measure the temperature in the crucible. The double-layered crucible includes an outer crucible body and an inner crucible body. The outer crucible body includes: a first front end part which has a cylindrical shape; a first main body part which has a cylindrical shape and has a diameter greater than a diameter of the first front end part; and a first conical part which integrally connects the first front end part to the first main body part to form a contiguous surface therebetween. The inner crucible body has the same shape as the shape of the outer crucible body and is fitted into the outer crucible body. The inner crucible body includes: a second front end part which has a diameter less than the diameter of the first front end; a second main body part which has a diameter less than the diameter of the first main body part; and a second conical part which integrally connects the second front end part to the second main body part to form a contiguous surface therebetween.

[5] The present invention is not restrictive and includes the following technical characteristics.

[6] The outer crucible body and the inner crucible body may be made of same material.

Preferably, the material of the inner and outer crucible bodies may be one selected from a group including W (tungsten), Pt (platinum), Pt-Rh, Rh-Ir and Pt-Zr or an alloy of two or more selected from the group. Furthermore, the double-layered crucible may further include a cover which seals the upper end of the inner crucible body and is concave towards the interior of the inner crucible body. Here, the material of the cover may be the same as the material of the inner crucible body. In addition, the double- layered crucible may further include one or more thermocouples welded to the circumferential outer surface of the outer crucible body. Preferably, the thermocouples may be attached to the outer crucible body by welding. The double-layered crucible may further include one or more thermocouples welded to the circumferential outer surface of the first front end part of the outer crucible body. The material of one of the thermocouples may be the same as the material of the outer crucible body. Furthermore, the double-layered crucible may further include a blocker which is inserted into the second front end part of the inner crucible body. The blocker has an annular shape such that an upper opening formed in an upper end thereof differs from a lower opening formed in a lower end thereof. The blocker can prevent a problem in which a grain boundary which grows from a remelting portion of the seed crystal progresses in the vertical direction so that it is present in a single crystal ingot and a crystalline structural defect thus occurs, resulting in reduced yield of the single crystal. Here, the diameter of the upper opening of the blocker may be greater than the diameter of the lower opening thereof. In other words, the blocker may be inserted into the second front end part of the inner crucible body such that the lower end of the blocker comes into contact with the upper end of the seed crystal inserted into the front end part of the inner crucible body. In addition, the circumferential inner surface of the inner crucible body which is inserted into the outer crucible body may be processed by polishing.

Advantageous Effects

[7] A double-layered crucible for single crystal growth according to the present invention can prevent a raw material melt from leaking during the step of growing a single crystal, thus innovatively raising the productivity, compared to the conventional single-layered crucible. Furthermore, the double-layered crucible can effectively withstand the pressure in the crucible during the single crystal growth step and prevent volatilization of PbO which is caused in the crucible. In addition, in the double-layered crucible of the present invention, the temperature in a front end part, that is, the seed holder, of the crucible can be precisely measured. Hence, single crystal growth can be realized even if a relatively small seed crystal is used. As well, the present invention can appropriately cope with variations in the temperature profile. Thanks to these advantages, waste of a seed crystal which is used as a seed can be prevented. Furthermore, because the double-layered crucible can be manufactured such that the seed holder is relatively short, waste of resources when manufacturing the crucible can be prevented. Moreover, the present invention may have a blocker of a simple structure. In this case, a grain boundary can be effectively removed at low costs, and a crystalline structural defect of the produced single crystal can be minimized. Brief Description of Drawings

[8] FIG. 1 is a sectional view of a single-layered crucible for growth of a single crystal, according to a conventional technique;

[9] FIG. 2 is a perspective view showing the coupling between an inner crucible body 20 and an outer crucible body 30 of a double-layered crucible for single crystal growth, according to an embodiment of the present invention;

[10] FIG. 3 is a sectional view of the double-layered crucible 100 which is configured such that the inner crucible body 20 is inserted into the outer crucible body 30 according to the present invention;

[11] FIG. 4 is a sectional view showing a double-layered crucible for single crystal growth, according to another embodiment of the present invention; and

[12] FIG. 5 is a partial enlarged view of the embodiment of FIG. 4.

Best Mode for Carrying out the Invention

[13] Hereinafter, embodiments of a double-layered crucible for single crystal growth according to the present invention will be described in detail with reference to the attached drawings.

[14] An outer crucible body 30 has the same shape as that of the conventional single- layered crucible. The outer crucible body 30 is thinner than the conventional crucible and, preferably, the thickness of the outer crucible body 30 is 1/10 to 3/10 of that of the conventional crucible. Furthermore, the outer crucible body 30 is made of one selected from a group including W (tungsten), Pt (platinum), Pt-Rh, Rh-Ir and Pt-Zr or an alloy of two or more selected from the group. The outer crucible body 30 includes a first front end part 31 having a cylindrical shape, and a first main body part 33 which has a cylindrical shape and has a diameter larger than that of the first front end part 31. The first main body part 33 is coaxial with the first front end part 31. The outer crucible body 30 further includes a first conical part 32 which integrally connects the first front end part 31 to the first main body part 33 to form a contiguous surface therebetween. The first front end part which is called a seed holder may be shorter than the seed holder of the conventional single-layered crucible, preferably, such that the length of the first front end part is 1/2 to 1/3 of that of the seed holder of the conventional single- layered crucible. One or more thermocouples 34 are attached to the circumferential outer surface of the outer crucible body 30 to measure the temperatures of several portions in the crucible (with respect to the longitudinal direction) in of the step of growing a single crystal. Preferably, one or more thermocouples 34 may be attached to the circumferential outer surface of the first front end part 31 of the outer crucible body. One of the thermocouples 34 is made of the same material as that of the outer crucible body. The thermocouples 34 are preferably attached to the outer crucible body by welding. Here, in the present invention, even if the characteristics of the material of the outer crucible body vary attributable to the attachment of the thermocouples, because the growth of the single crystal is conducted in the inner crucible body 20 which will be explained later, the present invention can avoid the problem of the single-layered crucible of the conventional technique in which a raw material melt leaks out due to a fine defect which may have resulted during a process of welding thermocouples to the outer surface of the crucible.

[15] Furthermore, the present invention can make single crystal growth possible despite using a seed holder which is shorter than that of the conventional single-layered crucible. The reason for this is that because the thermocouples 34 are directly attached to the seed holder, the temperature in the seed holder can be precisely measured, so that a melting temperature of a seed crystal inserted into the seed holder can be precisely measured, unlike the conventional single-layered crucible in which the seed crystal is melted at an imprecise temperature. The outer crucible body 30 is configured such that the lower end of the first front end part 31 is closed and the upper end of the first main body part 33 is open. Therefore, the inner crucible body 20 is inserted into the outer crucible body 30 through the upper end of the first main body part 33.

[16] Meanwhile, the inner crucible body 20 of the present invention has the same shape as that of the outer crucible body 30. The inner crucible body 20 includes a second front end part 21 which has a second front end part 21 which has a diameter less than that of the first front end part 31, a second main body part 23 which has a diameter less than that of the first main body part 33, and a second conical part 22 which integrally connects the second front end part 21 to the second main body part 23 to form a contiguous surface therebetween. Here, the term 'diameter is less' means, for example, that the outer diameter of the second main body part 23 is less than the inner diameter of the first main body part 33 such that the second main body part is forcibly fitted into the first main body part. Furthermore, it is preferable that the thickness of the inner crucible body is about 7/10 to 9/10 of that of the single-layered crucible for single crystal growth.

[17] When the second front end part 21 of the inner crucible body is inserted into the outer crucible body through the opening of the upper end of the outer crucible body by a mechanical or manual method, the circumferential outer surface of the inner crucible body 20 comes into surface contact with the circumferential inner surface of the outer crucible body 30 so that a gap therebetween is almost nonexistent. The length of the second main body part 23 of the inner crucible body is equal to or greater than that of the first main body part 33 of the outer crucible body. The crucible of the present invention may further include a cover 24 which is concave towards the interior of the inner crucible body and closes the opening of the upper end of the inner crucible body in the step of single crystal growth. The cover 24 of the inner crucible body seals the interior of the inner crucible body from the exterior to control volatilization of the raw material metal, thus minimizing variation in the composition of the single crystal that is growing. Here, preferably, the cover 24 of the inner crucible body is designed such that it is concave downwards to withstand the high temperature and high pressure in the inner crucible body. The cover is made of the same material as that of the inner crucible body. Preferably, the material of the inner crucible body 20 is the same as that of the outer crucible body 30. The circumferential inner surface of the inner crucible body 20 may be processed by polishing.

[18] Furthermore, the double-layered crucible for single crystal growth according to the present invention further includes a blocker 27 which has an annular shape and is configured such that a diameter of an opening formed in the upper end thereof differs from that of an opening formed in the lower end thereof. As is well known to those skilled in the art, in the case where single crystal growth is performed in the crucible provided with the seed holder having the constant diameter, a grain boundary which grows from a remelting portion of the seed crystal progresses in the vertical direction so that it is present in a single crystal ingot. Hereby, a crystalline structural defect occurs, and the yield of the single crystal is reduced. To prevent this, as shown in FIG. 5, the present invention provides the annular blocker 27 which is configured such that the diameter of the first opening 273 of the upper end 271 thereof differs from that of the second opening 274 of the lower end 272 thereof. The blocker 27 is fitted into the second front end part 21 of the inner crucible body. Preferably, the diameter of the first opening 273 is greater than that of the second opening 274. Because of the installation of the blocker 27, a grain boundary can be prevented from undesirably forming in the seed crystal during the single crystal growth process. Thereby, a crystalline structural defect of the single crystal can be prevented. In detail, the blocker 27 is fitted into the second front end part 21 of the inner crucible body and is disposed such that the lower end 272 of the blocker comes into contact with the upper surface of the seed crystal. In addition, the inner surface of the blocker which forms the first opening 273 and the second opening 274 is streamlined to prevent side effects which may occur during seed crystal growth.

[19] As such, the double-layered crucible for single crystal growth according to the present invention is configured such that the inner crucible body and the outer crucible body form the double-layered structure, thus more reliably preventing a raw material melt from leaking, compared to the single-layered crucible of the conventional technique.

[20] Furthermore, before the inner crucible body is inserted into the outer crucible body, one or more thermocouples 34 are welded to the circumferential outer surface of the outer crucible body. Hence, the inner crucible body which comes into direct contact with raw material can be prevented from being deformed by heat despite ensuring precise measurement of the temperature in the interior of the inner crucible body. In particular, one of the electrodes of one of the thermocouples is made of the same material as is the crucible. The electrode of the thermocouple that is made of the same material as that of the crucible is used in common. Only the other electrodes of the remaining thermocouples which are made of a material different from the crucible are welded to the crucible at positions at which the temperature in the crucible is measured. Therefore, the amount of material required to construct the thermocouples can be reduced, and the temperature in the crucible can be precisely measured at the correct positions at which the thermocouples are welded to the crucible.

[21] Furthermore, of the thermocouples 34 used in the double-layered crucible for single crystal growth according to the present invention, two or more thermocouples 34 are welded to the seed holder. Thus, the temperature in the seed holder can be maintained constant during the crystal growth. Hence, even though a relatively small seed crystal is used in the reduced seed holder, the temperature in the seed holder can be precisely measured, so that precisely the desired amount of seed single crystal can be melted. As a result, the size of the seed holder can be reduced when manufacturing the crucible, and a seeding failure attributable to excessive or insufficient melting can be prevented. Hereby, the productivity of the crucible can be enhanced. Moreover, in the present invention, the cover of the inner crucible body is configured such that it is concave inwards to withstand a high temperature of 1500⁰C or more and high pressure, thus improving the internal pressure conditions of the crucible. In addition, in the present invention, the annular blocker which is made of the same material as that of the inner and outer crucible bodies is installed in the seed holder, thus preventing aberrant crystal growth attributable to heterogeneous nucleation which is induced on the boundary surface. Therefore, a single crystal of high quality can be produced. Here, the annular blocker is configured such that a difference between the outer diameter thereof and the inner diameter of the seed holder of the inner crucible body is about 0.1 mm to prevent a heterogeneous crystal from passing through a space between the outer diameter of the annular blocker and the inner diameter of the seed holder of the inner crucible body.

Claims

[1] A double-layered crucible for growth of a single crystal, comprising: an outer crucible body comprising: a first front end part having a cylindrical shape; a first main body part having a cylindrical shape, the first main body part having a diameter greater than a diameter of the first front end part; and a first conical part integrally connecting the first front end part to the first main body part to form a contiguous surface therebetween; and an inner crucible body having a same shape as a shape of the outer crucible body, the inner crucible body being fitted into the outer crucible body and comprising: a second front end part having a diameter less than the diameter of the first front end; a second main body part having a diameter less than the diameter of the first main body part; and a second conical part integrally connecting the second front end part to the second main body part to form a contiguous surface therebetween.

[2] The double-layered crucible for growth of the single crystal according to claim 1, wherein the outer crucible body and the inner crucible body are made of same material.

[3] The double-layered crucible for growth of the single crystal according to claim 2, wherein the material of the inner and outer crucible bodies is one selected from a group including W (tungsten), Pt (platinum), Pt-Rh, Rh-Ir and Pt-Zr or an alloy of two or more selected from the group.

[4] The double-layered crucible for growth of the single crystal according to claim 1, further comprising: a cover sealing an upper end of the inner crucible body, the cover being concave towards an interior of the inner crucible body.

[5] The double-layered crucible for growth of the single crystal according to claim 4, wherein a material of the cover is the same as the material of the inner crucible body.

[6] The double-layered crucible for growth of the single crystal according to claim 1, further comprising: one or more thermocouples welded to a circumferential outer surface of the outer crucible body.

[7] The double-layered crucible for growth of the single crystal according to claim 6, further comprising: one or more thermocouples welded to a circumferential outer surface of the first front end part of the outer crucible body.

[8] The double-layered crucible for growth of the single crystal according to claim 6 or 7, wherein a material of one of the thermocouples is the same as the material of the outer crucible body. [9] The double-layered crucible for growth of the single crystal according to claim 1, further comprising: a blocker inserted into the first front end part of the inner crucible body, the blocker having an annular shape such that an upper opening formed in an upper end thereof differs from a lower opening formed in a lower end thereof. [10] The double-layered crucible for growth of the single crystal according to claim 9, wherein a diameter of the upper opening of the blocker is greater than a diameter of the lower opening thereof. [11] The double-layered crucible for growth of the single crystal according to claim

10, wherein an inner surface of the blocker which forms the upper and lower openings is streamlined. [12] The double-layered crucible for growth of the single crystal according to claim 1, wherein a circumferential inner surface of the inner crucible body is processed by polishing.