JP3825127B2 - Single crystal growth method and apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for growing a single crystal by a micro pulling method and an apparatus therefor.
[0002]
[Prior art]
Recently, as a method for growing an oxide single crystal, a method of forming a single crystal fiber by a so-called micro-pulling-down method has attracted attention. "Electrotechnical News" July 1993 (No. 522), pages 4-8, JP-A-4-280891 and JP-A-6-345588 disclose niobate, potassium, lithium (K ₃ Li) by this method. _2-2x Nb _{5 + x} O _{15 + x} (hereinafter referred to as KLN)) and the like have been disclosed.
[0003]
According to the above article of “Electrotechnical News”, electric power is supplied to a platinum cell or crucible and resistance heating is performed. A melt outlet is formed at the bottom of the cell, and a rod-like body called a melt feeder is inserted into the outlet, thereby supplying the melt to the outlet and the solid phase liquid phase. Control both the state of the interface. By adjusting the diameter of the melt outlet, the thickness of the feeder, the protruding length of the feeder from the outlet, etc., a small-diameter KLN single crystal fiber is continuously formed. According to this μ pulling-down method, a single crystal fiber having a diameter of 1 mm or less can be formed, heat distortion can be reduced, convection in the melt can be easily controlled, and the diameter of the single crystal fiber can be easily controlled. It is characterized by the ability to produce small high-quality single crystals suitable for second harmonic generation. Similarly, it is used for blue second harmonic generation devices, optical communication waveguide devices, surface acoustic wave filter devices, and the like. It is known that LiNbO ₃ , LiTaO _{3 and the} like can also be grown.
[0004]
Also, in single crystal growth in the micro-pulling down method, conventionally, in the process of cooling the grown crystal, in order to suppress the occurrence of cooling cracks due to strain immediately after the growth, temperature control (temperature gradient and Annealing etc. are being studied.
[0005]
[Problems to be solved by the invention]
However, for example, when a single crystal fiber made of a ferroelectric compound such as LiNbO ₃ , LiTaO ₃ , or KLN is grown, the obtained single crystal fiber has a multidomain structure, or it happens to be a single domain structure. As a result, it was found that results with good reproducibility could not be obtained. When such a single crystal fiber was used to produce a second harmonic (SHG) generating element and the SHG generation efficiency and the like were evaluated, overall efficiency could not be obtained. For this reason, it was indispensable to separately treat the single crystal once grown.
[0006]
However, in order to perform single-domain processing on a ferroelectric single crystal, it is necessary to apply a voltage by a platinum plate electrode, for example, by mediating LiNbO ₃ , LiTaO ₃ , or KLN powder. During this single domain treatment, the crystallinity of the single crystal is degraded. In addition, depending on the conditions for applying the voltage, there is damage such as cracking in the single crystal. Furthermore, it is necessary to perform a complicated process for single domain processing, and the entire manufacturing apparatus is also complicated.
[0007]
An object of the present invention is to be able to grow single-domain single crystals when continuously growing single crystals by the micro-pulling-down method. It is to eliminate the need for conversion. Further, this is to obtain a single-domain crystal having very good crystallinity.
[0008]
[Means for Solving the Problems]
The present invention relates to a method for growing a single crystal, wherein a single crystal is formed by cooling the melt of the single crystal material from the crucible, and the temperature range of the Curie point is ± 200 ° C. during the cooling process of the single crystal. In the method for growing a single crystal, wherein the single crystal is lowered and continuously made into a single domain by providing a temperature gradient perpendicular to the growth direction with respect to the single crystal. is there.
[0009]
The present invention also includes a crucible containing a melt of a single crystal material and provided with a drawing port for pulling down the melt, and a melting heating mechanism for melting the single crystal material in the crucible. A single crystal growth apparatus comprising a drive mechanism for pulling down the single crystal from the crucible, wherein the temperature gradient is perpendicular to the growth direction with respect to the single crystal in a temperature range of Curie point ± 200 ° C. The present invention relates to an apparatus for growing a single crystal, characterized in that a temperature control mechanism is provided for providing
[0010]
The present inventor has conducted experiments for growing single crystal fibers such as LiNbO ₃ , LiTaO ₃ , and KLN by the micro-pulling-down method, and further disclosed LiNbO ₃ , LiTaO ₃ by the micro-pulling-down method as disclosed in JP-A-8-259375. Experiments have been conducted to continuously grow single crystal plates such as KLN. In the course of this research, even if the experiment was conducted under the same manufacturing conditions, the inside of the single crystal fiber or plate was divided into multiple domains or almost single domains, and the domain structure was not constant. discovered.
[0011]
The present inventor has been researching to find out the cause of this, but in the process where the melt lowered from the crucible outlet is cooled and solidified, and further lowered downward, it passes the Curie temperature. The idea was that the temperature around the single crystal was not constant in this temperature range. In other words, if a certain temperature gradient is generated in the single crystal at this time, single-domaining proceeds according to this temperature gradient, and if the temperature gradient in the single crystal is small or does not exist, it is randomly divided. It is presumed that a domain structure occurs and a multidomain structure is generated.
[0012]
Based on this estimation, it was discovered that when the heating mechanism was controlled under conditions that would cause a temperature gradient in the single crystal, at least in the vicinity of the Curie temperature, the single crystal would always be in a single domain. And the present invention has been reached.
[0013]
Specifically, in order to control the temperature gradient inside the single crystal itself, the single crystal in the pulled state is in a direction substantially perpendicular to the pulling direction (growing direction) of the single crystal. It has been found that a single crystal having a single domain structure can be stably grown by making the temperature difference between both surfaces 10 ° C. or more. In other words, by controlling the temperature gradient near the Curie point so that a temperature difference of 10 ° C. or more is made on both sides of the grown single crystal plate perpendicular to the growth direction, the low temperature side is the negative surface and the high temperature side is the positive surface. It was found that a single crystal having a single domain structure was obtained. From this viewpoint, it is preferable that the temperature difference be 50 ° C. or more.
[0014]
On the other hand, by setting the temperature difference to 300 ° C. or less, it is possible to prevent the deterioration of crystallinity and the generation of cracks due to the internal stress of the single crystal. From this viewpoint, in order to further improve the crystallinity of the single crystal, it is preferable that the temperature difference is 200 ° C. or less.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The method and apparatus of the present invention can be applied to both fiber-like single crystals and plate-like single crystals. However, the single domain has progressed more reliably in the plate-like or plate-like single crystal than in the fiber-like single crystal. Specifically, it has been found that, under predetermined conditions, both sides of the grown single crystal plate are negatively polarized with higher certainty and reliability.
[0016]
The present invention is particularly suitable when producing a solid solution single crystal. Examples of the solid solution single crystal include LiNbO ₃ , LiTaO ₃ , Li (Nb, Ta) O ₃ , KLN (K ₃ Li _2-2x Nb _{5 + x} O _{15 + x} ), and KLTN [K ₃ Li _2-2x ( Ta _y Nb _1-y ) _{5 + x} O _{15 + x} ], Ba _1-X Sr _X Nb ₂ O ₆
[0017]
LiNbO ₃ , LiTaO ₃ , and KLN single crystals have recently attracted attention as optical materials, and in particular as single crystals for blue light second harmonic generation (SHG) elements for semiconductor lasers. This can be generated up to the 390 nm ultraviolet region, so by using such short wavelength light, a wide range of applications such as optical disk memory, medical use, photochemistry use, and various optical measurement applications are possible. is there.
[0018]
The present invention can also be applied to growth of oxide single crystals that undergo compositional segregation. For example, when neodymium is dissolved in LiNbO ₃ , since the segregation coefficient is not 1, only a smaller amount of neodymium than the amount of neodymium in the composition of the melt enters the single crystal. For example, even if about 1.0 mol of neodymium is contained in the melt, only about 0.3 mol is contained in the single crystal. However, by rapidly cooling the melt in the nozzle portion, a single crystal having the same composition as that of the melt can be produced without causing segregation. This can also be applied to other laser single crystals, for example, YVO ₄ substituted with Nd, Er, Yb, or YVO ₄ substituted with Nd, Er, Yb.
[0019]
In the present invention, the method for growing a single crystal, particularly an oxide single crystal, is not particularly limited as long as it is a micro pulling method. However, it is necessary that the growth apparatus includes a temperature control mechanism for providing a temperature gradient in the single crystal in the vicinity of the Curie point. Such temperature control in the vicinity of the Curie point of the single crystal needs to be carried out specifically within the range of the Curie point ± 150 ° C. Such a temperature control mechanism is not particularly limited, but the following can be preferably exemplified from the viewpoint of mass productivity.
[0020]
(1) In the cooling zone at the bottom of the growth furnace, heaters are provided on the left and right sides of the single crystal plate, respectively. Each heater has a structure in which the temperature gradient of each part in the vertical direction of the heater can be freely set.
[0021]
(2) Heaters are provided on the left and right sides of the single crystal plate in the cooling zone at the bottom of the growth furnace. And on one surface side of the single crystal, an auxiliary heater is provided outside the heater, and the auxiliary heater generates heat, whereby the temperature of one surface side of the single crystal is changed to the other surface side opposite to this. Increase the temperature by 10 ° C. or more.
[0022]
(3) In the cooling zone at the bottom of the growth furnace, heaters are provided on the left and right sides of the single crystal plate, respectively. The distance between the heater and one side of the single crystal is made relatively small, and the heater and the single crystal are brought closer to each other. At the same time, the distance between the heater and the other surface of the single crystal is relatively increased, and the heater and the single crystal are moved away from each other. As a result, the temperature on one side of the single crystal is made 10 ° C. or higher than the temperature on the other side opposite to this.
[0023]
In the case of a single crystal fiber, the methods (1) to (3) can be adopted.
[0024]
【Example】
Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings. FIG. 1 is a schematic sectional view showing a manufacturing apparatus for growing a single crystal.
[0025]
A crucible 7 is installed inside the furnace body. An upper furnace 1 is installed so as to surround the crucible 7 and its upper space 5, and a heater 2 is embedded in the upper furnace 1. A nozzle portion 13 extends downward from the lower end portion of the crucible 7, and a drawing port 13 a is formed at the lower end portion of the nozzle portion 13. The lower furnace 3 is installed so as to surround the nozzle portion 13 and the surrounding space 6, and the heater 4 is embedded in the lower furnace 3. Both the crucible 7 and the nozzle portion 13 are formed of a corrosion-resistant conductive material. Of course, the form of the heating furnace itself can be variously changed. For example, in FIG. 1, the heating furnace is divided into two zones, but the heating furnace can be divided into three or more zones.
[0026]
One electrode of the power source 10 is connected to the position A of the crucible 7 by the electric wire 9, and the other electrode of the power source 10 is connected to the lower end B of the crucible 7. One electrode of the power source 10 is connected to the position C of the nozzle portion 13 by the electric wire 9, and the other electrode is connected to the lower end D of the nozzle portion 13. These energization mechanisms are separated from each other, and are configured so that the voltage can be controlled independently.
[0027]
In the crucible 7, the intake pipe 11 extends upward, and an intake port 22 is provided at the upper end of the intake pipe 11. The intake 22 protrudes slightly from the bottom of the melt 8. The melt inlet can also be formed at the bottom of the crucible so as not to protrude from the bottom of the crucible. In this case, the intake pipe 11 is not provided. An after heater can be provided in the space 6 at an interval so as to surround the nozzle portion 13.
[0028]
The upper furnace 1, the lower furnace 3, and the after-heater are heated to appropriately determine the temperature distribution in the spaces 5 and 6, supply the raw material of the melt into the crucible 7, and supply power to the crucible 7 and the nozzle unit 13. Heat. In this state, in the single crystal growing portion 18 at the lower end portion of the nozzle portion 13, the melt slightly protrudes from the opening 13 a and is held by the surface tension to form a relatively flat surface.
[0029]
As a result, the single crystal plate 14 is continuously formed on the upper side of the seed crystal 15 and is drawn downward. In this embodiment, the seed crystal 15 and the single crystal plate 14 are fed by a roller 16.
[0030]
The single crystal plate 14 is cooled as it goes downward from the nozzle portion 13. In this embodiment, heaters 12 </ b> A and 12 </ b> B are further installed on both sides of the single crystal plate 14. The temperature of the single crystal plate 14 while passing through this region is usually set to be within the Curie temperature ± 200 ° C., preferably within 150 ° C. By controlling the temperature of each heater 12A, 12B, the temperature difference between the one surface 14a and 14b of the single crystal 14 is adjusted as described above.
[0031]
2A is a side view of the single crystal 14, and FIG. 2B is a plan view. For example, the plate 14 is grown in a single domain in the direction of arrow E.
[0032]
In addition, even when the single crystal fiber 20 is grown in FIG. 1, as shown in FIG. 2 (c), the single domain is formed in the direction of the arrow E in the fiber 20.
[0033]
FIG. 3 is a schematic cross-sectional view schematically showing a manufacturing apparatus according to another embodiment. The same parts as those in the apparatus of FIG. Further, the peripheral parts such as the upper furnace and the lower furnace shown in FIG. 1 are not shown in FIG. In the apparatus of FIG. 3, the electrode of the power source 10 </ b> A is connected to the upper end F and the substantially central portion G of the crucible 7, and the power source 10 </ b> B is connected to the substantially central portion G and the lower end portion H of the crucible 7. The electrode of AC power supply 10C is connected to the lower end H of the crucible 7 and the upper end I of the nozzle part. An AC power supply 10 </ b> D is connected to the nozzle unit 13. These energization mechanisms are separated from each other, and are configured so that the voltage can be controlled independently.
[0034]
The single crystal plate 14 is cooled as it goes downward from the nozzle portion 13. In this embodiment, heaters 12 </ b> A and 12 </ b> B are further installed on both sides of the single crystal plate 14. The temperature of the single crystal plate 14 while passing through this region is usually set to be within the Curie temperature ± 200 ° C., preferably within 150 ° C. Further, an auxiliary heater 19 is provided outside the heater 12A. By controlling the temperature of each heater 12A, 12B, 19 the temperature difference between the one surface 14a and 14b of the single crystal 14 is adjusted as described above.
[0035]
FIG. 4 is a schematic cross-sectional view showing an apparatus according to another embodiment of the present invention, and the same members as those shown in FIG. Heaters 25A and 25B are installed on both sides of the single crystal plate. The temperature of the single crystal plate 14 while passing through this region is usually set to be within the Curie temperature ± 200 ° C., preferably 150 ° C. or less. The distance m between the plate 14 and the heater 25A is relatively small on the one surface 14a side of the single crystal plate 14, and the distance l between the plate 14 and the heater 25B is relatively small on the other surface 14b side of the plate 14. Make it bigger. Thereby, the temperature difference between the surface 14a and the surface 14b can be adjusted within a certain range.
[0036]
Next, the form of the nozzle part that is particularly suitable for manufacturing a single crystal plate will be described. In the μ pulling-down method, the present inventor forms a planar flat surface corresponding to the cross section of the single crystal plate at the tip of the nozzle part, and forms a plurality of rows of melt flow holes in the nozzle part. It was confirmed that a single crystal plate could be formed by simultaneously pulling down the melt from the melt flow holes and integrating the melt pulled down from the flow holes along the flat surface.
[0037]
In this aspect, the whole nozzle part can be made into a flat plate shape. In addition, an extension portion can be provided at the tip of the tubular nozzle portion, and the tip surface of the extension portion can be a flat surface as described above. Alternatively, the nozzle portion can be constituted by a plurality of tubular members, and the tubular members can be joined together to form an integrated flat surface by the distal end surfaces of the tubular members.
[0038]
[ Example 1 ]
Hereinafter, more specific experimental results will be described.
A single crystal production apparatus as shown in FIG. 1 was used to produce a LiNbO ₃ single crystal plate 14 according to the present invention. The temperature in the entire furnace was controlled by the upper furnace 1 and the lower furnace 3. The temperature gradient in the vicinity of the single crystal growing portion 18 can be controlled by supplying power to the nozzle portion 13 and generating heat from the after heater. As a mechanism for pulling down the single crystal plate 14, a mechanism for pulling down the single crystal plate 14 was mounted while uniformly controlling the pulling speed within a range of 2 to 100 mm / hour in the vertical direction.
[0039]
Lithium carbonate and niobium oxide were blended at a composition ratio of 50:50 to produce a raw material powder. About 50 g of this raw material powder was supplied into a platinum crucible 7, and this crucible 7 was installed at a predetermined position. The temperature of the space 5 in the upper furnace 1 was adjusted to a range of 1250 to 1350 ° C., and the raw material in the crucible 7 was melted. The temperature of the space 6 in the lower furnace 3 was controlled to 700 ° C to 1000 ° C. Predetermined electric power was supplied to the crucible 7, the nozzle part 13, and the after heater, and single crystal growth was performed. At this time, the temperature of the single crystal growing part could be 1200 to 1300 ° C., and the temperature gradient in the single crystal growing part could be controlled to 10 to 150 ° C./mm.
[0040]
The shape of the cross section outside the nozzle portion 13 was a rectangle, and the size thereof was 1.0 mm × 30 mm. The length of the nozzle part 13 was 20 mm. Thirty melt flow holes were provided in the nozzle portion 13. The diameter of each melt flow hole was 0.2 mm. The crucible 7 had a circular planar shape, a diameter of 30 mm, and a height of 30 mm.
[0041]
The temperature difference between the surfaces 14a and 14b of the single crystal plate 14 was set to 50 ° C. A single crystal plate having a width of 50 mm and a thickness of 1.0 mm was pulled down in the a-axis direction at a speed of 20 mm / hour. An 80 mm cooling zone for cooling from around 1150 ° C. to around 1000 ° C. was provided, and this zone was passed over about 4 hours at a growth rate of 20 mm / hour.
[0042]
The single crystal plate was thermally decomposed with a 3: 1 mixture of hydrofluoric acid and nitric acid at 180 ° C. for 1 hour. As a result, the positive side was not etched at all, whereas the negative side was etched by about 10 μm, and etch pits were observed in the portion with poor crystallinity. As a result, it was confirmed that a complete single-domain crystal was obtained.
[0043]
Further, when the SHG generation efficiency of this LiNbO ₃ single crystal was measured, it was improved by about 30% compared with the single crystal subjected to the conventional single domain treatment. The cause is related to a decrease in distortion of the substrate, for example, an X-ray rocking curve is reduced by about 20% in half width and crystallinity is improved.
[0044]
【The invention's effect】
As described above, according to the present invention, when a single crystal is continuously grown by the micro-pulling down method, a single-domained single crystal can be grown, and the single-domain processing of the grown single crystal can be performed. It can be made unnecessary. As a result, a single-domain single crystal having extremely good crystallinity can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a manufacturing apparatus for growing a single crystal according to an embodiment of the present invention.
2A is a side view of the single crystal plate 14, FIG. 2B is a plan view of the single crystal plate 14, and FIG. 2C is a cross-sectional view of the single crystal fiber 20;
FIG. 3 is a schematic cross-sectional view showing a main part of a manufacturing apparatus for growing a single crystal according to another embodiment of the present invention.
FIG. 4 is a schematic sectional view showing a main part of a manufacturing apparatus for growing a single crystal according to still another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Upper furnace, 2 Upper furnace 1 heater, 3 Lower furnace, 4 Lower furnace 3 heater, 7 Crucible, 10, 10A, 10B, 10C, 10D power supply, 12A, 12B 2 zone heater, 13 Nozzle part, 13a Drawer port, 14 single crystal plate, 15 seed crystal, 18 single crystal growth section, 20 single crystal fiber, 19 auxiliary heater, 25, 25A heater

Claims (10)

A method for growing a single crystal in which a melt of a single crystal material is pulled down from a crucible and cooled to produce a single crystal, wherein the single crystal is cooled in the temperature range of the Curie point ± 200 ° C. during the cooling process of the single crystal. A method for growing a single crystal, characterized by providing a temperature gradient perpendicular to the growth direction to continuously pull down the single crystal and continuously make it a single domain. 2. The method for growing a single crystal according to claim 1, wherein a temperature gradient is provided perpendicularly to the growth direction with respect to the single crystal in a temperature range of Curie point ± 150 ° C. 3. The temperature difference caused by the temperature gradient at the same pulling position of the single crystal when pulling down the single crystal is 10 ° C to 300 ° C, The single crystal according to claim 1 or 2, Training method. The method for growing a single crystal according to claim 3, wherein the temperature difference caused by the temperature gradient is 50 ° C. to 200 ° C. 5. The method for growing a single crystal according to claim 1, wherein the single crystal is a ferroelectric substance. A crucible containing a melt of a single crystal material and provided with a drawing port for pulling down the melt, a melting heating mechanism for melting the single crystal material in the crucible, and the single unit from the crucible A device for growing a single crystal comprising a drive mechanism for pulling down the crystal, and temperature control for providing a temperature gradient perpendicular to the growth direction of the single crystal in a temperature range of Curie point ± 200 ° C An apparatus for growing a single crystal, comprising a mechanism. 7. The apparatus for growing a single crystal according to claim 6, wherein the temperature control mechanism provides a temperature gradient perpendicular to the growth direction with respect to the single crystal in a temperature range of Curie point ± 150 ° C. The temperature control mechanism is characterized in that a temperature difference caused by the temperature gradient at the same pulling position of the single crystal when pulling down the single crystal is 10 ° C to 300 ° C. The apparatus for growing a single crystal as described in 1. 9. The apparatus for growing a single crystal according to claim 8, wherein, in the temperature control mechanism, the temperature difference caused by the temperature gradient is set to 50 ° C. to 200 ° C. The apparatus for growing a single crystal according to claim 6, wherein the single crystal is a ferroelectric substance.

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