Near-stoichiometric lithium niobate crystal and growth method thereof

Abstract

The invention provides a near-stoichiometric lithium niobate crystal and a growth method thereof, and relates to the technical field of crystal growth. The growth method comprises the following steps that the interior of a crucible is divided into a first temperature zone and a second temperature zone from top to bottom, the first temperature zone is located on the upper portion, the second temperature zone is located on the lower portion, the second temperature zone is filled with a growth raw material with the Li source / Nb source ratio being 50 / 50, and the first temperature zone is filled with a lithium-rich raw material; heating the first temperature zone, wherein the heating temperature of the first temperature zone is higher than the melting point of the lithium-rich raw material; then necking, cooling and shouldering are sequentially carried out, and meanwhile, the second temperature zone is heated, so that the melting speed of the growth raw material with the Li / Nb ratio being 50 / 50 is equal to the growth speed in the shouldering process; and finally, performing equal-diameter growth to obtain the near-stoichiometric lithium niobate crystal. According to the invention, an external continuous feeding growth device is canceled, continuous feeding growth is realized through regional control heating, and high-quality NSLN crystals can be grown at low cost.

Description

Technical Field

[0001] This invention relates to the field of crystal growth technology, and more specifically, to a near-stoichiometric lithium niobate crystal and its growth method. Background Technology

[0002] LiNbO3 (LN) crystal is a widely used multifunctional optoelectronic crystal that has long been valued. It possesses excellent piezoelectric, electro-optic, nonlinear optical, and photorefractive properties, making it widely applicable in lasers, optoelectronics, communications, and optical storage. LN is a typical non-stoichiometric crystal, generally obtained by pulling from a homologous melt with Li / Nb = 48.6 / 51.4, known as congruent lithium niobate (CLN) crystal. Due to the absence of Li ions, CLN crystals contain numerous vacancies and antisite defects, severely impairing their physical properties. Studies have shown that increasing the Li content in LN crystals significantly improves its performance.

[0003] Given the above background, there is a strong desire to grow near-stoichiometric lithium niobate (NSLN) crystals with low defect concentrations. The growth and performance research of NSLN crystals has become an international hot topic. The melt pulling method has always been an important method for growing large-size LN crystals, so scientists have proposed several melt growth methods for near-stoichiometric LN crystals. In 1992, Japan reported the use of an automated powder feeding system with a double crucible technique to grow NSLN crystals with diameters of 50-100 mm. The basic principle is to fill two crucibles (with identical bottom holes) with raw materials of different compositions. The crystal is pulled from the lithium-rich melt in the inner crucible, and throughout the growth process, raw materials of the stoichiometric composition are continuously replenished to the outer crucible according to the amount of crystal grown. However, the double crucible method has inherent disadvantages: poor fluid convection between the inner and outer crucibles easily leads to overheating of the melt in the outer crucible; furthermore, the double crucibles are difficult to clean after the growth process, requiring the manufacture of new crucibles for each growth cycle, significantly increasing the crystal growth cost. The suspended crucible technology alleviates the melt convection problem and reduces crucible costs, but the continuous addition of powder affects crystal growth, and the crucible cost is slightly higher than that of the traditional Czochralski method. It also has the problem of overheating of the melt in the outer crucible, making it difficult to use for the growth of large-size NSLN crystals.

[0004] Chinese invention patent CN105002560A discloses a method for preparing near-stoichiometric lithium niobate crystals with controllable directional crystallization. This method sequentially forms a low-temperature zone, a high-temperature zone, a temperature gradient zone, and a holding zone within the furnace chamber of a single crystal furnace, allowing the crucible to pass through these four temperature zones in sequence. The specific steps include: 1) Pre-crystallization material preparation: preparing two mixtures AB of Li₂CO₃ and Nb₂O₅, where the Li / Nb molar ratio in mixture A is 1:1, and the Li / Nb molar ratio in mixture B is between 1:1 and 58.5 / 41.5; 2) Furnace loading: loading lithium niobate seed crystals into the seed crystal slot of the crucible, placing pre-crystallized material B into the crucible shoulder, and placing pre-crystallized material A into the equal-width portion of the crucible; 3) Material preparation; 4) Crystal introduction; 5) Shoulder formation; 6) Equal-width growth; 7) Finishing; 8) Annealing and cooling. This patent describes a directional solidification process, not an improvement on the Czochralski growth process. In this patent, the crystal shoulder angle is determined by the container; changing the shoulder angle requires recasting the container, and the crystal shape cannot be arbitrarily adjusted. The crystal grown in this patent is in direct contact with the container, and the coefficients of thermal expansion differ in different directions. During cooling, the crystal is simultaneously subjected to internal thermal stress and container deformation stress, which increases the likelihood of mechanical twinning and cracking.

[0005] Chinese invention patent CN102689927A discloses a method for preparing near-stoichiometric lithium niobate crystals, comprising: heating, melting, cooling, crystallizing, and pulverizing a mixture of lithium carbonate (23%–45% by mass) and niobium pentoxide to obtain lithium-rich polycrystalline powder A; placing A into the lower part of a high-temperature resistant crucible; placing lithium-deficient lithium niobate crystals to be diffused on A, covering the exposed A around the crystal with platinum sheets; then heating, melting, cooling, crystallizing, and pulverizing a mixture of lithium carbonate (10%–18% by mass) and niobium pentoxide to obtain lithium-poor polycrystalline powder B, placing B into the upper part of the crucible; and then heating the crucible in a high-temperature furnace for diffusion treatment at a diffusion temperature of 1000–1150°C. The diffusion time is determined according to the diffusion temperature and the thickness of the crystal; the higher the diffusion temperature and the smaller the crystal thickness, the shorter the diffusion time required, and vice versa. After diffusion treatment, high-quality near-stoichiometric lithium niobate crystals can be obtained. This patent pertains to weather diffusion transport. During the diffusion process, lithium-deficient wafers are prone to twinning and cracking. It is not an improvement on the Czochralski growth process. The patent has a slow growth rate, and the diffusion speed decreases sharply with the increase of crystal thickness, making it difficult to obtain large-thickness stoichiometric crystals.

[0006] Based on the problems existing in the prior art, this invention innovates and modifies the Czochralski method, proposing a new method for growing near-stoichiometric lithium niobate crystals, which can be used to grow high-quality NSLN crystals at low cost. Summary of the Invention

[0007] Continuous feeding is a key method to solve the difficulties in NSLN crystal growth. To address the issue of continuous feeding in the Czochralski method, this invention eliminates the external continuous feeding growth device and achieves continuous feeding growth through zoned heating control. Single-layer crucible growth is employed, reducing platinum usage and avoiding problems such as difficult crucible cleaning, melt convection difficulties, and overheating at the crucible edge, thus achieving low-cost growth of high-quality NSLN crystals.

[0008] The first aspect of this invention provides a method for growing near-stoichiometric lithium niobate crystals, the method comprising the following steps: S1: Divide the inside of the crucible into a first temperature zone and a second temperature zone from top to bottom. The first temperature zone is located above the crucible, and the second temperature zone is located below the crucible. Fill the second temperature zone inside the crucible with a Li source / Nb source molar ratio of 50 / 50, and fill the first temperature zone inside the crucible with a Li source / Nb source molar ratio of (55-59) / (41-45). The melting point of the growth material with a Li source / Nb source molar ratio of 50 / 50 is higher than that of the lithium-rich material. S2: Heat the first temperature zone to form a molten lithium-rich melt. The heating temperature of the first temperature zone is higher than the melting point of the lithium-rich raw material. Optionally, the heating rate can be increased from fast to slow, for example, by increasing the heating rate at a gradient of 90-120℃ / h, 50-70℃ / h, 20-40℃ / h, and 5-10℃ / h to ensure that the lithium-rich raw material is fully melted.

[0009] S3: The descending seed crystal contacts the upper surface of the molten lithium-rich melt to undergo a necking process; S4: After necking is completed, the temperature is lowered and the shoulders are formed. At the same time, the second region is heated so that the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 is equal to the growth rate of the shoulder forming process. S5: After the shoulder formation is completed, constant diameter growth is performed. The growth rate of constant diameter growth is equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50, resulting in near-stoichiometric lithium niobate crystals.

[0010] This invention employs a zone-based material addition process during crystal growth. The upper part of the crucible is filled with lithium-rich material, while the lower part is filled with growth material with a Li / Nb molar ratio of 50 / 50. Zone-based temperature control is then used. First, the area containing the lithium-rich material is heated to form a molten lithium-rich melt, while the lower part of the crucible with a 50 / 50 Li / Nb molar ratio approaches a molten state. As the crystal is pulled from the lithium-rich melt, the temperature of the lower part of the crucible with a 50 / 50 Li / Nb molar ratio is increased. Based on the zone melting principle, NSLN crystals with a Li / Nb ratio of 1:1 can be obtained, while maintaining the melt composition at a constant Li / Nb ratio of 52 / 48. The mass of the molten growth material equals the mass of the grown crystal, keeping the lithium-rich melt composition constant and ensuring stable crystal growth. This effectively achieves continuous feeding to maintain normal NSLN crystal growth, preventing the formation of a second phase due to component segregation, thus obtaining high-quality NSLN crystals. Meanwhile, the simple single-layer crucible structure reduces the amount of platinum used, avoids problems such as difficulty in crucible cleaning, difficulty in melt convection, and overheating of the crucible edge, and the high melting point of the growth raw material also ensures that the bottom of the melt will not solidify due to overcooling, and avoids the situation where the bottom of the crystal protrudes and touches the bottom of the crucible, causing the crystal to stop.

[0011] Optionally, in step S2, the heating temperature of the first temperature zone is greater than the melting point of the lithium-rich raw material, and the heating temperature of the first temperature zone is less than the melting point of the growth raw material with a Li source / Nb source molar ratio of 50 / 50.

[0012] Optionally, the melting point of the growth feedstock with a Li / Nb molar ratio of 50 / 50 is 1213-1223℃; the melting point of the lithium-rich feedstock is 1175-1185℃.

[0013] Optionally, in step S4, the growth rate of the shoulder formation process is 0.2-0.5 mm / h. Optionally, in step S4, the cooling rate of the shoulder formation process is 2-5℃ / h.

[0014] Optionally, in step S5, the growth rate of constant diameter growth is 0.1-0.2 mm / h.

[0015] Optional, the diameter of the shoulder strap should be no less than 3 inches; Optionally, the length of the constant diameter growth shall not be less than 30 mm.

[0016] Optionally, in step S3, the necking process includes the following steps: S31: Adjust the heating temperature of the first temperature zone to be 1-3°C higher than the melting point of the lithium-rich raw material. Lower the seed crystal to 3-5 mm away from the upper surface of the molten lithium-rich melt and hold for 30-50 min. S32: Lower the seed crystal into contact with the upper surface of the molten lithium-rich melt and maintain it for 30-50 minutes; S33: Adjust the heating temperature of the first temperature zone, increase the heating temperature by 3-5℃, and pull upwards to narrow the neck.

[0017] Optionally, in step S33, the heating rate is 1-5℃ / h.

[0018] Furthermore, when adjusting the temperature slightly above the melting point of the lithium-rich melt, lower the seed crystal to 3-5 mm above the melt and hold it for 30-50 minutes to preheat it. This is to ensure the seed crystal is sufficiently preheated, preventing thermal stress and dislocations caused by excessive temperature difference when the seed crystal contacts the melt. Then, slowly lower the seed crystal to contact the melt and hold it for 30-50 minutes. The appropriate temperature is reached when the seed crystal does not melt due to excessive temperature or expand due to excessive temperature, and a bright halo appears around the seed crystal. Slowly increase the temperature by 3-5°C and pull the seed crystal upwards to perform the necking process. Typically, the necking size is half the size of the seed crystal. If the necking size is too small, the crystal may become too large and fall during subsequent growth; if the necking size is too large, dislocations may remain.

[0019] Optionally, the Li source is lithium carbonate; Optionally, the Nb source is niobium oxide.

[0020] Optionally, the height H2 of the second temperature zone inside the crucible is greater than 2 to 3 times the height H1 of the first temperature zone inside the crucible.

[0021] A second aspect of the present invention provides a near-stoichiometric lithium niobate crystal.

[0022] Optionally, the near-stoichiometric ratio of Li / [Nb+Li] in the lithium niobate crystal is greater than or equal to 49.95%; the optical uniformity is less than 10. -4 .

[0023] Optionally, the diameter of near-stoichiometric lithium niobate crystals is not less than 3 inches.

[0024] Optionally, the length of the near-stoichiometric lithium niobate crystal is not less than 30 mm.

[0025] Compared with the prior art, the present invention achieves at least one of the following beneficial effects: (1) In the near stoichiometric lithium niobate crystal growth process of the present invention, an innovative improvement is made to the existing Czochralski method, eliminating the external continuous feeding growth device, and realizing continuous feeding growth by controlling heating in different regions, thereby achieving low-cost growth of high-quality NSLN crystals.

[0026] (2) The present invention uses a single-layer crucible for growth, which reduces the amount of platinum used and avoids problems such as difficulty in crucible cleaning, difficulty in melt convection and excessive heating of crucible edge.

[0027] (3) This invention is an improvement on the traditional Czochralski method, and the crystal shape can be adjusted at will without restriction.

[0028] (4) In this invention, the crystal only contacts the melt and the grown crystal does not directly contact the container. During cooling, it is only affected by internal thermal stress. The subsequent cooling process can avoid the influence of internal thermal stress. The grown crystal will not have mechanical twins or cracks.

[0029] (5) The present invention can obtain large-sized, long and equal-diameter near stoichiometric lithium niobate crystals with fast and stable growth rate. Attached Figure Description

[0030] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 An exemplary embodiment of the NSLN crystal growth temperature field structure of the present invention is shown.

[0031] Explanation of reference numerals in the attached figures: In the diagram: 1—NSLN crystal, 2—Li-rich melt with Li / Nb = (55-59) / (41-45), 3—Growth material with Li / Nb = 50 / 50, 4—Platinum crucible, 5—Low-power coil, 6—High-power coil, 7—Corundum, 8—Insulation material, 9—Quartz, 10, 11—Thermocouples. Detailed Implementation To more clearly illustrate the overall concept of the present invention, a detailed description will be provided below with reference to the accompanying drawings and examples.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0033] Example 1 In one exemplary embodiment of the present invention, reference is made to... Figure 1 As shown, the temperature field structure for growing near-stoichiometric lithium niobate crystals includes a thermal insulation structure, a platinum crucible 4, and a heating assembly. Both the thermal insulation structure and the platinum crucible 4 are cylindrical bodies with an open top and a closed bottom. During the growth process, the platinum crucible 4 is placed inside the thermal insulation structure, with the central axes of the platinum crucible and the thermal insulation structure coinciding. The insulation structure includes a circular cylinder and a bottom connected to the lower end of the cylinder. From the inside out, the cylinder includes a corundum layer 7, an insulation material layer 8, and a quartz layer 9. The insulation structure can be customized with appropriate insulation materials and thicknesses according to actual needs. The heating components are located on the outside of the insulation structure and do not directly contact it. The heating components include a low-power induction coil 5 and a high-power induction coil 6.

[0034] The near-stoichiometric lithium niobate crystal growth method includes the following steps: S1: Divide the interior of the platinum crucible 2 into a first temperature zone and a second temperature zone from top to bottom. The first temperature zone is located above the crucible, and the second temperature zone is located below the crucible. Fill the second temperature zone inside the crucible with growth material 3 of Li source / Nb source molar ratio of 50 / 50, and fill the first temperature zone inside the crucible with lithium-rich material 2 of Li source / Nb source molar ratio of (55-59) / (41-45). The melting point of the growth material of Li source / Nb source molar ratio of 50 / 50 is higher than that of the lithium-rich material. S2: Place the platinum crucible 4 filled with material in the insulation structure. The induction coil 5 is set on the outside of the cylinder and its position corresponds to the first temperature zone of the platinum crucible 4 to ensure precise heating of the first temperature zone. The induction coil 6 is set on the outside of the cylinder and its position corresponds to the second temperature zone of the platinum crucible 4 to ensure precise heating of the second temperature zone.

[0035] The first temperature zone is heated by starting coil 5. The heating temperature of the first temperature zone is greater than the melting point of lithium-rich raw material. Thermocouples 10 and 11 are respectively set in the first temperature zone and the second temperature zone. The temperature change is detected by the thermocouples at all times to form molten lithium-rich melt. S3: The seed crystal is lowered from directly above the crucible to contact the upper surface of the molten lithium-rich melt and perform the necking process; S4: After necking is completed, cooling and shoulder formation are carried out by controlling coil 5. At the same time, coil 6 is started to heat the second temperature zone. Temperature changes are monitored at all times by thermocouples to ensure that the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 is equal to the growth rate of the shoulder formation process. S5: After the shoulder is formed, constant diameter growth is carried out. By adjusting the power of the heating coil 6, the growth rate of constant diameter growth is made equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50. The end is then carried out. After the end is completed, the crystal is removed, cooled and cooled to obtain near stoichiometric lithium niobate crystal 1.

[0036] Example 2 In an exemplary embodiment of the present invention, the temperature field structure for growing near-stoichiometric lithium niobate crystals adopts the operating mode of Example 1, and the method for growing 3-inch near-stoichiometric lithium niobate crystals includes the following steps: S1: The platinum crucible is 100mm high. The growth material with a lithium carbonate / niobium oxide molar ratio of 50 / 50 (melting point of about 1218℃) is filled into the second temperature zone inside the crucible, with a height of 70mm. The lithium-rich material with a lithium carbonate / niobium oxide molar ratio of 58 / 42 (melting point of about 1180℃) is filled into the first temperature zone inside the crucible, with a height of 30mm.

[0037] S2: Place the platinum crucible filled with material in the heat-insulating structure, and heat the first temperature zone by starting coil 5. The heating rate is fast at first and then slows down, at 100℃ / h, 60℃ / h, 30℃ / h, and 10℃ / h respectively, to raise the temperature to 1185℃ and hold it for 5 hours to ensure that the lithium-rich raw material is fully melted to form a molten lithium-rich melt. The melting point of the raw material with Li / Nb=50 / 50 is lower than the heating temperature of the first temperature zone, so it remains solid. S3: Lower the seed crystal to 3 mm from the upper surface of the molten lithium-rich melt and hold for 30 min. Then lower the seed crystal to contact the upper surface of the molten lithium-rich melt and hold for 30 min. When you see that the seed crystal does not melt due to excessive temperature or expand due to excessive temperature, and a bright halo appears around the seed crystal, start heating. Increase the temperature by 3°C and pull it upward to perform the necking process. The heating rate is 1°C / h, and the necking size is 1 / 2 of the seed crystal size.

[0038] S4: After necking is completed, cooling and shoulder formation are performed by controlling coil 5 at a cooling rate of 2℃ / h. At the same time, coil 6 is started to heat the second temperature zone. Temperature changes are monitored by thermocouples at all times. The heating rate of the second temperature zone is 60℃ / h, and the temperature is raised to 1219℃. This makes the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 equal to the growth rate of the shoulder formation process. The growth rate of the shoulder formation process is 0.5mm / h.

[0039] S5: Cool down to a shoulder diameter of 80mm, then switch to constant diameter growth. By adjusting the heating power of coil 5, the temperature slowly decreases during the constant diameter growth process. The cooling rate is about 0.2℃ / h, which corresponds to the constant diameter growth rate of 0.2mm / h. This makes the growth rate of constant diameter growth equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50. After the end of the process, the crystal is removed, cooled down, and near-stoichiometric lithium niobate crystals are obtained.

[0040] Example 3 In an exemplary embodiment of the present invention, the temperature field structure for growing near-stoichiometric lithium niobate crystals adopts the operating mode of Example 1. The method for growing 6-inch near-stoichiometric lithium niobate crystals includes the following steps: S1: The platinum crucible is 120mm high and 200mm in diameter. The growth material with a lithium carbonate / niobium oxide (Li / Nb) molar ratio of 50 / 50 (melting point of about 1218℃) is filled in the second temperature zone inside the crucible, with a height of 90mm. The lithium-rich material with Li / Nb = 58 / 42 (melting point of about 1180℃) is filled in the remaining space of the crucible.

[0041] S2: Place the platinum crucible filled with material in the heat-insulating structure, and heat the first temperature zone by starting coil 5. The heating rate is fast at first and then slows down, at 100℃ / h, 60℃ / h, 30℃ / h, and 10℃ / h respectively, to raise the temperature to 1185℃ and hold it for 6 hours to ensure that the lithium-rich raw material is fully melted to form a molten lithium-rich melt. The melting point of the raw material with Li / Nb=50 / 50 is lower than the heating temperature of the first temperature zone, so it remains solid. S3: Lower the seed crystal to 5 mm from the upper surface of the molten lithium-rich melt and hold for 50 min. Then lower the seed crystal to contact the upper surface of the molten lithium-rich melt and hold for 40 min. When a bright halo appears around the seed crystal, start heating. Increase the temperature by 5°C and pull it upward to perform the necking process. The heating rate is 4°C / h, and the necking size is 1 / 2 of the seed crystal size.

[0042] S4: After necking is completed, the low coil 5 is adjusted to cool down and form a shoulder at a rate of 5℃ / h. At the same time, the high coil 6 is started to heat the second temperature zone. The temperature change is monitored by thermocouples at all times. The heating rate of the second temperature zone is 60℃ / h, and the temperature is raised to 1219℃. This makes the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 equal to the growth rate of the shoulder formation process. The growth rate of the shoulder formation process is 0.5mm / h.

[0043] S5: Cool down and grow the shoulder to a diameter of 155mm and a shoulder height of 20mm, then switch to constant diameter growth. By controlling coil 5, the temperature during constant diameter growth is slowly reduced, with a cooling rate of about 0.1℃ / h corresponding to the constant diameter growth rate of 0.1mm / h. This makes the growth rate of constant diameter growth equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50. After the constant diameter length reaches 40mm, the crystal is finished off. After finishing, the crystal is removed, cooled, and obtained near-stoichiometric lithium niobate crystal.

[0044] Example 4 In an exemplary embodiment of the present invention, the temperature field structure for growing near-stoichiometric lithium niobate crystals adopts the operating mode of Example 1. The method for growing 6-inch near-stoichiometric lithium niobate crystals includes the following steps: S1: The platinum crucible is 120mm high and 200mm in diameter. The growth material with a lithium carbonate / niobium oxide (Li / Nb) molar ratio of 50 / 50 (melting point of about 1218℃) is filled in the second temperature zone inside the crucible, with a height of 90mm. The remaining space of the crucible is filled with lithium-rich material with a Li / Nb ratio of 55 / 45 (melting point of about 1195℃).

[0045] S2: Place the platinum crucible filled with materials in the insulation structure, and heat the first temperature zone by starting coil 5. Due to the change in the composition of the lithium-rich raw material, the melting point increases, and the melting temperature needs to be increased accordingly. The heating rate is initially fast and then slows down, at 100℃ / h, 60℃ / h, 30℃ / h, and 10℃ / h respectively, to reach 1200℃ and hold for 6 hours to ensure that the lithium-rich raw material is fully melted to form a molten lithium-rich melt, while ensuring that the Li / Nb=50 / 50 raw material remains solid because its melting point (1218℃) is higher than this temperature.

[0046] S3: Lower the seed crystal to 5 mm from the upper surface of the molten lithium-rich melt and hold for 50 min for preheating. Then lower the seed crystal to contact the upper surface of the molten lithium-rich melt and hold for 40 min. When a bright halo is observed around the seed crystal, start heating, increasing the temperature by 3°C, and pull upwards to perform a necking process. The heating rate is 2°C / h, and the necking size is 1 / 2 of the seed crystal size.

[0047] S4: After necking is completed, cooling and shoulder formation are performed by controlling coil 5 at a rate of 3℃ / h. At the same time, coil 6 is started to heat the second temperature zone. Temperature changes are monitored by thermocouples. The heating rate of the second temperature zone is 60℃ / h, and the temperature is raised to 1219℃. This ensures that the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 is equal to the growth rate of the shoulder formation process, which is 0.5mm / h.

[0048] S5: Cool down and allow the shoulder to form a diameter of 155mm and a shoulder height of 20mm, then proceed to constant-diameter growth. By adjusting coil 5, the temperature during constant-diameter growth is slowly reduced at a rate of approximately 0.1℃ / h, corresponding to the constant-diameter growth rate of 0.1mm / h. This ensures that the growth rate of constant-diameter growth is equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50. After the constant-diameter length reaches 50mm, the crystal is finished off. After finishing, the crystal is removed, cooled, and a 6-inch near-stoichiometric lithium niobate crystal is obtained.

[0049] Comparative Example 1 Based on Example 2, the main difference is that in step S2, the heating temperature of the first temperature zone is 1220°C.

[0050] Comparative Example 2 Based on Example 2, the main difference is that in step S4, the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 is not equal to the growth rate of the shoulder formation process.

[0051] Comparative Example 3 Based on Example 2, the main difference is that in step S5, the growth rate of the constant diameter growth is not equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50.

[0052] Comparative Example 4 The main difference from Example 2 is that the lithium-rich raw material has a Li source / Nb source molar ratio of 62 / 38.

[0053] Comparative Example 5 The main difference from Example 2 is that the growth rate of the constant diameter growth is 0.3 mm / h.

[0054] Comparative Example 6 The main difference from Example 2 is that the growth rate of the shoulder formation process is 1 mm / h.

[0055] Test case The performance of the crystals grown in the examples and comparative examples was tested, as shown in Table 1.

[0056] Among them, optical uniformity was tested using a white light interferometer to measure the maximum deviation of the refractive index.

[0057] Table 1

[0058] Referring to Table 1, this application yields high-quality near-stoichiometric lithium niobate crystals with a Li / [Nb+Li] ratio exceeding 49.9%. Furthermore, the near-stoichiometric lithium niobate crystals obtained in this application exhibit optical uniformity <10. -4 By controlling heating in different regions to achieve continuous feeding growth, the defect density of near-stoichiometric lithium niobate crystals is greatly reduced, and the optical uniformity is improved. In addition, this application can obtain large-size (not less than 3 inches) near-stoichiometric lithium niobate crystals with equal diameter (equal diameter length not less than 40 mm).

[0059] In Comparative Example 1, when the heating temperature in the first temperature zone is higher than the melting point of the raw material (Li / Nb=50 / 50), seed crystal melting occurs, preventing growth.

[0060] In Comparative Example 2, if the melting rate of the growth feedstock with a Li / Nb molar ratio of 50 / 50 is not synchronized with the growth rate during the shoulder formation process, a second phase will appear during the shoulder formation process.

[0061] In Comparative Example 3, if the growth rate of the constant diameter growth is not synchronized with the melting rate of the growth raw material with a Li / Nb molar ratio of 50 / 50, a second phase will appear during the constant diameter process.

[0062] In Comparative Example 4, when the lithium-rich raw material with a Li source / Nb source molar ratio of 62 / 38 is used, this ratio of lithium-rich melt will result in the inability to obtain a single-crystal phase of lithium niobate.

[0063] In Comparative Example 5, when the growth rate of constant diameter growth is too fast, the Li ions at the solid-liquid interface do not have enough time to diffuse. The crystal composition formed by melt crystallization is Li / Nb≠50 / 50, and instead, a lithium-rich second phase, Li3NbO4, is formed. The resulting near-stoichiometric lithium niobate crystal has defects and poor optical uniformity.

[0064] In Comparative Example 6, the shoulder growth rate was too fast, resulting in twins or inclusions.

[0065] The above description is merely an embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for growing near-stoichiometric lithium niobate crystals, characterized in that, The growth method includes the following steps: S1: Divide the inside of the crucible into a first temperature zone and a second temperature zone from top to bottom. The first temperature zone is located above the crucible, and the second temperature zone is located below the crucible. Fill the second temperature zone inside the crucible with a Li source / Nb source molar ratio of 50 / 50, and fill the first temperature zone inside the crucible with a Li source / Nb source molar ratio of (55-59) / (41-45). The melting point of the growth material with a Li source / Nb source molar ratio of 50 / 50 is higher than that of the lithium-rich material. S2: The first temperature zone is heated to form a molten lithium-rich melt. The heating temperature of the first temperature zone is greater than the melting point of the lithium-rich raw material. S3: The descending seed crystal contacts the upper surface of the molten lithium-rich melt to undergo a necking process; S4: After necking is completed, the temperature is lowered and the shoulders are formed. At the same time, the second temperature zone is heated so that the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50 is equal to the growth rate of the shoulder forming process. S5: After the shoulder formation is completed, constant diameter growth is performed. The growth rate of constant diameter growth is equal to the melting rate of the growth material with a Li / Nb molar ratio of 50 / 50, resulting in near-stoichiometric lithium niobate crystals.

2. The growth method according to claim 1, characterized in that, In step S2, the heating temperature of the first temperature zone is greater than the melting point of the lithium-rich raw material, and the heating temperature of the first temperature zone is less than the melting point of the growth raw material with a Li source / Nb source molar ratio of 50 / 50.

3. The growth method according to claim 1, characterized in that, The melting point of the growth feedstock with a Li / Nb molar ratio of 50 / 50 is 1213-1223℃; the melting point of the lithium-rich feedstock is 1175-1185℃.

4. The growth method according to claim 1, characterized in that, In step S4, the growth rate of the shoulder formation process is 0.2-0.5 mm / h.

5. The growth method according to claim 1, characterized in that, In step S5, the growth rate of constant diameter growth is 0.1-0.2 mm / h.

6. The growth method according to claim 1, characterized in that, The diameter of the shoulder pads should be no less than 3 inches; And / or the length of the isodiameter growth is not less than 30 mm.

7. The growth method according to claim 1, characterized in that, In step S3, the necking process includes the following steps: S31: Adjust the heating temperature of the first temperature zone to be 1-3°C higher than the melting point of the lithium-rich raw material. Lower the seed crystal to 3-5 mm away from the upper surface of the molten lithium-rich melt and hold for 30-50 min. S32: Lower the seed crystal into contact with the upper surface of the molten lithium-rich melt and maintain it for 30-50 minutes; S33: Adjust the heating temperature of the first temperature zone, increase the heating temperature by 3-5℃, and pull upwards to narrow the neck.

8. The growth method according to claim 1, characterized in that, The Li source is lithium carbonate; And / or the Nb source is niobium oxide.

9. The growth method according to claim 1, characterized in that, The height H2 of the second temperature zone inside the crucible is 2-3 times the height H1 of the first temperature zone inside the crucible.

10. A near-stoichiometric lithium niobate crystal prepared by the near-stoichiometric lithium niobate crystal growth method according to any one of claims 1-9.

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