High-cleanliness large-size silicon carbide crystal growth method

Abstract

The invention relates to the technical field of crystal growth, and discloses a high-cleanliness large-size silicon carbide crystal growth method, which comprises the following steps: putting SiC powder into a graphite crucible for growth, and carrying out pretreatment on the SiC powder before the SiC powder is put into the graphite crucible: controlling the acceptor impurity total concentration Na and donor impurity concentration Nd in the SiC powder to meet the condition that Na is greater than or equal to 0.8 Nd and less than or equal to 1.2 Nd; according to the invention, the problems of carbon particle wrapping and harmful element introduction in the existing silicon carbide growth technology can be solved, so that the silicon carbide crystal with high purity and large size can be grown.

Description

Technical Field

[0001] This invention relates to the field of crystal growth technology, and specifically to a method for growing high-purity, large-size silicon carbide crystals. Background Technology

[0002] Silicon carbide crystals, due to their high refractive index (2.6-2.7), high hardness (Mohs hardness 9.5), excellent visible light transmittance, and chemical stability, have become key raw materials for AR eyeglass lenses and moissanite. The core requirements for crystal quality in these two applications are: 1. High clarity: In the field of AR glasses lenses, the core requirement is low optical loss, requiring the crystal to have a transmittance of ≥80% before coating and ≥95% after coating in the 400-700nm visible light band. This indicator relies on the absence of obvious impurity scattering sources inside the crystal—it is necessary to avoid both macroscopic inclusion defects and control microscopic harmful element doping to prevent light scattering or absorption inside the crystal, ensuring the clarity and brightness of the AR display.

[0003] In the field of moissanite: Jewelry-grade appearance and color uniformity are paramount; crystals must achieve a gemological "internally flawless (IF)" grade. Specific quantitative requirements include: total impurity content ≤ 10¹ 6 atoms / cm³, and the density of carbon particle inclusions, which are the main defects, is ≤10² cm³. - ². The presence of impurities and inclusions can cause color banding and cloud-like flaws in moissanite, damaging the uniformity and transparency of its "fire" and reducing its jewelry value.

[0004] 2. Large size: In the field of AR eyeglasses: AR eyeglasses need to be processed into optical lenses of specific shapes. Using silicon carbide substrates of 6 inches or larger, more qualified lenses can be cut at once, which greatly reduces the raw material loss and processing cost of a single lens, meeting the stringent cost control requirements of the consumer electronics industry.

[0005] In the field of moissanite: the mainstream demand in the jewelry market is for large moissanite particles of 3 carats and above. This requires precise control of crystal size during the growth of silicon carbide single crystals. Not only must the volume requirements corresponding to the weight be met, but also the uniformity of internal properties of large crystals must be ensured, with no local defects concentrated, so as to ensure the quality stability of the finished jewelry products.

[0006] Currently, the mainstream industrial method for growing silicon carbide crystals is the physical vapor transport (PVT) method. This method achieves single crystal growth through a process of raw material sublimation, vapor transport, and seed crystal deposition at high temperatures. However, this method has two major problems: 1. Defects caused by carbon particle encapsulation: In the high-temperature environment (2200-2500℃) of PVT growth, silicon carbide raw materials undergo a decomposition reaction (SiC→Si(g)+C(s)). The resulting solid carbon particles cannot be transported with the gas phase and are easily suspended in the growth area. At the same time, the graphite crucible used for growth will undergo surface volatilization at high temperature, and the resulting carbon impurity particles will also enter the crystal growth interface and be captured by the growing silicon carbide single crystal, forming carbon particle inclusion defects.

[0007] This defect has a direct and fatal impact on two types of applications: for AR glasses lenses, carbon particles will strongly scatter visible light, causing a significant decrease in lens transmittance, making it impossible to meet the ≥95% requirement; for moissanite, carbon particles will form black or gray blemishes, destroying its "internally flawless" quality grade, while also causing "fire" distortion, thus losing its jewelry-grade value.

[0008] 2. Introduction of harmful elements: During the growth process, harmful elements such as aluminum (Al), nitrogen (N2), and boron (B) enter the silicon carbide crystal through multiple pathways, forming doping contamination. The sources mainly include three aspects: first, raw material contamination, such as impurities like Al2O3 and B4C contained in silicon carbide powder; second, residual atmosphere, where N2 not completely removed from the vacuum growth system will participate in crystal growth at high temperatures; and third, crucible diffusion, where trace amounts of Al and B elements contained in the graphite crucible will diffuse into the crystal interior at high temperatures.

[0009] The doping of these elements can lead to targeted performance degradation: nitrogen (N), as a typical donor impurity, will cause silicon carbide crystals to have a distinct yellow tint, directly destroying the colorless and transparent properties of moissanite; aluminum (Al) and boron (B), as acceptor impurities, will cause local light absorption phenomena in the crystal due to their uneven distribution. On the one hand, this will cause uneven color of moissanite, with color bands or dark areas, and on the other hand, it will increase the optical loss of AR glasses lenses and reduce light transmission efficiency, thus doubly restricting the feasibility of crystal applications.

[0010] In summary, solving the problems of carbon particle encapsulation defects and the introduction of harmful elements to achieve large-size crystal growth has become the core technological breakthrough for promoting the large-scale application of silicon carbide crystals in AR eyeglass lenses and moissanite. Summary of the Invention

[0011] The present invention aims to provide a method for growing high-purity, large-size silicon carbide crystals to solve the problems of carbon particle encapsulation and the introduction of harmful elements in existing silicon carbide growth technologies, thereby growing high-purity, large-size silicon carbide crystals.

[0012] To achieve the above objectives, the present invention adopts the following technical solution: A method for growing high-purity, large-size silicon carbide crystals involves placing SiC powder in a graphite crucible for growth. The SiC powder undergoes pretreatment before being placed in the graphite crucible: controlling the total concentration of acceptor impurities N in the SiC powder. a With donor impurity concentration N d Satisfy: 0.8N d ≤ N a ≤ 1.2N d .

[0013] Preferably, as an improvement, the acceptor impurities include Al and B, and the ratio of Al to B doping in the SiC powder is 1:1.2.

[0014] Preferably, as an improvement, during growth, one of argon, hydrogen, or a mixture of argon and hydrogen is introduced.

[0015] Preferably, as an improvement, the gas is filtered to remove impurities before being introduced into the graphite crucible, and the N2 content in the atmosphere is controlled to be ≤0.5ppm.

[0016] Preferably, as an improvement, when introducing the mixed gas, the proportion of hydrogen in the mixed gas is controlled to be 5%-10%.

[0017] Preferably, as an improvement, the temperature of the raw material zone is 70℃-150℃ higher than the temperature of the seed crystal zone during growth.

[0018] Preferably, as an improvement, during growth, the growth rate is controlled at 0.6-1.0 mm / h, the temperature difference of the radial temperature field of the graphite crucible is ≤3℃, and the temperature is gradually reduced to room temperature at a rate of 5℃ / h in the later stage of growth.

[0019] Preferably, as an improvement, a conical guide ring is provided inside the graphite crucible, the guide ring being located between the raw material zone and the seed crystal zone, and the cone angle of the guide ring being 60°-80°.

[0020] Preferably, as an improvement, a porous graphite sheet is disposed below the flow guide ring, the porous graphite sheet is coated with TaC, and a C coating is disposed on the inner surface of the graphite crucible, the C coating thickness is 3-8μm, and the density is ≥99.8%.

[0021] Preferably, as an improvement, an annular baffle is provided below the porous graphite sheet, and the annular baffle is fixed to the inner wall of the graphite crucible.

[0022] The principles and beneficial effects of this solution are as follows: 1. This solution precisely controls the doping ratio of Al and B, the two acceptor impurities, in SiC powder to 1:1.2, and strictly controls the total N concentration of both. a With donor impurity concentration N dSatisfying 0.8N d ≤ N a ≤ 1.2N d By utilizing the synergistic electrical compensation matching relationship between acceptor and donor impurities, the technical pain points of uneven local light absorption, poor color consistency, and increased optical loss caused by single acceptor impurity doping are effectively offset. On the other hand, the yellow tone shift of the crystal caused by the presence of donor impurities alone is significantly suppressed. Ultimately, the prepared SiC crystal achieves a high level of 80% transmittance before coating and 96.5% after coating in the 400-700nm visible light band, and the moissanite color grade reaches the D color (colorless) standard, realizing the synergistic optimization of optical performance and appearance quality.

[0023] 2. During the crystal growth stage, argon, hydrogen, or a mixture of both are selected as the growth atmosphere. A high-precision gas filter is used to perform a secondary deep impurity removal process on the selected atmosphere, strictly controlling the nitrogen (N2) impurity content to ≤0.5ppm. This design effectively reduces the interference of impurities on the seed crystal growth process. On the one hand, it significantly improves the quality and stability of seed crystal growth, providing a good environmental foundation for the uniform growth of large-size crystals; on the other hand, it further optimizes the light transmittance and color uniformity of the crystal by reducing the light scattering and light absorption effects caused by impurities. Specifically, a mixture of argon and hydrogen is preferred as the growth atmosphere, with the hydrogen content precisely controlled at 5%-10%. This ratio design has dual technical advantages: it utilizes the reducing properties and thermal conductivity of hydrogen to promote the migration and arrangement of atoms on the crystal surface, improving the growth rate and crystal density; and it avoids the flammability and explosion risks caused by excessive hydrogen content, ensuring both improved growth effect and crystal performance while maintaining the safety and controllability of the production process.

[0024] 3. This scheme provides crucial support for the growth of large-size, high-integrity crystals through the synergistic control of multi-dimensional temperature field and growth parameters: During crystal growth, the temperature of the raw material zone is precisely controlled to be 70℃-150℃ higher than that of the seed crystal zone, while the growth rate is stabilized at 0.6-1.0 mm / h, and the temperature difference in the radial temperature field of the graphite crucible is strictly controlled to ≤3℃. In the later stages of growth, a step-wise cooling process is implemented at a rate of 5℃ / h to room temperature. The core advantage of this multi-parameter synergistic design lies in providing a stable driving force for the sublimation-transport-deposition process of the raw material through a reasonable temperature gradient between the raw material zone and the seed crystal zone. Combined with a suitable growth rate and a uniform radial temperature field, it effectively avoids stress concentration caused by uneven temperature distribution during crystal growth. The step-wise cooling strategy in the later stages of growth can slowly release the thermal stress accumulated inside the crystal, further ensuring the integrity of the crystal structure. Among them, the temperature difference control between the raw material zone and the seed crystal zone is particularly critical: if the temperature difference is too small, it will lead to insufficient sublimation rate of raw materials and lack of transport force, directly limiting the crystal growth size; if the temperature difference is too large, although it can improve the raw material transport efficiency to expand the crystal size, it will cause a significant thermal stress gradient to form inside the crystal, resulting in more internal defects and increased brittleness, making it very easy to crack during the growth process or cooling stage.

[0025] 4. In addition to precise control of process parameters, this solution also innovatively improves the structure of the graphite crucible: a conical guide ring is added inside the graphite crucible, and the cone angle of the guide ring is precisely limited to 60°-80°. The core function of this structural design is to construct a directional airflow channel from the raw material area to the seed crystal area. The conical guide surface guides the stable transport and uniform distribution of airflow, which effectively suppresses the disordered diffusion of carbon particles and avoids the formation of carbon encapsulation defects during crystal growth. On the other hand, it provides a stable and clean growth environment for the seed crystal, ensuring the continuous growth of large-size crystals and reducing the damage of defects to the integrity of the crystal structure. At the same time, it significantly reduces light scattering and light absorption caused by defects, further optimizing the light transmittance and color uniformity of the crystal.

[0026] The cone angle parameter of the flow guiding ring has a crucial impact on the flow guiding effect and growth quality: if the cone angle is too large, the tilt angle of the flow guiding slope formed by the sidewall of the flow guiding ring is insufficient, failing to form an effective airflow guiding effect, leading to airflow turbulence and uncontrolled carbon particle diffusion, directly affecting the normal growth of the seed crystal; if the cone angle is too small, although the tilt angle of the flow guiding slope increases, the effective length of the flow guiding slope will be significantly shortened due to the limited internal space of the crucible and the design requirement to avoid covering the seed crystal, making it difficult to achieve stable long-distance airflow guidance, resulting in uneven distribution of airflow when it reaches the seed crystal surface, which in turn leads to inconsistent crystal growth rates, increased defects, and other problems, seriously affecting crystal quality. This scheme locks in the optimal cone angle range of 60°-80°, ensuring both the effective guiding length of the flow guiding slope and achieving directional and stable airflow transport, forming a synergistic effect with the process parameters, ultimately achieving the efficient preparation of large-size, high-integrity, and high-optical-performance crystals.

[0027] 5. Further, a dense C coating with a thickness of 3-8 μm and a density ≥99.8% is prepared on the inner surface of the graphite crucible, and a porous graphite sheet is added between the raw material area and the seed crystal area. A TaC coating is selectively coated on the surface of the porous graphite sheet. This composite structure design precisely suppresses the generation and diffusion of free carbon particles through a dual protection mechanism: the dense C coating can effectively reduce the carbon volatilization loss of the graphite crucible body and reduce free carbon impurities formed by carbonization and shedding of the crucible wall; while the TaC coating on the surface of the porous graphite sheet, with its high stability and low volatility, can further intercept carbon particles generated during the sublimation of the raw material and prevent the porous graphite sheet itself from carbonizing and generating new impurities, thereby reducing the formation of carbon encapsulation defects from the source and providing a clean environment for crystal growth.

[0028] More importantly, this solution employs a differentiated design of "selective coating": the TaC coating is applied only to the porous graphite sheet, while the remaining inner surface of the crucible is still coated with C. Compared to the solution where the entire inner surface of the crucible is coated with TaC, this approach offers significant technical and economic advantages. On the one hand, TaC material is expensive, and selective coating can significantly reduce the amount of this valuable material used, effectively controlling production costs. On the other hand, if the TaC coating is directly close to the seed crystal area, it may adversely affect the nucleation quality and crystal structure integrity of the seed crystal growth. This solution, through differentiated coating layout, avoids direct contact between the TaC coating and the seed crystal area, thus preserving its core role in impurity interception while ensuring the purity and stability of the seed crystal growth environment. Ultimately, this achieves the synergistic goals of increasing crystal size, improving structural integrity, and optimizing production costs.

[0029] 6. Since the graphite heater配套with the graphite crucible is arranged around the outer wall of the graphite crucible (prior art), the temperature at the side wall of the graphite crucible is relatively high, and the temperature in the middle is relatively low. Therefore, more carbon particles are likely to be generated in the raw materials near the side wall of the graphite crucible. If these carbon particles directly float up, it will cause the premature blockage of the edge of the porous graphite sheet, thereby affecting the filtering effect of the porous graphite sheet. In this solution, an annular baffle is arranged below the porous graphite sheet, and the annular baffle is arranged along the inner wall of the graphite crucible, which can effectively block the carbon particles at the side wall and prevent them from directly rising to the porous graphite sheet to cause premature blockage of the edge of the porous graphite sheet.

[0030] Description of the drawings.

[0031] Figure 1 It is a schematic structural diagram of the graphite crucible in the embodiment of the present invention.

[0032] The reference numerals in the drawings of the specification include: cover plate 1, seed crystal 2, flow guiding ring 3, porous graphite sheet 4, annular baffle 5, SiC ultrafine powder 6, SiC powder 7, pot body 8. Detailed implementation manners

[0033] The following further describes the present invention in detail with reference to embodiments. Overview of the solution: A method for growing high-purity large-size silicon carbide crystals uses Figure 1 the described graphite crucible for growth. The graphite crucible includes a pot body 8, a cover plate 1 is buckled on the top of the pot body 8, the bottom of the cover plate 1 is used to fix the seed crystal 2 to form a seed crystal area, the growth area for the growth of the seed crystal 2 is below the seed crystal area, and the raw material area for placing raw materials is below the growth area. A flow guiding ring 3 is integrally formed on the inner wall of the graphite crucible, the flow guiding ring 3 is located in the growth area, and the cone angle of the flow guiding ring 3 is 60° - 80°. A porous graphite sheet 4 is installed below the flow guiding ring 3 through a mounting frame, a TaC coating is coated on the porous graphite sheet 4, an annular baffle 5 is arranged below the porous graphite sheet 4, and the outer wall of the annular baffle 5 is fixedly fitted with the inner wall of the graphite crucible. The inner surface of the graphite crucible is plated with a C coating, the thickness of the C coating is 3 - 8 μm, and the density is ≥ 99.8%.

[0034] It is controlled that the SiC powder includes acceptor impurities and donor impurities. The acceptor impurities include Al and B, and the donor impurity is N2. The SiC powder is pretreated before being put into the graphite crucible: the ratio of the doping amounts of Al and B in the SiC powder is controlled to be 1:1.2, so that the total concentration N a of the two as acceptor impurities d satisfies: 0.8N d ≤ N a ≤ 1.2N d .

[0035] During growth, one of the following gases—argon, hydrogen, or a mixture of argon and hydrogen—is introduced as the growth atmosphere. The atmosphere is filtered to remove impurities before being introduced into the graphite crucible, and the N2 content in the atmosphere is controlled to be ≤0.5ppm. If a mixed gas is chosen as the growth atmosphere, the proportion of hydrogen in the mixed gas must be controlled to be 5%-10%.

[0036] During growth, the temperature of the raw material zone is 70℃-150℃ higher than that of the seed crystal zone, the growth rate is controlled at 0.6-1.0mm / h, the temperature difference of the radial temperature field of the graphite crucible is ≤3℃, and the temperature is gradually reduced to room temperature at a rate of 5℃ / h in the later stage of growth.

[0037] Example 1: A method for growing high-purity, large-size silicon carbide crystals includes the following steps: 1. Equipment and raw material preparation Equipment: PVT growth furnace (equipped with a multi-zone temperature control system, accuracy ±1℃), three-stage vacuum system (ultimate vacuum 10). - 5 Pa), gas purification device, in this embodiment the gas purification device adopts palladium membrane filter.

[0038] Raw material: 6N grade SiC powder (initial Al content 5×10¹) 5 atoms / cm³, B content 4×10¹ 5 (atoms / cm³), 4H-SiC seed crystal (φ200mm, 4° deflection).

[0039] Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). - The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N d =N a The content of Al, B and N2 elements in SiC powder is detected by existing technology. If the concentration does not meet the requirements, the corresponding elements are added. Al and B are solid substances and can be added directly to the raw materials in the early stage. N2 is a gas and needs to be introduced in a corresponding amount during the subsequent growth process.

[0040] 2. Crucible processing After ultrasonic cleaning, the graphite crucible (inner diameter φ230mm, height 270mm) is placed in a deposition furnace, and C2H4, C2H2, or C3H8 gas is introduced. Deposition is carried out at 1600℃ and 5kPa for 4-10 hours to form a 5μm thick C coating. The cone angle of the flow guide ring 3 in the graphite crucible is 70°.

[0041] 3. Crystal growth SiC powder 7 is loaded into the raw material area, and a layer of SiC ultrafine powder 6 is spread on top of SiC powder 7. After the raw materials are loaded into the crucible, a vacuum of 5×10 is drawn. -5 Pa, ultra-high purity argon gas (purity 99.9999%, N2 content ≤0.5ppm) purified by a filter is introduced, argon gas flow rate is 100sccm, and the pressure inside the furnace is maintained at 80mbar; The temperature of the raw material zone is raised to 120°C higher than that of the seed crystal zone, i.e., 2350°C for the raw material zone and 2250°C for the seed crystal zone. The temperature is held for 1 hour to allow Al and B to fully volatilize and mix. The growth rate is controlled at 0.8-1.2 mm / h and continued for 150 hours (growing an 8-inch single crystal). Then, the temperature is lowered to room temperature at a rate of 5°C / h to obtain a φ200mm silicon carbide single crystal.

[0042] 4. Perform performance testing on the obtained silicon carbide single crystals. Laser scattering tomography (LST) analysis: Carbon particle encapsulation defect density 38 cm⁻¹ - ²; Optical performance: 82.1% transmittance before coating in the 400-700nm wavelength range, 96.5% transmittance after coating, and the moissanite color reaches D grade (colorless). Example 2: The difference between this embodiment and Embodiment 1 is that: Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). - The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N a =0.8N d .

[0043] Example 3: The difference between this embodiment and Embodiment 1 is that: Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). - The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N a =1.2N d .

[0044] Example 4: The difference between this embodiment and Embodiment 1 is that: The temperature of the raw material zone is raised to 70°C higher than that of the seed crystal zone, i.e., 2400°C for the raw material zone and 2250°C for the seed crystal zone. The temperature is held for 1 hour to allow Al and B to fully volatilize and mix. The growth rate is controlled at 0.8-1.2 mm / h and continued for 150 hours (growing an 8-inch single crystal). Then the temperature is lowered to room temperature at a rate of 5°C / h to obtain a φ200mm silicon carbide single crystal.

[0045] Example 5: The difference between this embodiment and Embodiment 1 is that: The temperature of the raw material zone is raised to 150°C higher than that of the seed crystal zone, i.e., 2370°C for the raw material zone and 2250°C for the seed crystal zone. The temperature is held for 1 hour to allow Al and B to fully volatilize and mix. The growth rate is controlled at 0.8-1.2 mm / h and continued for 150 hours (growing an 8-inch single crystal). Then, the temperature is lowered to room temperature at a rate of 5°C / h to obtain a φ200mm silicon carbide single crystal.

[0046] Example 6: The difference between this embodiment and Embodiment 1 is that the cone angle of the flow guide ring 3 in the graphite crucible is 60°.

[0047] Example 7: The difference between this embodiment and Embodiment 1 is that the cone angle of the flow guide ring 3 in the graphite crucible is 80°.

[0048] Comparative Example 1: The difference between this comparative example and Example 1 is that: Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). -The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N a =0.7N d .

[0049] Comparative Example 2: The difference between this comparative example and Example 1 is that: Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). - The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N a =0.4N d .

[0050] Comparative Example 3: The difference between this comparative example and Example 1 is that: Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). - The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N a =1.3N d .

[0051] Comparative Example 4: The difference between this comparative example and Example 1 is that: Raw material pretreatment: 6N grade SiC powder is calcined under vacuum at 1700-1900℃ (vacuum degree ≤10). -The powder was subjected to a concentration of 3 Pa for 2 hours to remove adsorbed N2 and volatile Al and B compounds (such as AlCl3 and B2O3) from the powder surface. Subsequently, the content of Al, B, and N2 elements in the 6N-grade SiC powder was measured, and the ratio of Al to B doping in the SiC powder was controlled at 1:1.2 to ensure that both form the total concentration of N2 as acceptor impurities. a With donor impurity concentration N d Satisfy: N a =1.8N d .

[0052] Comparative Example 5: The difference between this comparative example and Example 1 is that: The temperature of the raw material zone is raised to 50°C higher than that of the seed crystal zone, i.e., 2330°C for the raw material zone and 2250°C for the seed crystal zone. The temperature is held for 1 hour to allow Al and B to fully volatilize and mix. The growth rate is controlled at 0.8-1.2 mm / h and continued for 150 hours (growing an 8-inch single crystal). Then, the temperature is lowered to room temperature at a rate of 5°C / h to obtain a φ200mm silicon carbide single crystal.

[0053] Comparative Example 6: The difference between this comparative example and Example 1 is that: The temperature of the raw material zone is raised to 170°C higher than that of the seed crystal zone, i.e., 2420°C for the raw material zone and 2250°C for the seed crystal zone. The temperature is held for 1 hour to allow Al and B to fully volatilize and mix. The growth rate is controlled at 0.8-1.2 mm / h and continued for 150 hours (growing an 8-inch single crystal). Then, the temperature is lowered to room temperature at a rate of 5°C / h to obtain a φ200mm silicon carbide single crystal.

[0054] Comparative Example 7: The difference between this comparative example and Example 1 is that the cone angle of the guide ring 3 in the graphite crucible is 50°.

[0055] Comparative Example 8: The difference between this comparative example and Example 1 is that the cone angle of the flow guide ring 3 in the graphite crucible is 90°.

[0056] Crystals were grown using Examples 1-7 and Comparative Examples 1-8, and their performance was tested. The results are shown in the table below (only the varied parameters are shown in the table; the other parameters are the same as in Example 1 and are not shown in the table):

[0057] As shown in Table 1: 1. It can be seen that N a With N d The relationship between the crystal, the temperature difference between the raw material zone and the seed crystal zone, and the cone angle of the current-guiding ring 3 all have a significant impact on the crystal's transmittance, color, and electrical insulation. Especially N...a With N d The relationship is such that once the deviation between the two exceeds ±20% (N) a <0.8N d or N a >1.2N d The light transmittance will drop below 75%, the color will deviate from Grade D, and the electrical insulation performance will also deteriorate accordingly.

[0058] 2. The cone angle of the guide ring 3 is stable and excellent when it is between 60-80°, with a light transmittance of 78.2%-81.2%, color grade D, and insulation "excellent". When it deviates from the range of 60-80°, such as to 50° or 90°, the light transmittance drops to below 65.4%, the color deteriorates to grade H, and the insulation becomes "medium".

[0059] 3. The optimal process parameters for preparing wafers with high light transmittance (≥80%), excellent color (Grade D), and high electrical insulation (excellent) are N. d =N a Temperature difference 120℃, cone angle of guide ring 3 70°.

[0060] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for growing high-purity, large-size silicon carbide crystals, characterized in that: SiC powder was placed in a graphite crucible for growth. The SiC powder underwent pretreatment before being placed in the crucible: the total concentration of acceptor impurities N in the SiC powder was controlled. a With donor impurity concentration N d Satisfy: 0.8N d ≤ N a ≤ 1.2N d .

2. The method for growing high-purity, large-size silicon carbide crystals according to claim 1, characterized in that: The acceptor impurities include Al and B, and the doping ratio of Al to B in SiC powder is 1:1.

2.

3. The method for growing high-purity, large-size silicon carbide crystals according to claim 2, characterized in that: During growth, one of the following gases is introduced: argon, hydrogen, or a mixture of argon and hydrogen.

4. The method for growing high-purity, large-size silicon carbide crystals according to claim 3, characterized in that: The gas is filtered to remove impurities before being introduced into the graphite crucible, with the N2 content in the atmosphere controlled to be ≤0.5ppm.

5. The method for growing high-purity, large-size silicon carbide crystals according to claim 4, characterized in that: When introducing the mixed gas, control the proportion of hydrogen in the mixed gas to be 5%-10%.

6. The method for growing high-purity, large-size silicon carbide crystals according to claim 5, characterized in that: During growth, the temperature in the raw material zone is 70℃-150℃ higher than that in the seed crystal zone.

7. A method for growing high-purity, large-size silicon carbide crystals according to claim 6, characterized in that: During growth, the growth rate is controlled at 0.6-1.0 mm / h, the temperature difference of the radial temperature field of the graphite crucible is ≤3℃, and the temperature is gradually reduced to room temperature at a rate of 5℃ / h in the later stage of growth.

8. A method for growing high-purity, large-size silicon carbide crystals according to any one of claims 1-7, characterized in that: A conical flow guide ring is set inside the graphite crucible, located between the raw material zone and the seed crystal zone, with a cone angle of 60°-80°.

9. A method for growing high-purity, large-size silicon carbide crystals according to claim 8, characterized in that: A porous graphite sheet is placed below the flow guide ring, and a TaC coating is placed on the porous graphite sheet. A C coating is placed on the inner surface of the graphite crucible, with a thickness of 3-8 μm and a density of ≥99.8%.

10. The method for growing high-purity, large-size silicon carbide crystals according to claim 9, characterized in that: An annular baffle is placed below the porous graphite sheet and is fixed to the inner wall of the graphite crucible.

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