Double-layer porous carbon coated silicon carbide, preparation method thereof and growth method of silicon carbide crystal

Abstract

The invention belongs to the technical field of silicon carbide crystal growth, and particularly relates to double-layer porous carbon coated silicon carbide, a preparation method thereof and a growth method of a silicon carbide crystal. Compared with the prior art, the sublimation process of a silicon carbide raw material is accurately regulated and controlled through a double-layer porous carbon coating structure, the gradual decomposition of a carbon layer at a high temperature and the sublimation rate of a silicon carbide core are dynamically matched, and in the growth process, a carbon source is continuously supplemented through the slow release effect of the carbon layer, so that a gas phase component is always maintained in an ideal range; by means of the dynamic balance mechanism, a crystal growth interface is kept in a stable step flow mode, spiral dislocation and stacking fault caused by component imbalance are reduced, silicon steam erosion of the inner wall of the crucible at the high temperature can be reduced, and the wrapping defect formed by secondary deposition of carbon particles is avoided; in addition, even if local density difference exists in raw material filling, abnormal sublimation caused by local overheating can be weakened through uniform coating of the carbon layer, and polycrystalline nuclei occurring in the initial growth stage are prevented from being diffused to a single crystal area.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of silicon carbide crystal growth, and particularly relates to a double-layer porous carbon-coated silicon carbide and a preparation method and a growth method of the silicon carbide crystal. BACKGROUND

[0002] There are various typical defects in the growth of silicon carbide crystals, mainly including dislocations, microtubules and carbon inclusions. The dislocations can be divided into screw dislocations, edge dislocations and mixed dislocations, wherein the screw dislocations can reduce the reverse voltage performance of the device, the edge dislocations are easy to cause the leakage current of the Schottky diode to increase, and the mixed dislocations can cause the high-frequency characteristics of the device to attenuate; the microtubule is a hollow tubular defect penetrating through the crystal, which can greatly reduce the breakdown voltage of the device; and the porous carbon inclusion as a local carbon enrichment area can destroy the lattice continuity, resulting in device leakage or failure.

[0003] In the field of physical vapor transport (PVT) method for growing silicon carbide crystals, the prior art has carried out a number of studies around reducing defects and stabilizing growth: by introducing a mixed gas of carbon source gas such as methane and argon to adjust the silicon-carbon ratio of the system to maintain the stoichiometric balance of the growth interface; using a porous filter layer or a silicon carbide fiber layer to intercept carbon particles in the gas phase to reduce surface contamination; optimizing the temperature field design to stabilize the growth environment; improving the purity and stability of the raw material; at the same time, using isomorphous seed crystal and controlling the growth rate to reduce the formation of polymorphism and polycrystals. These technologies provide multiple paths for improving the quality of silicon carbide crystal growth.

[0004] When growing silicon carbide crystals by the traditional PVT method, in the high-temperature environment of the PVT method (usually > 2000℃), the silicon carbide raw material undergoes a decomposition reaction. This reaction is an endothermic reaction. If the temperature distribution is uneven or the gas flow is disturbed, the silicon sublimation rate will instantaneously exceed the carbon sublimation rate, forming a silicon-rich gas phase (Si / C > 1), which will cause carbon particle contamination and graphite crucible erosion, resulting in a disordered temperature field distribution; at the same time, the edge of the seed crystal is a sensitive area for polymorphism and polycrystal nucleation due to the large radial temperature gradient and the concentration of residual mechanical stress, which is easy to form penetrating defects, significantly reducing the proportion of single crystal area. SUMMARY

[0005] Therefore, the technical problem to be solved by the present application is to provide a double-layer porous carbon-coated silicon carbide and a preparation method and a growth method of the silicon carbide crystal. Through the core-shell structure design of the carbon-coated silicon carbide raw material, the carbon layer is used to control the silicon-carbon ratio balance and the physical barrier effect, to inhibit the formation of silicon-rich gas phase and the transmission of carbon particles, to optimize the temperature field gradient and gas flow distribution in the PVT process, to reduce the thermal stress and mechanical stress of the edge of the seed crystal, and to reduce the polymorphism and polycrystal nucleation points from the source, to solve the problems of polymorphism and polycrystal at the edge of the seed crystal in the initial stage of crystal growth, to improve the proportion of single crystal area and the overall quality of the crystal, and to meet the demand for high-integrity single crystal for large-scale production of silicon carbide substrates.

[0006] The present application provides a double-layer porous carbon-coated silicon carbide, comprising: a silicon carbide core; a first carbon layer wrapped outside the silicon carbide core; a second carbon layer wrapped outside the first carbon layer;

[0007] The average pore size of the first carbon layer is smaller than the average pore size of the second carbon layer;

[0008] The porosity of the first carbon layer is higher than the porosity of the second carbon layer.

[0009] Preferably, the average pore size of the first carbon layer is 1-10 nm;

[0010] And / or, the average pore size of the second carbon layer is 50-200 nm.

[0011] Preferably, the particle size of the silicon carbide core is 100-300 μm;

[0012] And / or, the total thickness of the first carbon layer and the second carbon layer is 360-540 nm;

[0013] And / or, the thickness ratio of the first carbon layer to the second carbon layer is 1:(0.5-2).

[0014] Preferably, the first carbon layer is formed from a first carbon source; the first carbon source is a small molecule saturated hydrocarbon; the number of carbon atoms of the small molecule saturated hydrocarbon is less than or equal to 2;

[0015] And / or, the second carbon layer is formed from a second carbon source; the second carbon source includes an unsaturated hydrocarbon.

[0016] Preferably, the second carbon source further includes a macromolecular saturated hydrocarbon; the number of carbon atoms of the macromolecular saturated hydrocarbon is greater than or equal to 3;

[0017] The volume ratio of the macromolecular saturated hydrocarbon to the unsaturated hydrocarbon is (2-4):1.

[0018] The present application also provides a preparation method of the above-mentioned double-layer porous carbon-coated silicon carbide, comprising the following steps:

[0019] S1) loading silicon carbide into a fluidized bed reactor, passing carrier gas to make the silicon carbide in a suspended state, then heating to a first target temperature, and then passing carrier gas and a first carbon source to deposit a first carbon layer on the surface of the silicon carbide;

[0020] S2) stopping the passage of the first carbon source, heating to a second target temperature under the condition of only passing carrier gas, and then passing carrier gas and a second carbon source to deposit a second carbon layer on the surface of the first carbon layer to obtain a double-layer carbon-coated precursor;

[0021] S3) annealing the double-layer carbon-coated precursor to obtain a double-layer porous carbon-coated silicon carbide.

[0022] Preferably, in step S1), the amount of silicon carbide loaded is 1 / 4 to 1 / 3 of the effective volume of the fluidized bed reactor;

[0023] In step S1), when the carrier gas is introduced to keep the silicon carbide in a suspended state, the flow rate of the carrier gas is 1.2 to 1.5 times the minimum fluidization rate.

[0024] The first target temperature is 900℃~1000℃;

[0025] In step S1), the volume ratio of carrier gas to the first carbon source is (10~15):1;

[0026] In step S1), the deposition of the first carbon layer is carried out under conditions of 500~1000 Pa; the deposition time of the first carbon layer is 2~4 h.

[0027] In step S1), the gas porosity of the fluidized bed reactor during the deposition of the first carbon layer is 25% to 35%.

[0028] Preferably, the second target temperature in step S2) is 1100℃~1200℃;

[0029] In step S2), the volume ratio of carrier gas to the second carbon source is (4~6):1;

[0030] In step S2), the deposition of the second carbon layer is carried out under conditions of 500-700 Pa; the deposition time of the second carbon layer is 2-3 h.

[0031] In step S2), the gas porosity of the fluidized bed reactor during the deposition of the second carbon layer is 45%~60%.

[0032] The annealing temperature is 900℃~1200℃; the annealing time is 2~4 h.

[0033] The present invention also provides a method for growing silicon carbide crystals, comprising the following steps:

[0034] The above-mentioned double-layer porous carbon-coated silicon carbide is mixed with silicon carbide particles to obtain a mixed raw material;

[0035] Silicon carbide single crystals were prepared using the aforementioned mixed raw materials via physical vapor transport.

[0036] Preferably, the mass ratio of the double-layer porous carbon-coated silicon carbide to silicon carbide particles is (10~30):(70~90).

[0037] The present invention provides a double-layer porous carbon-coated silicon carbide, comprising: a silicon carbide core; a first carbon layer wrapped around the silicon carbide core; a second carbon layer wrapped around the first carbon layer; wherein the average pore size of the first carbon layer is smaller than the average pore size of the second carbon layer; and the porosity of the first carbon layer is higher than the porosity of the second carbon layer. Compared with existing technologies, this invention achieves precise control over the sublimation process of silicon carbide raw materials through a double-layer porous carbon coating structure. The gradual decomposition of the carbon layer at high temperature dynamically matches the sublimation rate of the silicon carbide nuclei, constructing a stable silicon-carbon gas phase environment. During growth, the slow-release effect of the carbon layer continuously replenishes the carbon source, avoiding drastic fluctuations in the silicon-carbon ratio during the sublimation of traditional raw materials, and keeping the gas phase composition within an ideal range. Furthermore, this dynamic balance mechanism not only maintains a stable step flow pattern at the crystal growth interface, reducing spiral dislocations and stacking faults caused by compositional imbalance, but also reduces silicon vapor erosion of the crucible inner wall at high temperatures, avoiding encapsulation defects caused by secondary deposition of carbon particles. In addition, even if there are local density differences in the raw material packing, the uniform coating of the carbon layer can weaken abnormal sublimation caused by local overheating, preventing polycrystalline nuclei from diffusing into the single crystal region in the early stage of growth. Using this double-layer porous carbon-coated silicon carbide to grow silicon carbide single crystals can significantly improve the single crystal yield of 4H-SiC crystals and greatly reduce the production cost per unit crystal. Attached Figure Description

[0038] Figure 1 A schematic diagram of the structure of double-layer porous carbon-coated silicon carbide provided by the present invention;

[0039] Figure 2 A schematic diagram illustrating a specific preparation process for double-layer porous carbon-coated silicon carbide provided by the present invention;

[0040] Figure 3 This is a schematic diagram of the preparation of bilayer porous carbon-coated silicon carbide using fluidized bed vapor deposition, as provided by the present invention. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0042] The present invention provides a double-layer porous carbon-coated silicon carbide, comprising: a silicon carbide core; a first carbon layer wrapped around the silicon carbide core; a second carbon layer wrapped around the first carbon layer; wherein the average pore size of the first carbon layer is smaller than the average pore size of the second carbon layer; and the porosity of the first carbon layer is higher than the porosity of the second carbon layer.

[0043] See Figure 1 ,Figure 1 This is a schematic diagram of the structure of double-layer porous carbon-coated silicon carbide provided by the present invention.

[0044] In one specific embodiment of the present invention, the particle size of the silicon carbide core is preferably 100~300μm; optionally, the particle size of the silicon carbide core is 100 μm, 150 μm, 200 μm, 250 μm, 300 μm or any two of the above values.

[0045] The surface of the silicon carbide core is sequentially coated with a first carbon layer and a second carbon layer; the total thickness of the first carbon layer and the second carbon layer is preferably 360~540 nm; optionally, the total thickness of the first carbon layer and the second carbon layer is 360 nm, 380 nm, 400 nm, 420 nm, 440 nm, 460 nm, 480 nm, 500 nm, 520 nm, 540 nm or any two of the above values; the thickness ratio of the first carbon layer to the second carbon layer is preferably 1:(0.5~2); optionally, the thickness ratio of the first carbon layer to the second carbon layer is 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2 or any two of the above values.

[0046] The first carbon layer has small and numerous pores, making it a dense carbon layer dominated by micropores and / or small mesopores, resulting in high porosity. In a specific embodiment of the present invention, the average pore size of the first carbon layer is preferably 1 to 10 nm. Optionally, the average pore size of the first carbon layer is 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 8 nm, 9 nm, 10 nm, or any two of the above values.

[0047] In a specific embodiment of the present invention, the first carbon layer is formed by a first carbon source; the first carbon source is preferably a small-molecule saturated hydrocarbon; the number of carbon atoms in the small-molecule saturated hydrocarbon is preferably less than or equal to 2; specifically, the first carbon source is methane. Using a small-molecule saturated hydrocarbon as the first carbon source has the following advantages: simple structure: short carbon chain and single functional group (mainly CH), which can avoid excessive entanglement of large molecular chains leading to enlarged pore size; suitable decomposition temperature (medium and low temperature): can be stably decomposed in the range of 900℃~1000℃, avoiding violent reorganization of carbon chains at high temperatures to form large pores; slow and controllable deposition rate: the carbon source decomposition activity is moderate, ensuring that carbon atoms are deposited gradually to form a uniform and dense carbon skeleton, rather than rapid accumulation leading to pore blockage; good compatibility with carrier gas: can be uniformly mixed with inert gases such as argon, achieving no dead angle coating of SiC particles in a fluidized bed.

[0048] The second carbon layer has large and sparse pores, making it a loose carbon layer dominated by large mesopores and / or macropores, with low porosity. In a specific embodiment of the present invention, the average pore size of the second carbon layer is preferably 50~200 nm. Optionally, the average pore size of the second carbon layer is 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, or any two of the above values.

[0049] Since the second carbon layer is a loose carbon layer, the requirements for the carbon source are as follows: complex molecular structure or high activity: macromolecular saturated hydrocarbons or highly active carbon sources (such as unsaturated hydrocarbons) are preferred, with long carbon chains or containing unsaturated bonds (double / triple bonds), which are easy to decompose violently at high temperatures to form large interconnected pores; high decomposition temperature (high temperature): rapid decomposition is required in the range of 1100℃~1200℃, utilizing the violent recombination of carbon chains and the concentrated release of volatiles at high temperatures to promote the formation of macropores; fast deposition rate: the carbon source has high decomposition activity and can quickly accumulate to form a loose carbon skeleton, avoiding excessive densification; good compatibility with the first carbon layer: the carbon free radicals generated by decomposition at high temperatures can bond with the surface of the first carbon layer to ensure that the two carbon layers are tightly bonded.

[0050] In one specific embodiment of the present invention, the second carbon layer is formed by a second carbon source; the second carbon source includes unsaturated hydrocarbons; the unsaturated hydrocarbons may be alkenes or alkynes, without any particular limitation, and in the present invention, the unsaturated hydrocarbons are preferably alkynes; the number of carbon atoms in the unsaturated hydrocarbons is preferably 2 to 4, more preferably 2 to 3, and even more preferably 2.

[0051] In another specific embodiment of the present invention, the second carbon source preferably further includes a macromolecular saturated hydrocarbon; the number of carbon atoms in the macromolecular saturated hydrocarbon is preferably greater than or equal to 3, more preferably 3 to 6, even more preferably 3 to 4, and most preferably 3; the volume ratio of the macromolecular saturated hydrocarbon to the unsaturated hydrocarbon is preferably (2 to 4): 1; optionally, the volume ratio of the macromolecular saturated hydrocarbon to the unsaturated hydrocarbon is 2:1, 2.5:1, 3:1, 3.5:1, 4:1 or any two of the above ratios.

[0052] This invention achieves precise control over the sublimation process of silicon carbide raw materials through a double-layer porous carbon coating structure. The gradual decomposition of the carbon layer at high temperature dynamically matches the sublimation rate of the silicon carbide nuclei, constructing a stable silicon-carbon gas phase environment. During growth, the slow-release effect of the carbon layer continuously replenishes the carbon source, avoiding drastic fluctuations in the silicon-carbon ratio during the sublimation of traditional raw materials, and keeping the gas phase composition within an ideal range. Furthermore, this dynamic balance mechanism not only maintains a stable step flow pattern at the crystal growth interface, reducing spiral dislocations and stacking faults caused by compositional imbalance, but also reduces silicon vapor erosion of the crucible inner wall at high temperatures, avoiding encapsulation defects caused by secondary deposition of carbon particles. In addition, even if there are local density differences in the raw material packing, the uniform coating of the carbon layer can weaken abnormal sublimation caused by local overheating, preventing polycrystalline nuclei from diffusing into the single crystal region in the early stage of growth. Using this double-layer porous carbon-coated silicon carbide to grow silicon carbide single crystals can significantly improve the single crystal yield of 4H-SiC crystals and greatly reduce the production cost per unit crystal.

[0053] The present invention also provides a method for preparing the above-mentioned double-layer porous carbon-coated silicon carbide, comprising the following steps: S1) filling silicon carbide into a fluidized bed reactor, introducing a carrier gas to suspend the silicon carbide, then heating to a first target temperature, and then introducing a carrier gas and a first carbon source to deposit a first carbon layer on the surface of the silicon carbide; S2) stopping the introduction of the first carbon source, heating to a second target temperature under the condition of only introducing a carrier gas, and then introducing a carrier gas and a second carbon source to deposit a second carbon layer on the surface of the first carbon layer to obtain a double-layer carbon-coated precursor; S3) annealing the double-layer carbon-coated precursor to obtain double-layer porous carbon-coated silicon carbide.

[0054] See Figure 2 , Figure 2 This is a schematic diagram of a specific preparation process for double-layer porous carbon-coated silicon carbide provided by the present invention.

[0055] See Figure 3 , Figure 3 This is a schematic diagram of the preparation of bilayer porous carbon-coated silicon carbide using fluidized bed vapor deposition, as provided by the present invention.

[0056] In this invention, there are no special restrictions on the source of any raw materials; they can be commercially available.

[0057] In one specific embodiment of the present invention, the particle size of the silicon carbide is preferably 100~300 μm; optionally, the particle size of the silicon carbide core is 100 μm, 150 μm, 200 μm, 250 μm, 300 μm or any two of the above values.

[0058] In a specific embodiment of the present invention, the silicon carbide is preferably pretreated. The pretreatment method can be any method well-known to those skilled in the art and is not particularly limited. Specifically, the silicon carbide is immersed in a hydrofluoric acid solution to remove the surface oxide layer, then washed and dried. The volume concentration of hydrofluoric acid in the hydrofluoric acid solution is preferably 2%~8%, more preferably 4%~6%, and even more preferably 5%. The immersion time is preferably 10~30 min, more preferably 15~25 min, even more preferably 18~22 min, and most preferably 20 min. The washing is preferably ultrasonic washing. The ultrasonic power is preferably 100~500 W, more preferably 200~400 W, even more preferably 250~350 W, and most preferably 300 W. The washing is preferably performed sequentially using acetone, isopropanol, and water. The number of acetone washes is preferably 1~3 times, more preferably 2 times. The time for each acetone wash is preferably 5~20 min, more preferably 5~15 min, and even more preferably 10 minutes. The isopropanol washing is preferably performed 1 to 3 times, more preferably 2 times; the washing time for each isopropanol washing is preferably 5 to 20 minutes, more preferably 5 to 15 minutes, and even more preferably 10 minutes; the water washing is preferably performed multiple times, and the washing time for each water washing is preferably 5 to 30 minutes, more preferably 10 to 20 minutes, and even more preferably 15 minutes; in this invention, the water washing is preferably performed until the pH value of the washing solution is 6 to 7; the drying is preferably vacuum drying; the drying temperature is preferably 80℃ to 120℃, more preferably 90℃ to 110℃, even more preferably 95℃ to 105℃, and most preferably 100℃; the vacuum degree of the drying is preferably -0.05 to -0.1 MPa, more preferably -0.08 to -0.1 MPa, and even more preferably -0.09 MPa; the drying time is preferably 1 to 5 hours, more preferably 2 to 4 hours, and even more preferably 3 hours.

[0059] The pretreated silicon carbide is loaded into the fluidized bed reactor; the amount of silicon carbide loaded is 1 / 4 to 1 / 3 of the effective volume of the fluidized bed reactor.

[0060] After loading silicon carbide, it is preferable to first perform gas replacement using a carrier gas; the carrier gas can be any carrier gas known to those skilled in the art, and there are no special restrictions. In this invention, argon is preferred, and high-purity argon is more preferred; the flow rate of the carrier gas during gas replacement is preferably 100~500 sccm, more preferably 200~400 sccm, even more preferably 250~350 sccm, and most preferably 300 sccm; the gas replacement time is preferably 10~60 min, more preferably 20~50 min, and even more preferably 30~40 min; in this invention, it is preferable to perform gas replacement until the oxygen content of the tail gas is less than or equal to 5 ppm.

[0061] After gas replacement, a carrier gas is introduced to suspend the silicon carbide. The flow rate of the carrier gas at this time is greater than the flow rate during gas replacement. In a specific embodiment of the present invention, the flow rate of the carrier gas when suspending the silicon carbide is preferably 1.2 to 1.5 times the minimum fluidization velocity. Optionally, the flow rate of the carrier gas when suspending the silicon carbide is 1.2, 1.3, 1.4, or 1.5 times the minimum fluidization velocity, or any two of the above values. The expansion rate of the bed when the silicon carbide is in suspension is preferably 1.5 to 2 times. Optionally, the expansion rate of the bed when the silicon carbide is in suspension is 1.5, 1.6, 1.7, 1.8, 1.9, or 2 times, or any two of the above values.

[0062] After the silicon carbide is in a suspended state, it is heated to a first target temperature; the heating rate is preferably 5~10℃ / min; optionally, the heating rate is 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min or any two of the above values; the first target temperature is preferably 900℃~1000℃; optionally, the first target temperature is 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃ or any two of the above values. Deposition within this temperature range allows for a balance between carbon source decomposition efficiency and carbon layer density.

[0063] After heating to the first target temperature, a carrier gas and a first carbon source are introduced to deposit a first carbon layer on the silicon carbide surface. The types of the carrier gas and the first carbon source are the same as described above and will not be repeated here. The total flow rate of the carrier gas and the first carbon source is preferably 500~800 sccm; optionally, the total flow rate is 500 sccm, 600 sccm, 700 sccm, 800 sccm, or any two of the above values. The volume ratio of the carrier gas to the first carbon source is preferably (10~15):1; optionally, the volume ratio is 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, or any two of the above ratios. The deposition of the first carbon layer is preferably performed under conditions of 500~1000 Pa; optionally, the deposition of the first carbon layer is performed at 500 Pa, 600 Pa, 700 Pa, 800 Pa, 900 Pa, or 1000 Pa. The deposition is carried out under conditions of Pa or any two of the above values; the gas porosity of the fluidized bed reactor during the deposition of the first carbon layer is preferably 25% to 35%, more preferably 28% to 32%, and even more preferably 30%; the deposition time of the first carbon layer is preferably 2 to 4 h; optionally, the deposition time of the first carbon layer is 2 h, 2.5 h, 3 h, 3.5 h, 4 h or any two of the above values.

[0064] After the first carbon layer is deposited, the first carbon source is stopped, and the temperature is raised to the second target temperature while only the carrier gas is supplied. During this process, the carrier gas flow rate is preferably kept constant. The heating rate is preferably 5~10℃ / min; optionally, the heating rate is 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any two of the above values. The second target temperature is preferably 1100℃~1200℃; optionally, the first target temperature is 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 2000℃, or any two of the above values.

[0065] In one specific embodiment of the present invention, after heating to the second target temperature, it is preferable to hold the temperature for 5 to 20 minutes to stabilize the temperature, and then introduce the carrier gas and the second carbon source; optionally, the holding time is 5 minutes, 10 minutes, 15 minutes, 20 minutes or any two of the above values.

[0066] In a specific embodiment of the present invention, the types of the carrier gas and the second carbon source are the same as described above, and will not be repeated here; the total flow rate of the carrier gas and the second carbon source is preferably 600~1000 sccm; optionally, the total flow rate of the carrier gas and the second carbon source is 600 sccm, 700 sccm, 800 sccm, 900 sccm, 1000 sccm or any two of the above values; the volume ratio of the carrier gas to the second carbon source is preferably (4~6):1; optionally, the volume ratio of the carrier gas to the second carbon source is 4:1, 5:1, 6:1 or any two of the above ratios; the deposition of the second carbon layer is preferably carried out under conditions of 500~700 Pa; optionally, the deposition of the second carbon layer is carried out under conditions of 500 Pa, 550 Pa, 600 Pa, 650 Pa, 700 Pa. The deposition is carried out under conditions of Pa or any two of the above values; the gas porosity of the fluidized bed reactor during the deposition of the second carbon layer is preferably 45% to 60%, more preferably 45% to 55%, and even more preferably 50%; the deposition time of the second carbon layer is preferably 2 to 3 h; optionally, the deposition time of the second carbon layer is 2 h, 2.5 h, 3 h or any two of the above values.

[0067] In one specific embodiment of the present invention, the total flow rate of the carrier gas and the second carbon source is greater than the total flow rate of the carrier gas and the first carbon source during the deposition of the first carbon layer, thereby increasing the expansion rate of the bed. More specifically, the expansion rate of the bed during the deposition of the second carbon layer is preferably 1.8 to 2.2 times, more preferably 1.9 to 2.2 times. By adjusting the total flow rate of the carrier gas and the second carbon source and the gas porosity, the formation of the perforated structure can be promoted.

[0068] After the second carbon layer is deposited, it is preferable to stop the supply of the second carbon source and cool down to the third target temperature while the carrier gas is supplied, to obtain a double-layer carbon-encapsulated precursor. The flow rate of the carrier gas during cooling is preferably 500~800 sccm; optionally, the flow rate of the carrier gas during cooling is 500 sccm, 600 sccm, 700 sccm, 800 sccm or any two of the above values; the cooling rate is preferably 1~10℃ / min; optionally, the cooling rate is 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min or any two of the above values; the third target temperature is preferably room temperature to 100℃, more preferably room temperature to 80℃.

[0069] The double-layer carbon-coated precursor is annealed; the annealing temperature is preferably 900℃~1200℃; optionally, the annealing temperature is 900℃, 1000℃, 1100℃, 1200℃ or any two of the above values; the annealing time, i.e., the holding time, is preferably 2~4 h; optionally, the annealing time is 2 h, 2.5 h, 3 h, 3.5 h, 4 h or any two of the above values; the heating rate of the annealing is preferably 1~10℃ / min; optionally, the heating rate of the annealing is 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min or any two of the above values. Annealing can enhance the interfacial bonding force between the carbon layer and silicon carbide particles; specifically, the bonding strength between the carbon layer and silicon carbide is greater than or equal to 6 MPa.

[0070] In one specific embodiment of the present invention, the annealing temperature is 1000℃; the annealing time is 3 h; at this time, the bonding strength between the carbon layer and silicon carbide is greater than or equal to 8 MPa.

[0071] In a specific embodiment of the present invention, after annealing, the silicon carbide is preferably sieved to obtain a double-layer porous carbon-coated silicon carbide; the sieving is preferably carried out using a vibrating sieve; after sieving, the particle size deviation of the double-layer porous carbon-coated silicon carbide is preferably less than or equal to 8%.

[0072] The present invention also provides a method for growing silicon carbide crystals, comprising the following steps: mixing the above-mentioned double-layer porous carbon-coated silicon carbide with silicon carbide particles to obtain a mixed raw material; and using the mixed raw material to prepare silicon carbide single crystals by physical vapor transport method.

[0073] In a specific embodiment of the present invention, the preferred mass ratio of the double-layer porous carbon-coated silicon carbide to silicon carbide particles is (10~30):(70~90); optionally, the mass ratio of the double-layer porous carbon-coated silicon carbide to silicon carbide particles is 10:90, 15:85, 20:80, 25:75, 30:70 or any two of the above ratios; the silicon carbide particles are preferably high-purity silicon carbide particles; the purity of the silicon carbide particles is 99.99%; the particle size of the silicon carbide particles is preferably 100~300 μm; optionally, the particle size of the silicon carbide particles is 100 μm, 150 μm, 200 μm, 250 μm, 300 μm or any two of the above values.

[0074] In a specific embodiment of the present invention, the mixing of double-layer porous carbon-coated silicon carbide and silicon carbide particles is preferably carried out using a three-dimensional mixer; the mixing speed is preferably 10-50 rpm; optionally, the mixing speed is 10 rpm, 20 rpm, 25 rpm, 30 rpm, 35 rpm, 40 rpm, 45 rpm, 50 rpm or any two of the above values; the mixing time is preferably 1-3 hours; optionally, the mixing time is 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours or any two of the above values. Mixing can improve the uniformity of the mixed raw materials.

[0075] In one specific embodiment of the present invention, the total carbon-silicon ratio (C / Si) in the mixed raw materials is precisely controlled at 1.02 ± 0.01.

[0076] In a specific embodiment of the present invention, silicon carbide single crystals are prepared by physical vapor transport method using the mixed raw materials. Specifically, the mixed raw materials are filled into a graphite crucible, a seed crystal is fixed on the top of the crucible, the crucible is sealed and an exhaust hole is reserved; the sealed crucible is placed in a growth device, a vacuum is drawn to remove impurities, and then a protective gas is introduced to a preset growth pressure. Crystal growth is carried out under preset power conditions to obtain silicon carbide single crystals.

[0077] In one specific embodiment of the present invention, the seed crystal is preferably a 4H-SiC seed crystal; the phase deviation of the seed crystal is less than or equal to 0.5°.

[0078] In a specific embodiment of the present invention, the protective gas can be any protective gas known to those skilled in the art, and there are no special limitations. It includes, but is not limited to, one or more of argon, hydrogen, nitrogen and helium, preferably nitrogen and argon. The flow ratio of nitrogen to argon is preferably (1~10):90, more preferably (2~8):90, even more preferably (3~6):90, and most preferably 4:90.

[0079] In a specific embodiment of the present invention, the preset growth pressure is preferably 400~600 Pa, more preferably 450~550 Pa, and even more preferably 500 Pa; the preset power is preferably 8~12 kW, more preferably 9~11 kW, and even more preferably 10 kW.

[0080] In a specific embodiment of the present invention, the pulling speed during crystal growth is preferably 0.8~1.5 mm / h, more preferably 0.9~1.4 mm / h, even more preferably 1~1.3 mm / h, and most preferably 1.2 mm / h; the crystal growth time is preferably 40~60 h, more preferably 42~55 h, even more preferably 45~52 h, and most preferably 48 h.

[0081] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a double-layer porous carbon-coated silicon carbide, its preparation method, and the growth method of silicon carbide crystals provided by the present invention.

[0082] All reagents used in the following examples are commercially available. This example is based on the preparation of raw materials required for the growth of 4H-SiC single crystals by the PVT method, and uses a fluidized bed process to prepare a double-layer porous carbon-coated silicon carbide raw material.

[0083] Example 1

[0084] SiC particles with a particle size of 100-300 μm (purity 99.99%) were selected and soaked in a 5% hydrofluoric acid solution at room temperature for 20 minutes to remove the surface oxide layer. Then, they were ultrasonically cleaned twice with acetone (300 W power) for 10 minutes each time, followed by ultrasonic cleaning twice with isopropanol (300 W power) for 10 minutes each time. Subsequently, they were transferred to deionized water and ultrasonically cleaned three times (15 minutes each time, 300 W power) until the pH of the cleaning solution was 6-7. Finally, they were dried in a vacuum drying oven at 100℃ (vacuum degree -0.09 MPa) for 3 hours to obtain clean SiC particles.

[0085] Pretreated SiC particles were added to a quartz fluidized bed reactor with an inner diameter of 10 cm, filling one-third of the reactor's effective volume. After shutting down the reactor, high-purity argon gas (99.999% purity) was introduced at a flow rate of 300 sccm for 30 minutes to replace the air in the furnace. The oxygen content in the tail gas was measured to be ≤5 ppm. Subsequently, the argon gas flow rate was increased to 450 sccm (apparent gas velocity 0.225 m / s, bed expansion rate 1.6 times), and the temperature was increased to 900℃ at a rate of 8℃ / min.

[0086] After reaching the target temperature, a mixture of methane and argon gas (total flow rate 600 sccm) was introduced, with a methane to argon volume ratio of 1:14 (methane accounting for 6.7%). The pressure inside the reactor was controlled at 1000 Pa, and deposition was maintained for 2 hours (carbon layer deposition rate 120 nm / h, target thickness 240 nm). During the deposition process, entrained fine particles were recovered using a top cyclone separator, with a recovery rate exceeding 95%.

[0087] After the first layer deposition is completed, maintain an argon flow rate of 450 sccm and heat to 1150℃ at a rate of 10℃ / min.

[0088] After reaching the target temperature, switch to a mixed gas of acetylene and argon (total flow rate 750 sccm), with a volume ratio of acetylene to argon of 1:4 (acetylene accounting for 20%). Control the pressure inside the reactor to 600 Pa and maintain deposition for 1.5 hours (carbon layer deposition rate 180 nm / h, target thickness 270 nm).

[0089] During the second deposition process, the formation of macroporous structures was promoted by adjusting the porosity of the gas distribution plate (increasing it from 30% to 50% in the first layer) and the apparent gas velocity (0.375 m / s, bed expansion rate 2.2 times).

[0090] After deposition, the carbon source gas was shut off, and the argon flow rate was maintained at 700 sccm. The temperature was lowered to 80°C at a rate of 5°C / min, and the bottom discharge valve was opened to collect the product. The product was transferred to an argon-protected tube furnace, heated to 1000°C at a rate of 5°C / min, and held for 3 hours for annealing to enhance the bonding force between the double carbon layers and the interface. After annealing, the product was screened by a vibrating screener to obtain double-layer porous carbon-coated silicon carbide particles with a particle size deviation ≤8%.

[0091] Double-layer porous carbon-coated silicon carbide particles and high-purity SiC powder (purity ≥99.999%, particle size 100~200 μm) were mixed at a mass ratio of 20:80 and added to a three-dimensional mixer and mixed at 40 rpm for 2 hours to ensure homogeneity. The total carbon-silicon ratio (C / Si) of the mixed raw materials was precisely controlled at 1.02±0.01.

[0092] Before growth, a vacuum is required to remove impurities from the furnace, followed by the introduction of N2 and Ar gases (N2:Ar flow rate ratio = 4 sccm: 90 sccm) and maintaining a low pressure of 500 Pa. The single crystal furnace power is set to 10 kW, the crystal pulling speed is 1.2 mm / h, and then the crystal is continuously grown for 48 h.

[0093] The prepared carbon-coated silicon carbide raw material was tested and found that the uniformity deviation of the carbon layer thickness was ≤8%, and the bonding strength between the carbon layer and the SiC core reached 8.5 MPa (indentation test).

[0094] After the crystal was cut into wafers, the test results showed that the microtube defect density decreased from 5.2 defects / cm² (using the same growth process, with only high-purity SiC powder as raw material) to 1.5 defects / cm², the dislocation defect density decreased to 3000 defects / cm², and there were no obvious inclusion defects on the crystal surface.

[0095] Experiment Example 2

[0096] SiC particles with a particle size of 150~250 μm (purity 99.99%) were selected and soaked in 5% hydrofluoric acid solution at room temperature for 20 minutes to remove the surface oxide layer. Then, they were ultrasonically cleaned twice with acetone (300 W power) for 10 minutes each time, followed by ultrasonic cleaning twice with isopropanol (300 W power) for 10 minutes each time. Subsequently, they were transferred to deionized water and ultrasonically cleaned three times (15 minutes each time, 300 W power) until the pH of the cleaning solution was 6~7. Then, they were placed in a vacuum drying oven at 100℃ (vacuum degree -0.09MPa) for 3 hours to obtain clean SiC particles.

[0097] The pretreated particles were added to a quartz fluidized bed reactor with an inner diameter of 12 cm, filling the reactor to 1 / 3 of its effective volume, ensuring uniform particle packing height (±5 mm). After shutting down the reactor, high-purity argon gas (99.999%) was introduced at a flow rate of 350 sccm for 30 minutes to replace the air in the furnace. The oxygen content of the exhaust gas was measured to be ≤4 ppm.

[0098] Increase the argon flow rate to 600 sccm to bring the particles into a stable fluidized state (apparent gas velocity 0.28 m / s, bed expansion rate 1.5 times, no local accumulation observed through the viewing window), and raise the temperature to 950℃ at a rate of 8℃ / min.

[0099] After reaching the target temperature, a mixture of methane and argon gas was introduced at a total flow rate of 700 sccm (methane:argon = 1:13). The pressure inside the reactor was controlled at 800 Pa, and deposition was maintained for 3 hours (carbon layer deposition rate 110 nm / h, target thickness 330 nm). During the deposition process, a top cyclone separator (separation efficiency 96%) recovered entrained fine particles (≤150 μm), achieving a recovery rate of 95%.

[0100] After the first layer deposition is completed, maintain an argon flow rate of 600 sccm and heat to 1100℃ at a rate of 10℃ / min, then hold at that temperature for 10 minutes to stabilize the temperature.

[0101] Switch the flow rate to a mixture of acetylene-propane mixed carbon source (volume ratio 1:3) and argon, with a total flow rate of 800 sccm (mixed carbon source: argon = 1:6). Control the reactor pressure to 700 Pa and maintain deposition for 2 hours (carbon layer deposition rate 180 nm / h, target thickness 360 nm). At this point, the bed expansion rate increases to 1.9 times (apparent gas velocity 0.35 m / s), and the particle fluidization state is observed to be more intense through the viewing window.

[0102] After deposition, the mixed carbon source was shut off, and the argon flow rate was maintained at 800 sccm. The temperature was lowered to 80°C at a rate of 5°C / min. The bottom discharge valve was opened to collect the product (including recovered particles), yielding 3.02 kg of crude product (loss rate 1.3%). The product was transferred to an argon-protected tube furnace and annealed at 1050°C at a rate of 5°C / min for 3.5 hours to enhance the interlayer bonding between the bilayer carbon layers and the interface bonding with the SiC core.

[0103] After annealing, the samples were screened using a vibrating sieve (sieve size 150~250 μm, tolerance ±8%).

[0104] Double-layer carbon-coated silicon carbide particles and high-purity SiC powder (150~250 μm, purity 99.99%) were mixed at a mass ratio of 25:75 and added to a three-dimensional mixer (45 rpm) and mixed for 2.5 hours to ensure uniformity.

[0105] The specific growth conditions are the same as in Specific Example 1.

[0106] The test results of the prepared carbon-coated silicon carbide raw material showed that the carbon layer thickness was 380~420 nm (SEM cross-sectional analysis, deviation ±5%), and the interfacial bonding strength between the carbon layer and the SiC core was 9.2 MPa (indentation test).

[0107] After the crystal was cut into wafers, the test results showed that the microtube defect density decreased from 5.2 defects / cm² to 0.7 defects / cm², the dislocation defect density decreased to 2700 defects / cm², and there were no obvious inclusion defects on the crystal surface.

[0108] Example 3

[0109] SiC particles with a particle size of 150~250 μm (purity 99.99%) were selected and soaked in 5% hydrofluoric acid solution at room temperature for 20 minutes to remove the surface oxide layer. Then, they were ultrasonically cleaned twice with acetone (300 W power) for 10 minutes each time, followed by ultrasonic cleaning twice with isopropanol (300 W power) for 10 minutes each time. Subsequently, they were transferred to deionized water and ultrasonically cleaned three times (15 minutes each time, 300 W power) until the pH of the cleaning solution was 6~7. Then, they were placed in a vacuum drying oven at 100℃ (vacuum degree -0.09 MPa) for 3 hours to obtain clean SiC particles.

[0110] The pretreated particles were added to a quartz fluidized bed reactor with an inner diameter of 12 cm, filling the reactor to 1 / 3 of its effective volume, ensuring uniform particle packing height (±5 mm). After shutting down the reactor, high-purity argon gas (99.999%) was introduced at a flow rate of 350 sccm for 30 minutes to replace the air in the furnace. The oxygen content of the exhaust gas was measured to be ≤4 ppm.

[0111] Increase the argon flow rate to 600 sccm to bring the particles into a stable fluidized state (apparent gas velocity 0.28 m / s, bed expansion rate 1.5 times, no local accumulation observed through the viewing window), and heat to 950℃ at a rate of 8℃ / min.

[0112] After reaching the target temperature, a mixture of methane and argon gas was introduced at a total flow rate of 700 sccm (methane:argon = 1:13). The pressure inside the reactor was controlled at 800 Pa, and deposition was maintained for 3 hours (carbon layer deposition rate 110 nm / h, target thickness 330 nm). During the deposition process, a top cyclone separator (separation efficiency 96%) recovered entrained fine particles (≤150 μm), achieving a recovery rate of 95%.

[0113] After the first layer deposition is completed, maintain an argon flow rate of 600 sccm and heat to 1100℃ at a rate of 10℃ / min, then hold at that temperature for 10 minutes to stabilize the temperature.

[0114] Switch the flow rate to a mixture of acetylene-propane mixed carbon source (volume ratio 1:3) and argon, with a total flow rate of 800 sccm (mixed carbon source: argon = 1:6). Control the reactor pressure to 700 Pa and maintain deposition for 2 hours (carbon layer deposition rate 180 nm / h, target thickness 360 nm). At this point, the bed expansion rate increases to 1.9 times (apparent gas velocity 0.35 m / s), and the particle fluidization state is observed to be more intense through the viewing window.

[0115] After deposition, the mixed carbon source was shut off, and the argon flow rate was maintained at 800 sccm. The temperature was lowered to 80°C at a rate of 5°C / min. The bottom discharge valve was opened to collect the product (including recovered particles), yielding 3.05 kg of crude product (loss rate 1.0%). The product was transferred to an argon-protected tube furnace and annealed at 1050°C at a rate of 5°C / min for 3.5 hours to enhance the interlayer bonding between the bilayer carbon layers and the interface bonding with the SiC core.

[0116] After annealing, the samples were screened using a vibrating sieve (sieve size 150~250 μm, tolerance ±8%).

[0117] Double-layer carbon-coated silicon carbide particles and high-purity SiC powder (150~250 μm, purity 99.99%) were mixed at a mass ratio of 40:60 and added to a three-dimensional mixer (45 rpm) and mixed for 2.5 hours to ensure uniformity.

[0118] The specific growth conditions are the same as in Specific Example 1.

[0119] The test results of the prepared carbon-coated silicon carbide raw material showed that the carbon layer thickness was 370~430 nm (SEM cross-sectional analysis, deviation ±7%), and the interfacial bonding strength between the carbon layer and the SiC core was 9.5 MPa (indentation test).

[0120] The carbon-silicon ratio of the mixed raw materials was controlled at 1.05±0.02 (higher than 1.02±0.01 for the 25:75 ratio); crystal properties: the density of microtube defects in the crystal decreased from 5.2 defects / cm² to 0.5 defects / cm² (better than 0.7 defects / cm² for the 25:75 ratio), but a small number of carbon inclusion defects appeared on the crystal surface (density 0.3 defects / cm²), and the radial resistivity deviation increased to 6% (≤5% for the 25:75 ratio).

[0121] Compared to a 25:75 ratio, the 40:60 high-ratio double-layer carbon-coated particles showed a 14% further reduction in microtubule defect density due to a more abundant carbon source. However, excessive carbon source led to localized carbon enrichment, resulting in a small number of inclusion defects, and a slight decrease in resistivity uniformity. The results indicate that the proportion of double-layer carbon-coated particles needs to be controlled within a reasonable range (25%–35%) to optimize defect performance while avoiding the negative impacts of excessive carbon.

[0122] Comparative Example 1

[0123] SiC particles with a particle size of 100~300 μm (purity 99.99%) were selected and soaked in a 5% hydrofluoric acid solution at room temperature for 20 minutes to remove the surface oxide layer. Then, they were ultrasonically cleaned twice with acetone (300 W power) for 10 minutes each time, followed by ultrasonic cleaning twice with isopropanol (300 W power) for 10 minutes each time. Subsequently, they were transferred to deionized water and ultrasonically cleaned three times (15 minutes each time, 300 W power) until the pH of the cleaning solution was 6~7. Finally, they were dried in a vacuum drying oven at 100℃ (vacuum degree -0.09 MPa) for 3 hours to obtain clean SiC particles.

[0124] Pretreated SiC particles were added to a quartz fluidized bed reactor with an inner diameter of 10 cm, filling the reactor to one-third of its effective volume. After shutting down the reactor, high-purity argon gas (99.999% purity) was introduced at a flow rate of 300 sccm for 30 minutes to replace the air in the furnace. The oxygen content in the exhaust gas was measured to be ≤5 ppm.

[0125] The argon flow rate was then increased to 450 sccm (apparent gas velocity 0.225 m / s, bed expansion rate 1.6 times), and the temperature was increased to 900℃ at a rate of 8℃ / min.

[0126] After reaching the target temperature, a mixture of methane and argon gas (total flow rate 600 sccm) was introduced, with a methane to argon volume ratio of 1:14 (methane accounting for 6.7%). The pressure inside the reactor was controlled at 1000 Pa, and deposition was maintained for 4.25 hours (ensuring the total thickness was consistent with the 510 nm total thickness of the double layer in Example 1, and the deposition rate was 120 nm / h). During the deposition process, entrained fine particles were recovered using a top cyclone separator, with a recovery rate of over 95%.

[0127] After deposition, the carbon source gas was shut off, and the argon flow rate was maintained at 700 sccm. The temperature was lowered to 80°C at a rate of 5°C / min, and the bottom discharge valve was opened to collect the product. The product was transferred to an argon-protected tube furnace, heated to 1000°C at a rate of 5°C / min, and held for 3 hours for annealing. After annealing, the product was screened by a vibrating screen to obtain single-layer carbon-coated silicon carbide particles with a particle size deviation ≤8%.

[0128] The single-layer carbon-coated silicon carbide particles were mixed with high-purity SiC powder (purity ≥99.999%, particle size 100~200 μm) at a mass ratio of 20:80. The mixture was added to a three-dimensional mixer and mixed at 40 rpm for 2 hours. The total carbon-silicon ratio (C / Si) of the mixed raw materials was controlled at 1.02±0.01.

[0129] The specific growth conditions are the same as in Specific Example 1.

[0130] The test results showed that the carbon layer structure was as follows: the cross-sectional morphology observed by scanning electron microscopy (SEM) showed that the carbon layer thickness was 490~530 nm (uniformity deviation ±7.8%), the whole layer was a microporous structure (average pore size 3.3 nm), and there was no obvious macropore distribution; the bonding strength between the carbon layer and the SiC core reached 8.2 MPa (slightly lower than 8.5 MPa in Example 1).

[0131] The test results after the crystal was cut into wafers showed that the microtube defect density was 3.1 defects / cm² (significantly higher than 1.5 defects / cm² in Example 1); carbon inclusion defects appeared on the crystal surface, and the diffusion of gaseous carbon species was hindered and locally enriched due to the excessively dense microporous structure; during the growth process, the power of the single crystal furnace fluctuated by ±14% (±8% in Example 1), and the volatile matter escape rate was unstable.

[0132] Comparative Example 2

[0133] SiC particles with a particle size of 100-300 μm (purity 99.99%) were selected and soaked in a 5% hydrofluoric acid solution at room temperature for 20 minutes to remove the surface oxide layer. Then, they were ultrasonically cleaned twice with acetone (300 W power) for 10 minutes each time, followed by ultrasonic cleaning twice with isopropanol (300 W power) for 10 minutes each time. Subsequently, they were transferred to deionized water and ultrasonically cleaned three times (15 minutes each time, 300 W power) until the pH of the cleaning solution was 6-7. Finally, they were dried in a vacuum drying oven at 100℃ (vacuum degree -0.09 MPa) for 3 hours to obtain clean SiC particles.

[0134] Pretreated SiC particles were added to a quartz fluidized bed reactor with an inner diameter of 10 cm, filling one-third of the reactor's effective volume. After shutting down the reactor, high-purity argon gas (99.999% purity) was introduced at a flow rate of 300 sccm for 30 minutes to replace the air in the furnace. The oxygen content in the tail gas was measured to be ≤5 ppm. Subsequently, the argon gas flow rate was increased to 450 sccm (apparent gas velocity 0.225 m / s, bed expansion rate 1.6 times), and the temperature was directly raised to 1150℃ at a rate of 8℃ / min (skipping the 900℃ stage of the first layer deposition).

[0135] After reaching the target temperature, a mixture of acetylene and argon gas (total flow rate 750 sccm) was introduced, with a volume ratio of acetylene to argon of 1:4 (acetylene accounting for 20%). The pressure inside the reactor was controlled at 600 Pa, and deposition was maintained for 2.83 hours (ensuring the total thickness was consistent with the 510 nm total thickness of the double layer in the example, with a deposition rate of 180 nm / h). During deposition, the porosity of the gas distribution plate was maintained at 50%, and the apparent gas velocity was 0.375 m / s (bed expansion rate 2.2 times). Entrained fine particles were recovered through a top cyclone separator, with a recovery rate of over 95%.

[0136] After deposition, the carbon source gas was shut off, and the argon flow rate was maintained at 700 sccm. The temperature was lowered to 80°C at a rate of 5°C / min, and the bottom discharge valve was opened to collect the product. The product was transferred to an argon-protected tube furnace, heated to 1000°C at a rate of 5°C / min, and held for 3 hours for annealing. After annealing, the product was screened by a vibrating screen to obtain single-layer carbon-coated silicon carbide particles with a particle size deviation ≤8%.

[0137] Monolayer carbon-coated silicon carbide particles and high-purity SiC powder (purity ≥99.999%, particle size 100~200 μm) were mixed at a mass ratio of 20:80 and added to a three-dimensional mixer and mixed at 40 rpm for 2 hours to ensure uniformity. The total carbon-silicon ratio (C / Si) of the mixed raw materials was controlled at 1.02±0.01.

[0138] The specific growth conditions are the same as in Specific Example 1.

[0139] The test results showed that the carbon layer thickness of the prepared monolayer carbon-coated silicon carbide raw material was 490~530 nm (uniformity deviation ±8.2%), and the whole layer had a macroporous structure (average pore size 88 nm). The bonding strength between the carbon layer and the SiC core was 6.2 MPa (significantly lower than 8.5 MPa in the example).

[0140] The test results after the crystal was cut into wafers showed that: the microtube defect density was 2.8 defects / cm² (higher than 1.5 defects / cm² in the example); there were no obvious inclusion defects on the crystal surface; growth stability: the power fluctuation of the single crystal furnace was ±18% (±10% in the example), and the fluctuation of the raw material sublimation rate caused the crystal growth interface to be unstable; the total carbon-silicon ratio deviation: the C / Si fluctuation during the actual growth process was ±0.06 (±0.02 in the example).

[0141] Comparative analysis of Example 1 and Comparative Examples 1-2 reveals that while a single-layer coating structure using only a second carbon source can achieve gaseous species diffusion through macropores, it suffers from two major drawbacks: first, rapid high-temperature deposition leads to insufficient bonding strength between the carbon layer and the SiC core, making it prone to detachment during growth; second, the lack of a dense inner structure to regulate the volatilization rate results in an imbalance in the Si / C ratio and deterioration of growth stability. This result further demonstrates that a dual-layer carbon source coating design combining "dense inner layer regulation + loose outer layer diffusion" is necessary to meet the high-performance raw material requirements for growing 4H-SiC single crystals using the PVT method.

[0142] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A double-layer porous carbon-coated silicon carbide, characterized in that, include: silicon carbide core; The first carbon layer surrounding the silicon carbide core; A second carbon layer surrounding the first carbon layer; The average pore size of the first carbon layer is smaller than the average pore size of the second carbon layer; The porosity of the first carbon layer is higher than that of the second carbon layer.

2. The double-layer porous carbon-coated silicon carbide according to claim 1, characterized in that, The average pore size of the first carbon layer is 1~10 nm; And / or, the average pore size of the second carbon layer is 50~200 nm.

3. The double-layer porous carbon-coated silicon carbide according to claim 1, characterized in that, The particle size of the silicon carbide core is 100~300 μm; And / or, the total thickness of the first carbon layer and the second carbon layer is 360~540 nm; And / or, the thickness ratio of the first carbon layer to the second carbon layer is 1:(0.5~2).

4. The double-layer porous carbon-coated silicon carbide according to claim 1, characterized in that, The first carbon layer is formed from a first carbon source; the first carbon source is a small molecule saturated hydrocarbon; the small molecule saturated hydrocarbon has less than or equal to 2 carbon atoms; And / or, the second carbon layer is formed from a second carbon source; the second carbon source includes unsaturated hydrocarbons.

5. The double-layer porous carbon-coated silicon carbide according to claim 4, characterized in that, The second carbon source also includes macromolecular saturated hydrocarbons; the macromolecular saturated hydrocarbons have 3 or more carbon atoms; The volume ratio of the macromolecular saturated hydrocarbon to the unsaturated hydrocarbon is (2~4):

1.

6. A method for preparing double-layer porous carbon-coated silicon carbide as described in claim 1, characterized in that, Includes the following steps: S1) Silicon carbide is loaded into a fluidized bed reactor, a carrier gas is introduced to suspend the silicon carbide, and then the temperature is raised to the first target temperature. The carrier gas and the first carbon source are then introduced to deposit the first carbon layer on the surface of the silicon carbide. S2) Stop the first carbon source from entering, and heat to the second target temperature while only the carrier gas is introduced. Then, introduce the carrier gas and the second carbon source to deposit the second carbon layer on the surface of the first carbon layer to obtain a double-layer carbon-encapsulated precursor. S3) The double-layer carbon-coated precursor is annealed to obtain double-layer porous carbon-coated silicon carbide.

7. The preparation method according to claim 6, characterized in that, In step S1), the amount of silicon carbide loaded is 1 / 4 to 1 / 3 of the effective volume of the fluidized bed reactor; In step S1), when the carrier gas is introduced to keep the silicon carbide in a suspended state, the flow rate of the carrier gas is 1.2 to 1.5 times the minimum fluidization rate. The first target temperature is 900℃~1000℃; In step S1), the volume ratio of carrier gas to the first carbon source is (10~15):1; In step S1), the deposition of the first carbon layer is carried out under conditions of 500~1000 Pa; the deposition time of the first carbon layer is 2~4 h. In step S1), the gas porosity of the fluidized bed reactor during the deposition of the first carbon layer is 25% to 35%.

8. The preparation method according to claim 6, characterized in that, The second target temperature in step S2) is 1100℃~1200℃; In step S2), the volume ratio of carrier gas to the second carbon source is (4~6):1; In step S2), the deposition of the second carbon layer is carried out under conditions of 500-700 Pa; the deposition time of the second carbon layer is 2-3 hours. In step S2), the gas porosity of the fluidized bed reactor during the deposition of the second carbon layer is 45%~60%. The annealing temperature is 900℃~1200℃; the annealing time is 2~4 h.

9. A method for growing silicon carbide crystals, characterized in that, Includes the following steps: Mix the double-layer porous carbon-coated silicon carbide as described in any one of claims 1 to 5 or the double-layer porous carbon-coated silicon carbide prepared by any one of claims 6 to 8 with silicon carbide particles to obtain a mixed raw material; Silicon carbide single crystals were prepared using the aforementioned mixed raw materials via physical vapor transport.

10. The growth method according to claim 9, characterized in that, The mass ratio of the double-layer porous carbon-coated silicon carbide to silicon carbide particles is (10~30):(70~90).

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