Silicon carbide crystal growth method

Abstract

The invention discloses a silicon carbide crystal growth method, which relates to the technical field of silicon carbide growth, and comprises the following steps: S1, immersing a silicon carbide seed crystal into an ultrasonic treatment tank filled with a mixed solution; s2, activating treatment of a thiofluorinated precursor: performing amino functionalization on the surface of the treated seed crystal through an amino silane solution, and then pre-activating a surface dangling bond in argon / hydrogen mixed plasma; s3, pulse modulation SF6 plasma gradient film forming: a double-frequency plasma source is adopted, and gradient passivation films with Si-F bonds as inner layers and Si-S bonds as outer layers are generated layer by layer through pulse modulation of power and air pressure in a dynamically adjusted SF6 / Ar mixed atmosphere; s4, performing boron doping reinforcement on the sulfur-fluorine passivation layer; and S5, packaging and protecting the graphene quantum dots. According to the process, multi-stage cooperative treatment is adopted, a multi-layer protection system is formed, the comprehensive performance of the passivation layer is remarkably improved, and the balance of high-temperature stability and oxygen barrier is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide growth, and particularly relates to a method for growing silicon carbide crystals. Background Art

[0002] Physical vapor transport method is a commonly used technical method for preparing silicon carbide crystals. It refers to the process of depositing or crystallizing silicon carbide crystals on a substrate through physical vapor transport or chemical vapor reaction of gases or vapors containing carbon and silicon elements under specific temperature, pressure and atmosphere conditions. The process usually uses a seed crystal as a carrier. The seed crystal is not only the "seed" for crystal growth in the preparation of silicon carbide, but also the core element for controlling crystal form, reducing defects and improving performance. Its function runs through the whole process from gas phase transport to epitaxial growth, directly affecting the final crystal quality and application potential.

[0003] To ensure that the seed crystal as a carrier can grow high-quality silicon carbide crystals in the subsequent process, the seed crystal is usually treated with hydrofluoric acid and boiling water, which can remove the oxide layer on the surface of the seed crystal (the existence of this oxide layer will hinder the epitaxial growth of silicon carbide crystals, resulting in interface defects such as stacking faults and microtubes) and form a hydrogen passivation layer to protect its surface from air pollution, and provide an ideal starting interface for the subsequent growth of silicon carbide crystals. However, the above treatment process has the following problems:

[0004] First, deep oxidation is difficult to remove: the removal efficiency of the subsurface oxide layer (depth > 5nm) is low (only about 70%);

[0005] Second, the stability of the hydrogen passivation layer is poor: the bond energy of the Si-H bond is low and it is easily broken under high temperature (>400°C) or ultraviolet irradiation, resulting in re-oxidation of the surface;

[0006] Third, short lifespan: the hydrogen passivation layer gradually fails during storage, and the oxygen impurity penetration rate increases with time (the oxygen content rises to more than 0.5at% after 6 months);

[0007] Fourth, high surface roughness: the surface roughness (Ra) rises from the initial 0.8nm to 2.3nm, which easily induces dislocation defects;

[0008] Therefore, the present application provides a method for growing silicon carbide crystals to meet the requirements. Summary of the Invention

[0009] The purpose of the present application is to provide a method for growing silicon carbide crystals, which is used to solve the technical problems that the hydrogen passivation layer formed by the existing process has poor stability, short lifespan, difficult to remove deep oxidation, and high surface roughness, resulting in the inability to grow high-quality silicon carbide crystals subsequently.

[0010] To achieve the above object, the present application provides the following technical solution: A method for growing a silicon carbide crystal, characterized by comprising the following steps,

[0011] S1. Multi-stage oxide layer etching and nano-porosity filling: Immerse the silicon carbide seed crystal into an ultrasonic treatment tank containing a mixed solution composed of nitric acid, hydrofluoric acid, ammonium fluoride, a surfactant, and a nano-diamond suspension, and use ultrasonic treatment to remove the subsurface oxide layer and fill the surface micro-cracks;

[0012] Utilize the cooperation of nitric acid, hydrofluoric acid, ammonium fluoride, a surfactant, and ultrasonic waves to remove the surface oxide layer of the seed crystal, and at the same time use nano-diamond particles to fill the surface defects of the seed crystal, reduce the surface roughness, reduce the uneven corrosion rate caused by the roughness difference, and is beneficial to the uniformity of subsequent coating.

[0013] S2. Activation treatment of thiofluoride precursor: Perform amino-functionalization on the surface of the seed crystal through an amino-silane solution, and then pre-activate the surface dangling bonds in an argon / hydrogen mixed plasma;

[0014] The purpose is to ensure the uniformity, reactivity of surface chemical modification, and long-term stability of the passivation layer.

[0015] S3. Pulse-modulated SF 6 Plasma gradient film formation: Use a dual-frequency plasma source, in a dynamically adjusted SF 6 / Ar mixed atmosphere, through pulse modulation of power and pressure, make a gradient passivation film with Si-F bonds as the inner layer and Si-S bonds as the outer layer grow layer by layer on the surface of the seed crystal;

[0016] S4. Boron doping enhancement of the sulfur-fluorine passivation layer: Introduce diborane into the SF 6 plasma for in-situ boron doping, and form a Si-S-B-F four-element bonding structure through high-temperature annealing;

[0017] S5. Graphene quantum dot encapsulation protection: Self-assemble carboxylated graphene quantum dots on the surface of the passivation layer in a supercritical CO 2 environment, and achieve covalent fixation through a silane coupling agent.

[0018] This process adopts multi-stage collaborative treatment, combines chemical etching, plasma film formation, doping enhancement, and quantum dot encapsulation to form a multi-layer protection system, significantly improves the comprehensive performance of the passivation layer, and realizes the balance of high-temperature stability and oxygen barrier through the stratification of the Si-F inner layer (high bond energy) and the Si-S outer layer (thermal oxidation resistance).

[0019] As a preferred embodiment in this example, the mixed solution in S1 contains: 8 - 13 wt% nitric acid, 3 - 8 wt% hydrofluoric acid, 1 - 3 wt% ammonium fluoride, 0.05 - 0.15 wt% sodium perfluorooctanesulfonate, and 0.02 - 0.1 wt% nanodiamond suspension with a particle size of 3 - 5 nm;

[0020] The ultrasonic treatment sequentially includes a low - frequency stage: 15 - 25 kHz, temperature 60 °C, treatment for 5 minutes;

[0021] An intermediate - frequency transition stage: 100 - 300 kHz, temperature linearly rising from 60 °C to 80 °C, treatment for 3 minutes;

[0022] A high - frequency stage: 500 kHz - 2 MHz, temperature 80 °C, treatment for 5 minutes.

[0023] 8 - 13 wt% nitric acid: If the concentration is too high (>13%), it will cause excessive corrosion and damage the silicon carbide lattice; if it is too low (<8%), the oxide layer removal is incomplete (oxygen residue > 0.1 at%);

[0024] 3 - 8 wt% hydrofluoric acid: It etches silicon oxide synergistically with nitric acid. If it is >8%, it will corrode the silicon carbide body (etching rate > 10 nm / min), and if it is <3%, it cannot remove the subsurface oxide layer;

[0025] 1 - 3 wt% ammonium fluoride: As a buffer to stabilize the activity of HF. If it is >3%, it will generate excessive NH 4 + resulting in the colloidization of the solution, and if it is <1%, it will lose the buffering effect;

[0026] 0.05 - 0.15 wt% sodium perfluorooctanesulfonate (which can reduce the surface tension of the solution, make the etching solution evenly cover the surface of the seed crystal, and reduce the wetting difference between the edge and the center): If it is >0.15 wt%, excessive micelle formation will occur, encapsulating hydrofluoric acid and nanodiamonds, and the etching rate will decrease by 40%; The pyrolysis of PFOS - Na produces residues of perfluorooctanesulfonic acid (PFOS) (the proportion of F atoms detected by XPS > 5 at%), hindering the thiofluorination reaction (the formation rate of Si - S bonds decreases by 50%); If it is <0.05 wt%, the solution cannot penetrate into the subsurface micropores (the residual rate of the oxide layer with a depth > 5 nm > 30%) and the control of oxygen impurities fails, with the oxygen residue > 0.2 at%, resulting in a 30% decrease in the bonding strength of the subsequent passivation layer;

[0027] 0.05 - 0.1 wt% nanodiamond suspension (after filling the surface defects, it reduces the surface roughness of the substrate and reduces the uneven corrosion rate caused by roughness differences): If it is >0.1%, it will cause surface scratches (Ra > 1 nm), and if it is <0.05%, it cannot effectively fill the microcracks;

[0028] Low frequency 15 - 25 kHz: The cavitation bubble size is large, suitable for stripping the macroscopic oxide layer; > 25 kHz, the cavitation energy is insufficient, < 15 kHz, it will damage the surface.

[0029] Medium frequency 100 - 300 kHz can not only penetrate the surface microcracks (depth 5 - 10 nm), but also maintain sufficient energy density to effectively remove the subsurface oxide layer.

[0030] The temperature linearly rises from 60 °C to 80 °C, generating a thermal migration drive. The increasing temperature promotes the migration of nanodiamonds along the temperature gradient direction, and the filling rate (i.e., the entry of nanodiamond particles into deep pores) increases from 70% to 90%. Moreover, the increasing temperature accelerates the buffering effect of ammonium fluoride, inhibiting local over - etching (surface roughness fluctuation < ±0.1 nm).

[0031] High frequency 500 kHz - 2 MHz: The acoustic streaming effect removes the subsurface oxide layer. > 2 MHz, the energy attenuation is serious, < 500 kHz, it cannot penetrate the deep - layer structure.

[0032] The low - frequency → medium - frequency → high - frequency gradually refines the processing scale (macroscopic → subsurface → nanoscale), achieving the full - dimensional removal of the oxide layer. At the same time, the heating process drives the synergistic effect of chemical etching and physical filling, avoiding the performance bottleneck at a single temperature.

[0033] Through the three - stage design of removing the macroscopic oxide layer by low frequency, penetrating the subsurface by medium frequency, and nanoscale refinement by high frequency, combined with the temperature - gradient - driven thermal migration and chemical etching, the comprehensive optimization of oxygen residue, surface roughness, and microcrack filling rate is achieved.

[0034] As a preferred implementation method in this embodiment, the amino functionalization treatment in S2 is specifically: immersing the seed crystal in an ethanol solution of 3 - aminopropyltriethoxysilane, and the content of 3 - aminopropyltriethoxysilane is 1 - 3 wt%, and treating at 60 °C for 30 minutes.

[0035] The conditions of plasma pre - activation are: argon / hydrogen volume ratio 3 - 4:1, power 100 - 200 W, gas pressure 20 - 30 Pa, and treatment time 3 minutes.

[0036] 3 - aminopropyltriethoxysilane 1 - 3 wt%: > 3 wt% leads to multilayer adsorption (thickness > 2 nm), hindering subsequent reactions; < 1 wt%, the amino density is insufficient.

[0037] Argon / hydrogen 3 - 4:1: > 4:1, the hydrogen is insufficient, and the dangling bond density < 10 12 cm -2 ; < 3:1, the hydrogen etching effect is too strong (surface roughness increases).

[0038] Power 100 - 200W: >200W leads to surface amorphization (Raman D peak intensity ratio >0.5); <100W results in insufficient activation.

[0039] As a preferred implementation in this embodiment, the pulse modulation parameters in S3 include: the low frequency of 2MHz and the high frequency of 13.56MHz of the dual - frequency plasma source act alternately, with the power density modulated between 100 - 300W / cm 2 interval, and the duty cycle is 40 - 60%;

[0040] SF 6 / Ar volume ratio is 1:2 - 5, and the dynamic pressure regulation range is 10 - 50Pa;

[0041] During gradient film formation, the thickness of the inner Si - F bond - dominated region is 1 - 2nm, and the thickness of the outer Si - S bond network is 2 - 3nm.

[0042] Dual - frequency 2MHz + 13.56MHz: The low frequency controls the ion energy, and the high frequency maintains the plasma density; a single frequency results in insufficient energy or density;

[0043] Power density 100 - 300W / cm 2 : <100W cannot excite high - energy F + ions (the Si - F layer is discontinuous); >300W leads to surface sputtering (thickness fluctuation >±10%);

[0044] Duty cycle 40 - 60%: <40% results in insufficient ion energy, F + ion energy <100eV (it is necessary to be ≥150eV to break through the surface oxide layer), resulting in the Si - F bond ratio <50%, and the oxygen permeability rises to 1×10 -15 cm 2 / s (original 3×10 -16 cm 2 / s), and the reaction gas is not dissociated sufficiently, the dissociation rate of SF 6 is <30% (it is necessary to be ≥50%), and the unreacted SF 6 forms sulfur deposition clusters (particle size >10nm) during Toff, and the film roughness Ra >1nm;

[0045] >60% causes matrix overheating and damage. Continuous ion bombardment leads to local temperature >900℃, graphitization occurs on the surface of silicon carbide, the dislocation density increases, and film stress accumulates. The internal stress in the film under high duty cycle >2GPa, triggering cracking (crack density >10 3 / cm 2 ), and the oxygen permeation path increases;

[0046] SF 6 / Ar 1:2 to 5:>1:2 The sulfur deposition rate is too fast (S atomic clusters), <1:5 The fluorination efficiency is reduced (the proportion of Si-F bonds <60%).

[0047] Gradient film thickness: Inner layer Si-F <1nm protection is insufficient, >3nm stress accumulation; Outer layer Si-S <2nm coverage is low, >3nm hinders epitaxial growth.

[0048] As a preferred implementation mode in this embodiment, in the S4: The flow rate of diborane introduced is 2 to 6 sccm, and the molar ratio with SF 6 is 1:20;

[0049] The annealing conditions are: nitrogen atmosphere, heating rate 10°C / s, holding at 800°C for 10 minutes, and the crystallinity of the passivation layer ≥85%.

[0050] Diborane 2 to 6 sccm: >6 sccm boron concentration >10 at%, forming B-S bonds weakens the thermal stability; <2 sccm doping is ineffective (B <1 at%).

[0051] Annealing at 700 to 900°C: >900°C causes sulfur volatilization (S content drops >50%), <700°C crystallinity <70% (XRD full width at half maximum >0.3°);

[0052] Excessive boron: The brittleness of the passivation layer increases (fracture toughness <1 MPa·m 1 / ) 2 )

[0053] Insufficient annealing: Many lattice defects, resistivity reduced to <10 5 Ω·cm.

[0054] As a preferred implementation mode in this embodiment, supercritical CO 2 The conditions are a pressure of 10 to 20 MPa and a temperature of 50°C;

[0055] The particle size of carboxylated graphene quantum dots is 3 to 5 nm, and Si-O-C covalent bonds are formed on the surface of the passivation layer through trimethylchlorosilane for fixation.

[0056] Supercritical CO 2 10 to 20 MPa: >20 MPa the equipment cost surges, <10 MPa cannot drive the close packing of quantum dots (coverage <70%).

[0057] Quantum dot particle size 3 to 5 nm: >5 nm the oxygen diffusion channel increases (permeability >1×10 -15 cm 2 / s), <3 nm serious agglomeration (film layer light transmittance <80%);

[0058] If the quantum dot size is too large, the oxygen barrier will fail; if the pressure is insufficient, the encapsulation layer will become porous and the oxygen permeability will increase.

[0059] As a preferred implementation in this embodiment, in S5, a vertical magnetic field is applied with a magnetic field strength of 0.5 to 1.5 T to arrange the quantum dots along the magnetic field direction, and the aspect ratio of the quantum dots is controlled to be 3:1 to 5:1 with a spacing of 10 to 20 nm.

[0060] Carboxylated graphene quantum dots are prone to random distribution or agglomeration due to Brownian motion and van der Waals forces, and are difficult to form an ordered arrangement structure. Since graphene quantum dots have anisotropic magnetic susceptibility, when a perpendicular magnetic field is applied, the quantum dots will be arranged along the magnetic field direction (long axis) to reduce the system energy.

[0061] When the magnetic field strength is lower than 0.5T, the quantum dots are disordered, the oxygen diffusion path is not tortuous enough (tortuosity factor <2), and the oxygen permeability is >1×10 -16 cm 2 / s; When the temperature is higher than 1.5T, the quantum dots are over-aggregated, the transmittance is less than 90%, the transmittance of the film layer decreases, and the optical performance of the device deteriorates;

[0062] When the aspect ratio is <3:1, the quantum dots are approximately spherical, which cannot effectively extend the oxygen diffusion path, and the oxygen barrier efficiency decreases; when the aspect ratio is >5:1, the quantum dots are easy to bend and break, the film layer becomes more brittle, and is prone to cracking;

[0063] Spacing <10nm: Van der Waals forces between quantum dots lead to agglomeration, and transmittance is <85%; Spacing >20nm: The width of the oxygen diffusion channel is >2nm, and the oxygen permeability increases.

[0064] As a preferred implementation in this example, the magnetic field strength increases linearly from 0.5 T to 1.5 T at a rate of 0.2 T / min, inducing the quantum dots to be arranged in a gradient along the long axis direction.

[0065] Linearly increasing the magnetic field strength allows the quantum dots to gradually adjust their orientation, avoiding stress concentration and interface cracks caused by sudden changes in the magnetic field. Linearly increasing the magnetic field strength allows the quantum dots to be evenly distributed, avoiding local over-crowding or over-sparseness. When the rate is too fast, the quantum dots will not have time to adjust their orientation, resulting in local disordered accumulation. When the rate is too slow, the process time is too long, and the quantum dots may deviate from the predetermined orientation due to thermal motion. When the rate is 0.2T / min, the quantum dot orientation deviation angle can be <±1° while ensuring process efficiency.

[0066] As a preferred implementation in this embodiment, during the application of the vertical magnetic field, the temperature is reduced from 50° C. to 30° C., and the cooling rate is controlled at 0.5-1° C. / min.

[0067] Lowering the temperature from 50°C to 30°C can inhibit the orientation deviation caused by the thermal motion of quantum dots. If the temperature drops too quickly, the thermal motion of quantum dots suddenly decreases, resulting in incomplete arrangement and disordered accumulation in local areas; if the temperature drops too slowly, the process time is prolonged, and the quantum dots may deviate from the predetermined orientation due to long-term thermal motion.

[0068] The cooling rate (0.5 - 1°C / min) is matched with the rate of 0.2T / min to ensure that the quantum dots gradually stabilize their orientation during the cooling process and avoid random distribution caused by thermal motion.

[0069] In summary, the technical effects and advantages of the present invention are as follows:

[0070] The structure of the present invention is reasonable. This process adopts multi-level collaborative processing, combines chemical etching, plasma film formation, doping strengthening, and quantum dot encapsulation to form a multi-layer protection system, significantly improving the comprehensive performance of the passivation layer, and achieving a balance between high-temperature stability and oxygen barrier through the stratification of the inner Si-F layer (high bond energy) and the outer Si-S layer (thermal oxidation resistance). BRIEF DESCRIPTION OF THE DRAWINGS

[0071] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0072] Figure 1 It is a comparison chart of the data of the seed crystal processed by the traditional process and the seed crystal processed by the present process. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0073] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0074] Example 1:

[0075] S1. Multi-level oxide layer etching and nano-pore filling: Immerse the silicon carbide seed crystal in an ultrasonic treatment tank containing a mixed solution composed of nitric acid, hydrofluoric acid, ammonium fluoride, surfactant, and nano-diamond suspension. The ultrasonic treatment removes the subsurface oxide layer and fills the surface micro-cracks.

[0076] The mixed solution contains: 8 wt% nitric acid, 3 wt% hydrofluoric acid, 1 wt% ammonium fluoride, 0.05 wt% sodium perfluorooctanesulfonate, and 0.05 wt% of a nanodiamond suspension with a particle size of 3 nm; The ultrasonic treatment sequentially includes a low-frequency stage: 15 kHz, a temperature of 60 °C, and a treatment time of 5 minutes;

[0077] An intermediate-frequency transition stage: 100 kHz, the temperature linearly rises from 60 °C to 80 °C, and the treatment time is 3 minutes;

[0078] A high-frequency stage: 500 kHz, a temperature of 80 °C, and a treatment time of 5 minutes;

[0079] S2. Thiofluorination precursor activation treatment: The surface of the treated seed crystal is subjected to amino-functionalization through an amino-silane solution, and then the surface dangling bonds are pre-activated in an argon / hydrogen mixed plasma;

[0080] The amino-functionalization treatment is specifically: The seed crystal is immersed in an ethanol solution of 3-aminopropyltriethoxysilane, and 3-aminopropyltriethoxysilane is 1 wt%, and the treatment is carried out at 60 °C for 30 minutes;

[0081] The plasma pre-activation conditions are: an argon / hydrogen volume ratio of 3:1, a power of 100 W, a gas pressure of 20 Pa, and a treatment time of 3 minutes;

[0082] S3. Pulse-modulated SF 6 Plasma gradient film formation: Using a dual-frequency plasma source, in a dynamically adjusted SF 6 / Ar mixed atmosphere, through the pulse modulation of power and gas pressure, a gradient passivation film with an inner layer of Si-F bonds and an outer layer of Si-S bonds is formed layer by layer on the surface of the seed crystal;

[0083] The pulse modulation parameters include: The low frequency of 2 MHz and the high frequency of 13.56 MHz of the dual-frequency plasma source act alternately, and the power density is modulated between 100 W / cm 2 with a duty cycle of 40%;

[0084] SF 6 / Ar volume ratio of 1:2, and the dynamic adjustment range of the gas pressure is 10 Pa;

[0085] During gradient film formation, the thickness of the inner Si-F bond dominant region is 1 nm, and the thickness of the outer Si-S bond network is 2 nm;

[0086] S4. Boron doping strengthening of the sulfur-fluorine passivation layer: Diborane is introduced into the SF 6 plasma for in-situ boron doping, and a Si-S-B-F quaternary bonding structure is formed through high-temperature annealing;

[0087] The flow rate of the introduced diborane is 2 sccm, and it is mixed with SF 6The molar ratio is 1:20;

[0088] The annealing conditions are as follows: in a nitrogen atmosphere, a heating rate of 10 °C / s, holding at 700 °C for 10 minutes, and the crystallinity of the passivation layer is ≥85;

[0089] S5. Encapsulation and protection with graphene quantum dots: In a supercritical CO 2 environment, carboxylated graphene quantum dots are self-assembled on the surface of the passivation layer and covalently fixed through a silane coupling agent;

[0090] Supercritical CO 2 The conditions are a pressure of 10 MPa and a temperature of 50 °C;

[0091] The particle size of the carboxylated graphene quantum dots is 3 nm, and Si-O-C covalent bonds are formed on the surface of the passivation layer through trimethylchlorosilane for fixation.

[0092] Example 2:

[0093] S1. Multi-stage oxide etching and nanopore filling: Immerse the silicon carbide seed crystal in an ultrasonic treatment tank containing a mixed solution composed of nitric acid, hydrofluoric acid, ammonium fluoride, a surfactant, and a nanodiamond suspension to remove the subsurface oxide layer by ultrasonic treatment and fill the surface microcracks;

[0094] The mixed solution contains: 13 wt% nitric acid, 8 wt% hydrofluoric acid, 3 wt% ammonium fluoride, 0.15 wt% sodium perfluorooctanesulfonate, and 0.1 wt% nanodiamond suspension with a particle size of 5 nm;

[0095] The ultrasonic treatment sequentially includes a low-frequency stage: 25 kHz, a temperature of 60 °C, and a treatment time of 5 minutes;

[0096] An intermediate-frequency transition stage: 300 kHz, the temperature linearly rises from 60 °C to 80 °C, and the treatment time is 3 minutes;

[0097] A high-frequency stage: 2 MHz, a temperature of 80 °C, and a treatment time of 5 minutes;

[0098] S2. Activation treatment of the thiofluorination precursor: Perform amino-functionalization on the surface of the treated seed crystal through an amino-silane solution, and then pre-activate the surface dangling bonds in an argon / hydrogen mixed plasma;

[0099] The specific amino-functionalization treatment is as follows: Immerse the seed crystal in an ethanol solution of 3-aminopropyltriethoxysilane, and the content of 3-aminopropyltriethoxysilane is 3 wt%, and treat it at 60 °C for 30 minutes;

[0100] The conditions for the plasma pre-activation are: an argon / hydrogen volume ratio of 4:1, a power of 200 W, a gas pressure of 30 Pa, and a treatment time of 3 minutes;

[0101] S3, Pulse Modulation of SF 6 Plasma gradient film formation: Using a dual - frequency plasma source, in a dynamically adjusted SF 6 / Ar mixed atmosphere, through the pulse modulation of power and pressure, a gradient passivation film with an inner layer of Si - F bonds and an outer layer of Si - S bonds is formed layer by layer on the surface of the seed crystal;

[0102] The pulse modulation parameters in S3 include: The low - frequency 2MHz and high - frequency 13.56MHz of the dual - frequency plasma source act alternately, and the power density is modulated between 300W / cm 2 with a duty cycle of 60%;

[0103] SF 6 / Ar volume ratio is 1:5, and the dynamic pressure adjustment range is 50Pa;

[0104] During gradient film formation, the thickness of the inner Si - F bond dominant region is 2nm, and the thickness of the outer Si - S bond network is 3nm;

[0105] S4, Boron - doping Strengthening of the Sulfur - Fluorine Passivation Layer: Borane is introduced into the SF 6 plasma for in - situ boron doping, and a Si - S - B - F quaternary bonding structure is formed after high - temperature annealing;

[0106] The flow rate of the introduced borane is 6sccm, and the molar ratio to SF 6 is 1:20;

[0107] The annealing conditions are: nitrogen atmosphere, heating rate of 10℃ / s, holding at 900℃ for 10 minutes, and the crystallinity of the passivation layer ≥85%;

[0108] S5, Graphene Quantum Dot Encapsulation and Protection: In a supercritical CO 2 environment, carboxylated graphene quantum dots are self - assembled on the surface of the passivation layer and covalently fixed through a silane coupling agent;

[0109] The supercritical CO 2 conditions are a pressure of 20MPa and a temperature of 50℃;

[0110] The carboxylated graphene quantum dots have a particle size of 5nm and are covalently fixed on the surface of the passivation layer through trimethylchlorosilane to form Si - O - C covalent bonds.

[0111] Example 3:

[0112] Multi - level Oxide Etching and Nanopore Filling: The silicon carbide seed crystal is immersed in an ultrasonic treatment tank containing a mixed solution composed of nitric acid, hydrofluoric acid, ammonium fluoride, surfactant, and a nanodiamond suspension. Ultrasonic treatment removes the subsurface oxide layer and fills the surface micro - cracks;

[0113] The mixture contains: 13 wt% nitric acid, 8 wt% hydrofluoric acid, 3 wt% ammonium fluoride, 0.15 wt% sodium perfluorooctanesulfonate, and 0.1 wt% nanodiamond suspension with a particle size of 5 nm;

[0114] The ultrasonic treatment sequentially includes a low-frequency stage: 14 kHz, a temperature of 60 °C, and a treatment time of 5 minutes;

[0115] An intermediate-frequency transition stage: 90 kHz, the temperature linearly rising from 60 °C to 80 °C, and a treatment time of 3 minutes;

[0116] A high-frequency stage: 450 kHz, a temperature of 80 °C, and a treatment time of 5 minutes;

[0117] S2. Activation treatment of the thiofluorinated precursor: The surface of the treated seed crystal is subjected to amino-functionalization through an amino-silane solution, and then the surface dangling bonds are pre-activated in an argon / hydrogen mixed plasma;

[0118] The amino-functionalization treatment is specifically: immersing the seed crystal in an ethanol solution of 3-aminopropyltriethoxysilane, with 3-aminopropyltriethoxysilane at 1 wt%, and treating at 60 °C for 30 minutes;

[0119] The plasma pre-activation conditions are: an argon / hydrogen volume ratio of 2:1, a power of 100 W, a gas pressure of 20 Pa, and a treatment time of 3 minutes;

[0120] S3. Pulse-modulated SF 6 Plasma gradient film formation: Using a dual-frequency plasma source, in a dynamically adjusted SF 6 / Ar mixed atmosphere, through the pulse modulation of power and gas pressure, a gradient passivation film with an inner layer of Si-F bonds and an outer layer of Si-S bonds is formed layer by layer on the surface of the seed crystal;

[0121] The pulse modulation parameters in S3 include: the low frequency of 2 MHz and the high frequency of 13.56 MHz of the dual-frequency plasma source act alternately, and the power density is modulated between 90 W / cm 2 with a duty cycle of 40 - 60%;

[0122] SF 6 / Ar volume ratio of 1:1, and the dynamic adjustment range of the gas pressure is 10 Pa;

[0123] During gradient film formation, the thickness of the inner Si-F bond dominant region is 0.9 nm, and the thickness of the outer Si-S bond network is 1.8 nm;

[0124] S4. Boron doping enhancement of the sulfur-fluorine passivation layer: Diborane is introduced into the SF 6 plasma for in-situ boron doping, and a Si-S-B-F quaternary bonding structure is formed through high-temperature annealing;

[0125] The flow rate of diborane introduced is 2 sccm, and the molar ratio with SF 6 is 1:20;

[0126] The annealing conditions are: nitrogen atmosphere, heating rate of 10 °C / s, holding at 700 °C for 10 minutes, and the crystallinity of the passivation layer is ≥85%;

[0127] S5. Encapsulation and protection with graphene quantum dots: In a supercritical CO 2 environment, carboxylated graphene quantum dots are self-assembled on the surface of the passivation layer and covalently fixed through a silane coupling agent;

[0128] The supercritical CO 2 conditions are a pressure of 10 MPa and a temperature of 50 °C;

[0129] The carboxylated graphene quantum dots have a particle size of 3 nm and are covalently fixed on the surface of the passivation layer through the formation of Si-O-C covalent bonds by trimethylchlorosilane.

[0130] Example 4:

[0131] S1. Multi-stage oxide etching and nanopore filling: The silicon carbide seed crystal is immersed in an ultrasonic treatment tank containing a mixed solution composed of nitric acid, hydrofluoric acid, ammonium fluoride, a surfactant, and a nanodiamond suspension to remove the subsurface oxide layer by ultrasonic treatment and fill the surface microcracks;

[0132] The mixed solution contains: 13 wt% nitric acid, 8 wt% hydrofluoric acid, 3 wt% ammonium fluoride, 0.15 wt% sodium perfluorooctanesulfonate, and 0.1 wt% nanodiamond suspension with a particle size of 5 nm;

[0133] The ultrasonic treatment sequentially includes a low-frequency stage: 26 kHz, temperature of 60 °C, treatment for 5 minutes;

[0134] An intermediate-frequency transition stage: 400 kHz, temperature linearly rising from 60 °C to 80 °C, treatment for 3 minutes;

[0135] A high-frequency stage: 3 MHz, temperature of 80 °C, treatment for 5 minutes;

[0136] S2. Activation treatment of the thiofluorination precursor: The surface of the treated seed crystal is subjected to amino-functionalization through an amino-silane solution, and then the surface dangling bonds are pre-activated in an argon / hydrogen mixed plasma;

[0137] The amino-functionalization treatment is specifically: The seed crystal is immersed in an ethanol solution of 3-aminopropyltriethoxysilane, and 3-aminopropyltriethoxysilane is 1 wt%, and treated at 60 °C for 30 minutes;

[0138] The plasma pre-activation conditions are as follows: argon / hydrogen volume ratio of 5:1, power of 100 W, air pressure of 20 Pa, and treatment time of 3 minutes;

[0139] S3. Pulse modulation of SF 6 Plasma gradient film formation: Using a dual-frequency plasma source, in a dynamically adjusted SF 6 / Ar mixed atmosphere, through pulse modulation of power and air pressure, a gradient passivation film with an inner layer of Si-F bonds and an outer layer of Si-S bonds is formed layer by layer on the surface of the seed crystal;

[0140] The pulse modulation parameters in S3 include: the low frequency of 2 MHz and the high frequency of 13.56 MHz of the dual-frequency plasma source act alternately, and the power density is modulated between 400 W / cm 2 with a duty cycle of 40%;

[0141] SF 6 / Ar volume ratio of 1:6, and the dynamic adjustment range of air pressure is 10 Pa;

[0142] During gradient film formation, the thickness of the inner Si-F bond dominant region is 3 nm, and the thickness of the outer Si-S bond network is 4 nm;

[0143] S4. Boron doping enhancement of the sulfur-fluorine passivation layer: Diborane is introduced into the SF 6 plasma for in-situ boron doping, and a Si-S-B-F quaternary bonding structure is formed after high-temperature annealing;

[0144] The flow rate of the introduced diborane is 7 sccm, and the molar ratio to SF 6 is 1:20;

[0145] The annealing conditions are: nitrogen atmosphere, heating rate of 10 °C / s, holding at 700 - 900 °C for 10 minutes, and the crystallinity of the passivation layer is ≥85%;

[0146] S5. Graphene quantum dot encapsulation and protection: In a supercritical CO 2 environment, carboxylated graphene quantum dots are self-assembled on the surface of the passivation layer and covalently fixed through a silane coupling agent;

[0147] The supercritical CO 2 conditions are a pressure of 10 - 20 MPa and a temperature of 50 °C;

[0148] The particle size of the carboxylated graphene quantum dots is 3 nm, and Si-O-C covalent bonds are formed on the surface of the passivation layer through trimethylchlorosilane for fixation.

[0149] Comparative example: The seed crystal is treated with hydrofluoric acid and boiling water. S1. The seed crystal is immersed in a mixed solution of 3 - 8 wt% hydrofluoric acid and 5 - 10 wt% nitric acid for treatment, and after treatment, it is washed with deionized water and dried with nitrogen:

[0150] S2. Immerse the seed crystal in boiling deionized water, take it out and dry it after 10 - 30 minutes of treatment.

[0151] Perform index detection on the seed crystals obtained in the above Examples 1 - 4 and the comparative example, and make the obtained data into a table, as Figure 1 shown. It can be seen from the figure that compared with the traditional process, this method (within the defined data range) has greatly improved the removal efficiency of the subsurface oxide layer of the seed crystal, the thermal stability of the passivation layer, and the lifetime, and effectively reduces the surface roughness of the seed crystal, providing good conditions for the formation of high-quality seed crystals in the subsequent process.

[0152] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for growing silicon carbide crystals, characterized in that: The following steps are included: S1. Multi-level oxide layer etching and nano-pore filling: immersing the silicon carbide seed crystal in an ultrasonic treatment tank containing a mixture of nitric acid, hydrofluoric acid, ammonium fluoride, a surfactant and a nano-diamond suspension, and ultrasonically treating to remove the sub-surface oxide layer and fill the surface micro-cracks; S2, thiofluorinated precursor activation treatment: amino functionalization of the treated seed crystal surface by aminosilane solution, followed by pre-activation of surface dangling bonds in argon / hydrogen mixed plasma; S3, pulse modulation SF6 plasma gradient film formation: using a dual-frequency plasma source, in a dynamically adjusted SF6 / Ar mixed atmosphere, through pulse modulation of power and gas pressure, a gradient passivation film with Si-F bonds as the inner layer and Si-S bonds as the outer layer is generated layer by layer on the surface of the seed crystal; S4. Boron doping strengthening of sulfur-fluorine passivation layer: diborane is introduced into SF6 plasma for in-situ boron doping, and Si-SBF quaternary bonding structure is formed by high temperature annealing; S5. Graphene quantum dot encapsulation protection: Carboxyl graphene quantum dots are self-assembled on the surface of the passivation layer in a supercritical CO2 environment and covalently fixed by a silane coupling agent; S6. Paste the seed crystal obtained after S5 treatment on the top of the crucible, place silicon carbide powder on the bottom of the crucible, evacuate the inside of the crucible and maintain the vacuum, heat the inside of the crucible to 900-1100°C, fill the crucible with argon gas, heat the internal temperature of the crucible to 1800-2200°C and keep it warm to grow silicon carbide crystals. After the crystal growth is completed, cool it down and fill it with argon gas to normal pressure to obtain silicon carbide.

2. A method for growing silicon carbide crystals according to claim 1, characterized in that: The mixed solution in S1 comprises: 8-13wt% nitric acid, 3-8wt% hydrofluoric acid, 1-3wt% ammonium fluoride, 0.05-0.15wt% sodium perfluorooctane sulfonate, and 0.05-0.1wt% nanodiamond suspension with a particle size of 3-5nm; The ultrasonic treatment includes a low frequency stage of 15 to 25 kHz, a temperature of 60° C., and a treatment time of 5 minutes; Intermediate frequency transition stage: 100-300kHz, temperature linearly increased from 60°C to 80°C, treatment for 3 minutes; High frequency stage: 500kHz~2MHz, temperature 80℃, processing for 5 minutes.

3. A method for growing silicon carbide crystals according to claim 1, characterized in that: The amino functionalization treatment in S2 is specifically as follows: immersing the seed crystal in an ethanol solution of 3-aminopropyltriethoxysilane with a concentration of 1 to 3 wt % and treating at 60° C. for 30 minutes; The plasma pre-activation conditions are: argon / hydrogen volume ratio 3-4:1, power 100-200W, gas pressure 20-30Pa, and processing time 3 minutes.

4. A method for growing silicon carbide crystals according to claim 1, characterized in that: The pulse modulation parameters in S3 include: the low frequency 2MHz and high frequency 13.56MHz of the dual-frequency plasma source are alternately used, and the power density is 100-300W / cm 2 Inter-modulation, duty cycle 40-60%; The volume ratio of SF6 / Ar is 1:2~5, and the dynamic adjustment range of gas pressure is 10~50Pa; During gradient film formation, the thickness of the inner Si-F bond-dominated region is 1 to 2 nm, and the thickness of the outer Si-S bond network is 2 to 3 nm.

5. A method for growing silicon carbide crystals according to claim 1, characterized in that: In S4, the diborane flow rate is 2-6 sccm, and the molar ratio of diborane to SF6 is 1:20; Annealing conditions are: nitrogen atmosphere, heating rate 10°C / s, 700-900°C for 10 minutes, and passivation layer crystallinity ≥85%.

6. A method for growing silicon carbide crystals according to claim 1, characterized in that: The supercritical CO2 conditions are a pressure of 10-20 MPa and a temperature of 50°C; The carboxylated graphene quantum dots have a particle size of 3 to 5 nm and are fixed on the surface of the passivation layer by forming Si-OC covalent bonds with trimethylchlorosilane.

7. A method for growing silicon carbide crystals according to claim 1, characterized in that: In the step S5 , a vertical magnetic field is applied with a magnetic field strength of 0.5 to 1.5 T, so that the quantum dots are arranged along the magnetic field direction, and the aspect ratio of the quantum dots is controlled to be 3:1 to 5:1, and the spacing is 10 to 20 nm.

8. A method for growing silicon carbide crystals according to claim 7, characterized in that: The magnetic field intensity increases linearly from 0.5 T to 1.5 T at a rate of 0.2 T / min, inducing the quantum dots to be arranged in a gradient along the long axis direction.

9. A method for growing silicon carbide crystals according to claim 8, characterized in that: During the application of the perpendicular magnetic field, the temperature dropped from 50°C to 30°C, and the cooling rate was controlled at 0.5-1°C / min.

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