Method for preparing n-type SiC single crystal

Abstract

The present invention provides a method for preparing an n-type SiC single crystal, comprising the following steps: (1) placing a flux in a graphite crucible and fixing a SiC seed crystal on a lifting rod above the graphite crucible; (2) placing the graphite crucible in a growth furnace and then evacuating the growth furnace; (3) heating the graphite crucible until the flux is completely melted to form a melt, introducing a mixed gas containing nitrogen to control the gas pressure in the growth furnace and maintain a constant nitrogen partial pressure; (4) pushing down the lifting rod so that the seed crystal contacts the melt, thereby growing an n-type SiC single crystal. The method of the present invention can effectively grow high-quality, uniformly doped n-type SiC single crystals by a liquid phase method, and can achieve precise control of the nitrogen doping concentration over a wide range. At the same time, the method also has the advantages of low growth temperature, high growth rate, and low growth cost.

Description

Technical Field

[0001] The present invention belongs to the field of materials and crystal growth. Specifically, the present invention relates to a method for preparing an n-type SiC single crystal. Background Art

[0002] Silicon carbide (SiC) is a third-generation semiconductor material with advantages such as a wide bandgap, high breakdown field strength, high saturated electron mobility, high thermal conductivity, and excellent thermal stability. It is an ideal material for high-frequency, high-voltage, high-temperature, and high-power devices. Silicon carbide has significant application prospects in new energy vehicles, high-speed rail transit, aerospace, high-voltage smart grids, and clean energy, and has garnered widespread attention. SiC single crystal substrates are the fundamental material for manufacturing SiC-based devices. Their high crystalline quality and low manufacturing cost play a decisive role in promoting the large-scale application of SiC-based devices. Exploring and researching high-quality, large-scale, and low-cost SiC single crystal substrates is crucial for realizing the potential of SiC applications. In recent years, as more and more automakers incorporate SiC-based devices into new energy vehicles, the interest and demand for SiC substrates, particularly n-type SiC substrates with low resistivity and high crystalline quality, has rapidly increased in academia and industry.

[0003] At present, n-type SiC single crystal substrates grown by physical vapor transport (PVT) have achieved large-scale industrial production, but n-type SiC single crystals grown by PVT have a high dislocation density (generally 10 2 ~10 4 pieces / cm 2 ), low yield, and high prices. Compared to the currently well-established PVT method, the liquid phase method for growing SiC single crystals offers advantages such as lower growth temperatures (generally below 2000°C), a growth state close to thermodynamic equilibrium, and a stable growth environment. Growing n-type SiC single crystals using the liquid phase method can further improve the crystal quality, thereby enhancing the performance and lifespan of SiC devices.

[0004] Chinese patent CN 103210127 B discloses a method for preparing n-type SiC single crystals by liquid phase method. According to the description, the inventors first extract the air in the furnace chamber at high temperature (above 1100°C) to make the vacuum degree reach 1.0×10 -1Pa or less, and then a mixed gas containing nitrogen and rare gases is introduced into the furnace chamber to achieve liquid phase growth of n-type SiC single crystals in such an atmosphere. The method provided by the above patent has the following limitations: on the one hand, it does not take into account the solubility of nitrogen in the alloy melt. Since the crystal growth interface is completely covered by the alloy melt during the liquid phase growth of SiC single crystals, the nitrogen in the atmosphere can only be doped into the SiC crystal if it dissolves in the alloy melt. Otherwise, liquid phase growth of n-type SiC single crystals cannot be achieved; on the other hand, it does not take into account the uneven N doping caused by the continuous consumption of nitrogen in the furnace chamber during crystal growth. If there is no continuous nitrogen replenishment during the growth process, the N doping degree in the SiC crystal will be uneven.

[0005] In addition, during the liquid phase growth of SiC, the properties of the alloy melt (such as saturated carbon concentration, viscosity, surface tension, etc.) and elemental composition will have an important influence on the growth and doping of SiC crystals. Many studies have shown that adding Al to the flux can significantly optimize the stability of the crystal growth interface (see prior art MITANI T, KOMATSU N, TAKAHASHI T, et al. Effect of aluminum addition on the surface step morphology of 4H–SiC grownfrom Si–Cr–C solution[J]. Journal of Crystal Growth, 2015, 423(0):45-9; KOMATSUN, MITANI T, HAYASHI Y, et al. Modification of the surface morphology of 4H-SiCby addition of Sn and Al in solution growth with SiCr solvents[J]. Journal of Crystal Growth, 2017, 458(0):37-43; and SUZUKI K, TAISHI T. The effect of Aladdition to a Cr solvent without molten Si on the surface morphology in asolution growth of SiC[J]. Japanese Journal of Applied Physics, 2020, 59(025504): 1-6.). Therefore, the addition of Al is very important for the liquid phase growth of high-quality SiC single crystals. However, since the atomic radius of Al and Si is very close, Al is easily doped into the SiC lattice to form p-type doped SiC single crystals (SHIRAI T, DANNOK, SEKI A, et al. Solution growth of p-type 4H-SiC bulk crystals with low resistivity[J]. Materials Science Forum, 2014, 778-780(0): 75-8.).Although n-type SiC single crystals can also be grown by liquid phase method by co-doping with Al and N, studies have shown that this will increase the resistivity of n-type SiC crystals (MITANI T, KOMATSU N, TAKAHASHI T, et al. 4H-SiC Growth from Si-Cr-C Solution under Al and N Co-Doping Conditions [J]. Materials Science Forum, 2015, 821-823 (0): 9-13.). Therefore, the addition of Al is not conducive to the liquid phase growth of n-type SiC single crystals. However, if Al is simply removed, the crystallization quality of the crystal will be seriously affected, which brings difficulties to the liquid phase method for growing n-type SiC single crystals.

[0006] Therefore, there is an urgent need for a method that does not contain Al flux and can grow n-type SiC single crystals with low resistivity and high crystalline quality by liquid phase method. Summary of the Invention

[0007] The present invention aims to provide a method for growing n-type silicon carbide single crystals using a liquid phase process. This method can grow n-type SiC single crystals with high crystalline quality and low resistivity without the use of an Al flux. This method also ensures uniform doping during long-term growth and allows for wide-range control of the crystal's carrier concentration.

[0008] The above-mentioned object of the present invention is achieved through the following technical solutions.

[0009] The present invention provides a method for preparing an n-type SiC single crystal, comprising the following steps:

[0010] (1) placing flux in a graphite crucible and fixing a SiC seed crystal on a lifting rod above the graphite crucible;

[0011] (2) placing the graphite crucible in a growth furnace, and then evacuating the growth furnace;

[0012] (3) heating the graphite crucible until the flux is completely melted to form a melt, and introducing a mixed gas containing nitrogen to control the gas pressure in the growth furnace and maintain a constant nitrogen partial pressure;

[0013] (4) pushing down the lifting rod so that the seed crystal contacts the melt, thereby growing an n-type SiC single crystal;

[0014] Wherein, the flux is an alloy that does not contain Al;

[0015] The alloy consists of the elements Si, M, X and Z;

[0016] The element M is selected from one or more of Cr, Ti, Sc, Ce, Co, Ni and Ce;

[0017] The element X is selected from one or more of Fe, Co, Ni and Cu;

[0018] The element Z is selected from one or more of Ga, In, Sn, Ge, Y, La and Pr.

[0019] In the present invention, M is used to improve the carbon solubility of the flux to promote SiC crystal growth, X is used to improve the nitrogen solubility of the melt to increase the amount of nitrogen incorporated, and Z is used to regulate the solid-liquid interface energy and melt viscosity to improve the crystal growth interface.

[0020] In the present invention, an alloy that does not contain Al is used as a flux for liquid phase growth of n-type SiC single crystals, which can prevent the incorporation of Al into the SiC crystal. At the same time, the addition of elements with good nitrogen solubility, such as Fe, Co, Ni or Cu, is beneficial for obtaining low-resistivity n-type conductive SiC single crystals. The addition of Z, such as Ga, In, Sn, Ge, Y, La or Pr, is beneficial for regulating the solid-liquid interface energy and melt viscosity to improve the crystal growth interface.

[0021] In a specific embodiment of the present invention, the method provided herein can achieve liquid-phase growth of high-quality n-type SiC single crystals. Because this method utilizes an Al-free flux, the effects of Al doping on the electrical properties of the SiC single crystal are avoided, resulting in a lower resistivity for the grown n-type SiC. Furthermore, because the method ensures the long-term stability of the nitrogen partial pressure in the growth system, uniformly doped n-type SiC single crystals can be grown, and the doping concentration can be precisely controlled over a wide range.

[0022] The inventors of the present application unexpectedly discovered that when the flux does not contain Al, by adding X and Z to the flux and simultaneously maintaining the partial pressure of nitrogen at a constant state during the growth of the single crystal, the preparation of n-type SiC single crystals with high crystalline quality and low resistivity can be achieved.

[0023] Preferably, in the method of the present invention, the atomic molar ratio of Si, M, X and Z in the alloy is (10-70):(30-60):(0.5-10):(0.5-10).

[0024] In the embodiments of the present invention, the atomic molar ratios of Si, M, X, and Z in the alloy are preferably within the aforementioned preferred ranges. Excessively high Z content reduces the melt's carbon solubility, adversely affecting both crystal growth rate and quality, and easily leading to the formation of groove-like defects and inclusions in the crystals. Excessively low Z content leads to insufficient optimization of solid-liquid interface properties, unstable crystal growth interfaces, and the potential for unacceptable effects such as phase transformation and polycrystalline growth. Similarly, excessively high X content not only reduces the melt's carbon solubility but also results in unacceptably high N doping concentrations. Excessively low X content significantly hinders N incorporation.

[0025] Preferably, in the method of the present invention, the mixed gas is formed by mixing nitrogen with one or more gases selected from oxygen, hydrogen, helium and argon.

[0026] Preferably, in the method of the present invention, the volume of nitrogen in the mixed gas accounts for 10% to 99%, preferably 35% to 75%.

[0027] Preferably, in the method of the present invention, controlling the gas pressure in the growth furnace in step (3) is performed under the condition that the gas pressure in the growth furnace is controlled to be 0.1 to 2 atm.

[0028] In a specific embodiment of the present invention, maintaining a constant nitrogen partial pressure enables SiC crystals to grow in an atmosphere with a constant nitrogen partial pressure, ensures the uniformity of N doping during long-term crystal growth, and precisely controls the N doping concentration to achieve wide-range and precise control of the resistivity of n-type SiC single crystals.

[0029] In a specific embodiment of the present invention, the gas pressure can be kept constant by a nitrogen partial pressure control system, that is, while continuously introducing a nitrogen-containing mixed gas, the gas is continuously pumped out, and the dynamic stabilization of the gas pressure is achieved by controlling the pumping and releasing rate while ensuring a constant nitrogen partial pressure.

[0030] In a specific embodiment of the present invention, the partial pressure of nitrogen can be kept constant by a nitrogen partial pressure control system. The nitrogen partial pressure control system includes three parts: an inflation device, a pressure detection and feedback device, and an exhaust device. The inflation device includes a gas source, a gas flow meter, and a gas mixing chamber, and a mixed gas containing nitrogen is filled into the furnace cavity according to a set gas flow rate and mixing ratio. The pressure detection and feedback device can monitor the air pressure in the furnace cavity in real time and feed it back to the exhaust device. The exhaust device is a vacuum pump with adjustable exhaust speed, which can change the exhaust speed under the control of the pressure detection and feedback device to ensure that the air pressure in the furnace cavity is constant. The main working principle of the nitrogen partial pressure control system is as follows: nitrogen and other gases enter the mixing chamber through the gas flow meter at a set flow rate for full mixing, and then are filled into the furnace cavity from the top of the furnace cavity; the exhaust device continuously exhausts gas from the bottom of the furnace cavity; the pressure detection and feedback control device monitors the gas pressure in the furnace cavity in real time through a vacuum gauge, and converts the difference between the actual gas pressure and the set gas pressure into a feedback signal to adjust the exhaust speed of the exhaust device to ensure that the gas pressure in the furnace cavity always remains at the set gas pressure value; since the proportion of nitrogen in the mixed gas is fixed, the nitrogen partial pressure in the furnace cavity always remains unchanged.

[0031] Preferably, in the method of the present invention, the introduction of the mixed gas containing nitrogen in step (3) is carried out under the condition that the flow rate of the mixed gas is 10 to 200 sccm.

[0032] Preferably, in the method of the present invention, the step (2) of evacuating the growth furnace is to evacuate the growth furnace to a temperature less than or equal to 10 -3 Pa.

[0033] Preferably, in the method of the present invention, the SiC seed crystal is a SiC wafer with a bias angle of 0°, 4° or 8°.

[0034] Preferably, in the method of the present invention, the diameter of the SiC seed crystal is 2 to 8 inches.

[0035] Preferably, in the method of the present invention, the growing of the n-type SiC single crystal in step (4) is carried out by a method comprising the following steps:

[0036] (i) controlling the temperature during the growth of the n-type SiC single crystal so that the temperature at the seed crystal is 1700° C. to 2000° C., the melt gradually heats from the surface near the seed crystal to the bottom of the graphite crucible with a temperature gradient of 3 to 30° C. / cm, and the temperature of the melt at the bottom of the graphite crucible is 1800 to 2100° C.; and / or

[0037] (ii) The seed crystal and the graphite crucible are periodically accelerated and decelerated while the seed crystal is slowly pulled up.

[0038] Preferably, in the method of the present invention, the periodic acceleration and deceleration rotation is carried out under the following conditions: the graphite crucible and the seed crystal are periodically accelerated and decelerated in opposite directions, the rotation speed is ±0 to 200 r / min, and the rotation acceleration is ±0 to 30 r / min. 2 .

[0039] Preferably, in the method of the present invention, the pulling is performed at a rate of 1 to 3000 μm / h. In the present invention, it is important to ensure that the seed crystal and the melt do not separate during pulling to prevent interruption of crystal growth.

[0040] Preferably, in the method described in the present invention, the inner wall of the graphite crucible is porous, honeycomb-shaped or multi-grooved to increase the contact area between the melt and the graphite crucible, greatly improve the dissolution rate of C by the melt, and improve the growth rate and quality of the crystal.

[0041] Preferably, in the method of the present invention, the inner diameter of the graphite crucible is 10 to 100 mm larger than the diameter of the seed crystal.

[0042] The present invention has the following beneficial effects:

[0043] The method of the present invention can effectively grow high-quality, uniformly doped n-type SiC single crystals through a liquid phase method, and can achieve precise control of the N doping concentration over a wide range. Furthermore, the method has the advantages of low growth temperature, high growth rate, and low growth cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which:

[0045] Figure 1 Schematic diagram of a device for growing n-type SiC single crystals by a liquid phase method according to a specific embodiment of the present invention;

[0046] Figure 2 A physical photograph of the n-type SiC crystal grown in Example 1 and its X-ray rocking curve test results;

[0047] Figure 3 The Hall effect test results of the n-type SiC crystal grown in Example 1 are reported;

[0048] Figure 4 The Hall effect test results of the n-type SiC crystal grown in Example 2 are reported;

[0049] Figure 5 This is a physical photo of the n-type SiC crystal grown in Example 3; Figure 5 The surface of the crystal becomes brighter.

[0050] Figure 6 The following are photos of the microscopic morphology of the growth surface of the crystal grown in Comparative Example 1 and the macroscopic morphology of the grown crystal;

[0051] Figure 7 The microscopic morphology of the growth surface of the crystal grown in Comparative Example 2;

[0052] Figure 8 This is a macroscopic optical photograph of the crystal grown in Comparative Example 3; Figure 8 It shows that the crystal surface has not grown completely due to insufficient solute supply;

[0053] Reference numerals:

[0054] 1-Gas flow meter; 2-Gas valve; 3-Cavity housing; 4-Graphite shaft connected to seed crystal; 5-Graphite crucible; 6-Induction coil; 7-Graphite support for bonding SiC seed crystal; 8-SiC seed crystal; 9-Al-free flux; 10-Thermal insulation material; 11-Crucible tray; 12-Crucible support shaft; 13-Vacuum gauge. DETAILED DESCRIPTION

[0055] The present invention will be further described in detail below in conjunction with specific embodiments. The examples given are only for illustrating the present invention, not for limiting the scope of the present invention.

[0056] The following is combined with Figure 1 , the method for growing silicon carbide single crystal by liquid phase method in the present invention is described and explained in detail. Figure 1 The heating method in the schematic diagram shown is induction heating, but the heating method is not limited to this. Other heating methods such as resistance heating can also be selected in combination with specific processes.

[0057] Example 1

[0058] The inner diameter of the graphite crucible used in this embodiment is 130 mm, the height is 150 mm, and the wall thickness is 20 mm. The seed crystal used is a 4-inch n-type SiC single crystal crystal with a 4° deflection angle. The flux used is Si 60 Cr 30 Fe8Sn2. The total amount of flux is controlled to make the melt thickness 30mm.

[0059] First, the four raw materials were mixed evenly in an atomic molar ratio of Si:Cr:Fe:Sn = 60:30:8:2, placed in a graphite crucible and compacted. Then, the crucible was placed in a single crystal growth furnace, and the seed crystal was fixed on the seed crystal rod and connected to the upper rotating pull rod of the single crystal growth furnace. After closing the furnace chamber, the furnace chamber was pumped to 10 -5Pa. The nitrogen partial pressure control system is then turned on to maintain the gas pressure in the furnace chamber at 0.1atm, where the composition of the mixed gas in the furnace chamber is 60% argon and 40% nitrogen, and the gas flow rate of the mixed gas is set to 100sccm. The crucible is then heated, and the temperature field is controlled so that the temperature of the seed crystal at the melt surface is lower than the temperature at the bottom of the crucible, the axial temperature gradient is 4°C / cm, and the temperature at the seed crystal is 1800°C. During the growth process, the seed crystal rotates clockwise at a speed of 30r / min, and the crucible rotates counterclockwise at a speed of 5r / min. The seed crystal is pulled upward at a rate of 60μm / h. After the crystal has grown for 60h, the seed crystal is pulled upward at a speed of 10mm / h, so that it is completely disconnected from the liquid surface and then slowly cooled to room temperature, and finally the crystal is taken out.

[0060] In this embodiment, the addition of Fe element promotes the doping of N element, and the addition of Sn element improves the stability of the crystal growth interface, effectively growing n-type SiC crystal. The actual optical photograph and X-ray rocking curve test results of the crystal are shown in Figure 2 . Figure 2 The half-height width of the crystal X-ray rocking curve is 21.6 arcsec, indicating that the crystal quality is very high. The Hall test results of the crystal are shown in Figure 3 The results show that its Hall coefficient is -0.000898m 3 / C, proving that the carrier type of the grown SiC single crystal is n-type, and its carrier concentration is 7.719×10 18 cm -3 The resistivity of the crystal was 0.0054 Ω·cm. The crystal was sectioned longitudinally, and SIMS analysis was performed to characterize the longitudinal distribution of the N concentration. The results showed a uniform distribution of N concentration in the vertical direction, indicating that the N doping concentration does not change with crystal growth. This example demonstrates that the method provided by the present invention can achieve liquid-phase growth of high-quality n-type SiC single crystals and ensure long-term stability of the N doping concentration.

[0061] Example 2

[0062] In this embodiment, the pressure of the mixed gas is increased to 0.2 atm, and the ratio of argon to nitrogen is changed to 30% argon and 70% nitrogen. Other steps are the same as in embodiment 1.

[0063] The crystals grown in this embodiment have smooth surfaces and high crystal quality. Figure 4 This is the Hall test result of the crystal, and the result shows that its Hall coefficient is -0.000313m 3 / C, proving that the carrier type of the crystal is n-type, and the carrier concentration is increased to 2.497×10 19 cm -3, the resistivity is reduced to 0.0048Ω·cm. This embodiment shows that the method provided by the present invention can achieve regulation of the N doping concentration and resistivity of the crystal by changing the nitrogen partial pressure.

[0064] Example 3

[0065] In this embodiment, the composition of the flux melt is changed to: Si 60 Cr 30 Fe6Ge2Sn2, the seed crystal used is a 2-inch n-type SiC single crystal crystal with no bias angle, and the other steps are the same as Example 1.

[0066] The optical photograph of the crystal grown in this example is shown in Figure 5 Since Ge has a low melting point and viscosity, the viscosity of the melt is significantly reduced after adding Ge at the same growth temperature, which is more conducive to the diffusion of the solute in the melt. The surface of the grown crystal is very bright and the crystal quality is very high.

[0067] Example 4

[0068] In this embodiment, the temperature gradient in the melt is increased to 10° C. / cm, and the pulling speed of the seed crystal is increased to 120 μm / h. Other steps are the same as in Example 1.

[0069] In this example, by increasing the temperature gradient in the melt and the seed crystal pulling speed, the transport rate of C in the melt was accelerated due to the increased temperature gradient. Simultaneously, the faster pulling speed increased the heat dissipation rate of the seed crystal, accelerating the precipitation rate of SiC on the seed crystal. Consequently, the crystal growth rate increased by 1.5 times compared to Example 3.

[0070] Example 5

[0071] In this embodiment, the side wall of the crucible is processed into a serrated shape, and the inner bottom surface of the crucible is processed into a porous shape. The other steps are the same as those in Example 3.

[0072] In this embodiment, the serrated structure of the crucible side wall and the porous structure of the bottom increase the contact area between the melt and the graphite crucible, accelerate the dissolution rate of C by the melt, and improve the growth rate and quality of the crystal.

[0073] Example 6

[0074] In this embodiment, the rotation mode of the seed crystal is changed to an accelerated forward and reverse periodic rotation mode, with a maximum forward and reverse rotation speed of ±120r / min and a rotation acceleration of ±12r / min. 2 , the other steps are the same as those in Example 3.

[0075] In this embodiment, by changing the seed crystal's rotational pattern, the acceleration, deceleration, and forward and reverse rotations, on the one hand, increased the melt's convection intensity and enhanced solute transport; on the other hand, the high rotational speed reduced the thickness of the solute boundary layer at the growth interface, making the crystal growth interface more stable. Consequently, both the crystal growth rate and crystal quality were improved.

[0076] Comparative Example 1

[0077] The flux used in this comparative example is composed of Si 62 Cr 30 Fe8, other conditions remain the same as in Example 1, and crystal growth comparison is performed.

[0078] In this comparative example, since the flux does not contain the element for improving the crystal growth interface (i.e., Z in the element composition of the present invention), the crystal growth interface is very unstable. Figure 6 ) It can be clearly seen that the step flow on the crystal growth surface produces serious coalescence phenomenon, and there are many small cones on the step surface caused by two-dimensional nucleation, which is very unfavorable to the crystal quality and stability of the crystal form. Figure 6 (b) shows the macroscopic morphology of the crystal grown in this comparative example. It can be seen that the crystal surface is very rough and has many polycrystalline particles, indicating that the crystal quality is very poor.

[0079] Comparative Example 2

[0080] The flux used in this comparative example is composed of Si 62 Cr 33 Sn5, other conditions are consistent with Example 1, and crystal growth comparison is performed.

[0081] In this comparative example, since the flux does not contain any element that can improve the melt's ability to dissolve nitrogen (i.e., X in the elemental composition of the present invention), it is difficult for nitrogen in the atmosphere to dissolve into the melt, limiting the doping of nitrogen in the crystal. The Hall test shows that the carrier concentration of the crystal is low, at 2.497×10 17 cm -3 , the resistivity is relatively high, 0.2Ω·cm. From the microscopic morphology image of the crystal growth surface ( Figure 7 ) It can be clearly seen that the step flow distribution on the crystal growth surface is very uniform, and there is no two-dimensional nucleation. This is because the addition of Sn element improves the stability of the crystal growth interface.

[0082] Comparative Example 3

[0083] The flux used in this comparative example is composed of Si 60 Cr 10 Fe 15 Sn 15, other conditions remain the same as in Example 1, and a crystal growth comparison is performed.

[0084] The optical photograph of the crystal grown in this comparative example is shown in Figure 8 , it can be seen that due to insufficient solute supply, the crystal does not grow a complete plane. This is because the excessive addition of Fe (i.e., X in the elemental composition described in the present invention) to the flux to improve the melt's ability to dissolve nitrogen and Sn (i.e., Z in the elemental composition described in the present invention) to improve interfacial stability has significantly reduced the content of Cr (i.e., M in the elemental composition described in the present invention) in the flux to improve the melt's ability to dissolve carbon, thereby significantly reducing the carbon concentration in the melt, severely limiting the growth rate and quality of the crystal.

[0085] Comparative Example 4

[0086] In this comparative example, the furnace chamber was evacuated to 1×10 -5 After Pa, the furnace chamber was filled with a mixed gas of 0.1 atm, the mixed gas composition of 60% argon and 40% nitrogen. During the growth process, no nitrogen partial pressure control device was used, the furnace chamber was in a completely closed state, and the other crystal growth processes were consistent with those in Example 1. A crystal growth comparison was performed.

[0087] The crystals grown in this comparative example were cut longitudinally, and SIMS (Secondary Ion Mass Spectrometry) tests and analyses were performed on the early, middle, and late stages of crystal growth. The results showed that the N content in the early, middle, and late stages of the crystals was significantly different, and the content gradually decreased. The test results were as follows: the N concentration in the early stage was 8.96×10 18 atoms / cm 3 The N concentration in the mid-term was 1.35×10 18 atoms / cm 3 , the N concentration in the later stage was 3.25×10 17 atoms / cm 3 This is because without a nitrogen partial pressure control device, continuous nitrogen replenishment cannot be achieved. Therefore, as the crystal grows, the nitrogen content in the furnace chamber will gradually decrease due to doping consumption, so the doping concentration of the N element in the crystal will also gradually decrease, resulting in uneven doping concentration.

[0088] It should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit them. While specific process parameters can be optimized and adjusted, the two core concepts of the present invention and the basic structure of the growth apparatus are clear. Those skilled in the relevant art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and are intended to be encompassed by the claims of the present invention.

Claims

1. A method for preparing an n-type SiC single crystal, comprising the following steps: (1) Place the flux in a graphite crucible and fix the SiC seed crystal on the lifting rod above the graphite crucible; (2) placing the graphite crucible in a growth furnace, and then evacuating the growth furnace; (3) heating the graphite crucible until the flux is completely melted to form a melt, and introducing a mixed gas containing nitrogen to control the gas pressure in the growth furnace and maintain a constant nitrogen partial pressure; (4) pushing down the lifting rod so that the seed crystal contacts the melt, thereby growing an n-type SiC single crystal; Wherein, the flux is an alloy that does not contain Al; The alloy consists of the elements Si, M, X and Z; The element M is selected from one or more of Cr, Ti, Sc, Ce, Co, Ni and Ce; The element X is selected from one or more of Fe, Co, Ni and Cu; The element Z is selected from one or more of Ga, In, Sn, Ge, Y, La and Pr; The atomic molar ratio of Si, M, X and Z in the alloy is (10-70):(30-60):(0.5-10):(0.5-10); The mixed gas is formed by mixing nitrogen with one or more gases selected from oxygen, hydrogen, helium and argon; The volume of nitrogen in the mixed gas accounts for 35% to 75%.

2. The method according to claim 1, wherein The step (3) of controlling the gas pressure in the growth furnace is performed under the condition that the gas pressure in the growth furnace is controlled to be 0.1 to 2 atm.

3. The method according to claim 1, wherein The introduction of the mixed gas containing nitrogen in step (3) is carried out under the condition that the flow rate of the mixed gas is 10 to 200 sccm.

4. The method according to claim 1, wherein The step (2) of evacuating the growth furnace is to evacuate the growth furnace to a temperature less than or equal to 10 -3 Pa.

5. The method according to claim 1, wherein The SiC seed crystal is a SiC wafer with a bias angle of 0°, 4° or 8°.

6. The method according to claim 1, wherein The SiC seed crystal has a diameter of 2 to 8 inches.

7. The method according to claim 1, wherein The growth of the n-type SiC single crystal in step (4) is carried out by a method comprising the following steps: (i) controlling the temperature during the growth of the n-type SiC single crystal so that the temperature at the seed crystal during the growth of the single crystal is between 1700°C and 2000°C, the melt gradually heats from the surface near the seed crystal to the bottom of the graphite crucible with a temperature gradient of 3°C / cm to 30°C / cm, and the temperature of the melt at the bottom of the graphite crucible is between 1800°C and 2100°C; and / or (ii) The seed crystal and the graphite crucible are periodically accelerated and decelerated while the seed crystal is slowly pulled up.

8. The method according to claim 7, wherein: The periodic acceleration and deceleration rotation is carried out under the following conditions: the graphite crucible and the seed crystal are periodically accelerated and decelerated in opposite directions, the rotation speed is ±0 to 200 r / min, and the rotation acceleration is ±0 to 30 r / min 2 .

9. The method according to claim 7, wherein: The pulling is performed at a rate of 1 to 3000 μm / h.

10. The method according to claim 1, wherein The inner wall of the graphite crucible is porous, honeycomb-shaped or multi-grooved.

11. The method according to claim 1, wherein The inner diameter of the graphite crucible is 10 to 100 mm larger than the diameter of the seed crystal.

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