Method for preparing n-type 4H-SiC single crystal

Abstract

The present invention provides a method for preparing n-type 4H-SiC single crystals, which sequentially includes the following steps: (1) placing a metal raw material containing Si, Al, and a transition metal in a graphite crucible, and fixing a SiC seed crystal on a graphite seed crystal rod; (2) placing the graphite crucible in a growth furnace, and then evacuating the growth furnace; (3) introducing a gas and controlling the air pressure in the growth furnace; (4) heating the graphite crucible so that the metal raw material is completely melted to form a melt; (5) lowering the seed crystal so that the seed crystal contacts the melt, thereby growing an n-type 4H-SiC single crystal; wherein, the transition metal is selected from one or more of Cr, Sc, V, Mn, and Cu. The method of the present invention can effectively reduce the interfacial energy between 4H-SiC and the melt, stabilize the 4H crystal form, improve the crystallization quality, and can greatly increase the crystal growth rate and reduce the cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquid-phase growth of SiC single crystals. Specifically, the present invention relates to a method for preparing n-type 4H-SiC single crystals. Background Art

[0002] Silicon carbide (SiC) is one of the wide-bandgap semiconductor materials that have received extensive attention. It has the advantages of low density, large bandgap width (at room temperature, the bandgap of 4H-SiC is 3.2 eV), high breakdown field strength (about 10 times that of Si), high saturated electron mobility (about 2 times that of Si), high thermal conductivity (3 times that of Si, 10 times that of GaAs), and good chemical stability. It is an ideal substrate material for making high-frequency, high-voltage, high-power devices and blue light-emitting diodes. In recent years, with the gradual increase in the application fields of SiC-based devices, the attention and demand in the academic and industrial circles for SiC substrates, especially n-type conductive SiC substrates with low resistivity and high crystal quality, are increasing rapidly.

[0003] The current main growth method of SiC is physical vapor transport method, but it has problems such as high defect density, difficult diameter expansion and p-type doping. The liquid-phase method has a low growth temperature, a relatively stable growth environment, a growth process close to thermodynamic equilibrium conditions, good crystal quality, and has good prospects in terms of diameter expansion and p-type doping, and has received widespread attention in the academic and industrial circles in recent years.

[0004] Chinese Patent Application CN115821362A discloses a method for preparing n-type SiC single crystals by the liquid-phase method. According to the description, the inventor selects a raw material formula as a mixture of four elemental substances, Si, M, X, and Z, with an atomic molar ratio of (10-70):(30-60):(0.5-10):(0.5-10), and the raw materials do not contain Al. After mixing the elemental raw materials evenly, they are placed in a graphite crucible and compacted. The crucible is placed in a single-crystal growth furnace, the seed crystal is fixed on the seed crystal rod, and the furnace cavity is pumped to 10 -5 Pa by a mechanical pump and a molecular pump. Then, the nitrogen partial pressure control system is turned on to keep the air pressure in the furnace cavity at a certain level. The crucible is heated to make the temperature of the seed crystal at the melt liquid surface lower than the temperature at the bottom of the crucible. During the growth process, the seed crystal and the crucible rotate at a certain speed, and the seed crystal is pulled up at a certain speed. After the growth is completed, the seed crystal is pulled up to completely disconnect it from the liquid surface and then slowly cooled to room temperature, and finally the crystal is taken out. The method provided by the above patent has the following limitations: According to the description of the inventor, the thickness of the ingot obtained after 60 hours of growth is about 3-6 mm, and the growth rate is 50-100 μm / h. This prior art has the disadvantages of low single-crystal growth rate, poor crystal growth efficiency, and high cost.

[0005] In addition, during the growth of n-type 4H-SiC by the liquid phase method, N enters the lattice as a doping element to provide electron carriers, and nitrogen gas needs to be introduced into the growth system. However, the addition of nitrogen gas will change the properties of the melt. On the one hand, N atoms will combine with metal atoms to form clusters and even some refractory compounds, increasing the viscosity of the melt and reducing the convection and diffusion rates of solutes in the melt. This will bring a series of problems, such as the instability of the growth interface resulting in cellular growth and inclusion defects, the generation of polytypes, and the reduction of the crystal growth rate. On the other hand, when nitrogen dissolves into the melt, it will change the surface tension of the melt and the solid-liquid interfacial energy between the melt and SiC, thereby affecting the stability of 4H-SiC and easily causing polytype transformation. Therefore, the properties and elemental composition of the alloy melt have an important impact on the stable growth of SiC crystals.

[0006] Many studies have shown that adding Al to the metal raw material can optimize the stability of the crystal growth interface (see the prior art MITANI T, KOMATSU N, TAKAHASHI T, et al. Effect of aluminum addition on the surface step morphology of 4H-SiC grown from Si-Cr-C solution [J]. Journal of Crystal Growth, 2015, 423(0):45-9; KOMATSU N, MITANI T, HAYASHI Y, et al. Modification of the surface morphology of 4H-SiC by 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 Al addition to a Cr solvent without molten Si on the surface morphology in a solution growth of SiC [J]. Japanese Journal of Applied Physics, 2020, 59(025504):1-6.). However, since the atomic radii of Al and Si are very close, Al is easily doped into the SiC lattice to form a p-type doped SiC single crystal (SHIRAI T, DANNO K, 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.). Therefore, the addition amount of Al cannot be too much. Some researchers have grown n-type SiC single crystals by liquid phase method through co-doping of Al and N, but the results show that this will increase the resistivity of the n-type SiC crystal, and with the increase of the doping concentration, the crystal quality decreases.(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.)

[0007] Therefore, there is an urgent need for a method for growing n-type 4H-SiC crystals by a liquid-phase method, which can effectively stabilize the 4H crystal form, improve the crystallization quality, and can greatly increase the crystal growth rate and reduce the cost. Summary of the Invention

[0008] The object of the present invention is to provide a method for preparing n-type 4H-SiC single crystals, which can solve the problems existing in the prior art. Specifically, this method can effectively stabilize the 4H crystal form, improve the crystallization quality, and can greatly increase the crystal growth rate and reduce the cost.

[0009] The above object of the present invention is achieved by the following technical solutions.

[0010] The present invention provides a method for preparing n-type 4H-SiC single crystals, which successively includes the following steps:

[0011] (1) Place a metal raw material containing Si, Al, and a transition metal in a graphite crucible, and fix a SiC seed crystal on a graphite seed crystal rod;

[0012] (2) Place the graphite crucible in a growth furnace, and then evacuate the growth furnace;

[0013] (3) Introduce a gas and control the air pressure in the growth furnace;

[0014] (4) Heat the graphite crucible to completely melt the metal raw material to form a melt;

[0015] (5) Lower the seed crystal so that the seed crystal contacts the melt, and then grow an n-type 4H-SiC single crystal;

[0016] Wherein, the transition metal is selected from one or more of Cr, Sc, V, Mn, and Cu.

[0017] The inventors of the present application unexpectedly found that when the metal raw material of the present invention contains Si, Al, and the transition metal claimed by the present invention at the same time, and the mixed gas contains nitrogen, an n-type 4H-SiC single crystal can be prepared. If the type of the transition metal is changed, the n-type 4H-SiC single crystal claimed by the present invention cannot be obtained.

[0018] Without wishing to be bound by theory, this combination of the present invention (i.e., the metal raw material contains Si, Al and the transition metal claimed in the present invention at the same time, and the mixed gas contains nitrogen) can improve the growth interface morphology, reduce the interface energy between 4H-SiC and the melt, stabilize the 4H crystal form, and improve the growth rate and crystal quality.

[0019] In the present invention, a smaller addition amount of Al combined with a transition metal element with stronger nitrogen dissolution ability can increase the nitrogen concentration in the melt and improve the doping amount of nitrogen. It weakens the influence of Al doping on the electrical properties of SiC single crystals, and the resistivity of the grown n-type SiC is lower.

[0020] The inventors of the present application also unexpectedly found that when a small amount of Al is added to the metal raw material, by combining some other transition metal elements and optionally rare earth elements as raw materials, the interface energy between the growth surface of 4H-SiC and the melt can be significantly reduced, while increasing the interface energy between other common polytypes (such as 3C-SiC and 6H-SiC, etc.) and the melt, thereby stabilizing the 4H crystal form, preventing crystal form transformation, and improving crystal quality. However, the content of Al should be within the range described in the present invention. If the addition amount of Al is too small, the effect of reducing the interface energy between the growth surface of 4H-SiC and the melt after combination with other transition metal elements and rare earth elements is not significant, and the 4H-SiC crystal form cannot be stabilized; if the addition amount of Al is too large, an n-type conductive 4H-SiC crystal cannot be obtained.

[0021] In the present invention, refractory compounds with a melting point higher than the SiC single crystal growth temperature are not formed between the metals in the metal raw material.

[0022] Preferably, in the method described in the present invention, the atomic molar ratio of Si, Al and the transition metal in the metal raw material is Si:Al:transition metal = (30 - 70):(0.5 - 4):(30 - 60).

[0023] Preferably, in the method described in the present invention, the metal raw material further contains rare earth metals.

[0024] Preferably, in the method described in the present invention, the rare earth metal is selected from one or more of La, Ce, Pr and Nd.

[0025] Preferably, in the method described in the present invention, the atomic molar ratio of Si, Al, the transition metal and the rare earth metal in the metal raw material is Si:Al:transition metal:rare earth metal = (30 - 70):(0.5 - 4):(30 - 60):(0.01 - 20).

[0026] Preferably, in the method described in the present invention, the gas consists of nitrogen, or is composed of nitrogen mixed with one or two of argon and helium.

[0027] Preferably, in the method of the present invention, the partial pressure of nitrogen in the gas is 10% to 100% of the total pressure, preferably 30% to 60%, and the total pressure is 0.1 to 2 atm, preferably 0.2 to 1.2 atm.

[0028] Preferably, in the method of the present invention, the SiC seed crystal is a 2-6 inch SiC wafer with a 0°, 4°, or 8° angle deviation.

[0029] Preferably, in the method of the present invention, the SiC seed crystal is a semi-insulating SiC single crystal substrate or an n-type SiC single crystal substrate.

[0030] Preferably, in the method of the present invention, the graphite crucible is made of high-purity graphite with a purity greater than or equal to 99.95%.

[0031] Preferably, in the method of the present invention, the inner diameter of the graphite crucible is 10 to 150 mm larger than the diameter of the SiC seed crystal.

[0032] Preferably, in the method of the present invention, the wall thickness of the graphite crucible is greater than or equal to 10 mm.

[0033] Preferably, in the method of the present invention, the density of the graphite crucible is 1.7 to 2.0 g / cm 3 .

[0034] Preferably, in the method of the present invention, evacuating the growth furnace in step (2) evacuates the growth furnace to less than or equal to 5×10 -4 Pa.

[0035] Preferably, in the method of the present invention, growing the n-type 4H-SiC single crystal in step (5) is carried out by a method including the following steps:

[0036] (i) Controlling the temperature at the seed crystal to be 1700°C to 1900°C, gradually increasing the temperature of the melt from the surface near the SiC seed crystal to the bottom of the graphite crucible with a temperature gradient of 2 to 20°C / cm, and the temperature of the melt at the bottom of the graphite crucible being 1850°C to 2050°C;

[0037] (ii) Periodically accelerating and decelerating the rotation of the SiC seed crystal and the graphite crucible, while slowly pulling up the SiC seed crystal.

[0038] Preferably, in the method of the present invention, the periodic acceleration and deceleration rotation are carried out under the following conditions: the SiC seed crystal and the graphite crucible rotate periodically in opposite directions, the rotation speed is ±0 to 300 r / min, and the rotational acceleration is ±0 to 40 r / min 2 .

[0039] Preferably, in the method of the present invention, the pulling is carried out at a rate of 1 to 2000 μm / h.

[0040] The method of the present invention ensures the long-term stability of the nitrogen partial pressure in the growth system, can grow n-type SiC single crystals with uniform doping, and can also precisely control the doping concentration within a wide range.

[0041] In a specific embodiment of the present invention, keeping the partial pressure of nitrogen constant can achieve the growth of SiC crystals in an atmosphere with a constant nitrogen partial pressure, ensure the uniformity of N doping during the long-term growth of the crystals, and can precisely control the doping concentration of N to achieve precise control of the resistivity of n-type 4H-SiC single crystals within a wide range.

[0042] In a specific embodiment of the present invention, the nitrogen partial pressure control system includes three parts: an inflation device, a pressure detection and feedback device, and an air extraction device. The inflation device includes a gas source, a gas flow meter, and a gas mixing chamber. The mixed gas containing nitrogen is filled into the furnace cavity according to the set gas flow rate and mixing ratio. The pressure detection and feedback device can monitor the pressure in the furnace cavity in real time and feedback it to the air extraction device. The air extraction device adjusts the pumping speed in different ways (butterfly valve, needle valve, frequency converter, etc.) to ensure that the 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 gas mixing chamber through the gas flow meter according to the set flow rates respectively for full mixing, and then are filled into the furnace cavity from the top of the furnace cavity; the air extraction device continuously extracts air from the bottom of the furnace cavity; the pressure detection and feedback control device monitors the pressure in the furnace cavity in real time through a vacuum gauge and adjusts the pumping speed of the air extraction device to ensure that the pressure in the furnace cavity always remains at the set pressure value; since the proportion of nitrogen in the mixed gas remains fixed, the nitrogen partial pressure in the furnace cavity always remains unchanged.

[0043] The present invention has the following beneficial effects:

[0044] The method of the present invention can effectively reduce the interfacial energy between 4H-SiC and the melt, stabilize the 4H crystal form, improve the crystallization quality of growing n-type 4H-SiC crystals by the liquid phase method, and can greatly increase the crystal growth rate and reduce costs. Description of the Drawings

[0045] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings, wherein:

[0046] Figure 1 Schematic diagram of a device showing a specific embodiment of the present invention;

[0047] Figure 2 Flow chart of a method for improving the growth rate and quality of n-type 4H-SiC single crystals provided by a specific embodiment of the present invention;

[0048] Figure 3 Photo of the n-type 4H-SiC crystal grown in Example 1;

[0049] Figure 4 Microscopic morphology of the growth surface of the crystal grown in Example 1;

[0050] Figure 5 X-ray rocking curve test results and Raman test results of the crystal grown in Example 1;

[0051] Figure 6 Hall test and resistivity test results of the crystal grown in Example 1;

[0052] Figure 7 High-temperature in-situ contact angle test results of the metal raw materials used in Example 1 with different crystal forms of SiC;

[0053] Figure 8 Photo of the n-type 4H-SiC crystal grown in Example 2;

[0054] Figure 9 Photo of the n-type 4H-SiC crystal grown in Example 3;

[0055] Figure 10 Photo of the n-type 4H-SiC crystal grown in Example 4;

[0056] Figure 11 Photo of the n-type 4H-SiC crystal grown in Example 5;

[0057] Figure 12 Photo of the SiC crystal grown in Comparative Example 1;

[0058] Figure 13 Microscopic morphology of the growth surface of the crystal grown in Comparative Example 1;

[0059] Figure 14 X-ray rocking curve test results and Raman test results of the crystal grown in Comparative Example 1;

[0060] Figure 15 High-temperature in-situ contact angle test results of the metal raw materials used in Comparative Example 1 with different crystal forms of SiC;

[0061] Figure 16Photograph showing the SiC crystal grown in Comparative Example 2;

[0062] Figure 17 Test results of the high-temperature in-situ contact angle of the metal raw material used in Comparative Example 2 with different crystal forms of SiC are shown;

[0063] Figure 18 Photograph showing the SiC crystal grown in Comparative Example 3;

[0064] Figure 19 Photograph showing the SiC crystal grown in Comparative Example 4;

[0065] Figure 20 Data summary of the raw material composition, growth rate, polytype condition, rocking curve full width at half maximum, Hall coefficient, carrier concentration, and average resistivity of the SiC crystals grown in each example and comparative example is shown;

[0066] Among them, reference numerals:

[0067] 1 - seed crystal axis; 2 - seed crystal rod; 3 - heat insulation material; 4 - graphite crucible; 5 - metal raw material containing Si and Al; 6 - induction coil; 7 - crucible tray; 8 - air extraction port; 9 - air extraction device (such as a mechanical pump); 10 - air pressure detection and feedback device; 11 - gas charging device; 12 - air inlet; 13 - seed crystal holder; 14 - seed crystal; 15 - single crystal growth furnace cavity. Detailed Description of the Invention

[0068] The present invention will be further described in detail below in conjunction with the specific embodiments. The examples given are only for clarifying the present invention and not for limiting the scope of the present invention.

[0069] Refer to Figure 1 and Figure 2 , the method for growing silicon carbide single crystals by the liquid phase method in the present invention will be described and explained in detail. It should be noted that although Figure 1 the heating method shown in the schematic diagram is induction heating, other heating methods such as resistance heating can also be selected in combination with the specific process.

[0070] Example 1

[0071] In this example, the inner diameter of the graphite crucible used is 150 mm, the height is 200 mm, and the wall thickness is 15 mm. The seed crystal used is a 4-inch n-type 4H-SiC single crystal wafer with a 0° tilt angle. The metal raw material used is Si 60 Cr 35 Al3Nd2.

[0072] First, the elemental particles of the four raw materials are mixed evenly at an atomic molar ratio of Si:Cr:Al:Nd = 60:35:3:2 and then placed in a graphite crucible. Then the crucible is put into a single crystal furnace, and the seed crystal is fixed on the seed crystal rod. After closing the furnace chamber, the furnace chamber is evacuated to 5×10 -5 Pa using a mechanical pump and a molecular pump. Then the nitrogen partial pressure control system is turned on to keep the air pressure in the furnace chamber at 0.2 atm. 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 at 200 sccm. The crucible is 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 5 °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 100 r / min, and the crucible rotates counterclockwise at a speed of 2 r / min. The seed crystal is pulled up at a rate of 50 μm / h. After 60 h of crystal growth, the seed crystal is pulled up at a speed of 5 mm / h until it is completely separated from the liquid surface and then slowly cooled to room temperature. Finally, the crystal is taken out.

[0073] In this embodiment, the addition of Cr element is used to promote the dissolution of C and N, enhance the supply of solutes, and weaken the influence of the increase in resistivity caused by the entry of Al atoms. By the cooperation of Al with Cr and Nd elements, the interfacial energy between the growth surface of 4H-SiC and the melt is reduced, while the interfacial energy between 3C-SiC and 6H-SiC and the melt is increased. Furthermore, the 4H crystal form is stabilized, which plays a role in preventing crystal form transformation and improving crystal quality. A high-quality n-type 4H-SiC crystal is grown, and the crystal photograph is as Figure 3 shown. Figure 3 It is shown that the crystal surface is smooth, without macroscopic defects such as cracks and inclusions. The growth rate reaches 160 μm / h, as shown in Figure 20 . The microscopic morphology of the crystal growth surface is as Figure 4 shown. It can be seen that the step flow is dense, the step height is low, there are no giant steps, and the growth surface is stable.

[0074] The X-ray rocking curve test results and Raman test results of the crystal are as Figure 5 shown. The full width at half maximum of the crystal X-ray rocking curve is 17.89 arcsec, and the Raman peaks at different positions of the crystal are all characteristic peaks of 4H-SiC, indicating that there is no polytype phase transformation in the crystal.

[0075] The Hall test and resistivity test results of the crystal are as Figure 6 shown. The results show that the Hall coefficient at room temperature is -1.5935×10 -6 m 3 / C, proving that the carrier type of the grown 4H-SiC single crystal is n-type, and its carrier concentration is 5.93×10 18 cm -3, the average resistivity of the wafer after crystal processing is about 0.05 Ω·cm. From Figure 6 It can be seen that the resistivity distribution is relatively uniform, indicating that the doping in the grown crystal is relatively uniform.

[0076] The test results of the high-temperature in-situ contact angle of the metal raw materials used in this example with different crystal forms of SiC are as Figure 7 shown. Figure 7 It is shown that the contact angle of the metal raw materials used in this example with 4H-SiC is less than that of 3C-SiC and 6H-SiC with the metal raw materials, indicating that the solid-liquid interfacial energy of the metal raw materials used in this example with 4H-SiC is less than that of 3C-SiC and 6H-SiC with the metal raw materials. Under this metal raw material system, 4H-SiC has an advantage in energy compared to other polytypes, playing a role in stabilizing the 4H crystal form.

[0077] This example proves that the method provided by the present invention can realize the liquid-phase growth of high-quality n-type 4H-SiC single crystals. Compared with other methods, this method can effectively reduce the interfacial energy between 4H-SiC and the melt, stabilize the 4H crystal form, improve the crystallization quality, and can greatly increase the crystal growth rate and reduce the cost.

[0078] Example 2

[0079] In this example, the pressure of the mixed gas is increased to 0.5 atm, and the ratio of argon to nitrogen is changed to 40% argon and 60% nitrogen. Other steps are the same as in Example 1.

[0080] As Figure 8 shown, the surface of the crystal grown in this example is smooth and has a high crystallization quality. As Figure 20 shown, the Hall test results show that its Hall coefficient is -7.3464×10 -5 m 3 / C, proving that the carrier type of the crystal is n-type, and the carrier concentration is increased to 2.11×10 19 cm -3 , and the resistivity is reduced to 0.031 Ω·cm. This example shows that the method provided by the present invention can regulate the carrier concentration and resistivity of the crystal by changing the nitrogen partial pressure.

[0081] Example 3

[0082] In this example, the composition of the metal raw material melt is changed to: Si 60 Cr 36 Al2Nd2. Other steps are the same as in Example 1.

[0083] As Figure 9 shown, the surface of the crystal grown in this example is smooth and has a high crystallization quality. AsFigure 20 As shown, the Hall test results show that its Hall coefficient is -4.8261×10 -5 m 3 / C, proving that the carrier type of this crystal is n-type, and the carrier concentration is increased to 2.42×10 19 cm -3 , and the resistivity is reduced to 0.029 Ω·cm. This example shows that the method provided by the present invention can achieve the regulation of the carrier concentration and resistivity of the crystal by changing the Al content.

[0084] Example 4

[0085] In this example, the composition of the molten metal raw material is changed to: Si 60 Mn 30 Al3Ce7. The temperature gradient in the melt is increased to 10 °C / cm, and at the same time, the rotation speed of the seed crystal is increased to 200 rpm, and the pulling speed is increased to 120 μm / h. Other steps are the same as those in Example 1.

[0086] The photo of the crystal grown in this example is as Figure 10 shown. Since the melting point of Mn is relatively lower than that of Cr, the viscosity of the formed alloy melt is lower, the fluidity is better, and the temperature gradient in the melt and the rotation speed of the seed crystal are increased, strengthening the thermal convection and forced convection in the melt, making the solute supply more sufficient. At the same time, the acceleration of the pulling speed leads to enhanced heat dissipation of the crystal. These three factors result in a significantly faster crystal growth rate compared to Example 1, but grooved defects are generated in the later stage of growth.

[0087] Example 5

[0088] In this example, the composition of the molten metal raw material is changed to: Si 60 Sc 35 Al3Pr2. The axial temperature gradient in the melt is controlled to 2 °C / cm. Other steps are the same as those in Example 1.

[0089] The photo of the crystal grown in this example is as Figure 11 shown. Since the axial temperature gradient in the melt is reduced, the solute supersaturation at the seed crystal is reduced. Compared with Example 1, the growth rate is slightly reduced. The crystal surface is smooth and has a high crystallization quality. As Figure 20 shown, the Raman test results show that the prepared crystal is of 4H crystal form, and the Hall test results show that its Hall coefficient is -2.0453×10 -6 m 3 / C, proving that the carrier type of this crystal is n-type, and the carrier concentration is 7.93×10 18 cm -3 , and the resistivity is 0.047 Ω·cm.

[0090] Comparative Example 1

[0091] The metal raw material used in this comparative example has a composition of Si 60 Cr 38 Nd2. Other conditions are the same as those in Example 1, and crystal growth comparison is carried out.

[0092] The crystal photograph of the crystal grown in this comparative example is as Figure 12 shown. Since Al is not added to the metal raw material, the height of the crystal growth steps is high, the growth interface is very unstable, and the crystal growth surface is densely covered with groove-like defects. From the microscopic morphology image of the crystal growth surface ( Figure 13 ), it can be clearly seen that the step flow has a serious coalescence phenomenon, and two-dimensional nucleation occurs on the growth surface, which is very unfavorable for the crystal crystallization quality and the stability of the crystal crystal form.

[0093] The X-ray rocking curve test results and Raman test results are as Figure 14 shown. Figure 14 It shows that compared with the crystal grown in Example 1, the crystal grown in this comparative example has poor crystallization quality and is accompanied by polytype phase changes such as 3C and 6H.

[0094] The high-temperature in-situ contact angles of the metal raw material used in this comparative example with different polytypes of SiC were tested, and the results are as Figure 15 shown. It is found that the contact angles of the metal raw material used in this comparative example with 4H-SiC, 3C-SiC, and 6H-SiC with the metal raw material are almost equal, indicating that the solid-liquid interfacial energy of the metal raw material used in this comparative example with 4H-SiC, 3C-SiC, and 6H-SiC is close. Under this metal raw material system, 4H-SiC has no advantage in energy compared with other polytypes and is prone to polytype phase changes.

[0095] Comparative Example 2

[0096] The metal raw material used in this comparative example has a composition of Si 60 Ti 38 Nd2. Other conditions are the same as those in Example 1, and crystal growth comparison is carried out.

[0097] The crystal photograph of the crystal grown in this comparative example is as Figure 16 shown. Figure 16 It shows that there are groove defects on the surface of the crystal grown in this comparative example, the crystallization quality is poor, and it is accompanied by a 15R polytype phase change.

[0098] The test results of the high-temperature in-situ contact angles of the metal raw material used in this comparative example with different polytypes of SiC are as Figure 17As shown, the results indicate that the contact angle between the metal raw materials used in this comparative example and 15R-SiC is smaller than that between 4H-SiC and the metal raw materials. The solid-liquid interfacial energy between the metal raw materials used in this example and 15R-SiC is smaller than that between 4H-SiC and the metal raw materials, indicating that under this metal raw material system, 15R-SiC has an advantage in terms of energy compared to 4H-SiC. Therefore, the polytype phase transformation of 15R will occur.

[0099] Comparative Example 3

[0100] The composition of the metal raw materials used in this comparative example is Si 60 Ni 37 Al1Ce2, and other conditions are the same as those in Example 1 for crystal growth comparison.

[0101] The crystal photograph grown in this comparative example is as Figure 18 shown. It can be seen that the growth surface is relatively flat and smooth, but the growth rate is low, only 36 μm / h. As Figure 20 shown, the Raman test results show that the prepared crystal is of the 3C polytype. This indicates that using Ni as the transition metal flux in this comparative example cannot prepare 4H-SiC, indicating that the solid-liquid interfacial energy between the metal raw materials in this comparative example and 3C-SiC is smaller than that between 4H-SiC and the metal raw materials, indicating that under this metal raw material system, 3C-SiC has an advantage in terms of energy compared to 4H-SiC. Therefore, the polytype phase transformation of 3C will occur.

[0102] Comparative Example 4

[0103] The composition of the metal raw materials used in this comparative example is Si 60 Cr 33 Al5Nd2, and other conditions are the same as those in Example 1 for crystal growth comparison.

[0104] The crystal photograph grown in this comparative example is as Figure 19 shown. It can be seen that the growth surface is relatively flat and smooth, but the growth rate is low, only 40 μm / h. As Figure 20 shown, the Raman test results show that the prepared crystal is of the 4H polytype. The Hall test and resistivity test results show that its Hall coefficient at room temperature is 6.9143×10 -5 m 3 / C, proving that the carrier type of the grown 4H-SiC single crystal is p-type, and its carrier concentration is 6.98×10 18 cm -3 . This comparative example shows that if the addition amount of Al is too much, an n-type 4H-SiC crystal cannot be obtained.

[0105] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although specific process parameters can be optimized and adjusted, the two core ideas of the present invention and the basic framework of the growth device are clear. Those skilled in the relevant art should understand that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing n-type 4H-SiC single crystal, which successively includes the following steps: (1) Place a metal raw material containing Si, Al and a transition metal in a graphite crucible, and fix a SiC seed crystal on a graphite seed crystal rod; (2) Place the graphite crucible in a growth furnace, and then evacuate the growth furnace; (3) Introduce a gas and control the gas pressure in the growth furnace; (4) Heat the graphite crucible to completely melt the metal raw material to form a melt; (5) Lower the seed crystal to make the seed crystal contact with the melt, and then grow an n-type 4H-SiC single crystal; Among them, The transition metal is selected from one or more of Cr, Sc, V, Mn and Cu; The atomic molar ratio of Si, Al and the transition metal in the metal raw material is Si:Al:transition metal = (30 - 70):(0.5 - 4):(30 - 60); The gas consists of nitrogen, or is composed of nitrogen mixed with one or two of argon and helium.

2. The method according to claim 1, wherein, The metal raw material further contains a rare earth metal.

3. The method according to claim 2, wherein The rare earth metal is selected from one or more of La, Ce, Pr and Nd.

4. The method according to claim 2, wherein The atomic molar ratio of Si, Al, the transition metal and the rare earth metal in the metal raw material is Si:Al:transition metal:rare earth metal = (30 - 70):(0.5 - 4):(30 - 60):(0.01 - 20).

5. The method according to claim 1, wherein The partial pressure of nitrogen in the gas is 10% - 100% of the total pressure, and the total pressure is 0.1 - 2 atm.

6. The method according to claim 5, wherein, The partial pressure of nitrogen in the gas is 30% - 60% of the total pressure.

7. The method according to claim 5, wherein, And the total pressure is 0.2 - 1.2 atm.

8. The method according to claim 1, wherein, The SiC seed crystal is a 2 - 6 inch SiC wafer with a 0°, 4° or 8° deviation angle.

9. The method according to claim 1, wherein The SiC seed crystal is a semi-insulating SiC single crystal substrate or an n-type SiC single crystal substrate.

10. The method according to claim 1, wherein The graphite crucible is made of high-purity graphite with a purity of greater than or equal to 99.95%.

11. The method according to claim 1, wherein, The inner diameter of the graphite crucible is 10 - 150 mm larger than the diameter of the SiC seed crystal.

12. The method according to claim 1, wherein, The wall thickness of the graphite crucible is greater than or equal to 10 mm.

13. The method according to claim 1, wherein The density of the graphite crucible is 1.7 to 2.0 g / cm 3 .

14. The method according to claim 1, wherein The evacuation of the growth furnace in step (2) evacuates the growth furnace to less than or equal to 5×10 -4 Pa.

15. The method according to claim 1, wherein The growth of the n-type 4H-SiC single crystal in step (5) is carried out by a method including the following steps: (i) Control the temperature at the seed crystal to be 1700°C - 1900°C, the melt gradually heats up from the surface near the SiC seed crystal to the bottom of the graphite crucible with a temperature gradient of 2 - 20°C / cm, and the temperature of the melt at the bottom of the graphite crucible is 1850°C - 2050°C; (ii) Carry out periodic acceleration and deceleration rotation on the SiC seed crystal and the graphite crucible, and at the same time, slowly lift the SiC seed crystal.

16. The method according to claim 15, wherein, The periodic acceleration and deceleration rotation are carried out under the following conditions: the SiC seed crystal and the graphite crucible rotate periodically with opposite directions, the rotation speed is ±0 to 300 r / min, and the rotational acceleration is ±0 to 40 r / min 2 .

17. The method according to claim 15, wherein, The lifting is carried out at a rate of 1 - 2000 μm / h.

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