Method for growing SiC single crystal by solution method

Abstract

A method for growing SiC single crystals by the solution method, comprising: (1) placing a raw material block at the bottom of a crucible and placing a flux above the raw material block; wherein the composition of the raw material block satisfies a molar ratio of Si to C of 1:1; (2) placing the crucible in a growth furnace, fixing an SiC seed crystal on a lifting rod above the crucible, and then evacuating the growth furnace; (3) introducing a functional gas to control the air pressure in the growth furnace, and then heating the crucible to completely melt the flux to form a flux melt and maintaining the flux in a specific temperature field; (4) pushing down the lifting rod to bring the seed crystal into contact with the flux melt, and rotating and lifting the seed crystal and the crucible and dynamically adjusting the relative positions among the heat source, the crucible and the seed crystal, so that the relative positions and temperatures among the crystal growth interface, the flux melt and the upper surface of the raw material block remain unchanged. The method of the present invention can achieve continuous, stable and rapid growth of high-quality SiC single crystals.

Description

Technical Field

[0001] The present invention belongs to the fields of materials and crystal growth. The present invention relates to a method for growing SiC single crystals by a solution method. Background Art

[0002] SiC is a wide-bandgap semiconductor material (4H-SiC: E g = 3.23 eV), which has the advantages of high breakdown field strength, high saturated electron mobility, high thermal conductivity, high thermal stability and high chemical stability, etc. It has broad application prospects in the fields of new energy vehicles, high-speed rail transit, aerospace, high-voltage smart grid, and clean energy, and has received extensive attention from academia and industry in various countries.

[0003] At present, the relatively high manufacturing cost of SiC single crystal substrates is the main factor restricting their application. Exploring a method for growing high-quality, large-size and low-cost SiC single crystals is of great significance for giving full play to the huge potential of SiC.

[0004] SiC single crystal growth usually adopts the physical vapor transport method (PVT). This method is relatively mature and can currently supply SiC single crystal substrates to the market. However, the PVT method has the disadvantages of unstable growth environment, high defect density, low yield, high cost, difficulty in increasing the diameter, and difficulty in p-type doping.

[0005] Compared with the PVT method, the high-temperature liquid phase method is a method for growing SiC crystals under thermodynamic equilibrium conditions, which has the advantages of low growth temperature, low cost, easy p-type doping, and easy diameter expansion. Therefore, in recent years, the high-temperature liquid phase method has received increasing attention from academia and industry.

[0006] At present, the mainstream method for growing SiC single crystals by the liquid phase method is the Top Seeded Solution Growth (TSSG). The basic principle is as follows: A flux with a high carbon solubility and Si are placed together in a high-purity graphite crucible. A seed crystal connected to a seed crystal rod is placed above the crucible. The crucible is heated to completely melt the flux and Si in it. After the temperature stabilizes, the seed crystal rod is pushed down. When the seed crystal comes into contact with the high-temperature solution, the growth of SiC single crystals begins by rotation and pulling (see the references SUNOKI K, OKADA N, KAMEI K, et al. Top-seeded solution growth of three-inch-diameter 4H-SiC using convection control technique[J]. Journal of Crystal Growth, 2014, 395:68-73; AIKOKU H, KADO M, SEKI A, et al. Solution Growth on Concave Surface of 4H-SiC Crystal[J]. Crystal Growth&Design, 2016, 16(3):1256-60; and YOON J-Y, LEE M-H, KIM Y, et al. Enhancement in the rate of the top seeded solution growth of SiC crystals via a roughening of the graphite surface[J]. Japanese Journal of Applied Physics, 2017, 56(6), as well as the patent applications CN 110747504 A, CN102203330 A and the patent CN 101796227 B). In the prior art, the graphite crucible provides a carbon source for crystal growth, and the high-temperature alloy solution containing Si provides a Si source for crystal growth. As the crystal growth proceeds, the content of Si in the high-temperature solution continuously decreases, resulting in a change in the thermodynamic properties of the high-temperature solution. At the same time, the wall of the graphite crucible will also be gradually corroded and thinned, making it difficult to accurately calibrate the liquid level of the melt. These two problems will directly affect the dynamic stability and durability of crystal growth and the quality of SiC crystals. In addition, to increase the single growth amount of SiC single crystals, it is necessary to increase the mass and volume of the melt. At this time, the convection in the melt is enhanced, and a large number of spontaneous nucleations of SiC will occur at the liquid surface, which is not conducive to the formation of a stable growth interface and hinders the growth of single crystals.

[0007] In view of this, it is urgent to explore and develop a new growth method to overcome the above problems, give full play to the advantages of growing SiC crystals by the high-temperature liquid phase method, and obtain high-quality, large-sized and low-cost SiC single crystal substrates. Summary of the Invention

[0008] The object of the present invention is to provide a method for growing SiC single crystals by the solution method. The method of the present invention constructs a continuous and stable growth system, which can realize the continuous, stable and rapid growth of high-quality SiC single crystals.

[0009] To achieve the above object, the present invention is realized by the following technical solutions.

[0010] In the context of the present invention, the term "stable thermodynamic state" means that during the crystal growth process, the temperature, solute supersaturation and interfacial energy at the contact interface between the melt and the seed crystal (i.e., the growth interface) always remain in the state at the initial growth without change.

[0011] The present invention provides a method for growing SiC single crystals by the solution method, which comprises the following steps:

[0012] (1) Place the raw material block at the bottom of the crucible, and place the flux above the raw material block; wherein the composition of the raw material block satisfies that the molar ratio of Si to C is 1:1;

[0013] (2) Place the crucible in the growth furnace, fix the SiC seed crystal on the lifting rod above the crucible, and then evacuate the growth furnace;

[0014] (3) Introduce a functional gas and control the air pressure in the growth furnace, and then heat the crucible to completely melt the flux to form a flux melt and make the flux melt in a specific temperature field;

[0015] (4) Push down the lifting rod to make the seed crystal contact with the flux melt, and rotate and lift the seed crystal and the crucible and dynamically adjust the relative positions among the heat source, the crucible and the seed crystal, so that the relative positions and temperatures among the crystal growth interface, the flux melt and the upper surface of the raw material block remain unchanged, so as to realize that the raw material block is continuously and uniformly dissolved layer by layer into the flux melt from the upper surface in contact with the flux melt to the lower surface of the raw material block, and continuously precipitate SiC single crystals on the crystal growth interface;

[0016] Among them, the specific temperature field is as follows: the temperature of the upper surface of the flux melt in contact with the seed crystal is 1500 - 2100 °C, the temperature of the flux melt increases with a temperature gradient of 2 - 30 °C per centimeter thickness of the flux melt from the upper surface to the lower surface, the temperature of the upper surface of the raw material block in contact with the flux melt is equal to the temperature of the lower surface of the flux melt, and the temperature of the raw material block decreases with a temperature gradient greater than 30 °C per centimeter thickness of the raw material block from the upper surface to the lower surface.

[0017] In the method of the present invention, the functions of the crucible, the flux, and the raw material are specialized. The crucible with high temperature resistance and resistance to flux corrosion is only responsible for containing the flux and the raw material, providing a stable space environment for crystal growth, and no longer providing a C source for crystal growth. The flux melt only dissolves, transports, and re-precipitates the Si and C solutes, and no longer provides a Si source for the growth of SiC crystals. The Si and C sources required for the growth of SiC crystals are completely provided by the raw material block placed at the bottom of the crucible. This can ensure the long-term stability of the crystal growth system, and can also ensure that the ratio of the Si and C contents in the flux melt remains constant during the growth process, which is beneficial to the long-term stable growth of the crystal.

[0018] In the method of the present invention, the high-temperature zone (the zone where the temperature is higher than the melting point of the flux melt) in the growth system is very narrow (as shown in the temperature distribution schematic diagram on the left). Figure 1 The high-temperature zone only covers a small part of the block below the upper surface of the raw material block. In this way, it can effectively inhibit the strong preferential dissolution of the local part of the raw material block, and can ensure that the raw material block can be uniformly dissolved layer by layer from top to bottom. In this way, on the one hand, it can ensure that the melt thickness remains unchanged, which is conducive to controlling the relative positions and temperatures among the crystal growth interface, the melt, and the upper surface of the raw material block, so that the crystal growth interface always maintains a stable thermodynamic state; on the other hand, it can reduce the amount of the flux melt to shorten the distance between the seed crystal and the raw material, reduce the transmission distance of Si and C, improve the transmission efficiency of Si and C, and increase the growth rate; reducing the amount of the flux melt can also prevent the generation of strong convection caused by too large a melt thickness (the convection intensity of the melt growing vertically is proportional to the cube of the melt thickness), which is beneficial to stabilizing the solute concentration distribution in the melt, preventing spontaneous nucleation at the melt surface during the growth process, and thus stabilizing the crystal growth interface. In addition, the narrow high-temperature zone can also greatly reduce the heating cost, which is beneficial to reducing the crystal growth cost.

[0019] The inventors of the present application unexpectedly found that based on the above advantages of the solution method for growing SiC single crystals provided by the present invention, the method of the present invention can achieve the long-term stable growth of high-quality SiC single crystals, can effectively improve the crystal growth rate and quality, obtain high-quality SiC crystals with a bright surface and no flux wrapping, and can reduce the crystal growth cost.

[0020] In a specific embodiment of the present invention, the heating method for heating the crucible can be induction heating, resistance heating, or optical heating.

[0021] In a specific embodiment of the present invention, the temperature of the upper surface of the raw material block is always higher than the temperature at the crystal growth interface of the melt surface.

[0022] Preferably, in the method of the present invention, the flux is a combination of one or more of the elements Cr, Fe, Cu, Y, Co, Ni, Sc, Ti, Ce, La, Pr, Al, Ga, In, Ge, and Sn and the Si element. The flux of the present invention has a large solubility for SiC at high temperatures.

[0023] Preferably, in the method of the present invention, the diameter of the raw material block is equal to the inner diameter of the crucible and the thickness of the raw material block is 10 - 300 mm, preferably 50 - 300 mm.

[0024] Preferably, the thickness of the flux melt is 10 - 50 mm.

[0025] Preferably, in the method of the present invention, the evacuation of the growth furnace in step (2) is to evacuate the growth furnace to less than or equal to 10 -3 Pa.

[0026] Preferably, in the method of the present invention, the functional gas is selected from one or several of argon, helium, nitrogen, oxygen, and hydrogen.

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

[0028] Preferably, in the method of the present invention, the rotation of the seed crystal and the crucible in step (4) is a periodic acceleration and deceleration rotation of the seed crystal and the crucible.

[0029] Preferably, in the method of the present invention, the periodic acceleration and deceleration rotation is carried out under the following conditions: the crucible and the seed crystal are periodically accelerated and decelerated in opposite directions, the rotation speed is ±0 - 200 r / min, and the rotational acceleration is ±0 - 30 r / min 2 . More specifically, a single cycle includes four stages, and the time and rotational acceleration of each stage are T1 and a1, T2 and a2, T3 and a3, and T4 and a4 respectively. The initial rotational speed in a single cycle is ±0 - 200 r / min, the time of each stage is 3 - 60 min, and the rotational acceleration is ±0 - 30 r / min 2 .

[0030] Preferably, in the method of the present invention, the pulling is carried out at a rate of 1 to 3000 μm / h.

[0031] Preferably, in the method of the present invention, the upper surface of the raw material block is multi-grooved, multi-conical or honeycomb-shaped. Making the upper surface of the raw material block body into shapes such as multi-grooved, multi-conical or honeycomb can increase the contact area and improve the dissolution rate of Si and C.

[0032] Preferably, in the method of the present invention, the crucible is a tungsten carbide or tantalum carbide crucible, or a tungsten crucible or tantalum crucible with an inner wall treated by carbonization, or a graphite crucible with a high-temperature and flux-corrosion-resistant coating on the inner wall.

[0033] Preferably, in the method of the present invention, the seed crystal size is 2 to 8 inches, and the inner diameter of the crucible is 0.5 to 2 inches larger than the diameter of the seed crystal.

[0034] In a specific embodiment of the present invention, the steps for growing SiC single crystal of the present invention are as follows:

[0035] Crucible pretreatment: Put graphite powder in a tantalum crucible or tungsten crucible, then heat it to 2200 °C and keep it for a period of time, so that the inner wall of the crucible is fully carbonized and a tantalum carbide or tungsten carbide layer is formed, thereby obtaining a crucible with a high-temperature and flux-corrosion-resistant inner wall.

[0036] Preparation of raw material block: First, purify the SiC powder, and then use the high-temperature sintering method to make it into a cylinder with the same inner diameter as the crucible; it is also possible to mix high-purity silicon and graphite in a molar ratio of 1:1 and press them into a cylinder with the same inner diameter as the crucible.

[0037] Loading into the furnace: According to the structural schematic diagram as Figure 1 shown, first fix the pre-prepared raw material block at the bottom of the crucible, then place the prepared flux above the raw material block, then load the crucible into the furnace, and extend the seed crystal into the crucible to a certain height above the material surface.

[0038] Heating: Heat the crucible according to the temperature field as Figure 1 shown, completely melt the flux and let it stand for 1 h, so that the flux dissolves the raw material block and reaches saturation; where Figure 1 T1 in it represents the crystal growth temperature at the interface between the flux and the seed crystal.

[0039] Starting growth: Slowly push down the seed crystal until the seed crystal is completely in contact with the melt, and then the crystal starts to grow. During the growth process, the crucible and the seed crystal always rotate in the manner as Figure 1 shown, and the rotation mode can be set accordingly according to the needs of the process;

[0040] Dynamic regulation: During the growth process, the crucible and the seed crystal are slowly lifted to keep the growth conditions at the crystal growth interface unchanged.

[0041] End of growth: After the growth is completed, the seed crystal is slowly lifted upward at a certain speed to disconnect the crystal from the liquid surface and terminate the growth. After the crystal is slowly cooled to room temperature, the crystal is taken out.

[0042] The present invention can solve the following three important problems faced by the traditional liquid-phase method for growing SiC single crystals.

[0043] First, the existing high-temperature liquid-phase method uses a high-purity graphite crucible as the carbon source for crystal growth. As the crystal growth progresses, the inner wall of the graphite crucible is continuously corroded, causing the inner diameter of the crucible to increase, resulting in a continuous decrease in the liquid level during the growth process. It is difficult to accurately calibrate the liquid level position, and a stable crystal growth interface cannot be formed, which is not conducive to growing high-quality crystals. In addition, if the growth time is too long, the crucible will be corroded and melted through, resulting in the termination of crystal growth.

[0044] The growth method of the present invention does not have the problem of crucible corrosion. The inner diameter of the crucible remains unchanged during the growth process. There is an exact corresponding relationship between the growth rate of the SiC crystal and the descent rate of the liquid level. By using the dynamic regulation technology, the crystal can grow for a long time in a stable growth state. By putting sufficient raw materials in the crucible, the stable and continuous growth of the SiC crystal can be achieved.

[0045] Second, in the existing high-temperature liquid-phase growth technology, only the C source is replenished without the replenishment of an additional Si source, resulting in a continuous decrease in the Si content in the melt, causing the thermodynamic properties of the melt to change continuously, which is not conducive to the long-term stable growth of high-quality SiC crystals.

[0046] In the growth method of the present invention, the raw material block can continuously and stably provide Si and C elements for crystal growth. When the growth process reaches dynamic equilibrium, the growth amount of the SiC crystal per unit time is always consistent with the consumption amount of Si and C elements in the raw material block, and the composition of the flux melt always remains unchanged, which is very important for growing high-quality SiC crystals for a long time.

[0047] Third, in the existing high-temperature liquid-phase method for growing SiC crystals, to increase the single growth amount of SiC crystals, it is necessary to increase the total amount of the melt to increase the Si content under the condition of keeping the melt ratio constant. This will increase the transmission distance of the C source from the bottom of the crucible to the seed crystal, which is not conducive to the rapid, continuous and stable transmission of C. At the same time, the increase in the total amount of the melt will also increase the longitudinal temperature difference of the melt, resulting in strong convection, which is not conducive to the stability of the crystal growth interface, and even a large number of SiC spontaneous nuclei will be formed on the upper surface of the melt, seriously affecting the quality of the crystal.

[0048] In the method of the present invention, the flux melt no longer serves as the Si source for crystal growth, and its composition remains unchanged all the time. This can significantly reduce the usage amount of the flux, shorten the distance between the seed crystal and the raw material, and improve the growth quality and speed of the crystal. At the same time, because the method of the present invention requires less flux raw material and has a narrow high-temperature zone, it can reduce the consumption of energy and flux raw material, and greatly reduce the cost of crystal growth.

[0049] In addition, in the present invention, by making the upper surface of the raw material block into a serrated or honeycomb shape, etc., the contact area between the flux melt and the raw material block is increased, the dissolution rate of the solute is improved, and the growth rate of the crystal is enhanced.

[0050] By adjusting the growth process, the growth method of the present invention can prepare large-size SiC single crystal substrates, can obtain SiC single crystals of different conduction types, and can also grow SiC epitaxial films. Brief Description of the Drawings

[0051] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings, wherein:

[0052] Figure 1 is a schematic diagram of the device for a specific embodiment of growing SiC single crystals of the present invention; Figure 1 The temperature distribution schematic diagram on the left shows that the high-temperature zone (the area where the temperature is higher than the melting point of the flux melt) is only limited to a small range near the flux melt;

[0053] Figure 2 is an optical photograph of a 4-inch SiC ingot grown in Example 1 of the present invention;

[0054] Figure 3 is a photograph of the graphite crucible and the grown crystal after growth in Comparative Example 1;

[0055] Figure 4 is a schematic diagram of the temperature field configuration of Comparative Example 2 and the state inside the crucible after growing for 20 h under this temperature field configuration;

[0056] Figure 5 is an optical photograph of a 4-inch SiC ingot grown in Comparative Example 3 and a topographic photograph of the corresponding melt surface;

[0057] Reference Signs:

[0058] 1 - furnace cavity housing; 2 - graphite shaft connecting the seed crystal; 3 - thermal insulation material; 4 - high-temperature resistant crucible; 5 - silicon carbide seed crystal; 6 - flux melt; 7 - induction coil; 8 - high-purity raw material block fixed at the bottom of the crucible; 9 - crucible support shaft. Detailed Description of the Specific Embodiment

[0059] The present invention will be further described in detail below in conjunction with specific embodiments. The provided embodiments are only for clarifying the present invention and not for limiting the scope of the present invention.

[0060] The technical solution of the present invention will be described and explained in detail below in conjunction with the attached Figure 1 drawings. It should be noted that although Figure 1 the heating method shown is medium-frequency induction heating, the heating method is not limited thereto, and other heating methods such as resistance heating and optical heating can also be selected in combination with specific processes.

[0061] Example 1

[0062] In this embodiment, the crucible used is a tantalum carbide crucible with an inner diameter of 150 mm and a height of 200 mm. The seed crystal is a 4-inch semi-insulating SiC single crystal without a tilt angle, with the C plane as the growth plane. The raw material block is a SiC polycrystalline block with a diameter of 150 mm and a height of 50 mm. The flux ratio used is: Si 0.3 Cr 0.65 Al 0.05 , and the thickness of the melt after the flux melts is 30 mm.

[0063] After loading the furnace, the vacuum degree of the growth chamber is pumped to 10 -4 Pa, and then high-purity Ar at 0.1 atm is filled into the furnace chamber. The crucible is heated to make the temperature at the liquid surface of the flux reach 1800 °C, and the temperature field is controlled so that the temperature of the lower surface of the flux melt is higher than the temperature of the upper surface of the melt, with a temperature gradient of 10 °C / cm. The upper surface of the raw material block is in full contact with the lower surface of the flux melt, and the temperature is equal to the temperature of the lower surface of the flux melt, which is 1830 °C / cm. The temperature of the raw material block decreases rapidly from top to bottom, with a temperature gradient of 50 °C / cm. During the growth process, the seed crystal rotates clockwise at a speed of 30 r / min, and the crucible rotates counterclockwise at a speed of 10 r / min. The pulling speed of the seed crystal is 300 μm / h, and the pulling speed of the crucible is 240 μm / h.

[0064] After growing for 60 h, the crystal is pulled up at a speed of 3 mm / h, disconnected from the liquid surface, slowly cooled to room temperature, and the crystal taken out is the desired SiC single crystal.

[0065] Results and Analysis of the Embodiment

[0066] Figure 2 is the optical photograph of the 4-inch SiC ingot grown in Example 1 of the present invention; among them, Figure 2 (a) and Figure 2(b) are respectively a top view and an oblique view of a 4-inch SiC ingot grown by Example 1. The figure shows that the surface of the crystal is smooth and flat, without groove-shaped defects or solvent wrapping, and the crystal presents a regular hexagon, indicating that the crystal growth environment is close to a thermodynamic equilibrium state. This example proves that the method provided by the present invention can effectively obtain high-quality SiC single crystals.

[0067] Example 2

[0068] The crucible used in this embodiment is a tantalum crucible with an inner diameter of 150 mm and a height of 200 mm after the inner wall is carburized. The seed crystal is a 4-inch SiC wafer with no deflection angle, with the C surface as the growth surface. The raw material block is a SiC polycrystalline block with a diameter of 150 mm and a height of 50 mm. The flux ratio used is: Si 0.3 Cr 0.65 Al 0.05 , the thickness of the melt after the flux is melted is 30mm.

[0069] After loading the furnace, the vacuum degree of the growth chamber was evacuated to 10 -4 Pa, and then fill the furnace chamber with 0.5atm of high-purity Ar. Heat the crucible so that the temperature at the flux liquid surface reaches 1800℃, and control the temperature field so that the temperature of the lower surface of the melt is higher than the temperature of the upper surface of the melt, with a temperature gradient of 5℃ / cm. The upper surface of the raw material block is in full contact with the lower surface of the melt, and the temperature is equal to the temperature of the lower surface of the melt, which is 1815℃ / cm. The temperature of the raw material block decreases rapidly from top to bottom, with a temperature gradient of 50℃ / cm. During the growth process, the seed crystal rotates clockwise at a speed of 30r / min, and the crucible rotates counterclockwise at a speed of 10r / min. The pulling speed of the seed crystal is 200μm / h, and the pulling speed of the crucible is 180μm / h. After 60h of growth, pull the crystal out of the melt, the growth is terminated, and the crystal is taken out after slowly cooling to room temperature.

[0070] Implementation Results and Analysis

[0071] The surface of the crystal obtained in this example becomes brighter and the crystal quality is higher, but the crystal growth rate decreases, which shows that reducing the temperature gradient in the melt can improve the crystal quality of the crystal, but will reduce the growth rate.

[0072] Example 3

[0073] The inner wall of the crucible is a tungsten crucible with a high temperature carburization treatment, an inner diameter of 150mm, and a height of 150mm. The seed crystal is a 4-inch SiC wafer with an 8-degree deflection angle, with the C surface as the growth surface. The raw material block is a block made by sintering a mixture of pure Si and C in a molar ratio of 1:1, with a diameter of 150mm and a height of 50mm. The flux ratio used is: Si 0.3 Cr 0.6 5Al0.05 The total amount of the flux is controlled so that the thickness of the melt after the flux melts is reduced to 10 mm.

[0074] After loading into the furnace, the vacuum degree of the growth chamber is pumped to 10 -4 Pa. Then, high-purity argon gas at 0.5 atm is filled into the furnace chamber. The crucible is heated to make the temperature at the liquid surface of the flux reach 1800 °C, and the temperature field is controlled so that the temperature of the lower surface of the melt is higher than that of the upper surface of the melt, and the temperature gradient is 10 °C / cm. The upper surface of the raw material block is in full contact with the lower surface of the melt, and the temperature is equal to that of the lower surface of the melt, which is 1810 °C / cm. The temperature of the raw material block decreases rapidly from top to bottom, and the temperature gradient is 50 °C / cm. During the growth process, the seed crystal rotates clockwise at a speed of 30 r / min, and the crucible rotates counterclockwise at a speed of 10 r / min. The pulling speed of the seed crystal is 300 μm / h, the pulling speed of the crucible is 200 μm / h, and the growth time is 60 h.

[0075] Implementation Results and Analysis

[0076] Compared with the crystal obtained in Example 1, the surface of the SiC crystal grown by this example is smoother, and at the same time, the growth rate has also been significantly improved. This is because reducing the thickness of the flux melt can achieve a smaller supersaturation to improve the crystal crystallization quality while reducing the solute transport distance and improving the solute transport efficiency, thereby increasing the crystal growth rate. In addition, after reducing the thickness of the flux melt and changing to a raw material block made of a mixture of Si and C sintered, the cost required for this crystal growth is reduced by about 35%.

[0077] Example 4

[0078] The crucible used is a graphite crucible with an inner diameter of 150 mm and 240 mm and a tantalum carbide coating on the inner wall. A 4-inch high-quality SiC single crystal with an inclination angle of 8 degrees is used as the seed crystal, and the C plane is used as the growth surface. The diameter of the raw material block is 150 mm, and the thickness is increased to 100 mm. The flux ratio used is: Si 0.3 Cr 0.6 Ni 0.05 Al 0.05 The total amount of the flux is controlled so that the thickness of the melt after the flux melts is about 10 mm, and the growth time is extended to 120 h. Other growth processes are the same as those in Example 3 for crystal growth.

[0079] Implementation Results and Analysis

[0080] In this example, by increasing the thickness of the raw material block, the crystal growth for a longer time is achieved, and at the same time, the crystal quality remains at a good level. The single crystal growth time and the single crystal growth amount are both increased by about 1 time.

[0081] Example 5

[0082] In this embodiment, the rotation process of the seed crystal is changed to a periodic acceleration rotation mode. A single period contains four stages, and the time and rotational acceleration of each stage are T1 and a1, T2 and a2, T3 and a3, T4 and a4 respectively. The initial rotation speed of the seed crystal is 60 r / min, T1 = T3 = 20 min, -a1 = a3 = 6 r / min 2 , T2 = T4 = 10 min, a2 = a4 = 0 r / min 2 . Keep other growth processes the same as those in Embodiment 3 and carry out crystal growth.

[0083] Implementation Results and Analysis

[0084] After changing to the periodic acceleration rotation process of the seed crystal, the fluidity of the melt is enhanced, the solute transport rate is increased, and the crystal growth rate is further improved. In addition, the higher rotation speed reduces the thickness of the solute boundary layer at the growth interface, which is beneficial to improving the stability of the crystal growth interface. Therefore, the crystal surface becomes smoother and the crystal crystallization quality is improved.

[0085] Embodiment 6

[0086] In this embodiment, with other growth process parameters being exactly the same as those in Embodiment 4, the upper surface of the raw material block is processed into a serrated structure for the growth of SiC crystals.

[0087] Implementation Results and Analysis

[0088] In this embodiment, by processing the surface of the raw material block into a serrated structure, the contact area between the flux melt and the raw material block is effectively increased, the dissolution rate of the solute is greatly increased, and the crystal growth rate is increased by about 20%.

[0089] Comparative Example 1

[0090] In this comparative example, the existing top-seed method is used for the growth of SiC single crystals. A graphite crucible with an inner diameter of 150 mm and a height of 150 mm is used, and a flux with a ratio of Si 0.3 Cr 0.65 Al 0.05 is loaded into the graphite crucible, and the total amount of the flux is controlled so that the thickness of the melt after melting is 30 mm. The seed crystal is a 4-inch SiC wafer with an 8-degree inclination angle, and the C plane is used as the growth surface.

[0091] After loading into the furnace, the vacuum degree of the growth chamber is pumped to 10 -4Pa, and then fill the furnace chamber with 0.5atm of high-purity Ar. Heat the crucible so that the temperature at the flux surface reaches 1800℃, and control the temperature field so that the temperature of the lower surface of the melt is higher than the temperature of the upper surface of the melt. The temperature gradient is 10℃ / cm, and the temperature at the bottom of the crucible is 1830℃ / cm. During the growth process, the seed crystal rotates clockwise at a speed of 30r / min, and the crucible rotates counterclockwise at a speed of 10r / min. The pulling speed of the seed crystal is 200μm / h, and the pulling speed of the crucible is 180μm / h. After 60h of growth, pull the crystal out of the melt, the growth is terminated, and the crystal is taken out after slowly cooling to room temperature.

[0092] Figure 3 This is a photo of the graphite crucible and the grown crystal after the growth in Comparative Example 1; Figure 3 (a) is a longitudinal cross-sectional photograph of the graphite crucible after the growth of this comparative example. Figure 3 (b) is a photograph of the crystal morphology grown in this comparative example. Figure 3 (a) shows that the inner wall of the graphite crucible is corroded very seriously and unevenly, which will cause the inner diameter of the crucible to change, and then make the liquid level drop during the growth process uncontrollable and difficult to accurately calibrate. In addition, since there is no Si supplementation during the growth process, the Si content in the melt will continue to decrease as the crystal grows, causing the properties of the melt to change and affecting crystal growth. The above two unstable factors will have a very adverse effect on crystal growth, especially causing a large number of defects in the later stage of crystal growth. Figure 3 (b) shows the morphology of the crystal grown in this comparative example. There are a large number of groove defects on the surface of the crystal. Figure 3 (c)) It can be seen that the defects in the late stage of crystal growth are obviously more than those in the early stage of growth. This comparative example shows that the unstable factors existing in the existing technology will have a very serious impact on crystal growth, which is very unfavorable for the long-term stable growth of crystals.

[0093] Comparative Example 2 (When the high temperature zone is too wide)

[0094] This comparative example adopts the preparation steps of Example 1, except that the step of Example 1 "the temperature of the raw material block decreases rapidly from top to bottom, and the temperature gradient is 50°C / cm" is not included. Specifically, the temperature of the raw material block of this comparative example gradually increases from top to bottom, and the temperature gradient is 10°C / cm. Figure 4 The temperature field configuration shown in (a) is that the temperature of the raw material block gradually increases from top to bottom without a rapid decrease. At this time, the high temperature zone is very wide, covering the melt and the entire raw material block.

[0095] Figure 4 Schematic diagram of the temperature field configuration of comparative example 2 and the state inside the crucible after 20 hours of growth under the temperature field configuration; wherein Figure 4(b) is a schematic diagram of the crucible after 20 h of crystal growth in this comparative example. Since the high-temperature zone is very wide, the temperature of the raw material block gradually increases from top to bottom. The higher the temperature, the easier the raw material block is dissolved. Therefore, the melt tends to continuously dissolve the raw material block downward. In addition, since the crucible wall is a direct heat source and has a relatively high temperature, the raw material block near the crucible wall will be preferentially dissolved, thus forming Figure 4 the corrosion morphology shown in (b). This will cause a large amount of melt to flow into the gap between the crucible side wall and the raw material block, resulting in an abnormal decrease in the melt level. In severe cases, it will cause the separation of the melt level and the crystal, terminating the crystal growth. Therefore, if the special temperature field form proposed in the present invention is not adopted, the long-term stable growth of SiC crystals cannot be achieved, and the technical effects expected by the present invention cannot be obtained.

[0096] Comparative Example 3 (when the melt thickness is too large)

[0097] In this comparative example, the thickness of the flux melt was increased to 60 mm, and other processes were exactly the same as those in Example 1 for crystal growth comparison.

[0098] In this comparative example, due to the too large melt thickness, the temperature difference between the upper surface and the lower surface of the melt is as high as 60 °C, which will cause strong convection in the melt. At the same time, it will lead to too large supersaturation formed at the melt level, resulting in serious spontaneous nucleation at the liquid level, which is very unfavorable for the growth of single crystals. Figure 5 are the optical photograph of the 4-inch SiC ingot grown in Comparative Example 3 and the morphology photograph of the corresponding upper surface of the melt; where Figure 5 (a) is the crystal morphology photograph obtained in this comparative example, and Figure 5 (b) is the morphology photograph of the upper surface of the melt after growth. Figure 5 (a) shows that there are many groove-like defects on the surface of the crystal, and at the same time, there are many polycrystalline SiC particles. Figure 5 (b) shows the morphology of the upper surface of the melt after growth, and it can be seen that there are many polycrystalline SiC particles caused by spontaneous nucleation at the liquid level.

[0099] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although specific process parameters can be optimized and adjusted, the core guiding ideology 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 by the scope of the claims of the present invention.

Claims

1. A method for growing SiC single crystals by the solution method, which comprises the following steps: (1) Place the raw material block at the bottom of the crucible, and place the flux above the raw material block; wherein the composition of the raw material block satisfies that the molar ratio of Si to C is 1:1; (2) Place the crucible in the growth furnace, fix the SiC seed crystal on the lifting rod above the crucible, and then evacuate the growth furnace; (3) Introduce the functional gas and control the air pressure in the growth furnace, and then heat the crucible to completely melt the flux to form a flux melt and make the flux melt in a specific temperature field; (4) Push down the lifting rod to make the seed crystal contact with the flux melt, and rotate and lift the seed crystal and the crucible and dynamically adjust the relative positions among the heat source, the crucible and the seed crystal, so that the relative positions and temperatures among the crystal growth interface, the flux melt and the upper surface of the raw material block remain unchanged, so as to realize that the raw material block is continuously and uniformly dissolved layer by layer into the flux melt from the upper surface in contact with the flux melt to the lower surface of the raw material block, and continuously precipitate SiC single crystals on the crystal growth interface; Wherein, the specific temperature field is: the temperature of the upper surface of the flux melt in contact with the seed crystal is 1500 - 2100 °C, the temperature of the flux melt increases with a temperature gradient of 2 - 30 °C per centimeter-thick flux melt from the upper surface to the lower surface, the temperature of the upper surface of the raw material block in contact with the flux melt is equal to the temperature of the lower surface of the flux melt, and the temperature of the raw material block decreases with a temperature gradient of more than 30 °C per centimeter-thick raw material block from the upper surface to the lower surface; The thickness of the flux melt is 10 - 50 mm; The diameter of the raw material block is equal to the inner diameter of the crucible; In the step (4), the rotation of the seed crystal and the crucible is to perform periodic acceleration and deceleration rotation on the seed crystal and the crucible.

2. The method according to claim 1, wherein The flux is a combination of one or more of the elements Cr, Fe, Cu, Y, Co, Ni, Sc, Ti, Ce, La, Pr, Al, Ga, In, Ge and Sn and Si element.

3. The method according to claim 1, wherein, The thickness of the raw material block is 10 - 300 mm.

4. The method according to claim 3, wherein The thickness of the raw material block is 50 - 300 mm.

5. 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 10 -3 Pa.

6. The method according to claim 1, wherein The functional gas is argon and / or helium.

7. The method according to claim 1, wherein, The control of the air pressure in the growth furnace in the step (3) is carried out under the condition that the air pressure in the growth furnace is controlled to be 0.1 - 2 atm.

8. The method according to claim 1, wherein The periodic accelerating and decelerating rotation is carried out under the following conditions: the crucible and the seed crystal rotate periodically with opposite directions, the rotation speed is ±0 to 200 r / min, and the rotational acceleration is ±0 to 30 r / min 2 .

9. The method according to claim 1, wherein The lifting is carried out at a rate of 1 - 3000 μm / h.

10. The method according to claim 1, wherein, The upper surface of the raw material block is multi-grooved, multi-conical or honeycomb-shaped.

11. The method according to claim 1, wherein The crucible is a tungsten carbide or tantalum carbide crucible, or a tungsten or tantalum crucible with an inner wall treated by carbonization, or a graphite crucible with a high-temperature resistant and flux-corrosion resistant coating on the inner wall.

12. The method according to claim 1, wherein, The size of the seed crystal is 2 - 8 inches, and the inner diameter of the crucible is 0.5 - 2 inches larger than the diameter of the seed crystal.

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