Growth method for improving quality of gallium nitride crystal on silicon carbide substrate

Abstract

The invention provides a growth method for improving the quality of gallium nitride crystals on a silicon carbide substrate. According to the method, gallium nitride grows on a silicon carbide substrate through the MOCVD technology, a special buffer layer structure is adopted, and a special cooling process matched with the structure is adopted for optimization. And the buffer layer structure adopts a multi-layer structure with aluminum nitride-aluminum gallium nitride-gallium nitride-aluminum gallium nitride as a repetitive structure unit. In the cooling process, segmented cooling is adopted, and the segmented cooling comprises slow cooling by introducing hydrogen, wave type cooling, heat preservation at the set temperature and finally natural cooling to the room temperature. According to the method, the quality of the gallium nitride crystal is effectively improved, the stress and dislocation density in the gallium nitride crystal are reduced, and the surface appearance is improved, so that the performance and reliability of a gallium nitride device are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor material growth, in particular to a growth method for improving the quality of a gallium nitride crystal on a silicon carbide substrate. BACKGROUND

[0002] GaN, as a prominent representative of the third-generation semiconductor material, is gradually causing a profound revolution in the global semiconductor field. It boasts many outstanding characteristics, such as a large bandgap (about 3.4 eV), which enables it to efficiently excite shorter-wavelength high-energy photons in the field of optoelectronic devices, providing a solid foundation for the development of blue and ultraviolet light emitting devices; a high electron mobility (about 800 cm² / (V·s)), which means that electrons can complete high-speed migration under the action of an electric field in a very short time, enabling GaN-based electronic devices to have ultra-fast response and signal processing capabilities, meeting the stringent demands of modern high-frequency communication and fast pulse circuits; and an extremely fast saturated electron drift speed (about 2.7×10 7 cm / s), which enables it to exhibit extraordinary transmission and conversion performance in high-power and high-frequency applications, effectively avoiding signal delay and loss. For this reason, GaN has shown broad application prospects in the fields of optoelectronic devices and electronic devices. In the field of optoelectronic devices, it has promoted the advent of high-brightness blue and ultraviolet light emitting diodes (LEDs), which not only bring more vibrant color presentation, higher brightness, and lower energy consumption in display technology, but also shine in professional fields such as curing and medical lighting. In the field of electronic devices, GaN-based high-frequency and high-power devices, such as high electron mobility transistors (HEMTs), perform well in high-frequency application scenarios such as 5G communication and radar systems, and are suitable for power management systems of new energy vehicles and high-power power converters in industrial applications.

[0003] Currently, the growth of GaN materials faces a diversified selection of substrates. GaN, Si, sapphire (Al2O3), SiC, ZnO, GaAs, and other substrates each have their own advantages and disadvantages. However, among the many substrates, silicon carbide (SiC) substrates stand out with unique advantages and have become the ideal choice for GaN growth. SiC substrates exhibit excellent stability at high temperatures, and their crystal structure is less likely to distort or decompose under high-temperature conditions, providing a stable base for the growth of GaN epitaxial layers. They also have excellent electrical conductivity and thermal conductivity, and efficient thermal conductivity can effectively dissipate the heat generated by GaN devices during operation, effectively reducing the risk of performance degradation and failure due to overheating, and ensuring the reliability of devices under high-power and long-time operation conditions.

[0004] From the perspective of lattice matching, the lattice mismatch between SiC and GaN is only 3.4%, which helps to reduce the lattice distortion and defects caused by the difference in lattice constants during epitaxial growth. In terms of thermal expansion coefficient, SiC and GaN have similar thermal expansion coefficients, which makes their size change trends relatively consistent during temperature changes, reducing the thermal stress caused by the large difference in thermal expansion coefficients, thereby reducing the risk of cracks and delamination in the epitaxial layer, effectively ensuring the stability and service life of the device.

[0005] However, despite the many advantages of SiC substrates, there is still a certain difference in the thermal expansion coefficients of silicon carbide and gallium nitride. During high-temperature growth, the silicon carbide substrate and the gallium nitride epitaxial layer expand due to the increase in temperature, but due to the difference in thermal expansion coefficients, the expansion of the two is inconsistent. When the growth is completed and enters the cooling stage, the silicon carbide substrate and the gallium nitride epitaxial layer shrink due to the decrease in temperature, but again due to the difference in thermal expansion coefficients, the shrinkage speed and degree of the two are not the same. This mismatched shrinkage can cause complex thermal stress in the epitaxial layer, mainly in the form of tensile stress or compressive stress. Taking tensile stress as an example, when the shrinkage degree of the GaN epitaxial layer is less than that of the SiC substrate, the GaN layer will be stretched, resulting in tensile stress. If this tensile stress exceeds the bearing limit of GaN material, it will initiate and propagate cracks in the epitaxial layer, causing a significant decrease in the crystalline quality of the material. In terms of device performance, cracks can damage the lattice integrity of GaN crystals, increase the scattering centers of charge carriers, and hinder the migration of electrons and holes, reducing the electrical performance of the device, such as reduced mobility and increased resistance, which in turn leads to increased leakage current. The increase in leakage current not only consumes additional electrical energy and reduces the energy efficiency of the device, but also can cause overheating and other problems. At the same time, cracks can weaken the insulation performance of the device, reducing the breakdown voltage and limiting the performance of the device in high-voltage and high-power applications, seriously affecting the reliability of the device, shortening its service life, and hindering the further expansion of GaN devices in high-end application fields.

[0006] Some research focuses on improving the substrate material itself, trying to reduce the difference in thermal expansion coefficient through special treatment or doping, but this method often faces problems such as complex process, high cost, and limited effect. Some other research focuses on optimizing epitaxial growth process parameters, such as adjusting growth temperature, pressure, gas flow, etc., aiming to reduce the generation of defects during growth, but the stress control during the cooling stage has not been fundamentally solved. Therefore, adopting a new buffer layer structure and combining the key link of the cooling process, developing an effective optimization method is of great significance for improving the quality of GaN crystals on silicon carbide substrates, and further improving the performance and reliability of the device, and is also the key to promoting GaN-based devices to more widely used application fields. SUMMARY

[0007] In view of the problem of thermal expansion coefficient difference of gallium nitride hetero-epitaxial growth on silicon carbide substrate, the application provides a growth method for improving the quality of gallium nitride crystal on silicon carbide substrate. The method of the application relates to growing gallium nitride on silicon carbide substrate by MOCVD technology, adopting a special buffer layer structure, and adopting a special cooling process suitable for the structure for optimization. The buffer layer structure adopts a multilayer structure with aluminum nitride-aluminum gallium nitride-gallium nitride-aluminum gallium nitride as a repeating structural unit. The cooling process adopts segmented cooling, which includes slow cooling by hydrogen gas, wave-like cooling, and holding at a set temperature, and finally natural cooling to room temperature. The method effectively improves the quality of gallium nitride crystal, reduces the internal stress and dislocation density of gallium nitride crystal, and improves the surface morphology, thereby significantly improving the performance and reliability of gallium nitride devices.

[0008] The technical scheme of the application is as follows: A growth method for improving the quality of gallium nitride crystal on silicon carbide substrate, comprising the following steps: (1) Substrate cleaning: the silicon carbide substrate is sequentially washed with acetone, deionized water, hydrochloric acid solution, and deionized water, and dried to obtain a cleaned substrate; (2) Substrate high-temperature cleaning: the cleaned substrate is placed in a MOCVD reaction chamber, and hydrogen gas is introduced for pretreatment to remove the oxide layer on the surface of the substrate and activate the surface, thereby obtaining a pretreated substrate; (3) Buffer layer growth: growing a buffer layer on the pretreated substrate; the structure of the buffer layer is a multilayer structure with aluminum nitride (AlN)-aluminum gallium nitride (AlGaN)-gallium nitride (GaN)-aluminum gallium nitride (AlGaN) as a repeating structural unit; in the repeating structural unit, from bottom to top, there are aluminum nitride, aluminum gallium nitride, gallium nitride, and aluminum gallium nitride; (4) Gallium nitride layer growth: growing a gallium nitride layer on the buffer layer; (5) First step of cooling: after the gallium nitride layer is grown, hydrogen gas is immediately introduced, and the cooling rate is controlled to be 5-10 ℃ / min, and the temperature is reduced to 800-950 ℃; the slow cooling in this stage helps to release the thermal stress accumulated during growth, reduces the stress concentration in the crystal, and improves the quality of the gallium nitride crystal.

[0009] (6) Second step of cooling: continue to introduce hydrogen gas, hold at 800-950 ℃ for 5-15 minutes, control the cooling rate to be 10-15 ℃ / min, reduce by 30-60 ℃, increase by 20-30 ℃ at the same rate, and repeat the cooling and heating process until 400-500 ℃, then hold at 400-500 ℃ for 5-15 minutes. The wave-like cooling in this stage helps to further release the thermal stress accumulated during growth, reduces the stress concentration in the crystal, ensures the stability of the crystal structure, and effectively improves the quality of the gallium nitride crystal.

[0010] (7) The third step of cooling: continue to pass hydrogen, and naturally cool to room temperature.

[0011] According to the application, preferably, in step (1), the silicon carbide substrate is soaked in acetone for ultrasonic cleaning for 5-15 minutes to remove oil stains and organic impurities on the surface of the substrate; then, the substrate is sequentially soaked in deionized water for ultrasonic cleaning for 5-15 minutes, and in a 5%-10% hydrochloric acid aqueous solution for 5-15 minutes to remove particles and metal ion impurities; finally, the substrate is cleaned with deionized water; and high-purity nitrogen is used for blowing dry, so that the surface of the substrate is clean and flawless, and a cleaned substrate is obtained.

[0012] According to the application, preferably, in step (2), the flow rate of hydrogen is 2500-3500 sccm; the pretreatment temperature is 1000-1200℃, and the pretreatment time is 10-20 minutes.

[0013] According to the application, preferably, in step (3), the thickness of the buffer layer is 80-150 nm, the thickness of the aluminum nitride layer is 1-5 nm, the thickness of the aluminum gallium nitride layer is 10-20 nm, and the thickness of the gallium nitride layer is 1-5 nm.

[0014] According to the application, preferably, in step (3), the growth of the buffer layer is performed in a MOCVD reaction chamber; the reaction chamber pressure is 20-200 mbar, and the growth temperature is 1000-1200℃; the raw material for the growth of the aluminum nitride layer is trimethylaluminum and ammonia; the raw material for the growth of the aluminum gallium nitride layer is trimethylaluminum, trimethylgallium and ammonia; the raw material for the growth of the gallium nitride layer is trimethylgallium and ammonia; the flow rate of trimethylaluminum is 10-100 sccm, the flow rate of trimethylgallium is 20-200 sccm, and the flow rate of ammonia is 1000-5000 sccm.

[0015] According to the application, preferably, in step (4), the thickness of the gallium nitride layer is 1-5 μm.

[0016] According to the application, preferably, in step (4), the growth of the gallium nitride layer is performed in a MOCVD reaction chamber; the reaction chamber pressure is 20-200 mbar, the growth temperature is 1000-1200℃, the flow rate of trimethylgallium is 50-200 sccm, and the flow rate of ammonia is 2000-6000 sccm.

[0017] According to the application, preferably, in step (5), the cooling rate increases with the increase of the thickness of the gallium nitride layer.

[0018] According to the application, preferably, in step (6), in one cooling and heating cycle process, the heating amplitude is less than the cooling amplitude.

[0019] The technical features and beneficial effects of the application are as follows: (1) Improve crystal quality: by using multiple layers of aluminum nitride-aluminum gallium nitride-gallium nitride-aluminum gallium nitride reciprocating structure buffer layer and optimizing the cooling process, effectively reducing the thermal stress inside the GaN crystal, reducing the generation of dislocations and cracks, thereby improving the quality and performance of the crystal. This optimization can significantly reduce the generation of cracks caused by thermal mismatch. Compared with the comparative sample without using the cooling technology and buffer layer of the present application, the dislocation density of the epitaxial layer is reduced by 15%-25%, which improves the overall performance of the gallium nitride epitaxial layer.

[0020] (2) Strong repeatability: the method of the present application has strong repeatability. It can be stably reproduced in different equipment and environment, providing reliable guarantee for large-scale production and application of gallium nitride materials.

[0021] (3) Good process compatibility: the cooling optimization method and buffer layer growth method of the present application are highly compatible with existing gallium nitride growth processes. No additional new equipment needs to be purchased or complex process modification needs to be carried out. Only simple adjustment of the existing MOCVD growth process can realize the optimization of the cooling process and the growth of the buffer layer. This greatly reduces the threshold and cost of technology application, making it easy to popularize and apply in industrial production, and helps to promote the large-scale commercial production of gallium nitride-based devices.

[0022] (4) The optimization of the cooling process also needs to consider the influence of the gas environment. In the cooling process, specific gas (such as hydrogen) is introduced to protect the crystal and prevent it from reacting with oxygen in the air at high temperature. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The growth structure of the device of the present application; wherein 1 is a silicon carbide substrate, 2 is a buffer layer, and 3 is a gallium nitride layer.

[0024] Figure 2 The cooling process of the present application is shown in the figure.

[0025] Figure 3 The XRD test data of the sample grown by the method of Example 1 of the present application and the sample grown by the method of Comparative Example 1. DETAILED DESCRIPTION

[0026] The present application will be further described below in conjunction with the embodiments and drawings, but is not limited thereto.

[0027] Example 1 A growth method for improving the quality of gallium nitride crystal on a silicon carbide substrate, referring to Figure 1 and Figure 2 , gallium nitride is grown on a 4-inch silicon carbide substrate using a MOCVD device, which specifically includes the following steps: (1) Substrate cleaning and pretreatment: 4-inch silicon carbide substrates are immersed in an acetone solution and ultrasonically cleaned for 10 minutes to remove oil and organic impurities on the surface of the substrate. Subsequently, the substrate is immersed in deionized water and ultrasonically cleaned for 10 minutes, and then immersed in a 10% hydrochloric acid aqueous solution for 10 minutes to remove particulate and metal ion impurities. Finally, the substrate is rinsed with deionized water and dried with high-purity nitrogen gas to ensure that the surface of the substrate is clean and free of defects, thereby obtaining a cleaned substrate.

[0028] (2) High-temperature cleaning of the substrate: The cleaned substrate is placed in an MOCVD reaction chamber, hydrogen gas is introduced (flow rate of 3000 sccm), and the pretreatment temperature is set to 1100°C for 15 minutes.

[0029] (3) Buffer layer growth: A buffer layer 2 is grown on the pretreated substrate 1 in the MOCVD reaction chamber, and the buffer layer 2 is a multilayer structure with aluminum nitride (AlN)-aluminum gallium nitride (AlGaN)-gallium nitride (GaN)-aluminum gallium nitride (AlGaN) as the repeating structural unit, and the number of repeating structural units is 3. The reaction chamber pressure is set to 50 mbar, and the growth temperature is set to 1050°C. Trimethylaluminum is introduced at a flow rate of 50 sccm, and ammonia is introduced at a flow rate of 3000 sccm to grow an aluminum nitride layer with a thickness of 5 nm on the substrate. Then, trimethylaluminum is introduced at a flow rate of 50 sccm, trimethylgallium is introduced at a flow rate of 100 sccm, and ammonia is introduced at a flow rate of 3000 sccm to grow an aluminum gallium nitride layer with a thickness of 15 nm on the aluminum nitride layer. Then, trimethylgallium is introduced at a flow rate of 100 sccm, and ammonia is introduced at a flow rate of 3000 sccm to grow a gallium nitride layer with a thickness of 5 nm on the aluminum gallium nitride layer. Then, trimethylaluminum is introduced at a flow rate of 50 sccm, trimethylgallium is introduced at a flow rate of 100 sccm, and ammonia is introduced at a flow rate of 3000 sccm to grow an aluminum gallium nitride layer with a thickness of 15 nm on the gallium nitride layer. The above growth process is repeated for a total of 3 cycles, and the total thickness of the final buffer layer 2 is 120 nm.

[0030] (4) Gallium nitride layer 3 growth: The gallium nitride layer 3 is continuously grown on the buffer layer 2 in the MOCVD reaction chamber. The temperature and pressure of the reaction chamber are kept basically unchanged, trimethylgallium is adjusted to a flow rate of 150 sccm, ammonia is adjusted to a flow rate of 4000 sccm, and a gallium nitride layer 3 with a thickness of 2 μm is grown.

[0031] (5) First step of cooling: After the growth of the gallium nitride layer 3 is completed, hydrogen gas is immediately introduced, and the cooling rate is controlled at 5°C / min, and the temperature is slowly reduced from 1050°C to 900°C.

[0032] (6) Second step of cooling: continue to pass hydrogen, keep 900 °C for 10 minutes, control to reduce 40 °C at the rate of 10 °C / min, then increase 20 °C at the same rate of cooling, cycle the above cooling, heating process, until 460 °C, keep 10 minutes. Further stabilize the crystal structure, relieve the stress mismatch problem caused by the difference of thermal expansion coefficient.

[0033] (7) Third step of cooling: after keeping, continue to pass hydrogen, turn off the power, let the reaction chamber naturally cool to room temperature.

[0034] Example 2 A method for improving the quality of gallium nitride crystal grown on silicon carbide substrate, as described in Example 1, except that: In step (3), the thickness of the aluminum nitride layer in the repeated structure unit of the buffer layer is 5 nm, the thickness of the aluminum gallium nitride layer is 20 nm, and the thickness of the gallium nitride layer is 5 nm; the number of repeated structure units is 3, and the total growth thickness of the buffer layer is 150 nm.

[0035] In step (5), the temperature is slowly reduced from 1050 °C to 850 °C.

[0036] In step (6), continue to pass hydrogen, keep 850 °C for 10 minutes, control to reduce 50 °C at the rate of 15 °C / min, then increase 25 °C at the same rate of cooling, cycle the above cooling, heating process, until 400 °C, keep 10 minutes.

[0037] Other steps and conditions are the same as in Example 1.

[0038] Example 3 A method for improving the quality of gallium nitride crystal grown on silicon carbide substrate, as described in Example 1, except that: In step (3), the thickness of the aluminum nitride layer in the repeated structure unit of the buffer layer is 1 nm, the thickness of the aluminum gallium nitride layer is 10 nm, and the thickness of the gallium nitride layer is 1 nm; the number of repeated structure units is 4, and the total growth thickness of the buffer layer is 88 nm.

[0039] Other steps and conditions are the same as in Example 1.

[0040] Example 4 A method for improving the quality of gallium nitride crystal grown on silicon carbide substrate, as described in Example 1, except that: In step (5), the temperature is slowly reduced from 1050 °C to 800 °C.

[0041] In step (6), hydrogen gas is continuously introduced, and the temperature is kept at 800°C for 10 minutes. The temperature is decreased by 30°C at a rate of 10°C / min, and then increased by 20°C at the same rate. The above-mentioned decreasing and increasing processes are repeated until the temperature reaches 400°C, and the temperature is kept at 400°C for 10 minutes.

[0042] The other steps and conditions are the same as in Example 1.

[0043] Comparative Example 1 A method for growing a gallium nitride crystal on a silicon carbide substrate, the steps of which are as follows: Steps (1) and (2) are the same as in Example 1. Step (3) is as follows: Buffer layer growth: a buffer layer is grown on the pretreated substrate in the MOCVD reaction chamber. The buffer layer is an aluminum gallium nitride buffer layer with a thickness of 80 nm. The pressure in the reaction chamber is set to 50 mbar, and the growth temperature is set to 1050°C. Trimethylaluminum, trimethylgallium, and ammonia are introduced, and an aluminum gallium nitride buffer layer with a thickness of 80 nm is grown on the pretreated substrate.

[0044] Step (4) is the same as in Example 1. Step (5) is as follows: After the gallium nitride layer is grown, hydrogen gas is immediately introduced, the power is turned off, and the temperature is decreased to room temperature using natural cooling.

[0045] Comparative Example 2 A method for growing a gallium nitride crystal on a silicon carbide substrate, as described in Example 1, except that: Step (3) is as follows: Buffer layer growth: a buffer layer is grown on the pretreated substrate in the MOCVD reaction chamber. The buffer layer is a multilayer structure with aluminum nitride (AlN)-aluminum gallium nitride (AlGaN) as the repeating structural unit, and the number of repeating structural units is 4. The pressure in the reaction chamber is set to 50 mbar, and the growth temperature is set to 1050°C. Trimethylaluminum is introduced at a flow rate of 50 sccm, and ammonia is introduced at a flow rate of 3000 sccm. An aluminum nitride layer with a thickness of 5 nm is grown on the substrate. Then, trimethylaluminum is introduced at a flow rate of 50 sccm, trimethylgallium is introduced at a flow rate of 100 sccm, and ammonia is introduced at a flow rate of 3000 sccm. An aluminum gallium nitride layer with a thickness of 15 nm is grown on the aluminum nitride layer. The above-mentioned growth process is repeated a total of 4 times. The total thickness of the final buffer layer is 80 nm.

[0046] The other steps and conditions are the same as in Example 1.

[0047] Comparative Example 3 A method for growing a gallium nitride crystal on a silicon carbide substrate, as described in Example 1, except that: Step (6) is omitted; after the temperature is decreased to 900°C in step (5), hydrogen gas is continuously introduced, the power is turned off, and the reaction chamber is naturally cooled to room temperature.

[0048] Other procedures and conditions are the same as Example 1.

[0049] Test Example The length of edge cracks of the samples prepared in the examples and comparative examples were observed under a metallographic microscope. The full width at half maximum of XRD of the samples prepared in the examples and comparative examples were tested. The length of edge cracks and the full width at half maximum of XRD of the examples and comparative examples are as follows.

[0050] Compared with Comparative Example 1, the length of edge cracks of the sample prepared in Example 1 was shortened from 10 mm to 3 mm under a metallographic microscope.

[0051] The XRD patterns of the samples prepared in Example 1 and Comparative Example 1 are shown in Figure 3 Compared with Comparative Example 1, the full width at half maximum of XRD of the sample prepared in Example 1 was reduced from 153 arcsec to 121 arcsec, the dislocation density was reduced by 21%, and the crystal quality was obviously improved.

Claims

1. A method for improving the quality of gallium nitride crystals on a silicon carbide substrate, comprising the following steps: (1) Substrate cleaning: The silicon carbide substrate is washed sequentially with acetone, deionized water, hydrochloric acid aqueous solution and deionized water, and then dried to obtain the cleaned substrate; (2) High-temperature cleaning of substrate: The cleaned substrate is placed in the MOCVD reaction chamber and hydrogen is introduced for pretreatment to obtain a pretreated substrate; (3) Buffer layer growth: A buffer layer is grown on the pretreated substrate; the structure of the buffer layer is a multilayer structure with aluminum nitride (AlN)-aluminum gallium nitride (AlGaN)-gallium nitride (GaN)-aluminum gallium nitride (AlGaN) as repeating structural units; in the repeating structural units, from bottom to top, they are aluminum nitride, aluminum gallium nitride, gallium nitride, and aluminum gallium nitride; (4) Gallium nitride layer growth: A gallium nitride layer is grown on the buffer layer; (5) First step of cooling: After the gallium nitride layer growth is completed, hydrogen gas is immediately introduced and the cooling rate is controlled at 5-10℃ / min to reduce the temperature to 800-950℃; (6) Second step of cooling: continue to introduce hydrogen gas, keep at 800-950℃ for 5-15 minutes, control the cooling rate to drop by 30-60℃ at a rate of 10-15℃ / min, and raise the temperature by 20-30℃ at the same rate of cooling. Repeat the cooling and heating process until 400-500℃, and then keep at 400-500℃ for 5-15 minutes. (7) Third step of cooling: continue to introduce hydrogen gas and let it cool naturally to room temperature.

2. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (1), the silicon carbide substrate is immersed in acetone and ultrasonically cleaned for 5-15 minutes; then, the substrate is successively immersed in deionized water and ultrasonically cleaned for 5-15 minutes, and then immersed in hydrochloric acid aqueous solution with a mass concentration of 5%-10% for 5-15 minutes; finally, it is rinsed clean with deionized water; and then dried with high-purity nitrogen to obtain the cleaned substrate.

3. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (2), the flow rate of hydrogen is 2500-3500 sccm; the pretreatment temperature is 1000-1200℃; and the pretreatment time is 10-20 minutes.

4. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (3), the thickness of the buffer layer is 80-150nm, the thickness of the aluminum nitride layer is 1-5nm, the thickness of the aluminum gallium nitride layer is 10-20nm, and the thickness of the gallium nitride layer is 1-5nm.

5. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (3), the growth of the buffer layer is carried out in the MOCVD reaction chamber; the reaction chamber pressure is 20-200 mbar and the growth temperature is 1000-1200℃; the raw materials used for the growth of the aluminum nitride layer are trimethylaluminum and ammonia; the raw materials used for the growth of the aluminum gallium nitride layer are trimethylaluminum, trimethylgallium and ammonia; the raw materials used for the growth of the gallium nitride layer are trimethylgallium and ammonia; the flow rate of trimethylaluminum is 10-100 sccm, the flow rate of trimethylgallium is 20-200 sccm, and the flow rate of ammonia is 1000-5000 sccm.

6. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (4), the thickness of the gallium nitride layer is 1-5 μm.

7. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (4), the growth of the gallium nitride layer is carried out in an MOCVD reaction chamber; the reaction chamber pressure is 20-200 mbar, the growth temperature is 1000-1200℃, the trimethylgallium flow rate is 50-200 sccm, and the ammonia flow rate is 2000-6000 sccm.

8. The method for improving the quality of gallium nitride crystals on a silicon carbide substrate according to claim 1, characterized in that, In step (6), during a cooling and heating cycle, the heating amplitude is less than the cooling amplitude.

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