A gallium nitride epitaxial wafer and a preparation method thereof

Abstract

This application belongs to the field of semiconductor technology. To solve the problem of warping that easily occurs in existing epitaxial wafers due to lattice mismatch or stress, a gallium nitride epitaxial wafer and a preparation method thereof are disclosed. The gallium nitride epitaxial wafer includes: a substrate, and a low-temperature AlN buffer layer, a high-temperature AlN buffer layer, a porous AlGaN layer, an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer sequentially disposed on the substrate. The low-temperature AlN buffer layer and the high-temperature AlN buffer layer can buffer the lattice mismatch between the substrate and the gallium nitride transition layer. The porous AlGaN layer can play a role in stress relief. The aluminum gradient layer can improve the lattice matching of the upper gallium nitride layer. And the gallium nitride epitaxy can further reduce the stress generated by the lattice mismatch between the substrate and the epitaxial layer through the gallium nitride transition layer, and can prevent the warping of large-size epitaxial wafers due to lattice mismatch or stress. The gallium nitride epitaxial layer can be prepared by CVD equipment.

Description

Technical Field

[0001] This application belongs to the field of semiconductor technology, and specifically relates to a gallium nitride epitaxial wafer and a preparation method thereof. Background Art

[0002] Gallium nitride (GaN), as a third-generation wide-bandgap semiconductor material, has high application value in the field of microwave devices due to its good physical properties and stability, etc., and is more expected to play an important role in aviation, high-temperature radiation, radar, communication, automotive electronics, etc. However, the preparation of gallium nitride single crystal substrates is relatively difficult and is usually formed by heteroepitaxy. In this way, the epitaxial wafer is prone to warping due to lattice mismatch or stress at large sizes, and as the substrate size increases, the warping problem becomes more prominent. Therefore, it is urgent to solve the stress and warping problems caused by lattice mismatch in the prior art. Summary of the Invention

[0003] In order to solve the warping problem of the existing epitaxial wafer easily caused by lattice mismatch or stress, a gallium nitride epitaxial wafer and a preparation method are proposed.

[0004] To achieve the above object, the present application adopts the following technical solutions:

[0005] A gallium nitride epitaxial wafer includes a substrate, and a low-temperature AlN buffer layer, a high-temperature AlN buffer layer, a porous AlGaN layer, an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer are sequentially arranged on the substrate.

[0006] Preferably, the porous AlGaN layer is formed by depositing an AlInGaN layer on the high-temperature AlN buffer layer and then completely precipitating indium.

[0007] Preferably, after the porous AlGaN layer is formed, nitrogen is used to purge the porous AlGaN layer to remove all the indium precipitated from the AlInGaN layer.

[0008] Preferably, the substrate includes a sapphire substrate; the thickness of the low-temperature AlN buffer layer is 10 - 15 nm, and the formation temperature is 500 - 650 degrees Celsius; the thickness of the high-temperature AlN buffer layer is 40 - 55 nm, and the formation temperature is 800 - 1100 degrees Celsius; the thickness of the AlInGaN layer is 100 - 300 nm; the thickness of the aluminum gradient layer is 80 - 150 nm; the thickness of the gallium nitride transition layer is 50 - 100 nm.

[0009] Preferably, the gallium nitride epitaxial layer is composed of a first gallium nitride epitaxial layer and a second gallium nitride epitaxial layer, and the total thickness of the gallium nitride epitaxial layer is 1.5 - 2 microns, and the thickness of the first gallium nitride epitaxial layer is 100 - 180 nm.

[0010] Based on the above method for preparing a gallium nitride epitaxial wafer, the method comprises the following steps: sequentially disposing a low-temperature AlN buffer layer, a high-temperature AlN buffer layer, and an AlInGaN layer on a substrate, completely precipitating indium in the AlInGaN layer to form a porous AlGaN layer, and then sequentially depositing an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer on the porous AlGaN layer.

[0011] Specifically, the preparation method comprises the following steps:

[0012] Step S1: Place the substrate into a reaction chamber;

[0013] Step S2: Introduce an aluminum source and a nitrogen source into the reaction chamber, and control the temperature in the reaction chamber to be a first temperature to form a low-temperature AlN buffer layer on the substrate;

[0014] Step S3: Raise the temperature in the reaction chamber to a second temperature, and continue to introduce the aluminum source and the nitrogen source to form a high-temperature AlN buffer layer on the low-temperature AlN buffer layer; the second temperature is greater than the first temperature;

[0015] Step S4: Keep introducing the aluminum source and the nitrogen source, and introduce an indium source and a gallium source into the reaction chamber to form an AlInGaN layer on the high-temperature AlN buffer layer; then simultaneously stop introducing the aluminum source, the nitrogen source, the indium source, and the gallium source. After all the indium in the AlInGaN layer has precipitated, a porous AlGaN layer is formed. Use nitrogen to purge the surface of the porous AlGaN layer to remove all the precipitated indium from the AlInGaN layer;

[0016] Step S5: Control the pressure and temperature in the reaction chamber to remain unchanged, reduce the introduction of the aluminum source, and form an aluminum gradient layer on the surface of the porous AlGaN layer;

[0017] Step S6: Stop introducing the aluminum source, and form a gallium nitride transition layer on the surface of the aluminum gradient layer;

[0018] Step S7: Form a gallium nitride epitaxial layer on the gallium nitride transition layer.

[0019] Preferably, the gallium nitride epitaxial layer comprises: a first gallium nitride epitaxial layer and a second gallium nitride epitaxial layer; in the growth stage of the first gallium nitride epitaxial layer, both the temperature and pressure in the reaction chamber are greater than those in the growth stage of the second gallium nitride epitaxial layer, but the growth rate of gallium nitride in the growth stage of the first gallium nitride epitaxial layer is less than that in the growth stage of the second gallium nitride epitaxial layer.

[0020] Specifically, in step S2: the nitrogen source is ammonia, the aluminum source is trimethylaluminum, the first temperature is 500 - 650 degrees Celsius, and the thickness of the formed low-temperature AlN buffer layer is 10 - 15 nm;

[0021] Step S3: The first temperature is 800 - 1100 °C, and the thickness of the formed high-temperature AlN buffer layer is 40 - 55 nm; Step S4: The gallium source is trimethylgallium, the indium source is trimethylindium, the pressure in the reaction chamber is 150 - 200 mbar, the temperature is 1050 - 1100 °C, and at the same time, the feeding of the aluminum source, nitrogen source, indium source, and gallium source is suspended for 30 - 60 s; Control the feeding of nitrogen and the direction of nitrogen blowing to blow away the droplet-shaped indium from the surface of the AlInGaN layer, and collect the blown-away droplet-shaped indium in a recovery device.

[0022] First, a low-temperature AlN buffer layer is formed adjacent to the substrate. When the fine and dense grain-shaped aluminum nitride forms the low-temperature AlN buffer layer, there will be a certain gap between the island-shaped grains; In order to reduce the surface energy, the grains will deform to close the gap, and a high-temperature AlN buffer layer is provided on the low-temperature AlN buffer layer. A dense high-temperature AlN buffer layer can be formed at high temperature, which can improve the formation quality of the epitaxial wafer.

[0023] The formed loose AlGaN layer has an irregular porous microstructure, which can play a role in stress relaxation. The aluminum gradient layer can improve the lattice matching of the upper gallium nitride layer. And depositing a gallium nitride transition layer between the upper part of the aluminum gradient layer and the gallium nitride epitaxial layer can further reduce the stress generated by the lattice mismatch between the substrate and the epitaxial layer, and can prevent the warping of large-size epitaxial wafers due to lattice mismatch or stress.

[0024] It should be noted that the "low-temperature" and "high-temperature" in the "low-temperature AlN buffer layer" and "high-temperature AlN buffer layer" described in this application are names adopted for the convenience of distinguishing the AlN buffer layers generated at two different temperatures. Here, the high temperature and low temperature do not refer to the temperature range. The "low-temperature AlN buffer layer" and "high-temperature AlN buffer layer" can also be correspondingly replaced by the "first AlN buffer layer" and "second AlN buffer layer", and the formation temperature of the first AlN buffer layer is lower than that of the second AlN buffer layer. Brief Description of the Drawings

[0025] Figure 1 It is a schematic cross-sectional structure diagram of the gallium nitride epitaxial wafer structure according to the embodiment of the present application.

[0026] Figure 2 It is a schematic growth process diagram of the preparation of the gallium nitride epitaxial layer according to the embodiment of the present application.

[0027] Illustration: 1 - Substrate, 2 - Low-temperature AlN buffer layer, 3 - High-temperature AlN buffer layer, 4 - Loose AlGaN layer, 5 - Aluminum gradient layer, 6 - Gallium nitride transition layer, 7 - Gallium nitride epitaxial layer. Detailed Embodiment

[0028] The above solution will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are for illustrating the present application and are not intended to limit the scope of the present application. The implementation conditions adopted in the embodiments can be further adjusted according to the conditions of specific manufacturers, and the implementation conditions not specified are usually those in conventional experiments.

[0029] Embodiment 1. As Figure 1 shown, a gallium nitride epitaxial wafer includes a substrate 1, and a low-temperature AlN buffer layer 2, a high-temperature AlN buffer layer 3, a porous AlGaN layer 4, an aluminum gradient layer 5, a gallium nitride transition layer 6, and a gallium nitride epitaxial layer 7 are sequentially arranged on the substrate.

[0030] The gallium nitride epitaxy can play a role in stress relief through the porous AlGaN layer. The aluminum gradient layer can improve the lattice matching of the upper gallium nitride layer. The gallium nitride transition layer can reduce the stress generated by the lattice mismatch between the substrate and the epitaxial layer, and can prevent warping of large-size epitaxial wafers due to lattice mismatch or stress. The gallium nitride epitaxial layer can be prepared by a CVD device.

[0031] Embodiment 2. A method for preparing a gallium nitride epitaxial wafer includes the following steps: sequentially arranging a low-temperature AlN buffer layer, a high-temperature AlN buffer layer, and an AlInGaN layer on a substrate, precipitating all indium in the AlInGaN layer to form a porous AlGaN layer, and using nitrogen to purge the surface of the porous AlGaN layer, and then sequentially depositing an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer on the porous AlGaN layer.

[0032] A preferred implementation manner: The schematic diagram of the growth process for preparing the gallium nitride epitaxial layer is as Figure 2 shown.

[0033] First, place the substrate into the reaction chamber. The substrate can be a sapphire substrate. Before placing the substrate into the reaction chamber, a cleaning step is preferably performed. Cleaning can remove the contamination particles and impurities on the surface of the substrate. The cleaning includes wet cleaning and dry cleaning. First, perform wet cleaning. Use deionized water to clean the surface of the substrate, and then use dry cleaning, such as purging the surface of the substrate with nitrogen, and perform a drying treatment on the substrate to reduce the possible influence of moisture.

[0034] Secondly, control the temperature in the reaction chamber at 500 - 650 °C, introduce an aluminum source and a nitrogen source to form a low-temperature AlN buffer layer with a thickness of 10 - 15 nm; then raise the temperature in the reaction chamber to between 800 - 1100 °C and continue to introduce the aluminum source and the nitrogen source to form a high-temperature AlN buffer layer with a thickness of 40 - 55 nm, where the aluminum source is trimethylaluminum and the nitrogen source is ammonia; in the present invention, a low-temperature AlN buffer layer is first formed on the surface of the substrate. Since the sapphire substrate is made of alumina and has a good lattice match with AlN, the stress between AlN and the substrate is small. And for the AlN layer formed at a lower temperature, because the temperature range is between 500 - 650 °C, multiple small and dense grain-like aluminum nitrides are formed. When forming the aluminum nitride layer, there will be certain gaps between the island-like grains, forming a polycrystalline AlN layer. At high temperatures, a dense high-temperature aluminum nitride layer can be formed as a single-crystal AlN layer with a dense structure, which can improve the formation quality of the epitaxial wafer.

[0035] After forming the high-temperature AlN buffer layer, keep introducing the aluminum source and the nitrogen source, and introduce an indium source and a gallium source into the reaction chamber to form an AlInGaN layer on the high-temperature AlN buffer layer. The gallium source is trimethylgallium and the indium source is trimethylindium. The pressure in the reaction chamber is 150 - 200 mbar, the temperature is 1050 - 1100 °C, and the thickness of the formed AlInGaN layer is 100 - 300 nm. The formation of the AlInGaN layer is for the subsequent formation of a porous AlGaN layer. When forming a certain thickness of the AlInGaN layer, when its thickness is greater than 300 nm, indium in the AlInGaN layer is difficult to precipitate at high temperatures and is prone to form an In - AlGaN alloy. For an AlInGaN layer with a thickness less than 100 nm, after forming the porous AlGaN layer, it is difficult to better relieve stress through the porous structure. Therefore, in the present invention, when forming the AlInGaN layer, control the thickness in the range of 100 - 300 nm, which can facilitate the formation of the porous AlGaN layer and can utilize its porous structure to play a role in stress relief.

[0036] Then simultaneously stop introducing the aluminum source, the nitrogen source, the indium source, and the gallium source, and the stopping time is 30 - 60 s. While stopping the introduction of the reaction source gas, due to the high temperature in the reaction chamber (above 1000 °C near the deposition temperature), indium in it is prone to precipitate. During the pause time of introduction, indium will precipitate from the AlInGaN, and all the indium in the AlInGaN layer can be precipitated during this period.

[0037] Keep the pressure and temperature in the reaction chamber unchanged, reduce the introduction of the aluminum source, and form an aluminum gradient layer.

[0038] Stop introducing the aluminum source to form a gallium nitride transition layer on the surface of the aluminum gradient layer;

[0039] Subsequently, form a gallium nitride epitaxial layer on the gallium nitride transition layer.

[0040] In a preferred embodiment, after all the indium in the AlInGaN layer is precipitated, purge the AlInGaN layer with nitrogen, control the introduction of nitrogen and the direction of nitrogen blowing, blow away the droplet-shaped indium from the surface of the AlInGaN layer after precipitation, and collect the blown-away droplet-shaped indium in a recovery device to prevent contamination of other layer structures. Due to the precipitation of indium, it will adhere to the surface of the porous AlGaN layer to form droplet-shaped indium. During subsequent deposition, it is easy to form an alloy layer with the subsequent indium structure. For example, when a GaN layer is formed subsequently, if indium is present, indium will form an In-GaN alloy with the GaN layer. After forming the alloy layer, on the one hand, it is difficult to separate the epitaxial wafer from the substrate wafer, and on the other hand, due to the presence of indium, it will change the conductivity of the GaN layer. Whether it is used as an LED material or a substrate material for a laser, it will affect the light-emitting performance.

[0041] Form an AlInGaN layer on the high-temperature AlN layer. When forming, when forming the AlN, an aluminum source and a nitrogen source are introduced. After forming a high-temperature AlN layer with a predetermined thickness, keep introducing the aluminum source and the nitrogen source, and introduce an indium source and a gallium source into the reaction chamber. The aluminum source is trimethylaluminum, the gallium source is trimethylgallium, the indium source is trimethylindium, and the nitrogen source is NH3. The pressure in the reaction chamber is 150 - 200 mbar, and the temperature is 1050 - 1100 degrees Celsius to form the AlInGaN layer. Then, simultaneously stop introducing the aluminum source, the nitrogen source, the indium source, and the gallium source, and the pause time for introduction is 30 - 60S.

[0042] After the AlInGaN layer is deposited, pause the growth, and the pause time for growth is 30 - 60S. Since indium is prone to precipitation at high temperatures (the temperature for forming the AlInGaN layer is 1050 - 1100 degrees Celsius, and the temperature will be maintained near the growth temperature during the pause in growth), the precipitated indium will adhere to the surface of the AlInGaN layer. After the indium in the AlInGaN precipitates, the AlInGaN layer will form a porous AlGaN layer structure. By controlling the pause time for growth, all the indium in the AlInGaN layer is precipitated, so that a porous AlGaN layer can be formed. Due to its porous structure, the porous AlGaN layer can act as a stress buffer layer to achieve stress buffering.

[0043] After pausing the introduction of the reaction source for a preset time, control the temperature in the reaction chamber to be 1150 - 1200 °C (the formation temperature is higher than the temperature for forming the AlInGaN layer), introduce an aluminum source, a gallium source, and a nitrogen source into the reaction chamber, and control the introduction amounts of the aluminum source, the gallium source, and the nitrogen source to form Al xGa 1-x AlxGa1-xN aluminum gradient layer, where x is the content of Al, 0 ≤ x < 1, and during the introduction process, the introduction amount of the Al source is continuously reduced until the Al source is completely not introduced. When the Al source is completely not introduced, the formed layer is the GaN layer. The gallium source is trimethylgallium, and the flow rate of trimethylgallium is 150 - 200 sccm. The aluminum source is trimethylaluminum, and the flow rate of trimethylaluminum is 150 - 200 sccm. The nitrogen source is ammonia. The time from reducing the introduction of the Al source to completely not introducing the Al source is 10 - 25 min, and the thickness of the formed AlxGa1-xN layer is 80 - 150 nm. After forming the porous AlGaN layer, although the stress can be relieved, due to the porous gaps inside the AlGaN layer, when growing the gallium nitride layer epitaxially above it, because the crystallization quality of the underlying porous structure is poor, it will affect the crystallization quality of the epitaxial layer and have a great impact on the quality of the epitaxial wafer. Improving the formation of Al x Ga 1-x The formation temperature of the AlxGa1-xN aluminum gradient layer can improve the crystallization quality of the aluminum gradient layer, and with the decrease of the aluminum content, the formed aluminum gradient layer has good lattice matching with the upper gallium nitride epitaxial layer to be formed, preventing the stress caused by lattice mismatch.

[0044] After gradually reducing the introduction amount of the Al source until the Al source is completely not introduced, when the Al source is not introduced, continuously maintain the temperature and pressure in the reaction chamber, and continue to introduce the nitrogen source and the gallium source to form a gallium nitride transition layer. At this time, there is no need to change the reaction conditions, and only need to continue to maintain the reaction conditions and continue to introduce the reaction sources. After stopping the introduction of the Al source, maintain for 2 - 10 min to form a gallium nitride transition layer with a thickness of 50 - 100 nm. This part of the gallium nitride transition layer is mainly to improve the crystallization quality of the subsequent gallium nitride epitaxial wafer. Since it is formed under high temperature and high pressure and the epitaxial speed is slow, therefore, a certain thickness of the gallium nitride transition layer can improve the crystallization quality of the upper epitaxial wafer, and there is no need to form a thicker thickness, which can improve the production effect.

[0045] Then form a gallium nitride epitaxial layer on the gallium nitride transition layer. The subsequent steps of forming the gallium nitride epitaxial layer can refer to the specific steps of this invention.

[0046] Form a gallium nitride epitaxial layer with a total thickness between 1.5 - 2 microns on the gallium nitride transition layer. This step includes: The formation of the gallium nitride epitaxial layer includes two stages,

[0047] Form the first gallium nitride epitaxial layer and the second gallium nitride epitaxial layer. The specific formation process is as follows:

[0048] The first stage: Control the temperature of the reaction chamber. The temperature of the reaction chamber is controlled at 1080 - 1150 degrees Celsius. Introduce a gallium source and a nitrogen source. The gallium source is trimethylgallium, and the flow rate of trimethylgallium is 150 - 200 sccm. The nitrogen source is NH3. Control the flow rates of the gallium source and the nitrogen source, and control the pressure of the reaction chamber. The pressure is controlled at 300 - 350 mbar. Control the growth rate of GaN to be 10 - 15 nm / min. Control the growth time. The thickness of the GaN grown in the first stage is 100 - 180 nm. In the first stage, by controlling the high-pressure and high-temperature state inside the reaction chamber during growth and growing GaN at a slow speed, a dense GaN layer with a well-matched lattice constant and good quality can be formed on the transition GaN layer, which can further reduce the stress between the upper epitaxial layer and the lower layer structure and improve the overall quality of the externally grown GaN. However, the growth in this stage is slow, and the growth thickness should not be too thick, which can save the growth time.

[0049] In this method, when forming a GaN epitaxial wafer, it is formed in two stages. In the first stage, the temperature and pressure are relatively high, and the growth rate is slow, so that a bottom epitaxial wafer with high quality can be formed.

[0050] In the second stage, control the growth rate, and a thicker epitaxial wafer can be quickly formed. And because the quality of the first epitaxial layer below is high, the quality of the epitaxial layer above is also very high, and the stress is small, preventing warping and improving the yield of the epitaxial wafer product.

[0051] Advantages of the present invention: 1. First, form a low-temperature AlN buffer layer and a high-temperature AlN buffer layer on the substrate. The two buffer layers can relieve the stress generated between the upper GaN layer and the substrate due to lattice mismatch. Then, first form a loose AlGaN layer on the high-temperature AlN buffer layer to further buffer the stress. The loose AlGaN layer is formed after indium is precipitated from the AlInGaN layer. After forming the loose AlGaN layer, since the precipitation of indium is likely to affect the device structure, use nitrogen to purge the liquid indium on the surface of the loose AlGaN layer to remove the liquid indium on the surface;

[0052] 2. Increase the temperature on the loose AlGaN layer to form an aluminum gradient layer, which can improve the crystallization quality of the upper layer structure and prevent more crystallization defects brought by the loose layer;

[0053] 3. After the aluminum gradient layer, keep the formation conditions unchanged to form a gallium nitride transition layer. The gallium nitride transition layer formed under high temperature and high pressure has good crystallization quality. When forming a gallium nitride epitaxial layer above, it can improve the crystallization quality of the epitaxial layer. And due to the stress relief of the two buffer layers and the loose AlGaN layer, it can well reduce the stress of the epitaxial wafer and prevent the epitaxial wafer from warping.

[0054] The above embodiments are only used to illustrate the technical concept and features of the present application. The purpose is to enable those skilled in the art to understand the content of the present application and implement it accordingly, and it should not be used to limit the protection scope of the present application. Any equivalent transformation or modification made in accordance with the spirit of the present application should be covered within the protection scope of the present application.

Claims

1. A gallium nitride epitaxial wafer, comprising a substrate, characterized in that: A low-temperature AlN buffer layer, a high-temperature AlN buffer layer, a porous AlGaN layer, an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer are sequentially disposed on the substrate from bottom to top; Among them, the thickness of the gallium nitride epitaxial layer is 1.5 - 2 micrometers, and the thickness of the gallium nitride transition layer is 50 - 100 nm; The preparation of the porous AlGaN layer includes: introducing an aluminum source, a nitrogen source, an indium source, and a gallium source into the reaction chamber, controlling the pressure in the reaction chamber to be 150 - 200 mbar and the temperature to be 1050 - 1100 °C to form an AlInGaN layer; then simultaneously stopping the introduction of the aluminum source, the nitrogen source, the indium source, and the gallium source for 30 - 60 s to form the porous AlGaN layer; The aluminum gradient layer is Al x Ga 1-x N aluminum gradient layer, and the preparation of the Al x Ga 1-x N aluminum gradient layer includes: controlling the temperature in the reaction chamber to be 1150 - 1200 °C, introducing an aluminum source, a gallium source, and a nitrogen source into the reaction chamber to form an Al x Ga 1-x N aluminum gradient layer with a thickness of 80 - 150 nm; where x is the content of Al, 0 ≤ x < 1, and during the introduction process, continuously reduce the introduction amount of the aluminum source until the aluminum source is completely not introduced, and the time from reducing the introduction of the aluminum source to completely not introducing the aluminum source is 10 - 25 min; The low-temperature AlN buffer layer, the high-temperature AlN buffer layer, and the porous AlGaN layer are formed in the same reaction chamber.

2. The gallium nitride epitaxial wafer according to claim 1, wherein: After forming the porous AlGaN layer, the porous AlGaN layer is purged with nitrogen to remove all the indium precipitated from the AlInGaN layer.

3. The gallium nitride epitaxial wafer according to claim 1, characterized in that: The substrate includes a sapphire substrate; the thickness of the low-temperature AlN buffer layer is 10 - 15 nm, and the formation temperature is 500 - 650 degrees Celsius; the thickness of the high-temperature AlN buffer layer is 40 - 55 nm, and the formation temperature is 800 - 1100 degrees Celsius; the thickness of the AlInGaN layer is 100 - 300 nm.

4. A gallium nitride epitaxial wafer according to any one of claims 1 to 3, characterized in that: The gallium nitride epitaxial layer is composed of a first gallium nitride epitaxial layer and a second gallium nitride epitaxial layer, where the thickness of the first gallium nitride epitaxial layer is 100 - 180 nm.

5. A method for preparing a gallium nitride epitaxial wafer for preparing the gallium nitride epitaxial wafer as claimed in claim 1, characterized in that: Deposit a low-temperature AlN buffer layer, a high-temperature AlN buffer layer, a porous AlGaN layer, an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer on the substrate in sequence; Among them, the low-temperature AlN buffer layer, the high-temperature AlN buffer layer, the porous AlGaN layer, and the aluminum gradient layer are formed in the same reaction chamber.

6. The preparation method of a gallium nitride epitaxial wafer according to claim 5, characterized in that: The preparation method includes the following steps: sequentially dispose a low-temperature AlN buffer layer, a high-temperature AlN buffer layer, and an AlInGaN layer on the substrate from bottom to top, precipitate all the indium in the AlInGaN layer to form a porous AlGaN layer, and then deposit an aluminum gradient layer, a gallium nitride transition layer, and a gallium nitride epitaxial layer on the porous AlGaN layer in sequence.

7. The manufacturing method of a gallium nitride epitaxial wafer as described in claim 5, characterized in that: The preparation method includes the following steps: Step S1: Place the substrate in the reaction chamber; Step S2: Introduce an aluminum source and a nitrogen source into the reaction chamber, and control the temperature in the reaction chamber to be a first temperature to form a low-temperature AlN buffer layer on the substrate; Step S3: Raise the temperature in the reaction chamber to a second temperature, and continue to introduce an aluminum source and a nitrogen source to form a high-temperature AlN buffer layer on the low-temperature AlN buffer layer; the second temperature is greater than the first temperature; Step S4: Keep introducing the aluminum source and the nitrogen source, and introduce the indium source and the gallium source into the reaction chamber to form an AlInGaN layer on the high-temperature AlN buffer layer; then stop introducing the aluminum source, the nitrogen source, the indium source, and the gallium source simultaneously. After all the indium in the AlInGaN layer precipitates, a porous AlGaN layer is formed. Use nitrogen to purge the surface of the porous AlGaN layer to remove all the precipitated indium from the AlInGaN layer. Step S5: Keep the pressure and temperature in the reaction chamber unchanged, reduce the introduction of the aluminum source, and form an aluminum gradient layer on the surface of the porous AlGaN layer. Step S6: Stop introducing the aluminum source and form a gallium nitride transition layer on the surface of the aluminum gradient layer. Step S7: Form a gallium nitride epitaxial layer on the gallium nitride transition layer.

8. The preparation method of a gallium nitride epitaxial wafer according to claim 5, wherein: The gallium nitride epitaxial layer includes: a first gallium nitride epitaxial layer and a second gallium nitride epitaxial layer; during the growth stage of the first gallium nitride epitaxial layer, the temperature and pressure in the reaction chamber are both higher than those in the growth stage of the second gallium nitride epitaxial layer, but the growth rate of gallium nitride during the growth stage of the first gallium nitride epitaxial layer is lower than that during the growth stage of the second gallium nitride epitaxial layer.

9. The method for preparing a gallium nitride epitaxial wafer according to claim 7, wherein: Step S2: The nitrogen source is ammonia, the aluminum source is trimethylaluminum, the first temperature is 500 - 650 degrees Celsius, and the thickness of the formed low-temperature AlN buffer layer is 10 - 15 nm. Step S3: The second temperature is 800 - 1100 degrees Celsius, and the thickness of the formed high-temperature AlN buffer layer is 40 - 55 nm. Step S4: The gallium source is trimethylgallium, the indium source is trimethylindium, the pressure in the reaction chamber is 150 - 200 mbar, the temperature is 1050 - 1100 degrees Celsius, and the introduction of the aluminum source, the nitrogen source, the indium source, and the gallium source is paused for 30 - 60 s. Control the introduction of nitrogen and the direction of nitrogen blowing to blow away the droplet-shaped indium from the surface of the AlInGaN layer and collect the blown-away droplet-shaped indium in a recovery device.

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