A non-polar a-plane gallium nitride epitaxial structure and preparation method thereof

Abstract

The present invention relates to the field of semiconductor device technology, and in particular to a non-polar a-plane gallium nitride epitaxial structure and a preparation method thereof. After providing an r-plane sapphire substrate and few-layer graphene, the few-layer graphene is transferred to the growth surface of the substrate, and the few-layer graphene is bonded to the growth surface to prepare an epitaxial substrate. The area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate accounts for less than or equal to 1%, and the maximum thickness is any value between 0.3nm and 0.7nm. Metal organic compound chemical vapor deposition is performed on the surface of the epitaxial substrate to obtain a non-polar a-plane gallium nitride epitaxial structure. The few-layer graphene reduces the lattice mismatch problem between the substrate and the gallium nitride epitaxial layer obtained by epitaxy, ensures that the electrostatic potential of the substrate can act on the epitaxial layer through the few-layer graphene, and also reduces the local stress or gaps caused by the wrinkles, which helps to improve the quality of the epitaxial layer.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor devices, and in particular to a non-polar a-plane gallium nitride epitaxial structure and a preparation method thereof. Background Art

[0002] Gallium nitride (GaN) is a direct-gap semiconductor with a wide bandgap and high hardness. Its excellent electron mobility and saturated drift velocity enable it to perform exceptionally well in radio frequency and microwave electronics, and it is currently widely used in various semiconductor devices. Currently, mass production of polar GaN epitaxial structures grown along the c-plane has been achieved. However, the combined effects of spontaneous polarization and piezoelectric polarization on the c-axis of the GaN crystal form an internal electric field. This field significantly perturbs the spatial distribution of electron and hole wave functions through the quantum trapped Stark effect (QCSE), significantly reducing the probability of carrier recombination and luminescence efficiency. This also causes a redshift in the emission wavelength, which severely restricts the performance of optoelectronic devices such as light-emitting diodes (LEDs) and lasers. In contrast, GaN grown on non-polar surfaces exhibits no polarization effect and exhibits no band bending. Therefore, growing GaN on non-polar surfaces can avoid the QCSE and improve the performance of semiconductor devices.

[0003] However, non-polar GaN materials are currently difficult to obtain through epitaxial growth. The reason lies in the complex contradiction between the crystallographic properties and growth dynamics of GaN. First, the lattice mismatch between the non-polar surface of GaN and the commonly used substrate is large, and the periodicity of the atomic arrangement is weak, which makes it difficult to form a continuous and smooth two-dimensional growth front in the early stage of epitaxy. It is easy to convert into a three-dimensional island growth mode, which significantly increases the surface roughness and defect density. Secondly, the surface energy of the non-polar surface of GaN is high, and the atomic migration barrier is large. Higher activation energy is required to maintain orderly diffusion, which leads to the degradation of the buffer layer preparation quality and the narrowing of the high-temperature epitaxial window, and the tolerance for process parameter control is low. In addition, the in-plane anisotropic stress is significant during the growth of the non-polar surface of GaN, which can easily induce wafer bending, crack initiation and dislocation aggregation, further exacerbating structural instability.

[0004] At present, the main research directions for improving the quality of non-polar GaN epitaxial layers include selective growth on the surface of patterned substrates, epitaxial lateral overgrowth, setting intermediate layers and forming multilayer films. However, these methods have limited effects on improving the quality of GaN epitaxial layers and are still difficult to meet the needs of practical applications. Summary of the Invention

[0005] The object of the present invention is to provide a non-polar a-plane gallium nitride epitaxial structure with high crystal quality and a preparation method thereof.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for preparing a non-polar a-plane gallium nitride epitaxial structure, comprising:

[0008] Provide r-plane sapphire substrate and few-layer graphene;

[0009] Transferring the few-layer graphene to the growth surface of the substrate and laminating the few-layer graphene to the growth surface to prepare an epitaxial substrate, wherein a ratio of an area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate to a total area is less than or equal to 1%, and a maximum thickness of the few-layer graphene in the epitaxial substrate is any value between 0.3 nm and 0.7 nm;

[0010] Metal organic compound chemical vapor deposition is performed on the surface of the epitaxial substrate to obtain a non-polar a-plane gallium nitride epitaxial structure.

[0011] Optionally, the few-layer graphene is grown by chemical vapor deposition on the surface of copper foil, and

[0012] Transferring the few-layer graphene to the growth surface of the substrate comprises:

[0013] Polymethyl methacrylate is coated on the surface of the grown few-layer graphene to form a transfer medium layer with a thickness of any value between 8 nm and 12 nm. The copper foil with the transfer medium layer is placed in a ferric chloride solution, and after the copper foil is corroded and removed, it is rinsed with deionized water. The transfer medium layer and the few-layer graphene are transferred together to the growth surface of the substrate, so that the transfer medium layer is adhered to the growth surface. After heating to a temperature of any value between 90° C. and 110° C., the substrate connected with the few-layer graphene is immersed in an acetone solution to transfer the few-layer graphene to the growth surface.

[0014] Optionally, the growth conditions of the few-layer graphene are to raise the temperature to any value between 950°C and 1020°C under argon protection, and then introduce hydrogen with a flow rate of any value between 45sccm and 50sccm and methane with a flow rate of any value between 8sccm and 10sccm, and the growth time is any value between 25min and 30min.

[0015] Optionally, in the metal organic compound chemical vapor deposition, nitrogen and hydrogen are used as carrier gases, trimethylgallium and ammonia are used as process gases, and after a first period of low-temperature deposition, a second period of high-temperature deposition is performed. The temperature of the low-temperature deposition is any value between 750°C and 850°C, and the temperature of the high-temperature deposition is any value between 900°C and 1000°C.

[0016] Optionally, in the low-temperature deposition, the total flow rate of the carrier gas and the process gas is any value between 2800 sccm and 2850 sccm, the flow rate ratio of the carrier gas to the process gas is any value between (8 and 9):1, the proportion of nitrogen in the carrier gas is any value between 85% and 91%, the flow rate ratio of trimethylgallium and ammonia is any value between 1:(80 and 85), and the first time is any value between 13 minutes and 17 minutes;

[0017] The total flow rate of the carrier gas and the process gas in the high-temperature deposition is any value between 6550 sccm and 6600 sccm, the flow ratio of the carrier gas and the process gas is any value between (30 and 38):1, the proportion of nitrogen in the carrier gas is any value between 85% and 91%, the flow ratio of trimethylgallium and ammonia is any value between 1:(16 and 18), and the second time is any value between 80 min and 90 min.

[0018] Optionally, laminating the few-layer graphene to the growth surface includes:

[0019] After preheating at a first temperature for a third time in an inert atmosphere, the epitaxial substrate is obtained by heating to a second temperature and annealing for a fourth time, wherein the first temperature is any value between 300°C and 400°C, the third time is any value between 30 minutes and 45 minutes, the second temperature is any value between 700°C and 900°C, and the fourth time is any value between 60 minutes and 90 minutes.

[0020] Optionally, before the metal organic compound chemical vapor deposition is performed on the epitaxial substrate, the integrity of the few-layer graphene is verified by atomic force microscopy.

[0021] Optionally, after the substrate is chemically polished to a surface roughness of any value between 0.1 nm and 0.5 nm, the surface is functionalized by ammonia at a temperature of any value between 300° C. and 500° C. for a treatment time of any value between 30 min and 60 min, and then the few-layer graphene is transferred.

[0022] The present invention also provides a non-polar a-plane gallium nitride epitaxial structure, which is prepared using the above-mentioned method for preparing the non-polar a-plane gallium nitride epitaxial structure.

[0023] The beneficial effects of the present invention are: a few-layer graphene is laid on an r-plane sapphire substrate, and the electrostatic potential of the substrate acts on the epitaxial layer through the few-layer graphene. At this time, the substrate has a much greater effect on the epitaxial layer than the few-layer graphene, and the crystal orientation of the epitaxial layer is determined by the substrate. Due to the presence of the few-layer graphene, an incompletely corresponding epitaxial relationship is formed between the epitaxial layer and the substrate, alleviating the stress caused by lattice mismatch. Compared with van der Waals epitaxy, this epitaxial method enhances the interaction between graphene and the epitaxial layer, alleviating the problems of in-plane angles and large-angle grain boundaries in van der Waals epitaxy, while retaining the advantages of the epitaxial layer being peelable and alleviating the lattice mismatch and thermal mismatch between the substrate and the epitaxial layer in van der Waals epitaxy. By setting the few-layer graphene and regulating the epitaxial growth parameters, gallium nitride with an a-plane crystal plane is grown on the surface of the few-layer graphene to form an epitaxial layer, thereby realizing the construction of a non-polar a-plane gallium nitride epitaxial structure. In the process of transferring the few-layer graphene, wrinkles are easily formed. The wrinkled areas of few-layer graphene can lead to uneven contact between the graphene and the substrate, resulting in local stress or voids that interfere with the epitaxial growth process. These uneven interfaces can cause defects, cracks, or irregular lattice structures in the epitaxial layer when it grows on the graphene surface, which in turn affects the crystal quality and optoelectronic properties of the epitaxial layer. In addition, wrinkles may also introduce additional interface defects, such as dislocations or grain boundaries, further reducing the uniformity and stability of the epitaxial layer. By constraining the wrinkle ratio on the surface of few-layer graphene, it helps to improve the quality of non-polar a-plane gallium nitride epitaxial structure. The thickness of few-layer graphene directly affects the interface spacing between the substrate and the epitaxial layer, and thus affects the ability of the substrate to influence the epitaxial layer. When the local thickness of the few-layer graphene is too large, the crystal orientation of the substrate at the corresponding position is difficult to influence the crystal orientation of the epitaxial layer, which is not conducive to the uniformity of the gallium nitride crystal orientation in the epitaxial layer.

[0024] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a flow chart of a method for preparing a non-polar a-plane gallium nitride epitaxial structure as shown in the first embodiment of the present invention;

[0026] Figure 2 Electron microscope images of the nucleation layer of the epitaxial structure in Example 1 and Comparative Example 1 of the present invention;

[0027] Figure 3 X-ray diffraction patterns of the epitaxial structures in Example 1 and Comparative Example 1 of the present invention;

[0028] Figure 4 Analytical diagram of the migration of Ga atoms and N atoms on the epitaxial substrate in Example 1 and Comparative Example 1 of the present invention;

[0029] Figure 5 Transmission electron microscope images and selected area electron diffraction patterns of the cross sections of the epitaxial structures in Example 1 and Comparative Example 1 of the present invention;

[0030] Figure 6 1 is a bright field selected area electron diffraction pattern of the cross section of the epitaxial substrate in Example 1 and Comparative Example 1 of the present invention;

[0031] Figure 7 Partial Raman spectra of the epitaxial structures in Example 1 and Comparative Example 1 of the present invention;

[0032] Figure 8 TEM images of the interface of the epitaxial substrate of the epitaxial structure in Example 1 and Comparative Example 1 of the present invention;

[0033] Figure 9 A partial Raman spectrum of the epitaxial structure in Comparative Example 1 of the present invention;

[0034] Figure 10 These are Raman spectra of the epitaxial structures in Example 1 and Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0035] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0037] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0038] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0039] The present invention applies to protect a method for preparing a non-polar a-plane gallium nitride epitaxial structure, comprising:

[0040] S1. Provide an r-plane sapphire substrate and few-layer graphene.

[0041] S2. Transferring the few-layer graphene to the growth surface of the substrate and laminating the few-layer graphene to the growth surface to prepare an epitaxial substrate, wherein the ratio of the area of ​​the wrinkles on the surface of the few-layer graphene in the epitaxial substrate to the total area is less than or equal to 1%, for example, it can be any value among 0.3%, 0.5%, 0.7%, 0.9% and 1%, and the maximum thickness of the few-layer graphene in the epitaxial substrate is any value among 0.3nm~0.7nm, for example, it can be any value among 0.3nm, 0.38nm, 0.46nm, 0.64nm and 0.7nm.

[0042] S3. Performing metal organic chemical vapor deposition on the surface of the epitaxial substrate to obtain a non-polar a-plane gallium nitride epitaxial structure.

[0043] Few-layer graphene is deposited on an r-plane sapphire substrate. The substrate's electrostatic potential acts on the epitaxial layer through the few-layer graphene. At this point, the substrate exerts a much greater influence on the epitaxial layer than on the few-layer graphene, and the crystal orientation of the epitaxial layer is determined by the substrate. Due to the presence of the few-layer graphene, an incomplete epitaxial relationship is formed between the epitaxial layer and the substrate, alleviating stress caused by lattice mismatch. Compared to van der Waals epitaxy, this epitaxial growth method enhances the interaction between the graphene and the epitaxial layer, alleviating the issues of in-plane rotations and large-angle grain boundaries encountered in van der Waals epitaxy. While retaining the advantages of van der Waals epitaxy, which allows for the epitaxial layer to be peeled off and mitigates lattice and thermal mismatch between the substrate and epitaxial layer, the epitaxial layer is grown on the surface of the few-layer graphene by regulating the epitaxial growth parameters. This allows the construction of a non-polar a-plane GaN epitaxial structure. Wrinkles are easily formed during the transfer of the few-layer graphene. Wrinkled regions in few-layer graphene can lead to uneven contact between the graphene and the substrate, creating localized stresses or voids that interfere with the epitaxial growth process. These uneven interfaces can cause defects, cracks, or irregular lattice structures in the epilayer grown on the graphene surface, compromising the epilayer's crystal quality and optoelectronic properties. Furthermore, wrinkles can introduce additional interface defects, such as dislocations or grain boundaries, further reducing the uniformity and stability of the epilayer. By constraining the wrinkle ratio on the surface of few-layer graphene, the quality of non-polar a-plane GaN epitaxial structures can be improved. The thickness of few-layer graphene directly affects the interfacial spacing between the substrate and the epilayer, and thus the substrate's ability to influence the epilayer. When the few-layer graphene is locally too thick due to wrinkles or other factors, the substrate's crystal orientation at the corresponding location becomes less able to influence the epilayer's crystal orientation, hindering the uniformity of the GaN crystal orientation within the epilayer. By constraining the maximum thickness of few-layer graphene, the quality of a-plane GaN can be improved and the formation of multiple crystal orientations can be prevented.

[0044] In some embodiments, the few-layer graphene is grown by chemical vapor deposition on a copper foil surface, and

[0045] Transferring few-layer graphene to the growth surface of a substrate includes:

[0046] Polymethyl methacrylate is coated on the surface of the grown few-layer graphene to form a transfer medium layer with a thickness of any value between 8 nm and 12 nm, for example, any value between 8 nm, 9 nm, 10 nm, 11 nm and 12 nm. A copper foil with the transfer medium layer is placed in a ferric chloride solution, and after the copper foil is corroded and removed, it is rinsed with deionized water. The transfer medium layer and the few-layer graphene are transferred together to the growth surface of the substrate, so that the transfer medium layer is adhered to the growth surface. After heating to a temperature of any value between 90° C. and 110° C., for example, any value between 90° C., 95° C., 100° C., 105° C. and 110° C., the substrate connected with the few-layer graphene is immersed in an acetone solution to transfer the few-layer graphene to the growth surface.

[0047] Few-layer graphene has high mechanical strength but poor flexibility. Using polymethyl methacrylate (PMMA) to support the structure of few-layer graphene can prevent the few-layer graphene from wrinkling, tearing, or agglomerating, reduce mechanical damage caused by transfer, and reduce wrinkles on the surface of the epitaxial substrate. PMMA can also reduce the solution's infiltration and erosion of the graphene surface with its hydrophobicity, preventing the graphene from curling or breaking due to surface tension. Removing the copper foil through chemical etching to expose the few-layer graphene surface helps reduce copper residue and prevents structural damage caused by tearing and separating the few-layer graphene. This helps improve the structural integrity of the separated few-layer graphene film, reduces the risk of damage to the few-layer graphene, and thus reduces costs. After etching is completed, PMMA gives the few-layer graphene an appropriate buoyancy and surface tension balance, carrying the few-layer graphene evenly suspended on the surface of the solution, facilitating its transfer to the substrate surface through van der Waals forces and electrostatic forces. During further heat treatment, PMMA undergoes a glass transition, forming a viscous fluid dynamic that fills the microscopic gaps between the graphene and substrate, effectively releasing interfacial stress and displacing air bubbles. Upon cooling, the PMMA solidifies into a rigid interlayer, preventing separation between the substrate and few-layer graphene during subsequent acetone treatment. Because PMMA and graphene only interact weakly in π-π fashion, it can be selectively dissolved and removed using acetone, eliminating the need for separation methods such as ultrasonic cleaning and preventing damage to the graphene lattice caused by strong mechanical forces. This orthogonal dissolution behavior ensures the cleanliness and intrinsic properties of the transferred graphene.

[0048] In some embodiments, the growth conditions of the few-layer graphene are as follows: heating the temperature to any value between 950°C and 1020°C under argon protection, for example, any value between 950°C, 960°C, 970°C, 990°C, 1010°C, and 1020°C, and then introducing hydrogen at a flow rate of any value between 45 sccm and 50 sccm and methane at a flow rate of any value between 8 sccm and 10 sccm. The hydrogen flow rate can be, for example, any value between 45 sccm, 46 sccm, 47 sccm, 48 sccm, 49 sccm, and 50 sccm, and the methane flow rate can be, for example, any value between 8 sccm, 9 sccm, and 10 sccm. The growth time is any value between 25 min and 30 min, for example, any value between 25 min, 26 min, 27 min, 28 min, 29 min, and 30 min. By controlling the deposition conditions, it is ensured that the deposited graphene has a single-layer structure.

[0049] In some embodiments, in metal organic chemical vapor deposition, nitrogen and hydrogen are used as carrier gases, and trimethylgallium and ammonia are used as process gases. After a first period of low-temperature deposition, a second period of high-temperature deposition is performed. The temperature of the low-temperature deposition is any value between 750°C and 850°C, for example, it can be any value between 750°C, 770°C, 790°C, 810°C, 830°C and 850°C. The temperature of the high-temperature deposition is any value between 900°C and 1000°C, for example, it can be any value between 900°C, 920°C, 940°C, 960°C, 980°C and 1000°C.

[0050] When depositing on the surface of the epitaxial substrate, nitrogen and hydrogen are used as carrier gases to accelerate diffusion, improve gas uniformity and reaction speed, and protect the surface to be deposited, inhibiting the occurrence of side reactions. Trimethylgallium and ammonia are used as process gases to react and produce gallium nitride. Because few-layer graphene is prepared using the copper foil transfer method, it is difficult to completely remove the copper element on the surface of the few-layer graphene. At high temperatures, copper can act as a metal catalyst to catalyze the decomposition of ammonia to produce active nitrogen compounds with strong oxidizing ability. At high temperatures, graphene oxide generates defects such as carbon-oxygen single bonds and carbon-oxygen double bonds. These defects can lead to stress concentration, induce dislocation nucleation, provide defect sources, promote dislocation proliferation, change surface energy, and affect dislocation dynamics. Ultimately, this can lead to a significant increase in dislocations in the epitaxial layer, resulting in low epitaxial layer quality.

[0051] In some embodiments, the total flow rate of the carrier gas and the process gas in the low temperature deposition is any value in the range of 2800 sccm to 2850 sccm, for example, any value in the range of 2800 sccm, 2810 sccm, 2820 sccm, 2830 sccm, 2840 sccm, and 2850 sccm, and the flow ratio of the carrier gas to the process gas is any value in the range of (8 to 9):1, for example, any value in the range of (8:1), (8.2:1), (8.4:1), (8.6:1), (8.8:1), and (9:1). The proportion of nitrogen in the carrier gas is any value between 85% and 91%, for example, it can be any value between 85%, 87%, 89% and 91%. The flow ratio of trimethylgallium and ammonia is any value between 1: (80 and 85), for example, it can be any value between (1:80), (1:81), (1:82), (1:83), (1:84) and (1:85). The first time is any value between 13 minutes and 17 minutes, for example, it can be any value between 13 minutes, 14 minutes, 15 minutes, 16 minutes and 17 minutes.

[0052] The total flow rate of the carrier gas and the process gas in the high temperature deposition is any value in the range of 6550 sccm to 6600 sccm, for example, it can be any value in the range of 6550 sccm, 6560 sccm, 6570 sccm, 6580 sccm, 6590 sccm and 6600 sccm. The flow ratio of the carrier gas to the process gas is any value in the range of (30 to 38):1, for example, it can be any value in the range of (30:1), (32:1), (34:1), (36:1) and (38:1). The ratio of nitrogen in the carrier gas is 2.5. For example, it can be any value between 85% and 91%, for example, it can be any value between 85%, 87%, 89% and 91%, the flow ratio of trimethylgallium and ammonia is any value between 1: (16 and 18), for example, it can be any value between (1:16), (1:16.5), (1:17), (1:17.5) and (1:18), the second time is any value between 80min and 90min, for example, it can be any value between 80min, 82min, 84min, 86min, 88min and 90min.

[0053] By regulating the flow of carrier gas and process gas, the growth rate of gallium nitride epitaxy can be controlled, which helps to maintain the uniformity of the epitaxial layer and improve the epitaxial quality.

[0054] In some embodiments, aligning the few-layer graphene with the growth surface includes:

[0055] After preheating at a first temperature for a third time in an inert atmosphere, the substrate is heated to a second temperature and annealed for a fourth time to obtain an epitaxial substrate, wherein the first temperature is any value between 300°C and 400°C, for example, it can be any value between 320°C, 340°C, 360°C, 380°C and 400°C, the third time is any value between 30min and 45min, for example, it can be any value between 30min, 35min, 40min and 45min, the second temperature is any value between 700°C and 900°C, for example, it can be any value between 700°C, 740°C, 780°C, 820°C, 860°C and 900°C, and the fourth time is any value between 60min and 90min, for example, it can be any value between 60min, 65min, 70min, 75min, 80min, 85min and 90min.

[0056] By preheating the substrate and few-layer graphene in direct contact, water molecules and other volatile impurities adsorbed on the surface of the few-layer graphene are removed. High-temperature annealing then allows atomic interdiffusion between the few-layer graphene and the substrate, forming a tighter bond. This also repairs some defects in the graphene created during the transfer process, improving the adhesion between the graphene and substrate, and ultimately producing an epitaxial matrix. During metal-organic chemical vapor deposition (MOCVD), the substrate exerts a much greater influence on the epitaxial layer than on the few-layer graphene, primarily relying on the substrate's electrostatic potential to penetrate the few-layer graphene and reach the epitaxial layer. At this stage, the few-layer graphene is "transparent." When the interlayer is too thick, the interfacial spacing between the substrate and epitaxial layer becomes excessive, completely isolating the substrate from any interaction. High-temperature annealing helps strengthen the bond between the few-layer graphene and the substrate, reduces the interfacial spacing between the substrate and epitaxial layer, and ensures that the substrate's electrostatic potential field can reach the epitaxial layer, influencing its growth. This results in the production of a-plane GaN, helping to ensure the quality of the epitaxial layer. Furthermore, during the annealing process, carbon atoms within the graphene gain sufficient energy to undergo thermally activated migration, alleviating residual stress introduced by lattice mismatch or the transfer process. This thermal relaxation transforms the graphene from a high-energy, twisted structure to a low-energy, flat structure, thereby reducing wrinkles on the surface of the few-layer graphene in the epitaxial substrate. Furthermore, at high temperatures, graphene's flexibility is enhanced, flattening local wrinkles originally caused by the substrate surface fluctuations or the transfer operation, thereby reducing dislocations in the epitaxially grown non-polar a-plane GaN.

[0057] In some embodiments, before metal organic chemical vapor deposition (MOCVD) is performed on the epitaxial substrate, the integrity of the few-layer graphene is verified using an atomic force microscope to help improve the epitaxial quality.

[0058] In some embodiments, after the substrate is chemically polished to a surface roughness of any value between 0.1 nm and 0.5 nm, for example, any value between 0.1 nm, 0.2 nm, 0.3 nm, 0.4 nm and 0.5 nm, the surface is functionalized by ammonia gas at a temperature of any value between 300°C and 500°C, for example, any value between 300°C, 350°C, 400°C, 450°C and 500°C, and the treatment time is any value between 30 min and 60 min, for example, any value between 30 min, 35 min, 40 min, 45 min, 50 min, 55 min and 60 min, and then the few-layer graphene is transferred.

[0059] Chemical mechanical polishing reduces the roughness of the substrate surface, providing a smoother bonding surface. This reduces stress concentration between the graphene and the substrate, preventing wrinkles in the few-layer graphene due to surface unevenness during the transfer process. Simultaneously, the introduction of amino groups through ammonia treatment enhances the adhesion of the few-layer graphene to the substrate, allowing the few-layer graphene to adhere evenly to the substrate surface, reducing bubble formation and uneven adhesion of the few-layer graphene, and effectively suppressing the formation of wrinkles. This combined treatment provides ideal conditions for high-quality transfer of few-layer graphene by optimizing substrate surface smoothness and enhancing interfacial adhesion.

[0060] The present invention also provides a non-polar a-plane gallium nitride epitaxial structure, which is prepared using the above-mentioned method for preparing the non-polar a-plane gallium nitride epitaxial structure.

[0061] It should be noted that the present invention reduces the risk of excessive surface wrinkling or localized overthickness of the few-layer graphene after transfer by surface treating the substrate before transferring the few-layer graphene, using PMMA to assist in the transfer of the few-layer graphene, and performing preheating and annealing after the transfer of the few-layer graphene. Even without using these methods, an epitaxial substrate with fewer wrinkles and a smaller maximum thickness of the few-layer graphene can be obtained. This method reduces the risk of substandard epitaxial substrates, thereby reducing costs and helping to improve the yield of the resulting non-polar a-plane gallium nitride epitaxial structure.

[0062] Please refer to the following examples for details.

[0063] Example 1:

[0064] A method for preparing a non-polar a-plane gallium nitride epitaxial structure as shown in a preferred embodiment of the present application includes:

[0065] S1. Provide an r-plane sapphire substrate and few-layer graphene.

[0066] S2. Transferring the few-layer graphene to the growth surface of the substrate and laminating the few-layer graphene to the growth surface to prepare an epitaxial substrate, wherein the ratio of the area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate to the total area is less than or equal to 1%, and the maximum thickness of the few-layer graphene in the epitaxial substrate is any value between 0.3 nm and 0.7 nm.

[0067] S3. Performing metal organic chemical vapor deposition on the surface of the epitaxial substrate to obtain a non-polar a-plane gallium nitride epitaxial structure.

[0068] Specifically, step S1 includes:

[0069] S110: Cut the sapphire into an r-Al2O3 wafer with an r-plane crystal face as the substrate. One side of the wafer serves as the growth surface. The growth surface is chemically polished using a polishing solution containing aluminum oxide. In this embodiment, a suspension of aluminum oxide with an average particle size of 0.5 μm is used as the polishing solution. The surface roughness of the growth surface is polished to approximately 0.35 nm. The polished growth surface is then subjected to a surface functionalization treatment using ammonia gas at 370°C for 40 minutes, thereby introducing amino groups into the growth surface.

[0070] S120, placing the copper foil in a CVD device, raising the temperature to 1000° C. under argon protection, then introducing hydrogen with a flow rate of 50 sccm and methane with a flow rate of 10 sccm, and growing high-quality few-layer graphene on the surface of the copper foil for 30 minutes.

[0071] Step S2 includes:

[0072] S210. Coating polymethyl methacrylate (PMMA) on the surface of the grown few-layer graphene to form a transfer medium layer, placing a copper foil with the transfer medium layer in a ferric chloride solution, corroding and removing the copper foil, rinsing with deionized water, and transferring the transfer medium layer and the few-layer graphene to the growth surface of the substrate so that the transfer medium layer is adhered to the growth surface. After heating to a temperature of any value between 90°C and 110°C, immersing the substrate connected with the few-layer graphene in an acetone solution to transfer the few-layer graphene to the growth surface.

[0073] S220 , preheating at the first temperature in an inert atmosphere for a third time, then heating to the second temperature and annealing for a fourth time to obtain an epitaxial substrate, wherein the few-layer graphene is adhered to the growth surface.

[0074] S230. Observe the surface morphology of the epitaxial substrate using an atomic force microscope to confirm that the few-layer graphene is intact and undamaged, and calculate the area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate and the maximum thickness of the few-layer graphene in the epitaxial substrate. In this embodiment, the ratio of the area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate to the total area is 0.4%, and the maximum thickness of the few-layer graphene in the epitaxial substrate is 0.59 nm. The test is qualified and can be used for subsequent metal organic compound chemical vapor deposition.

[0075] In step S210, a transfer medium layer is formed by spin coating PMMA with a thickness of 10 nm. When the copper foil is corroded and removed, the ferric chloride solution gradually changes from yellow to blue-green and stabilizes, and the film is suspended in the solution. When there is no residual copper foil when the edge of the sample is lightly touched, it can be judged that the copper foil has been removed. After observing under a microscope that there is no copper sheet remaining on the edge of the film, the film is taken out and rinsed with deionized water. After heating to 100°C, the milky white PMMA gradually becomes transparent or translucent, indicating that it has softened and adhered. Cooling and cooling, observe whether there are visible bubbles between PMMA and the substrate. If there are bubbles, heat again and cool until there are no bubbles. When dissolving by immersion in acetone, replace the acetone every 30 minutes until the acetone no longer becomes turbid, and the substrate surface is completely exposed with no oily residue. At this time, it can be judged that the PMMA has been completely removed and the few-layer graphene is adhered to the substrate surface. After standing and drying, confirm that there is no PMMA residue by microscopic observation.

[0076] In step S220, the first temperature is 330°C, the third time is 40 minutes, the second temperature is 790°C, and the fourth time is 75 minutes. Preheating can remove water molecules and other volatile impurity molecules adsorbed on the surface of the few-layer graphene, and can also convert the amino groups modified on the surface of the substrate into ammonia gas to escape, thereby preventing the subsequent metal organic chemical vapor deposition from being affected. High-temperature annealing allows the atoms between the few-layer graphene and the substrate to diffuse into each other, forming a tighter bond, while repairing some defects generated in the graphene during the transfer process, improving the fit between the graphene and the substrate, and improving the ability of the substrate's electrostatic potential to penetrate the few-layer graphene, which helps to ensure the uniform crystal orientation of the gallium nitride obtained by subsequent deposition.

[0077] Step S3 includes:

[0078] S310, using nitrogen and hydrogen as carrier gases and trimethylgallium and ammonia as process gases, performing low-temperature deposition for a third time to form a nucleation layer, wherein the temperature of the low-temperature deposition is any value between 750° C. and 850° C.

[0079] S320: After the low-temperature deposition is completed, a fourth period of high-temperature deposition is performed, wherein the temperature of the high-temperature deposition is any value between 900° C. and 1000° C.

[0080] In step S310 , the low temperature deposition temperature is 800° C., the flow rate of ammonia is 2500 sccm, the flow rate of trimethyl gallium is 30 sccm, the flow rate of nitrogen is 250 sccm, the flow rate of hydrogen is 35 sccm, and the deposition time is 15 minutes.

[0081] In step S320, high-temperature deposition is performed at a temperature of 1000°C, with a flow rate of 6000 sccm of ammonia, 350 sccm of trimethyl gallium, 200 sccm of nitrogen, and 20 sccm of hydrogen. The deposition time is 90 minutes. After the deposition is complete, an a-GaN epitaxial layer is obtained.

[0082] The epitaxial structure deposited in this embodiment is named a-GaN / SLG / r-Al2O3.

[0083] Comparative Example 1:

[0084] The only difference between this comparative example and Example 1 is that in this comparative example, the growth surface of the substrate is directly used as the epitaxial base, and low-temperature deposition and high-temperature deposition are performed. The obtained non-polar a-plane gallium nitride epitaxial structure is named a-GaN / r-Al2O3.

[0085] a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3 were tested separately.

[0086] In Example 1 and Comparative Example 1, step S320 was not performed, and the nucleation layers of the obtained a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3 epitaxial layers were exposed to the surface. The surface morphology of the nucleation layers was observed by electron microscopy, and the crystal orientation was analyzed by X-ray diffraction (XRD). Figure 2 , it can be seen that in the nucleation layers of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3, the GaN grains are all present as hexagonal prisms with side edges close to the substrate surface, and a few GaN grains have merged. In addition, the GaN grains in the nucleation layer of a-GaN / SLG / r-Al2O3 are larger than those in the nucleation layer of a-GaN / r-Al2O3, and the nucleation density is lower. See Figure 3 It can be seen that the GaN in the nucleation layer of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3 is [11-20] plane GaN, that is, a-plane GaN, and there is no other crystal orientation. This shows that the r-plane sapphire substrate and the few-layer graphene-r-plane sapphire composite substrate we used can be used to epitaxially grow single-oriented non-polar a-plane GaN.

[0087] On the epitaxial substrates of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3, migration path 1 along the c-axis and migration path 2 along the m-axis were set up respectively. Nine and eight intermediate points were taken on the two migration paths respectively to calculate the migration energies of Ga atoms and N atoms on each migration path. For path settings and calculation results, please refer to Figure 4, along the c-axis direction, the maximum migration barrier of Ga atoms on the epitaxial substrate in comparative example one is 0.456eV, and the maximum migration barrier of N atoms is 1.6eV. The maximum migration barrier of Ga atoms on the epitaxial substrate in embodiment one is 0.238eV, and the maximum migration barrier of N atoms is 1.377eV. Along the m-axis direction, the maximum migration barrier of Ga atoms on the epitaxial substrate in comparative example one is 0.625eV, and the maximum migration barrier of N atoms is 1.925eV. The maximum migration barrier of Ga atoms on the epitaxial substrate in embodiment one is 0.257eV, and the maximum migration barrier of N atoms is 1.549eV. According to the calculation results, it can be seen that no matter which direction is along, the migration energy on the epitaxial substrate in embodiment one is always lower. According to common knowledge in the field, the lower the migration energy, under the same experimental conditions, the atoms are more likely to migrate on the substrate surface, gather together to form larger grains, and also reduce the surface nucleation density. At the same time, it can also be found that the migration energy along migration path 1, that is, the c-axis direction, is smaller than the migration energy along migration path 2, that is, the m-axis direction, indicating that GaN tends to grow in two dimensions along the c-axis direction, and eventually forms long GaN grains. The two-dimensional layered growth mode can reduce the dislocations and defects caused by grain merging in three-dimensional island growth. The directional arrangement of the long grains further inhibits the propagation of defects and can also disperse the lattice mismatch stress between the substrate and the epitaxial layer through elastic deformation, reducing crack formation. The calculation results confirm that optimizing the epitaxial substrate using the method of the present invention helps to reduce crystal defects in the a-GaN epitaxial structure, improve material quality, and thus form a uniform and flat single crystal epitaxial layer.

[0088] The cross sections of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3 were observed using a transmission electron microscope (TEM) and selected area electron diffraction (SAED). The Al2O3 reciprocal lattice is marked in orange, and the GaN reciprocal lattice is marked in blue. Figure 5 It can be seen that whether or not a few-layer graphene is provided does not affect the epitaxial relationship between a-GaN and r-Al2O3, and it can be seen that the a-GaN deposited in the present invention has a single crystal structure.

[0089] SAED bright field images of the cross sections of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3 epitaxial substrates are shown in Figure 2. Figure 6 , it can be seen that the a-GaN / r-Al2O3 epitaxial layer contains many defects (D), and the diffraction spots in the yellow box area are elongated, indicating the presence of stacking faults. In contrast, the a-GaN / SLG / r-Al2O3 epitaxial layer has almost no defects and no elongated diffraction spots, indicating that there are almost no dislocations. This proves that the epitaxial substrate obtained after the treatment of this invention effectively reduces the generation of defects in heteroepitaxial growth.

[0090] Due to the mismatch between the lattice constant and thermal expansion coefficient of sapphire and gallium nitride, stress exists in the GaN epitaxial layer. Raman spectrometer was used to perform line scans along the c-axis of GaN grains of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3, respectively. The scanned data was pre-processed and Gaussian fitted to obtain Raman spectra. Figure 7 It can be seen that the E2 mode high frequency peak of a-GaN / SLG / r-Al2O3 is located at 568.3cm -1 , and the high-frequency peak of the E2 mode of a-GaN / r-Al2O3 is located at an average of 568.85 cm -1 . The lattice contraction is quantified by the Raman shift in the Raman spectrum to determine the compressive stress in the epitaxial layer. By comparison, it can be seen that the compressive stress in the epitaxial layer in a-GaN / SLG / r-Al2O3 is less, which is due to the change in the connection relationship between the substrate and the epitaxial layer. Sapphire has a larger thermal expansion coefficient and shrinks more when cooled after epitaxy. When the epitaxial layer is grown directly on the sapphire substrate, the epitaxial layer is tightly bonded to the substrate and cannot shrink freely, causing the epitaxial layer to be compressed and forming in-plane biaxial compressive stress. In the present invention, a few-layer graphene is introduced between the epitaxial layer and the substrate, so that there is only a weak interaction between the substrate and the epitaxial layer, and no non-covalent bonding is used, thereby alleviating the stress caused by the heteroepitaxial lattice mismatch.

[0091] The interfaces of a-GaN / SLG / r-Al2O3 and a-GaN / r-Al2O3 epitaxial substrates were analyzed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to further explore the strain release mechanism in remote epitaxy. Figure 8 , the distribution of GaN atoms at the interface of a-GaN / r-Al2O3 is arranged in an undulating manner like a "hill", and the part within the dotted line shows a certain periodicity. If the part within the frame is magnified locally, it can be found that lattice matching areas and mismatching areas appear alternately. The GaN atoms in the lattice mismatching area are arranged disorderly, no longer presenting a standard six-membered ring, and lattice stretching and compression distortions also appear. This is because in traditional heteroepitaxial growth, mismatch dislocations are generated at the epitaxial interface to release the elastic strain energy accumulated in the epitaxial layer due to lattice mismatch. Based on the structure of a-GaN / r-Al2O3, a dislocation heterojunction structure is established and the interface binding energy is calculated. The obtained interface binding energy is positive, indicating that this system is very unstable. In comparison, the GaN atoms at the interface of a-GaN / SLG / r-Al2O3 are arranged neatly, and the existence of geometric mismatch dislocations is almost invisible. This means that processing the substrate using the method of the present invention to obtain an epitaxial base can reduce the strain and defects of the epitaxial layer caused by lattice mismatch, and can also weaken the interface slip barrier. During the cooling process after epitaxy, GaN is more likely to slip and is less likely to generate residual stress.

[0092] The thickness and dislocation density of a-GaN / SLG / r-Al2O3 were detected and quantified by XRD. The thickness of the epitaxial layer was found to be 3 μm and the dislocation density was 6×10 9 cm -2 .

[0093] Comparative Example 2:

[0094] The only difference between this comparative example and Example 1 is that in step S110 of this comparative example, the substrate was not surface-functionalized with ammonia. In step S230, the ratio of the surface wrinkles of the few-layer graphene in the epitaxial substrate to the total surface area was measured to be 1.7%, and the maximum thickness of the few-layer graphene in the epitaxial substrate was 1.18 nm. That is, the few-layer graphene in this comparative example was unqualified. After proceeding to step S3 to obtain the epitaxial structure, the Raman spectrum of the epitaxial structure in this comparative example was obtained by Raman spectrometry. Please refer to Figure 9 It can be seen that the high-frequency peak of the E2 mode of the epitaxial structure in this comparative example is located at an average of 568.65 cm -1 Compared with the test results of the epitaxial structure in Example 1 and Comparative Example 1, it can be seen that the compressive stress in the epitaxial layer of the epitaxial structure in this comparative example is greater than that of a-GaN / SLG / r-Al2O3 and less than that of a-GaN / r-Al2O3. The dislocation density of the epitaxial layer of the epitaxial structure in this comparative example is 1×10 10 cm -2 , indicating that surface functionalization with ammonia can reduce defects in the epitaxial layer. SAED reveals that the epitaxial layer of the epitaxial structure in this comparative example is partially doped with polycrystalline orientation, and the location of the doping corresponds to the location of wrinkles at the maximum thickness of the few-layer graphene. This shows that surface functionalization with ammonia can reduce wrinkles, help reduce defects in the epitaxial layer, and inhibit the generation of polycrystalline orientation.

[0095] Comparative Example 3:

[0096] The only difference between this comparative example and Example 1 is that the high temperature deposition temperature in this comparative example is 1050°C. Figure 10 Comparing the Raman spectra of the epitaxial structure obtained in Example 1 with that obtained in this comparative example, it can be seen that the Raman spectrum corresponding to Example 1 has a higher defect (D) peak, and obvious graphene peaks, namely G peak and 2D peak, can be seen, indicating that the few-layer graphene is easily damaged during the deposition process. In the Raman spectrum of this comparative example, the graphene peak disappears, indicating that the few-layer graphene is completely destroyed. XRD detection shows that the thickness of the epitaxial layer in this comparative example is also 3μm, and the dislocation density is 2×10 10 cm -2, which is significantly improved compared with Example 1, proving that controlling the temperature of high-temperature deposition helps to protect the few-layer graphene, thereby reducing the dislocation density.

[0097] Comparative Example 4:

[0098] The only difference between this comparative example and Example 1 is that step S220 is not performed in this comparative example. In step S230, the ratio of the area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate to the total area is measured to be 0.8%, and the maximum thickness of the few-layer graphene in the epitaxial substrate is 0.66 nm. Low-temperature deposition and high-temperature deposition are directly performed on the surface of the epitaxial substrate. XRD detection of the non-polar a-plane gallium nitride epitaxial structure obtained in this comparative example shows that the thickness of the epitaxial layer is 3 μm and the dislocation density is 8×10 9 cm -2 .

[0099] In summary, while research has explored the epitaxial growth of GaN thin films on r-plane sapphire, it has long faced problems such as loss of crystal orientation control, difficulty in nucleation, and low epitaxial quality. In particular, attempts to obtain a-oriented GaN structures have faced difficulties in effectively controlling crystal orientation and polarity. The present invention improves the epitaxial growth method to form high-quality a-oriented GaN epitaxial layers. Furthermore, while the technique of using two-dimensional materials as semi-transparent interlayers to achieve epitaxial growth through electrostatic interactions has been applied in other crystal orientation systems, those skilled in the art have not yet realized that this technique can be used to induce GaN to preferentially grow along specific non-polar directions, such as the [11-20] direction, and that this technique can have a synergistic effect when coupled with r-plane sapphire. Therefore, the present invention proposes for the first time a stable epitaxial growth path for a-oriented GaN by introducing two-dimensional graphene onto r-plane sapphire and leveraging the electrostatic potential of the r-plane sapphire to act through the few-layer graphene on the epitaxial layer. This overcomes the technical bottleneck that has hindered the high-quality growth of traditional non-polar GaN and contributes to improved performance of related semiconductor devices.

[0100] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0101] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for preparing a non-polar a-plane gallium nitride epitaxial structure, characterized in that: include: Provide r-plane sapphire substrate and few-layer graphene; Transferring the few-layer graphene to the growth surface of the substrate, preheating the few-layer graphene at a first temperature for a third time in an inert atmosphere, and then heating the few-layer graphene to a second temperature for annealing for a fourth time to obtain an epitaxial substrate, wherein the first temperature is any value between 300° C. and 400° C., the third time is any value between 30 min and 45 min, the second temperature is any value between 700° C. and 900° C., and the fourth time is any value between 60 min and 90 min. The ratio of the area of ​​wrinkles on the surface of the few-layer graphene in the epitaxial substrate to the total area is less than or equal to 1%, and the maximum thickness of the few-layer graphene in the epitaxial substrate is any value between 0.3 nm and 0.7 nm. Metal organic compound chemical vapor deposition is performed on the surface of the epitaxial substrate to obtain a non-polar a-plane gallium nitride epitaxial structure.

2. The method for preparing a non-polar a-plane gallium nitride epitaxial structure according to claim 1, wherein: The few-layer graphene is grown by chemical vapor deposition on the surface of copper foil, and Transferring the few-layer graphene to the growth surface of the substrate comprises: Polymethyl methacrylate is coated on the surface of the grown few-layer graphene to form a transfer medium layer with a thickness of any value between 8 nm and 12 nm. The copper foil with the transfer medium layer is placed in a ferric chloride solution, and after the copper foil is corroded and removed, it is rinsed with deionized water. The transfer medium layer and the few-layer graphene are transferred together to the growth surface of the substrate, so that the transfer medium layer is adhered to the growth surface. After heating to a temperature of any value between 90° C. and 110° C., the substrate connected with the few-layer graphene is immersed in an acetone solution to transfer the few-layer graphene to the growth surface.

3. The method for preparing a non-polar a-plane gallium nitride epitaxial structure according to claim 2, wherein: The growth conditions of the few-layer graphene are as follows: raising the temperature to any value between 950°C and 1020°C under argon protection, then introducing hydrogen with a flow rate of any value between 45sccm and 50sccm and methane with a flow rate of any value between 8sccm and 10sccm, and the growth time is any value between 25min and 30min.

4. The method for preparing a non-polar a-plane gallium nitride epitaxial structure according to claim 1, wherein: In the metal organic compound chemical vapor deposition, nitrogen and hydrogen are used as carrier gases, trimethylgallium and ammonia are used as process gases, and after a first period of low-temperature deposition, a second period of high-temperature deposition is performed. The temperature of the low-temperature deposition is any value between 750°C and 850°C, and the temperature of the high-temperature deposition is any value between 900°C and 1000°C.

5. The method for preparing a non-polar a-plane gallium nitride epitaxial structure according to claim 4, wherein: In the low-temperature deposition, the total flow rate of the carrier gas and the process gas is any value between 2800 sccm and 2850 sccm, the flow rate ratio of the carrier gas to the process gas is any value between (8 and 9):1, the proportion of nitrogen in the carrier gas is any value between 85% and 91%, the flow rate ratio of trimethylgallium and ammonia is any value between 1:(80 and 85), and the first time is any value between 13 minutes and 17 minutes; The total flow rate of the carrier gas and the process gas in the high-temperature deposition is any value between 6550 sccm and 6600 sccm, the flow ratio of the carrier gas and the process gas is any value between (30 and 38):1, the proportion of nitrogen in the carrier gas is any value between 85% and 91%, the flow ratio of trimethylgallium and ammonia is any value between 1:(16 and 18), and the second time is any value between 80 min and 90 min.

6. The method for preparing a non-polar a-plane gallium nitride epitaxial structure according to claim 1, wherein: Before the metal organic compound chemical vapor deposition is performed on the epitaxial substrate, the integrity of the few-layer graphene is verified by atomic force microscopy.

7. The method for preparing a non-polar a-plane gallium nitride epitaxial structure according to claim 1, wherein: After the substrate is chemically polished to a surface roughness of any value between 0.1 nm and 0.5 nm, a surface functionalization treatment is performed using ammonia gas at a temperature of any value between 300° C. and 500° C. for a treatment time of any value between 30 min and 60 min, and then the few-layer graphene is transferred.

8. A non-polar a-plane gallium nitride epitaxial structure, characterized in that: The non-polar a-plane gallium nitride epitaxial structure is prepared by the method for preparing the non-polar a-plane gallium nitride epitaxial structure according to any one of claims 1 to 7.

Images