Method for preparing GAN single crystal substrate

Abstract

Disclosed is a method for preparing a GaN single crystal substrate, which comprises the following steps: sequentially growing a barrier layer and a first bonding layer on a base plate, and sequentially growing a GaN epitaxial thin film and a second bonding layer on a substrate; bonding the first bonding layer and the second bonding layer, forming a bonding layer, removing the substrate, and obtaining, from the bottom up, the base plate, the barrier layer, the bonding layer, and the GaN epitaxial thin film; by means of wet etching, causing a surface of the side of the GaN epitaxial thin film away from the bonding layer to form a GaN island having a (10-11) crystal face; on the GaN island, growing GaN at 800-1000°C by means of HVPE to form a GaN thick film; removing the bonding layer by means of HF or BOE etching, and separating the base plate from the GaN thick film; and performing surface grinding and polishing on the GaN thick film. The base plate and the GaN thick film of the present invention have consistent coefficients of thermal expansion, no mismatch is present, cracking is not prone to occurring when the film grows thicker, and a large-size GaN single crystal can be prepared.

Description

A method for preparing a GaN single crystal substrate Technical Field The invention relates to a GaN manufacturing method, in particular to a method for preparing a GaN single crystal substrate. Background Art GaN-based III-V nitrides are important direct bandgap wide bandgap semiconductor materials, and have broad application prospects in optoelectronic devices such as light-emitting diodes (LEDs), laser diodes (LDs) and ultraviolet detectors, as well as microelectronic power devices such as microwaves and power electronics. The current preparation method of GaN single crystal substrate generally uses sapphire, SiC, Si and other materials as heterogeneous single crystal substrates, performs heteroepitaxial growth on them to obtain GaN thick film materials, and then uses laser lift-off technology or self-separation technology to remove the heterogeneous substrate to obtain GaN single crystal substrate. However, the mismatch between the lattice and thermal expansion parameters between the heterogeneous substrate and the GaN material often causes large residual stress in the GaN thick film during the growth process, resulting in more material defects in the thick film. When the substrate is peeled off, it is easily broken by the impact force generated by the decomposition of GaN. Generally, only 2-4 inch substrates can be made, and large-size substrates cannot be made. Summary of the invention Purpose of the invention: In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a method for preparing a large-size, high-quality, and stress-reduced GaN single crystal substrate. Technical solution: A method for preparing a GaN single crystal substrate according to the present invention comprises the following steps: Step 1, sequentially growing a barrier layer and a first bonding layer on a substrate, and sequentially growing a GaN epitaxial film and a second bonding layer on a substrate; Step 2: Bonding the first bonding layer and the second bonding layer by van der Waals force to form a bonding layer, removing the substrate, and obtaining a substrate, a barrier layer, a bonding layer, and a GaN epitaxial film connected in sequence; Step 3, forming a GaN island having a (10-11) crystal plane on the surface of the GaN epitaxial film away from the bonding layer by wet etching; Step 4: On the GaN island, grow GaN at 800-1000° C. using HVPE (hydride vapor phase epitaxy) to form a GaN thick film; Step 5: Remove the bonding layer by etching with HF (hydrofluoric acid) or BOE (buffered oxide etchant) to separate the substrate from the GaN thick film; Step six, using CMP (chemical mechanical polishing) to grind and polish the surface of the GaN thick film to obtain a GaN single crystal substrate. Furthermore, in step 1, the substrate is an AlN ceramic substrate, and the first bonding layer is a SiO 2 or Si 3 N4 The total thickness of the growth barrier layer and the first bonding layer is 1 to 3 μm. The growth barrier layer is grown by LPCVD (low pressure chemical vapor deposition) or PECVD (plasma enhanced chemical vapor deposition), the growth temperature of the LPCVD method is 600 to 800°C, preferably 700°C, and the growth temperature of the PECVD method is 300 to 500°C; the growth temperature of the first bonding layer is 300 to 500°C, preferably 350°C. Furthermore, in step 2, the GaN epitaxial film is grown by MOCVD (metal organic chemical vapor deposition) at 900-1100°C with a thickness of 1-4 μm. The second bonding layer is grown by PECVD (plasma enhanced chemical vapor deposition) at 300-500°C with a thickness of 0.5-1.5 μm, and the second bonding layer is made of the same material as the first bonding layer. Furthermore, in step three, when removing the substrate, thinning is first performed and then dry etching is performed. Furthermore, in step 4, wet etching is performed by using a potassium hydroxide solution with a concentration of 5 to 30% by mass to etch the GaN epitaxial film at 25 to 80°C, preferably 60°C. Wet etching can dissolve the material on the surface of the crystal, effectively remove line dislocations, and obtain high-quality crystals. Line dislocations are a type of defect in the crystal lattice caused by lattice misalignment. When the chemicals in the etching solution react with the crystal surface, it will cause the dissolution of the material near the crystal surface, thereby causing the line dislocations to disappear. Furthermore, in step 5, the thickness of the GaN thick film is 800 μm to 2 mm, preferably 1 mm, and the growth rate of the growing GaN is 10 to 50 μm / h. Large-sized GaN (gallium nitride) single crystals have the following advantages over small-sized single crystals: (1) Improved device performance: Large-sized GaN single crystals can provide a larger area, so that the prepared devices have higher power handling capabilities and lower resistance. This is very important for high-frequency, high-power applications such as RF power amplifiers and high-efficiency power converters. (2) Reduce material loss: When preparing GaN devices, it is necessary to obtain a GaN single crystal substrate of appropriate size through epitaxial growth technology. Large-sized single crystals can provide more crystal planes, reduce crystal defect density, and reduce material loss. Improve preparation efficiency: Large-sized GaN single crystals can prepare multiple devices at the same time, improving preparation efficiency and output. This is very important for large-scale commercial production. (3) Improve device reliability: Large-sized GaN single crystals have better structural consistency and crystal quality, which can improve the reliability and long-term stability of devices. (4) Cost reduction: Although large-sized GaN single crystals usually face technical challenges such as growth difficulty and impurity control, once the efficient production of large-sized single crystals is successfully achieved, the material cost and device preparation cost can be reduced, promoting the commercial application of GaN devices. Preparation principle: Wet etching involves two main processes: diffusion and reaction. Diffusion process: The chemicals in the etching solution diffuse into the interior of the crystal through the dissolution layer near the crystal surface. This diffusion process is achieved through ion exchange or molecular diffusion on the crystal surface. Reaction process: Once the chemicals in the etching solution reach the interior of the crystal, they will react chemically with the crystal material. In the process of reacting with the material near the crystal surface, the chemicals in the etching solution will dissolve the material in the crystal. For line dislocations, the chemicals in the etching solution will preferentially react with the material in the line dislocation area, causing it to dissolve, thereby removing the line dislocation. To achieve effective line dislocation removal, it is necessary to select a suitable etching solution and adjust the etching conditions. This includes the composition, temperature, immersion time, etc. of the etching solution. By optimizing these parameters, selective etching of line dislocations can be achieved, that is, only line dislocations are eliminated while leaving the rest of the crystal intact. Beneficial effects: Compared with the prior art, the present invention has the following significant features: 1. The thermal expansion coefficients of the AlN ceramic substrate and the GaN thick film are consistent, and there is no mismatch, which helps to eliminate the accumulation of stress during the growth of thick GaN layers. It is not easy to break when growing thick, and large-sized GaN single crystals can be successfully prepared; 2. Wet etching can effectively remove line dislocations and obtain high-quality crystals; 3. AlN ceramic substrates can be reused, which helps save costs. BRIEF DESCRIPTION OF THE DRAWINGS FIG1 is a schematic structural diagram of the product obtained in step S1 of the present invention; FIG2 is a schematic structural diagram of the product obtained in step S2 of the present invention; FIG3 is a schematic structural diagram of the product obtained in step S3 of the present invention; FIG4 is a schematic structural diagram of the product obtained in step S4 of the present invention; FIG5 is a schematic structural diagram of the product obtained in step S5 of the present invention; FIG6 is a schematic structural diagram of the product obtained in step S6 of the present invention; FIG7 is a scanning electron microscope image after wet etching in step S4 of the present invention; Figure 8 is an X-ray diffraction diagram of GaN thick films on different substrates, where a is a GaN thick film on a composite substrate made of an AlN ceramic substrate.

[0002] a is a comparison diagram of the X-ray half-width of the crystal image and the crystal image of GaN thick film

[0002] on sapphire substrate. b is a comparison diagram of the X-ray half-width of the crystal image of GaN thick film

[0102] on composite substrate made of AlN ceramic substrate and the crystal image of GaN thick film

[0002] on sapphire substrate. DETAILED DESCRIPTION In the following examples, the PECVD method uses Corial Series of devices. Example 1 A method for preparing a 6-inch GaN single crystal substrate comprises the following steps: S1, as shown in FIG1, first, a barrier layer 2 is grown on an AlN ceramic substrate 1 by LPCVD at a growth temperature of 600°C. The barrier layer 2 is SiO 2 Next, a first bonding layer 3 is grown on the surface of the barrier layer 2 by using a PECVD method at a growth temperature of 300°C. S1.1, Gas injection: inject the required deposition gas (N 2 O、N 2 、SiH 4 , Ar or other gases or their mixtures) carrying TEOS (ethyl silicate) are introduced into the PECVD reaction chamber. S1.2, establish gas pressure: ensure that the gas pressure in the reaction chamber reaches 1800mTorr by controlling the gas flow and the opening and closing of the gate. S1.3, establishing plasma: by connecting a high-frequency power supply to the electrodes of the reaction chamber, an alternating electric field is generated on the dielectric layer to form plasma. S1.4, cleaning the surface of the AlN ceramic substrate 1: starting a plasma reaction to generate sufficient energy to clean the surface of the AlN ceramic substrate 1 and the barrier layer 2 to remove surface impurities and oxides. S1.5, depositing the first bonding layer 3: the plasma power is 80 W, the organic precursor gas is decomposed and deposited onto the surface of the barrier layer 2 to form the first bonding layer 3. S1.6, end the reaction: when the preset deposition thickness is reached, stop the gas supply, disconnect the power supply, and end the plasma reaction. S1.7, cooling: After the AlN ceramic substrate 1 is cooled to room temperature, it is taken out. The first bonding layer 3 is SiO 2 The total thickness of the growth barrier layer 2 and the first bonding layer 3 is 1 μm. S2, as shown in FIG2, first, a GaN epitaxial film 5 with a thickness of 1 μm is grown on a Si substrate 4 by MOCVD, including an AlN nucleation layer and a GaN layer, which are alternately grown in layers, wherein the GaN layer is located on the top, and the growth temperature is 900-1100°C. Next, a second bonding layer 6 is grown on the surface of the GaN epitaxial film 5 by PECVD, the growth temperature is 300°C, the thickness is 2 μm, and the second bonding layer 6 is made of the same material as the first bonding layer 3. S3, as shown in FIG3, is flipped so that the first bonding layer 3 of S1 and the second bonding layer 6 of S2 are first processed by CMP, cleaning, surface activation, etc., and then bonded by van der Waals force to form a bonding layer 7. First thinning, then dry etching, remove the Si substrate 4, and obtain the AlN ceramic substrate 1, barrier layer 2, bonding layer 7, and GaN epitaxial film 5 from bottom to top. S4, as shown in FIG4, a GaN island 8 having a (10-11) crystal plane is formed on the surface of the GaN epitaxial film 5 away from the bonding layer 7 by wet etching. A potassium hydroxide solution (KOH) is used with a concentration of 5 to 30% by mass to etch the GaN epitaxial film 5 at an etching temperature of 60°C. S5, as shown in FIG5, GaN is grown on the GaN island 8 at 800-1000°C by HVPE method at a growth rate of 10 μm / h to form a GaN thick film 9 with a thickness of 800 μm. S6, as shown in FIG6, the bonding layer 7 and the barrier layer 2 are removed by etching through the BOE method, and the AlN ceramic substrate 1 and the GaN thick film 9 are separated. S6.1, preparation: put the GaN thick film 9 containing the AlN ceramic substrate 1 from which the bonding layer 7 needs to be removed into the soaking tank. BOE will also corrode the GaN thick film 9 if soaked for a long time, so it is necessary to cover the surface with a blue film for protection before etching. S6.2, corrosion treatment: pour the prepared BOE solution into an immersion container. Usually, a solution with a concentration of 2.5 vol% is used to ensure that the GaN thick film 9 of the AlN ceramic substrate 1 is completely immersed in the solution. The corrosion process takes 18 hours. S6.3, monitoring: Regularly check the GaN thick film 9 on the AlN ceramic substrate 1 to ensure that the solution etches away the bonding layer 7 from the side, which helps to improve the etching effect. S6.4, rinsing and neutralization: After the etching is completed, the separated GaN thick film 9 and AlN ceramic substrate 1 are taken out from the BOE solution and placed in another tank respectively. They are first rinsed with 0.1% BOE to remove the original liquid adhering to the GaN thick film 9 and the AlN ceramic substrate 1, and then rinsed in another tank with sufficient pure water to completely remove the residual acid solution. S6.5, drying and inspection: Use nitrogen purge or spin dryer to thoroughly dry the GaN thick film 9 and AlN ceramic substrate 1 in S6.4. After completion, the surface of the GaN thick film 9 and the AlN ceramic substrate 1 can be inspected using a microscope or other equipment to confirm whether the bonding layer 7 and the barrier layer 2 have been successfully removed. S7, using CMP method to grind and polish the surface of GaN thick film 9 to obtain GaN single crystal substrate. Select a suitable polishing pad and polishing liquid, the polishing pad is made of polyurethane material and has a certain hardness. During operation, the polishing pad rotates and applies force to polish the sample surface. S7.1, grinding: Place the GaN thick film 9 on the CMP equipment, select polishing liquid and additives, generally use water-based polishing liquid as the processing medium, use deionized water as the solvent, add abrasive (such as SiO 2 、Al 2 O 3 、C、ZrO 2 Additives include dispersants, pH regulators, oxidants and other components, which have the function of changing the tension of the abrasive particles to make them evenly suspended in the polishing liquid; increasing the wettability of the abrasive particles; changing the pH value of the polishing liquid, etc. Polishing liquid is usually an alkaline or acidic liquid, which is used to remove defects on the surface of the material and unify the thickness. S7.2, rotation: the polishing pad and the polishing head start to rotate, and the polishing pad speed is 30rpm. At this time, the polishing pad rotates fast, the hardness of the polishing pad is relatively high, the diameter of the abrasive particles in the polishing liquid is large (0.5-2μm), and the pH of the polishing liquid is alkaline. After grinding, the surface of the GaN thick film 9 is relatively rough. S7.3, polishing: At this time, the polishing pad rotates slowly, the hardness of the polishing pad is relatively low, the diameter of the abrasive particles in the polishing liquid is small (10-100nm), the pH of the polishing liquid is neutral, and the flow rate of the polishing liquid is 100ml / min. After polishing, the surface of the GaN thick film 9 is smoother. S7.4, cleaning: put the polished GaN thick film 9 into 0.1% BOE cleaning solution for cleaning, and remove the alkaline polishing solution and the abrasive particles adhering to its surface. Use a cleaning device with a sponge stick to wipe the GaN thick film 9 clean, and further absorb the particles and cleaning solution on the surface of the GaN thick film 9; then rinse the surface of the GaN thick film 9 with clean water for secondary cleaning. Immerse the GaN thick film 9 in IPA (isopropyl alcohol) solvent for cleaning again. IPA is used to clean and dry the surface of the GaN thick film 9. In the process of drying the GaN thick film 9, the surface moisture and other contaminants are removed to ensure that the surface of the GaN thick film 9 is clean and flat. In step S1 of this embodiment, the LPCVD method for growing the barrier layer 2 can be replaced by the PECVD method, and the growth temperature is 300°C. Example 2 A method for preparing an 8-inch GaN single crystal substrate comprises the following steps: S1, first, a barrier layer 2 is grown on an AlN ceramic substrate 1 by LPCVD at a growth temperature of 700°C, and the barrier layer 2 is SiN. Next, a first bonding layer 3 is grown on the surface of the barrier layer 2 by PECVD at a growth temperature of 350°C. S1.1, Gas injection: inject the required deposition gas (N 2 O、N 2 、SiH 4 , Ar or other gases or their mixtures) carrying TEOS (ethyl silicate) are introduced into the PECVD reaction chamber. S1.2, establish gas pressure: ensure that the gas pressure in the reaction chamber reaches 2000mTorr by controlling the gas flow and the opening and closing of the gate. S1.3, establishing plasma: by connecting a high-frequency power supply to the electrodes of the reaction chamber, an alternating electric field is generated on the dielectric layer to form plasma. S1.4, cleaning the surface of the AlN ceramic substrate 1: starting a plasma reaction to generate sufficient energy to clean the surface of the AlN ceramic substrate 1 and the barrier layer 2 to remove surface impurities and oxides. S1.5, depositing the first bonding layer 3: the plasma power is 100 W, the organic precursor gas is decomposed and deposited onto the surface of the barrier layer 2 to form the first bonding layer 3. S1.6, end the reaction: when the preset deposition thickness is reached, stop the gas supply, disconnect the power supply, and end the plasma reaction. S1.7, cooling: After the AlN ceramic substrate 1 is cooled to room temperature, it is taken out. The first bonding layer 3 is SiO 2 The total thickness of the growth barrier layer 2 and the first bonding layer 3 is 2 μm. S2, first, a GaN epitaxial film 5 with a thickness of 3 μm is grown on a Si substrate 4 by MOCVD, including an AlN nucleation layer, a stress adjustment layer containing AlGaN, and a GaN layer, which are alternately grown in layers, wherein the GaN layer is located on the top, and the growth temperature is 900-1100°C. Next, a second bonding layer 6 is grown on the surface of the GaN epitaxial film 5 by PECVD, the growth temperature is 350°C, and the thickness is 2 μm. The second bonding layer 6 is made of the same material as the first bonding layer 3. S3, flip, so that the first bonding layer 3 of S1 and the second bonding layer 6 of S2 are first processed by CMP, cleaning, surface activation, etc., and then bonded by van der Waals force to form a bonding layer 7. First thinning, then dry etching, remove the Si substrate 4, and obtain the AlN ceramic substrate 1, barrier layer 2, bonding layer 7, and GaN epitaxial film 5 from bottom to top. S4, wet etching is performed to form GaN islands 8 having (10-11) crystal planes on the surface of the GaN epitaxial film 5 away from the bonding layer 7. Potassium hydroxide solution (KOH) is used to etch the GaN epitaxial film 5 at a concentration of 5-30% by mass at a temperature of 60°C. S5. On the GaN island 8, GaN is grown at 800-1000°C by HVPE method at a growth rate of 30 μm / h to form a GaN thick film 9 with a thickness of 1 mm. S6, removing the bonding layer 7 and the barrier layer 2 by etching with a BOE method, and separating the AlN ceramic substrate 1 from the GaN thick film 9. S6.1, preparation: put the GaN thick film 9 containing the AlN ceramic substrate 1 from which the bonding layer 7 needs to be removed into the soaking tank, and cover the surface of the GaN thick film 9 with a blue film for protection before soaking to prevent the GaN thick film 9 from being corroded due to long soaking time. BOE will also corrode the GaN thick film 9 if soaked for a long time, so it is necessary to cover the surface with a blue film for protection before corrosion. S6.2, corrosion treatment: pour the prepared BOE solution into an immersion container. Usually, a solution with a concentration of 3 vol% is used to ensure that the GaN thick film 9 of the AlN ceramic substrate 1 is completely immersed in the solution. The corrosion process takes 16 hours. S6.3, Monitoring: Regularly check the GaN thick film 9 on the AlN ceramic substrate 1 to ensure that the solution etches away the bonding layer 7 from the side, which helps to improve the etching effect. S6.4, rinsing and neutralization: After the etching is completed, the separated GaN thick film 9 and AlN ceramic substrate 1 are taken out from the BOE solution and placed in another tank respectively. They are first rinsed with 0.1% BOE to remove the original liquid adhering to the GaN thick film 9 and the AlN ceramic substrate 1, and then rinsed in another tank with sufficient pure water to completely remove the residual acid solution. S6.5, drying and inspection: Use nitrogen purge or spin dryer to thoroughly dry the GaN thick film 9 and AlN ceramic substrate 1 in S6.4. After completion, the surface of the GaN thick film 9 and the AlN ceramic substrate 1 can be inspected using a microscope or other equipment to confirm whether the bonding layer 7 and the barrier layer 2 have been successfully removed. S7, using CMP method to grind and polish the surface of GaN thick film 9 to obtain GaN single crystal substrate. Select a suitable polishing pad and polishing liquid, the polishing pad is made of polyurethane material and has a certain hardness. During operation, the polishing pad rotates and applies force to polish the sample surface. S7.1, grinding: Place the GaN thick film 9 on the CMP equipment, select polishing liquid and additives, generally use water-based polishing liquid as the processing medium, use deionized water as the solvent, add abrasive (such as SiO 2 、Al 2O 3 、C、ZrO 2 Additives include dispersants, pH regulators, oxidants and other components, which have the function of changing the tension of the abrasive particles to make them evenly suspended in the polishing liquid; increasing the wettability of the abrasive particles; changing the pH value of the polishing liquid, etc. Polishing liquid is usually an alkaline or acidic liquid, which is used to remove defects on the surface of the material and unify the thickness. S7.2, rotation: the polishing pad and the polishing head start to rotate, and the polishing pad speed is 65rpm. At this time, the polishing pad rotates fast, the hardness of the polishing pad is relatively high, the diameter of the abrasive particles in the polishing liquid is large (0.5-2μm), and the pH of the polishing liquid is alkaline. After grinding, the surface of the GaN thick film 9 is relatively rough. S7.3, polishing: At this time, the polishing pad rotates slowly, the hardness of the polishing pad is relatively low, the diameter of the abrasive particles in the polishing liquid is small (10-100nm), the pH of the polishing liquid is neutral, and the polishing liquid flow rate is 150ml / min. After polishing, the surface of the GaN thick film 9 is smoother. S7.4, cleaning: put the polished GaN thick film 9 into 0.1% BOE cleaning solution for cleaning, and remove the alkaline polishing solution and the abrasive particles adhering to its surface. Use a cleaning device with a sponge stick to wipe the GaN thick film 9 clean, and further absorb the particles and cleaning solution on the surface of the GaN thick film 9; then rinse the surface of the GaN thick film 9 with clean water for secondary cleaning. Immerse the GaN thick film 9 in IPA (isopropyl alcohol) solvent for cleaning again. IPA is used to clean and dry the surface of the GaN thick film 9. In the process of drying the GaN thick film 9, the surface moisture and other contaminants are removed to ensure that the surface of the GaN thick film 9 is clean and flat. As shown in FIG. 7 , the scanning electron microscope image after the wet etching in step S4 shows that the wet etching can dissolve the material on the surface of the crystal, effectively remove the line dislocation, and obtain a high-quality crystal. Example 3 A method for preparing a 6-inch GaN single crystal substrate comprises the following steps: S1, first, a barrier layer 2 is grown on an AlN ceramic substrate 1 by LPCVD at a growth temperature of 800°C. The barrier layer 2 is SiO 2 Next, a first bonding layer 3 is grown on the surface of the barrier layer 2 by using a PECVD method at a growth temperature of 500°C. S1.1, Gas injection: inject the required deposition gas (N 2 O、N 2 、SiH 4 , Ar or other gases or their mixtures) carrying TEOS (ethyl silicate) are introduced into the PECVD reaction chamber. S1.2, establish gas pressure: ensure that the gas pressure in the reaction chamber reaches 2200mTorr by controlling the gas flow and the opening and closing of the gate. S1.3, establishing plasma: by connecting a high-frequency power supply to the electrodes of the reaction chamber, an alternating electric field is generated on the dielectric layer to form plasma. S1.4, cleaning the surface of the AlN ceramic substrate 1: starting a plasma reaction to generate sufficient energy to clean the surface of the AlN ceramic substrate 1 and the barrier layer 2 to remove surface impurities and oxides. S1.5, depositing the first bonding layer 3: the plasma power is 120 W, the organic precursor gas is decomposed and deposited onto the surface of the AlN ceramic substrate 1 to form the first bonding layer 3. S1.6, end the reaction: when the preset deposition thickness is reached, stop the gas supply, disconnect the power supply, and end the plasma reaction. S1.7, cooling: After the AlN ceramic substrate 1 is cooled to room temperature, it is taken out. The first bonding layer 3 is SiO 2 The total thickness of the growth barrier layer 2 and the first bonding layer 3 is 3 μm. S2, first, a GaN epitaxial film 5 with a thickness of 4 μm is grown on a Si substrate 4 by MOCVD, including an AlN nucleation layer, a stress adjustment layer containing AlGaN, and a GaN layer, wherein the GaN layer is located on the top, and the growth temperature is 900-1100°C. Next, a second bonding layer 6 is grown on the surface of the GaN epitaxial film 5 by PECVD, the growth temperature is 500°C, and the thickness is 2 μm. The second bonding layer 6 is made of the same material as the first bonding layer 3. S3, flip, so that the first bonding layer 3 of S1 and the second bonding layer 6 of S2 are first processed by CMP, cleaning, surface activation, etc., and then bonded by van der Waals force to form a bonding layer 7. First thinning, then dry etching, remove the Si substrate 4, and obtain the AlN ceramic substrate 1, barrier layer 2, bonding layer 7, and GaN epitaxial film 5 from bottom to top. S4, wet etching is performed to form GaN islands 8 having (10-11) crystal planes on the surface of the GaN epitaxial film 5 away from the bonding layer 7. Potassium hydroxide solution (KOH) is used to etch the GaN epitaxial film 5 at a concentration of 5-30% by mass at a temperature of 60°C. S5. On the GaN island 8, GaN is grown at 800-1000°C by HVPE method at a growth rate of 50 μm / h to form a GaN thick film 9 with a thickness of 2 mm. S6, removing the bonding layer 7 and the barrier layer 2 by etching with a BOE method, and separating the AlN ceramic substrate 1 from the GaN thick film 9. S6.1, preparation: put the GaN thick film 9 containing the AlN ceramic substrate 1 from which the bonding layer 7 needs to be removed into the soaking tank, and cover the surface of the GaN thick film 9 with a blue film for protection before soaking to prevent the GaN thick film 9 from being corroded due to long soaking time. BOE will also corrode the GaN thick film 9 if soaked for a long time, so it is necessary to cover the surface with a blue film for protection before corrosion. S6.2, corrosion treatment: pour the prepared BOE solution into an immersion container. Usually, a solution with a concentration of 3.5 vol% is used to ensure that the GaN thick film 9 of the AlN ceramic substrate 1 is completely immersed in the solution. The corrosion process takes 14 hours. S6.3, monitoring: Regularly check the GaN thick film 9 on the AlN ceramic substrate 1 to ensure that the solution etches away the bonding layer 7 from the side, which helps to improve the etching effect. S6.4, rinsing and neutralization: After the etching is completed, the separated GaN thick film 9 and AlN ceramic substrate 1 are taken out from the BOE solution and placed in another tank respectively. They are first rinsed with 0.1% BOE to remove the original liquid adhering to the GaN thick film 9 and the AlN ceramic substrate 1, and then rinsed in another tank with sufficient pure water to completely remove the residual acid solution. S6.5, drying and inspection: Use nitrogen purge or spin dryer to thoroughly dry the GaN thick film 9 and AlN ceramic substrate 1 in S6.4. After completion, the surface of the GaN thick film 9 and the AlN ceramic substrate 1 can be inspected using a microscope or other equipment to confirm whether the bonding layer 7 and the barrier layer 2 have been successfully removed. S7, using CMP method to grind and polish the surface of GaN thick film 9 to obtain GaN single crystal substrate. Select a suitable polishing pad and polishing liquid, the polishing pad is made of polyurethane material and has a certain hardness. During operation, the polishing pad rotates and applies force to polish the sample surface. S7.1, grinding: Place the GaN thick film 9 on the CMP equipment, select polishing liquid and additives, generally use water-based polishing liquid as the processing medium, use deionized water as the solvent, add abrasive (such as SiO 2 、Al 2 O 3 、C、ZrO 2 Additives include dispersants, pH regulators, oxidants and other components, which have the function of changing the tension of the abrasive particles to make them evenly suspended in the polishing liquid; increasing the wettability of the abrasive particles; changing the pH value of the polishing liquid, etc. Polishing liquid is usually an alkaline or acidic liquid, which is used to remove defects on the surface of the material and unify the thickness. S7.2, rotation: the polishing pad and the polishing head start to rotate, and the polishing pad speed is 100rpm. At this time, the polishing pad rotates fast, the hardness of the polishing pad is relatively high, the diameter of the abrasive particles in the polishing liquid is large (0.5-2μm), and the pH of the polishing liquid is alkaline. After grinding, the surface of the GaN thick film 9 is relatively rough. S7.3, polishing: At this time, the polishing pad rotates slowly, the hardness of the polishing pad is relatively low, the diameter of the abrasive particles in the polishing liquid is small (10-100nm), the pH of the polishing liquid is neutral, and the polishing liquid flow rate is 200ml / min. After polishing, the surface of the GaN thick film 9 is smoother. S7.4, cleaning: put the polished GaN thick film 9 into 0.1% BOE cleaning solution for cleaning, and remove the alkaline polishing solution and the abrasive particles adhering to its surface. Use a cleaning device with a sponge stick to wipe the GaN thick film 9 clean, and further absorb the particles and cleaning solution on the surface of the GaN thick film 9; then rinse the surface of the GaN thick film 9 with clean water for secondary cleaning. Immerse the GaN thick film 9 in IPA (isopropyl alcohol) solvent for cleaning again. IPA is used to clean and dry the surface of the GaN thick film 9. In the process of drying the GaN thick film 9, the surface moisture and other contaminants are removed to ensure that the surface of the GaN thick film 9 is clean and flat. Example 4 A method for preparing a GaN single crystal substrate comprises the following steps: S1, first, a barrier layer 2 is grown on an AlN ceramic substrate 1 by LPCVD at a growth temperature of 600°C, and the barrier layer 2 is SiN. Next, a first bonding layer 3 is grown on the surface of the barrier layer 2 by PECVD at a growth temperature of 300°C. S1.1, Gas injection: inject the required deposition gas (N 2 O、N 2 、SiH 4 , Ar or other gases or their mixtures) carrying TEOS (ethyl silicate) are introduced into the PECVD reaction chamber. S1.2, establish gas pressure: ensure that the gas pressure in the reaction chamber reaches 500mTorr by controlling the gas flow and the opening and closing of the gate. S1.3, establishing plasma: by connecting a high-frequency power supply to the electrodes of the reaction chamber, an alternating electric field is generated on the dielectric layer to form plasma. S1.4, cleaning the surface of the AlN ceramic substrate 1: starting a plasma reaction to generate sufficient energy to clean the surface of the AlN ceramic substrate 1 and remove surface impurities and oxides. S1.5, depositing the first bonding layer 3: the plasma power is 20 W, the organic precursor gas is decomposed and deposited onto the surface of the AlN ceramic substrate 1 and the barrier layer 2 to form the first bonding layer 3. S1.6, end the reaction: when the preset deposition thickness is reached, stop the gas supply, disconnect the power supply, and end the plasma reaction. S1.7, cooling: After the AlN ceramic substrate 1 is cooled to room temperature, it is taken out. The first bonding layer 3 is Si 3 N 4 The total thickness of the growth barrier layer 2 and the first bonding layer 3 is 1 μm. S2, first, a GaN epitaxial film 5 with a thickness of 1 μm is grown on a Si substrate 4 by MOCVD, including an AlN nucleation layer and a GaN layer, wherein the GaN layer is located on the top, and the growth temperature is 900-1100°C. Next, a second bonding layer 6 is grown on the surface of the GaN epitaxial film 5 by PECVD, the growth temperature is 300°C, and the thickness is 2 μm. The second bonding layer 6 is made of the same material as the first bonding layer 3. S3, flip, so that the first bonding layer 3 of S1 and the second bonding layer 6 of S2 are first processed by CMP, cleaning, surface activation, etc., and then bonded by van der Waals force to form a bonding layer 7. First thinning, then dry etching, remove the Si substrate 4, and obtain the AlN ceramic substrate 1, barrier layer 2, bonding layer 7, and GaN epitaxial film 5 from bottom to top. S4, by wet etching, a GaN island 8 with a (10-11) crystal plane is formed on the surface of the GaN epitaxial film 5 away from the bonding layer 7. A potassium hydroxide solution (KOH) is used, with a concentration of 5-30% by mass, to etch the GaN epitaxial film 5 at a temperature of 25°C. S5. On the GaN island 8, GaN is grown at 800-1000°C by HVPE method at a growth rate of 10 μm / h to form a GaN thick film 9 with a thickness of 800 μm. S6, removing the bonding layer 7 and the barrier layer 2 by HF etching, and separating the AlN ceramic substrate 1 from the GaN thick film 9. S6.1, preparation: put the GaN thick film 9 containing the AlN ceramic substrate 1 from which the bonding layer 7 needs to be removed into the soaking tank, and cover the surface of the GaN thick film 9 with a blue film for protection before soaking to prevent the GaN thick film 9 from being corroded due to long soaking time. BOE will also corrode the GaN thick film 9 if soaked for a long time, so it is necessary to cover the surface with a blue film for protection before corrosion. S6.2, corrosion treatment: pour the prepared HF solution into the immersion container, usually using a solution with a concentration of 1 vol%, to ensure that the GaN thick film 9 of the AlN ceramic substrate 1 is completely immersed in the solution. The corrosion process takes 30 hours. S6.3, monitoring: Regularly check the GaN thick film 9 on the AlN ceramic substrate 1 to ensure that the solution etches away the bonding layer 7 from the side, which helps to improve the etching effect. S6.4, rinsing and neutralization: After the etching is completed, the separated GaN thick film 9 and AlN ceramic substrate 1 are taken out from the HF solution and placed in another tank respectively. They are first rinsed with 0.1% BOE to remove the original liquid adhering to the GaN thick film 9 and the AlN ceramic substrate 1, and then rinsed in another tank with sufficient pure water to completely remove the residual acid solution. S6.5, drying and inspection: Use nitrogen purge or spin dryer to thoroughly dry the GaN thick film 9 and AlN ceramic substrate 1 in S6.4. After completion, the surface of the GaN thick film 9 and the AlN ceramic substrate 1 can be inspected using a microscope or other equipment to confirm whether the bonding layer 7 and the barrier layer 2 have been successfully removed. S7, using CMP method to grind and polish the surface of GaN thick film 9 to obtain GaN single crystal substrate. Select a suitable polishing pad and polishing liquid, the polishing pad is made of polyurethane material and has a certain hardness. During operation, the polishing pad rotates and applies a pressure of 0.5PSI to polish the sample surface. S7.1, grinding: Place the GaN thick film 9 on the CMP equipment, select polishing liquid and additives, generally use water-based polishing liquid as the processing medium, use deionized water as the solvent, add abrasive (such as SiO 2 、Al 2 O 3 、C、ZrO 2 Additives include dispersants, pH regulators, oxidants and other components, which have the function of changing the tension of the abrasive particles to make them evenly suspended in the polishing liquid; increasing the wettability of the abrasive particles; changing the pH value of the polishing liquid, etc. Polishing liquid is usually an alkaline or acidic liquid, which is used to remove defects on the surface of the material and unify the thickness. S7.2, rotation: the polishing pad and the polishing head start to rotate, and the polishing pad speed is 30rpm. At this time, the polishing pad rotates fast, the hardness of the polishing pad is relatively high, the diameter of the abrasive particles in the polishing liquid is large (0.5-2μm), and the pH of the polishing liquid is alkaline. After grinding, the surface of the GaN thick film 9 is relatively rough. S7.3, polishing: At this time, the polishing pad rotates slowly, the hardness of the polishing pad is relatively low, the diameter of the abrasive particles in the polishing liquid is small (10-100nm), the pH of the polishing liquid is neutral, and the flow rate of the polishing liquid is 100ml / min. After polishing, the surface of the GaN thick film 9 is smoother. S7.4, cleaning: put the polished GaN thick film 9 into 0.1% BOE cleaning solution for cleaning, and remove the alkaline polishing solution and the abrasive particles adhering to its surface. Use a cleaning device with a sponge stick to wipe the GaN thick film 9 clean, and further absorb the particles and cleaning solution on the surface of the GaN thick film 9; then rinse the surface of the GaN thick film 9 with clean water for secondary cleaning. Immerse the GaN thick film 9 in IPA (isopropyl alcohol) solvent for cleaning again. IPA is used to clean and dry the surface of the GaN thick film 9. In the process of drying the GaN thick film 9, the surface moisture and other contaminants are removed to ensure that the surface of the GaN thick film 9 is clean and flat. Example 5 A method for preparing a GaN single crystal substrate comprises the following steps: S1, first, a barrier layer 2 is grown on an AlN ceramic substrate 1 by PECVD at a growth temperature of 500°C, and the barrier layer 2 is SiN. Next, a first bonding layer 3 is grown on the surface of the barrier layer 2 by PECVD at a growth temperature of 500°C. S1.1, Gas injection: inject the required deposition gas (N 2 O、N 2 、SiH 4 , Ar or other gases or their mixtures) carrying TEOS (ethyl silicate) are introduced into the PECVD reaction chamber. S1.2, establish gas pressure: ensure that the gas pressure in the reaction chamber reaches 3000mTorr by controlling the gas flow and the opening and closing of the gate. S1.3, establishing plasma: by connecting a high-frequency power supply to the electrodes of the reaction chamber, an alternating electric field is generated on the dielectric layer to form plasma. S1.4, cleaning the surface of the AlN ceramic substrate 1: starting a plasma reaction to generate sufficient energy to clean the surface of the AlN ceramic substrate 1 and the barrier layer 2 to remove surface impurities and oxides. S1.5, depositing the first bonding layer 3: the plasma power is 200 W, the organic precursor gas is decomposed and deposited onto the surface of the barrier layer 2 to form the first bonding layer 3. S1.6, end the reaction: when the preset deposition thickness is reached, stop the gas supply, disconnect the power supply, and end the plasma reaction. S1.7, cooling: After the AlN ceramic substrate 1 is cooled to room temperature, it is taken out. The first bonding layer 3 is SiO 2 The total thickness of the growth barrier layer 2 and the first bonding layer 3 is 3 μm. S2, first, a GaN epitaxial film 5 with a thickness of 4 μm is grown on a Si substrate 4 by MOCVD, including an AlN nucleation layer, a stress adjustment layer containing AlGaN, and a GaN layer, wherein the GaN layer is located on the top, and the growth temperature is 900-1100°C. Next, a second bonding layer 6 is grown on the surface of the GaN epitaxial film 5 by PECVD, the growth temperature is 500°C, and the thickness is 2 μm. The second bonding layer 6 is made of the same material as the first bonding layer 3. S3, flip, so that the first bonding layer 3 of S1 and the second bonding layer 6 of S2 are first processed by CMP, cleaning, surface activation, etc., and then bonded by van der Waals force to form a bonding layer 7. First thinning, then dry etching, remove the Si substrate 4, and obtain the AlN ceramic substrate 1, barrier layer 2, bonding layer 7, and GaN epitaxial film 5 from bottom to top. S4, by wet etching, a GaN island 8 with a (10-11) crystal plane is formed on the surface of the GaN epitaxial film 5 away from the bonding layer 7. A potassium hydroxide solution (KOH) is used with a concentration of 5-30% by mass to etch the GaN epitaxial film 5 at an etching temperature of 80°C. S5. On the GaN island 8, GaN is grown at 800-1000°C by HVPE method at a growth rate of 50 μm / h to form a GaN thick film 9 with a thickness of 2 mm. S6, removing the bonding layer 7 and the barrier layer 2 by etching with a BOE method, and separating the AlN ceramic substrate 1 from the GaN thick film 9. S6.1, preparation: put the GaN thick film 9 containing the AlN ceramic substrate 1 from which the bonding layer 7 needs to be removed into the soaking tank, and cover the surface of the GaN thick film 9 with a blue film for protection before soaking to prevent the GaN thick film 9 from being corroded due to long soaking time. BOE will also corrode the GaN thick film 9 if soaked for a long time, so it is necessary to cover the surface with a blue film for protection before corrosion. S6.2, corrosion treatment: pour the prepared BOE solution into an immersion container. Usually, a solution with a concentration of 5 vol% is used to ensure that the GaN thick film 9 of the AlN ceramic substrate 1 is completely immersed in the solution. The corrosion process takes 2 hours. S6.3, monitoring: Regularly check the GaN thick film 9 on the AlN ceramic substrate 1 to ensure that the solution etches away the bonding layer 7 from the side, which helps to improve the etching effect. S6.4, rinsing and neutralization: After the etching is completed, the separated GaN thick film 9 and AlN ceramic substrate 1 are taken out from the BOE solution and placed in another tank respectively. They are first rinsed with 0.1% BOE to remove the original liquid adhering to the GaN thick film 9 and the AlN ceramic substrate 1, and then rinsed in another tank with sufficient pure water to completely remove the residual acid solution. S6.5, drying and inspection: Use nitrogen purge or spin dryer to thoroughly dry the GaN thick film 9 and AlN ceramic substrate 1 in S6.4. After completion, the surface of the GaN thick film 9 and the AlN ceramic substrate 1 can be inspected using a microscope or other equipment to confirm whether the bonding layer 7 and the barrier layer 2 have been successfully removed. S7, using CMP method to grind and polish the surface of GaN thick film 9 to obtain GaN single crystal substrate. Select a suitable polishing pad and polishing liquid, the polishing pad is made of polyurethane material and has a certain hardness. During operation, the polishing pad rotates and applies a pressure of 10PSI to polish the sample surface. S7.1, grinding: Place the GaN thick film 9 on the CMP equipment, select polishing liquid and additives, generally use water-based polishing liquid as the processing medium, use deionized water as the solvent, add abrasive (such as SiO 2 、Al 2 O 3 、C、ZrO 2 Additives include dispersants, pH regulators, oxidants and other components, which have the function of changing the tension of the abrasive particles to make them evenly suspended in the polishing liquid; increasing the wettability of the abrasive particles; changing the pH value of the polishing liquid, etc. Polishing liquid is usually an alkaline or acidic liquid, which is used to remove defects on the surface of the material and unify the thickness. S7.2, rotation: the polishing pad and the polishing head start to rotate, and the polishing pad speed is 100rpm. At this time, the polishing pad rotates fast, the hardness of the polishing pad is relatively high, the diameter of the abrasive particles in the polishing liquid is large (0.5-2μm), and the pH of the polishing liquid is alkaline. After grinding, the surface of the GaN thick film 9 is relatively rough. S7.3, polishing: At this time, the polishing pad rotates slowly, the hardness of the polishing pad is relatively low, the diameter of the abrasive particles in the polishing liquid is small (10-100nm), the pH of the polishing liquid is neutral, and the polishing liquid flow rate is 200ml / min. After polishing, the surface of the GaN thick film 9 is smoother. S7.4, cleaning: put the polished GaN thick film 9 into 0.1% BOE cleaning solution for cleaning, and remove the alkaline polishing solution and the abrasive particles adhering to its surface. Use a cleaning device with a sponge stick to wipe the GaN thick film 9 clean, and further absorb the particles and cleaning solution on the surface of the GaN thick film 9; then rinse the surface of the GaN thick film 9 with clean water for secondary cleaning. Immerse the GaN thick film 9 in IPA (isopropyl alcohol) solvent for cleaning again. IPA is used to clean and dry the surface of the GaN thick film 9. In the process of drying the GaN thick film 9, the surface moisture and other contaminants are removed to ensure that the surface of the GaN thick film 9 is clean and flat. Among the above embodiments, Embodiment 2 is the best embodiment. Comparative Example 1 The remaining steps of this comparative example are the same as those of Example 2, except that the wet etching step of S4 is omitted and GaN is directly grown in S5. It is found that there are line dislocations and the quality of the obtained GaN single crystal is poor. Comparative Example 2 This comparative example adopts the traditional method in the background technology to perform heteroepitaxial growth on a sapphire substrate to obtain a GaN thick film. After X-ray diffraction testing, as shown in Figure 8, it is found that: the X-ray half-peak width (FWHM) of the GaN thick film

[0002] crystal image of the composite substrate made of the AlN ceramic substrate is 120.2 arcsec, and the X-ray half-peak width (FWHM) of the GaN thick film

[0002] crystal image when the sapphire substrate is used is 362.7 arcsec; the X-ray half-peak width (FWHM) of the GaN thick film

[0102] crystal image of the composite substrate made of the AlN ceramic substrate is 289.3 arcsec, and the X-ray half-peak width (FWHM) of the GaN thick film

[0002] crystal image when the sapphire substrate is used is 750.0 arcsec; the larger the FWHM value, the worse the crystal quality. It can be seen that the crystal quality obtained by using the AlN ceramic substrate to make the composite substrate is significantly better than that of the traditional sapphire substrate.

Claims

1. A method for preparing a GaN single crystal substrate, characterized in that: The following steps are involved: Step 1: sequentially growing a barrier layer (2) and a first bonding layer (3) on a substrate (1), and sequentially growing a GaN epitaxial thin film (5) and a second bonding layer (6) on a substrate (4); Step 2: bonding the first bonding layer (3) of step 1 to the second bonding layer (6) of step 2 to form a bonding layer (7), removing the substrate (4), and obtaining a substrate (1), a barrier layer (2), a bonding layer (7), and a GaN epitaxial film (5) connected in sequence; Step 3, forming a GaN island (8) having a (10-11) crystal plane on the surface of the GaN epitaxial film (5) away from the bonding layer (7) by wet etching; Step 4: growing GaN on the GaN island (8) at 800-1000° C. using the HVPE method to form a GaN thick film (9); Step 5, removing the bonding layer (7) by etching with HF or BOE, and separating the substrate (1) from the GaN thick film (9); Step 6, using CMP method to grind and polish the surface of GaN thick film (9) to obtain GaN single crystal substrate; In the step three, the wet etching is performed by etching the GaN epitaxial film (5) with a potassium hydroxide solution at 25 to 80° C., wherein the concentration of the potassium hydroxide solution is 5 to 30% by mass. When the potassium hydroxide solution reacts with the crystal surface, it causes the dissolution of the material near the crystal surface, thereby causing the line dislocation to disappear.

2. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 1, the substrate (1) is an AlN ceramic substrate, and the first bonding layer (3) is SiO2 or Si3N4.

3. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 1, the total thickness of the growth barrier layer (2) and the first bonding layer (3) is 1 to 3 μm.

4. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 1, the barrier layer (2) is grown by LPCVD or PECVD, the growth temperature of the LPCVD method is 600-800° C., and the growth temperature of the PECVD method is 300-500° C.; the first bonding layer (3) is grown by PECVD, and the growth temperature of the first bonding layer (3) is 300-500° C.

5. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 1, the GaN epitaxial film (5) is grown at 900-1100° C. by MOCVD method, with a thickness of 1-4 μm.

6. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 1, the second bonding layer (6) is grown at 300-500° C. by PECVD method, with a thickness of 0.5-1.5 μm. The second bonding layer (6) is made of the same material as the first bonding layer (3).

7. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 2, when removing the substrate (4), thinning is first performed and then dry etching is performed.

8. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 4, the thickness of the GaN thick film (9) is 800 μm to 2 mm.

9. The method for preparing a GaN single crystal substrate according to claim 1, characterized in that: In the step 4, the growth rate of growing GaN is 10 to 50 μm / h.

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