Method for epitaxy of uniform graphene on silicon carbide substrate

Abstract

The invention relates to the technical field of semiconductor materials, in particular to a method for epitaxy of uniform graphene on a silicon carbide substrate, which comprises the following steps of: (1) removing processing defects on the surface of the silicon carbide substrate through hydrogen etching to form a regular silicon carbide step structure; (2) the silicon carbide substrate is subjected to two-step annealing in argon, in the first-step annealing, heat preservation is conducted for 0.5-1 h at the temperature of 1530-1630 DEG C, and the pressure is kept at 800-900 mbar; and in the second annealing step, the temperature is kept at 1650-1750 DEG C for 0.5-1 hour, and the pressure is kept at 800-900 mbar. Compared with a conventional method of directly heating to the graphene growth temperature, the method has the advantages that the temperature stability condition in the graphene growth process is controlled in a segmented manner, the surface step morphology is obviously improved, irregular step coalescence in the growth process is successfully inhibited, and the stable and uniform morphology is more beneficial to the layer number uniformity of the graphene of the whole substrate.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor materials, and particularly to a method for epitaxially growing uniform graphene on a silicon carbide substrate. Background Art

[0002] Graphene has a unique two-dimensional structure. Due to its unique Dirac cone band structure, graphene exhibits excellent electrical and optical properties and has broad application prospects in the fields of future integrated circuits and optoelectronic devices. The SiC pyrolysis method for preparing epitaxial graphene has natural advantages. It can achieve the controllable growth of large-area uniform graphene, does not require transfer, is compatible with the current Si-based semiconductor process, and SiC, as a third-generation wide-bandgap semiconductor material, has excellent electrical and thermal properties. This method is considered to be one of the most promising methods for graphene preparation. The principle of epitaxially growing graphene by the SiC pyrolysis method is to place the SiC substrate in a graphite crucible and perform high-temperature annealing in a growth furnace. Since the saturated vapor pressure of Si is higher than that of C, Si atoms will preferentially sublime and detach from the substrate in the form of gaseous components, and then the remaining free C atoms will nucleate, diffuse and recombine on the SiC surface to form graphene. The epitaxial growth of graphene on the SiC substrate usually includes two stages: the hydrogen etching stage and the stage of annealing in argon to form graphene.

[0003] CN 102936009 A discloses a method for fabricating a low-layer graphene film on a silicon carbide substrate. Argon is introduced into the reaction chamber and the pressure in the reaction chamber is increased to inhibit the sublimation of Si atoms in the SiC substrate during the heating process. The preparation of the low-layer graphene film is achieved by controlling the annealing time and pressure during the growth process, but it is difficult to ensure the uniformity of graphene on the entire substrate. CN 107344868 A discloses a method for preparing buffer-layer-free single-layer graphene on a SiC substrate. In this method, the buffer layer is annealed in hydrogen at 800-1200 °C, and hydrogen atoms are inserted into the Si atoms of the substrate to break the covalent bond between the substrate and the buffer layer, converting the buffer layer into single-layer graphene. However, the surface step morphology reported in this patent is still relatively disordered, and the buffer layer on the SiC substrate reported so far will open a certain bandgap, which has great application potential in semiconductor devices, and in some applications, it is not expected to destroy this buffer layer structure.

[0004] It can be seen that the epitaxial graphene on the SiC substrate prepared by the existing technology still has problems such as an incomplete regular surface morphology, uneven graphene thickness, and many grain boundary defects, which have a great impact on the electrical and optical properties of graphene. During the growth process of graphene, directly heating to the graphene growth temperature at too fast a heating rate and a relatively high growth temperature will cause step coalescence on the surface after graphene growth, resulting in large irregular steps, causing an irregular surface morphology. At the same time, the large irregular steps will hinder the decomposition of the substrate of adjacent small steps and the formation of graphene, thereby affecting the uniformity of graphene. Summary of the Invention

[0005] Aiming at the technical problem of uneven surface steps of graphene existing in the existing method for epitaxially growing graphene on a SiC substrate, the present invention provides a method for epitaxially growing uniform graphene on a silicon carbide substrate.

[0006] The technical solution of the present invention is as follows: A method for epitaxially growing uniform graphene on a silicon carbide substrate includes the following steps: (1) The silicon carbide substrate is first etched with hydrogen to remove processing defects on the substrate surface, such as scratches, etc., to form a regular silicon carbide step structure; (2) The silicon carbide substrate is annealed in two steps in argon. The first annealing is to keep the temperature at 1530 - 1630 °C for 0.5 - 1 h, and the pressure is maintained at 800 - 900 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first annealing; the second annealing is to keep the temperature at 1650 - 1750 °C for 0.5 - 1 h, and the pressure is maintained at 800 - 900 mbar. Through the second annealing, near-equilibrium decomposition graphitization will occur on the substrate surface, thereby obtaining high-quality graphene.

[0007] Further, the silicon carbide substrate used in step (1) can be 4H-SiC, 6H-SiC, etc. The thickness of the silicon carbide substrate is 350 - 500 μm, the surface roughness is <0.2 nm, and the flatness is <10 μm. It is obtained through the following pretreatment method: First, polish the surface of the silicon carbide substrate. The polishing method preferably uses chemical mechanical polishing, and then clean the silicon carbide substrate using the standard RCA process.

[0008] Further, in step (1), the silicon carbide substrate is loaded into the growth chamber, and a mechanical pump or / and a vacuum pump are used to pump the growth chamber to a vacuum degree of less than 10 -5 Pa to exclude the air components introduced into the growth chamber when assembling the silicon carbide substrate; in order to further remove the water vapor and impurity gases adsorbed in the thermal insulation material and the crucible wall, when the vacuum degree reaches less than 10 -5 Pa, first heat up to bake the growth chamber, and the baking temperature is 800 - 1000 °C.

[0009] Further, in step (1), the growth Si surface of the silicon carbide substrate is placed downward.

[0010] Further, in step (1), the temperature of hydrogen etching is 1400 - 1500 °C, the time is 10 - 30 min, and the pressure is maintained at 800 - 900 mbar. A chemical reaction occurs between hydrogen and the silicon carbide substrate, etching a regular step morphology on the surface. Through hydrogen etching pretreatment, scratches on the surface of the silicon carbide substrate are removed, and the substrate steps are broadened, enabling a surface suitable for high-quality graphene growth to be obtained.

[0011] Further, before the two-step annealing in step (2), the gas composition introduced into the growth chamber is switched, and the gas in the growth chamber is converted into an argon atmosphere.

[0012] Further, after the growth of graphene in step (2) is completed, the temperature of the growth chamber is lowered while maintaining gas introduction to carry away the decomposed Si component gas on the surface layer and prevent it from flowing back to the graphene surface. Then, the temperature in the growth chamber slowly drops to room temperature, ending the growth of graphene.

[0013] The beneficial effects of the present invention are as follows: The present invention optimizes the surface step morphology of graphene through a two-step annealing method to obtain a more uniform number of graphene layers. First, a buffer layer is preferentially formed at 1530 - 1630 °C and stabilized at this temperature for 0.5 - 1 h, which plays a role in stabilizing the surface step morphology, and the surface morphology does not change significantly after the second-step annealing for growing graphene. Compared with directly heating to the graphene growth temperature conventionally, the present invention controls the temperature stability during the graphene growth process in segments, significantly improving the surface step morphology, successfully suppressing the coalescence of irregular steps during the growth process, and the stable and uniform morphology is more conducive to the uniformity of the number of graphene layers on the entire substrate. Description of the Drawings

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0015] Figure 1 It is an AFM surface morphology diagram of the graphene surface morphology epitaxially grown in Example 1, where (a) is the morphology diagram of the buffer layer prepared after the first-step annealing, and (b) is the morphology diagram of the graphene prepared after the second-step annealing.

[0016] Figure 2 It is an optical morphology diagram of the graphene obtained by two-step annealing epitaxial growth of the graphene prepared after the second-step annealing in Example 1.

[0017] Figure 3 It is the nine-point Raman spectrum of the buffer layer on the Si surface of the SiC substrate prepared by the first-step annealing in Example 1.

[0018] Figure 4 It is the nine-point Raman spectrum of the single-layer graphene on the Si surface of the SiC substrate prepared by the second-step annealing in Example 1.

[0019] Figure 5 It is the optical morphology diagram of the graphene prepared by directly heating and annealing at one time in Comparative Example 1.

[0020] Figure 6 It is the nine-point Raman spectrum of the sample on the Si surface of the SiC substrate prepared by directly heating and annealing at one time in Comparative Example 1.

[0021] Figure 7 It is the AFM morphology diagram of the graphene grown at a lower temperature in the first-step annealing in Comparative Example 2.

[0022] Figure 8 It is the AFM morphology diagram of the graphene grown at a higher temperature in the first-step annealing in Comparative Example 3. Detailed implementation manners

[0023] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0024] Example 1 Epitaxially grow uniform graphene on a 4H-SiC substrate. The thickness of the 4H-SiC is 500 μm, and the specification is 10×10 mm. The specific method includes the following steps: (1) First, chemically mechanically polish the surface of the silicon carbide substrate, and then clean the silicon carbide substrate using the standard RCA process to obtain a silicon carbide substrate with a surface roughness <0.2 nm and a flatness <10 μm.

[0025] Place the silicon carbide substrate in the growth chamber with the growth Si surface facing down. In order to exhaust the air components in the growth chamber, use a combination of a mechanical pump and a vacuum pump to pump the growth chamber to a vacuum of less than 10 -5 Pa; In order to further remove the water vapor and impurity gases adsorbed in the thermal insulation material and the crucible wall, raise the temperature to 1000 °C in the high-vacuum stage and bake the growth chamber.

[0026] To remove surface contamination, the subsurface damaged layer, and improve the substrate surface topography, hydrogen gas is introduced into the growth chamber, and at the same time, the temperature is continuously raised to 1450 °C. The hydrogen etching time is 20 min, and the pressure is maintained at 800 mbar during this period. A chemical reaction occurs between hydrogen and the silicon carbide substrate, etching away defects such as scratches on the surface of the silicon carbide substrate to form a regular silicon carbide step structure.

[0027] (2)Switch the gas composition introduced into the growth chamber to convert the gas in the growth chamber to an argon atmosphere, and then perform two-step annealing. The first annealing is to hold the temperature at 1550 °C for 1 h, and the pressure is maintained at 800 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first annealing; then, the growth temperature is slowly raised to the second annealing temperature of 1650 °C and held for 30 min, and the pressure is maintained at 800 mbar during this period. Through the second annealing, near-equilibrium decomposition graphitization occurs on the substrate surface, thereby obtaining high-quality graphene.

[0028] After the growth of graphene is completed, the heating current is turned down, the temperature of the growth chamber is lowered, and at the same time, gas is continuously introduced to carry away the decomposed Si component gas on the surface layer to prevent it from flowing back to the graphene surface. Then, the temperature of the growth chamber is slowly lowered to room temperature to end the growth of graphene, and the sample is taken out to obtain epitaxial graphene with a uniform surface step topography and a controllable number of layers.

[0029] Use an atomic force microscope and an optical microscope to observe the morphology of graphene after the first annealing and the second annealing in step (2). The results are as Figure 1 and Figure 2 shown. Figure 1 (a) shows the surface morphology of the graphene buffer layer after the first annealing. Figure 1 (b) shows the surface morphology of graphene after the second annealing. The graphene buffer layer after the first annealing exhibits a uniform and regular step morphology, and the subsequent second annealing still maintains the original morphology without obvious changes, indicating that the stable stage of the buffer layer formed during the first annealing in the growth process plays an important role in the stable control of the surface step morphology. By segmentally controlling the temperature stability during the graphene growth process in two steps, the surface step morphology is significantly improved, and the irregular step coalescence during the growth process is successfully suppressed. Using this two-step annealing method, graphene with a more uniform and regular step morphology can be obtained, which is more conducive to the uniformity of the number of graphene layers. Figure 2 The large-scale graphene morphology map still shows an overall uniform, regular, and flat step morphology.

[0030] Perform Raman tests on the buffer layer and graphene samples prepared by two-step annealing. The test results are as Figure 3 and Figure 4 shown. Only a buffer layer is formed after the first annealing.Figure 3 There is no Raman 2D peak of graphene; the nine-point Raman spectra of graphene at different positions on the Si surface of the SiC substrate after the second annealing show that the 2D peaks of graphene can all be fitted by a single Lorentz peak, and the full width at half maximum is 30 - 40 cm -1 , which proves that the prepared graphene is monolayer graphene. The intensity of the D peak of graphene is basically 0, indicating that the prepared graphene has few defects and high quality.

[0031] Example 2 Uniform graphene is epitaxially grown on a 6H-SiC substrate. The thickness of the 6H-SiC is 350 μm, and the specification is 10×10 mm. The specific method includes the following steps: (1) First, the surface of the silicon carbide substrate is chemically mechanically polished, and then the silicon carbide substrate is cleaned by the standard RCA process to obtain a silicon carbide substrate with a surface roughness <0.2 nm and a flatness <10 μm.

[0032] The silicon carbide substrate is loaded into the growth chamber, and the growth Si surface of the silicon carbide substrate is placed downward. In order to evacuate the air components in the growth chamber, a mechanical pump and a vacuum pump are used in combination to pump the growth chamber to a vacuum degree of 10 -5 Pa or less; in order to further remove the water vapor and impurity gases adsorbed in the thermal insulation material and the crucible wall, the temperature is raised to 800 °C in the high vacuum stage, and the growth chamber is baked.

[0033] In order to remove surface contamination, the surface sub-damage layer and improve the surface morphology of the substrate, hydrogen is introduced into the growth chamber, and at the same time, the temperature is continuously raised to 1500 °C. The hydrogen etching time is kept at 10 min, and the pressure is kept at 900 mbar during this period. The hydrogen reacts chemically with the silicon carbide substrate to etch and remove defects such as scratches on the surface of the silicon carbide substrate, forming a regular silicon carbide step structure.

[0034] (2) Switch the gas components introduced into the growth chamber to convert the gas in the growth chamber into an argon atmosphere, and then perform two-step annealing. The first annealing is to keep the temperature at 1600 °C for 0.5 h, and the pressure is kept at 900 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first annealing; then the growth temperature is slowly raised to the second annealing temperature of 1750 °C and kept for 1 h, and the pressure is kept at 900 mbar during this period. Through the second annealing, near-equilibrium decomposition graphitization occurs on the substrate surface, thereby obtaining high-quality graphene.

[0035] After the growth of graphene is completed, reduce the heating current to lower the temperature of the growth chamber. Meanwhile, keep the gas flowing in to carry away the decomposed Si component gas on the surface layer and prevent it from flowing back to the graphene surface. Then, slowly lower the temperature of the growth chamber to room temperature to end the growth of graphene. Take out the sample to obtain epitaxial graphene with a uniform surface step morphology and a controllable and uniform number of layers.

[0036] Characterize the graphene material prepared by two-step annealing epitaxial growth using Raman spectroscopy and atomic force microscopy. The morphology of the graphene is uniform, and the full width at half maximum of the Raman 2D peak of the graphene is generally in the range of 50 - 65 cm -1 , indicating that the graphene is uniform bilayer graphene. The intensity of the D peak of the graphene is basically 0, indicating that the prepared graphene has few defects and high quality.

[0037] Example 3 A method for epitaxially growing uniform graphene on a 4-inch 4H-SiC substrate. According to the same growth method as in Example 1, the difference is that the processed 4-inch 4H-SiC substrate is placed with the Si face down in the growth crucible. The first annealing temperature is 1600 °C and it is kept warm for 1 h; the second annealing temperature is 1700 °C and it is kept warm for 0.5 h for graphene growth. Characterize the prepared graphene material using Raman spectroscopy and atomic force microscopy. The morphology of the graphene is uniform, and the full width at half maximum of the Raman 2D peak of the graphene is generally in the range of 30 - 55 cm -1 , indicating that the number of graphene layers is uniform, generally 1 - 2 layers as a whole. The intensity of the Raman D peak of the graphene is basically 0, indicating that the prepared graphene has few defects and high quality.

[0038] Comparative Example 1 Epitaxially grow graphene on a 4H-SiC substrate by a one-step direct temperature-raising annealing method. The thickness of the 4H-SiC is 500 μm and the specification is 10×10 mm. The specific method includes the following steps: (1) First, chemically mechanically polish the surface of the silicon carbide substrate, and then clean the silicon carbide substrate using the standard RCA process to obtain a silicon carbide substrate with a surface roughness <0.2 nm and a flatness <10 μm.

[0039] Place the silicon carbide substrate into the growth chamber with the growth Si face of the silicon carbide substrate facing down. In order to exhaust the air components in the growth chamber, use a combination of a mechanical pump and a vacuum pump to pump the growth chamber to a vacuum degree of less than 10 -5 Pa; in order to further remove the water vapor and impurity gases adsorbed in the thermal insulation material and the crucible wall, raise the temperature to 1000 °C in the high vacuum stage and bake the growth chamber.

[0040] To remove surface contamination, the subsurface damaged layer, and improve the surface topography of the substrate, hydrogen gas is introduced into the growth chamber, and at the same time, the temperature is continuously raised to 1450 °C. The hydrogen etching time is maintained at 20 min. A chemical reaction occurs between hydrogen and the silicon carbide substrate, etching away defects such as scratches on the surface of the silicon carbide substrate to form a regular silicon carbide step structure.

[0041] (2)Switch the gas composition introduced into the growth chamber, convert the gas in the growth chamber to an argon atmosphere, and then directly and slowly heat it up to the graphene growth temperature, hold it at 1800 °C for 2 h, and keep the pressure at 800 mbar to grow graphene.

[0042] After the graphene growth is completed, reduce the heating current, lower the temperature of the growth chamber, and at the same time keep the gas flowing in to carry away the decomposed Si component gas on the surface layer to prevent it from flowing back to the graphene surface. Then slowly lower the temperature of the growth chamber to room temperature, end the graphene growth, and take out the sample.

[0043] From Figure 5 and Figure 6 it can be seen that only through a single heating process for direct annealing, irregular coalescence of steps occurs on the graphene surface, the step morphology becomes uneven, and carbon atom reconstruction and nucleation often occur preferentially at the large steps formed by step coalescence to form graphene. The 2D peak is detected at the large steps in the nine-point Raman spectrum, and the full width at half maximum is 30 - 37 cm -1 , which is monolayer graphene; while only a buffer layer is often formed on the uncoalesced small steps at the edges of the large steps, and the 2D peak of graphene is not detected by Raman testing. The height of the large steps formed by step coalescence exceeds 25 nm, and the adjacent large steps inhibit the growth of graphene on the small steps, resulting in uneven graphene growth. In addition, compared with the two-step growth of graphene, directly annealing to grow graphene through a single heating process requires a higher nucleation temperature and a longer growth time.

[0044] Comparative Example 2 In this Comparative Example 2, graphene is epitaxially grown on a 4H-SiC substrate according to the same growth method as in Example 1. The difference is that in the first step of annealing in this Comparative Example 2, a lower annealing temperature is used, specifically holding at 1500 °C for 1 h; the annealing temperature in the second step is 1700 °C, and graphene growth is carried out by holding for 0.5 h. Atomic force microscopy and Raman spectroscopy are used to characterize the prepared graphene material, and the AFM morphology is as Figure 7 shown. Due to the too low annealing temperature in the first step, the buffer layer is not fully covered after annealing, resulting in uneven graphene morphology and more curved and disordered step morphology after the second step of annealing. The full width at half maximum of the Raman 2D peak of graphene is generally in the range of 30 - 40 cm -1 , indicating that most of the graphene is monolayer.

[0045] Comparative Example 3 Comparative Example 3 epitaxially grew graphene on a 4H-SiC substrate according to the same growth method as in Example 1, except that in the first annealing step of Comparative Example 3, a higher annealing temperature was used, specifically, it was held at 1650 °C for 1 h; the annealing temperature in the second step was 1700 °C, and it was held for 0.5 h for graphene growth. The prepared graphene material was characterized by atomic force microscopy and Raman spectroscopy. The AFM morphology is as Figure 8 shown. Due to the too high annealing temperature in the first step, large step coalescence reappeared during the growth process, and the surface morphology was uneven. Single-layer graphene was detected at the position of the coalesced large steps. After the second annealing, the full width at half maximum of the Raman 2D peak of graphene was generally in the range of 30 - 55 cm -1 . At the position of the large steps, due to the preferential nucleation of graphene, the number of graphene layers grown compared to the adjacent small steps was uneven.

[0046] Comparing the graphene prepared in Examples 1 - 2 and Comparative Examples 2 - 3, it can be seen that the key point for obtaining graphene with uniform layer numbers by the two-step annealing method lies in the control of the growth temperature when forming the buffer layer in the first annealing step. When the temperature is significantly lower than 1530 °C, the buffer layer grows incompletely, resulting in a disordered surface morphology after growth; when the temperature is significantly higher than 1630 °C, step coalescence will occur. Graphene will preferentially grow at the coalesced large steps, and the large step height fluctuations will inhibit the decomposition of silicon carbide at the adjacent small steps and the growth of graphene. It can be seen that the buffer layer, as an intermediate layer, plays a role in structural adaptation and energy transition during the stable stage of the first annealing, and plays an important role in stabilizing the surface step morphology. The method of the present invention precisely controls the temperature and time conditions in the two-step annealing process. By the growth in the first step, the uniform and regular surface step morphology is stabilized. Subsequently, when the growth temperature is increased in the second step, the surface uniformity morphology after the first step growth can still be maintained, and 100% coverage of graphene and controllable and uniform layer numbers can be ensured.

[0047] Although the present invention has been described in detail by referring to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, those of ordinary skill in the art can make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions should all be within the scope covered by the present invention. / Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, and all should be covered within the protection scope of the present invention.

Claims

1. A method for epitaxially growing uniform graphene on a silicon carbide substrate, characterized in that: The steps include: (1) The silicon carbide substrate is first etched with hydrogen to remove processing defects on the substrate surface and form a regular silicon carbide step structure; (2) The silicon carbide substrate is annealed in two steps in argon. The first step is to keep the temperature at 1530~1630℃ for 0.5~1 h and keep the pressure at 800~900 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first step annealing. The second step is to keep the temperature at 1650~1750℃ for 0.5~1 h and keep the pressure at 800~900 mbar. Through the second step annealing, the substrate surface will undergo near-equilibrium decomposition and graphitization to obtain graphene.

2. The method according to claim 1, characterized in that The silicon carbide substrate used in step (1) has a thickness of 350-500 μm, a surface roughness of <0.2 nm, and a flatness of <10 μm.

3. The method according to claim 2, characterized in that The silicon carbide substrate with a surface roughness of <0.2 nm and a flatness of <10 μm is obtained by the following pretreatment method: first, the surface of the silicon carbide substrate is chemically mechanically polished, and then the silicon carbide substrate is cleaned by a standard RCA process.

4. The method according to claim 1, wherein: In step (1), the silicon carbide substrate is placed in the growth chamber, and the growth chamber is evacuated to a vacuum degree of 10 -5 Pa, and then high-purity hydrogen is introduced.

5. The method according to claim 1, characterized in that In step (1), the growth Si surface of the silicon carbide substrate is placed facing downward.

6. The method according to claim 1, wherein: In step (1), the temperature of hydrogen etching is 1400~1500°C, the time is 10~30 min, and the pressure is maintained at 800~900 mbar.

7. The method according to claim 1, characterized in that Before performing the two-step annealing in step (2), the gas composition introduced into the growth chamber is switched to convert the gas in the growth chamber into an argon atmosphere.

8. The method according to claim 1, characterized in that After the graphene growth in step (2) is completed, the temperature of the growth chamber is lowered while the gas is kept flowing, and the temperature in the growth chamber is lowered to room temperature, thereby ending the growth of graphene.

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