A method of epitaxially growing uniform graphene on a silicon carbide substrate

Abstract

The present application relates to the technical field of semiconductor material, in particular to a method for epitaxial uniform graphene on silicon carbide substrate, comprising the following steps: (1) the silicon carbide substrate is first subjected to hydrogen etching to remove processing defects on the surface of the substrate, and a regular silicon carbide step structure is formed; (2) the silicon carbide substrate is subjected to two-step annealing in argon, wherein the first step annealing is heat preservation at 1530-1630 DEG C for 0.5-1 h, and the pressure is kept at 800-900 mbar; the second step annealing is heat preservation at 1650-1750 DEG C for 0.5-1 h, and the pressure is kept at 800-900 mbar. Compared with the conventional direct heating to the graphene growth temperature, the temperature stability in the graphene growth process is controlled by the present application in stages, the surface step morphology is obviously improved, the irregular step coalescence in the growth process is successfully inhibited, and the stable and uniform morphology is more conducive to the layer uniformity of the graphene on the whole substrate.

Description

Technical Field

[0001] This invention relates to the field of semiconductor materials technology, and more specifically to a method for epitaxially growing uniform graphene on a silicon carbide substrate. Background Technology

[0002] Graphene possesses a unique two-dimensional structure. Due to its distinctive Dirac cone band structure, graphene exhibits excellent electrical and optical properties, showing broad application prospects in future integrated circuits and optoelectronic devices. The SiC pyrolysis method for preparing epitaxial graphene has natural advantages, enabling the controlled growth of large-area, uniform graphene without the need for transfer, and is compatible with current Si-based semiconductor processes. Furthermore, SiC, as a third-generation wide-bandgap semiconductor material, possesses excellent electrical and thermal properties, making this method considered one of the most promising approaches for graphene preparation. The principle of SiC pyrolysis for epitaxial graphene involves placing a SiC substrate in a graphite crucible and annealing it at high temperature in a growth furnace. Because the saturated vapor pressure of Si is higher than that of C, Si atoms preferentially sublimate and detach from the substrate in gaseous form, leaving behind enriched free C atoms that nucleate, diffuse, and recombine on the SiC surface to form graphene. The epitaxial growth of graphene on SiC substrates typically involves two stages: a hydrogen etching stage and an annealing stage in argon to form graphene.

[0003] CN 102936009 A discloses a method for fabricating low-layer-count graphene films on silicon carbide substrates. Argon gas is introduced into the reaction chamber, and the pressure inside the chamber is increased to suppress the sublimation of Si atoms in the SiC substrate during the heating process. The low-layer-count graphene film is prepared by controlling the annealing time and pressure during the growth process. However, this method has difficulty guaranteeing the uniformity of graphene across the entire substrate. CN 107344868 A discloses a method for preparing a single-layer graphene without a buffer layer on a SiC substrate. This method involves annealing the buffer layer in hydrogen gas at 800-1200°C, inserting hydrogen atoms into the Si atoms of the substrate, thus breaking the covalent bonds between the substrate and the buffer layer, and transforming the buffer layer into a single-layer graphene. However, the surface step morphology reported in this patent is still relatively disordered. Furthermore, it has been reported that buffer layers on SiC substrates can open a certain degree of band gap, which has great application potential in semiconductor devices. In some applications, it is not desirable to destroy this buffer layer structure.

[0004] It is evident that existing technologies for preparing SiC-based epitaxial graphene still suffer from problems such as incompletely regular surface morphology, uneven graphene thickness, and numerous grain boundary defects, which significantly impact the electrical and optical properties of graphene. Directly heating to the graphene growth temperature during growth, with an excessively rapid heating rate and high growth temperature, can lead to step aggregation on the graphene surface after growth, generating irregular large steps and resulting in an irregular surface morphology. Furthermore, these large, irregular steps hinder the decomposition of the substrate adjacent to smaller steps and the formation of graphene, thus affecting the uniformity of the graphene. Summary of the Invention

[0005] To address the technical problem of uneven graphene surface morphology in existing methods for epitaxial growth of graphene on SiC substrates, this invention provides a method for epitaxial growth of uniform graphene on silicon carbide substrates.

[0006] The technical solution of this invention is as follows:

[0007] A method for epitaxially growing uniform graphene on a silicon carbide substrate includes the following steps:

[0008] (1) The silicon carbide substrate is first etched with hydrogen to remove the processing defects on the substrate surface, such as scratches, to form a regular silicon carbide step structure.

[0009] (2) The silicon carbide substrate is annealed in two steps in argon atmosphere. The first step of annealing is to hold at 1530~1630℃ for 0.5~1 h and maintain the pressure at 800~900 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first step of annealing. The second step of annealing is to hold at 1650~1750℃ for 0.5~1 h and maintain the pressure at 800~900 mbar. Through the second step of annealing, the substrate surface will undergo near-equilibrium decomposition and graphitization, thereby obtaining high-quality graphene.

[0010] Furthermore, 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 by the following pretreatment method: First, the surface of the silicon carbide substrate is polished, preferably by chemical mechanical polishing, and then the silicon carbide substrate is cleaned by standard RCA process.

[0011] Furthermore, in step (1), the silicon carbide substrate is placed into the growth chamber, and the growth chamber is evacuated to a vacuum level of 10 using a mechanical pump and / or a vacuum pump. -5 The vacuum level is kept below 10 Pa to remove air components introduced into the growth chamber during silicon carbide substrate assembly; to further remove water vapor and impurity gases adsorbed in the insulation material and crucible walls, the vacuum level is increased to 10 Pa. -5When the temperature is below Pa, the growth chamber is first heated to bake it at a temperature of 800~1000℃.

[0012] Furthermore, in step (1), the growth Si side of the silicon carbide substrate is placed downwards.

[0013] Furthermore, in step (1), the hydrogen etching temperature is 1400~1500℃, the time is 10~30 min, and the pressure is maintained at 800~900 mbar. The hydrogen reacts chemically with the silicon carbide substrate, etching a regular step morphology onto the surface. Through hydrogen etching pretreatment, scratches on the surface of the silicon carbide substrate are removed, and the substrate steps are widened, resulting in a surface suitable for high-quality graphene growth.

[0014] Furthermore, 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.

[0015] Furthermore, after the graphene growth in step (2) is completed, the temperature of the growth chamber is reduced while gas is introduced to remove the Si component gas after surface decomposition and prevent it from flowing back to the graphene surface. Then the temperature in the growth chamber is slowly reduced to room temperature to end the graphene growth.

[0016] The beneficial effects of this invention are as follows:

[0017] This invention optimizes the surface step morphology of graphene through a two-step annealing method, thereby achieving a more uniform graphene layer count. First, a buffer layer is preferentially formed at 1530–1630 °C and stabilized at this temperature for 0.5–1 h, stabilizing the surface step morphology. Furthermore, the surface morphology remains largely unchanged after the second annealing step and subsequent graphene growth. Compared to conventional methods that directly heat to the graphene growth temperature, this invention significantly improves the surface step morphology by segmentally controlling the temperature stability during graphene growth. This successfully suppresses irregular step aggregation during growth, and the stable, uniform morphology further contributes to the uniformity of the graphene layer count across the entire substrate. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The images are AFM surface morphology diagrams of the graphene surface morphology obtained by epitaxial growth in Example 1, where (a) is the morphology diagram of the buffer layer prepared after the first annealing step, and (b) is the morphology diagram of the graphene prepared after the second annealing step.

[0020] Figure 2 This is an optical morphology image of the graphene obtained by two-step annealing epitaxial growth after the second step of annealing in Example 1.

[0021] Figure 3 This is a nine-point Raman spectrum of the buffer layer on the Si surface of the SiC substrate prepared by the first step of annealing in Example 1.

[0022] Figure 4 This is a nine-point Raman spectrum of monolayer graphene on the Si surface of the SiC substrate prepared by annealing in the second step of Example 1.

[0023] Figure 5 This is an optical morphology image of the graphene prepared by direct heating and annealing in Comparative Example 1.

[0024] Figure 6 The image shows the nine-point Raman spectrum of the Si surface sample of the SiC substrate prepared by direct heating and annealing in Comparative Example 1.

[0025] Figure 7 The graphene AFM morphology image is shown in Comparative Example 2, which was grown at a lower temperature during the first step of annealing.

[0026] Figure 8 The graphene AFM morphology image is shown in Comparative Example 3, which was grown at a higher temperature during the first step of annealing. Detailed Implementation

[0027] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0028] Example 1

[0029] Uniform graphene was epitaxially grown on a 4H-SiC substrate with a thickness of 500 μm and dimensions of 10 × 10 mm. The specific method included the following steps:

[0030] (1) First, the surface of the silicon carbide substrate is chemically and mechanically polished, and then the silicon carbide substrate is cleaned using the standard RCA process to obtain a silicon carbide substrate with a surface roughness of <0.2 nm and a flatness of <10 μm.

[0031] A silicon carbide substrate is placed inside the growth chamber with the Si side of the substrate facing down. To remove air from the growth chamber, a combination of a mechanical pump and a vacuum pump is used to evacuate the chamber to a vacuum level of 10.-5 Below Pa; in order to further remove water vapor and impurity gases adsorbed in the insulation material and crucible wall, the temperature is raised to 1000℃ in the high vacuum stage to bake the growth chamber.

[0032] To remove surface contamination, the sub-damaged surface layer, and improve the surface morphology of the substrate, hydrogen gas was introduced into the growth chamber while the temperature was continuously increased to 1450°C. The hydrogen etching time was 20 minutes, during which the pressure was maintained at 800 mbar. The hydrogen gas reacted chemically with the silicon carbide substrate, etching away scratches and other defects on the surface of the silicon carbide substrate to form a regular silicon carbide step structure.

[0033] (2) Switch the gas composition 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 step of annealing is to hold at 1550℃ for 1 h and maintain the pressure at 800 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first step of annealing. Then, the growth temperature is slowly increased to the second step annealing temperature of 1650℃ and held for 30 min. During this period, the pressure is maintained at 800 mbar. Through the second step of annealing, the substrate surface will undergo near-equilibrium decomposition and graphitization, thereby obtaining high-quality graphene.

[0034] After the graphene growth is complete, the heating current is reduced to lower the temperature of the growth chamber while gas is introduced to remove the Si component gas after surface decomposition and prevent it from flowing back to the graphene surface. Then, the temperature of the growth chamber is slowly reduced to room temperature to end the graphene growth. The sample is then removed to obtain epitaxial graphene with uniform surface step morphology and controllable number of layers.

[0035] The morphology of graphene after the first and second annealing in step (2) was observed using atomic force microscopy and optical microscopy, and the results are as follows: Figure 1 and Figure 2 As 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, while the continued second annealing still maintains the original morphology without significant change. This indicates that the stable stage of the buffer layer formed by the first annealing during the growth process plays an important role in the stable control of the surface step morphology. By controlling the temperature stability during the graphene growth process in two steps, the surface step morphology is significantly improved, and the irregular step aggregation 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 images still show an overall uniform, regular, and flat step-like morphology.

[0036] Raman spectroscopy was performed on the buffer layer and graphene samples prepared by two-step annealing. The test results are as follows: Figure 3 and Figure 4 As shown. The first annealing step only forms a buffer layer. Figure 3 No 2D Raman peaks of graphene were observed in the first step; however, after the second annealing, nine-point Raman spectra of graphene at different positions on the Si surface of the SiC substrate showed that the 2D peaks of graphene could all be fitted by a single Lorentz peak, with a half-width of 30-40 cm⁻¹. -1 This proves that the prepared graphene is a single-layer graphene, and 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 2

[0038] Uniform graphene was epitaxially grown on a 6H-SiC substrate with a thickness of 350 μm and dimensions of 10 × 10 mm. The specific method included the following steps:

[0039] (1) First, the surface of the silicon carbide substrate is chemically and mechanically polished, and then the silicon carbide substrate is cleaned using the standard RCA process to obtain a silicon carbide substrate with a surface roughness of <0.2 nm and a flatness of <10 μm.

[0040] A silicon carbide substrate is placed inside the growth chamber with the Si side of the substrate facing down. To remove air from the growth chamber, a combination of a mechanical pump and a vacuum pump is used to evacuate the chamber to a vacuum level of 10. -5 Below Pa; in order to further remove water vapor and impurity gases adsorbed in the insulation material and crucible wall, the temperature is raised to 800°C in the high vacuum stage to bake the growth chamber.

[0041] To remove surface contamination, the sub-damaged surface layer, and improve the surface morphology of the substrate, hydrogen gas was introduced into the growth chamber while the temperature was continuously increased to 1500°C. The hydrogen etching time was maintained at 10 min, and the pressure was maintained at 900 mbar. The hydrogen gas reacted chemically with the silicon carbide substrate, etching away scratches and other defects on the surface of the silicon carbide substrate to form a regular silicon carbide step structure.

[0042] (2) Switch the gas composition 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 step of annealing is to hold at 1600 °C for 0.5 h and maintain the pressure at 900 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first step of annealing. Then, the growth temperature is slowly increased to the second step annealing temperature of 1750 °C and held for 1 h. During this period, the pressure is maintained at 900 mbar. Through the second step of annealing, the substrate surface will undergo near-equilibrium decomposition and graphitization, thereby obtaining high-quality graphene.

[0043] After the graphene growth is complete, the heating current is reduced to lower the temperature of the growth chamber while gas is introduced to remove the Si component gas after surface decomposition and prevent it from flowing back to the graphene surface. Then, the temperature of the growth chamber is slowly reduced to room temperature to end the graphene growth. The sample is then removed to obtain epitaxial graphene with uniform surface step morphology and controllable number of layers.

[0044] The graphene material prepared by two-step annealing epitaxial growth was characterized by Raman spectroscopy and atomic force microscopy. The graphene had a uniform morphology, and the full width at half maximum (FWHM) of the Raman 2D peak was generally located in the range of 50–65 cm⁻¹. -1 The results indicate that the graphene is a uniform bilayer graphene, and the intensity of the graphene D peak is basically 0, indicating that the prepared graphene has few defects and high quality.

[0045] Example 3

[0046] The method for epitaxially growing uniform graphene on a 4-inch 4H-SiC substrate follows the same growth method as in Example 1, except that the prepared 4-inch 4H-SiC substrate is placed with the Si side facing down in a growth crucible. The first annealing temperature is 1600℃, held for 1 h; the second annealing temperature is 1700℃, held for 0.5 h for graphene growth. The prepared graphene material was characterized by Raman spectroscopy and atomic force microscopy. The graphene morphology was uniform, and the full width at half maximum (FWHM) of the Raman 2D peak was generally located between 30 and 55 cm⁻¹. -1 The results indicate that the graphene has a uniform number of layers, with an overall density of 1 to 2 layers. The Raman D peak intensity of the graphene is basically 0, indicating that the prepared graphene has few defects and is of high quality.

[0047] Comparative Example 1

[0048] Graphene was epitaxially grown on a 4H-SiC substrate using a single direct heating annealing method. The 4H-SiC substrate had a thickness of 500 μm and a size of 10 × 10 mm. The specific method included the following steps:

[0049] (1) First, the surface of the silicon carbide substrate is chemically and mechanically polished, and then the silicon carbide substrate is cleaned using the standard RCA process to obtain a silicon carbide substrate with a surface roughness of <0.2 nm and a flatness of <10 μm.

[0050] A silicon carbide substrate is placed inside the growth chamber with the Si side of the substrate facing down. To remove air from the growth chamber, a combination of a mechanical pump and a vacuum pump is used to evacuate the chamber to a vacuum level of 10. -5 Below Pa; in order to further remove water vapor and impurity gases adsorbed in the insulation material and crucible wall, the temperature is raised to 1000℃ in the high vacuum stage to bake the growth chamber.

[0051] To remove surface contamination, the sub-damaged surface layer, and improve the surface morphology of the substrate, hydrogen gas is introduced into the growth chamber while the temperature is continuously increased to 1450°C. The hydrogen etching time is maintained at 20 min. The hydrogen gas reacts chemically with the silicon carbide substrate, etching away scratches and other defects on the surface of the silicon carbide substrate to form a regular silicon carbide step structure.

[0052] (2) Switch the gas composition introduced into the growth chamber, convert the gas in the growth chamber into an argon atmosphere, and then directly and slowly heat it to the graphene growth temperature. Keep it at 1800℃ for 2 h and maintain the pressure at 800 mbar to grow graphene.

[0053] After the graphene growth is complete, the heating current is reduced to lower the temperature of the growth chamber. At the same time, gas is kept flowing in to remove the Si component gas after the surface decomposition and prevent it from flowing back to the graphene surface. Then, the temperature of the growth chamber is slowly reduced to room temperature to end the graphene growth and the sample is taken out.

[0054] Depend on Figure 5 and Figure 6 It can be seen that direct annealing through a single heating process results in irregular step-like aggregation on the graphene surface, leading to an uneven step morphology. Larger steps in this aggregation often preferentially undergo carbon atom recombination to form graphene. Nine-point Raman spectroscopy detected 2D peaks at these large steps, with a full width at half maximum (FWHM) of 30–37 cm. -1 The first step is a single-layer graphene; however, the un-agglomerated small steps at the edge of the large step often only have a buffer layer formed, and Raman spectroscopy did not detect the 2D peak of graphene. The height of the large step with aggregated steps 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 two-step graphene growth, direct annealing growth of graphene through a single heating process requires a higher nucleation temperature and a longer growth time.

[0055] Comparative Example 2

[0056] Comparative Example 2 uses the same growth method as Example 1 to epitaxially grow graphene on a 4H-SiC substrate. The difference is that the first annealing step in Comparative Example 2 uses a lower annealing temperature, specifically 1500℃ for 1 h; the second annealing step is performed at 1700℃ for 0.5 h to grow the graphene. The prepared graphene material was characterized using atomic force microscopy (AFM) and Raman spectroscopy, with the AFM morphology as shown below. Figure 7 As shown, due to the excessively low temperature in the first annealing step, the buffer layer was not fully covered after annealing, resulting in uneven graphene morphology and a more curved and disordered step morphology after the second annealing step. The full width at half maximum (FWHM) of the Raman 2D peak of graphene was generally located between 30 and 40 cm. -1 This indicates that graphene is mostly a single layer.

[0057] Comparative Example 3

[0058] Comparative Example 3 uses the same growth method as Example 1 to epitaxially grow graphene on a 4H-SiC substrate. The difference is that the first annealing step in Comparative Example 3 uses a higher annealing temperature, specifically 1650℃ for 1 h; the second annealing step is performed at 1700℃ for 0.5 h. The prepared graphene material is characterized by atomic force microscopy (AFM) and Raman spectroscopy, with the AFM morphology as shown below. Figure 8 As shown, due to the excessively high temperature in the first annealing step, large step-like aggregations reappeared during the growth process, resulting in uneven surface morphology. Monolayer graphene was also detected at these large step-like aggregation sites. After the second annealing step, the full width at half maximum (FWHM) of the Raman 2D peak of the graphene was generally between 30 and 55 cm⁻¹. -1 At the large step, graphene preferentially nucleates, resulting in an uneven number of graphene layers compared to those growing on adjacent small steps.

[0059] Comparing the graphene prepared in Examples 1-2 and Comparative Examples 2-3, it can be seen that the key to obtaining graphene with a uniform number of layers through the two-step annealing method lies in the control of the growth temperature during the first annealing step to form the buffer layer. When the temperature is significantly lower than 1530℃, the buffer layer grows incompletely, resulting in a disordered surface morphology after growth. When the temperature is significantly higher than 1630℃, it leads to step aggregation. Graphene preferentially grows at the large aggregated steps, and the large step height fluctuations inhibit the decomposition of silicon carbide at 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 stabilization stage of the first annealing, and plays an important role in stabilizing the surface step morphology. The method of this invention precisely controls the temperature and time conditions during the two-step annealing process. The first step of growth stabilizes the uniformity and regularity of the surface step morphology, and the subsequent second step, which increases the growth temperature, still maintains the uniform surface morphology after the first step and ensures 100% graphene coverage and uniform and controllable number of layers.

[0060] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also 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, Includes the following steps: (1) The silicon carbide substrate is first etched with hydrogen to remove the processing defects on the substrate surface, forming a regular silicon carbide step structure; The silicon carbide substrate has a thickness of 350~500 μm, a surface roughness of <0.2 nm, and a flatness of <10 μm; The silicon carbide substrate is placed with the Si side facing down; (2) The silicon carbide substrate is annealed in two steps in argon gas. The first step of annealing is to hold the substrate at 1530~1630℃ for 0.5~1 h and maintain the pressure at 800~900 mbar. A uniform buffer layer is formed on the surface of the silicon carbide substrate through the first step of annealing. The second step of annealing is to hold the substrate at 1650~1750℃ for 0.5~1 h and maintain the pressure at 800~900 mbar. Through the second step of annealing, the substrate surface will undergo near-equilibrium decomposition and graphitization, thereby obtaining graphene.

2. The method as described in claim 1, characterized in that, Silicon carbide substrates with a surface roughness of <0.2 nm and a flatness of <10 μm are obtained by the following pretreatment method: first, the surface of the silicon carbide substrate is chemically and mechanically polished, and then the silicon carbide substrate is cleaned using the standard RCA process.

3. The method as described in claim 1, characterized in that, In step (1), the silicon carbide substrate is placed into the growth chamber, and the growth chamber is evacuated to a vacuum level of 10. -5 The pressure is below Pa, and then high-purity hydrogen gas is introduced.

4. The method as described in claim 1, characterized in that, In step (1), the temperature of hydrogen etching is 1400~1500℃, the time is 10~30 min, and the pressure is maintained at 800~900 mbar.

5. The method as described in 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.

6. The method as described in claim 1, characterized in that, After the graphene growth in step (2) is completed, the temperature of the growth chamber is lowered while gas is kept flowing in. The temperature in the growth chamber is lowered to room temperature, and the graphene growth is terminated.

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