A method for preparing a gallium nitride film based on a patterned graphene mask

Abstract

This invention relates to a method for preparing gallium nitride (GaN) thin films based on patterned graphene masks. The method involves growing multilayer graphene layers on the surface of a substrate using plasma-enhanced chemical vapor deposition (PECVD); fabricating a patterned hexagonal graphene mask structure by photolithography and etching; inorganically cleaning the mask structure to remove residual oxides in the window areas; and then growing a GaN epitaxial layer in two steps using metal-organic chemical vapor deposition (MOCVD). In the first step, GaN nucleates and grows within the window of the hexagonal mask, and in the second step, it grows laterally towards the center of the mask to obtain a complete GaN thin film. Using the technical solution provided by this invention, the growth mode of GaN is altered by controlling growth parameters such as temperature, pressure, and V / III ratio. The grown GaN eventually merges laterally, resulting in a high-quality GaN thin film. Results show that the patterned hexagonal graphene mask effectively reduces the high dislocation density caused by GaN mismatch, relaxes GaN stress, and improves the crystal quality of GaN.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor technology, specifically relating to a method for growing gallium nitride using a patterned hexagonal graphene mask. Background Technology

[0002] Gallium nitride (GaN), a representative of third-generation semiconductor materials, possesses excellent properties such as wide bandgap, high breakdown electric field, and high electron mobility, making it widely used in optoelectronic devices, radio frequency devices, and power devices. However, GaN does not exist in nature and must be artificially synthesized. Due to cost considerations and the lack of homogeneous substrates for GaN, it is usually obtained through heteroepitaxial growth. However, this leads to a problem: the difference in lattice constant and coefficient of thermal expansion between the heterosubstrate and GaN significantly affects the crystal quality of GaN, resulting in high dislocation density and stress levels, which hinders the performance and development of GaN-based devices. Inserting a mask layer can effectively reduce the dislocation density of GaN. Mask layers are often fabricated on a substrate as single-layer masks made of silicon dioxide or silicon nitride and other dielectric materials, or as multilayer dielectric mask structures combining multiple materials. While these masks effectively reduce dislocations, the thickness and amorphous morphology of the dielectric mask introduce small-angle grain boundaries and additional stress into the GaN. Furthermore, the good stability of the dielectric mask means it remains within the sample after growth, which affects the performance of gallium nitride (GaN), such as its electrical and thermal conductivity. On the other hand, there is also lateral epitaxy technology using patterned heterosubstrates, where the substrate is etched into a periodic patterned structure with varying depths. In these structures, the substrate itself acts as a mask, and the obstruction of the substrate and the lateral merging of dislocations are the main reasons for the reduction in GaN dislocation density. The degree of dislocation density reduction is influenced by the groove depth. Therefore, we propose a method for growing GaN using a patterned hexagonal graphene mask. Due to the unique dangling bond-free surface of graphene, GaN is difficult to nucleate on it and can be used as a mask layer. The weak van der Waals forces between graphene and the GaN layer can overcome the mismatch effect. After growth, the dislocation density and stress of GaN are significantly reduced, improving the crystal quality of GaN and showing broad application prospects. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a method for preparing gallium nitride thin films with good uniformity based on patterned graphene masks. This method can effectively improve the crystal quality of gallium nitride thin films on heterogeneous substrates and reduce the high dislocation density and high stress of heteroepitaxial gallium nitride.

[0004] The technical solution to achieve the purpose of this invention is to provide a method for preparing gallium nitride thin films based on patterned graphene masks. Multilayer graphene layers are grown on the surface of a substrate using plasma-enhanced chemical vapor deposition (PECVD). Patterned hexagonal graphene mask structures are prepared by photolithography and etching of the graphene layers. The mask structure is inorganically cleaned to remove residual oxides from the etching window area. A gallium nitride epitaxial layer is then grown in two steps using metal-organic chemical vapor deposition (MOCVD). In the first step, gallium nitride nucleates and grows at the window of the hexagonal mask. In the second step, gallium nitride grows laterally towards the center of the mask, resulting in a complete gallium nitride thin film.

[0005] The substrate layer described in this invention is sapphire-based gallium nitride.

[0006] This invention provides a method for preparing gallium nitride thin films based on patterned graphene masks. The process conditions for growing multilayer graphene layers by plasma-enhanced chemical vapor deposition are: ion source power 80W, growth temperature 800℃, growth time 90min, and growth gases methane, hydrogen, and argon.

[0007] This invention provides a method for fabricating gallium nitride thin films based on patterned graphene masks. The method employs photolithography and oxygen plasma etching to prepare a patterned hexagonal graphene mask structure. The oxygen plasma etching process conditions are: 200 sccm oxygen flow rate, 400 W power, and 80 s etching time. The hexagonal graphene mask structure has a window width of 3–5 micrometers and a mask width of 15–25 micrometers.

[0008] This invention provides a method for preparing gallium nitride (GaN) thin films based on patterned graphene masks. The first step involves the nucleation and growth of GaN within the window of a hexagonal mask under the following conditions: growth temperature 950–1000℃, pressure 450–550 Torr, V / III 2000–2500, ammonia 15–20 slm, and trimethylgallium 25–35 sccm. The second step involves the lateral growth and polymerization of GaN towards the center of the mask under the following conditions: growth temperature 1000–1100℃, pressure 250–350 Torr, V / III 3500–4000, ammonia 50–60 slm, and trimethylgallium 55–65 sccm.

[0009] The present invention provides a method for preparing gallium nitride thin films based on patterned graphene masks, wherein the sample is placed in hydrochloric acid at a temperature of 60°C and a concentration of 38% and immersed for 10 minutes to perform inorganic cleaning on the mask structure.

[0010] In this invention, complete multilayer graphene is first obtained through plasma-enhanced chemical vapor deposition. Then, patterned hexagonal graphene masks are fabricated from the multilayer graphene layers using photolithography and etching processes. Finally, gallium nitride is grown using a two-step growth parameter. The advantages of this invention are mainly divided into the following three parts:

[0011] 1. This invention uses plasma-enhanced chemical vapor deposition to directly grow graphene on the target substrate, effectively avoiding graphene damage caused by pollution and human factors.

[0012] 2. Due to the inherent advantages of graphene, the lack of dangling bonds on its surface makes it an excellent mask material, effectively blocking gallium nitride dislocations in the substrate. The van der Waals forces between the graphene mask layer and the epitaxial gallium nitride layer can overcome the effects of lattice mismatch and improve the stress of gallium nitride. Furthermore, due to the excellent physicochemical properties of graphene, it has good electrical and thermal conductivity, which can effectively improve the stability and heat dissipation performance of gallium nitride-based devices. At the same time, the ultrathin thickness of graphene does not cause small-angle grain boundary problems in gallium nitride.

[0013] 3. This invention employs a patterned hexagonal mask structure. Compared to a one-dimensional grating strip mask, the patterned hexagonal mask has six window regions, allowing for faster gallium nitride film formation. Due to the anisotropy of gallium nitride, the surface of gallium nitride films grown on one-dimensional grating strip masks may exhibit unevenness. The patterned hexagonal graphene mask provided by this invention allows for multi-directional controlled growth, resulting in gallium nitride films with excellent uniformity. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of a graphene mask for growing gallium nitride thin films, provided in an embodiment of the present invention.

[0015] Figure 2 A schematic diagram of the planar structure of a graphene mask layer with a hexagonal pattern provided in an embodiment of the present invention;

[0016] Figure 3 A schematic diagram illustrating the window and mask width structure of the mask layer provided for an embodiment of the invention;

[0017] Wherein: 1 is the gallium nitride epitaxial layer, 2 is the graphene mask layer, 3 is the gallium nitride / sapphire composite substrate layer, 4 is the graphene mask region, and 5 is the window region;

[0018] Figure 4 The Raman spectra of the mask region and window region of the patterned hexagonal graphene mask structure provided in the embodiments of the present invention;

[0019] Figure 5 Electron microscopy images of the patterned hexagonal graphene mask structure provided in the embodiments of the present invention after the first step of gallium nitride growth;

[0020] Figure 6The scanning electron microscope image (a) and the cathodoluminescence spectrum image (b) of the patterned hexagonal graphene mask structure provided in the embodiments of the present invention after the second step of gallium nitride growth are not merged.

[0021] Figure 7 Raman spectrum of graphene in the mask region after the second step of gallium nitride growth of the patterned hexagonal graphene mask structure provided in the embodiments of the present invention;

[0022] Figure 8 Raman spectra of the patterned hexagonal graphene mask structure provided in this embodiment of the invention after growth of the substrate gallium nitride and the epitaxial gallium nitride;

[0023] Figure 9 Raman spectra of wet-transfer graphene provided in an embodiment of the present invention;

[0024] Figure 10 An optical mirror image of a wet-transfer graphene mask structure provided in an embodiment of the present invention;

[0025] Figure 11 The scanning electron microscope image (a) and the cathodoluminescence spectrum image (b) of gallium nitride after wet transfer graphene mask growth provided in the embodiments of the present invention are shown in Figure (a). Detailed Implementation

[0026] This invention grows gallium nitride (GaN) on a patterned hexagonal graphene mask using metal-organic chemical vapor deposition (MOCVD). First, multilayered, complete graphene is grown on a GaN / sapphire composite substrate using plasma-enhanced chemical vapor deposition (PECVD). Next, the multilayered graphene is fabricated into a patterned hexagonal mask structure using photolithography and etching processes. Finally, GaN is grown on the patterned hexagonal graphene mask using MOCVD. Trimethylgallium (TMGa) and ammonia (NH3) are used as Ga and N sources, respectively, to synthesize GaN. The GaN growth parameters are divided into two steps: the first step selects a low growth temperature, a high growth pressure, and a low V / III ratio to promote the three-dimensional growth of GaN. The second step, based on the first step, increases the growth temperature and V / III ratio while decreasing the growth pressure to promote the lateral growth and merging of GaN. In this invention, the patterned graphene mask is hexagonal in shape, with a window area width of 4 μm and a mask area width of 20 μm. For different growth methods, the shape of the graphene mask and the aspect ratio of the window to the mask can be adjusted appropriately.

[0027] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0028] Example 1: This example provides a method for growing gallium nitride based on a patterned hexagonal graphene mask, the steps of which are as follows:

[0029] Commercially available sapphire-based gallium nitride was routinely cleaned and dried. Complete multilayer graphene was obtained on the gallium nitride surface by plasma-enhanced chemical vapor deposition. The growth conditions for the graphene were: ion source power of 60W, growth temperature of 800℃, growth gases of methane, hydrogen and argon at rates of 10sccm, 2sccm and 2sccm respectively, and growth time of 90min.

[0030] The sample was photolithographically and etched to prepare a patterned hexagonal graphene mask. The specific steps are as follows: (1) Spin coating and pre-baking: AZ5214 photoresist was applied to the surface of the graphene layer and the sample was vacuum-adsorbed. Spin coating was performed at a spin coating rate of 600 rad / min forward and 4000 rad / min backward for 30s. The sample was then placed on a hot plate at 95°C for pre-baking for 90s. (2) Exposure and development: The sample was exposed for 6.5s in hard mode using an MA6 UV lithography machine. After exposure, the sample was developed using a developer solution for AZ5214 photoresist for 40s. (3) Etching of the graphene in the window area: The graphene in the window area was etched using oxygen plasma from a March stripper. The oxygen flow rate and power were set to 200 sccm and 400W, and the etching time was set to 80s. A hexagonal graphene mask layer was formed on the surface of the gallium nitride substrate.

[0031] Gallium nitride epitaxial layers were grown on graphene masks using metal-organic chemical vapor deposition (MOCVD) with two-step growth parameters. The first step involved a growth temperature of 970℃, a pressure of 500 Torr, a V / III ratio of 2100, an ammonia atmosphere of 16.1 slm, a trimethylgallium atmosphere of 30 sccm, and a growth time of 25 minutes. The second step involved a growth temperature of 1050℃, a pressure of 300 Torr, a V / III ratio of 3600, an ammonia atmosphere of 55 slm, a trimethylgallium atmosphere of 60 sccm, and a growth time of 180 minutes.

[0032] See appendix Figure 1 This is a schematic diagram of the structure of gallium nitride thin film grown on a graphene mask provided in this embodiment. It includes a gallium nitride epitaxial layer 1, a graphene mask layer 2, and a gallium nitride / sapphire composite substrate layer 3. Multilayer complete graphene is grown on the gallium nitride / sapphire composite substrate 3 by plasma-enhanced chemical vapor deposition. The multilayer graphene is fabricated into a patterned hexagonal graphene mask layer 2 by photolithography, etching and other processes. The gallium nitride epitaxial layer 1 is grown on the patterned hexagonal graphene mask by metal-organic chemical vapor deposition.

[0033] See appendix Figure 2 , 3These are schematic diagrams of the planar structure of the graphene mask layer arranged in a hexagonal pattern and the window and mask width structure of the mask layer provided in this embodiment. The black covered area is the graphene mask area 4, and the white exposed area is the window area 5. Figure 3 As can be seen, the hexagonal graphene mask layer structure provided in this embodiment has a window area width of 4 micrometers and a mask area width of 20 micrometers.

[0034] See appendix Figure 4 The image shows the graphene Raman images of the mask region and the window region in this embodiment. The three characteristic peaks of graphene (D peak, G peak and 2D peak) are clearly visible in the mask region, while the signals of these three characteristic peaks are not present in the window region, indicating that the mask structure has been successfully fabricated.

[0035] See appendix Figure 5 The image shows an electron microscope image of gallium nitride after the first step of growth parameters. The gallium nitride epitaxial layer preferentially nucleates and grows in the window region and covers the entire window region.

[0036] See appendix Figure 6 (a) is an electron microscope image after epitaxial layer growth, and (b) is a cathodoluminescence spectrometer image. As shown in (a), after the second growth step, gallium nitride grows laterally and aggregates towards the center of the mask region. The distribution of dislocations penetrating the gallium nitride surface is consistent with the distribution of the window. Many dislocations are still visible in the unaggregated substrate region, indicating that the graphene mask successfully functions as a mask, effectively blocking dislocations from the substrate. Based on the number of dislocation black spots in the cathodoluminescence spectrum in (b), the dislocation density of gallium nitride is estimated to be approximately 8.5 × 10⁻⁶. 7 cm -2 .

[0037] See appendix Figure 7 The image shows the Raman spectrum of graphene in the mask region after gallium nitride growth using the second-step growth parameters. Figure 7 It can be seen that the characteristic peak signal of graphene is no longer present in the Raman spectrum, indicating that the graphene decomposed during the growth process and is not present in the sample.

[0038] See appendix Figure 8 The figures show the Raman spectra of substrate gallium nitride (GaN) and epitaxial GaN. The E2 (high) peak of GaN is highly sensitive to stress, allowing for direct detection of stress in GaN. The E2 (high) peak of stress-free GaN is located at 568 cm⁻¹. -1 After growth, the E2 (high) peak of gallium nitride increased from 570.44 cm⁻¹. -1 Reduced to 569.5cm -1 The stress in gallium nitride was relaxed, decreasing from 0.57 GPa to 0.35 GPa.

[0039] Example 2: After routine cleaning and drying of commercially available sapphire-based gallium nitride, multilayer graphene deposited on copper foil was wet-transferred to the target substrate using chemical vapor deposition. The wet transfer process was as follows: (1) Polymethyl methacrylate (PMMA) was spin-coated onto the surface of the copper foil covering the graphene; (2) The sample coated with PMMA was immersed in an ion etchant to completely etch off the copper foil; (3) The PMMA / graphene was lifted with a silicon substrate, repeatedly washed in deionized water, and then transferred to the target substrate; (4) The sample was dried until the graphene adhered tightly to the target substrate; (5) After the dried sample was immersed in an acetone solution to remove PMMA, the sample was placed in an ethanol solution to remove acetone, dried again, and the transfer was completed.

[0040] See appendix Figure 9 The image shows the Raman spectrum of wet-transfer graphene. Figure 9 As can be seen, graphene exhibits three Raman characteristic peaks: the D peak, the G peak, and the 2D peak. Compared to multilayer graphene obtained by plasma-enhanced chemical vapor deposition, wet-transferred graphene has better crystal quality, exhibiting a lower intensity defect D peak.

[0041] The sample was subjected to photolithography and etching processes according to the conditions provided in Example 1, and the wet-transfer graphene layer was prepared into a patterned hexagonal mask structure with a window width of 4 micrometers and a mask width of 20 micrometers.

[0042] See appendix Figure 10 The image shows a light microscope image of the completed hexagonal mask. It can be seen that there are differences in contrast on the mask surface, which indicates that the surface morphology of the mask is uneven and incomplete. This is caused by contamination during the transfer process or human operation.

[0043] The sample was grown using a two-step method under the conditions provided in Example 1. The growth results are shown in the appendix. Figure 11 (a) is a scanning electron microscope image, and (b) is a cathodoluminescence spectrum image. As can be seen from the figures, gallium nitride (GaN) growth is not in a lateral epitaxial mode, but rather the mask region and window region grow together. Due to the inhomogeneity of the graphene in the mask region, the surface morphology of the grown GaN is not smooth, leaving many pits. On the other hand, the cathodoluminescence spectrum image shows that the surface-penetrating dislocations of GaN have no regularity, indicating that the graphene did not act as a mask.

[0044] This invention utilizes plasma-enhanced chemical vapor deposition (PECVD) to grow multilayer, intact graphene, avoiding graphene contamination and human-caused damage. The graphene is fabricated as a patterned hexagonal mask layer, upon which a gallium nitride (GaN) epitaxial layer is grown. The GaN epitaxial layer preferentially nucleates in the window region and grows vertically in three dimensions. Subsequently, under controlled growth parameters, GaN grows laterally and aggregates towards the mask center. Results show that the graphene mask decomposes during GaN growth, but GaN still grows in a lateral epitaxial mode. The surface dislocation distribution of GaN is consistent with the window distribution, and the dislocation density is reduced to 10-1. 7 Orders of magnitude. On the other hand, the stress in gallium nitride showed significant relaxation, with the E2 (high) peak decreasing from 570.44 cm⁻¹. -1 Reduced to 569.5cm -1 The stress was reduced from 0.57 GPa to 0.35 GPa. Therefore, this invention significantly improves the high dislocation density and high stress problems of heteroepitaxial gallium nitride, effectively enhancing the performance of gallium nitride-based devices and showing promising applications in the semiconductor industry.

Claims

1. A method for fabricating a gallium nitride thin film based on a patterned graphene mask, the method comprising: The multilayer graphene layer is grown on the surface of the substrate layer by plasma enhanced chemical vapor deposition; the graphene layer is prepared into a patterned hexagonal graphene mask structure by photolithography and etching, the patterned hexagonal graphene mask structure comprises a retained graphene pattern and a window region formed by the patterned removal of graphene, wherein the graphene pattern is a hexagonal structure, the window region is located at the periphery of the graphene pattern, and the window region is a hexagonal structure surrounding the graphene pattern; the mask structure is inorganic cleaned to remove the etching residue oxide in the window region; a gallium nitride epitaxial layer is grown by two-step metal organic chemical vapor deposition, the gallium nitride is first nucleated and grown in the window of the hexagonal mask in the first step, and the gallium nitride is then laterally grown and aggregated to the center of the mask in the second step, so as to obtain a complete gallium nitride film. ​ 2. The method for preparing GaN thin film based on patterned graphene mask according to claim 1, characterized in that: The substrate layer is a sapphire-based gallium nitride. 3.The method of claim 1, wherein the method further comprises: forming a graphene mask on the substrate; and forming a mask pattern on the graphene mask. The process conditions of the plasma enhanced chemical vapor deposition for growing the multilayer graphene layer are as follows: ion source power 80 W, growth temperature 800 ℃, growth time 90 min, and growth gas methane, hydrogen and argon.

4. The method of claim 1, wherein the method comprises: The patterned hexagonal graphene mask structure is prepared by photolithography and oxygen plasma etching, and the process conditions of the oxygen plasma etching are as follows: oxygen gas flow flux 200 sccm, power 400 W, and etching time 80 s. ​ 5. The method of claim 1, wherein the method comprises: The window width of the hexagonal graphene mask structure is 3-5 microns, and the mask width is 15-25 microns. ​ 6. The method of claim 1, wherein the method comprises: The process conditions of the first step of nucleated growth of the gallium nitride in the window of the hexagonal mask are as follows: growth temperature 950-1000 ℃, pressure 450-550 Torr, V / III 2000-2500, ammonia 15-20 slm, and trimethyl gallium 25-35 sccm; and the process conditions of the second step of lateral growth and aggregation of the gallium nitride to the center of the mask are as follows: growth temperature 1000-1100 ℃, pressure 250-350 Torr, V / III 3500-4000, ammonia 50-60 slm, and trimethyl gallium 55-65 sccm.

7. The method of claim 1, wherein the method comprises: depositing a graphene film on a substrate; and patterning the graphene film to form a graphene mask. The sample is placed in hydrochloric acid with a concentration of 38% and a temperature of 60 ℃ for 10 minutes for inorganic cleaning of the mask structure.

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