A wafer-level single-crystal graphene heterojunction and a direct growth method thereof

Abstract

The present invention discloses a wafer-level single-crystal graphene heterojunction and a direct growth method thereof, and belongs to the technical field of two-dimensional material heterojunction preparation. The present invention solves the problem of difficulty in preparing existing wafer-level single-crystal graphene / hexagonal boron nitride / graphene heterojunction films. The present invention adopts a chemical vapor deposition method to first grow hexagonal boron nitride on the surface of a single-crystal metal catalyst, and then grow graphene on its upper surface and lower surface respectively to form a single-crystal graphene / hexagonal boron nitride / graphene heterojunction film, and the grown hexagonal boron nitride and graphene are both single-crystal structures, and the number of layers can be controlled from a single layer to multiple layers, and the size of the graphene heterojunction is determined by the size of the metal catalyst, which can reach the wafer level.

Description

Technical Field

[0001] The invention relates to a wafer-level single-crystal graphene heterojunction and a direct growth method thereof, and belongs to the technical field of two-dimensional material heterojunction preparation. Background Art

[0002] Emerging two-dimensional materials such as graphene, hexagonal boron nitride, and molybdenum disulfide have broad application prospects in national defense, military, aerospace, reconnaissance and monitoring due to their excellent optical, electrical, mechanical and thermal properties, and have received extensive attention and high attention from academia and industry. The excellent performance and wide application of two-dimensional materials are based on high-quality and large-scale materials, so the preparation technology of two-dimensional materials is particularly important. As representatives of conductor and insulator two-dimensional materials, graphene and hexagonal boron nitride, respectively, have been studied for more than ten years. At present, the preparation of 4-inch, single crystal, and single-layer films has been achieved. However, there has been no breakthrough in the direct growth technology of heterojunction single crystal structures of the two, and it is still impossible to directly grow wafer-level single crystal heterojunction films.

[0003] At present, there are three main methods for preparing heterojunctions composed of graphene and hexagonal boron nitride. One is to construct a graphene heterojunction by layer-by-layer transfer, first transfer a layer of graphene on the target substrate, and then transfer a layer of hexagonal boron nitride on the graphene, or transfer layer by layer in the opposite order; according to this method, the cost of preparing the heterojunction is high, and environmental factors and mechanical damage during the transfer process will seriously affect the quality of the heterojunction. The second is to directly form a film on a substrate with a thin film grown using chemical vapor deposition (CVD), and use CVD to prepare a hexagonal boron nitride film on a copper foil on which graphene has been grown, or prepare a graphene / hexagonal boron nitride heterojunction in the opposite order. Although this method can reduce the impact of environmental factors and mechanical damage, the controllability of the growth process is poor. The third is to dissolve carbon atoms in the metal in advance through the method of pre-dissolution combined with segregation, and generate a graphene / boron nitride heterojunction during the cooling process. However, the heterojunctions prepared by the pre-dissolution combined with segregation method generally produce polycrystalline graphene / hexagonal boron nitride heterojunctions, and single-crystalline graphene / hexagonal boron nitride / graphene heterojunctions have not been successfully prepared. Therefore, it is very necessary to provide a wafer-level single-crystalline graphene / hexagonal boron nitride / graphene heterojunction film preparation technology. Summary of the invention

[0004] Aiming at the problem that it is difficult to prepare the existing wafer-level single-crystal graphene / hexagonal boron nitride / graphene heterojunction film, the present invention provides a wafer-level single-crystal graphene heterojunction and a direct growth method thereof.

[0005] The technical solution of the present invention:

[0006] One of the purposes of the present invention is to provide a method for directly growing a wafer-level single-crystal graphene heterojunction, the method comprising the following steps:

[0007] S1, pre-treating a polycrystalline metal foil to obtain a single crystal metal foil;

[0008] S2, growing hexagonal boron nitride single crystal domains with consistent orientation on the surface of the single crystal metal foil by crystal orientation induction, further extending the growth time to make the single crystal domains seamlessly connected, and forming a single crystal hexagonal boron nitride film with the same size as the single crystal metal foil substrate;

[0009] S3, under high temperature conditions, using metal vapor evaporated from the surface of the single crystal metal foil to catalytically crack the carbon precursor, and epitaxially grow a graphene film on the upper surface of the hexagonal boron nitride to form a hexagonal boron nitride / graphene vertical heterojunction film;

[0010] S4, a catalytic cracking method is used to dissolve the carbon atoms cracked from the carbon source inside the single crystal metal foil, precipitate from the interface between the single crystal metal foil and hexagonal boron nitride during the cooling process, and grow epitaxially under the crystal orientation induction of the hexagonal boron nitride, forming a single crystal graphene film between the single crystal metal foil and the hexagonal boron nitride film, thereby achieving the growth of a single crystal graphene / hexagonal boron nitride / graphene vertical heterojunction film on a single crystal metal foil substrate, referred to as graphene heterojunction.

[0011] It is further defined that the pretreatment of the polycrystalline metal foil in S1 is high temperature annealing under tensile stress.

[0012] It is further defined that the thickness of the polycrystalline metal foil in S1 is 0.025-0.5 mm.

[0013] It is further defined that the pretreatment tensile stress in S1 is 0.5-5 MPa, the high temperature annealing temperature is 600°C-2000°C, and the treatment time is 2-10h.

[0014] It is further defined that the polycrystalline metal foil in S1 is a single substance consisting of one element of copper, nickel, iron, tungsten and molybdenum, or an alloy consisting of multiple elements.

[0015] It is further defined that the method for forming a single crystal hexagonal boron nitride film in S2 is as follows: in a two-temperature zone horizontal tube furnace, 5 mg of a nitrogen-boron source is placed in temperature zone 1 upstream of the air flow, and a piece of a single crystal metal foil is placed in temperature zone 2 downstream of the air flow; hydrogen and argon with a flow ratio of 50 sccm:250 sccm are used to purge the quartz tube and exhaust the original air; then, temperature zone 2 is raised to 1065°C at a heating rate of 10-20°C / min and kept warm for 20-60 minutes; temperature zone 1 is then heated to 60-90°C at a rate of 0.5-2°C / min to evaporate the raw material and grow hexagonal boron nitride for 10-60 minutes, and the flow ratio of hydrogen and argon during growth is 10 sccm:100 sccm; after the reaction is completed, heating of temperature zone 1 is stopped, and temperature zone 2 is cooled to room temperature at a rate of 5-18°C / min to form a single layer or multiple layers of hexagonal boron nitride on the surface of the single crystal metal foil.

[0016] It is further defined that the nitrogen and boron source is one of borazine, borane ammonia complex, a combination of boric acid and ammonia, and a combination of nitrogen and iron-boron alloy.

[0017] It is further defined that the method for forming a hexagonal boron nitride / graphene heterojunction film in S3 is: after S2 is completed, the nitrogen-boron source in temperature zone 1 is replaced with a carbon precursor, and the temperatures of temperature zones 1 and 2 are set to 900-1050°C, the carrier gas flow rate is set to a hydrogen and argon ratio of 10sccm:100sccm, the reaction is carried out for 10-60min, and a single crystal graphene film is grown on the upper surface of the hexagonal boron nitride, thereby forming a hexagonal boron nitride / graphene vertical heterojunction.

[0018] It is further defined that the carbon precursor in S3 is a low temperature cracking substance such as methane, ethane or butane.

[0019] It is further defined that the method for forming a single crystal graphene film between the single crystal metal foil and the hexagonal boron nitride film in S4 is: after the reaction in S3 is completed, the gas pressure of the two-temperature zone horizontal tube furnace reaction chamber is adjusted to 1-10Pa, and then under the protection of hydrogen with a flow rate of 1-10sccm, the temperature of temperature zone 1 and temperature zone 2 are simultaneously reduced to 800-950°C, and then the hydrogen flow rate is reduced to 1-10sccm, and the carbon source is loaded through hydrogen, and the reaction is carried out for 0.5-5h; finally, temperature zone 1 and temperature zone 2 are simultaneously reduced to room temperature at a rate of 5-10°C / min.

[0020] It is further defined that the carbon source in S4 is acetylene or ethanol.

[0021] Compared with the prior art, the present invention also has the following beneficial effects:

[0022] (1) The present invention converts the polycrystalline metal foil into a single crystal metal foil by subjecting the large-sized polycrystalline metal foil substrate to high-temperature and long-term annealing treatment under stress, and uses the polycrystalline metal foil as the growth substrate of hexagonal boron nitride, and grows multi-layer and single-crystalline hexagonal boron nitride of the same size as the substrate by regulating the growth process parameters. The problem that the hexagonal boron nitride grown by using polycrystalline metal foil as a catalyst substrate is a single-layer polycrystalline structure when the size is the same as the substrate size, and the hexagonal boron nitride grown by using liquid metal foil or single crystal metal foil can be a single crystal, but still a single-layer structure, is effectively solved. This makes it possible to generate graphene on the surface of hexagonal boron nitride. When the hydrogen used and the growth temperature of 900 to 1050°C are used, the hydrogen has a strong etching effect on the hexagonal boron nitride, causing the single-layer hexagonal boron nitride to disappear, and it is impossible to form a hexagonal boron nitride / graphene heterojunction on the upper surface of the hexagonal boron nitride, thereby laying a foundation for the successful preparation of single-crystalline graphene / hexagonal boron nitride / graphene heterojunction.

[0023] (2) The present invention reduces the etching effect on the lower hexagonal boron nitride by selecting a carbon precursor and optimizing the graphene growth process parameters. Under high temperature conditions, the vapor evaporated from the metal foil is used to catalytically crack the carbon precursor, and graphene is epitaxially grown on the upper surface of the hexagonal boron nitride. Since the hexagonal boron nitride is a single crystal, the lattice mismatch between it and the graphene is only 1.8%. Although under the mechanism of van der Waals epitaxial growth, the upper graphene still has a consistent orientation relationship with the hexagonal boron nitride, and the epitaxial growth relationship between the graphene domains and the lower hexagonal boron nitride is maintained, so that when the graphene domains are connected together, a single crystal graphene film is formed, that is, a single crystal graphene / hexagonal boron nitride heterojunction is obtained on the upper surface of the hexagonal boron nitride.

[0024] (3) When preparing graphene between hexagonal boron nitride and metal foil, the present invention selects a substance with a lower cracking temperature as a carbon source, which is cracked at a lower temperature. The formed carbon radicals or carbon atoms first dissolve into the surface layer of the metal foil, and in the process of slow cooling, due to the supersaturation of carbon in the metal foil, a small amount of carbon atoms begin to precipitate at the interface between the metal foil and the hexagonal boron nitride. Under the epitaxial effect of the hexagonal boron nitride, the precipitated carbon atoms gradually nucleate and grow into graphene domains with a straight orientation. When the temperature is continuously lowered, the graphene domains gradually grow until they are connected into a single crystal graphene film, thereby obtaining a single crystal graphene / hexagonal boron nitride heterojunction on the lower surface of the hexagonal boron nitride.

[0025] (5) The hexagonal boron nitride and graphene prepared by the present invention are both single crystal structures, and the number of layers can be controlled from a single layer to multiple layers. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The process flow chart of preparing wafer-level single-crystal graphene heterojunction thin film of the present invention;

[0027] Figure 2 This is a physical picture of the wafer-level single-crystal graphene heterojunction film obtained in Example 1;

[0028] Figure 3 This is a selected area electron diffraction pattern in a transmission electron microscope. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art unless otherwise specified, and can be obtained through commercial channels by those skilled in the art. Specific embodiments

[0032] The process flow of preparing the wafer-level single-crystal graphene heterojunction film in this embodiment is as follows: Figure 1 As shown, the specific process is carried out in the following steps:

[0033] Step 1, pre-treating the polycrystalline copper foil to obtain a single crystal copper foil;

[0034] Specifically, a copper foil having a size of 50 mm×50 mm is subjected to a high temperature annealing treatment under a tensile stress of 2 MPa, wherein the annealing temperature is 1080° C. and the time is 5 hours to obtain a single crystal copper foil.

[0035] This step involves high-temperature annealing of commercial polycrystalline copper foil under tensile stress to prefabricate lattice defects and internal stress conditions inside the copper foil; then high-temperature remelting and solidification is performed at a temperature close to the melting point of copper to obtain large-sized single-crystalline copper foil, providing high-quality copper foil substrate material for the growth of wafer-level single-crystalline hexagonal boron nitride.

[0036] Step 2, preparing a single crystal hexagonal boron nitride film on the surface of the single crystal copper foil;

[0037] In a two-temperature-zone horizontal tube furnace, 5 mg of borane-ammine complex as raw material was placed in temperature zone 1 upstream of the gas flow, and a single crystal copper foil was placed in temperature zone 2 downstream of the gas flow; hydrogen / argon (50:250 sccm) was used to purge the quartz tube and exhaust the original air; thereafter, temperature zone 2 was raised to 1065°C at a heating rate of 20°C / min and maintained at this temperature for 20 minutes; temperature zone 1 where the borane-ammine complex was located was then heated to 85°C at a rate of 2°C / min for raw material evaporation and hexagonal boron nitride growth, and the growth time was 60 minutes, at which time the hydrogen / argon was (10:100 sccm); then the heating of temperature zone 1 was stopped, the raw material evaporation stopped, and at the same time, temperature zone 2 was cooled to room temperature at a rate of 18°C / min.

[0038] This step uses borane-ammine complex as a precursor, and grows hexagonal boron nitride single crystal domains with consistent orientation on the surface of single crystal copper foil by crystal orientation induction. The growth time is further extended to seamlessly connect the single crystal domains, forming a single crystal hexagonal boron nitride film with the same size as the copper foil substrate.

[0039] Step 3, preparing a single crystal graphene film on the upper surface of the single crystal hexagonal boron nitride film;

[0040] Using the hexagonal boron nitride obtained in step 2 as the substrate, directly switch the raw material in the reaction device to methane, adjust the temperature of temperature zone 1 and temperature zone 2 to 1050°C, use hydrogen / argon (10:100 sccm) as the carrier gas, and react for 30 minutes. A single crystal graphene film can be grown on the upper surface of the hexagonal boron nitride, thereby forming a hexagonal boron nitride / graphene vertical heterojunction.

[0041] This step uses methane as a carbon precursor, and uses copper vapor evaporated from the surface of the copper foil under high temperature conditions to catalytically crack methane, and epitaxially grows graphene on the upper surface of hexagonal boron nitride to form a hexagonal boron nitride / graphene heterojunction film. Since hexagonal boron nitride is a single crystal, its lattice mismatch with graphene is only 1.8%. Under the mechanism of van der Waals epitaxial growth, the upper graphene still has a consistent orientation relationship with the hexagonal boron nitride. Therefore, the graphene grown on the upper surface of hexagonal boron nitride is still a single crystal film.

[0042] Step 4, forming a single crystal graphene film between the single crystal copper foil and the single crystal hexagonal boron nitride film;

[0043] After the reaction in step 3 is completed, the pressure of the reaction chamber of the two-zone horizontal tube furnace is pumped down to 5Pa, and then, under the protection of hydrogen (10sccm), the temperature of zones 1 and 2 is lowered to 950°C, the hydrogen flow rate is reduced to 1sccm, and the carbon source is loaded through it, and the reaction time is maintained for 3 hours, so that the carbon atoms can fully diffuse and dissolve on the surface of the copper foil; finally, the temperature of both zones 1 and 2 is lowered to room temperature at a rate of 5°C / min. The carbon source is continuously precipitated at the interface between the copper foil and hexagonal boron nitride, and single-crystalline graphene is formed under the epitaxy of hexagonal boron nitride.

[0044] This step uses easily cracked ethanol as a carbon source. The lower copper foil is catalytically cracked to dissolve carbon atoms inside the copper foil. The carbon atoms are precipitated from the interface between the copper foil and hexagonal boron nitride during the cooling process. The orientation relationship between the precipitated carbon atoms and the existing hexagonal boron nitride is controlled to obtain single-crystalline graphene domains. Then, under the crystal orientation induction of the hexagonal boron nitride, van der Waals epitaxial growth is generated to obtain a single-crystalline graphene film.

[0045] The wafer-level single-crystal graphene heterojunction film obtained in this embodiment is as follows Figure 2 As shown, the size can reach 50mm×50mm (about the size of a 2-inch wafer), and the transmission electron microscope characterizes that the orientation of each part within its range is consistent, and the heterojunction film is a single crystal structure, such as Figure 3 shown.

[0046] Although the present invention has been disclosed as above in the preferred embodiment, it is not intended to limit the present invention. Anyone familiar with this technology can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the definition of the claims.

Claims

1. A method for directly growing a wafer-level single-crystal graphene heterojunction, characterized in that: The following steps are involved: S1, pre-treating the polycrystalline metal foil to obtain a single crystal metal foil; The polycrystalline metal foil in S1 is a single element consisting of one element of copper, nickel, iron, tungsten, and molybdenum, or an alloy consisting of multiple elements; S2, growing hexagonal boron nitride single crystal domains with consistent orientation on the surface of the single crystal metal foil by crystal orientation induction, further extending the growth time to make the single crystal domains seamlessly connected, and forming a single crystal hexagonal boron nitride film with the same size as the single crystal metal foil substrate; In a two-temperature-zone horizontal tube furnace, 5 mg of nitrogen-boron source is placed in temperature zone 1 upstream of the gas flow, and a piece of single crystal metal foil is placed in temperature zone 2 downstream of the gas flow to evaporate the raw materials and grow hexagonal boron nitride; S3, under high temperature conditions, using metal vapor evaporated from the surface of the single crystal metal foil to catalytically crack the carbon precursor, and epitaxially grow a graphene film on the upper surface of the hexagonal boron nitride to form a hexagonal boron nitride / graphene vertical heterojunction; The method for forming a hexagonal boron nitride / graphene heterojunction film in S3 is as follows: after S2 is completed, the nitrogen-boron source in temperature zone 1 is replaced with a carbon precursor, and the temperature of temperature zone 1 and temperature zone 2 is set to 900-1050° C., the carrier gas flow rate is set to a hydrogen-argon ratio of 10 sccm:100 sccm, the reaction is carried out for 10-60 minutes, and a single crystal graphene film is grown on the upper surface of the hexagonal boron nitride, thereby forming a hexagonal boron nitride / graphene vertical heterojunction; The carbon precursor in S3 is methane, ethane or butane; S4, using a catalytic cracking method to dissolve the carbon atoms cracked by the carbon source inside the single crystal metal foil, precipitate from the interface between the single crystal metal foil and hexagonal boron nitride during the cooling process, and grow epitaxially under the crystal orientation induction of the hexagonal boron nitride, forming a single crystal graphene film between the single crystal metal foil and the hexagonal boron nitride film, forming a graphene / hexagonal boron nitride heterojunction film, thereby realizing the growth of a single crystal graphene / hexagonal boron nitride / graphene vertical heterojunction film on a single crystal metal foil substrate, referred to as a graphene heterojunction; The method for forming a single crystal graphene film between the single crystal metal foil and the hexagonal boron nitride film in S4 is as follows: after the reaction in S3 is completed, the gas pressure of the reaction chamber of the two-temperature zone horizontal tube furnace is adjusted to 1-10Pa, and then, under the protection of hydrogen with a flow rate of 1-10sccm, the temperature of temperature zone 1 and temperature zone 2 are simultaneously reduced to 800-950°C, and then the hydrogen flow rate is reduced to 1-10sccm, and a carbon source is loaded through hydrogen, and the reaction is carried out for 0.5-5h; finally, temperature zone 1 and temperature zone 2 are simultaneously reduced to room temperature at a rate of 5-10°C / min; The carbon source in S4 is acetylene or ethanol.

2. The method for direct growth of wafer-level single-crystal graphene heterojunction according to claim 1, characterized in that: The pretreatment of the polycrystalline metal foil in S1 is high temperature annealing under tensile stress.

3. The method for direct growth of wafer-level single-crystal graphene heterojunction according to claim 2, characterized in that: In S1, the pretreatment tensile stress is 0.5-5Mpa, the high temperature annealing temperature is 600℃-2000℃, and the treatment time is 2-10h.

4. The method for direct growth of wafer-level single-crystal graphene heterojunction according to claim 1, characterized in that: The method for forming a single crystal hexagonal boron nitride film in S2 is as follows: in a two-temperature zone horizontal tube furnace, 5 mg of a nitrogen-boron source is placed in temperature zone 1 upstream of the gas flow, and a piece of a single crystal metal foil is placed in temperature zone 2 downstream of the gas flow; hydrogen and argon with a flow ratio of 50 sccm:250 sccm are used to purge the quartz tube to exhaust the original air; then, temperature zone 2 is heated to 1065°C at a heating rate of 10-20°C / min and kept warm for 20-60 minutes; temperature zone 1 is then heated to 60-90°C at a rate of 0.5-2°C / min to evaporate the raw material and grow hexagonal boron nitride for 10-60 minutes, and the flow ratio of hydrogen and argon during growth is 10 sccm:100 sccm; after the reaction is completed, heating of temperature zone 1 is stopped, and temperature zone 2 is cooled to room temperature at a rate of 5-18°C / min to form a single layer or multiple layers of hexagonal boron nitride on the surface of the single crystal metal foil.

5. The method for direct growth of wafer-level single-crystal graphene heterojunction according to claim 4, characterized in that: The nitrogen and boron source is one of borazine and borane ammonia complex.

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