Flexible boron nitride homojunction pn and method of fabrication

Abstract

The application discloses a flexible boron nitride homojunction pn and a preparation method, wherein a sulfur-doped n-type boron nitride film is grown on a sapphire substrate through a low-pressure chemical vapor deposition technology; a magnesium-doped p-type boron nitride film is grown on a copper substrate through a low-pressure chemical vapor deposition technology; the sulfur-doped n-type boron nitride film and the magnesium-doped p-type boron nitride film are transferred to a flexible substrate through a polymethyl methacrylate assisted liquid phase exfoliation technology, and heat treatment is conducted in an argon atmosphere to obtain a flexible boron nitride homojunction pn. The application realizes efficient n-type and p-type doping of the boron nitride film, and prepares a boron nitride-based homojunction pn on the flexible substrate through a large-area film exfoliation and transfer technology. The homojunction structure has high lattice matching degree and low junction formation energy, further develops the preparation and research of van der Waals homojunctions in the field of ultrawide bandgap materials, and has great application prospect in the fields of optoelectronic devices and high-power power electronic devices.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional semiconductor thin film material doping growth and pn junction preparation technology, specifically relating to a flexible boron nitride homogeneous pn junction and its preparation method. Background Technology

[0002] Hexagonal boron nitride (hBN) films are two-dimensional layered materials with a graphene-like structure, possessing both an ultra-wide optical bandgap and high-voltage, high-frequency resistance. Therefore, hBN materials have significant application potential in high-power power electronic devices. In particular, van der Waals heterojunctions formed by hBN materials with graphene, transition metal sulfides (TMDs), and other two-dimensional materials exhibit high carrier mobility, high carrier concentration, and sensitive switching characteristics in field-effect transistor (FET) applications due to the negligible impurity scattering effect caused by surface dangling bonds. However, the optical bandgap of TMDs and other two-dimensional materials is generally less than 2.5 eV. These narrow-bandgap two-dimensional materials are prone to degradation under high voltage and high power conditions, leading to device failure and severely limiting the application of hBN-based van der Waals heterostructures in high-power power electronic devices. Various ultra-wide bandgap materials capable of withstanding high frequencies and high voltages (such as diamond, gallium oxide, and aluminum nitride) are used to form heterostructures with hBN. Although the two-dimensional layered structure of hBN has been shown to significantly suppress impurity scattering effects caused by interface states, constructing such hBN-based ultra-wide bandgap heterostructures faces complex fabrication processes and instrumentation requirements. Furthermore, the interfacial lattice mismatch and differences in thermal expansion coefficients in the ultra-wide bandgap heterostructure can cause hBN film fragmentation and detachment during van der Waals epitaxy. Currently, the process challenges of directly growing and fabricating efficient hBN-based ultra-wide bandgap heterostructures have not yet been solved.

[0003] Currently, the fabrication of hBN-based ultrawide wide-bandgap heterostructures is almost entirely achieved through film exfoliation and transfer processes. However, the high heterojunction formation energy caused by interfacial lattice mismatch is difficult to meet through film transfer. Furthermore, hBN materials have near-insulating properties, limiting their use to simple insulating dielectric layers and failing to fully realize their advantages in high-power power electronic devices. Therefore, achieving efficient n / p doping of hBN thin films, enabling the application of doped hBN materials as channel layers in field-effect transistor devices, can simultaneously leverage their high carrier mobility and high-frequency, high-voltage electrical characteristics. Simultaneously, designing and constructing boron nitride homogeneous pn junctions, using this homogeneous structure as a structural unit for further designing power devices such as JFETs, MOSFETs, and HEMTs, can meet the application requirements of ultra-high frequency and ultra-high voltage applications. Furthermore, hBN-based van der Waals homogeneous pn junctions possess high interfacial lattice matching and low formation energy, allowing for the construction of high-performance boron nitride homogeneous pn junctions on flexible substrates using PMMA-assisted liquid-phase exfoliation and transfer techniques, further expanding their application scenarios. For hBN epitaxial processes, the epitaxial growth temperature of the metal substrate limits the generation of boron vacancies and boron atoms in the epitaxially grown hBN crystal. x O y The composite material enables intrinsic hBN thin films to exhibit p-type semiconductor characteristics. Combined with the negative electron affinity of hBN thin films, it is easier to achieve efficient p-type doping compared to materials such as gallium oxide and aluminum nitride. However, efficient n-doping of hBN materials remains a technical challenge, including the large atomic radius of n-type dopants leading to higher substitutional formation energies and requiring higher doping growth temperatures; the need to further improve the crystal quality of hBN to reduce impurity compensation effects; and the requirement for low impurity ionization energies in the donor levels introduced by n-type doping.

[0004] In summary, the selection of substrate, selection of dopant atoms, precursor supply rate, growth temperature, and other process parameters are all crucial for achieving n-type doping of hBN thin films. Summary of the Invention

[0005] To overcome the severe interface lattice mismatch and surface state-induced impurity scattering effects in existing ultra-wide bandgap heterostructures, the present invention aims to provide a flexible boron nitride homogeneous pn junction and its preparation method. This method achieves efficient n-type and p-type doping of hBN thin films using LPCVD technology, and constructs a homogeneous pn junction of hBN on a flexible PI substrate using PMMA-assisted liquid phase exfoliation and transfer technology, thereby realizing the preparation of an hBN-based van der Waals homostructure with high interface lattice matching.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a flexible boron nitride homogeneous pn junction includes the following steps:

[0008] Sulfur-doped n-type boron nitride thin films were grown on sapphire substrates using low-pressure chemical vapor deposition.

[0009] Magnesium-doped p-type boron nitride thin films were grown on copper substrates using low-pressure chemical vapor deposition.

[0010] Using polymethyl methacrylate-assisted liquid-phase exfoliation technology, sulfur-doped n-type boron nitride thin films and magnesium-doped p-type boron nitride thin films were transferred onto a flexible substrate and then heat-treated in an argon atmosphere to obtain a flexible boron nitride homogeneous pn junction.

[0011] Furthermore, sulfur-doped n-type boron nitride thin films are prepared by the following process:

[0012] Under a nitrogen and argon atmosphere, the sapphire substrate inside the container is heated to 1400-1450℃, and B / N and S sources are introduced into the container. The temperature is maintained for 30-60 minutes at a pressure of 800-1000 mtorr, and then cooled to form a sulfur-doped n-type boron nitride thin film on the sapphire substrate.

[0013] Furthermore, ammonia borane is heated to 115°C to form sublimation vapor and pyrolysis products of ammonia borane, which serve as a B / N source; sulfur powder is heated to 115°C to form sublimation vapor of sulfur, which serves as a S source.

[0014] Furthermore, the cooling rate is 3-5℃ / min.

[0015] Furthermore, when heating to 1400-1450℃ and cooling down, the volume ratio of nitrogen to argon is 1:3, and the flow rate of nitrogen is 5-10 sccm.

[0016] During heat preservation, the volume ratio of nitrogen to argon is 1:3, and the flow rate of nitrogen is 20-30 sccm.

[0017] Furthermore, magnesium-doped p-type boron nitride thin films are prepared via the following process:

[0018] In a nitrogen and argon atmosphere, the copper substrate in the container is heated to 1000-1050℃, and B / N source and Mg source are introduced into the container. The temperature is maintained at 300-500 mtorr for 15-20 min, and then cooled to form a magnesium-doped boron nitride thin film on the copper substrate.

[0019] Furthermore, during the preparation of magnesium-doped boron nitride thin films, ammonia borane is heated to 115°C to form sublimation vapor and pyrolysis products of ammonia borane, which serve as the B / N source. Magnesium nitride powder is heated to 1000-1050°C to form sublimation vapor and pyrolysis products of magnesium nitride, which serve as the Mg source. The cooling rate is 2°C / min. The volume ratio of nitrogen to argon is 1:3, and the nitrogen flow rate is 5-10 sccm.

[0020] Furthermore, sulfur-doped n-type boron nitride thin films and magnesium-doped p-type boron nitride thin films are transferred onto a flexible substrate through the following process:

[0021] PMMA aqueous solution was spin-coated onto a sulfur-doped n-type boron nitride film and cured by heating. Then, the sulfur-doped n-type boron nitride film was immersed in hydrochloric acid aqueous solution, peeled off from the sapphire substrate, transferred to a flexible substrate, and dried to obtain an hBN:S / PI structure.

[0022] A PMMA aqueous solution was spin-coated onto a magnesium-doped p-type boron nitride film and cured by heating. Then, the magnesium-doped p-type hBN was immersed in an ammonium sulfite solution. The magnesium-doped p-type boron nitride film was peeled off from the copper substrate and transferred to the hBN:S / PI structure. After drying, the hBN:Mg / hBN:S / PI structure was obtained.

[0023] Furthermore, the heat treatment temperature is 300-350℃, and the time is 30-40 minutes.

[0024] A flexible boron nitride homogeneous pn junction prepared according to the method described above.

[0025] Compared with the prior art, the method for preparing a flexible boron nitride homogeneous pn junction of the present invention has the following beneficial effects:

[0026] This invention utilizes low-pressure chemical vapor deposition (LPCVD) technology to perform sulfur doping and magnesium doping of boron nitride (BN) thin films on sapphire and copper substrates, respectively, achieving highly efficient n-type and p-type doping of BN thin films. This efficient doping of BN thin films overcomes the limitation of BN materials in power devices, which can only be used as insulating dielectric layers, and expands the application prospects of BN materials as channel layers in high-power devices. Based on the successful epitaxial growth of n-type and p-type doped BN thin films, a flexible homogeneous BN pn junction is fabricated using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology. The flexible homogeneous BN pn junction prepared by this invention has the same hexagonal layered lattice structure and lattice constant for both the n-type and p-type BN thin films. Compared with other ultra-wide bandgap heterostructures, the proposed hBN:S / hBN:Mg structure pn junction has a higher lattice matching degree and a lower junction formation energy, allowing for the fabrication of homogeneous BN pn junctions through film exfoliation and transfer processes. Using this homogeneous structure as a structural unit to design power devices such as JFETs, MOSFETs, and HEMTs can meet the application requirements of power devices with ultra-high frequency and ultra-high voltage resistance. The film transfer process can also realize the change of the torsion angle when two layers are stacked in the hBN-based van der Waals homogeneous structure, providing a new research direction for the study of optical waveguide transmission and optical polarization in the ultraviolet band. The flexible boron nitride homogeneous pn junction prepared in this invention uses n-type and p-type doped hBN thin films, which are currently the only layered structure material among ultra-wide bandgap materials with no dangling bonds. Compared with other ultra-wide bandgap heterostructures, the interface state-induced scattering effect of this homogeneous pn junction can be significantly suppressed. In power device applications, both carrier mobility and carrier density are greatly improved. Therefore, the development of related power devices can simultaneously meet the requirements of high voltage and high frequency resistance and high switching characteristics.

[0027] Furthermore, as a van der Waals homostructure of an ultrawide bandgap material, this invention improves the interfacial contact quality through heat treatment at 300℃, thereby achieving high interfacial contact quality of hBN-based van der Waals homostructures. Attached Figure Description

[0028] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0029] Figure 1 This is a flowchart illustrating the fabrication process of a flexible boron nitride homogeneous pn junction.

[0030] Figure 2 X-ray diffraction pattern of sulfur-doped boron nitride thin film epitaxially formed on sapphire substrate;

[0031] Figure 3A scanning electron microscope (SEM) image of a sulfur-doped boron nitride thin film epitaxially grown on a sapphire substrate.

[0032] Figure 4 X-ray photoelectron spectra of sulfur-doped boron nitride thin films epitaxial on sapphire substrates, including S2p, B1s, and N1s; where (a) is S2p, (b) is B1s, and (c) is N1s.

[0033] Figure 5 The surface elemental distribution of a sulfur-doped boron nitride thin film epitaxially grown on a sapphire substrate includes S, B, and N elements; where (a) represents S, (b) represents B, and (c) represents N.

[0034] Figure 6 The valence band spectrum of a sulfur-doped boron nitride thin film epitaxially grown on a sapphire substrate;

[0035] Figure 7 X-ray photoelectron spectroscopy of magnesium-doped boron nitride thin films epitaxially grown on copper substrates, including Mg Auger , B1s, N1s; where (a) is Mg Auger (b) is B1s, and (c) is N1s;

[0036] Figure 8 The valence band spectrum of a magnesium-doped boron nitride thin film epitaxially grown on a copper substrate;

[0037] Figure 9 Optical micrograph of a homogeneous boron nitride pn junction with hBN:S / hBN:Mg structure prepared by PMMA-assisted liquid phase exfoliation and transfer method;

[0038] Figure 10 The IV characteristic curves are for a homogeneous pn junction with an hBN:S / hBN:Mg structure. Detailed Implementation

[0039] To make the technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific implementation examples and the accompanying drawings.

[0040] This invention discloses a method for fabricating a flexible boron nitride homogeneous pn junction. The method utilizes chemical vapor deposition (CVD) to epitaxially grow n-type and p-type hBN thin films. Using PMMA-assisted liquid-phase exfoliation, sulfur-doped hBN and magnesium-doped hBN thin films are sequentially transferred to a flexible PI substrate. Interfacial heat treatment is then performed to improve the interfacial contact quality, thereby fabricating a boron nitride homogeneous pn junction on the flexible substrate. The method specifically includes the following steps:

[0041] 1) A sulfur-doped n-type boron nitride (hBN:S) thin film was grown on a sapphire substrate using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0042] A sapphire substrate was placed in the central reaction region of a tube furnace. Low-pressure chemical vapor deposition (LCV) was used, and the sapphire substrate was heated to 1400-1450°C in a nitrogen and argon atmosphere (nitrogen and argon were used as carrier gases). Ammonia borane was heated to 115°C to form sublimation vapor and pyrolysis products of ammonia borane, which served as the B / N source. Sulfur powder was heated to 115°C to form sublimation vapor of sulfur, which served as the S source. The sublimation vapor and pyrolysis products of ammonia borane and the sublimation vapor of sulfur were introduced into the reaction region of the tube furnace along with the carrier gas to provide the B / N source and S source. The pressure inside the tube furnace was set to 800-1000 mtorr and maintained for 30-60 min to perform sulfur doping of hBN and epitaxial growth of the n-type hBN film. The temperature was then lowered (at a rate of 3-5°C / min) to obtain a sulfur-doped boron nitride film deposited on the sapphire substrate.

[0043] During heating and cooling, the volume ratio of nitrogen to argon is 1:3, and the flow rate of nitrogen is 5-10 sccm.

[0044] During thin film growth, the volume ratio of nitrogen to argon in the carrier gas is 1:3, and the flow rate of nitrogen is 20-30 sccm.

[0045] 2) Magnesium-doped p-type boron nitride thin films, i.e., hBN:Mg films, are grown on copper substrates using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0046] A copper substrate is placed in the central reaction region of the furnace tube and heated to 1000-1050℃ in a nitrogen and argon atmosphere (nitrogen and argon as carrier gases). Ammonia borane is heated to 115℃ to form sublimation vapor and pyrolysis products of ammonia borane, which serve as the B / N source. Magnesium nitride powder is heated to 1000-1050℃ to form sublimation vapor and pyrolysis products of magnesium nitride, which serve as the Mg source. The sublimation vapor and pyrolysis products of ammonia borane and magnesium nitride are introduced into the reaction region of the tube furnace along with the carrier gas to provide the B / N source and Mg source, respectively. The pressure inside the tube furnace is set to 300-500 mtorr and maintained for 15-20 min. The temperature is then lowered (at a rate of 2℃ / min) to obtain a magnesium-doped boron nitride thin film on the copper substrate.

[0047] During the preparation of magnesium-doped boron nitride thin films, the volume ratio of nitrogen to argon is 1:3, and the flow rate of nitrogen is 5-10 sccm.

[0048] 3) Using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology, hBN:S and hBN:Mg films were sequentially transferred onto a polyimide (PI) flexible substrate, and then heat-treated at 300℃ for 30 min under an argon atmosphere to improve the interfacial contact quality, resulting in a high-quality flexible boron nitride homogeneous pn junction. The specific process is as follows:

[0049] PMMA aqueous solution was spin-coated onto a sulfur-doped n-type boron nitride (hBN) film using a PMMA spin-coating process, and then heated and dried for curing. The sulfur-doped n-type hBN was then immersed in a stripping solution (1 mol / L hydrochloric acid aqueous solution). Under PMMA protection, the hBN:S film was peeled from the sapphire substrate and transferred to a flexible PI substrate. It was dried at 80°C for 20 min, and the PMMA organic layer was removed in acetone solution. Acetone residue was washed away in isopropanol solution, and the film was dried to obtain the hBN:S / PI structure. Similarly, PMMA aqueous solution was spin-coated onto a magnesium-doped p-type boron nitride (pBN) film and heated and dried for curing. The magnesium-doped p-type hBN was then immersed in a stripping solution (1.5 mol / L ammonium sulfite solution). Under PMMA protection, the hBN:Mg film was peeled from the copper substrate and transferred to the hBN:S / PI substrate. It was dried at 80°C for 20 min, and the PMMA organic layer was removed in acetone solution. Acetone residue was washed away in isopropanol solution. The prepared hBN:Mg / hBN:S / PI structure was heated at 300-350℃ for 30-40 min in an argon atmosphere to improve the interfacial contact quality.

[0050] The PMMA spin coating process involves holding the spin at 500 rpm for 10 seconds, followed by holding it at 3000 rpm for 30 seconds.

[0051] The hBN:S thin film epitaxially grown on the sapphire substrate was prepared using a 1 mol / L hydrochloric acid aqueous solution as the stripping solution.

[0052] The hBN:Mg thin film epitaxially formed on a copper substrate was prepared using a substrate etching solution of 1.5 mol / L ammonium sulfite aqueous solution.

[0053] A flexible boron nitride homogeneous pn junction prepared according to the method described above achieves efficient n-type and p-type doping of the hBN thin film. The effective area of ​​the pn junction on the flexible substrate is 0.5 cm². 2 .

[0054] Example 1

[0055] See Figure 1 A method for preparing a flexible boron nitride homogeneous pn junction includes the following steps:

[0056] 1) A sulfur-doped n-type boron nitride (hBN:S) thin film was grown on a sapphire substrate using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0057] Step 1.1: Cleaning the sapphire substrate. (The text abruptly ends here, seemingly mid-s 2A sapphire substrate of a certain size was immersed in an aqueous solution of 311 (sulfuric acid, hydrogen peroxide, and water in a volume ratio of 3:1:1) and heated at 130°C for 1 hour to remove residual organic impurities from the surface. Subsequently, the substrate was ultrasonically cleaned with water, ethanol, acetone, and isopropanol solutions for 20 minutes each. The sapphire sample cleaned with isopropanol was dried in an oven.

[0058] Step 1.2: Heating the sapphire substrate in a nitrogen / argon atmosphere. Place the cleaned sapphire substrate in the central heating zone of a tube furnace. Evacuate the furnace to 20 mtorr. Use argon as the furnace atmosphere cleaning gas, introducing argon into the furnace until atmospheric pressure is reached. Then, evacuate the furnace to 20 mtorr again, repeating this process three times to ensure no air remains inside the furnace. In a mixed atmosphere of nitrogen and argon at a volume ratio of 1:3 (nitrogen flow rate 10 sccm), heat the sapphire substrate to 1400℃ at a heating rate of 10℃ / min and hold at that temperature for 30 min.

[0059] Step 1.3: Precursor heating for epitaxial growth of hBN:S thin films on sapphire substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.98% pure sulfur powder was used as the S source. Both precursors were solid powders at room temperature. 60 mg of ammonia borane powder and 20 mg of sulfur powder were weighed and thoroughly mixed in a quartz crucible, then placed in the precursor heating zone. The precursor heating zone was separated from the tube furnace heating zone but connected via a carrier gas path. The precursor heating zone was heated to 115°C at a heating rate of 10°C / min. The carrier gas path is opened, with a nitrogen to argon volume ratio of 1:3 (nitrogen flow rate 30 sccm). The sublimation vapors of ammonia borane and sulfur, along with their pyrolysis products, enter the tube furnace with the carrier gas. In the central heating zone (1400℃), B, N, and S atoms are fully pyrolyzed, maintaining the furnace pressure at 800 mtorr. Subsequently, a crosslinking reaction occurs on the sapphire substrate to achieve the epitaxy of an hBN:S thin film.

[0060] Step 1.4: Cooling to obtain the hBN:S thin film on the sapphire substrate. After growth, the carrier gas path is closed, and the flow rate of the gas flowing into the tube furnace is reduced to 10 sccm N2:30 sccm Ar. The central heating area of ​​the tube furnace is cooled to room temperature at a rate of 5℃ / min to achieve epitaxial growth of a large-area, highly crystalline sulfur-doped boron nitride thin film.

[0061] In this embodiment, by applying an ultra-high reaction temperature (1400℃) to meet the high sulfur impurity atom substitution formation energy during the hBN thin film doping process, the epitaxial growth of a large-area hBN;S thin film with high crystallinity on a sapphire substrate was achieved. The high crystallinity of the film can be achieved by... Figure 2 As confirmed by XRD results, the large-area continuity of the thin film can be achieved by... Figure 3 Confirmed by surface SEM.

[0062] Successful sulfur doping in hBN thin films can be achieved by… Figure 4 The XPS results shown in (a), (b) and (c) confirm that the sulfur atom doping content is 1.21%.

[0063] Figure 5 EDS diagrams of the hBN:S thin film are shown in (a), (b), and (c), which confirm the uniform distribution of sulfur atoms in the hBN thin film.

[0064] In this embodiment, the hBN:S thin film synthesized on a sapphire substrate exhibits an ultrawide optical bandgap of 5.79 eV and significant n-type conductivity. These properties can be obtained from... Figure 6 This was confirmed by the valence band spectrum results.

[0065] The above results confirm that the process method of Example 1 successfully achieves efficient S doping of hBN thin films.

[0066] 2) Magnesium-doped p-type boron nitride thin films, i.e., hBN:Mg films, are grown on copper substrates using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0067] Step 2.1: Thermal Annealing of the Copper Substrate. Before epitaxial growth of hBN thin films on the copper substrate, thermal annealing of the copper is performed to reduce grain boundaries and surface roughness. An LPCVD system is used for thermal annealing. The copper substrate is placed in the central heating zone of a tube furnace, with argon gas used as the protective gas. 50 sccm of argon gas is introduced, and the furnace pressure is adjusted to 500 mtorr. The copper substrate is heated to 950°C at a heating rate of 10°C / min and held for 30 min. It is then cooled to room temperature at a cooling rate of 10°C / min to obtain a copper substrate with large grain domains and high surface smoothness.

[0068] Step 2.2: Precursor heating for epitaxial growth of hBN:Mg thin films on copper substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.5% pure magnesium nitride powder was used as the Mg source. Both precursors were solid powders at room temperature. 30 mg of ammonia borane powder and 10 mg of magnesium nitride powder were weighed and placed in the precursor heating area and the central heating area of ​​the tube furnace, respectively. The heat-annealed copper substrate was placed in the central heating area of ​​the tube furnace, and heated to 1050°C at a rate of 10°C / min in a mixed atmosphere with an N2 / Ar flow ratio of 1:3 (nitrogen flow rate of 10 sccm). Simultaneously, while the tube furnace temperature was 950°C, the precursor heating area was heated to 115°C at a rate of 10°C / min. The carrier gas path is opened, and a 1:3 mixture of nitrogen and argon is introduced as the carrier gas (nitrogen flow rate of 5 sccm). The sublimation vapor of ammonia borane and its pyrolysis products are introduced into the tube furnace along with the carrier gas, and the furnace pressure is maintained at 300 mtorr. In the central heating region of the tube furnace, the magnesium nitride powder undergoes complete pyrolysis, providing Mg atoms to the system. The pyrolyzed B and N atoms dissolve into the copper lattice at high temperature. Epitaxial growth of the hBN:Mg thin film is then performed at 1050℃ for 20 min.

[0069] Step 2.3: Cooling to obtain the hBN:Mg thin film on the copper substrate. After growth, the carrier gas path is shut off. The central heating region of the tube furnace is cooled to room temperature at a rate of 2℃ / min. During the cooling process, B and N atoms dissolved into the copper lattice undergo segregation. Mg atoms on the copper substrate surface cross-link with B and N atoms, thereby epitaxially growing the hBN:Mg thin film on the copper substrate.

[0070] In this embodiment, magnesium nitride powder was used as the magnesium source to successfully achieve the epitaxial growth of magnesium-doped boron nitride thin films on a copper substrate. The successful substitutional doping of magnesium impurity atoms can be achieved by… Figure 7 The XPS results of the hBN:Mg thin film are shown in (a), (b), and (c).

[0071] In this embodiment, a magnesium-doped boron nitride thin film is synthesized on a copper substrate, through... Figure 8 The valence band spectrum test results of the thin film determined the position of its Fermi level, and the results showed that the hBN:Mg thin film has obvious p-type conductivity.

[0072] 3) Using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology, hBN:S and hBN:Mg films were sequentially transferred onto a polyimide (PI) flexible substrate and then heat-treated to obtain a high-quality flexible boron nitride homogeneous pn junction. The specific process is as follows:

[0073] Step 3.1: Peeling and Transfer of the hBN:S Thin Film on the Sapphire Substrate. A PMMA aqueous solution was spin-coated onto the sapphire / hBN:S thin film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The sapphire / hBN:S / PMMA sample was immersed in a 1 mol / L hydrochloric acid solution for one day, after which the hBN:S / PMMA layer was successfully peeled off. It was transferred to a polyimide (PI) flexible substrate and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S thin film to the PI flexible substrate.

[0074] Step 3.2: Stripping and Transfer of the hBN:Mg Film on a Copper Substrate. A PMMA aqueous solution was spin-coated onto the copper / hBN:Mg film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The copper substrate of the copper / hBN:Mg / PMMA sample was placed face down and suspended on the surface of a 1.5 mol / L ammonium sulfite solution. After approximately 6 h, the copper substrate was completely etched away, leaving the remaining hBN:Mg / PMMA layer suspended on the solution surface. This was transferred to a PI flexible substrate / hBN:S structure and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / hBN:Mg / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S / hBN:Mg heterostructure onto the PI flexible substrate.

[0075] Step 3.3: Interfacial heat treatment of PI / hBN:S / hBN:Mg. The PI flexible substrate selected in this invention can withstand a high temperature of 500℃, meeting the temperature requirements for interfacial heat treatment. The PI / hBN:S / hBN:Mg prepared by transfer is placed in the central heating area of ​​a tube furnace, and 50 sccm of argon gas is introduced while maintaining the furnace pressure at 5 torr. The temperature is increased to 300℃ at a heating rate of 10℃ / min and held for 30 min to improve the interfacial contact quality. Finally, the temperature is reduced to room temperature at a cooling rate of 10℃ / min to obtain a flexible boron nitride homopolymer PN junction with high interfacial contact quality.

[0076] This invention uses sapphire and copper substrates to grow large-area, high-quality sulfur-doped boron nitride (BN) and magnesium-doped BN films, respectively, via low-pressure chemical vapor deposition (LPCVD). Subsequently, a PMMA-assisted liquid-phase lift-off and transfer process is used to fabricate a flexible BN homojunction with high interfacial contact quality on a flexible PI substrate.

[0077] from Figure 9 It can be seen that there is a clear boundary between the two layers. The low contrast on both sides of the boundary proves that the transfer and interface heat treatment process can achieve a tight interface contact between the two layers. The above results confirm the successful peeling and transfer of the film and the successful realization of the hBN:S / hBN:Mg homogeneous structure.

[0078] Figure 10 The IV curves of this pn junction are presented, showing significant rectification characteristics, proving that this homogeneous structure can achieve high-quality interfacial contact characteristics under heat treatment at 300℃. The fabrication of this boron nitride homogeneous pn junction enables the fabrication of ultra-wide bandgap van der Waals diodes. Using this diode structure as a structural design unit, the fabrication of high-power power electronic devices, deep ultraviolet light emission, and photodetectors can be further realized. The selection of flexible substrates and the fabrication of flexible devices can expand the application scenarios of related devices.

[0079] Example 2

[0080] 1) A sulfur-doped n-type boron nitride (hBN:S) thin film was grown on a sapphire substrate using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0081] Step 1.1: Cleaning the sapphire substrate. (The text abruptly ends here, seemingly mid-s 2 A sapphire substrate of a certain size was immersed in an aqueous solution of 311 (sulfuric acid, hydrogen peroxide, and water in a volume ratio of 3:1:1) and heated at 130°C for 1 hour to remove residual organic impurities from the surface. Subsequently, the substrate was ultrasonically cleaned with water, ethanol, acetone, and isopropanol solutions for 20 minutes each. The sapphire sample cleaned with isopropanol was dried in an oven.

[0082] Step 1.2: Nitrogen / Argon Atmosphere Heating of Sapphire Substrate. The cleaned sapphire substrate was placed in the central heating zone of a tube furnace. The vacuum level inside the furnace was evacuated to 20 mtorr. Argon was used as the furnace atmosphere purging gas. Argon was introduced into the tube furnace until atmospheric pressure was reached, and then the vacuum level was evacuated back to 20 mtorr. This process was repeated three times to ensure no air remained inside the furnace. Under a mixed atmosphere of nitrogen and argon in a volume ratio of 1:3 (nitrogen flow rate 5 sccm), the sapphire substrate was heated to 1400℃ at a heating rate of 10℃ / min and held at that temperature for 60 min.

[0083] Step 1.3: Precursor heating for epitaxial growth of hBN:S thin films on sapphire substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.98% pure sulfur powder was used as the S source. Both precursors were solid powders at room temperature. 60 mg of ammonia borane powder and 20 mg of sulfur powder were weighed and thoroughly mixed in a quartz crucible, then placed in the precursor heating zone, which was separated from the tube furnace heating zone but connected via a carrier gas path. The precursor heating zone was heated to 115°C at a heating rate of 10°C / min. The carrier gas path was opened, with a nitrogen to argon volume ratio of 1:3 (nitrogen flow rate 27 sccm). The sublimation vapors of ammonia borane and sulfur, along with their pyrolysis products, entered the tube furnace with the carrier gas and were fully pyrolyzed in the central heating zone of the tube furnace to release B, N, and S atoms, maintaining a furnace pressure of 800 mtorr. Subsequently, a crosslinking reaction occurs on a sapphire substrate to achieve the epitaxy of an hBN:S thin film.

[0084] Step 1.4: Cooling to obtain the hBN:S thin film on the sapphire substrate. After growth, the carrier gas path is closed, and the flow rate of the gas flowing into the tube furnace is reduced to 10 sccm N2:30 sccm Ar. The central heating area of ​​the tube furnace is cooled to room temperature at a rate of 3℃ / min, achieving epitaxial growth of a large-area, highly crystalline sulfur-doped boron nitride thin film.

[0085] 2) Magnesium-doped p-type boron nitride thin films, i.e., hBN:Mg films, are grown on copper substrates using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0086] Step 2.1: Thermal Annealing of the Copper Substrate. Before epitaxial growth of hBN thin films on the copper substrate, thermal annealing of the copper is performed to reduce grain boundaries and surface roughness. An LPCVD system is used for thermal annealing. The copper substrate is placed in the central heating zone of a tube furnace, with argon gas used as the protective gas. 50 sccm of argon gas is introduced, and the furnace pressure is adjusted to 500 mtorr. The copper substrate is heated to 950°C at a heating rate of 10°C / min and held for 30 min. It is then cooled to room temperature at a cooling rate of 10°C / min to obtain a copper substrate with large grain domains and high surface smoothness.

[0087] Step 2.2: Precursor heating for epitaxial growth of hBN:Mg thin films on copper substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.5% pure magnesium nitride powder was used as the Mg source. Both precursors were solid powders at room temperature. 30 mg of ammonia borane powder and 10 mg of magnesium nitride powder were weighed and placed in the precursor heating area and the central heating area of ​​the tube furnace, respectively. The heat-annealed copper substrate was placed in the central heating area of ​​the tube furnace, and heated to 1050°C at a rate of 10°C / min in a mixed atmosphere with an N2 / Ar flow ratio of 1:3 (nitrogen flow rate of 10 sccm). Simultaneously, while the tube furnace temperature was 950°C, the precursor heating area was heated to 115°C at a rate of 10°C / min. The carrier gas path is opened, and a 1:3 mixture of nitrogen and argon is introduced as the carrier gas (nitrogen flow rate of 5 sccm). The sublimation vapor of ammonia borane and its pyrolysis products are introduced into the tube furnace along with the carrier gas, and the furnace pressure is maintained at 300 mtorr. In the central heating region of the tube furnace, the magnesium nitride powder undergoes complete pyrolysis, providing Mg atoms to the system. The pyrolyzed B and N atoms dissolve into the copper lattice at high temperature. Epitaxial growth of the hBN:Mg thin film is then performed at 1050℃ for 20 min.

[0088] Step 2.3: Cooling to obtain the hBN:Mg thin film on the copper substrate. After growth, the carrier gas path is shut off. The central heating region of the tube furnace is cooled to room temperature at a rate of 2℃ / min. During the cooling process, B and N atoms dissolved into the copper lattice undergo segregation. Mg atoms on the copper substrate surface cross-link with B and N atoms, thereby epitaxially growing the hBN:Mg thin film on the copper substrate.

[0089] 3) Using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology, hBN:S and hBN:Mg films were sequentially transferred onto a polyimide (PI) flexible substrate and then heat-treated to obtain a high-quality flexible boron nitride homogeneous pn junction. The specific process is as follows:

[0090] Step 3.1: Peeling and Transfer of the hBN:S Thin Film on the Sapphire Substrate. A PMMA aqueous solution was spin-coated onto the sapphire / hBN:S thin film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The sapphire / hBN:S / PMMA sample was immersed in a 1 mol / L hydrochloric acid solution for one day, after which the hBN:S / PMMA layer was successfully peeled off. It was transferred to a polyimide (PI) flexible substrate and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S thin film to the PI flexible substrate.

[0091] Step 3.2: Stripping and Transfer of the hBN:Mg Film on a Copper Substrate. A PMMA aqueous solution was spin-coated onto the copper / hBN:Mg film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The copper substrate of the copper / hBN:Mg / PMMA sample was placed face down and suspended on the surface of a 1.5 mol / L ammonium sulfite solution. After approximately 6 h, the copper substrate was completely etched away, leaving the remaining hBN:Mg / PMMA layer suspended on the solution surface. This was transferred to a PI flexible substrate / hBN:S structure and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / hBN:Mg / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S / hBN:Mg heterostructure onto the PI flexible substrate.

[0092] Step 3.3: Interfacial heat treatment of PI / hBN:S / hBN:Mg. The PI flexible substrate selected in this invention can withstand a high temperature of 500℃, meeting the temperature requirements for interfacial heat treatment. The PI / hBN:S / hBN:Mg prepared by transfer is placed in the central heating area of ​​a tube furnace, and 50 sccm of argon gas is introduced while maintaining the furnace pressure at 5 torr. The temperature is increased to 340℃ at a heating rate of 10℃ / min and held for 32 min to improve the interfacial contact quality. Finally, the temperature is reduced to room temperature at a cooling rate of 10℃ / min to obtain a flexible boron nitride homopolymer pn junction with high interfacial contact quality.

[0093] Example 3

[0094] 1) A sulfur-doped n-type boron nitride (hBN:S) thin film was grown on a sapphire substrate using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0095] Step 1.1: Cleaning the sapphire substrate. (The text abruptly ends here, seemingly mid-s 2 A sapphire substrate of a certain size was immersed in an aqueous solution of 311 (sulfuric acid, hydrogen peroxide, and water in a volume ratio of 3:1:1) and heated at 130°C for 1 hour to remove residual organic impurities from the surface. Subsequently, the substrate was ultrasonically cleaned with water, ethanol, acetone, and isopropanol solutions for 20 minutes each. The sapphire sample cleaned with isopropanol was dried in an oven.

[0096] Step 1.2: Nitrogen / Argon Atmosphere Heating of Sapphire Substrate. The cleaned sapphire substrate was placed in the central heating zone of a tube furnace. The vacuum level inside the furnace was evacuated to 20 mtorr. Argon was used as the furnace atmosphere purging gas. Argon was introduced into the tube furnace until atmospheric pressure was reached, and then the vacuum level was evacuated back to 20 mtorr. This process was repeated three times to ensure no air remained inside the furnace. Under a mixed atmosphere of nitrogen and argon in a volume ratio of 1:3 (nitrogen flow rate 8 sccm), the sapphire substrate was heated to 1430°C at a heating rate of 10°C / min and held at that temperature for 40 min.

[0097] Step 1.3: Precursor heating for epitaxial growth of hBN:S thin films on sapphire substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.98% pure sulfur powder was used as the S source. Both precursors were solid powders at room temperature. 60 mg of ammonia borane powder and 20 mg of sulfur powder were weighed and thoroughly mixed in a quartz crucible, then placed in the precursor heating zone, which was separated from the tube furnace heating zone but connected via a carrier gas path. The precursor heating zone was heated to 115°C at a heating rate of 10°C / min. The carrier gas path was opened, with a nitrogen to argon volume ratio of 1:3 (nitrogen flow rate 22 sccm). The sublimation vapors of ammonia borane and sulfur, along with their pyrolysis products, entered the tube furnace with the carrier gas and were fully pyrolyzed in the central heating zone of the tube furnace to release B, N, and S atoms, maintaining a furnace pressure of 1000 mtorr. Subsequently, a crosslinking reaction occurs on a sapphire substrate to achieve the epitaxy of an hBN:S thin film.

[0098] Step 1.4: Cooling to obtain the hBN:S thin film on the sapphire substrate. After growth, the carrier gas path is closed, and the flow rate of the gas flowing into the tube furnace is reduced to 10 sccm N2:30 sccm Ar. The central heating area of ​​the tube furnace is cooled to room temperature at a rate of 4℃ / min to achieve epitaxial growth of a large-area, highly crystalline sulfur-doped boron nitride thin film.

[0099] 2) Magnesium-doped p-type boron nitride thin films, i.e., hBN:Mg films, are grown on copper substrates using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0100] Step 2.1: Thermal Annealing of the Copper Substrate. Before epitaxial growth of hBN thin films on the copper substrate, thermal annealing of the copper is performed to reduce grain boundaries and surface roughness. An LPCVD system is used for thermal annealing. The copper substrate is placed in the central heating zone of a tube furnace, with argon gas used as the protective gas. 50 sccm of argon gas is introduced, and the furnace pressure is adjusted to 500 mtorr. The copper substrate is heated to 950°C at a heating rate of 10°C / min and held for 30 min. It is then cooled to room temperature at a cooling rate of 10°C / min to obtain a copper substrate with large grain domains and high surface smoothness.

[0101] Step 2.2: Precursor heating for epitaxial growth of hBN:Mg thin films on copper substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.5% pure magnesium nitride powder was used as the Mg source. Both precursors were solid powders at room temperature. 30 mg of ammonia borane powder and 10 mg of magnesium nitride powder were weighed and placed in the precursor heating area and the central heating area of ​​the tube furnace, respectively. The heat-annealed copper substrate was placed in the central heating area of ​​the tube furnace, and heated to 1050°C at a rate of 10°C / min in a mixed atmosphere with an N2 / Ar flow ratio of 1:3 (nitrogen flow rate of 10 sccm). Simultaneously, while the tube furnace temperature was 950°C, the precursor heating area was heated to 115°C at a rate of 10°C / min. The carrier gas path is opened, and a 1:3 mixture of nitrogen and argon is introduced as the carrier gas (nitrogen flow rate 10 sccm). The sublimation vapor of ammonia borane and its pyrolysis products are introduced into the tube furnace along with the carrier gas, maintaining the furnace pressure at 300 mtorr. In the central heating region of the tube furnace, the magnesium nitride powder undergoes complete pyrolysis, providing Mg atoms to the system. The pyrolyzed B and N atoms dissolve into the copper lattice at high temperature. Epitaxial growth of the hBN:Mg thin film is then performed at 1050℃ for 20 min.

[0102] Step 2.3: Cooling to obtain the hBN:Mg thin film on the copper substrate. After growth, the carrier gas path is shut off. The central heating region of the tube furnace is cooled to room temperature at a rate of 2℃ / min. During the cooling process, B and N atoms dissolved into the copper lattice undergo segregation. Mg atoms on the copper substrate surface cross-link with B and N atoms, thereby epitaxially growing the hBN:Mg thin film on the copper substrate.

[0103] 3) Using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology, hBN:S and hBN:Mg films were sequentially transferred onto a polyimide (PI) flexible substrate and then heat-treated to obtain a high-quality flexible boron nitride homogeneous pn junction. The specific process is as follows:

[0104] Step 3.1: Peeling and Transfer of the hBN:S Thin Film on the Sapphire Substrate. A PMMA aqueous solution was spin-coated onto the sapphire / hBN:S thin film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The sapphire / hBN:S / PMMA sample was immersed in a 1 mol / L hydrochloric acid solution for one day, after which the hBN:S / PMMA layer was successfully peeled off. It was transferred to a polyimide (PI) flexible substrate and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S thin film to the PI flexible substrate.

[0105] Step 3.2: Stripping and Transfer of the hBN:Mg Film on a Copper Substrate. A PMMA aqueous solution was spin-coated onto the copper / hBN:Mg film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The copper substrate of the copper / hBN:Mg / PMMA sample was placed face down and suspended on the surface of a 1.5 mol / L ammonium sulfite solution. After approximately 6 h, the copper substrate was completely etched away, leaving the remaining hBN:Mg / PMMA layer suspended on the solution surface. This was transferred to a PI flexible substrate / hBN:S structure and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / hBN:Mg / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S / hBN:Mg heterostructure onto the PI flexible substrate.

[0106] Step 3.3: Interfacial heat treatment of PI / hBN:S / hBN:Mg. The PI flexible substrate selected in this invention can withstand a high temperature of 500℃, meeting the temperature requirements for interfacial heat treatment. The PI / hBN:S / hBN:Mg prepared by transfer is placed in the central heating area of ​​a tube furnace, and 50 sccm of argon gas is introduced while maintaining the furnace pressure at 5 torr. The temperature is increased to 320℃ at a heating rate of 10℃ / min and held for 35 min to improve the interfacial contact quality. Finally, the temperature is reduced to room temperature at a cooling rate of 10℃ / min to obtain a flexible boron nitride homopolymer pn junction with high interfacial contact quality.

[0107] Example 4

[0108] 1) A sulfur-doped n-type boron nitride (hBN:S) thin film was grown on a sapphire substrate using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0109] Step 1.1: Cleaning the sapphire substrate. (The text abruptly ends here, seemingly mid-s2 A sapphire substrate of a certain size was immersed in an aqueous solution of 311 (sulfuric acid, hydrogen peroxide, and water in a volume ratio of 3:1:1) and heated at 130°C for 1 hour to remove residual organic impurities from the surface. Subsequently, the substrate was ultrasonically cleaned with water, ethanol, acetone, and isopropanol solutions for 20 minutes each. The sapphire sample cleaned with isopropanol was dried in an oven.

[0110] Step 1.2: Nitrogen / Argon Atmosphere Heating of Sapphire Substrate. The cleaned sapphire substrate was placed in the central heating zone of a tube furnace. The vacuum level inside the furnace was evacuated to 20 mtorr. Argon was used as the furnace atmosphere purging gas. Argon was introduced into the tube furnace until atmospheric pressure was reached, and then the vacuum level was evacuated back to 20 mtorr. This process was repeated three times to ensure no air remained inside the furnace. Under a mixed atmosphere of nitrogen and argon in a volume ratio of 1:3 (nitrogen flow rate 7 sccm), the sapphire substrate was heated to 1410°C at a heating rate of 10°C / min and held at that temperature for 50 min.

[0111] Step 1.3: Precursor heating for epitaxial growth of hBN:S thin films on sapphire substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.98% pure sulfur powder was used as the S source. Both precursors were solid powders at room temperature. 60 mg of ammonia borane powder and 20 mg of sulfur powder were weighed and thoroughly mixed in a quartz crucible, then placed in the precursor heating zone, which was separated from the tube furnace heating zone but connected via a carrier gas path. The precursor heating zone was heated to 115°C at a heating rate of 10°C / min. The carrier gas path was opened, with a nitrogen to argon volume ratio of 1:3 (nitrogen flow rate 25 sccm). The sublimation vapors of ammonia borane and sulfur, along with their pyrolysis products, entered the tube furnace with the carrier gas and were fully pyrolyzed in the central heating zone of the tube furnace to release B, N, and S atoms, maintaining a furnace pressure of 900 mtorr. Subsequently, a crosslinking reaction occurs on a sapphire substrate to achieve the epitaxy of an hBN:S thin film.

[0112] Step 1.4: Cooling to obtain the hBN:S thin film on the sapphire substrate. After growth, the carrier gas path is closed, and the flow rate of the gas flowing into the tube furnace is reduced to 10 sccm N2:30 sccm Ar. The central heating area of ​​the tube furnace is cooled to room temperature at a rate of 3℃ / min, achieving epitaxial growth of a large-area, highly crystalline sulfur-doped boron nitride thin film.

[0113] 2) Magnesium-doped p-type boron nitride thin films, i.e., hBN:Mg films, are grown on copper substrates using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0114] Step 2.1: Thermal Annealing of the Copper Substrate. Before epitaxial growth of hBN thin films on the copper substrate, thermal annealing of the copper is performed to reduce grain boundaries and surface roughness. An LPCVD system is used for thermal annealing. The copper substrate is placed in the central heating zone of a tube furnace, with argon gas used as the protective gas. 50 sccm of argon gas is introduced, and the furnace pressure is adjusted to 500 mtorr. The copper substrate is heated to 950°C at a heating rate of 10°C / min and held for 30 min. It is then cooled to room temperature at a cooling rate of 10°C / min to obtain a copper substrate with large grain domains and high surface smoothness.

[0115] Step 2.2: Precursor heating for epitaxial growth of hBN:Mg thin films on copper substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.5% pure magnesium nitride powder was used as the Mg source. Both precursors were solid powders at room temperature. 30 mg of ammonia borane powder and 10 mg of magnesium nitride powder were weighed and placed in the precursor heating area and the central heating area of ​​the tube furnace, respectively. The heat-annealed copper substrate was placed in the central heating area of ​​the tube furnace, and heated to 1050°C at a rate of 10°C / min in a mixed atmosphere with an N2 / Ar flow ratio of 1:3 (nitrogen flow rate of 10 sccm). Simultaneously, while the tube furnace temperature was 950°C, the precursor heating area was heated to 115°C at a rate of 10°C / min. The carrier gas path is opened, and a 1:3 mixture of nitrogen and argon is introduced as the carrier gas (nitrogen flow rate 6 sccm). The sublimation vapor of ammonia borane and its pyrolysis products are introduced into the tube furnace along with the carrier gas, and the furnace pressure is maintained at 300 mtorr. In the central heating region of the tube furnace, the magnesium nitride powder undergoes complete pyrolysis, providing Mg atoms to the system. The pyrolyzed B and N atoms dissolve into the copper lattice at high temperature. Epitaxial growth of the hBN:Mg thin film is then performed at 1050℃ for 20 min.

[0116] Step 2.3: Cooling to obtain the hBN:Mg thin film on the copper substrate. After growth, the carrier gas path is shut off. The central heating region of the tube furnace is cooled to room temperature at a rate of 2℃ / min. During the cooling process, B and N atoms dissolved into the copper lattice undergo segregation. Mg atoms on the copper substrate surface cross-link with B and N atoms, thereby epitaxially growing the hBN:Mg thin film on the copper substrate.

[0117] 3) Using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology, hBN:S and hBN:Mg films were sequentially transferred onto a polyimide (PI) flexible substrate and then heat-treated to obtain a high-quality flexible boron nitride homogeneous pn junction. The specific process is as follows:

[0118] Step 3.1: Peeling and Transfer of the hBN:S Thin Film on the Sapphire Substrate. A PMMA aqueous solution was spin-coated onto the sapphire / hBN:S thin film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The sapphire / hBN:S / PMMA sample was immersed in a 1 mol / L hydrochloric acid solution for one day, after which the hBN:S / PMMA layer was successfully peeled off. It was transferred to a polyimide (PI) flexible substrate and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S thin film to the PI flexible substrate.

[0119] Step 3.2: Stripping and Transfer of the hBN:Mg Film on a Copper Substrate. A PMMA aqueous solution was spin-coated onto the copper / hBN:Mg film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The copper substrate of the copper / hBN:Mg / PMMA sample was placed face down and suspended on the surface of a 1.5 mol / L ammonium sulfite solution. After approximately 6 h, the copper substrate was completely etched away, leaving the remaining hBN:Mg / PMMA layer suspended on the solution surface. This was transferred to a PI flexible substrate / hBN:S structure and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / hBN:Mg / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S / hBN:Mg heterostructure onto the PI flexible substrate.

[0120] Step 3.3: Interfacial heat treatment of PI / hBN:S / hBN:Mg. The PI flexible substrate selected in this invention can withstand a high temperature of 500℃, meeting the temperature requirements for interfacial heat treatment. The PI / hBN:S / hBN:Mg prepared by transfer is placed in the central heating area of ​​a tube furnace, and 50 sccm of argon gas is introduced while maintaining the furnace pressure at 5 torr. The temperature is increased to 300℃ at a heating rate of 10℃ / min and held for 40 min to improve the interfacial contact quality. Finally, the temperature is reduced to room temperature at a cooling rate of 10℃ / min to obtain a flexible boron nitride homopolymer pn junction with high interfacial contact quality.

[0121] Example 5

[0122] 1) A sulfur-doped n-type boron nitride (hBN:S) thin film was grown on a sapphire substrate using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0123] Step 1.1: Cleaning the sapphire substrate. (The text abruptly ends here, seemingly mid-s2 A sapphire substrate of a certain size was immersed in an aqueous solution of 311 (sulfuric acid, hydrogen peroxide, and water in a volume ratio of 3:1:1) and heated at 130°C for 1 hour to remove residual organic impurities from the surface. Subsequently, the substrate was ultrasonically cleaned with water, ethanol, acetone, and isopropanol solutions for 20 minutes each. The sapphire sample cleaned with isopropanol was dried in an oven.

[0124] Step 1.2: Heating the sapphire substrate in a nitrogen / argon atmosphere. Place the cleaned sapphire substrate in the central heating zone of a tube furnace. Evacuate the furnace to 20 mtorr. Use argon as the furnace atmosphere cleaning gas, introducing argon into the furnace until atmospheric pressure is reached. Then, evacuate the furnace to 20 mtorr again, repeating this process three times to ensure no air remains inside the furnace. In a mixed atmosphere of nitrogen and argon at a volume ratio of 1:3 (nitrogen flow rate 9 sccm), heat the sapphire substrate to 1450°C at a heating rate of 10°C / min and hold at that temperature for 30 min.

[0125] Step 1.3: Precursor heating for epitaxial growth of hBN:S thin films on sapphire substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.98% pure sulfur powder was used as the S source. Both precursors were solid powders at room temperature. 60 mg of ammonia borane powder and 20 mg of sulfur powder were weighed and thoroughly mixed in a quartz crucible, then placed in the precursor heating zone, which was separated from the tube furnace heating zone but connected via a carrier gas path. The precursor heating zone was heated to 115°C at a heating rate of 10°C / min. The carrier gas path was opened, with a nitrogen to argon volume ratio of 1:3 (nitrogen flow rate 20 sccm). The sublimation vapors of ammonia borane and sulfur, along with their pyrolysis products, entered the tube furnace with the carrier gas and were fully pyrolyzed in the central heating zone of the tube furnace to release B, N, and S atoms, maintaining the furnace pressure at 950 mtorr. Subsequently, a crosslinking reaction occurs on a sapphire substrate to achieve the epitaxy of an hBN:S thin film.

[0126] Step 1.4: Cooling to obtain the hBN:S thin film on the sapphire substrate. After growth, the carrier gas path is closed, and the flow rate of the gas flowing into the tube furnace is reduced to 10 sccm N2:30 sccm Ar. The central heating area of ​​the tube furnace is cooled to room temperature at a rate of 5℃ / min to achieve epitaxial growth of a large-area, highly crystalline sulfur-doped boron nitride thin film.

[0127] 2) Magnesium-doped p-type boron nitride thin films, i.e., hBN:Mg films, are grown on copper substrates using low-pressure chemical vapor deposition (LPCVD). The specific process is as follows:

[0128] Step 2.1: Thermal Annealing of the Copper Substrate. Before epitaxial growth of hBN thin films on the copper substrate, thermal annealing of the copper is performed to reduce grain boundaries and surface roughness. An LPCVD system is used for thermal annealing. The copper substrate is placed in the central heating zone of a tube furnace, with argon gas used as the protective gas. 50 sccm of argon gas is introduced, and the furnace pressure is adjusted to 500 mtorr. The copper substrate is heated to 950°C at a heating rate of 10°C / min and held for 30 min. It is then cooled to room temperature at a cooling rate of 10°C / min to obtain a copper substrate with large grain domains and high surface smoothness.

[0129] Step 2.2: Precursor heating for epitaxial growth of hBN:Mg thin films on copper substrates. 95% pure ammonia borane (BH3NH3) was used as the B / N source, and 99.5% pure magnesium nitride powder was used as the Mg source. Both precursors were solid powders at room temperature. 30 mg of ammonia borane powder and 10 mg of magnesium nitride powder were weighed and placed in the precursor heating area and the central heating area of ​​the tube furnace, respectively. The heat-annealed copper substrate was placed in the central heating area of ​​the tube furnace, and heated to 1050°C at a rate of 10°C / min in a mixed atmosphere with an N2 / Ar flow ratio of 1:3 (nitrogen flow rate of 10 sccm). Simultaneously, while the tube furnace temperature was 950°C, the precursor heating area was heated to 115°C at a rate of 10°C / min. The carrier gas path is opened, and a 1:3 mixture of nitrogen and argon is introduced as the carrier gas (nitrogen flow rate 7 sccm). The sublimation vapor of ammonia borane and its pyrolysis products are introduced into the tube furnace along with the carrier gas, maintaining the furnace pressure at 300 mtorr. In the central heating region of the tube furnace, the magnesium nitride powder undergoes complete pyrolysis, providing Mg atoms to the system. The pyrolyzed B and N atoms dissolve into the copper lattice at high temperature. Epitaxial growth of the hBN:Mg thin film is then performed at 1050℃ for 20 min.

[0130] Step 2.3: Cooling to obtain the hBN:Mg thin film on the copper substrate. After growth, the carrier gas path is shut off. The central heating region of the tube furnace is cooled to room temperature at a rate of 2℃ / min. During the cooling process, B and N atoms dissolved into the copper lattice undergo segregation. Mg atoms on the copper substrate surface cross-link with B and N atoms, thereby epitaxially growing the hBN:Mg thin film on the copper substrate.

[0131] 3) Using polymethyl methacrylate (PMMA) assisted liquid-phase exfoliation technology, hBN:S and hBN:Mg films were sequentially transferred onto a polyimide (PI) flexible substrate and then heat-treated to obtain a high-quality flexible boron nitride homogeneous pn junction. The specific process is as follows:

[0132] Step 3.1: Peeling and Transfer of the hBN:S Thin Film on the Sapphire Substrate. A PMMA aqueous solution was spin-coated onto the sapphire / hBN:S thin film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The sapphire / hBN:S / PMMA sample was immersed in a 1 mol / L hydrochloric acid solution for one day, after which the hBN:S / PMMA layer was successfully peeled off. It was transferred to a polyimide (PI) flexible substrate and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S thin film to the PI flexible substrate.

[0133] Step 3.2: Stripping and Transfer of the hBN:Mg Film on a Copper Substrate. A PMMA aqueous solution was spin-coated onto the copper / hBN:Mg film using a spin-coating process. The spin-coating speed was 500 rpm for 10 s, followed by 3000 rpm for 30 s. After spin-coating, the PMMA film was dried and cured on an 80°C hot plate for 20 min. The copper substrate of the copper / hBN:Mg / PMMA sample was placed face down and suspended on the surface of a 1.5 mol / L ammonium sulfite solution. After approximately 6 h, the copper substrate was completely etched away, leaving the remaining hBN:Mg / PMMA layer suspended on the solution surface. This was transferred to a PI flexible substrate / hBN:S structure and dried on an 80°C hot plate for 20 min to remove residual solution. The PI / hBN:S / hBN:Mg / PMMA sample was then immersed in acetone solution for 10 min to remove the PMMA organic layer, successfully transferring the hBN:S / hBN:Mg heterostructure onto the PI flexible substrate.

[0134] Step 3.3: Interfacial heat treatment of PI / hBN:S / hBN:Mg. The PI flexible substrate selected in this invention can withstand a high temperature of 500℃, meeting the temperature requirements for interfacial heat treatment. The PI / hBN:S / hBN:Mg prepared by transfer is placed in the central heating area of ​​a tube furnace, and 50 sccm of argon gas is introduced while maintaining the furnace pressure at 5 torr. The temperature is increased to 350℃ at a heating rate of 10℃ / min and held for 30 min to improve the interfacial contact quality. Finally, the temperature is reduced to room temperature at a cooling rate of 10℃ / min to obtain a flexible boron nitride homopolymer pn junction with high interfacial contact quality.

[0135] Finally, it should be stated that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a flexible boron nitride homogeneous pn junction, characterized in that, Includes the following steps: Sulfur-doped n-type boron nitride thin films were grown on sapphire substrates using low-pressure chemical vapor deposition. Magnesium-doped p-type boron nitride thin films were grown on copper substrates using low-pressure chemical vapor deposition. Using polymethyl methacrylate-assisted liquid-phase exfoliation technology, sulfur-doped n-type boron nitride thin films and magnesium-doped p-type boron nitride thin films were transferred onto a flexible substrate and then heat-treated in an argon atmosphere to obtain a flexible boron nitride homogeneous pn junction. Sulfur-doped n-type boron nitride thin films are prepared by the following process: Under a nitrogen and argon atmosphere, the sapphire substrate in the container is heated to 1400-1450℃, and the B / N source and S source are introduced into the container. The temperature is maintained for 30-60 minutes at a pressure of 800mtorr-1000mtorr, and then cooled to form a sulfur-doped n-type boron nitride thin film on the sapphire substrate. Magnesium-doped p-type boron nitride thin films are prepared by the following process: Under a nitrogen and argon atmosphere, the copper substrate in the container is heated to 1000-1050℃, and B / N source and Mg source are introduced into the container. The temperature is maintained at 300-500 mtorr for 15-20 min, and then cooled to form a magnesium-doped boron nitride thin film on the copper substrate. Sulfur-doped n-type boron nitride films and magnesium-doped p-type boron nitride films are transferred onto a flexible substrate using the following process: PMMA aqueous solution was spin-coated onto a sulfur-doped n-type boron nitride film and cured by heating. Then, the sulfur-doped n-type boron nitride film was immersed in hydrochloric acid aqueous solution, peeled off from the sapphire substrate, transferred to a flexible substrate, and dried to obtain an hBN:S / PI structure. A PMMA aqueous solution was spin-coated onto a magnesium-doped p-type boron nitride film and cured by heating. The magnesium-doped p-type boron nitride film was then immersed in an ammonium sulfite solution. The magnesium-doped p-type boron nitride film was peeled off from the copper substrate and transferred to an hBN:S / PI structure. After drying, the hBN:Mg / hBN:S / PI structure was obtained.

2. The method for preparing a flexible boron nitride homogeneous pn junction according to claim 1, characterized in that, Ammonia borane is heated to 115°C to form sublimation vapor and pyrolysis products of ammonia borane, which serve as the B / N source; sulfur powder is heated to 115°C to form sublimation vapor of sulfur, which serves as the S source.

3. The method for preparing a flexible boron nitride homogeneous pn junction according to claim 1, characterized in that, The cooling rate during the preparation of sulfur-doped n-type boron nitride thin films is 3-5 °C / min.

4. The method for preparing a flexible boron nitride homogeneous pn junction according to claim 1, characterized in that, When preparing sulfur-doped n-type boron nitride thin films, the volume ratio of nitrogen to argon is 1:3 during heating to 1400-1450℃ and during cooling, and the flow rate of nitrogen is 5-10 sccm. During heat preservation, the volume ratio of nitrogen to argon is 1:3, and the flow rate of nitrogen is 20-30 sccm.

5. The method for preparing a flexible boron nitride homogeneous pn junction according to claim 1, characterized in that, In the preparation of magnesium-doped boron nitride thin films, ammonia borane is heated to 115℃ to form sublimation vapor and pyrolysis products of ammonia borane, which serve as the B / N source. Magnesium nitride powder is heated to 1000-1050℃ to form sublimation vapor and pyrolysis products of magnesium nitride, which serve as the Mg source. The cooling rate is 2℃ / min. The volume ratio of nitrogen to argon is 1:3, and the flow rate of nitrogen is 5-10 sccm.

6. The method for preparing a flexible boron nitride homogeneous pn junction according to claim 1, characterized in that, The heat treatment temperature is 300-350℃ and the time is 30-40 minutes.

7. A flexible boron nitride homogeneous pn junction, prepared according to the method for preparing a flexible boron nitride homogeneous pn junction according to any one of claims 1-6.

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