Hexagonal boron nitride thin films, their preparation methods, bandgap modulation methods, and broadband ultraviolet photodetectors

Abstract

This invention relates to the preparation of wide bandgap semiconductor materials using chemical vapor deposition (CVD) technology. It provides hexagonal boron nitride (h-BN) thin films, their preparation methods, bandgap modulation methods, and a broadband ultraviolet photodetector. Using magnesium nitride as the dopant source, the dopant source is simultaneously introduced into the epitaxial layer of the h-BN thin film via CVD, achieving in-situ doping of magnesium in the h-BN lattice. By controlling the magnesium doping concentration, the bandgap of the h-BN thin film is modulated from 5.91 eV to 3.38 eV. Based on this magnesium-doped h-BN thin film, a broadband ultraviolet photodetector is also prepared, broadening the ultraviolet absorption range to the 185-365 nm band, achieving wide-spectrum, high-sensitivity ultraviolet detection. The doping process of this invention is controllable, and the bandgap modulation effect is significant, breaking through the spectral response limitations of intrinsic h-BN-based detectors and promoting the application of optoelectronic devices in environmental monitoring, biochemical analysis, and other fields.

Description

Technical Field

[0001] This invention relates to the preparation of wide bandgap semiconductor materials using chemical vapor deposition (CVD) technology, and particularly to a method for preparing hexagonal boron nitride thin films and bandgap modulation, as well as their application in the preparation of broadband ultraviolet photodetectors. Background Technology

[0002] Hexagonal boron nitride (h-BN) is considered an ideal deep ultraviolet (wavelength <280nm) optoelectronic material and insulating substrate material for high-power electronic devices due to its excellent chemical stability, high temperature stability, high thermal conductivity, and ultrawide bandgap (~6.0 eV). However, h-BN is an ultrawide bandgap semiconductor with a bandgap of approximately 6 eV and an intrinsic response wavelength of approximately 215 nm, which prevents it from responding to other wavelengths. Its excessively wide bandgap also limits its effective absorption of ultraviolet light with wavelengths greater than 220 nm. As a result, photodetectors based on intrinsic h-BN can typically only operate in a very narrow deep ultraviolet band and cannot respond to a wider ultraviolet spectrum (such as UVA: 315-400 nm, UVB: 280-315 nm). Therefore, boron nitride cannot currently be directly used in the fabrication of broadband optoelectronic devices, which limits its application scope in fields such as environmental monitoring, biochemical analysis, and flame early warning.

[0003] To control the band structure of h-BN, researchers have explored various methods, including elemental doping (such as heteroatoms like carbon, silicon, and oxygen), introducing defects, and applying strain. Elemental doping is the most direct approach. However, dopant atoms in wide and ultra-wide bandgap semiconductors have high activation energies, and the unintentionally introduced background defect concentration in the material itself exhibits a strong self-compensation effect. These factors often make it difficult for existing doping techniques to effectively reduce the bandgap. Furthermore, maintaining good crystal quality while effectively doping is also crucial. For example, carbon doping may introduce deep energy levels into the bandgap, leading to increased nonradiative recombination and deterioration of photoelectric properties. Doping typically only produces limited photoresponse, cannot effectively adjust the bandgap, results in high dark current in the thin film, and significantly affects the lattice structure. Achieving controllable and synchronous introduction of dopants during epitaxial growth to obtain thin films with stable photoelectric properties remains a significant technical challenge.

[0004] In addition, traditional mechanical exfoliation methods for preparing h-BN have inherent limitations such as low yield and uncontrollable layer number. Although chemical vapor deposition (CVD) can synthesize highly crystalline, large-area h-BN films on metal substrates, this method has stringent requirements for substrate quality, and the grown films are tightly attached to the metal substrate. Subsequent characterization and device fabrication require transfer, which can lead to contamination and mechanical damage. Dielectric substrates are ideal substrate choices for optoelectronic devices due to their excellent insulation and thermal stability, but the direct preparation of h-BN films on them still faces core challenges such as low nucleation density, limited growth kinetics, and defects induced by lattice mismatch.

[0005] Therefore, developing a doping method and new materials that can effectively and controllably adjust the bandgap of h-BN thin films and are suitable for preparing broadband ultraviolet photodetectors is of great scientific significance and application value. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing a hexagonal boron nitride thin film, a bandgap modulation method, and a high-performance broadband ultraviolet photodetector based on the thin film.

[0007] To achieve the above-mentioned objectives, the first aspect of this invention provides a method for preparing hexagonal boron nitride thin films, which employs chemical vapor deposition (CVD) technology to simultaneously introduce magnesium doping sources, particularly magnesium nitride (Mg3N2), during the growth process, thereby achieving in-situ doping of magnesium elements in the hexagonal boron nitride lattice to the degree of alloying.

[0008] The present invention provides a method for preparing a hexagonal boron nitride thin film, comprising the following steps:

[0009] A certain amount of boron-containing precursor was used as the boron source and placed together with a sapphire substrate loaded with graphene oxide film in the second temperature zone of the chemical vapor deposition reaction chamber. A certain amount of magnesium-containing dopant source was weighed and placed in the upstream first temperature zone of the chemical vapor deposition reaction chamber as the dopant source, with a mass ratio of boron-containing precursor to magnesium-containing dopant source of 8:1. Subsequently, the reaction chamber was evacuated. When the growth temperature of the second temperature zone rose to greater than 800℃ but less than 1100℃, a mixed gas of nitrogen-containing precursor and carrier gas with a flow rate ratio of 0.5-1.5:1 was introduced into the reaction chamber, and the total flow rate of the mixed gas was controlled at 300 sccm. After the second temperature zone was kept at a constant temperature for a certain period of time to allow the graphene oxide to completely disappear, the first temperature zone was heated to 800-900℃ to promote the decomposition of the magnesium-containing dopant source, which was then carried to the second temperature zone by the carrier gas. The reaction conditions were then maintained and the reaction continued for a certain period of time. After the growth was completed, the mixture was naturally cooled to room temperature to obtain a light gray, semi-transparent magnesium-doped hexagonal boron nitride film attached to the sapphire substrate.

[0010] Preferably, the sapphire substrate loaded with the graphene oxide film shown is prepared by the following method:

[0011] (1) First, a certain amount of graphene oxide powder was added to deionized water and ultrasonically treated to make it completely dispersed; then the supernatant was collected by centrifugation; the supernatant was ultrasonically treated again and the supernatant was collected by centrifugation a second time; finally, the supernatant was ultrasonically treated and allowed to stand to obtain a stable aqueous solution of graphene oxide.

[0012] (2) Take a certain amount of graphene oxide aqueous solution and add it to a glass dish. Slowly stack a certain amount of pentane along the bottle wall to form a clear pentane-water two-phase interface. Then inject a certain amount of anhydrous ethanol, cover the dish and let it stand at room temperature to allow the graphene oxide sheets to fully self-assemble at the liquid-liquid interface. After standing, a uniformly floating graphene oxide film gradually forms at the interface. After the cleaned sapphire substrate is gently touched to the floating graphene oxide film at the interface to complete the transfer, the pentane and ethanol are allowed to evaporate naturally. Finally, a clean graphene oxide film with no solvent residue is obtained.

[0013] Preferably, the boron-containing precursor is at least one of borane, boron oxide, boron halide, or borazane; the magnesium-containing dopant source is magnesium nitride powder; and the nitrogen-containing precursor is at least one of ammonia or nitrogen.

[0014] As a preferred option, the growth temperature in the second temperature zone is 1000℃.

[0015] Preferably, the total film growth time is not less than 90 minutes.

[0016] A second aspect of the present invention provides a magnesium-doped hexagonal boron nitride thin film prepared by the above method, wherein the band gap of the thin film is significantly reduced compared to intrinsic hexagonal boron nitride, decreasing from 5.91 eV to 3.38 eV.

[0017] A third aspect of this invention provides a method for bandgap modulation of a hexagonal boron nitride thin film, comprising the following steps:

[0018] A certain amount of boron-containing precursor was used as a boron source and placed together with a sapphire substrate loaded with graphene oxide film in the second temperature zone of the chemical vapor deposition reaction chamber. A certain amount of magnesium-containing dopant source was weighed and placed in the upstream first temperature zone of the chemical vapor deposition reaction chamber as a dopant source, with a mass ratio of boron-containing precursor to magnesium-containing dopant source of 8:0.2-1. Subsequently, the reaction chamber was evacuated. When the growth temperature in the second temperature zone rose to greater than 800 but less than 1100°C, a flow rate of 0.5-1.5 was introduced into the reaction chamber. A mixture of nitrogen-containing precursor and carrier gas was prepared, with the total flow rate of the mixture controlled at 300 sccm. After maintaining constant temperature growth in the second temperature zone for a certain period of time to allow the graphene oxide to completely disappear, the temperature of the first temperature zone was increased to 800-900℃ to promote the decomposition of the magnesium-containing dopant source, which was then carried to the second temperature zone by the carrier gas. The reaction conditions were then maintained to continue the reaction and growth for a certain period of time. After the growth was completed, the mixture was naturally cooled to room temperature to obtain a magnesium-doped hexagonal boron nitride thin film with an optical bandgap tuned between 5.02 eV and 3.38 eV.

[0019] The preparation method of the sapphire substrate loaded with graphene oxide film is the same as the preparation method of the sapphire substrate loaded with graphene oxide film in the above-mentioned preparation method of hexagonal boron nitride film.

[0020] Preferably, the boron-containing precursor is at least one of borane, boron oxide, boron halide, or borazane; the magnesium-containing dopant source is magnesium nitride powder; and the nitrogen-containing precursor is at least one of ammonia or nitrogen.

[0021] As a preferred option, the growth temperature in the second temperature zone is 1000℃.

[0022] Preferably, the mass ratio of the boron-containing precursor to the magnesium-containing dopant source is any one of 8:0.2, 8:0.4, 8:0.6, 8:0.8, or 8:1, and the corresponding optical band gaps of the magnesium-doped hexagonal boron nitride thin films are 5.02 eV, 4.83 eV, 4.48 eV, 4.10 eV, 3.90 eV, and 3.38 eV.

[0023] The magnesium-doped hexagonal boron nitride thin film prepared by this invention can be used to prepare broadband ultraviolet photodetectors.

[0024] A fourth aspect of the present invention provides a broadband ultraviolet photodetector based on the above-mentioned magnesium-doped hexagonal boron nitride thin film, wherein the above-mentioned magnesium-doped hexagonal boron nitride thin film is used as a light absorption layer; a first electrode and a second electrode are disposed on the light absorption layer, and a gap is provided between the first electrode and the second electrode to expose a portion of the light absorption layer for receiving light.

[0025] The broadband ultraviolet photodetector has a working spectral response range of 185 nm to 365 nm, covering the deep ultraviolet to mid-ultraviolet bands.

[0026] The beneficial effects of this invention include:

[0027] This invention selects graphene oxide (GO) as an interfacial self-assembly template. Its surface is rich in defects (ID / IG=1.6) and possesses hydrophilicity (contact angle 43.5°). The defect sites provide ample nucleation active centers, while the hydrophilicity ensures uniform precursor spreading, synergistically solving the problems of low nucleation density and lattice mismatch of h-BN on non-metallic substrates. A combined process of "ultrasonic dispersion - two-step centrifugation - ethanol-assisted liquid - liquid-interfacial self-assembly" is employed, with multi-step purification and assembly to form a dense GO film. The process control is highly precise, and the template purity and uniformity are superior to traditional methods. The optimal growth conditions (temperature 1000℃, NH3:Ar=1.5:1) are determined using the controlled variable method, achieving large-area continuous, high-crystallinity h-BN film growth. The E2g peak of h-BN in the Raman spectrum has the narrowest full width at half maximum (FWHM), indicating optimal crystallinity. Furthermore, the GO template serves only as a nucleation auxiliary medium and is completely etched away in situ during h-BN growth, leaving no residual carbon contamination and ensuring the high purity of the final h-BN film. The APCVD method is used to grow sapphire substrates directly without the need for metal catalysts and subsequent thin film transfer steps, avoiding damage and contamination during the transfer process. The process is simple and suitable for the direct fabrication of optoelectronic devices.

[0028] This invention employs an in-situ stepwise doping strategy: the boron source, substrate, and Mg3N2 doping source are controlled independently in separate areas. The initial h-BN film is grown first (GO is completely decomposed) before doping is started, so as to achieve uniform and stable doping of Mg atoms and avoid the problem of insufficient uniformity caused by simultaneous doping and growth.

[0029] This invention utilizes Mg3N2 as a doping source and, by adjusting the proportion of Mg3N2, successfully reduced the bandgap of h-BN from 5.91 eV to approximately 3.38 eV, laying the foundation for the application of h-BN in a wider spectral range. This method has good process compatibility, and continuous and precise control of doping concentration and film bandgap can be achieved by adjusting the precursor ratio.

[0030] This invention is based on a photodetector fabricated from a doped thin film, which breaks through the limitation of intrinsic h-BN detectors that only respond to deep ultraviolet light. It achieves wide-spectrum, high-sensitivity detection of ultraviolet light from 185nm to 365nm, maintains low dark current (around 7pA) with low doping amount, and balances response range and insulation characteristics. It is suitable for ultraviolet photodetection applications and greatly expands the application scenarios of h-BN-based optoelectronic devices. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the apparatus for preparing magnesium-doped h-BN thin films in an embodiment of the present invention. In the figure, 1 is Mg3N2, 2 is B2O3, 3 is a sapphire substrate loaded with a GO thin film, and 4 is an Ar / NH3 mixed gas.

[0032] Figure 2 The image shows a comparison of the UV-Vis absorption spectra of the magnesium-doped h-BN film prepared in Example 1 of this invention and the intrinsic h-BN film, as well as the band gap diagram calculated by the Taucplot method.

[0033] Figure 3 The transmittance curve of the magnesium-doped h-BN thin film prepared in Example 1 of this invention in the visible light band is shown.

[0034] Figure 4 This is a schematic diagram of the structure of the broadband ultraviolet photodetector prepared in Example 3 of the present invention.

[0035] Figure 5 The photocurrent response spectra of the broadband ultraviolet photodetector prepared in Example 3 of the present invention under ultraviolet light irradiation at different wavelengths are shown. Detailed Implementation

[0036] Example 1

[0037] This embodiment provides a method for preparing a hexagonal boron nitride thin film, comprising the following steps:

[0038] (1) Preparation of graphene oxide (GO) aqueous solution:

[0039] Uniform dispersion and purification of GO were achieved through gentle ultrasonic treatment. First, 60 mg of GO powder was added to 20 mL of deionized water and ultrasonicated for 1 hour to ensure complete dispersion. Then, a floor-standing centrifuge was used to centrifuge at 3000 rpm for 30 minutes to remove a large amount of undispersed precipitate and impurities, and the supernatant was collected. The supernatant was ultrasonicated again for 30 minutes, then transferred to centrifuge tubes and precisely balanced, and centrifuged at 9000 rpm for 30 minutes for further purification, with the supernatant collected a second time. Finally, the purified supernatant was ultrasonicated for 3 hours and allowed to stand for 24 hours. A stable GO aqueous solution was successfully prepared using this two-step centrifugation method.

[0040] (2) Preparation of GO thin films by interfacial self-assembly:

[0041] GO films were prepared using an ethanol-assisted liquid-liquid interface self-assembly method. 10 mL of GO aqueous solution was added to a 4 cm diameter glass dish, and 5 mL of pentane (analytical grade, 99.5%) was slowly stacked along the dish wall to form a clear pentane-water two-phase interface. Then, 5 mL of anhydrous ethanol (analytical grade, 99.9%) was rapidly injected, the dish was capped, and the mixture was allowed to stand at room temperature for 30 min to allow the GO sheets to fully self-assemble at the liquid-liquid interface. After standing, a uniformly floating GO film gradually formed at the interface. The cleaned sapphire substrate was gently touched to the floating GO film to complete the transfer. Taking advantage of the low boiling points of pentane and anhydrous ethanol, which allow for complete evaporation under ventilation, the sample was placed in a fume hood (maintaining stable airflow and avoiding dust contamination) to allow the pentane to evaporate naturally for 24 h, thoroughly removing residual ethanol and pentane. Finally, a clean GO film with no solvent residue was obtained.

[0042] (3) Epitaxial growth of h-BN thin films:

[0043] Magnesium-doped h-BN films were epitaxially grown on sapphire substrates using atmospheric pressure chemical vapor deposition (APCVD). Figure 1 As shown, 400 mg of B2O3 powder was placed in the corundum boat as a boron source and together with the sapphire substrate loaded with GO film was placed in the second temperature zone (i.e., the film growth temperature zone) of the tubular CVD reactor; 50 mg of Mg3N2 powder was weighed and placed in the corundum boat as a dopant source and placed in the upstream first temperature zone (i.e., the dedicated heating zone for the dopant source), so as to realize the independent control of the boron source, substrate and dopant source, and provide conditions for subsequent stepwise reaction. The reaction system was then started. After the system was evacuated, when the second temperature zone reached a growth temperature of 1000℃, a mixture of ammonia (NH3) and argon (Ar) with a flow ratio of 1.5:1 was introduced into the reaction chamber as a reaction atmosphere and protective medium. Ammonia (NH3) was used as the reactant gas, and argon (Ar) was used as the carrier gas. The total flow rate of the mixed gas was controlled at 300 sccm. The second temperature zone was kept at a constant temperature of 1000℃ for 30 minutes. After the GO completely disappeared, the first temperature zone was heated to 900℃ to induce the controlled thermal decomposition of Mg3N2 powder to generate Mg atoms and N2, which were carried to the high-temperature zone by the Ar carrier gas. The reaction conditions were then maintained and the reaction continued for another 60 minutes, making the total film growth time 90 minutes. After the growth was completed, the precursor was shut off and the film was naturally cooled to room temperature in an argon atmosphere to obtain a light gray, semi-transparent magnesium-doped h-BN film attached to a sapphire substrate.

[0044] The obtained films were subjected to UV-Vis absorption spectroscopy. The absorption edge of the intrinsic h-BN film was located at approximately 210 nm (corresponding to ~5.91 eV), while the absorption edge of the magnesium-doped h-BN film prepared in this invention was red-shifted to approximately 365 nm (corresponding to ~3.38 eV), demonstrating a significant reduction in its band gap. X-ray photoelectron spectroscopy detected characteristic peaks of N-Mg bonds, confirming that Mg does not exist in a free state but is incorporated into the crystal lattice through chemical bonding. Secondary ion mass spectrometry results eliminated surface adsorption interference, showing that Mg is uniformly distributed in the bulk of the film. The two results corroborate each other, verifying the effective doping and uniform distribution of Mg.

[0045] The obtained h-BN epitaxial films were comprehensively characterized using Raman spectroscopy, infrared spectroscopy, and TEM. Only the E2g vibrational mode of h-BN was observed in the Raman spectrum, and no characteristic peak of GO was detected, proving that GO was completely etched away during film growth, leaving only a pure-phase h-BN film on the substrate. The E2g peak of h-BN had the narrowest full width at half maximum (FWHM) at 1000℃, reflecting optimal crystal quality. However, when the temperature was further increased to 1100℃, the FWHM of the E2g peak broadened significantly, indicating that excessively high temperatures led to lattice distortion or increased defects in the h-BN crystal structure, resulting in severe surface wrinkling and decreased structural integrity.

[0046] Example 2

[0047] This embodiment provides a method for bandgap modulation of a hexagonal boron nitride thin film, comprising the following steps:

[0048] (1) Preparation of graphene oxide (GO) aqueous solution:

[0049] Same as in Example 1, and will not be repeated here;

[0050] (2) Preparation of GO thin films by interfacial self-assembly:

[0051] Same as in Example 1, and will not be repeated here;

[0052] (3) Epitaxial growth of h-BN thin films:

[0053] 400 mg of B₂O₃ powder was placed in an alumina boat as a boron source, and together with a sapphire substrate loaded with a GO thin film, it was placed in the second temperature zone of a tubular CVD furnace. 10-50 mg of Mg₃N₂ powder of varying masses was placed in the alumina boat as a dopant source, positioned in the upstream first temperature zone. The different masses of Mg₃N₂ powder allowed the optical bandgap of the h-BN thin film to be tuned between 5.02 eV and 3.38 eV. The reaction system was started, and after evacuation, when the second temperature zone reached the growth temperature of 1000 °C, a mixture of ammonia (NH₃) and argon (Ar) at a flow rate ratio of 1.5:1 was introduced into the reaction chamber as the reaction atmosphere and protective medium. Ammonia (NH3) was used as the reactant gas, and argon (Ar) was used as the carrier gas. The total flow rate of the mixed gas was controlled at 300 sccm. The second temperature zone was kept at 1000℃ for 30 min. After the GO completely disappeared, the first temperature zone was raised to 900℃ to induce the controlled thermal decomposition of Mg3N2 powder to generate Mg atoms and N2, which were carried to the high-temperature zone by the Ar carrier gas. The reaction conditions were then maintained and the reaction continued for 60 min, making the total film growth time 90 min. After the growth was completed, the precursor was turned off and the film was naturally cooled to room temperature in an argon atmosphere to obtain light gray semi-transparent magnesium-doped h-BN films with different energy bands attached to the sapphire substrate.

[0054] The mass of Mg3N2 powder can be 10mg, 15mg, 20mg, 25mg, 30mg, 35mg, 40mg, 45mg, or 50mg.

[0055] The obtained thin film was subjected to ultraviolet-visible absorption spectroscopy, such as... Figure 2 As shown. The visible light transmission spectrum of the test film, as shown. Figure 3 As shown, the intrinsic h-BN has an average transmittance of over 99% in the visible light region (380-780nm), while the transmittance of the magnesium-doped h-BN film in the 380-780nm visible light band remains above 70%, indicating that the film still has good transmittance in the visible spectrum region. The effective doping threshold of Mg in h-BN:Mg thin films should be controlled within 50 mg of Mg3N2. Within this range, stable doping of Mg in the h-BN lattice can be achieved while maintaining the basic order of the lattice structure. When the amount of Mg3N2 precursor is in the range of 10-50 mg, the film can form a clear Mg-BN bond structure, and the lattice distortion is within a controllable range. When the amount of Mg3N2 doping exceeds 50 mg, the introduction of excess Mg causes severe degradation of the h-BN lattice structure. At this time, the Raman spectral characteristics change significantly, and the intrinsic E2g characteristic peak of h-BN completely merges with the Mg-related vibrational peak, forming a broadened bulging band with a full width at half maximum (FWHM) exceeding 100 cm⁻¹. -1This indicates that the lattice order has been greatly destroyed. This spectral feature usually corresponds to two structural evolutions: one is that the h-BN lattice undergoes an amorphization transformation due to excessive doping, and the long-range order disappears; the other is that the excess Mg element cannot be incorporated into the lattice and phase separation occurs, forming Mg element or Mg-based impurity phase (such as Mg3N2). The superposition of the vibrational peaks of the two phases leads to the broadening and blurring of the spectral peaks.

[0056] The defect density of the thin film was evaluated by the full width at half maximum (FWHM) of the X-ray diffraction rocking curve, indicating that the intrinsic h-BN epitaxial film prepared in this invention has a low dislocation density of approximately ~5 × 10⁻⁶. 14 cm -3 The XRD diffraction peaks of the doped sample were significantly broadened, indicating that the defect density of the sample increased significantly.

[0057] Example 3

[0058] This embodiment provides a broadband ultraviolet photodetector based on a magnesium-doped hexagonal boron nitride thin film, the preparation method of which is as follows;

[0059] First, a magnesium-doped h-BN thin film was prepared on a sapphire substrate using the method described in Example 1. Then, according to the predetermined shape and spacing of the first and second electrodes, a 70 nm thick titanium (Ti) layer was sputtered onto the surface of the magnesium-doped h-BN thin film using magnetron sputtering. A 20 nm thick gold (Au) layer was then deposited on the titanium (Ti) layer to suppress electrode oxidation, forming interdigitated electrodes for the first and second electrodes. The sample was then placed in an argon atmosphere and rapidly annealed at 430 °C for 30 min to ensure good ohmic contact between the electrodes and the sample, resulting in a photodetector device. The prepared interdigitated electrodes contain four pairs of interdigitates with a spacing of 50 μm and a length of 1 mm, providing an effective testing area of ​​2.5 × 10⁻⁶. -4 cm 2 .

[0060] Device testing:

[0061] In a dark room, a 20V bias voltage is applied to the device. A xenon lamp, in conjunction with a monochromator, generates monochromatic light of different wavelengths (from 185nm to 385nm) to illuminate the photosensitive area of ​​the device, and the photocurrent is measured. The structure is as follows: Figure 4 As shown. The results are as follows. Figure 5 As shown, the device exhibits a significant photocurrent response under ultraviolet light irradiation from 185 nm to 365 nm, with the peak response located at approximately 185 nm. When the illumination wavelength is greater than 365 nm (energy below 3.4 eV), the photocurrent rapidly decreases to the dark current level. This indicates that the detector effectively covers a broad spectral range from deep ultraviolet to mid-ultraviolet, verifying the effect of magnesium doping in reducing the h-BN band gap and broadening the spectral response.

[0062] When the bandgap of the thin film decreased to 3.38 eV, the sample was able to absorb photon energy of near-ultraviolet light at a wavelength of 365 nm, exhibiting obvious near-ultraviolet light response characteristics, confirming that Mg doping can successfully extend the ultraviolet light response range of h-BN thin films. As the optical bandgap of the h-BN:Mg thin film continuously narrowed from 5.91 eV in the pure phase to 3.38 eV, the response wavelength of the device simultaneously redshifted, extending from the deep ultraviolet band of 185 nm to the near-ultraviolet band of 365 nm, achieving effective control of the ultraviolet response range. At the same time, the maximum photoelectric responsivity of the device showed a significant decreasing trend, from 11.12 mA / W in the undoped state to 0.28 mA / W in the highly doped state, with the response performance showing a significant decay as the bandgap narrowed.

[0063] Comparative example:

[0064] An intrinsic h-BN thin film was prepared and a photodetector with the same structure was fabricated under the same conditions as in Example 1, but without the addition of a Mg3N2 source. Tests showed that the detector only responded to deep ultraviolet light with wavelengths less than approximately 200 nm, and showed no response to longer wavelengths of ultraviolet light.

[0065] The above embodiments demonstrate that this invention, by using Mg3N2 as a doping source, successfully achieved magnesium doping of h-BN thin films during the CVD process, effectively tuning its bandgap from 5.91 eV to 3.38 eV. The photodetector fabricated based on this material exhibits excellent broadband ultraviolet detection capabilities. This invention features a controllable and reproducible process, providing a novel material platform and technical pathway for developing novel h-BN-based optoelectronic devices.

Claims

1. A method for preparing a hexagonal boron nitride thin film, characterized in that: Includes the following steps: A boron-containing precursor was used as the boron source and placed together with a sapphire substrate loaded with graphene oxide film in the second temperature zone of a chemical vapor deposition (CVD) reaction chamber. A magnesium-containing dopant source was weighed and placed in the upstream first temperature zone of the CVD reaction chamber, with a mass ratio of boron-containing precursor to magnesium-containing dopant source of 8:

1. Subsequently, the reaction chamber was evacuated. When the growth temperature in the second temperature zone rose to above 800°C but below 1100°C, a mixture of nitrogen-containing precursor and carrier gas with a flow rate ratio of 0.5-1.5:1 was introduced into the reaction chamber, and the total flow rate of the mixed gas was controlled at 300 sccm. After the second temperature zone was kept at a constant temperature to allow the graphene oxide to completely disappear, the first temperature zone was heated to 800-900°C to promote the decomposition of the magnesium-containing dopant source, which was then carried to the second temperature zone by the carrier gas. The reaction conditions were then maintained to continue the reaction and growth. After the growth was completed, the mixture was naturally cooled to room temperature to obtain a light gray, semi-transparent magnesium-doped hexagonal boron nitride film attached to the sapphire substrate.

2. The method for preparing a hexagonal boron nitride thin film according to claim 1, characterized in that: The sapphire substrate with graphene oxide film loaded shown was prepared by the following method: (1) First, graphene oxide powder was added to deionized water and ultrasonically treated to disperse it completely; then the supernatant was collected by centrifugation; the supernatant was ultrasonically treated again and centrifuged a second time to collect the supernatant; finally, the supernatant was ultrasonically treated and allowed to stand to obtain a stable aqueous solution of graphene oxide. (2) Add the aqueous solution of graphene oxide to a glass dish, and slowly stack pentane along the wall of the bottle to form a clear pentane-water two-phase interface; then inject anhydrous ethanol, cover and let stand at room temperature to allow the graphene oxide sheets to fully self-assemble at the liquid-liquid interface; after standing, a uniformly floating graphene oxide film gradually forms at the interface; after the cleaning sapphire substrate is gently touched to the floating graphene oxide film at the interface to complete the transfer, and then allow the pentane and ethanol to evaporate naturally; finally, a clean graphene oxide film with no solvent residue is obtained.

3. The method for preparing a hexagonal boron nitride thin film according to claim 1, characterized in that: The boron-containing precursor is at least one of borane, boron oxide, boron halide, or borazane; the magnesium-containing dopant source is magnesium nitride powder; and the nitrogen-containing precursor is at least one of ammonia or nitrogen.

4. The method for preparing a hexagonal boron nitride thin film according to claim 1, characterized in that: The growth temperature in the second temperature zone is 1000℃; the total film growth time is not less than 90 minutes.

5. A hexagonal boron nitride thin film, characterized in that: The magnesium-doped hexagonal boron nitride thin film prepared according to any one of claims 1-4 has a band gap of 3.38 eV.

6. The application of the magnesium-doped hexagonal boron nitride thin film according to claim 5, characterized in that: It can be used to prepare a broadband ultraviolet photodetector, wherein the working spectral response range of the broadband ultraviolet photodetector is from 185 nm to 365 nm in the ultraviolet band.

7. A method for bandgap modulation of a hexagonal boron nitride thin film, characterized in that: Includes the following steps: A boron-containing precursor was used as the boron source and placed together with a sapphire substrate loaded with graphene oxide film in the second temperature zone of a chemical vapor deposition (CVD) reaction chamber. A magnesium-containing dopant source was weighed and placed in the upstream first temperature zone of the CVD reaction chamber, with a mass ratio of boron-containing precursor to magnesium-containing dopant source of 8:0.2-1. Subsequently, the reaction chamber was evacuated. When the growth temperature in the second temperature zone rose to above 800°C but below 1100°C, a mixture of nitrogen-containing precursor and carrier gas with a flow rate ratio of 0.5-1.5:1 was introduced into the reaction chamber, and the total flow rate of the mixed gas was controlled at 300 sccm. After the second temperature zone was kept at a constant temperature to allow the graphene oxide to completely disappear, the first temperature zone was heated to 800-900°C to promote the decomposition of the magnesium-containing dopant source, which was then carried to the second temperature zone by the carrier gas. The reaction conditions were then maintained to continue the growth. After the growth was completed, the mixture was naturally cooled to room temperature to obtain a magnesium-doped hexagonal boron nitride film with an optical bandgap tuned between 5.02 eV and 3.38 eV.

8. The bandgap modulation method for a hexagonal boron nitride thin film according to claim 7, characterized in that: The boron-containing precursor is at least one of borane, boron oxide, boron halide, or borazane; the magnesium-containing dopant source is magnesium nitride powder; the nitrogen-containing precursor is at least one of ammonia or nitrogen; and the growth temperature of the second temperature zone is 1000℃.

9. The bandgap modulation method for a hexagonal boron nitride thin film according to claim 7, characterized in that: The optical band gaps of magnesium-doped hexagonal boron nitride thin films are 5.02 eV, 4.83 eV, 4.48 eV, 4.10 eV, 3.90 eV, and 3.38 eV when the mass ratio of boron-containing precursor to magnesium-containing doped source is any one of 8:0.2, 8:0.4, 8:0.6, 8:0.8, or 8:

1.

10. A broadband ultraviolet photodetector based on a magnesium-doped hexagonal boron nitride thin film, characterized in that: The hexagonal boron nitride thin film of claim 5 is used as the light absorption layer; a first electrode and a second electrode are disposed on the light absorption layer, and a gap is formed between the first electrode and the second electrode to expose a portion of the light absorption layer for receiving light.

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