Hexagonal boron nitride thin film, preparation method thereof, energy band regulation and control method and wide-spectrum ultraviolet photoelectric detector

Abstract

The invention relates to a chemical vapor deposition technology for preparing a wide bandgap semiconductor material, and provides a hexagonal boron nitride film, a preparation method thereof, an energy band regulation and control method and a wide spectrum ultraviolet photoelectric detector. Magnesium nitride is used as a doping source, and the doping source is synchronously introduced to the epitaxy of an h-BN film by adopting a chemical vapor deposition method; in-situ doping of the magnesium element in an h-BN crystal lattice is achieved, and the forbidden band width of the h-BN film is regulated from 5.91 eV to 3.38 eV by controlling the magnesium doping concentration. According to the magnesium-doped h-BN film, a wide-spectrum ultraviolet photoelectric detector is prepared based on the magnesium-doped h-BN film, the ultraviolet light absorption range is widened to the wave band of 185-365 nm, and wide-spectrum and high-sensitivity ultraviolet detection is achieved. The doping process is controllable, the energy band regulation and control effect is remarkable, the spectral response limitation of an intrinsic h-BN-based detector is broken through, and the application of a photoelectric device in the fields of environmental monitoring, biochemical analysis and the like is promoted.

Description

TECHNICAL FIELD

[0001] The present application relates to the preparation of wide bandgap semiconductor materials by chemical vapor deposition technology, in particular to a method for preparing and bandgap regulating of hexagonal boron nitride film and its application in preparing wide-spectrum ultraviolet photodetectors. BACKGROUND

[0002] Hexagonal boron nitride (h-BN) is considered as an ideal deep ultraviolet (wavelength <280nm) photovoltaic material and high-power electronic device insulating substrate material due to its excellent chemical stability, high-temperature stability, high thermal conductivity and ultra-wide bandgap (~6.0eV). However, h-BN is an ultra-wide bandgap semiconductor with a bandgap of about 6eV and an intrinsic response wavelength of about 215nm, which cannot respond to other wavebands. Its ultra-wide bandgap also limits its effective absorption of ultraviolet light with a wavelength greater than 220nm, resulting in that the photodetector based on intrinsic h-BN can only work in a very narrow deep ultraviolet waveband and cannot respond to a wider ultraviolet spectrum (such as UVA: 315-400nm, UVB: 280-315nm). Therefore, boron nitride cannot be directly used for the preparation of wide-spectrum photovoltaic devices at present, which limits its application range in the fields of environmental monitoring, biochemical analysis, flame warning, etc.

[0003] In order to regulate the energy band structure of h-BN, researchers have tried methods such as element doping (such as carbon, silicon, oxygen and other heteroatoms), introducing defects, and applying strain. Among them, element doping is a direct method. However, the activation energy of the doped atoms in wide bandgap and ultra-wide bandgap semiconductor materials is very high, and the background defect concentration of the material itself is high, and there is a strong self-compensation effect. These factors make it difficult for existing technology to effectively reduce the bandgap. In addition, it is also crucial to maintain good crystal quality while effectively doping. For example, carbon doping may introduce deep levels in the bandgap, leading to increased non-radiative recombination and degraded photoelectric performance. Doping can only produce limited light response and cannot effectively adjust the bandgap. The dark current of the film is very high, and the impact on the lattice is also great. How to achieve the controllable and synchronous introduction of dopants during epitaxial growth and obtain a film with stable photoelectric properties is still a technical difficulty.

[0004] In addition, the traditional mechanical exfoliation method for preparing h-BN has inherent limitations such as low yield and uncontrollable layer number; although the chemical vapor deposition (CVD) method can synthesize high-crystallinity and large-area h-BN thin films on metal substrates, the method has strict requirements for the quality of the substrate, and the grown thin film is closely attached to the metal substrate, so that subsequent characterization and device fabrication need to be transferred, which will cause pollution and mechanical damage; the dielectric substrate is an ideal choice for the optoelectronic device field due to its excellent insulation and thermal stability, but there are still core challenges such as low nucleation density, limited growth kinetics and lattice mismatch induced defects in directly preparing h-BN thin films on the dielectric substrate.

[0005] Therefore, it is of great scientific significance and application value to develop a doping method and new material which can effectively and controllably adjust the band gap of the h-BN thin film and is suitable for preparing a wide-spectrum ultraviolet photodetector. SUMMARY

[0006] In order to solve the above technical problems, the application provides a preparation method of a hexagonal boron nitride thin film, a band control method, and a high-performance wide-spectrum ultraviolet photodetector based on the thin film.

[0007] In order to achieve the above application purposes, the first aspect of the application provides a preparation method of a hexagonal boron nitride thin film, which adopts a chemical vapor deposition (CVD) technology and synchronously introduces a magnesium-containing doping source, especially magnesium nitride (Mg3N2), during the growth process, so as to realize in-situ doping of magnesium elements in the hexagonal boron nitride lattice and reach the level of alloy.

[0008] The application provides a preparation method of a hexagonal boron nitride thin film, which comprises the following steps:

[0009] A certain amount of boron-containing precursor is taken as a boron source and is placed in a second temperature zone in a chemical vapor deposition reaction cavity together with a sapphire substrate loaded with a graphene oxide film; a certain amount of magnesium-containing doping source is taken as a doping source and is placed in an upstream first temperature zone in the chemical vapor deposition reaction cavity, and the mass ratio of the boron-containing precursor to the magnesium-containing doping source is 8:1; then, the reaction cavity is vacuumized, when the growth temperature of the second temperature zone is increased to greater than 800 and less than 1100 DEG C, a mixed gas of a nitrogen-containing precursor and a carrier gas with a flow ratio of 0.5-1.5:1 is introduced into the reaction cavity, and the total flow of the mixed gas is controlled to be 300 sccm; after the graphene oxide completely disappears by keeping the second temperature zone at a constant temperature for a certain time, the first temperature zone is heated to 800-900 DEG C, so as to promote the decomposition of the magnesium-containing doping source and carry it to the second temperature zone by the carrier gas, and then the reaction conditions are maintained to continue the reaction and growth for a certain time; after the growth is completed, the temperature is naturally cooled to room temperature, and a light gray translucent magnesium-doped hexagonal boron nitride thin film attached to the sapphire substrate is obtained.

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

[0011] (1) Firstly, a certain amount of graphene oxide powder is added into deionized water, and is completely dispersed by ultrasonic treatment; then the supernatant is collected by centrifugation; the obtained supernatant is ultrasonically treated again, and the supernatant is collected by centrifugation for the second time; finally, the supernatant is ultrasonically treated and then is left to stand, so that a stable graphene oxide aqueous solution is obtained;

[0012] (2) A certain amount of graphene oxide aqueous solution is taken into a glass dish, and a certain amount of pentane is slowly added along the bottle wall to form a clear pentane-water two-phase interface; then a certain amount of anhydrous ethanol is injected, and the dish is left to stand at room temperature after being covered, so that the graphene oxide layers are fully self-assembled at the liquid-liquid interface; after the standing is completed, the interface gradually forms a uniform floating graphene oxide film; the cleaned sapphire substrate is lightly touched with the floating graphene oxide film on the interface to complete the transfer, and then the pentane and ethanol are naturally volatilized; finally, a clean graphene oxide film without solvent residue is obtained.

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

[0014] As preferred, the growth temperature of the second temperature zone is 1000℃.

[0015] As preferred, the total growth time of the film is not less than 90min.

[0016] The second aspect of the present application provides a magnesium-doped hexagonal boron nitride film prepared by the above method, and the band gap of the film is significantly reduced from 5.91eV to 3.38eV compared with intrinsic hexagonal boron nitride.

[0017] The third aspect of the present application provides a band control method of a hexagonal boron nitride film, which comprises the following steps:

[0018] A certain amount of boron-containing precursor is taken as a boron source, and is placed in a second temperature zone in a chemical vapor deposition reaction cavity together with a sapphire substrate loaded with a graphene oxide film; a certain amount of magnesium-doped source is weighed as a doping source and is placed in an upstream first temperature zone in the chemical vapor deposition reaction cavity, and the mass ratio of the boron-containing precursor to the magnesium-doped source is 8:0.2-1; then, the reaction cavity is vacuumized, when the growth temperature of the second temperature zone is increased to greater than 800 and less than 1100 DEG C, mixed gas of a nitrogen-containing precursor and a carrier gas with a flow ratio of 0.5-1.5:1 is introduced into the reaction cavity, and the total flow of the mixed gas is controlled to be 300 sccm; after the graphene oxide is completely disappeared by keeping the second temperature zone constant for a certain time, the first temperature zone is increased to 800-900 DEG C to promote the decomposition of the magnesium-doped source and carry it to the second temperature zone by the carrier gas, and then the reaction condition is maintained to continue the reaction for a certain time; after the growth is completed, the natural cooling to room temperature is performed, and a magnesium-doped hexagonal boron nitride film with an optical band gap between 5.02 eV and 3.38 eV is obtained.

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

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

[0021] As preferred, the growth temperature of the second temperature zone is 1000 DEG C.

[0022] As preferred, the mass ratio of the boron-containing precursor to the magnesium-doped source is any one of 8:0.2, 8:0.4, 8:0.6, 8:0.8 and 8:1, and the optical band gap of the magnesium-doped hexagonal boron nitride film corresponding to each mass ratio is 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 film prepared by the method can be used to prepare a wide-spectrum ultraviolet photodetector.

[0024] The fourth aspect of the present application provides a wide-spectrum ultraviolet photodetector based on the above-mentioned magnesium-doped hexagonal boron nitride film, and the above-mentioned magnesium-doped hexagonal boron nitride film is used as a light absorption layer; a first electrode and a second electrode are arranged on the light absorption layer, and the first electrode and the second electrode have a gap to expose part of the light absorption layer for receiving light.

[0025] The working spectral response range of the wide-spectrum ultraviolet photodetector is 185 nm to 365 nm, covering the deep ultraviolet to the middle ultraviolet band.

[0026] The beneficial effects of the present application include:

[0027] The present application selects graphene oxide (GO) as an interface self-assembly template, which is rich in defects (ID / IG=1.6) and has hydrophilicity (contact angle 43.5°), the defect sites provide sufficient nucleation active centers, and the hydrophilicity ensures uniform spreading of the precursor, which solves the problems of low nucleation density and lattice mismatch of h-BN on non-metallic substrates. By adopting the combined process of "ultrasonic dispersion-two-step centrifugation-ethanol assisted liquid-liquid interface self-assembly", a dense GO film is formed through multi-step purification and assembly, the process control precision is high, and the purity and uniformity of the template are better than those of the traditional method. By controlling the variable method, the best growth conditions (temperature 1000℃, NH3:Ar=1.5:1) are determined, and large-area continuous and high-crystallinity h-BN film growth is realized, the E2g peak half-width of h-BN in Raman spectrum is the narrowest, and the crystalline quality is the best, and the GO template is only used as a nucleation auxiliary medium and is completely etched and removed in situ during h-BN growth, without residual carbon pollution, which ensures the high purity of the final h-BN film. By using the APCVD method, the h-BN film is directly grown on the sapphire substrate without the need for metal catalysts and subsequent film transfer steps, avoiding damage and pollution during the transfer process, and the process is simple and suitable for the direct preparation requirements of optoelectronic devices.

[0028] The present application adopts an in-situ step-by-step doping strategy: the boron source, the substrate and the Mg3N2 doping source are independently regulated in different zones, the initial h-BN film (GO is completely decomposed) is first grown, and then the doping is started, realizing uniform and stable doping of Mg atoms, and avoiding the problem of insufficient uniformity caused by simultaneous doping and growth.

[0029] The present application uses Mg3N2 as a doping source, and by adjusting the proportion of Mg3N2, the band gap of h-BN is successfully reduced from 5.91eV to about 3.38eV, which lays a foundation for the application of h-BN in a wider spectral range, and the method has good process compatibility, and the doping concentration and film band gap can be continuously and accurately controlled by adjusting the precursor ratio.

[0030] The photoelectric detector prepared based on the doped film breaks through the limitation of the intrinsic h-BN detector only responding to deep ultraviolet, realizes wide-spectrum and high-sensitivity detection of ultraviolet light from 185nm to 365nm, maintains low dark current (about 7pA) under low doping amount, balances the response range and insulation characteristics, is suitable for ultraviolet photoelectric detection application, and greatly expands the application scenarios of h-BN-based photoelectric devices. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 It is a schematic diagram of the preparation device of the magnesium-doped h-BN film in the embodiments of the present application. In the figure, 1 is Mg3N2, 2 is B2O3, 3 is a sapphire substrate loaded with a GO film, and 4 is an Ar / NH3 mixed gas.

[0032] Figure 2 The UV-visible absorption spectrum of the magnesium-doped h-BN film prepared in Embodiment 1 of the present application and the intrinsic h-BN film, and the band gap diagram calculated by the Tauc plot method are compared.

[0033] Figure 3 The light transmittance curve of the magnesium-doped h-BN film prepared in Embodiment 1 of the present application in the visible light band.

[0034] Figure 4 The structure diagram of the wide-spectrum ultraviolet photodetector prepared in Embodiment 3 of the present application.

[0035] Figure 5 The photocurrent response spectrum of the wide-spectrum ultraviolet photodetector prepared in Embodiment 3 of the present application under irradiation of different wavelengths of ultraviolet light. DETAILED DESCRIPTION

[0036] Embodiment 1,

[0037] The embodiment provides a preparation method of a hexagonal boron nitride film, and the method comprises the following steps:

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

[0039] The uniform dispersion and purification of GO are realized by mild ultrasonic treatment. First, 60 mg of GO powder is added to 20 mL of deionized water, and ultrasonic treatment is performed for 1 h to completely disperse it; then, a floor type centrifuge is used, and first, centrifugation is performed at a speed of 3000 r / min for 30 min to remove a large amount of undispersed precipitate and impurities in the system, and the supernatant is collected; the obtained supernatant is ultrasonically treated for 30 min again, and then is loaded into a centrifuge tube and strictly leveled, and centrifugation is performed at a speed of 9000 r / min for 30 min for further purification, and the supernatant is collected twice; finally, the purified supernatant is ultrasonically treated for 3 h, and is left to stand for 24 h. The GO aqueous solution with excellent stability is successfully prepared by using the two-step centrifugation method.

[0040] (2) Preparation of GO film by interfacial self-assembly method:

[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 thin film is subjected to ultraviolet-visible absorption spectrum test, the absorption edge of intrinsic h-BN is located at about 210 nm (corresponding to ~5.91 eV), and the absorption edge of the magnesium-doped h-BN thin film prepared by the application is red-shifted to about 365 nm (corresponding to ~3.38 eV), which proves that the band gap is significantly reduced. The X-ray photoelectron spectroscopy detects the N-Mg bond characteristic peak, which confirms that Mg does not exist in the free state, but is integrated into the lattice through chemical bond action; the secondary ion mass spectrometry test result excludes the surface adsorption interference, shows that Mg is uniformly distributed in the film body, and the two results mutually confirm each other, verifying the effective doping and uniform distribution of Mg element.

[0045] Raman and infrared, TEM and the like are used to comprehensively characterize the obtained h-BN epitaxial film. Only the E2g vibration mode of h-BN appears in the Raman spectrum, and no characteristic peak of GO is detected, which proves that GO has been completely etched and removed in the film growth process, and only pure-phase h-BN film is left on the substrate. The E2g peak half-width of h-BN at 1000℃ is the narrowest, which reflects the optimal crystalline quality; when the temperature is further increased to 1100℃, the half-width of the E2g peak is significantly widened, which indicates that the high temperature leads to lattice distortion or defect increase of h-BN crystal structure, and the film surface is severely wrinkled, and the structural integrity is reduced.

[0046] Example 2,

[0047] The energy band modulation method for the hexagonal boron nitride thin film provided in the embodiment comprises the following steps:

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

[0049] The same as example 1, which will not be repeated here;

[0050] (2) Preparation of GO film by interfacial self-assembly method:

[0051] The same as example 1, which will not be repeated here;

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

[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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