Method for remarkably improving yield of high-quality hexagonal boron nitride single crystal at high temperature and high pressure

Abstract

The invention belongs to the technical field of single crystal material preparation, and provides a barium-magnesium bimetal boron nitride compound-based solvent (Ba-Mg DMB), which significantly improves the yield of hexagonal boron nitride (h-BN) single crystals under high temperature and high pressure (HPHT) conditions, and further improves the crystal quality. Compared with the yield of about 12% of a barium-based solvent, the yield of the Ba-Mg DMB solvent is increased by 4.8 times, and the stable yield of about 57.3% is achieved. Researches find that under the HPHT condition, the magnesium element in Ba-Mg DMB can effectively remove carbon and oxygen impurities, which plays a key role in promoting the obtaining of h-BN single crystals with excellent quality. The high-quality characteristic is verified through detailed characterization of Raman spectrum, X-ray diffraction, X-ray photoelectron spectroscopy and the like, and is particularly reflected in photoluminescence and cathode luminescence characteristics. Through the high carrier mobility shown in the single-layer graphene, it is directly proved that the h-BN single crystal derived from Ba-Mg DMB can be used as an ideal packaging material of a two-dimensional device.

Description

Technical Field

[0001] This invention belongs to the field of single-crystal material preparation technology. Specifically, this invention relates to a method for significantly improving the yield of high-quality hexagonal boron nitride single crystals under high temperature and high pressure. Background Technology

[0002] Hexagonal boron nitride (h-BN) belongs to the wide-bandgap III-V group compound semiconductors and is widely recognized as a high-performance layered van der Waals crystal with broad applications in various fields. Due to its chemical inertness and atomically smooth surface (free of dangling bonds), h-BN has become an ideal dielectric and protective layer for two-dimensional (2D) materials. In fact, encapsulating 2D materials in h-BN has proven to be an effective strategy for improving their performance, enabling in-depth research into many novel phenomena, including the fractional quantum Hall effect, the Hofstadter butterfly effect in moiré superlattices, and superconductivity in twisted bilayer graphene. Furthermore, its ultra-wide bandgap (~5.9 eV) and high quantum yield make h-BN a promising material for fabricating vacuum ultraviolet light-emitting devices and high-performance solar-blind detectors. In addition, as a naturally hyperbolic material, h-BN exhibits low-loss phonon polaron properties, making it an ideal candidate material for cutting-edge applications such as negative refraction, nanolithography, and thermal radiation enhancement. In all these applications, the performance of the device or material is highly dependent on the quality of the h-BN material used. Currently, h-BN thin films prepared by techniques such as chemical vapor deposition, molecular beam epitaxy, and sputtering deposition have high-density grain boundaries and defects, making it difficult to meet the quality standards required for high-performance applications.

[0003] Currently, high-quality h-BN single crystals are mainly produced through the atmospheric pressure high-temperature flux method (APHT), high pressure high-temperature method (HPHT), or polymer derivative crystal method (PDC), characterized by E 2g The full width at half maximum (FWHM) of the Raman vibration modes is extremely narrow (< 9 cm). -1 Regarding HPHT synthesis, T. Taniguchi and J. Cheng et al. have achieved significant results in synthesizing h-BN single crystals, successfully preparing crystals with a size of approximately 2 mm, high purity, and low defect density. Sodium-based, magnesium-based, barium-based, and strontium-based solvents have all been successfully used to obtain high-quality HPHT h-BN single crystals. HPHT h-BN single crystals prepared by T. Taniguchi and K. Watanabe of the National Institute for Materials Science (NIMS) in Japan using barium-based solvents are currently in use in over 200 research institutions worldwide. In fact, the yield of the HPHT process is severely limited by the size and yield of the high-pressure, high-temperature reaction chamber. When using a cubic anvil high-pressure apparatus, the typical yield of barium-based solvents is approximately 12%–17% (h-BN single crystal / h-BN precursor by mass), resulting in extremely high production costs for h-BN single crystals.

[0004] One of the most critical challenges at present is the development of preparation technologies that combine high yield and high quality, so as to provide strong support for the growing demand for two-dimensional materials research. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a method for significantly improving the yield of high-quality hexagonal boron nitride single crystals under high temperature and high pressure. Compared with traditional methods, this method increases the yield of h-BN single crystals by 4.8 times, achieving a stable yield of approximately 57.3%, greatly reducing costs, and enabling the preparation of high-quality h-BN single crystals.

[0006] The above-mentioned objective of this invention is achieved through the following technical solution:

[0007] In the context of this invention, the term "heating and pressurization treatment" refers to the simultaneous application of temperature and pressure in a reactor.

[0008] In the context of this invention, the term "heat treatment" refers to applying temperature in a reactor.

[0009] In the context of this invention, the term "oxygen content" refers to the weight percentage of oxygen in hexagonal boron nitride powder.

[0010] This invention provides a method for significantly improving the yield of high-quality hexagonal boron nitride single crystals under high temperature and high pressure, comprising the following steps:

[0011] Hexagonal boron nitride single crystals were prepared by heating and pressurizing barium-magnesium bimetallic boron-nitrogen compound solvent and deoxidized hexagonal boron nitride powder.

[0012] The inventors of this application unexpectedly discovered that when a barium-magnesium bimetallic boron-nitrogen compound is used as a co-solvent, the co-solvent meets all the characteristics required of a suitable solvent, including (i) sufficient solubility for boron and nitrogen; (ii) high thermodynamic stability during the reaction; (iii) the ability to remove impurities such as oxygen and carbon in the hBN growth environment; and (iv) minimal contamination from the solvent components remaining in the h-BN growth process. The inventors of this application further unexpectedly discovered that by using this co-solvent, the yield of high-quality h-BN single crystals can be increased by 4.8 times, reaching a stable yield of approximately 57.3%.

[0013] Preferably, in the method described in this invention, the main components of the barium-magnesium bimetallic boron-nitrogen compound solvent are barium boron nitride and magnesium nitride.

[0014] Preferably, in the method described in this invention, the oxygen content of the hexagonal boron nitride powder is less than or equal to 0.1% by weight.

[0015] In this invention, when barium-magnesium bimetallic boron nitride compound solvent and hexagonal boron nitride powder are subjected to heating and pressure treatment to obtain h-BN single crystals, commercially available h-BN powder is deoxidized. The lower the oxygen content, the more beneficial it is to the synthesis of high-quality h-BN single crystals.

[0016] Preferably, in the method described in this invention, the barium-magnesium bimetallic boron-nitrogen compound solvent is prepared by a method comprising the following steps:

[0017] Barium nitride powder and deoxidized hexagonal boron nitride powder were heated to obtain a barium-based flux. The obtained barium-based flux was then thoroughly ground and mixed with magnesium nitride powder in a mortar in a glove box to obtain a barium-magnesium bimetallic boron-nitrogen compound solvent.

[0018] Preferably, in the method described in this invention, the deoxidized hexagonal boron nitride powder is prepared by a method comprising the following steps:

[0019] Under a protective atmosphere, hexagonal boron nitride powder was treated at a temperature of 2000℃~2100℃ for 2h~8h.

[0020] Preferably, in the method described in this invention, the heat treatment is carried out under the following conditions: the temperature of the heat treatment is 1000℃~1100℃, the time of the heat treatment is 12h~48h, and the heat treatment is carried out under a protective atmosphere.

[0021] Preferably, in the method described in this invention, the barium-based co-solvent and magnesium nitride powder are mixed at a mass ratio of 1:3 and ground in a mortar for 1 to 2 hours.

[0022] Preferably, in the method described in this invention, the protective atmosphere is selected from one or more of nitrogen, hydrogen, and argon.

[0023] Preferably, in the method described in this invention, the flow rate of the protective atmosphere is 100 sccm to 200 sccm.

[0024] Preferably, in the method described in this invention, the mass ratio of barium-magnesium bimetallic boron nitride compound solvent to hexagonal boron nitride powder is (3~5):1.

[0025] Preferably, in the method described in this invention, the heating and pressurizing treatment is carried out under the following conditions: the pressure of the heating and pressurizing treatment is 4 GPa to 5 GPa, the temperature of the heating and pressurizing treatment is 1450℃ to 1500℃, and the time of the heating and pressurizing treatment is 20h to 30h.

[0026] This invention also provides applications of the above-described method in the preparation of substrates, packaging materials, thermally conductive materials, solar-blind detectors, and deep ultraviolet light sources.

[0027] Beneficial effects

[0028] Compared to the yield of approximately 12% with classic barium-based solvents, barium-magnesium bimetallic boron-nitrogen compound solvents increased the yield by 4.8 times, achieving a stable yield of approximately 57.3%, which greatly reduced the cost of HPHT h-BN single crystal synthesis.

[0029] Under HPHT conditions, magnesium in the barium-magnesium bimetallic boron-nitrogen compound solvent can effectively remove carbon and oxygen impurities, further optimizing the quality of h-BN single crystals.

[0030] Regarding crystal quality, analysis of X-ray diffraction, Raman characterization, cathode and photoluminescence spectroscopy, and X-ray photoelectron spectroscopy confirms that the method of this invention yields h-BN single crystals comparable to, or even superior to, those prepared using barium-based systems. The h-BN single crystals obtained by this invention show promising applications in substrates, packaging materials, thermally conductive materials, solar-blind detectors, and deep ultraviolet light sources. Attached Figure Description

[0031] Figure 1 To compare the yields of h-BN single crystals synthesized in Ba- and Ba-Mg DMB solvent systems, the top-left inset shows a schematic diagram of the experimental setup, illustrating the assembly process of the solvent and hexagonal boron nitride powder in the high-pressure reaction chamber. The bottom-left and top-right insets present photographs of hexagonal boron nitride single crystals obtained in a single synthesis using Ba- and Ba-Mg DMB-based solvents, respectively.

[0032] Figure 2 The photos are for repeatability verification experiments, showing the process of weighing the obtained h-BN single crystals on a high-precision balance after single synthesis using Ba- and Ba-Mg DMB solvents respectively; (a) is Ba-h-BN, (b) is Ba-Mg-h-BN;

[0033] Figure 3 Characterization of cubic boron nitride crystals synthesized using Ba-Mg DMB solvent. (a) Crystal photograph of cubic boron nitride obtained from a single synthesis experiment; (b) Raman spectrum of the cubic boron nitride crystal.

[0034] Figure 4 Characterization results for bulk h-BN single crystals. (a) XRD diffraction patterns at 2θ angles of 20°–90°. (b) Enlarged views of XRD diffraction patterns at 2θ angles of 86.5°–88.5°. (c) Representative Raman spectra. (d) Raman spectra E0 of 20 randomly selected samples. 2gThe FWHM distribution. (e) shows a low-magnification transmission electron microscope image and the corresponding selected area electron diffraction pattern. (f) shows a typical high-resolution transmission electron microscope image;

[0035] Figure 5 Raman spectra of 20 randomly selected single-crystal samples derived from h-BN crystals based on Ba-Mg DMB solvent are shown. The FWHM distribution of the corresponding Raman peaks is as follows. Figure 4 As shown in d;

[0036] Figure 6 The crystallization uniformity of h-BN single crystals synthesized in Ba-Mg DMB solvent is shown. (a)-(d) are diffraction patterns of four h-BN thin films;

[0037] Figure 7 Characterization of h-BN sheets. (a) Optical microscope image of the exfoliated h-BN sheets on a SiO2 (285 nm) / Si substrate. (b) Atomic force microscope image of the h-BN sheets; the inset shows the height profile corresponding to the blue curve. (c) and (d) Raman peak position and intensity mapping of the h-BN sheets. Green lines indicate the boundaries of the test area;

[0038] Figure 8 Elemental analysis of h-BN crystals synthesized using Ba-Mg DMB solvent. (a) XPS broad scan spectrum; (b) C 1s; (c) O 1s; (d) Ba 3d; (e) Mg 1s and (f) B 1s and N 1s high-resolution XPS spectra.

[0039] Figure 9 The luminescence properties of bulk h-BN single crystals are characterized. (a) is the cathodoluminescence spectrum on logarithmic coordinates at room temperature (300K). The photoluminescence spectra of bulk hexagonal boron nitride single crystals on logarithmic coordinates at 5K correspond to (b) 5.7-6.0 eV, (c) 5.0-6.0 eV and (d) 3.5-6.0 eV, respectively.

[0040] Figure 10 Characterization of a monolayer graphene device encapsulated in a Ba-Mg DMB solvent-synthesized h-BN single crystal (T=1.5K). (a) Optical microscope image of the Hall strip device. Pink, blue, white, and yellow dashed lines indicate the top h-BN, bottom h-BN, monolayer graphene, and top gate, respectively. (b) Schematic side view of the device. (c) R xx (Pink) and σ xx (Blue) Curve as a function of n. (d) is σ in a double logarithmic coordinate system. xx Curve showing the variation with n. The dashed line indicates the extracted value of n*. (e) is the curve showing the variation of carrier mobility μ with n;

[0041] Figure 11 The image shows the XRD pattern of the residue after HPHT synthesis using Ba-Mg DMB solvent. The bottom left inset is a photograph of the residue. Detailed Implementation

[0042] Example 1

[0043] In a specific embodiment of the present invention, the method of the present invention may include the following steps:

[0044] Step 1: Deoxidation method based on hexagonal boron nitride powder

[0045] (1-1) Prepare commercially available hexagonal boron nitride powder;

[0046] (1-2) Under a protective atmosphere, the prepared hexagonal boron nitride powder is deoxidized in a deoxidation furnace at a temperature of 2100℃ for 8 hours, so that the oxygen content of the deoxidized hexagonal boron nitride powder is less than or equal to 0.1wt%; the protective atmosphere is selected from nitrogen; the flow rate of the protective atmosphere is 200 sccm.

[0047] Step 2: Method for preparing barium-based co-solvents based on barium nitride

[0048] (2-1) Prepare commercially available barium nitride powder and previously obtained deoxygenated hexagonal boron nitride powder;

[0049] (2-2) Barium nitride powder and deoxidized hexagonal boron nitride powder were thoroughly mixed at a mass ratio of 2:1 and then placed in a boron nitride crucible for heating to prepare a barium-based flux. The heating temperature was 1100℃ for 48 hours, and the process was carried out under a protective atmosphere. The protective atmosphere used was nitrogen gas with a flow rate of 200 sccm.

[0050] Step 3: Method for further preparing barium-magnesium bimetallic boron nitrogen compound solvent based on the prepared barium-based co-solvent

[0051] (3-1) Prepare the prepared barium-based co-solvent and commercially available magnesium nitride powder;

[0052] (3-2) The barium-based co-solvent and magnesium nitride powder are thoroughly ground and mixed evenly to obtain a barium-magnesium bimetallic boron nitrogen compound solvent; the mass ratio of the barium-based co-solvent to the magnesium nitride powder is 1:3, the grinding time in the mortar is 2 hours, and the entire process is carried out in a glove box.

[0053] Step 4: Preparation of high-quality hexagonal boron nitride single crystals based on barium-magnesium bimetallic boron nitride compound solvents

[0054] (4-1) The previously obtained barium-magnesium bimetallic boron nitride compound solvent and deoxygenated hexagonal boron nitride powder are sequentially loaded into a molybdenum crucible or a tantalum crucible; the mass ratio of barium-magnesium bimetallic boron nitride compound solvent to deoxygenated hexagonal boron nitride powder is 3:1;

[0055] (4-2) Insert the molybdenum crucible or tantalum crucible filled with raw materials into the high-pressure assembly;

[0056] (4-3) Place the complete high-pressure assembly into a 2000-ton two-stage propulsion multi-faceted anvil press, apply high pressure and heat, and after a sufficiently long holding time, turn off the heating and slowly release the high pressure, removing the molybdenum crucible. The heating and pressurization treatment pressure was 4.5 GPa, the heating and pressurization treatment temperature was 1500℃, and the heating and pressurization treatment time was 30 hours. The specific operation procedure is as follows: First, raise the pressure to 4.5 GPa and keep it constant within 2.5 hours; then, raise the temperature to 1500℃ within 1 hour, while simultaneously holding the temperature and pressure for 30 hours. After synthesis, rapidly lower the temperature to room temperature within 1 minute. Finally, begin depressurization, slowly reducing the pressure to zero within 5 hours.

[0057] (4-4) The removed molybdenum crucible was placed in aqua regia for etching in order to facilitate the recovery of hexagonal boron nitride single crystals.

[0058] In a specific embodiment of the present invention, the obtained barium-based solvent, barium-magnesium bimetallic boron nitride compound solvent, and hexagonal boron nitride powder are stored in a glove box.

[0059] In a specific embodiment of the present invention, the high-voltage component includes: a tungsten carbide hammerhead, an octahedron, a boron nitride cylinder, a graphite cylinder, a molybdenum sheet, a molybdenum column, pyrophyllite, and zirconium oxide;

[0060] Example 2

[0061] Figure 1 The yields of h-BN single crystals obtained using Ba-based solvents (preparation method presented in step 2 of Example 1) and Ba-Mg DMB solvents (preparation method presented in step 3 of Example 1) were compared. The results showed that both solvent systems were favorable for forming smooth, colorless, transparent, plate-like h-BN single crystals. The crystal size ranged from 200 to 1000 μm, comparable to crystals reported from Ba-based solvents at the National Institute for Materials Science, Japan, and Sr-based solvents used in our laboratory. Notably, there was a significant difference in h-BN single crystal yields between the two solvent systems. The yield of the Ba-Mg DMB-based solvent system was approximately 4.8 times that of the Ba-based solvent system (see...). Figure 1 First bar chart and inset). To verify this finding, multiple repeated experiments were conducted, with consistent results, thus confirming the effectiveness and high reproducibility of the Ba-Mg DMB solvent-based approach (see...). Figure 1The second histogram and Figure 2 Furthermore, considering previous studies have shown that higher oxygen content in the feedstock can promote the growth of cubic boron nitride (c-BN), we selected an h-BN powder with relatively high oxygen content (0.66 wt%, Saint-Gobain) as a precursor to further evaluate the effectiveness of the Ba-Mg DMB-based system in improving yield. The results showed a significant increase in the yield of cubic boron nitride (c-BN). Figure 3 This confirms that Ba-Mg DMB-based solvents are highly efficient solvent systems. Furthermore, the quality of the synthesized cubic boron nitride crystals was evaluated using Raman spectroscopy. Figure 3 (b) The results show that its quality is comparable to that of cubic boron nitride crystals synthesized using barium-based solvents, previously reported by the National Institute for Materials Science in Japan.

[0062] Example 3

[0063] Figure 4 Figure a shows the X-ray diffraction (XRD) spectra of h-BN single crystals synthesized using Ba-Mg DMB solvent. For comparison, the XRD patterns of h-BN single crystals synthesized using Ba-based solvents are presented, including samples prepared in our laboratory and samples from the National Institute for Materials Science, Japan (batch number: M987). All samples exhibit strong diffraction peaks corresponding to h-BN(0002), (0004), and (0006) (JCPDF: 34-0421). Weak diffraction peaks belonging to the (10-12) crystal plane were observed in the 46°–60° range, possibly due to slight tilting of a few crystals during XRD measurements. Notably, no impurity phases such as rhombohedral boron nitride (r-BN) or Bernal stacked boron nitride (b-BN) were detected in any of the samples. Figure 4 As shown in b, the (0006) diffraction peaks of h-BN single crystals prepared by Ba-based and Ba-Mg DMB-based solvents both exhibit Gaussian line shapes, with full width at half maximum (FWHM) of 0.07° and 0.09°, respectively. These values ​​are significantly lower than the FWHM (~0.15°) of crystals obtained by the Mg-based solvent method reported in the literature, and are comparable to high-quality crystals synthesized by the National Institute for Materials Science in Japan using Ba- (FWHM ~0.06°) and Sr-based (FWHM ~0.05°) solvents used in our laboratory.

[0064] Figure 4 c shows the Raman spectra of h-BN single crystals prepared using Ba-Mg DMB-based solvent (pink curve), Ba-based solvent from our laboratory (blue curve), and Ba-based solvent from the National Institute for Materials Science, Japan (orange curve), respectively. All samples are at 1366 cm⁻¹. -1 A single symmetrical peak is observed at this point, corresponding to the E peak between boron and nitrogen atoms within the h-BN layer.2g The vibration modes, with FWHM values ​​of 8.72 cm, were observed. -1 8.68 cm -1 and 8.71 cm -1 These FWHM values ​​are similar to those reported for h-BN single crystals grown using high-temperature high-pressure and atmospheric-pressure high-temperature flux methods. To further evaluate the uniformity of growth, we randomly selected 20 single crystal samples from h-BN single crystals grown in Ba-Mg DMB-based solvents for Raman spectroscopy analysis. Figure 5 ). Figure 4 Figure d shows the FWHM distribution of these 20 single-crystal Raman spectral peaks, with a narrow distribution range (8.74 ± 0.04 cm⁻¹). -1 This confirms that the prepared h-BN single crystals have excellent batch uniformity.

[0065] Figure 4 The transmission electron microscope (TEM) image shown in image e reveals a clear lattice structure. Figure 4 The interlayer spacing of the lattice fringes in f is 0.209 nm and 0.216 nm, which matches the (0-111) and (1-100) interplanar spacings of h-BN. Selected area electron diffraction (SAED) pattern ( Figure 4 The illustration (e.g., the inset) shows clear and sharp hexagonal diffraction spots. SAED analysis of four randomly selected crystals (…) Figure 6 This further demonstrates that the single crystal exhibits excellent batch uniformity.

[0066] Example 4

[0067] To further evaluate the crystal quality, we characterized the samples using atomic force microscopy (AFM) and Raman mapping. Figure 7 Image a is an optical microscope image of the h-BN sheet prepared by mechanical exfoliation, showing a smooth, uniform surface free of cracks. AFM measurement results ( Figure 7 b) shows that the thickness of the thin film is 114.4 nm, and the surface roughness is approximately 3.3 nm. Raman mapping results indicate that the Raman peak position ( Figure 7 c) and peak intensity ( Figure 7 d) The uniform distribution throughout the entire sample area further confirms the excellent crystal quality of the exfoliated h-BN flakes. These results collectively demonstrate that h-BN single crystals grown using Ba-Mg DMB solvent exhibit high uniformity and excellent quality, showing great potential in device applications such as two-dimensional material packaging.

[0068] Example 5

[0069] To further evaluate the purity and chemical composition of the crystals, we performed X-ray photoelectron spectroscopy (XPS) analysis on h-BN single crystals grown using Ba-Mg DMB solvent. Wide-scan XPS spectra ( Figure 8 a) shows that the sample mainly consists of B 1s and N 1s signals, with only weak C 1s and O 1s signals detected. High-resolution XPS spectroscopy further reveals that C 1s ( Figure 8 b) and O1s ( Figure 8 c) The signal did not exhibit characteristics of bonding with the h-BN lattice (such as CN or CB bonds), indicating that these elements originated from surface physical adsorption caused by exposure to the atmosphere during sample storage. It is noteworthy that in Ba 3d ( Figure 8 d) and Mg 1s ( Figure 8 No corresponding signal peaks were detected in the characteristic energy regions of e), confirming that the concentrations of impurity elements such as C, O, Mg, and Ba are below the detection limit of XPS. Figure 8 As shown in f, both the B 1s peak at 190.6 eV and the N 1s peak at 398.0 eV exhibit narrow and highly symmetrical peak shapes, which are typical characteristics of sp² hybridized BN bonds. These results fully demonstrate that the prepared sample is a high-purity h-BN single crystal.

[0070] Example 6

[0071] Figure 9 This figure shows the room-temperature cathodoluminescence (CL) spectra of h-BN single crystals grown using three different solvent systems: Ba-Mg DMB-based solvent (pink curve), our laboratory's Ba-based solvent (blue curve), and NIMS Ba-based solvent (orange curve). All samples exhibit a strong deep ultraviolet (DUV) emission peak at 5.76 eV, which can be attributed to the inherent longitudinal phonon (LO)-assisted emission in high-quality h-BN single crystals. Notably, the point defect emission (~4.1 eV) signal associated with residual impurities such as carbon and oxygen is extremely weak in the h-BN single crystals grown in the Ba-Mg DMB solvent system. Calculations showed that the integrated intensity ratios of LO phonon-assisted emission (~5.76 eV) to point defect emission (~4.1 eV) for the crystals prepared in the three solvent systems were 2.565, 2.164, and 2.195, respectively, with the h-BN single crystal grown in the Ba-Mg DMB solvent system achieving the highest ratio.

[0072] As an indirect bandgap semiconductor, h-BN requires phonon scattering to maintain momentum during photon emission and absorption. Therefore, the observation of a clear phonon-assisted emission peak in the energy range of 5.7–6.0 eV is a crucial indicator of its excellent crystal quality. To this end, we measured the photoluminescence (PL) spectra of h-BN single crystals grown in Ba-based and Ba-Mg DMB solvents at a low temperature (5 K), and the results are as follows: Figure 9 As shown in b (vertical axis is logarithmic scale), both samples exhibit five distinct intrinsic phonon-assisted emission peaks above 5.7 eV, with energies of 5.76 eV, 5.79 eV, 5.86 eV, 5.89 eV, and 5.93 eV, corresponding to LO, transverse optical phonon (TO), longitudinal phonon (LA), transverse phonon (TA), and out-of-plane transverse phonon (ZA), respectively. Compared to Ba-based solvents, h-BN single crystals grown using Ba-Mg DMB solvent exhibit a sharper and more pronounced doublet fine structure in each phonon-assisted emission peak. Furthermore, a series of weaker, broad emission bands (peaks at 5.62, 5.56, 5.47, and 5.3 eV) were observed below 5.7 eV, originating from recombination localized at extended defects (such as dislocations) and stacking faults. Figure 9 c). In the 3.0–4.4 eV emission region associated with point defects ( Figure 9 d) The h-BN single crystals prepared by Ba-Mg DMB-based solvent exhibited only extremely weak characteristic peaks, the intensity of which could only be resolved on logarithmic coordinates, and were far lower than those of the samples grown by Ba-based solvent. This result is highly consistent with the room temperature CL spectral data. The above spectroscopic evidence indicates that the h-BN single crystals synthesized by the Ba-Mg DMB solvent method have lower concentrations of stacking faults, point defects, and impurities, and their crystal quality is comparable to the best results reported in the literature. Combined with XRD and Raman spectroscopy analysis (both the E2g vibrational mode and the (0002) diffraction peak have extremely narrow FWHM), it can be confirmed that both Ba-based and Ba-Mg DMB solvents can grow h-BN single crystals with excellent crystal quality, and the Ba-Mg DMB solvent has the significant advantage of improving yield while ensuring high quality.

[0073] Example 7

[0074] h-BN single crystals grown in Ba-Mg DMB solvent have demonstrated excellent performance in device applications. Figure 10 a and 10b are optical microscope images and structural schematic diagrams of the fabricated h-BN / monolayer graphene / h-BN heterojunction Hall bar device, respectively. Typically, the free-weighted wave height (FWHM) of the resistivity peak at the charge neutral point (CNP) can be used to estimate charge inhomogeneities formed by electron-hole aggregation regions at low carrier densities. Figure 10c shows the resistance R of the graphene device at a temperature of T=1.5 K. xx and conductivity σ xx The relationship between R and carrier concentration n, where the value of n is obtained by Hall resistance measurement. xx It exhibits a sharp peak at CNP, with a FWHM of δn ≈ 5.5 × 10⁻⁶. 10 cm -2 This value can be used as an upper limit for charge inhomogeneity n*. This value represents an improvement of nearly three orders of magnitude compared to graphene devices on traditional SiO2 substrates, and is comparable in performance to low-disorder graphene devices encapsulated by h-BN single crystals grown using Ba-based solvents at NIMS. To quantify the carrier density inhomogeneity n*, we... Figure 10 d plots the σ near CNP xx A double logarithmic curve varying with n. According to the theoretical model, n* is defined as the intersection of the two linear fitting curves, corresponding to σ under saturation. min Compared to σ(n) at high carrier concentration n, at T = 1.5 K, we measured n* ≈ 2 × 10⁻⁶. 10 cm -2 The low inhomogeneity indicates that the sample has low disorder and high homogeneity. Figure 10 e further illustrates the relationship between carrier mobility μ and n, where mobility is given by the Drude model μ = σ xx The values ​​were calculated using / en. When |n| > n*, the hole mobility and electron mobility reach approximately 2.8 × 10⁻⁶. 5 cm 2 V -1 s -1 and 1.9 × 10 5 cm 2 V -1 s -1 It is worth noting that, since the electron mean free path is limited by device size rather than impurity scattering, and carrier scattering mainly occurs at the boundaries of the conductive channels rather than in the internal regions, the measured μ value only represents the lower limit of this device. Analysis of n* and μ clearly shows that the hexagonal boron nitride single crystal synthesized based on Ba-Mg DMB solvent can serve as an encapsulation layer for van der Waals heterojunctions, and its performance is comparable to that of the h-BN single crystal grown based on Ba-based solvent reported by the National Institute for Materials Science in Japan, and the isotopic h-BN single crystal grown by Li et al. using the APHT method. 10 BN and 11 It is comparable to BN.

[0075] Example 8

[0076] Based on the above experimental results, the Ba-Mg DMB solvent system exhibits higher yield and better crystal quality in the synthesis of h-BN single crystals. To investigate the specific role of Mg3N2 in this system, we used XRD to characterize the phase residue after the reaction in the high-pressure chamber. Figure 10 The results showed that the main phases of the residual powder were Ba3N2, h-BN, and unreacted Mg3N2, while newly formed phases of Mg2C3, MgO, and MgB2 were also detected. No diffraction peaks of any Ba-Mg intermetallic compounds or mixed phases were observed in the spectra, indicating that the relevant magnesium components played an independent role in this process and did not form an intermediate phase with barium, thus preliminarily ruling out a synergistic catalytic mechanism at the atomic scale. The formation of Mg2C3 and MgO confirmed that Mg3N2 chemically reacted with residual carbon and oxygen impurities in the precursor, effectively purifying the crystal growth environment. The relevant chemical reaction equations are as follows:

[0077]

[0078] The mobility data obtained through encapsulation experiments on monolayer graphene directly demonstrates the high quality of the single crystal. Therefore, this purification mechanism ensures high-yield and high-quality h-BN single crystal growth.

Claims

1. A method for significantly improving the yield of high-quality hexagonal boron nitride single crystals under high temperature and high pressure, characterized in that, Includes the following steps: High-quality hexagonal boron nitride single crystals were obtained by heating and pressurizing barium-magnesium bimetallic boron-nitrogen compound solvent and deoxidized hexagonal boron nitride powder.

2. The method according to claim 1, characterized in that, The oxygen content of the deoxidized hexagonal boron nitride powder is less than or equal to 0.1 wt%.

3. The method according to claim 1, characterized in that, The barium-magnesium bimetallic boron nitrogen compound solvent is prepared by a method comprising the following steps: Barium nitride powder and deoxidized hexagonal boron nitride powder were heated to obtain a barium-based flux; then the barium-based flux was thoroughly ground and mixed with magnesium nitride powder to obtain a barium-magnesium bimetallic boron nitrogen compound solvent.

4. The method according to claim 1, wherein, The deoxidized hexagonal boron nitride powder is prepared by a method including the following steps: Under a protective atmosphere, hexagonal boron nitride powder was treated at a temperature of 2000℃ ~ 2100℃ for 2h ~ 8h.

5. The method according to claim 3, wherein, The heat treatment is performed at a temperature of 1000℃~1100℃ for a duration of 12h~48h under a protective atmosphere.

6. The method according to claim 3, wherein, The mass ratio of the barium-based co-solvent to the magnesium nitride powder is 1:3, the grinding time is 1-2 hours, and the entire process is carried out in a glove box.

7. The method according to claim 4 or 5, wherein, The protective atmosphere is selected from one or more of nitrogen, hydrogen and argon; the flow rate of the protective atmosphere is 100 sccm to 200 sccm.

8. The method according to claim 1, wherein, The mass ratio of barium-magnesium bimetallic boron nitride compound solvent to deoxygenated hexagonal boron nitride powder is (3~5):

1.

9. The method according to claim 1, wherein, The heating and pressurizing treatment is carried out under the following conditions: the pressure of the heating and pressurizing treatment is 4 GPa to 5 GPa, the temperature of the heating and pressurizing treatment is 1450℃ to 1500℃, and the time of the heating and pressurizing treatment is 20h to 30h.

10. The application of the method according to any one of claims 1 to 9 in the preparation of substrates, packaging materials, thermally conductive materials, solar-blind detectors, and deep ultraviolet light sources.

Images