Self-supporting large-area hexagonal boron nitride single crystal and preparation method thereof

Abstract

A self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof belong to the field of single crystal material preparation and semiconductor detectors. The specific steps of the preparation method include: mechanical alloying to prepare a boron source; high-temperature melting; segregation synthesis of hexagonal boron nitride; chemical corrosion and stripping to obtain a self-supporting large-area hexagonal boron nitride single crystal. The method achieves the preparation of a large-area hexagonal boron nitride single crystal by prefabricating a boron source and introducing protective gas and reaction gas in sections during a high-temperature experiment. At present, the main problem in the research and application of hexagonal boron nitride is that the single crystal size is not large enough, the quality is not high enough, and commercial use is impossible. This method can obtain hexagonal boron nitride single crystals with a lateral size of up to several centimeters, which is of great significance for future commercial applications on hexagonal boron nitride. It also has certain inspiration for the synthesis, research and application of other semiconductor materials or two-dimensional materials.

Description

Technical Field

[0001] The invention belongs to the field of single crystal material preparation and semiconductor detectors, and in particular relates to a self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof. Background Art

[0002] With the development of the semiconductor industry, silicon-based transistors are constantly upgraded, and their scale has approached the physical limit. Traditional silicon-based materials seem to be unable to meet people's requirements for device miniaturization and high integration. Against this background, scientists have launched a research boom on two-dimensional materials, and hexagonal boron nitride, as a representative material with ultra-wide bandgap in two-dimensional materials, has naturally attracted much attention. Due to its unique properties, it has great application potential in deep ultraviolet optoelectronic devices, neutron detectors, single photon emitters and supercapacitors.

[0003] However, to realize the practical application of boron nitride, especially in integrated devices, high-quality, large-size single crystal growth is a prerequisite. At present, the methods for obtaining high-quality hexagonal boron nitride single crystals mainly include chemical vapor deposition, high temperature and high pressure method, and metal co-solvent method. Chemical vapor deposition is to use volatile substances containing boron and nitrogen elements as reaction precursors, deposit them on the substrate surface to react and synthesize hexagonal boron nitride. Among them, the use of catalytic transition metals as substrates is the preferred method for preparing high-quality hexagonal boron nitride single crystals with a single atomic layer and / or a few atomic layers. The high temperature and high pressure method is similar to the preparation method of artificial diamond. It requires the use of large-scale high-pressure equipment. The raw materials containing boron source and nitrogen source are continuously subjected to GPa-level hydrostatic pressure for several hours at high temperature to achieve nucleation growth under the action of catalysts and seeds. However, due to the limitation of high temperature and high pressure chambers, the size of the hexagonal boron nitride single crystals prepared is usually very small, which limits the commercial application potential of boron nitride. The metal flux rule takes full account of the thermodynamic stability of hexagonal boron nitride at high temperature and atmospheric pressure. After continuous improvement of solvents and experimental conditions, it has been developed in recent years. In 2021, Cheng Xiaolong, Wang Wenjun and others from the Institute of Physics, Chinese Academy of Sciences, used copper-chromium alloy as a solvent to grow hexagonal boron nitride single crystals with a thickness of 20μm and a single crystal size of 6mm (Zhang, NF Yang, NJ Wang, WJ Zhong, X. Chen, XL, Growth of hexagonal boron nitride crystals at atmospheric pressure from Cu-Cr flux, Journal of Crystal Growth, 2021, 562, 126074.). In the same year, James H. Edgar's research group at Kansas State University in the United States used iron as a solvent to prepare hexagonal boron nitride single crystals with a single crystal size of more than 1.5 cm and a thickness of 10-20 μm (Li, JH Wang, JY Zhang, XTElias, C. Ye, GHE vans, D. Eda, G. Redwing, JM Casabois, G. Gil, B. Valvin, P. He, R. Bin, L. Edgar, JH, Hexagonal Boron Nitride Crystal Growth from Iron, a Single Component Flux, 2021, 15, 7032.). Although the metal flux method can obtain high-quality and large hexagonal boron nitride single crystals while reducing experimental costs, the alloy and boron nitride systems will be affected by their respective thermal expansion coefficients and crystal constants during the cooling process, resulting in the generation of stress and the appearance of grain boundaries and macro cracks.In addition, after the reaction is cooled, the precipitated boron nitride film usually needs to be peeled off from the alloy block. The current treatment methods include mechanical peeling, chemical peeling, etc., which will not only damage the single crystal or introduce impurities, but also cause loss of single crystal size, resulting in further reduction of the available size. Therefore, how to prepare single crystals with high crystallinity, low defect density, lateral size above centimeters and thickness, excellent homogeneity is still one of the key issues that need to be solved in the field of hexagonal boron nitride single crystal preparation, and it is also the basis for boron nitride to achieve real device applications. Summary of the invention

[0004] In view of this, the present invention provides a self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof, which inhibits the deposition of metal nitride impurities by alloying the boron source and introducing protective gas and reaction gas in stages during the melting process and the cooling and segregation process, improves the growth efficiency and scale of the single crystal, and successfully synthesizes a large-area hexagonal boron nitride single crystal. A large-area, complete and continuous self-supporting boron nitride single crystal film can be obtained by simple chemical etching. The method has a simple process, safe and cheap raw materials, and has very good implementation and application prospects. The prepared boron nitride single crystal film has a large lateral size, which can reach the centimeter level, has a low impurity content, good insulation, and controllable thickness. It can be used for neutron detectors, optoelectronic devices, heat dissipation layers, two-dimensional material growth templates, etc.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] Self-supporting large-area hexagonal boron nitride single crystal and preparation method thereof, the hexagonal boron nitride single crystal has the following excellent properties: the overall crystal plane is (002), the side edges are [1-100] and [11-20] directions, and the single crystal is excellent; the lateral size exceeds 3 cm, and the area exceeds 5 cm 2 , and the area can be increased with the increase of equipment and container volume, the thickness can be 5-100μm, and the size of a single crystal domain is more than 1mm; the transmittance from short-wave infrared to near-ultraviolet light is more than 85%; the electrical insulation is good, and the surface resistivity is 10 15 Ω(1000TΩ) level.

[0007] The method for preparing the self-supporting large-area hexagonal boron nitride single crystal comprises:

[0008] Step 1, preparing a boron source by mechanical alloying;

[0009] Step 2: high temperature melting;

[0010] Step 3, segregation synthesis of hexagonal boron nitride;

[0011] Step 4: Chemical etching and peeling to obtain a self-supporting large-area hexagonal boron nitride single crystal.

[0012] The specific method for preparing the boron source by mechanical alloying in the step 1 is as follows: metallic nickel and chromium are placed in a ball mill at a mass ratio of 1:0.1-1.5, and the two metals and elemental boron powder are placed in a ball mill at a mass ratio of 1:0.1-0.4, and the ball-to-powder weight ratio is 1.5-2.5:1. The ball mill is evacuated to vacuum, filled with argon gas to normal pressure, and ball-milled at a speed of 600-1000 rpm for 20-50 hours to finally obtain nickel-chromium-boron alloy powder.

[0013] Preferably, the mass ratio of metallic nickel, metallic chromium and elemental boron powder is 4:1:1.

[0014] Preferably, during the ball milling process, the ball-to-powder weight ratio is controlled at 2:1, the rotation speed is controlled at 700 rpm, and the ball milling time is 30 h.

[0015] The specific method of high-temperature melting in the step 2 is as follows: quickly transfer the prepared nickel-chromium-boron alloy powder to a high-temperature crucible, place the crucible in the heating zone of a tubular furnace, evacuate the furnace tube to a vacuum, pass a hydrogen / argon mixed gas to normal pressure, and repeat 3 to 5 times; then heat the furnace temperature to 1400 to 1700°C in a hydrogen / argon mixed atmosphere with a flow rate of 50 to 200 sccm, and maintain it for 1 to 4 hours after the temperature stabilizes.

[0016] Preferably, the temperature of the tube furnace is raised to 1500° C. in a hydrogen / argon mixed atmosphere with a gas flow rate of 100 sccm, and then kept at this temperature for 4 hours.

[0017] The specific method for segregating and synthesizing hexagonal boron nitride in step three is as follows: after the heat preservation is completed, nitrogen and hydrogen / argon mixed gas are introduced at a gas flow ratio of 1:0.5-1.2, with a total flow rate of 200-300sccm, and then the temperature is reduced to 1300-1600°C at a rate not higher than 6°C / h, and then naturally cooled to room temperature to obtain a hexagonal boron nitride single crystal film on the upper surface of the alloy block.

[0018] Preferably, nitrogen and hydrogen / argon mixed gas are introduced at rates of 150 sccm and 100 sccm, and the temperature is lowered to 1450° C. at a rate of 2° C. / h.

[0019] The specific method of chemically etching and stripping to obtain a self-supporting large-area hexagonal boron nitride single crystal in step 4 is as follows: the cooled alloy block is placed in a mixed solution of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1 and heated, and the temperature is controlled at 40-60°C until the boron nitride film on the upper surface falls off naturally, thereby obtaining a complete and continuous self-supporting large-area hexagonal boron nitride single crystal film.

[0020] The hexagonal boron nitride single crystal film prepared by the above method can be cut, peeled and thinned into any size and shape as needed.

[0021] The hexagonal boron nitride single crystal film prepared by the above method can be used for the insulating gate of the device, the semiconductor layer of the deep ultraviolet photodetector, the growth template of the gas two-dimensional material, etc.

[0022] The invention provides the Raman spectrum characteristics of the hexagonal boron nitride single crystal, which proves that the sample is pure hexagonal boron nitride and has no other impurity phases.

[0023] The invention provides XRD diffraction spectrum characteristics of the hexagonal boron nitride single crystal, further proving that the crystallinity of the large-area hexagonal boron nitride single crystal is extremely excellent.

[0024] The beneficial effects of the present invention are mainly reflected in:

[0025] (1) The present invention alloys the boron source in advance, pre-alloys the boron with metallic nickel and metallic chromium, and fully forms a mixed state of boron, nickel and chromium. On the one hand, the temperature of the eutectic is reduced, and on the other hand, the combination of different elements improves the stability of the precursor during the heating process.

[0026] (2) The present invention shortens the high-temperature melting time from the usual 20 to 30 hours to 1 to 4 hours by prefabricating the boron source, thereby avoiding the evaporation of the metal in a long-term high-temperature melting state. At the same time, it also reduces the time the equipment works at high temperature, reduces energy consumption, and improves productivity.

[0027] (3) The present invention adopts a design of gas segmentation, and the atmosphere in the high-temperature melting and segregation synthesis steps is different. In the high-temperature melting process, only a hydrogen / argon mixed gas is introduced, and no nitrogen-containing gas source is introduced, thereby avoiding the inhibitory effect of nitrogen on boron solubility. In the segregation synthesis step of hexagonal boron nitride, nitrogen is introduced, which can provide a nitrogen source for synthesizing hexagonal boron nitride on the one hand, and on the other hand, nitrogen changes the solubility ratio of boron and promotes the segregation of boron on the surface.

[0028] (4) The boron source prefabrication and segmented introduction of the high-temperature experimental gas proposed in the present invention ensure that no difficult-to-remove metal nitride impurities are generated at the interface between the hexagonal boron nitride single crystal and the alloy. The prepared single crystal is large and continuous in size, high in purity, and has an extremely low impurity content. A complete, continuous, self-supporting large-area hexagonal boron nitride single crystal can be peeled off using a chemical etching method, which is convenient for cutting and thinning to any size and shape as needed, and is easy for subsequent integration and application.

[0029] (5) The present invention uses metal nickel, metal chromium, elemental boron powder, gaseous nitrogen and hydrogen / argon mixed gas. The raw materials are stable in nature, easy to obtain, transport and store, and the production process is green, environmentally friendly, highly safe and inexpensive. At the same time, the chemical corrosion stripping method can easily obtain a large-area self-supporting, continuous and complete hexagonal boron nitride single crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flow chart of the preparation method of the present invention;

[0031] Figure 2 Photo of the chemical etching stripping process

[0032] Figure 3 This is a photo of peeling a large piece of hexagonal boron nitride single crystal;

[0033] Figure 4 This is a photo of a torn hexagonal boron nitride sheet;

[0034] Figure 5 This is the Raman spectrum of hexagonal boron nitride single crystal;

[0035] Figure 6 This is the XRD image of hexagonal boron nitride single crystal;

[0036] Figure 7 Photo of the sample prepared in Comparative Example 1

[0037] Figure 8 Photo of the sample prepared for Comparative Example 2 DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0039] Example 1

[0040] A self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof, wherein metal nickel, metal chromium, elemental boron powder, nitrogen, argon, and hydrogen / argon mixed gas are prepared before the experiment. Figure 1The experimental process comprises the following steps: firstly, metallic nickel, metallic chromium and elemental boron powder are placed in a ball mill of a planetary ball mill in a mass ratio of 4:1:1, the weight ratio of the ball powder is controlled at 2:1, the ball mill is evacuated to a vacuum to remove the air attached to the metal and elemental boron powder, and then argon is used as a protective gas to fill the ball mill to normal pressure, the ball mill is installed on the ball mill, the speed is set to 700 rpm, the ball milling time is 30 hours, and a nickel-chromium-boron alloy is obtained after the ball milling, and the nickel-chromium-boron alloy is quickly Transfer to a high-temperature crucible, place the high-temperature crucible in the heating zone of a tube furnace, evacuate the furnace tube, and then introduce a hydrogen / argon mixed gas with a hydrogen content of 5% to normal pressure, repeat 3 times, then introduce a hydrogen / argon mixed atmosphere with a flow rate of 100sccm to raise the temperature of the tube furnace to 1500℃ at a rate of 5℃ / min, maintain for 4h, then introduce nitrogen with a gas flow rate of 150sccm, and start cooling to 1450℃ at a rate of 2℃ / h, and finally cool naturally to room temperature. The cooled alloy block is placed in a mixed solution of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1, and finally a whole piece of hexagonal boron nitride single crystal is obtained by chemical etching and peeling at 50℃.

[0041] Figure 2 A photo of the chemical etching stripping process.

[0042] Figure 3 The hexagonal boron nitride single crystal peeled by chemical etching clearly shows that the single crystal size has reached more than 3 cm, and the single crystal size can be increased as the volume of the equipment and container increases.

[0043] Hexagonal boron nitride single crystal slices of suitable size for device preparation can be cut or torn by hand from the complete hexagonal boron nitride single crystal for later application. Figure 4 This is a manually torn hexagonal boron nitride sheet. You can see that the lateral size of the sheet is at the millimeter level, there are almost no cracks or grooves visible to the naked eye, and the sheet is extremely transparent.

[0044] Figure 5 This is the Raman spectrum of the sample. The Raman peak shown in the figure is located at 1366cm -1 , which belongs to E of hexagonal boron nitride single crystal 2g Vibration peak, half peak width less than 10cm -1 , indicating that the crystal quality is relatively good.

[0045] Figure 6 They are the XRD images of hexagonal boron nitride single crystals. In the figure, only (002) and (004) peaks can be seen, and there are no other redundant peaks, which means that the hexagonal boron nitride single crystal grown by this method is strictly perpendicular to the c-axis, and the (002) peak width is very narrow, only 0.027 degrees.

[0046] Example 2

[0047] A self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof, wherein metal nickel, metal chromium, elemental boron powder, nitrogen, argon, and hydrogen / argon mixed gas are prepared before the experiment. Figure 1 The experimental process comprises the following steps: firstly, metallic nickel, metallic chromium and elemental boron powder are placed in a ball mill of a planetary ball mill in a mass ratio of 2:2:1, the weight ratio of the ball powder is controlled at 2.5:1, the ball mill is evacuated to a vacuum to remove the air attached to the metal and elemental boron powder, and then argon is used as a protective gas to fill the ball mill to normal pressure, the ball mill is installed on the ball mill, the speed is set to 800 rpm, the ball milling time is 30 hours, and a nickel-chromium-boron alloy is obtained after the ball milling is completed, and the nickel-chromium-boron alloy is quickly Gold was transferred to a high-temperature crucible, and the high-temperature crucible was placed in the heating zone of a tube furnace. The furnace tube was evacuated to vacuum, and then a hydrogen / argon mixed gas with a hydrogen content of 5% was introduced to normal pressure, and repeated 4 times. Then, a hydrogen / argon mixed atmosphere with a flow rate of 100sccm was introduced to raise the temperature of the tube furnace to 1550℃ at a rate of 5℃ / min, and maintained for 4h. Then, nitrogen was introduced with a gas flow rate of 120sccm, and the temperature was lowered to 1450℃ at a rate of 4℃ / h, and finally cooled naturally to room temperature. The cooled alloy block was placed in a mixed solution of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1, and finally a whole piece of hexagonal boron nitride single crystal was obtained by chemical etching and stripping at 40℃.

[0048] Example 3

[0049] A self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof, wherein metal nickel, metal chromium, elemental boron powder, nitrogen, argon, and hydrogen / argon mixed gas are prepared before the experiment. Figure 1 The experimental process comprises the following steps: firstly, metallic nickel, metallic chromium and elemental boron powder are placed in a ball mill of a planetary ball mill in a mass ratio of 4:1:1, the weight ratio of the ball powder is controlled at 1.5:1, the ball mill is evacuated to a vacuum to remove the air attached to the metal and elemental boron powder, and then argon is used as a protective gas to fill the ball mill to normal pressure, the ball mill is installed on the ball mill, the speed is set to 600 rpm, the ball milling time is 40 hours, and a nickel-chromium-boron alloy is obtained after the ball milling is completed, and the nickel-chromium-boron alloy is quickly Gold was transferred to a high-temperature crucible, and the high-temperature crucible was placed in a heating zone in a tubular furnace. The furnace tube was evacuated to vacuum, and then a hydrogen / argon mixed gas with a hydrogen content of 5% was introduced to normal pressure, and repeated 5 times. Then, a hydrogen / argon mixed atmosphere with a flow rate of 100sccm was introduced to raise the temperature of the tubular furnace to 1600℃ at a rate of 4℃ / min, and maintained for 2h. Then, nitrogen was introduced with a gas flow rate of 170sccm, and the temperature was lowered to 1550℃ at a rate of 2℃ / h, and finally cooled naturally to room temperature. The cooled alloy block was placed in a mixed solution of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1, and finally a whole piece of hexagonal boron nitride single crystal was obtained by chemical etching and stripping at 40℃.

[0050] Example 4

[0051] A self-supporting large-area hexagonal boron nitride single crystal and a preparation method thereof, wherein metal nickel, metal chromium, elemental boron powder, nitrogen, argon, and hydrogen / argon mixed gas are prepared before the experiment. Figure 1 The experimental process comprises the following steps: firstly, metallic nickel, metallic chromium and elemental boron powder are placed in a ball mill of a planetary ball mill in a mass ratio of 4:1:1, the weight ratio of the ball powder is controlled at 2:1, the ball mill is evacuated to a vacuum to remove the air attached to the metal and elemental boron powder, and then argon is used as a protective gas to fill the ball mill to normal pressure, the ball mill is installed on the ball mill, the speed is set to 900 rpm, the ball milling time is 20 hours, and a nickel-chromium-boron alloy is obtained after the ball milling, and the nickel-chromium-boron alloy is quickly Transfer to a high-temperature crucible, place the high-temperature crucible in the heating zone of a tube furnace, evacuate the furnace tube, and then introduce a hydrogen / argon mixed gas with a hydrogen content of 5% to normal pressure, repeat 3 times, then introduce a hydrogen / argon mixed atmosphere with a flow rate of 100sccm to raise the temperature of the tube furnace to 1600℃ at a rate of 4℃ / min, maintain for 2h, then introduce nitrogen with a gas flow rate of 150sccm, and start cooling to 1500℃ at a rate of 2℃ / h, and finally cool naturally to room temperature. The cooled alloy block is placed in a mixed solution of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1, and finally a whole piece of hexagonal boron nitride single crystal is obtained by chemical etching and stripping at 55℃.

[0052] Comparative Example 1

[0053] In Example 1, no mechanical alloying was performed on metallic nickel, metallic chromium, and elemental boron powder, and other conditions remained unchanged, so a self-supporting large-area hexagonal boron nitride single crystal could not be obtained.

[0054] Figure 7 This is the sample prepared in Comparative Example 1.

[0055] Comparative Example 2

[0056] In Example 1, the protective gas and the reaction gas are not introduced in stages, and the system is always in a nitrogen / hydrogen / argon mixed atmosphere during the high-temperature melting and low-temperature segregation processes. Other conditions remain unchanged, and a self-supporting large-area hexagonal boron nitride single crystal cannot be obtained.

[0057] Figure 8 This is the sample prepared in Comparative Example 2.

Claims

1. Preparation method of self-supporting large-area hexagonal boron nitride single crystal, It is characterized in that The hexagonal boron nitride single crystal is a complete, continuous, self-supporting large-area hexagonal boron nitride single crystal film, with an overall crystal plane of (002), sides of [1-100] and [11-20] directions, a lateral size of more than 3 cm, and an area of ​​more than 5 cm 2 , thickness is 5~100μm, and the lateral size of a single crystal domain is more than 1 mm; high transparency, with a transmittance of more than 85% from short-wave infrared to near-ultraviolet light band; Good electrical insulation, surface resistivity is 10 15 Ω level; the preparation method comprises: Step 1: preparing nickel-chromium-boron alloy powder by mechanical alloying: wherein the mass ratio of metallic nickel and chromium is 1:0.1-1.5, and the mass ratio of the two metals to elemental boron powder is 1:0.1-0.4; Step 2, high temperature melting: quickly transfer the prepared nickel-chromium-boron alloy powder to a high temperature crucible, place the crucible in the heating temperature zone of a tube furnace, evacuate the furnace tube to vacuum, pass hydrogen / argon mixed gas to normal pressure, and repeat 3 to 5 times; then heat the furnace temperature to 1400~1700°C in a hydrogen / argon mixed atmosphere with a flow rate of 50~200sccm, and maintain it for 1~4h after the temperature stabilizes; Step 3, segregation synthesis of hexagonal boron nitride: nitrogen and hydrogen / argon mixed gas are introduced at a gas flow ratio of 1:0.5~1.2, with a total flow rate of 200~300 sccm, and the temperature is reduced to 1300~1600℃ at a rate not higher than 6℃ / h, and naturally cooled to room temperature to synthesize a hexagonal boron nitride single crystal film on the upper surface of the alloy block; Step 4: Chemical etching and stripping to obtain a self-supporting large-area hexagonal boron nitride single crystal: Place the cooled alloy block in a mixed solution of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3:1 and heat it at 40-60°C until the boron nitride film on the upper surface falls off naturally to obtain a complete and continuous self-supporting large-area hexagonal boron nitride single crystal film.

2. According to the method for preparing a self-supporting large-area hexagonal boron nitride single crystal as described in claim 1, It is characterized in that In the step 1, nickel-chromium-boron alloy powder is prepared by mechanical alloying: metallic nickel, chromium and elemental boron powder are placed in a ball mill with a ball-to-powder weight ratio of 1.5-2.5:1, the ball mill is evacuated to vacuum, filled with argon gas to normal pressure, and ball milled at a speed of 600-1000 rpm for 20-50 hours to finally obtain nickel-chromium-boron alloy powder.

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