Preparation method of multilayer hexagonal boron nitride single crystal film

Abstract

The invention belongs to the field of preparation of hexagonal boron nitride materials, and discloses a preparation method of a multilayer hexagonal boron nitride single crystal film, which comprises the following steps: (1) preparing an asymmetric substrate, introducing a solid film of a boron source material on the substrate, and carrying out low-pressure nitridation reaction and subsequent high-temperature annealing to obtain a boron nitride single crystal film; therefore, a plurality of discrete and directionally distributed hexagonal boron nitride single-crystal crystal domain films are formed on the substrate; and (2) carrying out MOCVD epitaxial growth on the substrate on which the plurality of discrete and directionally distributed hexagonal boron nitride single-crystal domain films are formed, so as to continuously grow by taking the hexagonal boron nitride single-crystal domain films as nucleation points to obtain a continuous and multilayer hexagonal boron nitride single-crystal film. The whole process of the preparation method is improved, a two-step method is utilized, and the growth of the hexagonal boron nitride single crystal multilayer film is completed by adopting two boron sources.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of hexagonal boron nitride materials, and more specifically, relates to a method for preparing a multilayer hexagonal boron nitride single crystal film. The obtained multilayer hexagonal boron nitride single crystal film can be particularly applied to the field of optoelectronics. Background Art

[0002] Hexagonal boron nitride (h-BN) is a III-V compound with a graphene-like layered structure in which boron atoms and nitrogen atoms are arranged alternately in a hexagonal pattern. It is a wide bandgap (bandgap width is about 5.9 eV) electronic device material. Hexagonal boron nitride has strong absorption in the deep ultraviolet band and excellent dielectric properties. It can be used in the insulating layer of MOSFET and HEMT structures, the absorption layer of deep ultraviolet detectors, and the active region of deep ultraviolet LEDs. It has great application potential in the fields of optoelectronic materials and microelectronic devices. Due to its unique layered two-dimensional structure and excellent chemical inertness, it is an ideal substrate for van der Waals epitaxy or remote epitaxy to prepare high-temperature single-crystalline electronic materials, and can be used to make high-performance flexible electronic devices. Despite all the above excellent properties, the preparation of high-crystalline quality films is still the main existing problem.

[0003] In the preparation of high crystalline quality boron nitride, there are currently several main growth methods:

[0004] 1. Chemical vapor deposition (CVD). This method uses high temperature to decompose precursors containing boron and nitrogen (such as boric acid, ammonia, etc.) to grow boron nitride on the substrate surface. This method requires very high temperatures (often up to 1500°C to 1700°C), but in recent years, some auxiliary technologies such as radio frequency plasma and ion beam can effectively reduce the temperature to promote the reaction. The main problem is that the prepared hexagonal boron nitride has poor uniformity, most of which are single layers, and a considerable part are two or three layers of boron nitride. It is difficult to prepare hexagonal boron nitride single crystal films with controllable thickness.

[0005] 2. Physical vapor deposition. In a high vacuum environment, high-purity hexagonal boron nitride is evaporated, and an ion beam is used to bombard the substrate surface to deposit a hexagonal boron nitride film. This method includes evaporation and sputtering. The film prepared by this method has high purity and good density, but the equipment is complex and costly, and it is also difficult to prepare a hexagonal boron nitride single crystal film with controllable thickness.

[0006] In recent years, the growth of single-layer single-crystal hexagonal boron nitride thin films has been achieved through chemical vapor deposition. However, due to the self-limiting effect of the growth of hexagonal boron nitride, it is difficult to grow multi-layer single-crystal boron nitride thin films, and only a single-layer hexagonal boron nitride single-crystal thin film can be obtained. Metal organic chemical vapor deposition (MOCVD) overcomes the self-limiting effect of the growth of hexagonal boron nitride by alternating the reaction of boron source and nitrogen source pulse mode, and realizes the growth of multi-layer boron nitride thin films. However, this film is polycrystalline, and it is difficult to grow multi-layer single-crystal boron nitride thin films. Summary of the invention

[0007] In view of the above defects or improvement needs of the prior art, the object of the present invention is to provide a method for preparing a multi-layer hexagonal boron nitride single crystal film, wherein the overall process of the preparation method is improved, and the growth of the hexagonal boron nitride single crystal multi-layer film is completed by a two-step method and two boron sources.

[0008] To achieve the above object, according to one aspect of the present invention, a method for preparing a multilayer hexagonal boron nitride single crystal thin film is provided, characterized in that it comprises the following steps:

[0009] (1) preparing an asymmetric substrate, introducing a solid film of a boron source material on the substrate, and forming a plurality of discrete and directionally distributed hexagonal boron nitride single crystal domain films on the substrate through a low-pressure nitridation reaction and subsequent high-temperature annealing; wherein:

[0010] When the asymmetric substrate is from an inert substrate, the inert substrate is specifically an R-plane sapphire single crystal substrate, an M-plane sapphire single crystal substrate, a C-plane sapphire single crystal substrate with a bevel of 0.2-2° from the A-plane, or a C-plane sapphire single crystal substrate with a bevel of 0.2-2° from the M-plane. When the asymmetric substrate is from a non-inert substrate, the non-inert substrate is specifically a (111)-plane single crystal nickel substrate, a (111)-plane single crystal copper substrate, a (111)-plane single crystal cobalt substrate, a (111)-plane copper-nickel alloy substrate, a (111)-plane nickel-cobalt alloy substrate, or a (111)-plane copper-cobalt alloy substrate.

[0011] The low-pressure nitriding reaction is carried out by heating under a pressure lower than 760 torr with the participation of ammonia as the nitriding gas and a protective gas; the protective gas is selected from hydrogen, nitrogen, argon and helium;

[0012] The annealing temperature used in the high temperature annealing is 1000-1750°C;

[0013] (2) performing MOCVD epitaxial growth on the substrate having a plurality of discrete and directionally distributed hexagonal boron nitride single crystal domain thin films obtained in step (1), thereby continuously growing a continuous, multi-layer hexagonal boron nitride single crystal thin film with the hexagonal boron nitride single crystal domain thin films as nucleation points; wherein:

[0014] The MOCVD epitaxy is carried out under the conditions of a pressure of 20-100 torr and a reaction temperature of 1000-1500° C. by alternately introducing a boron source gas and a nitrogen source gas in a pulsed manner; the boron source gas is triethyl boron, and the nitrogen source gas is ammonia.

[0015] As a further preferred embodiment of the present invention, the plurality of discrete and directionally distributed hexagonal boron nitride single crystal domain thin films formed in step (1) are 1 to 3 layers;

[0016] Step (2) obtains a continuous, multi-layer hexagonal boron nitride single crystal film of more than 3 layers.

[0017] As a further preferred embodiment of the present invention, in step (1), in the low-pressure nitridation reaction, the pressure condition is specifically 20-50 torr, the flow rate of ammonia is 100-1000 sccm; the heating corresponding nitridation temperature is 600-1000° C., and the insulation time is more than 10 minutes;

[0018] The annealing temperature used in the high temperature annealing is 1100-1650° C., and the annealing atmosphere used in the high temperature annealing is selected from N2, Ar, NH3, and H2; preferably, the holding time of the high temperature annealing is more than 10 minutes.

[0019] As a further preferred embodiment of the present invention, in step (1), the solid film introduced with the boron source material is obtained by introducing a liquid film containing the boron source material by spin coating or dipping, and then drying;

[0020] Preferably, the spin coating or dipping method uses a boron source solution with a boron source material concentration of 0.001-1 mol / L, the boron source material is selected from boric acid or borax, and the corresponding solid film of the boron source material obtained is specifically a boric acid solid film or a borax solid film.

[0021] As a further preferred embodiment of the present invention, the boron source solution further contains sodium hydroxide and / or potassium hydroxide, and the ratio of the total amount of sodium hydroxide and potassium hydroxide to the amount of the boron source material is 0.001-0.5:1.

[0022] As a further preferred embodiment of the present invention, in step (1), the solid film of the introduced boron source material is formed by evaporation or sputtering;

[0023] Preferably, the solid film of the boron source material is a boron trioxide solid film, and the thickness of the boron trioxide solid film is 1-50 nm;

[0024] More preferably, the solid film of the boron source material is a patterned film, which is formed by shielding with a patterned mask during evaporation or sputtering.

[0025] As a further preferred embodiment of the present invention, in step (2), the flow rate of triethyl boron used in the MOCVD epitaxy is 10-50 μmol / min, the flow rate of ammonia is 300-3000 sccm, and the molar ratio of the amount of ammonia to triethyl boron is 1000-3000:1.

[0026] As a further preferred embodiment of the present invention, in step (2), the pulse period used in the MOCVD epitaxy is 6 s; in one pulse period, the duration T of introducing triethylborane is 三乙基硼 is 4s, and the duration of ammonia introduction is T 氨气 2s;

[0027] Alternatively, the pulse period used is 3s; in one pulse period, the duration T of passing triethylborane is 三乙基硼 is 2s, and the duration of ammonia introduction is T 氨气 is 1s.

[0028] As a further preferred embodiment of the present invention, in step (1), when the asymmetric substrate is an M-plane sapphire single crystal substrate, a C-plane sapphire single crystal substrate with a bevel of 0.2-2° to the A-plane, or a C-plane sapphire single crystal substrate with a bevel of 0.2-2° to the M-plane, the substrate has been pre-heat treated; the heat treatment is specifically heat treated at 1000-1500°C for 2-10 hours in an air atmosphere.

[0029] Through the above technical scheme conceived by the present invention, compared with the prior art, the present invention uses a two-step method and two boron sources to complete the growth of hexagonal boron nitride single crystal multilayer film, wherein, in the first step, firstly, an asymmetric substrate is prepared, and the boron source is formed on the substrate by spin coating, dipping, evaporation or magnetron sputtering to form a solid film of the boron source material, and then ammonia is passed under low pressure, and a nitridation reaction is carried out and a high-temperature thermal annealing treatment is performed to form a discrete and directionally distributed hexagonal single crystal boron nitride film; in the second step, a continuous and uniform hexagonal single crystal boron nitride multilayer film is prepared by MOCVD process using the discrete and directionally distributed hexagonal single crystal boron nitride as the nucleation site, triethyl boron as the boron source, and ammonia as the nitrogen source.

[0030] Specifically, the present invention can achieve the following beneficial effects:

[0031] 1) The present invention can obtain a multi-layer single-crystalline hexagonal boron nitride continuous film (the number of layers is greater than 3 and the thickness is greater than 1.1 nm). Compared with the one-step CVD method that can only obtain a few-layer single-crystalline hexagonal boron nitride continuous film (the number of layers is 1-3 and the thickness ranges from 0.3 nm to 1.1 nm), the method of the present invention overcomes the self-limiting effect of the growth of hexagonal boron nitride and obtains a multi-layer continuous film.

[0032] 2) Compared with the one-step MOCVD method which can only obtain a multilayer polycrystalline boron nitride film (in the one-step MOCVD method, the nucleation points are random, and the arrangement direction of the initial crystal domains corresponding to the nucleation points is also random, and the crystal orientation distribution between different crystal domains is irregular, so a polycrystalline film is finally formed), the multilayer single-crystal hexagonal boron nitride film obtained by the present invention uses a two-step reaction. The first step of the reaction generates a discrete, directionally distributed hexagonal single-crystal boron nitride film (the crystal orientation distribution between different crystal domains remains consistent). These discrete, directionally distributed hexagonal single-crystal boron nitrides will serve as nucleation sites during the second step of MOCVD epitaxy, thereby obtaining a continuous and uniform hexagonal single-crystal boron nitride film.

[0033] 3) In the method of the present invention, the first step of the low-pressure nitridation reaction is based on an asymmetric substrate (e.g., the substrate is an R-plane sapphire single crystal substrate, an M-plane sapphire single crystal substrate after heat treatment, a C-plane sapphire single crystal substrate with an A-plane bevel of 0.2-2° after heat treatment, and a C-plane sapphire substrate with an M-plane bevel of 0.2-2° after heat treatment, and the heat treatment is specifically heat treatment at 1000-1500°C for 2-10 hours in an air atmosphere; or, the substrate is a (111)-plane single crystal nickel substrate, a (111)-plane single crystal copper substrate, a (111)-plane single crystal cobalt substrate, a (111)-plane copper-nickel alloy substrate, a (111)-plane nickel-cobalt alloy substrate, or a (111)-plane copper-cobalt alloy substrate), and the substrate surface has asymmetry, so that it is more conducive to obtaining a discrete and directionally distributed hexagonal single crystal boron nitride film through the interaction between the substrate and the reactants in the low-pressure nitridation reaction.

[0034] 4) Moreover, compared with other methods, the present invention can accurately control the thickness of the multi-layer single crystal hexagonal boron nitride film by controlling the second step MOCVD epitaxy, which is beneficial to industrial production and large-scale batch preparation. In actual preparation, according to the different designed thicknesses, the present invention can accurately control the thickness of the multi-layer single crystal hexagonal boron nitride film by adjusting the number of MOCVD pulses. The present invention can obtain large-sized, high-quality multi-layer single crystal hexagonal boron nitride. Since the method of the present invention can obtain an adjustable, multi-layer, high-crystallization quality single crystal boron nitride film, it has great potential in practical applications.

[0035] 5) The present invention prepares a multilayer single-crystal hexagonal boron nitride continuous film by a two-step method, wherein the first step is to use an asymmetric substrate to perform a low-pressure nitridation reaction under a pressure condition lower than 760 torr, and cooperate with subsequent high-temperature annealing at a temperature of 1000-1750°C, so that a discrete, directionally distributed hexagonal single-crystal boron nitride film can be obtained in the first step. This directionally distributed film is greatly affected by the substrate, low-pressure conditions, and annealing temperature, and it is necessary to strictly control the asymmetry of the substrate used in the first step, the low-pressure conditions, and the annealing temperature; accordingly, a few layers (1-3 layers) of multiple discrete and directionally distributed hexagonal boron nitride single crystal domain films (thickness ranges from 0.3nm to 1.1nm) are obtained. The second step MOCVD method relies on the discrete and directionally distributed hexagonal single crystal boron nitride film obtained in the first step to continue to obtain a multi-layer single crystal hexagonal boron nitride continuous film (the number of layers is greater than 3 layers, and the thickness is greater than 1.1nm), which can combine the advantages of MOCVD industrial production (the sample size prepared by MOCVD can reach the wafer level), and is conducive to the rapid and stable growth of wafer-level multi-layer single crystal hexagonal boron nitride continuous film. The two-step method of the present invention for preparing a multi-layer single crystal hexagonal boron nitride continuous film is more conducive to realizing process control and realizing the growth of a wafer-level multi-layer single crystal hexagonal boron nitride continuous film. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is an optical microscope image of hexagonal single crystal boron nitride after low-pressure nitridation of the C / A plane 0.5° sapphire substrate obtained in Example 1 (i.e., a sapphire substrate with a C plane beveled 0.5° from the A plane).

[0037] Figure 2 This is the Raman spectrum of the hexagonal single crystal boron nitride obtained in Example 1 after low-pressure nitridation of the C / A plane 0.5° sapphire substrate.

[0038] Figure 3 This is an AFM image of the multi-layer single crystal boron nitride film grown by MOCVD epitaxially on the C / A plane 0.5° sapphire substrate obtained in Example 1.

[0039] Figure 4 This is a TEM image of a multilayer single crystal boron nitride film grown by MOCVD epitaxially on a C / A plane 0.5° sapphire substrate obtained in Example 1.

[0040] Figure 5 This is the Raman spectrum of the multi-layer single crystal boron nitride film grown by MOCVD epitaxially on the C / A plane 0.5° sapphire substrate obtained in Example 1.

[0041] Figure 6 Schematic diagram of the preparation process of Example 1. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0043] Take the introduction of a solid film of a boron source material onto a substrate using a spin coating method of a boron source solution as an example. Figure 6 As shown, the method for preparing a multilayer single crystal hexagonal boron nitride film of the present invention may include the following steps:

[0044] 1) Prepare an asymmetric substrate

[0045] Asymmetric substrates can be used directly. For example, if an R-plane sapphire single crystal substrate is used, it can be used directly because it is asymmetric in itself. For another example, if a metal substrate is used, such as a (111)-plane single crystal nickel substrate, a (111)-plane single crystal copper substrate, a (111)-plane single crystal cobalt substrate, a (111)-plane copper-nickel alloy substrate, a (111)-plane nickel-cobalt alloy substrate, or a (111)-plane copper-cobalt alloy substrate, they can be used directly because they are asymmetric in themselves.

[0046] For substrates that do not exhibit asymmetry, the substrate can be annealed first, and an asymmetric substrate can be obtained by heat treatment. For example, if an inert substrate is used, such as a sapphire substrate with a C / A plane of 0.5° and an M plane, the sapphire substrate can be placed in a muffle furnace, heat treated at 1000-1500°C for 2-10 hours in an air atmosphere, and then taken out for use, thereby obtaining an asymmetric substrate.

[0047] Of course, a stacked substrate (e.g., single crystal nickel (111) / sapphire stacked substrate) may also be used. For example, nickel, copper, and cobalt metal films may be first evaporated on the surface of a sapphire substrate. The evaporated sample is then heat treated at 1000-1500°C for 2-10 hours in an argon-hydrogen mixed atmosphere (e.g., hydrogen content 5%; volume percentage, the same below) to obtain a (111) surface metal substrate to be taken out for use.

[0048] 2) Substrate pretreatment

[0049] The substrate is ultrasonically cleaned in acetone, isopropanol, and deionized water for 5-10 minutes, blown dry with nitrogen, and then treated with an oxygen plasma treatment machine for 5-10 minutes;

[0050] 3) Prepare the boron source solution

[0051] The boron source solution (boron source is boric acid, borax) is configured with a content of 0.001-1 mol / L, and more preferably with a content of 0.001-0.5 mol / L. In addition, in order to improve the reaction activity, sodium hydroxide (or potassium hydroxide) can be added as a catalyst (the catalyst is optional), and the molar ratio to the boron source can be, for example, 0.001-0.5.

[0052] 4) Boron source solution spin coating

[0053] Add the prepared boric acid solution to the substrate, spin-coat at 800-1500 rpm for 20-60 seconds, then spin-coat at 2000-3000 rpm for 20-90 seconds and air-dry naturally;

[0054] 5) Low pressure nitriding and high temperature annealing

[0055] Taking the use of MOCVD equipment for low-pressure nitridation reaction as an example, the spin-coated substrate can be placed in an MOCVD reaction chamber, and under a pressure condition lower than 760 torr (such as a pressure of 20-50 torr), in a hydrogen or nitrogen atmosphere, the temperature is raised to 600-1000° C., 100-2000 sccm of ammonia is introduced, and the temperature is kept for 10-30 minutes; after the temperature is kept, the temperature is continued to be raised to 1000-1750° C. (such as 1100-1650° C., 1100-1500° C.), and in an ammonia atmosphere, the temperature is kept for 10-30 minutes, and high-temperature annealing is performed.

[0056] 6) MOCVD epitaxial growth

[0057] At a temperature of 1000-1500°C, maintaining a pressure of 20-100 torr (e.g., 50-100 torr), triethylboron and ammonia are alternately introduced in pulses, the flow rate of triethylboron is 10-50 μmol / min, the flow rate of ammonia is 300-3000 sccm, and the pulse period is T 三乙基硼 / T 氨气 =4s / 2s or 2s / 1s (one pulse cycle includes the time T of passing triethylborane alone) 三乙基硼 And the time T for ammonia to be introduced alone 氨气 , period T = T 三乙基硼 +T 氨气 ; T 三乙基硼 / T 氨气 = 4s / 2s, for example, T 三乙基硼 =4s, T 氨气 =2s), the molar ratio of ammonia to triethylboron is in the range of 1000-3000. According to the different thickness requirements of the multilayer boron nitride single crystal film, different pulse times can be set based on the pulse period to achieve the required thickness.

[0058] In addition to the solid film of the boron source material introduced on the substrate by the boron source solution spin coating method, the solid film of the boron source material can also be formed by the dipping method. Similar to the spin coating method, a 0.001-1mol / L boron source solution (boron source is boric acid, borax; sodium hydroxide, potassium hydroxide can be optionally added as a catalyst) can be used to immerse and remove the solid film and then dry it. Of course, in addition to the solid film of the boron source material introduced by the solution method with the subsequent drying treatment, the solid film of the boron source material (such as boron trioxide, boric acid, borax) can also be formed by evaporation or sputtering, and the thickness can be controlled to be 1-50nm. In addition, the low-pressure nitridation reaction in the above step 5 can be carried out in a low-pressure CVD device in addition to using MOCVD equipment.

[0059] The following are specific embodiments:

[0060] Example 1

[0061] 1) preparing a 2-inch C / A plane 0.5° sapphire substrate, placing the C / A plane 0.5° sapphire substrate in a muffle furnace, heat treating it at 1200° C. for 4 hours in an air atmosphere, and then taking it out for use;

[0062] 2) Weigh 3.09 g of boric acid and 0.2 g of sodium hydroxide, add deionized water, and prepare a 0.5 mol / L boric acid solution;

[0063] 3) The substrate was ultrasonically cleaned in acetone, isopropanol, and deionized water for 5 minutes, and dried with nitrogen; then treated with an oxygen plasma treatment machine at a power of 50W for 5 minutes;

[0064] 4) Add the prepared boric acid solution onto the substrate, spin coat it at 1000 rpm for 20 seconds, then spin coat it at 2000 rpm for 70 seconds and then air dry it naturally.

[0065] 5) Place the spin-coated substrate in a MOCVD reaction chamber, raise the temperature to 900°C under a hydrogen atmosphere at a pressure of 50 torr, introduce 200 sccm ammonia, and keep warm for 10 minutes; after keeping warm, continue to raise the temperature to 1200°C, keep warm for 30 minutes in an ammonia atmosphere, and perform high-temperature annealing.

[0066] 6) At the high temperature annealing temperature in step 5, the pressure is kept constant at 50 torr, and triethylboron and ammonia are alternately introduced in pulses, the flow rate of triethylboron is 18 μmol / min, the flow rate of ammonia is 1000 sccm, and the pulse period is T 三乙基硼 / T 氨气 =4s / 2s, and the molar ratio of ammonia to triethylboron is 2300. The time of pulse MOCVD epitaxy in this step is 1h.

[0067] In the same batch of experiments in Example 1, an experimental sample was taken after step 5 was completed and characterized. The optical microscope image of the sample in step 5 is as follows: Figure 1 As shown, it can be seen that discrete single crystal boron nitride (independent of each other) is formed on the substrate, and these single crystal boron nitrides have a fixed and consistent orientation (such as Figure 1 As shown, these single crystal films have a triangular shape, and the arrangement directions of each triangular shape are consistent with each other). At the same time, the sample of step 5 is subjected to Raman testing, and the results are as follows Figure 2 As shown, it can be seen that the obtained product is hexagonal boron nitride (BN).

[0068] Another experimental sample was taken and characterized after step 6. The AFM test results were as follows: Figure 3 As shown in FIG. 1 , it can be seen that the discrete, oriented single-crystal hexagonal boron nitride film obtained after low-pressure nitridation and high-temperature heat treatment is subjected to MOCVD epitaxy to obtain a continuous, multi-layer single-crystal hexagonal boron nitride film. At the same time, the sample in step 6 is subjected to TEM test, and the results are as follows Figure 4 As shown in FIG. 1 , the continuous, multi-layer single crystal hexagonal boron nitride film obtained by MOCVD epitaxy has a thickness of 4 to 5 nm. In addition, the sample in step 6 was subjected to Raman testing, and the results are shown in FIG. Figure 5 shown.

[0069] Example 2

[0070] 1) placing a sapphire substrate with a 200 nm nickel film deposited thereon in a tube furnace, in an argon-hydrogen mixed atmosphere (hydrogen content 5%), maintaining a mixed gas flow rate of 200 sccm, heat-treating at 1450° C. for 4 hours, and then taking it out for use;

[0071] 2) Weigh 3.09 g of boric acid and add deionized water to prepare a 0.5 mol / L boric acid solution;

[0072] 3) The substrate was ultrasonically cleaned in acetone, isopropanol, and deionized water for 5 min, and then dried with nitrogen;

[0073] 4) Add the prepared boric acid solution onto the substrate, spin coat it at 1000 rpm for 20 seconds, then spin coat it at 2000 rpm for 70 seconds and then air dry it naturally.

[0074] 5) Place the spin-coated substrate in a MOCVD reaction chamber, raise the temperature to 900°C under a hydrogen atmosphere at a pressure of 50 torr, introduce 200 sccm ammonia, and keep warm for 10 minutes; after keeping warm, continue to raise the temperature to 1000°C, keep warm for 30 minutes in an ammonia atmosphere, and perform high-temperature annealing.

[0075] 6) At the high temperature annealing temperature in step 5, the pressure is kept constant at 50 torr, and triethylboron and ammonia are alternately introduced in pulses, the flow rate of triethylboron is 18 μmol / min, the flow rate of ammonia is 1000 sccm, and the pulse period is T 三乙基硼 / T 氨气 =4s / 2s, the molar ratio of ammonia to triethylborane is 2300.

[0076] Example 3

[0077] 1) placing a sapphire substrate with a 200 nm nickel film deposited thereon in a tube furnace, in an argon-hydrogen mixed atmosphere (hydrogen content 5%), maintaining a mixed gas flow rate of 200 sccm, heat-treating at 1450° C. for 4 hours, and then taking it out for use;

[0078] 2) The substrate was ultrasonically cleaned in acetone, isopropanol, and deionized water for 5 min, and then dried with nitrogen;

[0079] 3) Using laser direct writing to make a hollow mask with micro-nano patterns, and placing the mask on the metal surface of the substrate for shielding;

[0080] 4) Weigh 5 g of boron trioxide powder, place it in an electron beam evaporation device, and evaporate a 30 nm boron trioxide film.

[0081] 5) Place the substrate with the patterned boron trioxide film evaporated in the MOCVD reaction chamber, heat it to 900°C under a hydrogen atmosphere at a pressure of 20 torr, introduce 200 sccm ammonia, and keep it warm for 10 minutes; after keeping warm, continue to heat it to 1000°C, keep it warm for 30 minutes in an ammonia atmosphere, and perform high temperature annealing.

[0082] 6) At the high temperature annealing temperature in step 5, the pressure is kept constant at 20 torr, and triethylboron and ammonia are alternately introduced in pulses, the flow rate of triethylboron is 18 μmol / min, the flow rate of ammonia is 1000 sccm, and the pulse period is T 三乙基硼 / T 氨气 =4s / 2s, the molar ratio of ammonia to triethylborane is 2300.

[0083] Example 4

[0084] 1) Place a C / A surface 0.5° sapphire substrate in a muffle furnace, heat treat it at 1200°C for 4 hours in an air atmosphere, and then take it out for use;

[0085] 2) Weigh 19.05 g of borax and 0.2 g of sodium hydroxide, add deionized water, and prepare a 0.5 mol / L borax solution;

[0086] 3) The substrate was ultrasonically cleaned in acetone, isopropanol, and deionized water for 5 minutes, and dried with nitrogen; then treated with an oxygen plasma treatment machine at a power of 50W for 5 minutes;

[0087] 4) Add the prepared boric acid solution onto the substrate, spin coat it at 1000 rpm for 20 seconds, then spin coat it at 2000 rpm for 70 seconds and then air dry it naturally.

[0088] 5) Place the spin-coated substrate in a MOCVD reaction chamber, raise the temperature to 900°C under a hydrogen atmosphere at a pressure of 50 torr, introduce 200 sccm of ammonia, keep the temperature for 10 minutes, and then take out the substrate;

[0089] 6) Place the substrate in a high temperature annealing furnace, raise the temperature to 1650°C, keep it warm for 1 hour in a nitrogen atmosphere, and take it out after high temperature annealing.

[0090] 7) Place the substrate processed in step 6 in the MOCVD reaction chamber, raise the temperature to 1300°C, set the reaction pressure to 50 torr, and alternately introduce triethylboron and ammonia in pulses, with a triethylboron flow rate of 36 μmol / min, an ammonia flow rate of 2000 sccm, and a pulse period of T 三乙基硼 / T 氨气 =2s / 1s, the molar ratio of ammonia to triethylborane is 2300.

[0091] Example 5

[0092] 1) Place a C / A surface 0.5° sapphire substrate in a muffle furnace, heat treat it at 1200°C for 4 hours in an air atmosphere, and then take it out for use;

[0093] 2) Weigh 6.18 g of boric acid and add deionized water to prepare a 1 mol / L boric acid solution;

[0094] 3) The substrate was ultrasonically cleaned in acetone, isopropanol, and deionized water for 5 min, dried with nitrogen, and then treated with an oxygen plasma treatment machine at a power of 50 W for 5 min;

[0095] 4) The substrate is immersed in the boric acid solution by the Dipping method, and after confirming that the surface of the substrate is wetted to form a liquid film, it is taken out and naturally air-dried.

[0096] 5) Place the spin-coated substrate in a MOCVD reaction chamber, raise the temperature to 900°C under a hydrogen atmosphere at a pressure of 100 torr, introduce 200 sccm ammonia, and keep warm for 10 minutes; after keeping warm, continue to raise the temperature to 1000°C, keep warm for 30 minutes in an ammonia atmosphere, and perform high-temperature annealing.

[0097] 6) At the high temperature annealing temperature of step 5, the pressure is maintained at 100 torr, and triethylboron and ammonia are introduced alternately in pulses, the flow rate of triethylboron is 50 μmol / min, the flow rate of ammonia is 3500 sccm, the pulse period is Ttriethylboron / Tammonia=4s / 2s, and the molar ratio of the amount of ammonia introduced to triethylboron is 3000.

[0098] The above embodiments are only examples. For example, if the substrate used is different (eg, the material and crystal plane of the substrate are different), then although the crystal domains generated by the first step of the low-pressure nitridation reaction are still discrete and directionally distributed, the specific arrangement direction will change.

[0099] In the present invention, C / A 0.2-2° sapphire substrate refers to a sapphire substrate with the C plane beveled 0.2-2° to the A plane, and C / M0.2-2° sapphire substrate refers to a sapphire substrate with the C plane beveled 0.2-2° to the M plane. The R plane, M plane, C plane, and A plane of sapphire meet the conventional definition.

[0100] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a multilayer hexagonal boron nitride single crystal film, characterized in that: The following steps are involved: (1) preparing an asymmetric substrate, introducing a solid film of a boron source material on the substrate, and forming a plurality of discrete and directionally distributed hexagonal boron nitride single crystal domain films on the substrate through a low-pressure nitridation reaction and subsequent high-temperature annealing; wherein: When the asymmetric substrate is from an inert substrate, the inert substrate is specifically an R-plane sapphire single crystal substrate, an M-plane sapphire single crystal substrate, a C-plane sapphire single crystal substrate with a bevel of 0.2-2° from the A-plane, or a C-plane sapphire single crystal substrate with a bevel of 0.2-2° from the M-plane. When the asymmetric substrate is from a non-inert substrate, the non-inert substrate is specifically a (111)-plane single crystal nickel substrate, a (111)-plane single crystal copper substrate, a (111)-plane single crystal cobalt substrate, a (111)-plane copper-nickel alloy substrate, a (111)-plane nickel-cobalt alloy substrate, or a (111)-plane copper-cobalt alloy substrate. The low-pressure nitriding reaction is carried out by heating under a pressure lower than 760 torr with the participation of ammonia as the nitriding gas and a protective gas; the protective gas is selected from hydrogen, nitrogen, argon and helium; The annealing temperature used in the high temperature annealing is 1000-1750°C; (2) performing MOCVD epitaxial growth on the substrate having a plurality of discrete and directionally distributed hexagonal boron nitride single crystal domain thin films obtained in step (1), thereby continuously growing a continuous, multi-layer hexagonal boron nitride single crystal thin film with the hexagonal boron nitride single crystal domain thin films as nucleation points; wherein: The MOCVD epitaxy is carried out under the conditions of a pressure of 20-100 torr and a reaction temperature of 1000-1500° C. by alternately introducing a boron source gas and a nitrogen source gas in a pulsed manner; the boron source gas is triethyl boron, and the nitrogen source gas is ammonia.

2. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: The plurality of discrete and directionally distributed hexagonal boron nitride single crystal domain thin films formed in step (1) are 1 to 3 layers; Step (2) obtains a continuous, multi-layer hexagonal boron nitride single crystal film of more than 3 layers.

3. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: In step (1), in the low-pressure nitridation reaction, the pressure condition is specifically 20-50 torr, the flow rate of ammonia is 100-1000 sccm; the heating corresponding nitridation temperature is 600-1000° C., and the insulation time is more than 10 minutes; The annealing temperature used in the high temperature annealing is 1100-1650° C., and the annealing atmosphere used in the high temperature annealing is selected from N2, Ar, NH3, and H2; preferably, the holding time of the high temperature annealing is more than 10 minutes.

4. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: In step (1), the solid film introduced with the boron source material is obtained by introducing a liquid film containing the boron source material by spin coating or dipping, and then drying; Preferably, the spin coating or dipping method uses a boron source solution with a boron source material concentration of 0.001-1 mol / L, the boron source material is selected from boric acid or borax, and the corresponding solid film of the boron source material obtained is specifically a boric acid solid film or a borax solid film.

5. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 4, characterized in that: The boron source solution also contains additional sodium hydroxide and / or potassium hydroxide dissolved therein, and the ratio of the total amount of sodium hydroxide and potassium hydroxide to the amount of the boron source material is 0.001-0.5:

1.

6. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: In step (1), the solid film of the introduced boron source material is formed by evaporation or sputtering. Preferably, the solid film of the boron source material is a boron trioxide solid film, and the thickness of the boron trioxide solid film is 1-50 nm; More preferably, the solid film of the boron source material is a patterned film, which is formed by shielding with a patterned mask during evaporation or sputtering.

7. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: In step (2), the flow rate of triethyl boron used in the MOCVD epitaxy is 10-50 μmol / min, the flow rate of ammonia is 300-3000 sccm, and the molar ratio of the amount of ammonia to triethyl boron is 1000-3000:

1.

8. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: In step (2), the pulse period used in the MOCVD epitaxy is 6 s; in one pulse period, the duration T of introducing triethylborane is 三乙基硼 is 4s, and the duration of ammonia introduction is T 氨气 2s; Alternatively, the pulse period used is 3s; in one pulse period, the duration T of passing triethylborane is 三乙基硼 is 2s, and the duration of ammonia introduction is T 氨气 is 1s.

9. The method for preparing a multilayer hexagonal boron nitride single crystal thin film according to claim 1, characterized in that: In step (1), when the asymmetric substrate is an M-plane sapphire single crystal substrate, a C-plane 0.2-2° beveled sapphire single crystal substrate, or a C-plane 0.2-2° beveled sapphire single crystal substrate, the substrate has been pre-heat treated; the heat treatment is specifically heat treated at 1000-1500°C for 2-10 hours in an air atmosphere.

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