Method for preparing multilayer single crystal rhombohedral boron nitride film

Abstract

The invention provides a method for preparing a multilayer single crystal rhombohedral boron nitride film. The method comprises the following steps: (1) respectively placing a growth source and a single crystal nickel foil in an upstream temperature zone and a downstream temperature zone of a chemical vapor deposition system; (2) introducing carrier gas into the chemical vapor deposition system, and heating the downstream temperature zone to 1300-1450 DEG C for annealing to remove an oxide layer on the surface of the nickel foil; the temperature of the upstream temperature zone is kept to be less than 60 DEG C; (3) the downstream temperature zone is cooled to 1200-1350 DEG C at a constant rate, heat preservation is conducted, so that parallel steps are formed on the surface of the nickel foil, and the parallel steps are composed of platform faces and inclined faces; (4) heating the upstream temperature zone to form a single-orientation multilayer rhombohedral boron nitride crystal nucleus; and then, the temperature of the upstream temperature zone is kept unchanged, the carrier gas is fixed, and meanwhile, the temperature of the downstream temperature zone is increased to 1300-1450 DEG C so as to grow the multi-layer single crystal rhombohedral boron nitride thin film. According to the method, in-plane lattice orientation consistency and interlayer rhombohedral stacking configuration are effectively controlled, and preparation of the large-size single crystal rhombohedral boron nitride thin film is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of two-dimensional material growth. Specifically, the present invention relates to a method for preparing multi-layer single-crystal rhombohedral boron nitride thin films. More specifically, the present invention relates to a method for growing multi-layer single-crystal rhombohedral boron nitride thin films by locking the step angle. Background Art

[0002] Due to its many excellent properties such as low dielectric constant and dielectric dissipation, large electrical bandgap, ultra-high chemical stability, excellent thermal conductivity, and no dangling bonds on the surface, layered boron nitride is regarded as an ideal two-dimensional chip-level insulator material. While rhombohedral boron nitride has these properties, the interlayer parallel stacking configuration gives it a broken inversion symmetry in both in-plane and out-of-plane directions, enabling it to achieve efficient second-order nonlinear optical frequency conversion in a wide wavelength range and exhibit non-volatile interlayer slip ferroelectricity at the atomic scale. It is a key material for new application fields such as future integrated photonics chips, ferroelectric field-effect transistors, and computing-in-memory devices.

[0003] Preparing large-area single-crystal pure-phase and controllable-thickness rhombohedral boron nitride thin films is a prerequisite for its practical application. At present, the development of boron nitride synthesis methods mainly focuses on controlling the lattice orientation of single-layer boron nitride and the thickness of multi-layer boron nitride, that is, using the rich surface structure of the substrate to regulate the orientation of single-layer boron nitride, or based on the growth mode of the growth source dissolving and precipitating in a high-solubility metal substrate to prepare multi-layer boron nitride thin films with different thicknesses.

[0004] However, so far, there is still no reliable solution for regulating the interlayer stacking mode of multi-layer boron nitride. The main reason is that the Coulomb interaction between boron nitride layers makes the antiparallel stacking configuration (i.e., the hexagonal phase) in the lowest energy state. Although traditional high-temperature and high-pressure synthesis methods can prepare rhombohedral boron nitride powder polycrystals, due to the harsh conditions required in this process, the crystal phase purity of the prepared products is relatively low, and thin film samples cannot be prepared.

[0005] Therefore, there is an urgent need for a method that can effectively control the in-plane lattice orientation and interlayer rhombohedral stacking configuration simultaneously, and realize the preparation of large-size, controllable-thickness single-crystal rhombohedral boron nitride thin films. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing multi-layer single-crystal rhombohedral boron nitride thin films. This method can effectively control the in-plane lattice orientation and interlayer rhombohedral stacking configuration simultaneously, and can realize the preparation of large-size, controllable-thickness single-crystal rhombohedral boron nitride thin films.

[0007] The above object of the present invention is achieved by the following technical solutions.

[0008] The present invention provides a method for preparing a multi-layer single-crystal rhombohedral boron nitride film, which sequentially includes the following steps:

[0009] (1) Place the growth source and the single-crystal nickel foil in the upstream temperature zone and the downstream temperature zone of the chemical vapor deposition system respectively;

[0010] (2) Introduce a carrier gas into the chemical vapor deposition system, and heat the downstream temperature zone to 1300 - 1450 °C for annealing to remove the oxide layer on the surface of the nickel foil; the temperature of the upstream temperature zone is maintained at less than 60 °C;

[0011] (3) Cool the downstream temperature zone to 1200 - 1350 °C at a constant rate and keep it warm, so that parallel steps are formed on the surface of the nickel foil, and the parallel steps are composed of a platform surface and an inclined surface;

[0012] (4) Heat the upstream temperature zone to form single-oriented multi-layer rhombohedral boron nitride crystal nuclei on the surface of the nickel foil with parallel steps; subsequently, keep the temperature of the upstream temperature zone unchanged, and fix the carrier gas, while heat the downstream temperature zone to 1300 - 1450 °C to grow a multi-layer single-crystal rhombohedral boron nitride film;

[0013] (5) Optionally, after the growth is completed, turn off the heating power supply, maintain the gas flow rate in the tube furnace unchanged, and naturally cool to room temperature to obtain a multi-layer single-crystal rhombohedral boron nitride film.

[0014] The inventors of the present application unexpectedly found that when the single-crystal nickel foil located in the downstream temperature zone experiences a temperature increase (heating to 1300 - 1450 °C), and then cools down at a constant rate (cooling to 1200 - 1350 °C), parallel steps with a certain height composed of periodic platform surfaces and inclined surfaces can be formed on the single-crystal nickel foil. Through these parallel steps, the present invention realizes the preparation of a large-area single-crystal pure-phase and thickness-adjustable rhombohedral boron nitride film.

[0015] The present invention mainly realizes the preparation of a large-area single-crystal pure-phase and thickness-adjustable rhombohedral boron nitride film by designing and constructing parallel steps on the surface of the metal substrate, where the included angle between the platform surface and the inclined surface of the steps is greater than 90°, and at the same time locking the consistent lattice orientation of each layer of boron nitride and the fixed slip vector between adjacent layers, laying a material foundation for its applications in many fields such as future photonic chips, high-density non-volatile storage, and memory-computation integrated devices.

[0016] In the present invention, the parallel steps in step (3) can have a certain height, and the parallel steps of the present invention are similar to the shape of a "staircase".

[0017] Preferably, in the method of the present invention, the method further includes the following steps before step (1): annealing industrial nickel foil in a carrier gas at 1300-1450 °C for 1-20 h to obtain the single-crystal nickel foil. In the present invention, if industrial nickel foil is used, the industrial nickel foil needs to be annealed in a carrier gas (such as hydrogen) to remove impurities such as carbon elements.

[0018] Preferably, in the method of the present invention, the carrier gas in step (2) includes argon with a flow rate of 10-1000 sccm and hydrogen with a flow rate of 0-100 sccm.

[0019] Preferably, in the method of the present invention, the annealing in step (2) is carried out for 10-60 min.

[0020] Preferably, in the method of the present invention, the constant rate in step (3) is 1-20 °C / min.

[0021] Preferably, in the method of the present invention, the heat preservation in step (3) is carried out for 1-8 h.

[0022] Preferably, in the method of the present invention, the formation of single-oriented multi-layer rhombohedral boron nitride crystal nuclei on the surface of the nickel foil with parallel steps in step (4) is carried out under the following conditions: heating the upstream temperature zone to 60-90 °C, and adjusting the volume percentage of hydrogen in the carrier gas and controlling the nucleation time.

[0023] Preferably, in the method of the present invention, the adjustment of the volume percentage of hydrogen in the carrier gas is to adjust the volume percentage of hydrogen to more than 1%.

[0024] Preferably, in the method of the present invention, the nucleation time is 0.1-2 h.

[0025] Preferably, in the method of the present invention, the growth of the multi-layer single-crystal rhombohedral boron nitride film in step (4) is carried out for 1-10 h.

[0026] Preferably, in the method of the present invention, the angle between the platform surface and the inclined surface of the parallel steps is 90° < θ < 180°. In the present invention, the angle between the platform surface and the inclined surface of the parallel steps is as Figure 1 shown by θ in.

[0027] Preferably, in the method of the present invention, the crystal plane index of the single-crystal nickel foil is Ni(hk0).

[0028] Preferably, in the method of the present invention, the size of the single-crystal nickel foil is above the centimeter scale.

[0029] Preferably, in the method of the present invention, the growth source is borane ammonia and / or borazine.

[0030] The present invention has the following beneficial effects:

[0031] 1. In the present invention, the proportion of the prepared multi-layer single-crystal rhombohedral boron nitride crystal nuclei with consistent orientation is greater than 99%, the area is above the centimeter scale, and the thickness can be achieved at 1-15 nm. The multi-layer single-crystal rhombohedral boron nitride prepared by the present invention has adjustable thickness and high quality, and has good application prospects.

[0032] 2. In the present invention, commercially available polycrystalline nickel foil can be selected. Only simple surface pretreatment (high-temperature annealing) is required to obtain a corresponding large-size single-crystal substrate, and the production cost is low.

[0033] 3. The single-crystal nickel foil selected in the present invention has crystal plane indices on the surface including but not limited to (hk0), which reduces the cost of crystal plane selection and has strong universality.

[0034] 4. The step angle locking mode proposed in the present invention has broad reference value for growing other rhombohedral two-dimensional materials (such as graphene, transition metal chalcogenides, etc.).

[0035] 5. Compared with the extreme environment (such as ultra-high temperature and pressure) required for the preparation of traditional boron nitride crystals, the preparation temperature and pressure used in the present invention can be achieved in a common tube furnace, which reduces energy consumption and preparation cost and has the prospect of large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings, wherein:

[0037] Figure 1 is a schematic diagram of the principle of growing a multi-layer single-crystal rhombohedral boron nitride thin film by locking the step angle in the present invention (left) and the difference in the binding energy between different stacked crystal nuclei and the step edge (right).

[0038] Figure 2 is a physical diagram of the single-crystal nickel substrate used in Example 1 of the present invention (left), X-ray diffraction data for characterizing the crystal plane indices (middle), and atomic force microscope images of the surface step morphology and the corresponding angle statistics results (right).

[0039] Figure 3 is a scanning electron microscope image of the growth of rhombohedral boron nitride multi-layer crystal domains along the surface steps of a single-crystal nickel in Example 1 of the present invention (left), and atomic resolution scanning transmission electron microscope images of the plane (middle) and cross-section (right) of the prepared sample.

[0040] Figure 4It is the electron microscope image of the rhombohedral boron nitride single - orientation multi - layer crystal domains obtained at different positions on the surface of single - crystal nickel in Example 1 of the present invention.

[0041] Figure 5 It is the atomic force microscope characterization image of the single - crystal rhombohedral boron nitride thin film prepared in Example 1 of the present invention.

[0042] Figure 6 It is the scanning electron microscope image (left), cross - sectional scanning transmission electron microscope image (middle) and the photo transferred to a silicon wafer (right) of the 6 - nanometer - thick single - crystal rhombohedral boron nitride continuous film prepared in Example 2 of the present invention.

[0043] Figure 7 It is the atomic force microscope characterization image of the single - crystal rhombohedral boron nitride thin films with different thicknesses obtained by using different growth times in Example 3 of the present invention.

[0044] Figure 8 It is the scanning electron microscope photo, low - energy electron diffraction pattern and transmission electron microscope photo of the hexagonal boron nitride prepared in Comparative Example 1 of the present invention. Detailed implementation manners

[0045] The present invention will be further described in detail below in conjunction with the detailed implementation manners. The examples given are only for clarifying the present invention, rather than limiting the scope of the present invention.

[0046] In the following implementation manners, unless otherwise specified, the methods are all conventional methods; unless otherwise specified, the raw materials can all be obtained from public commercial channels.

[0047] Example 1:

[0048] (1) Place 30 mg of borane ammonia in the upstream temperature zone of the chemical vapor deposition system, and put the single - crystal nickel (520) into the downstream temperature zone of the system;

[0049] (2) Introduce 500 sccm of argon and 50 sccm of hydrogen into the system, heat the downstream temperature zone to 1450 °C, and anneal the single - crystal nickel foil for 1 h; then cool the downstream temperature zone to 1350 °C at a constant rate of 20 °C / min and maintain for 5 h; at this time, the temperature of the upstream temperature zone is kept below 60 °C;

[0050] (3) Rapidly heat the upstream temperature zone to 65 °C, adjust argon to 50 sccm, adjust hydrogen to 30 sccm, and use a mechanical pump to reduce the pressure in the corundum tube to 200 Pa, with a nucleation time of 2 h; then keep the temperature of the upstream temperature zone unchanged, fix the carrier gas, and rapidly heat the downstream temperature zone to 1450 °C to grow a multi - layer single - crystal rhombohedral boron nitride thin film, where the growth time is 10 h;

[0051] (4) After the growth is completed, turn off the heating power supply, keep the gas flow rate in the tubular furnace unchanged, and cool it naturally to room temperature to obtain multi-layer single-crystal rhombohedral boron nitride.

[0052] Figure 2 The physical picture (left) of the single-crystal nickel substrate used in this example, the X-ray diffraction data (middle) for characterizing the crystal plane index, and the atomic force microscope image of the surface step morphology and the corresponding included angle statistical results (right) are shown.

[0053] Figure 3 The scanning electron microscope image (left) showing the growth of rhombohedral boron nitride multi-layer domains along the surface steps of the single-crystal nickel in this example, and the planar (middle) and cross-sectional (right) atomic-resolution scanning transmission electron microscope images of the prepared sample are shown. In step (3), the multi-layer rhombohedral boron nitride domains nucleate and grow along the parallel steps on the nickel surface (as shown in Figure 3 (left)), and lock their in-plane orientation and interlayer stacking through the step included angle. Figure 3 The planar (middle) and cross-sectional (right) atomic-resolution scanning transmission electron microscope images confirm that the obtained boron nitride domains have a perfect rhombohedral stacking configuration.

[0054] Figure 4 It is the electron microscope image of the single-orientation multi-layer domains of rhombohedral boron nitride obtained at different positions on the single-crystal nickel surface in this example. Figure 4 It shows that the proportion of the multi-layer rhombohedral boron nitride crystal nuclei with consistent orientation prepared is greater than 99%.

[0055] Figure 5 It is the atomic force microscope characterization image of the single-crystal rhombohedral boron nitride thin film prepared in this example. Figure 5 It shows that the thickness of this single-crystal rhombohedral boron nitride thin film is 12 nanometers.

[0056] Example 2:

[0057] (1) Place 15 mg of borazine in the upstream temperature zone of the chemical vapor deposition system, and put the single-crystal nickel (520) into the downstream temperature zone of the system;

[0058] (2) Introduce 100 sccm of argon and 30 sccm of hydrogen into the system, heat the downstream temperature zone to 1350 °C, and anneal the single-crystal nickel foil for 10 min; then cool the downstream temperature zone to 1250 °C at a constant rate of 5 °C / min and keep it for 2 h; at this time, the temperature of the upstream temperature zone is kept below 60 °C;

[0059] (3) Rapidly heat the upstream temperature zone to 75 °C, adjust argon to 20 sccm, adjust hydrogen to 80 sccm, and use a mechanical pump to reduce the air pressure in the corundum tube to 200 Pa. The nucleation time is 30 min; then keep the temperature of the upstream temperature zone unchanged, fix the carrier gas, and rapidly heat the downstream temperature zone to 1350 °C to grow a multi-layer single-crystal rhombohedral boron nitride film, where the growth time is 2 h;

[0060] (4) After the growth is completed, turn off the heating power supply, maintain the gas flow rate in the tube furnace unchanged, and naturally cool to room temperature to obtain multi-layer single-crystal rhombohedral boron nitride.

[0061] Figure 6 are the scanning electron microscope image (left), cross-sectional scanning transmission electron microscope image (middle) and the photo transferred to a silicon wafer (right) of the 6-nm-thick single-crystal rhombohedral boron nitride continuous film prepared in this example.

[0062] Example 3:

[0063] Preparation of Multilayer Single-Crystal Rhombohedral Boron Nitride Films with Different Thicknesses

[0064] This example uses the same preparation method as Example 1, except that the growth times in step (3) are adjusted to 2 hours, 4 hours, and 7 hours respectively.

[0065] Figure 7 are the atomic force microscope characterization images of the single-crystal rhombohedral boron nitride films with different thicknesses obtained by the present invention using different growth times. It can be seen that the thickness of the multi-layer single-crystal rhombohedral boron nitride prepared by the present invention is adjustable.

[0066] Comparative Example 1:

[0067] The preparation method of this comparative example is the same as that of Example 1, except that the temperature of the downstream temperature zone in step (2) is lowered to 1100 °C.

[0068] Figure 8 It shows that lowering the downstream temperature zone to 1100 °C will cause the appearance of common hexagonal boron nitride. It can be seen from this figure that the lattice orientations of each layer of the grown boron nitride sample are opposite, so the electron diffraction and transmission electron microscope photos can reflect the central inversion symmetry of the lattice.

Claims

1. A method for preparing a multi-layer single-crystal rhombohedral boron nitride film, which sequentially includes the following steps: (1) Place the growth source and the single-crystal nickel foil in the upstream temperature zone and the downstream temperature zone of the chemical vapor deposition system respectively; (2) Introduce a carrier gas into the chemical vapor deposition system, and heat the downstream temperature zone to 1300 - 1450 °C for annealing to remove the oxide layer on the surface of the nickel foil; the temperature of the upstream temperature zone is maintained at less than 60 °C; (3) Cool the downstream temperature zone to 1200 - 1350 °C at a constant rate and keep it warm, so that parallel steps are formed on the surface of the nickel foil, and the parallel steps are composed of a platform surface and an inclined surface; (4) Heat the upstream temperature zone to form single-oriented multi-layer rhombohedral boron nitride crystal nuclei on the surface of the nickel foil with parallel steps; subsequently, keep the temperature of the upstream temperature zone unchanged, and fix the carrier gas, while heat the downstream temperature zone to 1300 - 1450 °C to grow the multi-layer single-crystal rhombohedral boron nitride film.

2. The method according to claim 1, wherein, The method further includes the following step before step (1): Anneal the industrial nickel foil in a carrier gas at 1300 - 1450 °C for 1 - 20 h to obtain the single-crystal nickel foil.

3. The method according to claim 1, wherein The carrier gas in step (2) contains argon with a flow rate of 10 - 1000 sccm and hydrogen with a flow rate of 0 - 100 sccm.

4. The method according to claim 1, wherein The annealing in step (2) is carried out for 10 - 60 min.

5. The method according to claim 1, wherein The constant rate in step (3) is 1 - 20 °C / min.

6. The method according to claim 1, wherein, The heat preservation in step (3) is carried out for 1 - 8 h.

7. The method according to claim 1, wherein The formation of single-oriented multi-layer rhombohedral boron nitride crystal nuclei on the surface of the nickel foil with parallel steps in step (4) is carried out under the following conditions: heat the upstream temperature zone to 60 - 90 °C, and adjust the volume percentage of hydrogen in the carrier gas and control the nucleation time.

8. The method according to claim 7, wherein, The adjustment of the volume percentage of hydrogen in the carrier gas is to adjust the volume percentage of hydrogen to more than 1%.

9. The method according to claim 7, wherein The nucleation time is 0.1 - 2 h.

10. The method according to claim 1, wherein The growth of the multi-layer single-crystal rhombohedral boron nitride film in step (4) is carried out for 1 - 10 h; Preferably, the angle between the platform surface and the inclined surface of the parallel steps is 90° < θ < 180°. Preferably, the crystal plane index of the single-crystal nickel foil is Ni(hk0); Preferably, the size of the single-crystal nickel foil is above the centimeter scale; Preferably, the growth source is borane ammonia and / or borazine.

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