Large-area hexagonal boron nitride heteroepitaxial film on sapphire substrate and preparation method thereof

Abstract

The application belongs to the technical field of semiconductor material science and technology, and particularly relates to a large-area hexagonal boron nitride heteroepitaxial film on a sapphire substrate and a preparation method thereof. The technical scheme provides a large-area hexagonal boron nitride epitaxial film directly grown on a sapphire substrate. The preparation method adopts an ion beam assisted sputtering method. In the growth process, the main and auxiliary ion sources give high-energy ion sputtering respectively, so that high-energy B / N atoms cross the energy barrier of the preferred orientation between the B / N atoms and the substrate surface, form covalent bonding with the substrate surface, induce the hexagonal grid plane of the hexagonal boron nitride to extend vertically to the substrate surface, and thus obtain a large-area vertically-oriented hexagonal boron nitride epitaxial film. Compared with the common Van der Waals epitaxial growth method, the epitaxial method can effectively release stress, so that the bonding force between the hexagonal boron nitride film and the substrate surface is stronger, and the stability is better, and the hexagonal boron nitride film is suitable for being used as a template, a transition layer, a heat dissipation layer and the like of optoelectronic devices and electronic devices.

Description

Technical Field

[0001] This invention relates to the field of semiconductor materials science and technology, specifically to a large-area hexagonal boron nitride heteroepitaxial film on a sapphire substrate and its preparation method. Background Technology

[0002] The past few decades have witnessed the rise of group III nitride semiconductors, which laid the foundation for the development of modern optoelectronic and electronic devices. As a typical member of the group III nitride semiconductor family, hexagonal boron nitride (h-BN) has received widespread attention and importance in the field of optoelectronics and electronic devices due to its unique atomic structure and ultra-wide bandgap electronic properties. It has been used in various device schemes as a substrate, template or buffer layer, encapsulation layer, tunnel barrier or dielectric layer. Currently, compared with the large-area two-dimensional monolayer or few-layer h-BN atomic crystals that have been widely studied and realized [Nature, 579, 219-223 (2020)][Nature, 570, 91-95 (2019)][Nano Lett. 16, 3360-3366 (2016)][Nature, 606, 88-93 (2022)], the epitaxial growth of large-area h-BN films still faces certain challenges.

[0003] Due to the lack of large-size homogeneous single crystals, existing fabrication methods typically utilize chemical vapor deposition (CVD) and ion beam sputtering to epitaxially grow h-BN films on sapphire substrates. Due to energy-advantage orientation, the epitaxial film usually grows along the c-axis, and the resulting two-dimensional hexagonal mesh planar layer is generally parallel to the substrate surface. However, during heteroepitaxial growth, lattice and thermal mismatches between sapphire and h-BN lead to the accumulation of stress (including thermal stress) within the grown film. The strong in-plane covalent bonds make effective stress release difficult. As the film continues to thicken, exceeding a critical thickness, stress relaxation can cause cracking and even detachment, significantly impacting film quality. To address this, researchers have first deposited a layer of amorphous boron nitride (a-BN) of a certain thickness on a sapphire substrate at a relatively low temperature (800℃) as a buffer to avoid stress caused by dislocations between h-BN and the sapphire substrate [Appl. Phys. Lett. 100, 061121 (2012)][Appl. Phys. Lett. 98, 211110 (2011)]. However, the deposition of the transition layer increases the complexity of the growth process to some extent. If the hexagonal grid plane of h-BN is perpendicular to the substrate surface for heteroepitaxial growth, the weak van der Waals interactions between layers along the horizontal direction of the epitaxial film can effectively release the stress generated in the horizontal direction, thus making the growth of the epitaxial film more orderly and stable. In addition, the vertically arranged hexagonal grid h-BN epitaxial layer bonds to the substrate through strong covalent bonds, which is more conducive to improving the adhesion and stability of the device structure on the substrate compared to the van der Waals force bonding when the hexagonal grid is grown in parallel. Currently, this heteroepitaxial method has not been reported.

[0004] In summary, to meet the current development needs of optoelectronic devices and electronic devices, developing a method for controllably fabricating large-area vertically oriented hexagonal boron nitride heteroepitaxial films on sapphire substrates is a key technical problem that urgently needs to be solved. Summary of the Invention

[0005] In view of this, the purpose of this invention is to overcome the shortcomings of the prior art and provide a large-area hexagonal boron nitride heteroepitaxial film on a sapphire substrate and its preparation method.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a large-area hexagonal boron nitride heteroepitaxial film on a sapphire substrate. The epitaxial film is characterized by hexagonal boron nitride grid planar layers arranged perpendicular to the substrate, with the epitaxial relationship being: h-BN(-1100)[11-20] / / Al2O3(0001)[11-20]. The film has low stress between the film and the substrate, an ultra-wide bandgap, high optical transmittance, high surface flatness, a lateral diameter of two inches, and a thickness that can increase with deposition time.

[0008] This invention provides a method for preparing a large-area hexagonal boron nitride heteroepitaxial film on a sapphire substrate, comprising the following steps:

[0009] (1) Prepare a sapphire substrate;

[0010] (2) The substrate is transferred to the ion beam sputtering chamber, which contains a pure boron target, a main ion source and an auxiliary ion source; the ion beam sputtering chamber is evacuated.

[0011] (3) A high-momentum argon ion beam generated by ionization and acceleration of the main ion source is used to bombard a pure boron target to sputter boron atoms to the substrate surface. The auxiliary ion source is used to ionize and generate nitrogen ions and argon ions, which are accelerated and sputtered to the substrate surface. The high-momentum nitrogen ions and boron atoms that reach the substrate surface are directly deposited onto the sapphire substrate surface to grow a hexagonal boron nitride film.

[0012] (4) After growth is complete, the epitaxial film is cooled to room temperature in an argon atmosphere.

[0013] The purity of the pure boron target material in step (2) is greater than 99.5%.

[0014] Preferably, the back vacuum in step (2) is 9×10 -6 Below Pa.

[0015] Preferably, the working gas pressure for growing the hexagonal boron nitride heteroepitaxial film in step (3) is 4 × 10⁻⁶. -2 Pa to 5×10 -2 Pa.

[0016] Preferably, the deposition temperature for growing the hexagonal boron nitride heteroepitaxial film in step (3) is 1000℃.

[0017] Preferably, in step (3), the argon gas flow rate introduced into the main ion source is 6 sccm, and the ion beam density of the main ion source is 2 mA / cm. 2 The ion acceleration voltage is 1250 eV; the flow rates of nitrogen and argon gas introduced into the auxiliary ion source are 13 sccm and 2 sccm, respectively, and the ion beam density of the auxiliary ion source is 3 mA / cm². 2 The ion acceleration voltage is 280 eV.

[0018] Beneficial effects of the present invention

[0019] (1) The main ion source and auxiliary ion source used in this invention independently provide boron atoms and nitrogen atoms for the deposition process, which can achieve precise control of growth parameters.

[0020] (2) The high-energy ion sputtering method used in the growth process of this invention enables B / N atoms to overcome the energy barrier of the dominant growth orientation (i.e., the hexagonal grid plane is parallel to the sapphire c-plane) and form a covalent bond with the substrate surface, thereby inducing the hexagonal grid plane of hexagonal boron nitride to grow longitudinally along a path perpendicular to the substrate surface.

[0021] (3) Compared with the common van der Waals epitaxy, the hexagonal boron nitride heteroepitaxial method with vertically arranged hexagonal grid obtained by the present invention is more conducive to the effective release of stress caused by lattice mismatch and thermal mismatch in heteroepitaxialization. The epitaxial film has stronger bonding force with the substrate, better stability, and is not easy to fall off, which is suitable for the robustness and durability required by electronic devices.

[0022] (4) The vertically oriented hexagonal boron nitride epitaxial film obtained by the present invention can also be used as a growth template or buffer material for other group III compound semiconductors such as AlN, GaN and InN. It can overcome the common problems of high epitaxial layer defects, high stress and film cracking caused by lattice mismatch and thermal mismatch between heterogeneous substrates and group III compounds. It has more potential in the device design of high-efficiency blue / green / ultraviolet light-emitting diodes (LEDs) and laser diodes (LDs). Attached Figure Description

[0023] Figure 1 The image shows the Raman spectrum of the hexagonal boron nitride heteroepitaxial film prepared in this invention.

[0024] Figure 2 This is a high-resolution transmission electron microscope image of the hexagonal boron nitride heteroepitaxial film prepared in this invention.

[0025] Figure 3 This is a scanning electron microscope image of the hexagonal boron nitride heteroepitaxial film prepared in this invention.

[0026] Figure 4 This is a scanning electron microscope image of the hexagonal boron nitride heteroepitaxial film prepared in this invention.

[0027] Figure 5 This is an atomic force microscope image of the hexagonal boron nitride heteroepitaxial film prepared in this invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments.

[0029] Example

[0030] This invention utilizes a dual-ion-beam assisted sputtering method. The high-energy ion sputtering employed during the growth process enables high-energy B / N atoms to overcome the energy barrier of their dominant growth orientation (i.e., the hexagonal mesh plane is parallel to the sapphire c-plane), forming covalent bonds with the substrate surface. Due to the saturation characteristic of covalent bonds, this further induces the hexagonal boron nitride mesh to epitaxially grow along its longitudinal direction perpendicular to the substrate during the nucleation and growth stages. The specific operation steps are as follows:

[0031] Step 1: Prepare the sapphire substrate. The surface of the sapphire single crystal substrate used should be oriented c-axis, with a thickness typically ranging from 300 to 500 micrometers and a size of two inches. For ease of subsequent characterization, especially in determining the epitaxial relationship between the film and the substrate, the substrate needs to be cut along its low crystal index direction, resulting in (11-20) and (1-100) planes, and marked accordingly. To remove any organic contaminants and inorganic impurities that may be present on the substrate surface, it needs to be cleaned sequentially with acetone, deionized water, hydrogen peroxide + sulfuric acid, deionized water, and ethanol. After cleaning, the substrate is dried using nitrogen gas.

[0032] Step 2: The cleaned sapphire substrate is transferred to the ion beam sputtering chamber, which contains a pure boron target, a main ion source, and an auxiliary ion source. The ion beam sputtering chamber is then evacuated to a background vacuum of 9 × 10⁻⁶. -6 Below Pa.

[0033] Step 4: Heat the substrate to reach the growth temperature of 1000℃. After the temperature stabilizes, argon gas at a flow rate of 6 sccm is introduced into the main ion source and ionized and accelerated, with an ion beam density of 2 mA / cm². 2 The ion acceleration voltage was 1250V, causing argon plasma to bombard the pure boron target at a specific angle, sputtering boron atoms onto the substrate surface. Simultaneously, the auxiliary ion source was supplied with 13 sccm of nitrogen and 2 sccm of argon gas. The ion source parameters were adjusted to ionize and accelerate the two gases, achieving an ion beam density of 3 mA / cm². 2 The ion acceleration voltage is 280V, causing the plasma to bombard the substrate surface directly at a specific angle. After the main and auxiliary ion beams stabilize, the substrate is transferred to the sputtering station to begin sputtering. High-energy boron and nitrogen ions reaching the substrate surface first bond with the surface and then nucleate perpendicular to the substrate surface longitudinally, growing along this direction. The bombardment by the auxiliary argon plasma helps to make the film denser and also breaks down the boron atom clusters reaching the substrate surface into single atoms. The working pressure for the deposition process is 5 × 10⁻⁶. -2Pa, deposition time was 120 min.

[0034] Step 5: After deposition, turn off the ion source and the nitrogen gas valve, and keep the argon gas flowing into the chamber to allow the sample to cool down to room temperature in the argon atmosphere, thus completing the sample preparation.

[0035] Based on the above implementation methods, large-area vertically oriented hexagonal boron nitride heteroepitaxial films can be directly grown on sapphire substrates. The stress between the film and the substrate is low, the film has high optical transmittance, an ultra-wide bandgap, high surface flatness, and a lateral dimension of two inches. Figure 1 The image shows the Raman scattering spectrum of the hexagonal boron nitride heteroepitaxial thin film directly grown on a sapphire substrate according to this invention, located at 1369.5 cm⁻¹. -1 A very strong peak appears at this point, corresponding to the E of hexagonal boron nitride. 2g The vibration mode indicates that the prepared hexagonal boron nitride heteroepitaxial thin film has good crystal quality. Figure 2 This is a high-resolution transmission electron microscope image of the interface between the hexagonal boron nitride heteroepitaxial thin film on the sapphire substrate prepared in this invention. The lattice structure of the substrate and the thin film can be clearly seen. The boron nitride hexagonal grid is arranged perpendicularly on the sapphire substrate, and the epitaxial relationship between the two is h-BN(-1100)[11-20] / / Al2O3(0001)[11-20]. Figure 3 The UV-Vis absorption spectrum and Tauc diagram of the hexagonal boron nitride heteroepitaxial thin film prepared in this invention show that it has high transparency in the visible light range and exhibits an ultrawide bandgap of up to 6.1 eV. Figure 4 The image shows a scanning electron microscope image of the hexagonal boron nitride heteroepitaxial thin film prepared in this invention. It can be seen that the prepared film is dense and uniform overall, with a smooth surface and no wrinkles or cracks. Figure 5 The image shown is an atomic force microscope image of the hexagonal boron nitride heteroepitaxial film prepared in this invention. The root mean square roughness of the surface is only 0.572 nm, indicating that the surface of the prepared heteroepitaxial film is smooth and uniform.

[0036] Comparative Example 1

[0037] In this embodiment, the argon ion acceleration voltage of the main ion source changes to 800V during the growth process, and the other steps are the same as in the embodiment. However, it is not possible to obtain the hexagonal boron nitride epitaxial film with the hexagonal grid arranged perpendicular to the substrate.

[0038] Comparative Example 2

[0039] In this embodiment, the acceleration voltage of the auxiliary ion source nitrogen ions and argon ions during the growth process is changed to 180V. The other steps are the same as in the embodiment, and it is impossible to grow a hexagonal boron nitride epitaxial film with a hexagonal grid arranged perpendicular to the substrate.

[0040] Comparative Example 3

[0041] In this embodiment, the auxiliary ion source does not bombard the film with argon ions during the growth process, and the other steps are the same as in the embodiment. It is impossible to grow a hexagonal boron nitride epitaxial film with a hexagonal grid arranged perpendicular to the substrate, and the surface roughness of the obtained film is much higher than that of the embodiment.

[0042] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing a large-area hexagonal boron nitride heteroepitaxial film on a sapphire substrate, characterized by, The preparation method is a dual-ion beam assisted sputtering method, comprising the following steps: (1) preparing a sapphire substrate with a c-oriented surface; (2) conveying the substrate to an ion beam sputtering chamber, wherein the ion beam sputtering chamber comprises a pure boron target, a main ion source and an auxiliary ion source; and the ion beam sputtering chamber is subjected to vacuum treatment; (3) using the main ion source to ionize and accelerate high-momentum argon ions to bombard the pure boron target to sputter boron atoms to the substrate surface; the auxiliary ion source ionizes nitrogen ions and argon ions and accelerates them to sputter to the substrate surface; the nitrogen atoms and boron atoms reaching the substrate surface are directly deposited on the sapphire substrate surface to grow a hexagonal boron nitride film; The working pressure for depositing the boron nitride film in step (3) is 4 x 10 -2 Pa-6 x 10 -2 Pa, and the deposition temperature is 800-1000°C. The argon flow rate of the main ion source in step (3) is 4-8 sccm, the ion beam density of the main ion source is 1.5-2.5 mA / cm 2 , and the ion acceleration voltage is 1000-1500 V; the nitrogen and argon flow rates of the auxiliary ion source are 10-32 sccm and 1-3 sccm respectively, the ion beam density of the auxiliary ion source is 1.5-4.0 mA / cm 2 , and the ion acceleration voltage is 200-310 V; (4) after the growth is completed, the epitaxial film is cooled in an argon atmosphere until room temperature; The epitaxial relationship between the hexagonal boron nitride heteroepitaxial film and the sapphire substrate is h-BN(-1100)[11-20] / / Al2O3(0001)[11-20], and the hexagonal grid surface of the hexagonal boron nitride is perpendicular to the substrate plane.

2. The method of claim 1, wherein the method further comprises the steps of: The purity of the pure boron target in step (2) is greater than 99.5%. ​ 3. The method of claim 1, wherein the method further comprises the step of: The background vacuum in step (2) is 9 x 10 -6 Pa or below.

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