Composite crucible for large-size AlN single crystal growth and preparation method thereof

Abstract

The invention discloses a composite crucible for large-size AlN single crystal growth and a preparation method thereof, and belongs to the technical field of PVT method crystal growth. A heat conduction device is arranged in a cavity of the composite crucible and is composed of a plurality of high-purity graphite plates, the high-purity graphite plates are distributed at equal included angles to form a wing-shaped structure, the inner surface of the high-purity graphite crucible and the surfaces of the high-purity graphite plates are covered with a metal tantalum Ta layer or a TaC coating, and the high-purity graphite plates and the high-purity graphite plates are subjected to heat conduction under the temperature environment of 1800-2600 DEG C, so that the heat conduction effect of the high-purity graphite plates is improved. And TaC / C material layers are formed on the inner surface of the high-purity graphite crucible and the high-purity graphite plate. The heat conduction efficiency of the large-size crucible can be greatly improved, the problem that heat transfer from the side wall of the crucible to the source powder center is insufficient in the powder burning process can be remarkably solved, the uniformity of large-size AlN crystals is improved, thermal stress in crystal growth is relieved, and favorable conditions are created for preparing large-size and high-quality AlN single crystals.

Description

Technical Field

[0001] The present invention is applied to the field of crystal growth using a PVT method, and is particularly suitable for a composite crucible required for preparing large-size AlN single crystals using an induction heating PVT device. Background Art

[0002] Aluminum nitride (AlN), as a third-generation wide bandgap semiconductor, is an ideal substrate material for high-power devices due to its high stability, high thermal conductivity, high breakdown field strength, and low dielectric constant. Since AlN single crystal substrates and GaN materials have a smaller lattice mismatch, the dislocation density generated in GaN epitaxial AlN can be significantly reduced during the preparation of GaN-based devices, further improving the performance of GaN-based devices. With the development of AlN single crystals, the continuous improvement of wafer size and crystal quality, AlN single crystal materials are expected to become the core material of future electronic devices. In recent years, my country has made significant progress in the research of AlN single crystals. The PVT method is one of the most promising ways to grow large-sized, high-quality AlN single crystals. By heterogeneously growing AlN single crystals on large-sized, high-quality SiC seed crystals, it is possible to obtain large-sized AlN single crystal substrates.

[0003] However, when using induction-heated PVT equipment, large-sized graphite crucibles are required to prepare large-sized AlN crystals, especially those 4 inches and larger. During the induction heating process of the graphite crucible, due to the skin effect and eddy current heating, the heat is mainly stored on the side of the crucible near the coil, and is transferred to the interior of the crucible through heat conduction and heat radiation. In large crucibles, especially when preparing AlN larger than 4 inches, the AlN source powder on the side wall will sublime first due to the high temperature of the side wall, resulting in a cavity between the source powder and the side wall of the crucible, which hinders the further transfer of heat to the center of the source powder. This problem not only leads to insufficient sintering of the center of the AlN source powder during the powder sintering stage, but also causes uneven sublimation of the central source powder and the source powder near the side wall during crystal growth, which is not conducive to controlling the crystal shape, optimizing crystal quality, and reducing thermal stress. Especially in the later stages of long-term growth, the size of the cavity between the source powder and the crucible side wall further increases, resulting in the inability to conduct heat to the central area, further reducing the sublimation rate of the source powder, or even stopping it. Compared with resistance-heating PVT equipment, which can more conveniently optimize the temperature field for crystal growth by simply adjusting the relative position of the heater and crucible, the heater in induction heating is the graphite crucible itself. Adjusting the temperature field requires changing the insulation structure and the relative position of the coil and crucible.

[0004] To address the above issues, the temperature field structure at the corresponding position can be adjusted by changing the coil turn spacing. However, as the crucible size increases and the insulation conditions change, the problem of uneven sublimation of the source powder on the sidewalls and the source powder in the center has not been significantly resolved, and the adjustment cost is too high. Therefore, in the growth of large-scale AlN single crystals, especially AlN crystals of 4 inches and above, insufficient source powder sintering, uneven source powder sublimation during the crystal growth stage, and a rapid drop in the growth rate in the later stages of long-term growth are urgent problems that need to be solved. In addition, to prevent the graphite crucible from contaminating the AlN source powder, the graphite crucible cannot directly contact the source powder, and a protective layer that does not react with AlN must be formed on the inner wall of the crucible. However, during the PVT induction growth of AlN crystals, many commonly used high-temperature resistant crucible materials cannot be used due to harsh conditions such as high growth temperature (1800℃-2200℃), highly corrosive atmosphere of active Al vapor, and all-carbon heating environment. For example, tungsten or niobium crucibles will react with graphite heaters, and ceramic crucibles such as boron nitride or oxides are resistant to temperature differences and react with Al. Currently, only tantalum carbide (TaC) layers have been found to fully meet the above conditions. Summary of the Invention

[0005] In order to solve the problem of uneven radial heat conduction in the preparation of large-sized AlN crystals by the PVT equipment in the above-mentioned induction heating, the present invention designs a composite crucible with a heat conduction device and a preparation method thereof. This can effectively solve the problem of insufficient heat conduction from the side wall of the crucible to the center of the source powder in the large-sized graphite induction heating PVT equipment, thereby improving the sublimation efficiency of the AlN source powder. It can also significantly reduce the radial temperature gradient, alleviate the problem of uneven sublimation of the source powder caused by the large radial temperature gradient, and improve the radial growth uniformity of large-sized AlN crystals, thereby regulating the shape of the AlN crystals, alleviating thermal stress during crystal growth, and improving the quality of the AlN crystals.

[0006] To achieve the above objectives, the present invention provides the following technical solutions:

[0007] A composite crucible for growing large-size AlN single crystals, the composite crucible being a high-purity graphite crucible. The crucible is characterized in that a heat conducting device is provided within the cavity of the high-purity graphite crucible. The heat conducting device is composed of multiple high-purity graphite plates, which are distributed at equal angles to form a wing-shaped structure. A TaC / C material layer is provided on the inner surface of the high-purity graphite crucible and on the surfaces of the high-purity graphite plates.

[0008] Furthermore, the high-purity graphite crucible and the high-purity graphite plate are made of a graphite material with a purity of 99.99% that can withstand high temperature environments.

[0009] Furthermore, the number of the high-purity graphite plates is 6-10.

[0010] Furthermore, the maximum outer diameter of the high-purity graphite crucible is in the range of 180-220 mm, the outer wall thickness of the high-purity graphite crucible is in the range of 10-20 mm, and the height of the high-purity graphite crucible is less than 200 mm.

[0011] Furthermore, the ratio of the length of the high-purity graphite plate radially extending to the interior of the high-purity graphite crucible to the inner diameter of the high-purity graphite crucible is L s1 / L d =0.2-0.4, the ratio of the axial height of the high-purity graphite plate to the height inside the high-purity graphite crucible is L s2 / L h =0.5-0.7, and the thickness of the high-purity graphite plate is 3-5 mm.

[0012] Furthermore, the high-purity graphite plate is in a rectangular, trapezoidal or triangular shape.

[0013] The present invention further provides a method for preparing a composite crucible for growing large-sized AlN single crystals, comprising the following steps:

[0014] 1) The high-purity graphite plate is fixed to the inner wall of the high-purity graphite crucible by inserting the groove, or the high-purity graphite plate is fixed to the inner wall of the high-purity graphite crucible by mechanical processing;

[0015] 2) Repeatedly spraying TaC spray powder at least three times on the inner surface of the high-purity graphite crucible and the high-purity graphite plate by spraying, or covering the inner surface of the high-purity graphite crucible and the high-purity graphite plate with metal tantalum Ta foil by stamping;

[0016] 3) Under a temperature environment of 1800-2600° C., a TaC / C material layer is formed on the inner surface of the high-purity graphite crucible and the surface of the high-purity graphite plate.

[0017] Further, step 3) specifically includes the following steps:

[0018] 3-1) Place the high-purity graphite crucible into the isostatic hot pressing equipment and use a vacuum pump to pump air to 10 -3 When the pressure is below 70000 Pa, fill with argon gas to 70000 Pa, and then continue to pump and fill repeatedly to exhaust the air in the equipment;

[0019] 3-2) Inject argon gas and increase the pressure to 100 MPa. Within the working range of the chiller, start the heating system and heat to 1800-2600℃ at a heating rate not exceeding 25℃ / min. Keep the temperature and pressure for 2-4 hours.

[0020] 3-3) Stop the heating system, cool to room temperature, and then release the pressure. When the pressure and temperature return to normal, remove the composite crucible.

[0021] Furthermore, in step 2), the thickness of the metal tantalum Ta layer is in the range of 50-500 μm, and the surface roughness Ra of the metal tantalum Ta layer is less than 1.6 μm.

[0022] The present invention has the following advantages compared with the prior art:

[0023] 1) The present invention employs a heat conduction device placed inside the crucible to regulate the source powder temperature field. The device is composed of multiple high-purity graphite plates arranged at equal angles to form a wing-like structure. This device increases the contact area between the central source powder and the crucible, improving the heat transfer efficiency from the crucible inner wall to the source powder during induction heating. By adjusting the size ratio of the heat conduction device, the temperature field distribution in the source powder can be more precisely adjusted, improving the uniformity of AlN source powder sublimation, which is beneficial for optimizing the crystal growth shape, reducing thermal stress during crystal growth, and optimizing AlN crystal quality.

[0024] 2) The heat conduction device used in this invention, placed inside the crucible to regulate the source powder temperature field, provides a thermal buffering effect that mitigates thermal shock damage to PVT equipment during high-temperature growth, extending the crucible's service life compared to conventional methods of adjusting insulation structures. This reduces energy loss in the induction heating system, meaning that at the same power, the crucible heated with the heat conduction device achieves superior heating performance.

[0025] 3) The present invention provides a TaC / C layer on the heat conducting device and the inner wall of the high-purity graphite crucible, which can effectively prevent carbon from contaminating the AlN source powder and improve the high temperature resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 1 is a cross-sectional view of a composite crucible with a heat conducting device according to the present invention; wherein:

[0027] 1—High-purity graphite crucible; 2—High-purity graphite plate

[0028] L S1 L is the length of the high-purity graphite plate extending radially to the inside of the high-purity graphite crucible; s2 is the axial height of the high-purity graphite plate; L d is the inner diameter of the high-purity graphite crucible. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the accompanying drawings through examples, which however do not limit the scope of the present invention in any way.

[0030] Example 1

[0031] like Figure 1As shown, this solution uses a mechanical processing process to fix a high-purity graphite plate to the inner wall of a high-purity graphite crucible. The high-purity graphite plate is in close contact with the inner wall and bottom surface of the high-purity graphite crucible. A stamping method is used to cover the inner surface of the crucible and the surface of the high-purity graphite plate with metal tantalum (Ta) foil. The high-purity graphite crucible and high-purity graphite plate are 99.99% pure graphite materials that can withstand high-temperature environments. Under a temperature environment of 1800-2600°C, the metal tantalum (Ta) is carbonized into TaC, and the carbon atoms of the high-purity graphite crucible and high-purity graphite plate penetrate from the interface to form a TaC / C material layer. The specific steps of the preparation method include:

[0032] S1. Use mechanical processing to process the high-purity graphite crucible into the following dimensions: outer diameter 200mm, inner diameter 180mm, crucible bottom thickness 10mm, inner height 190mm, the heat conduction device consists of 6 high-purity graphite plates, the high-purity graphite plates are distributed at equal angles to form a wing-shaped structure, and the L of the high-purity graphite plates is 1 / 4 of the height. s1 55mm, L s2 The diameter is 115mm and the thickness is 4mm.

[0033] S2. Select a punch head that matches the shape of the heat conduction device inside the crucible, and pre-apply 1mm of fluororesin coating release agent evenly to the area where the punch head contacts the 1mm thick Ta foil to ensure that the Ta foil does not fall off when the mold is separated. The demolding angle is 4°, and the Ta foil is pressed tightly against the crucible and the heat conduction device using a stamping method. Punch multiple times to exhaust the gas in the Ta foil and the graphite crucible, so that the gap between the two components is in a nearly negative pressure state. The tolerance between the outer dimension of the metal tantalum Ta foil crucible and the inner dimension of the crucible is -0.06-0mm;

[0034] S3. Under a temperature environment of 1800-2600°C, the metal tantalum Ta is carbonized to form TaC. A close bond is formed between the inner surface of the high-purity graphite crucible and the interface between the graphite material and the metal tantalum Ta on the high-purity graphite plate. The C concentration penetrates from the interface to form a TaC / C material layer. The specific steps include:

[0035] ① Loading the furnace: Place the crucible into the isostatic hot pressing equipment;

[0036] ②Gas cleaning furnace: Use vacuum pump to extract air to 10 -3 When the pressure is below 70000 Pa, fill with argon gas to 70000 Pa, and then continue to pump and fill, repeating the pumping and filling for 3 times to exhaust the air in the equipment;

[0037] ③ Boost pressure: inject argon gas to increase the pressure to 100MPa;

[0038] ④Temperature rise and pressure maintenance: within the working range of the chiller, start the heating system, heat to 1100℃ at a heating rate not exceeding 25℃ / min, and maintain temperature and pressure for 2 hours;

[0039] ⑤Depressurization and cooling: Stop the heating system, cool to room temperature and then release the pressure. After the pressure and temperature are reduced to normal, take out the TaC / C composite crucible.

[0040] Example 2

[0041] This solution uses mechanical processing to prepare a high-purity graphite crucible with a bottom groove. By inserting a high-purity graphite plate that fits the groove, a high-purity graphite crucible with a heat conduction device is formed. TaC spray powder is sprayed on the inner surface of the high-purity graphite crucible and the high-purity graphite plate. The high-purity graphite crucible and the high-purity graphite plate are graphite materials with a purity greater than or equal to 99.99% that can withstand high-temperature environments. Under a temperature environment of 1800-2600°C, the graphite material on the inner surface of the high-purity graphite crucible and the high-purity graphite plate forms a tight bond with the TaC spray material, and the carbon atoms of the high-purity graphite crucible and the high-purity graphite plate penetrate from the interface to form a TaC / C material layer. The specific steps include:

[0042] S1. Using a CNC machine tool and a diamond tool, a 4mm thick groove structure is machined on the bottom surface of the high-purity graphite crucible to facilitate the subsequent insertion of high-purity graphite plates. Rough machining is performed first, with a large knife quickly removing the excess, leaving a finishing allowance of about 0.5mm. A small knife is then used to trim the surface at a low speed to ensure the groove size and finish. The heat conduction device consists of 6 high-purity graphite plates, which are distributed at equal angles to form a wing-like structure. The high-purity graphite plates are fixed to the inner wall of the high-purity graphite crucible by plugging through the grooves. The high-purity graphite plates are in close contact with the inner wall and bottom surface of the high-purity graphite crucible.

[0043] S2. Spraying TaC powder on the inner surface of the crucible and the heat conducting device, specifically including:

[0044] ① Configuration of coating liquid

[0045] Prepare a carbon-tandem composite slurry by mixing the following ingredients by mass: 60% TaC powder (particle size 1-5 μm), sintering aid SiC powder (particle size ≤ 5 μm), 5% polyethanol butyral organic linker, and 15% anhydrous ethanol. Place the mixed slurry in a ball mill and ball mill at 200-400 r / min for 4-8 hours to form a uniform suspension with a viscosity controlled at 200-500 mPa·s.

[0046] ②Electrostatic spray deposition

[0047] Fix the high-purity graphite crucible on the turntable of the electrostatic spraying device, ensure that the inner surface of the crucible is exposed to the spraying area by using a clamp, the slurry flow rate is 70mL / min, start the electrostatic spray gun, and spray in layers along the axial direction of the inner wall of the crucible. Repeat the spraying for at least three times. The thickness of each layer is controlled at 10-30μm. The spraying interval is 5-10 minutes to evaporate the solvent.

[0048] ③ Place the sprayed crucible in a vacuum drying oven and cure at 100°C for 3 hours to remove the organic solvent. In an inert Ar / N2 atmosphere, heat to 700°C at a rate of 5-10°C / min and hold for 1.5 hours to decompose the residual organic matter and initially form a TaC coating.

[0049] S3. Under a temperature environment of 1800-2600°C, a close bond is formed between the inner surface of the high-purity graphite crucible and the interface between the graphite material and the TaC coating on the high-purity graphite plate. The C concentration penetrates from the interface to form a TaC / C material layer. The specific steps include:

[0050] ① Loading the furnace: Place the high-purity graphite crucible into the high-temperature furnace;

[0051] ②Gas cleaning furnace: Use vacuum pump to extract air to 10 -3 When the pressure is below 70000 Pa, fill with argon gas to 70000 Pa, and then continue to pump and fill, repeating the pumping and filling for 3 times to exhaust the air in the equipment;

[0052] ③ Boost pressure: inject argon gas to increase the pressure to 100MPa;

[0053] ④ Temperature increase and pressure maintenance: within the working range of the chiller, start the heating system and increase the temperature to 1600℃ at 10℃ / min. Keep the temperature for 4 hours;

[0054] ⑤ Depressurization and cooling: Stop the heating system, cool to room temperature and then release the pressure. After the pressure and temperature are reduced to normal, take out the TaC / C composite crucible. The porosity of the TaC / C layer is ≤5%, and the interface bonding strength is ≥15MPa.

[0055] The above embodiments are only some preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any changes that adopt the design principles of the present invention and are made through non-creative work on this basis should fall within the scope of protection of the present invention.

Claims

1. A composite crucible for growing large-size AlN single crystals, the composite crucible being a high-purity graphite crucible, characterized in that: A heat conducting device is provided in the cavity of the high-purity graphite crucible. The heat conducting device is composed of multiple high-purity graphite plates. The high-purity graphite plates are distributed at equal angles to form a wing-shaped structure. A TaC / C material layer is provided on the inner surface of the high-purity graphite crucible and the surface of the high-purity graphite plates.

2. The composite crucible for growing large-size AlN single crystals according to claim 1, wherein: The ratio of the length of the high-purity graphite plate radially extending to the interior of the high-purity graphite crucible to the inner diameter of the high-purity graphite crucible is L s1 / L d =0.2-0.4, the ratio of the axial height of the high-purity graphite plate to the height inside the high-purity graphite crucible is L s2 / L h =0.5-0.7, and the thickness of the high-purity graphite plate is 3-5 mm.

3. The composite crucible for growing large-size AlN single crystals according to claim 1, wherein: The high-purity graphite crucible and the high-purity graphite plate are made of graphite material with a purity of 99.99% that can withstand high-temperature environments.

4. The composite crucible for growing large-size AlN single crystals according to claim 1, wherein: The number of the high-purity graphite plates is 6-10.

5. The composite crucible for growing large-size AlN single crystals according to claim 1, wherein: The maximum outer diameter of the high-purity graphite crucible is in the range of 180-220 mm, the outer wall thickness of the high-purity graphite crucible is in the range of 10-20 mm, and the height of the high-purity graphite crucible is less than 200 mm.

6. The composite crucible for growing large-size AlN single crystals according to claim 1, wherein: The high-purity graphite plate is in the shape of a rectangle, a trapezoid or a triangle.

7. A method for preparing a composite crucible for growing large-size AlN single crystals according to claim 1, comprising the following steps: 1) The high-purity graphite plate is fixed to the inner wall of the high-purity graphite crucible by inserting the groove, or the high-purity graphite plate is fixed to the inner wall of the high-purity graphite crucible by mechanical processing; 2) Repeatedly spraying TaC spray powder at least three times on the inner surface of the high-purity graphite crucible and the high-purity graphite plate by spraying, or covering the inner surface of the high-purity graphite crucible and the high-purity graphite plate with metal tantalum Ta foil by stamping; 3) Under a temperature environment of 1800-2600° C., a TaC / C material layer is formed on the inner surface of the high-purity graphite crucible and the surface of the high-purity graphite plate.

8. The preparation method according to claim 7, wherein Step 3) The specific steps include: 3-1) Place the composite crucible into the isostatic hot pressing equipment and use a vacuum pump to evacuate the air to 10 -3 When the pressure is below 70000 Pa, fill with argon gas to 70000 Pa, and then continue to pump and fill repeatedly to exhaust the air in the equipment; 3-2) Inject argon gas and increase the pressure to 100 MPa. Within the working range of the chiller, start the heating system and heat to 1800-2600°C at a heating rate not exceeding 25°C / min. Keep the temperature and pressure for 2-4 hours. 3-3) Stop the heating system, cool to room temperature, and then release the pressure. When the pressure and temperature return to normal, remove the composite crucible.

9. The preparation method according to claim 7, wherein The depth of the groove in step 1) is 3-5 mm, and the interference fit between the high-purity graphite plate and the groove is 0.03 mm to 0.1 mm.

10. The preparation method according to claim 7, wherein The thickness of the metal tantalum Ta layer in step 2) is in the range of 50-500 μm, and the surface roughness Ra of the metal tantalum Ta layer is less than 1.6 μm.

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