A method for improving crystal growth quality

Abstract

The present invention provides a method for improving the quality of crystal growth. In the whole growth process, boron (B) is doped into silicon carbide raw materials at a certain ratio, and the concentration of nitrogen (N) in the growth atmosphere is controlled to change in a gradient, thereby realizing co-doping of nitrogen and boron elements. In this way, a high-doping concentration area in the bottom nucleation area can be formed to transition to the expansion area and then to the fast-growing part, and the concentration of mixed elements in the fast-growing part meets the performance requirements of the conductive silicon carbide substrate.

Description

Technical Field

[0001] The invention belongs to the technical field of silicon carbide crystal growth, relates to the preparation of silicon carbide, and in particular to a method for improving crystal growth quality, which is simple and effective, does not cause waste, can flexibly control the formation of a strain layer, and improves the efficiency and quality of crystal growth. Background Art

[0002] See also Figure 3 In the prior art, the initial growth stage is usually 3-5 mm, the nucleation zone is 0.5-1.0 mm, and the expansion zone is 2.0-4.0 mm. Depending on the diameter and expansion angle required for expansion, the growth rate is slow.

[0003] The stable growth zone is usually 10-100mm, and the diameter of the equal diameter growth zone remains unchanged, slightly larger than the required diameter. If 150mm is required, the diameter of the equal diameter zone is 156-160mm. The growth rate is high, 0.3-0.5mm / hr, 80-300hr→20-100mm.

[0004] With the growth process, dislocations in the seed crystal, such as TSD, BPD, and TED, are copied from the slow-growing part into the fast-growing part. On the other hand, at the beginning of growth, the surface of the seed crystal is subjected to thermal corrosion, and various dislocations are easily formed in the initial nucleation. The dislocation density is reduced through rearrangement during the slow growth process.

[0005] There are some technologies for controlling dislocation density, such as reducing dislocation inheritance by selecting high-quality seed crystals, or using specific growth conditions (such as increasing the depressurization time) to control the generation and replication of dislocations during crystal growth. However, these solutions have some limitations, such as relying on high-quality seed crystals, complex growth conditions, or sacrificing production capacity, which limits the efficiency and quality of crystal growth, and still cannot solve the problem of high substrate dislocation density. The application of electric vehicle main drive inverter requires SiC substrate to have 3000 / cm 2 The dislocation density below is 5000-8000 / cm 2 (See Figure 6 ). Summary of the invention

[0006] In order to solve the above problems, the present invention provides a method for improving the quality of crystal growth. The present invention can use low-quality seed crystals with high dislocation density to grow high-quality and low-dislocation crystals, reducing the dependence on high-quality seed crystals. The replication and proliferation of dislocations are suppressed by forming different strain layers, thereby achieving the growth of crystals with lower dislocation density from seed crystals, thereby improving the quality of the crystals. The method of the present invention is simple and effective, and will not cause waste. The formation of strain layers can be flexibly controlled, thereby improving the efficiency and quality of crystal growth.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for improving crystal growth quality, the method comprising the following steps:

[0009] 1) preparing the required seed crystal, powder, doping element material, growth substrate and heating crucible, and mixing the powder and doping element material evenly to obtain mixed powder;

[0010] 2) Place the mixed powder into a heated crucible, wipe the seed crystal, and place the seed crystal on the upper part of the heated crucible to ensure a height difference between the upper surface of the mixed powder and the seed crystal; seal;

[0011] 3) Place the heating crucible into a heating furnace and evacuate the furnace;

[0012] 4) Start heating and continue to fill with high-purity argon gas;

[0013] 5) After stabilization for a period of time, increase the heating power, fill with nitrogen, and maintain a certain ratio of argon to nitrogen;

[0014] 6) Continue to increase the power so that the temperature difference between the upper and lower parts of the crucible is maintained at 70-100°C;

[0015] 7) Gradually reduce the pressure in the heating furnace and increase the nitrogen concentration;

[0016] 8) Continue to increase the power to the growth temperature and gradually reduce the nitrogen concentration within 5-15 hours;

[0017] 9) After the crystal growth is completed, slowly increase the pressure in the furnace to 600mbar, slowly cool the temperature in the furnace to room temperature, and take out the crystal.

[0018] As a preferred embodiment of the present invention, in step 1), the doping element is B, the doping element material is boron powder, and the mass ratio of boron powder to silicon carbide is 1.5-2.5g:2.5-4.5kg.

[0019] As a preferred embodiment of the present invention, in step 2), the height difference between the upper surface of the mixed powder and the seed crystal is 80-110 mm.

[0020] As a preferred embodiment of the present invention, in step 5), the flow ratio of argon to nitrogen is 500:15-30, and the flow rate of nitrogen is 10-20 mL / min.

[0021] As a preferred embodiment of the present invention, in step 6), the temperature difference between the upper temperature and the lower temperature of the crucible is maintained at 80-100°C.

[0022] As a preferred embodiment of the present invention, in step 7), the pressure in the furnace is 2-7 mbar, the flow ratio of argon to nitrogen is 80-100:5-8, and the nitrogen concentration is 5*10 18 / cm 3 -5*10 20 / cm 3 .

[0023] Preferably, the flow ratio of argon to nitrogen is 85-90:6-7.

[0024] As a preferred embodiment of the present invention, in step 8), the lower temperature is between 2250°C and 2350°C, and the upper temperature is between 2140°C and 2260°C.

[0025] As a preferred embodiment of the present invention, in step 8), the nitrogen concentration is reduced to 5*10 17 / cm 3 -5*10 18 / cm 3 , the ratio of argon to nitrogen is 90-110:3-5.

[0026] As a preferred solution of the present invention, a strain layer with a high doping concentration is formed in the nucleation stage, and the thickness of the strain layer is 0.5-2 mm.

[0027] Preferably, the thickness of the strain layer is 0.5-1.5 mm.

[0028] During the entire growth process, the present invention adds boron (B) to the silicon carbide raw material at a certain ratio, and controls the concentration of nitrogen (N) in the growth atmosphere to change in a gradient, thereby achieving co-doping of nitrogen and boron elements, hereinafter referred to as mixed elements. In this way, a high-doping concentration area from the bottom nucleation area to the expansion area and then to the fast-growing part can be formed, and the mixed element concentration of the fast-growing part meets the performance requirements of the conductive silicon carbide substrate. The N element doping runs through the entire growth process, and the concentration of the high-concentration strain layer is 5*10 18 / cm 3 -5*10 20 / cm 3， Preferably in 5*10 19 / cm 3 , the concentration in the fast-growing zone is 5*10 17 / cm 3 -5*10 18 / cm 3 Boron doping is only introduced into the high concentration strain region, and the mass ratio of boron to silicon carbide powder needs to be controlled at 0-2.5g:2.5-4.5kg, preferably 1.5-2.5g:2.5-4.5kg.

[0029] By controlling the sudden change of the concentration of mixed elements, a strain layer with high doping concentration can be formed in the nucleation stage. The difference between this strain layer and the previous technology is that the addition of mixed elements, especially nitrogen, can make the lattice constant a of this layer smaller than the lattice constant of the seed crystal and the lattice constant of the fast growing part. The change of lattice constant causes the high concentration area to form compressive stress on the expansion area and the fast growing area. During the crystal growth process, dislocations tend to reduce their dislocation length and reduce their mirror force, tending to be stress-free or have the lowest energy (Dislocation conversion in 4H silicon carbideepitaxy. Journal of Crystal Growth, 244 (3-4), 257–266 | 10.1016 / s0022-0248 (02) 01706-2), and the compressive stress formed by high concentration N doping can promote this dislocation transformation trend, so that most of the dislocations change their inheritance direction (not along the 0001 direction) in the early stage, achieving the purpose of suppressing dislocations. Therefore, under the action of compressive stress, the inheritance of dislocation density along the growth direction is suppressed, thereby blocking the inheritance of dislocation from seed crystal to crystal to a certain extent. The effect of a small amount of doping of B element is: the high N concentration in the early stage will cause a large number of C atoms in the strained layer to be replaced by N atoms, causing the problem of lattice mismatch between it and the seed crystal area, the fast growth area and graphite, and easily generating mismatch stress and causing other defects such as stacking faults. The atomic radius of B element is slightly larger than that of C atom, and a small amount of doping can relatively increase its lattice constant, achieving the purpose of suppressing the lattice matching problem. At the same time, it is found in the practice of the method of the present invention that even if the dosage of B doped is extremely low or even not doped with B, the effect of reducing dislocations by high concentration N still exists.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1) The present invention can form a high-concentration strain layer in the early stage of growth, forming compressive stress on the seed crystal, thereby inhibiting the dislocation of the seed crystal from being inherited into the crystal during the growth of the ingot, thereby growing a low-dislocation, high-quality silicon carbide crystal.

[0032] 2) The present invention inhibits the replication and proliferation of dislocations by forming different strain layers (compressive stress and tensile stress), thereby achieving the goal of blocking the inheritance of dislocations from seed crystals to crystals and reducing dislocations in crystals, thereby growing crystals with lower dislocation density. Compared with the prior art method that relies on high-quality seed crystals, the present invention is no longer limited by the problem of high cost of obtaining and using high-quality seed crystals.

[0033] 3) The present invention can use low-quality seed crystals with high dislocation density to grow high-quality low-dislocation crystals, which changes the problem in the current technology that the quality of crystals strongly depends on the quality of seed crystals, and will improve the efficiency and quality of crystal growth.

[0034] 4) The strain layer in the present invention can be removed during the crystal processing process because it is the expanded diameter part and the diameter that does not reach the required diameter does not cause waste. Compared with the method in the prior art that requires complex growth conditions to control the generation and replication of dislocations, the present invention is simpler and more effective and will not cause waste.

[0035] 5) The strained layer of the present invention can flexibly control the formation of the strained layer by adjusting the change of the lattice constant by adjusting the doping concentration. Compared with the method in the prior art that relies on specific growth conditions to control the generation and replication of dislocations, the present invention has higher controllability and flexibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the present invention.

[0037] Figure 2 is a dislocation diagram.

[0038] Figure 3 It is a schematic diagram of the prior art.

[0039] Figure 4 This is a dislocation comparison diagram.

[0040] Figure 5 It is a schematic diagram of the principle of the present invention.

[0041] Figure 6 is the dislocation density of the SiC substrate obtained by the prior art. DETAILED DESCRIPTION

[0042] In order to make the technical means, creative features, purpose and efficacy of the present invention easy to understand, the present invention is further described below in conjunction with specific embodiments, but the following embodiments are only preferred embodiments of the present invention, not all. Based on the embodiments in the implementation mode, other embodiments obtained by those skilled in the art without creative work all belong to the protection scope of the present invention. The experimental methods in the following embodiments, unless otherwise specified, are conventional methods, and the materials, reagents, etc. used in the following embodiments, unless otherwise specified, can all be obtained from commercial channels.

[0043] See also Figure 3 In the prior art, the initial growth period is usually 3-5mm, the nucleation zone is 0.5-1.0mm, and the diameter expansion zone is 2.0-4.0mm. The growth rate is slow depending on the diameter and the angle of the expansion. The stable growth zone is usually 10-100mm, and the diameter of the equal diameter growth zone remains unchanged, slightly larger than the required diameter. If 150mm is required, the diameter of the equal diameter zone is 156-160mm. The growth rate is high, 0.3-0.5mm / hr, 80-300hr→20-100mm.

[0044] Therefore, the present invention provides a method for improving the quality of crystal growth, see Figure 1 In the present invention, boron (B) is doped into the silicon carbide raw material in a certain proportion during the entire growth process, and the concentration of nitrogen (N) in the growth atmosphere is controlled to change in a gradient, thereby achieving co-doping of nitrogen and boron elements, hereinafter referred to as mixed elements. In this way, a high-doping concentration area from the bottom nucleation area to the expansion area and then to the fast-growing part can be formed, and the mixed element concentration of the fast-growing part meets the performance requirements of the conductive silicon carbide substrate. The N element doping runs through the entire growth process, and the concentration of the high-concentration strain layer is 5*10 18 / cm 3 -5*10 20 / cm 3 , preferably 5*10 19 / cm 3 , the concentration in the fast-growing zone is 5*10 17 / cm 3 -5*10 18 / cm 3 Boron doping is only introduced into the high concentration strain region, and the mass ratio of boron to silicon carbide powder needs to be controlled at 0-2.5g:2.5-4.5kg, preferably 1.5-2.5g:2.5-4.5kg.

[0045] The inventors named the method of the present invention SIDE (strain-induced dislocation elimination), see Figure 5 , high concentration nitrogen can form a high concentration strain layer: during the growth process, the doped material will cause a change in the lattice constant. According to the difference in doping concentration, a strain layer with a high N concentration is formed. The introduction of N will replace the C atoms in silicon carbide and connect with four silicon atoms. Since the radius of the N atom is smaller than that of the C atom, the lattice constant a of the strain layer is smaller than the lattice constant of the seed crystal and smaller than the lattice constant of the fast growing part. Among them, high-concentration nitrogen atoms replace carbon lattice sites, resulting in a smaller lattice constant, which has been confirmed in many documents (Materials Science Forum Vols.717-720 (2012) pp481-484; Acta Phys. Sin. Vol.63, No.5 (2014) 053102; Cryst. Growth Des. 2015, 15, 3131-3136;). At the same time, a small amount of B element in the early stage will replace the silicon atoms in silicon carbide and connect with four carbon atoms (the radius of B atoms is larger than that of C atoms). Its role is to prevent the high nitrogen concentration from causing the lattice constant to become smaller, resulting in lattice mismatch problems in the early silicon carbide crystals and silicon carbide crystals with low nitrogen concentration in the rapid growth period.

[0046] Under the action of compressive stress, dislocation replication and proliferation are suppressed: the high-concentration strain layer forms compressive stress with the expansion zone and the fast growth zone, thereby suppressing the replication and proliferation of dislocations. During the crystal growth process, dislocations tend to reduce their dislocation length and reduce their mirror force, tending to be stress-free or have the lowest energy (Dislocation conversion in 4H siliconcarbide epitaxy. Journal of Crystal Growth, 244 (3-4), 257–266 | 10.1016 / s0022-0248 (02) 01706-2), and the high-concentration compressive stress can promote this trend, so that most of the dislocations change their inheritance direction (not along the 0001 direction) in the early stage, achieving the purpose of suppressing dislocations.

[0047] The present invention controls the doping of different elements and different concentrations during the growth process, forms a high-concentration strain layer in the nucleation stage, forms compressive stress on the seed crystal, thereby inhibiting the dislocation of the seed crystal from being inherited into the crystal during the growth of the ingot, and grows a low-dislocation, high-quality silicon carbide crystal. Figure 2 and Figure 4 .

[0048] Example 1

[0049] This embodiment provides a method for improving the quality of crystal growth. In order to form a high-concentration strain layer, the following steps may be performed:

[0050] 1) Prepare materials: prepare the required seed crystals, uniformly dope the element material [B] in the powder at a certain concentration, a growth substrate, and graphite materials such as a heating crucible. The mass ratio of boron powder to silicon carbide is 0-2.5g:2.5-4.5kg, preferably 1.5-2.5g:2.5-4.5kg.

[0051] 2) Load the mixed powder into a heated crucible, wipe the seed crystal, load the seed crystal into the upper part of the heated crucible, ensure that the height difference between the upper surface of the powder and the seed crystal is between 80-110 mm, preferably controlled at 90-100 mm; and tighten and seal the crucible cover and the crucible.

[0052] 3) Place the crucible in a heating furnace and evacuate the furnace to remove excess impurities.

[0053] 4) Start the heater and increase the power so that the temperature of the top and bottom of the crucible reaches 1100°C at the same time, and continue to fill with high-purity argon gas to make the pressure in the furnace reach 600mbar.

[0054] 5) After stabilization for 5 hours, continue to increase the power until the upper and lower temperatures reach 1700C at the same time, and start filling with nitrogen. The ratio of argon to nitrogen is maintained at 500:15-30, and the nitrogen flow rate is 10-20mL / min.

[0055] 6) Increase the power to keep the lower temperature of the crucible at 2100-2150°C and the upper temperature at 2000-2050°C, with the upper and lower temperature difference maintained at 70-100°C, preferably 80-100°C.

[0056] 7) Gradually reduce the pressure in the furnace to 2-7 mbar, preferably 3-5 mbar, and at the same time gradually increase the nitrogen concentration, preferably 5*10 18 / cm 3 -5*10 20 / cm 3 , the argon-nitrogen ratio is gradually increased from 500:15-30 to 80-100:5-8, preferably 80:5, 90:8 or 100:8, and this flow ratio is stabilized for 10-20 hours after the pressure reaches the target pressure.

[0057] 8) Continue to increase the power to the growth temperature, the lower temperature is 2250℃-2350℃, the upper temperature is 2140℃-2260℃, and the growth temperature is maintained. At the same time, the nitrogen concentration is gradually reduced by 5*10 within 5-15h. 17 / cm 3 -5*10 18 / cm 3 , so that the argon-nitrogen ratio is controlled at 90-110:3-5.

[0058] 9) After the crystal growth is completed, slowly increase the pressure in the furnace to 600mbar, and slowly cool the temperature in the furnace to room temperature, and take out the crystal.

[0059] The present invention can only dope nitrogen without doping B, but too high N concentration in the high concentration strain zone will cause lattice mismatch with the seed crystal, the fast growth zone and graphite, generate mismatch stress, and cause other defects such as stacking faults.

[0060] Therefore, the present invention adopts the method of adding boron (B) into the silicon carbide raw material in a certain proportion, and controlling the concentration of nitrogen (N) in the growth atmosphere to change in a gradient, thereby realizing the co-doping of nitrogen and boron elements, so as to form a high-doping concentration area in the bottom nucleation area to transition to the expansion area and then to the fast-growing part, and the concentration of mixed elements in the fast-growing part meets the performance requirements of the conductive silicon carbide substrate.

[0061] In the present invention, N element doping is performed throughout the entire growth process, and the concentration of the high concentration strain layer is 5*10 18 / cm 3 -5*10 20 / cm3 , preferably 5*10 19 / cm 3 , the concentration in the fast-growing zone is 5*10 17 / cm 3 -5*10 18 / cm 3 .

[0062] Boron doping is only introduced in the high concentration strain area, and the mass ratio of boron to silicon carbide powder needs to be controlled at 0-2.5g:2.5-4.5kg, preferably 1.5-2.5g:2.5-4.5kg.

[0063] The thickness of the strain layer should be controlled at 0.5-2 mm, preferably 0.8-1.5 mm.

[0064] The above is only a preferred embodiment of the present invention, and is not any formal or substantial limitation of the present invention. It should be pointed out that ordinary technicians in this technical field can make several improvements and supplements without departing from the method of the present invention, and these improvements and supplements should also be regarded as the protection scope of the present invention. Any technician familiar with this profession, without departing from the spirit and scope of the present invention, can make some changes, modifications and evolutions of the technical content disclosed above, which are equivalent embodiments of the present invention; at the same time, any changes, modifications and evolutions of any equivalent changes made to the above embodiments based on the essential technology of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for improving crystal growth quality, characterized in that: The method comprises the following steps: 1) preparing the required seed crystal, powder, doping element material, growth substrate and heating crucible, and mixing the powder and doping element material evenly to obtain mixed powder; the doping element is B, the doping element material is boron powder, and the mass ratio of boron powder to silicon carbide is 1.5-2.5g:2.5-4.5kg; 2) Place the mixed powder into a heated crucible, wipe the seed crystal, place the seed crystal on the upper part of the heated crucible, ensure the height difference between the upper surface of the mixed powder and the seed crystal, and seal it; 3) Place the heating crucible into a heating furnace and evacuate the furnace; 4) Start heating and continue to fill with high-purity argon gas; 5) After stabilization for a period of time, increase the heating power, fill with nitrogen, and maintain a certain ratio of argon to nitrogen; the flow ratio of argon to nitrogen is 500:15-30, and the nitrogen flow rate is 10-20mL / min; 6) Continue to increase the power to keep the temperature of the lower part of the crucible at 2100-2150°C, the temperature of the upper part at 2000-2050°C, and the temperature difference between the upper and lower parts of the crucible at 70-100°C; a strain layer with a high doping concentration is formed in the nucleation stage; 7) Gradually reduce the pressure in the heating furnace and increase the nitrogen concentration; the pressure in the furnace is 2-7mbar, the flow ratio of argon to nitrogen is 80-100:5-8, and the nitrogen concentration is 5×10 18 / cm 3 -5×10 20 / cm 3 ; 8) Continue to increase the power to the growth temperature, the lower temperature is 2250℃-2350℃, the upper temperature is 2140℃-2260℃, and the growth temperature is maintained, and the nitrogen concentration is gradually reduced within 5-15h; the nitrogen concentration is reduced to 5×10 17 / cm 3 -5×10 18 / cm 3 , the ratio of argon to nitrogen is 90-110:3-5; 9) After the crystal growth is completed, slowly increase the pressure in the furnace to 600mbar, slowly cool the temperature in the furnace to room temperature, and take out the crystal.

2. A method for improving crystal growth quality according to claim 1, characterized in that: In step 2), the height difference between the upper surface of the mixed powder and the seed crystal is 80-110 mm.

3. A method for improving crystal growth quality according to claim 1, characterized in that: In step 6), the temperature difference between the upper temperature and the lower temperature of the crucible is maintained at 80-100°C.

4. A method for improving crystal growth quality according to any one of claims 1 to 3, characterized in that: The thickness of the strain layer is 0.5-2 mm.

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