Method for preparing monocrystal diamond based on carbon dioxide / methane mixed gas carbon source

Abstract

The invention discloses a method for preparing single crystal diamond based on a carbon dioxide / methane mixed gas carbon source, which adopts a microwave plasma chemical vapor deposition method and takes CH4 / CO2 mixed gas as the carbon source to prepare the high-quality single crystal diamond, and obviously improves the growth rate of the single crystal diamond. The method solves the problems that the existing carbon dioxide methanation conversion rate is not high, and the carbon dioxide / methane mixed gas output after conversion cannot be directly converted into the high-value single crystal diamond, and the method has important significance for converting the carbon dioxide into the diamond.

Description

Technical Field

[0001] The invention belongs to the field of diamond preparation, and in particular relates to a method for preparing single crystal diamond based on a carbon dioxide / methane mixed gas carbon source. Background Art

[0002] With the increasing pressure of global warming and the full recovery of the economy in the post-epidemic era, a large amount of greenhouse gases have been emitted, and the global average temperature has risen rapidly. As one of the main greenhouse gases, the annual increase in CO2 emissions has become a major challenge that all mankind needs to face. CCU technology, which captures carbon dioxide and utilizes it as a resource, is considered to be one of the most promising technical approaches due to its good economic and social benefits.

[0003] Diamond is a carbon single substance with high hardness, high thermal conductivity, extremely low thermal expansion coefficient, low friction coefficient, good chemical stability, high acoustic propagation velocity, doping-induced semiconductor properties, high optical transmittance from ultraviolet light to infrared light, and extremely high hole mobility, which are difficult to match with other functional materials. It has very broad application prospects in the precision processing of superhard materials, high-energy radiation detectors, deep ultraviolet optoelectronic devices, wide bandgap and ultra-wide bandgap semiconductor materials. If the carbon in carbon dioxide can be used to transform it into high-value diamond, it will provide strong technical support for the "dual carbon" goal and is a carbon resource utilization path with great development prospects.

[0004] However, carbon dioxide alone cannot be used as the carbon source required for the growth of single crystal diamonds. This is because carbon dioxide dissociates to produce a large amount of oxygen ions, which will etch diamonds, thereby affecting the growth rate and crystal quality of diamonds. At present, high-purity methane is generally used as a carbon source in diamond CVD technology. Microwaves are used as an energy source to excite methane and hydrogen into a plasma state to grow single crystal diamonds on a substrate. Therefore, before converting carbon dioxide into diamond, it is necessary to methanize carbon dioxide. However, the current conversion rate of carbon dioxide methanization technology is not high, about 70%, and the output gas after conversion is a carbon dioxide / methane mixture.

[0005] In recent years, studies have used carbon dioxide as an auxiliary gas, methane, hydrogen or methane, hydrogen, and nitrogen as the main gas source, to prepare diamond polycrystalline films on microwave plasma chemical vapor deposition equipment. However, there has been no report on the preparation of high-quality single-crystalline diamond using a carbon dioxide / methane mixed gas carbon source. This is because carbon dioxide in a carbon dioxide / methane mixed gas is more difficult to ionize than methane, and the ionization rate is low. Most of the carbon dioxide exists in the plasma as a neutral gas molecule, which affects the collision between hydrogen plasma and methane molecules and reduces the content of C2 groups in the plasma. It is difficult to directly use the mixed gas to prepare high-quality single-crystalline diamond. Therefore, it is necessary to use other technical means to strip carbon dioxide multiple times and purify methane to increase the methane concentration. This process is cumbersome and will cause a waste of resources. In summary, there is an urgent need for a method for preparing single-crystalline diamond based on a carbon dioxide / methane mixed gas source. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source. The present invention adopts microwave plasma chemical vapor deposition (MPCVD) method, takes CH4 / CO2 mixed gas as carbon source, and prepares high-quality single crystal diamond by adjusting parameters such as seed crystal pretreatment method, microwave power, growth pressure, etc., and significantly improves the growth rate of single crystal diamond. The method is of great significance for the conversion of carbon dioxide into diamond.

[0007] The technical solution of the present invention is as follows:

[0008] A method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source, comprising the following steps:

[0009] (1) Diamond seed crystal pretreatment

[0010] Use an optical microscope to select seed crystals with clean surfaces, perform ultrasonic cleaning, and set aside;

[0011] The seed crystal is cut by laser and mechanically polished by a friction machine, and the surface roughness Ra value is less than 20nm;

[0012] The specific operation of ultrasonic cleaning is: first use ethanol ultrasonic cleaning for 3 to 5 times, each time for 30 minutes, and then use deionized water ultrasonic cleaning for 3 to 5 times, each time for 30 minutes;

[0013] (2) Etching

[0014] The seed crystal treated in step (1) is placed on a microwave CVD substrate. After the equipment is evacuated, the reaction chamber is flushed with hydrogen. After the flushing is completed, vacuum is evacuated, and then hydrogen is introduced. When the pressure in the microwave CVD equipment reaches 4 to 8 Torr, microwaves (microwave frequency 2.45 GHz) are input to form plasma; the amount of hydrogen introduced is gradually increased to raise the pressure in the reaction chamber to 130 to 150 Torr, and the microwave power is gradually increased to 3 to 5 kW while increasing the hydrogen flow rate. As the pressure and microwave power increase, the seed crystal temperature rises to 800 to 900° C., and etching is performed for 30 to 60 minutes;

[0015] The specific operation is as follows: placing the seed crystal treated in step (1) on a microwave CVD substrate, flushing the reaction chamber with 30-80 sccm (preferably 30 sccm) of hydrogen after the equipment is evacuated, flushing 2-3 times, and evacuating to 0.004-0.010 Torr (preferably 0.004 Torr) after flushing, then introducing 30-50 sccm (preferably 30 sccm) of hydrogen, and when the gas pressure in the microwave CVD equipment reaches 4-8 Torr (preferably 5 Torr), input 0.6 kW microwave to form plasma; increasing the pressure by 15 sccm every 1 min. The hydrogen flow rate gradually increases the hydrogen flow rate to increase the pressure in the reaction chamber. The microwave power is gradually increased while increasing the hydrogen flow rate. When the hydrogen flow rate reaches 250-300 sccm (preferably 250 sccm), the pressure in the reaction chamber rises to 130-150 Torr (preferably 140 Torr), and the microwave power rises to 3-5 kW (preferably 3.5-4.5 kW). As the pressure and microwave power increase, the seed temperature rises to 800-900° C. (preferably 850° C., with a heating rate of 5-30° C. / min), and etching is performed for 30-60 min (preferably 30 min).

[0016] (3) Growth

[0017] After etching, a methane / carbon dioxide mixture is introduced, and the growth is carried out for 6 hours in an environment with a hydrogen flow rate of 250-300 sccm (preferably 300 sccm), a microwave power of 3-6 kW, a seed temperature of 900-1000° C. (preferably 950° C.), and a gas pressure of 130-150 Torr (preferably 140 Torr);

[0018] The preferred methane / carbon dioxide mixed gas flow rate is 24 sccm, and the volume fraction of carbon dioxide in the mixed gas is 6.25-25% (preferably 16.7%);

[0019] (4) Cooling

[0020] After the growth is completed, the introduction of the methane / carbon dioxide mixed gas is stopped and the amount of hydrogen introduced is gradually reduced, the temperature is lowered by a gradient of 50 to 100° C. (preferably 50° C.), and the temperature is kept at each gradient node for 5 to 10 minutes (preferably 5 minutes), and then the temperature is lowered to room temperature, the microwave is turned off, and the introduction of hydrogen is terminated to obtain a single crystal diamond;

[0021] The preferred cooling rate is 15 to 30°C / min.

[0022] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in:

[0023] 1. Single crystal diamond is directly prepared by using carbon dioxide / methane mixed gas, without the need to use other technical means to strip carbon dioxide, which greatly reduces the difficulty of carbon dioxide resource utilization, and has significant economic value and high-tech characteristics;

[0024] 2. Single crystal diamond is directly prepared by using carbon dioxide / methane mixed gas. The surface of the prepared single crystal diamond has obvious step flow growth morphology, which can reduce the defect density of the sample and has higher crystal quality;

[0025] 3. The growth rate of single crystal diamond is improved by rationally controlling the volume fraction of carbon dioxide in the mixed gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : Optical microscope image of the single crystal diamond obtained in Example 1.

[0027] Figure 2 : Raman spectrum of the single crystal diamond obtained in Example 1.

[0028] Figure 3 : Optical microscope image of the single crystal diamond obtained in Example 2.

[0029] Figure 4 : Raman spectrum of the single crystal diamond obtained in Example 2.

[0030] Figure 5 : Optical microscope image of the single crystal diamond obtained in Example 3.

[0031] Figure 6 : Raman spectrum of the single crystal diamond obtained in Example 3.

[0032] Figure 7 : Optical microscope image of the single crystal diamond obtained in Example 4.

[0033] Figure 8 : Raman spectrum of the single crystal diamond obtained in Example 4.

[0034] Fig. 9: Optical microscope image of the single crystal diamond obtained in Example 5.

[0035] Fig.10 : Raman spectrum of the single crystal diamond obtained in Example 5. DETAILED DESCRIPTION

[0036] The present invention is further described below by means of specific embodiments, but the protection scope of the present invention is not limited thereto.

[0037] In the following embodiments,

[0038] CVD single crystal diamond seed crystals were purchased from Dongguan Jinyue New Material Technology Co., Ltd., 5 mm × 5 mm × 0.3 mm.

[0039] The microwave CVD equipment was purchased from Shenzhen Super Magnetic Robotics Technology Co., Ltd., model MPCVD-6A.

[0040] Example 1

[0041] Select (5mm×5mm×0.3mm) CVD single crystal diamond seed crystals. Observe the morphology of the seed crystal growth surface under an optical microscope, and remove unqualified seed crystals with obvious stains and scratches on the surface. Put the qualified seed crystals after screening into 95% ethanol ultrasonic cleaning for 30 minutes, wash three times, and then put them into deionized water ultrasonic cleaning for 30 minutes, wash three times, to obtain the pretreated seed crystals.

[0042] Place the pretreated seed crystal on a flat molybdenum substrate in the microwave CVD equipment, and then place the substrate in the reaction chamber. Use hydrogen to flush the reaction chamber, evacuate the chamber to below 0.004 Torr, open the hydrogen valve, set the hydrogen flow value to 30 sccm, and when the pressure in the chamber reaches 6 Torr, turn on the vacuum pump, evacuate to below 0.004 Torr, and circulate hydrogen to flush the chamber 3 times.

[0043] Open the hydrogen valve and continue to set the hydrogen flow rate to 30sccm. After the pressure in the cavity rises to 5Torr, input microwaves and set the microwave power to 600W to form plasma. Start to increase the reaction chamber pressure and microwave power. Increase the hydrogen flow rate by 15sccm every 1min. For every 10Torr increase in pressure, the power increases by 250W. Repeat the pressure and power increase rule step by step to increase the temperature. When the hydrogen flow rate is 250sccm, the pressure rises to 140Torr, the sample temperature rises to 850℃, and the microwave power ranges from 3.5kW to 5kW to maintain a constant etching temperature. Etch for 30min.

[0044] After etching, the hydrogen flow rate is increased and a carbon dioxide / methane mixture with a carbon dioxide volume fraction of 6.25% is introduced, wherein the hydrogen increases by 20 sccm every 2 minutes, and the methane / carbon dioxide mixture increases by 1.5 sccm every 2 minutes. With the increase in gas flow, the microwave power increases by 250 W every 2 minutes. When the hydrogen flow rate is 300 sccm, the mixed gas flow rate is 24 sccm, the pressure is maintained at 140 Torr, the seed temperature is maintained at 950°C, and the microwave power is 4.5 kW-5.5 kW to maintain a growth temperature of 950°C for 6 hours.

[0045] After the growth is completed, the introduction of methane / carbon dioxide mixed gas is stopped and the hydrogen flow rate is gradually reduced. After keeping warm for 5 minutes, follow the principle of "reducing power first and then reducing pressure". For every 100W reduction, the pressure is reduced by 5Torr. Keep warm for 5 minutes and slowly cool to room temperature. When the hydrogen flow rate is 30sccm, the pressure reaches 10Torr, and the microwave power is 600W, turn off the microwave and stop introducing hydrogen to obtain single crystal diamond with a diamond growth rate of 4.55μm / h.

[0046] Figure 1 The surface morphology of single crystal diamond prepared by the mixed gas carbon source with a volume fraction of 6.25% carbon dioxide. It can be seen from the figure that step flow growth appears on the surface of the sample, the steps are sparse and blurred with only a small height difference, and there are obvious "black spot" areas, that is, non-epitaxial grains. After a long period of growth, pyramid nucleation will occur and then polycrystalline diamond or graphite phase will be produced, which will have a negative impact on the purity and surface flatness of the single crystal diamond.

[0047] Figure 2 This is the Raman spectrum of a single crystal diamond prepared from a mixed gas carbon source with a carbon dioxide volume fraction of 6.25%. By analyzing the Raman peak position of the sample, it was found that the sample had a peak at 1332.4 cm -1 The diamond Raman peak exists at , proving that the prepared sample is a diamond of high purity. By fitting the characteristic peak of the grown diamond, its half-peak width is 2.90cm -1 , indicating that the prepared single crystal diamond has a higher crystal quality.

[0048] Example 2

[0049] Select (5mm×5mm×0.3mm) CVD single crystal diamond seed crystals. Observe the morphology of the seed crystal growth surface under an optical microscope, and remove unqualified seed crystals with obvious stains and scratches on the surface. Put the qualified seed crystals after screening into 95% ethanol ultrasonic cleaning for 30 minutes, wash three times, and then put them into deionized water ultrasonic cleaning for 30 minutes, wash three times, to obtain the pretreated seed crystals.

[0050] Place the pretreated seed crystal on a flat molybdenum substrate in the microwave CVD equipment, and then place the substrate in the reaction chamber. Use hydrogen to flush the reaction chamber, evacuate the chamber to below 0.004 Torr, open the hydrogen valve, set the hydrogen flow value to 30 sccm, and when the pressure in the chamber reaches 6 Torr, turn on the vacuum pump, evacuate to below 0.004 Torr, and circulate hydrogen to flush the chamber 3 times.

[0051] Open the hydrogen valve and continue to set the hydrogen flow rate to 30sccm. After the pressure in the cavity rises to 5Torr, input microwaves and set the microwave power to 600W to form plasma. Start to increase the reaction chamber pressure and microwave power. Increase the hydrogen flow rate by 15sccm every 1min. For every 10Torr increase in pressure, the power increases by 250W. Repeat the pressure and power increase rule step by step to increase the temperature. When the hydrogen flow rate is 250sccm, the pressure rises to 140Torr, the sample temperature rises to 850℃, and the microwave power ranges from 3kW to 5kW to maintain a constant etching temperature. Etch for 30min.

[0052] After etching, the hydrogen flow rate is increased and a carbon dioxide / methane mixture with a carbon dioxide volume fraction of 8.3% is introduced, wherein the hydrogen increases by 20 sccm every 2 minutes, and the methane / carbon dioxide mixture increases by 3 sccm every 2 minutes. With the increase in gas flow, the microwave power increases by 250 W every 2 minutes. When the hydrogen flow rate is 300 sccm, the mixed gas flow rate is 24 sccm, the pressure is maintained at 140 Torr, the seed temperature is maintained at 950°C, and the microwave power is 3kW-4kW to maintain a growth temperature of 950°C for 6 hours.

[0053] After the growth is completed, the introduction of methane / carbon dioxide mixed gas is stopped and the hydrogen flow rate is gradually reduced. After keeping warm for 5 minutes, follow the principle of "reducing power first and then reducing pressure". For every 100W reduction, the pressure is reduced by 5Torr. Keep warm for 5 minutes and slowly cool to room temperature. When the hydrogen flow rate is 30sccm, the pressure reaches 10Torr, and the microwave power is 600W, turn off the microwave and stop introducing hydrogen to obtain single crystal diamond. The diamond growth rate is 9.44μm / h, which is much greater than the diamond growth rate of 5.83μm / h in Example 5 when no carbon dioxide is added.

[0054] Figure 3 This is the surface morphology of the single crystal diamond prepared by the mixed gas carbon source with a carbon dioxide volume fraction of 8.3%. It can be seen from the figure that there is a more obvious raised step flow on the surface of the sample, the steps are dense but the flatness is poor.

[0055] Figure 4This is the Raman spectrum of the single crystal diamond prepared from the mixed gas carbon source with a volume fraction of 8.3% carbon dioxide. By analyzing the Raman peak position of the sample, it was found that the sample had a peak at 1333cm -1 The diamond Raman peak is found, proving that the prepared sample is a diamond of high purity. By fitting the characteristic peak of the grown diamond, its half-peak width is 2.42 cm -1 , indicating that the prepared single crystal diamond has a higher crystal quality.

[0056] Example 3

[0057] Select (5mm×5mm×0.3mm) CVD single crystal diamond seed crystals. Observe the morphology of the seed crystal growth surface under an optical microscope, and remove unqualified seed crystals with obvious stains and scratches on the surface. Put the qualified seed crystals after screening into 95% ethanol ultrasonic cleaning for 30 minutes, wash three times, and then put them into deionized water ultrasonic cleaning for 30 minutes, wash three times, to obtain the pretreated seed crystals.

[0058] Place the pretreated seed crystal on a flat molybdenum substrate in the microwave CVD equipment, and then place the substrate in the reaction chamber. Use hydrogen to flush the reaction chamber, evacuate the chamber to below 0.004 Torr, open the hydrogen valve, set the hydrogen flow value to 30 sccm, and when the pressure in the chamber reaches 6 Torr, turn on the vacuum pump, evacuate to below 0.004 Torr, and circulate hydrogen to flush the chamber 3 times.

[0059] Open the hydrogen valve and continue to set the hydrogen flow rate to 30sccm. After the pressure in the cavity rises to 5Torr, input microwaves and set the microwave power to 600W to form plasma. Start to increase the reaction chamber pressure and microwave power. Increase the hydrogen flow rate by 15sccm every 1min. For every 10Torr increase in pressure, the power increases by 250W. Repeat the pressure and power increase rule step by step to increase the temperature. When the hydrogen flow rate is 250sccm, the pressure rises to 140Torr, the sample temperature rises to 850℃, and the microwave power ranges from 4kW to 5kW to maintain a constant etching temperature. Etch for 30min.

[0060] After etching, the hydrogen flow rate is increased and a carbon dioxide / methane mixture with a carbon dioxide volume fraction of 16.7% is introduced, wherein the hydrogen increases by 25 sccm every 2 minutes, and the methane / carbon dioxide mixture increases by 2 sccm every 2 minutes. With the increase in gas flow, the microwave power increases by 250W every 2 minutes. When the hydrogen flow rate is 300 sccm, the mixed gas flow rate is 24 sccm, the pressure is maintained at 140 Torr, the seed temperature is maintained at 950°C, and the microwave power is 3.5kW-5kW to maintain a growth temperature of 950°C for 6 hours.

[0061] After the growth is completed, the introduction of methane / carbon dioxide mixed gas is stopped and the hydrogen flow rate is gradually reduced. After keeping warm for 5 minutes, follow the principle of "reducing power first and then reducing pressure". For every 100W reduction, the pressure decreases by 5Torr. Keep warm for 5 minutes and slowly cool to room temperature. When the hydrogen flow rate is 30sccm, the pressure reaches 10Torr, and the microwave power is 600W, turn off the microwave and stop introducing hydrogen to obtain single crystal diamond with a diamond growth rate of 6.33μm / h.

[0062] Figure 5 This is the surface morphology of the single crystal diamond prepared by the mixed gas carbon source with a carbon dioxide volume fraction of 16.7%. It can be seen from the figure that the sample surface has obvious step flow growth, the step flow is uniform and has a high flatness, the step direction is consistent, and there is no polycrystalline on the surface.

[0063] Figure 6 The Raman spectrum of the single crystal diamond prepared from the mixed gas carbon source with a volume fraction of 16.7% carbon dioxide. By analyzing the Raman peak position of the sample, it was found that the sample had a peak at 1332.9 cm -1 The diamond Raman peak is found, proving that the prepared sample is a diamond of high purity. By fitting the characteristic peak of the grown diamond, its half-peak width is obtained to be 2.44 cm -1 , indicating that the prepared single crystal diamond has a higher crystal quality.

[0064] Example 4

[0065] Select (5mm×5mm×0.3mm) CVD single crystal diamond seed crystals. Observe the morphology of the seed crystal growth surface under an optical microscope, and remove unqualified seed crystals with obvious stains and scratches on the surface. Put the qualified seed crystals after screening into 95% ethanol ultrasonic cleaning for 30 minutes, wash three times, and then put them into deionized water ultrasonic cleaning for 30 minutes, wash three times, to obtain the pretreated seed crystals.

[0066] Place the pretreated seed crystal on a flat molybdenum substrate in the microwave CVD equipment, and then place the substrate in the reaction chamber. Use hydrogen to flush the reaction chamber, evacuate the chamber to below 0.004 Torr, open the hydrogen valve, set the hydrogen flow value to 30 sccm, and when the pressure in the chamber reaches 6 Torr, turn on the vacuum pump, evacuate to below 0.004 Torr, and circulate hydrogen to flush the chamber 3 times.

[0067] Open the hydrogen valve and continue to set the hydrogen flow rate to 30sccm. After the pressure in the cavity rises to 5Torr, input microwaves and set the microwave power to 600W to form plasma. Start to increase the reaction chamber pressure and microwave power. Increase the hydrogen flow rate by 15sccm every 1min. For every 10Torr increase in pressure, the power increases by 250W. Repeat the pressure and power increase rule step by step to increase the temperature. When the hydrogen flow rate is 250sccm, the pressure rises to 140Torr, the sample temperature rises to 850℃, and the microwave power ranges from 3.5kW to 4.5kW to maintain a constant etching temperature. Etch for 30min.

[0068] After etching, the hydrogen flow rate is increased and a carbon dioxide / methane mixture with a carbon dioxide volume fraction of 25% is introduced, wherein the hydrogen increases by 15 sccm every 1 min, and the methane / carbon dioxide mixture increases by 2 sccm every 4 min. With the increase in gas flow, the microwave power increases by 250 W every 2 min. When the hydrogen flow rate is 300 sccm, the mixed gas flow rate is 24 sccm, the pressure is maintained at 140 Torr, the seed temperature is maintained at 950°C, and the microwave power is 4.5 kW-5.5 kW to maintain a growth temperature of 950°C for 6 hours.

[0069] After the growth is completed, the introduction of methane / carbon dioxide mixed gas is stopped and the hydrogen flow rate is gradually reduced. After keeping warm for 5 minutes, follow the principle of "reducing power first and then reducing pressure". For every 100W reduction, the pressure is reduced by 5Torr. Keep warm for 5 minutes and slowly cool to room temperature. When the hydrogen flow rate is 30sccm, the pressure reaches 10Torr, and the microwave power is 600W, turn off the microwave and stop introducing hydrogen to obtain single crystal diamond with a diamond growth rate of 2.17μm / h. This is mainly because when the carbon dioxide content is high, the concentration of oxygen-containing groups in the plasma increases. While etching the non-diamond phase, the etching effect on the diamond phase becomes more significant, causing the etching rate to be close to the growth rate, which in turn leads to a decrease in the diamond growth rate.

[0070] Figure 7 This is the surface morphology of the single crystal diamond prepared by the mixed gas carbon source with a volume fraction of 25% carbon dioxide. It can be seen from the figure that a dense step flow growth appears on the surface of the sample, and the steps are consistent in direction but the growth steps are not uniform.

[0071] Figure 8 This is the Raman spectrum of a single crystal diamond prepared from a mixed gas carbon source with a carbon dioxide volume fraction of 25%. By analyzing the Raman peak position of the sample, it was found that the sample had a peak at 1332.9 cm -1 The diamond Raman peak is found, proving that the prepared sample is a diamond of high purity. By fitting the characteristic peak of the grown diamond, its half-peak width is obtained to be 2.44 cm -1, indicating that the prepared single crystal diamond has a higher crystal quality.

[0072] Example 5

[0073] Select (5mm×5mm×0.3mm) CVD single crystal diamond seed crystals. Observe the morphology of the seed crystal growth surface under an optical microscope, and remove unqualified seed crystals with obvious stains and scratches on the surface. Put the qualified seed crystals after screening into 95% ethanol ultrasonic cleaning for 30 minutes, wash three times, and then put them into deionized water ultrasonic cleaning for 30 minutes, wash three times, to obtain the pretreated seed crystals.

[0074] Place the pretreated seed crystal on a flat molybdenum substrate in the microwave CVD equipment, and then place the substrate in the reaction chamber. Use hydrogen to flush the reaction chamber, evacuate the chamber to below 0.004 Torr, open the hydrogen valve, set the hydrogen flow value to 30 sccm, and when the pressure in the chamber reaches 6 Torr, turn on the vacuum pump, evacuate to below 0.004 Torr, and circulate hydrogen to flush the chamber 3 times.

[0075] Open the hydrogen valve and continue to set the hydrogen flow rate to 30sccm. After the pressure in the cavity rises to 5Torr, input microwaves and set the microwave power to 600W to form plasma. Start to increase the reaction chamber pressure and microwave power. Increase the hydrogen flow rate by 15sccm every 1min. For every 10Torr increase in pressure, the power increases by 250W. Repeat the pressure and power increase rule step by step to increase the temperature. When the hydrogen flow rate is 250sccm, the pressure rises to 140Torr, the sample temperature rises to 850℃, and the microwave power ranges from 3.5kW to 4.5kW to maintain a constant etching temperature. Etch for 30min.

[0076] After etching, the hydrogen flow rate is increased and methane gas is introduced, wherein the hydrogen increases by 20 sccm every 2 minutes, and the methane gas increases by 1.5 sccm every 2 minutes. With the increase in gas flow rate, the microwave power increases by 250W every 2 minutes. When the hydrogen flow rate is 300 sccm, the methane gas flow rate is 24 sccm, the pressure is maintained at 140 Torr, the seed temperature is maintained at 950°C, and the microwave power is 4.5kW-5.5kW to maintain the growth temperature of 950°C, and the growth is carried out for 6 hours.

[0077] After the growth is completed, the methane gas is stopped and the hydrogen flow rate is gradually reduced. After keeping warm for 5 minutes, follow the principle of "reducing power first and then reducing pressure". For every 100W reduction, the pressure is reduced by 5Torr. Keep warm for 5 minutes and slowly cool to room temperature. When the hydrogen flow rate is 30sccm, the pressure reaches 10Torr, and the microwave power is 600W, turn off the microwave and stop passing hydrogen to obtain single crystal diamond with a diamond growth rate of 5.83μm / h.

[0078] Fig. 9 This is the surface morphology of single crystal diamond prepared with pure methane gas as the carbon source. As can be seen from the figure, there are very sparse step flows on the surface of the sample, with obvious height differences and different step widths. There are "black spots" in some areas of the surface. Continuing to grow this area will affect the surface morphology of the sample and reduce the surface flatness of the single crystal diamond.

[0079] Fig.10 This is the Raman spectrum of single crystal diamond prepared with pure methane gas as the carbon source. By analyzing the Raman peak position of the sample, it was found that the sample has a peak at 1333.1cm -1 The diamond Raman peak is found, proving that the prepared sample is a diamond of high purity. By fitting the characteristic peak of the grown diamond, its half-peak width is 2.54 cm -1 , indicating that the prepared single crystal diamond has a higher crystal quality.

Claims

1. A method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source, characterized in that: The steps include: (1) Diamond seed crystal pretreatment Use an optical microscope to select seed crystals with clean surfaces, perform ultrasonic cleaning, and set aside; (2) Etching The seed crystal treated in step (1) is placed on a microwave CVD substrate. After the equipment is evacuated, the reaction chamber is flushed with hydrogen. After the flushing is completed, the reaction chamber is evacuated, and then hydrogen is introduced. When the gas pressure in the microwave CVD equipment reaches 4 to 8 Torr, microwaves are input to form plasma. The amount of hydrogen introduced is gradually increased to raise the gas pressure in the reaction chamber to 130 to 150 Torr. While increasing the hydrogen flow rate, the microwave power is gradually increased to 3 to 5 kW. As the gas pressure and microwave power increase, the seed crystal temperature is raised to 800 to 900° C., and etching is performed for 30 to 60 minutes. (3) Growth After etching, a methane / carbon dioxide mixture is introduced, and the cells are grown for 6 hours in an environment with a hydrogen flow rate of 250-300 sccm, a microwave power of 3-6 kW, a seed temperature of 900-1000°C, and a pressure of 130-150 Torr. (4) Cooling After the growth is completed, the introduction of the methane / carbon dioxide mixture is stopped and the amount of hydrogen introduced is gradually reduced, the temperature is lowered in a gradient of 50 to 100°C, and the temperature is kept at each gradient node for 5 to 10 minutes, then cooled to room temperature, the microwave is turned off, and the introduction of hydrogen is terminated to obtain a single crystal diamond.

2. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: In step (1), the surface roughness Ra value of the seed crystal is less than 20 nm.

3. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: In step (1), the operation of ultrasonic cleaning is: first use ethanol to ultrasonically clean for 3 to 5 times, each time for 30 minutes, and then use deionized water to ultrasonically clean for 3 to 5 times, each time for 30 minutes.

4. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: In step (2), the microwave frequency is 2.45 GHz.

5. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: The operation of step (2) is as follows: placing the seed crystal processed in step (1) on a microwave CVD substrate, flushing the reaction chamber with 30-80 sccm hydrogen after the equipment is evacuated, and flushing is performed 2-3 times. After flushing, the temperature is evacuated to 0.004-0.010 Torr, and then 30-50 sccm hydrogen is introduced. When the gas pressure in the microwave CVD equipment reaches 4-8 Torr, 0.6 kW microwave is input to form plasma; the hydrogen flow rate is gradually increased at a rate of increasing the hydrogen flow rate by 15 sccm per minute to increase the gas pressure in the reaction chamber, and the microwave power is gradually increased while increasing the hydrogen flow rate. When the hydrogen flow rate reaches 250-300 sccm, the gas pressure in the reaction chamber is increased to 130-150 Torr, and the microwave power is increased to 3-5 kW. As the gas pressure and microwave power increase, the seed crystal temperature is increased to 800-900° C., and etching is performed for 30-60 minutes.

6. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: In step (3), the flow rate of the methane / carbon dioxide mixed gas is 24 sccm.

7. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: The volume fraction of carbon dioxide in the mixed gas in step (3) is 6.25-25%.

8. The method for preparing single crystal diamond based on carbon dioxide / methane mixed gas carbon source as claimed in claim 1, characterized in that: The volume fraction of carbon dioxide in the mixed gas in step (3) is 16.7%.

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