Deposition of diamond films by microwave plasma CVD on 4H-SiC substrates

Diamond films were deposited on 4H-SiC substrates by microwave plasma chemical vapor deposition (MPCVD). The substrate pretreatment method of electrostatic adsorption of seed crystals by nanodiamond suspensions was used, and the nucleation density of diamond on the substrate surface reached 1010/cm2 compared with ultrasonic seed crystals of diamond micro-powder suspensions, and continuous dense diamond films were formed in a shorter growth time. Scanning electron microscopy and Raman spectroscopy were used to characterize the changes of diamond grain morphology and quality with methane concentration, deposition time and substrate temperature during the growth process. The experimental results show that the methane concentration, deposition time and substrate temperature are the key factors affecting the grain shape and quality of diamond. And the best quality of diamond film is obtained at 850 °C substrate temperature.


Introduction
Silicon carbide (SiC) materials are a major development in the power semiconductor industry and are used to make high power devices that can significantly improve power utilization [1].The deposition of diamond films allows passivation of SiC devices, reduces leakage currents, avoids premature device breakdown, and also optimizes the heat dissipation problem of the device, improves its performance level and extends its lifetime [2].
At present, the main methods of diamond film preparation are hot filament chemical vapor deposition (HFCVD), DC-assisted plasma chemical vapor deposition (DC-PACVD), microwave plasma chemical vapor deposition (MPCVD), DC arc plasma jet chemical vapor deposition (DC Arc Plasma Jet CVD), etc MPCVD utilizes microwave energy to excite the collision of gas molecules to generate a plasma.The working gas generates a large number of active groups due to the excitation of microwave energy.This microwave energy excitation produces a plasma with a higher density of active particles and a more stable plasma.The excitation method directly utilizes microwave energy coupling without introducing other auxiliary devices, such as tantalum wire (or tungsten wire) in hot wire equipment.Therefore, when using microwave equipment, other impurity atoms will not deposit on the substrate during the thin film deposition, which affects the quality of the diamond film [3].Heterogeneous substrate materials commonly used to prepare diamond include silicon, ceramic materials (aluminum carbide, silicon carbide, aluminum nitride, etc) and high melting point metals (molybdenum, titanium, etc) [4], among which 4H-SiC has a crystal structure similar to diamond and the thermal expansion coefficients of the two do not differ much, which is also suitable for growing low-stress, highquality diamond films.
In recent years, many scholars have studied chemical vapor deposition methods for depositing diamond films on SiC substrates.Junya Yaita [5] achieved heterogeneous epitaxial nucleation of diamond on 3C-SiC (001)/Si(001) substrates using antenna-edge microwave plasma chemical vapor deposition, and obtained a 3 × 10 9 diamond-shaped nucleation density after 2 h of growth.Debarati Musketeer [2] used hot filament chemical vapor deposition (HFCVD) method to deposit diamond films on SiC substrates, using diamond particles of different diameters ground on suspensions in the substrate pretreatment stage and then growing diamond films at different temperatures and different methane concentrations, the results show that a lower growth temperature (500 °C) induces lateral growth of diamond crystals compared to a higher temperature (600 °C).
Wang He [6] used microwave plasma chemical vapor deposition (MPCVD) to prepare diamond films on SiC substrates with different microwave power, gas reaction pressure and methane concentration process parameters, and showed that by increasing the methane concentration as well as the reaction pressure, the diamond content in the films decreased, the graphite content increased, and the self-lubricating properties of the films were enhanced.Xiufei Hu [7] used microwave plasma chemical vapor deposition (MPCVD) to deposit diamond films on surface-polished, ground and slotted 4H-SiC substrates respectively, and showed that the nucleation density of ground substrates increased by three orders of magnitude compared with surface-polished substrates, and methane concentration was a key factor affecting nucleation density, grain size and growth orientation.
As mentioned above, the common method of substrate pretreatment is to make the surface 'rough' by grinding in order to promote the formation of diamond nucleation sites, but the nucleation density obtained is generally low, and there is a need to find a better pretreatment method to increase the nucleation density of diamond, which can form in a shorter growth time.In this paper, the MPCVD method is used to improve the nucleation density of diamond and to form continuous and dense diamond films.Therefore, this paper adopts MPCVD method to deposit diamond films on 4H-SiC substrates using methane and hydrogen as reaction gases to study the effects of different substrate pretreatment methods on diamond nucleation density, to find the best pretreatment method, to study the effects of different process parameters on the growth mechanism of diamond, and to characterize and analyze the growth morphology and quality of diamond, and finally to obtain the results of the preparation of high quality diamond on 4H-SiC substrates.

Experiment
As shown in figure 1, 4H-SiC and diamond have similar crystal structures, and from the crystal structures of both [8], the silicon and carbon atoms of 4H-SiC are arranged in a tetrahedral stacking manner to form a tetrahedral lattice structure, and each carbon atom of diamond forms an orthotetrahedral structure with the four adjacent carbon atoms.However, there are certain differences in the lattice constants as well as thermal expansion coefficients between the two, and the problem of detachment between the diamond film and the 4H-SiC substrate may occur during the deposition process.
4H-SiC has two polar surfaces, the silicon surface and the carbon surface, the C atom at the carbon surface can directly connect with the hydrocarbon groups in the plasma to form a C-C bond for the growth of diamond phase, therefore the nucleation and growth of diamond on the carbon surface can be considered as homoepitaxial.Bo Wang [9] conducted comparative nucleation density experiments on the carbon and silicon surfaces of 4H-SiC, and the results showed that the nucleation density of diamond on the carbon surface was 2-3 orders of magnitude higher than that on the silicon surface, so we chose to deposit diamond on the carbon surface in our subsequent experiments.In addition, 4H-SiC material is similar to 'glass' and its surface is smooth, so generally speaking, without 'seed crystal' pretreatment, the nucleation density of diamond on the smooth substrate surface is low.Therefore, it is necessary to pretreat the substrate surface before depositing diamond films to promote the formation of diamond nucleation sites.

Substrate pretreatment
A 10 mm × 10 mm 4H-SiC with a thickness of 500 ± 20 μm and a surface roughness of less than 0.2 nm (5 μm × 5 μm) was used as a substrate.The current seed crystal pretreatment on heterogeneous substrates by grinding the substrate surface with diamond micronized powder or ultrasonic treatment of the substrate surface with diamond suspension has achieved extremely high nucleation density on silicon substrates and successfully realized the heterogeneous epitaxial growth of polycrystalline diamond [10].Diamond nucleation is essential for diamond growth and affects the shape and quality of diamond films.Therefore, we propose the pretreatment methods of ultrasonic seed crystals of diamond micro-powder suspensions and electrostatic adsorption of seed crystals by nanodiamond suspensions on the carbon surface of 4H-SiC substrates to compare the effects of the two pretreatment methods on diamond nucleation density.
The pretreatment process of diamond micro-powder suspension ultrasonic seed crystal is as follows: 1. Immerse the substrate into 50% HF acid solution for 5 min, and then shake the substrate in the ultrasonic cleaner for 3 min using ethanol solution and deionized water in turn; 2. Configure diamond micro-powder suspension: diamond micro-powder (particle size of 50 μm) and acetone solution are mixed in a certain ratio and stirred well; 3. The suspension is oscillated in the ultrasonic cleaning machine for 30 min.
The pretreatment process of adsorption of seed crystals by nanodiamonds solution is as follows: 1. Immerse the substrate into 50% HF acid solution for 5 min, and then use ethanol solution and deionized water to oscillate in the ultrasonic cleaner for 3 min in turn; 2. Immerse the substrate in a Poly (sodium 4-styrenesulfonate) solution for 10 min, allowing the substrate surface to be modified with a layer of negatively charged ions, resulting in a negative charge on the substrate surface.3. Immerse the substrate in a Positive Zeta Potential Nano diamond Suspension solution (diamond particle size of 5 nm) for 2 min, using the principle of 'electrostatic adsorption' , the schematic diagram is shown in figure 2, the positively charged nanodiamonds particles are 'adsorbed' to the negatively charged substrate surface to achieve the purpose of seed crystal [11].
The substrates with different pretreatment methods were put into the MPVCD reaction chamber at the same time, among which the substrates without any pretreatment were only cleaned.The nucleation experiments were carried out under the process conditions of 160 Torr, 4500 W microwave power and 4% methane concentration, and the nucleation stage was started with the introduction of methane gas for 8 min.The purpose is to allow the nuclei to grow properly to facilitate observation and calculation of the nucleation density of the diamond.
The SEM images of diamond nucleation density results under different substrate pretreatment methods as shown in figure 3, it can be seen that the substrate pretreatment is the key factor affecting the diamond nucleation density.In order to better calculate the number and density of nucleation in SEM images, different magnification ratios were used for different sample images.Randomly select several regions in figures 3(a) ∼ (c), calculate the average number of diamond nucleation, and then divide by the area of the selected region to obtain an approximate nucleation density.The experimental scheme and results are shown in table 1.For the 4H-SiC substrate without any pretreatment method, diamond is difficult to nucleate on its surface, and the nucleation density is only 10 4 ∼ 10 5 cm −2 , but in the diamond micro-powder suspension ultrasonic seeding pretreatment method, it can effectively promote the nucleation of diamond, and the nucleation density is 10 7 ∼ 10 8 cm −2 at this time.In addition, compared with the diamond micro-powder suspension ultrasonic seeding, the nucleation density of diamond on the surface of 4H-SiC substrate was significantly increased by using the electrostatic adsorption seeding method of nanodiamond suspension, and the nucleation density could reach 10 9 ∼ 10 10 /cm 2 .This pretreatment method can solve the problem of low nucleation density of diamond on the smooth   surface of 4H-SiC substrate.Therefore, the method of electrostatic adsorption of seed crystals by nanodiamond suspensions was used in all subsequent growth experiments.

MPCVD system
The MPCVD device used in this experiment is a 2.45 GHz cylindrical single-mode resonant cavity device.The schematic structure of the device is shown in figure 4. The experimental CVD device is mainly composed of microwave system, power supply system, vacuum system, water circulation cooling system and other parts.The principle of the device is to place a quartz glass plate in the middle of the cavity to separate the microwave system and the vacuum system, and pass the reaction gases methane and hydrogen into the reaction cavity through the gas flow controller.The microwave system emits high energy microwaves to decompose the methane gas and hydrogen and excite a spherical plasma consisting of carbon groups and hydrogen atoms on the abutment [12].
The system is capable of setting the relevant process parameters (mainly gas flow rate, microwave power, growth time and reaction pressure) for epitaxial growth of diamond on heterogeneous substrates, where the vacuum system controls the reaction pressure and the adjustment of microwave power and reaction pressure controls the substrate temperature of diamond, and the substrate temperature is detected by infrared light.The process of growing diamond by MPCVD is as follows: the substrate is placed into the reaction chamber, vacuum is started, and H 2 is introduced when the vacuum reaches 10 −4 Torr, and then the microwave source is turned on to excite hydrogen plasma to clean the substrate for 30 min to further remove the oxides on the substrate surface.In the process of hydrogen plasma cleaning, the reaction air pressure as well as the microwave power are adjusted to make the substrate warm up, and after approaching the target temperature, methane gas is started to be introduced, at which time a chemical reaction occurs inside the cavity, and the diamond produced is adsorbed by the substrate to become seed crystals, in addition to the substrate after the adsorption of seed crystals by the nanodiamond solution, the nanodiamond particles attached to its surface will likewise adsorb the atoms in the reaction gas (this is also the nucleation process), which then grows and expands to form 'islands' and finally a continuous diamond film [13].

Growth experiment
Previous research results have shown that the CH 4 /H 2 flux ratio suitable for growing diamond films is about 2% ∼ 10% and the substrate temperature is 600 ∼ 1100 °C.In this paper, CH 4 /H 2 flow ratio of (10,20,30,40)/500, growth time of 0 min,15 min, 30 min, 60 min and substrate temperature of 650 °C,750 °C,850 °C,950 °C are selected for experimental study.The pretreated 4H-SiC substrate is put into MPCVD equipment for diamond film growth, and the reaction gas methane and hydrogen in certain ratio continuously into the reaction chamber, and the CH 4 /H 2 flow rate ratio is the methane concentration.Different growth process parameters, i.e., methane concentration, growth time, and substrate temperature, were used to prepare diamond films on 4H-SiC substrates, and the specific growth process parameters are listed in table 2.

Results and analysis
The pretreatment method of electrostatic adsorption of seed crystals by nanodiamond suspensions led to a significant increase in the nucleation density on the surface of 4H-SiC substrates, and the prepared diamond films were well dense.The cross-sectional morphology of diamond on 4H-SiC shown in figure 5 indicates that the pretreatment method successfully achieved the growth of diamond on 4H-SiC substrate, and the film adheres well to the substrate surface, and there is no detachment and cracking holes at the interface between diamond and substrate, while the damage to 4H-SiC is greatly reduced, and then the diamond film is characterized in terms of morphology and quality.Subsequently, the diamond films were characterized in terms of shape and quality.
A Sigma 500 (SEM) high-resolution field emission scanning electron microscope (SEM resolution: 1.0 nm @ 30 kV STEM, 1.0 nm @ 15 kV, 1.8 nm @ 1 kV) from Zeiss, Germany, was used to observe the surface morphology as well as the cross-sectional morphology of the diamond films.An Alpha300 confocal Raman spectrometer (spectral resolution 0.02 cm −1 ) with an excitation wavelength of 532.287 nm from Germany was used to characterize the diamond films for the presence of graphite and non-diamond equivalent impurities, and thus to examine the growth quality of the diamond films.

Effect of different methane concentrations on diamond films
Methane concentration is a very important parameter for the growth of diamond films, and different methane concentrations will affect various aspects of diamond films, including grain morphology and purity.In this section, the effect of different methane concentrations on the growth of diamond films will be investigated.The methane concentrations were set to 2%, 4%, 6% and 8% for four groups of comparative experiments, and the other process parameters were kept the same, so as to find out the most suitable methane density for the growth of high-quality diamond films through the characterization of the film in terms of morphology and quality.The specific experimental parameters are shown in table 3.In the process of diamond film growth, the methane concentration is positively correlated with the content of carbon-containing active groups in the plasma, and the content of carbon-containing active groups can be  changed by adjusting the methane concentration.As shown in figure 6, the SEM images of diamond surface morphology prepared on 4H-SiC substrate with different methane concentrations, it can be seen that the methane concentration has a great influence on the morphology of diamond film growth, and the grain size of the film increases firstly and then decreases with the increase of methane concentration.When the methane concentration is 2%, as shown in figure 6(a), the diamond grains are dense, the average size of the grains is about 1 μm, the grain outline is not obvious, and some regions of the grains begin to appear aggregation phenomenon, and at this time, the content of carbon-containing reactive groups in the plasma is relatively low, when the methane concentration increases to 4%, as shown in figure 6(b), the diamond grains gradually increase, the average size of the grains is about 4 μm, and the grain boundaries gradually become clear, and the grains begin to have the outline.When the methane concentration is increased to 6%, as shown in figure 6(c), with the increase of the content of carbon-containing reactive groups in the plasma, it leads to an increase in the probability of secondary nucleation of diamond, and some of the grains continue to increase up to 7 μm, with a narrow and long shape, which also hinders the growth of other grains.When the methane concentration continued to increase to 8%, as shown in figure 6(d), the grains began to refine, and the size of individual grains tended to be homogeneous, and their average size was about 5 μm.
In order to further investigate the effect of methane concentration on the quality of diamond films, Raman spectroscopy was used to characterize the purity of diamond phase in the films, and figure 7 shows the Raman spectroscopy results of diamond films prepared under different methane concentrations.The first characteristic Raman peak of defect-free natural diamond is located at 1332 cm −1 , and from the Raman characterization results, the samples prepared with different methane concentrations from 2% to 8% showed obvious characteristic peaks of diamond phase, and the diamond peaks were gradually broadened with the increase of the concentration of methane concentration, which indicated that the content of the graphite phase gradually increased, and the poorer the quality of the films.This is because with the increase of methane concentration, the concentration of active hydrogen atoms in the deposition process will be low, in which the etching effect of hydrogen atoms on the sp 2 structure carbon is much higher than that of the sp 3 structure carbon, and it can inhibit the formation of the initially generated graphite phase, and the rate of hydrogen ions in the plasma to etch the graphite phase is reduced, which also makes the content of diamond phase decrease significantly, and the content of non-diamond increase [14].When the methane concentration is 2%, as shown in figure 7(a), the half-peak width (FWHM) is 6.2 cm −1 , and the intensity of the diamond characteristic peak is extremely large, which indicates that the crystallization quality of diamond is excellent.When the methane concentration is increased to 4%, as shown in figure 7(b), the intensity of the diamond peak is still large, but the Raman spectra show that the background impurity content gradually increases, indicating that the SP 3 bonds in the film begin to transform to the SP 2 bonds, and the content of the graphite phase increases, and the half-peak width (FWHM) of the characteristic peaks of the diamond at this time is 7.5 cm −1 , which indicates that the crystalline quality of the diamond is good.When the methane concentration was increased to 6%, as shown in figure 7(c), the intensity of the diamond peak began to weaken, the intensity of the background impurity peak continued to increase, and the diamond characteristic peak gradually broadened, and the half-peak width of the diamond characteristic peak at this time was 9.1 cm −1 , which indicated that the rate of the hydrogen ions in the plasma etching of the non-diamond phase was gradually greater than the rate of its generation, which resulted in the obvious increase in the content of the non-diamond phase in the film, and the crystallization quality of diamond began to deteriorate.The quality of diamond began to deteriorate.When the methane concentration is increased to 8%, as shown in figure 7(d), the intensity of the diamond peak is further weakened, the diamond characteristic peak continues to broaden, and the intensity of the diamond peak continues to weaken, and at this time, the half-peak width of the diamond characteristic peak is 14.2 cm −1 , which indicates that the proportion of the nondiamond phases in the films rises sharply, the purity of diamond decreases, and the quality of crystallization continues to deteriorate [15].

Effect of different deposition times on diamond films
The SEM images of diamond prepared under different deposition times as shown in figure 8, the average size of a single higher grain is about 200 ∼ 400 nm (15 min),0.5 ∼ 1 μm(30 min)and 1.5 ∼ 3 μm(1 h), the specific deposition process parameters are shown in table 4.
It can be seen that the deposition time is a key factor affecting the diamond grain size.The grain size of diamond gradually becomes larger with time during the first hour when the methane gas starts to pass.As shown in figure 8(a), the film shows obvious holes in the first 15 min after the start of deposition, when the diamond film does not completely cover the surface of the 4H-SiC substrate.As shown in figure 8(b), the gap or hole of the diamond film is significantly reduced when the deposition time increases to 30 min, and the diamond film has completely covered the substrate surface, which indicates that a continuous dense diamond film can be formed in a shorter growth time.
As shown in figure 9, the Raman spectra of diamond prepared under different deposition times, the characteristic peak of 4H-SiC substrate appears near the position of 961.3 cm −1 in the first 15 min of the    beginning of deposition, indicating that the diamond film has certain gaps or holes during the deposition process, and a continuous dense film is not formed at this time.By extending the deposition time to 1 h, the characteristic peak of 4H-SiC substrate gradually started to weaken, and the characteristic peak of diamond located near 1332 cm −1 became sharp, and the Raman spectrum showed that the background peak gradually weakened, indicating that the crystalline quality of diamond gradually became better.

Effect of different substrate temperatures on diamond films
Like the process parameter of methane concentration, different substrate temperatures will affect the grain morphology and purity of diamond films.In this section, the effect of different substrates on the growth of diamond films will be investigated.The substrate temperatures were set at 650 °C, 750 °C, 850 °C and 950 °C for four sets of comparison experiments, while other process parameters were kept consistent, in order to find out the most suitable substrate temperature for the growth of high-quality diamond films through the characterization of the film's morphology and quality.The specific experimental parameters are shown in table 5.
As shown in figure 10, the SEM images of diamond surface morphology prepared on 4H-SiC substrate with different substrate temperatures, it can be seen that the substrate temperature is the key factor affecting the morphology of diamond grains.When the substrate temperature is 650 °C, as shown in figure 10(a), the silicon carbide substrate shows that there are only sporadic diamond particles, and there is no formation of a continuous dense diamond film, and the individual grains grow in isolation, and the average size of the grains at this time is 4 μm, which is due to the lack of energy of the carbon-containing reactive groups in the adsorption plasma on the surface of the silicon carbide substrate, which results in the lack of the active sites on the surface of the substrate and makes it difficult to form the active groups and substrate surface, and it is difficult to form the active groups and substrate surface.groups are difficult to form a bond with the substrate surface, so the lower substrate temperature is not conducive to the growth of continuous and dense diamond films.
As can be seen from figures 10(b) ∼ (c), in the range of substrate temperatures from 750 °C to 950 °C, the growth of continuous and dense diamond films can be better, and the grain size increases with the increase of substrate temperature.When the substrate temperature is 750 °C, as shown in figure 10(b), the grain profile is not obvious, and the average size of the grains is small about 1 μm.When the substrate temperature is increased  to 850 °C, as shown in figure 10(c), the grain profile starts to appear, and the size of the grains starts to increase, and the size of the grains is more uniform, and the average size of the grains is about 5 μm, and the average size is about 5 μm, and the energy gained by carbon-bearing reactive group increases, and the carbon-containing group increases.In addition, with the increase of substrate temperature, the energy obtained by the carboncontaining active groups in the plasma increases, and the collision between the carbon-containing groups and the silicon carbide substrate starts to be frequent, leading to the phenomenon of secondary nucleation of diamond.When the substrate temperature is increased to 850 °C, as shown in figure 10(d), some grains continue to grow, and the grain size increases to 6.5 μm, and the growth principle of this part of the grains also restricts the growth of other grains, which leads to the deterioration of the uniformity of the grains [16].
In order to further investigate the effect of substrate temperature on the quality of diamond films, Raman spectroscopy was used to characterize the quality of the films, and figure 11 shows the Raman spectral results of the diamond films prepared at different substrate temperatures.As shown in figure 11, obvious characteristic peaks of diamond phase appeared under the substrate temperature preparation from 650 °C to 950 °C, and the diamond peaks became narrower and then wider with the increase of 4H-SiC substrate temperature, which indicated that the content of diamond phase increased and then decreased, and the quality of the films became better and then worse.
When the substrate temperature is 650 °C, as shown in figure 11(a), the characteristic peaks of 4H-SiC substrate are displayed at the position of 960 cm −1 , accompanied by strong background impurity peaks, and the half-peak width of the diamond peaks (FWHM) is 15.6 cm −1 , which indicates that the crystalline quality of diamond is not good at this substrate temperature, and therefore, both from the perspective of the film morphology characterization and Raman spectroscopy characterization, the lower substrate temperature is not favorable for the growth of diamond films.When the substrate temperature is 750 °C, as shown in figure 11(b), the half peak width (FWHM) of the diamond peak decreases to 12.2 cm −1 , and the background impurity peak is still obvious, which is due to the fact that although the energy of the carbon-containing reactive groups rises, and the ability of the hydrogen atoms to etch the nondiamond phase starts to appear, the etching rate is still smaller than the efficiency of the generation of the nondiamond phase.When the substrate temperature is increased to 850 °C, as shown in figure 11(c), the signal intensity of the diamond peak is enhanced, and its half-peak width (FWHM) continues to decrease to 7.5 cm −1 , and the intensity of the background impurity peak is weakened, which indicates that with the increase of the substrate temperature, the activity of the carbon-containing reactive groups and hydrogen atoms is improved, and the ability of hydrogen atoms to etch the nondiamond phases is strengthened, which inhibits the formation of a large number of nondiamond phases.The formation of a large number of non-diamond phases is inhibited, and the film quality is further improved.When the substrate temperature continues to increase to 950 °C, as shown in figure 11(d), the signal of the diamond peak is still strong, and its half-peak width (FWHM) begins to broaden to 9.5 cm −1 , and the background impurity peaks start to appear again, which is due to the fact that with the further increase in temperature, the hydrogen atoms on the surface of the film are gradually desorbed, so that the carbon atoms inside the chamber do not match with sufficiently large numbers of hydrogen atoms, and some of them will directly match with the neighboring carbon atoms.Some of the carbon atoms will be directly bonded with the neighboring carbon atoms, resulting in the formation of diamond phase SP 3 bonds to SP 2 bonds began to shift, the proportion of non-diamond phase gradually increased, so too high a substrate temperature can easily lead to the graphitization of diamond, and the quality of the film deteriorated.
In summary, the diamond thin films prepared by this experimental MPCVD device can be obtained that the methane concentration, growth time and substrate temperature are the key factors affecting the grain morphology size as well as the quality of the whole diamond film.Therefore, this paper proposes to use 4H-SiC as the substrate for heterogeneous epitaxial diamond, and the realization of this process is beneficial to guide the high quality growth of diamond on 4H-SiC substrate.By extending the growth time or using 'secondary growth' to obtain a high quality diamond film with a certain thickness, the surface of the diamond film is subsequently ground and polished, and finally the substrate is removed by a certain method to obtain self-supported diamond, which also provides a feasible way to prepare diamond heat sink materials.

Conclusion
In this paper, the MPCVD method is used to prepare diamond on 4H-SiC substrate.Compared with the pretreatment method of ultrasonic seeding of diamond powder suspension, the pretreatment method of adsorption of seed crystal by nanodiamonds suspension significantly increases the nucleation density of diamond on the surface of 4H-SiC substrate, and the nucleation density can reach 10 10 /cm 2 .
The growth experiment results indicate that methane concentration, deposition time, and substrate temperature are key factors affecting the grain morphology size and the quality of the entire diamond film.
(1) Within a certain range of methane concentration (2% ∼ 8%), with the increase of methane concentration, the diamond grain size firstly increases and then decreases, and the crystallization quality gradually deteriorates.The lower the methane concentration, the less impurities and the higher quality of the prepared diamond films; (2) In the first hour before the start of diamond deposition, the grain size of diamond gradually becomes larger with time.When the deposition time is 15 min, there are certain gaps or holes in the diamond film.By extending the deposition time, the grains begin to increase and gradually cover the substrate surface completely, and the crystalline quality is further improved; (3) At a lower substrate temperature (650 °C), it is difficult to obtain a complete diamond film.Within a certain range, as the substrate temperature increases, the grain size increases with the substrate temperature, and the quality of the film becomes better and then worse, among which the best diamond crystallization quality is obtained at the substrate temperature of 850 °C.

Figure 2 .
Figure 2. The schematic diagram of the principle of electrostatic adsorption seed crystal.

Figure 3 .
Figure 3. SEM images of nucleation results of different pretreatment seed crystal.(a) SEM images of diamond nucleation results without any pretreatment.(b) SEM images of the nucleation results of ultrasonic seed crystals of diamond micro-powder suspensions.(c) SEM images of nucleation results of electrostatic adsorption of seed crystals by nanodiamond suspensions.

Figure 6 .
Figure 6.SEM images of diamond prepared with different methane concentration.

Figure 7 .
Figure 7. Raman spectra of diamond prepared under different methane concentration.

Figure 8 .
Figure 8. SEM images of diamond prepared at different deposition times.

Figure 9 .
Figure 9. Raman spectra of diamonds prepared at different deposition times.

Figure 10 .
Figure 10.SEM images of diamond prepared at different substrate temperatures.

Figure 11 .
Figure 11.Raman spectra of diamond prepared at different substrate temperatures.

Table 1 .
Pretreatment methods and nucleation density results.

Table 2 .
Diamond film growth process parameters.

Table 3 .
Process parameters for growth at different methane concentrations.

Table 4 .
Process parameters for growth at different deposition time.

Table 5 .
Process parameters for growth at different substrate temperatures.