CVD Diamond Growth Enhanced by a Dynamic Magnetic Field

Abstract

A dynamic magnetic field (DMF) with different angular frequencies (50, 100, and 150 π rad/s) was introduced during diamond growth via hot filament chemical vapor deposition (HFCVD). The effects of the dynamic magnetic field on the growth rate, diamond quality, growth orientation, and deposition uniformity of large-area diamond films were investigated with scanning electron microscopy (SEM), X-ray diffractometry (XRD), and Raman spectroscopy. The correlation between diamond growth and angular frequency was discussed. The results showed that a faster growth rate (about 2.5 times) and higher diamond quality were obtained by increasing the angular frequency of the DMF. A (100) textured polycrystalline diamond film was achieved, and the preferential orientation was found to evolve from (110) to (100), while the expected uniform deposition of a large-area diamond film under DMF was not achieved. The enhancement effect of the DMF was ascribed to the activation of more gas molecules, which participated in CVD diamond growth.

1. Introduction

Hot-filament chemical vapor deposition (HFCVD) is one of the most important methods to synthesize diamonds at low pressure and high temperature [1]. This process involves resistively heating a hot filament to a temperature in the range of 2000–2400 °C, which catalytically decomposes the source gases (CH₄ and H₂) into active C_xH_y precursors and atomic hydrogen, respectively. A series of reactions in the gas phase as well as on the surface leads to the growth of a diamond phase, generally with a small admixture of the amorphous carbon phase. Due to its simplicity, HFCVD is the most commonly used method for the synthesis of diamond films [2,3].

Magnetic fields have a high and controllable energy concentration, which allows them to transfer kinetic energy to materials without contaminating them. Wen et al. [4] investigated diamond synthesis from carbon black catalyzed by Fe nanoparticles in a static high magnetic field of 10 T. Graphite-coated diamond nanoparticles of about 100 nm along with diamond-like carbon nanopowders were obtained at atmospheric pressure and 1100 °C. When increasing the intensity of the static magnetic field to 20 T and using Mo-Fe catalytic clusters as the catalyst, the synthesis temperature was decreased to 650 °C [5]. Little et al. concluded that magnetic fields can stabilize sp³-hybridized carbon states via the use of a molten metal catalyst. Then, higher temperatures and hydrogen atoms decreased rehybridization, leading to the undesired graphitic sp²-carbon states. They further proposed that the combined use of a strong magnetic field and low-pressure and high-temperature methods may accelerate the growth of large single-crystal diamond wafers [6]. Subsequently, Huang et al. [7] introduced a cyro-cooled superconductor magnet (4 T) during the gas-phase synthesis of diamond. Nanocrystalline diamond films with a mean surface roughness of 8.9 nm and a grain size of about 13 nm were produced.

You et al. [8] applied a periodic magnetic field with an intensity of 106 Gauss during routine HFCVD. A diamond film was successfully deposited at a low deposition temperature of 520 °C, and the quality of the diamond film was greater than that of the film deposited when using only HFCVD. After applying a magnetic field during HFCVD, Wu et al. [9] attempted to deposit boron-doped diamond on a Mo-Re alloy. Cyclic voltammograms showed that the deposited boron-doped diamond electrodes presented a wide window and a low baseline current. They continued to deposit a diamond film on a Re substrate by magnetic field-assisted HFCVD and investigated the field emission properties [10]. The turn-on voltage of the diamond film was lowered to 2.6 V/µm, while the emission current density was advanced to 21.5 µA/cm² (at 6.2 V/µm).

When deposited on a Si substrate, diamond films with (100) orientations can be obtained by using periodic magnetic field-assisted HFCVD [11]. Field emission tests showed that the film with a (110) orientation presented better emissivity than the (100) orientation. Long et al. [12] deposited nanocrystalline diamond films on a Si substrate with the assistance of a periodic magnetic field. These nanocrystalline diamond films exhibited excellent field emission properties, with a turn-on voltage of 2.9 V and a current density of 32.7 µA/cm² (at 6.5 V/µm). Furthermore, an electric field can be coupled with a magnetic field-assisted HFCVD to synthesize unique carbon nanocone arrays [13] and caterpillar-like structural carbon films [14,15]. Both of these carbon materials exhibited excellent field emission properties.

Overall, the previous literature has demonstrated that the use of a magnetic field can enhance the properties of CVD diamond films by modifying diamond growth behavior. However, little research has focused on the nature of CVD diamond growth under a magnetic field or clarified the enhancement effects of a magnetic field. Therefore, in this work, a dynamic magnetic field with an angular frequency of 50, 100, and 150 π rad/s was introduced during diamond growth during hot filament CVD. The enhancement effects of a dynamic magnetic field on the growth rate, diamond quality, growth orientation, and deposition uniformity were investigated by SEM, XRD, and Raman spectroscopy, and the correlation between diamond growth and angular frequency was discussed.

2. Materials and Methods

Experimental sets are schematically shown in Figure 1, where the cylindrical winding of a three-phase stator was applied externally to the HFCVD chamber. During HFCVD, by adjusting the frequency of the input current to the winding through a variable-frequency drive (VFD), a dynamic magnetic field (DMF) with different angular frequencies was excited on the deposition area. The magnetic flow Φ_m at each pole of the three-phase winding could not be held constant, resulting in a decrease in the magnetic intensity of DMF when the frequency exceeded the rated value. Therefore, typical angular frequencies (ω) of 50, 100, and 150 π rad/s below the rated value were chosen for CVD diamond deposition without considering variations in the magnetic intensity (B = 110 Gs).

Tungsten wire with a diameter of 0.5 mm was wrapped into seven coils and then mounted on Mo electrodes as the hot filament. A polished Si wafer with dimensions of 5 mm × 5 mm × 0.7 mm was used as the substrate and placed below the filament, with a vertical height to the filament of 10 mm. A thermocouple set on the center of the substrate back was used to monitor the deposition temperature. Unless otherwise specified, the deposition temperature was maintained at 1023 ± 50 K. Before diamond deposition, Si substrates were pretreated in an ultrasonic bath with an aqueous solution of diamond powder (particle size: 1 µm) to enhance the nucleation density. Methane (CH₄) diluted with hydrogen (H₂) to 2% was metered into the HFCVD chamber with a flow rate of 100 sccm (standard cubic centimeters per minute). The deposition pressure was balanced at 3 kPa by evacuation using a vacuum pump.

The morphology of the CVD diamond films was analyzed by field emission scanning electron microscopy (FE-SEM, Zeiss Sigma, Oberkochen, Germany) operated at a voltage of 15 kV, while the film thickness was measured by a spectroscopic ellipsometer (SE-L, Eeoptics, Wuhan, China). The growth quality of CVD diamond films was studied with a micro-Raman spectrometer (Lab Raman RH800, Horiba Jobin Yvon, Paris, France). The wavelength and power of the Raman spectrometer were 488 nm and 10 mW, respectively. The oriented growth of CVD diamond films was investigated by X-ray diffractometry (XRD, D8-Advance, Bruker, Karlsruhe, Germany) with a step width of 0.01° and a scan rate of 2 °/min. The wavelength of X-rays irritated by the Cu Kα radiation was 0.15418 nm.

3. Results and Discussion

3.1. Growth Rate

Figure 2 shows the growth rate of CVD diamond films under a dynamic magnetic field with different angular frequencies, where the growth rate was obtained by dividing the film thickness by the deposition time. Another group experiment with a deposition temperature of 1123 K was carried out to confirm the dependency of the growth rate on the angular frequency. As presented in Figure 2, the CVD diamond growth rates of the two groups were both significantly enhanced in the presence of a dynamic magnetic field. When deposited at 1023 K, the CVD diamond growth rate was only 0.62 μm/h at an angular frequency of 0 π rad/s (no applied dynamic magnetic field). Upon increasing the angular frequency, the diamond growth rate increased exponentially and finally reached 1.53 μm/h at 150 π rad/s. For a higher deposition temperature (1123 K), the diamond growth rate was generally higher than the diamond deposited at 1023 K, but the increase in the growth rate was not as significant as at 1023 K. However, it still increased with the angular frequency. The growth rates were 1.27 and 1.86 μm/h at an angular frequency of 0 and 150 π rad/s, respectively.

Interestingly, the CVD diamond growth rate at 1023 K and an angular frequency of 150 π rad/s was higher than at 1123 K and an angular frequency of 0 π rad/s. The result shows that the temperature needed to stimulate diamond growth can be increased by the energy from the dynamic magnetic field, which implies that CVD diamonds can be grown at low temperatures with the help of a dynamic magnetic field. It is important to deposit diamond at a low temperature since a high deposition temperature may damage the substrate or introduce tremendous thermal stresses.

The enhancement effect of the DMF on CVD diamond growth was attributed to electron-gas molecule collisional excitation. During HFCVD, the W filament was resistively heated to 2000–2400 °C to catalytically decompose the precursor source gas into an active species. As the number of electrons whose kinetic energy exceeded the work function gradually increased, many electrons escaped from the metal surface, resulting in thermionic emission. Without an external field, these emitted electrons experienced a series of collisions and eventually lost their energy or were captured by substrates or chamber walls. The use of a dynamic magnetic field confined the electrons. These electrons finally transferred their momentum to gas molecules by elastic collisions or to other molecules by inelastic collisions [8]. As a result, the gas molecules were activated and ionized, which then accelerated diamond growth.

Deducing electron motion without collisions under DMF can further clarify the correlation between the CVD growth rate and the angular frequency. The complex interactions between DMF, magnetic intensity B, and angular frequency ω moving on a static electron with a mass m and an electric quantity q can be simplified into the movement of an electron into the DMF with a velocity v = ωr by selecting the DMF as a reference (r is the distance of an electron from a magnetic field’s center of gyration). The final electron motion under DMF in a frame-of-earth reference can be obtained by adding the motion of the relationship reference to the electron motion in the DMF reference. The electron motion in a reference DMF will be a helix. The electron velocity of a component perpendicular to DMF drives an electron to move in a circular path with a radius:

R = mv_⊥/qB = ωrsinα/qB

(1)

where α is the angle between v and B. As presented in Equation (1), a higher angular frequency will drive the electron to traverse with a higher velocity and a larger radius. This means that the range of electron motion increases, and electron-molecule collisional excitations become more intense upon increasing the angular frequency of the DMF. Thus, more gas molecules can be activated to participate in CVD diamond growth and enhance the growth rate.

You et al. [8] investigated the effect of a periodic magnetic field on CVD diamond growth and obtained a 7-fold increase in the diamond growth rate. In our study, we did not obtain such a high increase, and the maximum enhancement was approximately 2.5 times (1.53 μm/h at 0 π rad/s versus 0.62 μm/h at 150 π rad/s) obtained at 1023 K. This discrepancy may be attributed to two reasons. One is that the deposition temperature in reference [8] was lower than ours. A lower deposition temperature resulted in a more obvious enhancement in the diamond growth rate. The other reason is the deposition duration. Anupam et al. [16] investigated the influence of seeding density on CVD diamond growth and noted that a sparsely seeded diamond presented a higher growth rate during the coalescence stage. After coalescence, the growth rate slowed and became similar for both sparse and dense seeding. The deposition duration in reference [8] was 1 h, while 5 h was adopted in this study. The shorter duration allowed diamond growth to occur during the coalescence stage, thus exhibiting a higher growth rate.

3.2. Diamond Quality

Defects such as sp² carbon, substitutional impurities, vacancies, interstitial dislocations, and twinning planes can lower the quality of CVD diamond. In this study, diamond quality was characterized through the ratio I_D/(I_D + I_G/50), where I_D and I_G are the integrated intensities of the diamond characteristic peak and graphitic G-band from the Raman spectrum, respectively. The I_G/50 term was introduced due to the different Raman scattering intensities. This ratio represents sp² carbon-related defects.

Figure 3 illustrates the quality of CVD diamond films obtained under a dynamic magnetic field with different angular frequencies (the corresponding Raman spectra can be seen in Figures S1 and S2, Supplementary Materials). In general, the two groups of diamond films deposited at all angular frequencies presented a high diamond quality, and the ratio was always higher than 0.97. Specifically, for the diamond film deposited at 1023 K, the diamond quality decreased slightly at an angular frequency of 50 π rad/s and then increased with the angular frequency. For the diamond film deposited at 1123 K, the diamond quality was higher than the diamond film deposited at 1023 K. Under most growth conditions, the ratio was higher than 0.99, except for a slight decrease at an angular frequency of 50 π rad/s. Similar to the results of the diamond growth rate, the quality of the diamond film deposited at 1023 K with an angular frequency of 150 π rad/s reached that of the diamond film deposited at 1123 K with an angular frequency of 0 π rad/s. These results verify that a dynamic magnetic field improved the diamond quality and that no more sp² carbon was generated after applying the DMF. Although the DMF drives electrons and ions similar to the action of an electric field via bias-enhanced nucleation, it did not induce these charged particles to persistently bombard the diamond growth front and cause graphitization.

The quality ratio decreased slightly at an angular frequency of 50 π rad/s for both diamond films deposited at 1023 K and 1123 K (Figure 3). This implies some general action under the DMF for this angular frequency. CVD diamond growth consists of sequential steps that establish surface radical sites by hydrogen abstraction, the adsorption of hydrocarbon species onto radical sites, and the incorporation of hydrocarbon species into the diamond lattice. When an adsorbed methyl group reacts with a neighboring adsorbed group instead of being incorporated into the diamond lattice, it will result in the formation of an sp²-ethylene-like group. Based on the above assumption, Goodwin et al. [17] proposed a model to quantify the sp² carbon-related defect formation, and the deduced defect fraction X_def was:

$$X_{def}\propto \frac{G}{{[H]}^2}$$

(2)

Equation (2) shows that the defect fraction in diamond films is proportional to the growth rate G and is also related to the reciprocal of the square of the atomic hydrogen concentration [H].

Since Goodwin’s mode mainly accounted for sp² carbon-related defects, it can be used to specifically discuss diamond quality, as characterized by I_D/(I_D + I_G/50) in our study. At an angular frequency of 50 π rad/s, the growth rate of the diamond film was higher than when deposited using normal HFCVD (Figure 2). According to Equation (2), a slight decrease in diamond quality means the concentration [H] was lower than the increased growth rate. At a higher angular frequency, DMF drove electrons to move over a greater area and enhanced electron-molecule collisional excitation. This increased [H], which improved the diamond quality again.

3.3. Growth Orientation

Figure 4 shows SEM images of the CVD diamond films grown under a dynamic magnetic field with different angular frequencies. The shapes of CVD diamond grains vary with the angular frequency. When deposited under routine HFCVD, the crystal grains mainly presented a pyramidal shape (Figure 4a). When a DMF with an angular frequency of 50 π rad/s was introduced during HFCVD, the crystal grains were larger, and more grains exhibited a pyramidal shape. When a higher angular frequency was adopted, some pyramidal grains transformed into cubic grains. At the highest angular frequency of 150 π rad/s, almost all grains presented cubic shapes with smooth (100) crystal surfaces.

The crystal shape and appearance of facets are a result of the dynamic growth competition of the diamond nucleus, which is determined by the growth parameter α (α = 3^1/2V₁₀₀/V₁₁₁, where V₁₀₀ and V₁₁₁ are the growth rates of the (100) and (111) crystal surfaces, respectively). Upon increasing the growth parameter α, the (100) crystal surface gradually decreased. When α ≤ 1, the diamond grain presented a cubic shape. When 1 < α < 3, it presented a cubic-octahedral shape. When α ≥ 3, the (100) crystal plane disappeared, and the grains presented an octahedral shape. Since the deposition parameters can alter the relative growth rates of the (100) and (111) surfaces and, thus, the growth parameter α, CVD diamond grains with cubic, cubic-octahedral, and octahedral shapes were obtained. This reveals that the growth parameters changed upon increasing the angular frequency.

Furthermore, CVD diamond nuclei follow the van der Drift competitive growth model to coalesce into a continuous film. Starting from randomly oriented nuclei, only those crystals with the fastest growth direction (perpendicular to the substrate surface) will survive and form a textured diamond film, whereas all other orientations are gradually buried [18]. Wild et al. [19] defined a film surface consisting of cubic grains with square (100) facets completely parallel to the substrate normal as a (100) textured diamond film. A film surface consisting of crystal grains with triangular (111) facets parallel to the substrate normal is a (111) textured diamond film, while a film surface exhibiting a pyramidal shape with a tilt angle τ₁₀₀ = 45° (τ₁₀₀ is related to the growth parameter α) is a (110) textured diamond film.

The SEM morphology of the CVD diamond presented in Figure 4 demonstrates a preferential growth orientation and texture formation in these polycrystalline diamond films. The XRD pattern in Figure 5 shows diffraction peaks at 44.2°, 75.5°, 91.6°, and 119.9°, which correspond to the (111), (220), (311), and (400) crystal planes of a diamond film, respectively. The diamond film deposited by HFCVD (angular frequency 0 π rad/s) presented a preferred (110) orientation since the ratio of integrated intensities of the (220) and (111) diffraction peaks was 36.9%, which was higher than that of the standard diamond sample (I₍₂₂₀₎/I₍₁₁₁₎ = 25%) [20]. When a dynamic magnetic field was applied to HFCVD and the angular frequency was 50 π rad/s, I₍₂₂₀₎/I₍₁₁₁₎ increased to 38.8%, indicating that the (110)-orientated texture of the diamond film was enhanced. When the angular frequency was increased to 100 π rad/s, a distinct diffraction peak of the (400) crystal plane appeared, indicating that the (110) textured film combined with the (100) textured film. When the angular frequency was 150 π rad/s, there was no significant change in the diffraction peak of the (400) crystal plane, but the diffraction intensity of the (220) plane greatly decreased. I₍₄₀₀₎/I₍₁₁₁₎ was calculated to be 33.8%, which was higher than that of standard diamond (I₍₄₀₀₎/I₍₁₁₁₎ = 8%), showing that the film deposited at 150 π rad/s was completely a (100) textured diamond film. By combining the SEM and XRD results, it can be found that a textured polycrystalline diamond film can be achieved, and the preferential orientation changes from (110) to (100) upon increasing the angular frequency of the DMF.

Lin et al. [21] reported that the preferential orientation growth of diamond film was closely related to the concentration of the hydrocarbon precursor. When the concentration of CH₃ above the substrate surface was significantly higher than that of C₂H₂, the (100) crystal plane disappeared and only the (111) crystal plane appeared. When C₂H₂ was dominant, only the (100) crystal plane was present. During HFCVD, H atoms or electrons can react with methane to form ∙CH₃, whereas C₂H₂ has to pass through a series of dehydrogenation reactions. The dissociation energy of the C-H bond in CH₄ is 104 kcal/mol, while that of the C-C bond in C₂H₂ is 230 kcal/mol and that of the C-H bond is 128 kcal/mol, indicating that CH₃ is more easily dissociated. When DMF was introduced into HFCVD, electron-molecule collisional excitation was enhanced, and CH₃ was more likely to further dissociate and generate C₂H₂. Upon increasing the angular frequency, DMF drove electrons to collide with the gas molecules more intensely, which ultimately raised the relative concentration of C₂H₂ and CH₃, promoting the texture of diamond film to evolve from (110) to (100).

3.4. Deposition Uniformity

Heat and mass transfer by gases is the most fundamental step in the process of hot filament chemical vapor deposition. Heat transfer is carried out by simultaneous heat conduction and convection, and mass transfer of the hydrocarbon precursor can be carried out by dispersion or convection. Many experimental research and simulation results have shown that heat conduction and diffusion are the main modes of heat and mass transfer, respectively, during HFCVD [22,23,24]. Gas temperature and atomic hydrogen relative density did not change significantly upon doubling the gas flow.

According to magnetic aerodynamic theory, the magnetic field force acting on an uncharged gas mixture per unit volume is [25]:

$$F_m=0.5{\mu }_0{\displaystyle \sum }_{i=1}^N{\chi }_i\left(T\right)T{Y}_i\rho \nabla H^2$$

(3)

where μ₀ is the vacuum permeability, χ_i is the volume magnetization of the ith gas, T is the temperature, Y_i is the mass fraction of the ith gas, ρ is the concentration of the gas, and H is the magnetic field strength. When the magnetic field force is non-conservative, i.e., $\nabla H\ne 0$, it will induce an eddy current, which strengthens the mass transport and heat transfer of the reaction gas and promotes the rapid homogenization of the temperature and concentration of the reaction gas. Therefore, uniform deposition of large area diamond films is anticipated under a dynamic magnetic field.

A larger Si wafer with dimensions of 15 mm × 15 mm × 0.7 mm was adopted as the substrate. After diamond deposition, positions 2 mm from the center of the Si substrate to the edge along the L direction indicated in Figure 1 were investigated by SEM observations and Raman spectroscopy. The growth rate and diamond quality were calculated to assess the deposition uniformity. Figure 6 presents CVD diamond growth rates at different positions, which shows that the growth rate decreased from the center of the Si substrate to the edge under all conditions. After normalizing the growth rates, the decreases were compared (Figure 7). For HFCVD, the normalized growth rate at the center quickly decreased to 0.30 at L = 6 mm. When DMF with an angular frequency of 50 π rad/s was applied, no significant change in the decreasing trend was observed, but it decreased linearly from 1 at the center to 0.36 at L = 6 mm. When the angular frequency was increased to 100 π rad/s, the decrease in the growth rate greatly slowed, indicating some improvement in the deposition uniformity. The normalized growth rate at L = 6 mm reached 0.66. However, when the angular frequency was advanced further to 150 π rad/s, the trend returned to the level observed at 50 π rad/s.

Figure 8 shows the corresponding diamond quality at different positions for all deposition conditions. Compared with the growth rate, the decrease in diamond quality from the center of the Si substrate to the edge was not apparent. At L = 6 mm, diamond quality worsened, but the ratio was still higher than 0.88. For the area between the center and L = 4 mm, the ratio decreased with the distance from the center but remained higher than 0.96, which indicated that the diamond film presented a desirable quality. When a DMF was applied, the diamond quality at L = 6 mm at all angular frequencies was distinctly higher than without a DMF.

Overall, the anticipated uniform deposition of the large-area diamond film under a DMF was not achieved. The main reason was attributed to the low magnetic field intensity of the DMF in our study, which prevented it from efficiently driving the motion of gas molecules to enhance heat and mass transfer. Generally, the substrate temperature depends mainly on filament heat irradiation via HFCVD, although lateral heat conduction of the substrate also occurs. Thus, the substrate temperature at the hot filament projection was the highest. When the distance between the hot filament and substrate surface increased, the substrate temperature decreased quickly. Under HFCVD, the substrate temperature at the center monitored by the thermocouple was 1023 K, while the edge temperature at L = 6 mm was 823 K, verifying the presence of a huge temperature fluctuation. When DMF was introduced into HFCVD, no significant change was observed in terms of the substrate temperature at the center and the edge, regardless of the angular frequency. This shows that the large temperature fluctuation was not equalized using a DMF to enhance heat transfer or convection. This finding is consistent with the growth rates shown in Figure 6 and the diamond quality shown in Figure 8. The active effect of DMF can work at the substrate with a low-temperature fluctuation. Follow-up work will be carried out by arranging more filaments to reduce the temperature fluctuation.

4. Conclusions

The CVD diamond growth rate increased exponentially with the angular frequency from 0.62 μm/h (0 π rad/s) to 1.53 μm/h (150 π rad/s). At a higher deposition temperature (1123 K), the growth rate increased with the angular frequency, although the enhancement was not as significant as at 1023 K. The higher growth rate upon increasing the angular frequency of DMF was ascribed to the activation of more gas molecules that could participate in CVD diamond growth.

A dynamic magnetic field introduced a positive effect on the diamond quality, and no more sp² carbon was generated after applying the DMF because it did not introduce charged particles to persistently bombard the diamond growth front and lead to graphitization. A slight decrease in diamond quality at 50 π rad/s was because an increase in the atomic hydrogen concentration [H] did not exactly correspond to the increase in the growth rate.

A textured polycrystalline diamond film was obtained, and the preferential orientation evolved from (110) to (100) upon increasing the angular frequency of DMF. DMF promoted more intense collisions between electrons and gas molecules and ultimately raised the relative concentration of C₂H₂ and CH₃, thus promoting the transformation of the preferred orientation.

The anticipated uniform deposition of a large area of diamond film under a DMF was not achieved, mainly because of the low magnetic field intensity of the DMF, which could not drive gas molecules to move and thus enhance heat and mass transfer.