Determination of optical properties of single crystal diamond substrates grown via welding-assisted microwave plasma enhanced chemical vapour deposition

Abstract

Lab grown single crystal diamonds (SCDs) offer unrivalled hardness, a wide range of optical transparency, and supremely high thermal conductivity reliable materials to be a part of devices run at high frequency, temperature, and power. Effective synthesis techniques are essential in enhancing the potential applications of high-quality SCDs. This study aims to decrease the thermal contact resistance between diamond seeds and the molybdenum holder by utilizing welding material. Quality of grown diamond substrates (plates) were assessed by analysing optical properties through Raman, UV-Vis, and FT-IR spectroscopic methods. The findings revealed that the grown single-crystal diamonds have excellent transmittance (>70%) and absorption at 270 nm. Calculations also showed an average absorption coefficient of 1.1 cm⁻¹, indicating the high quality of the grown SCDs with nitrogen impurities below 10 ppm. The absorption observed in the FTIR spectra, ranging from 1600 cm⁻¹ to 2700 cm⁻¹ with a peak at 2354.13 cm⁻¹, is referred to as the “two phonon region,” which is a distinctive feature of the diamond phase.

Graphical Abstract

Keywords:

• Single crystal diamonds (SCDs)
• microwave plasma enhanced chemical vapour deposition (MPCVD)
• optical properties
• welding method
• welding material

1. Introduction

Diamond possesses a face-centred cubic close-packed structure with the highest atomic number density among all materials (1.76 × 10²³ atoms/cm³). The carbon-carbon bonds are not especially strong but the tetragonal symmetry and high coordination number of cubic Group IV elements, in which carbon imparts give rise to mechanical properties through the number and symmetry of the bonds [1]. The structure of diamond accounts for its distinctive optical properties and its symmetry led to a pure covalent bonding, which elucidates the absence of infrared-active fundamental lattice vibrations to the first order. Therefore, the diamond is transparent throughout the infrared region. However, higher-order multi-photon absorptions become optically active in diamond cause considerable absorption from 1700 cm⁻¹ to 4000 cm⁻¹ spectral region. The optical-infrared quality of grown single crystal diamond can be appraised by directly ­measuring its absorption. By modifying the growth conditions, the infrared absorption in grown single crystal diamond through Microwave Plasma Enhanced Chemical Vapour Deposition (MPCVD) can approach that of naturally occurring diamond [2]. Single crystal diamonds can be grown using the MPCVD method in the presence of a controlled low level of nitrogen. The nitrogen concentration significantly influences the regulation of crystal defect formation, playing a pivotal role in attaining a diamond material with the essential attributes required. Elevated nitrogen levels in the growth process have been observed to result in unfavourable absorptions, potentially compromising the material’s characteristics. Additionally, heightened nitrogen concentrations can adversely impact the overall crystal quality of the material. The nitrogen quantity employed in the procedure is chosen to be adequate for preventing or minimizing the formation of defects caused by local strain. Simultaneously, it is kept at a low enough level to avoid or minimize unfavourable absorptions and the degradation of crystal quality [3]. The commercial availability and fairly low cost of single crystal diamond with sufficiently free of impurities and defects has stimulate intense interest in this material as a platform on which to build a new breed of optical, electrical, and quantum devices [4].

In the past decades, the use of diamond as an optical material was minimal to specialized applications, such as planetary radiometry from satellites and in high-pressure cells. Contemporary progress in the manufacturing of high-quality single crystal diamonds using the MPCVD process raised the demand for a variety of new commercial applications, including semiconducting diamond power electronic devices, diamond substrates as heat sinks, and as optical elements for a wide transmission range [2]. The Prime objective of this work is to improve heat conduction during the carbon deposition process using welding material along with SCD substrate to achieve a high-quality optical grade single crystal diamond with a minimum amount of nitrogen impurity. Earlier, it was reported in the conventional method (without the use of welding material) to keep the substrate temperature consistent during the growth, the deposition parameters such as gas pressure and input power in the deposition chamber have to be changed. Ultimately it affects the growth rate and is responsible for the poor quality of SCDs [5,6]. However, Bo Yang et al. [7] reported the reason for the substrate temperature increment during growth and proposed a theory of the accumulation of the graphitic carbons at the bottom of the grown single-crystal diamonds. All observations indicate that the main reason for the substrate temperature increment with growth time is the growth of graphitic carbons between the substrate and the substrate holder. Therefore, the thermal contact resistance must be reduced to realize substrate temperature stability during the growth process. Inserting materials at the interface involves filling the gap between the two contact surfaces so that point-to-point heat conduction is made possible [7]. To minimize the effect of the accumulated graphitic carbons on heat conduction, a thin copper sheet was introduced between the substrate and substrate holder to analyze the effect of this proximity alteration. To enhance the quality of the grown SCDs, the use of welding material is beneficial in reducing thermal contact resistance. A comparative study on the growth of SCDs using both conventional as well as welding method is described and discussed.

2. Experiments

An effective way to solve the problem of thermal instability in diamond seeds is the so-called “welding” growth method, as discussed. The homoepitaxial growth is carried out using the Seki Diamond MPCVD system (Model SDS 6K) operating at 2.45 GHz and pressures ranging from 100 Torr to 200 Torr. Commercially available, (100) oriented CVD grown SCD seeds with dimensions of 10 mm × 10 mm × 0.5 mm is used for the growth process of diamonds. Through meticulous and real time observations, the optimum growth parameters for quality single crystal diamond substrates are determined and a thorough analysis of their resulting optical characteristics is discussed.

2.1. Welding method

Bo Yang et al. [7] reported a comparison of several growth experiments using different welding materials and recommended welding is favourable to avoid the formation of graphitic carbon at the bottom of the substrate. During the growth of the diamond, the temperature at the bottom of the substrate is generally about 175°C lower than that of the upper surface. Therefore, to ensure the stability of welding during growth, the melting point of the selected welding material needs to be higher than the temperature of the lower surface. Meanwhile, it is essential to prevent the molybdenum substrate holder from experiencing high-temperature strain. To achieve these objectives, the used welding material must have a melting point lower than that of the substrate holder. Referring to the material properties of copper, mentioned in Table 1, it has a lower melting point and higher thermal conductivity than molybdenum. So, it can be an appropriate welding material to perform welding experiments. Before conducting a welding experiment, it is necessary to reduce the size of the welding material in comparison to the seed substrate. The copper foils (welding material) were inserted at the gap between the bottom of the substrate and the substrate holder [7].

Table 1. Material properties of copper and molybdenum.

Material	Melting Point (˚C)	Thermal Conductivity (W m^–^1 K^–1)
Molybdenum	2616	138
Copper	1082	397

2.2. Pre-growth treatment

A schematic representation of the pre-growth treatment of diamond seeds is shown in Figure 1 [8]. Prior to the deposition experiment, the single crystal diamond seeds underwent a cleaning process using a wet chemical mixture of nitric acid (HNO₃), sulfuric acid (H₂SO₄), and hydrochloric acid (HCl) to remove metallic and organic residues. A 100 ml mixture of the above acids was taken in a proportion of 2:1:1 to clean seeds at 300°C in a Pyrex beaker for approximately 1 h.

Figure 1. Schemetic representation of pre-growth treatment on (100) oriented CVD grown SCDs.

Finally, seeds were subjected to an ultrasonic treatment using acetone and methanol to clean any remaining residues for 30 min. Subsequently, the seeds were washed with deionized water. Before loading the samples into the reactor, the molybdenum substrate holder was cleaned with ethanol and then rinsed with deionized water.

2.3. Growth of single crystal diamond substrates

Subsequently, high-quality growth of SCDs was achieved by arranging the seeds on a substrate holder along with copper foils. In order to reduce thermal contact resistance, the 14-micron thin copper foil (welding material) was inserted between the seed and molybdenum holder. Figure 2 shows a schematic representation of seed substrates on a molybdenum holder. Copper (Cu) foils having dimensions 8 mm × 8 mm × 0.014 mm were used to fill the interface between the seed substrate and the molybdenum holder, which enabled heat flow at the contact interface. Figure 3 depicts the actual arrangement of copper foils on a molybdenum holder to achieve thermal stability during the growth of SCDs. Before conducting the welding experiment, the welding foil needs to be cut to a slightly smaller size than the seed to avoid melting out from the bottom of the seed. In our experiment, we cut copper foils into 8 mm × 8 mm dimensions, as the seed’s surface area was 10 mm × 10 mm. These copper foils were inserted between diamond seeds and the molybdenum holder in the plasma chamber. During the welding process, the temperature at the bottom surface of the seed substrate is lower than that at the upper surface of the seed substrate. When the seed substrate temperature reaches the growth temperature, the substrate temperature still needs to be progressively raised until the substrate temperature drops suddenly by about 50°C to 100°C, indicating that the welding material has melted successfully [7].

Figure 2. Schematic representation of the arrangement of seed substrates on Molybdenum substrate holder.

Figure 3. Arrangement of copper foils over Molybdenum holder.

Growth was initiated in the deposition chamber with clean hydrogen gas (99.9998%) and ionized through the electric field generated by resonance between the microwave and the chamber. The adequate substrate temperature was achieved to the desired growth conditions using the trial and error method by adjusting the gas flow, gas pressure, and input power. It was then etched for 30 min in the hydrogen plasma. Subsequently, methane with a purity of 99.9998% was introduced into the deposition chamber to initiate the growth of diamonds. The gas pressure in the deposition chamber during growth was maintained between 120 Torr-180 Torr, while the input power was adjusted within the range of 3000 W–5500W. Pure nitrogen gas (99.9998%) is used whereas introducing a small proportion of nitrogen into the reactive gas mixture has been shown to have an advantageous effect of creating favourable growth sites, enhancing the growth rate, and promoting {100} face growth [9–12]. The gas flow rate for hydrogen was set at 500 sccm and methane and nitrogen concentrations were up to 5% and 1% respectively, of the total flow rate. The substrate temperature was controlled between 900°C and 1100°C and it was measured using a two-color pyrometer (Williamson Make). After 8 h, the grown diamond surfaces were observed, followed by calculating the average growth rate [5].

2.4. Growth mechanism

Figure 4 shows a schematic representation of the growth mechanism and creation of species in the plasma. In the plasma atmosphere, species are responsible for the growth of diamonds, and their stages of creation are represented through various reactions over different spans of time. During this chemical vapor deposition, atomic hydrogen is produced through either the electron impact dissociation or thermal dissociation in the gas phase. After that, it attached itself to the unsatisfied high-energy sites to terminate them; their reaction with different species creates CH₃ radicals. It is concluded that surface reactions such as H atom abstraction and the addition of CH₃ radicals to the adjacent site of the diamond lattice are responsible for the growth of SCDs. Figure 4(a) illustrates the stepwise growth mechanism, which includes the production of atomic hydrogen through direct impact dissociation with electrons or thermal dissociation in the gas phase. As gas pressure increases with temperature, a significant amount of this atomic hydrogen produces hydrocarbon precursors that enable the growth of the diamond phase.

Figure 4. Schematic representation of growth mechanism and creation of species in plasma.

At comparatively high pressure, the production of CH₃ radical takes place as a result of the dissociation of a CH₄ molecule with atomic hydrogen, as seen in Figure 4(b). In Figure 4(c) abstraction of the H atom takes place, which attracts the nearest carbon radical to form a ring structure and leads to the locking of carbon atoms at the diamond lattice [6].

2.5. Post-growth treatment

Figure 5 presents a schematic representation of post growth treatment on as grown SCDs. In order to remove polycrystalline inclusions as well as graphitic carbons formed over the faces of SCDs, a chemical cleansing technique is applied. A stochiometric wet chemical mixture of 100 ml made using oxidizing agents such as concentrated sulfuric acid (H₂SO₄) and potassium dichromate (K₂Cr₂O₇) in a 5:1 ratio is used to oxidize polycrystalline carbon at elevated temperature.

Figure 5. Schematic representation of post growth treatment for grown SCDs.

During the oxidation process at 350°C (∼398°C, melting point K₂Cr₂O₇), a chemical reaction between the oxidizing reagent and the diamond produces CO or CO₂ [13]. Residues of polycrystalline inclusions found in a wet chemical mixture of oxidising agents as a mixture appeared black after this treatment led to enhancement in the quality of samples [14].

3. Result and discussion

3.1. Growth parameters

After conducting several growth runs using the trial-and-error approach, a summary of the different growth parameters of the samples is presented in Table 2. The average growth rates over the entire process cycle were calculated as the thickness gain divided by growth time [5]. In the conventional method, the variation in substrate temperature, ­measured using a two-color pyrometer, was approximately (950 ± 150°C). For the welding method, the substrate temperature variation was around (950 ± 100°C). It is noted that the growth rate of sample S2, grown using the welding method, is higher than the growth rate of sample S1 grown using the conventional method due to adequate heat conduction during the growth process. In Figure 6(b), the growth of polycrystalline inclusions over the faces of sample S1 was greater compared to sample S2, as we did not use welding material for the preparation of the S1 sample.

Figure 6. Reported growth of single crystal diamond seeds after (a) 130 h (b) 9 mm× 9 mm × 1.85 mm cubes grown in 171 h process cycle using welding method.

Table 2. Summary of growth parameters.

Sample	Microwave Power (kW)	Pressure (Torr)	Temperature (°C)	Time duration (hrs)	Obtained Thickness (mm)	Average growth rate (µm/hrs)
S1(conventional method)	3.5–5.5	150–170	950–1150	171	1.71	10–12
S2 (welding method)	3.5–5.5	160–180	950–1050	154	1.85	12–14

As shown in Figure 6, polycrystalline inclusions are encountered during the growth cycle of SCDs using a conventional method. Figure 7(a–c) shows the growth stages of SCDs using the welding MPCVD method. There was less growth of polycrystalline inclusions along edges during the entire process cycle in the welding method. Single crystal diamond having dimensions 9 mm× 9 mm × 1.85 mm are shown in Figure 7(d), grown in a process cycle of duration of 154 h with a growth rate of 12–14 µm/hr. These observations indicate that the reason for the difference in variations of substrate temperature over the process cycle for both methods is the reduction in roughness caused by reducing the growth of sp² carbon using welding material between the substrate and the substrate holder. As the variation in substrate temperature is less with the welding method, thermal contact resistance is reduced as compared to a conventional method. However, variation in substrate temperature increases the growth rate, but it also leads to more growth of polycrystalline inclusions at the edges [15]. No more variation in substrate temperature was observed due to the welding method. So, it can be observed that thermal contact resistance decreased by a considerable amount.

Figure 7. Reported growths of single crystal diamond seeds after (a) 18 h (b) 25 h (c) 130 h using (d) 9 mm× 9 mm × 1.85 mm cubes grown in 154 h process cycle using welding method.

3.2. Temperature distribution during growth cycles

The temperature distribution during different growth cycles is mentioned in Tables 3 and 4. In the conventional method variation in temperature during different process cycles is higher as compared to the welding method. The distribution of temperature during the growth cycle of the welding method is mentioned in below Table 4. It has been noted that during the 30–40 h process cycle temperature at the bottom of the substrate reached up to the melting point of copper 1082°C therefore a sudden drop in temperature of about 98°C noted it seems to be strong evidence of melting of copper foil during the same process cycle.

Table 3. Temperature distribution of conventional method for different process cycle.

Method	Conventional method
Process time (hrs)	0–10	10–20	20–30	30–40	40–50	50–60
Temperature (˚C)	950	1002	1040	1055	1135	1146

Table 4. Temperature distribution of welding method for different process cycle.

Method	Welding method
Process time (hrs)	0–10	10–20	20–30	30–40	40–50	50–60
Temperature (˚C)	950	1010	1038	1059	961	1012

In Figure 8 comparison for temperature distribution over a 60 h growth process cycle for conventional as well as the welding method shown. As stated earlier the range of substrate temperature during the whole process cycle was 950°C–1050°C and 950°C–1150°C for welding and conventional method respectively. In the graph of the welding method during the 40–50 h process cycle temperature was reduced up to 98°C as compared to the previous process cycle due to the melting of copper foils.

Figure 8. Comparison between substrate temperatures for different growth cycles for both methods.

3.3. Raman spectroscopy

Raman spectroscopy is most widely used to characterize grown single crystal diamonds by the MPCVD method because of its ability to distinguish between different forms of carbon like sp² bonded graphite, and sp³ bonded diamonds [16–18]. These show bands with a high-frequency cut-off near 1332 cm⁻¹ for the sp³ bonded carbon and around 1580 cm⁻¹ for the sp² bonded carbon [19]. The presence of graphitic carbons can be detected in the first-order region (1100 cm⁻¹–1800 cm⁻¹) and the second-order region (2200 cm⁻¹–3400 cm⁻¹) [17].

Figure 9 shows the Raman spectra of grown SCD, measured using the Horiba Micro-Raman Xplora Plus Model having resolution ±1 cm⁻¹utilizing at a 532 nm laser source for excitation. The presence of peaks near 1332 cm⁻¹ is referred to as First-order Raman peaks with full width at half maximum (FWHM) ranging from 5 cm⁻¹ to 10 cm⁻¹. The absence of peaks in the region 1500 cm⁻¹–1550 cm⁻¹ is generally considered strong evidence for the growth of the “pure” diamond phase without non-diamond components present [14]. In the Raman spectra of samples S1 and S2, peaks appeared at 1332.74 cm⁻¹ (10574.03 a.u.) with FWHM 5.85 cm⁻¹ and 1332.72 cm⁻¹ (2587.87 a.u.) with FWHM 5.40 cm⁻¹ respectively are responsible for the vibration of the sp³ bonded carbon atoms. Additionally, peaks at 1411.62 cm⁻¹ (1185.03 a.u.) and 1414.90 cm⁻¹ (98.91 a.u.) are caused by to presence of low-order graphitic carbons in the S1 and S2 samples respectively [17].

Figure 9. Raman Spectra taken from the samples; Excitation laser wavelength is 532 nm (a) for S1 sample grown using the conventional method (b) for S2 grown using welding method.

3.4. UV-Visible-NIR spectroscopy

Referring to the transmittance spectra measured using LAMBDA 1050+ Perklin Elmer UV-Vis spectrometer of sample S1 and S2, the absorption edge near 225 nm corresponds to the wide band gap of diamond material, and results from absorption as excitation of electrons from the valance band to the conduction band [20]. The transmittance spectrum shown in Figure 11(a) has an exceptionally high transmission over a very broad range of wavelengths from 200 nm to 2700 nm maximum of 72% at 412 nm for sample S2. Where as in the transmittance spectrum of sample S1 in Figure 10(a) 49% at 1336 nm. Figures 10 and 11(b) shows the absorbance spectrum of sample S1 and S2 showing a sharp peak at 220 nm and a peak at 282 nm respectively. This absorption is mainly attributable to nitrogen impurities [21]. The average absorption coefficient (Shreya Nad et al. earlier reported 0.77 cm⁻¹ for less than 1 ppm nitrogen impurity) calculated for the range 240 nm to 400 nm in which transmission approximately beyond 70% using the Sellmeier formula [5]. (1) $$\alpha =\frac{1}{d}\hspace{0.17em}\mathit{\text{ln}}\left[\frac{{(1-R)}^2}{(1-R^2)}\hspace{0.17em}\frac{1}{T}\right]$$(1) where d is the thickness of the sample, T is the transmission and R is the reflectance, $$T=\frac{2n}{\left(1+n^2\right)},R=\frac{{(n-1)}^2}{\left(n+1\right)}$$

Figure 10. (a) The transmittance spectrum shows maximum transmission at 1336 nm of S1 and (b) Absorbance spectrum shows peak at 220 nm of sample S1.

Figure 11. (a) Transmittance spectrum shows maximum transmission at 412 nm of S2 and (b) Absorbance spectrum shows a peak at 282 nm of sample S2.

The average absorption coefficient of sample S1 is 5.81 cm⁻¹. For sample S2 is 1.11 cm⁻¹ indicates the high quality of grown SCDs, which contain a minimum amount of nitrogen impurity in the range < 10 ppm.

The absorption coefficient calculated [22] using Equation (2)(2) $$\alpha \left({\mathit{\text{cm}}}^{-1}\right)=\frac{2.303\mathrm{ }A}{t}$$(2) (2) $$\alpha \left({\mathit{\text{cm}}}^{-1}\right)=\frac{2.303\mathrm{ }A}{t}$$(2) where, A is optical absorbance and t is the thickness in cm of grown SCDs, in spectra absorption peak at 502 nm shown in Figures 12 and 13(b) is responsible for the H3 defect related to nitrogen vacancy center [N-V-N] [23]. An absorption coefficient spectrum shown in Figures 12 and 13(a), absorption at 270 nm is usually used to estimate the Nitrogen content [24,25] using the following Equation (3)(3) $$\mathrm{N}=0.56\mathrm{\alpha }$$(3) , (3) $$\mathrm{N}=0.56\mathrm{\alpha }$$(3)

Figure 12. (a) Absorption coefficient spectra shows a peak at 220 nm (b) Absorption coefficient spectra shows zero phonon line at 502 nm of sample S1.

Figure 13. (a) Absorption coefficient spectra shows a peak at 280 nm (b) Absorption coefficient spectra shows zero phonon line at 502 nm of sample S2.

Where, N is the nitrogen concentration (in ppm) in the synthetic diamond, α (cm⁻¹) the absorption coefficient at 270 nm. In this study estimated nitrogen concentration 12.62 ppm and 7.60 ppm for sample S1 and S2 respectively.

3.5 Fourier transform infrared (FT-IR) spectroscopy

Figures 14 and 15 shows typical FT-IR spectra measured by Bruker Alpha spectrometer with a resolution 2 cm⁻¹ of the sample S1 and S2 respectively, and the type of diamonds can be known by analysing these spectra. Typically, nitrogen impurities show absorption in one phonon region 600 cm⁻¹ to 1500 cm⁻¹. The peaks are found at 719.14 cm⁻¹ and 1415 cm⁻¹ in the spectra of sample S1 and at 670.10 cm⁻¹ and 1470 cm⁻¹ for sample S2 attributing them to nitrogen impurities. Carbon interstitials respond to substitutional nitrogen atoms, producing a bond-centered nitrogen interstitial showing strong IR absorption between 1450 cm⁻¹ and 2000 cm⁻¹ [26]. The region between 1600 cm⁻¹ and 2700 cm⁻¹ having peak at 2351.13 cm⁻¹ (5.81 a.u.) and 2354.13 cm⁻¹ (4.02 a.u.) is known as the “two phonon region,” and it is one of the characteristics of the diamond phase [27]. It is an excellent marker and can be used to differentiate non diamond and diamond phases. Hydrogen can be detected in the three-phonon region between 2700 cm⁻¹ and 3600 cm⁻¹ [28,29].

Figure 14. (a) FT-IR spectra of sample S1 over the spectral range 500 cm⁻¹–4000 cm⁻¹ with a peak at 2351.13 cm⁻¹ in two phonon region.

Figure 15. (a) FT-IR spectra of sample S2 over the spectral range 500 cm⁻¹–4000 cm⁻¹ with a peak at 2354.13 cm⁻¹ in two phonon region.

In Figures 14 and 15 the peaks at 3282 cm⁻¹, 3620.16 cm⁻¹, 3645 cm⁻¹, 3719 cm⁻¹ and 3762.45 cm⁻¹ correspond to the OH-stretching vibration and the bending vibration bands [30–32]. Generally, the bands at 2850 cm⁻¹ and 2924 cm⁻¹ are for symmetric stretching vibration of CH₂ and asymmetric stretching vibration of CH₂ respectively [33,34]. These bands are absent in both samples. As shown in Figure 15(b), the weak bands near 1690 cm⁻¹ of broad absorption between 1658 cm⁻¹ and 1821 cm⁻¹ are responsible for C = O bonding, which results from the oxidation of acid boiling. FT-IR spectrum of sample S1 shows broad absorption near 1535 cm⁻¹–1595 cm⁻¹ with a sharp peak at 1582 cm⁻¹ indicates C = C stretching evidence for the growth of graphitic carbon and absorption due to the presence of phonon bands also shown near 1800 cm⁻¹–2600 cm⁻¹ in Figure 14(a) [35–39].

3.6. Comparasion of experimental findings obtained from MPCVD methods

A high quality and growth rate were observed for SCDs in the welding method compared to the conventional method using MPCVD. It was found that due to the welding material, thermal contact resistance decreases up to a certain extent as we obtain improved results in the welding method for SCDs. It was also found that there is less growth of polycrystalline diamond boundaries during the growth of SCDs by the welding method. In the case of a sample grown by the conventional method, Raman peaks are found at 1332.74 cm⁻¹ and 1411.62 cm⁻¹ with a FWHM of 5.85 cm⁻¹, which indicates poor quality of diamonds due to the inclusion of more sp² content. In the case of welding growth of SCDs, a first-order Raman peak appeared at 1132.72 cm⁻¹ with FWHM of 5.40 cm⁻¹ and no observable peaks in the region 1500 cm⁻¹–1550 cm⁻¹ which is generally considered strong evidence for the growth of the “pure” diamond phase. A UV-Vis spectra result for SCDs grown by the welding method shows the sample has remarkable transmission above 70%, which makes it a potential candidate for optical grade applications while sample S1 has comparatively very poor transmission around 49%. The average absorption coefficient of 1.1 cm⁻¹ indicates the high quality of grown SCDs, and calculations indicate a minimum amount of nitrogen impurity (< 10 ppm) in grown samples by the welding method. In FT-IR spectra of sample S1 broad absorption near 1535 cm⁻¹–1595 cm⁻¹ with a peak at 1582 cm⁻¹ indicates C = C stretching due to the growth of graphitic carbon.

4. Conclusion

Inconsistently, an increase in substrate temperature during the synthesis of SCDs is responsible for poor quality and the deposition of diamond-like carbon instead of homoepitaxial growth. To overcome this predicament, the welding method was employed, and the quality of the grown SCDs evaluated. A growth rate of 12–14 µm/h was observed during the process cycle using the welding method, whereas 10–12 µm/h was observed using the conventional method. The reason for the high growth rate encountered in the welding method is that the insertion of copper foil made heat conduction possible in order to hinder the growth of polycrystalline inclusion. In the Raman spectra, a peak near 1332 cm⁻¹ called the first-order Raman peak indicates the growth of the diamond phase. The absence of peaks in the region 1500 cm⁻¹–1550 cm⁻¹ refers to the absence of the non-diamond phase. By analyzing transmission spectra obtained using UV-Vis spectroscopy, sample S2 shows high transmission over a wide range of wavelengths from 200 nm to 2700 nm, with a maximum of 72% at 412 nm. 1.1 cm⁻¹ is the average absorption coefficient calculated using a formula that indicates the presence of a minimum amount of nitrogen impurity in the range < 10 ppm. By analysis of UV-VIS spectra, a peak at 502 nm was found due to the H3 defect related to the presence of nitrogen vacancy [N-V-N] center called zero-phonon line (ZPL). Our simple but effective findings indicate that the welding method will allow the production of high-quality CVD diamonds. In FTIR spectra, the region between 1600 cm⁻¹ to 2700 cm⁻¹ having a peak at 2351.13 cm⁻¹of sample S1 and 2354.13 cm⁻¹ of sample S2 is known as the “two phonon region,” and it is one of the characteristics of the diamond phase. There are several aspects of this method that still need to be investigated by choosing a suitable welding material. The appropriate welding of diamond seeds on a molybdenum holder using suitable material can minimize the accumulation of sp² carbons, which supports nucleation for high-quality SCDs. This work may provide insight into the growth technique through which technological-grade especially optical grade diamonds can be produced.

Acknowledgment

The authors would like to acknowledge the Sophisticated Instrumentation Centre for Applied Research and Testing Institute (SICART), Vallabh Vidyanagar, Gujarat, India for providing instrumentation facilities to study the optical properties of grown SCDs. We would also like to thank ABD Diamonds Pvt. Ltd, GIDC, Vatva, Ahmedabad for providing support to carry out experimental work related to the growth of SCDs.

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Funding

The author(s) reported there is no funding associated with the work featured in this article.