1. Introduction

2. Materials and methods

2.1. Substrates

Silicon and sapphire substrates, laser cut into 10×10 mm² sizes, were used for CVD diamond growth. Silicon substrates were cut out from commercially available p-type 0.5 mm thick 4-inch wafers with a resistivity of 20 ohm-cm. Sapphire substrates were single crystal wafers from the commercial supplier Kyocera. Both of them were seeded with nanodiamonds to enhance the diamond nucleation during CVD growth as described in section 2.1.1.

2.1.1. DND seeding

Water-based detonation nanodiamond (DND) slurry was spin-coated on top of the silicon and sapphire substrates with a rotating speed at 4000 rpm by a spin coater (Laurell Technologies Corporation, model WS-400B-6NPP/LITE) for 40 sec, along with brief rinsing with distilled water, until the surfaces become completely dry [41]. Such monolayer-seeded substrates were then loaded into the LA-MPCVD chamber. Figure 1a is the SEM image of the DND-seeded silicon substrate. The nucleation density was calculated to be 2×10¹⁰/cm². Figure 1b is the x-ray diffraction of the sapphire substrate used in this study.

2.1.2. CNT and EPD

Other than the conventional DND seeding procedure, two separate sets of silicon substrates were also prepared. One set of substrates was carbon nanotubes (CNT) deposited on silicon wafers by iron catalyst-assisted thermal CVD. The other set was prepared by further electrophoretically depositing diamond on top of such CNT/Si substrates.

The growth conditions of CNTs were a working pressure of 3 Torr, a growth time of 20 min at 750℃ with gas flow rates of NH₃ and C₂H₂ of 10 sccm and 40 sccm, respectively. Figure 1c shows the SEM image of the top surface of the CNT/Si substrate. The length of CNTs was found to be 510 µm (inset).

In order to protect the CNTs from plasma damage during deposition of the diamond film, the diamond nanoparticles were coated on top of the CNTs by electrophoretic deposition (EPD). In the EPD process, water based nanodiamond solution (Nanoseed 18 by Micro Diamant AG Switzerland) was used. The earlier prepared CNTs/Si substrates were biased at about +20 V with respect to the reference electrode, the Pt, for 20 s. Figure 1d shows the SEM photograph of the diamond nanoparticles on top of the CNT/Si substrate. The nucleation density was estimated at about 4.7×10¹⁰/cm².

Figure 1. (a) SEM image of the DND seeded silicon substrate, (b) XRD of sapphire substrate, (c) CNT coated silicon substrate, inset SEM showing cross-sectional CNT length and (d) top view of the EPD of diamond nanoparticles on the CNT/Si substrate.

Figure 1. (a) SEM image of the DND seeded silicon substrate, (b) XRD of sapphire substrate, (c) CNT coated silicon substrate, inset SEM showing cross-sectional CNT length and (d) top view of the EPD of diamond nanoparticles on the CNT/Si substrate.

3. Results and discussion

3.1. First hour of LA-MPCVD growth

3.1.1. Silicon substrates

Figure 3 compares the NCD growth patterns after 15, 30 and 60 minutes of LA-MPCVD runs. Figure 3a, Figure 3b and Figure 3d show gradual increase in the size of the diamond nanocrystals with increasing deposition time. After the first 15 minutes of LA-MPCVD, the nanodiamond seed crystals, which are typically 4-6 nm in sizes or their agglomerates (Figure 1a), grew into bigger sizes but did not become large enough to effectively touch each other in order to form connecting network of diamond nanocrystals (Figure 3a – the current SEM image magnification was not high enough to determine the respective NCD sizes after 15 minutes of LA-MPCVD growth). As the deposition time is increased to 30 minutes and further into 60 minutes of LA-MPCVD, the diamond nanocrystals start touching each other but still not enough to completely cover the underlying silicon substrate. The black contrasting substrate features are gradually found to diminish in the corresponding SEM images (Figure 3a, Figure 3b and Figure 3d, respectively). It was easier to calculate the uncovered substrate surface areas than the size of the individual NCDs grown over them after 15 minutes of LA-MPCVD. The average lengths of the uncovered silicon substrate areas were more than 270 nm (Figure 3a) after 15 minutes of LA-MPCVD run, and were reduced but not enough to let the nanodiamond crystals touch each other after 30 minutes of LA-MPCVD, as shown in Figure 3e. The sizes of the nanodiamond seed crystals is found to grow to about 30 nm isolated nanocrystals or to their agglomerates of 60-70 nm sizes in Figure 3e. Even after 60 minutes of LA-MPCVD, 150-200 nm long uncovered silicon areas were still visible in Figure 3d. It is important to notice that the 30 nm or 70 nm NCDs, shown in Figure 3e are actually agglomerations of tinier (about 10 nm) nanoparticles.

The Raman spectra from the diamond nanocrystals after 15 minutes of LA-MPCVD is found to be very noisy (Figure 3f) with a small sp³ peak at 1335 cm^-1 in contrast to disordered graphite (D) band at 1351 cm^-1. There are also Raman peaks at 1448, 1553 and 1582 cm^-1, which correspond to trans-poly-acetylene (TPA) and graphite crystals (G) respectively in the growing NCD film. On the other hand, the sp³ Raman signal after 60 minutes of LA-MPCVD diamond growth is found to be very strong (I_sp3= 156) with FWHM of 18 cm^-1 (Figure 3c). The corresponding non-diamond Raman peaks for TPA (1140 and 1480 cm^-1) and G-band (1553 cm^-1 and hump thereafter) are also present but at much-reduced level (I_sp2=60), indicating growth of good quality diamond nanocrystals. The sp³ Raman peak signal enhancement in Figure 3c in comparison to Figure 3f is due to the combined effect of increased LA-MPCVD deposition time, which also helped to achieve higher substrate temperatures. It is important to note that the NCD growth temperature is dynamic and continuously increasing by the deposition time over which the stage was allowed to be heated by the microwave plasma. The longer the CVD time period, the higher the substrate temperature - until the substrate temperature becomes gradually stable with the progressing time. 15-60 minutes of heating by the plasma does not allow the substrate temperature to become stabilised (Figure 2a).

Figure 3. NCD crystals on DND seeded silicon substrates, (a) SEM of the sample grown after first 15 minutes, (b) SEM of the sample grown after 30 minutes at 20kX magnification, (c) Raman spectra of the sample grown after 60 minutes, (d) SEM of the sample grown after 60 minutes, (e) SEM of the sample grown after 30 minutes at 200kX magnification and (f) Raman spectra after of the sample grown 15 minutes of LA-MPCVD growth.

Figure 3. NCD crystals on DND seeded silicon substrates, (a) SEM of the sample grown after first 15 minutes, (b) SEM of the sample grown after 30 minutes at 20kX magnification, (c) Raman spectra of the sample grown after 60 minutes, (d) SEM of the sample grown after 60 minutes, (e) SEM of the sample grown after 30 minutes at 200kX magnification and (f) Raman spectra after of the sample grown 15 minutes of LA-MPCVD growth.

3.1.2. Sapphire substrates

Figure 4 shows the corresponding Raman spectra from the NCDs grown over DND-seeded sapphire substrates, kept alongside the silicon substrates as shown in Figure 2d. Here it is also found that the sp³ Raman peak in and around 1331 cm^-1 is the sharpest with a FWHM of 15 cm^-1 (Figure 4a), from the NCD grown over the longer time period of 60 minutes. In fact, NCDs grown over sapphire substrate have better FWHM values than NCDs grown over silicon substrates (Figure 3c). The sp³ Raman signals become gradually weaker (figures 4b and 4c) and other non-diamond peaks (TPA - 1491, graphitic D - 1350 and G – 1600 cm^-1) become stronger due to the presence of smaller NCDs, with the reduction in LACVD time periods.

3.1.3. CNT and EPD substrates

The SEM and Raman spectroscopy results from the CNT/Si and EPD-diamond/CNT/Si substrates kept alongside the silicon and sapphire DND seeded substrates are shown in Figure 5, after 15 minutes of LA-MPCVD processing. Longer CVD growth periods of 30 and 60 minutes, were completely etching the CNTs due to the oxidation by the microwave plasma recipe of H₂:CH₄:CO₂=89:5:6. The well connected CNT network shown in Figure 1c is found to be etched completely by the LA-MPCVD plasma even after 15 minutes into the deposition period, as shown in figures 5a and 5d. However, the EPD treated CNT/Si substrate is found to be less affected by the LA-MPCVD plasma, as the length of individual CNTs are found to be longer (>20 µm) in Figure 5a than that in Figure 5d (<5 µm). Figure 5b is the higher magnification image of Figure 5a, taken from the intermediate region of the individual CNTs. It shows that the substrate surface is fully covered with carbonaceous film. The Raman signal of such film is shown in Figure 5c with stronger sp³ signal at 1335 cm^-1 along with another less intense peak at 1559 cm^-1 for graphitic carbon. The image 5b consists of tiny individual NCD grains (20-60 nm) present all over the EPD-diamond/CNT/Si plasma treated surface along with square shaped holes (140-280 nm). Such rectangular or square holes are also present on the CNT/Si plasma treated surface (Figure 5e) but much smaller in sizes (50-100 nm). Correspondingly, the individual diamond nanoparticles that are present all over the substrate surface in Figure 5e, are also smaller in sizes (20-40 nm) than those found in Figure 5b. Similarly, the sp³ (1334 cm^-1) and graphitic (1588 cm^-1) carbon signals from the plasma treated CNT/Si surface (Figure 5f) are also weaker, along with the presence of peaks at 1453 cm^-1 which is due to TPA. The presence of sp³ Raman signal from the LA-MPCVD plasma treated CNT/Si substrates confirms the fact that even without nanodiamond seeding it was possible to deposit NCD on top of CNTs.

There are few white particles that are found to be present inside the rectangular holes (Figure 5b and Figure 5e). The holes might have been created with the dislodgment of such white particles. Thus, it is found that the EPD treated CNTs are burnt to a less extent by the LA-MPCVD plasma (comparing Figure 5a and Figure 5b). Moreover, it is also possible to grow CVD diamond onto CNT/Si substrate without the need of nanodiamond seeding (Figure 5f).

Figure 5. EPD-diamond /CNT/Si substrate after 15 minutes of LA-MPCVD plasma treatment, (a) SEM at 500X magnification, (b) 20kX magnification, (c) Raman signal. EPD-diamond CNT/Si substrate after 15 minutes of LA-MPCVD plasma treatment, (d) SEM at 500X magnification, (e) 20kX magnification, (f) Raman signal. CNT/Si substrate after 15 minutes of LA-MPCVD plasma treatment.

Figure 5. EPD-diamond /CNT/Si substrate after 15 minutes of LA-MPCVD plasma treatment, (a) SEM at 500X magnification, (b) 20kX magnification, (c) Raman signal. EPD-diamond CNT/Si substrate after 15 minutes of LA-MPCVD plasma treatment, (d) SEM at 500X magnification, (e) 20kX magnification, (f) Raman signal. CNT/Si substrate after 15 minutes of LA-MPCVD plasma treatment.

3.2. Low MW power

The substrate temperatures that can be achieved inside the LA-MPCVD reactor, solely by means of plasma heating, depends on the input MW power and also on the stage to antenna distances. Therefore, in the next set of experiments, the MW power was reduced to 1100 W and also the stage to antenna distances were varied from 5 to 6.5 cm in order to see its effect on the growth of nanodiamond crystals during the early periods of LA-MPCVD. The lowest possible substrate temperature (15 hours of deposition) that Izak et al. [14] could achieve was 250^oC by lowering the input microwave power to 1200 W. They also used 1700 W and 2500 W of higher input MW powers. However, they did not mention the substrate to antenna distance used in their experiments. Reportedly [11,16,22] it is kept at 5-7 cm optimal distances to achieve 400-450^oC stable substrate temperature by plasma heating. Closer distances like 4 cm could produce 550^oC substrate temperatures. In order to achieve higher substrate temperatures (>600^oC), it is usually done by resistive heater underneath the substrate.

The substrate temperature continues to rise from the room temperature to 242^oC, until the end of one-hour long LA-MPCVD run (Table 1, samples # LA200520-1). Surprisingly, at the lower MW of 1100 W, the NCDs were found to almost cover the entire silicon substrate (Figure 6a), which was not observed even at 1500 W (Figure 3d). Higher magnification SEM image in Figure 6b, reveals the NCD sizes to be around 40-100 nm. Still there are intermediate bare substrate areas visible in between the NCD grains. There is no evidence of secondary nucleation. The NCD seems to grow in lateral direction in order to cover the substrate surface by a single layer coalescing film.

Figure 6c (sapphire sample # LA200519-1) and Figure 6d (sapphire sample # LA200520-1) compare the effect of lowering the input MW power on the diamond growth behaviour during the first 60 minutes of LA-MPCVD processing with identical stage to antenna distance of 5 cm. 1500 W input MW power seems only to heat the substrate to a higher temperatures of 284^oC, starting from room temperatures, but higher power did not help in effective grain coalescence - as it seems to be possible with 1100 W power as shown in Figure 6d. The sizes of the NCDs are comparable in between Figure 6c (approximately 140 nm NCDs scattered and isolated from each other) and 6d (about 120 nm NCD agglomerates). However, at lower power the grains are more coalesced and interconnected (Figure 6d). It appears that lower input power is beneficial to lateral growth of the diamond nanocrystals. There is still no sign of secondary-nucleation.

Figure 6. Nearly coalesced NCD crystals grown by LA-MPCVD reactor at reduced power level of 1100 W on silicon substrate for 60 minutes, (a) 20kX and (b) 50kX magnification SEM images. Comparison of diamond nanocrystals coalescence on sapphire substrates after 60 minutes of LA-MPCVD at MW power levels of (c) isolated, at 1500 W and (d) connected, at 1100 W.

Figure 6. Nearly coalesced NCD crystals grown by LA-MPCVD reactor at reduced power level of 1100 W on silicon substrate for 60 minutes, (a) 20kX and (b) 50kX magnification SEM images. Comparison of diamond nanocrystals coalescence on sapphire substrates after 60 minutes of LA-MPCVD at MW power levels of (c) isolated, at 1500 W and (d) connected, at 1100 W.

3.4. Fully coalesced NCD films

It was found, that, even after 60 minutes of LA-MPCVD processing, at higher power of 1500 W, or lower power of 1100 W, or even with pulse mode MW power, at a stage to antenna distance of 5 cm, none of the processing conditions, could produce continuous diamond films on both the DND seeded silicon and sapphire substrates. Had this been conventional resonant cavity microwave plasma CVD reactor, it would have produced continuous micron thick high quality NCD films with H₂/CH₄/CO₂ = 89/5/6 process recipe after 1 hour of growth [1]. Next, it was attempted to lower the substrate stage to 6.5 cm distance away from the linear antenna quartz tube, in order to lower the substrate temperature. In effect, it was as to promote less etching of the deposited films nearer to the antenna, and producing a continuous film formation on the substrate surface - even if it will be poorer in diamond quality.

Lowering the stage from 5 cm to 6.5 cm is beneficial to the substrate surface. Figure 8a and its inset which were the diamond film SEM images on silicon substrate after 1 hr LA-MPCVD in pulse mode at 5 cm distance, still show dark contrasting uncovered silicon substrate areas underneath, but the nanodiamond grains are more or less connected with each other without any sign of isolated NCDs. It appears from the Figure 8a that the pulse mode was more effective processing condition in covering the substrate surface with single layer of NCD film. But if the distance is increased to 6.5 cm (Figure 8b and inset), there is sign of secondary nucleation which may produce poor quality diamond. It concludes that wider stage to antenna distance helped in secondary nucleation to take place and produced almost fully covered silicon substrate. The Raman signal from such semi-continuous NCD film on sapphire substrate shows very high FWHM of 40 cm^-1 for the sp³ peak at 1333 cm^-1, along with appearance of TPA peak at 1502 and graphite G peaks at 1606 and 1552 cm^-1. The high value of FWHM confirms the presence of secondary nuclei, as evident also from the SEM image. The end temperature of the second 60-minute LA-MPCVD run was somewhat higher at 307^oC, as its starting temperature was also higher at 126^oC - after the end of first experiment of 60 minutes LA-MPCVD which started from room temperature of 21^oC. But both of them were not performed long enough to make the substrate temperature reaching a stable flat curve. The substrate temperature was continuously changing (increasing) during both the runs – initially at a faster pace (steep slope) and later on at slower (flatter slope) heating rates.

It was found that for the samples # LA200520-2, grown at longer substrate to antenna distance of 6.5 cm, and grown for longer duration of 2 hrs, LA-MPCVD process conditions could produce a flat substrate temperature curve as shown in Figure 2c. The longer distance of 6.5 cm promoted in producing secondary nucleation of diamond crystals (wide range of NCD grain sizes from 20-120nm – Figure 8f) and longer time of LA-MPCVD allowed the films to coalesce to form full substrate coverage as shown in figures 8d-f. Earlier it was also found that lower power of 1100 W contributed to produce better film coverage (Figure 6a) than 1500 W on silicon substrate (Figure 3a). Figure 6d is the NCD film after 1 hr LA-MPCVD on sapphire, showing underlying bare substrate but if the LA-MPCVD was allowed to proceed for another 1 hr, it was found to fully cover the sapphire substrate surface (Figure 8e and Figure 8f). Figure 6d shows a cluster of 12 diamond nanoparticles, with 120 nm total diameter. Although every grain is not of equal dimension, but it may be estimated that the individual nanodiamond crystals are 5-10 nm in size. This was the case after 1 hr of LA-MPCVD on sapphire substrate, but when the LA-MPCVD was proceeded for another 60 minutes, it is found to produce secondary nucleation with variable nanodiamond sizes of 120, 60 or 30 nm of individual grain sizes (Figure 8f). Therefore, longer periods of LA-MPCVD helped to grow the film both laterally and in three dimensions due to secondary nucleation. Researchers have earlier reported [22] that there are high pressure growth regime which promotes diamond re-nucleation and low pressure growth regime which promotes lateral growth. Low electron temperature inside LA-MPCVD is essential for continuous re-nucleation of diamond in hydrogen rich plasma [42].

Figure 8. Coalescence and secondary nucleation of NCDs. SEM of (a) interconnected NCD on Si, pulse mode at 5 cm, (b) secondary nucleated NCDs on Si in pulse mode at 6.5 cm, (c) Raman spectra from secondary nucleated NCD after 1 hr LA-MPCVD in pulse mode at 6.5 cm. SEM images of NCDs after 2 hr LA-MPCVD in CW mode at 6.5 cm, 1100 W power (d) on Si, (e) sapphire substrates and (f) 50 kX magnification of image in 8e.

Figure 8. Coalescence and secondary nucleation of NCDs. SEM of (a) interconnected NCD on Si, pulse mode at 5 cm, (b) secondary nucleated NCDs on Si in pulse mode at 6.5 cm, (c) Raman spectra from secondary nucleated NCD after 1 hr LA-MPCVD in pulse mode at 6.5 cm. SEM images of NCDs after 2 hr LA-MPCVD in CW mode at 6.5 cm, 1100 W power (d) on Si, (e) sapphire substrates and (f) 50 kX magnification of image in 8e.

3.5. LA-MPCVD growth without CO₂

3.5.1. EPD-diamond/CNT/Si substrates

It was found that the EPD treated CNT/Si substrates were also etching out fast under the LA-MPCVD plasma recipe of H₂:CH₄:CO₂=89:5:6. Therefore, in order to reduce the effect of oxidation, in the next set of experiments, CO₂ gas was removed from the process recipe. Figure 9a and 9b show the Raman spectra of the EPD seeded CNT/Si substrates after 15 and 30 minutes of LA-MPCVD plasma treatment without CO₂ in the recipe of H₂:CH₄=97:3. It was found that even after 30 minutes of LA-MPCVD, CNT survived the plasma conditions (inset Figure 9d). The corresponding Raman signal of the sp³ peak was weaker than those of the graphitic D-band (Figure 9a) after 15 minutes of LA-MPCVD, however, the Raman spectra started producing fluorescent signals after 30 minutes of LA-MPCVD as shown in Figure 9b. There lies a definitive signal (not prominent enough due to background fluorescence) for diamond at 1330 cm^-1 which was taken from the sample in Figure 9d. There are loosely scattered diamond nanoparticles, of around 80 nm diameter or less, all throughout the images in 9c and 9d. There is some occasional appearance of square or rectangular holes in the plasma treated EPD-diamond/CNT/Si substrate surface. Sizes of those holes vary from 250-350 nm. It is interesting to see that some of the holes were occupied by bright whitish nanoparticles - its dimensions was measured to be 250 nm. It can be assumed that dislodgment of such white particles resulted in the creation of those square or rectangular holes. The SEM images of figures 9c and 9d were taken from the intermediate regions of the individual CNTs (inset image of Figure 9d). The sizes of the some of the CNTs are found to be as big as 25 µm. If the CNT lengths in Figure 9d are compared with those in figures 5a (20 µm after15 minutes LA-MPCVD), it is concluded that even after 30 minutes of LA-MPCVD plasma treatment, EPD seeded CNT/Si survived diamond deposition conditions without complete etching, due to the absence of CO₂ in the recipe.

3.5.2. Silicon and sapphire substrates

3.5.2. a. Effect of CH₄

Section 3.1.1. and Section 3.1.2. have already shown that the early periods of low temperature LA-MPCVD run could not produce fully covered, otherwise perfectly DND seeded silicon and sapphire substrates (even after 60 minutes of deposition). One of the reasons of the slow growth of diamond crystals, could have been rapid etching of the nanodiamond films by the CO₂ present in the plasma recipe. Therefore, it was further investigated to see the effect of removing CO₂ from the recipe during the LA-MPCVD nanodiamond growth. Researchers [43] have shown that at the low CO₂ concentration (less than 0.5%), there is formation of SiC in competition with the growth of boron doped NCD during the LA-MPCVD process, which is due to the silicon contamination from the quartz tube. Moreover CO₂ (less than 1%) enhances the NCD growth rate (16 nm/hr) in continuous mode LA-MPCVD. But an increase in CO₂ gas over CH₄ gas percentages reduced [11,13] the growth rate (7 nm/hr). But there is no report so far about the effect of CH₄ percentages on the NCD growth pattern, specifically during the unstable periods of LA-MPCVD run, i.e., until the stable substrate temperature is achieved.

Figure 10a, Figure 10b and Figure 10c are the SEM images of the NCD crystals grown over the Si substrates with the increasing methane percentages (3% - 5%), only after first 15 minutes of LA-MPCVD. It is found that the individual DND seed crystals (Figure 1a) grew in sizes in figures 10. The diamond nanoparticle density is calculated to be equal to 0.85×10¹⁰/cm² in figures 11c for 5% CH₄ percentage over silicon substrates, to some extent less than the that calculated from the Figure 1a for the DND seeds (2×10¹⁰/cm²). It demonstrates that during the early growth stage, the DND seeds only grow laterally in size, also with the disappearance of some seed crystals due to plasma etching. The Raman signal from the NCD film on Si substrate, grown at 3% CH₄, but after 30 minutes of LA-MPCVD (Figure 10d), is shown to produce quite sharp sp³ peak with FWHM of 22 cm^-1. There are also Raman peaks in the Figure 10d corresponding to non-diamond carbon depositions at 1541 and 1600 cm^-1, respectively. It is observed that the dark contrasting Si substrate areas in the figures 10a, 10b and 10c are progressively diminished - indicating that with the increase in the CH₄ percentages, the DND seed crystals grow bigger in sizes, within a given LA-MPCVD time period.

If the NCD crystal densities are compared in between silicon and sapphire substrates after 15 minutes of LA-MPCVD growth at 4% CH₄ without CO₂ (Figure 11a and 11b), it is found that there are 10 NCDs within an area of 200 nm × 200 nm square (Figure 11a) on Si and there are about 20 NCDs on sapphire substrate (Figure 11b). Hence it may be inferred that sapphire substrates are more effective in growing the DND seeds. Similar trend of double the number of NCD crystals present on the sapphire substrate in comparison to the silicon substrate after the 15 minutes of LA-MPCVD growth, at 5% CH₄ without CO₂ (Figure 11c and Figure 11d), is also observed.

Now if we compare the NCD crystal densities onto Si substrate with increasing CH₄ percentages, it is observed that there are more NCDs, 15 in number, within an area of 200 nm × 200 nm square (Figure 11c). Similarly increase in CH₄ percentages from 4% to 5% on the sapphire substrate also increases the NCD crystal densities from 20 to 30 NCDs present within an area of 200 nm × 200 nm square (Figure 11b and Figure 11d) of the sapphire substrate after 15 minutes of LA-MPCVD growth. Thus, it can be concluded that the increasing methane percentages promoted LA-MPCVD growth of NCDs.

Figure 10. LA-MPCVD growth of NCD crystals on DND seeded silicon substrates without CO₂ in the recipe. SEM images showing early stages of CVD diamond growth at (a) 3%, (b) 4% and (c) 5% CH₄ after 15 minutes of growth at 1500 W input average power. (d) Raman signal from such NCD crystals.

Figure 10. LA-MPCVD growth of NCD crystals on DND seeded silicon substrates without CO₂ in the recipe. SEM images showing early stages of CVD diamond growth at (a) 3%, (b) 4% and (c) 5% CH₄ after 15 minutes of growth at 1500 W input average power. (d) Raman signal from such NCD crystals.

Figure 11. Comparison of LA-MPCVD growth (1500 W, 5 cm, 15 minutes) of NCD crystals on DND seeded silicon and sapphire substrates without CO₂ in the recipe. NCD crystal (samples # LA200605-1) SEM 50 kX magnification images on (a) Si and (b) sapphire substrates at 4% CH₄. NCDs (samples # LA200605-2) on (c) Si and (d) sapphire substrates at 5% CH₄.

Figure 11. Comparison of LA-MPCVD growth (1500 W, 5 cm, 15 minutes) of NCD crystals on DND seeded silicon and sapphire substrates without CO₂ in the recipe. NCD crystal (samples # LA200605-1) SEM 50 kX magnification images on (a) Si and (b) sapphire substrates at 4% CH₄. NCDs (samples # LA200605-2) on (c) Si and (d) sapphire substrates at 5% CH₄.

3.5.2. b. Effect of deposition time

Growth rates of linear antenna CVD is slower in comparison to conventional microwave plasma CVD growth of diamond crystals inside resonant cavity reactors. The typical growth rates for MPCVD is in the order of microns per hour [1,44], whereas inside LA-MPCVD, NCD grows by 5-20 nm per hour [13] - depending on the presence of CO₂ percentages in the recipe and also on the use of pulse mode, which favours higher growth rates. Hence, it is understood that longer LA-MPCVD deposition time is required to fully cover the substrates NCD films. The available literature reports only about the long deposition periods (mostly 8, 15 and 20 hours of LA-MPCVD). There is little or no information about the growth of NCDs during the initial periods of LA-MPCVD, when the substrate temperature is not stabilised yet (Figure 1a) and is continuously increasing by heating with plasma. Figure 1b is the cooling-heating cycle for the samples # LA200605-3, with 11 minutes of short LA-MPCVD deposition period, shown with a sudden rise in substrate temperature (from 88^o to 270^oC) at 2800 W input average power and 5 cm stage to antenna distance. The experiment # LA200605-1 started from the room temperature with 4% CH₄ and the plasma heating lasted for 15 minutes, which continuously increased the substrate temperature up to 170^oC at 1500 W input average power and at 5 cm stage to antenna distance. Thereafter, the reactor was let to cool down below 140^oC (temperature-safety-limit of the reactor) before it was to re-open to unload and reload the next set of 4-substrates. In this way, the starting temperature for the next set of samples # LA200605-2 became 70^oC (not restarting from the room temperature) and again with 15 minutes of heating with 5% CH₄ in the hydrogen plasma, it could heat up to a little bit of higher substrate temperature of 200^oC at 1500 W input average power and 5 cm stage to antenna distance. Again, the experiment was stopped and the reactor was allowed to cool down to 88^oC (shown in Figure 1b – depending on the time one takes to load and unload samples) before restarting # LA200605-3 for 11 minutes of plasma heating up to 270^oC, but at 2800 W power and 5 cm distance. The important point to be noticed is that the heating rates were, 9.9 ^oC/min for # LA200605-1, 8.6 ^oC/min for # LA200605-2, and 16.5^oC/min for # LA200605-3. The third LA-MPCVD heating rate was higher because of the very high input MW power level of 2800 W.

Figure 12a and Figure 12c compare the diamond film growth behaviour on silicon substrates after 15 min. (1500 W) and 11min. (2800 W) of LA-MPCVD heating with 5% CH₄ in the hydrogen plasma recipe, respectively. There are almost equal numbers (about 45 diamond nanoparticles inside 500 nm × 500 nm square) of whitish bright NCD particles scattered on the dark contrasting silicon substrate in both the images. Although, there was a 4-minute reduction in deposition time with concomitant increase in MW input power by 1300 W, the Figure 12c shows somewhat sharper white spots of NCDs on Si, which may be due to its wider substrate temperature range of 88^o-270^oC. On the other hand, sapphire substrate has more numbers (approximately 70 diamond nanoparticles inside 500 nm × 500 nm square) of NCDs scattered in the Figure 12b. Moreover, there are also signs of nanodiamond agglomeration in Figure 12b. It may be concluded that sapphire substrate favoured more lateral growth of DND seed crystals than silicon substrate. It may be attribute to a tendency for silicon to form carbide which reduces the formation of diamond nanoparticles. The number of NCD particles present inside 200 nm × 200 nm square area is about 20 in the Figure 12d, for the LA-MPCVD growth on sapphire substrate, for 11 minutes with 5% CH₄ at 2800 W power and at 5 cm distance - which is almost equal in number of the NCD particle density found in Figure 11b for 15 minutes of LA-MPCVD with 4% CH₄ at 5 cm distance. The number of diamond nanoparticles was 30 in Figure 11d inside 200 nm × 200 nm square area for 15 minutes of LA-MPCVD with 5% CH₄ at 5 cm distance. So, this decrease in NCD density number (30 to 20) with identical LA-MPCVD recipe is mainly due to the decrease in deposition time from 15 to 11 minutes. It is important to note that although the MW input power was much higher (2800 W) - leading to higher substrate temperatures (88^o-270^oC), but it could not become effective in increasing the diamond nanoparticle density at shorter LA-MPCVD periods of 11 minutes.

Now, if the LA-MPCVD was allowed to proceed from 15 minutes to 30 minutes at 3% CH₄ in the process recipe without CO₂, with simultaneous lowering of the stage from 5 to 6.5 cm distance away from the quartz tube antenna, it was found (inside 500 nm × 500 nm squares) that the NCD crystal sizes grow bigger in sizes with concomitant touch with each other to form some kind of agglomeration (Figure 13a and Figure 13c) on silicon substrate surfaces. On the other hand, the NCD crystals are found already to be much bigger on sapphire substrates (Figure 13b and Figure 13d) under identical LA-MPCVD processing conditions than on the silicon substrates, which is again due to tendency of silicon to form carbide favourably over nanodiamond formation. There are about 15 NCD crystals/agglomerates inside 200 nm × 200 nm squares in Figure 13b in comparison to approximately 10 NCD crystals/agglomerates inside 200 nm × 200 nm squares in Figure 13d. The NCD sizes vary from as small as 25 nm individual crystals to as big an agglomeration size as 100 nm in Figure 13d, whereas, the smallest NCD in Figure 13b is about 20 nm and the biggest agglomeration is found to be as big as 50-60 nm. Therefore, it may be concluded that longer deposition time allowed the NCD to grow in the lateral direction in covering more the underlying sapphire substrate, gradually with time. The NCD crystals are found to be 40-45 nm in size (Figure 13c) on silicon substrate after 30 minutes of LACVD with 3% methane in hydrogen plasma, occasionally touching each other, whereas, in Figure 13a (15 minutes) they remained isolated from each other with smaller (30 – 35 nm) diamond nanoparticle sizes.

4. Conclusion

References

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