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Diamond single crystal growth in hot filament CVD
Diamond & Related Materials 15 (2006) 536 – 541 www.elsevier.com/locate/diamond
Diamond single crystal growth in hot filament CVD J. Hirmke ⁎, S. Schwarz, C. Rottmair, S.M. Rosiwal, R.F. Singer Lehrstuhl für Werkstoffkunde und Technologie der Metalle (WTM), Universität Erlangen-Nürnberg, Martensstr. 5, D-91058 Erlangen, Germany Available online 20 February 2006
Abstract The synthesis of diamond crystals is of particular interest due to the material's outstanding physical and mechanical properties. In hot filament CVD (HFCVD) we found a new process parameter window where the growth of single diamond volume crystals can be stabilized without the use of monocrystalline substrates. These CVD parameters are far beyond growth conditions for HFCVD diamond coating processes. Extremely low methane contents in the feed gas along with high substrate temperatures allow single diamond nuclei of a sufficiently large size to grow stabile. Crystals up to 80 μm in diameter were successfully synthesized. The morphology of the crystals is cubo-octaedric. According to our proposed growth model [S. Schwarz, C. Rottmair, J. Hirmke, S. Rosiwal, R.F. Singer, J. Cryst. Growth 271 (2004) 425.], the observed growth defects are primarily caused by the gas phase conditions during the CVD process. The aim of this work was to exclude a further possible formation of growth defects due to the employed diamond seed particles. The early growth stage was investigated by tracking distinct monocrystalline diamond seeds. It is shown that cubo-octaedric crystals with CVD typical smooth faces of high quality can be grown from micrometer-sized particles. Seed imperfections are therefore not considered as a major reason for growth defects of the larger crystals. © 2006 Elsevier B.V. All rights reserved. Keywords: Hot filament CVD; Single crystal growth; Nucleation; Diamond crystal
1. Introduction Diamond single crystals are in great demand for mechanical and electrical applications. Various techniques for synthesizing high quality diamond of well defined material properties are pursued by numerous research groups. Regarding large single diamond crystals for mechanical purposes, where impurities are of minor interest, high pressure – high temperature (HPHT) synthesis [2] is currently the only reasonable alternative to natural diamond. Concerning electronic applications, where single crystallinity, high purity and the possibility for doping is demanded, diamond synthesis by CVD layer growth is pursued [3]. In this field, synthesis by plasma CVD has prevailed. The trend in this method is clearly towards employing higher and higher process pressures along with increasing methane contents in order to increase growth rate [4,5]. This high growth rate synthesis, however, can lead to stacking faults [6]. Moreover, in plasma CVD, alike at the HPHT method, reactor sizes are limited and such is the amount of diamond that can be synthesized in a single process. In Ref. [1] we reported on a possible way to grow ⁎ Corresponding author. Tel.: +49 9131 8527520; fax: +49 9131 8527515. E-mail address: [email protected] (J. Hirmke). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.01.003
single diamond volume crystals in hot filament CVD (HFCVD). With this CVD method, the size of the reactor is virtually unlimited. Large area deposition of homogenous polycrystalline diamond coatings is already industrial state of the art. Therefore we intend to get a broader understanding of our observed single crystal growth in order to be able to evaluate the possibility for upscaling this process. Upscaling would allow growing scores of diamond crystals on large deposition areas simultaneously. This could highlight a new way of economic diamond crystal production even under the HFCVD typical moderate growth rates. These moderate growth rates are moreover considered less likely to cause stacking faults, a high diamond material quality is therefore expected. As reported in [1], in first experiments crystals up to 100μm have successfully been synthesized (Fig. 1). An advanced growth model regarding stable growth of volume crystals instead of layer growth and regarding the obtained morphology was given. Our further work aims towards revealing the causes for growth defects such as excrescences on the {100}-faces and defective island growth with increased sp2 ratios on the {111}-surfaces (Fig. 1). In this work, the particular influence of the employed seed crystals on growth defects such as the applicability of different seed types is investigated. As elucidated in [1], our applied CVD growth conditions of low
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Fig. 1. SEM-pictures of free standing diamond crystals up to 100 μm in diameter grown on tungsten carbide substrates. The cubo-octaedric crystals feature high quality {100}-faces with growth spirals and rough {111}-faces of lower diamond quality. Excrescences can be found on the {100}-faces. Secondary nucleation during CVD growth caused the formation of smaller crystals among them.
methane content and high substrate temperatures lead to an exclusive growth of crystals that are larger than a certain critical cluster size. This is due to a lowered super-saturation of growth species in the gas phase above the substrate compared to diamond layer growth conditions, where higher methane contents and lower substrate temperatures are employed. Therefore, in stating CH4 content↓ þ TSub ↑→super saturation↓ →critical cluster size↑ the growth conditions regarding nucleation in our single crystal growth processes can be summarized. The critical cluster at single crystal growth conditions was estimated to contain nearly 3000 atoms (equals a cluster radius of approximately 2.2 nm) compared to some 100 atoms at standard coating conditions [1]. So, when applying relatively large monocrystalline diamond particles of 1–20μm size as seed crystals, all applied seeds should be able to grow stable. Correspondingly, the undesired forming of further critical clusters during the CVD process, which leads ultimately to layer growth, should be avoided by the sufficiently enhanced critical cluster size. Therefore the use of rather large seed particles seems reasonable. On the other hand, employing larger and larger seed crystals for the observed volume growth the shape and surface morphology of the seed particles are expected to have a rising influence on the CVD diamond's morphology. This could lead to the formation of growth defects already in the early growth stages. In order to evaluate a suitable seed size, monocrystalline diamond particles of the sizes 1, 5 and 20μm were investigated. By tracking distinct crystals before and after growth, the morphology of the resulting CVD crystals was observed. Further, the applicability of the seeding methods for providing sufficiently low seed densities to allow single crystals volume growth was examined.
patterns of 1200 square boxes (100 × 100 μm each, see Fig. 4a) were applied on the substrate's surfaces by a Nd-YAG laser. For evaluation, 5 distinct seeds of each of the 3 grain sizes were tracked before and after CVD growth. The CVD growth was conducted in a state of the art HFCVD reactor with tungsten filaments. The presented crystals were grown in a HFCVD reactor at substrate temperatures N 900°C and 0.3% methane in feed gas. Characterization of the crystals was made by scanning electron microscopy (SEM) and Raman spectroscopy. In SEM the crystals' shape is specified by determining the alpha-factor as defined in Ref. [7]. The alpha-factor is given by pffiffiffi a ¼ 3dvð100Þ=vð111Þ ð1Þ where v (100) is the growth velocity of the (100) surface and v (111) is the growth velocity of the (111) surface. Its value for a CVD grown crystal can be derived from measuring the length of the edge between two (100)-faces and the length of the edge between a (100)- and a (111)-face by geometrical consideration according to Ref. [8]. The alpha-factor represents the crystal's shape from cubic (α = 1) via cubo-octahedral (α = 1.5) to octahedral (α = 3) [7,8] and indicates to which ratio the slowest grown surfaces terminate the crystal. From the Raman spectra a diamond quality factor q was derived according to Ref. [9] by employing q¼
75dId P d100% 75Id þ Ind
ð2Þ
nd
where the area below the diamond peak Id and non-diamond peaks Ind in Raman spectra are considered. The factor 75 takes the more effective Raman scattering on sp2-structures for the wavelength of the used laser (Ar+-laser, λ = 514 nm) into account [10].
2. Experimental 3. Results and discussion The evaluated diamond particles for seeding were monocrystalline synthetic diamond powders of the grain size 1 μm (0.75–1.25μm), 5 μm (4–6μm) and 20μm (15–25μm). The powders where diluted in ethanol and applied to tungsten substrates. The employed mean seed densities on the substrates were determined to 5.5 · 104/cm2, 1.1 · 104/cm2 and 250/cm2, respectively. Higher dilutions of the seeds on ethanol allowed also lower seed densities. For tracking distinct crystals, grid
Fig. 2 displays SEM images of representative diamond particles of the grain sizes 1, 5 and 20 μm before and after HFCVD growth. For the grain sizes 1 and 5 μm (Fig. 2a,b) the CVD grown crystals show a defined morphology with smooth, flat {100}- and {111}-faces. The resulting α-factor (as defined in Ref. [7]) of the crystals is between 1.2 and 1.3. This indicates that the seed diamond particles' irregular shape determined by
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Fig. 2. SEM-pictures of representative diamond seed particles of (a) 1μm, (b) 5μm and (c) 20μm grain size before CVD growth on the right and corresponding crystals developed in CVD growth on the left. From diamond particles of 1 and 5 μm size, cubo-octaedric CVD crystals of defined morphology can develop. The 20 μm diamond particles are too large to not affect the forming of growth surfaces and so the crystals' shape.
cleavage planes from milling processes does not disturb the development of crystals whose faces are terminated by the slowest growing surfaces, hence leading to a defined α-factor of the crystal. Also, the rough seed's surfaces are overgrown by CVD deposited material to form smooth flat surfaces after the early CVD growth stages. Further, the seed surface imperfections are not observed to be the cause for the forming of excrescences, like present on larger crystals after longer growth times (e.g. Fig. 1). From the largest investigated diamond particles of 20μm grain size seeds (Fig. 2c), no crystals of defined morphology could be grown. The seed particles shape and surface roughness lead to the growth of CVD crystals of undefined morphology with twinning planes and shapes that are clearly determined by the form of the seed itself. At the grain sizes 1 and 5μm an overgrowing of stabile surfaces regarding the existing CVD conditions over originally present surfaces of the seed particles is noticed. This aspect becomes even more apparent, when tracking distinct diamond particles through the first hours of CVD growth. Fig. 3 shows two representative diamond seed particles of 1μm grain size before and after 2, 4 and 10h cumulative time of CVD growth. In the very early stages of growth, the rough surfaces of the
original particles are smoothened. Subsequently the characteristic {100}- and {111}-faces emerge, exclusively following crystallographic orientations but discounting the original particle shape. Considered as a promising grain size to be small enough to not evoke defective growth but on the other hand large enough to not agglomerate during substrate pre-treatment, the focus was put on the 5 μm grain size particles. Fig. 4 shows a representative crystal grown from a 5 μm diamond grain on a laser structured tungsten substrate. The CVD conditions apparently allow the exclusive growth of the applied seeds, since secondary nucleation is only scarcely detected even after more than 10 h CVD growth time. Even though the substrate's surface is roughened by the laser-applied grid structure, there are no locations of favoured nucleation next to the applied seeds detected. Under the tilted angle view of Fig. 4b one can see the volume growth of the crystal in all three directions in space. The surfaces appear absolutely flat, no growth defects such as excrescences are found. Even under magnifications of 100,000× in the SEM, on the {100}-faces no surface structure could be detected. The {111}-faces show in a slight contrast of a pattern of lines including an angle of 120°. Regarding the evaluation of
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Fig. 3. Two representative 1μm diamond particles viewed in SEM before and after 2, 4 and 10h cumulative CVD growth time. From both particles develop {100}- and {111}-faces regarding the crystal's orientation and the CVD conditions. The original particles surfaces do not determine the formation of CVD surfaces nor suppress the development of a defined morphology.
a suitable seed type we concluded: the diamond particles of 5 μm grain size appeared to be large enough not to agglomerate. Therefore they allow a homogenous, sufficient low seed density of the substrates. At the same time, they were found to be small enough to allow the forming of morphologically well defined CVD crystals. Imperfections in morphology and surface roughness of these 5 μm seeds are not found to cause defective growth. Therefore, the 5 μm diamond particles were considered as suitable seeds for our CVD growth. Applying the 5 μm monocrystalline seeds, larger diamond crystals were synthesized (Fig. 5). On the tungsten substrate a region of interest of 1 cm 2 with a crystal density of approximately 6 · 10 3/cm2 was determined. Well facetted cubo-octaedric crystals have grown predominantly. Among these also some multiple twinned crystals and distinctively
smaller crystals, which nucleated statistically during CVD growth, were found. Fig. 5 displays a diamond crystal with more than 80μm length of edge representative for ten well facetted ones, that were characterised by SEM. The mean growth rate for the crystals was determined to be between 1.0 and 1.2μm/h. In contrast to previously presented crystals (Fig. 1) [1], the {100}-faces appear smooth and flat without indications of growth spirals or excrescences. In Raman analysis these {100}-faces exhibit a diamond quality factor of 99.9% regarding the diamond / graphite ratio (Fig. 5). The rough appearing {111}-faces show a comparably lower quality factor of 96.1%. Growth conditions for the crystals of Fig. 5 have been more stable compared to conditions for crystals show in Fig. 1. The type of seeds applied was the same. The forming of excrescence on the {100}-faces observed in earlier
Fig. 4. (a) Tilted angle view on the laser structured grid on a tungsten substrate employed for crystal identification. The crystal of after CVD growth is located in the middle and is magnified in (b). (c), and (d): Even under 100,000× magnification in SEM, its {100}-faces appear absolutely flat. The {111}-faces have not developed the typical rough {111}-surface of larger crystals (e.g. Fig. 1) yet.
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Fig. 5. SEM-picture and Raman spectra of a high quality diamond crystal grown from a 5μm diamond particle. The {100}-face features a diamond quality factor of 99.9% in Raman characterization, the {111}-face a quality of 96.1%. The edge length of the crystal is more than 80μm.
from this (growth mechanism described in our former work [1]) is intended to be characterised by X-ray analysis of larger crystals. For this, we have to show in further work if the volume growth of our crystals, meaning the simultaneous growth of (111)- and (100)-faces, can be stabilised to sufficiently large crystal sizes. To have the perspective to become competitive to the plasma CVD method in terms of process yield, the main advantage of the HFCVD against plasma CVD processes, which is the possibility of upscaling to diamond growth on large areas, has to be realised. That way it would be possible with HFCVD to reach Carat/h numbers competitive to plasma CVD. Homogenous growth conditions in HFCVD over large areas are necessary for such processes. But the possibility for that has recently been shown by under the aspect of coating 300 mm silicon wafers by [16]. Especially for applications where high amounts of single crystal diamonds up to a few hundred micrometers of defined size and quality are demanded, the HFCVD is more suitable than plasma CVD. 4. Conclusion
experiments (Fig. 1) such as the rough island growth on {111}-surfaces containing rather high sp2 ratios are therefore concluded not to derive from seed imperfections. They rather evolve at later growth stages due to CVD growth conditions. Therefore, ensuring well defined and stable CVD process parameters as gas composition and substrate temperature is presumed to be predominantly vital for permitting defectless growth. The presented results show that by employing suitable monocrystalline seeds it is possible to grow high quality diamond single crystals in our HFCVD parameter window. Compared to the recent outstanding results of single crystal diamond growth in plasma CVD by Yan et al. [4,11], the growth rates in HFCVD are significantly lower. But the very high growth rates in plasma CVD are only observed under very high process pressures of 160–300 mbar. We believe that the hot filament method employing lower pressures (7–30 mbar) has considerable advantages concerning purity and quality of diamond growth. Since growth rates are moderate, the formation of stacking faults observed at high pressure plasma CVD growth [6] is less likely. Additionally, there are indications that the purity of the grown diamond is expected to be higher in hot filament CVD. Concerning boron, it was found that the incorporation into the diamond lattice as electrically active impurity is strongly dependent on process pressure [12]. At lower pressures less boron is built in. It is further reported that in hot filament CVD nitrogen impurities are less likely incorporated into diamond [13]. This is caused by the fact that the strong N–N bond of nitrogen is less likely to be activated in hot filament gas phase [13–15] than in plasma CVD gas phase. A further aspect that has to be mentioned when comparing our hot filament growth to plasma CVD growth is, that at processes of Yan the relatively high CH4 content in the feed gas and the high process pressure stabilise the growth exclusively in (100)direction. Polycrystalline grown material on the edges of each crystal has to be removed by laser cutting. In our process, (111)and (100)-faces grow simultaneously. The resulting quality
In [1] we have presented a process parameter window in HFCVD that stabilizes diamond volume crystal growth. A model for the observed growth aspects was presented. In this work we investigated the influence of seed particles on our crystal growth. From tracing particular diamond particles during CVD growth we conclude that imperfections in form of rough surface structures of monocrystalline seeds up to 5μm grain size do not impede the development of flat smooth surfaces, desired for single crystalline growth. Monocrystalline diamond particles of 5 μm grain size were found to be suitable for seeding. Crystals larger than 80 μm of high diamond quality without the growth defects observed in former experiments have been synthesized (Fig. 5). In former works the growth of crystals up to sizes of N100 sμm in this HFCVD process has been shown (Fig. 1). Further work will focus on CVD process engineering regarding stable growth conditions as well as on concepts for growth rate enhancement. The basic idea behind our efforts is to realize single diamond growth on large areas at HFCVD moderate growth rates, leading to a reasonable process yield as well as to high diamond quality. A superior diamond quality is believed to be possible by slower diamond growth compared to the high rate diamond growth in plasma CVD. References [1] S. Schwarz, C. Rottmair, J. Hirmke, S. Rosiwal, R.F. Singer, J. Cryst. Growth 271 (2004) 425. [2] Yu.N. Pal'yanov, Yu. N., Yu.M. Borzdov, et al., Diam. Rel. Mat. 7 (1998) 916. [3] J. Isberg, J. Hammersberg, D.J. Twitchen, A.J. Whitehead, Diam. Rel. Mat. 13 (2) (2004) 320. [4] C.S. Yan, Y.K. Vohra, H.-K. Mao, R.J. Hemley, PNAS 99 (20) (2002) 12523. [5] T. Bauer, M. Schreck, H. Sternschulte, B. Stritzker, Diam. Rel. Mat. 14 (3–7) (2005) 266. [6] S. Delclos, D. Dorignac, F. Phillipp, F. Silva, A. Gicquel, Diam. Rel. Mat. 7 (2–5) (1998) 222.
J. Hirmke et al. / Diamond & Related Materials 15 (2006) 536–541 [7] C. Wild, R. Kohl, N. Herres, W. Müller-Sebert, P. Koidl, Diam. Rel. Mat. 3 (1994) 373. [8] J. Bühler, Y. Prior, J. Cryst. Growth 2009 (2000) 788. [9] F. Silva, A. Gicquel, A. Tardieu, P. Cledat, T. Chauveau, Diam. Rel. Mat. 5 (1996) 338. [10] R.E. Shroder, R.J. Nemanich, J.T. Glass, Phys. Rev. B 41 (6) (1990) 3738. [11] C.S. Yan, H.K. Mao, W. Li, J. Qian, Y. Zhao, R.J. Hemley, PHYS: Stat: Sol. (a) 201 (2004) R24. [12] T. Tsubota, T. Fukui, M. Kameta, T. Saito, et al., Diam. Rel. Mat. 8 (1999) 1079.
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[13] S.T. Kshirsagar, R.B. Kshirsagar, P.S. Patil, A.V. Kulkarni, et al., Diam. Rel. Mat. 14 (2005) 232. [14] Yu.A. Mankelevich, N.V. Suetin, J.A. Smith, M.N.R. Ashfold, Diam. Rel. Mat. 11 (2002) 567. [15] R.S. Tsang, C.A. Rego, P.W. May, M.N.R. Ashfold, K.N. Rosser, Diam. Rel. Mat. 6 (1997) 247. [16] J. Zimmer, K.V. Ravi, Diam. Rel. Mat. 2005 in press, available online.
Diamond single crystal growth in hot filament CVD J. Hirmke ⁎, S. Schwarz, C. Rottmair, S.M. Rosiwal, R.F. Singer Lehrstuhl für Werkstoffkunde und Technologie der Metalle (WTM), Universität Erlangen-Nürnberg, Martensstr. 5, D-91058 Erlangen, Germany Available online 20 February 2006
Abstract The synthesis of diamond crystals is of particular interest due to the material's outstanding physical and mechanical properties. In hot filament CVD (HFCVD) we found a new process parameter window where the growth of single diamond volume crystals can be stabilized without the use of monocrystalline substrates. These CVD parameters are far beyond growth conditions for HFCVD diamond coating processes. Extremely low methane contents in the feed gas along with high substrate temperatures allow single diamond nuclei of a sufficiently large size to grow stabile. Crystals up to 80 μm in diameter were successfully synthesized. The morphology of the crystals is cubo-octaedric. According to our proposed growth model [S. Schwarz, C. Rottmair, J. Hirmke, S. Rosiwal, R.F. Singer, J. Cryst. Growth 271 (2004) 425.], the observed growth defects are primarily caused by the gas phase conditions during the CVD process. The aim of this work was to exclude a further possible formation of growth defects due to the employed diamond seed particles. The early growth stage was investigated by tracking distinct monocrystalline diamond seeds. It is shown that cubo-octaedric crystals with CVD typical smooth faces of high quality can be grown from micrometer-sized particles. Seed imperfections are therefore not considered as a major reason for growth defects of the larger crystals. © 2006 Elsevier B.V. All rights reserved. Keywords: Hot filament CVD; Single crystal growth; Nucleation; Diamond crystal
1. Introduction Diamond single crystals are in great demand for mechanical and electrical applications. Various techniques for synthesizing high quality diamond of well defined material properties are pursued by numerous research groups. Regarding large single diamond crystals for mechanical purposes, where impurities are of minor interest, high pressure – high temperature (HPHT) synthesis [2] is currently the only reasonable alternative to natural diamond. Concerning electronic applications, where single crystallinity, high purity and the possibility for doping is demanded, diamond synthesis by CVD layer growth is pursued [3]. In this field, synthesis by plasma CVD has prevailed. The trend in this method is clearly towards employing higher and higher process pressures along with increasing methane contents in order to increase growth rate [4,5]. This high growth rate synthesis, however, can lead to stacking faults [6]. Moreover, in plasma CVD, alike at the HPHT method, reactor sizes are limited and such is the amount of diamond that can be synthesized in a single process. In Ref. [1] we reported on a possible way to grow ⁎ Corresponding author. Tel.: +49 9131 8527520; fax: +49 9131 8527515. E-mail address: [email protected] (J. Hirmke). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.01.003
single diamond volume crystals in hot filament CVD (HFCVD). With this CVD method, the size of the reactor is virtually unlimited. Large area deposition of homogenous polycrystalline diamond coatings is already industrial state of the art. Therefore we intend to get a broader understanding of our observed single crystal growth in order to be able to evaluate the possibility for upscaling this process. Upscaling would allow growing scores of diamond crystals on large deposition areas simultaneously. This could highlight a new way of economic diamond crystal production even under the HFCVD typical moderate growth rates. These moderate growth rates are moreover considered less likely to cause stacking faults, a high diamond material quality is therefore expected. As reported in [1], in first experiments crystals up to 100μm have successfully been synthesized (Fig. 1). An advanced growth model regarding stable growth of volume crystals instead of layer growth and regarding the obtained morphology was given. Our further work aims towards revealing the causes for growth defects such as excrescences on the {100}-faces and defective island growth with increased sp2 ratios on the {111}-surfaces (Fig. 1). In this work, the particular influence of the employed seed crystals on growth defects such as the applicability of different seed types is investigated. As elucidated in [1], our applied CVD growth conditions of low
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Fig. 1. SEM-pictures of free standing diamond crystals up to 100 μm in diameter grown on tungsten carbide substrates. The cubo-octaedric crystals feature high quality {100}-faces with growth spirals and rough {111}-faces of lower diamond quality. Excrescences can be found on the {100}-faces. Secondary nucleation during CVD growth caused the formation of smaller crystals among them.
methane content and high substrate temperatures lead to an exclusive growth of crystals that are larger than a certain critical cluster size. This is due to a lowered super-saturation of growth species in the gas phase above the substrate compared to diamond layer growth conditions, where higher methane contents and lower substrate temperatures are employed. Therefore, in stating CH4 content↓ þ TSub ↑→super saturation↓ →critical cluster size↑ the growth conditions regarding nucleation in our single crystal growth processes can be summarized. The critical cluster at single crystal growth conditions was estimated to contain nearly 3000 atoms (equals a cluster radius of approximately 2.2 nm) compared to some 100 atoms at standard coating conditions [1]. So, when applying relatively large monocrystalline diamond particles of 1–20μm size as seed crystals, all applied seeds should be able to grow stable. Correspondingly, the undesired forming of further critical clusters during the CVD process, which leads ultimately to layer growth, should be avoided by the sufficiently enhanced critical cluster size. Therefore the use of rather large seed particles seems reasonable. On the other hand, employing larger and larger seed crystals for the observed volume growth the shape and surface morphology of the seed particles are expected to have a rising influence on the CVD diamond's morphology. This could lead to the formation of growth defects already in the early growth stages. In order to evaluate a suitable seed size, monocrystalline diamond particles of the sizes 1, 5 and 20μm were investigated. By tracking distinct crystals before and after growth, the morphology of the resulting CVD crystals was observed. Further, the applicability of the seeding methods for providing sufficiently low seed densities to allow single crystals volume growth was examined.
patterns of 1200 square boxes (100 × 100 μm each, see Fig. 4a) were applied on the substrate's surfaces by a Nd-YAG laser. For evaluation, 5 distinct seeds of each of the 3 grain sizes were tracked before and after CVD growth. The CVD growth was conducted in a state of the art HFCVD reactor with tungsten filaments. The presented crystals were grown in a HFCVD reactor at substrate temperatures N 900°C and 0.3% methane in feed gas. Characterization of the crystals was made by scanning electron microscopy (SEM) and Raman spectroscopy. In SEM the crystals' shape is specified by determining the alpha-factor as defined in Ref. [7]. The alpha-factor is given by pffiffiffi a ¼ 3dvð100Þ=vð111Þ ð1Þ where v (100) is the growth velocity of the (100) surface and v (111) is the growth velocity of the (111) surface. Its value for a CVD grown crystal can be derived from measuring the length of the edge between two (100)-faces and the length of the edge between a (100)- and a (111)-face by geometrical consideration according to Ref. [8]. The alpha-factor represents the crystal's shape from cubic (α = 1) via cubo-octahedral (α = 1.5) to octahedral (α = 3) [7,8] and indicates to which ratio the slowest grown surfaces terminate the crystal. From the Raman spectra a diamond quality factor q was derived according to Ref. [9] by employing q¼
75dId P d100% 75Id þ Ind
ð2Þ
nd
where the area below the diamond peak Id and non-diamond peaks Ind in Raman spectra are considered. The factor 75 takes the more effective Raman scattering on sp2-structures for the wavelength of the used laser (Ar+-laser, λ = 514 nm) into account [10].
2. Experimental 3. Results and discussion The evaluated diamond particles for seeding were monocrystalline synthetic diamond powders of the grain size 1 μm (0.75–1.25μm), 5 μm (4–6μm) and 20μm (15–25μm). The powders where diluted in ethanol and applied to tungsten substrates. The employed mean seed densities on the substrates were determined to 5.5 · 104/cm2, 1.1 · 104/cm2 and 250/cm2, respectively. Higher dilutions of the seeds on ethanol allowed also lower seed densities. For tracking distinct crystals, grid
Fig. 2 displays SEM images of representative diamond particles of the grain sizes 1, 5 and 20 μm before and after HFCVD growth. For the grain sizes 1 and 5 μm (Fig. 2a,b) the CVD grown crystals show a defined morphology with smooth, flat {100}- and {111}-faces. The resulting α-factor (as defined in Ref. [7]) of the crystals is between 1.2 and 1.3. This indicates that the seed diamond particles' irregular shape determined by
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Fig. 2. SEM-pictures of representative diamond seed particles of (a) 1μm, (b) 5μm and (c) 20μm grain size before CVD growth on the right and corresponding crystals developed in CVD growth on the left. From diamond particles of 1 and 5 μm size, cubo-octaedric CVD crystals of defined morphology can develop. The 20 μm diamond particles are too large to not affect the forming of growth surfaces and so the crystals' shape.
cleavage planes from milling processes does not disturb the development of crystals whose faces are terminated by the slowest growing surfaces, hence leading to a defined α-factor of the crystal. Also, the rough seed's surfaces are overgrown by CVD deposited material to form smooth flat surfaces after the early CVD growth stages. Further, the seed surface imperfections are not observed to be the cause for the forming of excrescences, like present on larger crystals after longer growth times (e.g. Fig. 1). From the largest investigated diamond particles of 20μm grain size seeds (Fig. 2c), no crystals of defined morphology could be grown. The seed particles shape and surface roughness lead to the growth of CVD crystals of undefined morphology with twinning planes and shapes that are clearly determined by the form of the seed itself. At the grain sizes 1 and 5μm an overgrowing of stabile surfaces regarding the existing CVD conditions over originally present surfaces of the seed particles is noticed. This aspect becomes even more apparent, when tracking distinct diamond particles through the first hours of CVD growth. Fig. 3 shows two representative diamond seed particles of 1μm grain size before and after 2, 4 and 10h cumulative time of CVD growth. In the very early stages of growth, the rough surfaces of the
original particles are smoothened. Subsequently the characteristic {100}- and {111}-faces emerge, exclusively following crystallographic orientations but discounting the original particle shape. Considered as a promising grain size to be small enough to not evoke defective growth but on the other hand large enough to not agglomerate during substrate pre-treatment, the focus was put on the 5 μm grain size particles. Fig. 4 shows a representative crystal grown from a 5 μm diamond grain on a laser structured tungsten substrate. The CVD conditions apparently allow the exclusive growth of the applied seeds, since secondary nucleation is only scarcely detected even after more than 10 h CVD growth time. Even though the substrate's surface is roughened by the laser-applied grid structure, there are no locations of favoured nucleation next to the applied seeds detected. Under the tilted angle view of Fig. 4b one can see the volume growth of the crystal in all three directions in space. The surfaces appear absolutely flat, no growth defects such as excrescences are found. Even under magnifications of 100,000× in the SEM, on the {100}-faces no surface structure could be detected. The {111}-faces show in a slight contrast of a pattern of lines including an angle of 120°. Regarding the evaluation of
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Fig. 3. Two representative 1μm diamond particles viewed in SEM before and after 2, 4 and 10h cumulative CVD growth time. From both particles develop {100}- and {111}-faces regarding the crystal's orientation and the CVD conditions. The original particles surfaces do not determine the formation of CVD surfaces nor suppress the development of a defined morphology.
a suitable seed type we concluded: the diamond particles of 5 μm grain size appeared to be large enough not to agglomerate. Therefore they allow a homogenous, sufficient low seed density of the substrates. At the same time, they were found to be small enough to allow the forming of morphologically well defined CVD crystals. Imperfections in morphology and surface roughness of these 5 μm seeds are not found to cause defective growth. Therefore, the 5 μm diamond particles were considered as suitable seeds for our CVD growth. Applying the 5 μm monocrystalline seeds, larger diamond crystals were synthesized (Fig. 5). On the tungsten substrate a region of interest of 1 cm 2 with a crystal density of approximately 6 · 10 3/cm2 was determined. Well facetted cubo-octaedric crystals have grown predominantly. Among these also some multiple twinned crystals and distinctively
smaller crystals, which nucleated statistically during CVD growth, were found. Fig. 5 displays a diamond crystal with more than 80μm length of edge representative for ten well facetted ones, that were characterised by SEM. The mean growth rate for the crystals was determined to be between 1.0 and 1.2μm/h. In contrast to previously presented crystals (Fig. 1) [1], the {100}-faces appear smooth and flat without indications of growth spirals or excrescences. In Raman analysis these {100}-faces exhibit a diamond quality factor of 99.9% regarding the diamond / graphite ratio (Fig. 5). The rough appearing {111}-faces show a comparably lower quality factor of 96.1%. Growth conditions for the crystals of Fig. 5 have been more stable compared to conditions for crystals show in Fig. 1. The type of seeds applied was the same. The forming of excrescence on the {100}-faces observed in earlier
Fig. 4. (a) Tilted angle view on the laser structured grid on a tungsten substrate employed for crystal identification. The crystal of after CVD growth is located in the middle and is magnified in (b). (c), and (d): Even under 100,000× magnification in SEM, its {100}-faces appear absolutely flat. The {111}-faces have not developed the typical rough {111}-surface of larger crystals (e.g. Fig. 1) yet.
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Fig. 5. SEM-picture and Raman spectra of a high quality diamond crystal grown from a 5μm diamond particle. The {100}-face features a diamond quality factor of 99.9% in Raman characterization, the {111}-face a quality of 96.1%. The edge length of the crystal is more than 80μm.
from this (growth mechanism described in our former work [1]) is intended to be characterised by X-ray analysis of larger crystals. For this, we have to show in further work if the volume growth of our crystals, meaning the simultaneous growth of (111)- and (100)-faces, can be stabilised to sufficiently large crystal sizes. To have the perspective to become competitive to the plasma CVD method in terms of process yield, the main advantage of the HFCVD against plasma CVD processes, which is the possibility of upscaling to diamond growth on large areas, has to be realised. That way it would be possible with HFCVD to reach Carat/h numbers competitive to plasma CVD. Homogenous growth conditions in HFCVD over large areas are necessary for such processes. But the possibility for that has recently been shown by under the aspect of coating 300 mm silicon wafers by [16]. Especially for applications where high amounts of single crystal diamonds up to a few hundred micrometers of defined size and quality are demanded, the HFCVD is more suitable than plasma CVD. 4. Conclusion
experiments (Fig. 1) such as the rough island growth on {111}-surfaces containing rather high sp2 ratios are therefore concluded not to derive from seed imperfections. They rather evolve at later growth stages due to CVD growth conditions. Therefore, ensuring well defined and stable CVD process parameters as gas composition and substrate temperature is presumed to be predominantly vital for permitting defectless growth. The presented results show that by employing suitable monocrystalline seeds it is possible to grow high quality diamond single crystals in our HFCVD parameter window. Compared to the recent outstanding results of single crystal diamond growth in plasma CVD by Yan et al. [4,11], the growth rates in HFCVD are significantly lower. But the very high growth rates in plasma CVD are only observed under very high process pressures of 160–300 mbar. We believe that the hot filament method employing lower pressures (7–30 mbar) has considerable advantages concerning purity and quality of diamond growth. Since growth rates are moderate, the formation of stacking faults observed at high pressure plasma CVD growth [6] is less likely. Additionally, there are indications that the purity of the grown diamond is expected to be higher in hot filament CVD. Concerning boron, it was found that the incorporation into the diamond lattice as electrically active impurity is strongly dependent on process pressure [12]. At lower pressures less boron is built in. It is further reported that in hot filament CVD nitrogen impurities are less likely incorporated into diamond [13]. This is caused by the fact that the strong N–N bond of nitrogen is less likely to be activated in hot filament gas phase [13–15] than in plasma CVD gas phase. A further aspect that has to be mentioned when comparing our hot filament growth to plasma CVD growth is, that at processes of Yan the relatively high CH4 content in the feed gas and the high process pressure stabilise the growth exclusively in (100)direction. Polycrystalline grown material on the edges of each crystal has to be removed by laser cutting. In our process, (111)and (100)-faces grow simultaneously. The resulting quality
In [1] we have presented a process parameter window in HFCVD that stabilizes diamond volume crystal growth. A model for the observed growth aspects was presented. In this work we investigated the influence of seed particles on our crystal growth. From tracing particular diamond particles during CVD growth we conclude that imperfections in form of rough surface structures of monocrystalline seeds up to 5μm grain size do not impede the development of flat smooth surfaces, desired for single crystalline growth. Monocrystalline diamond particles of 5 μm grain size were found to be suitable for seeding. Crystals larger than 80 μm of high diamond quality without the growth defects observed in former experiments have been synthesized (Fig. 5). In former works the growth of crystals up to sizes of N100 sμm in this HFCVD process has been shown (Fig. 1). Further work will focus on CVD process engineering regarding stable growth conditions as well as on concepts for growth rate enhancement. The basic idea behind our efforts is to realize single diamond growth on large areas at HFCVD moderate growth rates, leading to a reasonable process yield as well as to high diamond quality. A superior diamond quality is believed to be possible by slower diamond growth compared to the high rate diamond growth in plasma CVD. References [1] S. Schwarz, C. Rottmair, J. Hirmke, S. Rosiwal, R.F. Singer, J. Cryst. Growth 271 (2004) 425. [2] Yu.N. Pal'yanov, Yu. N., Yu.M. Borzdov, et al., Diam. Rel. Mat. 7 (1998) 916. [3] J. Isberg, J. Hammersberg, D.J. Twitchen, A.J. Whitehead, Diam. Rel. Mat. 13 (2) (2004) 320. [4] C.S. Yan, Y.K. Vohra, H.-K. Mao, R.J. Hemley, PNAS 99 (20) (2002) 12523. [5] T. Bauer, M. Schreck, H. Sternschulte, B. Stritzker, Diam. Rel. Mat. 14 (3–7) (2005) 266. [6] S. Delclos, D. Dorignac, F. Phillipp, F. Silva, A. Gicquel, Diam. Rel. Mat. 7 (2–5) (1998) 222.
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