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The effect of ion bombardment on the nucleation of CVD diamond
Diamond and Related Materials 8 (1999) 1414–1417 www.elsevier.com/locate/diamond
The effect of ion bombardment on the nucleation of CVD diamond X.S. Sun, N. Wang, H.K. Woo, W.J. Zhang *, G. Yu, I. Bello, C.S. Lee, S.T. Lee Centre of Super-Diamond and Advanced Films and Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue 83, Kowloon, Hong Kong Accepted 30 November 1998
Abstract The nucleation effect of CVD diamond by ion bombardment was studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. It was found that after the ion bombardment, an amorphous layer embedded with nano-crystalline diamond particles formed on the Si substrate. These nano-crystalline diamond particles were proposed to serve as the nucleation centers for the growth in the second step. The nucleation density depended strongly on the ion dosage and a nucleation density of up to 2×109 cm−2 could be achieved under optimized conditions. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond films; Interface; Ion bombardment; Nucleation
1. Introduction Owing to its various outstanding properties, such as extreme hardness, good thermal conductivity, excellent transparency, chemical inertness and wide band gap, diamond is a potential candidate for tribological, optical, thermal and electronic applications. The application of diamond films in semiconductor devices is, of course, the most attractive goal, but the preparation of large area, high-quality single crystal diamond films is yet to be achieved. The understanding of the mechanism of diamond nucleation is critical for controlling the film quality. Single crystal Si wafer, from the micro-electronic technological viewpoint, is the most suitable substrate material. However, from the viewpoint of crystal growth, Si is not a suitable candidate because there is a large lattice mismatch between diamond and Si (52%). Furthermore, their surface energies also differ quite significantly (experimentally determined values of fracture surface energies are 6 and 1.5 J m−2 for diamond and the Si (111) plane, respectively) [1,2]. Both factors contribute to a low sticking probability of reactant precursors which may form diamond nuclei on the substrates, and cause a low nucleation density especially * Corresponding author. Fax: +852 27887830. E-mail address: [email protected] ( W.J. Zhang)
on a mirror-polished Si wafer. To overcome this crucial difficulty, many ex-situ methods for enhancing nucleation (e.g. pre-scratching of Si wafers [2,3] and ion implantation [4] etc.) and an in-situ pretreatment of bias-enhanced-nucleation (BEN ) [5] were employed. Among these nucleation enhancement methods, the BEN technique has received much attention and widely recognized as an effective method of enhancing nucleation. Jiang et al. [6 ] and Stoner et al. [7] reported the successful growth of oriented diamond film on (001) Si and (001) b-SiC substrates by introducing a BEN process in microwave chemical vapor deposition (CVD) of diamond films. However, the mechanism of nucleation enhancement is not yet clear to date. Shigesato et al. [8] proposed that the application of a negative substrate bias could increase the concentration of atomic hydrogen in the plasma, which caused the nucleation enhancement as a result of the increased etching rate of the sp2-bonded carbon. On the contrary, Beckmann et al. [9] found that the increase of atomic hydrogen in the plasma was too small to account for the nucleation enhancement. Moreover, it was suggested that the substrate biasing led to the extraction of ion species from plasma and thus increased the arrival rate of reactant radicals and thus enhancing nucleation [10]. Recently, there were some reports showing that the BEN process was due to the bombardment of substrate by positive ions [11,12]. In fact, ion bombardment is invariably
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 01 2 - 6
X.S. Sun et al. / Diamond and Related Materials 8 (1999) 1414–1417
present in the BEN process, however, the mechanism of how ion bombardment enhances nucleation is not yet clearly understood. In this paper, the evidence for the effect of ion bombardment on diamond nucleation was directly studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. The mechanism of CVD diamond nucleation; particularly the role of ion bombardment on diamond nucleation was discussed.
2. Experimental A vacuum chamber equipped with a Kaufman ion source (3.0-1500-1000, ION. TECH, INC. USA) was used for the first step of nucleation in present experiment. The equipment setup was shown elsewhere [13]. Mirror-polished n-type (001) Si wafers were used as the substrates. Before ion bombardment, Si wafers were rinsed with acetone and ethanol, etched with a 5% HF solution for 1 min, and then cleaned with de-ionized water. The ion bombardment process was carried out in the vacuum chamber with a base pressure of 2×10−8 Torr. A mixture of methane, hydrogen and argon (CH :H :Ar=1:4:4) was introduced into the ion 4 2 source as the working gas. The total flow rate was 2 sccm and the working pressure was kept at about 8×10−4 Torr. The ions generated in the ion source were accelerated to bombard the substrates perpendicularly with an accelerating voltage of 100 V. The ion dosing rate was measured with a Faraday cup prior to ion bombardment. The substrate temperature was 780°C as measured with an infrared thermometer. After ion bombardment, the samples were moved into a hot filament CVD (HFCVD) reactor and a subsequent growth was performed by conventional HFCVD. The HFCVD growth conditions were as follows: CH /H =2:98, reac4 2 tant pressure was maintained at 30 Torr, filament and substrate temperatures were 2100 and 850°C, respectively. In order to investigate the effect of ion bombardment on nucleation enhancement, the growth time was maintained at 15 min. High-resolution transmission electron microscopy (HRTEM ), scanning electron microscopy (SEM ), atomic force microscopy (AFM ), selected-area electron diffraction pattern (SAEDP), and micro-Raman spectroscopy were used to characterize the samples after nucleation and growth steps.
3. Results and discussion Fig. 1a is a cross-sectional HRTEM image of the sample after ion bombardment. The ion bombarding
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energy was 100 eV and the total ion dose was 1×1019 cm−2. It can be seen that an amorphous layer, which was determined by Raman spectroscopy to be amorphous carbon, was deposited on the silicon substrate. The thickness of the amorphous film was about 400 nm measured by cross-section TEM. An interlayer of SiC was formed between the Si substrate and the amorphous carbon layer, the presence of SiC was revealed by Raman spectroscopy as well as by SAEDP as shown in Fig. 1b. It is interesting that nano-crystalline particles were found to be dispersed in an amorphous matrix, as indicated in Fig. 1c. These nano-particles were 20~30 nm in size. After image filter processing by Fourier transform (see the inset in the upper-left of Fig. 1c), the lattice spacing of the nano-crystals was ˚ which is in good agreement with the d-spacing about 2 A of diamond {111}. Combined with the electron diffraction pattern, these nano-particles were confirmed to be nano-crystalline diamond powders. The 111 directions of these nano-crystalline particles are randomly oriented. It should be noticed that only the particles with their {111} crystal faces parallel to the electron beam direction can be observed in the TEM image. The total density of the nano-crystalline particles in the amorphous matrix was possibly higher than that observed in the TEM image. A subsequent diamond growth was performed by conventional HFCVD for 15 min on the sample after ion bombardment. Fig. 2 shows a SEM image of the sample after ion bombardment and growth by HFCVD for 15 min. Diamond grains were deposited with an average size of about 200 nm. The grains have not coalesced to form a continuous film, so the nucleation density can be counted and is about 2×109 cm−2. The similar two-step process was carried out for various ion doses and different reactant gases. The resulting nucleation density was measured. Fig. 3 depicts the relationship between nucleation density and the ion bombardment dosage. The nucleation density was strongly dependent on the ion dosage. In the case of using CH , H and Ar as the working gas, an increase 4 2 of nucleation density with increasing ion dose was observed, and the nucleation density increased to about 109 cm−2 when the ion dose reached more than 1018 cm−2. The use of Ar was for the stability of ion source. The contribution of bombardment by argon ions was studied by feeding only argon gas into the ion gun. The nucleation density also showed a tendency to increase up to 106 cm−2 at an ion dose of 1×1019 cm2, which was higher than the case of diamond growth on mirror-polished Si substrate without any pretreatment (nucleation density ~104 cm−2). The increase of nucleation density may be caused by ion-bombardmentinduced surface damages which serve as active nucleation sites during nucleation. The degree of enhancement by Ar ion bombardment, however, is three orders of
1416
X.S. Sun et al. / Diamond and Related Materials 8 (1999) 1414–1417
Fig. 1. (a) HRTEM image near the interface between Si substrate and deposited amorphous carbon layer of the sample after ion bombardment; (b) SAEDP of the ion bombarded sample. (1: 11) reflections are marked by thin arrows, (111) and (220) reflections are marked by SiC diamond diamond double and single fat arrows, respectively. (c) HRTEM image demonstrating the formation of nanocrystalline diamond particles in the amorphous layer.
Fig. 2. SEM image of a sample after 1019 ions cm−2 ion beam bombardment and growth by conventional HFCVD for 15 min.
magnitude lower than that obtained in a CH /H /Ar 4 2 mixture was used as the reactant gas. This result suggests that hydrocarbon and hydrogen ions are predominantly responsible for the nucleation enhancement in the ion bombardment process. In addition, when the mixture of hydrogen and argon was used as the working gas (1:1), the nucleation enhancement effect was minimal.
Fig. 3. Diamond nucleation density versus ion dosage at 100 V accelerating voltage.
The result is similar to that reported by Backmann et al. [14]. Based on the experimental observation above, it can be imagined that during the ion bombardment process,
X.S. Sun et al. / Diamond and Related Materials 8 (1999) 1414–1417
a SiC layer was first formed on the substrate, and then an amorphous carbon film was deposited on the SiC layer. In our experiments, the kinetic energy of the ions bombarding the substrate was about 100 eV. Considering that the subplantation efficiency reached a maximum for ion energies between 80–100 eV [15], first, we suggest that ion bombardment in our experiments causes subplantation of hydrocarbon ions into Si substrate forming a SiC layer. Second, the hydrocarbon clusters migrate and aggregate on the substrate surface, and an amorphous carbon layer was deposited subsequently. The deposition of amorphous carbon films by ion beam deposition has been reported by many researchers before. However, it is important that the formation of nano-crystalline diamond particles by ion bombardment was directly observed for the first time. Although the mechanism is not well understood, the following factors are proposed to be favorable to diamond formation. First, the substrate was continuously bombarded with energetic ions. sp3 carbon clusters particles were formed in the amorphous film as a result of the ion-bombardment-induced stress and energy fluctuation. If the size of such a carbon cluster is larger than the critical size, it can survive and grow to form nano-crystalline diamond particles. Second, the presence of hydrogen ions during the ion bombardment process plays two important roles: saturating the carbon dangling bonds, and preferentially etching the sp2 component, both of which are favorable for the formation of diamond. Moreover, the high substrate temperature was also considered to be an important factor. When the CVD growth of diamond was subsequently performed on the ion-bombarded sample, the amorphous carbon was selectively etched away by atomic hydrogen in the CVD plasma and the nano-crystalline diamond particles were exposed. These exposed particles could possibly serve as nuclei in the subsequent diamond growth. Remarkably, the nucleation density counted after 15 min growth ( Fig. 2) is one order of magnitude lower than the nano-particle density of about 1010 cm−2 in the amorphous carbon film after ion bombardment (Fig. 1c). This suggests that most of the nanocrystalline diamond particles did not survive in the growth environment and only a very small portion of these particles could grow and serve as the nuclei in the growth process. Detailed study of these nano-crystalline diamond particles during the initial growth period is ongoing.
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4. Conclusion The effects of ion bombardment on diamond nucleation were directly studied by an ion beam deposition technique. During the ion bombardment process, a SiC layer was formed on the silicon substrate and then an amorphous carbon film was deposited on the SiC. Nanosized crystalline diamond particles were observed to disperse in the amorphous carbon matrix. These nanodiamond-crystals are proposed to serve as nuclei during the subsequent growth process by conventional HFCVD. The nucleation density increased with increasing ion bombardment dose of the substrate. A nucleation density higher than 109 cm−2 was achieved with an ion dose of 1019 cm−2.
Acknowledgement This research work is financially supported by Hong Kong Research Council CERG project 9040195.
References [1] J.E. Field (Ed.), The Properties of Diamond, Academic Press, London, 1979, p. 302. [2] K. Ishibashi, S. Furukawa, Jpn. J. Appl. Lett. 24 (1985) 912. [3] W.A. Yarbrough, R. Messier, Science 247 (1990) 688. [4] J. Yang, X. Su, Q. Chen, Z. Lin, Appl. Phys. Lett. 66 (1995) 3284. [5] S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 1036. [6 ] X. Jiang, C.P. Klages, Diamond Relat. Mater. 2 (1992) 1112. [7] B.R. Stoner, J.T. Glass, Appl. Phys. Lett. 60 (1992) 698. [8] Y. Shigesato, R.E. Boekenhauer, B.W. Sheldon, Appl. Phys. Lett. 63 (1993) 314. [9] R. Beckmann, B. Sobisch, W. Kulisch, C. Rau, Diamond Relat. Mater. 3 (1994) 555. [10] S.D. Wolter, B.R. Stoner, J.T. Glass, P.J. Ellis, D.S. Buhaenko, C.E. Jenkins, P. Southworth, Appl. Phys. Lett. 62 (1993) 1215. [11] X. Jiang, W.J. Zhang, M. Paul, C.P. Klages, Appl. Phys. Lett. 68 (1996) 1927. [12] S. Sattel, J. Gerber, H. Ehrhardt, Phys. Stat. Sol. (a) 154 (1996) 141. [13] X.S. Sun, PhD thesis, City University of Hong Kong, 1999. [14] P.K. Backmann, R. Weimer, W. Drawl, Y. Liou, R. Messier, Diamond Technology Initiative Symposium, SDIO/IST-ONR, Crystal city, VA, July 12–14, 1988. [15] J. Gerber, S. Sattel, K. Jung, H. Ehrhardt, J. Robterson, Diamond Relat. Mater. 4 (1995) 559.
The effect of ion bombardment on the nucleation of CVD diamond X.S. Sun, N. Wang, H.K. Woo, W.J. Zhang *, G. Yu, I. Bello, C.S. Lee, S.T. Lee Centre of Super-Diamond and Advanced Films and Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue 83, Kowloon, Hong Kong Accepted 30 November 1998
Abstract The nucleation effect of CVD diamond by ion bombardment was studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. It was found that after the ion bombardment, an amorphous layer embedded with nano-crystalline diamond particles formed on the Si substrate. These nano-crystalline diamond particles were proposed to serve as the nucleation centers for the growth in the second step. The nucleation density depended strongly on the ion dosage and a nucleation density of up to 2×109 cm−2 could be achieved under optimized conditions. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond films; Interface; Ion bombardment; Nucleation
1. Introduction Owing to its various outstanding properties, such as extreme hardness, good thermal conductivity, excellent transparency, chemical inertness and wide band gap, diamond is a potential candidate for tribological, optical, thermal and electronic applications. The application of diamond films in semiconductor devices is, of course, the most attractive goal, but the preparation of large area, high-quality single crystal diamond films is yet to be achieved. The understanding of the mechanism of diamond nucleation is critical for controlling the film quality. Single crystal Si wafer, from the micro-electronic technological viewpoint, is the most suitable substrate material. However, from the viewpoint of crystal growth, Si is not a suitable candidate because there is a large lattice mismatch between diamond and Si (52%). Furthermore, their surface energies also differ quite significantly (experimentally determined values of fracture surface energies are 6 and 1.5 J m−2 for diamond and the Si (111) plane, respectively) [1,2]. Both factors contribute to a low sticking probability of reactant precursors which may form diamond nuclei on the substrates, and cause a low nucleation density especially * Corresponding author. Fax: +852 27887830. E-mail address: [email protected] ( W.J. Zhang)
on a mirror-polished Si wafer. To overcome this crucial difficulty, many ex-situ methods for enhancing nucleation (e.g. pre-scratching of Si wafers [2,3] and ion implantation [4] etc.) and an in-situ pretreatment of bias-enhanced-nucleation (BEN ) [5] were employed. Among these nucleation enhancement methods, the BEN technique has received much attention and widely recognized as an effective method of enhancing nucleation. Jiang et al. [6 ] and Stoner et al. [7] reported the successful growth of oriented diamond film on (001) Si and (001) b-SiC substrates by introducing a BEN process in microwave chemical vapor deposition (CVD) of diamond films. However, the mechanism of nucleation enhancement is not yet clear to date. Shigesato et al. [8] proposed that the application of a negative substrate bias could increase the concentration of atomic hydrogen in the plasma, which caused the nucleation enhancement as a result of the increased etching rate of the sp2-bonded carbon. On the contrary, Beckmann et al. [9] found that the increase of atomic hydrogen in the plasma was too small to account for the nucleation enhancement. Moreover, it was suggested that the substrate biasing led to the extraction of ion species from plasma and thus increased the arrival rate of reactant radicals and thus enhancing nucleation [10]. Recently, there were some reports showing that the BEN process was due to the bombardment of substrate by positive ions [11,12]. In fact, ion bombardment is invariably
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 01 2 - 6
X.S. Sun et al. / Diamond and Related Materials 8 (1999) 1414–1417
present in the BEN process, however, the mechanism of how ion bombardment enhances nucleation is not yet clearly understood. In this paper, the evidence for the effect of ion bombardment on diamond nucleation was directly studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. The mechanism of CVD diamond nucleation; particularly the role of ion bombardment on diamond nucleation was discussed.
2. Experimental A vacuum chamber equipped with a Kaufman ion source (3.0-1500-1000, ION. TECH, INC. USA) was used for the first step of nucleation in present experiment. The equipment setup was shown elsewhere [13]. Mirror-polished n-type (001) Si wafers were used as the substrates. Before ion bombardment, Si wafers were rinsed with acetone and ethanol, etched with a 5% HF solution for 1 min, and then cleaned with de-ionized water. The ion bombardment process was carried out in the vacuum chamber with a base pressure of 2×10−8 Torr. A mixture of methane, hydrogen and argon (CH :H :Ar=1:4:4) was introduced into the ion 4 2 source as the working gas. The total flow rate was 2 sccm and the working pressure was kept at about 8×10−4 Torr. The ions generated in the ion source were accelerated to bombard the substrates perpendicularly with an accelerating voltage of 100 V. The ion dosing rate was measured with a Faraday cup prior to ion bombardment. The substrate temperature was 780°C as measured with an infrared thermometer. After ion bombardment, the samples were moved into a hot filament CVD (HFCVD) reactor and a subsequent growth was performed by conventional HFCVD. The HFCVD growth conditions were as follows: CH /H =2:98, reac4 2 tant pressure was maintained at 30 Torr, filament and substrate temperatures were 2100 and 850°C, respectively. In order to investigate the effect of ion bombardment on nucleation enhancement, the growth time was maintained at 15 min. High-resolution transmission electron microscopy (HRTEM ), scanning electron microscopy (SEM ), atomic force microscopy (AFM ), selected-area electron diffraction pattern (SAEDP), and micro-Raman spectroscopy were used to characterize the samples after nucleation and growth steps.
3. Results and discussion Fig. 1a is a cross-sectional HRTEM image of the sample after ion bombardment. The ion bombarding
1415
energy was 100 eV and the total ion dose was 1×1019 cm−2. It can be seen that an amorphous layer, which was determined by Raman spectroscopy to be amorphous carbon, was deposited on the silicon substrate. The thickness of the amorphous film was about 400 nm measured by cross-section TEM. An interlayer of SiC was formed between the Si substrate and the amorphous carbon layer, the presence of SiC was revealed by Raman spectroscopy as well as by SAEDP as shown in Fig. 1b. It is interesting that nano-crystalline particles were found to be dispersed in an amorphous matrix, as indicated in Fig. 1c. These nano-particles were 20~30 nm in size. After image filter processing by Fourier transform (see the inset in the upper-left of Fig. 1c), the lattice spacing of the nano-crystals was ˚ which is in good agreement with the d-spacing about 2 A of diamond {111}. Combined with the electron diffraction pattern, these nano-particles were confirmed to be nano-crystalline diamond powders. The 111 directions of these nano-crystalline particles are randomly oriented. It should be noticed that only the particles with their {111} crystal faces parallel to the electron beam direction can be observed in the TEM image. The total density of the nano-crystalline particles in the amorphous matrix was possibly higher than that observed in the TEM image. A subsequent diamond growth was performed by conventional HFCVD for 15 min on the sample after ion bombardment. Fig. 2 shows a SEM image of the sample after ion bombardment and growth by HFCVD for 15 min. Diamond grains were deposited with an average size of about 200 nm. The grains have not coalesced to form a continuous film, so the nucleation density can be counted and is about 2×109 cm−2. The similar two-step process was carried out for various ion doses and different reactant gases. The resulting nucleation density was measured. Fig. 3 depicts the relationship between nucleation density and the ion bombardment dosage. The nucleation density was strongly dependent on the ion dosage. In the case of using CH , H and Ar as the working gas, an increase 4 2 of nucleation density with increasing ion dose was observed, and the nucleation density increased to about 109 cm−2 when the ion dose reached more than 1018 cm−2. The use of Ar was for the stability of ion source. The contribution of bombardment by argon ions was studied by feeding only argon gas into the ion gun. The nucleation density also showed a tendency to increase up to 106 cm−2 at an ion dose of 1×1019 cm2, which was higher than the case of diamond growth on mirror-polished Si substrate without any pretreatment (nucleation density ~104 cm−2). The increase of nucleation density may be caused by ion-bombardmentinduced surface damages which serve as active nucleation sites during nucleation. The degree of enhancement by Ar ion bombardment, however, is three orders of
1416
X.S. Sun et al. / Diamond and Related Materials 8 (1999) 1414–1417
Fig. 1. (a) HRTEM image near the interface between Si substrate and deposited amorphous carbon layer of the sample after ion bombardment; (b) SAEDP of the ion bombarded sample. (1: 11) reflections are marked by thin arrows, (111) and (220) reflections are marked by SiC diamond diamond double and single fat arrows, respectively. (c) HRTEM image demonstrating the formation of nanocrystalline diamond particles in the amorphous layer.
Fig. 2. SEM image of a sample after 1019 ions cm−2 ion beam bombardment and growth by conventional HFCVD for 15 min.
magnitude lower than that obtained in a CH /H /Ar 4 2 mixture was used as the reactant gas. This result suggests that hydrocarbon and hydrogen ions are predominantly responsible for the nucleation enhancement in the ion bombardment process. In addition, when the mixture of hydrogen and argon was used as the working gas (1:1), the nucleation enhancement effect was minimal.
Fig. 3. Diamond nucleation density versus ion dosage at 100 V accelerating voltage.
The result is similar to that reported by Backmann et al. [14]. Based on the experimental observation above, it can be imagined that during the ion bombardment process,
X.S. Sun et al. / Diamond and Related Materials 8 (1999) 1414–1417
a SiC layer was first formed on the substrate, and then an amorphous carbon film was deposited on the SiC layer. In our experiments, the kinetic energy of the ions bombarding the substrate was about 100 eV. Considering that the subplantation efficiency reached a maximum for ion energies between 80–100 eV [15], first, we suggest that ion bombardment in our experiments causes subplantation of hydrocarbon ions into Si substrate forming a SiC layer. Second, the hydrocarbon clusters migrate and aggregate on the substrate surface, and an amorphous carbon layer was deposited subsequently. The deposition of amorphous carbon films by ion beam deposition has been reported by many researchers before. However, it is important that the formation of nano-crystalline diamond particles by ion bombardment was directly observed for the first time. Although the mechanism is not well understood, the following factors are proposed to be favorable to diamond formation. First, the substrate was continuously bombarded with energetic ions. sp3 carbon clusters particles were formed in the amorphous film as a result of the ion-bombardment-induced stress and energy fluctuation. If the size of such a carbon cluster is larger than the critical size, it can survive and grow to form nano-crystalline diamond particles. Second, the presence of hydrogen ions during the ion bombardment process plays two important roles: saturating the carbon dangling bonds, and preferentially etching the sp2 component, both of which are favorable for the formation of diamond. Moreover, the high substrate temperature was also considered to be an important factor. When the CVD growth of diamond was subsequently performed on the ion-bombarded sample, the amorphous carbon was selectively etched away by atomic hydrogen in the CVD plasma and the nano-crystalline diamond particles were exposed. These exposed particles could possibly serve as nuclei in the subsequent diamond growth. Remarkably, the nucleation density counted after 15 min growth ( Fig. 2) is one order of magnitude lower than the nano-particle density of about 1010 cm−2 in the amorphous carbon film after ion bombardment (Fig. 1c). This suggests that most of the nanocrystalline diamond particles did not survive in the growth environment and only a very small portion of these particles could grow and serve as the nuclei in the growth process. Detailed study of these nano-crystalline diamond particles during the initial growth period is ongoing.
1417
4. Conclusion The effects of ion bombardment on diamond nucleation were directly studied by an ion beam deposition technique. During the ion bombardment process, a SiC layer was formed on the silicon substrate and then an amorphous carbon film was deposited on the SiC. Nanosized crystalline diamond particles were observed to disperse in the amorphous carbon matrix. These nanodiamond-crystals are proposed to serve as nuclei during the subsequent growth process by conventional HFCVD. The nucleation density increased with increasing ion bombardment dose of the substrate. A nucleation density higher than 109 cm−2 was achieved with an ion dose of 1019 cm−2.
Acknowledgement This research work is financially supported by Hong Kong Research Council CERG project 9040195.
References [1] J.E. Field (Ed.), The Properties of Diamond, Academic Press, London, 1979, p. 302. [2] K. Ishibashi, S. Furukawa, Jpn. J. Appl. Lett. 24 (1985) 912. [3] W.A. Yarbrough, R. Messier, Science 247 (1990) 688. [4] J. Yang, X. Su, Q. Chen, Z. Lin, Appl. Phys. Lett. 66 (1995) 3284. [5] S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 1036. [6 ] X. Jiang, C.P. Klages, Diamond Relat. Mater. 2 (1992) 1112. [7] B.R. Stoner, J.T. Glass, Appl. Phys. Lett. 60 (1992) 698. [8] Y. Shigesato, R.E. Boekenhauer, B.W. Sheldon, Appl. Phys. Lett. 63 (1993) 314. [9] R. Beckmann, B. Sobisch, W. Kulisch, C. Rau, Diamond Relat. Mater. 3 (1994) 555. [10] S.D. Wolter, B.R. Stoner, J.T. Glass, P.J. Ellis, D.S. Buhaenko, C.E. Jenkins, P. Southworth, Appl. Phys. Lett. 62 (1993) 1215. [11] X. Jiang, W.J. Zhang, M. Paul, C.P. Klages, Appl. Phys. Lett. 68 (1996) 1927. [12] S. Sattel, J. Gerber, H. Ehrhardt, Phys. Stat. Sol. (a) 154 (1996) 141. [13] X.S. Sun, PhD thesis, City University of Hong Kong, 1999. [14] P.K. Backmann, R. Weimer, W. Drawl, Y. Liou, R. Messier, Diamond Technology Initiative Symposium, SDIO/IST-ONR, Crystal city, VA, July 12–14, 1988. [15] J. Gerber, S. Sattel, K. Jung, H. Ehrhardt, J. Robterson, Diamond Relat. Mater. 4 (1995) 559.