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Influence on diamond nucleation of the carbon concentration near the substrate surface
Diamond and Related Materials. 2 (1993) 19-23
19
Influence on diamond nucleation of the carbon concentration near the substrate surface D. Michau, B. Tanguy and G. Demazeau Lahoratoire de ('himie du Solide du C N R S , Unit,ersitk Bordeaux 1,351 Cours de la Liberation, 33405 Talence Cedex (France)
M. Couzi and R. Cavagnat Laboratoire de Spectroscopic Molkculaire et Cristalline, UniversiN~de Bordeaux I, 351 Cours de la Libkration, 33405 Talence Cedex ( France /
(Received July 24, 1992: accepted in final form October 5, 1992)
Abstract A study of the influence on diamond nucleation of the carbon concentration in [or near the surface of) the substrate in a hotfilament chemical vapour deposition system is presented. The role of chemically bonded carbon (as carbide) and the role of additional "free" carbon on the surface of the substrate [before diamond deposition) are discussed versus the density of nucleation and the crystal quality of diamond deposited.
1. Introduction For the main applications of diamond films, dense diamond nucleation and a film substrate with good adhesion are required. To promote nucleation, the most useful method is to scratch the substrate with diamond paste. Despite the number of papers describing this procedure, the effect of this pretreatment on the nucleation step is not well known (modification of the surface energy, role of small diamond seeds [1] etc.). Many papers have described some effects of the substrate on nucleation but only through new pretreatment processes (predecarburization [2], laser irradiation I-3], pre-etching with a hydrogen plasma [4], hydrocarbon oil [5]). For the synthesis of diamond thin films, a key question is: "In the thermodynamical conditions where diamond is not stable, what are the best factors able to facilitate the growth of a thin film?" In a first approach "the role of the substrate" could be very important. In fact, the difference in energy AG between the two structural forms of carbon (graphite and diamond) is very weak (0.016 eV) compared with the activation barrier (3.545 eV) [6]. In the case of chemical vapour deposition (CVD), the reactional carbonated species are in the gaseous flux but the substrate, through its physicochemical characteristics, can act to lower this activation barrier and increase the nucleation; then in a second step the growth of a diamond thin film occurs {like a catalysis process). If we consider the nucleation as resulting from an
0925 9635,'93/$6.00
interaction between carbonated gaseous species and the substrate, a diamond nucleus can be induced only if carbon supersaturation is reached locally. This chemical state can occur as a result of different processes: if the local concentration in carbon is higher as the richer carbide form; if the substrate cannot form a carbide, then it is possible to reach the upper limit of carbon solubility under the experimental CVD conditions.
2. Objective of this work The objective of our work was to underline the role of carbon in (or near) the surface layers of the substrate on the diamond nucleation, without pretreatment by diamond paste. Three main substrates were selected: tungsten leading to the two carbides ~-W2C and WC [7]; molybdenum f o r m i n g M o 2 C and under some conditions a richer carbide MoC1 x [8]; copper which has no stable carbide. The diamond depositions were carried out using conventional hot-filament equipment described previously [9]. The experimental conditions were constant for all the following experiments. The substrate and the filament temperature were 850 °C and 2000 °C, the total pressure was maintained at about 50 Torr and the total flow rates near 300 cm 3 min-1. The gas flow used was a mixture of 1% methane in hydrogen and the experiment lasted 4 h. In order to evaluate the effect of carbon in the top layers of the substrate on diamond nucleation, different
,c 1993 - - Elsevier Sequoia. All rights reserved
D. Michau et al. / Influence of C concentration on nucleation
20
processes were developed for inducing carbon supersaturation: use of a precarburized substrate; deposition of graphite onto the substrate.
3. Diamond nucleation on a tungsten-based substrate
When we compare diamond deposition onto metallic and precarburized tungsten (~-W2C) we observe an increase in nucleation density when the carbon is preexistent in the substrate as a carbide phase. The Raman spectra of the two deposits (Fig. 1) show the typical line of diamond at 1334cm -1 (within our resolution of about 3 cm ~, this value is equivalent to the standard value of 1332 cm 1), but for the precarburized substrate, the background underlines the presence of a small amount of Csp2 in the diamond deposit. In order to compare the role of chemically bonded carbon in a carbide phase and that of free carbon on the surface of the substrate at the beginning of nucleation, we used a metallic substrate covered with graphite (using two
L
i
i
I
I
covering processes, the first based on sputtering and the second using a graphite spray). After diamond deposition, comparison with the previous experiments shows an increase in the nucleation density. The differences in crystallite morphology resulting from the two different carbon predepositions could be explained by the greater uniformity of the graphite layer produced by sputtering. In the case of the graphite spray sample (Fig. 2), the diamond crystals show a "cauliflower" morphology, probably induced by improvement in the formation of Csp2. The Raman spectra are characteristic of diamond (Fig. 1) [10] and can be compared with the results of Morrish and Pehrsson [5] obtained under the same conditions. This phenomenon has also been observed when the mixture of reactive gases is too rich in methane. In order to verify the hypothesis of carbon excess, the same experiment was carried out using a graphite disc as substrate. The same type of diamond deposit was found (Fig. 3(a)) and the Raman spectrum (Fig. 3(b)) shows a large band suggesting a mixture of Csp2 and Csp3 [11]. These different results lead to two conclusions. Carbon participating in the surface composition of the substrate plays an important role since the nucleation density is enhanced for the precarburized substrate, compared with the metallic substrate. Free carbon deposited onto the substrate seems to induce a higher density of diamond nucleation.
)
with a graphite spray
with sputtarafl graphite
precarhurized tungsten
metalli= tungsten
I
I 1600
I 1500
I 1400
I 1300
I Wavenumber (cm -I)
Fig. I. Micro-Raman spcctra of diamond dcposited onto tungstenbased substratcs.
Fig. 2. Scanning electron micrograph of diamond nucleation on tungsten covered with a graphite spray.
D. M i c h a u et ,_tl. / l , ttuence
_~
k'
~,/C
2I
c m w e n t r a t i o n or1 m w l e a t i o n
stability domain of MoC1 .,, and near the surface it could promote the nucleation of diamond. Experiments were carried out successively on metallic molybdenum, precarburized molybdenum and molybdenum with a thick layer of graphite (produced by sputtering). Scanning electron microscopy shows that the precarburized molybdenum substrate leads to a denser nucleation (Fig. 4(a)), and the Raman spectrum (Fig. 4(b)) exhibits only a sharp diamond line (1334 cm 11. This result suggests that in a first step, the top layers of the substrate M o , C turn into the richer carbide MoCI ,.: in a second step carbon excess could
'k_
'w
¸
Qo,
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~y :
/
"
%:
/
(al
;(f"7
-f
j-
/ / /
"\ \ \
( 1700
t 1600
15'00
I
1400
I
1300
,
\
1200
"[
-- ' -1
Wavenumber (cm
)
(b)
(a)
Fig. 3. (a) Scanning electron micrograph and (b) micro-Raman spectrum of diamond deposited onto a graphite disc. In all cases, after diamond deposition, X-ray diffraction analysis confirms the formation of tungsten carbides ~-W2C and WC (~-W2C with a hexagonal structure and WC with a hexagonal structure) at the interface between tungsten and the diamond film.
[[ i
!, 4. Diamond substrate
deposition
using
a molybdenum-based
Molybdenum is an interesting substrate since it can form the carbide Mo2C and another carbide with compositional domain MoC1 x [8]. In the latter case, a continuous range of carbon may be available in the
I
I 1600
1500
1400
I 1300
[ W a v e n u m b e r (cm -I
(b)
Fig. 4. (a) Scanning electron micrograph and (b) micro-Raman spectrum of diamond deposited onto a precarburized molybdenum substrate.
22
D. Michau et al. / Influence of C concentration on nucleation
help diamond nucleation, but the higher diffusion coefficient of carbon in molybdenum (10 -11 cm 2 s -1) compared with that in tungsten (10 13 c m 2 S - 1 ) [12] favours diamond nucleation and prevents carbon excess at the surface. After diamond deposition, X-ray diffraction analysis only detects the formation of Mo2C. The experimental differences between tungsten and molybdenum substrates illustrate the carbide competition, the difference in chemical bonding M-C ( M - W , Mo), and also the difference in carbon diffusion in these metals.
5. Use of copper as substrate
In order to specify the role of carbon on the surface, some experiments were carried out on copper, a metal that does not form carbides. Figure 5 shows the density of nucleation respectively on metallic copper (Fig. 5(a)) and on copper with a thick layer of graphite on its surface (Fig. 5(b)). In the second case scanning microscopy shows significant increase in diamond nucleation. This makes clear the role of local supersaturation of the surface in the case of carbon that is not chemically bonded. The main results of these experiments using four different substrates are summarized in Table 1.
(a)
6. Conclusion
In the top layers of substrates, carbon plays an important role via local supersaturation inducing diamond nucleation. Nevertheless, carbon excess favours the "cauliflower" morphology as observed in the gas phase. Comparison of the experimental results with carburized substrates and with thick layers of graphite TABLE 1. Summary of results Substrate
Tungsten Metallic Precarburized Graphite spray Carbon sputtered Graphite disc Molybdenum Metallic Precarburized Carbon sputtered Copper Metallic Carbon sputtered
Nucleation density
Crystal quality
Low Medium High High Very high
Facetted Facetted Ball like Facetted Ball like
Very low Very high Medium
Facetted Facetted Facetted
Medium High
Facetted Facetted
(b) Fig. 5. Diamond nucleation on (a) metallic copper and (b) metallic copper with a thin carbon film at the surface.
D. Michau et al.
InItuence ~!! C concentration on nucleation
suggests that three factors can play an important role: (a) the diffusion rate of carbon in the substrate, (b) the degree of chemical bonding of carbon (M C), (c) the uniformity and thickness of carbon. All these experiments can be compared with the results of other depositions using a thick layer of a-C:H [13] or carbon clusters (C~o and C~o) [14] to promote the nucleation of diamond. The mechanism governing diamond nucleation still needs to be clarified. Three main hypotheses can be given to explain the role of carbon near the substrate surface: a chemical role correlated with the local chemical potential: a structural role because local structures of superficial carbides assist epitaxy; modification of the surface energy or local chemical bonding improving nucleation sites.
Acknowledgments The authors wish to thank Pascal Bradu and the D.R.E.T. (Groupe III) for fruitful scientific discussions and financial support.
23
References I S. lijima, Y. Aikawa and K. Baba, .l. Mater Res., 6(7) (1991) 1491 1497. 2 K. Saijo, M. Yagi, K. Shibuki and S. Takatsu, Surll Coat. Technol., 43 44 11990~ 30 40. 3 J. Narayan, V. P. Godbole, G. Matera and R. K. Singh, J. Appl, Phys., 7l(2) (1992) 966 971. 4 S Nakao, M. Noda. H. Watatani and S. Maruno, .I. Appl. Phys,. 30(7A) 119911 El 195-L1198. 5 A. Morrish and P. E. Pehrsson, Appl. Phys. Lett, 59(4) (1991) 417 419. 6 T. R. Anthony, Vacuum, 4l(4 6) (1990) 1356 I359. 7 M. Soussi El Begrani, Thesis, Universit6 Libre de Bruxelles, 1992. 8 Kacim, Thesis. Universitd Libre de Bruxelles, 1990. 9 K. E. Spear, J. Am. Ceram. Sot.. 72~2) (1989) 171 191. 10 T. J. Dines, D. Tither, A. Dehbi and A. Matthews, Carbon, 29(2) (1991) 225 231. I 1 W. Zhu, C. A. Randall, A. R. Badzian and R. Messier, J. Vac. Sci. Technol. ,t, 7(3) (1989) 2315 2324. 12 B. Lux and R. Haubner, in R. g. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamond like Films and Coatings, (Nato-Asi. Series B. Physics J, Vol. 266, Plenum, New York, 1991, pp. 579-609. 13 J.J. Dubray, C. G. Pantano, M. Meloncelli and E. Bertran, J. Vuc. Sci. Technol. A. 9(6) (1991) 3012 3018. 14 R. J. Meilunas, R. P. H. Chang, S. kiu and M. M. Kappes, Appl. Phys. Lett.,59(26)(1991)3461 3463.
19
Influence on diamond nucleation of the carbon concentration near the substrate surface D. Michau, B. Tanguy and G. Demazeau Lahoratoire de ('himie du Solide du C N R S , Unit,ersitk Bordeaux 1,351 Cours de la Liberation, 33405 Talence Cedex (France)
M. Couzi and R. Cavagnat Laboratoire de Spectroscopic Molkculaire et Cristalline, UniversiN~de Bordeaux I, 351 Cours de la Libkration, 33405 Talence Cedex ( France /
(Received July 24, 1992: accepted in final form October 5, 1992)
Abstract A study of the influence on diamond nucleation of the carbon concentration in [or near the surface of) the substrate in a hotfilament chemical vapour deposition system is presented. The role of chemically bonded carbon (as carbide) and the role of additional "free" carbon on the surface of the substrate [before diamond deposition) are discussed versus the density of nucleation and the crystal quality of diamond deposited.
1. Introduction For the main applications of diamond films, dense diamond nucleation and a film substrate with good adhesion are required. To promote nucleation, the most useful method is to scratch the substrate with diamond paste. Despite the number of papers describing this procedure, the effect of this pretreatment on the nucleation step is not well known (modification of the surface energy, role of small diamond seeds [1] etc.). Many papers have described some effects of the substrate on nucleation but only through new pretreatment processes (predecarburization [2], laser irradiation I-3], pre-etching with a hydrogen plasma [4], hydrocarbon oil [5]). For the synthesis of diamond thin films, a key question is: "In the thermodynamical conditions where diamond is not stable, what are the best factors able to facilitate the growth of a thin film?" In a first approach "the role of the substrate" could be very important. In fact, the difference in energy AG between the two structural forms of carbon (graphite and diamond) is very weak (0.016 eV) compared with the activation barrier (3.545 eV) [6]. In the case of chemical vapour deposition (CVD), the reactional carbonated species are in the gaseous flux but the substrate, through its physicochemical characteristics, can act to lower this activation barrier and increase the nucleation; then in a second step the growth of a diamond thin film occurs {like a catalysis process). If we consider the nucleation as resulting from an
0925 9635,'93/$6.00
interaction between carbonated gaseous species and the substrate, a diamond nucleus can be induced only if carbon supersaturation is reached locally. This chemical state can occur as a result of different processes: if the local concentration in carbon is higher as the richer carbide form; if the substrate cannot form a carbide, then it is possible to reach the upper limit of carbon solubility under the experimental CVD conditions.
2. Objective of this work The objective of our work was to underline the role of carbon in (or near) the surface layers of the substrate on the diamond nucleation, without pretreatment by diamond paste. Three main substrates were selected: tungsten leading to the two carbides ~-W2C and WC [7]; molybdenum f o r m i n g M o 2 C and under some conditions a richer carbide MoC1 x [8]; copper which has no stable carbide. The diamond depositions were carried out using conventional hot-filament equipment described previously [9]. The experimental conditions were constant for all the following experiments. The substrate and the filament temperature were 850 °C and 2000 °C, the total pressure was maintained at about 50 Torr and the total flow rates near 300 cm 3 min-1. The gas flow used was a mixture of 1% methane in hydrogen and the experiment lasted 4 h. In order to evaluate the effect of carbon in the top layers of the substrate on diamond nucleation, different
,c 1993 - - Elsevier Sequoia. All rights reserved
D. Michau et al. / Influence of C concentration on nucleation
20
processes were developed for inducing carbon supersaturation: use of a precarburized substrate; deposition of graphite onto the substrate.
3. Diamond nucleation on a tungsten-based substrate
When we compare diamond deposition onto metallic and precarburized tungsten (~-W2C) we observe an increase in nucleation density when the carbon is preexistent in the substrate as a carbide phase. The Raman spectra of the two deposits (Fig. 1) show the typical line of diamond at 1334cm -1 (within our resolution of about 3 cm ~, this value is equivalent to the standard value of 1332 cm 1), but for the precarburized substrate, the background underlines the presence of a small amount of Csp2 in the diamond deposit. In order to compare the role of chemically bonded carbon in a carbide phase and that of free carbon on the surface of the substrate at the beginning of nucleation, we used a metallic substrate covered with graphite (using two
L
i
i
I
I
covering processes, the first based on sputtering and the second using a graphite spray). After diamond deposition, comparison with the previous experiments shows an increase in the nucleation density. The differences in crystallite morphology resulting from the two different carbon predepositions could be explained by the greater uniformity of the graphite layer produced by sputtering. In the case of the graphite spray sample (Fig. 2), the diamond crystals show a "cauliflower" morphology, probably induced by improvement in the formation of Csp2. The Raman spectra are characteristic of diamond (Fig. 1) [10] and can be compared with the results of Morrish and Pehrsson [5] obtained under the same conditions. This phenomenon has also been observed when the mixture of reactive gases is too rich in methane. In order to verify the hypothesis of carbon excess, the same experiment was carried out using a graphite disc as substrate. The same type of diamond deposit was found (Fig. 3(a)) and the Raman spectrum (Fig. 3(b)) shows a large band suggesting a mixture of Csp2 and Csp3 [11]. These different results lead to two conclusions. Carbon participating in the surface composition of the substrate plays an important role since the nucleation density is enhanced for the precarburized substrate, compared with the metallic substrate. Free carbon deposited onto the substrate seems to induce a higher density of diamond nucleation.
)
with a graphite spray
with sputtarafl graphite
precarhurized tungsten
metalli= tungsten
I
I 1600
I 1500
I 1400
I 1300
I Wavenumber (cm -I)
Fig. I. Micro-Raman spcctra of diamond dcposited onto tungstenbased substratcs.
Fig. 2. Scanning electron micrograph of diamond nucleation on tungsten covered with a graphite spray.
D. M i c h a u et ,_tl. / l , ttuence
_~
k'
~,/C
2I
c m w e n t r a t i o n or1 m w l e a t i o n
stability domain of MoC1 .,, and near the surface it could promote the nucleation of diamond. Experiments were carried out successively on metallic molybdenum, precarburized molybdenum and molybdenum with a thick layer of graphite (produced by sputtering). Scanning electron microscopy shows that the precarburized molybdenum substrate leads to a denser nucleation (Fig. 4(a)), and the Raman spectrum (Fig. 4(b)) exhibits only a sharp diamond line (1334 cm 11. This result suggests that in a first step, the top layers of the substrate M o , C turn into the richer carbide MoCI ,.: in a second step carbon excess could
'k_
'w
¸
Qo,
>
~y :
/
"
%:
/
(al
;(f"7
-f
j-
/ / /
"\ \ \
( 1700
t 1600
15'00
I
1400
I
1300
,
\
1200
"[
-- ' -1
Wavenumber (cm
)
(b)
(a)
Fig. 3. (a) Scanning electron micrograph and (b) micro-Raman spectrum of diamond deposited onto a graphite disc. In all cases, after diamond deposition, X-ray diffraction analysis confirms the formation of tungsten carbides ~-W2C and WC (~-W2C with a hexagonal structure and WC with a hexagonal structure) at the interface between tungsten and the diamond film.
[[ i
!, 4. Diamond substrate
deposition
using
a molybdenum-based
Molybdenum is an interesting substrate since it can form the carbide Mo2C and another carbide with compositional domain MoC1 x [8]. In the latter case, a continuous range of carbon may be available in the
I
I 1600
1500
1400
I 1300
[ W a v e n u m b e r (cm -I
(b)
Fig. 4. (a) Scanning electron micrograph and (b) micro-Raman spectrum of diamond deposited onto a precarburized molybdenum substrate.
22
D. Michau et al. / Influence of C concentration on nucleation
help diamond nucleation, but the higher diffusion coefficient of carbon in molybdenum (10 -11 cm 2 s -1) compared with that in tungsten (10 13 c m 2 S - 1 ) [12] favours diamond nucleation and prevents carbon excess at the surface. After diamond deposition, X-ray diffraction analysis only detects the formation of Mo2C. The experimental differences between tungsten and molybdenum substrates illustrate the carbide competition, the difference in chemical bonding M-C ( M - W , Mo), and also the difference in carbon diffusion in these metals.
5. Use of copper as substrate
In order to specify the role of carbon on the surface, some experiments were carried out on copper, a metal that does not form carbides. Figure 5 shows the density of nucleation respectively on metallic copper (Fig. 5(a)) and on copper with a thick layer of graphite on its surface (Fig. 5(b)). In the second case scanning microscopy shows significant increase in diamond nucleation. This makes clear the role of local supersaturation of the surface in the case of carbon that is not chemically bonded. The main results of these experiments using four different substrates are summarized in Table 1.
(a)
6. Conclusion
In the top layers of substrates, carbon plays an important role via local supersaturation inducing diamond nucleation. Nevertheless, carbon excess favours the "cauliflower" morphology as observed in the gas phase. Comparison of the experimental results with carburized substrates and with thick layers of graphite TABLE 1. Summary of results Substrate
Tungsten Metallic Precarburized Graphite spray Carbon sputtered Graphite disc Molybdenum Metallic Precarburized Carbon sputtered Copper Metallic Carbon sputtered
Nucleation density
Crystal quality
Low Medium High High Very high
Facetted Facetted Ball like Facetted Ball like
Very low Very high Medium
Facetted Facetted Facetted
Medium High
Facetted Facetted
(b) Fig. 5. Diamond nucleation on (a) metallic copper and (b) metallic copper with a thin carbon film at the surface.
D. Michau et al.
InItuence ~!! C concentration on nucleation
suggests that three factors can play an important role: (a) the diffusion rate of carbon in the substrate, (b) the degree of chemical bonding of carbon (M C), (c) the uniformity and thickness of carbon. All these experiments can be compared with the results of other depositions using a thick layer of a-C:H [13] or carbon clusters (C~o and C~o) [14] to promote the nucleation of diamond. The mechanism governing diamond nucleation still needs to be clarified. Three main hypotheses can be given to explain the role of carbon near the substrate surface: a chemical role correlated with the local chemical potential: a structural role because local structures of superficial carbides assist epitaxy; modification of the surface energy or local chemical bonding improving nucleation sites.
Acknowledgments The authors wish to thank Pascal Bradu and the D.R.E.T. (Groupe III) for fruitful scientific discussions and financial support.
23
References I S. lijima, Y. Aikawa and K. Baba, .l. Mater Res., 6(7) (1991) 1491 1497. 2 K. Saijo, M. Yagi, K. Shibuki and S. Takatsu, Surll Coat. Technol., 43 44 11990~ 30 40. 3 J. Narayan, V. P. Godbole, G. Matera and R. K. Singh, J. Appl, Phys., 7l(2) (1992) 966 971. 4 S Nakao, M. Noda. H. Watatani and S. Maruno, .I. Appl. Phys,. 30(7A) 119911 El 195-L1198. 5 A. Morrish and P. E. Pehrsson, Appl. Phys. Lett, 59(4) (1991) 417 419. 6 T. R. Anthony, Vacuum, 4l(4 6) (1990) 1356 I359. 7 M. Soussi El Begrani, Thesis, Universit6 Libre de Bruxelles, 1992. 8 Kacim, Thesis. Universitd Libre de Bruxelles, 1990. 9 K. E. Spear, J. Am. Ceram. Sot.. 72~2) (1989) 171 191. 10 T. J. Dines, D. Tither, A. Dehbi and A. Matthews, Carbon, 29(2) (1991) 225 231. I 1 W. Zhu, C. A. Randall, A. R. Badzian and R. Messier, J. Vac. Sci. Technol. ,t, 7(3) (1989) 2315 2324. 12 B. Lux and R. Haubner, in R. g. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamond like Films and Coatings, (Nato-Asi. Series B. Physics J, Vol. 266, Plenum, New York, 1991, pp. 579-609. 13 J.J. Dubray, C. G. Pantano, M. Meloncelli and E. Bertran, J. Vuc. Sci. Technol. A. 9(6) (1991) 3012 3018. 14 R. J. Meilunas, R. P. H. Chang, S. kiu and M. M. Kappes, Appl. Phys. Lett.,59(26)(1991)3461 3463.