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Combined effect of nitrogen and pulsed microwave plasma on diamond growth
ELSEVIER
DIAMOND AND RE T|D TER|AL$
Diamond and Related M ateriais 6 ( 1997 ) 505- 510
Combined effect of nitrogen and pulsed microwave plasma on diamond growth H a s s a n C h a t e i a.,.~, J a m a l B o u g d i r a a, M i c h e l R e m y ", P a t r i c k A l n o t J a n K. K r t i g e r b
a,
C h r i s t i a n B r u c h b,
a Laboratoire de Physique des Milieux lonis~s (CNRS URA 835), Universitk Henri Pohwar~, Nano, L BP 239, F-54506 Vandoeuvre les Nancy, Cede.,:, France b Fachrichtung Experimentalphysik 10.2, Universitdt ties Saarlandes, Bau 38, Postfach 151150, D-66041 Saarbracken, Germany
Abstract Deposition of diamond layers from a CH4-H 2 microwave discharge operating in pulse mode was achieved. The resulting diamond layers showed less graphite contamination as compared with that obtained in continuous discharge. Furthermore the addition of nitrogen to the discharge operating in this pulse mode was shown to further decrease the graphite contamination and to improve crystallisation due to reduction of secondary germination. However using nitrogen in the gas mixture resulted also in a decrease in nucleation density and in discontinuous formation of diamond layers. © 1997 Elsevier Science S.A.
Keywords: Diamond; Nitrogen; Raman spectroscopy; Pulsed microwave discharge
1. Introduction Since 1983 microwave discharges have been used [1] for the deposition of diamond films by plasma-assisted chemical vapour deposition (MPACVD). The basic gas mixture consists of methane and hydrogen. Usually a continuous wave (cw)-source is applied to generate the plasma from the gas having a pressure below the atmospheric one and a temperature below 1270 K. In order to ameliorate the diamond quality, much effort is made to improve (i) the plasma composition and (ii) the reactor design including its working conditions. Because of its high reactivity, supersaturated atomic hydrogen plays a prominent role for the MPACVD deposition of diamond. The dominant effect of atomic hydrogen results from its etching properties for different species of carbon [2]. It turns out that the etching rate for graphite is about 100 times larger than that for diamond, resulting in a preferential etching of graphite during the deposition process. This yields an elegant method to improve the diamond contribution to the whole deposited film. Consequently, many attempts are
* Corresponding author. tOn leave from LPTP, Facult6 des Sciences. Universite Mohamed 1, Oujda, Marocco. 0925-9635/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-9635 (96 }00656-5
made to optimise the rate of hydrogen and/or to find alternative gas mixtures in order to amplify the effect of atomic hydrogen. Bou [3] has pointed out that the production of high quality diamond films in C-H systems mainly stems from the high density of high-energy electrons within the plasma and a low concentration of hydrocarbon in the gas inlet. Both conditions favour the production of atomic hydrogen and the preferential formation of CHx (x <4) radicals. The increase of the atomic hydrogen rate can be correlated to that of the concentration of carbon-hydrogen radicals such as CHx which possibly are precursors of CVD-diamond. As a matter of fact the addition of easily ionised rare gases to the discharge is known to increase the density of electrons and the concentration of H and CHx, (x<4). Thomas [4] has shown that the atomic hydrogen production reaches it maximum for a ratio [H2]/[Ar] = 1.5 in the gas inlet. The effect of nitrogen on the diamond film properties has been investigated by several authors [5,6]. A comparative study of the chemical reactivity of the plasma on one hand and its preferential etching influence on the other has been discussed recently [7] for the discharge of H2-CH4-N2 gas mixtures. As mentioned above, the quality of the deposition process for diamond
506
H. Chatei et aL / Diamond and Related Materials 6 (1997) 505-510
also can be influenced by changing the working parameters. The aim of this article is to present some new studies of the effect of pulsed microwave excitation of the MPACVD-reactor. In this context we compare, for three different experimental conditions, the plasma characteristics and the properties of the related diamond films for three different experimental conditions: cw-microwave discharge in a H 2 - C H 4 gas mixture, pulsed microwave discharge in the same gas, pulsed microwave discharge in a H2-CH4-N2 gas mixture.
2. Experimental
Electrically pulsed MPACVD has been used to produce an intermittent plasma. The pulse signal had a rectangular shape, switching periodically between zero and a maximum value. It is worth noting that the average temperature of the plasma depends on the applied time-averaged power. Similar studies have already been carried out for the deposition of diamond in a pulsed electron cyclotron resonance plasma reactor [8] and in a pulsed microwave discharge, activated by a magnetic field [9]. Hatta et al. [8] reported an increase of the diamond growth rate when carrying out a pulsed discharge with a modulation frequency of 500 Hz and a duty cycle l:l. We have limited our investigations to these values, previously optimised. The experiments have been carried out in a microwave plasma reactor working at 2.45 GHz either in continuous or pulsed mode. The reaction chamber consists of a quartz tube having a diameter of 50 mm. Details are given in Ref. [7]. The deposition process is carried out on p-Si (100) substrates ultrasonically seeded with l-~tm diamond powder. For the preparation of the three samples in question we applied the following common experimental conditions: total pressure, 60 Torr; substrate temperature, 900°C; growth duration, 4 h. The experimental conditions deviating for the three samples are presented in Table I. The average microwave power was adjusted in order to achieve the same substrate temperature for all samples.
3. Results and discussions
The properties of the plasma for the three differe~lt working conditions are characterised by optical emissic, n spectroscopy (OES). At this stage of our investigatio1~s we have not carried out time-resolved OES, thus, the measured line intensities are time averaged. However the relations between the relative intensities of the emission lines for the averaged power (experiments 2 and 3) are comparable to those being obtained at a continuous illumination of the plasma (experiment 1). The optical emission spectra obtained for the three experiments cited above are shown in Fig. l(a-c). The main spectral lines identified in the H 2 - C H 4 discharge during the continuous deposition of sample no. 1 (Fig. 1(a)) are as follows: the CH system (390.0 and 431.4 nm), the C2 Swan band (516.5 nm), the Balmer series of atomic hydrogen (H~ at 656.3 nm, Hp at 486.1 nm, H7 at 434.1 nm) and several emission lines from electronically excited molecular hydrogen (centred at 463 and 581 nm). Fig. 1(b) shows a spectrum for the same composition of the gas mixture as before but recorded during a pulsed discharge (sample no. 2). Compared to Fig. 1(a), the emission lines of atomic hydrogen of sample no. 2 (Fig. 1(b)) obviously have a higher intensity in relation to those of molecular hydrogen. Moreover the intensity ratio I(H)/I(H2) increases but I(CH)/I(H) remains 12
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400
450
500
550
600
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Wavelength(nm) Fig. 1. Typical emission spectra in the range 300-700 n m f r o m a m i c r o wave p l a s m a used for the g r o w t h of: (a) sample i, ( b ) s a m p l e 2, (c) sample 3. O p e r a t i n g c o n d i t i o n s o f the discharge are listed in Table 1.
Table 1 Sample no.
Excitation m o d e
Power p e a k / a v e r a g e d ( W )
Gaseous mixture
l 2 3
Continuous Pulsed Pulsed
300 544/272 544/272
0.4% C H 4 - 9 9 . 6 % H 2 0.4% CH4--99.6% H 2 0.36% N 2 - 0 . 4 % C H 4 - 9 9 . 2 4 % H 2
H. Chatei et al. / Diamond and Related Materials 6 (1997) 505-510
almost constant, a possible explanation could be that a high peak-power promotes the production of atomic hydrogen as well as of radicals such as CH, which in turn, can be considered as precursors for diamond growth [3]. As a result, we expect an increasing growth rate during the film depostion and an increasing content of diamond within the film. However, the above explanations cannot be considered to be exclusive since the emission line intensities can also be related to an increase in excitation cross-sections or electron density. These predictions have been verified by the examination of samples 1 and 2 using scanning electron microscopy (SEM) (Fig. 2(a,b)) as well as micro-Raman spectroscopy ( Fig. 3 (a,b)). The SEM investigations give clear-cut information about the surface topography of the deposited films. Fig. 2(a) clearly shows triangular ( 111 ) facets. However, due to hetero-germination each (l 11 ) facet is partially covered by disordered small crystals. The general topography of sample 2 (Fig. 2(b)) is similar to that of sample l, but dominated by i l l l) facets which are almost parallel to the film surface. Moreover, the grain size is larger and hetero-nucleation has almost disappeared. The increase of the crystal size is linked to a decrease of the nucleation density and/or to a serious reduction of secondary germination. The SEM investigations may be compared with those of our micro-Raman studies. Fig. 3(a) shows the spectrum of sample I. It is worth noting that this spectrum looks very cumbersome. Comparing our result with data from the literature, we give a tentative assignment of the peaks. From Fig. 3(a), we report here each peak position and corresponding full-width at halfmaximum ( F W H M ) both expressed in cm-1:519.8 and 4.1 (first-order Si Raman peak), 719.1 and 41.3 (unknown), 958.2 and 49.5 (second-order Si Raman peak), 1147.1 and 29.2 (nanocrystalline diamond particle) [10,11], 1325.1 and 300 ("D" carbon peak), 1330.9 and 13.7 (diamond peak), 1376.1 and 25.5 (unknown), 1490.2 and 26.9 (diamond precursor) [12], 1524.6 and 245.7 ("G" peak carbon), 1717.4 and 73.0 (unknown), 1939.1 and 326 (unknown), 2358.9 and 18.1 (unknown). In addition to the usual spectral features of graphite, silicon and diamond, we found peaks at 1147 and 1490 cm -1, which have been attributed in the literature [10,11] to the appearance of nanocrystalline diamond or of diamond precursor. The 1490-cm- I peak has been attributed to C6o fullerene molecules [ 12]. The effect of a pulsed excitation of the plasma on the micro-Raman spectra (sample 2) is shown in Fig. 3(b). Compared to Fig. 3(a) this spectrum is much more clear-cut, since a lot of peaks have disappeared. Apart from the peaks of silicon and diamond, the remaining peaks are difficult to assign. In this context it is interesting to note that, for this case, we had no evidence for the spectral line indicative for nano-particles of dia-
507
mond. On the other hand, the peak at 1518 cm -i attributed to the presence of the precursor state of diamond, is still present within this spectrum. Adding a small amount of nitrogen to the gas composition of sample 2, but using otherwise the same experimental parameters, we obtained sample 3. The micro-Raman spectrum (Fig. 3(c)) looks again as simple as Fig. 2(b) and consists of only five peaks, which could be attributed to silicon, diamond, graphite (the small one at 1500cm -~) and an unknown peak at 205 2 c m - t. In order to evaluate the concentration of diamond within the samples, we introduce a quality factor R=
sp
t. +t, p3+ k
defined elsewhere [7]. This factor takes into account spurious intensities Ik at the position of the diamond line which originate from adjacent Raman lines within the spectrum. Values of R close to 1 give a clear indication for a high degree of diamond. The different values of this quality factor R, together with the respective FWHM of the diamond peak are: (a) 0.25 and 13.7cm -t for sample 1; (b) 0.57 and 7.2cm -t for sample 2; (c) 0.68 and 6.7 cm -~ for sample 3. From these results we conclude that the use of a pulsed discharge improves the diamond to graphite balance. In the case of sample 3, we have tested the effect of nitrogen enrichment of the gas inlet on the plasma properties during the pulsed discharge. The corresponding emission spectrum is shown in Fig. 1(c). In addition to the emission lines already observed in Fig. 1(a,b), the emission lines of nitrogen and of nitrogen containing radicals appear. Particularly, the second positive system of molecular nitrogen, a line of the NH radical (336 nm). and the very intense violet band of the CN radical are present in the spectrum. It turns out that addition of nitrogen to the parent gas within the reactor reduces all emission lines (H2, H, CH), but not that of species containing nitrogen. However the intensity ratios I( Ha)//( H2), I(H~.)/I(H2) and I(CH)/I(H) are slightly higher than in experiment no. 2, the ratio I(H~)/I(H2) being still smaller. Thus an effect of the nitrogen on the dissociation of the hydrogen molecules and on the average energy of the free electrons may be anticipated, but not proved without complementary measurements. The production of CN can happen directly in the gas phase, or can be due to the etching of the deposited film. In both cases the CN radical results from the interaction of atomic nitrogen or of nitrogen-containing species with carbon-containing radicals of the gas phase, or with carbon deposited on the substrate. These reactions can produce directly CN radicals or C - N bonds
508
H. Chatei et al. / Diamond and Related Materials 6 (1997) 505-510
~) Fig. 2. SEM micrographs of diamond films corresponding to: (a) sample 1, (b) sample 2, (c) sample 3.
H. Chatei et al. / Diamond and Related Materials 6 (1997) 505-510 8000
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509
nucleation density thus enlarging the size of the grains. These phenomena have already been established in a permanent discharge [7]. In fact the SEM picture (Fig. 2(c)) clearly shows large isolated particles with a cubo-octahedral shape, different from those observed in Fig. 2(a,b). This is due to a modification of the growth rate of the (100) and ( I l l ) facets. It is possible to quantify these modifications by the growth parameter ~=3~/'3(Vloo/Vlll) previously defined by Wild et al. [13] where V,oo and V111 are the growth rates of the (100) and (I I I) facets,respectively. In our case l <~<1.5, I/'ioois thus smaller than VII, indicating the selective reactivityof the different facets with regard to the adsorbed species.
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It was shown that operating the microwave plasma in the pulse mode at 500 Hz resulted in an enhancement of the production of atomic hydrogen inducing an improvement of diamond-to-graphite balance. The addition of nitrogen to the pulse-modulated plasma further improved the deposited diamond crystallites, reducing graphite contamination and the secondary germination. This fact occurs in parallel with reduction of nucleation density thought to be due to the etching of nanodiamond particles. In order to explain our results concerning diamond growth process in the pulsed microwave discharge, time-resolved optical measurements would have to be performed to probe the active species in the gas phase.
_~ 150oo Acknowledgement
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10000
The authors are grateful to Prof. Dr. H.-G. Unruh, who made the micro-Raman studies possible, and to J.M. Claude, A. Kohler, and J. Joffran for the technical support in scanning electron microscopy. One of us would like to thank Prof. J. Derkaoui for his support.
5000
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.
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.
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Fig. 3. Raman spectra of a deposited diamond film corresponding to: (a) sample l, (b) sample 2, (c) sample 3.
containing compounds, which can further dissociate to produce CN. CN is produced in the gas from carbonated species containing either C-H bonds or C-C bonds. The former can be considered as diamond deposition precursors, the second as responsible for graphite deposition [3]. It can thus be speculated that the introduction of nitrogen in the pulsed discharge drastically reduces the growth rate of both diamond and graphite, and decreases the
References [1] M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Crystal. Growth, 62 (1983) 642. [2] N. Setaka, J. Mater. Res., 4 (1989) 664. [3] P. Bou, L. Vandenbulke and R. Herbin, Diamond Relat. Mater., 1 (1992) 933. [4] L. Thomas, M.J. Cinelli, J.L. Jauberteau, J. Aubreton and A. Catherinot, Diamond Relat. Mater., 3 (1994) 560. [5] T.M. Hong, S.H. Chen, Y.S. Chiou and C.F. Chen, Appi. Phys. Lett., 67 (1995) 2149-2151. [6] R. Locher, C. Wild, N. Herres, D. Behrand and P. Koidi, Appl. Phrs. Lett., 65 (1994) 34. [7] H." Chatei, J. Bougdira, M. Remy, P. Alnot, C. Bruch and J.K. Kruger, Diamond Relat. Mater., 6 (1997) 107-119.
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H. Chatei et aL / DhonopM and Related Materials 6 (1997) 505-510
[8] A. Hatta, K. Kadota, Y. Mori, T. Ito, T. Sasaki, A. Hiraki and S. Okada, AppL Phys. Lett., 66 (1995) 1602-1604. [9] Z. Ring, T.D. Mantei, S. Tiali and H.E. Jackson, Appl Phys. Lett., 66 (1995) 3380-3382. [10] E.D. Obraztsova, K.G. Korotushenko, S.M. Pimeno~, V.G. Ralchenko, A.A. Smolin, V.i. Konov and E.N. Loubnin. Nanostruct. Mater., 6 (1995) 827-830.
[!1] W.N. Wang, N.A. Fox, P.W. May, M.P. Knapper, G. Meaden, P.G. Partridge, M.N.R. Ashfold, J.W. Steeds, I.P. Haywardand G.D. Pitt, Phys. Stat. Solidi A: Appl. Res., 154 (1996) 255-268. [12] P.V. Huong, J. Mol. Struct., 292 (1993) 81-87. [13] C. Wild, P. Koidl, W. Mtiller-Sebert, R. Kohl, N. Herres, R. Locher and R. Samlenski, Diamond Relat. Mater., 2 (1993;) 158.
DIAMOND AND RE T|D TER|AL$
Diamond and Related M ateriais 6 ( 1997 ) 505- 510
Combined effect of nitrogen and pulsed microwave plasma on diamond growth H a s s a n C h a t e i a.,.~, J a m a l B o u g d i r a a, M i c h e l R e m y ", P a t r i c k A l n o t J a n K. K r t i g e r b
a,
C h r i s t i a n B r u c h b,
a Laboratoire de Physique des Milieux lonis~s (CNRS URA 835), Universitk Henri Pohwar~, Nano, L BP 239, F-54506 Vandoeuvre les Nancy, Cede.,:, France b Fachrichtung Experimentalphysik 10.2, Universitdt ties Saarlandes, Bau 38, Postfach 151150, D-66041 Saarbracken, Germany
Abstract Deposition of diamond layers from a CH4-H 2 microwave discharge operating in pulse mode was achieved. The resulting diamond layers showed less graphite contamination as compared with that obtained in continuous discharge. Furthermore the addition of nitrogen to the discharge operating in this pulse mode was shown to further decrease the graphite contamination and to improve crystallisation due to reduction of secondary germination. However using nitrogen in the gas mixture resulted also in a decrease in nucleation density and in discontinuous formation of diamond layers. © 1997 Elsevier Science S.A.
Keywords: Diamond; Nitrogen; Raman spectroscopy; Pulsed microwave discharge
1. Introduction Since 1983 microwave discharges have been used [1] for the deposition of diamond films by plasma-assisted chemical vapour deposition (MPACVD). The basic gas mixture consists of methane and hydrogen. Usually a continuous wave (cw)-source is applied to generate the plasma from the gas having a pressure below the atmospheric one and a temperature below 1270 K. In order to ameliorate the diamond quality, much effort is made to improve (i) the plasma composition and (ii) the reactor design including its working conditions. Because of its high reactivity, supersaturated atomic hydrogen plays a prominent role for the MPACVD deposition of diamond. The dominant effect of atomic hydrogen results from its etching properties for different species of carbon [2]. It turns out that the etching rate for graphite is about 100 times larger than that for diamond, resulting in a preferential etching of graphite during the deposition process. This yields an elegant method to improve the diamond contribution to the whole deposited film. Consequently, many attempts are
* Corresponding author. tOn leave from LPTP, Facult6 des Sciences. Universite Mohamed 1, Oujda, Marocco. 0925-9635/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-9635 (96 }00656-5
made to optimise the rate of hydrogen and/or to find alternative gas mixtures in order to amplify the effect of atomic hydrogen. Bou [3] has pointed out that the production of high quality diamond films in C-H systems mainly stems from the high density of high-energy electrons within the plasma and a low concentration of hydrocarbon in the gas inlet. Both conditions favour the production of atomic hydrogen and the preferential formation of CHx (x <4) radicals. The increase of the atomic hydrogen rate can be correlated to that of the concentration of carbon-hydrogen radicals such as CHx which possibly are precursors of CVD-diamond. As a matter of fact the addition of easily ionised rare gases to the discharge is known to increase the density of electrons and the concentration of H and CHx, (x<4). Thomas [4] has shown that the atomic hydrogen production reaches it maximum for a ratio [H2]/[Ar] = 1.5 in the gas inlet. The effect of nitrogen on the diamond film properties has been investigated by several authors [5,6]. A comparative study of the chemical reactivity of the plasma on one hand and its preferential etching influence on the other has been discussed recently [7] for the discharge of H2-CH4-N2 gas mixtures. As mentioned above, the quality of the deposition process for diamond
506
H. Chatei et aL / Diamond and Related Materials 6 (1997) 505-510
also can be influenced by changing the working parameters. The aim of this article is to present some new studies of the effect of pulsed microwave excitation of the MPACVD-reactor. In this context we compare, for three different experimental conditions, the plasma characteristics and the properties of the related diamond films for three different experimental conditions: cw-microwave discharge in a H 2 - C H 4 gas mixture, pulsed microwave discharge in the same gas, pulsed microwave discharge in a H2-CH4-N2 gas mixture.
2. Experimental
Electrically pulsed MPACVD has been used to produce an intermittent plasma. The pulse signal had a rectangular shape, switching periodically between zero and a maximum value. It is worth noting that the average temperature of the plasma depends on the applied time-averaged power. Similar studies have already been carried out for the deposition of diamond in a pulsed electron cyclotron resonance plasma reactor [8] and in a pulsed microwave discharge, activated by a magnetic field [9]. Hatta et al. [8] reported an increase of the diamond growth rate when carrying out a pulsed discharge with a modulation frequency of 500 Hz and a duty cycle l:l. We have limited our investigations to these values, previously optimised. The experiments have been carried out in a microwave plasma reactor working at 2.45 GHz either in continuous or pulsed mode. The reaction chamber consists of a quartz tube having a diameter of 50 mm. Details are given in Ref. [7]. The deposition process is carried out on p-Si (100) substrates ultrasonically seeded with l-~tm diamond powder. For the preparation of the three samples in question we applied the following common experimental conditions: total pressure, 60 Torr; substrate temperature, 900°C; growth duration, 4 h. The experimental conditions deviating for the three samples are presented in Table I. The average microwave power was adjusted in order to achieve the same substrate temperature for all samples.
3. Results and discussions
The properties of the plasma for the three differe~lt working conditions are characterised by optical emissic, n spectroscopy (OES). At this stage of our investigatio1~s we have not carried out time-resolved OES, thus, the measured line intensities are time averaged. However the relations between the relative intensities of the emission lines for the averaged power (experiments 2 and 3) are comparable to those being obtained at a continuous illumination of the plasma (experiment 1). The optical emission spectra obtained for the three experiments cited above are shown in Fig. l(a-c). The main spectral lines identified in the H 2 - C H 4 discharge during the continuous deposition of sample no. 1 (Fig. 1(a)) are as follows: the CH system (390.0 and 431.4 nm), the C2 Swan band (516.5 nm), the Balmer series of atomic hydrogen (H~ at 656.3 nm, Hp at 486.1 nm, H7 at 434.1 nm) and several emission lines from electronically excited molecular hydrogen (centred at 463 and 581 nm). Fig. 1(b) shows a spectrum for the same composition of the gas mixture as before but recorded during a pulsed discharge (sample no. 2). Compared to Fig. 1(a), the emission lines of atomic hydrogen of sample no. 2 (Fig. 1(b)) obviously have a higher intensity in relation to those of molecular hydrogen. Moreover the intensity ratio I(H)/I(H2) increases but I(CH)/I(H) remains 12
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.......
350
400
450
500
550
600
650
(a). 700
Wavelength(nm) Fig. 1. Typical emission spectra in the range 300-700 n m f r o m a m i c r o wave p l a s m a used for the g r o w t h of: (a) sample i, ( b ) s a m p l e 2, (c) sample 3. O p e r a t i n g c o n d i t i o n s o f the discharge are listed in Table 1.
Table 1 Sample no.
Excitation m o d e
Power p e a k / a v e r a g e d ( W )
Gaseous mixture
l 2 3
Continuous Pulsed Pulsed
300 544/272 544/272
0.4% C H 4 - 9 9 . 6 % H 2 0.4% CH4--99.6% H 2 0.36% N 2 - 0 . 4 % C H 4 - 9 9 . 2 4 % H 2
H. Chatei et al. / Diamond and Related Materials 6 (1997) 505-510
almost constant, a possible explanation could be that a high peak-power promotes the production of atomic hydrogen as well as of radicals such as CH, which in turn, can be considered as precursors for diamond growth [3]. As a result, we expect an increasing growth rate during the film depostion and an increasing content of diamond within the film. However, the above explanations cannot be considered to be exclusive since the emission line intensities can also be related to an increase in excitation cross-sections or electron density. These predictions have been verified by the examination of samples 1 and 2 using scanning electron microscopy (SEM) (Fig. 2(a,b)) as well as micro-Raman spectroscopy ( Fig. 3 (a,b)). The SEM investigations give clear-cut information about the surface topography of the deposited films. Fig. 2(a) clearly shows triangular ( 111 ) facets. However, due to hetero-germination each (l 11 ) facet is partially covered by disordered small crystals. The general topography of sample 2 (Fig. 2(b)) is similar to that of sample l, but dominated by i l l l) facets which are almost parallel to the film surface. Moreover, the grain size is larger and hetero-nucleation has almost disappeared. The increase of the crystal size is linked to a decrease of the nucleation density and/or to a serious reduction of secondary germination. The SEM investigations may be compared with those of our micro-Raman studies. Fig. 3(a) shows the spectrum of sample I. It is worth noting that this spectrum looks very cumbersome. Comparing our result with data from the literature, we give a tentative assignment of the peaks. From Fig. 3(a), we report here each peak position and corresponding full-width at halfmaximum ( F W H M ) both expressed in cm-1:519.8 and 4.1 (first-order Si Raman peak), 719.1 and 41.3 (unknown), 958.2 and 49.5 (second-order Si Raman peak), 1147.1 and 29.2 (nanocrystalline diamond particle) [10,11], 1325.1 and 300 ("D" carbon peak), 1330.9 and 13.7 (diamond peak), 1376.1 and 25.5 (unknown), 1490.2 and 26.9 (diamond precursor) [12], 1524.6 and 245.7 ("G" peak carbon), 1717.4 and 73.0 (unknown), 1939.1 and 326 (unknown), 2358.9 and 18.1 (unknown). In addition to the usual spectral features of graphite, silicon and diamond, we found peaks at 1147 and 1490 cm -1, which have been attributed in the literature [10,11] to the appearance of nanocrystalline diamond or of diamond precursor. The 1490-cm- I peak has been attributed to C6o fullerene molecules [ 12]. The effect of a pulsed excitation of the plasma on the micro-Raman spectra (sample 2) is shown in Fig. 3(b). Compared to Fig. 3(a) this spectrum is much more clear-cut, since a lot of peaks have disappeared. Apart from the peaks of silicon and diamond, the remaining peaks are difficult to assign. In this context it is interesting to note that, for this case, we had no evidence for the spectral line indicative for nano-particles of dia-
507
mond. On the other hand, the peak at 1518 cm -i attributed to the presence of the precursor state of diamond, is still present within this spectrum. Adding a small amount of nitrogen to the gas composition of sample 2, but using otherwise the same experimental parameters, we obtained sample 3. The micro-Raman spectrum (Fig. 3(c)) looks again as simple as Fig. 2(b) and consists of only five peaks, which could be attributed to silicon, diamond, graphite (the small one at 1500cm -~) and an unknown peak at 205 2 c m - t. In order to evaluate the concentration of diamond within the samples, we introduce a quality factor R=
sp
t. +t, p3+ k
defined elsewhere [7]. This factor takes into account spurious intensities Ik at the position of the diamond line which originate from adjacent Raman lines within the spectrum. Values of R close to 1 give a clear indication for a high degree of diamond. The different values of this quality factor R, together with the respective FWHM of the diamond peak are: (a) 0.25 and 13.7cm -t for sample 1; (b) 0.57 and 7.2cm -t for sample 2; (c) 0.68 and 6.7 cm -~ for sample 3. From these results we conclude that the use of a pulsed discharge improves the diamond to graphite balance. In the case of sample 3, we have tested the effect of nitrogen enrichment of the gas inlet on the plasma properties during the pulsed discharge. The corresponding emission spectrum is shown in Fig. 1(c). In addition to the emission lines already observed in Fig. 1(a,b), the emission lines of nitrogen and of nitrogen containing radicals appear. Particularly, the second positive system of molecular nitrogen, a line of the NH radical (336 nm). and the very intense violet band of the CN radical are present in the spectrum. It turns out that addition of nitrogen to the parent gas within the reactor reduces all emission lines (H2, H, CH), but not that of species containing nitrogen. However the intensity ratios I( Ha)//( H2), I(H~.)/I(H2) and I(CH)/I(H) are slightly higher than in experiment no. 2, the ratio I(H~)/I(H2) being still smaller. Thus an effect of the nitrogen on the dissociation of the hydrogen molecules and on the average energy of the free electrons may be anticipated, but not proved without complementary measurements. The production of CN can happen directly in the gas phase, or can be due to the etching of the deposited film. In both cases the CN radical results from the interaction of atomic nitrogen or of nitrogen-containing species with carbon-containing radicals of the gas phase, or with carbon deposited on the substrate. These reactions can produce directly CN radicals or C - N bonds
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~) Fig. 2. SEM micrographs of diamond films corresponding to: (a) sample 1, (b) sample 2, (c) sample 3.
H. Chatei et al. / Diamond and Related Materials 6 (1997) 505-510 8000
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nucleation density thus enlarging the size of the grains. These phenomena have already been established in a permanent discharge [7]. In fact the SEM picture (Fig. 2(c)) clearly shows large isolated particles with a cubo-octahedral shape, different from those observed in Fig. 2(a,b). This is due to a modification of the growth rate of the (100) and ( I l l ) facets. It is possible to quantify these modifications by the growth parameter ~=3~/'3(Vloo/Vlll) previously defined by Wild et al. [13] where V,oo and V111 are the growth rates of the (100) and (I I I) facets,respectively. In our case l <~<1.5, I/'ioois thus smaller than VII, indicating the selective reactivityof the different facets with regard to the adsorbed species.
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It was shown that operating the microwave plasma in the pulse mode at 500 Hz resulted in an enhancement of the production of atomic hydrogen inducing an improvement of diamond-to-graphite balance. The addition of nitrogen to the pulse-modulated plasma further improved the deposited diamond crystallites, reducing graphite contamination and the secondary germination. This fact occurs in parallel with reduction of nucleation density thought to be due to the etching of nanodiamond particles. In order to explain our results concerning diamond growth process in the pulsed microwave discharge, time-resolved optical measurements would have to be performed to probe the active species in the gas phase.
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The authors are grateful to Prof. Dr. H.-G. Unruh, who made the micro-Raman studies possible, and to J.M. Claude, A. Kohler, and J. Joffran for the technical support in scanning electron microscopy. One of us would like to thank Prof. J. Derkaoui for his support.
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Fig. 3. Raman spectra of a deposited diamond film corresponding to: (a) sample l, (b) sample 2, (c) sample 3.
containing compounds, which can further dissociate to produce CN. CN is produced in the gas from carbonated species containing either C-H bonds or C-C bonds. The former can be considered as diamond deposition precursors, the second as responsible for graphite deposition [3]. It can thus be speculated that the introduction of nitrogen in the pulsed discharge drastically reduces the growth rate of both diamond and graphite, and decreases the
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[8] A. Hatta, K. Kadota, Y. Mori, T. Ito, T. Sasaki, A. Hiraki and S. Okada, AppL Phys. Lett., 66 (1995) 1602-1604. [9] Z. Ring, T.D. Mantei, S. Tiali and H.E. Jackson, Appl Phys. Lett., 66 (1995) 3380-3382. [10] E.D. Obraztsova, K.G. Korotushenko, S.M. Pimeno~, V.G. Ralchenko, A.A. Smolin, V.i. Konov and E.N. Loubnin. Nanostruct. Mater., 6 (1995) 827-830.
[!1] W.N. Wang, N.A. Fox, P.W. May, M.P. Knapper, G. Meaden, P.G. Partridge, M.N.R. Ashfold, J.W. Steeds, I.P. Haywardand G.D. Pitt, Phys. Stat. Solidi A: Appl. Res., 154 (1996) 255-268. [12] P.V. Huong, J. Mol. Struct., 292 (1993) 81-87. [13] C. Wild, P. Koidl, W. Mtiller-Sebert, R. Kohl, N. Herres, R. Locher and R. Samlenski, Diamond Relat. Mater., 2 (1993;) 158.