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Influence of oxygen and nitrogen addition during growth of CVD diamond on pure cobalt substrates
Diamond & Related Materials 15 (2006) 465 – 471 www.elsevier.com/locate/diamond
Influence of oxygen and nitrogen addition during growth of CVD diamond on pure cobalt substrates M.A. Neto *, E. Pereira Department of Physics, University of Aveiro, 3810 Aveiro, Portugal Available online 2 November 2005
Abstract It was shown that in MPCVD reactors the use of common H and CH4 flows, can lead to the formation of free-standing diamond films directly on pure cobalt substrates. The characteristic low adhesion of these films can be explained by the formation of a graphitic layer between the substrate and the film. Scanning electron microscopy analysis shows special areas at the film grain boundaries that were subsequently identified as cobalt inclusions by EDS. In this work we have studied the influence of oxygen and nitrogen addition on the film’s growth morphology. Once again SEM and EDS were used to access respectively the films growth morphology and cobalt content. Our results clearly indicate that the addition of oxygen during the initial stages of diamond nucleation prevents diffusion of cobalt into the film. Further, we believed that this effect is likely due to the formation of a cobalt-oxide layer between the film and the substrate. Also, the addition of nitrogen and oxygen the later during nucleation or during growth leads to an increase in the film’s growth rate. A preferential (111) surface morphology with clear octahedron facets and films with dominant (100) crystallographic planes are detected. A model is presented for the growth of CVD diamond on cobalt substrates with the addition of oxygen and nitrogen. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond growth and characterisation; Cobalt; Morphology; Free-standing diamond films
1. Introduction The use of CVD diamond films in a greater number on commercial and industrial applications is essentially limited by the characteristic presence of imperfections which can lead to variations in the properties of the material and by the poor adhesion that sometimes is observed between the film and the substrate. During the last years there was a growing interest for the deposition of diamond on cobalt containing substrates such as: cobalt cemented tungsten carbide (WC-Co) substrates [1 – 9] due to their applications on drilling, polishing and cutting machinery and tools; and pure cobalt (1000) substrates [10 – 12] which are good candidates for epitaxial growth with only 1.2% mismatch between diamond and cobalt [13]. However it is well established that Co present at the substrate surface has a negative effect on the nucleation of diamond as it catalyses the formation of graphite and other non-diamond carbon
phases [3 –7], resulting in very thin and low adherent films [6]. Nevertheless it was shown that with well controlled MPCVD growth parameters in CH4 – H2 plasma it is possible to grow ¨200 Am thick diamond films directly on pure cobalt substrates even though their adhesion remains very low [10]. These films contain a considerable amount of cobalt specially located at the back surface forming a layer of graphite and cobalt [10,11]. Cobalt also appears on the growth surface in the form of droplets, similar to what is observed in WC-Co substrates, and at crystallite grain boundaries (GB) in the form of tunnels that grow with the film [11]. In order to study the effect of oxygen and nitrogen addition on the film’s morphology, adhesion and cobalt incorporation, we have grown several samples with and without the presence of such gases. 2. Experimental details 2.1. Sample preparation
* Corresponding author. Present address: Physics Department, University of Aveiro. Tel.: +351 234 370 356; fax: +351 234 424 965. E-mail address: [email protected] (M.A. Neto). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.015
Highly pure (99.9%) monocrystalline (1000) cobalt sheet of 1 mm thick, was selected as the substrate material. This sheet
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Table 1 MPCVD growth parameters Film ref.
H2 (sccm)
CH4 (sccm)
O2 (sccm)
Air (sccm)
Time (h)
Thickness (Am)
Growth-rate (Am/h)
Co1 Co2 Co3 Co4 Co5 Co6
470 470 470 470 470 470
25 25 25 25 25 25
– – – 1.5 (N + G) 1.5 (N + G) –
– – – – 0.5 (G) 0.5 (G)
5 25 68 5 23 6
18 65 190 25 172 50
3.6 2.6 2.8 5.0 7.5 8.3
N—Nucleation; G—Growth.
was cut in 5 5 mm square pieces to be used as individual substrates. Prior to the CVD process, a polishing technique was applied on all substrates: they were first polished with a sequence of SiC sand papers with 600, 1200 and 2400 grid and then polished with 3 Am diamond paste and ultrasonic cleaned in acetone. This procedure, usually enhances the density of nucleation sites on the surface where it is applied resulting in faster diamond nucleation [14]. Also, the application of this technique just before CVD deposition creates a clean and ‘‘fresh’’ cobalt surface. The films were grown using the Microwave Power Chemical Vapour Deposition technique in a ASTeX PDS 18 reactor with a constant pressure of 90 Torr and a 2400 W microwave power for all depositions. This system allows the control of input gases namely hydrogen, methane, oxygen and nitrogen. This later was fed into the system via air flow at atmospheric pressure. The growth of CVD diamond films starts with diamond nucleation. This phase is of major importance because it creates on the substrate surface the nucleation sites necessary for diamond growth. The CVD process starts with the formation of hydrogen plasma in the reactor’s chamber. After a period of 10 min, methane is added slowly to the system. For samples Co4 and Co5, oxygen was fed into the
reactor at the same time as the methane. Also the air flow was switched on only 30 min after the beginning of the CVD process. While oxygen was added to the system during nucleation and growth phases, nitrogen (as air) was added only during growth. In the case of Co6 sample, oxygen and nitrogen were added only during growth. Three different thick films were grown with same growth conditions but without the addition of these two gases. They will serve as references for the SEM, EDS and Raman analysis. Table 1 shows the growth parameters of these depositions. 2.2. Sample analysis The films morphology and crystallite size were examined by scanning electron microscopy (SEM) using the FEG – SEM Hitachi S4100 system. The presence of cobalt in these films was studied by energy disperse X-ray spectroscopy (EDS) available in the SEM system. Prior to SEM analysis, the films were coated with carbon and in a few cases with Pt/Au, in order to avoid charge effect. Finally the presence of diamond and other forms of carbon was studied by micro-Raman analysis with the use of a Jobin Yvon T64000 spectrometer, with a spectral resolution of 0.45 cm 1 and with an Ar+ laser using
Fig. 1. Typical SEM images of the film’s back surface.
M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
467
substrates [15 – 19]. All films became free-standing due to the self-delamination from the substrates after post-deposition cooling to room temperature. This observation is consistent with our previous work [20,21] and other results on cobalt cemented tungsten carbide (WC/Co) substrates [22 –26]. The film’s poor adhesion can be explained by the absence of carbide formation as well as lattice and thermal expansion coefficient mismatch. More, it is known that the presence of cobalt at the substrate surface catalyses the formation of nondiamond carbon phases specially graphite, which are deposited at the interface and therefore reducing the adhesion [25]. We believed that the adhesion is even lower when oxygen and nitrogen are added to the deposition gases, because for these two films delamination occurs during the deposition process. 3.1. Film’s back surface
Fig. 2. Raman spectra of the film’s back surface immediately after growth.
the 514.5 nm excitation line with 2 mW laser power on the sample. 3. Results and discussion From the data presented in Table 1, it is evident that the addition of oxygen alone or in the presence of nitrogen, increases the film’s growth-rate. This is consistent with previous results on the deposition of diamond on different
Fig. 1 shows the characteristic SEM pictures of the film’s back surface. Raman spectra taken on these surfaces (Fig. 2) show the formation of graphite (D and G bands) at the beginning of film growth only when there is no oxygen in the system during nucleation (samples Co2 and Co6). For the other films the Raman line of diamond is observed. This means that oxygen probably reacts with the substrate surface reducing strongly the formation of graphite at the interface. In fact it is known that cobalt reacts with oxygen giving rise to Co3O4 and CoO when heated at temperatures below and above 900 -C, respectively. Considering the usual CVD growth temperature (around 900 -C) we expect the formation of both oxides. If indeed these oxides are formed cobalt concentration will decrease in the film’s back surface. This study is presented in Fig. 3 where we compare EDS spectra of samples grown with and without the presence of oxygen. It is obvious that the decrease in cobalt content corresponds to an increase of oxygen. This proves the formation of a cobalt oxide layer onto the substrate preventing diffusion of nonbounded cobalt to the film and formation of graphite. This
Fig. 3. EDS spectra of the film’s back surface.
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Fig. 4. Typical SEM images of the film’s growth surface.
cobalt oxide layer could also explain the lower adhesion of these films and therefore the self-delamination during the CVD process. 3.2. Film’s growth surface The film’s growth morphology was studied using SEM images on the film’s growth surface (Fig. 4). In this figure we compare films with approximately the same thickness which will allow us to know the effect of oxygen and nitrogen on the growth surface morphology. It is evident the effect of these gases on the structure of diamond crystallites. Fig. 4 shows an increase in the crystallite size when O2 is added during the nucleation phase (Co4 and Co6). On the other hand the addition of O2 together with N2 in the growth stage creates well Table 2 Results from Raman analysis conducted on the film’s growth surface Diamond peak
Fig. 5. Raman spectra of the film’s growth surface.
Film
Q (%)
Position (cm 1)
FWHM (cm 1)
Co1 Co2 Co3 Co4 Co5 Co6
96 98 99 95 100 89
1332.9 1332.1 1333.4 1334.7 1333.4 1334.4
4.3 3.2 5.4 4.2 3.6 11.9
M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
Fig. 6. EDS spectra of the film’s growth surface.
defined (111) octahedron facets (Co6) and dominant (100) crystallites (Co5), whether O2 was respectively absent and included during nucleation. Raman analysis was performed on this surface to access the quality of the diamond layer (Fig. 5). The position and FWHM of the diamond peak were determined as well the quality factor Q given by Eq. (1) [27]. The results presented in Table 2 clearly show that Q changes very little except for Co6. More, for this particular film the value of FWHM is particularly high. The values between 3 and 5 are typical for this kind of films. This means that even though the surface of Co6 shows well orientated crystallites they should contain sufficient defects at
469
Fig. 8. Possible model for deposition of diamond on pure cobalt substrates.
the grain boundaries. The position of the diamond peak shifts to higher wavenumbers for the samples grown with oxygen and nitrogen. This is indication of stress present in the film possibly in a thin cobalt oxide layer or in grains. Q¼
ID ID þ
IC 233
100
ð1Þ
This surface was also studied in terms of impurity content using once again EDS. The spectra of Fig. 6 were obtained on a large square area on the growth surface, while for Fig. 7 a
Fig. 7. EDS spectra and SEM images taken at the crystallites grain boundaries: a) inside a DS for films Co1, Co2 and Co3; b) between crystallites on Co6; c) on an aggregate of Co6 sample.
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M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
nanometer square region was used on the crystallites grain boundaries. This analysis shows similar results on the surface and on the GB. Only sample Co6 appear to contain a considerable amount of Co and O homogeneously distributed as aggregates and at the crystallites facets. Both Co4 and Co5 samples are virtually free of these elements. As observed previously [12] the grain boundaries of films Co1, Co2 and Co3 contains the so-called dark spots (DS) which are essentially made of cobalt and silicon and have the form of tunnels that grow with the film. These results suggest that the addition of O2 during nucleation blocks cobalt diffusion to the film, whereas if added only during growth it will react with cobalt creating small cobalt oxide aggregates that precipitate on the growing surface. 3.3. Proposed model In Fig. 8 we present a possible model that could explain the effect of oxygen and nitrogen addition during the different stages of diamond deposition on pure cobalt substrates. Prior to any deposition all substrates were prepared in the same way using the polishing technique already described (step 1). When oxygen is present in the beginning of the CVD process, it will react with weakly bounded cobalt on the substrate surface leading to the formation of cobalt oxide and therefore preventing carbon diffusion to the substrate and the formation of a thick graphite layer (step 2). The presence of atomic hydrogen will steadily transform non-diamond sp2 carbon on sp3 diamond above the cobalt oxide layer (step 3). This layer prevents any cobalt transport to the plasma and diffusion into the growing film (step 4). These films are relatively free of cobalt. For these films the addition of oxygen and nitrogen during growth don’t change cobalt incorporation but could influence their final surface morphology. When nucleation occurs in the absence of oxygen a graphite layer is formed on the substrate that contains a considerable amount of cobalt (step 2). At the same time some cobalt is transported to the plasma. The beginning of film growth is characterized by diamond crystallites separated by cobalt regions. Then the addition of oxygen creates cobalt oxide between the crystallites preventing further Co transport to the plasma and GB diffusion (step 3). Further growth in these conditions precipitate the remaining Co in the plasma in the form of cobalt oxide aggregates onto the substrate especially at the GB (step 4). 4. Conclusions With this work we have found important changes on diamond films grown with oxygen and nitrogen compared to the ones obtained without the presence of these gases. As expected there was an increase in the film’s growth-rate and a decrease in the film adhesion to the substrate. This lower adhesion results when O2 is present during nucleation leading to the formation of a cobalt oxide layer instead of graphite. Also we believe that this oxide layer prevents Co transport to the reactor’s chamber and diffusion through the film. As a
result the dark spots (DS), observed in similar films grown without oxygen and nitrogen are no longer observed. In fact these films are relatively cobalt-free, except when oxygen is added during growth. But in this case cobalt is no longer localised in nanometer sized regions (DS) instead it is homogeneously distributed on the surface in the form of oxide aggregates located between the crystallites. The film’s growth morphology became much more diverse depending in which stage oxygen and nitrogen are introduced. From randomly oriented to dominant (100) and even (111) growth was observed. Acknowledgements M. A. Neto thanks FCT—Fundac¸a˜o para a Cieˆncia e a Tecnologia for the grant PRAXIS XXI/BD/16161/98. The authors would like to acknowledge A. J. S. Fernandes for the precious help on the CVD equipment and any relevant discussions. References [1] M. Murakawa, S. Takeuchi, Surface and Coatings Technology 49 (1991) 359. [2] J. Oakes, X.X. Pan, R. Bichler, R. Haubner, B. Lux, Surface and Coatings Technology 47 (1991) 600. [3] R. Haubner, B. Lux, Journal de Physique C5 (1989) 169. [4] R. Haubner, A. Lindlbauer, B. Lux, Diamond and Related Materials 2 (1993) 1505. [5] R. Haubner, S. Kubelka, B. Lux, M. Griesser, M. Grasserbauer, Journal de Physique C5 (1995) 753. [6] H. Matsubara, J. Kihara, in: S. Saito, O. Fukunaga, M. Yoshikawa (Eds.), Science, Technology of New Diamond, KTK Scientific Publishers, Tokyo, 1990, p. 89. [7] T.H. Huang, C.T. Kuo, C.S. Chang, C.T. Kao, H.Y. Wen, Diamond and Related Materials 1 (1992) 594. [8] R. Cremer, R. Mertens, D. Neushu¨tz, O. Lemmer, M. Frank, T. Leyendecker, Thin Solid Films 355 – 356 (1999) 127. [9] A. Fernandes, A. Neves, R.F. Silva, M.H. Nazare´, Diamond and Related Materials 6 (1997) 769. [10] M.A. Neto, Qi Hua Fan, E. Pereira, Diamond and Related Materials 10 (2001) 316. [11] Wei Liu, Denise A. Tucker, Peichun, Yang, Jeffrey T. Glass, Journal of Applied Physics 78 (2) (1995) 1291. [12] M. A. Neto, E. Pereira, Journal of Phase Equilibria and Diffusion, in press. [13] D. Lide (Ed.), Handbook of Chemistry and Physics, 71st edR, CRC, Boca Raton, 1990, p. 12. [14] K. Miotsuda, Y. Kojima, T. Yoshida, K. Akashi, Journal of Materials Science 22 (1987) 1557. [15] Chia-Fu Chen, Y.C. Huang, S. Hosomi, I. Yoshida, Materials Research Bulletin 24 (1989) 87. [16] T. Kawato, K. Kondo, Japanese Journal of Applied Physics 26 (1987) 1429. [17] Y. Hirose, S. Amanuma, N. Okada, K. Komaki, in: J.P. Dismukes, A.J. Purdes, J.C. Angus, R.F. Davis, B.M. Meyerson, K.E. Spear, M. Yoder (Eds.), Diamond and Diamond-like Films, The Electrochemical Society, Pennington, NJ, 1989, p. 80. [18] S. Matsumoto, in: J.P. Dismukes, A.J. Purdes, J.C. Angus, R.F. Davis, B.M. Meyerson, K.E. Spear, M. Yoder (Eds.), Diamond and Diamond-like Films, The Electrochemical Society, Pennington, NJ, 1989, p. 50. [19] S. Jin, T.D. Moustakas, Applied Physics Letters 65 (4) (1994) 403. [20] M.A. Neto, Qi Hua Fan, E. Pereira, Diamond and Related Materials 10 (2001) 316.
M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471 [21] M.A. Neto, E. Pereira, 1st International Conference on Diffusion in Solids and Liquids, July 6 – 8, Aveiro, Portugal, 2005. [22] R. Haubner, B. Lux, Journal de Physique C5 (1989) 169. [23] R. Haubner, A. Lindlbauer, B. Lux, Diamond and Related Materials 2 (1993) 1505. [24] R. Haubner, S. Kubelka, B. Lux, M. Griesser, M. Grasserbauer, Journal de Physique C5 (1995) 753.
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[25] H. Matsubara, J. Kihara, in: S. Saito, O. Fukunaga, M. Yoshikawa (Eds.), Science, Technology of New Diamond, KTK Scientific Publishers, Tokyo, 1990, p. 89. [26] T.H. Huang, C.T. Kuo, C.S. Chang, C.T. Kao, H.Y. Wen, Diamond and Related Materials 1 (1992) 594. [27] C.D.O. Pickard, T.J. Davis, W.N. Wang, J.W. Steeds, Diamond and Related Materials 7 (1998) 238.
Influence of oxygen and nitrogen addition during growth of CVD diamond on pure cobalt substrates M.A. Neto *, E. Pereira Department of Physics, University of Aveiro, 3810 Aveiro, Portugal Available online 2 November 2005
Abstract It was shown that in MPCVD reactors the use of common H and CH4 flows, can lead to the formation of free-standing diamond films directly on pure cobalt substrates. The characteristic low adhesion of these films can be explained by the formation of a graphitic layer between the substrate and the film. Scanning electron microscopy analysis shows special areas at the film grain boundaries that were subsequently identified as cobalt inclusions by EDS. In this work we have studied the influence of oxygen and nitrogen addition on the film’s growth morphology. Once again SEM and EDS were used to access respectively the films growth morphology and cobalt content. Our results clearly indicate that the addition of oxygen during the initial stages of diamond nucleation prevents diffusion of cobalt into the film. Further, we believed that this effect is likely due to the formation of a cobalt-oxide layer between the film and the substrate. Also, the addition of nitrogen and oxygen the later during nucleation or during growth leads to an increase in the film’s growth rate. A preferential (111) surface morphology with clear octahedron facets and films with dominant (100) crystallographic planes are detected. A model is presented for the growth of CVD diamond on cobalt substrates with the addition of oxygen and nitrogen. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond growth and characterisation; Cobalt; Morphology; Free-standing diamond films
1. Introduction The use of CVD diamond films in a greater number on commercial and industrial applications is essentially limited by the characteristic presence of imperfections which can lead to variations in the properties of the material and by the poor adhesion that sometimes is observed between the film and the substrate. During the last years there was a growing interest for the deposition of diamond on cobalt containing substrates such as: cobalt cemented tungsten carbide (WC-Co) substrates [1 – 9] due to their applications on drilling, polishing and cutting machinery and tools; and pure cobalt (1000) substrates [10 – 12] which are good candidates for epitaxial growth with only 1.2% mismatch between diamond and cobalt [13]. However it is well established that Co present at the substrate surface has a negative effect on the nucleation of diamond as it catalyses the formation of graphite and other non-diamond carbon
phases [3 –7], resulting in very thin and low adherent films [6]. Nevertheless it was shown that with well controlled MPCVD growth parameters in CH4 – H2 plasma it is possible to grow ¨200 Am thick diamond films directly on pure cobalt substrates even though their adhesion remains very low [10]. These films contain a considerable amount of cobalt specially located at the back surface forming a layer of graphite and cobalt [10,11]. Cobalt also appears on the growth surface in the form of droplets, similar to what is observed in WC-Co substrates, and at crystallite grain boundaries (GB) in the form of tunnels that grow with the film [11]. In order to study the effect of oxygen and nitrogen addition on the film’s morphology, adhesion and cobalt incorporation, we have grown several samples with and without the presence of such gases. 2. Experimental details 2.1. Sample preparation
* Corresponding author. Present address: Physics Department, University of Aveiro. Tel.: +351 234 370 356; fax: +351 234 424 965. E-mail address: [email protected] (M.A. Neto). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.015
Highly pure (99.9%) monocrystalline (1000) cobalt sheet of 1 mm thick, was selected as the substrate material. This sheet
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M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
Table 1 MPCVD growth parameters Film ref.
H2 (sccm)
CH4 (sccm)
O2 (sccm)
Air (sccm)
Time (h)
Thickness (Am)
Growth-rate (Am/h)
Co1 Co2 Co3 Co4 Co5 Co6
470 470 470 470 470 470
25 25 25 25 25 25
– – – 1.5 (N + G) 1.5 (N + G) –
– – – – 0.5 (G) 0.5 (G)
5 25 68 5 23 6
18 65 190 25 172 50
3.6 2.6 2.8 5.0 7.5 8.3
N—Nucleation; G—Growth.
was cut in 5 5 mm square pieces to be used as individual substrates. Prior to the CVD process, a polishing technique was applied on all substrates: they were first polished with a sequence of SiC sand papers with 600, 1200 and 2400 grid and then polished with 3 Am diamond paste and ultrasonic cleaned in acetone. This procedure, usually enhances the density of nucleation sites on the surface where it is applied resulting in faster diamond nucleation [14]. Also, the application of this technique just before CVD deposition creates a clean and ‘‘fresh’’ cobalt surface. The films were grown using the Microwave Power Chemical Vapour Deposition technique in a ASTeX PDS 18 reactor with a constant pressure of 90 Torr and a 2400 W microwave power for all depositions. This system allows the control of input gases namely hydrogen, methane, oxygen and nitrogen. This later was fed into the system via air flow at atmospheric pressure. The growth of CVD diamond films starts with diamond nucleation. This phase is of major importance because it creates on the substrate surface the nucleation sites necessary for diamond growth. The CVD process starts with the formation of hydrogen plasma in the reactor’s chamber. After a period of 10 min, methane is added slowly to the system. For samples Co4 and Co5, oxygen was fed into the
reactor at the same time as the methane. Also the air flow was switched on only 30 min after the beginning of the CVD process. While oxygen was added to the system during nucleation and growth phases, nitrogen (as air) was added only during growth. In the case of Co6 sample, oxygen and nitrogen were added only during growth. Three different thick films were grown with same growth conditions but without the addition of these two gases. They will serve as references for the SEM, EDS and Raman analysis. Table 1 shows the growth parameters of these depositions. 2.2. Sample analysis The films morphology and crystallite size were examined by scanning electron microscopy (SEM) using the FEG – SEM Hitachi S4100 system. The presence of cobalt in these films was studied by energy disperse X-ray spectroscopy (EDS) available in the SEM system. Prior to SEM analysis, the films were coated with carbon and in a few cases with Pt/Au, in order to avoid charge effect. Finally the presence of diamond and other forms of carbon was studied by micro-Raman analysis with the use of a Jobin Yvon T64000 spectrometer, with a spectral resolution of 0.45 cm 1 and with an Ar+ laser using
Fig. 1. Typical SEM images of the film’s back surface.
M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
467
substrates [15 – 19]. All films became free-standing due to the self-delamination from the substrates after post-deposition cooling to room temperature. This observation is consistent with our previous work [20,21] and other results on cobalt cemented tungsten carbide (WC/Co) substrates [22 –26]. The film’s poor adhesion can be explained by the absence of carbide formation as well as lattice and thermal expansion coefficient mismatch. More, it is known that the presence of cobalt at the substrate surface catalyses the formation of nondiamond carbon phases specially graphite, which are deposited at the interface and therefore reducing the adhesion [25]. We believed that the adhesion is even lower when oxygen and nitrogen are added to the deposition gases, because for these two films delamination occurs during the deposition process. 3.1. Film’s back surface
Fig. 2. Raman spectra of the film’s back surface immediately after growth.
the 514.5 nm excitation line with 2 mW laser power on the sample. 3. Results and discussion From the data presented in Table 1, it is evident that the addition of oxygen alone or in the presence of nitrogen, increases the film’s growth-rate. This is consistent with previous results on the deposition of diamond on different
Fig. 1 shows the characteristic SEM pictures of the film’s back surface. Raman spectra taken on these surfaces (Fig. 2) show the formation of graphite (D and G bands) at the beginning of film growth only when there is no oxygen in the system during nucleation (samples Co2 and Co6). For the other films the Raman line of diamond is observed. This means that oxygen probably reacts with the substrate surface reducing strongly the formation of graphite at the interface. In fact it is known that cobalt reacts with oxygen giving rise to Co3O4 and CoO when heated at temperatures below and above 900 -C, respectively. Considering the usual CVD growth temperature (around 900 -C) we expect the formation of both oxides. If indeed these oxides are formed cobalt concentration will decrease in the film’s back surface. This study is presented in Fig. 3 where we compare EDS spectra of samples grown with and without the presence of oxygen. It is obvious that the decrease in cobalt content corresponds to an increase of oxygen. This proves the formation of a cobalt oxide layer onto the substrate preventing diffusion of nonbounded cobalt to the film and formation of graphite. This
Fig. 3. EDS spectra of the film’s back surface.
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M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
Fig. 4. Typical SEM images of the film’s growth surface.
cobalt oxide layer could also explain the lower adhesion of these films and therefore the self-delamination during the CVD process. 3.2. Film’s growth surface The film’s growth morphology was studied using SEM images on the film’s growth surface (Fig. 4). In this figure we compare films with approximately the same thickness which will allow us to know the effect of oxygen and nitrogen on the growth surface morphology. It is evident the effect of these gases on the structure of diamond crystallites. Fig. 4 shows an increase in the crystallite size when O2 is added during the nucleation phase (Co4 and Co6). On the other hand the addition of O2 together with N2 in the growth stage creates well Table 2 Results from Raman analysis conducted on the film’s growth surface Diamond peak
Fig. 5. Raman spectra of the film’s growth surface.
Film
Q (%)
Position (cm 1)
FWHM (cm 1)
Co1 Co2 Co3 Co4 Co5 Co6
96 98 99 95 100 89
1332.9 1332.1 1333.4 1334.7 1333.4 1334.4
4.3 3.2 5.4 4.2 3.6 11.9
M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
Fig. 6. EDS spectra of the film’s growth surface.
defined (111) octahedron facets (Co6) and dominant (100) crystallites (Co5), whether O2 was respectively absent and included during nucleation. Raman analysis was performed on this surface to access the quality of the diamond layer (Fig. 5). The position and FWHM of the diamond peak were determined as well the quality factor Q given by Eq. (1) [27]. The results presented in Table 2 clearly show that Q changes very little except for Co6. More, for this particular film the value of FWHM is particularly high. The values between 3 and 5 are typical for this kind of films. This means that even though the surface of Co6 shows well orientated crystallites they should contain sufficient defects at
469
Fig. 8. Possible model for deposition of diamond on pure cobalt substrates.
the grain boundaries. The position of the diamond peak shifts to higher wavenumbers for the samples grown with oxygen and nitrogen. This is indication of stress present in the film possibly in a thin cobalt oxide layer or in grains. Q¼
ID ID þ
IC 233
100
ð1Þ
This surface was also studied in terms of impurity content using once again EDS. The spectra of Fig. 6 were obtained on a large square area on the growth surface, while for Fig. 7 a
Fig. 7. EDS spectra and SEM images taken at the crystallites grain boundaries: a) inside a DS for films Co1, Co2 and Co3; b) between crystallites on Co6; c) on an aggregate of Co6 sample.
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M.A. Neto, E. Pereira / Diamond & Related Materials 15 (2006) 465 – 471
nanometer square region was used on the crystallites grain boundaries. This analysis shows similar results on the surface and on the GB. Only sample Co6 appear to contain a considerable amount of Co and O homogeneously distributed as aggregates and at the crystallites facets. Both Co4 and Co5 samples are virtually free of these elements. As observed previously [12] the grain boundaries of films Co1, Co2 and Co3 contains the so-called dark spots (DS) which are essentially made of cobalt and silicon and have the form of tunnels that grow with the film. These results suggest that the addition of O2 during nucleation blocks cobalt diffusion to the film, whereas if added only during growth it will react with cobalt creating small cobalt oxide aggregates that precipitate on the growing surface. 3.3. Proposed model In Fig. 8 we present a possible model that could explain the effect of oxygen and nitrogen addition during the different stages of diamond deposition on pure cobalt substrates. Prior to any deposition all substrates were prepared in the same way using the polishing technique already described (step 1). When oxygen is present in the beginning of the CVD process, it will react with weakly bounded cobalt on the substrate surface leading to the formation of cobalt oxide and therefore preventing carbon diffusion to the substrate and the formation of a thick graphite layer (step 2). The presence of atomic hydrogen will steadily transform non-diamond sp2 carbon on sp3 diamond above the cobalt oxide layer (step 3). This layer prevents any cobalt transport to the plasma and diffusion into the growing film (step 4). These films are relatively free of cobalt. For these films the addition of oxygen and nitrogen during growth don’t change cobalt incorporation but could influence their final surface morphology. When nucleation occurs in the absence of oxygen a graphite layer is formed on the substrate that contains a considerable amount of cobalt (step 2). At the same time some cobalt is transported to the plasma. The beginning of film growth is characterized by diamond crystallites separated by cobalt regions. Then the addition of oxygen creates cobalt oxide between the crystallites preventing further Co transport to the plasma and GB diffusion (step 3). Further growth in these conditions precipitate the remaining Co in the plasma in the form of cobalt oxide aggregates onto the substrate especially at the GB (step 4). 4. Conclusions With this work we have found important changes on diamond films grown with oxygen and nitrogen compared to the ones obtained without the presence of these gases. As expected there was an increase in the film’s growth-rate and a decrease in the film adhesion to the substrate. This lower adhesion results when O2 is present during nucleation leading to the formation of a cobalt oxide layer instead of graphite. Also we believe that this oxide layer prevents Co transport to the reactor’s chamber and diffusion through the film. As a
result the dark spots (DS), observed in similar films grown without oxygen and nitrogen are no longer observed. In fact these films are relatively cobalt-free, except when oxygen is added during growth. But in this case cobalt is no longer localised in nanometer sized regions (DS) instead it is homogeneously distributed on the surface in the form of oxide aggregates located between the crystallites. The film’s growth morphology became much more diverse depending in which stage oxygen and nitrogen are introduced. From randomly oriented to dominant (100) and even (111) growth was observed. Acknowledgements M. A. Neto thanks FCT—Fundac¸a˜o para a Cieˆncia e a Tecnologia for the grant PRAXIS XXI/BD/16161/98. The authors would like to acknowledge A. J. S. Fernandes for the precious help on the CVD equipment and any relevant discussions. References [1] M. Murakawa, S. Takeuchi, Surface and Coatings Technology 49 (1991) 359. [2] J. Oakes, X.X. Pan, R. Bichler, R. Haubner, B. Lux, Surface and Coatings Technology 47 (1991) 600. [3] R. Haubner, B. Lux, Journal de Physique C5 (1989) 169. [4] R. Haubner, A. Lindlbauer, B. Lux, Diamond and Related Materials 2 (1993) 1505. [5] R. Haubner, S. Kubelka, B. Lux, M. Griesser, M. Grasserbauer, Journal de Physique C5 (1995) 753. [6] H. Matsubara, J. Kihara, in: S. Saito, O. Fukunaga, M. Yoshikawa (Eds.), Science, Technology of New Diamond, KTK Scientific Publishers, Tokyo, 1990, p. 89. [7] T.H. Huang, C.T. Kuo, C.S. Chang, C.T. Kao, H.Y. Wen, Diamond and Related Materials 1 (1992) 594. [8] R. Cremer, R. Mertens, D. Neushu¨tz, O. Lemmer, M. Frank, T. Leyendecker, Thin Solid Films 355 – 356 (1999) 127. [9] A. Fernandes, A. Neves, R.F. Silva, M.H. Nazare´, Diamond and Related Materials 6 (1997) 769. [10] M.A. Neto, Qi Hua Fan, E. Pereira, Diamond and Related Materials 10 (2001) 316. [11] Wei Liu, Denise A. Tucker, Peichun, Yang, Jeffrey T. Glass, Journal of Applied Physics 78 (2) (1995) 1291. [12] M. A. Neto, E. Pereira, Journal of Phase Equilibria and Diffusion, in press. [13] D. Lide (Ed.), Handbook of Chemistry and Physics, 71st edR, CRC, Boca Raton, 1990, p. 12. [14] K. Miotsuda, Y. Kojima, T. Yoshida, K. Akashi, Journal of Materials Science 22 (1987) 1557. [15] Chia-Fu Chen, Y.C. Huang, S. Hosomi, I. Yoshida, Materials Research Bulletin 24 (1989) 87. [16] T. Kawato, K. Kondo, Japanese Journal of Applied Physics 26 (1987) 1429. [17] Y. Hirose, S. Amanuma, N. Okada, K. Komaki, in: J.P. Dismukes, A.J. Purdes, J.C. Angus, R.F. Davis, B.M. Meyerson, K.E. Spear, M. Yoder (Eds.), Diamond and Diamond-like Films, The Electrochemical Society, Pennington, NJ, 1989, p. 80. [18] S. Matsumoto, in: J.P. Dismukes, A.J. Purdes, J.C. Angus, R.F. Davis, B.M. Meyerson, K.E. Spear, M. Yoder (Eds.), Diamond and Diamond-like Films, The Electrochemical Society, Pennington, NJ, 1989, p. 50. [19] S. Jin, T.D. Moustakas, Applied Physics Letters 65 (4) (1994) 403. [20] M.A. Neto, Qi Hua Fan, E. Pereira, Diamond and Related Materials 10 (2001) 316.
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