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Free-standing diamond films grown on cobalt substrates
Diamond and Related Materials 10 Ž2001. 316᎐321
Free-standing diamond films grown on cobalt substrates M.A. NetoU , Qi Hua Fan, E. Pereira Department of Physics, Uni¨ ersity of A¨ eiro, 3810 A¨ eiro, Portugal
Abstract Diamond films were grown directly on cobalt substrates, using microwave plasma-assisted chemical vapour deposition. Although cobalt is known to inhibit the nucleation of diamond and enhancing the formation of graphite, we were able to grow relatively thick films Ž; 190 m.. The films were easily detached from the substrates. The poor adhesion allows the possibility of obtaining free-standing diamond films without chemical etching. Micro-Raman spectroscopy showed the 1332 cmy1 characteristic Raman peak of diamond and the 1580 cmy1, 1360 cmy1 bands of graphite, on the growth surface and backside of the films, respectively. Through scanning electron microscopy and X-ray diffraction we were able to monitor film thickness and morphology with growth evolution. The results showed the Ž111. preferential growth morphology for the film with higher growth rate. By energy dispersive X-ray spectroscopy it was only possible to detect cobalt in the back of the films, but not in the surface. The role of cobalt in the film growth is discussed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond growth and characterization; Free-standing diamond films; Cobalt; Chemical vapour deposition
1. Introduction The exceptional properties of diamond arise from two basic facts: Ža. carbon atoms are relatively small and light; and Žb. when the carbon atoms bind together in the diamond structure, they form very strong sp 3 covalent bonds. For example, approximately 5.5 eV is required to excite an electron from the valence band to the conduction band, compared with 1.1 eV for Si and 0.7 eV for Ge. Diamond is, therefore, a wide bandgap material. Furthermore, due to the high vibration energy of carbon atoms in diamond, the frequency of infrared absorption is abnormally high. As the one phonon transition is forbidden in the diamond lattice, only two phonon absorption and one phonon defect induced absorption are observed. These led to a broad optical transparency for diamond from the deep UV to
U
Corresponding author. Tel.: q351-234-370-278; fax: q351-234424-965. E-mail address: [email protected] ŽM.A. Neto..
the far IR region of the electromagnetic spectrum. Therefore, together with its mechanical properties, diamond is no longer a promising material but a reality for optical w1x, electronic w2x and mechanical applications w3x. One can then understand why chemical vapour deposition ŽCVD. diamond has an unquestionable leading position over natural and synthetic high pressure high temperature ŽHPHT. diamond. For mechanical applications we are interested to have good adherent diamond films on hard materials. Much work has been done on WCŽCo. tools. The presence of up to 12% of cobalt in tungsten carbide tools has generally been considered to prevent good adhesion between the film and the substrate w4᎐6x. For other applications Ži.e. optical., however, one must have transparent free-standing films. Usually the chosen substrate is silicon that requires the use of chemical etching. In this work we report the synthesis of good quality free-standing diamond films directly on cobalt substrates. To the author’s knowledge, there are very few reports on this subject so far w7x. The choice of
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 6 0 - 4
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
cobalt substrates arise from the fact that cobalt dissolves carbon, leading to the formation of graphite and amorphous carbon between the substrate and the film which in turn compromises adhesion. Changing the growth parameters we were able to monitor the growth rates and morphologies of the films thereby obtained. Since CVD diamond is formed at relatively high temperatures Ž; 800⬚C., it is possible that cobalt incorporates in CVD diamond similarly to what happens in HPHT diamond w8x. The effect of this eventual incorporation on the optical properties, have to be taken into account. 2. Experimental details Diamond films were grown using ASTeX PDS 18 microwave plasma CVD equipment on 99.9% cobalt substrates Ž5 = 5 = 1 mm.. All substrates were first polished with a sequence of SiC sand papers of 600, 1200 and 2400 grid. After this the substrates were polished with 3 m diamond paste and then ultrasonic cleaned in acetone. Finally and before the deposition the substrates were polished with 3 m diamond powder and again ultrasonically cleaned. The process gases used were hydrogen ŽH 2 . and methane ŽCH 4 .. The process parameters employed were as follows: microwave power, 2500 W; gas pressure, 90 torr; H 2 flow rate, 470 ; 500 sccm; CH 4 flow rate 20 ; 25 sccm ; deposition temperature, ; 800⬚C. The morphologies and crystallite sizes of the CVD diamond films were examined by scanning electron microscopy ŽSEM. using the FEG-SEM Hitachi S4100 system. The presence of cobalt in these films was done by energy dispersive X-ray spectroscopy ŽEDS. available in the SEM system. Prior to SEM analysis, the films were coated with PtrAu, in order to avoid charge effect. The presence of diamond and other forms of carbon was done by micro-Raman analysis with the use of a Jobin Yvon T64000 spectrometer, with a spectral resolution of 0.45 cmy1 and with an Arq laser using the 514.5-nm excitation line with 2 mW laser power on the sample. X-Ray diffraction ŽXRD. analysis was also used to confirm the synthesis of diamond in all the films. The XRD analysis was carried out on a Philips X’Pert equipment with a scanning step of 0.05⬚ and 0.5 s of integration time. Photoluminescence spectra were obtained with the samples at constant temperature Ž15 K. using the 325nm line of a He-Cd laser, as the excitation source and a detection system as described elsewhere w9x. 3. Results and discussion Diamond films with thickness ranging from 6 to 190
317
Table 1 The growth time, film thickness and growth rate of diamond on cobalt substrates Sample
Time Žh.
Thickness Žm.
Growth rate Žmrh.
Co1 Co2 Co3 Co4
3 5 25 68
6 18 65 190
2 3.6 2.6 2.8
m were successfully grown on Co substrates as summarized in Table 1. During post-deposition cooling the diamond films grown on Co did not crack, although the thermal expansion coefficient for cobalt is significantly higher as compared with diamond. This is because the films exhibited no adhesion. The films self delaminated from the Co substrates, similar to what happens on copper substrates w10x, without the use of any chemical etching. 3.1. Film morphology The morphologies of CVD diamond films are very diverse and very much a function of the growth conditions. Substrate temperature, carbon super-saturation and impurities have strong effects on morphology. Also, most properties and applications of CVD diamond are strongly related to growth morphology. So, if we understand the relationships between growth conditions and morphology it becomes possible to design diamond films for specific applications. The most stable growth planes of diamond are the octahedral Ž111. faces, followed by the cube Ž100. faces and the Ž110. faces w11x. So, under conditions close to equilibrium and during growth of polycrystalline diamond films, triangular Ž111. faces are the expected growth morphology. As the growth process moves away from equilibrium, the Ž100. cube faces appear, which results in a mixture of growth faces. A further movement away from equilibrium can lead to a dominant Ž100. cube morphology in the later stages of growth evolution, unless the Ž110. faces become energetically favourable. Fig. 1 shows the change of the films grain size and morphology when subject to different growth conditions. In sample Co2 we observe preferential Ž111. growth morphology with clear triangular facets parallel to the plane of the film. The next two films do not show this morphology, instead they can be characterized by having very sharp angular faceted surfaces with twins and stacking faults. Some Ž110. facets are also seen. In sample Co1 and Co2 we detected small particles on the diamond growth surface that we have identified as cobalt particles by EDS. These particles were not observed on the thicker films.
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M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
Fig. 1. SEM images showing the films grain size and morphologies. a. Co1; b. Co2; c. Co3; d. Co4.
Most CVD diamond films develop columnar grains in the direction of growth and show strong, preferred orientation as the film thickens. These columnar grains are a natural consequence of the growth processes. In Fig. 2 we present the films cross-section SEM images. It is very clear the formation of such columnar grains.
It is evident from the XRD spectra presented in Fig. 3 the characteristic X-ray diffraction angles for Ž111., Ž220. and Ž311. planes of diamond with 2 equal to 43.9⬚, 75.3⬚ and 91.5⬚, respectively. For sample Co2 we see a preferential Ž111. crystal orientation, while for samples Co3 and Co4 there is a mixture of Ž111., Ž220.
Fig. 2. SEM images showing the diamond films cross-section and thickness. a. Co1; b. Co2; c. Co3; d. Co4.
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
319
Fig. 3. X-Ray diffraction spectra of diamond films grown on cobalt substrates.
and Ž311. crystallographic planes. The fact that the Ž220. planes appear instead of the Ž400. ones is indicative that the Ž110. planes are energetically more favourable than the Ž100. under the current deposition conditions.
This proved that the diamond film Žsample Co4. is indeed of good homogeneity. The diamond line was fitted to a Lorentzian function giving a FWHM of 6.3 cmy1 , typical of good quality CVD diamond. 3.3. EDS analysis
3.2. Raman spectra Raman spectroscopy is one important qualitative technique for thin diamond film analysis. It offers the advantage of sensitivity not only to crystalline material, but also to the various possible non-crystalline phases. The first order Raman peak of well crystallized diamond, is at approximately 1332 cmy1 with a typical 1.5 cmy1 full width at half maximum ŽFWHM.. For CVD diamond films the typical FWHM is 7 cmy1 . Graphite displays a single Raman first-order active mode at 1580 cmy1 , labelled the G band while for disordered polycrystalline graphite one can find a second sharp band at approximately 1360 cmy1 , the D band. Amorphous carbon phases may give rise to broad ill-defined bands with a high-frequency cut-off near 1332 cmy1 for the sp 3-bonded carbon, and at approximately 1580 cmy1 for the sp 2-bonded carbon. The intensity and position of these broad bands depend on the growth conditions used and the wavelength of the exciting photon w12᎐16x. Fig. 4 shows Raman spectra measured from both sides of sample Co4 before and after etching in H 2 plasma. The formation of a graphite layer is evident in the beginning of the growth process. The graphite bands obtained from the film backside before the etching are very much the only observed ones ŽFig. 4a. at 1360 cmy1 and 1580 cmy1 , respectively. This initial graphite layer accounts for the poor adhesion of the films. After the etching the diamond peak starts to appear ŽFig. 4b., which confirms that diamond nucleated on a graphite layer. The Raman analysis made on the sample cross-section a few micrometers away from the filmrsubstrate interface gave spectra similar to the one measured from the film surface as shown in Fig. 4c.
To determine whether cobalt is present in the films, EDS analysis was carried out on sample Co4 Žthe thickest one.. In Fig. 5 we present EDS spectra taken on both sides of the film before and after the H 2 plasma etching. Besides the peaks due to contamination of gold and platinum, the peaks related to cobalt clearly indicate the presence of this element in the film backside. The
Fig. 4. Raman spectra of sample Co4 taken on: Ža. backside; Žb. backside after etching; and Žc. growth surface.
320
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
Fig. 5. EDS analysis taken on Co4 sample: Ža. backside; Žb. backside after etching and; Žc. growth surface.
quantitative analysis done in the film backside, gives a percentage of cobalt of approximately 27% and 3% before and after the etching, respectively. Although we did not observe any trace of cobalt in either the growth surface or the film cross-section for the two thickest samples, we cannot conclude that cobalt is not there, as EDS sensitivity requires at least 1% of the element. The presence of Co in the diamond lattice was confirmed by photoluminescence. In Fig. 6 we present the photoluminescence spectrum of sample Co4, whose luminescence band is very similarly to that of non-annealed HPHT samples w17x.
tion, from a preferential Ž111. face at the beginning to a mixture of Ž111. and Ž110. faces after a certain stage. Table 1 also shows the calculated growth rates. The highest growth rate obtained for sample Co2 can be understood if one looks at the XRD spectra and SEM pictures. This sample has Ž111. growth morphology, which means that the growth conditions are very close to the equilibrium ones. From the Raman spectra taken on the film backside before and after H 2 etching we can conclude that there is a graphite layer between the diamond film and the substrate very common on films, which exhibit poor adhesion. We can also say that diamond nucleation on cobalt started only after the saturation of the substrate surface by a layer of graphite and amorphous carbon, similar to what happens on iron substrates w18x. Both EDS and photoluminescence confirm the fact that Co is incorporated in the lattice, at least to a few micrometers thickness. We can conclude from the 18-m-thick Co2 film, that the Co penetrates deep into the diamond film. Cobalt particles are present on the crystallites grain boundaries and Co is also incorporated in the lattice. This leads us to the conclusion that cobalt probably diffuses into the diamond film through preferential Ž111. planes very much like what happens in HPHT diamond w8x. Further work, namely polishing of the films is under way to access the optical transparency.
Acknowledgements 4. Conclusions We have shown that free-standing diamond films can be easily grown on cobalt metal surfaces, even though the carburization of cobalt inhibits the nucleation of diamond. The film morphology changes with the growth evolu-
Fig. 6. PL spectrum taken on the growth surface of sample Co4.
This work was financially supported by the ‘Sub-Programa Ciencia e Tecnologia do 2⬚ Quadro Comunitario ˆ ´ de Apoio’. References w1x A.T. Collins, in: P. Vincenzini ŽEd.., New Diamond and Diamond-Like Films, Techna, Faenza, 1995, p. 315. w2x B.A. Fox, M.L. Hartsell, D.M. Malta et al., Diamond Relat. Mater. 4 Ž1995. 622. w3x I. Reineck, S. Soderberg, P.-E. Ekholm, K. Westergren, Surf. Coat. Technol. 57 Ž1993. 47. w4x S. Soderberg, A. Gerendas, M. Sjostrand, Vacuum 41 Ž1990. 1317. w5x T.H. Huang, C.-T. Kuo, T.S. Lin, Surf. Coat. Technol. 56 Ž1993. 47. w6x X. Peng, Z. Gan, Thin Solid Films 239 Ž1994. 47. w7x R. Cremer, R. Mertens, D. Neushutz, ¨ O. Lemmer, M. Frank, T. Leyendecker, Thin Solid Films 355r356 Ž1999. 127. w8x X. Jia, H. Kagi, S. Hayakawa, M. Wakatsuky, Y. Gohshi, in: S. Saito, N. Fujimori, O. Fukunaga, M. Kamo, K. Kobashi, M. Yoshicawa ŽEds.., Advances in New Diamond Science and Technology, MY, Tokyo, 1994, p. 525. w9x E. Pereira, L. Santos, Diamond Relat. Mater. 4 Ž1994. 26. w10x M.L. Hartsell, L.S. Plano, J. Mater. Res. 9 Ž1994. 921.
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321 w11x T.H. Huang, C.-T. Kuo, T.S. Lin, Surf. Coat. Technol. 56 Ž1993. 105. w12x J. Wagner, M. Ramsteiner, Ch. Wild, P. Koild, Phys. Rev. B 40 Ž1989. 1817. w13x M. Yoshikawa, G. Ktagiri, H. Ishida, A. Ishitani, M. Ono, K. Matsumura, Appl. Phys. Lett. 55 Ž1989. 2608. w14x J. Wagner, C. Wild, P. Koild, Appl. Phys. Lett. 59 Ž1991. 779.
321
w15x J. Wagner, C. Wild, W. Muller-Sebert, P. Koild, Appl. Phys. ¨ Lett. 61 Ž1992. 1284. w16x C.D. Clark, C.B. Dikerson, J. Phys. D. 4 Ž1992. 869. w17x S.C. Lawson, H. Kanda, K. Watanable, I. Kiflawi, Y. Sato, J. Appl. Phys. 79 Ž8. Ž1996. 4348. w18x Q.H. Fan, J. Gracio, E. Pereira, Diamond Relat. Mater. 7 ´ Ž1998. 603.
Free-standing diamond films grown on cobalt substrates M.A. NetoU , Qi Hua Fan, E. Pereira Department of Physics, Uni¨ ersity of A¨ eiro, 3810 A¨ eiro, Portugal
Abstract Diamond films were grown directly on cobalt substrates, using microwave plasma-assisted chemical vapour deposition. Although cobalt is known to inhibit the nucleation of diamond and enhancing the formation of graphite, we were able to grow relatively thick films Ž; 190 m.. The films were easily detached from the substrates. The poor adhesion allows the possibility of obtaining free-standing diamond films without chemical etching. Micro-Raman spectroscopy showed the 1332 cmy1 characteristic Raman peak of diamond and the 1580 cmy1, 1360 cmy1 bands of graphite, on the growth surface and backside of the films, respectively. Through scanning electron microscopy and X-ray diffraction we were able to monitor film thickness and morphology with growth evolution. The results showed the Ž111. preferential growth morphology for the film with higher growth rate. By energy dispersive X-ray spectroscopy it was only possible to detect cobalt in the back of the films, but not in the surface. The role of cobalt in the film growth is discussed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond growth and characterization; Free-standing diamond films; Cobalt; Chemical vapour deposition
1. Introduction The exceptional properties of diamond arise from two basic facts: Ža. carbon atoms are relatively small and light; and Žb. when the carbon atoms bind together in the diamond structure, they form very strong sp 3 covalent bonds. For example, approximately 5.5 eV is required to excite an electron from the valence band to the conduction band, compared with 1.1 eV for Si and 0.7 eV for Ge. Diamond is, therefore, a wide bandgap material. Furthermore, due to the high vibration energy of carbon atoms in diamond, the frequency of infrared absorption is abnormally high. As the one phonon transition is forbidden in the diamond lattice, only two phonon absorption and one phonon defect induced absorption are observed. These led to a broad optical transparency for diamond from the deep UV to
U
Corresponding author. Tel.: q351-234-370-278; fax: q351-234424-965. E-mail address: [email protected] ŽM.A. Neto..
the far IR region of the electromagnetic spectrum. Therefore, together with its mechanical properties, diamond is no longer a promising material but a reality for optical w1x, electronic w2x and mechanical applications w3x. One can then understand why chemical vapour deposition ŽCVD. diamond has an unquestionable leading position over natural and synthetic high pressure high temperature ŽHPHT. diamond. For mechanical applications we are interested to have good adherent diamond films on hard materials. Much work has been done on WCŽCo. tools. The presence of up to 12% of cobalt in tungsten carbide tools has generally been considered to prevent good adhesion between the film and the substrate w4᎐6x. For other applications Ži.e. optical., however, one must have transparent free-standing films. Usually the chosen substrate is silicon that requires the use of chemical etching. In this work we report the synthesis of good quality free-standing diamond films directly on cobalt substrates. To the author’s knowledge, there are very few reports on this subject so far w7x. The choice of
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 6 0 - 4
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
cobalt substrates arise from the fact that cobalt dissolves carbon, leading to the formation of graphite and amorphous carbon between the substrate and the film which in turn compromises adhesion. Changing the growth parameters we were able to monitor the growth rates and morphologies of the films thereby obtained. Since CVD diamond is formed at relatively high temperatures Ž; 800⬚C., it is possible that cobalt incorporates in CVD diamond similarly to what happens in HPHT diamond w8x. The effect of this eventual incorporation on the optical properties, have to be taken into account. 2. Experimental details Diamond films were grown using ASTeX PDS 18 microwave plasma CVD equipment on 99.9% cobalt substrates Ž5 = 5 = 1 mm.. All substrates were first polished with a sequence of SiC sand papers of 600, 1200 and 2400 grid. After this the substrates were polished with 3 m diamond paste and then ultrasonic cleaned in acetone. Finally and before the deposition the substrates were polished with 3 m diamond powder and again ultrasonically cleaned. The process gases used were hydrogen ŽH 2 . and methane ŽCH 4 .. The process parameters employed were as follows: microwave power, 2500 W; gas pressure, 90 torr; H 2 flow rate, 470 ; 500 sccm; CH 4 flow rate 20 ; 25 sccm ; deposition temperature, ; 800⬚C. The morphologies and crystallite sizes of the CVD diamond films were examined by scanning electron microscopy ŽSEM. using the FEG-SEM Hitachi S4100 system. The presence of cobalt in these films was done by energy dispersive X-ray spectroscopy ŽEDS. available in the SEM system. Prior to SEM analysis, the films were coated with PtrAu, in order to avoid charge effect. The presence of diamond and other forms of carbon was done by micro-Raman analysis with the use of a Jobin Yvon T64000 spectrometer, with a spectral resolution of 0.45 cmy1 and with an Arq laser using the 514.5-nm excitation line with 2 mW laser power on the sample. X-Ray diffraction ŽXRD. analysis was also used to confirm the synthesis of diamond in all the films. The XRD analysis was carried out on a Philips X’Pert equipment with a scanning step of 0.05⬚ and 0.5 s of integration time. Photoluminescence spectra were obtained with the samples at constant temperature Ž15 K. using the 325nm line of a He-Cd laser, as the excitation source and a detection system as described elsewhere w9x. 3. Results and discussion Diamond films with thickness ranging from 6 to 190
317
Table 1 The growth time, film thickness and growth rate of diamond on cobalt substrates Sample
Time Žh.
Thickness Žm.
Growth rate Žmrh.
Co1 Co2 Co3 Co4
3 5 25 68
6 18 65 190
2 3.6 2.6 2.8
m were successfully grown on Co substrates as summarized in Table 1. During post-deposition cooling the diamond films grown on Co did not crack, although the thermal expansion coefficient for cobalt is significantly higher as compared with diamond. This is because the films exhibited no adhesion. The films self delaminated from the Co substrates, similar to what happens on copper substrates w10x, without the use of any chemical etching. 3.1. Film morphology The morphologies of CVD diamond films are very diverse and very much a function of the growth conditions. Substrate temperature, carbon super-saturation and impurities have strong effects on morphology. Also, most properties and applications of CVD diamond are strongly related to growth morphology. So, if we understand the relationships between growth conditions and morphology it becomes possible to design diamond films for specific applications. The most stable growth planes of diamond are the octahedral Ž111. faces, followed by the cube Ž100. faces and the Ž110. faces w11x. So, under conditions close to equilibrium and during growth of polycrystalline diamond films, triangular Ž111. faces are the expected growth morphology. As the growth process moves away from equilibrium, the Ž100. cube faces appear, which results in a mixture of growth faces. A further movement away from equilibrium can lead to a dominant Ž100. cube morphology in the later stages of growth evolution, unless the Ž110. faces become energetically favourable. Fig. 1 shows the change of the films grain size and morphology when subject to different growth conditions. In sample Co2 we observe preferential Ž111. growth morphology with clear triangular facets parallel to the plane of the film. The next two films do not show this morphology, instead they can be characterized by having very sharp angular faceted surfaces with twins and stacking faults. Some Ž110. facets are also seen. In sample Co1 and Co2 we detected small particles on the diamond growth surface that we have identified as cobalt particles by EDS. These particles were not observed on the thicker films.
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M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
Fig. 1. SEM images showing the films grain size and morphologies. a. Co1; b. Co2; c. Co3; d. Co4.
Most CVD diamond films develop columnar grains in the direction of growth and show strong, preferred orientation as the film thickens. These columnar grains are a natural consequence of the growth processes. In Fig. 2 we present the films cross-section SEM images. It is very clear the formation of such columnar grains.
It is evident from the XRD spectra presented in Fig. 3 the characteristic X-ray diffraction angles for Ž111., Ž220. and Ž311. planes of diamond with 2 equal to 43.9⬚, 75.3⬚ and 91.5⬚, respectively. For sample Co2 we see a preferential Ž111. crystal orientation, while for samples Co3 and Co4 there is a mixture of Ž111., Ž220.
Fig. 2. SEM images showing the diamond films cross-section and thickness. a. Co1; b. Co2; c. Co3; d. Co4.
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
319
Fig. 3. X-Ray diffraction spectra of diamond films grown on cobalt substrates.
and Ž311. crystallographic planes. The fact that the Ž220. planes appear instead of the Ž400. ones is indicative that the Ž110. planes are energetically more favourable than the Ž100. under the current deposition conditions.
This proved that the diamond film Žsample Co4. is indeed of good homogeneity. The diamond line was fitted to a Lorentzian function giving a FWHM of 6.3 cmy1 , typical of good quality CVD diamond. 3.3. EDS analysis
3.2. Raman spectra Raman spectroscopy is one important qualitative technique for thin diamond film analysis. It offers the advantage of sensitivity not only to crystalline material, but also to the various possible non-crystalline phases. The first order Raman peak of well crystallized diamond, is at approximately 1332 cmy1 with a typical 1.5 cmy1 full width at half maximum ŽFWHM.. For CVD diamond films the typical FWHM is 7 cmy1 . Graphite displays a single Raman first-order active mode at 1580 cmy1 , labelled the G band while for disordered polycrystalline graphite one can find a second sharp band at approximately 1360 cmy1 , the D band. Amorphous carbon phases may give rise to broad ill-defined bands with a high-frequency cut-off near 1332 cmy1 for the sp 3-bonded carbon, and at approximately 1580 cmy1 for the sp 2-bonded carbon. The intensity and position of these broad bands depend on the growth conditions used and the wavelength of the exciting photon w12᎐16x. Fig. 4 shows Raman spectra measured from both sides of sample Co4 before and after etching in H 2 plasma. The formation of a graphite layer is evident in the beginning of the growth process. The graphite bands obtained from the film backside before the etching are very much the only observed ones ŽFig. 4a. at 1360 cmy1 and 1580 cmy1 , respectively. This initial graphite layer accounts for the poor adhesion of the films. After the etching the diamond peak starts to appear ŽFig. 4b., which confirms that diamond nucleated on a graphite layer. The Raman analysis made on the sample cross-section a few micrometers away from the filmrsubstrate interface gave spectra similar to the one measured from the film surface as shown in Fig. 4c.
To determine whether cobalt is present in the films, EDS analysis was carried out on sample Co4 Žthe thickest one.. In Fig. 5 we present EDS spectra taken on both sides of the film before and after the H 2 plasma etching. Besides the peaks due to contamination of gold and platinum, the peaks related to cobalt clearly indicate the presence of this element in the film backside. The
Fig. 4. Raman spectra of sample Co4 taken on: Ža. backside; Žb. backside after etching; and Žc. growth surface.
320
M.A. Neto et al. r Diamond and Related Materials 10 (2001) 316᎐321
Fig. 5. EDS analysis taken on Co4 sample: Ža. backside; Žb. backside after etching and; Žc. growth surface.
quantitative analysis done in the film backside, gives a percentage of cobalt of approximately 27% and 3% before and after the etching, respectively. Although we did not observe any trace of cobalt in either the growth surface or the film cross-section for the two thickest samples, we cannot conclude that cobalt is not there, as EDS sensitivity requires at least 1% of the element. The presence of Co in the diamond lattice was confirmed by photoluminescence. In Fig. 6 we present the photoluminescence spectrum of sample Co4, whose luminescence band is very similarly to that of non-annealed HPHT samples w17x.
tion, from a preferential Ž111. face at the beginning to a mixture of Ž111. and Ž110. faces after a certain stage. Table 1 also shows the calculated growth rates. The highest growth rate obtained for sample Co2 can be understood if one looks at the XRD spectra and SEM pictures. This sample has Ž111. growth morphology, which means that the growth conditions are very close to the equilibrium ones. From the Raman spectra taken on the film backside before and after H 2 etching we can conclude that there is a graphite layer between the diamond film and the substrate very common on films, which exhibit poor adhesion. We can also say that diamond nucleation on cobalt started only after the saturation of the substrate surface by a layer of graphite and amorphous carbon, similar to what happens on iron substrates w18x. Both EDS and photoluminescence confirm the fact that Co is incorporated in the lattice, at least to a few micrometers thickness. We can conclude from the 18-m-thick Co2 film, that the Co penetrates deep into the diamond film. Cobalt particles are present on the crystallites grain boundaries and Co is also incorporated in the lattice. This leads us to the conclusion that cobalt probably diffuses into the diamond film through preferential Ž111. planes very much like what happens in HPHT diamond w8x. Further work, namely polishing of the films is under way to access the optical transparency.
Acknowledgements 4. Conclusions We have shown that free-standing diamond films can be easily grown on cobalt metal surfaces, even though the carburization of cobalt inhibits the nucleation of diamond. The film morphology changes with the growth evolu-
Fig. 6. PL spectrum taken on the growth surface of sample Co4.
This work was financially supported by the ‘Sub-Programa Ciencia e Tecnologia do 2⬚ Quadro Comunitario ˆ ´ de Apoio’. References w1x A.T. Collins, in: P. Vincenzini ŽEd.., New Diamond and Diamond-Like Films, Techna, Faenza, 1995, p. 315. w2x B.A. Fox, M.L. Hartsell, D.M. Malta et al., Diamond Relat. Mater. 4 Ž1995. 622. w3x I. Reineck, S. Soderberg, P.-E. Ekholm, K. Westergren, Surf. Coat. Technol. 57 Ž1993. 47. w4x S. Soderberg, A. Gerendas, M. Sjostrand, Vacuum 41 Ž1990. 1317. w5x T.H. Huang, C.-T. Kuo, T.S. Lin, Surf. Coat. Technol. 56 Ž1993. 47. w6x X. Peng, Z. Gan, Thin Solid Films 239 Ž1994. 47. w7x R. Cremer, R. Mertens, D. Neushutz, ¨ O. Lemmer, M. Frank, T. Leyendecker, Thin Solid Films 355r356 Ž1999. 127. w8x X. Jia, H. Kagi, S. Hayakawa, M. Wakatsuky, Y. Gohshi, in: S. Saito, N. Fujimori, O. Fukunaga, M. Kamo, K. Kobashi, M. Yoshicawa ŽEds.., Advances in New Diamond Science and Technology, MY, Tokyo, 1994, p. 525. w9x E. Pereira, L. Santos, Diamond Relat. Mater. 4 Ž1994. 26. w10x M.L. Hartsell, L.S. Plano, J. Mater. Res. 9 Ž1994. 921.
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