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amorphous carbon composite films
Diamond & Related Materials 13 (2004) 2071 – 2075 www.elsevier.com/locate/diamond
Structural investigation of nanocrystalline diamond/amorphous carbon composite films C. Popov a,*, W. Kulisch a,b, S. Boycheva a, K. Yamamoto c, G. Ceccone b, Y. Koga c a
Institute of Microstructure Technologies and Analytics (IMA), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany b Institute for Health and Consumer Protection, Joint Research Centre, Ispra, Italy c National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan Available online 15 June 2004
Abstract Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been prepared by microwave plasma chemical vapor deposition (MWCVD) from methane/nitrogen mixtures. The complex nature of the coatings required the application of a variety of complementary analytical techniques in order to elucidate their structure. The crystallinity of the samples was studied by selected-area electron diffraction (SAED). The diffraction patterns revealed the presence of diamond crystallites within the films. From the images taken by transmission electron microscopy (TEM) the crystallite size was determined to be on the order of 3 – 5 nm. The results were confirmed by X-ray diffraction (XRD) measurements exhibiting broad (111) and (220) peaks of diamond from which the average size of the crystallites was calculated. The grain boundary width is 1 – 1.5 nm as observed by TEM images which corresponds to a matrix volume fraction of about 40 – 50%. This correlates very well with the crystalline phase content of about 50% in the films estimated from their density (2.75 g/cm3 as determined by X-ray reflectivity). The bonding structure of the composite films was studied by electron energy loss spectroscopy (EELS) in the region of carbon core level. The spectra were dominated by a peak at 292 eV indicating the diamond nature of the investigated films. In addition, the spectra of NCD/a-C films possessed a shoulder at 284 eV due to the presence of a small sp2 bonded fraction. This phase was identified also by X-ray photoelectron spectroscopy (XPS). The sp2/sp3 ratio was on the order of 10% as determined by deconvolution of the C1s XPS peak. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond films; Microwave plasma CVD; Microstructure
1. Introduction The outstanding properties of diamond, such as extreme hardness, low friction coefficient, chemical inertness, high electrical resistance, excellent thermal conductivity, and good biocompatibility [1], make it of potential interest for a wide spectrum of applications including wear resistant and transparent protective coatings for optical components, heat spreaders, novel semiconductor devices, etc. However, the diamond coatings prepared by chemical vapor deposition techniques (CVD) are in most cases rough and nonuniform over large areas. The high surface roughness is a major * Corresponding author. Tel.: +49-561-804-4205; fax: +49-561-8044136. E-mail address: [email protected] (C. Popov). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.04.001
problem for wear resistant applications [2], for example in optical coatings, since it causes attenuation and scattering of the transmitted light. The deposition of diamond-related films, e.g. diamondlike carbon (DLC), tetrahedral amorphous carbon (ta-C) or nanocrystalline diamond (NCD) can overcome the roughness problem [3]. These films are much smoother and at the same time extremely hard, not reaching, however, the hardness of diamond. The first two types of films (DLC and ta-C) are prepared by ion-assisted methods leading to high compressive stresses [4]. The stress in the films directly affects their adhesion to the substrate and causes delamination. NCD films can be deposited by CVD techniques, i.e. without ion impact on the growing films. In such a way the stress problem can be solved to a great extent [5].
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In general, there are two main approaches towards deposition of NCD films, either in ‘‘pure’’ form or as nanocrystallites embedded in an amorphous matrix: (i) ‘‘the polycrystalline diamond route‘‘ based on standard low pressure techniques for diamond deposition (especially MWCVD) [6 –9]. In order to interrupt the crystal growth and to enhance the rate of secondary nucleation (leading finally to nanocrystallites), one or more of the process parameters should deviate from the standard values for polycrystalline diamond growth [9]: lowered working pressure; higher methane concentrations; partial or complete substitution of H2 by N2 or Ar; application of bias voltage during the entire process. (ii) ‘‘the DLC and ta-C route‘‘ based on ion-assisted techniques such as sputtering [10] or rf plasmaenhanced CVD [11]. By using high pressures, increased concentrations of Ar in the plasma as well as addition of H2 and providing sp3-rich underlayers, it is possible to achieve diamond nanocrystals in an amorphous matrix. In almost all papers concerning NCD up to now, irrespective of the deposition route used for its preparation, the films are not comprehensively characterized; this regards especially the fractions of the crystalline and the amorphous phase and how the nature of the matrix influences the properties of the nanocomposite material. In this paper we report on the results of a comprehensive investigation of the structure of NCD/a-C composite films by a variety of analytical techniques, which yield important information for the control of the basic and application relevant properties of these coatings.
2. Experimental Nanocrystalline diamond/amorphous carbon composite films were prepared by MWCVD with the deposition setup described in detail previously [12 –14]. Methane/nitrogen mixtures with CH4 concentrations up to 17% were used as precursors while the remaining process parameters were kept constant: substrate temperature 770 jC, working pressure 26 mbar, microwave plasma input power 800 W, deposition time 390– 420 min. All films were grown on monocrystalline (100) Si wafers, which were ultrasonically pretreated in a suspension of diamond powder (grain size up to 500 nm) and n-pentane in order to promote the nucleation of diamond. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) of the NCD/a-C films were carried out with an energy-filtering transmission electron microscope (Leo, EM-922). The electron acceleration voltage was 200 kV. The specimens for TEM observations were prepared by a conventional mechanical grinding and dimpling and subsequently thinned by ion milling with Ar+ at an energy of 5 keV. The incident angle of the ions was 4j to the
surface, which minimized the damage of the samples by the ion irradiation. The results concerning the crystallinity of the films were compared to X-ray diffraction (XRD) patterns taken with the 0.15418 nm CuKa line at normal as well as at grazing (5j) incidence. Further information about the bonding nature of the films was obtained by X-ray photoelectron spectroscopy (XPS). The analyses were performed with an AXIS ULTRA spectrometer (Krakos Analytical) equipped with a monochromatic Al Ka X-ray source (1486.6 eV). The analysis spot was 110 Am in diameter and the take off angle 90j relative to the surface.
3. Results and discussion 3.1. Crystallinity and structure of NCD/a-C films Selected-area electron diffraction (500 nm in diameter) taken from samples prepared from different CH4/N2 mixtures revealed similar ring patterns (Fig. 1); the rings were homogeneous indicating the absence of any texture. The calculated d-spacings match very well with the reference data for diamond, i.e. the crystalline phase in the composite films is diamond. The TEM images proved the nanocrystalline nature of the films. From the dark field images the crystallite size was determined to be on the order of 3– 5 nm (Fig. 2a). The bright field images show a fine network of bright lines which we identified with the grain boundary material surrounding the diamond nanocrystals (Fig. 2b). The grain boundary width was measured from bright field images with higher magnifications to be 1 –1.5 nm. The above results concerning the crystallinity of the NCD/a-C films were entirely corroborated by the XRD investigations. The XRD patterns of the films, taken at 5j grazing incidence, exhibited broad peaks attributed to the (111) and (220) reflections of diamond. The average crys-
Fig. 1. Electron diffraction pattern of a NCD/a-C film prepared with 17% CH4.
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mined by TEM. Continuous and thicker films were deposited with 17% CH4; in this case the deposition rate was more than five times higher (0.57 Am/h). Thus, from the TEM measurements it can be concluded that there are no great (if any) differences between the samples prepared from different precursor mixtures with respect to the microscopic structure (crystallite size, width of the grain boundaries), while major differences can be found in their macroscopic appearance, observed also during previous morphological and topographical investigations of the layers. Another important question which has to be addressed is the fraction of each phase (crystalline and amorphous) in the composite films. The volume fraction of the amorphous matrix was calculated on the base of the TEM results, assuming for simplicity cubic crystals of 3 – 5 nm size, separated by grain boundaries with a width of 1 – 1.5 nm. Matrix fractions of 40 – 50% in the NCD/a-C films were obtained. Further, the crystalline phase fraction was estimated also from the density of the films which was determined by X-ray reflectivity [16]. For the smooth films prepared with 17% methane it was 2.75 g/cm3 F 10%. Comparing this density to that of diamond, which is 3.49 g/cm3 and assuming a matrix density of about 2 g/cm3, one can conclude that the crystalline fraction is ca. 50% of the composite material, which supports the results for the matrix fraction. 3.2. Bonding nature of the NCD/a-C films
Fig. 2. TEM images of a NCD/a-C film prepared with 9% CH4: dark field (top) and bright field (bottom).
tallite size was determined using a modified Debye-Scherrer formula [15]: it was on the order of 3 –4 nm, irrespective of the methane concentration, i.e. the same as revealed by the TEM studies. Further information about the macrostructure of the samples was obtained from the TEM low magnification images. They show that the films prepared with 9% methane consist of features with diameters on the order of 500 nm. Some of these features form a kind of agglomerates but others are clearly separated by voids. As these images have been taken from in-plane view specimens, they are in full agreement with SEM and AFM results reported previously [12 – 14,16]. The morphology of the films prepared with 9% CH4 was composed of nodules, which did not coalesce to form a continuous film owing to the relatively low growth rate (0.11 Am/h), as shown by SEM. The AFM images of these films exhibited the rounded tops to the nodules with a diameter of about 500 nm, i.e. of the same size as deter-
Initially, the bonding structure of the composite films was investigated by EELS. The different carbon phases have very distinct K-shell absorption edge structures: diamond has a single feature with an onset at 289 eV, due to transitions from 1 s to j* electronic states, while graphitic and amorphous sp2bonded carbon have an additional EELS edge at about 284 eV, owing to the transition from 1s to antibonding k* states. Fig. 3 shows the EEL spectra of our NCD/a-C films together with spectra of other carbon materials for comparison. All spectra of the investigated NCD/a-C films were quite similar, dominated by a peak at 292 eV which indicates their diamond nature and including a dip around 300 eV, characteristic of the diamond band structure [17]. In contrast to the CVD diamond film, some spectra of the NCD films possess a shoulder at 284 eV due to the presence of amorphous and/or graphitic sp2bonded fractions in the matrix. Okada et al. [18] observed similar EEL spectra of nanocrystalline diamond prepared by inductively coupled plasma CVD; by chemical bonding mapping they established that the width of the material containing sp2-bonds was approximately 1 nm, i.e. on the order of the grain boundaries widths discussed above. The comparison of the EEL spectra of our NCD/a-C films prepared from different precursor mixtures show that the gas phase composition had (almost) no influence on the bonding structure of the films but a slight inhomogeneity is present in the films, either in the sp2/sp3 ratio in the matrix, or in the matrix fraction of the composite films.
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C. Popov et al. / Diamond & Related Materials 13 (2004) 2071–2075 Table 1 Results of the deconvolution of the C 1s core level spectra and comparison with literature data [19,20] (Wilson et al. gave no absolute values but only the differences with respect to the main diamond peak) Peak no.
Fig. 3. EELS spectra of NCD/a-C films prepared with different CH4 concentrations. The spectra of CVD diamond, a-C and graphite are also given for comparison.
The XPS spectra of NCD/a-C films were taken after very slight Ar+ ion etching for 45 s at a low energy (500 eV) and low current density (3 nA/mm2). The C 1s core spectrum of the film prepared from 9% CH4/N2 mixture is presented in Fig. 4; it was deconvoluted into four sub-peaks on the basis of literature data [19]. The results from the deconvolution are summarized in Table 1 which contains also literature data from Wilson et al. [19] for polycrystalline diamond and Xu et al. [20] for nanocrystalline diamond films. As can be seen from Table 1, the sp2/sp3 ratio in the NCD/a-C films is on the order of 10%, irrespective of the methane concentration in the gas phase. These results are in good agreement with the EELS measurements that sp2-bonded carbon is present within the films but its fraction is not very large. The
Fig. 4. XPS C 1s peak of a NCD/a-C film prepared with 9% CH4 after slight ion etching (45 s at 500 eV.) The inset shows the XPS valence band spectrum of the same sample.
1
2 2
3
3
4
CUO, CUOUC 19.7
CMO
Assignment
sp CUC
Fraction in NCD/a-C (9%CH4) Fraction in NCD/a-C (17%CH4) Position [eV] Wilson et al. [19] Xu et al. [20]
7.8
sp CUC, CUH 69.6
7.0
80.1
9.1
2.5
284.4 0 284.5
285.2 1.2 1.6 285.8
286.4 2.6 2.9 286.9
283.5 0.8 –
0.9
1.8
presence of oxygen containing units is expected, having in mind the oxygen concentration of about 5 at.% in the surface region of the samples [14,16]. The valence band spectra of the samples confirm the conclusions drawn from the C 1s core spectra: there are no great differences between the samples prepared from different precursor mixtures, the sp2/sp3 ratio in all films is relatively low and the surface of the layers contains some oxygen (inset in Fig. 4). Additional XPS measurements were performed after heavy Ar+ bombardment (5 keV, 0.11 AA/mm2, 1 min); the obtained C 1s spectrum was compared to that observed after light Ar+ bombardment discussed above (Fig. 5). As a consequence of the ion bombardment, the sub-peak at 284.4 eV strongly decreased, while the sub-peak at 283.5 eV increased, i.e. the sp2/sp3 ratio dramatically increased due to graphitization/ amorphization of the NCD/a-C films. These results corroborate the assignments of the sub-peaks at 283.5 eV and 284.4 eV to sp2 and sp3 carbon – carbon and carbon – hydrogen bonds, respectively.
Fig. 5. XPS C 1s peak of a NCD/a-C film prepared with 9% CH4: (a) after low energy (500 eV) Ar+ etching; (b) after high energy (5 keV) Ar+ etching.
C. Popov et al. / Diamond & Related Materials 13 (2004) 2071–2075
4. Conclusions Nanocrystalline diamond/amorphous carbon composite films were prepared by MWCVD from CH4/N2 mixtures. The films are composed of diamond crystallites of 3– 5 nm separated by grain boundaries of about 1 –1.5 nm. The crystalline phase/amorphous matrix ratio is close to unity and the matrix includes at about 10% sp2-bonded carbon. The microscopic structure of the films remains unchanged within the range of the process parameters investigated.
Acknowledgements The authors gratefully acknowledge the financial support of the German Ministry of Education and Research (BMBF) under the WTZ Program (Contract CZE 02/010), as well as of the German Research Society (DFG KON 633/2004). W.K. would like to thank the support through an European Community Marie Curie Fellowship (HPMF-CT-200201713) and S.B. the Alexander von Humboldt Foundation for the Research Fellowship.
References [1] W. Kulisch, Deposition of Superhard Diamond-Like Materials, Springer, Heidelberg, 1999. [2] A.K. Gangopadhyay, M.A. Tamor, Wear 169 (1993) 221. [3] M. Chhowalla, Y. Yin, G.A.J. Amaratunga, D.R. McKenzie, T. Frauenheim, Appl. Phys. Lett. 69 (1996) 2344.
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[4] R.G. Lacerda, F.C. Marques, Appl. Phys. Lett. 73 (1998) 617. [5] T. Sharda, M. Umeno, T. Soga, T. Jimbo, J. Appl. Phys. 89 (2001) 4874. [6] N. Jiang, K. Sugimoto, K. Eguchi, T. Inaoka, Y. Shintani, H. Makita, A. Hatta, A. Hiraki, J. Cryst. Growth 222 (2001) 591. [7] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [8] K. Wu, E.G. Wang, J. Chen, N.S. Xu, J. Vac. Sci. Technol., B 17 (1999) 1059. [9] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [10] S.N. Kundu, M. Basu, A.B. Maity, S. Chaudhuri, A.K. Pal, Mater. Lett. 31 (1997) 303. [11] G.A.J. Amaratunga, A. Putnis, K. Clay, W. Milne, Appl. Phys. Lett. 55 (1989) 634. [12] C. Popov, W. Kulisch, in: M. Allendorf, F. Maury, F. Teyssandier (Eds.), Proceedings of the CVD XVI and EUROCVD-14 Conference, The Electrochemical Society, Pennington, NJ, 2003, p. 1079. [13] C. Popov, W. Kulisch, S. Boycheva, M. Jelinek, in: P. Sandera (Ed.), Proceedings of the International Conference on Nanosciences, Nanotechnologies and Nanomaterials NANO ’03, VUT Publisher, Brno, Czech Republic, 2003, p. 148. [14] C. Popov, M. Jelinek, S. Boycheva, V. Vorlicek, W. Kulisch, J. Metastable, Nanocryst. Mater. (2004) (in press). [15] T.H. de Keijser, E.J. Mittermeijer, H.C. Rozendaal, J. Appl. Crystallogr. 16 (1983) 309. [16] C. Popov, W. Kulisch, P.N. Gibson, G. Ceccone, M. Jelinek, Diamond Relat. Mater. 13 (2004) 1371. [17] I. Gouzman, A. Hoffman, G. Comtet, L. Hellner, G. Dujardin, M. Petravic, Appl. Phys. Lett. 72 (1998) 2517. [18] K. Okada, K. Kimoto, S. Komatsu, S. Matsumoto, J. Appl. Phys. 93 (2003) 3120. [19] J.I.B. Wilson, J.S. Walton, G. Beamson, J. Electron Spectrosc. Relat. Phenom. 121 (2001) 183. [20] T. Xu, S. Yang, J. Lu, Q. Xue, J. Li, W. Guo, Y. Sun, Diamond Relat. Mater. 10 (2001) 1441.
Structural investigation of nanocrystalline diamond/amorphous carbon composite films C. Popov a,*, W. Kulisch a,b, S. Boycheva a, K. Yamamoto c, G. Ceccone b, Y. Koga c a
Institute of Microstructure Technologies and Analytics (IMA), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany b Institute for Health and Consumer Protection, Joint Research Centre, Ispra, Italy c National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan Available online 15 June 2004
Abstract Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been prepared by microwave plasma chemical vapor deposition (MWCVD) from methane/nitrogen mixtures. The complex nature of the coatings required the application of a variety of complementary analytical techniques in order to elucidate their structure. The crystallinity of the samples was studied by selected-area electron diffraction (SAED). The diffraction patterns revealed the presence of diamond crystallites within the films. From the images taken by transmission electron microscopy (TEM) the crystallite size was determined to be on the order of 3 – 5 nm. The results were confirmed by X-ray diffraction (XRD) measurements exhibiting broad (111) and (220) peaks of diamond from which the average size of the crystallites was calculated. The grain boundary width is 1 – 1.5 nm as observed by TEM images which corresponds to a matrix volume fraction of about 40 – 50%. This correlates very well with the crystalline phase content of about 50% in the films estimated from their density (2.75 g/cm3 as determined by X-ray reflectivity). The bonding structure of the composite films was studied by electron energy loss spectroscopy (EELS) in the region of carbon core level. The spectra were dominated by a peak at 292 eV indicating the diamond nature of the investigated films. In addition, the spectra of NCD/a-C films possessed a shoulder at 284 eV due to the presence of a small sp2 bonded fraction. This phase was identified also by X-ray photoelectron spectroscopy (XPS). The sp2/sp3 ratio was on the order of 10% as determined by deconvolution of the C1s XPS peak. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond films; Microwave plasma CVD; Microstructure
1. Introduction The outstanding properties of diamond, such as extreme hardness, low friction coefficient, chemical inertness, high electrical resistance, excellent thermal conductivity, and good biocompatibility [1], make it of potential interest for a wide spectrum of applications including wear resistant and transparent protective coatings for optical components, heat spreaders, novel semiconductor devices, etc. However, the diamond coatings prepared by chemical vapor deposition techniques (CVD) are in most cases rough and nonuniform over large areas. The high surface roughness is a major * Corresponding author. Tel.: +49-561-804-4205; fax: +49-561-8044136. E-mail address: [email protected] (C. Popov). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.04.001
problem for wear resistant applications [2], for example in optical coatings, since it causes attenuation and scattering of the transmitted light. The deposition of diamond-related films, e.g. diamondlike carbon (DLC), tetrahedral amorphous carbon (ta-C) or nanocrystalline diamond (NCD) can overcome the roughness problem [3]. These films are much smoother and at the same time extremely hard, not reaching, however, the hardness of diamond. The first two types of films (DLC and ta-C) are prepared by ion-assisted methods leading to high compressive stresses [4]. The stress in the films directly affects their adhesion to the substrate and causes delamination. NCD films can be deposited by CVD techniques, i.e. without ion impact on the growing films. In such a way the stress problem can be solved to a great extent [5].
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In general, there are two main approaches towards deposition of NCD films, either in ‘‘pure’’ form or as nanocrystallites embedded in an amorphous matrix: (i) ‘‘the polycrystalline diamond route‘‘ based on standard low pressure techniques for diamond deposition (especially MWCVD) [6 –9]. In order to interrupt the crystal growth and to enhance the rate of secondary nucleation (leading finally to nanocrystallites), one or more of the process parameters should deviate from the standard values for polycrystalline diamond growth [9]: lowered working pressure; higher methane concentrations; partial or complete substitution of H2 by N2 or Ar; application of bias voltage during the entire process. (ii) ‘‘the DLC and ta-C route‘‘ based on ion-assisted techniques such as sputtering [10] or rf plasmaenhanced CVD [11]. By using high pressures, increased concentrations of Ar in the plasma as well as addition of H2 and providing sp3-rich underlayers, it is possible to achieve diamond nanocrystals in an amorphous matrix. In almost all papers concerning NCD up to now, irrespective of the deposition route used for its preparation, the films are not comprehensively characterized; this regards especially the fractions of the crystalline and the amorphous phase and how the nature of the matrix influences the properties of the nanocomposite material. In this paper we report on the results of a comprehensive investigation of the structure of NCD/a-C composite films by a variety of analytical techniques, which yield important information for the control of the basic and application relevant properties of these coatings.
2. Experimental Nanocrystalline diamond/amorphous carbon composite films were prepared by MWCVD with the deposition setup described in detail previously [12 –14]. Methane/nitrogen mixtures with CH4 concentrations up to 17% were used as precursors while the remaining process parameters were kept constant: substrate temperature 770 jC, working pressure 26 mbar, microwave plasma input power 800 W, deposition time 390– 420 min. All films were grown on monocrystalline (100) Si wafers, which were ultrasonically pretreated in a suspension of diamond powder (grain size up to 500 nm) and n-pentane in order to promote the nucleation of diamond. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) of the NCD/a-C films were carried out with an energy-filtering transmission electron microscope (Leo, EM-922). The electron acceleration voltage was 200 kV. The specimens for TEM observations were prepared by a conventional mechanical grinding and dimpling and subsequently thinned by ion milling with Ar+ at an energy of 5 keV. The incident angle of the ions was 4j to the
surface, which minimized the damage of the samples by the ion irradiation. The results concerning the crystallinity of the films were compared to X-ray diffraction (XRD) patterns taken with the 0.15418 nm CuKa line at normal as well as at grazing (5j) incidence. Further information about the bonding nature of the films was obtained by X-ray photoelectron spectroscopy (XPS). The analyses were performed with an AXIS ULTRA spectrometer (Krakos Analytical) equipped with a monochromatic Al Ka X-ray source (1486.6 eV). The analysis spot was 110 Am in diameter and the take off angle 90j relative to the surface.
3. Results and discussion 3.1. Crystallinity and structure of NCD/a-C films Selected-area electron diffraction (500 nm in diameter) taken from samples prepared from different CH4/N2 mixtures revealed similar ring patterns (Fig. 1); the rings were homogeneous indicating the absence of any texture. The calculated d-spacings match very well with the reference data for diamond, i.e. the crystalline phase in the composite films is diamond. The TEM images proved the nanocrystalline nature of the films. From the dark field images the crystallite size was determined to be on the order of 3– 5 nm (Fig. 2a). The bright field images show a fine network of bright lines which we identified with the grain boundary material surrounding the diamond nanocrystals (Fig. 2b). The grain boundary width was measured from bright field images with higher magnifications to be 1 –1.5 nm. The above results concerning the crystallinity of the NCD/a-C films were entirely corroborated by the XRD investigations. The XRD patterns of the films, taken at 5j grazing incidence, exhibited broad peaks attributed to the (111) and (220) reflections of diamond. The average crys-
Fig. 1. Electron diffraction pattern of a NCD/a-C film prepared with 17% CH4.
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mined by TEM. Continuous and thicker films were deposited with 17% CH4; in this case the deposition rate was more than five times higher (0.57 Am/h). Thus, from the TEM measurements it can be concluded that there are no great (if any) differences between the samples prepared from different precursor mixtures with respect to the microscopic structure (crystallite size, width of the grain boundaries), while major differences can be found in their macroscopic appearance, observed also during previous morphological and topographical investigations of the layers. Another important question which has to be addressed is the fraction of each phase (crystalline and amorphous) in the composite films. The volume fraction of the amorphous matrix was calculated on the base of the TEM results, assuming for simplicity cubic crystals of 3 – 5 nm size, separated by grain boundaries with a width of 1 – 1.5 nm. Matrix fractions of 40 – 50% in the NCD/a-C films were obtained. Further, the crystalline phase fraction was estimated also from the density of the films which was determined by X-ray reflectivity [16]. For the smooth films prepared with 17% methane it was 2.75 g/cm3 F 10%. Comparing this density to that of diamond, which is 3.49 g/cm3 and assuming a matrix density of about 2 g/cm3, one can conclude that the crystalline fraction is ca. 50% of the composite material, which supports the results for the matrix fraction. 3.2. Bonding nature of the NCD/a-C films
Fig. 2. TEM images of a NCD/a-C film prepared with 9% CH4: dark field (top) and bright field (bottom).
tallite size was determined using a modified Debye-Scherrer formula [15]: it was on the order of 3 –4 nm, irrespective of the methane concentration, i.e. the same as revealed by the TEM studies. Further information about the macrostructure of the samples was obtained from the TEM low magnification images. They show that the films prepared with 9% methane consist of features with diameters on the order of 500 nm. Some of these features form a kind of agglomerates but others are clearly separated by voids. As these images have been taken from in-plane view specimens, they are in full agreement with SEM and AFM results reported previously [12 – 14,16]. The morphology of the films prepared with 9% CH4 was composed of nodules, which did not coalesce to form a continuous film owing to the relatively low growth rate (0.11 Am/h), as shown by SEM. The AFM images of these films exhibited the rounded tops to the nodules with a diameter of about 500 nm, i.e. of the same size as deter-
Initially, the bonding structure of the composite films was investigated by EELS. The different carbon phases have very distinct K-shell absorption edge structures: diamond has a single feature with an onset at 289 eV, due to transitions from 1 s to j* electronic states, while graphitic and amorphous sp2bonded carbon have an additional EELS edge at about 284 eV, owing to the transition from 1s to antibonding k* states. Fig. 3 shows the EEL spectra of our NCD/a-C films together with spectra of other carbon materials for comparison. All spectra of the investigated NCD/a-C films were quite similar, dominated by a peak at 292 eV which indicates their diamond nature and including a dip around 300 eV, characteristic of the diamond band structure [17]. In contrast to the CVD diamond film, some spectra of the NCD films possess a shoulder at 284 eV due to the presence of amorphous and/or graphitic sp2bonded fractions in the matrix. Okada et al. [18] observed similar EEL spectra of nanocrystalline diamond prepared by inductively coupled plasma CVD; by chemical bonding mapping they established that the width of the material containing sp2-bonds was approximately 1 nm, i.e. on the order of the grain boundaries widths discussed above. The comparison of the EEL spectra of our NCD/a-C films prepared from different precursor mixtures show that the gas phase composition had (almost) no influence on the bonding structure of the films but a slight inhomogeneity is present in the films, either in the sp2/sp3 ratio in the matrix, or in the matrix fraction of the composite films.
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C. Popov et al. / Diamond & Related Materials 13 (2004) 2071–2075 Table 1 Results of the deconvolution of the C 1s core level spectra and comparison with literature data [19,20] (Wilson et al. gave no absolute values but only the differences with respect to the main diamond peak) Peak no.
Fig. 3. EELS spectra of NCD/a-C films prepared with different CH4 concentrations. The spectra of CVD diamond, a-C and graphite are also given for comparison.
The XPS spectra of NCD/a-C films were taken after very slight Ar+ ion etching for 45 s at a low energy (500 eV) and low current density (3 nA/mm2). The C 1s core spectrum of the film prepared from 9% CH4/N2 mixture is presented in Fig. 4; it was deconvoluted into four sub-peaks on the basis of literature data [19]. The results from the deconvolution are summarized in Table 1 which contains also literature data from Wilson et al. [19] for polycrystalline diamond and Xu et al. [20] for nanocrystalline diamond films. As can be seen from Table 1, the sp2/sp3 ratio in the NCD/a-C films is on the order of 10%, irrespective of the methane concentration in the gas phase. These results are in good agreement with the EELS measurements that sp2-bonded carbon is present within the films but its fraction is not very large. The
Fig. 4. XPS C 1s peak of a NCD/a-C film prepared with 9% CH4 after slight ion etching (45 s at 500 eV.) The inset shows the XPS valence band spectrum of the same sample.
1
2 2
3
3
4
CUO, CUOUC 19.7
CMO
Assignment
sp CUC
Fraction in NCD/a-C (9%CH4) Fraction in NCD/a-C (17%CH4) Position [eV] Wilson et al. [19] Xu et al. [20]
7.8
sp CUC, CUH 69.6
7.0
80.1
9.1
2.5
284.4 0 284.5
285.2 1.2 1.6 285.8
286.4 2.6 2.9 286.9
283.5 0.8 –
0.9
1.8
presence of oxygen containing units is expected, having in mind the oxygen concentration of about 5 at.% in the surface region of the samples [14,16]. The valence band spectra of the samples confirm the conclusions drawn from the C 1s core spectra: there are no great differences between the samples prepared from different precursor mixtures, the sp2/sp3 ratio in all films is relatively low and the surface of the layers contains some oxygen (inset in Fig. 4). Additional XPS measurements were performed after heavy Ar+ bombardment (5 keV, 0.11 AA/mm2, 1 min); the obtained C 1s spectrum was compared to that observed after light Ar+ bombardment discussed above (Fig. 5). As a consequence of the ion bombardment, the sub-peak at 284.4 eV strongly decreased, while the sub-peak at 283.5 eV increased, i.e. the sp2/sp3 ratio dramatically increased due to graphitization/ amorphization of the NCD/a-C films. These results corroborate the assignments of the sub-peaks at 283.5 eV and 284.4 eV to sp2 and sp3 carbon – carbon and carbon – hydrogen bonds, respectively.
Fig. 5. XPS C 1s peak of a NCD/a-C film prepared with 9% CH4: (a) after low energy (500 eV) Ar+ etching; (b) after high energy (5 keV) Ar+ etching.
C. Popov et al. / Diamond & Related Materials 13 (2004) 2071–2075
4. Conclusions Nanocrystalline diamond/amorphous carbon composite films were prepared by MWCVD from CH4/N2 mixtures. The films are composed of diamond crystallites of 3– 5 nm separated by grain boundaries of about 1 –1.5 nm. The crystalline phase/amorphous matrix ratio is close to unity and the matrix includes at about 10% sp2-bonded carbon. The microscopic structure of the films remains unchanged within the range of the process parameters investigated.
Acknowledgements The authors gratefully acknowledge the financial support of the German Ministry of Education and Research (BMBF) under the WTZ Program (Contract CZE 02/010), as well as of the German Research Society (DFG KON 633/2004). W.K. would like to thank the support through an European Community Marie Curie Fellowship (HPMF-CT-200201713) and S.B. the Alexander von Humboldt Foundation for the Research Fellowship.
References [1] W. Kulisch, Deposition of Superhard Diamond-Like Materials, Springer, Heidelberg, 1999. [2] A.K. Gangopadhyay, M.A. Tamor, Wear 169 (1993) 221. [3] M. Chhowalla, Y. Yin, G.A.J. Amaratunga, D.R. McKenzie, T. Frauenheim, Appl. Phys. Lett. 69 (1996) 2344.
2075
[4] R.G. Lacerda, F.C. Marques, Appl. Phys. Lett. 73 (1998) 617. [5] T. Sharda, M. Umeno, T. Soga, T. Jimbo, J. Appl. Phys. 89 (2001) 4874. [6] N. Jiang, K. Sugimoto, K. Eguchi, T. Inaoka, Y. Shintani, H. Makita, A. Hatta, A. Hiraki, J. Cryst. Growth 222 (2001) 591. [7] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [8] K. Wu, E.G. Wang, J. Chen, N.S. Xu, J. Vac. Sci. Technol., B 17 (1999) 1059. [9] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [10] S.N. Kundu, M. Basu, A.B. Maity, S. Chaudhuri, A.K. Pal, Mater. Lett. 31 (1997) 303. [11] G.A.J. Amaratunga, A. Putnis, K. Clay, W. Milne, Appl. Phys. Lett. 55 (1989) 634. [12] C. Popov, W. Kulisch, in: M. Allendorf, F. Maury, F. Teyssandier (Eds.), Proceedings of the CVD XVI and EUROCVD-14 Conference, The Electrochemical Society, Pennington, NJ, 2003, p. 1079. [13] C. Popov, W. Kulisch, S. Boycheva, M. Jelinek, in: P. Sandera (Ed.), Proceedings of the International Conference on Nanosciences, Nanotechnologies and Nanomaterials NANO ’03, VUT Publisher, Brno, Czech Republic, 2003, p. 148. [14] C. Popov, M. Jelinek, S. Boycheva, V. Vorlicek, W. Kulisch, J. Metastable, Nanocryst. Mater. (2004) (in press). [15] T.H. de Keijser, E.J. Mittermeijer, H.C. Rozendaal, J. Appl. Crystallogr. 16 (1983) 309. [16] C. Popov, W. Kulisch, P.N. Gibson, G. Ceccone, M. Jelinek, Diamond Relat. Mater. 13 (2004) 1371. [17] I. Gouzman, A. Hoffman, G. Comtet, L. Hellner, G. Dujardin, M. Petravic, Appl. Phys. Lett. 72 (1998) 2517. [18] K. Okada, K. Kimoto, S. Komatsu, S. Matsumoto, J. Appl. Phys. 93 (2003) 3120. [19] J.I.B. Wilson, J.S. Walton, G. Beamson, J. Electron Spectrosc. Relat. Phenom. 121 (2001) 183. [20] T. Xu, S. Yang, J. Lu, Q. Xue, J. Li, W. Guo, Y. Sun, Diamond Relat. Mater. 10 (2001) 1441.