Papers

ICMCTF2002 Session D1-1: Synthesis, Characterization and Applications of Carbon Nitride, Boron Nitride and Nano-Structures

Monday, April 22, 2002 1:30 PM in Room Royal Palm Salon 1-3

Monday Afternoon

Start	Invited?	Item
1:30 PM		D1-1-1 Materials Science of Fullerene-like Carbon Nitride L. Hultman (Linköping University, Sweden) It is noteworthy that a material finds large-scale application a few years after being discovered. That is the case for CN_x (x≤0.3) phases¹ used, e.g., as top coats for hard discs. The material is resilient and has a more reactive surface compared to DLC. The alleged role of N is to promote the curving of graphite planes by substituting for C and lowering the energy barrier to form pentagon² that constitutes fullerene-like materials. N can also promote cross-linking between t he curved planes. The flux and energy of the species to the growth surface control the degree of folding and cross-linking and therefore the mechanical properties. This paper reviews growth-structure-property relationships for fullerene-like CN_x films with examples from reactive unbalanced magnetron sputtering, XPS, NMR, Raman, TEM, STM³, AFM, and nanoindentation. Topics include the nature of N bonding, chemical sputtering⁴, plastic deformation and fracture, and thermal stability.⁵ Findings are presented for a material that consists of cross-linked nano-onions. ^6,7 Growth of the onion shells takes place atom-by-atom and yields thin solid films during unbalanced magnetron sputtering of graphite target in N₂/Ar discharge. Energy-filtering TEM show that the core shell contains up to 20 at% N. Total energy calculations show the stability of C_60-2nN_2n aza-fullerenes⁷ up to and including C₄₈N₁₂. ¹ S. Sjöström, et al., Phys. Rev. Lett. 75 (1995) 1336. ² N. Hellgren, et al., Phys. Rev. B 59 (1999) 5162. ³ N. Lin, et al., Phys. Rev. B 61 (2000) 4898 ⁴ N. Hellgren, et al., Thin Solid Films 382 (2001) 146. ⁵ N. Hellgren, et al., J. Mater. Res., in press (2001) ⁶ Zs. Czigany, et al., Appl. Phys. Lett. 79 (2001) 2639 ⁷ L. Hultman, et al., "Phys. Rev. Lett. 87 (2001) 225503-1".
2:10 PM		D1-1-3 Plasma Analysis of a DC Sputtering Discharge in an N₂/Ar Atmosphere for the Deposition of Fullerene like Carbon Nitride J. Neidhardt (Linköping University, Sweden); B. Fritsche, R. Gago (Forschungszentrum Rossendorf, Dresden, Germany); L. Hultman (Linköping University, Sweden) Fullerene like carbon nitride (CN_x) is a nanostructured material consisting of bent and intersecting graphitic layers. It combines the unique properties of being hard and elastic at the same time, which results in a fracture though and compliant material. Homogeneous well-structured fullerene like CN_x thin films are commonly grown by DC reactive magnetron sputtering of graphite in an N₂/Ar mixture. The nitrogen content in the films is fairly constant, limited due to the formation of volatile CN species at the substrate surface. Even though the composition is similar, the structure and properties of these films can be varied over a very wide range in a comparatively narrow process window. This suggests that beside C and N ions and atoms other species like CN clusters might play an important role during growth. Here direct effects as growth templates or nucleation sites should be considered. Plasma analysis was carried out during this study in order to investigate the presence, number, and energy of clusters consisting of C and N and their possible effects upon the growing film. A number of techniques, such as spatially resolved wire and ion probe measurements and energy selective mass spectrometry were used to characterize the composition and energetic conditions of a N₂/Ar DC sputtering plasma in front of a single graphite target, depending on discharge current, total pressure and gas composition.
2:30 PM		D1-1-4 What Do We See in the Infrared Spectra of Carbon Nitride Films? S.E. Rodil (Instituto De Investigaciones En Materiales, Unam, Mexico); A.C. Ferrari (Cambridge University, United Kingdom) Ambiguities associated with the assignment of the vibrational modes in amorphous carbon nitride (CN) films are discussed. The infra-red (IR) spectra of CN show absorption bands in three regions of the vibrational spectrum. For hydrogenated CN samples, CH_x and NH_x groups give rise to stretching vibrations at 3000 and 3400 cm^-1, respectively. A weaker sharp band is observed around 2200 cm^-1 due to CN sp¹ bonds. Finally, there is a broad band between 1000-2000 cm^-1, whose explanation is still confusing. So far, the interpretation relies on the work of Kaufman et al [1]. They conclude that the effect of nitrogen into carbon films is to break the molecular symmetry of the sp² carbon bonds making the Raman "G" (graphitic) and "D" (disorder) modes IR active, so similar IR and Raman spectra are obtained. We will show that nitrogen is not necessary to have IR activity in the 1000-2000cm^-1 region and that the Raman spectra of carbon nitride films present variations in peak intensities and positions with respect to the laser exciting light in a similar way than carbon films. Therefore the similarity between the visible Raman and the IR of some carbon nitrides has to be considered, a priori, only a coincidence and indeed most of the times does not hold. We deposited CN films using different deposition methods to obtain a wide range of film properties and their correlation with the IR and multi-wavelength Raman spectrum is discussed. Our main conclusion is that the origin of the broad band is an electronic effect, which is not exclusive of CN films but also holds for amorphous carbon films with a system of delocalised π bonds with increasing conjugation. 1. J. H. Kaufman et al. Phys. Rev. B 39, 13053 (1989).
3:10 PM		D1-1-6 An NMR Spectroscopy Study on Amorphous Carbon Nitride (a-CN_x) thin films: ¹³C NMR Results Show Highly Elastic and Hard a-CN_x is an sp² Carbon Bonded Structure W.J. Gammon, D.I. Malyarenko (College of William & Mary); O. Kraft (Max-Planck-Institut für Metallforschung, Germany); G.L. Houtson, R.L. Vold, A.S. Reilly, B.C. Holloway (College of William & Mary) In this work, the chemical bonding of a-CN_x thin films was investigated with nanoindentation, X-ray photoelectron spectroscopy (XPS), and ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy. The films were deposited on heated Si(001) substrates by DC magnetron sputtering using an argon and nitrogen gas mixture. Previous XPS work has shown that the N(1s) spectra of highly elastic a-CN_x can be resolved into two main peaks positioned at ~ 398.5 and 401 eV. ^1,2 Based on XPS data and theoretical calculations, earlier work suggests that the N(1s) peak at 398.5 eV in hard and elastic a-CN_x, is due to nitrogen bonded to sp³ hybridized carbon.³ This interpretation supports the phenomenological model that the mechanical properties of hard a-CN_x are due to the cross-linking of graphitic planes through sp³ bonded carbon. However, we present ¹³C NMR spectroscopy results that demonstrate that highly elastic a-CN_x is an all sp² hybridized carbon structure. As demonstrated by nanoindentation testing, the a-CN_x films deposited at 300 °C and with high atomic percent nitrogen concentration (~20%) are highly compliant and exhibit a high elastic recovery. An sp² bonded carbon structure is consistent with the high compliance of this material but is not consistent with the observed high elastic recovery (which is not observed in graphite, the pedagogical sp² hybridized carbon structure). In recent work, Hellgren et al. demonstrated dependence of nitrogen concentration on the elastic recovery on a-CN_x thin films.² They showed that a-CN_x deposited at elevated deposition temperature ( >200 °C) but with low atomic nitrogen concentration (<10%) exhibit a graphitic microstructure but with a significant drop in elastic recovery.² In the present work, we have fabricated a-CN_x material deposited at 300 °C with ~10% atomic nitrogen concentration to better elucidate the bonding mechanism responsible for the high elastic recovery. To complement the ¹³C NMR study, ¹⁵N NMR experiments were conducted to show the dependence of nitrogen bonding on film fabrication conditions. ¹B.C. Holloway, O. Kraft, D.K. Shuh, M.A. Kelley, W.D. Nix, P. Pianetta, and S. Hagström, Appl. Phys. Lett., 74, 3290 (1999). ² N. Hellgren, M.P. Johansson, E. Broitman, L. Hultman, and J. Sundren, Phys. Rev. B, 59, 5162, (1999). ³ Å. Johansson and S. Stafström, J. Che. Phys., 111, 3203, (1999).
3:30 PM		D1-1-7 Nitrogen Bonding in CN_x Thin Films Studied by Soft X-ray Spectrocopy N. Hellgren, R. Haasch, J.E. Greene, I. Petrov (Frederick Seitz Materials Research Laboratory, University of Illinois); J.-H. Guo (Uppsala University, Sweden); Y. Luo, H. Ågren (Royal Inst. of Technology, Sweden) The nitrogen bonding structure in magnetron sputtered CN_x thin films has been studied by soft x-ray absorption (SXAS, or NEXAFS) and emission (SXES) spectroscopies, as well as by x-ray photoelectron spectroscopy (XPS). In addition, the experimental results have been compared with calculated spectra of N in different model systems. Based on the good agreement between experimental and calculated spectra, we are able to identify N in three main bonding environments: (i) nitrile bonds, with a sharp SXAS peak at 399.5 eV, (ii) Pyridine-like N (i.e., N bonded to two C atoms), with an x-ray absorption resonance at ~398.5 eV, and (iii) N substituted in graphite, possibly with one sp³ carbon as a neighbor (SXAS energy ~401 eV). The latter two bonding environments are those typically seen by XPS (binding energies ~398.2 and 400.7 eV, respectively). These bondings are present in all CN_x films analyzed, however, the relative intensities between the peaks may vary depending on film structure and nitrogen concentration. Differences in the coordination of the nearest or second nearest C neighbors, however, only cause slight changes in the peak positions and spectrum shape. Furthermore, by studying the polarization dependence of the photon beam in the SXAS experiments, valuable information about the texture of the films can also be obtained. The interpretation and correlation between SXAS, SXES, and XPS experimental results, and their relation to the film microstructure, will be discussed.
3:50 PM		D1-1-8 Deposition of c-BN Films by PVD Methods : Transition from the Hexagonal to the Cubic Phase M.A. Djouadi, A. Vasin, C. Nouveau (ENSAM, France); V. Mortet (Limburgs Universitair Cemtrum, Belgium); G. Nouet (ISMRA, France) A cross section TEM image of a cubic boron nitride film reveals besides the silicon substrate the well-known layered structure : amorphous BN, textured turbostratic BN with c-axis parallel to substrate, textured nanocrystalline (111) c-BN planes and the turbostratic BN upper layer. The thickness of the sp² layer depends on the deposition conditions and in the case of IBAD and triode techniques lies between 10 to 30 nm. A lack of knowledge about this turbostratic BN with c-axis parallel to substrate is mainly due to its thickness and to the fact that it's a buried layer. Nevertheless, we consider that in condition to perform the deposition at conditions as close as possible to the threshold conditions for h-BN to c-BN transition, it is possible to obtain films with the same features than the sp² buried layer. In the beginning, the depositions parameters were scanned and the boundaries between the h-BN and c-BN phases were found. Afterwards sp² films with thickness up to 200 nm were deposited and investigated using stress, density and IR spectroscopy (transmission and reflection) measurements. These sp² films exhibit a stress up to 10 GPa and density up to 3.2. It was also observed an up-shift of TO and LO of E1u mode, from 1413 cm^-1 and 1587 cm^-1 to 1440 cm^-1 and 1611 cm^-1, respectively. The stressed phase which occurs during the transition from sp² to sp³ structure may be explained as a transition from h-BN phase to r-BN one.
4:10 PM		D1-1-9 Collection of Data on Nucleation, Growth and Properties of c-BN Films W. Kulisch (University of Kassel, Germany); S. Ulrich (Forschungszentrum Karlsruhe, Germany) The existing literature on the deposition of c-BN thin films has been critically reviewed in order to establish an enlarged and renewed data base for the modelling of c-BN nucleation and growth. In this contribution, the first results of this data collection will be presented. The first part of the talk deals with the parameter spaces of c-BN nucleation and growth. The nucleation of c-BN is a complex function of ion bombardment (ion/neutral flux ratio, ion energy, ion mass, ion incidence angle) and substrate temperature. A sharp boundary between the non-nucleation and the nucleation regions exist which depends in an interdepend way on all five parameters. Nucleation at very low energies (50-70 eV) is possible at very high ion/neutral ratios (more than 100). Once c-BN has nucleated, a reduction of all five parameters is possible. The further growth of c-BN does not depend on temperature any more; however, again a sharp, interdependent boundary exists concerning the four ion bombardment parameters. The shape of this boundary in the flux ratio/energy diagramme, for example, is similar to that for the nucleation but shifted to lower flux ratio/energy values. In the second part, additional data and also film properties are presented, e.g. growth rates , the thickness of the nucleation layer and its parameter dependencies, the influence of the substrate material, incorporation of impurities, crystallite sizes and crystalline orientations, the density, and mechanical and elastical properties Finally, the deposition processes hitherto leading to films with thicknesses of more than 1 µm mm are critically discussed.
4:30 PM		D1-1-10 Improvement of the Adhesion of c-BN Films by Bias-Graded h-BN Interlayers R. Freudenstein, W. Kulisch (University of Kassel, Germany) In the first part of this contribution we report on the local characterization of the nucleation of c-BN films, deposited by ICP-CVD, by AFM measurements. It turned out that the h-BN nucleation layers are extremely rough with rms roughnesses on the order of more than one third on the total film thickness. From this and other observations we concluded that the growth of the nucleation layer takes place via the island type of nucleation. This is in agreement with the rather thick nucleation layers observed for the ICP technique, and also with the poor adhesion, since with island growth the cohesive forces are stronger than the adhesive. Additionally, we observed small particles on top of the nucleation layer which are inhomogeneously distributed. They increase in size with increasing deposition time while their number decreases. In a later stage, these particles begin to coalesce. It is speculated that these particles can be identified with just nucleated c-BN particles. In the second part it will be shown that the poor adhesion of our standard process can be improved considerably by graded interlayers. Two approaches have been investigated: i) stoichiometry-graded interlayers starting from boron rich films; ii) structure-graded interlayers deposited by gradually increasing the bias voltage until the c-BN region is reached. The second approach yielded the better results, allowing the deposition of c-BN films with thicknesses of 200-300 nm, which was previously not possible with our ICP-CVD method. The development of the bias-graded underlayers has been investigated by AFM. The measurements show a much smoother growth than that of our standard films, thus indicating that island growth does not play a role in this case. Nevertheless, even on such layers still a nucleation layer is deposited prior to c-BN nucleation. Underlayer, nucleation layer and the c-BN film at the top as well as the interfaces between the individual layers have been studied by TEM images. Finally, the density and the elastical properties of the c-BN top layer as well as those of the BN underlayers deposited at different bias voltages have been investigated by means of ERD and SAWS.
4:50 PM		D1-1-11 Preparation and Properties of BN/AlN Nanolaminates C.H. Lee, C.M. Hsu, T.S. Yang, C.L. Cheng, M.S. Wong (National Dong Hwa University, Taiwan) Nanolaminates of BN/AlN deposited using reactive DC magnetron sputtering technique, were exploited to overcome the thickness limitation of c-BN film over 500 nm. Monolithic films with high c-BN content over 90% were sucessfully prepared by sputtering technique, but the films were usually cracked due to high film stress. The interrelation of film processing, microstructure and hardness of the nanolaminates was studied by XRD, FTIR and Vickers microhardness. BN/AlN nanolaminates with the periods from 2.4 to 20 nm and with a total film thickness up to 5 micrometer were produced by varying the rotation speed of the sample holder disc from 10 to 0.5 rpm. At periods thicker than 6.8 nm, the nanolaminate films were primarily consisted of o-BN and w-AlN layers. The nanolaminate of BN and AlN layers were intermixed with each other, and apparent wurtzite-AlBN alloy was formed in the interfaces as the period below 3.4 nm. The hardness of the nanolaminates was relative to the microstructure obtained from XRD and FTIR results.