ICMCTF2003 Session D1-2: Carbon Nitride, Boron Nitride and Ternary Phase Materials

Monday, April 28, 2003 1:30 PM in Room Royal Palm 1-3
Monday Afternoon

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Start Invited? Item
1:30 PM D1-2-1 Effect of Diamond Films as Bufferlayer on Formation of Cubic Boron Nitride Films by Chemical Vapor Deposition
T.S. Yang, Y.P. Cheng, M.S. Wong (National Dong Hwa University, Taiwan, ROC)
Boron nitride films are produced by the reaction of B2H6 and NH3 in the hydrogen and argon mixture using microwave plasma-assisted chemical vapor deposition (MPCVD). The influence of diamond films of various grain sizes as bufferlayer on the formation of c-BN is investigated. The diamond films of grain sizes from micrometer to nanometer are prepared under procedures involving Ar addition or substrate biasing. As-grown BN/diamond films are characterized by FTIR, Raman, XRD, and SEM. The results indicate that the nature of diamond film affects the growth of c-BN content. Nanocrystalline diamond film can promote the c-BN formation, resulting in the c-BN content up to 90%. Thick c-BN/Diamond multilayer up to 5 micrometer in total thickness and with nearly pure c- BN content is synthesized.
1:50 PM D1-2-2 Mechanical and Structural Properties of BN Thin Films
Th. Pfeifer, I. Hermann (Chemnitz University of Technology, Germany); T. Wittkowski (University of Kaiserslautern, Germany); T. Chudoba (ASMEC Advanced Surface Mechanics, Germany); F. Richter (Chemnitz University of Technology, Germany)

Hexagonal and cubic boron nitride thin films (h-BN and c-BN, resp.) have been deposited by r.f. magnetron sputtering from an h-BN target in a nitrogen atmosphere. As substrates, (100) oriented single-crystal silicon wafers were used. The YOUNG's modulus of the films was measured by two different methods: Firstly, spherical indentation together with theoretical modeling of the stress and strain fields was used [1]. In this technique, load depth curves are measured with high accuracy and compared with the theoretical modeling of those curves. The YOUNG's modulus of the film can be obtained by fitting the experimental to the calculated data. Secondly, BRILLOUIN light scattering from surface phonons with wavelengths ranging from 0.3 to 1.0 µm was applied. Small differences in the microstructure of various films of different thickness are revealed by discontinuities in the dispersion curves. The stiffness tensor of the film material can be extracted in a fitting procedure. YOUNG's modulus is deduced from the elastic constants. In addition, the structure of the films was thoroughly characterized by high-resolution cross-section transmission electron microscopy together with selected area electron diffraction. The results of the two mechanical characterizations are compared and discussed together with the structural data. Conclusions are drawn both on the properties of the films investigated and on the peculiarities of the measuring methods.

[1] T. Chudoba, N. Schwarzer, F. Richter, Surf. Coat. Technol. 127 (2000) 9.

2:10 PM Invited D1-2-3 Theory of Structure Formation in Superhard C-N Coatings and Amorphous Ternary Si-C-N Compounds
T. Frauenheim, M. Amkreutz, T. Köhler, T. Niehaus (University of Paderborn, Germany)

An efficient density-functional-theory (DFT)-based approach to predictive simulations of nanoscale materials and properties is described. Structure formation under technological relevant conditions is controlled by Molecular-Dynamics simulations based on quantum-mechanically calculated interatomic forces. Successful applications to inorganic structures already cover a broad spectrum of problems, ranging from amorphous semiconductors through surface growth simulations, defect studies and interface problems.

Superhard materials in this regard are one particular focus. During the years, we have provided insight into diamond growth problems including nanocrystalline diamond surfaces and studying the electronic properties of grain boundaries. Further we have derived a model for nanodiamond nucleation by energetic species (for example in bias-enhanced nucleation). It involves spontaneous bulk nucleation of a diamond embryo cluster in a dense amorphous carbon matrix; stabilisation of the cluster by favourable boundary conditions of nucleation sites and hydrogen termination and ion-bombardment induced growth through a preferential displacement mechanism.

Extending our amorphous carbon studies to binary C-N- and ternary Si-C-N-compounds, we more recently have addressed atomistic structure formation of precursor-derived amorphous ceramics in the ternary system Si-C-N. We proposed two different model structures and calculated their structure factors and pair correlation functions from X-ray and neutron diffraction. These data were found to agree very well with the experimental results and the position of the ceramic in the ternary phase diagram of Si-C-N. Thus, the generated structures represent possible models for the amorphous state of the ceramic. It could be shown that in the final ceramic a phase separation into amorphous Si3\N4, amorphous SiC, and graphite-like amorphous carbon is a characteristic property. This phase separation is thaught to retard the crystallization process by hindering the thermal diffusion of the atoms upon annealing and in this way to improve their temperature stability.

New developments in time-dependent density-functional-theory (TD-DFT) further allow to couple the electron and ion dynamics via a generalized quantum-classical Lagrangian and to study the interaction of intense laser fields and energetic particles with growing surfaces.

2:50 PM D1-2-5 Characterization of Si-C-N Films Prepared by Means of Combined RF Magnetron Sputtering and Ion Beam Synthesis
M. Bruns, H. Lutz (Forschungszentrum Karlsruhe GmbH, Germany); M. Rudolphi, H. Baumann (Universitaet Frankfurt/Main, Germany); H. Schmidt, G. Borchardt (TU Clausthal-Zellerfeld, Germany)

Carbonitride and Silicon Carbonitride thin films have been the subject of great interest in recent years due to the expected improvement of surface properties for a lot of applications. RF magnetron sputtering as well as ion beam synthesis have been proven suitable means to achieve high-purity ternary systems of up to 57 at% nitrogen content, e.g. Si2CN4. But in case of high carbon content the sputtered films in general have a lack of nitrogen. The respective preparation via ion beam synthesis likewise leads to a sub-stoichiometric nitrogen content in the films. A very promising approach to carbon rich compound synthesis (i.e. SiC2N4) is the combination of both methods. Defined and reproducible Si/C ratios within the films (SiC2, SiC, and SiC2) can be obtained using co-sputter targets of different Si/C areas. In a second step surface modification by high fluence implantation of 15N ions into these Si-C films result in suitable nitrogen content up to the theoretical amount. Severalfold implantation at different energies calculated from Monte-Carlo-simulations enable us to synthesize layers with homogeneous element depth-distribution up to the surface.

The chemical composition of the Si-C-N films was characterized by means of X-ray photoelectron spectroscopy (XPS). In addition, Auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy were used to achieve a comprehensive characterization. For quantification XPS and AES data were calibrated with absolute concentration values from non-Rutherford backscattering spectrometry (n-RBS). Resonant nuclear reaction analysis (NRRA) provides non-destructive depth profiles of the layer constituents 15N and 13C, respectively. The morphology after subsequent annealing at temperatures up to 1700°C was studied by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM).

3:10 PM Invited D1-2-6 Characterisation of the Bonding Structure of Carbon Nitride and other B-C-N Materials with X-Ray Absorption Spectroscopy (XANES)
I. Jiménez (Instituto de Ciencia y Tecnología de Polímeros (CSIC), Spain)
Carbon nitrides (CNx) and Boron carbonitrides (BxCyNz) are materials commonly synthesised as amorphous or nanostructured thin films whose characterisation is complex. To understand the material grown it is crucial to discern if a single phase or mixture of segregated domains is obtained, what is the hybridisation of the atoms (sp2 or sp3), its bonding structure and the presence of textures or preferential orientation at the nanoscale. X-ray-absorption near-edge spectroscopy (XANES or NEXAFS) is a powerful tool to examine these questions, since it is sensitive to the bonding structure, gives unambiguous information about the presence π bonds and can reveal the orientation of such π bonds. Graphitic carbon nitrides lacking long range order have been studied in detail, with special emphasis on the analysis of the π bands, which indicate the presence of π bonds between C and N atoms. This analysis provides also information on the orientation, corrugation and cross-linking of basal planes, which is consistent with the mechanical properties of the films. The XANES study has been extended to ternary B-C-N compounds. In this case, the variety of possible bonding structures (trigonal, tetragonal, icosahedral) and compositions makes XANES an essential tool to understand the material actually obtained. .
3:50 PM D1-2-8 Effect of the Bias Voltage on the Structure of Carbon Nitride Films
A. Champi, F.C. Marques (UNICAMP, Brazil)
In this work we study the effect of the bias voltage on the mechanical, electrical and structural properties of amorphous carbon nitride films deposited by the plasma decomposition of methane (CH4) and nitrogen (N2) atmosphere. Two series of films were deposited under different bias voltage in order to investigate the effect of the nitrogen incorporation in a-C:H films with different structures. The first series of films was deposited under the condition in which diamond-like a-C:H films is obtained, i.e, bias of -200 V, and pressure of 1.0Pa. On the other hand, the second series of films was deposited under the condition graphitic-like films is obtained, i.e, bias of -800 V, and pressures of 10 Pa. The N2 partial pressure was the only parameter varied in both series. In order to investigate the effect of these conditions on the properties of the films, a number of analyses were undertaken: FTIR, nanohardness, Raman, EPR, SEM, EELS, field emission and stress. The efficiency of nitrogen incorporation was strongly affected by the initial condition adopted. However, it was observed that the incorporation of nitrogen reduces the deposition rate, band-gap, hardness, the elastic constant of the films, and increases the sp2/sp3 ratio, in both series of films. The use of high pressure and high bias allowed the preparation of stable and thick (~1 micron) nitrogen-carbon films, with high hardness (17 GPa), and low stress (0.8 GPa) deposited at relatively high deposition rate (0.3 nm/s).
4:10 PM D1-2-9 Investigation of Bonding in Hard and Elastic Amorphous Carbon Nitride Films using Multinuclear NMR Spectroscopy
W.J. Gammon (College of William and Mary); O. Kraft (Institu für Materialforschung II, Germany); G.L. Hoatson, A.C. Reilly, B.C. Holloway (College of William and Mary)

In this study, the chemical bonding in hard and elastic amorphous carbon nitride (a-CNx) films is investigated with 15N, 13C, and 1H nuclear magnetic resonance (NMR) spectroscopy. The films were deposited by DC Magnetron sputtering in a pure nitrogen discharge on Si(001) substrates at 300 °C. Nanoindentation tests reveal an elastic recovery of 80%, a hardness of 5 GPa, and an elastic modulus of 47 GPa. Our previous 13C NMR study demonstrated the lack of sp3 bonded carbon in this material.1 The 13C and 15N NMR data imply a film-bonding model that has an aromatic carbon structure with sp2 hybridized nitrogen incorporated in heterocyclic rings. Our data also indicates that the a-CNx films prepared for this study have low hydrogen content but are hydrophilic. Results from 15N and 13C cross polarization (CP) and 1H magic angle spinning (MAS) NMR experiments suggest that the nitrogen bonding sites are susceptible to protonation. The most likely source of protons is from water absorbed during sample preparation for the NMR experiments. The sensitivity of the surface of a-CNx to water absorption may impact tribological applications for this material.

The measured 15N chemical shifts of our hard and elastic a-CNx films are similar to the calculated values of graphitic C3N4.2 This suggests that vacancy defect structures found in graphitic C3N4 may also be found in a-CNx. Our experimental 15N data also indicate that nitrogen may be bonded in a pyrrole-like configuration. In accord with these results, we propose a structural model that incorporates nitrogen bonding along the perimeter of vacancy defect structures and that incorporates nitrogen bonding in pentagons. Incorporation of pentagons with substituted nitrogen would take into account the buckling of the graphitic sheets as observed by previous transmission electron microscopy (TEM) and as suggested by computational work.3 We have carried out total energy calculations on graphene sheets with vacancy defects and pentagons to test the plausibility of this structural model. We have also investigated various bonding situations (without sp3 hybridization) that may be responsible for the cross-linking of the buckled graphitic planes.

1W.J. Gammon, D.I. Malyarenko, O. Kraft, G.L. Hoatson, A.C. Reilly, and B.C. Holloway, Phys. Rev. B., 66, 153402 (2002). 2Y. Young-Gui, B.G. Pfrommer, F. Mauri, and S.G. Louie, Phys. Rev. Lett., 80, 3388 (1998). 3H. Sjostrom, S. Stafstrom, M. Boman, and J-E, Sundren, Phys. Rev. Lett., 75, 1336 (1995).

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