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.

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.

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).

In this study, the chemical bonding in hard and elastic amorphous carbon nitride (a-CN_x) 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-CN_x 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-CN_x to water absorption may impact tribological applications for this material.

The measured ¹⁵N chemical shifts of our hard and elastic a-CN_x 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-CN_x. 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).