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Oriented Growth of Silicide and Carbon in Sic-Based Sandwich Structures with Nickel

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Oriented Growth of Silicide and Carbon in Sic-Based Sandwich Structures with Nickel doi:10.1016/j.matchemphys.2008.02.009; Materials Chemistry and Physics 110 (2008) 303-310 Oriented growth of silicide and carbon in SiC-based sandwich structures with nickel A. Hähnel, V. Ischenko, J. Woltersdorf Max-Planck-Institut für Mikrostrukturphysik, D-06120 Halle, Weinberg 2, Germany Abstract We report on the formation kinetics and the metal-mediated structuring in nanoregions of silicide and carbon containing interlayers in SiC-based materials. The silicide formation and the graphite texturisation are determined by complex reactive diffusion processes. High resolution and analytical electron microscopy evidenced a €-Ni2Si growth with a <506> fibre texture in parallel orientation to the <0001> direction of the SiC substrate. The oriented growth of graphitic regions in the silicide hints to a diffusion controlled carbon precipitation from the silicide supersaturated with carbon, explaining the observed orientation relationships between graphite and silicon carbide: perpendicular and parallel to the {0001} silicon carbide surfaces. Keywords Thin films, textured growth, nickel silicide, graphitic carbon, silicon carbide, high resolution and analytical electron microscopy. Introduction Carbon containing interlayers with specially structured nanoregions have essential micromechanical and electronic relevance for SiC and Si-O-C-based high-tech materials; particularly the improvement of the mechanical properties of such composites demands the tailoring of their interlayers [1-6]. For SiC fibre-reinforced ceramics and glasses a controlling of the mechanical properties is possible by graphitic interlayers with basal plane alignment parallel to the fibre/matrix interface [7], which can result from appropriate chemical reactions during the composite processing [8,9]. An enhancement of the oriented growth of graphitic interlayer regions can be achieved via the catalytic graphitisation by transition metals [10-13]. The involved chemical reactions between metals and silicon carbide are also of interest in the context of both, the joining of metallic and ceramic structure materials and the formation of metallic contacts in electronic SiC-devices [14-17]. The present paper is mainly focused on the structuring of micromechanically relevant interlayers, but might also be of interest for the design of electronic SiC-devices. In our former studies the metal-mediated graphitisation was applied for a structuring of the interlayers between silicon carbide and borosilicate glasses using platinum or nickel depositions as catalysts [12, 13]. In catalyst-free composites of such types high resolution and analytical electron microscope methods revealed complex interlayer systems, mainly consisting of amorphous carbon and silica. A distinct increase in the degree of graphitisation and texturisation of the reaction layer was observed after deposition of nanolayers of Pt or Ni 2 on the silicon carbide surface, resulting in a cellular network of lamellae of graphite basal planes embedded in a silica matrix, where a large amount of the graphite nanoregions followed a basal plane alignement parallel to the {0006} surface of the 6H-SiC wafer. Such a texturisation of the graphitic carbon could be achieved by the reaction between the SiC and the transition metal, yielding particles of metal silicides located at the attacked surface of the SiC. The graphitic regions occurred on the surfaces of the particles, but also in particle-free areas of the reaction layer, especially on the silicon carbide surface. Thus, the following questions arise: Which processes determine the preferential orientation of the graphitic basal planes? How are they related to the graphitic growth on both, the silicide particles, and the silicon carbide surface? Is it possible to designate a dominating process, which might allow a control of the carbon orientation during the layer formation? The present paper deals with those questions, and we give an account of recent findings on (i) reaction couples of 6H-SiC and nickel foils of 125 µm thickness (microsystems), (ii) reaction couples with only 5 nm nickel layers (nanosystems), (iii) composites of SiC and borosilicate glass as investigated before. Concerning the systems with thicker metal partners several studies have been published to elucidate the solid state reactions (e.g. [14, 18-21]). Between SiC and a 3 mm thick disc of nickel, reaction layers have been observed after heating at 1223 K, consisting of alternating bands of pure silicide and of silicide regions, containing a high density of carbon precipitates [14]. Nickel atoms, reaching the interface between nickel and SiC, reacted with SiC to form silicides, which can solve carbon only to a restricted degree of about one percent. Thus, the so formed nickel silicide was supersaturated with carbon. Locally the thermodynamic activity of carbon exceeded the value necessary for graphite precipitation, and thereupon a preferential growth of graphite regions along their basal planes was possible, resulting in flake-like structures. Following these facts we try to elucidate the details of the graphite formation kinetics and its orientation dependence on the nickel silicide formation. As the graphite orientation is predetermined during the initial growth of the reaction layer immediately at the interface to the silicon carbide, in the following we discuss particularly the processes occurring in this interfacial region. Experimental Three different series of tests on 6H-SiC1) based sandwich structures were investigated: For the first series (microsystems) reaction couples were made by heat-treating pieces of N- doped (0001) 6H-SiC wafers (4 x 3 x 0.25 mm3) and discs of pure Ni (Ni: 99.99 percent, diameter: 4 mm, thickness: 125 µm). The isopropanol- and acetone-cleaned pieces of SiC and Ni were placed on a silicon support in an alumina crucible, pressed together by a cover of quartz glass, and finally positioned in a horizontal mullite tube furnace, which was equipped with a continuous gas-supplying system and a ZrO2 oxygen sensor including a Pt/PtRh thermocouple. Using this sensor near the sample both, the partial pressure of oxygen and the temperature could be monitored. All experiments were performed under argon flow. By positioning tantalum foils inside the tube, the partial pressure of oxygen could be reduced to about 10-11 Pa while heating-up the furnace to the maximum temperature of 1245K, which was kept for 3 h before cooling-down again. For the second series (nanosystems) HF-cleaned, N-doped {0001} 6H-SiC crystal slides were coated with a thin layer of nickel of about 5 nm in thickness and heat-treated at 1250K for 30 min under flowing argon, from which the oxygen was gettered in situ in the manner described above. 1 SiCRYSTAL AG, Erlangen, Germany 3 The third series comprises model composites, which were made by merging HF-cleaned single crystals of 6H-SiC and plates of borosilicate glass (Fiolax•2)). The sandwich size was typically 10 x 10 x 4.4 mm3, containing the SiC-crystal slides (3 x 3 x 0.38 mm3) cut from N- doped wafers with the polished wafer surfaces being C- or Si-terminated. Before merging with the glass at 1250 K for 30 min, the HF-cleaned 6H-SiC crystal slides were coated with a thin layer of nickel of about 5 nm in thickness. Specimens appropriate for high resolution transmission electron microscopy (HRTEM) have been made using a refined cross-section technique: After fixing the sandwiches with Si- blocks, slides of about 1 mm in thickness were cut, embedded in wax and divided into small parts, containing the SiC/Ni-interface of interest in the centre. After grinding and dimpling, these parts were subsequently glued on Cu-rings. The thinning to electron transparency has been performed by ion milling using the precision ion polishing system PIPS 691 (Gatan Inc.). For the microstructure investigations we used selected-area electron diffraction techniques (SATED) and HRTEM, carried out in a Philips CM 20 FEG (field emission gun) microscope, operating at 200 keV with a point to point resolution of 0.24 nm and equipped with both, a Gatan Imaging Filter (GIF 200) and an EDX-detector enabling the detection of light elements (IDFix-system, SAMx-Germany). Electron energy loss spectroscopy (EELS), esp. the analysis of the energy loss near-edge structure (ELNES), were performed with the microscope working in the scanning transmission mode (STEM), whereas the half width of the zero loss peak was about 0.8 to 1.2 eV. The chemical composition and the lateral distribution of the reactive interlayers on the SiC substrates have been imaged by the electron energy filter technique (EFTEM), which is based on EELS, yielding a two-dimensional element distribution. For that we performed the three window method by acquiring three images using those electrons, which were selected via energy windows, one behind the characteristic energy loss edge of interest (post-edge), and two in front of the edge onset (pre- edges 1, 2). The element specific images were calculated by subtracting the background, which was modelled with the two pre-edge images based on an exponential law, from the post-edge image (DigitalMicrograph software, Gatan Inc.). Results I. Microsystems Immediately at the surface of the 6H-SiC substrate a reaction layer develops, consisting of a polycrystalline matrix of nickel silicide, in which carbon precipitates are embedded. Fig. 1 demonstrates the related findings of the EFTEM-analyses in the interfacial region of SiC and the reaction layer, with SiC on the left, and the reaction layer on the right. Fig. 1: EFTEM images of the layer region next to the SiC, a) TEM-BF image, b) C-map (C-K edge), c) Si-map (Si-L2,3 edge), d) Ni-map (Ni-L2,3 edge). 2 Schott Glaswerke, Mainz, Germany 4 The elemental maps, showing the distribution of carbon, silicon and nickel, clearly indicate, that the reaction layer next to the SiC indeed consists of a nickel and silicon containing matrix, in which carbon agglomerates are embedded. In agreement with the EFTEM-results the HREM-image Fig.
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