doi:10.1016/j.matchemphys.2008.02.009; Materials Chemistry and Physics 110 (2008) 303-310

Oriented growth of silicide and in SiC-based sandwich structures with

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 carbide: perpendicular and parallel to the {0001} 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. 2 demonstrates the precipitation of graphitic carbon within the silicide- matrix of the reaction layer. In the centre a carbon-flake of (0002) graphitic basal planes can be seen, which is elongated in the direction of the lattice fringes corresponding to these planes. Moreover, the (0002)-fringes show the typical, nearly perpendicular orientation relation to the (0001)-fringes of the 6H-SiC, which can be seen in the lower part of the image by the alternating bright-dark contrasts with the distance of 1.51 nm corresponding to the sixfold stacking of the (0006) atomic planes (see [22] for details). In general, at the interface between the layer and the SiC no carbon in parallel orientation could be found.

Fig. 2: HREM-image of the layer region next to the 6H-SiC showing graphitic carbon within the silicide matrix.

Thus, the typical nanostructure of the interfacial region between silicon carbide and the reaction layer is characterized by a direct adjoining of SiC and silicide as illustrated by the HREM-image Fig. 3. The atomic structure of the interface shows even terraces (left on Fig. 3), or step-like structures (right on Fig. 3), which can percolate the reaction layer in dimensions of several 10 nm. In addition, another important feature of the reactive layer formation is obvious: The growth of silicide in an orientation-relationship with respect to the silicon carbide surface: Silicide lattice fringes with a distance of about 0.2 nm are parallel oriented to the fringes with a distance of 0.25 nm corresponding to the (0006)-planes of the 6H-polytype of SiC, indicating a texturisation of the silicide, the details of which are elucidated by the overview picture of Fig. 4 and the the set of diffraction patterns of Fig. 5, which were indexed using the crystallographic information on 6H-SiC and €-Ni2Si, given in Table 1. 5

Fig. 3: HREM-image of the interfacial region between the 6H-SiC and the silicide, demonstrating both, the absence of graphitic carbon at the interface, and the oriented growth of the silicide.

Table 1: Crystal structure parameters of €-Ni2Si, 6H-SiC and 2H-C. €-Ni2Si [23] 6H-SiC [24] 2H-C [25] Lattice symmetry: orthorhombic hexagonal hexagonal Space group: Pbnm P63mc P63/mmc Lattice parameters: a = 0.706 nm a = 0.3081 nm a = 0.2464 nm b = 0.499 nm b = 0.3081 nm b = 0.2464 nm c = 0.372 nm c = 1.5117 nm c = 0.6711 nm ‚ƒ= 90.0° ‚ˆ= 90.0° ‚‹= 90.0° „…= 90.0° „‰= 90.0° „Œ= 90.0° †‡= 90.0° †Š= 120.0° †•= 120.0°

The carbon precipitates in the TEM overview can clearly be distinguished from the silicide matrix by their bright contrast. Often they show an elongated shape and appear to be arranged in columns perpendicular to the interface with the silicon carbide, being related to orientation- relationships of both, the €-Ni2Si-matrix and the graphitic carbon, with respect to the 6H-SiC surface, which is demonstrated on the different SATED images of that region presented in Fig. 5: All the images a) - d) represent the same diffraction pattern of the reaction layer and the SiC and differ only in the indexing of the reflexes of the silicide. In effect, the original pattern is composed of a superposition of six individual diffraction patterns: 1. of the 6H-SiC in <1120> zone axis orientation, 2. of the graphitic carbon with the rings of the (0002)- and the (1010)-reflexes, and 3. - 6. of four silicide grains, which can be attributed to €-Ni2Si. The four shown images a) - d) are related to the four slightly different orientations of the concerned silicide grains, and have been indexed in relation to the corresponding zone axes as follows: a) <133>, b) <153>, c) <296>, d) <113>. As clearly demonstrated, all the shown 6

four images have in common the systematic row of the {301}Ni2Si -reflection, indicating the oriented growth of the {301}-planes of the silicide grains parallel to the (0006)-planes of the 6H-SiC during the reactive layer formation. The grains are azimuthally rotated around the <506>-direction, which is normal to the {301}-plane. Such a matter of fact can be adequately denoted by the term fibre texture which describes in the field of texture analysis the case of a one-dimensional texture, in which the crystallites are arranged radialsymmetrically in a certain crystallographic direction around a defined axis. Accordingly, a <506> fibre texture with the <506>Ni2Si direction parallel to the [0001]SiC direction has been generated on the reactive silicide formation, which we could verify in several specimens.

Fig. 4: Microstructure of the layer region near the interface with the 6H-SiC: The layer consists of silicide grains with embedded C-precipitates (for the corresponding diffraction pattern, see Fig. 5 a-d).

The next outstanding feature shown in Fig. 5 is indicated by the arcs in the (0002)-ring of the graphitic carbon in the layer, which are perpendicular aligned to the systematic row of the {301}Ni2Si : That demonstrates a preferential orientation of a number of the graphitic basal planes nearly perpendicular to both, the {301} planes of the €-Ni2Si, and the (0006) surface of the 6H-SiC, which is in agreement with the HREM results presented in Fig. 2. Besides the preferentially oriented growth of the graphitic basal planes there is a certain tendency to form half-closed shell-structures of (0002) graphite planes within the precipitates. 7

Fig. 5: Versions of the SATED-pattern of the reaction layer next to the 6H-SiC demonstrating the formation of a <506> fibre texture of the €-Ni2Si grains coupled with the oriented precipitation of graphitic carbon ( Ni2Si / SiC: <506>Ni2Si II [0001]SiC, {301}Ni2Si II (0006)SiC; C / SiC: (0002)C normal to (0006)SiC; C / Ni2Si: (0002)C normal to {301}Ni2Si; a)-d) four silicide grains with <506> fibre texture and zone axis: a) <133>, b) <153>, c) <296>, d) <113>.

II. Nanosystems

Unlike the continuous reaction layers, being formed on the SiC in the microsystems, in case of the nanosystems isolated €-Ni2Si particles and lamellae of graphitic carbon were grown on the SiC-substrate, as a result of solid state reactions. However, in both systems analogies in the reaction processes occur, as revealed by similar observations: The €-Ni2Si particles on the surface of the SiC show also in case of the nanosystems dominant orientation relationships to the 6H-SiC, which indeed indicate a parallel growth of the {301}Ni2Si planes with respect to the {0006}SiC planes. That holds for both types of SiC wafer termination, the Si-terminated (0006)SiC, and the C-terminated (0006)SiC. An example is demonstrated in Fig. 6, which shows a €-Ni2Si particle imaged in <236> zone axis orientation, grown on the 6H-SiC substrate in <1100> zone axis orientation. As can be recognised in the magnified cutouts, the lattice fringes corresponding to the {301}-planes of the €-Ni2Si are parallel oriented to the (0006)-planes of the 6H-SiC surface, which is equivalent to a parallel alignment of the <506>Ni2Si to the [0001]SiC. However, some other 8

orientation relationships can additionally be observed, such as the growth of both, {220}Ni2Si and {020}Ni2Si , parallel to the {0006}SiC surface. The main difference to the microsystems concerns the occurrence of carbon regions as the second product of the solid state reaction between SiC and Ni: In contrast to the microsystems, the most part of the carbon has been found to be precipitated on the silicide surface and not within the interior of the silicide particles, as demonstrated in the upper part of Fig. 6 by the nanolayer of parallel (0002) graphite basal planes, which follow the silicide particle surface with the atomic planes being surface-parallel arranged.

Fig. 6: HREM-image of the reaction products of a Ni nanolayer with a (0006) 6H-SiC substrate: €-Ni2Si and C: The magnified cutouts demonstrate the growth of the {301} planes of the €-Ni2Si parallel to the (0006) plane of the 6H-SiC. In contrast, the graphitic base planes do not show any orientation relationship to the €-Ni2Si but follow the surface of the silicide particle.

III. Sandwich systems of borosilicate glass and Ni-coated 6H-SiC

For an intended use of the recent findings to design reaction layers, the question arises, if the above described oriented silicide formation and the succeeding carbon precipitation are also features of the reactive layer formation in composite materials with nickel nanolayers: As shown later on, it applies indeed, although the case of our model composites made by merging HF-cleaned and Ni-coated single crystals of 6H-SiC and plates of borosilicate glass 9

is much more complex in comparison to that of the reaction couples of 6H-SiC and Ni. In the composites, the reactive layer formation is determined by both, the generation and the disintegration of silicides, which proceed parallel by the effect of oxygen (for details, cf. [12, 13])). However, also in these specimens the oriented growth of silicide particles has been found with a preference of the <506> fibre texture parallel to the <0001> of the 6H-SiC, as demonstrated in Fig 7: The TEM-image (above) shows a €-Ni2Si-particle with the typical, irregular plate-like shape, grown on the silicon carbide and penetrating the surface. The corresponding diffraction pattern presented in the lower part of Fig. 7, clearly identifies the orientation relationship: The silicide grows with the {301} plane in parallel orientation to the (0006) plane of the 6H-SiC. In addition, the carbon precipitation occurs mainly in the same way as in the reaction couples of SiC and nickel nanolayers, shown, e.g., in the upper part of Fig. 6, i.e., the basal planes follow the surface of the silicide particles. In both cases, the orientation of the graphitic planes is not specified by crystallographic features but by the shape of the silicide particle. Thus, at large, the shape and the distribution of the particles determine the texture of the carbon. Most of the silicide particles are irregularly plate-like shaped with the plate base being parallel oriented to the {0006}-planes of SiC (cf. Fig. 7). Thus, the fraction of the particle surface nearly parallel to these planes is relatively high, yielding the preferential orientation of the graphitic carbon, observed in the reaction layers. Accordingly, in the diffraction pattern of Fig. 7 appear bright arcs in the (0002) ring of the graphitic carbon in a segment around the systematic row of {301}Ni2Si.

Fig. 7: Interfacial region of the reaction layer in a Ni-SiC-glass composite with a €-Ni2Si-particle in the typical orientation relation to the SiC with the {301} planes in parallel orientation to the (0006) planes, which also determines the shape of the particle, elongated along the (0006) of the SiC (SATED: €-Ni2Si in <010> zone axis orientation, 6H-SiC in <1120>).

An oriented formation of graphitic basal planes within the €-Ni2Si-particles could only scarcely be found in case of the sandwich systems. An example is shown in Fig. 8 with the overview in Fig. 8a and the magnified cutout of the C-precipitation in Fig. 8b, demonstrating the parallel growth of the (0002)C planes with respect to the {210}Ni2Si planes in the silicide particle. The graphitic precipitation in the interior of the silicide indicates that carbon, formed by the reaction of SiC and Ni, was initially solved in the silicide. Then, a supersaturation of carbon locally developed, resulting in the carbon precipitation within the silicide. However, in most cases the carbon was found to diffuse out to the particle surfaces, where it segregated enveloping the particles with graphitic base planes as demonstrated on the left of the particle shown in Fig. 8a. 10

Fig. 8: a) TEM-image of a big, plate-like €-Ni2Si-particle with a C-lamella in its interior, formed at the rough 6H-SiC interface, b) magnified cutouts, demonstrating both, the oriented growth of the {301} planes of the €- Ni2Si parallel to the (0006) planes of the 6H-SiC, and an oriented precipitation of carbon from the C- supersaturated silicide. 11

Discussion

Considering the phase evolution at the interface between nickel and SiC, at temperatures between 973K and 1400K,Ž€-Ni2Si and graphitic carbon are formed in agreement with the Ni- Si-C phase diagram [15, 26, 27]. The presented investigations indicate that the growth features of the graphitic carbon strongly depend on the reactive diffusion processes leading to the formation of a product layer on the silicon carbide substrate. On the one hand, in the continuous reaction layers of the microsystems, graphitic base planes precipitate nearly perpendicular oriented to the {0006} planes of the 6H-SiC within the silicide matrix. On the other hand, in the very discontinuous layers of the nanosystems graphitic base planes have been found to cover the silicide particles and the silicide-free SiC surface, yielding a roughly parallel growth with respect to the {0006} planes of the 6H-SiC. Along with the common principle of surface energy minimisation, the direction of the diffusion flows of the atomic species in the chemical system will also play an important role in the texturisation mechanisms. Especially for thicker reaction product layers (as investigated in our microsystems), the reaction between Ni and SiC tends to be diffusion controlled [14, 17]. In the experiments on the growth of €-Ni2Si on silicon, nickel has been identified to be the dominant diffusion species in both, the grain boundaries and the lattice of €-Ni2Si [28, 29]. Moreover, in marker experiments nickel was identified to be virtually the only mobile species, from which a low diffusivity and solubility of carbon within the silicide is concluded [15]. There is no clear experimental information on the diffusion of carbon in €-Ni2Si at higher temperatures, however, the carbon SIMS profiles for NiSi:C films annealed at 450°C showed that about ~0.8-1.0 at.% carbon can be solved in NixSiy:C phases [30]. In the thin Ni-Si-C reaction product layers (up to 200 nm in thickness), the carbon may penetrate the complete reaction layer and accumulate on its surface. However, in the thick reaction product layers, the excessive carbon tends rather to precipitate within the reaction layer, as demonstrated by the uniform distribution of carbon and appearance of the depleted carbon zone near the surface in the 500-600 nm thick reaction layers [20]. In our experiments with the thicker reaction product layers we observed the precipitation of the excessive carbon from the supersaturated NixSiy:C solid solution within the product layer, supporting the supposition about the relatively low diffusion rate of carbon in nickel silicide. In our nanosystems, where the reaction products are formed in nanosized dimensions, the diffusion paths for carbon are, however, short enough to enable its precipitation outside the silicide phase, i.e. on the particle surface. In the nanosystems the high amount of surfaces of the silicide islands and of the silicide-free SiC surfaces appears to be an important presupposition for the texturised growth of graphite. The specific orientation of the latter is determined in this case essentially by the decrease of the surface energy of the graphitic layers, leading to the parallel orientation of the graphite base planes along the surfaces [31]. In the continuous silicide layers of the microsystems the carbon excess precipitates within the silicide phase, driven by the related reduction in the total energy due to (i) the relaxation of the strain and (ii) the crystallographic arrangement of both, the silicide and the graphitic carbon. The effects minimising the interfacial energy and the strain energy can contribute the preferred orientation of the graphitic carbon. In particular strain energy minimising processes have to be considered, as the volume increase during the layer formation on the rigid SiC- substrate should yield a biaxial compressive stress field in the layer with the plane of stress lying parallel to the interface with the substrate [32]. According to thermodynamic calculations on the strain energy relaxation in compressed graphitic carbon [33], the graphitic base planes might preferentially align in the stress field to reduce the strain energy. However, in that case the most energetically favoured orientation should be a 45° inclination of the 12

graphite basal planes to the plane of stress, which is in contrast to our observation of a rough 90°-orientation of the graphitic (0002) planes to the (0001) SiC-surface. As mentioned in [33, 34], plastic mechanisms in the graphitic carbon might finally yield that 90° alignment. But it is questionable whether these aligning mechanisms can be transferred to the graphitic stacks just growing in the stressed silicide matrix of our specimens. Thus, the structural findings shown on Fig. 2 and Fig. 4 contradict such mechanisms: There is not only the rough 90°- alignment of the graphitic base planes within the carbon particles, but also an elongation of many carbon precipitates in the same direction. That oriented growth of the graphitic flakes within the silicide appears to be essentially determined by the direction of the diffusion flows of the carbon atoms, which is orthonormal to the reaction plane. As elucidated in the results, in all specimens €-Ni2Si with a dominant <506> fibre texture was formed by the solid state reactions between 6H-SiC and nickel. In particular, the growth of textured €-Ni2Si has been found to occur independently on (i) the termination of the {0001} 6H-SiC surface, (ii) the thickness of the original nickel deposit, and (iii) the effect of oxygen. These common issues should be emphasised, as the morphologies of the so formed silicides vary from continuous layers in the sandwiches with thick Ni-deposits (microsystems) to individually separated particles in the sandwiches with Ni-nanolayers (nanosystems). Recently published results on reaction couples of (0001) 4H-SiC and 150 nm thick Ni-layers support our findings [35]: By use of two-dimensional X-ray microdiffraction the preferential growth of silicide crystallites with the {301} planes in parallel orientation to the (0001) 4H- SiC substrate could be proven in rapid thermal-annealed specimens, where the texturising reveales an oriented growth from a very early stage of the layer formation, indicating that the <506> fibre texture of the €-Ni2Si is a general feature of the reactive silicide formation on {0001} surfaces of hexagonal polytypes of SiC at temperatures below ca. 1250 K. But the question remains, which processes and driving forces control the texture formation. Within the €-Ni2Si the majority atoms (Ni) are the dominant diffusing species, which diffuse in their own sublattice via a vacancy mechanism [28]. The processes involved are being investigated in detail by us in the framework of a DFT-study. Some features concerning these nanomechanisms of structuring are finally presented here: According to our experimental findings in the microsystems (cf. Figs. 2, 5), the precipitated stacks of graphitic basal planes are nearly perpendicular arranged to the {301} of the €-Ni2Si. A detailed analysis of the €- Ni2Si crystal structure reveals preferential directions of diffusion of Ni and presumably also of C atoms parallel to <101> as demonstrated in the scheme of Fig. 9. The specific orientation of the graphitic base planes, being nearly perpendicularly oriented with respect to the reaction front at the interface between the silicide and the SiC, indicates a diffusion controlled growth of the graphitic planes. Actually, strong diffusion flows of Ni atoms to and C atoms from the reaction front are coupled with the layer formation by the reaction diffusion as illustrated in Fig. 10a. For the mechanism of the oriented graphite growth we suggest a preferential growth of the graphitic (0002) planes in the direction of the diffusion flow of the carbon atoms by adding atoms to the reactive edges of the plane fragments, being precipitated in the €-Ni2Si. Such a transport-supported growth of the carbon phase can be considered as analogy to the growth of the basal graphite lattice planes in the metal dusting on Ni-base and Fe-base alloys, where these atomic planes are arranged perpendicularly to the carbide or metal surface with its edges acting as active sites in the disintegration process [36]. 13

Fig. 9: Scheme of the €-Ni2Si crystal structure (viewing direction: [010]) in orientation relationship with the 6H-SiC (viewing direction: [1100]), and the graphitic carbon (viewing direction: [1100]) with the preferred direction of diffusion marked in the €-Ni2Si structure.

In the nanosystems the situation is somewhat different, as illustrated in Fig. 10b: The graphitic base planes grow in parallel orientation to the surface of the SiC, but also enveloping the irregularly shaped surfaces of the silicide particles. Contrary to the microsystems with the distinct diffusion flows perpendicular to the reaction front, in the nanosystems the reaction proceeds quasi two-dimensional with rather weak diffusion flows perpendicular to the reaction front. Thus, the diffusion will occur mainly in the reaction plane, driven by crystallite coarsening and segregation processes. In particular for nanosystems, the minimisation of surface energy is an important factor for the orientation of the graphitic base planes along silicide-free SiC surfaces and curved silicide particles.

Fig. 10: Simplified scheme of the reaction between SiC and Ni a) in the microsystems, b) in the nanosystems, with the directions of the diffusion flows marked (green arrows: Ni diffusion, blue arrows: C diffusion). 14

Conclusions

All the findings reported in the paper demonstrate the diffusion character of the processes forming both, silicide and carbon, on the 6H-SiC substrates in the microsystems, in the nanosystems, and in the sandwich systems of borosilicate glass and Ni-coated 6H-SiC as well. Summarizing the processes, the following reaction model can be proposed:

1. Initial stage For the very beginning of the reaction at the interface between the SiC-substrate and the Ni- deposit the generation of a metastable silicide phase is indicated, as the so formed silicide will be supersaturated with carbon: 2 Ni+ SiC = Ni2Si + [C]Ni2Si Then, the product layer grows at the interface between the SiC and the €-Ni2Si by the Ni- diffusion through the layer to the reaction front. In the silicide preferred directions of Ni- and C-diffusion are generated nearly perpendicular to the reaction front due to the formation of a <506> fibre texture of the €-Ni2Si on the {0001} 6H-SiC substrates.

2. Segregation stage The tendency to minimise both, the surface energy and the total energy - due to the formation of more ordered crystal structures of €-Ni2Si and graphitic carbon - results in the precipitation of graphitic flakes within (microsystems) or on the surface of (nanosystems) the originally supersaturated silicide. That is related to the carbon diffusion flow, which might be stronger directed perpendicular to the interface with the SiC in the microsystems as in the nanosystems. The specifics of the systems can be described as follows:

Microsystems: Within the silicide, fluctuations in the chemical potential of carbon finally cause a carbon precipitation, and the graphitic flakes grow in the gradient of the carbon potential. The special orientation of the graphitic planes nearly perpendicular to the interface with the SiC developes by the growth of plane fragments in the diffusion flow of carbon, by adding atoms to the reactive edges of these planes, being precipitated in the €-Ni2Si, which might occur along the preferential directions of diffusion in the €-Ni2Si. Nanosystems: As the carbon diffusion flow is relatively weak and occurs within the reaction plane, the graphitic base planes can grow parallel to the SiC surface. The additionally observed specific orientation of the graphitic carbon parallel to the surfaces of the silicide particles is assumed to be mainly driven by the minimisation of the free surface and of the chemical potential of carbon, coupled with the graphitic ordering.

Acknowledgement

The authors thank the Deutsche Forschungsgemeinschaft for financial support in the framework of the Sonderforschungsbereich 418. 15

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