Structure and composition studies of chemical vapour-deposited BCN fibre coatings

Dagmar Dietrich

Structure and composition studies of chemical vapour-deposited BCN fibre coatings

Dagmar Dietrich

2002, Analytical and Bioanalytical Chemistry

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The composition and structure of boron carbonitride (BCN) films were studied. The films were continuously deposited on fibres by atmospheric pressure CVD. The precursors were ammonia, trimethyl borate and toluene. The composition was determined by photoelectron spectra of boron 1s, nitrogen 1s, carbon 1s and oxygen 1s. By fixing the C 1s peak at 285 eV, the position of the B 1s peak and the N 1s peak in the BCN films was equal to BN films. The C content of the films increases from about 6 at% to 60 at%, leaving the stoichiometric boron/nitrogen ratio as well as the oxygen content below 10 at% unchanged. Generally, the carbon content in the films is lower than predicted by the precursor ratios. Obviously, the insertion of carbon into the film is decreased in the presence of ammonia, which is known to etch carbon. With a decreasing ammonia/toluene ratio, the undesired effect in the reaction is suppressed and the carbon deposition becomes considerable. Transmission electron microscopic studies were performed on cross-sections of the coated fibres. High-resolution images generally show a hexagonal turbostratic structure with different orientation preferences of the atomic lamellae similar to hexagonal turbostratic boron nitride and pyrolytic carbon. When a noticeable carbon concentration (20 at%) is reached, the atomic sheets become uniformly distributed in all directions in space.

Anal Bioanal Chem (2002) 374 : 712–714 DOI 10.1007/s00216-002-1452-2 S P E C I A L I S S U E PA P E R Dagmar Dietrich · Ulrich Roll · Sabine Stöckel · Kathrin Weise · Günter Marx Structure and composition studies of chemical vapour-deposited BCN fibre coatings Received: 23 February 2002 / Revised: 14 June 2002 / Accepted: 25 June 2002 / Published online: 2 August 2002 © Springer-Verlag 2002 Abstract The composition and structure of boron carbonitride (BCN) films were studied. The films were continuously deposited on fibres by atmospheric pressure CVD. The precursors were ammonia, trimethyl borate and toluene. The composition was determined by photoelectron spectra of boron 1s, nitrogen 1s, carbon 1s and oxygen 1s. By fixing the C 1s peak at 285 eV, the position of the B 1s peak and the N 1s peak in the BCN films was equal to BN films. The C content of the films increases from about 6 at% to 60 at%, leaving the stoichiometric boron/nitrogen ratio as well as the oxygen content below 10 at% unchanged. Generally, the carbon content in the films is lower than predicted by the precursor ratios. Obviously, the insertion of carbon into the film is decreased in the presence of ammonia, which is known to etch carbon. With a decreasing ammonia/toluene ratio, the undesired effect in the reaction is suppressed and the carbon deposition becomes considerable. Transmission electron microscopic studies were performed on cross-sections of the coated fibres. High-resolution images generally show a hexagonal turbostratic structure with different orientation preferences of the atomic lamellae similar to hexagonal turbostratic boron nitride and pyrolytic carbon. When a noticeable carbon concentration (20 at%) is reached, the atomic sheets become uniformly distributed in all directions in space. Keywords XPS · TEM · CVD fibre coatings · BCN films D. Dietrich (✉) · S. Stöckel · K. Weise · G. Marx Technische Universität Chemnitz, Physikalische Chemie, 09107 Chemnitz, Germany e-mail: [email protected] U. Roll Physical Electronics GmbH, Fraunhoferstr. 4, 85737 Ismaning, Germany Introduction Ternary compounds of the B/C/N system are non-oxidic ceramics. As advanced materials in material science, they are expected to exhibit outstanding properties as reaction barriers and low-friction coatings with improved oxidation resistance as well as high-temperature semiconductors and optoelectronic materials. Based on similar atomic radii and a similar bonding structure in the hexagonal network, the B/C/N system can form two-phase compounds or single-phase solid solutions. The semi-metallic property of graphite is due to layered C sheets of ABAB stacking sequence with a band overlap of the bonding and antibonding π-electrons. In insulating hexagonal boron nitride, the layered BN sheets, with atoms of one kind over the other, result in stronger localized electrons at N atoms that form the band gap and cause a better oxidation resistance. Hence, the electronic and chemical properties of the ternary compounds depend on the composition just as the atomic configuration in the layers [1, 2, 3]. The aim of this work was the investigation of composition and structure of BCN films, which were CVD-processed via a boron-organic precursor, ammonia and toluene. Experimental Boron carbonitride (BCN) films of about 200 nm were deposited by atmospheric pressure CVD onto multifilament-rovings in a continuous process. Carbon fibres (Toray T800) and alumina fibres (Nextel 440) were coated. Typical deposition parameters were an isothermal deposition temperature of 1100 °C, a roving velocity of 33 cm min–1 and a summary gas flow rate up to 2 L min–1. The precursors were ammonia and trimethyl borate commonly used for CVD BN fibre coatings [4] and toluene used for pyrolytic carbon coatings [5]. The precursor ratios in the reactor atmosphere were adjusted under the assumption of unchanged yields for pure carbon or boron nitride deposition. For composition and chemical state studies by photoelectron spectroscopic investigations (XPS), fibre bundles were attached to the specimen mount. The measurements were performed on a PHI Quantum 2000 Scanning ESCA Microprobe with a spherical capacitor energy analyser. The excitation source was a focused monochromated X-ray beam (Al Kα) with 100 µm diameter and 713 Table 1 Typical peak positions and standard relative sensitivity factors (RSF) Line B 1s C 1s N 1s O 1s Position (eV) RSF 190.9 0.171 285.0 0.314 398.4 0.499 532.6 0.733 24.2 W. The 1s photoelectron peaks of boron, nitrogen, carbon and oxygen were recorded with a pass energy of 29.35 eV and with a step size of 0.125 eV. Despite neutralising electrons with 1.9 eV (10 µA), a slight charging of the samples was observed; thus, the C 1s peak was fixed at 285 eV for charge correction. Using the database of Wagner [6] (Table 1), corrected for the analyser transmission function and the analyzer/X-ray source angle, the chemical composition of the films was determined. For transmission electron microscopy (TEM), a 200 kV Hitachi H-8100 was used. Cross-sections of separated fibres were embedded in epoxy (GATAN G1) between silicon slices, grinded on 3 M diamond foils to 10 µm thickness and polished with 2 keV Ar ions (GATAN PIPS) to electron transparency [7]. Fig. 1 Chemical composition of BCN films by XPS measurements: ■ expected carbon, ✶ boron, ▲ nitrogen, ✕ carbon, ● oxygen Results and discussion The positions of the B 1s and N 1s peaks in the studied BCN films are equal to the peak positions in BN films. By fixing the C 1s peak at 285 eV for charge correction, no remarkable chemical shift was detected dependent on the film composition. A slightly varying oxygen content, which does not exceed 10 at%, was expected in consideration of former investigations. The oxygen seems to be bonded to boron atoms on border positions of the atomic lamellae, which exhibit dimensions of about 10 nm. Therefore, the number of border atoms is large with respect to the total number of atoms forming the lamellae [8]. The N 1s peak is reported to split for N-C bonds to 400 eV and for N-B bonds to 396 eV [3]. For our BCN films, the shape of all B 1s, C 1s and N 1s peaks remains fairly narrow and nearly symmetrical despite the increasing carbon concentration. This may be due to the need for surface cleaning by argon ions. Therefore, the bonding structure in the hexagonal network, a two-phase compound and/or a single-phase solid solution cannot be derived. The carbon content of the films increases from 6 at% to 60 at% leaving the stoichiometric boron/nitrogen ratio unchanged (Fig. 1). Generally, the carbon content in the films is lower than predicted by the precursor ratios. Obviously, the insertion of carbon into the film is decreased in the presence of ammonia in the reaction atmosphere. Ammonia is used in excess to sustain hydrolytically stable BN films; otherwise it is known to form volatile hydrocarbons in the reactor gas phase and beyond it to etch carbon fibres and growing carbon films during deposition. With decreasing ammonia/toluene ratio, the undesired effect in the reaction is suppressed and the carbon deposition becomes considerable. TEM images with moderate magnification of crosssections show evidence of increasing smoothness of the film surface with increasing carbon concentration, which Fig. 2 Pyrolytic carbon film on a carbon fibre (marker: 10 and 50 nm) Fig. 3 Hexagonal turbostratic boron nitride film on an alumina fibre (marker: 10 and 50 nm) 714 Fig. 4 BCN film – 10 at% carbon (marker: 10 and 50 nm) and is known for hexagonal BN films on different substrate materials, that is carbon fibres, ceramic fibres, silicon wafers [9] and different deposition techniques [10]. In the case of a small carbon contents below 10 at%, the BCN films under investigation show a similar behaviour to turbostratic BN films. The atomic sheets are soft bended and extended to about 15 nm. The lamellae stacks, which are directed towards the film surface, form bent needles and the surface appears rough on the nm scale. With increasing carbon concentration, the atomic sheets orient uniformly in all room directions (Fig. 4) and become smaller and more turbostratic (Fig. 5), which seems to be a direct consequence of the carbon incorporation into the film. We suppose from this modified growth of the atomic sheets, that carbon atoms were at least partially incorporated into the lamellae. Otherwise, carbon impurities in hexagonal BN films are known to precipitate in small clusters with amorphous structure [11]. We observed by HRTEM amorphous clusters in the turbostratic matrix of samples with 20 at% carbon. Therefore, the BCN films under investigation may be a single-phase solid solution as well as a two-phase compound. In future, energy-filtered transmission electron microscopic investigations (EFTEM), especially the analysis of the electron energy-loss near edge structure (ELNES) of the individual elements, will be performed to exhibit in which way the boron, nitrogen and in particular the carbon atoms are arranged in the BCN layers. References Fig. 5 BCN film – 60 at% carbon (marker: 10 and 50 nm) was confirmed by means of high-resolution images. Generally, BCN films have a hexagonal turbostratic structure with a typical spacing of about 0.35 nm, but a different orientation preference of the atomic lamellae. The C atom sheets in pyrolytic carbon films are known to be generally aligned parallel to the substrate surface (Fig. 2). In contrast, the atomic sheets in hexagonal boron nitride films are commonly oriented nearly perpendicular to the surface when the film compensates internal stress (Fig. 3). This feature is due to the anisotropic Young’s modulus 1. Saugnac F, Teyssandier F, Marchand A (1992) J Am Ceram Soc 75:161 2. Kawaguchi M (1997) Adv Materials 9:615 3. Hegemann D, Riedel R, Dreßler W, Oehr C, Schindler B, Brunner H (1997) Chem Vap Deposition 3:257 4. Weise K, Marx G, Stoeckel S (2001) DE 199 31 257 A1, 11.01.2001 5. Nestler K, Martin PW, Dietrich D, Preidel A, Marx G, Zesch U, Wielage U (1999) In: Dimigen H, Paatsch W (eds) Werkstoffwoche 98, Band IX, Symposium 11 Oberflächentechnik, 55. Wiley-VCH 6. Wagner CD, Davis LE, Zeller MV, Taylor JA, Raymond RH, Gale LH (1981) Surf Interface Anal 3:211 7. Dietrich D, Marx G (2000) Microsc Anal Sept:7 8. Dietrich D, Stöckel S, Marx G (1998) Fresenius J Anal Chem (1998) 361:653 9. Dietrich D, Stöckel S, Weise K, Nestler K, Marx G (1999) Fresenius J Anal Chem (1999) 365:255 10. Röder M, Hahn J, Falke U, Schulze S, Richter F, Hietschold M (1997) Microchim Acta 125:283 11. Pippel E, Woltersdorf J, Dietrich D, Stöckel S, Weise K, Marx G (2000) J Eur Ceram Soc 20:1837

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