Composition of SiCN crystals consisting of a predominantly carbon-nitride network
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Composition of SiCN crystals consisting of a predominantly carbon-nitride network D. M. Bhusari Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
C. K. Chen Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
K. H. Chen and T. J. Chuang Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
L. C. Chen Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
M. C. Lin Department of Chemistry, Emory University, Atlanta, Georgia 30322 (Received 18 June 1996; accepted August 29, 1996)
We report here on the synthesis of large crystals of Si-containing carbon nitride, consisting of a predominantly C–N network, by microwave CVD. The Si content in this material varies from crystal to crystal and also with the deposition conditions and has been observed to be as low as less than 5 at. % in some crystals, wherein the Si atoms are believed to substitute for some of the C sites only. This is the first time that such large and well-faceted crystals consisting almost entirely of carbon-nitride network have been synthesized. Moreover, there is no obvious deposition of amorphous CN material.
In the last few years, there has been a considerable surge in the research activity aimed at experimental realization of covalent C–N solid preferably in the hypothetical b –C3 N4 structure, following the prediction by Liu and Cohen1 that such material may be harder than diamond. Since diamond is the hardest known material, a superior diamond would be tantamount to material revolution. However, it must be noted that although initial calculations have been performed only for b –C3 N4 , it may not be the only stable structure for covalent C–N crystalline solid. Indeed more recent theoretical studies2,3 have shown that the C3 N4 materials having defect zinc blend or rhombohedral graphite-like structure should have stabilities comparable to or greater than the b structure. Very recent calculations4 have also indicated that a –C3 N4 would be more stable than b –C3 N4 , and its bulk modulus has been estimated to be 1890 Kb. In fact, any C–N covalent solid comprising of tetrahedrally bonded carbon atoms is likely to be very hard due to the small atomic sizes of carbon and nitrogen, low ionicity of the C–N bond, and sufficiently high coordination number. Thus, mere synthesis of crystals of covalently bonded carbon and nitrogen, regardless of its phase, is likely to be an important achievement from the technological perspective. Several groups have, since the prediction by Liu and Cohen,1 attempted to synthesize b –C3 N4 in lab by employing a variety of thin film deposition techniques. Particularly, ion beam deposition, laser ablation of graphite 322
J. Mater. Res., Vol. 12, No. 2, Feb 1997
in nitrogen ambient, and reactive sputtering of graphite have received more attention. Although a C–N alloy could be realized by these techniques, the resulting material was invariably amorphous with the nitrogen co
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