Synthesis and Characterization of A Metastable (SiC)aN 4 Phase
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C. Uslu, B. Park and D. B. Poker* School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 *Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT A metastable C-Si-N compound has been synthesized by high dose N' implantation into polycrystalline f3-SiC (cubic phase). The thin films formed upon 100 keV implantations were characterized with respect to various ion doses and target temperatures. X-ray diffraction with a position-sensitive detector and cross-sectional transmission electron microscopy revealed that the as-implanted surfaces contained -0.15 /m thick continuously-buried amorphous layers. Rutherford backscattering spectroscopy showed that the peak concentration of nitrogen saturated up to approximately 54 at. % with increasing doses, suggesting a new phase formation. INTRODUCTION The synthesis of (3-C 3N4 , the homologue of f3-Si 3N4 , has gained a lot of importance since it was theoretically predicted to have a bulk modulus rivaling that of diamond [1]. Derivation of a scaling relationship and first-principles total-energy calculations predicted the bulk modulus of j3-C 3N4 to be in the range of 427 to 483 GPa (443 GPa for diamond) with a large cohesive energy of 5.8 eV/atom. To date, several attempts employing various conventional and non-conventional techniques have been made [2-10]. However, these have resulted in the formation of either nonstoichiometric (nitrogen-deficient) compounds or unidentified crystalline compounds (based on diffraction studies). The formation of a carbon-silicon-nitrogen compound, namely #3-(SiC)3N4, has been investigated by ion-implantation process and preliminary results were reported previously [11]. This is a plausible approach since f3-Si 3N4 is a well-established equilibrium phase whilef3-C3N4 is still hypothetical, probably due to its extreme metastability. Our objective is to synthesize stoichiometric f3- or a-(SiC) 3N4 compounds by transforming the f3-SiC phase (carbon in sp3 bond configuration) with N' implantation. (The notations of f3-and a-(SiC)3N4 are the same as #3-and a-Si 3N4 phases, having 14 and 28 atoms per hexagonal unit cell, respectively [12].) With added nitrogen in SiC phase, silicon atoms may keep carbon-to-nitrogen bonds in sp 3 configurations, instead of sp2 hybridization. This effect may make f3- or c-(SiC) 3N4 closer to a thermodynamically stable state, rather than a metastable state of hypothetical f3- or 0-C3N4. The 3- or U-(SiC) 3N4 phases with 57.14 at. % N would arrange atoms as to form a network of CN 4 and SiN 4 tetrahedra linked at the corners by three fold coordination of N atoms. Nucleation and growth processes will depend on the structural defects induced by ion irradiation, and ion energy and dose rate with different substrate temperature may control the complex synthesis processes. 195
Mat. Res. Soc. Symp. Proc. Vol. 354 0 1995 Materials Research Society
EXPERIMENTAL PROCEDURE The silicon-carbide substrates were polycrystaffine (-5 Am grain size), chemical vapor depo
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