Tem Study of Crystallization of a-SiC in Contact With Silver
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ABSTRACT Metal mediated crystallization of an amorphous phase has been found in many systems, in which eutectic phase diagrams are formed. It is interesting to know if an amorphous phase containing two elements, which can form compound, can react and form a crystalline compound at a low temperature in contact with metals. In this paper we studied the Ag/Sil-xCx system by transmission electron microscopy(TEM). It was found that amorphous silicon carbide was crystallized into cubic SiC at about 8000C in areas containing Ag. The silver diffused and segregated into the a-SiC phase upon annealing. The silver grains in the original deposited layer as well as those segregated act as nucleation sites for the crystallization of 13-SiC. A nucleation temperature as low as 8000 C was observed.
I. INTRODUCTION Metals which form eutectic phase diagrams with elements of amorphous phases are found to mediate their crystallization at lowered temperatures. For example, Konno found that the crystallization temperature for a-Si is about 410°C while in contact with Ag[ I] and about 175 0 C while in contact with Al[2], which are much lower than the usual crystallization temperature of aSi, about 600-7000C. This phenomenon was also found in the Co-C system[3]. It would be interesting to know if metals can mediate the crystallization and/or compound formation of a twoelement amorphous phase. Ag and Sil-xCx are chosen in this paper because silver does not form a compound with either Si or C, and they are supposed to form a pseudo-binary phase diagram. Besides, the Ag mediated crystallization of a-Si has been well studied before[I-2]. There is also technological importance of silicon carbide for its wide application in structure components and electronic devices where interface stability, interface strength and a-SiC stability should be known[4]. Moreover, the crystallization of pure a-SiC has been somewhat studied for a long time and can be used as a comparison. It is found that amorphous SiC will crystallize at about 900-1000 0 C[5-6], which strongly depends on deposition methods and conditions, and the carbon content in the film as well. Though SiC has many different crystal structures, cubic SiC(13-SiC) was crystallized in these films. The direct reaction of Si and graphite to form cubic SiC was also observed in-situ at 1400°C[7]. In the Ag/Si case, the Ag grains act as nucleation sites though the crystallization is due to the diffusion of Si atoms from a-Si to c-Si phase through the Ag grains[ 1]. Silver and graphite form a peritectic phase diagram with about 0.036 at.% solubility of carbon in silver at 9620 C[8]. The crystallization of a-C was not observed up to 950 0 C while in contact with silver, but there is evidence of Ag diffusion through a-C films[9]. Therefore the mechanism of crystallization, if there is any, in the Ag/SiC system may be quite complex.
39 Mat. Res. Soc. Symp. Proc. Vol. 382 © 1995 Materials Research Society
II. EXPERIMENTAL Ag/Sil-xCx films were deposited onto Si(001) substrates(with native oxide) by using
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