Radial distribution functions of amorphous silicon carbide
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Radial distribution functions of amorphous silicon carbide Manabu Ishimaru1, In-Tae Bae2, and Yoshihiko Hirotsu1 1 The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan 2 Department of Materials Science and Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT Atomistic structures of amorphous silicon carbide generated by energetic particles were examined by electron diffraction techniques in combination with imaging plates. Atomic pair distribution functions revealed that not only heteronuclear (silicon-carbon) bonds but also homonuclear (silicon-silicon and carbon-carbon) bonds exist in the first coordination shell of amorphous silicon carbide induced by ion- or electron-beam-irradiation. Structural changes from amorphous silicon carbide to amorphous silicon were observed under the electron irradiation.
INTRODUCTION Although extensive studies have been carried out on structural analyses of amorphous silicon carbide (a-SiC), the fundamental nature of short-range order in a-SiC remains a topic of debate. Based on results obtained experimentally and theoretically, two atomistic structural models have been proposed for a-SiC [1-12]. Some researchers characterized a-SiC networks as highly chemically ordered, consisting of heteronuclear (silicon-carbon) first nearest neighbor bonds [1-6]. a-SiC basically consists of the same tetrahedral coordinated units as crystalline SiC, wherein a silicon atom is surrounded approximately by four carbon atoms and vice versa. On the other hand, there is evidence for the formation of homonuclear bonds (silicon-silicon and carbon-carbon bonds) in a-SiC [7-12]. In the present study, we investigated atomistic structures of a-SiC generated by energetic particles using transmission electron microscopy (TEM).
EXPERIMENTAL Single crystal wafers of (0001)-oriented 6H-SiC (obtained from Cree, Inc., 4600 Silicon Drive, Durham, NC 27703) were irradiated with ions and electrons at room temperature. The details of the irradiation conditions are described below. These wafers were fabricated into samples appropriate for TEM studies. Irradiation-induced microstructures were observed using JEOL JEM-2010 and JEM-3000F TEM with incident energies of 200 and 300 keV, respectively.
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Atomistic structures of irradiation-induced amorphous layers were examined through atomic pair distribution functions by electron diffraction techniques. Halo patterns were recorded on an imaging plate (Eu2+-doped BaFBr). This has a higher sensitivity, wider dynamic range, and better linearity for electron-beam intensities compared with the commercial TEM film material [13]. The intensities of the halo pattern were analyzed quantitatively in an imaging plate processor, specifically a Digital Micro-Luminography FDL 5000 system (Fuji Film). The details of the electron diffraction analysis are described elsewhere [14,15].
RESULTS AND DISCUSSION Ion-beam-induced a-SiC An a-SiC layer, ~200 nm thick, on the surface of the SiC wafers
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