Effect of Ge on SiC Film Morphology in SiC/Si Films Grown by MOCVD
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It is believed that the Si is first absorbed on the surface, and then it migrates to the proper site
and is subsequently carbonized on either terraces or at step edges. 2 In an attempt to improve the quality of SiC films grown on Si at 10000 C, we added Ge in the form of GeH 2 during growth. We find that adding Ge promotes two-dimensional growth, improves film quality, and at high concentrations may induce a cubic-to-hexagonal polytype transformation. EXPERIMENT The SiC/Si films were grown in a commercial vertical rotating disk CVD reactor. All of the samples were grown with a constant Si/C ratio under identical conditions with the exception of the Ge concentration. Sill3, C3H8 , GeH 2, and H2 were reacted at 1000°C on Si (111) substrates. After growth the samples were characterized ex-situ by AFM, X-ray, RBS, and high resolution transmission electron microscopy (HRTEM). The TEM samples were prepared using tripod polishing and ion milling, and were observed in a JEOL 4000 FX operated at 300 KV. RESULTS Transmission electron microscopy was used to obtain high resolution lattice images and diffraction patterns from each sample. The first sample was grown with 0 sccm Ge, but due to 185 Mat. Res. Soc. Symp. Proc. Vol. 572 c 1999 Materials Research Society
the presence of GeH 2 in the CVD reactor, it was unintentionally doped with a trace amount of Ge. Figure 1 is the HRTEM image and diffraction pattern from this sample. The distinct spots in the (1To) diffraction pattern correspond to the Si substrate. The very light, diffuse ring seen in the diffraction pattern corresponds to the SiC film and indicates that the film is primarily amorphous. The elongated spots over the diffuse ring, however, indicate that some crystallites are formed and they have a preferred orientation with respect to the substrate. The SiC/Si interface in the lattice image is visible and denoted by the arrowheads. The amorphous-like contrast of the film in Fig. 1 could not be the result of ion milling since SiC is much harder than Si and the Si lattice in the image is intact.
Fig. 1 HRTEM image and diffraction pattern from the sample grown with a trace amount of Ge. Figure 2 shows that the crystallite size increases and the preferred orientation in the film improves with increasing GeH 2 flow. The lattice image of the film with 10 sccm Ge in Fig 2. shows regions with lattice fringes for SiC on a background of amorphous-like SiC. The elongated spots and rings in the (112) diffraction pattern shown as inset to Fig. 2 are less diffuse and sharper than the spots in Fig. 1. This result indicates that the film is polycrystalline 3C SiC with some preferred orientation of the crystallites with respect to the substrate. The SiC/Si interface in the lattice image is very abrupt and obvious. The TEM lattice image of the sample grown with 15 sccm Ge showed slightly improved crystallinity, but otherwise was not appreciably different from the sample with 10 sccm.
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Fig. 2 HRTEM image from the sample grown with 10 sccm Ge. The diffraction pattern indexes de
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