Polytypes of SiC

When Acheson found the hexagonal crystals in the voids, he sent some to B.W. Frazier, a professor at Lehigh University. Professor Frazier found that although the crystals were all silicon carbide, they differed in their crystalline structure. He had discovered the polytypism of SiC [18]. Polytypism will be explained in Section 1.3.2. 

The surface morphology of grains has been studied by secondary electron microscopy (SEM) (Hoppe et al., 1995). Such studies have been especially useful for pristine SiC grains that have not been subjected to any chemical treatment (Bernatowicz et al., 2003). Finally, the transmission electron microscope (TEM) played an important role in the discovery of presolar SiC (Bernatowicz et al., 1987) and internal TiC and other subgrains in graphite (Bernatowicz et al., 1991). It has also been successfully applied to the study of diamonds (Daulton et al., 1996) and of polytypes of SiC (Daulton et al., 2002, 2003). 

Figure 9.14. A. C MAS NMR spectra of hexagonal SiC polytypes, from Hartman et al. (1987). The spectrum of the cubic (zincblende) structure was not detected by these authors under a wide range of conditions. B. Angultu" dependence of the C NMR lines of single-crystal 6H polytype of SiC. Curve (a) corresponds to the resonance at 21.9 ppm in this crystal, curve (b) corresponds to the resonance at 17.2 ppm and curve (c) to the resonance at 25.4 ppm. From Richardson etal. (1992). Both figures used by permission of the American Chemical Society.

The cubic polytype of SiC, namely 3C-SiC, is the only form that can be grown hetero-epitaxially on Si substrates. However, there exists a 20 % lattice mismatch between these two crystal systems and the hope was that growth on a porous buffer layer might provide a means to reduce the defect density. This section presents preliminary research performed with this goal in mind. 

Various physical properties of several of the polytypes of SiC have been listed together with the relevant experimental conditions such as temperature, lattice direction and carrier density. 

This Datareview outlines the data on optical absorption and refractive index in the various polytypes of SiC. The optical transitions give rise to the characteristic colour of each polytype. Values for both the ordinary and extraordinary refractive indices versus wavelength are given. 

Refractive indices for several polytypes of silicon carbide have been measured [8-12]. The 6H polytype of SiC has been measured in the most detail [8]. For the hexagonal form the c axis is assumed to be perpendicular to the surface. Thus, a normal incidence wave can be used to measure transmission and/or reflection. This normal incidence wave is often called the ordinary ray. A prism with its sides perpendicular to the c-axis is used to determine both n0 and ne in the normal way (n0 = ordinary ray, ne = extraordinary ray). A short summary of the data is presented here for additional information please refer to [8,9]. 

The technique of Raman scattering (RS) to study vibrational spectra in the numerous polytypes of SiC will be described. An explanation of the various notations used to describe the stacking sequences in these polytypes will then be given. Section C discusses the various optical phonons studied by RS and the concept of a common phonon spectrum for all polytypes will be introduced. Raman studies are also used to assess crystalline structure and quality of epitaxial layers of SiC on Si and SiC substrates. Section D outlines several other excitations of interest, e.g. polaritons, plasmons, and electronic RS, as well as impurity and defect recognition in irradiated and ion implanted material. 

ESR has been used to characterize a number of impurity-related and structural defects in several polytypes of SiC. Most centres observed in SiC can be described by a simple spin-Hamiltonian ... 

Clearly, the most important impurity in all polytypes of SiC is nitrogen, which appears to primarily substitute for carbon and act as a shallow donor. There is some dispute over... 

Optically-Detected Magnetic Resonance (ODMR) has provided magnetic and hyperfine parameters for excited states of defects in SiC. Since ODMR combines EPR with photoluminescence, it gives a link between the information provided by each experiment. In SiC, most of the publications describe donor- and acceptor-resonances detected on distant donor-acceptor-pair (DAP) recombination. As in the other experiments, interesting effects occur due to the different symmetries which arise from the polytypism of SiC. 

Both 6H and 4H polytypes of SiC doped with B and N exhibit signals which were attributed to B [5,6,8]. The g-anisotropy is smaller than those for Al and Ga which is consistent with the greater depth of the B centres. Detailed resonance parameters were not quoted but the authors stated that the spectra were distinct from the B-centres observed in EPR. It is fair to conclude that these B-centres are not effective-mass acceptors. 

Similar etch-stop principles can be applied to all polytypes of SiC which can be etched photoelectrochemically. Etching and etch stops have been demonstrated in 6H-SiC recently [17]. 

Figure C2.16.2. The sequence of atoms in the two polytypes of SiC, zincblende and wurtzite, along the c-direction. The zincblende lattice has perfect tetrahedral angles.

Another material that can be grown in either the wur-tzite or zinc blende forms is SiC. The bonding here is mainly covalent (-88%) since both Si and C are group IV elements. SiC is special in that it is very difficult to produce in a single structure. It always has the chemical composition SiC, but tends to be a mixture of the two stacking sequences. The two structures are two of the polytypes of SiC. The cubic form of SiC is being produced as a diamond simulant known as moissanite. 

Silicon carbide is known to crystallize in several crystallographic modifications, all having the same a parameters (3.078 A) but different c parameters, known as polytypes. The structural reason for this phenomenon is a low stacking fault energy and the possibility to form different modes of stacking of two-dimensional, structural, compatible units along a definite direction. Some 200 polytypes of SiC are known to exist, but the most common are 3C,... 

Figure 50. Imaging of the 1. ) R polytype of SiC along the closely packed rows of atoms...

Many Raman scattering Unes are observed for SiC, reflecting zone folding effects in phonon dispersion curves. These lines can be used to identify the polytype of SiC crystals, as mentioned in Sec. III.A. From the shift of the Raman peaks and the discrepancy of the selection rules in optical transitions, information about the internal stress and crystallinity of SiC crystals, respectively, can be obtained, as mentioned in Sec. ni.B. 

WJ Choyke. Optical properties of polytypes of SiC Interband absorption and luminescence of nitro-gen-exciton complexes. Mater Res Bull 4 5141, 1969. 

Figure 7.31 Shows the four most common polytypes of SiC viewed from a <1120> direction. The positions of atoms in the A, B, and C stacking positions are indicated by the dashed lines. Note that where the stacking faults are located depends upon whether you consider the basic structure to be cubic or hexagonal.

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