Polytypes common

The bandgap varies depending on the polytype between 2.39 eV for 3C-SiC to 3.33 eV for 2H-SiC [2]. The most commonly nsed polytype is 4H-SiC, which has a band-gap of 3.265 eV [2]. The wide bandgap makes it possible to use SiC for very high-temperature operation. Thermal ionization of electrons from the valence band to the conduction band, which is the primary limitation of Si-based devices dnring high-temperature operation, is not a problem for SiC-based devices because of this wide bandgap. 

Figure 1.6 The three most common polytypes in SiC viewed in the [1120] plane. From left to right, 4H-SiC, 6H-SiC, and 3C-SiC k and h denote crystal symmetry points that are cubic and hexagonal, respectively.

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

Ca2Fe205 and oxides of the CaMn03 v family are good examples of such vacancy-ordered structures. Complex intergrowth phases (ordered as well as disordered) involving brownmillerite and other related phases are commonly found in some of the anion-deficient oxides. Some of them also show polytypism due to different modes of stacking of the hexagonal and cubic layers. 

Layer thicknesses are similar to those of the AFm phases, and it would be difficult to distinguish the two groups of phases by XRD if only the basal reflections could be observed. Polytypism is common for example, sjogrenite, with a two-layer structure, has a three-layer polytype, called pyroaurite. Three-layer structures are the ones normally formed at room temperature. Meixnerite, which is a three-layer form, has a = 0.30463 nm, c = 2.293 (3 X 0.764)nm, space group R3m, Z = 3, D = 1950kgm (K30). Thermal dehydration (B86) and IR spectroscopic (FI35) studies on Mg-Al phases of this group have been reported. 

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