Many compound materials exhibit polymorphism, that is they can exist in different structures called polymorphs. Silicon carbide (SiC) is unique in this regard as more than 250 polymorphs of silicon carbide had been identified by 2006,with some of them having a lattice constant as long as 301.5 nm, about one thousand times the usual SiC lattice spacings.
The polymorphs of SiC include various amorphous phases observed in thin films and fibers,as well as a large family of similar crystalline structures called polytypes
Regions of the wafer crystallography which are polycrystalline or of a different polytype material than the remainder of the wafer, such as 6H mixed in with a 4H type substrate. Foreign polytype regions frequently exhibit color changes or distinct boundary lines, and are judged in terms of area percent under diffuse illumination.