Raman Microscopy for Characterizing Defects in SiC

Silicon carbide (SiC) is a wide bandgap semiconductor that is being developed for use in high-power integrated circuits because its large electronic indirect bandgap enables it to carry high currents without overheating. SiC is actually an interesting material. Its cubic phase has the same tetrahedral structure as silicon (Si), diamond, and germanium (Ge), with the important difference being that the two atoms in the unit cell are different. However, of greater interest is that there are many related polymorphs in which differences in the stacking order of the cubic (111) planes of SiC bilayers (that can be visualized as rotations around the [111] axis of the cubic cell), result in hexagonal and rhombohedral phases—many of them depending on the periodicity of the repeating planes. The reason that so many phases are possible is that the nearest neighbors are always the same, which means that the nearest neighbor interactions are almost the same in all structures. This is academically quite interesting, but it also has important implications in growing single crystals or films on which devices can be built—there can be local islands or defects that would interfere with a device’s performance, in part because the bandgaps of different polymorphs are different, but also because defects will interfere with electron flow. Because there is a different Raman signature for each of the polymorphs as well as the contaminants, Raman microscopy is an ideal tool for analyzing the structure of these materials as well as identifying possible contaminants that would also interfere with performance. In addition, there are characteristics in the Raman signature that reveal doping levels when n > 2x1016, thus providing an additional source of information for qualifying materials for integrated circuits.