Polymorpism of Group-IV carbides: Structures, (meta)stability, vibrational and electronic properties

Motivated by the rich polymorphism of elemental carbon and SiC, herein we systematically investigate the space of crystal structures and their metastability for the entire set of group-IV mono-carbides (C, SiC, GeC, and SnC) using an ab-initio random structure sampling methodology. All known structures of elemental C and SiC are correctly identified as metastable polymorphs. Moreover, the only structures found for the studied carbides are the tetrahedrally bonded zinc-blende (s.g. no. 216), wurtzite (s.g. no. 186), and rhombohedral (s.g. no. 160) structures as well as the high-pressure, octahedrally coordinated rocksalt (s.g. no. 225) phase. Though SnC and GeC share the same family of polymorphs as SiC, the metastability of these structures is distinct. For GeC, the rocksalt structure, which is missing in the random sampling, is shown to be dynamically unstable. In SnC all four structures are found, but the phase separated solution is much lower in energy, consistent with the lack of experimental realization of SnC. However, wurtzite SnC is well lattice matched to InN suggesting epitaxial growth as a possible avenue to realize SnC and take advantage of its predicted exceptional electron mobility.