  Inorganic nitrides with wurtzite crystal structures are well-known
semiconductors used in optoelectronic devices. In contrast, rocksalt-based
nitrides are known for their metallic and refractory properties. Breaking this
dichotomy, here we report on ternary nitride semiconductors with rocksalt
crystal structures, remarkable optoelectronic properties, and the general
chemical formula Mg$_{x}$TM$_{1-x}$N (TM=Ti, Zr, Hf, Nb). These compounds form
over a broad metal composition range and our experiments show that Mg-rich
compositions are nondegenerate semiconductors with visible-range optical
absorption onsets (1.8-2.1 eV). Lattice parameters are compatible with growth
on a variety of substrates, and epitaxially grown MgZrN$_{2}$ exhibits
remarkable electron mobilities approaching 100 cm$^{2}$V$^{-1}$s$^{-1}$. Ab
initio calculations reveal that these compounds have disorder-tunable optical
properties, large dielectric constants and low carrier effective masses that
are insensitive to disorder. Overall, these experimental and theoretical
results highlight Mg$_{G-3}$TMN$_{G-2}$ rocksalts as a new class of
semiconductor materials with promising properties for optoelectronic
applications.
