Epitaxial nitride rocksalt metal/semiconductor superlattices are emerging as a novel class of artificially structured materials that have generated significant interest in recent years for their potential application in plasmonic and thermoelectric devices. Though most nitride metals are rocksalt, nitride semiconductors in general have hexagonal crystal structure. We report rocksalt aluminum scandium nitride (Al,Sc)N alloys as the semiconducting component in epitaxial rocksalt metal/semiconductor superlattices. The Al x Sc 1Àx N alloys when deposited directly on MgO substrates are stabilized in a homogeneous rocksalt (single) phase when x < 0.51. Employing 20 nm TiN as a seed layer on MgO substrates, the homogeneity range for stabilizing the rocksalt phase has been extended to x < 0.82 for a 120 nm film. The rocksalt Al x Sc 1Àx N alloys show moderate direct bandgap bowing with a bowing parameter, B ¼ 1.41 AE 0.19 eV. The direct bandgap of metastable rocksalt-AlN is extrapolated to be 4.70 AE 0.20 eV. The tunable lattice parameter, bandgap, dielectric permittivity, and electronic properties of rocksalt Al x Sc 1Àx N alloys enable high quality epitaxial rocksalt metal/Al x Sc 1Àx N superlattices with a wide range of accessible metamaterials properties. [3,4], and as a component in metal/dielectric superlattices that are currently under investigation as metamaterials for thermionic and plasmonic devices [5][6][7][8]. Although comprehensive bulk data are lacking, studies of thin films indicate that (Al x Sc 1Àx N) exists as an equilibrium phase at standard temperature and pressure with the rocksalt crystal structure for compositions rich in ScN. For compositions close to AlN, the stable phase adopts the wurtzite crystal structure. A two-phase region separates these two single-phase regions of the pseudobinary diagram. In thin-film form, Al x Sc 1Àx N is particularly attractive as a potential functional material, as it allows bandgap engineering in nitride semiconductors starting from