Calculated phonon band structure and density of states and interpretation of the Raman spectrum in rocksalt ScN

Abstract

As one of the simplest transition-metal nitrides, ScN is quite interesting. After this material was thought to be a semimetal for a long time,1 it was finally concluded to be a semiconductor with an indirect gap of about 1 eV and smallest direct gap of about 2 eV.2–7 It was recently found to be a potentially interesting magnetic semiconductor host by adding Mn as dopant.8,9 Its crystal growth in thin-film form was improved considerably in recent years.4,10–12 It was also predicted that alloys or superlattices with IIIa column elements Ga, Al, or In might have interesting properties involving a hexagonal form of ScN.13–15 Yet, several properties of this material are only poorly known. Here we study the vibrational properties. While calculated phonon band structures were reported previously for the hypothetical hexagonal form,14 we are only aware of one previous study of the phonons in rocksalt ScN.16 That paper does not discuss the comparison with the Raman spectra in any detail and furthermore uses a different pseudopotential from the one used here, which leads to significant differences. In the rocksalt structure, first-order Raman scattering is forbidden by symmetry. Yet, a Raman spectrum was observed and reported by Travaglini et al.1 and by Gu et al.12 It was interpreted as a disorder-induced first-order Raman spectrum representing the density of phonon states. In infrared absorption, one expects to see the TO and LO modes at the zone center because these are optically allowed, but the presence of free carriers complicates this picture. Travaglini et al.1 extracted TO to be 45 meV or 362 cm −1 from an analysis of the infrared reflectivity, which exhibits a plasmon LO-phonon-coupled mode because of the large free-carrier concentration in their samples. Here, we present phonon band structures and density of states DOS and compare carefully with the Raman data by Travaglini et al.1 We find that the latter shows not a pure but weighted density of states, with a larger intensity for the LO L modes. Several features can be accounted for with Van Hove singularities at the zone boundary. The emphasis of the LO L modes is explained in terms of their dominant role in the breathing-induced motions around the cation which exhibit the strongest electron-phonon coupling.