Electronic structure and optical spectra of the semimetal ScAs and of the indirect-band-gap semiconductors ScN and GdN

Local (spin) density functional calculations for ScN and GdN are complemented with estimated quasiparticle corrections and calculations of the optical response to evaluate whether these materials are semimetals as suggested by some transport measurements or semiconductors as suggested by optical measurements. The quasiparticle corrections are estimated by assuming that gap corrections are inversely proportional to the dielectric constant and using experimentally known results on the quasiparticle d-band shift in ErxSc1-xAs. Results for the optical response functions and band structures are presented for ScAs, ScN, and GdN. The conclusion is that whereas ScAs is a semimetal, ScN and GdN are both narrow gap (0.9 and 0.7-0.85 eV, respectively) indirect gap (T-X) semiconductors, with first direct gap at X at 2.0 and 1.1-1.2 eV, respectively. Due to the strong exchange interaction of 4f electrons with the d bands, GdN is predicted to have a magnetic-field-induced redshift of both the indirect and direct absorption edges of about 0.3 eV.