Direct energy gap in light rare earth nitrides: EuN and SmN

Full text: Due to their proximity in the periodic table, the light rare earth metals, samarium and europium, are of particular interest because of their antiparallel L and S moments according to Hund's rule. On the optical energy gaps, there are a few reports for SmN and EuN. The energy gap as obtained from X-ray absorption and emission (XAS/XES) Spectra of SmN thin films is 1.5 eV, noticeably larger than the 0.70 eV of bulk SmN crystals. The theoretically estimated direct gap at the X point is 0.81 eV and an indirect gap at Γ-X is 0.48 eV when T>TC. Similarly for EuN, experimental values of Eg = 0.76 eV and 0.90 eV are appreciably different from the theoretical value of 1.20 eV.The direct energy gap for SmN and EuN is determined for the first time by transmittance and reflectance spectra for polycrystalline thin films at room temperature. SmN shows a direct optical energy gap at 1.2±0.05 eV and for EuN the energy gap value is 0.90±0.05 eV. Both samples show large effects due to free carriers in the subgap region. The fundamental absorption edge for SmN coincides with that of DyN indicating that both materials have similar electronic structure. For the case of EuN, a QSGW band structure calculation engineered to the experimentally determined energy gap value predicts a semiconducting ground state for EuN.