Optical Properties of Rare Earth Nitrides

Abstract

<p>This experimental thesis uncovers the fundamental optical features of rare earth nitride compounds and relates them to their electronic structure. Experimental observations for the optical energy gaps for thin films of GdN, DyN, SmN and EuN are made for the first time. Thin films are grown by thermal evaporation in ultra high vacuum environment and are passivated by MgF₂ layers. Initial characterizations indicate the polycrystalline thin films of RENs are strongly textured along [111] direction. Optical characterization techniques, Fourier transform infrared and conventional UV/Vis spectrometers are used in conjunction with SQUID magnetometer and DC electrical resistivity. Transmission and reflection spectra for rare earth nitride thin films were obtained in the photon energy range 0.5 – 5.5 eV in their paramagnetic and ferromagnetic phases. Paramagnetic GdN has a direct energy gap of 1.30±0.05 eV which coincides well with theoretically predicted energy gap. A red-shift in the fundamental absorption edge of ferromagnetic GdN is observed along with onset of absorption at higher energy attributable to the exchange splitting of conduction and valence bands of GdN. The spin split joint density of states is in remarkable agreement with theoretically calculated spin polarized band structure of GdN. Similarly for DyN a consensus is found between theory and experiment on the energy gap of 1.20±0.05 eV at room temperature. However, in the case of SmN, an energy gap of 1.30±0.1 eV is underestimated by theory to 0.81 eV. For EuN, the experimentally determined value of energy gap is 0.97±0.05 eV. This value is used to tune the band structure calculation by QSGW theory which returns a ferromagnetic semiconducting solution for EuN.</p>