Paper EL-ThP2
Phonon Dispersion and Electronic Band Structure of NiO

Thursday, October 22, 2015, 6:00 pm, Room Hall 3

The phonon dispersion and the electronic band structure of bulk NiO were investigated at 300 K using spectroscopic ellipsometry from the mid-infrared (0.03 Ev) to the near-ultraviolet (6.5 Ev). Since NiO crystallizes in the rocksalt structure, we find a single transverse-optical (TO) phonon at 49 meV with a broadening of 2 meV. There is no evidence of mode-splitting (due to antiferromagnetic ordering, within the broadening of this phonon of 2 meV) or zone folding (due to the doubling of the unit cell). Instead, we find a modification of the reststrahlen band due to TA+TO two-phonon absorption occurring between the TO and LO phonon energies, similar to LiF. Using transmission and ellipsometry measurements, we clearly establish that the lowest direct band gap of NiO occurs at 0.85 Ev at room temperature. The valence band maximum (VBM) of NiO is made up of O (2p) states and the lower Hubbard band of the Ni (3d) states. Direct interband transitions at 0.85 Ev are possible from the valence band to the dispersive Ni (4s) conduction band at the Γ point. A strong peak in the absorption at 3.95 Ev is attributed to transitions from the valence band to the upper Hubbard band (the charge transfer gap of NiO). A derivative analysis of the ellipsometry spectra also shows weak peaks at intermediate energies (between 1.7 and 3.6 Ev), which are attributed to transitions from localized valence band states to the dispersive Ni (4s) band. The charge-transfer gap of NiO shows a temperature dependence very similar to the E₁ gap of Si. The absorption between 1 and 3 Ev has a very different behavior, however. The character of the pseudo-dielectric function of NiO changes completely between 700 and 800 K in UHV, where NiO is known to deteriorate due to sublimation. Cooling the sample does not restore the original shape of the dielectric function, but it can be recovered partially by annealing in oxygen.